The RAF/MEK clamp VS-6766 shows strong anti-tumor activity across multiple MAPK pathway alterations, with a preferential effect on KRAS G12V

Background: KRAS is the most frequently mutated RAS/RAF family member (mutated in 15% of cancers), followed by BRAF (5%) and NRAS (3%). Several BRAF and MEK inhibitors (MEKi) are approved for BRAF V600E cancers, and the G12C inhibitor sotorasib is approved for KRAS G12C non-small cell lung cancer (NSCLC). However, there is still a need for agents targeting other RAS/RAF mutations (mt), including KRAS G12 V which is mutated in ∼7%, ∼9% and ∼19% of NSCLC, colorectal and pancreatic cancers, respectively. VS-6766 is a unique RAF/MEK clamp that potently inhibits MEK kinase activity and induces a dominant negative RAF-MEK complex preventing phosphorylation of MEK by ARAF, BRAF and CRAF. This unique mechanism allows VS-6766 to block MEK signaling without the compensatory re-activation of MEK that appears to limit the efficacy of MEKi. Material and Methods: In vitro assays were performed to evaluate the anti-proliferative activity of VS-6766 in human tumor cell lines with MAPK pathway alterations and in RAS-less mouse embryonic fibroblasts (MEFs) stably transfected with various KRAS variants. The anti-tumor activity of VS-6766 was assessed in genetically engineered mouse models (GEMM) of KRAS G12 V/Trp53 KO and KRAS G12C/Trp53 KO NSCLC. Results: VS-6766 showed strong anti-proliferative potency across tumor cell lines carrying KRAS, BRAF, CRAF, NRAS or NF1 alterations. Among KRAS mt cell lines, the anti-proliferative potency of VS-6766 differed by KRAS variant, with greatest potency observed with G12 V > G12C > G12D, which is consistent with G12 V signaling through CRAF/MEK/ERK and G12D signaling more through PI3 K/AKT/mTOR. Similarly, in MEFs stably transfected with different KRAS variants, VS-6766 showed best potency with KRAS G12 V and G12C, and least potency with G12D and KRAS wildtype. Accordingly, in the KRAS G12 V NSCLC GEMM model, previously shown to be CRAF-dependent, VS-6766 monotherapy induced stronger tumor regression than in a corresponding G12C NSCLC GEMM model. Strikingly, the combination of VS-6766 with FAK inhibition conferred tumor regressions in 87% (27/31) and 71% (36/51) of all tumors in the KRAS G12 V and KRAS G12C NSCLC models, respectively. Clinically, objective responses to VS-6766 monotherapy occurred mainly in patients with KRAS G12 V with 4 out of the 7 responders bearing KRAS G12 V (Guo, 2020). In KRAS mt NSCLC patients treated with VS-6766 in combination with the FAK inhibitor defactinib, a G12 V preference was also observed with confirmed responses in 2/2 patients with KRAS G12 V and tumor reduction in 4/6 patients with KRAS G12C (Krebs, 2021). Conclusions: These data support the ongoing registration-directed study evaluating VS-6766 ± defactinib for patients with KRAS G12 V NSCLC (NCT04620330), as well as the clinical combinations of VS-6766 with the KRAS G12C inhibitors sotorasib (NCT05074810) or adagrasib (NCT05375994) for KRAS G12C NSCLC. No conflict of interest.