Phase 2 study of rituximab in the treatment of cladribine-failed patients with hairy cell leukemia.

Hairy cell leukemia (HCL) is an indolent B-cell neoplasm, strongly expressing CD20. Despite initial very high response rates following cladribine, many patients (pts) ultimately relapse. Having relapsed after prior treatment with cladribine, 24 HCL pts (21 male, 3 female) with a median age of 53.5 years were treated with rituximab at 375 mg/m2 intravenously weekly for 4 weeks. Of the pts, 3 (13%) achieved complete remissions and 3 (13%), partial responses. Thus, 6 (25%) of 24 pts achieved a response following rituximab. At a median follow-up of 14.6 months, 2 responders have relapsed; median time to relapse was not yet reached. The only grade III or IV toxicities demonstrated were culture-negative febrile neutropenia, transient and reversible disseminated intravascular coagulation related to rituximab administration, and a diverticular abscess, each in single patients. Of 18 nonresponders, 9 pts subsequently received other treatments; 5 pts were retreated with cladribine, 3 underwent splenectomy, and 1 received pentostatin. Follow-up data are available on 7 of these 9 patients; all 7 patients achieved improvements in hematologic parameters. Rituximab, administered at this dose and schedule, has only modest single-agent activity in cladribine-failed HCL patients when compared with other agents active in this disease.