Vemurafenib Plus Rituximab in Hairy Cell Leukemia: A Promising Chemotherapy-Free Regimen for Relapsed or Refractory Patients

BACKGROUND: Hairy cell leukemia (HCL) responds well to purine analogs, but up to 50% of patients relapse. We previously identified the BRAF-V600E mutation as the genetic lesion underlying HCL (NEJM 364:230-2315, 2011), and successfully targeted this mutation in the clinic with the oral BRAF inhibitor vemurafenib through an academic phase-2 multi-center Italian trial in HCL patients relapsed after or refractory to purine analogs (NEJM 373:1733-1747, 2015). In these heavily pre-treated patients, vemurafenib given for a median of 16 weeks produced 96% of responses, including 9/26 (35%) complete remissions (CR) and 16/26 (61%) partial remissions (PR), which were obtained after a median of 8 weeks of treatment. Even in complete responders, immunohistochemistry showed residual (~10%) bone marrow HCL cells at the end of treatment, and relapses were common, occurring at a median of 19 months and 6 months in CR and PR patients respectively. Residual HCL cells resisting vemurafenib treatment might be targeted by concomitant immunotherapy with an anti-CD20 monoclonal antibody, an attractive strategy to potentially achieve a more profound response and a better clinical outcome through a chemotherapy-free approach.

METHODS: We started an academic, phase-2, single-center trial (EudraCT 2014-003046-27) in relapsed/refractory HCL, which tests vemurafenib in combination with rituximab, another targeted non-myelotoxic drug with known single-agent activity in HCL. Eligibility was extended to patients relapsed also after monotherapy with a BRAF inhibitor. Vemurafenib was given at its standard dose (960 mg twice daily orally) for 8 weeks. Rituximab infusions (375 mg/m²intravenously) were given concomitantly with vemurafenib every 2 weeks, as well as sequentially (after the end of vemurafenib dosing) four times every 2 weeks.

RESULTS: We have so far enrolled 22 patients in 16 months. Adverse reactions were reversible, usually mild and consistent with the known toxicity profile of the two drugs when used alone. Notably, a CR was achieved by all 14 patients already evaluable for efficacy (100%), including 4 who had relapsed after a BRAF inhibitor and 1 previously refractory to rituximab. Furthermore, 12/14 patients (86%) obtained the CR as early as after 4 weeks of vemurafenib and 2 concomitant rituximab infusions. This CR rate appears higher than that observed by us and others using vemurafenib alone in BRAF inhibitor-naive patients relapsed after or refractory to purine analogs (CR rate 35-42%; NEJM 373:1733-1747, 2015). Moreover, minimal residual disease (MRD) was undetectable in the bone marrow biopsy and aspirate of 8/11 patients evaluated (73%), both by immunophenotyping and by allele-specific PCR (limit of detection: 0.05% BRAF-V600E copies). In 5 of these 8 patients, MRD clearing was reached even before sequential rituximab dosing post-vemurafenib. In the remaining 3/11 patients, MRD was at most 5% in 2 vemurafenib-naive patients, and 10% in 1 patient relapsed after prior BRAF-inhibitor treatment. In contrast, residual bone marrow disease was a constant feature of all 26 patients treated by us with vemurafenib alone for a longer time period (NEJM 373:1733-1747, 2015).

CONCLUSIONS: This study - which is the first one combining vemurafenib and rituximab in relapsed/refractory HCL - suggests that this non-myelotoxic regimen produces more numerous, faster and deeper CRs than vemurafenib alone. Enrollment continues.