Abstract CT025: Dabrafenib plus trametinib in BRAF V600E-mutant high-grade (HGG) and low-grade glioma (LGG)

Background: More effective treatments are needed to improve outcomes in HGG and LGG. Activating BRAF mutations occur in ~3% of glioblastomas and 15% of LGGs. BRAF inhibitor dabrafenib + MEK inhibitor trametinib combination is FDA-approved in BRAF V600-positive melanoma, NSCLC, and anaplastic thyroid cancer. Methods: We conducted a nonrandomized, open-label, phase 2 basket study (NCT02034110) of dabrafenib + trametinib in pts with BRAF V600E mutation-positive rare cancers; here we report results for the HGG and LGG cohorts. Adult pts with histologically confirmed recurrent/progressive HGG (Grade III, IV) or LGG (Grade I, II) per WHO 2007 classification received oral dabrafenib, 150 mg twice daily, and oral trametinib, 2 mg once daily, until unacceptable toxicity, disease progression, or death. The primary endpoint was investigator-assessed objective response rate (ORR) using RANO criteria. Secondary endpoints included progression-free survival (PFS), duration of response (DOR), overall survival (OS), and safety; molecular characterization of baseline tumor samples was an exploratory endpoint. Results: As of Sept 14, 2020, 45 pts (23 male) were enrolled in the HGG cohort; 35 discontinued, 6 remained on treatment, 4 were in follow up. The majority had glioblastoma (69%), followed by anaplastic pleomorphic xanthoastrocytoma and anaplastic astrocytoma (each 11%); of pts with known IDH/MGMT status, 3/29 pts had IDH1 mutations and 8/17 had MGMT promoter methylation. Prior therapies included radiotherapy (98%), surgery, and chemotherapy (93% each). Median (range) follow-up was 12.7 (1.1-56.1) months (mo); ORR was 33% (3 CR, 12 PR); median DOR was 36.9 mo (95% CI, 7.4-44.2). Median PFS and OS were 3.8 mo (95% CI, 1.8-9.2) and 17.6 mo (95% CI, 9.5-45.2), respectively. The LGG cohort enrolled 13 pts (4 male); 7 discontinued, 5 remained on treatment, 1 was in follow up. Most common histologies were ganglioglioma (31%), diffuse astrocytoma, and pleomorphic xanthoastrocytoma (each 15%); of pts with known IDH/MGMT status, 1/8 pts had IDH1 mutation and 0/2 had MGMT promoter methylation. Prior therapies included surgery (92%), radiotherapy (62%), and chemotherapy (38%). Median (range) follow-up was 32.2 (0.8-71.8) mo; ORR was 69% (1 CR, 6 PR, 2 MR); median DOR, PFS, and OS were not reached. Overall, 54/58 pts (93%) experienced adverse events (AEs) across cohorts, most commonly (≥30%) fatigue (50%), headache (43%), nausea (34%), and pyrexia (33%); 31 pts (53%) had grade ≥3