Long Term Follow-up of a Phase II Study of Cladribine with Concurrent Rituximab in Patients with Hairy Cell Leukemia Variant

Background Hairy cell leukemia (HCL) variant (HCLv) is considered a separate, more aggressive entity compared to classic HCL. HCLv responds poorly to single-agent purine analog with complete response (CR) rates below 10% and overall response rates under 50%. Rituximab combined with purine-analog can improve response rate and duration, but long-term data have not been reported for HCLv, particularly regarding minimal residual disease (MRD). We therefore update the results of a phase II trial with cladribine and concurrent rituximab in patients with HCLv, previously reported for 10 of the 20 patients enrolled. Methods Patients with HCLv with 0 to 1 prior courses of cladribine, and/or 0-1 prior courses of rituximab, received cladribine (0.15 mg/kg days 1-5), with 8 weekly doses of rituximab (375 mg/m2) beginning day 1. The primary endpoint was to determine CR rate and secondary endpoints included evaluating minimal residual disease (MRD) by blood and bone marrow aspirate flow cytometry, and bone marrow biopsy immunohistochemistry. Patients were able to receive a 2nd course of rituximab ≥ 6 months after the first, if and when MRD was detected in blood. Results Twenty patients were enrolled. Median age was 67 (range: 42-86) years. No patients had prior concurrent cladribine-rituximab. Eight were previously untreated, 1 had only splenectomy, 6 had prior cladribine, 1 had prior cladribine and splenectomy, 1 had prior rituximab, 1 had prior rituximab and splenectomy, 1 had cladribine, rituximab, and splenectomy, and 1 had combination rituximab-containing chemotherapy followed by cladribine. Out of 20 patients receiving concurrent cladribine-rituximab (CDAR), the CR rate was 95% (95% CI: 75-100%). This CR rate was superior to a historical control group of 3 of 39 HCLv patients who achieved CR to cladribine alone (p<0.0001). Sixteen (80%, 95% CI: 56-94%) of 20 patients became MRD-free at 6 months; median duration of MRD-free CR was 72.0 months, with 9 of 16 still MRD-free at 5-108 (median 29.1) months. With median potential follow up of 88 months (range: 7-123 months), 10 patients received delayed rituximab and 4 re-achieved MRD-free CR. Six patients required alternative treatment and 6 patients died, 5 with HCLv including 1 with HCLv limiting treatment for lung cancer, and 1 with Parkinson's disease but still MRD-free. Time from progression of HCLv to death was 5.9-30.0 (median 28.1) months. Achieving MRD-free CR by 6 months after CDAR (16 vs 4 patients) was important for median progression free survival [PFS, unreached vs 17.4 mo, hazard ratio (HR) 0.031, 95% CI 0.003-0.29, p<0.0001] and overall survival (OS, unreached vs 38.2 months, HR infinite since all 4 MRD+ deaths were prior to deaths of 2 patients who achieved MRD-free CR, p<0.0001). A significant relationship between prior purine analog therapy or unmutated IGHV4-34 (n=7) status and either PFS or OS has not yet been observed. Conclusion Concurrent cladribine with rituximab is highly effective in HCLv irrespective of prior purine-analog treatment or IGHV4-34 status and should replace purine analog monotherapy as treatment. Patients with long-term MRD-free CR are being followed to determine whether concurrent cladribine-rituximab as 1st-3rd line systemic therapy can permanently eradicate HCLv. Patients who progress have limited OS. This provides a rationale for the testing of higher intensity approaches up front and the identification of additional treatment options for HCLv. Disclosures Kreitman: Genentech: Research Funding. OffLabel Disclosure: Rituximab for hairy cell leukemia