Molecular correlates of clinical benefit in previously treated patients (pts) with BRAF V600E-mutant metastatic colorectal cancer (mCRC) from the BEACON study.

Background: In the BEACON study, encorafenib + binimetinib + cetuximab (enco/bini/cetux; triplet) and enco + cetux (doublet) improved OS and ORR vs standard of care in previously treated BRAF V600E-mutant mCRC pts. Molecular profiling of tumours from this study was performed to identify molecular correlates of outcome. Methods : Baseline tumour samples were analysed by whole-exome sequencing (WES) and whole transcriptome sequencing (WTS). BRAF-mutant (BM) and consensus molecular subtypes (CMS) were determined and pathway activities evaluated with gene set variation analysis. Tumour response was evaluated for each subtype. Additional association and interaction analyses between molecular features and outcomes by treatments are ongoing and will be presented. Results : Baseline tumour samples were analysed by WES and/or WTS for 79.2% of pts. Of the 460 pts analysed by WTS (73.7% in the triplet arm, 66.4% in the doublet arm, 67.4% in the control arm), 84.6% were classified as CMS1 or CMS4. Overall 32.2% of pts were classified as BM1 and the majority (84.5%) of these were CMS4. Many of those classified as BM2 (67.8%) were CMS1 (64.7%). In the BM1 and CMS4 tumours, expression of inflammatory response and epithelial mesenchymal transition genes were elevated, and expression of cell cycle genes was reduced. Response rate in pts with CMS4 and/or BM1 tumors was higher in the triplet arm (CMS4: 33.3% [95% CI: 21.7-46.7]; BM1: 33.3% [95% CI: 21.4-47.1]) vs the doublet arm (CMS4: 19.2% [95% CI: 9.6-32.5]; BM1: 14.9% [95% CI: 6.2-28.3]). Conclusions : Molecular characteristics and biological properties observed in BRAF V600E-mutant mCRC suggest a subset of pts with specific molecular features may derive greater clinical benefit from triplet vs doublet therapy. These findings support further investigating the biological landscape in BRAF-mutant mCRC to enable potential hypotheses for pt selection to improve clinical outcome in future studies.