Robert Lechleider and Ira Pastan
Immunotoxins composed of monoclonal antibodies linked to bacterial or plant protein toxins have been studied during the past 30 years as a potential targeted therapy for cancer. A series of refinements in the design of immunotoxins containing the bacterial toxin Pseudomonas exotoxin A (PE) led to the development of the current smaller, more selective, and more stable recombinant immunotoxins that are composed of a truncated form of PE fused to the variable domain of a monoclonal antibody. Immunotoxins targeting CD22, an antigen commonly expressed on B-cell malignancies, including BL22 (also known as CAT-3888) and its improved, higher affinity derivative, moxetumomab pasudotox (also known as HA22 or CAT-8015) are being evaluated in patients with hematologic malignancies. BL22 induced complete remissions in the majority of patients with chemoresistant hairy cell leukemia (HCL) during phase 1 and 2 studies, and showed promising antitumor activity in patients with chronic lymphocytic leukemia (CLL). Moxetumomab pasudotox is currently undergoing phase 1 testing in refractory HCL, as well as in CLL, non-Hodgkin lymphoma, and pediatric acute lymphoblastic leukemia (ALL). Thus far, clinical activity has been observed in approximately 80% of adult patients with HCL and complete remissions have been reported in 25% of children with ALL. No dose-limiting toxicities have been reported in the adult study in patients with HCL or in patients in the pediatric study who also received dexamethasone.
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