Safety and Preliminary Efficacy Results from a Phase II Study Evaluating Combined BRAF and MEK Inhibition in Relapsed/Refractory Multiple Myeloma (rrMM) Patients with Activating BRAF V600E Mutations: The GMMG-Birma Trial

Introduction: The treatment of patients (pts) with rrMM remains challenging and response is often limited in depth and duration. In contrast to many other malignant diseases, targeted therapy in MM is hampered by the limited number of actionable targets. Activating mutations of BRAF have been found in 2-4% of newly diagnosed MM and in up to 8% of rrMM. Several case reports have reported clinical efficacy of downstream pathway inhibition in MM. This phase II trial is evaluating the safety and efficacy of combined BRAF/MEK inhibition in rrMM pts with BRAF V600E mutation.

Aims: The primary objective of this study was to demonstrate the therapeutic efficacy of encorafenib in combination with binimetinib. The primary endpoint was the overall response rate (ORR), defined as best response within 1 year of treatment. Main secondary endpoints included progression-free survival (PFS) and duration of response. Safety analyses included all adverse events (AEs) with those of grade 1/2 only being assessed further if considered to be related to study medications by the investigator.

Methods: A total of 15 pts with rrMM who have failed at least 2 prior lines of therapy, including a proteasome inhibitor (PI) and an immunomodulatory drug (IMID), were planned to be enrolled in this open-label phase II multicenter trial. Key inclusion criteria were presence of a BRAF V600E/K mutation confirmed by both mutation-specific immunohistochemistry (IHC) and next-generation sequencing in more than 50% of MM cells. Exclusion criteria included plasma cell leukemia, CNS involvement, cardiac dysfunction, or a history of retinal vein occlusion. Pts received encorafenib 450mg p.o. daily and binimetinib 45mg p.o. twice daily. Responses were assessed using IMWG 2016 criteria. The primary endpoint was analyzed by testing the one-sided null hypothesis of ORR <= 20% using a one-sided binomial test at a significance level of 5%. Exploratory biomarker assessments include cytogenetics, genomic analysis (WGS, RNAseq) and phospho-IHC.

Results: As of July 2020, 12 pts have been enrolled and were evaluable for safety and 11 pts were evaluable for response as 1 pt had just completed cycle 1 with confirmation of response pending at the time of data lock. Subtypes of MM were IgG in 6 pts, IgA in 2 pts, and Bence Jones in 4 pts. Pts had received a median of 5 prior lines of therapy (range 2-14). All pts had failed previous treatment with both a PI and an IMID and in addition carfilzomib, pomalidomid, and/or anti-CD38 antibodies in 8/4/6 pts, respectively.

The study already reached its primary endpoint. The ORR was 82% (lower limit of the 95% CI 56.4%, one-sided exact binomial test, p<0.0001) with 9/11 pts achieving partial response (PR) or better, 6/11 pts had at least a very good partial response (VGPR, 55%) and 3 pts reached (near) complete response (nCR/CR, 27%). Responses occurred rapidly with 8/11 pts achieving at least a PR or better already after cycle 1. PFS remains immature and will be reported at the conference. Duration of response exceeding 1 year has been observed in individual pts.

Adverse events of all grades, assessed to be at least possibly related to a study drug, occurred in 9/12 pts and included blurred vision, macula edema, cramps, arthralgia, diarrhea, skin rash, and decreased left ventricular function. Grade 3 or 4 AEs irrespective of causality occurring in more than 1 pt were reported in 8 pts with anemia, hypertension and thrombocytopenia (3/ 3/ 2 pts, respectively). SAEs were reported in 2 pts, respiratory tract infection in both pts and tooth decay in 1 pt that was not related to the study medication. No deaths occurred while on treatment or within 30 days of the end of treatment.

Preliminary results of biomarker assessments reveal new RAS mutations and amplification of the BRAF locus at the time of relapse while on study treatment as potential markers of resistance. Analysis of pharmacodynamic markers by phospho-IHC reveals suppression of BRAF/MEK signaling at cycle 1 / day 28 and restoration of expression at the time of relapse.

Conclusion: Targeting activating BRAF mutations in rrMM by combining the BRAF inhibitor, encorafenib, and the MEK inhibitor, binimetinib, induces rapid and deep responses in the majority of pts. No new safety signal has been observed when compared to prior reports on these compounds. The study reached its primary endpoint with fewer pts than expected. The primary efficacy and safety analysis will be presented at the conference.