Encorafenib, cetuximab, and cytotoxic chemotherapy combinations in BRAF^V600E CRC murine models

145 Background: Based on promising results from latest trials, a crucial point is to evaluate whether encorafenib (E) plus cetuximab (C), alone or in combination with chemotherapy, can improve clinical outcomes relative to current standard of care in previously untreated BRAF V600E mutant mCRC. Considering the high number of BRAF V600E mutant mCRC patients who will never receive a second-line treatment, the rationale of this strategy is to maximize treatment outcome within the first-line setting. Methods: We performed an in vivo study using human BRAF V600E CRC cell line-derived xenografts in nude mice. We evaluated the efficacy of encorafenib (E) + cetuximab (C), FOLFOX, and FOLFIRI, both as individual regimens and in combinations. Mice were treated for 3 weeks and followed for an additional 8 weeks to evaluate durability of tumor control. Additionally, we validated our findings using 3 BRAF V600E mutated patient derived xenografts. Tumors progressing on single agent and combined treatment were profiled by RNA sequencing, protein extraction for RPPA/Western blot, and establishment of in vitro primary cell cultures for further analyses. Results: Our study showed across all 4 models both FOLFOX and FOLFIRI, each in combination with encorafenib plus cetuximab, having greater efficacy than encorafenib plus cetuximab or either chemotherapy alone. No significant change in toxicity was seen with the addition of chemotherapy. Interestingly, in the one model with long term treatment, FOLFOX + E +/- C performed greatest over the long-term, with significant endpoint separation against all other treatment arms (P < 0.05). Conclusions: Taken together, results from our study suggest that the addition of chemotherapy to BRAF+EGFR targeted therapy can further increase the magnitude of response in BRAF V600E mCRC and is a promising combination now being explored clinically. Additionally, this research will substantially contribute to our understanding of the genetic and molecular bases of resistance to target therapies and chemo-based approach in BRAF V600E context.