Monoclonal Antibodies Show Some Progress Against Hairy Cell Leukemia

NEW ORLEANS—Hairy cell leukemia (HCL) has the highest reported surface expression of CD20 of leukemias studied to date, according to research reviewed at the ASH meeting. In a symposium presentation, Deborah A. Thomas, MD, and colleagues from the M.D. Anderson Cancer Center in Houston, reported that the anti-CD20 monoclonal antibody rituximab (Rituxan) is active against HCL. Response appears to correlate with a decrease in serum interleukin-2 (IL-2) receptor levels. In a poster presentation, Robert J. Kreitman, MD, and colleagues from the National Cancer Institute reported that recombinant immunotoxins containing truncated Pseudomonas exotoxin and targeting either CD25 or CD22 can induce some major responses in patients with refractory HCL.

Rituximab Pilot Study

Currently, chlorodeoxyadenosine (CdA) is widely used in treatment of HCL, but patients beyond 7 years of follow-up have an increased risk of relapse, and therapy is accompanied by a high risk of infectious complications. Durable remissions are achieved in most patients, but up to 25% do not respond either initially or eventually, and many require chronic blood product support.

Dr. Thomas said that HCL has the highest surface expression of CD20 of any leukemia. Rituximab was an attractive therapeutic candidate because it is a monoclonal antibody directed at the CD20 antigen. Her research team enrolled 10 HCL patients in a pilot study of the antibody. All had prior CdA chemotherapy; two were refractory and eight had responded but relapsed. Five patients were heavily pre-treated, with two or more prior therapies.

Patients were treated with rituximab 375 mg/m² IV weekly for 8 weeks. At the time of the ASH meeting, 9 of 10 patients were evaluable for response, with a median follow-up of two months. Dr. Thomas reported an overall response rate of 67% (6/9), including a complete response rate of 33% (3/9) and a complete response with minimal residual disease in 22% (2/9).

Serum IL-2 Receptors

The researchers studied changes in serum IL-2 receptors, which are expressed at high levels in HCL. In patients who received all eight planned doses of rituximab and had complete remissions, serum IL-2 receptor levels returned to undetectable levels. Four patients received only 4 of the 8 planned doses, and serum IL-2 receptor levels did not drop to undectable level in these patients.

Toxicity of rituximab was generally mild, with no infections and mostly only mild fever and chills. One patient had grade 3 myalgia requiring narcotics after each dose of rituximab but was able to take all eight planned doses. “Rituximab appears promising with minimal toxicity in relapsed/refractory HCL and warrants further study,” Dr. Thomas concluded. “Rituximab may also have a role in treatment of minimal residual disease.”

Targeting CD25 and CD22

Dr. Kreitman’s group tested two recombinant immunotoxins targeting either CD25 (the IL-2 receptor alpha chain) or CD22. LMB-2 is composed of the variable domains of the anti-Tac monoclonal antibody to CD25, fused to bits of Pseudomonas immunotoxin. BL-22 is composed of the variable domains of the RFB4 monoclonal antibody to CD22, fused to the same Pseudomonas toxin. In this Phase I study, four patients were treated with LMB-2, and seven were treated with BL-22.

Toxicities included reversible transaminase elevations, fever, and hypoalbuminemia. Dr. Kreitman said that BL-22 toxicity has been blocked by anti-inflammatory agents, and similar toxicity prevention is planned for LMB-2. “Toxicity appears to be related to a systemic inflammatory response syndrome, which may be preventable by anti-inflammatory agents,” he said.

Dr. Kreitman reported that LMB-2 produced complete response (CR) in 1 of 4 patients and partial response (PR) in 3 of 4. BL-22 produced CR in 3 of 7 patients and PR in 2 of 7 patients. “LMB-2 and BL-22 are clinically active in patients with chemotherapy-refractory hematologic malignancies, particularly HCL, where the response rate is 100% in patients treated with 10-63 µ/kg QOD x 3 of LMB-2 or BL-22,” Dr. Kreitman concluded. “Complete responses appear durable. BL-22 appears to be clinically active in patients with the poor-prognosis variant HCL characterized by CD25-/CD22+ malignant cells.”