Proto-oncoprotein RET (rearranged during transfection) is a receptor tyrosine kinase and belongs to the cadherin superfamily. RET fusion proteins and gatekeeper mutations represent strong cancer drivers involved in the development of a variety of cancers. Although selective RET inhibitors selpercatinib (LOXO-292) and pralsetinib (BLU667) were recently marketed for patients with RET-dependent NSCLC and thyroid cancers, RET solvent front mutations G810R/S/C/V and other secondary mutations such as Y806C/N and V728A have been identified as mechanisms counting for acquired resistance to the two drugs and limits the applications of the kinase inhibitors. To provide novel RET inhibitors effective to a broader spectrum of RET fusions and mutations, we identified TY-1091 as a novel next-generation RET inhibitor through the in-house RET program and systemic screening of RET compound candidates. TY-1091 was characterized for its anti-tumor activity through in vitro and in vivo testing of a variety of RET-dependent tumor models including a panel of 17 engineered RET mutant Ba/F3 cell lines and 2 cancer cell models. The results show that TY-1091 inhibits wild type and major RET mutants (IC50, nM): RET G810S (9.5 nM), RET V804M/L/E (2.8-12.6 nM), and double mutants RET V804M/G810S (23.5 nM) and M918T/G810S (47.0 nM) through a biochemical screening against a panel of 17 engineered RET mutant Ba/F3 cell lines. Importantly, the inhibitory activity of TY-1091 is much higher than that of first-generation RET inhibitor Cabozantinib, and comparable to other second-generation compounds LOXO-292 and BLU-667. Consistent with its mode of action, TY-1091 grants extraordinary inhibition effects to tumor cell proliferation compared to its peer compounds LOXO-292 and BLU-667 (including TT (thyroid cancer, RET C634W) and LC2/ad (NSCLC, CCDC6-RET)), and the inhibition of the RET pathway activation, i.e., inhibition of RET phosphorylation (IC50 < 1 nM), SHC phosphorylation (IC50 = 4.6 nM) and ERK phosphorylation (IC50 = 9.8 nM) in Ba/F3-KIF5B-RET cells further validated the above in vitro phenotype. The therapeutic potential of TY-1091 was then further validated through mouse pharmacology in that TY-1091 demonstrated remarkable anti-tumor efficacy in a variety of xenograft models including Ba/F3 KIF5B-RET (wt), Ba/F3 KIF5B-RET (V804L), TT (thyroid cancer, RET C634W), and LC2/ad (NSCLC, CCDC6-RET). Detailed data will be presented. In summary, TY-1091 is a highly potent, orally available, and safe small molecule inhibitor to pan-RET mutations in cancer, and may attenuate SFMs-mediated resistance to existing RET therapy. The IND clearance from the US FDA was received and Phase I clinical investigations of TY-1091 shall be launched in the US soon. *To Whom Correspondence should be addressed to: Jun Li, Chengshan Niu and Yusheng Wu

Citation Format: Chengshan Niu, Maolin Zheng, Huan Wang, Kaige Ji, Meihua Li, Guohui Wang, Rongzhen Ni, Apeng Liang, Aishen Gong, Yazhen Zhang, Hui Su, Mingyu Jiang, Shaoqing Chen, Xiugui Chen, Jun Li, Yusheng Wu. TY-1091, a highly selective and potent second-generation RET inhibitor, demonstrates superior antitumor activity in multiple RET-mutant models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3419.