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of lymphoid-associated conflicting

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Cancer Research Center and the University Seattle, Washington, USA

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Opinion

for acute leukemia.

in Immunology

Introduction

1992, 4:641-646

geted cell, may solve both problems. Recent publications have described the development of new animal models of leukemia as well as the novel application of mAbs to the therapy of patients with leukemia and lymphoma.

The advent of monoclonal antibody (mAb) technology has influenced the understanding of the biology of leukemia and lymphoma as well as the management of patients suffering from these diseases in several ways. The development of antibodies that react with normal cells of hematopoietic origin and the subsequent char acterization of the antigens defined by these antibodies has been instrumental in the definition of the ontogeny of cells in each lineage, and in many cases has allowed eluci dation of the function of cell-surface molecules. The availability of mAbs that react with the leukemia or lymphoma cells of most patients, as well as the relatively ready access of the malignant cells to antibody in circulation, has made these diseases a promising target for the use of mAbs as carriers for cytotoxic molecules.

The use of mAbs for prognostication hematopoietic

in

malignancies

Recent studies using mAbs to detect the cellsurface expression of normal myeloid and lymphoid-associated antigens by leukemic cells have called into question previous findings suggesting that the expression of antigens normally associated with cells of the lymphoid lineage by acute myeloid leukemia (AM,) blasts predicts a poorer outcome [ 1,2]. By analyzing large numbers of children 13-1 and adults [4**] with AML, these studies clearly demonstrate that such markers have no prognostic value in children, but a better prognostic value in adults with AM,.

The past year has seen progress in the use of mAbs for determining the prognosis in leukemia as well as in the therapy of leukemia and lymphoma. Previous studies have demonstrated that the efficacy of unmodified antibodies alone to treat these diseases is generally poor, in part limited by the capacity of the host’s immune system to effectively destroy antibody-coated cells, and by the emergence of antigen-negative variant cells after therapy. The conjugation of antibodies to moieties themselves toxic to cells, such as the naturally occurring toxins of plant origin, is an attempt to overcome the first limitation, whereas the use of radioisotopes, which may kill cells within several cell diameters of a tar-

In the study by Smith et al. [3**] of the prognostic significance of lymphoid-associated antigens expressed by childhood AML blast cells, the leukemic blasts from 176 of 590 patients with AML treated on the Childrens Cancer Group Protocol #2l3 were evaluated for the cell-surface expression of antigens that are thought to be specific for the lymphoid lineage (CD2 = 9.4%, CD5 = 2.7%, CD19 = 34.5%, CD20 = 0.8%). There was no association between lymphoid-associated antigens and factors such

Abbreviations ALL-acute

lymphoid

dgA-deglycosylated

leukemia; ricin

SCID-severe

A

AML-acute

myeloid

leukemia;

chain; mAbmonoclonal

combined

immunodeficient; @

Current

bR-blocked

antibody; TBI-total

Biology

ricin; cGy-centigray;

PAP-pokeweed body irradiation;

Ltd ISSN 0952-7915

antiviral

protein;

WBC-white

CR-complete PR-partial

response; response;

blood cell.

641

642

Cancer

as white blood cell (WBC) count at diagnosis, age and FAB classification, or with measures of outcome such as response to induction therapy and event-free survival. A similar study examining the prognostic value of lymphoid-associated antigens (CD2 and CDl9) in 339 adults with AML was performed by Ball et al. [4**]. Twenty-one percent of patients had AML blast cells that expressed CD2 while 14% of patients had blast cells that expressed CD19. There was no difference in age, WBC at diagnosis, FAB classification, or incidence of extramedullary disease between patients with CD2 + CD19 + and CD2- CD19cells. CD2+CD19+ blast cells were associated with a higher incidence of karyotypic abnormalities commonly seen in AML. However, patients with CD2 +CD19+ cells had higher complete remission rates (75% versus 59%, p = 0.04), longer time to failure, and improved survival in comparison with patients with CD2- CD19cells. The prognostic value of the expression of myeloid-associated cell surface antigens by childhood acute lymphoid leukemic blasts is less clear. A recent study by Kurec et al. [ 5.1 examined 51 children with acute lymphoid leukemia (ALL) as diagnosed by standard morphological and cytochemical methods. Cell-surface antigens associated with the myeloid-lineage (CDl3, 14, 15, 33) were observed in 16% of patients. There was no difference in age, sex, WBC at diagnosis, FAB classification, or incidence of extramedullary disease between patients with myeloid-antigen-positive and -neg. ative cells; however, the length of the first remission and survival time was shorter in those patients whose blast cells expressed myeloid-associated antigens. These results are consistent with those presented by Wiersma et al [6] who demonstrated that myeloid-antigen expression on blast cells from children with ALL was an independent predictor of a poor response to chemotherapy. However, these studies are in contrast to the recent studies by Pui et al. [7,8] and Urbano-Ispizua et al. [9] that found no prognostic value for the presence of myeloid-associated antigens on ALL blast cells, Thus, the concept that a ‘biphenotypic’ AML carries a poorer prognosis is not supported by recent findings. The outcome of ‘biphenotypic’ ALL is still unclear, as demonstrated by dissimilar findings by different investigators. The apparent discrepancies in these studies may be explained in part by differences in the criteria for cellsurface antigen positivity as well as treatment of patients with different therapeutic regimens. It is clear, however, that mAbs in certain circumstances do add prognostic information and may delineate leukemias of particular biological interest. For example, Bassan et al. [lo] identified six patients, from 194 adult patients with acute leukemia, whose blast cells coexpressed CD13 and CD7 but could not be classified by morphology and cytochemistry. These patients had a poor prognosis as determined by their inability to rem spond to treatment, high rate of relapse, and progression of disease.

lmmunotoxins Immunotoxins, particularly ricin, have for the most part been used to treat patients with solid tumors, and this treatment has resulted in a poor clinical response with relatively high toxicity and with the frequent development of human anti-mouse antibody and/or human antiricin antibody. Recent animal and human trials treating hematological malignancies with ‘second generation’ immunotoxins, designed to increase potency and minimize toxicity, have been more promising. Several investigators have developed animal models of human leukemia by implanting human leukemic cell lines in various strains of severe combined immunodeficient (SCID) mice, with or without the use of additional irnmunosuppression. These mice develop leukemic or lynphomatous involvement of hematopoietic organs including the bone marrow, spleen, and blood, as well as the lymph nodes, liver, ovaries, central nervous system, and other organs. The availability of these leukemic mice aI_ lows evaluation of the efficacy of immunoconjugates in an in uivo setting, particularly with respect to their pharmacological properties, but these models are limited by the lack of reactivity of the antibody with normal cells, as is encountered in humans. Striking results were obtained by Ghetie et al [ 11’1, who used two deglycosylated ricin A chain (dgA) constructs to treat SCID mice with disseminated human B-cell lym phoma (produced by intravenous injection of the B-cell Daudi line). DgA conjugated to intact IgG anti-CD22 antibody was compared with dgA conjugated to the Fab’ fragment of the same antibody; dgA constructs have the theoretical advantage of being taken up less by the liver compared with intact ricin A chain. This treatment resulted in a 45 log cell kill with the IgGdgA anti-CD22 antibody and a 2 log kill with the Fab’dgA anti-CD22 reagent, compared with 1 log kill with unconjugated IgG alone and no kill with the unconjugated Fab’. Fishwild et al. [ 12.1 have established the human T-cell leukemia line CEM in cyclophosphamide-pretreated NIH-III mice, and demonstrated the ability of anti-CD7-ricin A chain* immunoconjugates to deplete leukemic cells 10@200fold, compared with a threefold depletion using anti-CD7 *antibody alone. Uckun et al. [13**] have used antiCDl0 antibody conjugated with pokeweed antiviral protein (PAP) immunotoxin in a mouse model of pre-B-cell ALL, in which SCID mice were injected with a highly aggressive subclone of the human pre-B-cell ALL line NALM6. The administration of three daily doses of B43 (antiCD19)-PAP intraperitoneally, after intravenous injection of tumor, reduced the death rate from 100% to 36%, suggesting a > ‘6 log kill of tumor cells. This group used the same immunotoxin (B43-PAP) to treat a human t(4;ll) leukemic cell line implanted intravenously or intraperitoneally into CR.17 SCID mice pretreated with 250 centigray (cGy) total body irradiation (TBI) [ 141. A similar three day treatment regimen resulted in no discernible tumor in the liver and spleen 33-34 days after treatment with B433PAP in mice that received tumor cells intravenously, and a 60% survival rate of mice that received tumor cells

Monoclonal

antibodies

intraperitoneally. In another model, this group has taken advantage of a significant homology between the human and mouse amino acid sequences of CD19, and treated a highly aggressive subclone of the mouse B-cell leukemia BCL-1 injected into BALB/c mice with the same B43-PAP immunotoxin [IS*]. Mice treated with high doses had prologed survival suggesting at least a 3 log kill of tu mor cells. Several Phase I studies of treatment of patients with hematological malignancies with immunotoxins were reported this year. Vitetta et al. [16*] have used Fab’RFB4dgA (anti-CD221 in 15 patients with refractory B-cell lymphoma, with 38% of the patients demonstrating a greater than a 50% reduction in tumor size. The major side elfect in this trial with dgA was a vascular leak syndrome. Four of 14 patients made human antiimouse antibodies and/or human anti-ricin antibodies. LeMaistre et al. [17*] have studied anti-CD5-ricin A chain conjugates (H65-RTA) in 14 patients with cutaneous ‘T-cell lymphoma, with partial response (PR) in four patients persisting for 3-8 months. Vascular leak syndrome was again the major dose-limiting side effect, and 10 of 12 patients that could be evaluated made antiH65-RTA antibodies, with seven of 11 patients making antibodies that blocked the binding site of the immunoconjugate (i.e. antiidiotype antibodies). An alternative approach for improving the efficacy and minimizing the toxicity of the ricin molecule is the use of ‘blocked’ ricin. The B chain of the native ricin molecule mediates the nonspecific binding of the molecule to galactose moieties on the surface of all eukaryotic cells. The two high-affinity galactose binding sites can be blocked covalently with natural ligands. This ‘blocked’ ricin (bR) has been conjugated to the mAb anti-B4 (antiCD19), resulting in an immunoconjugate which is highly cytotoxic, in a restricted fashion, to CD19+ cells; the retention of the blocked B chain appears to mediate transport of the immunotoxin across the cell membrane [ lW]. Grossbard et al. [ 19.1 have treated 23 refractory B-cell NHL patients with anti-B4-bR and have obtained a complete response (CR) in one, a PR in two, and transient or mixed responses in eight patients. It is of interest that the main toxic side effect in this phase I study was not the vascular leak syndrome seen with ricin A chain constructs, but a reversible elevation of hepatic transaminases without impairment of hepatic JOUIn~ UO 3x3 -3x :asms!a dxm!~yapounumq pau!qulo3 aJaA% ql!M a2gq u! UMOJQ euroqdux.Q g ueurnH paleu!wass!a p syxol -ountuuq zza3-gut-@I pue ,qed 30 .4q~!i~vJorump~ :ss VLL%L.IA‘,& MHfI ‘a S3NOr ‘I kIm3fl~ ‘I NOSm3M ‘W 31L3H~ OJU! “!t?q;, “! u!ey>

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