A phase II study of MEK162 (binimetinib [BINI]) in combination with imatinib in patients with untreated advanced gastrointestinal stromal tumor (GIST).

Abstract

11508 Background: ETV1 and KIT are lineage-specific master transcriptional and signaling survival factors in GIST. In preclinical models, dual lineage targeting of ETV1 by MEK inhibition with BINI and KIT by imatinib are synergistic in suppressing GIST tumorigenesis and progression. This single-arm phase II study is designed to test the efficacy of the BINI+imatinib as a first-line treatment in patients (pts) with advanced GIST. Methods: Adult pts with untreated advanced GIST received imatinib (400mg daily) plus BINI (30mg twice daily), 28-day cycles. The primary endpoint (EP) was RECIST1.1 objective response rate (ORR) (complete response [CR]+partial response [PR]). The study was designed to detect a 20% improvement in the ORR of imatinib alone (unacceptable rate of 45%; acceptable rate of 65%). A sample size of 44 patients was required, using an exact binomial test, one-sided type I error of 0.08 and type II error of 0.1. Confirmed PR in > 24 pts would be considered positive. Secondary EPs included RR by Choi and EORTC criteria, resectability conversion rate (RCR), progression free survival (PFS), overall survival (OS) and long-term AEs. Correlatives included characterization of tumor genomics by MSK-IMPACT, cfDNA by MSK-ACCESS, ETV1 protein levels and transcriptomes and signaling inhibition. Results: At data cutoff of Jan 31, 2020, 38/39 pts with advanced GIST of all genotypes, including 3 KIT/PDGFRA-wild type GIST pts, were evaluable for primary EP. Median age 60 (range 29-78), 29% female. 26/38 pts with confirmed PR; Best ORR was 68.4% (two-sided 95% CI, 51-83%; one-sided 90% CI, 57-100%). 8/9 pts became resectable after treatment; RCR was 88.9% (95% CI, 52-100%). 13 pts remain on trial (2-159 weeks [wks]). 9 pts discontinued trial due to disease progression (11-159 wks); one pt progressed within 3 months, indicating primary resistance. Grade 3/4 toxicity included CPK elevation (asymptomatic, 61%), neutrophil decrease (11%), maculopapular rash (8%), anemia (8%). No unexpected toxicities observed. Correlation of outcome with MSK-IMPACT, MSK-Access and paired tumor biopsies will be presented. Conclusions: This study met its primary endpoint. BINI plus imatinib is highly effective in treatment-naive advanced GIST, with expected and manageable long-term treatment-associated toxicities. The combination strategy warrants further evaluation in direct comparison with imatinib in the frontline treatment of GIST. Clinical trial information: NCT01991379 .