We conducted a phase 1 study of an anti-CD33 immunotoxin, humanized monoclonal antibody M195 conjugated to recombinant gelonin (HUM-195/rGEL), in patients with relapsed, refractory myeloid leukemias. Twenty-eight patients received the construct intravenously at four dose levels (12, 18, 28 and 40 mg/m2 per course) in a "3+3" study design. The dose-limiting toxicity was infusion-related allergic reaction including hypoxia and hypotension. The 28 mg/m2 total dose was considered the maximally tolerated dose. Four patients developed a reduction in peripheral blood blasts of at least 50%. Three patients treated with the 10, 12 and 28 mg/m2 doses showed a 38-50% reduction in bone marrow blasts. There was normalization of platelets in one patient treated with 40 mg/m2. Pharmacokinetic analysis demonstrated that the highest blood levels achieved were 200-300 ng/mL which cleared with a half-life of ~20 hours. Antigenicity was low with one patient at the 12 mg/m2 dose and one patient at the 18 mg/m2 dose (2/23, <10%) developing antibodies to the recombinant gelonin component after 28 days. We concluded that HUM-195/rGel can be safely administered in a multi-dose cycle to patients with advanced myeloid malignancies and warrants further investigation.
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