European Journal of

Nuclear Medicine

Review article

Gamma scintigraphy in the evaluation of pharmaceutical dosage forms S.S. Davis, J.G. Hardy, S.P. Newman, and I.R. Wilding 1 Pharmaceutical Profiles Limited, 2 Faraday Building, Highfields Science Park, Nottingham NG7 2QP, UK

Abstract. Gamma-scintigraphy is applied extensively in the development and evaluation of pharmaceutical drug delivery systems. It is used particularly for monitoring formulations in the gastrointestinal and respiratory tracts. The radiolabelling is generally achieved by the incorporation of an appropriate technetium-99m or indiu m - I l l labelled radiopharmaceutical into the formulation. In the case of complex dosage forms, such as entericcoated tablets, labelling is best undertaken by the addition of a non-radioactive tracer such as samarium-152 oxide or erbium-170 oxide followed by neutron activation of the final product. Systems investigated include tablets and multiparticulates for oral administration, enemas and suppositories, metered dose inhalers and nebulisers, and nasal sprays and drops. Gamma-scintigraphy provides information on the deposition, dispersion and movement of the formulation. The combination of such studies with the assay of drug levels in blood or urine specimens, pharmacoscintigraphy, provides information concerning the sites of drug release and absorption. Data acquired from the scintigraphic evaluation of pharmaceutical dosage forms are now being used increasingly at all stages of product development, from the assessment of prototype delivery systems to supporting the product licence application. Key words: Drug delivery - Pharmacoscintigraphy Neutron activation - Gastrointestinal tract - Respiratory tract - Tablets Capsules - Aerosols - Enemas Nasal sprays Eur J Nucl Med (1992) 19:971-986

Introduction Pharmaceutical preparations take many shapes and forms and are administered by a variety of routes. Simple solution formulations may be injected into the bloodstream, while more sophisticated systems can be taken Offprint requests to: S.P. Newman

orally or inhaled. Oral dosage forms comprise technologies designed to provide controlled drug release or sitespecific delivery. The advent of more complex drug molecules, particularly those resulting from recombinant DNA technology in the form of therapeutic peptides and proteins, has necessitated the design of drug delivery systems that can include various components and mechanisms. For some drugs it is perfectly satisfactory that they are absorbed rapidly into the blood and reach all tissues; however, for others, particularly in cancer chemotherapy, it is an advantage for the drug to be localised in some manner. Prototype drug delivery systems can be tested in vitro using various techniques designed to study drug release, either in a passive or active manner. The pharmacopoeial dissolution test for solid dosage forms is a well-known example. It is critical that such systems be evaluated in vivo. Animal models have some utility, but presently there is a growing tendency for new delivery systems to be tested, whenever possible, in human subjects in a so-called phase 1 clinical evaluation.

Phase 1 clinical evaluation The clinical performance of a drug can often be influenced, advantageously or disadvantageously, by the deposition and release characteristics of the pharmaceutical formulation. It can therefore be helpful if the in vivo performance of a drug delivery system can be evaluated in healthy volunteers or patients to ascertain, for example, whether the designed system is performing correctly in providing optimal deposition at the preferred site of absorption or action, or whether it is releasing the drug according to a prescribed pattern or the intended rationale. Tables 1 and 2 summarize the studies with which we have been involved for dosage forms administered to the gastrointestinal and respiratory tracts. In most cases, these investigations have been fully documented in publications within the scientific literature. Gamma-scintigraphy has allowed basic but important questions in drug delivery to be addressed, such as Where is the dosage form? What is it doing? Is the

© Springer-Verlag 1992

972 Table 1. Applications of gamma-scintigraphy to the evaluation of drug delivery to the gastrointestinal tract

Table 2. Applications of gamma-scintigraphy to the evaluation of drug delivery to the respiratory tract

Buceal cavity

Pulmonary delivery

• Release characteristics of controlled release systems to include matrix tablets and chewing gum (Davis et al. 1981, 1983; Christrup et al. 1990)

• Measurement of total and regional deposition of aerosolised drugs delivered by pressurised metered dose inhaler (MDI), e.g. bronchodilators, corticosteroids, drugs for asthma prophylaxis, mucolytics (Newman et al. 1981a, 1989a, 1991a, b; Hardy et al. 1991 b) • Lung deposition of propellant-soluble drug (Ashworth et al. 1991) • Effect of various inhalation modes on drug delivery from MDIs (Newman et al. 1982a) • Use of holding chambers and spacer attachments to the MDI mouthpiece (Newman et al. 1981b, 1984a, 1986, 1989a, 1991a,

Oesophagus • Transit studies of pharmaceutical dosage forms (Robertson and Hardy 1988) Transit measurements through the stomach and intestines • Simultaneous gastric emptying of food and dosage forms (Coupe et al. 1991a) • Gastric emptying of dosage forms (Davis et al. 1984a, 1987, 1988a; O'Reilly et al. 1987; Khosla and Davis 1989; 1990; Khosla et al. 1989; Coupe et al. 1991b) • Small-intestinal transit of dosage forms (Davis et al. 1986a) • Colonic transit of dosage forms (Hardy et al. 1985a; Parker et al. 1988; Price et al. 1991) • Studies of the physiological factors likely to affect gastrointestinal transit of dosage forms, e.g. age, posture, time of dosing, exercise, bed rest (Ollerenshaw et al. 1987; Mundy et al. 1989; Coupe et al. 1992a, b) • Testing of pharmaceutical strategies intended to prolong gastrointestinal transit, e.g. dosage form density (Bechgaard et al. 1985; Davis et al. 1986b) • Effects of pathophysiology on gastrointestinal transit, e.g. irritable bowel syndrome, inflammatory bowel disease (Hardy et al. 1988 ; Davis et al. 1991) • Combination of transit studies with measurements of pH (Hardy et al. 1987a) Dosage form evaluations in the stomach and intestines • Differential transit behaviour of single and multiple unit dosage forms (Davis et al. 1984b, c) • Performance of enteric-coated dosage forms (Hardy et al. 1987b, c, 1991a) • Disintegration rate of capsules and tablets (Daly et al. 1982; Wilding et al. 1991 a) • In vitro-in vivo correlation of drug release from controlled release dosage forms, to include osmotic pumps (Davis et al. 1988b; Wilding et al. 1991b) • Putative bioadhesives intended to change gastrointestinal transit (Khosla and Davis 1987) • Absorption characteristics of drugs, in particular the use of slow release and pulsatile systems to evaluate the relationship between position in the gastrointestinal tract and drug absorption (Fischer et al. 1987; Wilding et al. 1992a) • Correlation of pharmacokinetic data with gastrointestinal transit (Davis et al. 1986c, 1989, 1990; Davis and Feely 1989; Wilding etal. 1991c, 1992b, c)

c) • Effects of breath-actuated MDIs and other devices on drug deposition (Newman et al. 1990, 1991b) • Changes in metered volume, propellant vapour pressure and other physicochemical factors (Newman et al. 1982b, 1984b) • Comparison of nebuliser systems in terms of total and regional deposition patterns (Johnson et al. 1989; Simonds et al. 1989) • Assessment of nebulisers for delivery of inhaled antibiotics (Newman et al. 1988a) • Total and regional lung deposition from powder inhalers (Newman et al. 1989b) • Effects of different modes of inhalation on drug delivery from powder inhalers (Newman et al. 1991d) • Techniques for the administration of inhaled drugs to neonates (O'Callaghan et al. 1992) • Correlation of aerosol deposition with drug absorption (Borgstr6m et al. 1991) • Correlation of drug deposition with clinical efficacy (Johnson et al. 1989; Newman et al. 1991b, c, d) Nasal delivery • Nasal deposition patterns and clearance of radiolabelled aerosol from MDIs, aqueous pump sprays and dry powder insufflators (Newman et al. 1987a, b; Thorsson et al. 1992) • Effects of changes in spray characteristics, inhaler position and breathing mode on nasal deposition patterns (Newman et al. 1987a, 1988b) • Use of inert insoluble particles to measure mucociliary clearance (Millar et al. 1986; Smelt et al. 1987) • Effect of solution viscosity on nasal deposition and clearance (Pennington et al. 1988) • Comparison of the deposition and spreading of nasal sprays and drops (Hardy et al. 1985b) • Effects of pathophysiology on the clearance rates of nasal formulations (Bond et al. 1984; Lee et al. 1984)

Rectal drug delivery • Drug release and the spreading characteristics of enemas and suppositories (Hardy et al. 1987d)

s y s t e m d i s t r i b u t i n g / t a r g e t i n g / r e l e a s i n g c o r r e c t l y in acc o r d w i t h a n i n t e n d e d m e c h a n i s m o r design p r o c e d u r e ? T h e m a i n a p p l i c a t i o n s o f such studies relate to d r u g d e l i v e r y systems a d m i n i s t e r e d to the r e s p i r a t o r y o r gas-

t r o i n t e s t i n a l tracts. S c i n t i g r a p h y can also be used to e v a l u a t e the p e r f o r m a n c e o f p h a r m a c e u t i c a l d o s a g e f o r m s a d m i n i s t e r e d b y o t h e r routes, into the eye, the v a g i n a l cavity, etc. C o n s i d e r a t i o n s o f r a d i a t i o n dosim e t r y w o u l d n o r m a l l y r e n d e r the s t u d y o f l o n g - t e r m i m p l a n t s b y g a m m a - s c i n t i g r a p h y i n a p p r o p r i a t e in hum a n subjects. In such s i t u a t i o n s the use o f suitable anim a l m o d e l s c a n often be v e r y instructive.

973

Labelled dosage forms It must be stressed at the outset that in conducting studies in healthy volunteers and patients to follow the fate of a pharmaceutical dosage form, it is not normally possible to radiolabel the drug molecule. This is because, in general, drug molecules are compounds of carbon, hydrogen, oxygen and nitrogen, and none of these elements has isotopes suitable for gamma-camera studies. Instead of labelling the drug molecule, a radiolabel is usually incorporated into the dosage form and is used to define the in vivo performance of the system. In this way the distribution of the dosage form within the body can be followed (for example, the deposition of an aerosol system into the lungs or the gastrointestinal transit of a controlled release tablet), or the integrity of the system can be determined (for example the dispersion of a controlled release pellet system within the stomach), or the release characteristics of a tablet can be evaluated by monitoring its dissolution performance. Some illustrative examples are described below. In order to be assured that the marker provides the required information, it is essential to undertake in vitro investigations to establish that the marker is indeed representative of the phenomenon to be investigated in vivo. As will be discussed in more detail, these markers can be both soluble or insoluble materials and can be used to monitor deposition, transit and release processes.

Pharmacoscintigraphy In the scintigraphic evaluation of a pharmaceutical preparation, it is now normal practice to include the required drug within the labelled dosage form and to measure drug absorption by established analytical procedures. It is thereby possible to relate the pharmacokinetic performance of the drug to the derived scintigraphic information. This combined technique has been given the term "pharmacoscintigraphy". In this manner the performance of a variety of complex dosage forms, intended for different routes, has been optimised, for example, to obtain maximum deposition in the peripheral airways, prolonged retention in the nasal cavity, targetting to preferred sites (for absorption or therapy) within the gastrointestinal tract. Furthermore, dosing with two different radionuclides allows for the simultaneous evaluation of two events or processes. This procedure has been used to study, for example, the differential transit of single and multiparticulate units, the separate labelling of the different components of a complex delivery device or the study of the gastrointestinal transit of a dosage form and co-administered foods or liquids. Most scintigraphic studies of pharmaceutical dosage forms have been carried out in healthy volunteers. More limited studies have also been performed in patients. Clearly, it is not possible within the scope of this article to provide details on all these investigations. Therefore,

examples have been selected to illustrate the role of gamma-scintigraphy in the evaluation of dosage forms for drug delivery to the gastrointestinal and respiratory tracts.

Oral dosage forms Despite a rapid growth in the novel routes for drug delivery, the vast majority of therapeutic agents are still administered orally. A large proportion of oral dosage forms comprises tablets and capsules which are designed to disintegrate rapidly in the stomach. There is a growing tendency, however, to employ sophisticated systems that enable the controlled or timed release of a drug, thereby providing a better dosing pattern and greater convenience to the patient. These modified release systems are, by necessity, more complicated than conventional tablets and capsules and require new methods for their evaluation. Reliance on pharmacokinetic measurements alone is not reliable and can be confusing since they indicate solely drug absorption and not the mechanisms responsible for drug release. Regulatory authorities now require pharmaceutical manufacturers to provide information about the in vivo performance of sophisticated oral dosage forms within the gastrointestinal tract. Gamma-scintigraphy provides a non-invasive means of acquiring such information under normal physiological conditions (Davis et al. 1990; Hardy et al. 1991 a; Wilding et al. 1991 a, b). Radiolabelling can be achieved either by the direct incorporation of a radiolabelled compound into the preparation or by neutron activation of a dosage form that contains a non-radioactive tracer. The latter method avoids the need to handle radioactive materials during lengthy or complex formulation procedures and permits dosage form manufacture to be conducted under normal production conditions. The quantity of material needed to be incorporated into a formulation to render it suitable for use in a gamma-scintigraphic study is very small and does not compromise the performance characteristics of the delivery system. Examples are presented to illustrate the different labelling methodologies employed and the results obtained.

Example 1." Assessment of a multiparticulate sustained release formulation of diltiazem Diltiazem is an orally active calcium channel blocking agent known to be effective and well-tolerated in the prophylaxis and treatment of angina. It is typically administered three to four times daily in the form of an immediate release formulation. Recently, attempts have been made to develop sustained release preparations with extended clinical effects and a reduced dosing frequency (Geoghegan et al. 1988; Pool et al. 1989). The important features for the rational design of a

974 sustained release formulation of diltiazem are comparable bioavailability to the conventional release dosage form, minimum peak to trough variation in multipledose studies and the absence of any food effects on bioavailability. Reported half-times for the elimination phase of diltiazem have ranged from 2 to 7 h (average about 4.5 h) (Chaffman and Brogden 1985). Previous studies have also reported a reduced absorptive capacity for diltiazem from the distal regions of the gastrointestinal tract. Therefore, in order to achieve an optimal sustained release formulation of the drug, it was important to locate the best compromise between prolongation of release and subsequent bioavailability. With this in mind, a multiparticulate sustained release pellet formulation of diltiazem was designed with release properties that were largely pH independent over an 8 h time period. The aims of this study were to verify the extent of drug absorption and to assess the effect of food on the gastrointestinal transit of the pellet formulation.

Study design. The sustained release multiparticulate formulation of diltiazem was manufactured using conventional pellet technology which involved the building up of a drug layer on sugar cores. The release properties of the system were controlled by the subsequent application of a polymeric film coat. Commercially available sustained release pellet formulations tend to be hydrophobic due to the presence of the film coat, whilst ionexchange resin beads are hydrophilic. A placebo pellet formulation therefore was developed in which the diltiazero was replaced by anion exchange resin powder radiolabelled with 99mTc-pertechnetate. The model formulation mimicked exactly the physical characteristics of the drug-containing cores. The radiolabelled pellets were incorporated into hard gelatin capsules along with drug pellets. In vitro studies were carried out to demonstrate the stability of the radiolabelling procedure. Following an overnight fast, the pellets were administered to 8 healthy subjects on 2 occasions, once with and once without a heavy breakfast. Transit of the formulation through the stomach and intestines was monitored by recording anterior and posterior gamma-camera images of the abdomen at frequent intervals throughout the day. Regions of interest were defined around the stomach and colon to allow the quantification of gastric emptying and estimation of the colonic arrival of the tracer. On a separate occasion each volunteer drank a solution of diltiazem to provide reference data on the pharmacokinetics of the drug. Blood samples were taken during each study period and assayed for diltiazem. Results. The data for the study were expressed as the time for half of the tracer to leave the stomach and the time for half to arrive in the colon. Small-intestinal transit was calculated by subtracting the gastric emptying value from the corresponding value for colonic arrival. The median times for gastric emptying following dos-

100"

~

80"

"i

40

2

4

6

8

10

Time (hours)

Fig. 1. Mean gastric emptyingprofiles followingoral administration of the pellet formulationwith (o) and without (o) food

ing with and without breakfast were 190 min and 25 min, respectively. These values were in accord with previous studies on the gastric emptying of multiparticulate formulations (Davis et al. 1987). The emptying of the pellet formulation from the fasted stomach was very rapid and occurred in an approximately exponential manner (Fig. 1). In one subject, gastric emptying was particularly slow under fasted conditions, with a half-time of 480 min. In the fed volunteers, a lag phase was observed before the commencement of pellet emptying from the stomach. This is a common occurrence and is believed to reflect a redistribution of the pellets mixed with the food from the fundus to the antrum, and the process in which solid food is converted into chyme. Pellets initially remained in the upper half of the stomach, dispersed in the food (O'Reilly et al. 1987) and then tended to become spread throughout the stomach (Hunter et al. 1980), presumably as the food was redistributed. In a number of subjects, the pellets exhibited an almost linear pattern of gastric emptying after the lag phase. This linear profile is the characteristic emptying pattern of solid food (Tothill et al. 1978) and indicates that the pellets may have become mixed with at least some food prior to the commencement of emptying. In the remaining subjects, the pellets appeared to empty from the stomach as a series of small discrete boluses (Hunter et al. 1982). Similar results for the gastric emptying of model pellet systems have been reported in the literature (Devereux et al. 1990). Previous studies have suggested that the transit of different pharmaceutical dosage forms through the small intestine is largely independent of feeding conditions and physical properties of the system (Davis et al. 1986a). The average small-intestinal transit times in this study

975 Pharmacokineticdata followingoral dosing of diltiazem (mean_+SEM)

Table 3.

Parameter

Pellets (fasted) 240 mg dose

Pellets(fed) 240 mg dose

Solution 60 mg dose

AUC(hng/mI) Cm,x(ng/ml) T.... (h)

1229 _+183 83 +14 7.4__l.4

1155 _+234 92 +18 9.7_+1.1

219 +55 68 +17 0.9_+0.2

AUC, Area under the plasma concentrationcurve; Cm,x,maximum plasma concentration;T.... time to achievemaximumplasma concentration

were approximately 3-4 h and these are in good accord with those reported previously. The bioavailability of the sustained release formulation, as measured by the area under the plasma concentration curve, appears independent of food (Table 3); however, there was a wide variation in the data. Previous experience has shown that diltiazem is subject to a significant first-pass metabolism in the liver, and therefore, large intersubject differences in bioavailability, plasma concentrations, clearance and plasma half-time may exist due to the differing first-pass metabolic capacities (Hermann et al. 1983). The absolute bioavailability was approximately 40% and was again probably affected by first-pass effects. The pellet formulation reduced the rate, but not the extent, of absorption with the relative bioavailability (pellets to solution) being greater than 90%. The prolonged gastric residence time observed in one volunteer in the fasted state had no significant effect on the plasma concentration-time profile, indicating that the drug release was not impaired by gastric stasis and that released drug could still be absorbed. In conclusion, this study has shown that the gastric emptying of the pellet formulation was influenced significantly by the presence of food. Transit through the small intestine was unaffected by food, and in the majority of subjects, the formulation reached the caecum about 3 4 h after leaving the stomach. The postprandial administration of the sustained release pellet formulation did not affect the bioavailability of the drug.

Example 2: Evaluation of an enteric-coated naproxen tablet radiolabelled by neutron activation The most frequently reported side effects of non-steroidal anti-inflammatory drugs (NSAIDs), such as naproxen, are gastrointestinal discomfort, nausea and gastric bleeding (Haslock 1989). The primary cause is considered to be the direct effect of the NSAID on the gastric mucosa (Pemberton and Strand 1979; Caruso and Porro 1980). Enteric-coated NSAID tablets, designed to prevent direct contact between the drug and the mucosal tissue of the stomach, have been shown to reduce the

incidence of gastrointestinal side effects (Trondstad et al. 1985). Such dosage forms are coated with a polymer designed to withstand the acid environment of the stomach but which dissolves readily once the preparation encounters the higher pH levels of the small intestine. Pharmacoscintigraphic evaluation provides a direct insight into the in vivo fate of the delivery system and its integrity and allows the relationship between gastrointestinal transit, tablet disintegration and drug absorption to be examined. This type of clinical investigation is particularly of use for enteric-coated preparations where it is important to verify that the formulation complies fully with the selected product rationale. Conventional methods of labelling pharmaceutical dosage forms require that the radioactive marker be incorporated as late as possible so as to minimise the handling of radioactive materials. The recent trend to produce more complex dosage forms, such as enteric-coated dosage forms, necessitates the use of specialised manufacturing equipment and rather lengthy and complex production techniques. Therefore, in many cases, the manufacturing process must be scaled down to minimise the amount of radioactivity handled, and in the case of complicated delivery systems, this may significantly alter the physical properties of the dosage form. These problems can be overcome by the incorporation of a non-radioactive tracer with subsequent radiolabelling by neutron activation (Parr et al. 1987; Digenis et al. 1990; Hardy et al. 1991a; Wilding et al. 1992c). Two materials, samarium-152 oxide and erbium-170 oxide are commonly used as tracers. These oxides are not absorbed from the gastrointestinal tract, and both samarium-152 and erbium-170 undergo neutron activation to form isotopes which are suitable for imaging with a gamma-camera. Samarium-153 has a half-life of 47 h and a principal gamma-ray energy of 103 keV, and erbium-171 has a half-life of 7.5 h and principal gammaray energies of 296 and 308 keV. Radiolabelling using this approach enables radiation doses during manufacture to be minimised, quality assurance of the product to be maintained and complicated delivery systems to be labelled easily and efficiently. The purpose of this pharmacoscintigraphic study was to evaluate the gastrointestinal transit and disintegration characteristics of an enteric-coated naproxen formulation manufactured under normal production conditions.

Study design. Enteric-coated naproxen tablets were prepared, each containing 2 mg samarium-152 oxide. The tablets were irradiated for 4 min in a neutron flux of 1012 neutrons cm -2 s -1 to give 0.5 MBq samarium-153 per tablet at the time of dosing, 3 days later. The 3-day time delay allowed decay of the sodium-24 produced as a result of the drug being present as its sodium salt. In vitro studies demonstrated that neither the addition of the samarium oxide nor the neutron activation process affected the performance of the product or the stability of the drug.

976 Table 4. Transit and disintegration profile of enteric-coated naproxen tables (in minutes) following administration after either an overnight fast or a light breakfast Volunteer number

Fasted

Fed

Gastric residence Tablet

1

2 3 4 5 6 7 Mean SEM Median

Disintegration time (after gastric emptying) Tablet

Gastric residence Tablet

Disintegration time (after gastric emptying) Tablet

1

2

1

2

1

2

1

2

28 6 17 162 67 16 26

28 6 17 162 78 16 26

114 83 116 47 11 73 117

134 83 116 101 11 85 128

100 128 114 68 110 100 67

110 187 151 100 121 110 67

140 71 27 118 86 140 63

130 60 57 140 75 130 84

47 14 24

87 11 93

Two tablets were administered to each of 7 healthy subjects on separate occasions once following a light breakfast and once without breakfast. The tablets were taken with a drink radiolabelled with 99mTc-labelled diethylenetriaminepentaacetic acid (99mTc-DTPA) to outline the stomach. Gamma-camera images were recorded at frequent intervals throughout the day and blood samples taken for naproxen assay. Results. The gastrointestinal transit and disintegration results for the enteric-coated tablets are presented in Table 4. In the fasted state, the tablets remained in the stomach for a median of 24 min compared with 110 min following administration with food. All the tablets disintegrated in the small intestine (Fig. 2), on average 1.6 h and 1.4 h after emptying from the stomach, following dosing under fasted and fed conditions, respectively. The corresponding post-dose times for the first detection of naproxen in the blood were 2.0 h and 1.7 h. Overall, the blood level results correlated well with the scintigraphic data (Fig. 3). Postprandial administration led to prolonged residence in the stomach, and the time to onset of tablet disintegration was predominantly controlled by the gastric emptying time. Residence time in the small intestine, prior to tablet disintegration, was independent of food intake but exhibited considerable inter- and intrasubject variation. Under fasted conditions, the gastric emptying time of the enteric-coated tablets was determined largely by the migrating myoelectric complex, which occurs over a 2-h cycle (Phillips 1988). It is the contractions in the third phase of the cycle that are important for the gastric emptying of large tablets (Park et al. 1984), since they have the effect o f ' s w e e p i n g ' indigestible material from

110 9 110

94 10 85

the stomach through the open pylorus and into the small intestine (the so-called 'housekeeper wave'). It is interesting to note that the two tablets often emptied from the fed stomach at different times. Variable gastric emptying of large tablets, whilst the stomach is still in the digestive state, is not without precedent (Khosla et al. 1989; Khosla and Davis 1990). In conclusion, the study has shown that gastrointestinal transit, in vivo disintegration and drug absorption were as expected for an enteric-coated tablet formulation. The preparations remained intact in the stomach, and all reliably disintegrated in the small intestine. Overall, feeding caused a predictable delay in transit, but individual variation in transit was large. This is unlikely to be of any clinical significance for the treatment of chronic conditions with naproxen. The use of neutron activation labelling enabled the tablets to be manufactured and tested under normal industrial conditions, lending confidence that the results of the study would be representative of those expected from the commercially available product.

Rectal dosage forms The extent of spreading of rectally administered pharmaceuticals can be readily monitored by gamma-scintigraphy. Enema preparations, for example corticosteroids, are generally administered to provide topical treatment to the large bowel. Thus, it is important to ensure that the formulation will spread to the affected site. The spreading of solution enemas labelled with 99mTc-DTPA has been investigated in healthy volunteers. Following dosing with a 100-ml solution, the dispersion was highly variable, ranging from total retention

977 N•PNOSYN

EC

2.0

h

E~.2h

2.3

h

2.5h

Fig. 2. Enteric-coated naproxen tablets disintegrating in the small intestine

in the rectum and sigmoid colon to complete coverage of the whole of the large bowel (Wood et al. 1985). By 2 h after dosing, dispersion tends to have ceased. Reducing the volume to 50 ml resulted in a greater proportion of the dose being retained in the rectum and sigmoid colon, whilst the dispersion of a 200-ml solution was similar to that of a 100-ml dose (Hardy et al. 1986). Thus, 100 ml solution enemas provide a reliable means

of delivery of topically acting drugs to the descending colon. Solution enemas can be difficult to administer. Foam enemas, however, offer greater convenience to the patient. As with solutions, the extent of spreading of foam enemas is dependent on the quantity applied. Low volume foams are retained within the rectum and sigmoid colon (Wood et al. 1985), whereas more bulky prepara-

978 Gastric Emptying 200.

Tablet Disintegration

o= 150

ed administration

e..

= 100

8 ~

---....

5O

e-

i

0

~.

2

,

•

•

i

.

4

.

•

i

•

•

.

i

.

6 8 Time (hours)

.

,

i

.

.

10

.

i

12

Gastric Emptying

{ 200

Tablet Disintegration =

•~ 150

100 ?

Fed administration

-_

so

0

2

4

6 8 Time (hours)

10

12

Fig. 3. P h a r m a c o k i n e t i c profiles for a single subject following adm i n i s t r a t i o n o f enteric-coated n a p r o x e n tablets after either a n overn i g h t fast or a light b r e a k f a s t

tions exhibit comparable spreading to that achieved with 100-ml solution enemas (Fig. 4). Rectal administration also provides a route for systemic drug delivery. It is useful in patients who have swallowing difficulties, or as a means of avoiding unpleasant gastrointestinal effects, such as nausea, which may accompany oral dosing. It has been widely claimed that rectal administration offers the advantage for drugs with extensive first-pass metabolism in the liver of providing direct absorption into the systemic circulation. Since most formulations, however, spread beyond the lower rectum, it is likely that much of the absorbed drug will pass into the portal system. Jay etal. (1985) radiolabelled suppositories with 99mTc_hydroxymethyldiphosphonatetorepresentawatersoluble drug and found that spreading was confined to the lower and middle rectum. More extensive spreading, into the sigmoid colon, was detected for 99mTc-labelled5-aminosalicylic acid suppositories (Williams etal. 1987). Hardy et al. (1987d) investigated the dispersion of two components of rectal hard gelatin capsules, the lipid base and the particulate suspension. The bases were radiolabelled by the incorporation of iodine-123-labelled fatty acids, and the suspension particles were indium11 I-labelled ion exchange resin. The preparations spread throughout the rectum and occasionally into the sigmoid colon. There was a tendency for greater dispersion of the base than the suspended particles. In general, the extent of dispersion of rectally administered pharmaceuticals is volume dependent. Suppositories and low volume enemas are confined to the rectum and sigmoid colon, while doses of larger volume provide reliable drug delivery throughout the descending colon.

Fig. 4. S p r e a d i n g o f a f o a m e n e m a t h r o u g h out the descending colon

979

Drug delivery to the lungs In common with the evaluation of pharmaceutical dosage forms for oral drug delivery, the application of gamma-scintigraphy to inhaled drug delivery systems provides information that is either difficult or impossible to obtain by other means. The deposition pattern in the respiratory tract and oropharynx usually determines the efficacy of inhaled drugs, which must almost inevitably be delivered to the target site in sufficient quantity if the therapy is to be successful. In some instances, such as inhaled bronchodilators, the total amount of drug deposited in the lungs is probably the key factor, while in other cases, it is important to optimise delivery to a particular site within the lungs (e.g. pentamidine therapy in human immunodeficiency virus disease and drugs intended for systemic absorption). In some cases, for instance with inhaled corticosteroids, it is desirable to reduce the deposition in the oropharynx because of concerns about both local and systemic side effects. Three types of inhalation device may be used (Mor~n 1985), namely nebuliser, pressurised metered dose inhaler (MDI) and dry powder inhaler, and each type of device can be evaluated by gamma-scintigraphy, following an appropriate choice of labelling technique. Scintigraphic measurements are particularly appropriate to inhaled drug delivery systems, since it is very difficult to predict with any certainty the deposition patterns from these devices on theoretical grounds alone. This reflects in part the physical instability of therapeutic aerosols; for instance, the propellant droplets generated by pressurised MDIs are constantly shrinking by evaporation as they travel away from the canister, while the particle size of the cloud from powder inhalers is generally a function of the aggregation and de-aggregation of active drug and inert particulate carrier complexes. Information on drug delivery can be determined from pharmacokinetic studies, but these alone give either an imprecise measure of total lung deposition (Davies 1975) or require the simultaneous administration of an oral charcoal suspension to block gastrointestinal absorption of drug, followed by a 48-h urine collection (Borgstr6m and Nilsson 1990). Furthermore, pharmacokinetic techniques are unable to give any information about the regional deposition pattern within the lungs. Of the three types of device, nebulisers are easiest to evaluate, for example by monitoring the deposition of a solution of 99mTc-DTPA. By means of suitable calibration procedures, both the percentage of the dose deposited in the lungs and its fractionation within "central", "intermediate" and "peripheral" zones may be determined from planar views of the lung fields (Newman et al. 1988 a). Regional deposition within the lungs is usually determined by reference to a krypton-81 m ventilation image, which is used to define the lung edges. It is important to note, however, that simple planar images give a two-dimensional representation of a threedimensional distribution of radiotracer, so that the lung

zones do not correspond to any well-defined anatomical regions. Tomographic imaging of the lungs may produce more sensitive indices of regional deposition patterns (Phipps et al. 1989), but it is only applicable to tracers which are cleared from the lungs sufficiently slowly and generally requires the deposition of higher quantities of radioactivity than planar imaging. Nebulised drugs can be mixed with appropriate 99mTc-labelled solutions within the nebuliser reservoir, so that both drug and radiolabel are delivered in the spray (Dashe et al. 1974; Ruffin et al. 1978). This approach permits the simultaneous assessment of deposition and efficacy and has enabled information about the whereabouts of receptor sites to be determined for some inhaled compounds (Ruffin et al. 1978). Whilst the principles of scintigraphic measurements are similar for studies with all types of inhalation device, radiolabelling techniques for pressurised MDIs and powder inhalers are much more complex than those for nebulisers. Technetium-99m is generally used as a tracer in studies of drug delivery from pressurised MDIs and powder inhalers; this radionuclide has been used to label inert solid particles (Newman et al. ~981a) or liquid droplets (Dolovich et al. 1981) in placebo-pressurised MDIs and has been incorporated by physical inclusion into spray-dried sodium cromoglycate particles (Vidgren et al. 1987). More recently, this radionuclide has been mixed in a suitable chemical form with drug material in such a way that it acts as a valid marker for the presence of drug across a range of droplet sizes (K6hler et al. 1988; Newman et al. 1989a, b, 1991 b). The example below illustrates the application of the technique.

Example : Lung deposition of sodium cromoglycate from a metered dose inhaler Sodium cromoglycate is administered by inhalation from MDIs for the prophylaxis of asthma. Only drug deposited in the lungs causes a therapeutic effect. This study was undertaken firstly to investigate whether lung deposition could be enhanced by the addition of a 10-cm "spacer" tube attachment to the mouthpiece of the inhaler and secondly to monitor the effect of changing the inhalation flow rate through the spacer (Newman et al. 1989a). This spacer device (Syncroner, Fisons) had an open section in its upper surface and was designed initially as a training aid for using pressurised MDIs, primarily to ensure that the firing of the spray is coordinated or synchronised with inhalation. If patients fail to co-ordinate firing the MDI with their inhalation, the entire dose is seen to emerge from the open section of the spacer; thus patients are instructed to inhale from the spacer ~n co-ordinated fashion, such that the spray cloud is not seen.

Study design. Metered dose inhalers delivering 1 mg sodium cromoglycate per metered dose were radiolabelled

980

/-

Fig. 5a-d. Deposition patterns in a healthy volunteer who inhaled technetium99m-labelled sodium cromoglycate from (a) a conventional metered dose inhaler at a slow inhaled flow rate, (b) a metered dose inhaler plus tube spacer at a slow inhaled flow rate and (e) a metered dose inhaler plus tube spacer at a fast inhaled flow rate. The inset (d) shows division of the lung fields into central, intermediate and peripheral zones superimposed upon an krypton-81m ventilation scan. The images show the increased lung deposition and reduced oropharyngeal deposition with the spacer, together with a more "central" deposition pattern within the lungs for fast inhalation via the spacer

981

with 99myc. The radiolabelling procedure involved evaporating a 99mTc-labelled solution to dryness in an empty canister and subsequently adding a suspension of sodium cromoglycate in chlorofluorocarbon propellants at below - 6 0 ° C. A metering valve was then crimped onto the empty can. The radiolabelled sodium cromoglycate formulation was assessed in vitro in order to confirm that the distribution of 99mTCmatched that of the drug across a range of particle sizes. This was carried out by firing doses from an MDI into a multistage liquid impinger, which fractionates the aerosol according to size. By assaying the fractions for both drug and radioactivity, it was possible to confirm that the distribution of the radiolabel reflected that of the drug. Ten healthy volunteers then inhaled a single radiolabelled metered dose on 3 occasions in randomised order: (a) by conventional pressurised MDI at an inhaled flow rate of 25 1 rain -1, (b) by MDI plus 10-cm tube spacer attachment at 25 1 rain-1 and (c) by MDI plus spacer at 100 l min -1. On each occasion, firing of the spray was co-ordinated with inhalation, volunteers were instructed to inhale deeply from functional residual capacity to total lung capacity, and a 10 s breath-holding pause was maintained at the end of the inhalation manoeuvre.

Results. Deposition patterns in one subject are shown in Fig. 5 and are typical of the gamma-camera images obtained in studies of this type. The scintigraphic data derived in the study are summarised in Table 5. Using the conventional MDI alone, a mean 11.0% of the dose was deposited in the lungs, and this was increased significantly to means of 16.1% and 13.3% with slow and fast inhalations through the spacer, respectively. The deposition pattern with fast inhalation was significantly more "central", showing on the images as a concentration of deposited material in the major bronchi and being reflected in a low peripheral zone to central zone ratio. Thus, with fast inhalation there was a shift of deposition away from the small peripheral airways, which may be the major site of airways obstruction in asthmatic patients. Using the MDI alone, 75% of the Table 5. The effects of a tube spacer device (Syncroner, Fisons) upon the delivery of 1 mg sodium cromoglycate from a pressurised metered dose inhaler (MDI) in 10 healthy volunteers. The effects of slow (25 l m i n 1) and fast (100 i m i n -1) inhaled flow rates via the spacer were also determined. Data taken from Newman et al. (1989a) and are expressed as mean (SEM)

dose was deposited in the oropharyn×; when the spacer was used, drug not deposited in the lungs was divided between the oropharynx and the internal walls of the spacer. This study showed the importance of both appropriate delivery devices and correct choice of an inhalation manoeuvre in order to optimise delivery of therapeutic aerosols to their target site.

Assessment of dry powder inhalers The move away from the use of chlorofluorocarbon propellants, together with an increasing realisation of the difficulties that many patients experience in using pressurised MDIs correctly, has stimulated increased interest in the development of dry powder systems for drug delivery to the lungs. Such formulations may contain drug powder alone or the drug powder mixed with a carrier substance, usually lactose. Powder inhalers are "breathactuated", the powder being released into the airstream upon inhalation through the device. Labelling and scintigraphic techniques used to assess the deposition from powder inhalers resemble those used for pressurised MDIs, although the labelling method must be tailored precisely to the drug formulation. In general, the drug particles can be radiolabelled by the addition of 99mTCin a solvent in which the drug is insoluble. When formulations containing carrier particles are tested, the drug must be blended with the carrier before filling into the device. In vitro assessments similar to those carried out with pressurised MDIs are required to show that the size distribution of the radiolabel is representative of that of the drug. These techniques have proved invaluable in the assessment of terbutaline sulphate delivery from a new breath-actuated multi-dose powder inhaler, Turbuhaler (Newman etal. 1989b, 1991d). These studies have shown that the Turbuhaler has drug delivery characteristics similar to those of a correctly used pressurised MDI and that deposition patterns are dependent upon the

Whole lung (% of dose) Peripheral lung (% of dose) Peripheral/central ratio Oropharynx (% of dose) Actuator and spacer (% of dose) Losses from open part of spacer (% of dose) Exhaled air (% of dose)

MDI alone (slow flow)

MDI + spacer (slow flow)

MDI + spacer (fast flow)

11.0 (1.4) 4.7 (0.6) 1.37 (0.13) 75.0 (2.5) 13.9 (1.8)

16.1 (2.2)* 6.9 (0.9) 1.34 (0.11) 31.8 (1.9)* 46.1 (2.8) *

13.3 (1.7)* 4.3 (0.5) 0.81 (0.13)** 44.9 (1.9) *' ** 40.4 (2.3)*' **

5.7 (1.1)

t.1 (0.4)**

0.3 (0.1)

0.3 (0.1)*

N/A 0.2 (0.1)

* P < 0 . 0 5 between MDI and spacer; ** P < 0 . 0 5 between spacer (slow) and spacer (fast), using multiple comparison test N/A, Not applicable

982 patient's peak inhaled flow rate through the device. Scintigraphic measurements were coupled with measurements of lung function and showed that the increase in lung deposition brought about with the use of an optimal inhalation technique through the Turbuhaler did not result in a further improvement in the bronchodilatot response.

mucociliary clearance to the nasopharynx occurs at a velocity of about 0.5 cm min-1. The principal methods for assessing nasal mucociliary clearance involve either monitoring the movement of a suitable gamma-emitting radiotracer or measuring the time taken for a sweet taste to be detected after a saccharin tablet is placed on the turbinates (Proctor 1977).

Assessment of systemic drug delivery via the lungs

Example: Assessment of particle delivery from nasal inhalers

The lungs provide a large surface area via which drug absorption into the systemic circulation can occur, and this approach offers many exciting future prospects, notably for the delivery of proteins and peptides. At the same time, adequate drug delivery is difficult to achieve [it has been likened to spray-painting the inside of a room via the keyhole (Edgington 1991)]. Scintigraphic studies coupled with measurements of blood levels of absorbed drug, pharmacoscintigraphy, are ideal for evaluating this type of drug delivery system, either as a test of a final product or as an aid to product development.

Drug delivery to the nasal passages Drugs administered to the nasal passages from pressurised inhalers, aqueous spray devices, powder inhalers or as nasal drops can be delivered very efficiently, either for topical or for systemic therapy. Owing to its anatomy, comprising a narrow constriction at the nasal ostium and a pair of narrow passages in the nasal cavity, the nose acts as a very effective particle or droplet "filter". Drug absorption can occur via the highly vascularised nasal mucosa, the surface area of which is approximately 150 cm 2 (Mygind 1985). Nasal deposition and clearance are related, since the nose can be considered crudely as a two-compartment system, a non-ciliated region comprising nostrils, nasal ostium and the anterior surfaces of the turbinates, for which no rapid clearance mechanism exists, and a ciliated region comprising the posterior two-thirds of the nasal cavity from which Table 6. Comparison of nasal deposition and clearance of sprays from a pressurised metered dose inhaler (MDI) (Newman et al. 1987 a) and an aqueous nasal pump spray (Newman et al. 1988b).

The above features are exemplified by two studies performed to determine the deposition patterns from a pressurised nasal M D I (Newman et al. 1987a) and from an aqueous nasal spray (Newman et al. 1988b), using placebo inhalers into which 99mTc-labelled Teflon microspheres (mass median aerodynamic diameter 3 pm) had been incorporated. The data from these two studies are summarised in Table 6.

Nasal pressurised MDL For the pressurised MDI, the deposition pattern was concentrated in a single region in the anterior part of the nose, the area of which was approximately 20 pixels on a 64 x 64 pixel computer matrix. Approximately 20% of the dose was cleared by mucociliary action in the first 30 rain, while 80% remained at its initial deposition site. No difference in deposition pattern or in subsequent clearance could be detected according to whether two doses were fired with the inhaler held upright, or whether one dose was fired with the inhaler upright and another fired with the inhaler tilted by 30 ° in the sagittal plane. No radioactivity was detected in the lungs.

Nasal aqueous spray. By contrast, the aqueous spray covered approximately twice the deposition area of the spray from the pressurised MDI, sometimes as a single ~,,ion and sometimes as two distinct regions, one in the anterior part of the nose and another more posteriorly. The anterior portion of the dose (approximately 50% of the dose) remained in situ over the first 30 min, while Both inhalers were placebos filled with 99mTc-labetledTeflon particles (mass median aerodynamic diameter 3 gm)

Device

Pixels in initial view of nose

Retention in nose at 30 min (%)

(%) Dose in lungs

MDI (fired in one direction) a MDI (fired in two directions)b Pump (metered volume 50 gl, spray cone 35°) Pump (metered volume 50 ~tl, spray cone 60°) Pump (metered volume 100 gl, spray cone 60°)

21.4 (2.2) 19.2 (1.8) 53.2 (6.9) 43.7 (3.7) 57.5 (7.3)

78.2 (5.8) 81.1 (4.7) 48.7 (5.0) 57.1 (4.5) 46.5 (4.4)

ND ND ND ND ND

ND, Not detectable a Two doses fired with the inhaler held upright b One dose fired with the inhaler held upright, and another dose fired with the inhaler tilted by 30° in the sagittal plane

983 the more posterior 50% underwent mucociliary clearance to the nasopharynx. The results also suggested small differences in nasal deposition and clearance according to the angle of the spray cone emerging from the inhaler, and according to the metered dose size. Again, no material was detected in the lungs.

Other studies of nasal deposition Other scintigraphic studies have enabled the deposition and clearance of nasal sprays and drops to be compared (Hardy et al. 1985b) and have shown the effects of common pathologies such as nasal polyps (Lee et al. 1984) and the common cold (Bond et al. 1984) on the clearance of nasal sprays. Nasal administration is potentially a useful route for systemic drug delivery. The extent of drug absorption, however, is dependent on the nature of the formulation administered. Gamma-scintigraphy has an important role to play in the development of novel delivery systems. When a mixture of desmopressin and 99mTc-labelled albumin microspheres was delivered either by aqueous nasal spray or as drops (Harris et al. 1986), the drops covered a wider area of the nasal cavity and were removed more rapidly by mucociliary clearance than the spray. Since there was inadequate time for the drug contained in the drops to be absorbed into the blood before the solution was cleared to the nasopharynx by mucociliary action, the drop formulation resulted in lower blood levels of desmopressin than administration by nasal spray. By combining scintigraphy with measurements of blood levels of drug, this study was thus able to show that by delivering the drug over a wider surface area of the nasal cavity there was a lower bioavailability of drug, in contrast to the finding that might have been expected on a priori grounds.

Administrative arrangements The basic equipment required for gamma-scintigraphic studies of pharmaceutical products is to be found in most nuclear medicine departments. The studies, however, tend to be very time-consuming and are best performed using dedicated gamma-camera systems. This is particularly the case for studies of gastrointestinal transit, where the imaging periods may extend to 24 h or more. Additionally, most studies in healthy subjects involve dosing 6-10 volunteers on the same occasion. For many studies there is a requirement for overnight accomodation, either to allow monitoring during the night or to ensure that volunteers comply fully with the requirements of the protocol. There is an increasing trend for the pharmaceutical industry to use data from gamma-camera studies in support of product licence applications. The regulatory authorities, such as the Committee of Proprietary Medici-

hal Products (European Communities) and the Food and Drug Administration in the USA, place stringent demands on the companies to ensure the quality of the data used in licence submissions. The requirements of quality assurance are encompassed in the current guidelines of good clinical practice. As an aid to compliance with good clinical practice, each clinical research organisation must have standard operating procedures, which are documents describing all aspects of the scientific and medical undertakings. Compliance with these procedures is designed to ensure that the data generated are of appropriate quality. The well-being of the volunteers is of paramount importance. The Declaration of Helsinki lays down the ground rules. Each study must be undertaken in accordance with a protocol approved by a correctly constituted ethics committee. Authorisation to administer the radiopharmaceutical must be obtained from the Department of Health and a clinical trials exemption certificate issued before undertaking studies in patients with a pharmaceutical product not licensed for use in the UK. Before taking part in a clinical trial, the volunteers must be provided with sufficient information to allow them to give true informed consent. The volunteers must be informed that they are free to withdraw from the study at any time. No fault clinical trials insurance must be carried by the clinical research organisation in order to ensure adequate compensation arrangements in the event of injury attributable to participation in the study. Appropriate pre- and poststudy medical examinations must be carried out and the volunteers restricted as to the frequency of their participation in clinical studies. The demands placed on investigators by the pharmaceutical industry has increased in recent years. The volume of documentation required for the management of clinical studies has grown considerably. Gamma-scintigraphy is, however, playing an increasingly important role in the development and evaluation of pharmaceutical dosage forms. The requirements of such studies can best be met in specialised units, staffed by appropriately experienced medical and scientific personnel and established to comply fully with regulatory authority guidelines.

Conclusions Gamma-scintigraphy has an established role in the development and assessment of pharmaceutical preparations. The technique allows formulations to be monitored non-invasively and under normal physiological conditions. Although it is not usually possible to label the drug molecule with a radionuclide suitable for imaging with a gamma-camera, incorporation of an appropriate tracer into the formulation enables sites of deposition, rates of dispersion and transit times to be monitored. Since the formulations tend to remain relatively localised during the period of monitoring, the doses of

984

radionuclides administered are small and, in general, less than those employed in diagnostic nuclear medicine procedures. It is usual for the initial assessments of delivery systems to be undertaken in healthy subjects. Where, however, the condition to be treated is likely to affect the performance of the delivery system, for example in the delivery of drug to the lungs of asthmatics, it is necessary to carry out comparable studies in patients. The demand for gamma-camera studies for the assessment of drug delivery systems is increasing as the regulatory authorities demand more information in support of product licence applications. This in turn has resulted in the pharmaceutical industry placing more stringent controls over the ways in which the studies are undertaken. Gamma-camera studies provide information that cannot be obtained by other means and have an important role in the development of more efficacious products.

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