Efficacy of the anti-CD22 recombinant immunotoxin BL22 in chemotherapy-resistant hairy-cell leukemia

Background Hairy-cell leukemia that is resistant to treatment with purine a nalogues, including cladribine, has a poor prognosis. We tested the safety and efficacy of an immunotoxin directed against a surface antigen that is s trongly expressed by leukemic hairy cells. Methods RFB4(dsFv)-PE38 (BL22), a recombinant immunotoxin containing an ant i-CD22 variable domain (Fv) fused to truncated pseudomonas exotoxin, was ad ministered in a dose-escalation trial by intravenous infusion every other d ay for a total of three doses. Results Of 16 patients who were resistant to cladribine, 11 had a complete remission and 2 had a partial remission with BL22. The three patients who d id not have a response received low doses of BL22 or had preexisting toxin- neutralizing antibodies. Of the 11 patients in complete remission, 2 had mi nimal residual disease in the bone marrow or blood. During a median follow- up of 16 months ( range, 10 to 23), 3 of the 11 patients who had a complete response relapsed and were retreated; all of these patients had a second c omplete remission. In 2 of the 16 patients, a serious but completely revers ible hemolytic uremic syndrome developed during the second cycle of treatme nt with BL22. Common toxic effects included transient hypoalbuminemia and e levated aminotransferase levels. Conclusions BL22 can induce complete remissions in patients with hairy-cell leukemia that is resistant to treatment with purine analogues. (N Engl J M ed 2001; 345: 241-7.) Copyright (C) 2001 Massachusetts Medical Society.