Abstract 1464: Pre-clinical characterization of potent and selective next-generation RET inhibitors

In May 2020, selpercatinib became the first FDA-approved selective RET inhibitor, indicated for patients (pts) with RET fusion-positive NSCLC and thyroid cancer as well as RET-mutant medullary thyroid cancer. Despite the durable activity of selpercatinib, pts can eventually develop acquired resistance. Previous studies in pts treated with selpercatinib or pralsetinib have reported the shared emergence of recurrent RET G810 mutations at the solvent front of the ATP pocket, which lead to a steric clash and loss of binding potency for both drugs. In some pts, these RET G810 solvent front mutations have been observed to co-occur with RET V804 gatekeeper mutations, demonstrating the importance of identifying agents that can maintain potency against both solvent front and gatekeeper resistance mutations. While multi-kinase inhibitors with potency against RET G810 mutations have been reported, these agents lack activity against V804 mutations and also carry off-target toxicities that further limit efficacy. We have identified a series of potent and selective next-generation RET inhibitors to address the emerging unmet need of pts who relapse on selective RET inhibitors. To assess potency, we evaluated the molecules in HEK293 cell lines engineered to express an M918T RET mutation or a KIF5B-RET fusion, as well as G810S or V804M resistance mutations. One compound, LOX-18228, exhibited nanomolar potency across the M918T RET, KIF5B-RET wild type, KIF5B-RET G810S, KIF5B-RET V804M, and KIF5B-RET V804M/G810S cell lines, with cellular IC50 values of 1.2, 0.9, 5.8, 31 and 51 nM, respectively. LOX-18228 also demonstrated high selectivity for RET when compared to a broad enzyme panel of off-target kinases, followed by relevant cellular assay screening. LOX-18228 also showed robust selectivity against a diverse panel of receptors, transporters and enzymes at a concentration of 10 μM, and exhibited a hERG IC50 of >30 μM. LOX-18228 demonstrated in vitro ADME properties predictive of good in vivo exposure with low predicted intrinsic clearance (range, 2.1-6.6 µL/min/million cells) in human, mouse, rat, and dog hepatocytes, and a measured high bin permeability of Papp = 15 (10-6 cm/s) in an intestinal MDCKII permeability assay. As predicted, LOX-18228 demonstrated high oral exposures in mouse and rat. In a pt-derived xenograft (PDX) model harboring a CCDC6-RET G810S mutation, LOX-18228 demonstrated complete regression at doses ≥30 mg/kg. Similarly, in a PDX model harboring a CCD