Responses in refractory hairy cell leukemia to a recombinant immunotoxin

We report major responses in 4 of 4 patients with hairy cell leukemia (HCL) who have recently been treated on a phase I trial with the recombinant imm unotoxin LMB-2, The immunotoxin, designed to target CD25(+) malignancies, i s composed of the Fv portion of the anti-Tac (anti-CD25) antibody, fused to a 38-kD truncated form of Pseudomonas exotoxin A, and has previously been called anti-Tac(Fv)-PE38, All 4 HCL patients were resistant to standard and salvage therapies for HCL, including 2-chlorodeoxyadenosine (CdA) and inte rferon alpha, and all patients responded to LMB-2 after a single cycle, One patient treated with 2 cycles had a complete remission (CR), with regressi on of HCL cells from the blood and marrow and resolution of splenomegaly an d pancytopenia. As is typical for patients in CR after treatment with CdA, minimal residual disease was detectable by flow cytometry of the bone marro w aspirate. This patient has not relapsed after 11 months. Three other pati ents had 98% to 99.8% reductions in malignant circulating cells. These resu lts represent a proof of principal that targeted therapy with recombinant F v-containing proteins can be clinically useful. LMB-2 may be an effective n ew therapy for patients with chemotherapy-resistant CD25+ HCL, This is a US government work. There are no restrictions on its use.