The Novel RET Inhibitor SYHA1815 Inhibits RET-Driven Cancers and Overcomes Gatekeeper Mutations by Inducing G1 Cell-Cycle Arrest through c-Myc Downregulation

Rearranged during transfection (RET), an oncogenic driver, has been found in multiple tumor types and is thus a promising Anticancer therapeutic target. Novel selective RET inhibitors (RETi) that can overcome V804 gatekeeper mutations, endowing resistance to multikinase inhibitors (MKI) and, in particular, achieving VEGFR2/KDR/Flk-1 selectivity, are needed. In addition, the mechanisms underlying RET-inhibition-induced antiproliferative effects in the context of RET addiction are incompletely understood. This study describes a novel selective RETi, SYHA1815, which inhibited the kinase activity of RET wild type and V804 mutant with an IC₅₀ in the subnanomolar to nanomolar range. Notably, SYHA1815 exhibited approximately 20-fold selectivity for RET over VEGFR2/KDR/Flk-1, almost equivalent to that of the launched selective inhibitor pralsetinib. SYHA1815 had only a marginal inhibitory effect on cellular VEGFR2/KDR/Flk-1 signaling at a high (200 nmol/L) concentration, confirming the selectivity over VEGFR2/KDR/Flk-1. In addition, SYHA1815 exhibited a favorable selectivity profile, with greater than 100-fold selectivity for RET over 347 other kinases. It exhibited potent antitumor efficacy and overcame V804 mutations in vitro and in vivo by targeting RET. Then, using SYHA1815 as a probe, we found that RET inhibition suppressed RET-driven cell proliferation via G₁ cell-cycle arrest through downregulating c-Myc. Furthermore, disruption of c-Myc upon BRD4 Inhibitor treatment led to G₁ cell-cycle arrest and overrode RET-driven cell proliferation. Moreover, consistent with the marked in vivo efficacy of RET inhibition, the intratumoral c-Myc level was significantly decreased. In summary, SYHA1815 is a promising RETi for RET-aberrant Cancer treatment that is currently in a phase I trial.