HIGH EFFICACY OF BRIEF BRAF AND MEK COINHIBITION, ALONE AND COMBINED WITH ANTI-CD20 IMMUNOTHERAPY, IN RELAPSED/REFRACTORY HAIRY CELL LEUKEMIA: A PHASE-2 TRIAL

Abstract

Introduction: ∼50% of HCL patients (pts) relapse after purine analogs (PA) chemotherapy. Vemurafenib (VEM), an oral inhibitor of the BRAF-V600E kinase mutation (genetic cause of HCL; Tiacci et al. NEJM, 2011), proved active when given for a few months to R/R pts (∼35% complete remissions/CR—Tiacci, Park et al. NEJM, 2015). Still, minimal residual disease/MRD persisted in the bone marrow (BM) even in CR pts and the median progression-free survival (PFS) was ∼1 year. Moreover, BM HCL cells often showed ERK phosphorylation (pERK+) despite ongoing BRAF inhibition by VEM, suggesting bypass MEK/ERK reactivation downstream of BRAF as resistance mechanism. BRAF+MEK coinhibition with dabrafenib + trametinib, given to BRAF inhibitor-naïve pts for an indefinite duration (median ∼3 years), led to 66% CR but still rare MRD-negativity (16%; Kreitman et al. Blood, 2023); such a continuous therapy sustained a high PFS at 2 years (94%) but led to frequent serious toxicities (35% of pts). Adding rituximab/RTX (8 doses) to a short VEM course (8 weeks) yielded 87% CR, frequent MRD-negativity (60%), 78% PFS at ∼3 years of median follow-up, and few clinically relevant toxicities (3% of pts) (Tiacci et al. NEJM, 2021).

To improve these results, we tested in R/R HCL a brief BRAF+MEK coinhibition (with VEM + cobimetinib/COB) combined to obinutuzumab/OBI (potentially more active than RTX), after establishing the currently unknown efficacy and safety of VEM+COB in HCL.

Methods: We enrolled 19 R/R pts (median age 58 years; median of 3 previous therapies, including BRAF inhibitor ± RTX in 4 pts [21%]) in a phase-2 multicenter trial testing VEM (960 mg bid) + COB (60 mg daily, 21 days on/7days off) for 2–3 cycles (28 days each), followed at disease progression (PD) by another brief course of VEM+COB combined to OBI (8 doses over 6 cycles, starting together with VEM+COB).

Results: VEM+COB toxicity was as expected and manageable. Clinical events (i.e., not laboratory-only abnormalities) were mostly grade 1–2, consisting mainly in rash due to either drug; arthralgia and photosensitivity due to VEM; and diarrhoea, oral aphthosis and serous retinopathy due to COB.

17/19 pts responded (89%), and 13 of 18 evaluable pts had a CR (72%; 1/13 MRD-). BM pERK+ HCL cells were detected in only 1/9 (11%) evaluable pts. Median PFS was long, 42 months, and a sizable pts subset (4/19, 21%) is free from PD at > 5 years.

Of 12 pts with PD, 5 have not (or not yet) received VEM+COBI+OBI for various reasons, while 7 were treated with this short regimen 22–79 months after VEM+COB. All 7 pts (100%) obtained MRD- CR, and are free from cytopenia recurrence at 4–51 months (n = 6 pts) or had a cytopenia relapse at 44 months (n = 1 pt); only 2 clinically relevant toxicities were recorded.

Conclusions: Brief BRAF+MEK coinhibition with VEM+COB, especially if boosted by anti-CD20 immunotherapy through OBI, is very effective in R/R HCL (see Figure), and compares well with other strategies tested in this setting.

Encore Abstract: EHA 2025

Keywords: molecular targeted therapies; combination therapies; indolent non-Hodgkin lymphoma

No potential sources of conflict of interest.