Treatment with Combination of Dabrafenib and Trametinib in Patients with Recurrent/Refractory BRAF V600E-Mutated Hairy Cell Leukemia (HCL)

BACKGROUND: Hairy cell leukemia is a rare, indolent, B-cell lymphoproliferative disease characterized by the BRAF V600E mutation in 90% to 100% of patients. Few treatment options are available for patients who progress on first-line therapy with a purine analogue and/or rituximab. The efficacy of combined BRAF and MEK inhibition is well established in BRAF V600-mutated melanoma, non-small cell lung cancer, and anaplastic thyroid cancer. Here we report interim results of treatment with the combination of dabrafenib (BRAF inhibitor) and trametinib (MEK inhibitor) in patients with recurrent/refractory BRAF V600E-mutated HCL.

METHODS: In this phase 2, open-label trial (ROAR; NCT02034110), patients with BRAF V600E mutations in 9 rare tumor types, including HCL, received continuous dabrafenib (150 mg twice daily) and trametinib (2 mg once daily) until unacceptable toxicity, disease progression, or death. For the HCL cohort, eligible patients had histologically confirmed HCL (according to WHO classification) that was refractory to first-line treatment with a purine analogue or relapsed after ≥ 2 prior lines of treatment. The presence of a BRAF V600E mutation was assessed locally and confirmed by a central laboratory. The primary endpoint was investigator-assessed overall response rate (ORR) based on criteria adapted from NCCN guidelines. Secondary endpoints were duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety. Minimal residual disease (MRD) status was assessed by flow cytometry in both peripheral blood and bone marrow aspirates. For complete response (CR) without MRD, both peripheral blood and bone marrow aspirate samples had to be negative.

RESULTS: At the data cutoff (January 3, 2018), 43 patients with HCL had enrolled. Median age was 67 years (range, 40-81) and 38 (88%) were male. Twenty-one patients (49%) had received ≥ 4 prior treatments. At the time of data cutoff, 35 patients (81%) remained on study treatment, 5 (12%) in follow-up; 2 (5%) had withdrawn from study (loss to follow-up and consent withdrawal [n = 1 each]), and 1 (2%) had died. Eight patients had discontinued therapy due to adverse events (AEs; 5 [12%]), study withdrawal (2 [5%]), or disease progression (1 [2%]). Median time on study treatment was 17 months (range, 1-39). Among 41 patients evaluable for response, the investigator-assessed confirmed ORR was 78% (32/41; 95% CI, 62%-89%) with 20 (49%) having CR, (6 [15%] CR without MRD and 14 [34%] CR with MRD), and 12 (29%) partial response. All responses were ongoing at the data cutoff; 16 (50%) responses were lasting ≥ 18 months. PFS and OS rates at 12 months were both 97.6% (95% CI, 83.9%-99.7%). The most common AEs in patients with HCL were pyrexia (67%), chills (51%), hyperglycemia (44%), nausea (44%), peripheral edema (42%), cough (40%), and fatigue (40%). Grade 3/4 AEs were reported in 49% of patients, with the most common (> 5%) being hyperglycemia (9%), anemia (7%), and neutropenia (7%). AEs led to dose reduction and treatment interruption in 42% and 56% of patients, respectively. AEs led to permanent discontinuation in 5 patients (12%; headache and malaise [n = 1]; pyrexia, chills and palmar-plantar erythrodysesthesia syndrome [n = 1]; fat necrosis [n = 1]; hyperglycemia and pancreatic adenocarcinoma [n = 1]; Hodgkin lymphoma [n = 1]).

CONCLUSIONS: Dabrafenib + trametinib was well tolerated and demonstrated a high rate of durable responses in patients with heavily pretreated recurrent/refractory BRAF V600E-mutated HCL.