SWOG S1221: A phase 1 dose escalation study co-targeting MAPK-dependent and MAPK-independent BRAF inhibitor resistance in BRAF mutant advanced solid tumors with dabrafenib, trametinib, and GSK2141795 (ClinicalTrials.gov NCT01902173).

Abstract

2578 Background: Aberrant PI3K/AKT signaling in BRAF mutant cancers contributes to resistance to MAPK pathway blockade. We conducted parallel phase 1 dose escalation studies of the doublet of the BRAFi dabrafenib with the AKT inhibitor GSK2141795 and of the triplet of dabrafenib, the MEKi trametinib, and GSK2141795. Methods: Patients (pts) with BRAF-V600E/K mutant advanced solid tumors with adequate end-organ function were eligible regardless of prior BRAFi and MEKi exposure. All pts received dabrafenib at 150 mg twice daily (bid), in the doublet cohorts together with dose escalation (3 + 3 scheme) of GSK2141795 started at 50 mg daily (qd), and in the triplet cohorts with dose escalation of both trametinib starting at 1.5 mg qd and GSK2141795 starting at 25 mg qd. DLTs included significant grade 3 and 4 adverse events (CTCAE v4) within the first 56 days of treatment. Radiographic responses were assessed at 8-week intervals. Results: No DLTs were observed in the doublet cohorts (N = 8) up to dabrafenib 150 mg bid and GSK2141795 75 mg qd. In the triplet cohorts (N = 11), no DLTs were observed at doses of up to trametinib 1.5 mg daily with GSK2141795 75 mg daily. At the highest triplet dose with dabrafenib 150 mg bid, trametinib 2 mg qd with GSK2141795 75 mg qd, 1 of 2 evaluable pts had a DLT of grade 3 febrile neutropenia and grade 3 maculo-papular rash. 2/2 treatment-naïve in the doublet cohorts had PRs (1 melanoma and 1 thyroid) the latter lasting over 1 year. 1/6 BRAF inhibitor-refractory (melanoma) pts also had an objective response. In the triplet cohorts, 3 of 6 treatment-naïve pts had a PR (1 melanoma, 2 lung). One lung pt remains in PR at 2 months and the otherhas an uPR at 1.2 months. Conclusions: Inhibition of both MAPK and PI3K/AKT pathways was well tolerated, leading to durable objective responses in pts with metastatic melanoma, thyroid cancer, and lung cancer. Further study of dual pathway inhibition is warranted. Funding: Supported in part by NIH/NCI grants CA180888, CA180819; and in part by Novartis Pharmaceuticals Corporation and GlaxoSmithKline, LLC. Clinical trial information: NCT01902173.