Phase II trial of CAT-3888 (BL22) in chemo-resistant hairy cell leukemia

7095 Background: CAT-3888 (BL22) is a recombinant anti-CD22 immunotoxin which showed phase I activity in chemo-resistant hairy cell leukemia (HCL). Methods: Eligible patients had relapsed HCL < 4-years after prior cladribine (CdA) and needed additional treatment based on blood counts. Patients were stratified into 2 groups by most recent CdA response of < 1 year or 1–4 years. A 3rd group was added for patients with uncontrolled infection. The study used a 2-stage design of 10 patients each for the first two strata with expansion to 17–30 if hematological remission (HR, i.e. ANC, platelets and Hgb at least 1.5, 100, and 11, respectively) was achieved in at least 3/10. The third strata allowed up to 5 patients. Patients received 40 ug/Kg every other day (QOD) ×3 on cycle 1. Those achieving HR were observed. Patients without a HR and no neutralizing antibodies were retreated at 30 ug/Kg QOD ×3 every 4 weeks beginning at least 8 weeks after cycle 1. Results: 35 patients were enrolled including 25, 9 and 1 in ...