Backgroud: Oncogenic RET is an actionable target across a variety of cancers. Selective RET inhibitors selpercatinib and pralsetinib were recently approved by the FDA and EMA for patients with RET-dependent NSCLC and thyroid cancers. The solvent front mutations (SFMs) RET G810C/S/R have been identified as mechanisms of acquired resistance to both drugs. The gatekeeper RET V804 mutation cannot be inhibited by selected next-generation RET inhibitors in development. APS03118 is a novel next-generation RET inhibitor which is potent against a range of RET fusions and mutations including both SFMs and gatekeeper mutations.

Methods: The selectivity, anti-RET activity, and intracranial efficacy of APS03118 were confirmed in vitro and in vivo in a variety of RET-dependent tumor models.

Results: APS03118 was highly selective against a panel of 468 kinases and demonstrated 130-fold selectivity over VEGFR-2. In enzymatic assays, APS03118 showed low nanomolar potency against wild type RET and 25 RET mutations/fusions, including the inhibition of RET G810R/C/S (IC50 0.04-5 nM) and RET V804M/L/E (IC50 0.04-1 nM). APS03118 inhibited RET phosphorylation (IC50 <15 nM) in Ba/F3 engineered RET cells (WT, G810R, V804M, M918T). In cell proliferation assays, APS03118 potently inhibited KIF5B-RET Ba/F3 (WT, V804M, V804L, M918T), CCDC6-RET Ba/F3 (WT, V804M, S904F), LC2/ad (CCDC6-RET), TT (RET C634W) (IC50 < 10nM); Ba/F3 RET G810R and G810S IC50 (8-65 nM). APS03118 demonstrated marked anti-tumor efficacy in vivo in RET-driven cell-derived (Ba/F3 KIF5B-RET, V804M, TT (C634W)) and patient-derived (KIF5B-RET, CCDC6-RET, CCDC6-RET V804M) xenograft tumor models. At 10 mg/kg BID, tumor regression was observed in these xenograft models (TGI 87-108%). Tumors completely subsided in CCDC6-RET orthotopic brain model with a 100% survival rate. In the Ba/F3 KIF5B-RET G810R xenograft model, APS03118 30 mg/kg BID showed 90% TGI and was well tolerated, and RET G810 mutations often drive clinical progression on current RET inhibitors.

Conclusions: APS03118 is a novel highly selective next-generation RET inhibitor that possesses potent in vitro and in vivo activity against a diverse range of RET alterations, including SFMs-mediated resistance. A first-in-human phase 1 trial for patients with RET-driven solid tumors with activating RET alterations is planned for 2022.

Citation Format: Alexander Drilon, Jun Zhong, Ying Lu, Yongbo Liu, Hao Wang, Minchun Chen, Xiaohu Chen, John Zhu, Shun Lu, Vivek Subbiah. The preclinical selectivity and activity of APS03118, a highly selective and potent next-generation RET inhibitor [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 5363.