Leukemia & Lymphoma, June 2011; 52(S2): 79–81
Enhancing immunotoxin cell-killing activity via combination therapy with ABT-737
DAVID J. FITZGERALD1, ELIZABETH MOSKATEL1, GAL BEN-JOSEF1, ROBERTA TRAINI1, TARA TENDLER1, ASHIMA SHARMA1, ANTONELLA ANTIGNANI1, FRANCIS MUSSAI1,2, ALAN WAYNE2, ROBERT J. KREITMAN1, & IRA PASTAN1
1Laboratory of Molecular Biology and 2Pediatric Oncology Branch, CCR, National Cancer Institute, NIH, HHS, Bethesda, MD, USA
Abstract
Immunotoxins are antibody–toxin fusion proteins directed to kill cancer cells displaying specific target antigens on their surface. Remarkably, immunotoxins directed to CD22 on hairy cell leukemia have produced complete remissions in approximately 60% of patients enrolled in phase I/II trials. For reasons that are not yet clear, 40% of patients responded less well. In addition, patients with other CD22-positive malignancies have not yet achieved complete remissions. In trying to understand ‘resistance’ to immunotoxin therapy, a number of challenging issues have been raised. These include insufficient dosing, the production of neutralizing anti-immunotoxin antibodies, poor access to malignant cells, and resistance to toxin killing. In designing immunotoxins, we employ truncated Pseudomonas exotoxin, which enzymatically inactivates protein synthesis and produces cell death in sensitive cells. To begin to address toxin resistance we have explored combination therapy with the BH3-only mimetic, ABT-737. Our results indicate that immunotoxin–ABT combinations often exhibit greater killing activity than either compound alone and in some instances overcome resistance. Expression of high levels of prosurvival Bcl-2 proteins may contribute to toxin resistance.
Keywords: apoptosis, ABT-737, BH3-only, Pseudomonas exotoxin Introduction
Immunotoxins are antibody–toxin chimeric proteins targeted to kill cancer cells [1]. Because native antibodies, even when they bind cancer cells, are rarely cytotoxic, modifications are needed to increase potency. One strategy is to attach a bacterial toxin to the antibody, combining the binding specificity of the antibody with the cell-killing action of the toxin. Recombinant immunotoxins, composed of antibody fragments (Fv) joined to domains II and III of Pseudomonas exotoxin (PE), have shown great pro- mise for the treatment of hematologic malignancies, especially hairy cell leukemia (see Kreitman et al., this issue), but have exhibited less activity against cancers derived from epithelial cells. PE-derived immunotoxins are cytotoxic because they gain access
to the cell cytosol and inhibit protein synthesis via the enzymatic adenosine diphosphate (ADP)- ribosylation of elongation factor 2 (EF2). While the mechanism of toxin-mediated cell death is still under investigation, some features have been reported. Du et al. have reported Bak-dependent apoptosis [2], and several groups have noted that inhibition of protein synthesis results in the loss of the prosurvival protein, Mcl-1, potentially mediating apoptosis via this route [3]. Despite their potency, immunotoxins produce complete remissions infrequently when administered as single agents. Accordingly, various strategies have been proposed to increase effectiveness. Immuno- toxins constructed with bacterial toxins are fre- quently immunogenic, prompting strategies to eliminate major antigenic epitopes [4]. Reducing
Correspondence: David J. FitzGerald, Laboratory of Molecular Biology, CCR, National Cancer Institute, NIH, HHS, Bethesda, MD, USA. E-mail:
djpf@helix.nih.gov
ISSN 1042-8194 print/ISSN 1029-2403 online 2011 Informa UK, Ltd. DOI: 10.3109/10428194.2011.569961
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