tive in cell killing; protection includes maneuvers to lessen normal tissue injury from the cytotoxic therapies. While some aspects of biochemical modulation fall within this field, for example, modification of intracellular thiols, other aspects, such as the use of one drug to alter the metabolism of another, are not included. ‘ezThis article does not include the discussion of combined modality therapy using standard cytotoxic chemotherapy plus radiation therapy. However, the distinction between a drug being included as a chemical modifiers vs. being included in a combined modality therapy approach can be blurred since certain chemotherapeutic agents, for example cisplatin, may be used in schedules that minimize their cytotoxic effects and maximize their radiation sensitizing properties. This article updates and reiterates information contained in recent overviews of this subject 3-6and includes information from the most recent international Chemical Modifiers meeting in April, 1988. While the basic science aspects of this field remain to be fully understood, there are a wide range of clinical investigations evaluating this concept. Table 1 lists the currently active clinical approaches which are discussed along with their scientific rationale.

Radiation and Chemotherapy Sensitizers and Protectors C. Norman Coleman and Andrew T. Turrisi

ABSTRACT Radiosensitizers and radioprotectors are part of the chemical modifier approach to cancer therapy whereby the state of the tumor cells and/or normal tissues are modified such that a therapeutic gain is achieved using conventional radiation or chemotherapy. Radiosensitization can be achieved by the use of oxygen-mimetic compounds, agents that alter DNA sensitivity to irradiation, maneuvers that alter DNA repair processes, and manipulation of tissue oxygenation. Standard chemotherapeutic agents such as cisplatin can be utilized in a manner that optimizes the radiosensitization properties. Protection and sensitization can occur by altering the thiol status of the cell. The chemical modifiers field is both developing novel approaches to cancer treatment and increasing the understanding of basic cancer biology. I.

II. RADIATION LESION; IMPORTANCE OF OXYGEN AND THIOLS

INTRODUCTION

A. Radiation Lesion Therapeutic irradiation ejects electrons from the target tissues. Subsequently, many very short-lived free radicals are formed. These radicals are formed in water, DNA, as well as other cellular molecules. Damage in the DNA is called the direct effect while that in surrounding water which is transferable to the DNA is the indirect effect.’ It is generally accepted that the important lesions for cytotoxicity are double strand breaks in DNA.B The ionization itself can produce a double strand break, or the break may be formed, in part, enzymatically. Ward has referred to the critical injury sites in DNA as locally-multiply damaged sites (LMDS).’ Given a sufficient number of the critical lesions, cell death will occur.

Chemical modification of cancer treatment is a recent concept in oncology that encompasses the spectrum from basic cancer biology to clinical practice. The novelty of this concept is that the treatments or maneuvers used are not designed to be therapeutic by themselves but rather to modify the normal tissue and tumor responses to standard cytotoxic therapies. The ultimate utility of these approaches will depend on how they affect the therapeutic index, the ratio of efficacy to toxicity. For a modifier to be of benefit it must enhance curability more than toxicity. In clinical trials, the therapeutic ratio is defined by various end-points. Efficacy can be assessed in terms of improved local control, prolonged median disease-free survival or overall survival, an increase in the percentage of patients ultimately cured as assessed by the plateau of the survival curve, or an increase in quality of life without affecting survival parameters. Toxicity assessment can include acute, reversible treatment toxicity, late organ dysfunction, treatment-related mortality, and also increases in time and resources spent undergoing treatment. Some severe late toxicity may be tolerable or surgically remediable, thereby mitigating its clinical impact. Therefore, the determination of a therapeutic gain is made based on the efficacy and toxicity results of clinical trials and on the patients’ and physicians’ willingness to accept certain toxicity as a trade-off for potential gain in treatment outcome. In this article chemical modification includes modifiers of radiation and chemotherapy damage. Sensitization includes approaches to make the radiation and chemotherapy more effec-

B. Competition Model Although the precise nature of the critical lesion is not known, a useful model for understanding the development of sensitizers and protectors is the competition model. Figure 1 illustrates the competition model.3 DNA* represents a free radical of the DNA which can be formed by radiation

C. N. Coleman, M.D., is Chairman and Professor, The Joint Center for Radiation Therapy, Harvard Medical School. 50 Binney Street,. sociate Professor,

1990

Department

of Radiation

Oncology,

University

of

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Critical Reviews In Table 1 Current Clinical Uses of Chemical Modifiers Agent/maneuver

Clinical setting Radiation therapy Oxygen mimetic sensitizers Increased

oxygen

delivery

Modulation of intracellular thiols Decrease glutathione (GSH) Increase thiols: radioprotection Altered radiosensitivity of DNA Chemotherapy Chemosensitization Chemoprotection Thiol depletion Applicable to both Agents toxic to hypoxic cells

Hyperthermia

SR 2508 (Etanidazole) Ro-03-8799 Pefluorochemicals with hyperbaric oxygen Transfusion BSO

WR-2121 IUdR, BUdR Misonidazole, WR-272 1 BSO

SR 2508

Mitomycin-C and analogs Nitroimidazoles Non-n&o compounds, SR 4233 Various techniques

From Coleman, C. N., Glover, D. I., and Tutisi, A. T., Cancer Chemotherapy: Principles ad Practice, Chabner, B. A., Collins, J. M., and Myers, C. E., Eds., W. B. Saunders, Philadelphia, 1989, in press. With permission.

THE COMPETITION

MODEL

PROTECTION

SENSITIZATION rate -k

[mclirr]

*

DNA-SENSITIZER*

FIGURE 1. Competition model between sensitizers and protectors. The DNA radical (DNA*) formed by the ionizing irradiation may very rapidly undergo chemical restitution (called “protection”) or the lesion can become stabilized by oxygen, or in the absence of oxygen by an oxygen-mimetic radiosensitizer (called “sensitization”). DNA repair can theoretically affect the stabilized lesion.* (Reprinted with permission from Reference 3.)

interacting with the DNA itself (direct effect) or by interacting with water molecules in close proximity to the DNA. The radicals produced in water may diffuse a very short distance and produce a DNA lesion (indirect effect). There are two processes that rapidly compete for this radical called sensitization and protection in Figure 1. In protection, reducing spe-

226

ties such as thiols ( - SH groups) can chemically restore the DNh radical to undamaged DNA by hydrogen donation, or prevent it by acting as a free radical scavenger. Alternatively, the DNh radical may interact with oxygen, which contains unpaired electrons, to produce a peroxy-DNA radical, DNA00. This is a more stable intermediate which can later be repaired by specific repair enzymes. In theory, an oxygenmimetic hypoxic-cell sensitizer can take the place of oxygen leading to a stable lesion, DNA-sensitizer, although the precise chemistry of this interaction is not known.9 Since the oxygen is much more reactive than the sensitizer, the sensitizer will not increase the damage formed in cells in the presence of oxygen. The processes in which DNA radicals are stabilized by either oxygen or an oxygen-mimetic sensitizer are referred to as sensitization. For a sensitizer or protector to be effective in the competition model, it must be present only at the time of irradiation. However, due to drug distribution and excretion, it is not possible in vivo to have the modifier present only at the time of irradiation. Rather, there is a more prolonged contact between the cell and the modifier which has the potential to produce additional drug/tissue interactions that can further affect the sensitization process, as later described. Two of the primary clinical approaches to radiation modification are hypoxic cell sensitization and thiol protection. The initial clinical approaches were to either increase the amount of protector in a normal tissue or to develop an oxygen-mimetic sensitizer that has access to the hypoxic tumor cells. Recently, two new approaches have been developed. The drug buthonine sulfoximine (L-BSO),” which inhibits glutathione (GSH) synthesis, will deplete GSH which in turn can produce sensitization (Figure 1).3,L1-13 Additionally, agents are being developed with selective toxicity toward hypoxic cellsL4-L6in an attempt to directly eliminate that population. In theory, normal tissues are not hypoxic and, therefore, they would not be affected by the hypoxic cell sensitizers. There are probably some normal tissues that contain cells that have marginal oxygenation but sensitization of normal tissues has not been observed with certainty in the hyperbaric oxygen trials” or in the oxygenmimetic sensitizers studies. Hypoxic cells are relative radioresistant.* To achieve the same fraction of cell survival, approximately two to three times the radiation dose is required to kill hypoxic cells as compared to normally oxygenated ones (Figure 2). The ratio of dose required to kill hypoxic cells to the dose required for the same fraction of cell killing in ambient air is called the oxygen enhancement ratio (OER). The OER at clinically relevant fraction sizes of 180-200 cGy is approximately 2; it is 2.5 to 3 at higher doses.7*19*20~ The sensitizer enhancement ratio (SER), like the OER, is the dose of radiation required to produce a defined level of cell killing without sensitizer divided by the dose of radiation required for the same level of cell killing with sensitizer. Since oxygen is the best oxygen-like sensitizer, the

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administration. Consequently, the overall treatment administered in these radiation modifier trials did not fully exploit reoxygenation and this may account, in part, for the failure to show any improvements over standard radiation therapy. C. Hypoxia Model The classic concept of hypoxia is that of diffusion-limited chronic hypoxia.’ Oxygen can diffuse approximately 150 microns from a vessel. Its diffusion is limited by its uptake and consumption by the cells nearest to the vessel. Cells which are beyond the diffusion limit will be anoxic and ultimately will die. Cells surrounding the necrotic zone are chronically hypoxic and relatively radioresistant, yet remain viable (Figure 3, left hand side). In this model it is assumed that the nutrient vessel remains open and that the hypoxic cells are confined to the area surrounding the necrotic tumor.

10-l -

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4

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24

TOTAL DOSE ( GY 1 FIGURE 2. Radiation survival curves demonstrating the OER and SER at clinically relevant doses. Oxygen or sensitizer increase the hypoxic cell kiiling by irradiation. A protector (not illustrated) would shift the aerobic survival curve to the right. (Reprinted with permission from Reference 19.)

/tlvmuwC~~~ l

IncmnsaOxypsnOeliwry Tmnsfuskm Hyperbortc Oxygen Perflwmchemlo3l Altered Hgb Affinity for 02

SER should, at best, equal the OER. However, as later described, in certain settings it is conceivable that the SER might exceed the OER due to other effects of the sensitizers beyond the pure oxygen-mimetic effect.21*22 Given the greater sensitivity of oxygenated cells, it might be expected that following the first few radiation treatments the oxygenated cells would be depleted, markedly increasing the tumor’s hypoxic fraction. However, moxygenation has been observed in most, but by no means all, experimental tumors. By this process the proportion of hypoxic cells remains relatively constant during a course of treatment.7*23~24 The extent and mechanism of this process in human tumors has not been established. It is logical that this relatively rapid change in hypoxic fraction is due to blood flow changes as indicated in the hypoxia models to be described. Since oxygen is the best oxygen-like sensitizer, and since reoxygenation takes place following each radiation fraction, radiation therapy regimens using only a few large fractions fail to take best advantage of reoxygenation. Many of the early clinical investigations using hypoxia-targeted radiation sensitizers or hyperbaric oxygen used suboptimal radiotherapy fractionation regimens which were dictated by the toxicity of the sensitizer or the technical constraints of the hyperbaric oxygen

Hypoxic Call Sensitizers *miolmodification l Bioreductiw Agents Mitomycin-C Nitmimidazoles l

to4wt8~ . Altemd Oxygen Deliwry Likely tohow minimal impact Hypoxic Cell Sensitizers *Thiol modifiootion l Bioreductiw Agents Would need mp4d action under hypoxio l ? Hyperthermio l ? Chemothempy l

FIGURE 3. Chronic and intermittent hypoxia. Chronically hypoxic cells (left side) are “diffusion-limited”. Intermittently hypoxic cells (right side) are “perfusion-limited” in that they are hypoxic only when blood flow stops in their nutrient vessel. Possible methods of overcoming acute and chronic hypoxia are suggested. The extent and clinical importance of the different types of hypoxia, and the effkacy of the various therapeutic approaches remains to be established. (Reprinted with permission from Reference 5.)

A second type of hypoxia, acute, intermittent hypoxia, has been observed in animal tumors using cell sorting and image analysis techniques25-28as illustrated in Figure 3 (right hand side). This hypoxia results from intermittent blood flow in small vessels supplying the tumor. It is not known whether the mechanism of this intermittent hypoxia is caused by physical or humoral changes. Cells irradiated when blood flow is present will have the sensitivity of oxygenated cells. However, when 1990

227

Critical Reviews In blood flow transiently stops, oxygen and other nutrients such as glucose are rapidly exhausted thereby leaving the cells in a temporarily hypoxic state. If radiation is given when the cells are not perfused, these cells will behave radiobiologically as hypoxic cells. These acutely hypoxic cells can be conceived as having perfusion-limited hypoxia. There is much to be learned about the extent, mechanism, and periodicity of this type of hypoxia. The post-irradiation oxygenation status of the cell is also important for the ultimate cell survival observed.29 Cells irradiated in a low oxygen environment, GO. 1% O,, are relatively radioresistant, OER = 2.3. If such cells are maintained in that low oxygen environment for many hours (>8 h) there will be a decrease in the surviving fraction, i.e., their ultimate survival curve will move closer to that of cells treated in normobaric oxygen by virtue of the cells remaining for a prolonged period of time under hypoxia of SO. 1%; their OER = 1.2 rather than 2.3.29 The existence of two types of hypoxia has implications for attempts to overcome hypoxic radioresistance as depicted in Figure 3. Since chronically hypoxic cells are diffusion-limited, an increased in the effective diffusion distance of oxygen would improve oxygenation. However, within a day the tumor may proliferate and, thereby, adapt to its improved environment by reestablishing a zone of diffusion-limited hypoxic cells. The proportion of hypoxic cells will return to their pretreatment leve1.3o A diffusible, slowly-metabolized sensitizer would reach these cells as would oxygen. The acutely hypoxic cells present a different problem. While blood is flowing through the nutrient vessel the cells are wellperfused with oxygen, or hypoxic sensitizer. However, when the blood flow ceases, the oxygen content will quickly be depleted, while the concentration of other slowly-metabolized substances will remain at or near their preexisting level. Maneuvers that increase oxygen diffusion would not be expected to sensitize these cells, unless they were fortuitously within the diffusion distance of oxygen from a neighboring vessel. A slowly metabolized hypoxic cell sensitizer would be effective for these cells. At present, the extent and natural history of each type of hypoxia during a course of treatment remains unknown for human tumors. New techniques under development to address this critical issue may have an important impact on the clinical use of radiation modifiers.

D. Nitroreduction Under Hypoxic Conditions In addition to their oxygen-mimetic properties, the 2-nitroimidazole sensitizers have other cellular effects related to their metabolism under hypoxic conditions. Prolonged exposure of hypoxic cells to the 2-nitroimidazole compounds, leads to (1) a reduction in the ability of the cells to repair DNA damage (as manifested by a reduction in the size of the shoulder of the radiation survival curve), (2) direct cytotoxicity, (3) sensitization of cells to alkylating agents and nitrosoureas, and

228

(4) covalent binding to hypoxic cells.3,3’ Since these effects were first noted when the hypoxic cells were exposed to misonidazole prior to radiation, these direct pharmacologic actions of misonidazole have been called the “preincubation” effect.6.32 The mechanisms by which the prolonged hypoxic exposure to misonidazole produces shoulder reduction, cytotoxicity, or chemosensitization remains to be fully elucidated.3’.33,34 Under hypoxic conditions, the nitro ( - NO,) group is reduced to the amine ( - NH,) with a production of a number of intermediates including those shown in Figure 4.

R-NO2 ndroinidazok

d-e

R-NO> radical

e-

anion

R-N-0 ni(ros.3

_ ‘?: _ R-N,,O,,

!,‘_ _ R-N,,

hydroxylsmine

2

amire

FIGURE 4. Schema of the nitroreduction of the 2-nitroimidazole compounds under anoxic conditions. The nitroreductase activity is due to flavoproteins which are part of the NADPH-cytochrome C reductase system.27 All of the metabolites formed in the reduction of the nitro (NO,) to the amine (NH,) have not been identified. These products can enhance radiation and chemotherapy cell killing and can produce direct cytotoxicity. (Reprinted with permission from Reference 3.)

The reduction is believed to be due to flavoproteins termed “nitro-reductases” which are part of the NADPH-cytochrome C reductase system.6s35 Glucose-6-phosphate is oxidized through the pentose cycle producing NADPH which is the source of the electrons for the reduction of the nitro group.35.36 For the complete reduction of the nitro compound, six electrons are required. The various intermediates are believed to contribute to the cytotoxicity, chemopotentiation, and macromolecular binding seen under hypoxic conditions.33,34.37 Oxygen will inhibit the formation of these products, some of which may be capable of limited diffusion to neighboring cells.34 Of interest is recent data indicating that both the preincubation effect and the inhibition of repair of potentially lethal damage repair by 2-nitroimidazoles may enhance their therapeutic efficacy beyond that of the oxygen mimetic sensitization,*’ i.e., the SER may be greater than the OER. As indicated earlier, for a compound producing only an oxygen-mimetic effect the OER is greater than or equal to the SER since oxygen is the best sensitizer. Thus, given these additional potential mechanisms of sensitization, using sensitizers with many fractions of radiation*’ or using a prolonged exposure to sensitizer,” it may be possible to produce an enhancement of tumor cell killing by sensitizer at least as great as that predicted by the 0ER.6.38

E. Hypoxia-Related Chemotherapy Resistance Cells poorly perfused by oxygen and other nutrients will have difficulty being reached by chemotherapeutic agents. Additionally, the abnormal physiologic environment of the poorly perfused cells, such as low pH, low glucose concentration, etc., may alter the efficacy of certain drugs. Rice, Schimke, and colleagues39+o reported that hypoxia by itself produces

Volume 10. Issue 3

gene amplification and subsequent drug resistance. Following exposure to either chronic or intermittent hypoxia, they observed amplification of the dihydrofolate reductase gene and resitance to methotrexate in cells which had no prior exposure to the antifolate. It is, as yet, unknown whether other treatmentresistance related genes are amplified. Thus, it may be worthwhile to eliminate hypoxic zones to overcome both radiation and chemoptherapy resistance. F. Other lmplictions of Physiologic and Biochemical Heterogeneity While overcoming hypoxia has been the emphasis of the radiation sensitizer program, it is important to realize that the heterogeneous and dynamic variability of oxygen concentration can serve as a model for other forms of physiologic and biochemical heterogeneity.4 Metabolic properties of tumor cells depend on the availability or substrates, not necessarily on inherent cellular properties. 41The ability of cells to repair potentially lethal damage is dependent on the environmental PH.~* The energy status of a cell will effect its ability to repair sublethal radiation injury.43 Conceivably, intratumoral heterogeneity differences in DNA ploidy44 may be the result of environmental heterogeneity in a manner analagous to the DHFR gene amplification described previously.39.40 Part of the difficulty in understanding the effects of such heterogeneity may result from the wide variability that could theoretically occur and from the lack of markers to define the environmental conditions. G. Approaches to Identifying Hypoxic Cells Predictive clinical assays for identifying hypoxic cells would be extremely useful. Ideally, it would be possible to determine the degree to which tumor cells are affected by both chronic and intermittent hypoxia. While such assays are not yet available, important steps have been taken to develop hypoxia markers. Chapman 45 has demonstrated that the 2nitroimidazole sensitizer misonidazole, when radiolabeled, can indicate hypoxic cells. To do so the drug must undergo nitroreduction as in Figure 4 so that reactive intermediates are formed which can bind to macromolecules and remain within the cell following removal of the parent drug. This process requires (1) an adequate concentration of sensitizer so the radioactive grains can be detected, (2) a sufficient time period under hypoxia so that enough drug can undergo nitroreduction and subsequent macromolecular binding, (3) nitroreductase activity,36 and (4) a sufficient glucose concentration to provide the reducing equivalents. 3c.3646.47 Using 3H-misonidazole, Urtasun et al.48.49 studied the presence of hypoxic cells in 16 patients. Greater than 5% of hypoxic cells was seen in one of six patients with sarcoma or mesothelioma, neither of the two patients with a squamous cancer of the tongue or lung, and in all seven patients with small cell lung cancer. Ideally, it would be possible to assess the hypoxic state

1990

of the cell using intrinsic markers or properties of the cell. Additional techniques are being developed to measure hypoxia in situ using MRI techniques with fluorinated nitro compounds. 50-5231PNMR spectroscopy can assess hypoxia before and during treatment.53.54Problems with this technique include interfering signals from surrounding tissues and a limit to the volume of tumor that can be resolved, as the critical hypoxic cells could make up a very small proportion of the tumor. Hlatky et al. 55.56have observed intrinsic flourescence from hypoxic cells. The mechanism of production of this flourescence remains to be determined. Sutherland and colleagues have observed the production of stress-related proteins (oxygen or glucose regulated proteins)57-59by cells exposed to hypoxia. The latter techniques require tumor biopsy, but may permit quantification of the types of hypoxia using cell sorting, image analysis or molecular biologic techniques. One approach that has been used in the clinic is the assessment of hypoxia within the tumor prior to treatment using a Clark oxygen electrode. Gatenby and colleagues have positively correlated oxygen content in lymph nodes of patients with head and neck cancer prior to treatment with treatment outcome using radiation therapy. The mean p0, in the complete remission group was 20.6 mm Hg, compared to 8.8 mm Hg for the partial responders and 4.7 mm Hg for the non-respondersa While the ideal technique to assess hypoxia remains to be established, methods such as those described will be useful for the understanding of tumor biology and, possibly, for the tailoring of a treatment plan based on the pre-treatment characteristics of the tumor. The latter would include other metabolic parameters, estimates of radiation and chemotherapy sensitivity, cell cycle parameters, and so forth.

APPROACHES CLINIC III.

TO HYPOXIA IN THE

A. Hyperbaric Oxygen; Blood Transfusion Although there are reasons to believe that hypoxia is important in tumor curability by radiation therapy, this hypothesis has not yet been proven. Animal models have demonstrated that hyperbaric and normobaric oxygen, and carbogen can increase radiosensitivity using large radiation doses per fraction. Of clinical relevance, Hong et al. have recently demonstrated radiosensitization of a murine tumor using many small radiation fractions with the mice breathing normobaric oxygen for 2 min before and after treatment.6’ Suggestive, albeit conflicting, evidence has emerged from the hyperbaric oxygen trials indicate that the oxygen effect may be relevant in certain clinical settings. 17.62 Given the technical and practical limitations of radiation treatment with patients in hyperbaric chambers, these trials often used few-fraction radiation treatment regimens which do not optimize reoxygenation. Most trials contained a limited number of patients, making it less likely that a small benefit would be observed. Despite

229

Critical Reviews In these limitations, the use of hyperbaric oxygen produced positive therapeutic results in moderate-sized lesions of the cervix and head and neck. l7 Additionally, other authors61-63have suggested that patients with certain cervix, head and neck, and bladder cancers who had higher hemoglobin levels had better tumor control than those with lower levels. These data were sufficiently encouraging to lead to the development of other methods to overcome hypoxic radioresistance.

I’“’ 0

2 ^: c : -J\



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B. Metronidazole The class of compounds that has been of clinical interest for hypoxic cell radiosensitization has been the nitroimidazoles. The first drug tested was the 5nitroimidazole, metronidazole. In the classic randomized trial by Urtasun et a1.48patients with glioblastoma were treated with 330 cGy, three times a week for three weeks with either metronidazole plus radiation or radiation alone. The median survival of the sensitizer group, seven months, was superior to the three month median survival of the control (p = 0.02). However, almost all the patients died by one year and the sensitizer group was not superior to historical controls given standard fraction radiation therapy. This study indicated that an oxygen-mimetic sensitizer could demonstrate clinical activity but that it should be added to the best possible radiation scheme. C. Misonidazole; Desmethylmisonidazole The 2nitroimidazole compounds are more electron affinic than the 5nitroimidazoles; therefore, their development has been emphasized. Misonidazole was the first of this group evaluated in clinical trials for a wide range of tumor sites. With the exception of a few studies, the drug was administered orally. The maximum tolerated single dose was 4 to 5 g/m* due to nausea and vomiting. However, the total dose that could be administered was limited by nervous system toxicity, manifest by a peripheral sensory neuropathy of numbness, tingling and dysesthesias of the extremities, primarily the feeLsa Some patients also experienced hearing loss and, early in the trials, confusion and lethargy were observed. A cumulative dose of approximately 12 g/m’ produced neuropathy in half the patients. The relationship between hypoxic cell radiosensitization and sensitizer concentration, as derived from in viva laboratory data is illustrated in Figure 5. The SER, applicable only to the hypoxic cells within the tumor, increases with sensitizer concentration, plotted as the log of sensitizer concentration. At a concentration of 20 yg/ml (approximately O.lM) the SER is approximately 1.1, at 100 p,mg/ml it is 1.6 and, to achieve an SER of 2.0 requires a sensitizer concentration of 800 pg/ml. Ideally, the sensitizer should be used with each radiation treatment in a dose that would produce a high SER. It must be emphasized that the SER should not be applied to the overall tumor, just the hypoxic cells. Furthermore, many other possible mechanisms of radiation resistance such as proficient repair, are not addressed by the oxygen-mimetic sensitizers. Given the cumulative toxicity of misonidazole, clinical trials 230

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CONCENTRATION

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FIGURE 5. Relationship between the SER and sensitizer concentration. These data have been derived from murine systems. The SER applies only to the hypoxic cells in the tumor. (Reprinted with permission from Reference 38.)

used either a very small dose of misonidazole (400 mg/m*) with each treatment, a large dose (2 g/mz) with some of the treatments in a standard radiation regimen or with a large dose (2g/m2) with every radiation treatment in a few-fraction regimen using large single doses of irradiation. In the first approach, the SER per dose was about 1.1 for hypoxic cells, which might not be detectable in a clinical trial. Using a larger dose of misonidazole with only a few treatments in a standard regimen, 5 or 6 out of 25 to 30, would be leave 80% of the treatments unsensitized. Using the larger dose of misonidazole with each treatment of a large-fraction radiation scheme would not optimize reoxygenation, a problem analogous to that of the hyperbaric oxygen trials. In retrospect, it would have been surprising had misonidazole produced a major therapeutic benefit in the clinic. Dische reviewed the results of 33 trials with misonidazole, five of which showed some possible benefit to the use of the drug. Four of the 5 positive trials were from 12 head and neck cancer studies. A large randomized trial from Denmark suggested that misonidazole was of clinical benefit for male patients with pharyngeal cancer. ‘OPatients with higher hemoglobin values fared better than those with lower concentrations; within the misonidazole and control groups, the trend toward better local control with higher hemoglobin values was maintained. These data from a trial large enough to demonstrate a small therapeutic gain add encouragement to the pursuit of sensitizing hypoxic cells in the clinic. The next sensitizer used was desmethymisonidazole, a misonidazole metabolite which is more water-soluble than the parent compound. Desmethylmisonidazole was less neurotoxic; however, since it, too, produced neuropathy in approximately 40% of patients at a cumulative dose of 14 g/m2, it

Volume 10, Issue 3

was not sufficiently superior to misonidazole ceed to phase II trials.67

erated.‘* A drug-related allergic macular rash occurs in about 3% of patients. Occasionally, a patient may develop erythema and scaling of the palms which does not interfere with treatment. Although not observed in the phase I trial, two patients in the phase II trials have developed neutropenia after approximately 4 weeks of treatment. This was rapidly reversible with the discontinuation of drug and did not recur in one patient who resumed treatment with the sensitizer following recovery from neutropenia.‘* The prevalence of this finding is unknown but it is consistent with a decrease in CFU-C by misonidazole observed in the laboratory.83 The dose-limiting toxicity is peripheral neuropathy.7*,80.84 A retrospective analysis of the Phase I trial indicated that the risk for an individual patient developing neurotoxicity can be predicted from their pharmacokinetic profile. Since the AUC was constant through a course of treatment, a patient’s predicted drug exposure can be calculated by a single-dose AUC multiplied by the number of drug administrations (Total-AUC). As shown in Figure 6, the probability of developing neurotoxicity is related to the Total-AUC and duration of drug exposure. In the phase II trials, approximately one-third of the patients develop grade I neuropathy at the planned total dose of 34 g/m2 over a 5 to 6 week period; very few grade II or higher drug related neurotoxicities have been encountered. For

and did not pro-

D. SF?2508 (Etanidazole) Brown, Lee and co-workers synthesized a series of misonidazole analogs that were as potent as misonidazole in terms of radiation sensitization but which differed in lipid solubility.71-73 The less lipid soluble drugs (lower octanol:water partition coefficient) were less neurotoxic because of an inability to cross the blood-brain barrier. Furthermore, less-lipid soluble drugs undergo less metabolic degradation and are more rapidly excreted from the blood in their parent form. Since the critical element in oxygen-mimetic sensitization is the concentration of drug at the target only at the time of irradiation, rapid elimination is desirable since drug remaining after irradiation would produce toxicity without benefit. Decreasing lipophilicity excluded drug from the brain. However, there was a limit to this approach as 2-nitroimidazole analogs with very low partition coefficients were not taken up by the cell. The compound judged to be the best in Brown’s series was SR 2508 (etanidazole), a non-polar drug with an amide side chain.73 Another 2-nitroimidazole currently under clinical development in Europe is Ro-03-8799 (pimonidazo1e).74-77 Since this compound has a basic side chain it accumulates in the acidic environment of tumors, producing a tumor:plasma ratio of approximately 3: 1.

60

1. Clinical Pharmacology and Toxicity of SR 2508 For sensitization of external beam radiotherapy, SR 2508 is administered as an intravenous bolus over 10 min. The distribution half-life is 10 to 15 min and the excretion half-life is approximately 5.5 h. Seventy percent of the parent compound is excreted intact in the urine; the fate of the other 30% is unknown. The single dose used in most clinical trials is 2 g/ m2 with a planned total dose of 34 g/m*. The plasma concentration of SR 2508 is approximately 100 kg/ml 30 min after the start of a IO-min infusion and 70 kg/ml at 1 h.78 The predicted SER for the hypoxic cells at a tumor concentration in this range is approximately 1.6 (Figure 5).38 Limited tumor biopsy data indicate that the tumor rapidly achieves its peak sensitizer concentration. For this reason, the radiation is given 30 to 40 min following the start of the infusion, permitting some time for the drug to enter the cell from the interstitial space. ” Plasma pharmacokinetic profiles differ among patients and the assessment of the area under the concentration vs. time curve is useful in predicting the risk of an individual developing neuropathy.80 An average single-dose AUC is approximately 2 mM X h with some patients having a single dose exposure of up to 5 mM X h. In an ongoing developmental trial SR 2508 is being administered as a 48- to 96-h continuous infusion accompanying brachytherapy.22.8’ There is little acute toxicity, with nausea occuring in less than 5% of patients. A bolus dose of 3.7 g/m’ was well tol-

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00

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toxicity

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no toxicity

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FIGURE 6. Relationship between drug exposure and neurotoxicity for SR 2508. The drug exposure (Total-AUC) is calculated by multiplying the area

under the plasma concentration vs. time curve (AUC) times the number of doses. Tbe single dose AUC can vary among patients by a factor of 2 to 3 for the same administered dose. (Reprinted with permission from Reference 80.)

1990

231

Critical Reviews In those with a single dose AUC .+Cl

FIGURE 7. Selective cytotoxicity of SR 4233in vitro for anoxic cells compared to cells in air. The hypoxic ratio of 160 to 200 is based on the equivalent cell killing of 5 pkf under anoxia with that of 800 to 1000 pM in air. A much less pronounce effect is seen in vivo. (Reprinted with permission from Reference 109.)

perthermia. Another group of vasoactive drugs under investigation are the calcium channel blockers such as flunarazine, verapamil, and nifedipine. 1’3aWhile the specific agent and dose are important, these agents can increase tumor blood flow and increase radiosensitization to a large single dose of irradiation. ‘13’Whether or not this approach will be both effective and safe in the clinic if used with either a course of fractionated radiotherapy or with bioreductive chemotherapy remains to be seen. If this approach is able to eliminate the hypoxic compartment rather than sensitize the hypoxic cells it might not be necessary to administer it very often. This appears to be the case with hyperthermia which is effective as a radiation modifier when used once or twice weekly.

V. GLUTATHIONE MODULATION Glutathione (GSH), a ubiquitous tripeptide tbiol, has important functions in a wide range of cellular functions including metabolism, transport, and protection.‘0.36 The functions of interest for cancer therapy are the protective functions including free radical scavenging of radiation-induced lesions (Figure 1) and detoxification of electrophiles such as alkylating agents via glutathione S-transferase (Figure 8).3 Thus, there is currently an interest in glutathione depletion as a means of sensitizing cells to the effects of radiation and chemotherapy.3*6

Volume 10, Issue 3

R&H

GS-X krophite

i \

/

GSH S-van: srerase ’ \

GSH

lnhlbttlon by B-SO ._ -.

3

E”

= P E 8 c

GSSG GSHl reductase

cys

NADP

f X-glycys

/

\

/

“\__ y -glu

60 GSH peroxldase

5 w

NADPH

60

40

20

synthetase

cysteine

I

I

I

I

I

I

2

4

8

12

24

HOURS AFTER A SINGLE I p. DOSE OF ES0 I2 mmol!kg: L Isomer1

FIGURE 8. Enzymatic reactions involving GSH. GSH has a major detoxifying role as a substrate for glutathione S-transferase which can detoxify electrophilic compounds such as alkylating agents. GSH is also important in the detoxification of peroxides by glutathione peroxidase. The de novo synthesis of GSH can be inhibited by L-BSO. The rate of change of GSH concentration following administration of L-BSO will depend on the GSH turnover rate of that tissue. (Reprinted with permission from Reference 3.)

FIGURE 9. Time course for depletion and recovery of GSH concentrations following a single intraperitoneal injection of BSO in mice. The values are expressed as a percentage of controls. (Reprinted with permission from Refer&ce 138.)

Given the many roles of glutathione, it is likely to be involved in additional future therapeutic strategies. A. L-BSO Glutathione is a tripeptide, gammaglutamyl- cysteinly-glytine. Glutamate is combined with cysteine by the enzyme gamma-glutamylcysteine synthetase; glycine is then added to the product, gammaglutamylcysteine, by the enzyme glutathione synthetase. Meister has developed the compound buthione sulfoximide (L-BSO) which inhibits the enzyme gamma-glutamylcysteine synthetase. ‘O The biochemical effect of GSH depletion may vary for the different cellular processes. In the in vitro studies, depletion to very low concentrations has been shown to alter the activities of a wide range of GSH dependent processes. 36Therefore, in interpreting the effects of GSH depletion the extent to which GSH is depleted is a critical variable. 3 An important concept in the therapeutic use of L-BSO in the depletion of GSH is that the pharmacokinetics of L-BSO per se are less important than the concentration of GSH in the target tissues. The rate of GSH depletion following the administration of L-BSO will depend on the rate that the existing GSH is utilized and the rate of recovery of GSH synthesis as the L-BSO is cleared from the body. As of yet, L-BSO is not in clinical trials, with the largeanimal studies nearing completion. Kramer et al. have demonstrated that following a single dose of L-BSO to mice the time course for depletion and recovery of GSH concentration varies from tissue to tissue (Figure 9). 114Similarly, following chronic oral or intraperitoneal administration, the extent of GSH depletion varied among the different tissues (Figure lo).“” It will be critical to be able to monitor GSH concentration

FIGURE 10. (a) The effect of multiple intraperitoneal injections (MIP) or chronic oral dosing (PO) of BSO on GSH concentration in various mouse tissues. There is a greater decrease than after a single oral dose. (b) The effect of MIP on GSH concentration in bone marrow and in a resistant L1210 line. The decrease in tumor GSH was approximately 90% compared to a 40% decrease for bone marrow. (Reprinted with permission from Reference 138.)

within tumor and normal tissue for the proper utilization of GSH depletion. B. GSH Depletion and Radiation Therapy As indicated in Figure 1, the depletion of critical thiols could enhance radiation cell killing by directing the DNA* toward a permanent lesion in the presence of either oxygen or sensitizer. If GSH depletion is to be used clinically, it would be necessary to sensitize tumor cells to a greater extent than normal tissue. This issue remains uneresolved, although the data suggest that depletion to clinically achievable concentrations will selectively enhance tumor cell killing.3

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Critical Reviews In As indicated in Figure 8, GSH is a ubiquitous molecule involved in many enzymatic processes. Therefore, depletion of GSH to very low levels would inhibit the activity of a number of enzyme systems, in addition to affecting the competition model (Figure 1). Additionally, the therapeutic ratio achieved by GSH depletion greatly depends on the endogenous concentration of GSH and on the extent of depletion in the target tissue. This is particularly true with the interaction between GSH and oxygen-mimetic sensitizem.“5.“6 GSH depletion alone can sensitize hypoxic cells to cell killing by irradiation.“7”8 there was no sensitization of aerobic In these experiments, 1’7~1’8 cells; therefore, GSH depletion would produce a therapeutic advantage. However, other investigators have found aerobic sensitization following thiol depletion,36*119-‘2’a discrepancy due largely to the much greater degree of GSH depletion in the latter experiments. Since oxygen is much more electron affinic than misonidazole or its analogs and would compete favorably with GSH for the DNk radicals, a better potential use of GSH depletion with radiotherapy would be to enhance the hypoxic cell sensitizers, in other words, sensitize the sensitizer. This enhancement will be especially important in cells that have a high endogenous concentration of GSH, a phenomenon seen in a number of human tumor cell lines.“5~‘22*‘23As indicated in Figure 11,“’ L-BSO may enhance the sensitization of SR 2508 following only a modest depletion of GSH to approximately 30% of contro1.“5~1’6Thus, L-BSO could be effective clinically without requiring the severe GSH depletion which would perturb a wide variety of biochemical pathways.

1.6

-

1.6

-

1.4

-

0.1

r

0.5

5.0

1.0

W-2508

10.

[mM]

FIGURE 11. GSH depletion as a mechanism of enhancement of SR 2508 radiosensitization. Tumors that have a high inherent GSH concentration will be sensitized less by SR 2508 than those with lower GSH. (Reprinted with permission from Reference 115 .)

236

The relatively high GSH content of some of the human tumor lines studied is not necessarily reflective of the clinical situation. Established cell lines may have developed the higher GSH concentration since they had adapted to sustained growth in culture. Thus, they may not be representative of de now tumors. Since GSH is present in all tissues including red blood cells, assessing human tumor GSH concentration by homogenizing a biopsy section might not produce an accurate reflection of GSH concentration. Furthermore, when investigating single cell GSH concentration from rodent tumor lines it has been noted that an individual tumor might contain cells with a range of GSH concentrations.‘**.‘“’ Therefore, a single average GSH concentration may be of limited use. At present, the methodology to investigate single cell GSH content is being further developed using cell sorting and image analysis systems. If some human tumors have a high concentration of GSH, then hypoxic cell sensitizers might not work well due to the competition between the GSH and sensitizer within the hypoxic cells.‘05*‘“.‘24 As indicated, this is a setting in which GSH depletion would be of benefit. Of interest are recent data from Ling et al. indicating that GSH depletion also enhances the efficacy of SR 2508 as a hypoxic cell cytotoxic agent.‘*’ Therefore, L-BSO may enhance the oxygen-mimetic and direct cytotoxic effects of SR 2508, a finding that might produce an SER > OER, as discussed previously. C. L-BSO and Chemotherapy The mechanisms of drug resistance are numerous’26.‘27and remain to be clarified. The presence of an elevated GSH concentration may be a mechanism for drug resistance.3*‘28.‘29 GSH depletion with L-BSO has been reported to increase the cytotoxicity of a number of chemotherapeutic agents, particularly the alkylating agents. 127,‘30-139 The interaction between the alkylating agent and GSH is mediated by the enzyme glutathione S-transferase. ‘*’ While both GSH concentration and glutathione S-transferase activity are increased, it is the GSH concentration that appears to be important for the resistance to the alkylating agents. ‘40444 An increase in GSH concentration may occur as part of the expression of the multidrug resistance phenotype. However, such an elevation in GSH per se is not necessarily the mechanism for drug resistance with other classes of drugs. Depletion of GSH in cells which were multi-drug resistant did not enhance the cytotoxicity of adriamycin. 145.‘46However, depletion of GSH as a means of inhibiting the redox cycling of agents such adriamycin indicate the wide range of activities of GSH and its related enzymes and the complexity of the effect of GSH depletion. D. Cross-Resistance Between Radiation and Chemotherapy The possible interaction between radiation and chemotherapeutic agent resistance is receiving a good deal of attention. In general, continued exposure of cells to irradiation does not

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produce radiation resistance, while continued exposure to a chemotherapeutic agent is an excellent means for producing drug resistance in the laboratory. Human tumor cell lines do vary in their inherent radiosensitivity’47’148however, the precise mechanisms of relative resistance remain to be elucidated. The mechanisms of drug resistance are many, most involving drug transport, metabolism, activation and concentration of target proteins. 126*‘27 One mechanism of cross-resistance between radiation and chemotherapy that has been investigated is the cross-resistance of cells to alkylating agents and radiation by virtue of an increase in GSH concentration. Ovarian cancer cell lines made resistant to alkylating agents by stepwise incubation in the laboratory were more radioresistant than the parent lines. The alkylating agent-resistant lines had an increased concentration of GSH. Furthermore, treatment with L-BSO restored the radiation and alkylating agent sensitivity of the resistant lines. “‘.I49 In contrast to this are findings with ovarian cancer cell lines that were resistant to melphalan at the time of their establishment, i.e., they were not made resistant in the laboratory. Two such cell lines had high concentrations of GSH that could be reduced with L-BSO. However, in neither of these lines was the radiation sensitivity changed by L-BS0.‘49 Mitchell et al. studied radiosensitivity before and after treatment by L-BSO with a drug sensitive and multidrug resistant cell line. The resistant cell line had a twofold increase in GSH concentration. There was no difference in radiosensitivity of the drug-sensitive or drug-resistant cells and L-BSO did not alter radiation sensitivity. IsoThe biochemical and physiologic factors which exist in the poorly-perfused hypoxic zones may lead to drug resistance from pharmacologic or other reasons. The complex interaction of resistance between the two major cytotoxic cancer treatments, radiation and chemotherapy, remains to be clarified. Such knowledge may profoundly affect how combined modality treatment is used.

zation increases as the percentage of thymidine replacement increases. Kinsella et al demonstrated in vitro a direct correlation between percent thymidine replacement, reduction of radiation survival parameters and the production of DNA strand breaks using drug concentrations clinically achievable. 154For example, the enhancement factor for the production of double strand break formation was approximately 1.3 to 1.6. Since incorporation into DNA is critical for sensitization, the cells must be exposed to the sensitizer for a sufficient period of time so that the halogenated pyrimidine will be incorporated during DNA synthesis. The scheduling of drug administration is designed such that tumor incorporation is maximized while the dose-limiting myelosuppression is kept at an acceptable level. The earliest clinical trials with BUdR showed little efficacy and excessive normal tissue toxicity, the latter due, in part, to the disease site selected. Patients treated for head and neck cancer had increased mucosal reactions due to the rapid cell cycling of the mucosa which incorporated more of the BUdR than the tumor. 156Kinsella et al”’ conducted a phase I trial of BUdR utilizing a continuous 12-h infusion for each day of 14 d. The final dose achieved was 850 mg/mV12 h; pharmacokinetic data were available for seven patients. The mean steady-state arterial plasma concentrations was 7.0 X IO-’ M for an infusion of 350 mg/m* and 2.1 X 10 -6 M for an infusion of 700 mg/m2. At the dose range of 650 to 700 mg/m2/12 h the arterial plasma levels were in the range of 2 to 3 X 10m6it4 which produced an SER > 1.5 using the human bone marrow CFUc assay. 15’ There was a trend toward increasing SER with increasing dose of BUdR. IUdR later replaced BUdR due to the photosensitivity seen with the latter compound.153,‘58Kinsella and colleagues conducted a phase I study with IUdR using a dose range of 250 to 1200 mg/m2/12 h for 14 consecutive days. Two 14-d courses were administered. The arterial plasma levels achieved ranged from 1 to 8 x 10e6 M with no suggestion of there being saturation of the drug metabolism pathways. ‘59.‘60The steady state arterial concentration by dose was 500 mg/m2 - 2.9 FmoV 1; 1000 mg/m2 - 5.6 pmol/l: 1200 mg/m2 - 7.4 kmol/l. Total body clearance was the same for all drug dose levels. 149Using an anti-IUdR monoclonal antibody and immunohistochemistry, tumor incorporation of IUdR was studied in four patients. By visual inspection, 50 to 70% of the sarcoma cells had incorporated IUdR; a smaller proportion of glioblastoma cells incorporated the sensitizer in a single patient studied.‘59 It has been proposed that other halogenated pyrimidines, such as chlorodeoxycytidine, may be good radiation sensitizers, particularly when employed with agents such as tetrahydrouridine that alter the normal metabolic pathways of the halopyrimidine, 16’.162 Such an approach is still under development.

VI. NON-HYPOXIC SENSITIZERS A. Halogenated Pyrimidines An investigative approach to clinical radiosensitization that is not based on the concept of hypoxia is the use of halogenated pyrimidines, bromo-deoxyuridine (BUdR) and iodo-deoxyuridine (IUdR).6 These halogenated pyrimidines were designed as thymidine analogs which incorporate into the DNA of cycling cells. The methyl group in the 5-position of thymidine is replaced by the halogen. The substitution of the halogenated pyrimidine in the DNA in place of thymidine is thought to be due to the stereochemical similarity between the methyl group of thymidine (2.0 A) and bromide (1.95A) and iodine (2.15A) atoms of the halogenated pyrimidines.‘51~152 The mechanism of sensitization has not been fully elucidated and includes an influence on the induction of the radiation lesion and effects on repair.‘52-‘55The degree of radiosensiti-

1. Clinical Use The skin toxicity encountered with BUdR was not observed 1990

237

Critical Reviews In with IUdR.‘59 The dose-limiting toxicity for both sensitizers was myelosuppression, in particular, thrombocytopenia. Phase II trials are in progress for sarcomas and gliomas with some encouraging early results. 158a9 As yet, there is insufficient data to assess the clinical efficacy of the halogenated pyrimidines. B. Inhibitors of Potentially Lethal Damage Repair (PLDR) Classical radiobiology operationally defines two classes of damage repair, sublethal, and potentially lethal damage.‘63 Sublethal damage repair is defined as the process which restores the initial shoulder of the radiation survival curve between split doses of radiation. Potentially lethal damage (PLD) repair is defined as the process which results in increased cell survival over time after a single radiation treatment. If cells are irradiated and immediately assayed for survival a certain level of cell killing will be measured. Under the proper conditions, such as density-inhibited plateau phase growth, the surviving fraction will increase over time so that at 24 h after treatment the proportion of surviving cells will be higher than it was immediately after treatment. The mechanism of the different repair processes has not been defined. In general, PLDR is best seen after large radiation doses. The importance of PLDR in vivP-‘” and in clinical medicine is not known, although Weichselbaum et al. have shown a correlation between the extent of PLDR and the efficacy of clinical radiother148,167-169 apy. Despite the unknown mechanisms a number of agents have been under development as inhibitors of PLDR. Inhibition has been seen for aerobic cells with misonidazole170 and with 3’deoxyadenosine and caffeine. 165If PLDR is, indeed, important at standard radiation doses, and if normal tissues do not undergo a similar process, then this area of modification may be extremely useful. A better understanding of the mechanism of repair is needed in order to develop effective and selective inhibitors. C. Inhibitors of Poly(ADP-Ribose) Polymerase Poly(ADP-ribose) polymerase (also referred to as poly [ADPribose] synthetase or transferase), is a chromatin-bound enzyme which synthesizes poly(ADP-ribose) from NAD after activation by DNA strand breaks. It is felt that the polymer may facilitate DNA repair. 171-173 A number of investigators have demonstrated that inhibition of poly(ADP-ribose) polymerase by agents such as 3aminobenzamide and nicotimamide can enhance radiation cell killing. 174-177 The effect has been largely an inhibition of potentially lethal damage repair. The mechanism for the radiation sensitization is not known173as the inhibitors can perturb a number of cellular functions depending on the concentration of the inhibitor.171*172 Radiation sensitization was found to occur in some mouse normal tissues as well as tumors174~178~179 although further studies are needed on the selectivity of these agents. The inhibitors have also been 238

observed to enhance the cytotoxicity of bleomycin.171 Since the sensitization of tumors appears to be greater than that of normal tissues174,178*179 further preclinical work is in progress to understand the mechanism and to develop analogs of 3aminobenzamide and nicotinamide. 178 D. Differentiating Agent, N-Methylformamide, and Related Compounds Leith and coworkers have demonstrated that the polar solvent differentiating agent N-methylformamide (NMF) and the related compounds N,N-dimethylformamide (DMF) and sodium butyrate can function as radiation sensitizers.180-183The sensitization occurs in the low dose region of the survival curve, which would make this clinically relevant. In radiation survival experiments the shoulder of the curve is reduced in size. Depending on the specific agent used, changes have been observed in the expression of sublethal damage repair and of potentially lethal damage repair.181.182 Additional cell killing has been seen when chemotherapy has been added to the radiation-differentiating agent treatment. lso This class of compounds is currently under study as a chemotherapeutic agent as well.‘@ E. Altering Redox State The cellular response to numerous drugs and therapeutic agents is mediated by redox changes involving the NADPH/ NADP and GSHGSSG systems. These systems are responsible for maintaining the redox state of cells and perturbation of these systems results in “oxidative stress” which can result in widespread metabolic disturbance, ranging from enzyme inhibition by failure to maintain required protein thiols in the reduced state to depletion of nutrients in the process of maintaining NADPH reduced. Is5 The alteration of redox state of the cell is distinctly different from that of perturbation in GSH concentration. Alteration of the redox state has been shown to sensitize the cells to radiation by a reduction in the size of the shoulder and a steepening of the slope of the X-ray survival curve.‘53~186*187 Radiation sensitization under aerobic conditions is greater with GSH oxidation than with GSH depletion.153*1m Furthermore, the efficacy of redox modification was seen when the agent was given after the irradiation. Although the mechanism remains to be elucidated, the post-radiation sensitization suggests that diamide and related compounds sensitize by altering DNA repair, probably by affecting the activity of enzymes that require reduced - SH group~.‘~~ At present, work with these agents is confined to the laboratory where efforts are being made to define the mechanisms of action.

VII. CHEMOSENSITIZATION An empiric observation by Rose et a1.1s8led to the investigation of the use of misonidazole and its analogs as a clinical chemosensitizer. While the mechanism of chemosensitization by misonidazole remains to be elucidated some empiric and

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mechanistic data are available and will be presented in brief. Additional information can be found in recent reviews.3.5.189,190 The agents whose activity has been shown to be modifiable by misonidazole and its analogs are the alkylating agents, such as melphalan and cyclophosphamide, and the nitrosoureas.‘89+‘90Since chemomodification emerged from observations with misonidazole, most of the sensitizers studied have been in the 2-nitroimidazole class. Virtually all of the 2-nitroimidazole radiosensitizers described above have been shown to have chemosensitization properties.3 However, the sensitizers are not necessarily “interchangeable” as in the laboratory a particular sensitizer- chemotherapeutic drug combination may be superior to a different sensitizer with the same chemotherapeutic drug. From the data available, there is no one superior sensitizer for use with all agents. Misonidazole appears to be a very good chemosensitizer and since chemosensitization is given at an intermittent schedule (misonidazole dose 4 g/m2 every 3 to 4 weeks), the dose-limiting neuropathy may be less of a problem using misonidazole as a chemosensitizer than as a radiosensitizer. However, since three times the amount of SR 2508 can be administered compared to misonidazole, current trials are utilizing the newer sensitizer. The initial experiments investigating the efficacy of sensitizers used a large single dose of sensitizer. This produced a number of effects which led to the conclusion that a primary mechanism of action of the sensitizer is that of altering the pharmacokinetics of the chemotherapeutic agent. A large single dose of misonidazole causes a significant drop in body temperature of the mouse which could alter pharmacokinetics and drug activation, distribution, and metabolism. Brown and Hirst and others helped clarify the situation by using a multiple dose misonidazole schedule which closely mimicked the human phannacokinetics. 191*‘92 Sensitization was still seen, indicating that the large single dose of sensitizer was not necessary. Using such scheduling the pharmacokinetics of melphalan or cyclophosphamide were unaltered by misonidazole.‘93-195It is generally accepted that when used in a clinically relevant schedule, the sensitizers do not produce a major perturbation of the pharmacokinetics of the alkylating agents. What is not clear is the relationship between plasma concentration of sensitizer and chemosensitization for the alkylating agents. Randhawa et al. felt that to obtain chemosensitization of melphalan by misonidazole in mice a sensitizer concentration above 70 p,g/ml is needed.‘95 Although not investigating a threshold effect, other authors have observed sensitization with concentrations of approximately 100 pg/ml. 194Increasing the concentration of sensitizer produced only a small increase in chemosensitization.i96 These observations suggest that to obtain chemosensitization in the clinic it might not be necessary to give the maximally tolerated sensitizer dose with each treatment but rather enough drug should be given to obtain a certain sensitizer concentration. With the limited data available, it appears that a sensitizer concentration

1990

of approximately 100 p.g/ml would be a reasonable target. What is not known is the optimum duration of exposure. For chemosensitization to occur, there is a critical need for a low oxygen tension. A lower sensitizer concentration in the presence of hypoxia would produce sensitization while any sensitizer concentration without hypoxia would likely be ineffective. 19’Thus, chemosensitization will likely be dependent to an unknown degree on sensitizer concentration,‘97,198duration of exposure, ‘99.200and extent of exposure to hypoxia. Consideration must again be given to the two types of hypoxia. For the chronically hypoxic cells the sensitizer could undergo reductive metabolism throughout its duration of contact with the cell. However, for the intermittently hypoxic cells, reductive metabolism would only take place during the time of hypoxia. Therefore, some prolongation of drug exposure may be best for the intermittently hypoxic cells. Furthermore, it is conceivable that the effect of the nitroreduction in the intermittent periods of hypoxia may be cumulative. Some of the hypoxia-related metabolites of misonidazole can diffuse to other cells,33*34.37 therefore it is possible that some cells will benefit from the hypoxic reductive metabolism of misonidazole while not being hypoxic themselves. At present, the optimal pharmacokinetic approach to the use of misonidazole has not been defined and may vary with the clinical setting. In vitro data show that the presence of low oxygen concentration is required for chemosensitization to occur. 190,‘97,199 It should be recalled that under low oxygen conditions the sensitizer itself can become cytotoxic.3’ Comparison of the shape of the curves relating oxygen tension to cytotoxicity of the sensitizer, and oxygen tension to chemosensitization suggests that a similar process is ongoing. As indicated in Figure 4 a number of misonidazole reduction products are obtained and a variety of them could be involved in chemosensitization and cytotoxicity. The need for hypoxia for tumor sensitization in vivo can be inferred from data of Spooner et al. demonstrating no sensitization of micrometastases, a situation in which hypoxia is not expected to be present, while gross tumors of the same cell type were sensitized.‘99 Therefore, based on these data it would not be expected that misonidazole or its analogs would not enhance the efficacy of adjuvant chemotherapy of non-hypoxic, micrometastatic disease. Metabolism of misonidazole or its analogs under hypoxic conditions can produce an array of products which can lead to a number of cellular changes. These include macromolecular binding, GSH depletion, altered levels of intracellular calcium, lower threshold to oxidative stress, enhancement of DNA crosslinking, and possibly alteration in DNA repair.3 GSH depletion per se is not a major factor in chemosensitization, since matching the extent GSH depletion seem with misonidazole using the reagent diethlymaleate (DEM) resulted in only slight chemosensitization, even though it was accompanied by an equal extent of enhancement of melphalan binding to macromolecules.202 Alkylating agents first form a mono-adduct

Critical Reviews In prior to forming a crosslink; the crosslinking will not be maximal for a number of hours.202-204Data from both Taylor et al. 203*204 and Mu1cahy203indicate that there is no interference with monoadduct formation. The chemo-enhancement occurs after monoadduct formation and may involve inhibition of the repair of the monoadduct or alteration of the repair of the DNADNA crosslink. The precise mechanism of chemo-enhancement, and the misonidazole metabolite necessary for enhancement to occur remain to be elucidated. A. Clinical Trials Mulcahy has reviewed the data from trials utilizing misonidazole as a chemosensitizer (Table 3).205 The Phase I trials were not designed to investigate therapeutic efficacy; most of the Phase II trials did not demonstrate a benefit compared to historical controls. However, a randomized Phase II trial using intravenous melphalan + oral misonidazole for patients with non-small cell lung cancer demonstrated an improved response rate in the sensitizer group.2o6 The efficacy of chemomodification, and the optimal use of chemosensitizers remains to be established. At this time, current chemosensitizer trials are utilizing SR 2508 with alkylating agents in the Phase I setting. Table 3 Results of Clinical Trials of Chemosensitization Type

of Study II II I I I/II II I II I II I II Nore:

Cancer Type Breast Colorectal Colorectal Cilioma Glioma Colon Melanoma Renal Non-small-cell

lung

Sensitizer

Chemotherapeutic agent

METRO METRO MIS0 MIS0 MIS0 MIS0 BENZ0 BENZ0 MIS0 MIS0 MIS0 MIS0

MMC 5-FU s-FU BCNU CCNU CCNU CCNU CCNU CTX CTX L-PAM L-PAM

NB, no benefit; SB, significant

From Mulcahy, R. T. and Tromp, With permission.

Result NB NB

NB NB NB NB SB

benefit to combination.

D. L., J. Clin. Oncol.. 6, 569, 1989.

VIII. PROTECTORS Protectors represent the opposite side of the therapeutic-ratio coin. Sensitization is designed to increase the efficacy of anticancer treatments by increasing the efficiency of cell-kill. Such an increase in efficacy is worthwhile only if the adverse effect to normal tissues remains tolerable. Protectors propose to increase the normal tissue tolerance or to reduce the toxicity caused by antineoplastic agents. Normal tissues should be pref-

240

erentially protected. The result would be either the use of the current “standard treatments” with lower toxicity, or the use of an increased dose of treatment with the level of toxicity equal to that of the standard treatment. The effort to develop protectors was initiated in the 1940s when the atomic age and “cold war” brought with it a perceived need to protect against the lethal effects of whole body irradiation. In 1949, Patt, using rodents, reported that cysteine could protect against the lethality of whole body irradiation.“’ Shortly thereafter, Brandt and Griffin showed similar protection from the toxicity caused by alkylating agents.208As classified research, the military pursued the search for protectors into the 1960s; since then there has been a more wide-spread effort to develop radiation and chemotherapy protectors. The properties of an ideal protector depend on the situation in which it is to be used. For medical purposes, an ideal protector should (a) reach, in active form, all tissues requiring protection; (b) reduce the toxicity of standard treatment; (c) not protect tumors to an equal degree to which normal tissues are protected; (d) allow for the delivery of a sufficiently larger dose of anti-cancer therapy so that there will be an improvement in ultimate treatment outcome; and (e) be well tolerated clinically. Radioprotectors require frequent use so that the agent can accompany daily fractionated radiation. Chemotherapy protectors would be used intermittently, probably once every three to four weeks. Thus, it would be acceptable for a chemoprotector to have more acute toxicity than a radioprotector. Protection of both acute toxicity, e.g., mucositis, leukopenia, alopecia, as well as protection against late effects, e.g., fibrosis, necrosis, mutagenesis or carcinogenesis, is extremely important. For military purposes, the protector must not only be well tolerated but should have a long-duration of action after oral ingestion. The protection sought was primarily against the acute radiation syndrome produced by a large single dose of irradiation. Thus, given the differing needs for military and clinical use, it would have been surprising had an ideal clinical agent emerged from the military developmental programs. Nonetheless, a very reasonable agent, WR-2721 (WR = Walter Reed) was developed.209.2’0 The Armed Forces Radiologic Institute (AFRI) had screened thousands of sulfhydryl (thiol or -SH) compounds. Simple sulfhydryl compounds, cysteine, cysteamine, monoethanol amine, amino ethyl thiouronium, etc., have two main problems which make them of little use: toxicity and lack of differential normal tissue protection. Simple sulfhydryl compounds such as these are readily oxidized to form either symmetric disulfides (composed of the same thiol), or mixed disulfides (composed of the parent thiol combined with another thiol to make either a mixed non-protein or protein disulfide) . Given the limitation of simple thiols, attention was given to the development of more complex compounds. The aminothiol phosphorothioate , WR-272 1 (ethiophos) , [S2-(3 aminopropylamino)-ethyl phosphorothioic acid, has been

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extensively studied because of its protecting abilities and relatively low toxicity. The phosphorothioates contain phosphate moieties covering the sulfhydryl group which prevents the oxidation seen with free thiol groups. Since these are charged molecules, they cannot be transported across the cell membrane until the phosphate group is cleaved. This is accomplished enzymatically by alkaline phosphatase,212-214which is membrane bound in many cells. The dephosphorylated metabolite of WR-2721 is WR-1065. As with the Enhancement Ratios for oxygen, OER, and sensitizers, SER, the efficacy of a protector is described by a protection factor (PF), or a dose reduction factors (DRF). The latter are the ratio of dose radiation or chemotherapeutic to produce an effect in the presence of protector divided by the dose required to produce the identical effect (isoeffect) without the protector.

3.

4. 5.

6.

A. Differedal Protection Yuhas and Storer first demonstrated differential radiation protection with protection factors of 2.4 and 2.7 for murine skin and bone marrow without concommitant protection of a mammary tumor.209 Some experimental tumors are, in fact, protected,211~2’s~216 however, differential protection of normal tissues has been reported for most experimental tumors.209*Z’6.2’7 Because it penetrates the blood-brain barrier poorly, WR-2721 protects neither brain2’g nor spinal cord**O from damage. In animal models, WR-2721 selectively protects normal tissues against the renal tolerance to cisplatin.2z0 Bone marrow tolerance to cyclophosphamide, nitrogen mustard and L-phenylalanine mustard are improved. *10,211,*21-223 Thus, co&i&g WR-2721 with alkylating agent or cisplatin chemotherapy, or with radiation therapy could theoretically permit the administration of a higher cytotoxic dose, which would produce a greater fractional tumor cell kill without a concomitant increase in host toxicity.210~223~224 If this is, indeed, possible, this would allow the dose-intensity of the chemotherapy and/or radiation therapy to be increased.**’

radical-induced damage, hydrogen atom transfer is necessary to restore the DNA to native form.227 Oxygen depletion. As a consequence of thiol oxidation, oxygen is consumed, inducing a relative hypoxic state, which may convey radioresistance. 228 Enhancement of biochemical repair processes. 21o.229 Altered binding of chemotherapeutic agents to DNA. The cytotoxicity of cisplatin complexes can be reduced by thiol compounds, which inhibit platinum binding. Thiols may react with either the chloride or aquo platinum species to prevent DNA cross linking.230 Competitive hypothesis. This descriptive term may involve a number of the preceeding mechanisms. As noted in Figure 1, oxygen vies with thiols to either restore or make permanent the radiation lesion in DNA. The relative amounts of thiol and oxygen are important in this competition. In the absence of oxygen, addition of sulfhydryls does not improve resistance further. With excess oxygen there is no protection afforded with the addition of a thiol such as WR-2721. However, in relatively low oxygen tensions there is competition. Thus, the precise concentration of oxygen and thiols in tissue would be important in determining the outcome of the competition illustrated in Figure 1.

The interaction between oxygen and sullhydryls is complex. “K-values” and “K-curves” are conceptualizations of the oxygen effect. The K-value is defined as the concentration of oxygen in the extracellular media required to produce an oxygen enhancement ratio of 50% of the maximum. Figure 12231 Panel A demonstrates the surviving fraction of hypothetically irradiated mammalian cells in fully oxic and, in gradation, to fully anoxic states. Panel B plots the K curves derived from these irradiations. At 0.5% 0, concentration, the response is half-way between fully oxic and fully anoxic curves, and the

6. Mechanism of Action The mechanism(s) by which sulthydryl compounds protect against radiation and chemotherapy toxicity has not been clearly established, however, there are a number of hypotheses. 1.

2.

Scavenging of free radicals. The free-radicals, particularly the hydroxyl radical, produced by ionizing radiation or chemotherapeutic agents can cause macromolecular damage. Sulfhydryl scavengers can combine with the free radicals before DNA damage is produced. WR- 1065, the dephosphorylated tbiol, reduced the frequency of radiation-induced single- and double-strand breaks in CHO cells.226 Hydrogen donation. In the process of repair of DNA

2

001

OXYGEN

CONCENTbATION

100%

rnrn”Q

FIGURE 12. Survival curves for hypothetical mammalian cells irradiated in various oxygen concentrations, and the oxygen K curve derived from them. 0.5% oxygen gives a sensitivity halfway between that in nitrogen and oxygen. Adding an oxygen-mimetic sensitizer shifts the baseline upward. Adding a thiol protector shifts the K curves to higher oxygen concentrations. (Reprinted with permission from Reference 23 1.)

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Critical Reviews In OER is 50% of maximum. The addition of exogenous thiol (or the increase in endogenous thiol concentration) theoretically shifts this curve to the right, i.e., more oxygen is required to produce the same effect. However, in fully oxygenated conditions, thiols exert very little effect, i.e., the oxygen excess cannot be counter-balanced by the increase in sulfhydryls. Conversely, in total anoxia, there is also little effect of the addition of sulfhydryls. The maximum effect of altering thiol concentration occurs at the K-value. 231*232 Figure 12 also indicates that the oxygen mimetic sensitizers are not effective once the oxygen concentration is above a certain point. Normal tissues and tumors would be expected to be variable in respect to both their oxygen and thiol content. Microelectrode studies demonstrate tissue oxygen heterogeneity;233 heterogeneity of glutathione concentration in tumor has been described using cell sorting techniques.‘23 Thus, the relative and absolute amounts of oxygen, sensitizer, and thiol at the individual cellular level would influence the outcome of treatment with irradiation. As noted in Figure 11, the cells with a higher glutathione concentration are less readily sensitized.1’s*“6 While glutathione concentration by itself is not the sole determinant of treatment outcome, in 30 human lung cancer cell lines, lower levels of GSH were found in the clinically responsive than in the less responsive ones.234 C. Preclinical Pharmacology The phosphate charge and hydrophilicity of the aminophosphorothioate WR-2721 make the intact drug poorly transmissible through membranes. Intact WR-2721 is unable to protect various cultured cell 1ines.212~222~235 The dephosphorylated metabolite, WR-1065, is thought to be the active protective agent.*13 The presence of a membrane associated alkaline phosphatase214 in different tissue may help explain the differential protection. This plasma membrane enzyme214 is responsible for the hydrolysis of WR-2721 to WR-1065 and the subsequent uptake of WR-1065 by cells in vivo. A high concentration of alkaline phosphatase is found in the plasma membrane of endothelial cells in small blood vessels236and the brush border of the kidney proximal tubules.237 Tumor pH and drug hydrophilicity will also influence drug uptake. 213*23* The lower uptake in tumors than in normal tissues which is seen in animals is the result of a combination of (a) the slower conversion of WR-2721 to WR-1065 due to the lower inherent level of alkaline phosphatase and more acidic pH in tumors; (b) a decreased uptake of WR-1065 as a consequence of the more acidic pH in tumors;2’3 and (c) deficient tumor vasculature. Shortly after WR-2721 is administered, the very reactive free sulfhydryl compound, WR-1065, appears as the major non-protein bound metabolite. It is probable that WR-1065 is rapidly converted into other metabolites which can contribute to protection or toxicity including cysteamine, sulfinite and sulfonate oxidation products, mixed disulfides with low molecular weight substances (such as cysteine and glutathione), and proteins containing reactive sulfhydryl groups .239 242

Using 35S labeled drug, Yuhas*” demonstrated uptake by many normal tissues, but not brain or spinal cord. The observed PFs observed for radiation injury varied from 1.2 for lung to 2.7 for bone marrow. 211*240 There was no correlation with the proportionate uptake and the DMF, e.g., the kidney had a large uptake but a small DMF. 241*242 This discrepancy is due, in part, to the fact that the concentration of oxygen and endogenous thiols influence the degree to which a tissue can express protection. D. Blodistribution Several pharmacokinetic assay systems have been developed to measure WR-2721 and its metabolites.235~24’-248 When administered as 150 mg/m* IV bolus to patients with advanced malignancy, less then 10% of WR-2721 is in the plasma compartment 6 min following the injection. Pharmacokinetic analysis following the administration of 740 mg/m* WR-2721 given as a 15 min infusion produced parameters similar to those following a bolus injection. The average percent of the total administered WR-2721 excreted in the urine during a one hour pharmacologic study period were 1.05, 1.38%, and 4.2% for WR-2721, WR-1065, and WR-33278 (the disulfide of WR2721), respectively. These excretion data suggest that WR2721 is rapidly dephosphorylated and enters normal tissues as WR-1065, the active thiol metabolite.213*249 Utley et alw7 measured the tissue concentrations of WR1065 following a 500 mg/kg intravenous dose of WR-2721 to mice. Maximal tissue concentrations of WR-1065 were achieved 5 to 15 min following injection. Fifteen minutes after injection, WR-1065 acounted for half the total drug in all normal tissues. However, in the tumor, the concentration of WR-1065 was only one-third of the total drug concentration. The rate of decline in the concentration of WR- 1065 varied among tissues. Thirty minutes after maximal WR-1065 levels were achieved, there was an l&fold drop in WR-1065 tissue concentration in the kidney and a 6-fold drop in the lung. However, one hour after administration, WR- 1065 tissue levels only decreased by 17% in salivary gland and by 29% in cardiac muscle. Shaw et a1.239studied the distribution of WR-1065 in tumor bearing mice following a 365 mg/kg intraperitoneal injection of WR-2721. Within 10 min of injection maximal concentrations of WR-1065 were achieved in blood, liver, and kidney. WR-1065 concentrations continued to increase 30 min following injection in cardiac muscle and in two solid tumors studied. E. Clinical Toxicity In the phase I trial of WR-2721 the single dose was escalated from 25 to 1330 mg/m *.250The main toxicity was short-lived hypotension that resulted in termination of the infusion in 5% of patients. A second important side effect was nausea and vomiting. Minor toxicities included sneezing, a warm or flushed feeling, mild somnolence, hypocalcemia, and rarely allergic reactions. Although WR-2721 can produce transient hypocalcemia due to inhibition of parathyroid hormone secretion and

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5 to 6 weeks to a maximum administered daily dose of 450 mg/m2, found 340 mg/m2 to be the maximally tolerated dose.253 Nausea and vomiting were dose-related and cumulative. Nearly half the entered patients failed to complete treatment. About 25% stopped because of protector-related toxicity, and another 25% stopped due to patient preference. A trial is now in progress designed to evaluate the toxicity of escalating radiation doses plus 340 mg/m’ of WR-2721 four times a week for patients with pelvic malignancies. Recognizing the difficulty of administering daily intravenous doses of WR-272 1, alternative radiotherapy schedules, with less-frequent, larger fraction sizes, have been examined. Eight patients requiring paliative radiotherapy were given WR-272 1, 740 mg/m’ weekly for three weeks with an 8.0 Gy fraction of radiotherapy. Good palliation was achieved without excess normal tissue toxicity (M. M. Kligerman, personal communication). An RTOG group study using WR-272 1 to reduce the bone marrow toxicity of hemibody irradiation suggested that there was an improvement in mean granulocyte and platelet counts and an acceleration of time to marrow recovery.258 However, there was no report of protection against parotid injury, lung damage, or nausea and vomiting. It remains to be seen if a higher radiation dose can be administered with the protector and if this higher dose translates into an improved efficacy of treatment.

direct inhibition of bone resorption,251.252only three patients experienced symptoms of hypocalcemia. Using a multivariate analysis, three variables, dose, infusion duration (longer being more toxic), and tumor site (head and neck, and lung), were significantly associated with hypotension. Two variables, dose and infusion duration were associated with vomiting. 250The single dose of WR-2721 chosen for Phase II studies with chemotherapy was 910 mg/m2. For the high-risk lung and head and neck cancer patients, as well as patients with prior neck irradiation, hypercalcemia and esophageal cancer a dose of 740 mdLm2 will be utilized. In the multiple dose study, 340 mg/m’ given 4 times per week for 5 to 6 weeks is the maximum tolerated dose. Nausea, vomiting, and asthenia were dose-limiting; rarely, allergic reactions were seen.253 F. WR-2721 and Radiotherapy Since the postulated mechanisms for WR-2721 involve inactivation of free-radicals and/or their damage, the protector must be located intracellularly when radiation is administered.229 The extrapolation from animal data with radiolabeled drug predicts that a 15 to 30-min interval between protector infusion and radiotherapy is optimal. Longer intervals allow for passive diffusion into tumors with increasing tumor protection. 254 In order to optimize the use of radioprotectors a number of issues must be addressed. Most experimental animal data used a large single dose of radiotherapy. It will be important to establish the degree of radioprotection seen with fractionated radiotherapy. Travis et al observed a PF of 2.3 for bone marrow using a single radiation dose, however, the PF was only 1.3 when 4 fractions were used.255 While the protection is better with larger single doses, such a scheme fails to take advantage of reoxygenation and may lead to excessive injury to lateresponding normal tissues. 256Furthermore, most clinical radiation schedules were developed to avoid serious late side effects. Since it is unethical to intentionally cause serious sideeffects, it has been difficult to identify optimal end-points with which to assess the efficacy of WR-2721. Acute effects (mucositis, hematologic toxicity, diarrhea, etc.) usually limit the actual rate of delivery of the radiation while it is the chronic toxicity (necrosis, fibrosis, etc.) that limits the total dose that can be delivered. In general acute toxicities will heal without consequence and the presence or severity of an acute effect will not predict for late damage. In reviewing the efficacy of WR-2721, Milas et a1.257found PFs of 1.2 to 2.0 for a variety of acute-responding murine tissues (marrow, immune system, gastrointestinal tract, skin) and PFs of 1.5 to 1.6 for late-effects (colon, lung, leg contracture). The efficacy of WR-2721 on therapeutic index will have to take into account both late and early injury. In a manner similar to that used in the fractionation studies, careful dose-escalation trials will be required. The Phase I study which used four drug doses per week for

G. WR-2721 and Cisplatin Phase I trials have been conducted to establish the toxicity of WR-2721 given prior to escalating doses of cisplatin. Initially, patients were prehydrated, but mannitol or diuretics were not administered. Mild transient nephrotoxicity, as judged by elevated serum creatine concentration, was observed in only 2/15 courses of cisplatin at a dose of 80 to 100 mg/m’. However, at a dose of 120 mg/m2, five of nine patients developed transient nephrotoxicity therefore, the protocol was modified to include mannitol diuresis.259 The data from the WR-272Vcisplatin Phase I trials suggest that WR-2721 may provide some protection against cisplatin induced nephrotoxicity, neurotoxicity, and ototoxicity.260When Al-Sarraf et al.261 used hydration and mannitol diuresis with 100 mg/m2 of cisplatin, 32% of their 34 patients experienced renal dysfunction; one patient died with renal failure. The trial of WR-2721 and cisplatin with mannitol diuresis was designed to determine if cisplatin could be safely administered at doses above 120 mg/m’. Patients received 740 mg/m2 WR-2721 in a 15-min intravenous infusion followed by a 30-min infusion of cisplatin beginning 15 min after the WR-2721 infusion was complete. Treatment was given every three to four weeks with monitoring for hematologic, renal, auditory, and neurologic toxicity. At doses of 120 to 135 mg/m’ transient nephrotoxicity was observed in 9% of courses; 29% of courses at 150 mg/m2 produced toxicity (Table 4). In all cases, renal function returned

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Critical Reviews In Table 4 Incidence of Nephrotoxicity Following WR-2721 and Cisplatin with Yannitol Dluresis CiSplptill dose (r&m*) 120 135 150 Total

Number of cour!ses

Number of courses with nephrotoxicity

153 36 17 256

1l(78) 4(11%) 5(29%) 22(9%)

Maximum serum creatinine (mg/dl) median (range) 3.4 (2.14.6) 3.7 (3.0-4.3) 4.0 (2.9-6.8)

From Coleman, C. N., Glover, D. J., and Tunisi, A. T., Cancer Chemotherapy: Principles and Practice, Chabner, B. A., Collins, J. M., and Myers, C. E., F!ds., W. B., Saunders, Philadelphia, 1989, in press. With permission.

to baseline normal values within one to two weeks. Hematologic toxicity was mild and infrequent. Granulocyte counts less than 1000/mm3 and white blood cell nadirs less than 2000/ mm3 were seen in 9 and 10% courses, respectively. Only 6% of courses had a platelet nadir less than 50,000/mm3. No patient had an infectious complication or required platelet transfusions. Grade 1 to 2 peripheral neuropathies occurred in 14% of patients after a median cumulative cisplatin dose of 825 mg/m2.262 The results from this trial suggest that WR-2721 may protect against cisplatin-induced peripheral neuropathy (Table 5). Patients who received WR-2721 pretreatment had a significantly lower incidence of peripheral nerve dysfunction compared to patients treated with cisplatin alone or combined with other antineoplastic agents. Forty-seven percent of patients treated with Adriamycin, cyclophosphamide, and cisplatin (50 mg/m*) developed neuropathy following a cumulative cisplatin dose of 358 mg/m2. When patients received five daily doses of cisplatin (40 mg/m’) with hypertonic saline and cyclophosphamide, 100% of patients experienced significant peripheral nerve damage after a median cumulative cisplatin dose of 327 mg/m*. However, with WR-2721 pretreatment, only 14% of patients treated

with 120 to 150 mg/m* doses of cisplatin developed mild to moderate neuropathies after a mean cumulative cisplatin dose of 825 mg/m2.263 There was no clinical evidence that WR-2721 decreased the antitumor efficacy of cisplatin. In the Phase I trial of WR-2721 and cisplatin with mannitol diuresis partial or complete responses were observed in 47/105 (45%) of patients with measurable disease. Objective responses occurred in 25/53 (47%) patients with metastatic melanoma, 12/22 (55%) with locally recurrent or metastatic head and neck cancer, and 7/13 (54%) patients with metastatic breast cancer refractory to conventional therapy. Two patients with melanoma and two with head and neck cancer had a complete response.262*2M H. WR-2721 and Cyclophosphamide The preliminary data from the Phase I trial of WR-2721 and cyclophosphamide suggested that WR-2721 protected against cyclophosphamide-induced granulocytopenia.265 Since variable drug doses and infusion rates were used, a controlled Phase II trial was conducted using constant drug doses to establish more precisely the level of protection of WR-2721. Initially,

Table 5 Cisplatin (DDP) Neuropathy Incidence of

neuropathy (number of patients)

Treatment type Overall DDP, Adriamycin + Cytoxan DDP + Velban DDP + Cytoxan DDP + WR-2721 DDP ’ b

Significantly Significantly

different different

Mean DDP dose onset of neuropathy mg/M* + S.D.

48% (115) 47% (19) 63% (8) 100%” (7) 15% (78) 67% (3)

399 358 370 327 870 200

k + k ” f

188 98 165 175 161b

from overall mean incidence, p < 0.05. from overall mean dose at onset, p < 0.01.

From Coleman, C. N., Glover, D. J., and Tunisi, A. T., Cancer Chemotherapy: Principles and Practice, Chabner, B. A., Collins, J. M., and Myers, C. E., Eds., W. B. Saunders, Philadelphia, 1989, in press. With permission.

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21 patients received 1500 mg/m2 of cyclophosphamide alone and were retreated four weeks later after hematologic recovery was complete with 740 mg/mZ of WR-2721 immediately prior to the same dose of cyclophosphamide. With WR-2721 pretreatment, 19 of 21 (90%) patients had improved white count and granulocyte count nadirs. The mean white count increased from 1760/mm3 with cyclophosphamide alone to 2500/mm3 with WR-2721 pretreatment (pc.0005). The mean granulocyte count nadir increased from 540/mm3 with cyclophosphamide to 1250/mm3 with WR-2721 and cyclophosphamide (pC.005). Following cyclophosphamide alone, neutropenic fevers developed in three patients; no patient experienced a febrile episode following WR-2721 and the same dose of cyclophosphamide. Platelet nadirs below 100,000/mm3 were observed in two patients treated with cyclophosphamide alone; no patient experienced thmmbocytopenia when they were retreated with the same cyclophosphamide dose with WR-2721 pretreatment. These data suggest that WR-2721 provides protection against cyclophosphamide-induced hematologic toxicity.*& It is not known whether the antitumor response rate is affected by the WR-2721. What remains to be established in the chemoprotector trials is whether or not a larger dose of antineoplastic agent can be administered, and if so, will this result in a superior response rate and survival. The use of protector may spare one organ toxicity which may lead to the developement of another. As with radiation therapy, careful assessment of therapeutic index for both acute and long term effects is essential.

initial studies were prototypes, the preliminary data from clinical trials using the current approaches listed in Table 1 suggest that there will be some ultimate clinical utility from the field of chemical modification. It is not likely that the current agents are ideal. Yet the knowledge derived from the trials with these modifiers, coupled with the advances in the understanding of the basic biology involved in sensitization and protection will lead to the rational development of newer-generation sensitizers and protectors.

1. Berger, N. A., Cancer chemotherapy: new strategies for success, J. Clin. Invest., 78, 1131, 1986. 2. Martin, D. S., StolfI, R. L., Sawyer, R. C. et al., Application of biochemical modulation with a therapeutically inactive modulating agent in clinical trials in cancer chemotherapy, Cancer Treat. Rep., 69,421, 1985. 3. Coleman, C. N., Bump, E. A., Kramer, R. A., Chemical modifiers of cancer therapy, J. C/in. On&., 6, 709, 1988. 4. Coleman, C. N., Hypoxia in tumors: a paradigm for the approach to biochemical and physiologic heterogeneity, J. Narl. Cancer Inst., 80, 310, 1988. 5. Coleman, C. N., Chemical modification of radiation and chemotherapy, in Cancer: Principles and Pracrice of Oncology, Devita, V. T., Jr., Hellman, S., and Rosenberg, S. A., Eds., J. B. Lippincott, Philadelphia, 1989, 2436, 6. Coleman, C. N., Glover, D. J., Turrisi, A. T., Radiation and chemotherapy sensitizers and protectors, in Cancer Chemotherapy: Principles and Pracfice. Chabner, B. A., Collins, J. M., and Myers, C. E., Eds., W. B. Saunders, Philadelphia, 1989, in press. 7. Hall, E. J., Radiobiology for the Radiologist, J. B. Lippincott, Philadelphia, 1988, 137. 8. Ward, J. F., Mechanisms of DNA repair and their potential moditication for radiotherapy, Int. J. Radial. Oncol. Biol. Phys., 12, 1027, 1986. 9. Finklestein, E. and Glatstein, E., Seduced by oxygen, Inr. J. Radar. Oncol. Biol. Phys., 14, 205, 1988 (editorial). 10. Me&r, A., Selective modification of glutathione metabolism, Science, 220, 472, 1983. 11. Bump, E. A., Yu, N. Y., Taylor, Y. C., et al., Radiosensitization and chemosensitization by dientylmaleate, in Radioprotectors and Anticarcingens, Nygaard, O., and Simic, M. Eds., Academic Press, New York, 1983, 208. 12. Russo, A., Mitchell, J. B., Fiielatein, E., et al., The effects of cellular glutathione elevation on the oxygen enhancement ratio, Radiar. Res.. 103, 232, 1985. 13. Yu, N. Y., Brown, J. M., Depletion of glutathione in vivo as a method of improving the therapeutic ratio of misonidazole and SR 2508, Int. J. Radar. Oncol. Biol. Phys., 10, 1265, 1984. 14. SartorelB, A. C., Therapeutic attack of hypoxic cells of solid tumors: presidential address, Cancer Res., 48, 775. 1988. 15. Chnpli~~, D. J., Durand, R. E., Stratford, 1. J., The radiosensitizing and toxic effects of RN-1069 on hypoxic cells in a murine tumor, Inr. J. Radiat. Oncol. Biol. Phys., 12, 1091, 1986. 16. Reman, E. M., Brown, J. M., Lemmon, M. J., et al., SR-4233: a new bioreductive agent with high selective toxicity for hypoxic mammalian cells, Inf. J. Radar. Oncol. Biol. Phys., 12, 1239, 1986.

I. Cytokines For Radioprotection The recent efforts to develop cytokines for clinical use has produced interesting results regarding protection from treatNeta and colleagues have ment-induced myelotoxicity. 267.268 demonstrated that interleukin 1 can protect mice against the lethal effects of whole body irradiation.2ffl*270The mechanism of this protection, and the role of other cytokines and growth factors in the protection remains to be elucidated. Given the preliminary results from chemotherapy studies, this general approach may ultimately have applicability for treatments that have substantial myelosuppression such as combined modality therapy with simultaneous radiation and chemotherapy, largefield radiotherapy as is used with lymphomas and certain abdominal and pelvic malignancies, and bone marrow transplantation.

IX. CONCLUSIONS The field of sensitizers and protectors is rapidly evolving in the laboratory and the clinic. Although additional information regarding the mechanisms of action is needed, sufficient information is available to permit the development of agents for use in clinical trials. Despite the fact that the drugs used in the 1990

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Critical Reviews In 17. Henk, J. M. Dues hyperbaricoxygen have a future in radiation therapy? Int. J. Radiat. 0~01. Biol. Phys., 7, 1125, 1981.

18. Thomllnsun, R. IL, Gray, L. H., The histological stmcture of some human lung cancers and the possible implications for radiotherapy, Br. J. Cancer, 9, 539, 1955. 19. Brown, J. M., Yu, N. Y., Radiosensitization of hypoxic cells in viva by SR 2508 at low radiation doses, Int. J. Radiat. Oncol. Biol. Phys., 10, 1207, 1984. 20. Skarsgard, L. D., Harrison, I., Durand, R. E., et al., Radiosensitization of hypoxic cells at low doses, Int. J. Radar. Oncol. Biol. Phys., 12, 1075, 1986. 21. Taylor, Y. C., Brown, J. M., Radiosensitization in multifraction schedules. II. Greater sensitization by 2-nitroimidazoles than by oxygen, Radar. Res., 112, 134, 1987. 22. Fu, K., Hurst, S., Begg, A. C., Brown, J. M., The effects of misonidazole during continuous low dose rate irradiation, in Radiation Sensitizers, Their Use in the Clinical Management of Cancer, Brady, L., Ed., 1980, 267. 23. Howes, A. E., An estimation of the changes in the proportions and absolute numbers of hypoxic-cells after irradiation of transplanted C3H mouse mammary tumors, Br. J. Radiol., 42, 441, 1969. 24. W, R. P., Sensitizers and radiation dose fractionation: Results and interpretations, fnt. J. Radiat. Biol. Phys., 12, 1049, 1986. 25. Chaplin, D. J., Olive, P. L., Durand, R. E., Intermittent blood flow in a murine tumor: radiobiological effects, CancerRes., 47,597, 1987. 26. Chaplin, D. J., Durand, R. E., Olive, P. L., Acute hypoxia in tumors: Implications for modifiers of radiation effects, ht. J. Radiat.

Cancer Res.. 44, 1735, 1984. 40. Rice, G. C., Hoy, C. A., Schlmke, R. T., Transient hypoxia enhances the frequency of dihydrofolate reductase gene amplification in Chinese hamster ovary cells, Proc. Natl. Acad. Sci., 83, 5978, 1986. 41. Kdllnowskl, F., Vaupel, R., Runkel, S., et al., Glucose uptake, lactate release, ketone body turnover, metabolism micromilieu, and pH distributions in human breast cancer xenografts in nude rats, Cancer Res., 48, 7264, 1988. 42. Varnes, M. E., DeIhlefson, L. A., Biaglow, J. A., The effect of pH on potentially lethal and sublethal damage recovery in A549 cells, Radiat. Res., 108, 80, 1986. 43. Llng, C. G., Robinson, E., Shrieve, D. C., Repair of radiation induce damage- dependence on oxygen and energy status, Int. J. R&at. Oncol. Biol. Phys., 15, 1179, 1988.

44. Sasaki, K., Hasbhnoto, T., Kawachino, et al., Intratumoral regional differences in DNA ploidy of gastrointestinal carcinomas, Cancer, 62, 2569, 1988. 45. Chapman, J. D., The detection and measurement of hypoxic cells in solid tumors, Cancer, 54, 2441, 1984. 46. Ling, L., Streffer, C., Sutherland, R., Decreased hypoxic toxicity and binding of misonidazole by low glucose concentration, ht. J. Radiat. Oncol. Biol. Phys., 12, 123 1, 1986. 47. Raleigh, J. A., Franko, A. J., Treiber, E. O., et al., Covalent

binding of a fluorinated 2-nitroimidazole to EMT-6 tumors in Balb/C mice: detection by F-19 nuclear magnetic resonance at 2.35 T, Int. J. Radiat. 01~01. Biol. Phys., 12, 1243, 1986. 48. Urtasun, R. C., Chapman, J. D., Raleigh, J. A., et al., Binding

Oncol. Biol. Phys., 12, 1279, 1986. 27. Olive, P. L., Chaplin, D. J., Durand,

R. E., Pharmacokinetics, binding and distribution of Hoechst 33342 in spheroids and murine tumours, Br. J. Cancer, 52, 739, 1985. 28. Trotter, M. J., Chaplin, D. J., Olive, P. L., The use of fluorescent probes to identify regions of transient perfusion in murine tumors, ht. J. Radiat. Oncol. Biol. Phys., 16, 931, 1989. 29. Gupta, V., Belli, J. A., Enhancement of radiation sensitivity by postirradiation hypoxia: time course and oxygen concentration dependency, Radiat. Res., 116, 124, 1988. 30. Hlrst, D. G., Anemia: a problem or an opportunity in radiotherapy?

49.

50.

51.

Int. J. Radiat. Oncol. Biol. Phys., 12, 2009, 1986. 31. Stratford, I. J., Adams, G. E., Horsman, M. R., et al., The in-

teraction of misonidazole with radiation, chemotherapeutic agents, or heat, Cancer Clin. Trials, 3, 231, 1980. 32. Hall, E. J., Astor, M. A., B&low, J., et al., The enhanced sensitivity of mammalian cells to killing by X-rays after prolonged exposure to several nitroimidazoles, Int. J. Radiat. Oncol. Biol. Phys., 8, 447, 1982. 33. Panicucci, R., McClelland,

R. A., Rauth, A. M., Stable reduction products of misonidazole, Int. J. Radiat. Oncol. Biol. Phys., 12, 1227,

1986. 34. Laderoute, K. R., Eryavec,

E., McClelland, R. A., et al., The production of strand breaks in DNA in the presence of the hydoxylamine of SR-2508 (l-[N-(2-hydroxylethyl)acetamido]-2-nitroimidazole at neutral pH, Int. J. Radiat. Oncol. Biol. Phys., 12, 1215, 1986. 35. Biaglow, J. E., Cellular electron transfer and radical mechanisms for drug metabolism, Radar. Res., 86, 212, 1981. 36. B&glow, J. E., Vames, M. E., Rolzlu-Towle, L., et al., Biochemistry of reduction of nitroheterocycles, Biochem. Pharmacol., 35, 77,

1986. 37. Vargbese, R. J., Wbitmore, G. F., Identification of a reactive glutathione conjugate as a metabolite of SR-2508 in CHO cells, ht. J. Radiat. Oncol. Biol. Phys., 12, 1223, 1986.

38. Brown, J. M., Clinical trials of radiosensitizers: What should wi expect? Int. J. Radiat. Oncol. Biol. Phys., 10, 425, 1984. 39. Schlmke, R. T., Gene amplification, drug resistance, and cancer,

246

52.

of 3H-misonidazole to solid human tumors as a measure of tumor hypoxia, Int. J. Radar. Oncol. Biol. Phys., 12, 1263, 1986. Urtasun, R. C., Chapman, J. D., Raleigh, J. A., et al., Measurement of the hypoxic fraction in solid human tumors utilizing the Wmisonidazole binding “in vivo” technique. Proceedings of the Chemical Modifiers of Cancer Treatment, Paris, abstract l-10, 1988. Raleigh, J. A., Franko, A. J., Treiber, E. O., et al., Covalent binding of a fluorinated 2-nitroimidazole to EMT-6 tumors in BalblC mice. Detection by F-19 nuclear magnetic resonance at 2.35 T, Int. J. Radiat. Oncol. Biol. Phys., 12, 1243, 1986. Raleigh, J. A., Franko, A. J., Trimble, L. A., and Allen, P. S., Development of a 19F magnetic resonance spectroscopy method for measuring oxygen deficiency in tumor tissue. Proceedings of the Chemical Modifiers of Cancer Treatment, Paris, abstract l-2, 1988. Maxwell, R. J., Workman, R., and GrifRths, J. R., Demonstration of tumor-selective retention of fluorinated nitroimidazole probes by *g magnetic resonance spectroscopy in vivo, Int. J. Radar. Oncol. Biol.

Phys.. 16, 925, 1989. 53. Rofstad, E. K., DeMuth, P., Fenton, B. M., et al., “P NMR spec-

troscopy and HbO, cryospectrophotometry in prediction of tumor radioresistance caused by hypoxia, Int. J. Radar. Oncol. Biol. Phys., 16, 919, 1989. 54. Okunieff, P., Ramsay, J., Tokuhiro, T., et al., Estimation of tumor

oxygenation and metabolic rate using 31PMRS: correlation of longitudinal relaxation with tumor growth rate and DNA synthesis, Inr. J. Radiat. Oncol. Biol. Phys., 14, 1185, 1988. 55. Hlatky, L., Hong, C., and Sachs, R., Patterns of misonidazole binding as observed in the sandwich system, Int. J. Radiat. Oncol. Bid. Phys., 16, 943, 1989. 56. Matky, L., Hong, C., and Sachs, R., An intrinsic marker for hypoxia, Cancer Res., 49, 5162, 1989. 57. Sutherland, R. M., Importance of critical metabolites and cellular interactions in the biology of microregions of tumors, Cancer, 58, 1668, 1986. 58. Heacock, C. S., and Sutherland, R. M., Induction characteristics of oxygen regulated proteins, Int. J. Radiat. Oncol. Biol. Phys., 12, 1287,

1986.

Volume 10, Issue 3

59. Sutherland, R. M., Heacock, C. S., Keng, P. C., et al., Hypoxia-

Int. J. Radiat. Oncol. Biol. Phys., 10, 1749, 1984. 79. Brown, J. M., and Yu, N. Y., The optimum time for irradiation relative to tumour concentration of hypoxic cell sensitizers, Pr. J. Radiol., 53, 915, 1980. 80. Coleman, C. N., Halsey, J., Cox, R. S., et al., prediction of the neurotoxicity of the hypoxic cell radiosensitizer SR 2508 from the pharmacokinetic profile, Cancer Res., 47, 319, 1987. 81. Coleman, C. N., Nell, L., Howes, A. E., et al., Initial results of a phase I trial of continuous infusion SR 2508 (Etanidazole): a Radiation Therapy Oncology Group study, Int. J. Radiat. Oncol. Biol. Phys., 16, 1085, 1988.

induced enhanced synthesis of specific proteins, RNA and amplified DNA, ht. .I. Radiat. Oncol. Biol. Phys., 16, 957, 1989. 60. Gatenby, R. A., Kessler, II. B., Rosenblum, J. S., et al., Oxygen distribution in squamous cell carcinoma metastases and its relationship to outcome of radiation therapy, ht. J. Rudiut. Oncol. Biol. Phys., 14, 831, 1988. 61. Hong, A., Rojas, A., and Diihe, S., Normobaric oxygen as a radiosensitizer of hypoxic tumor cells, ht. J. Radiut. Oncol. Biol. Phys., 16, 1097, 1989. 62. Henk, J. M., Does hyperbaric oxygen have a future in radiation therapy? ht. J. Radiat. Oncol. Biol. Phys., 7, 1125, 1981. 63. Bush, R. S., The significance of anemia in clinical radiation therapy, Int. J. Radiat. Oncol. Biol. Phys., 12, 2047, 1986 (editorial). 64. Overgaard, J., Sand, H. H., Jorgensen, K., et al., Primary radiotherapy of larynx and pharynx carcinoma - an analysis of some factors influencing local control and survival, Int. J. Rudiat. Oncol. Biol. Phys., 12, 515, 1986. 65. Quilty, P. M., and Duncan, W., The influence of hemoglobin level on the regression and long term local control of transitional cell carcinoma of the bladder following photon irradiation, Int. J. Radiat. Oncol. Biol. Phys., 12, 1735, 1986. 66. Wasserman, T. H., Phillips, T. L., Johnson, R. J., et al., Initial United States clinical and pharmacologic evaluation of misonidazole (RO-07-0582), and hypoxic cell radiosensitizer, Int. J. R&at. Oncoi. Biol. Phys.. 5, 775, 1979. 67. Coleman, C. N., Wasserman, T. H., Phillips, T. L., et al., Initial pharmacology and toxicology of intravenous desmethylmisonidazole, Int. J. Radiat. Oncol. Biol. Phys., 8, 371, 1982. 68. Dlsche, S., Saunders, M. I., Flockhart, I. R., et al., Misonidazole - a drug for trial in radiotherapy and oncology, Int. J. Radiut. Oncol. Biol. Phys., 5, 851, 1979. 69. Diache, S., Chemical sensitizers for hypoxic cells: a decade of experience in clinical radiotherapy, R&other. Oncol.. 3, 97, 1985. 70. Overgaard, J., Hansen, H. S., Anderson, A. P., et al., Misonidazole combined with split course radiotherapy in the treatment of invasive carcinoma of larynx and pharynx, Int. J. R&at. Oncol. Biol. Phys., 16, 1065, 1989. 71. Brown, J. M., and Lee, W. W., Pharmacokinetic considerations in radiosensitizer development, in Radiation Sensitizers, Their Use in the Clinical Management of Cancer, Brady, L., Ed., 1980, 2. 72. Brown, J. M., Clinical perspectives for the use of new hypoxic cell sensitizers, Int. J. R&at. Oncol. Biol. Phys., 8, 1491, 1982. 73. Brown, J. M., Yu, N. Y., Brown, D. M., et al., SR-2508: a 2nitroimidazole amide which should be superior to misonidazole as a radiosensitizer for clinical use, fnt. J. Radiut. Oncol. Biol. Phys., 7, 695, 1981. 74. Honess, D. J., Wasserman, T. H., Workman, P., et al., Additivity of radiosensitization by the combination of SR 2508 (etanidazole) and Ro 03-8799 (pimonidazole) in a murine tumor system, Int. J. Rudiur. Oncol. Biol. Phys., 15, 671, 1988. 75. Saunders, M. I., Anderson, P. J., Bennett, M. H., et al., The clinical testing of Ro 03-8799 - pharmacokinetics, toxicology, tissue and tumor concentrations, Int. J. Radiat. Oncol. Biol. Phys., 10, 1759, 1984. 76. Roberts, J. T., Bleehan, N. M., Walton, J. I., et al., A clinical phase I toxicity study of Ro 03-8799: plasma, urine, tumour and normal brain pharmacokinetics, Br. J. Radial., 59, 107, 1986. 77. Minchiuton, A. I., and Stratford, M. R. L., A comparison of tumor and normal tissue levels of acidic, basic and neutral 2nitroimidazole radiosensitizers in mice, Int. J. Rudiat. Oncol. Biol. Phys.. 12, 1117, 1986. 78. Coleman, C. N., Urtasun, R. C., Wasserman, T. H., et al., Initial report of the phase I trial of the hypoxic cell radiosensitizer SR 2508,

1990

82. Coleman, C. N., Wasserman, T. H., Urtasun, R. C., et al., Final report of the phase I trial of the hypoxic cell radiosensitizer SR 2508 (etanidazole): Radiation Therapy Oncology Group 83-03, Int. J. Radiut. Oncol. Biol. Phys., 16, 1085, 1989. 83. Allahmis, M. J., Turner, A. R., Partington, J. P., and Urtasun, R. C., Effect of misonidazole therapy on human granulopoietic stem cells, Cancer Treat. Rep., 64, 1097, 1980. 84. Coleman, C. N., Wasserman, T. H., Urtasuu, R. C., et al., Phase I trial of the hypoxic cell radiosensitizer SR 2508: the results of the five to six week schedule, Int. J. Radiat. Oncol. Biol. Phys., 12, 1105, 1986. 85. Awwad, H. K., El Badawy, S., Zagloul, M., et ai., Pharmacokinetics of etanidazole (SR-2508) in bladder and cervical cancer: evidence of diffusion from urine, Int. J. Radiut. Oncol. Biol. Phys., 16, 1083, 1989. 86. Awwad, H. K., El Merzabani, M. M., El Badawy, S., et al., Misonidazole in the preoperative and radical radiotherapy of bladder cancer, in Radiation Sensitizers, Their Use in the Clinical Management of Cancer, Brady, L., Eds.,Masson publishing, USA, Inc.,New York, 1980, 381. 87. Newman, H., Bleehan, N. M., and Workman, P., A phase I study of the combination of two hypoxic cell radiosensitizers, Ro-O3-8799 and SR 2508: toxicity and pharmacokinetics, Int. J. R&at. Oncol. Biol. Phys.. 12. 1113, 1986. 88. Newman, H. F. V., Bleehan, N. M., Ward, R., and Workman, P., Hypoxic cell radiosensitization in the treatment of high grade gliomas: a new direction using combined Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole), Int. J. Rudiar. Oncol. Biol. Phys. 15, 677, 1988. 89. Newman, H. F. V., Ward, R., Workman, P., and Bleebau, N. M., The multi-dose clinical tolerance and pharmacokinetics of the combined radiosensitizers, Ro 03-8799 (pimonidazole) and SR 2508 (etanidazole), Int. J. Radiur. Oncol. Biol. Phys.. 15, 1073, 1988. 90. Stratford, I. J., O’Neill, P., Sheldon, P. W., et al., RSU 1069, a nitroimidazole containing an aziridine group, Biochem. Pharmac., 35, 105. 1986. 91. Ahmed, I., Jenkins, T. C., Walling, J. M. et al., Analogues of RSU-1069: radiosensitization and toxicity in vitro and in viva, Int. J. Radiat. Oncol. Biol. Phys., 12, 1079, 1986. 92. Deacon, J. M., Holliday, S. B., Ahmed, I., et al., Experimental pharmacokinetics of RSU-1069 and its analogs: high tumor/plasma ratios, Int. J. Radiat. Oncol. Biol. Phys., 12, 1087, 1986. 93. Melgaard, B., Hansen, H. S., Kamienlecka, Z. et al., Misonidazole neurotoxicity: a clinical, electrophysiological, and histological study, Ann. Neural., 12, 10, 1982. 94. Wasserman, T. H., Nelson, J. S., and VonGerlchton, D., Neuropathy of nitroimidazole radiosensitizers: clinical and pathological description, Int. J. Radiat. Oncol. Biol. Phys., 10, 1725, 1984. 95. Varnes, M. E. and Biagiow, J. E., Misonidazole-induced biochemical alterations of mammalian cells: effects on glycolysis, Int. J. Radiut. Oncol. Biol. Phys., 8, 683, 1982. 96. Stevenson, M. A., Calderwood, S. K., and Coleman, C. N., Effects of nitroimidazoles on neuronal cells in viva, Int. J. Radiat. Oncol. Biol. Phys. 16, 925, 1989.

247

Critical Reviews In 97. Tanasichuk,

Il., Urtasun, R. C., Fulton, D. S. et al., Misonidazole with dexamethasone rescue: an escalating dose toxicity study, Int. J. Radiar. Oncol. Biol. Phys., 10, 1735, 1984. 98. Coleman, C. N., Hirst, V. K., Brown, D. M. et al., Tbe effect of vitamin B, on the neurotoxicity and pharmacology of desmethyhnisonidazole and misonidazole: clinical and laboratory studies, Inr. J. Radiar. Oncol. Biol. Phys., 10, 1381, 1984. 99. Fischer, J. J., Rockwell, S., and Martin, D. F., Perfluorochemicals and hyperbaric oxygen in radiation therapy, Int. J. Radiar. Oncol. Biol. Phys., 12, 95, 1986. 100. Siemann, D. W., and Macler, I. M., Tumor radiosensitization thtougb reductions in hemoglobin affinity, Int. J. Radiar. Oncol.’ Biol. Phys., 12, 1295, 1986.

101. Teicher, B. A., and Rose, C. M., Effects of dose and scheduling on growth delay of the Lewis lung carcinoma produced by the perfluorochemical emulsion, Fluosol-DA, Inr. J. Radiar. Oncol. Biol. Phys., 12, 1311, 1986. 102. Moulder, J. E., and Fish, B. L., Intermittent use of a perfluorochemical emulsion (Fhrosol-DA 20%) and carbogen breathing with fractionated irradiation, Inr. J. Radiar. Oncol. Eiol. Phys., 15, 1193, 1988. 103. Rockwell, S., Irvin, C. G., and Kelley, M., I’mclinical studies of a perfluorochemical emulsion as an adjunct to radiotherapy, Inr. J. Radiar. Oncol. Biol. Phys., 15, 913, 1988.

104. Goodman, R. L., Moore, R. E., Davis, M. E. et al., Perfhuocarbon emulsions in cancer therapy: preliminary observations on presently available formulations, Inr. J. Radiar. Oncol. Biol. Phys., 10, 1421, 1984. 105. West, L., McIntosh, N., Geadler, S. et al., Effects of intravenously infused Fhrosol-DA 20% in rats, Inr. J. Raa’iar. Oncol. Biol. Phys., 12, 1319, 1986. 106. Rose, C., Lust&, R., McIntosh, N. et al., A clinical trial of Fh~osolDA 20% in advanced squamous cell carcinoma of the head and neck, Inr. J. Radiar. Oncol. Biol. Phys., 12, 1325, 1986. 107. Rockwell, S., Keyes, S. R., and Sartorelli, A. C., Reclinical studies of porfiimycin as an adjunct to radiotherapy, Radiar. Res., 116, 100,

1988. 108. Steel, G. G., The search for therapeutic gain in the combination of radiotherapy and chemotherapy, Radiorher. Oncol., 11, 31, 1988. 109. Zeman, E. M., Hirst, V. K., Lenunon, M. J. et al., Enhancement of radiation-induced tumor cell killing by the hypoxic cell toxin SR 4233, Radiorher. Oncol., 12, 209, 1988. 110. Baker, M. A., Zeman, E. M., Hirst, V. K., and Brown, J. M., Metabolism of SR 4233 by Chinese Hamster ovary cells: basis of selective hypoxic cytotoxicity, Cancer Res., 48, 5947, 1988. 111. Adams, G. E., Barnes, D. W. I-L, du Boulay, C., et al., Induction of hypoxia in normal and malignant tissues by changing the oxygen affinity of hemoglobin - implications for therapy, Inr. J. Radiar. Oncol. Biol. Phys., 12, 1299, 1986. 112. Chaplin, D. J., Postirradiation modification of tumor blood flow: a method to increase the effectiveness of chemical radiosensitizers, Radiar. Res., 115, 292, 1988. 113. Okunieff, P., Kallinowski, F., Vaupel, R., and Neuringer, I. J., Effects of hydrahuine-induced vasodilation on the energy metabolism of murine tumors studied by in vivo )*P-nuclear magnetic resonance spectroscopy, J. Narl. Cancer. Inst., 80, 745, 1988. 113a. Wood, P. J., and Hirst, D. G., Calcium antagonists as radiation modifiers: site specificity in relation to tumor response, Inr. J. Radiar. Oncol. Biol. Phys., 16, 1141, 1989. 114. Kramer, R. A., Greene., K., Ahmad, S. et al., Chemosensitization of L-phenylalanine mustard by the thiol-modulation agent buthione sulfoximine, Cancer Res., 47, 1593, 1987. 115. Phillips, T. L., Mitchell, J. B., DeGraff, W. et al., Variation in sensitizing efficiency for SR 2508 in human cells dependent on glu-

248

tathione content, Inr. J. Radiar. Oncol. Biol. Phys., 12, 1627, 1986. 116. Mitchell, J. B., Phillips, T. L., DeGraff, W. et al., The relationship of SR-2508 sensitizer enhancement ratio to cellular glutathione in human tumor cell lines, Inr. J. Radiar. Oncol. Biol. Phys., 12, 1143, 1986. 117. Bump, E. A., Yu, N. Y., and Brown, J. M., Radiosensitization of hypoxic tumor cells by depletion of intracellular glutathione, Science, 127, 544, 1982. 118. Evans, J. W., Taylor, Y. C., and Brown, J. M., The role of glutathione and DNA strand break repair in determining the shoulder of the radiation survival curve, Br. J. Cancer, 49(suppl. 6); 49, 1984. 119. Mitchell, J. B., Russo, A., Biilow, J. E. et al., Cellular glutathione depleton by diethylmaleate or buthionine sulfoximine: no effect of glutathione depletion on the oxygen enhancement ratio, Radiar. Res., 96, 422, 1983. 120. B&low, J. E., Varnes, M. E., Tuttle, S. W. et al., The effect of L-buthionine sulfoximine on the aerobic radiation response of A549 human lung carcinoma cells, fnr. J. Radiar. Oncol. Eiol. Phys., 12, 1139, 1986. 121. Van der Schaus, G. P., Vos, O., Roes-VerheU, W. S. D. et al., Tbe influence of oxygen on the induction of radiation damage in DNA in mammalian cells after sensitization by intracellular glutathione depletion, Inr. J. Radiar. Biol., 50, 453, 1986. 122. Rice, G. C., Bump, E. A., Shrieve, D. C. et al., Quantitative analysis of cellular glutathione by flow cytometry utilizing monochlorobimane: some applications to radiation and drug resistance in virro and in vivo, Cancer Res., 46, 6105, 1986. 123. Shrieve, D. C., Bump, E. A., and Rice, G. C., Heterogeneity of cellular glutathione among cells derived from a murine fibrosarcoma or a human renal cell carcinoma detected by flow cytometric analysis, J. Biol. Chem., 263, 14107, 1988. 124. Phillips, T. L., Mitchell, J. B., DeGraff, W. G. et al., Alteration in SR 2508 radiosensitization caused by chemical manipulation of glutathione (GSH) levels, Inr. J. Radiar. Oncol. Biol. Phys. , 16, 1335, 1989. 125. Ling, C. C., Wong, R. S. L., Basal, R. et al., Cytotoxicity due to BSO, SR 2508 and hypoxia, proceedings of the Chemical Modifiers of Cancer Treatment, Paris, abstract 7-21; 1988. 126. Chnbner, B. A., The oncologic end game, J. Clin. Oncol., 4, 625, 1986. 127. Ozols, R. F., Masuda, IL, and Hamilton, T. C., Keynote address: mechanisms of cross-resistance between radiation and anti-neoplastic drugs, NC1 Monogr., 6, 159, 1988. 128. Arriek, B. A. and Nathan, C. F., Glutathione metabolism as a determinant of therapeutic efficacy: a review, Cancer Res., 44, 4224, 1984. 129. Russo, A., Carmichael,

J., Friedman, N. et al., Tbe role of intracellular glutathione in antineoplastic chemotherapy, Inr. J. Radiar. Oncol. Biol. Phys., 12, 1347, 1986. 130. Bump, E. A., Yu, N. Y., Taylor, Y. C. et al., Radiosensitization and chemosensitization by diethylmaleate, in Radioprorecrors and Anricarcinogens, Nygaard, 0. and Sink, M., Eds., Academic press, New York, 1983, 297. 131. Shreive, D. C. and Harris, J. W., Effects of glutathione depletion by buthionine sulfoximine on the sensitivity of EMT6/SF cells to chemotherapy agents or X-radiation, Inr. J. Radiar. Oncol. Eiol. Phys., 12, 1171, 1986.

132. Andrews, P. A., Murphy, M. P., and Howell, S. B., Differential potentiation of alkylating and platinating agent cytotoxicity in human ovarian carcinoma cells by glutathione depletion, Cancer Res., 45, 6250, 1985. 133. Crook, T. R., Souhami,

R. L., Whyman, G. D. et al., Glutathione depletion as a determinate of sensitivity of human leukemia cells to cyclophosphamide, Cancer Res., 46, 5035, 1986.

Volume 10, issue 3

Oncology/Hematology 134. Green, J. A., Vi&a, D. T., Young, R. C. et al., Potentiation of melphalan cytotoxicity in human ovarian cancer cell lines by glutathione depletion, Cancer Rex, 44, 5427, 1984. 135. Russ-o, A. and Mitchell, J. B., Potentiation and protection of doxorubicin cytotoxicity by cellular glutathione modulation, Cancer Treat. Rep., 69, 1293, 1985. 136. Ono, K. and Shrieve, D. C., Enhancement of EMT6/SF tumor cell killing by mitomycin-C and cyclophosphamide following in viva administration of buthionine sulfoximine. fnr. J. Radiur. Oncol. Biol. Phys., 12, 1175. 1986. 137. Tsutsui, K., Komura, C., Ono, K. et al., Chemosensitization by buthionine sulfoximine in viva, fnt. J. Rudiur. Oncol. Biol. Phys., 12. 1183, 1986.

155.

156.

157.

158.

138. Kramer, R. A., Greene, K., Ahmad, S. et al., Chemosensitization of L-phenylalanine mustard (L-PAM) by the thiol modulation agent buthionine sulfoximine (BSO), Cancer Res., 47, 1593, 1987. 139. Russo, A., DeGraff, W., Friedman, N. et al., Selective modulation of glutathione levels in human normal versus tumor cells and subsequent differential response to chemotherapy drugs, Cancer Rex, 46, 2845, 1986. 140. Adams, D. J., Carmichael, J., and Wolf, C. R., Altered mouse bone marrow glutathione and glutathione transferase levels in response to cytotoxins, Cancer Res., 45, 1669, 1985. 141. Cannichsel, J., Adams, D. J., Ansell, J. et al. , Glutathione and glutathione transferase levels in mouse granulocytes following cyclophosphamide administration, Cancer Res., 46, 735, 1986. 142. Carmichael, J., Friedman, N., Tocbner, Z. et al., Inhibition of the protective effect of cyclophosphamide by pre-treatment with buthionine sulfoximine, Inr. J. Radiat. Oncol. Biol. Phys., 12, 1191, 1986. 143. Russo, A., Tochner, Z., Pblllips, T. et al., In vivo modulation of glutathione by buthionine sulfoximine: effect on marrow response to melphalan, Inr. J. Rudiuf. Oncol. Biol. Phys., 12, 1187, 1986. 144. Wang, A. L. and Tew, K. D., Increased glutathione-S-transferase activity in a cell line with acquired resistance to nitrogen mustards, Cancer Treaf. Rep., 69, 677, 1985. 145. Moscow, J. A. and Cowan, K. Ii., Multidrug resistance, J. Nurl. Cancer fnsr.. 80, 14, 1988. 146. Kramer, R. A., Zakher, J., and Kim, G., Role of glutathione redox cycle in acquired and de nova multidrug resistance, Science, 241, 694, 1988. 147. Fertil, 8. and M&&e, E. P., Intrinsic radiosensitivity of human cell lines is correlated with radioresponsiveness of human tumors: analysis of 101 published survival curves, Inr. J. Radiut. Oncol. Biol. Phys., 11, 1699, 1985. 148. WeichseIbaum, R. R., Dahlberg, W., and Little, J. B., Inherently radioresistant cells exist in some human tumors, Proc. Narl. Acud. Sci. USA, 82, 4732, 1985.

159.

160.

161.

162.

163. 164.

165.

166.

167.

168.

169.

170.

149. Louie, K. G., Behrens, B. C., Khsella, T. J. et al., Radiation survival parameters of antineoplastic drug-sensitive and -resistant human ovarian cancer cell lines and their modification by buthionine sulfoximine, Cancer Res.. 45, 2110, 1985. 150. Mitchell, J. B., Gamson, J., Russo, A. et al., Chinese Hamster Pleiotropic multi-drug resistant cells are not radioresistant, NCI Monogr., 6, 187. 1988. 151. Szybalski, W., X-ray sensitization by halopyrimidines, Cancer Chemother. Rep., 58. 539, 1974. 152. Kinsella, T. J., Mitchell, J. B., Russo, A. et al., The use of halogenated thymidine analogs as clinical radiosensitizers: rational, current status, and fuNR prospects: non-hypoxic cell sensitizers, Int. J. Radiur. Oncol. Biol. Phys. 10, 1399, 1984. 153. Mitchell, J. B., Russo, A., Kinsella, T. J. et al., The use of nonhypoxic cell sensitizers in radiobiology and radiotherapy, Int. J. Rudiat. Oncol. Biol. Phys., 12, 1513, 1986. 154. Kinsella, T. J., Dobson, P. P., Mitchell, J. B. et al., Enhancement

171. 172.

173. 174.

175.

1990

of X-ray induced DNA damage by pretreatment with halogenated pyrimidine analogs, Inr. J. Radiat. Oncol. Biol. Phys., 13, 733, 1987. Malaise, E. P., The non-hypoxic cell sensitizers: their use in radiobiology and radiotherapy, in Proc. 8th Int. Congress of Radiation Research, Fielden, E. M. et al. Eds., Taylor & Francis, London, 750, 1987. Bagshaw, M. A., Doggett, R. L. S., Smith, K. C. et al., Intraarterial 5bromodeoxyuridine and X-ray therapy, Am. J. Roentgenol., 99, 886, 1967. Kin&a, T. J., Russo, A., MItcheU, J. 8. et al., A phase I study of intermittent intravenous bromodeoxyuridine (BUdR) with conventional irradiation, Inr. J. Radiur. Oncol. Biol. Phys., 10, 69, 1984. Kinsella, T. J. and Glatstein, E., Clinical experience with intravenous radiosensitizers in unresectable sarcomas, Cancer. 59, 908, 1987. Kinsella, T. J., Russo, A., Mitchell, J. B. et al., A phase I study of intravenous iododeoxyuridine as a clinical radiosensitizer, Inr. J. Radiat. Oncol. Biol. Phys., 11, 1941, 1985. Klecker, R. W., Jenkins, J. F., Kin&a, T. J. et al., Clinical pharmacology of 5-iodo-2’-deoxyuridine and 5-iodouracil and endogenous pyrimidine modulation, Clin. Pharmucol. Ther., 38, 45, 1985. Perez, L. M. and Greer, S., Sensitization to X-ray by 5-chloro-2’deoxycyitidineco-administered with tetrahydrouridine in several mammalian cell lines and studies of 2’-chloro derivatives, Int. J. Radiar. Oncol. Biol. Phys., 12, 1523, 1984. Russell, K. J., Rice, G. C., and Brown, J. M., in vizro and in vivo radiation sensitization by he halogenated pyrimidine 5-chloro-2’-deoxycytidine, Cancer Res., 46. 2882. 1986. &II, E. J., Radiobiology for the Radiologist, Lippincott, Philadelphia, 107, 1988. Rasey, J. S., and Nelson, N. J., Discrepancies between patterns of potentially lethal damage repair in the RIF- I tumor system in virro and in viva. Radiar. Res., 93. 157, 1983. Nakatsugawa, S. and Dewey, W. C., The role in cancer therapy in inhibiting recovery for PLD induced by radiation or bleomycin, Inr. J. Radiar. Oncol. Biol. Phys., 10, 1425, 1984. Nakatsugawa, S., Kada, T., Nidaido, 0. et al., PLDR inhibitors: their biological and clinical implications, Br. J. Cancer, 49, (suppl. 6). 43, 1984. Weichselbaum, R. R. and Little, J. B., Repair of potentially lethal X-ray damage and possible applications to clinical radiotherapy, Int. J. Radial. Oncol. Biol. Phys., 9, 91. 1982. Weichselbaum, R. R., SchmIt, A., and Little, J. B., Cellular repair factors influencing radiocurability of human malignant tumours, Br. J. Cancer, 45, 10, 1982. Guicbard, M., Weichselbaum, R. R., Little, J. B. et al., Potentially lethal damage repair as a possible determinant of human tumour radiosensitivity, Radiother. Oncol., I, 263, 1984. Brown, D. M., Dionet, C., and Brown, J. M., Inhibition of X-ray induced potentially lethal damage (PLD) repair in aerobic plateau-phase Chinese hamster cells by misonidazole, Rudiar. Res., 97, 162, 1984. Berger, N. A., Poly(ADP-ribose) in the cellular response to DNA damage, Rudiut. Res., 101, 4. 1985. Cleaver, J. E., Mii, K. M., and Morgan, W. F., Do inhibitor studies demonstrate a role for Poly(ADP-ribose) in DNA repair? Radiut. Res.. 101. 16. 1985. Oleinick, N. L. and Evans, H. H., Poly(ADP-ribose) and the response of cells to ionizing radiation, Rudiar. Res., 101. 29, 1985. Brown, D. M., Evans, J. W., and Brown, J. M., The influence of inhibitors of Poly(ADP-ribose) polymerase on X-ray influenced potentially lethal damage, Br. J. Cancer, 49 (suppl 6), 27. 1984. Ben-Hur, E., Chen, C. C., and Elkind, M. M., Inhibitors of poly(adenosine diphosphoribose) synthetase. examination of metabolic perturbations, and enhancement of radiation response in Chinese hamster cells. Cancer Res.. 45, 2123. 1985.

249

Critical Reviews In 176. Tbraves, P. J., Mossman, K. L., Frazier, D. M. T. et al., Inhibition of potentially lethal radiation damage repair in normal and neoplastic cells by 3-aminobenzamide: an inhibitor of poly(ADP-ribosylation), ht. J. Radiat. Oncol. Biol. Phys., 12, 1541, 1986. 177. Jonson, G. G., wellen, E., Pero, R. W. et al., Radiosensitization effects of nicotinamide on malignant and normal mouse tissue, Cancer Res., 45, 3609, 1985. 178. Horsman, M. R., Brown, D. M., Lemmon, M. J. et al., Preferential tumor radiosensitization by analogs of nicotinamide and benzamide, Int. J. Radiat. 0~01. Biol. Phys. , 12, 1307, 1986. 179. Horsman, M. R., Chaplin, D. J., and Brown, J. M., Radiosensitization by nicotinamide in vivo: a greater enhancement of tumor damage compared to that of normal tissue, Radiat. Res., 109, 479, 1987.

180. L&h, J. T., Lee, E. S., L&e, D. V. et al., Enhanced X-ray sensitivity of human colon tumor cells by combination of N-methylformamide with chemotherapeutic agents, Int. J. Raa’iat. Oncol. Biol Phys., 12, 1423, 1986. 181. Arundel, C. M., Glicksman, A. S., and Leith, J. T., In vitro effects of N,N-dimetblyformamide on sublethal and potentially lethal damage recovery processes after X-irradiation in heterogenous human colon tumor cells, Cancer Res., 45, 5557, 1985. 182. Anmdel, C. M., Kenney, S. M., L&h, J. T. et al., Contrasting effects on the differentiating agent sodium butyrate on recovery processes after X-irradiation in heterogenous human colon tumor cells, Int. J. Radiat. Oncol. Biol. Phys., 12, 959, 1986. 183. Dexter, D. L., Lee, E. S., Bliven, S. F. et al., Enhancement by Nmethylfonuamide of the effect of ionizing radiation on a human colon tumor xenogmfted in nude mice, Cancer Res., 44, 4942, 1984. 184. Spremulli, E. N. and Dexter, D. L., Polar solvents: a novel class of antineoplastic agents, J. Clin. Oncol., 2, 227, 1984. 185. Brigeliw, R., Mixed disulfides: biological functions and increase in oxidative stress, in Oxidotive Stress, Sies, H., Ed., Academic Press, London, 1985, 243. 186. Bump, E. A., Jacobs, G. P., Lee, W. W. et al,, Radiosensitization by diamide analogs and arsenicals, Int. J. Radiat. Oncol. Biol. Phys., 12, 1533, 1986. 187. Harris, J. W., Power, J. A., and Koch, C. J., Radiosensitization of hypoxic mammalian cells by diamide. I. Effect of experimental conditions, Radiat. Res., 64, 270, 1975. 188. Rose, C. M., MiBar, J. L., Peacock, J. IL et al., Differential enhancement of melphalan cytotoxicity in tumor and normal tissue by misonidazole, in Radiation Sensitizers. Their Use in the Clinical Management of Cancer,Brady, L. W., Ed., Masson, New York, 1980, 250. 189. McNally, N. J., Enhancement of chemotherapy agents, Int. J. Radiat. Oncol. Biol. Phys. 8, 593, 1982. 190. Siemann, D. W., Modification of chemotherapy by nitroimidazoles, Int. J. Radiat. Oncol. Biol. Phys., 10, 1585, 1984. 191. Brown, J. M. and Hirst, D. G., Effect of clinical levels of misonidazole on the response of tumour and normal tissues in the mouse to alkylating agents, Br. J. Cancer, 45, 700, 1982. 192. McNally, N. J., Hmchliffe, M. and de Ronde, J., Enhancement of the action of alkylating agents by single high, or chronic low doses of misonidazole, Br. J. Cancer, 48, 271, 1983. 193. Hinchliffe, M., McNally, N. J., and Stratford, M. R. L., The effect of radiosensitizers on the pharmacokinetics of melphalan and cyclophosphamide in the mouse, Br. J. Cancer, 48, 375, 1983. 194. Horsman, M. R., Evans, J. W., and Brown, J. M., Enhancement of melphalan-induced tumour cell killing by misonidazole: an interaction of competing mechanisms, Br. J. Cancer, 50, 305, 1984. 195. Randhawa, V. S., Stewart, F. A., Denekamp, J. et al., Factors influencing the chemosensitization of melphalan by misonidazole, Br. J. Cancer, 51, 219, 1985. 1%. Hirst, D. G., Horsman, M. R., Brown, J. M. et al., The effect of

250

timing on chemosensitization by clinical levels of SR-2508, Inr. J. Radiat. Oncol. Biol. Phys., 10, 1641, 1984.

197. Durand, R. E., and Olive, P. L., Potentiation of CCNU toxicity by AF-2 in V79 spheroids: implications for mechanisms of chemosensitization, Int. J. Radiat. Oncol. Biol. Phys., 12, 1375, 1986. 198. Koch, C. J., Stobbe, C. C., and Beer, K. A., Metabolism induced binding of ‘T-misonidazole to hypoxic cells: kinetic dependence on oxygen concentration and misonidazole concentration, Int. J. Radiat. Oncol. Biol. Phys., 10, 1327, 1984. 199. Spooner, D., Peacock, J. H., and Stephens, T. C., Enhancement of cytotoxic drugs by misonidazole in Lewis lung tumors of different sizes, and in mouse bone marrow, Int. J. Radiat. Oncol. Biol. Phys.. 8, 643, 1982. 200. Mulcaby, R. T., Effect of oxygen on misonidazole chemosensitization and cytotoxicity in vitro, Cancer Res., 44, 4409, 1984.

201. Roizen-Towle, L., Hall, E. J., and Pirro, J. P., Oxygen dependence for chemosensitization by misonidazole, Br. J. Cancer, p. 919, 1986. 202. Taylor, Y. C., Evans, J. W., and Brown, J. M., Mechanism of sensitization of Chinese hamster ovary cells to melphalan by hypoxic treatment with misonidazole, Cancer Res., 43, 3175, 1983. 203. Mulcahy, R. T., Cross-link formation and chemopotentiation of EMT6/Ro cells exposed to miso after CCNU treatment in vitro, In?. J. Radiat. Oncol. Biol. Phys., 12, 1389, 1986. 204. Taylor, Y. C., Sawyer, J. M., Hsu, B. et al., Mechanism of melphalan crosslink enhancement by misonidazole pretreatment, In?. J. Radiat. Oncol. Biol. Phys., 10, 1603, 1984.

205. Mule&y, R. T. and Trump, D. L., Clinical chemosensitization by misonidazole and related compounds: a critical evaluation, J. Clin. Oncol., 6, 569, 1988 (editorial). 206. Coleman, C. N., Carlson, R. C., Halsey, J. et al., Enhancement of the clinical activity of melphalan by the sensitizer misonidazole, Cancer Res., 48, 3528, 1988. 207. Pa& H. M., Tyree, E. B., Straube, R. L. et al., Cysteine protection against X-irradiation, Science, 110, 213, 1949. 208. Brand& E. L. and GrifBn, A. C., Reduction of toxicity of nitrogen mustard by cysteine, Cancer, 4, 1030, 1951. 209. Yuhas, H. M. and Storer, J. B., Differential chemoprotectionof normal and malignant tissues, J. Natl. Cancer Inst., 42, 331, 1969. 210. Yuhas, J. M., On the potential application of radioprotective drugs in radiotherapy, in Radiation-Drug Interaction in Sokol, G. H., Ed., John Wiley & Sons, New York, 1980, 113. 211. Harris, J. and Phillips, T. L., Radiobiological and biochemical studies of thiophosphate radioprotective compounds related to cysteamine, Radiat. Res., 46, 362, 1971. 212. Mori, T., Nikaido, O., and Sugahara, T., Dephosphorylation of WR-2721 with mouse tissue homogenates, Int. J. Radia?. Oncol. Biol. Phys., 10. 1529, 1984.

213. Calabro-Jones, P. M., Aguilera, J. A., Ward, J. F. et al., Uptake of WR-2721 derivatives by cells in culture: identification of the transported form of the drug, Cancer Res., 48, 3634, 1988. 214. Nakamura, J., Shaw, L. M., and Brown, D. Q., Hydrolysis of WR2721 by mouse liver cell fractions, Radiat. Res., 109, 143, 1987. 215. Stewart, F., Rojas, A., and De&camp, J., Radioprotection of two mouse tumors by WR-2721 in single and fractionated treatments, In?. J. Radiat. Oncol. Biol. Phys. 9, 507, 1984. 216. Mb, L., Hunter, N., Itoh, H. et al., Effect of tumor type, size, and endpoint on tumor radioprotection by WR-2721, ht. J. Radiat. Oncol. Biol. Phys., 10, 41, 1984.

217. Phillips, T. L., Kane, L. J., and Utley, J. F., Radioprotection of tumor and normal tissues by thiophosphate compounds, Cancer, 32, 528, 1973. 218. Yuhas, J. M., Efficacy testing of WR-2721 in Great Britain or everything is black and white at the Gray Lab, Inr. J. Radiat. Oncol. Biol. Phys., 9. 595, 1983.

Volume 10, Issue 3

Oncology/Hematology 219. Washburn,

response of the mouse kidney by misonidazole and WR-272 1, fnt. J. Radiar. Oncol. Biol. Phys., 9, 1731, 1983. 241. Rasey, J. S., Synthesis, biodistribution, and autoradiography of ra-

220.

diolabeled S-2-(3-metbylaminopropylamino) ethylphosphorothioic acid (WR-3689), Radiat. Res., 106, 366, 1986. 242. Rasey, J. S., Spence, A. M., Krohn, K. A. et al., Specific protection of different normal tissues, Pharm. Ther.. 1988, in press. 243. Shaw, L. M., Banner, H. S., Turrisl, A. et al., A liquid chromatographic electrochemical assay for S-2-(3-aminopropylamino) ethylphosphorothioate (WR-2721) in human plasma, J. Liq. Chromnrogr.,

22 1.

222.

223.

224.

225.

L. C., Rafter, J. J., and Hayes, R. L., Prediction of the effective radioprotective dose of WR-2721 in humans through an intraspecies tissue distribution study, R&at. Res., 66, 100, 1976. Yuhas, J. M. and Culo, F., Selective inhibition of the nephmtoxicity of &-platinum without altering its antitumor effectiveness, Cancer Treat. Rep., 64, 57, 1980. Wasserman, T. H., Phillips, T. L., Ross, G. et al., Protection against cytotoxic chemotherapeutic effects on bone marrow colony forming units by the radioprotector, WR-2721, Cancer Clin. Triuls, 4,3, 1981. F’urdie, J. W., A comparative study of the radioprotective effects of cysteamine, WR-2721, and WR-1065 in cultured human cells, Rudiar. Res., 77. 303, 1979. Yubaa, J. M., Differential protection of normal and malignant tissues against the cytotoxic effects of mechloethamine, Cancer Treat. Rep., 63, 971, 1979. Yubaa, J. M., Spellman, J. M., and Calo, F., The role of WR-2721 in radiotherapy and/or chemotherapy, Cancer Clin. Trials, 3, 211, 1980. Hymluk, W. M., Average relative dose intensity and the impact on design of clinical trials, Sem. Oncol., 14, 65, 1987.

7, 2447, 1984. 244. Shaw, L. M., Banner, H. S., Turrlsi, A. et al., Measurement of S-

2-(3aminopropylamino)

ethanethiol (WR-1065) in blood and tissue,

J. Liq. Chromatogr., 9, 845, 1986. 245. Shaw, L. M. and Banner, H. S., Detection and measurement of the

disultide WR-33278 [NH, (CH,)lNHCH,CH,S-], in blood and tissues, J. Liq. Chromatogr., 10, 439, 1987. 246. Swynnerton, N. F., McGovern, E. P., and M-old,

D. J., An improved HPLC assay for S-2-[3-aminopropylamino] ethylphoshorothioate (WR-2721) in plasma, Int. J. Radiuf. One. Biof. Phys.. 10, 1521, 1984. 247. Utiey, J. F., Seaver, N., Newton, G. L. et al., Pharmacokinetics of WR-1065 in mouse tissue following treatment of WR-2721, Int. J. Radiat. Oncol. Biol. Phys., 10, 1525, 1984. 248. Rialey, J. M., Van Etten, R. L., Shaw, L. M. et al., Hydrolysis of S-2-3(aminopropylamino) ethylphosphomthioate (WR-2721), B&hem.

226. Murray, D., van Ankeren, S. C., Midal, L., and Meyn, R. E., Radioptotective action of WR-1065 on radiation-induced DNA strand breaks in cultured Chinese hamster ovary cells, Rudiut. Rex, 113, 155, 1988. 227. Wifaoa, R. L., Free radical repair mechanisms and the interactions of glutathione and vitamins C and E, in Radioprotectors und Anticarcinogens, Nygaard, 0. F. and Simic, M. G., Eds., Academic Press, New York, 1983, 1. 228. Pnrdie, J. W., Inhaber, E. R., Schneider, H., and LaBelle, J. C., Interaction of altered mammalian cells with WR-2721 and its thiol WR1065: implications for mechanisms of radioprotection, Int. J. Rad. Oncol. Biol. Phys. 43, 517, 1983. 229. Smduk, G. D., Fahey, R. C., Calabro-Jones, P. M. et al., Radioprotection of cells in culture by WE-2721 and derivatives: form of the drug responsible for protection, Cancer Res., 48, 3641, 1988. 230. Chabner, B. A., Ed., Pharmacologic Principles of Cancer Treatment, W. B. Saunders, Philadelphia, 1982, 309. 231, Denekamp, J., Rojas, A., and Stevens, G., Redox competition and radiosensitivity: implications for testing radioprotective compounds,

Pharmacol., 35, 1453, 1986. 249. Shaw, L. M., Turrisi, A. T., Glover, D. J. et al., Human pharmacokinetics of WR-2721, Int. J. Rudiaf. Oncol. Biol. Phys., 12,

1501, 1986. 250. Turrisi, A. T., Glover, D. J., Hurwitz, S. et al., The final report of the Phase I trial of single dose WR-2721 S-2-(3aminopropylamino) ethyl phosphorothioic acid, intravenous infusion, Cancer Treat. Rep., 70, 1389, 1986. 251. Glover, D., Riley, L. J., Jr., Carmichael, K. et al., Hypocalcemia and inhibition of paratbyroid hormone secretion following administration of WR-2721: a radio- and chemoprotective agent, N. J. Med., 309, 1137, 1983. 252. Glover, D., Shaw, L., Glick, J. H. et al., Treatment of hypercalcemia in parathyroid cancer with WR-2721 S-2-(3aminopropylamino) etbylphosphorothioic acid, Ann. fnt. Med., 103, 55, 1985. 253. Kligennan, M. M., Turrisi, A. T., Urtasun, R. C. et al., Final report on Phase I trial of WR-2721 before protracted fractionated radiation therapy, Inr. J. Rudiut. Oncol. Biol. Phys., 14, 1119, 1988. 254. Yuhas, J. M., Active versus passive absorption kinetics as the basis for selective protection of normal tissues by S-2-(3aminopropylamino) ethyl phoshorothioic acid, Cancer Res., 40, 1519, 1980. 255. Travis, E. L. and Fang, M. Z., Basic I: Protection of mouse bone marrow by WR-2721 after fractionated radiotherapy, Inr. J. Rudiaf. Oncol. Biol. Phys.. 15, 377, 1988. 256. Wlthers, H. R., Biologic basis for altered fractionation schemes, Cancer. 55, 2086, 1985. 257. MBas, L., Murray, D., Brock, W. A., and Meyn, R. E., Radioprotectots in tumor radiotherapy: factors and setting determining therapeutic ratio, Phurm. Ther., 39, 179, 1988. 258. Cottatine, L. S., Zagars, G., Rubm, P. et al., Protection by WR2721 of human bone marrow function following irradiation, Inf. J. Radiat. Oncol. Biol. Phys., 12, 1505, 1986. 259. Glover, D., Glick, J., Weller, C. et al., Phase I trials of WR-2721 and cisplatin, Znr. J. Rudiar. Oncof. Biol. Phys., 10, 1781, !984. 260. We&man, R. A., Glover, D., Schwartz, D. M. et al., Limitation of cisplatin ototoxicity by a protector agent WR-272 1, Am. Acad. Ofolaryng-Head and Neck Surgery, 1985 (abstract). 261. Al-Sarraf, M., Fletcher, W., Oishi, N. et al., Cisplatin hydration

Pharm Ther., 39, 59, 1988. 232. Rasey, J. S., Spence, A., Badger, C. C. et al., Specific protection of different normal tissues, Phurm. Ther., 39, 33, 1988. 233. Schuchhardt, S., Comparative physiology of the oxygen supply, in Oxygen Supply, Theoretical and Practical Aspect of Oxygen Supply and Microcirculation of Tissues, Kessler, M., Bruley, D. F., Clark,

L. C., Labbers, D. W., Silver, I. A., and Strause, .I., Eds., Munich, Urban and Schwangenberg, 1973, 223. 234. Carmicbeal, J., Mitchell, J. B., Friedman, N. et al., Glutathione and related enzyme activity in human cancer cell lines, Vr. J. Cancer. 58, 437, 1988. 235. Vos, O., Budke, L., and Grant, G. A., Modification of the radiation response of the mouse kidney by midonidazole and WR-272 1, Inr. J. Radiat. 0~01. Biol. Phys., 9, 173 1, 1976. 236. Hatoff, D. E., Toyata, N., Wang, C. et al., Rat liver alkaline phosphatases. Evidence of hepatocyte and portal triad enzymes differ, Dig. Dis. Sci.. 30, 564, 1985. 237. McComb, R. B., Bowers, G. N. Jr., aud Posen, S., Alkaline Phos-

phatase, Plenum Press, New York, 1979, chap. 3. 238. Yubaa, J. M., Davis, M. E., Clover, D. et al., Circumvention of the tumor membrane barrier to WR-2721 absorption by reduction of drug hydrophilicity, Inr. J. Radiar. Oncol. Biol. Phys., 8, 519, 1982. 239. Shaw, L. M., Glover, D. J., Turrlsi, A. T., et al., Pharmacokinetics of WR-2721, Pharmacol. Ther.. 1988, in press. 240. Williams, M. V. and Denekamp, J., Modification of the radiation

1990

351 w-v

Critical Reviews In

262. 263.

264.

265.

266.

267. 268.

269. 270.

252

with and without &to1 diuresis in refractory disseminated malignant melanoma: a Southern Oncology Group Study, Cancer Treat Rep., 66, 31, 1982. Clover, D., Glick, J., Weiler, C. et al., Phase I/II trials of WR-2721 and cis-platinum, ht. J. R&at. Oncol. Biol. Phys., 12, 1509, 1986. MoIhuan, J. E., Glover, D. J., Hogan, W. M. et al., Cis-platinum neuropathy: risk factors and a possible protective agent, Neurology, 35 (suppl. 1). 80, 1985. Glover, D. J., G&k, J. EL, Weller, C. et al., WR-2721 and high dose cis-platin: an active combination in me&static melanoma, J. Clin. enc., 5, 574, 1987. Glick, J., Clover, D., WeiIer, C. et al., Phase I controlled trials of WR-2721 and Cyclophosphamide, Int. J. Radiar. Oncol. Biol. Phys., 10, 1777, 1984. Clover, D., Glick, J., Weiler, C. et al., WR-2721 protects against the hematologic toxicity of Cyclophosphamide: a controlled Phase II trial, J. Clin. Oncol., 4, 584, 1986. Clark, S. C. and Kamen, R., The human hematopoietic colonystimulating growth factors, Science, 236, 1229, 1987. Brandt, S. J., Peters, W. P., Atwater, S. K. et al., Effect of recombinant human granulocyte-macrophage colony- stimulating factor on hematopoietic reconstitution after high-dose chemotherapy and autologous bone marrow transplantation, N. Engl. J. Med., 318, 869, 1988. Neta, R., Douches, S., and Oppenheim, J. J., Interleukin 1 is a radioprotector, J. Immunol., 136, 2483, 1986. Neta, R., Oppenbeim, J. J., and Douches, S. D., Interdependence of the radioprotective effects of human recombinant Interleukin-la, tumor necrosis factor a, granulocyte colony stimulating factor and murine recombinant granulocyte-macmphage colony stimulating factor, J. lmmunol., 140, 108, 1988.

Volume 10, Issue 3

Radiation and chemotherapy sensitizers and protectors.

Radiosensitizers and radioprotectors are part of the chemical modifier approach to cancer therapy whereby the state of the tumor cells and/or normal t...
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