Abstract LB521: Antitumor activity of potent RAF inhibitors in solid tumors with activated RAS-RAF axis

Abstract FDA approved three RAF inhibitors for the treatment of tumors containing BRAFV600 mutations, but one of the major drawbacks of these type I RAF inhibitors is to activate MAPK signaling pathway, instead of inhibiting signaling, which is referred to as paradoxical activation. Such undesired paradoxical activation not only leads to renewed tumor growth but also spurs additional cancer growth in non-cancerous wild-type BRAF tissue. Plus, these first-generation RAF inhibitors targeting BRAFV600 mutants are unable to inhibit oncogenic RAF dimers. This has led to the development of type II RAF inhibitors such as belvarafenib and day101 to block the activity of multiple forms of RAF while avoiding paradoxical activation. Two lead-like stage compounds 1 and 2 were specifically designed as type II RAF inhibitors to have activity across RAF isoforms including BRAFV600E, BRAF wild-type, and CRAF, showing higher potency than competitors. Compounds 1 and 2 potently inhibited the growth of BRAFV600E melanoma cells and NRAS or KRAS mutant cancer cells. Compounds 1 and 2 promoted the formation of BRAF/CRAF heterodimers by directly binding to the RAF kinase domain like other type II RAF inhibitors and inhibited phosphorylation of downstream effectors MEK and ERK in a dose-dependent manner in RAS mutant cancer cells, suggesting less paradoxical activation liability. Compound 2 showed superior on-target inhibitory activity for BRAFV600E, BRAF wild-type, and CRAF than belvarafenib through RAF immunoprecipitation (IP) kinase assay. Inhibition of the RAF downstream signaling was also confirmed by quantifying the level of phospho-ERK in NRAS or KRAS mutant cancer cells. In HCT116 (KRASG13D) subcutaneous xenograft model, compound 1 showed tumor growth inhibition efficacy, suggesting a potential to address RAS mutant driven- as well as BRAFV600 mutant driven tumors. Also, combination treatment with MEK inhibitor and/or immune checkpoint inhibitor would further improve the therapeutic activity and expand target indication for unmet medical needs. Citation Format: Kwangwoo Hwang, SeoHyun Jo, Jieun Choi, Ga-young Choi, Jiseon Choi, Ji-Hye Kwon, Dong-Guk Shin, Jiyeon Kim, Se-Hyuk Kim, Haelee Kim, Ha Yeon Cho, Jung Beom Son, Nam Doo Kim, Hwan Geun Choi, Daekwon Kim, Sunghwan Kim. Antitumor activity of potent RAF inhibitors in solid tumors with activated RAS-RAF axis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr LB521.