Real-Life Efficacy and Safety of Vemurafenib Plus Rituximab (V+R) in Relapsed or Refractory Hairy Cell Leukemia: A Multi-Center Retrospective Study (HCL-PG03R)

BACKGROUND

Hairy cell leukemia (HCL) is a BRAFV600E-mutated indolent B-cell neoplasm (Tiacci et al., NEJM 2011) often relapsing after chemotherapy with purine analogs. BRAF inhibition is very active in relapsed/refractory (R/R) HCL, but produces mostly partial remissions (PR) and never clears minimal residual disease (MRD) (Tiacci, Park, et al., NEJM 2015). In a recent phase-2 single-center study on 30 R/R patients (pts) with a median of 3 prior therapies, a short chemotherapy-free regimen of the BRAF inhibitor vemurafenib (V) + rituximab (R; Mabthera) led to 87% complete remissions (CR), 60% MRD-negativity and 85% relapse-free survival (RFS) at a median follow-up of 34 months, with a favorable toxicity profile (Tiacci et al., NEJM 2021).

METHODS
A multi-center retrospective study was conducted to validate in a real-world setting the off-label use of V+R in R/R HCL needing therapy due to cytopenia(s).

RESULTS
From 6/2019 to 12/2021, 35 pts (median age 56 years, range 42-80) were treated at 14 Italian centers with VR (V: 960 mg bid for 8 weeks; R: 375 mg/m2 every 2 weeks for 8 doses); biosimilar R was used in 89% of pts. Prior therapies (median 2; range 0-11) included chemotherapy with cladribine or pentostatin (94% of pts), interferon-alpha (29%), rituximab (31%), splenectomy (6%), BRAF inhibitor monotherapy (6%) and zanubrutinib (3%).
Median blood counts across the 35 pts were: platelets 69000/mmc, neutrophils 860/mmc and hemoglobin 11.6 g/dl (with 20% of pts, i.e. 7/35, requiring transfusions). Eleven pts (31%) had active (n=3) or latent (n=8) infections, including: ongoing systemic atypical mycobacteriosis (n=1); ongoing miliary tuberculosis (n=1; a 80-year old pt who was the only one receiving V+R as first-line therapy); ongoing large cerebral abscess (n=1); HIV infection controlled by anti-viral therapy (n=1); and latent tuberculosis (n=1) or HBV (n=6) infections requiring anti-microbial prophylaxis.
Toxicity was as expected, mostly of grade 1-2 and always reversible (Table), allowing high relative dose intensities (RDI) for both drugs (V, median 97%; interquartile range/IQR 73-100%; R, 100% in 85% of pts). Similarly, among the 33/35 (94%) pts who received V for a substantial duration (≥35 days), and were thus evaluable for dose density, 20 pts (61%) completed V without ever suspending the drug, including 14/33 pts (42%) without any dose reduction. Vemurafenib toxicities were mainly represented by cutaneous rash, asymptomatic liver and pancreatic laboratory abnormalities, arthralgia, warts and photosensitivity.
Three pts (9%) were not evaluable for response as they died prematurely due to atypical mycobacteriosis, complications of neurosurgery for cerebral abscess and sudden death unrelated to study drugs.
Recovery of platelets (≥100000/mmc), neutrophils (≥1500/mmc) and hemoglobin (≥ 11 g/dl) occurred quickly, after a median of 2, 4 and 7 weeks after starting treatment, respectively.
A CR was observed in 30/32 evaluable cases (94%; or 86%, 30/35 pts, by intention to treat/ITT). CR was obtained in all evaluable cases refractory to chemotherapy (n=8) or rituximab (n=3), or previously splenectomized (n=2) or unable to reach CR with a prior BRAF inhibitor (n=2). The only 2 evaluable pts not achieving a CR (1 PR; 1 short-lived resolution of cytopenias) received V at only 16%/50% RDI and R at only 13%/34% RDI due to drug toxicities.
Analysis of MRD by allele-specific PCR for BRAF-V600E in the bone marrow aspirate was performed in 30 pts and showed MRD clearance (<0.05% mutant alleles) at a high rate: 79% (22/28 CR pts; or 65%, 22/35 pts, by ITT), which raised to 82% (66% by ITT) when including 1 additional CR case untested by PCR which had negative bone marrow flow cytometry.
At a median follow-up of 21 months after the end of treatment, RFS was high among the 31 responding pts, with just 1 relapse (3%) in the only case obtaining a PR post-therapy (Figure).

CONCLUSIONS
This study validates in a real-life context the high efficacy and tolerability of the V+R chemo-free regimen delivered to R/R HCL pts, including the cost-saving use of biosimilar rituximab.