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

Abstract

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 CCDC6-RET V804M mutation, LOX-18228 exhibited dose-dependent tumor inhibition with 100% tumor growth inhibition observed at 60 mg/kg. These data suggest that LOX-18228, as well as closely related compounds, represent promising next-generation RET inhibitor candidates that could be used to further extend durable disease control for pts with RET-altered cancers following the development of acquired resistance to current agents. An IND is planned for 2021. Citation Format: Gabrielle R. Kolakowski, Erin D. Anderson, Joshua A. Ballard, Barbara J. Brandhuber, Kevin R. Condroski, Eliana B. Gomez, Thomas C. Irvin, Manoj Kumar, Nisha A. Patel, Faith D. Watson, Steven W. Andrews. Pre-clinical characterization of potent and selective next-generation RET inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1464.

Precision oncology for RET-related tumors

“…SYHA1815 has an approximately 20-fold selectivity for RET over VEGFR2 and is being studied in a phase I trial in China ( 83 ). Other potential drug compounds, such as LOX-18228, LOX-19260, BOS172738 (DS-5010), and SL-1001 ( 84 – 86 ), are still in the preclinical stage. There are also research efforts to obtain mutant-selective inhibitors that may offer clinical advantages.…”

Personalized Systemic Therapies in Hereditary Cancer Syndromes

“…SYHA1815 has an approximately 20-fold selectivity for RET over VEGFR2 and is being studied in a phase I trial in China [ 199 ]. Some other selective RET inhibitors, such as BOS172738 (NCT03780517), LOX-18228 [ 200 ], LOX-19260 (NCT05241834) and SL-1001, are still in the preclinical stage.…”

RET signaling pathway and RET inhibitors in human cancer

“…HM06, another investigational compound, selectively inhibits RET across multiple tumor types in vitro and in vivo , circumvents RET-V804X gatekeeper mutation and RET-G810X resistance mutations ( 284 , 285 ), and is currently in Phase I/II clinical trials in patients with RET-altered tumors (NCT04683250). Lastly, LOX-18228 is a novel compound that potently inhibits RET, RET fusions, and can even inhibit RET-V804X and RET-G810X mutations singly or in tandem ( 286 ), suggesting that LOX-18228 may be effective in targeting tumors with resistance to first-generation RET inhibitors. LOX-18228 is expected to enter Phase I clinical trials in 2022 ( 286 ).…”

“…Lastly, LOX-18228 is a novel compound that potently inhibits RET, RET fusions, and can even inhibit RET-V804X and RET-G810X mutations singly or in tandem ( 286 ), suggesting that LOX-18228 may be effective in targeting tumors with resistance to first-generation RET inhibitors. LOX-18228 is expected to enter Phase I clinical trials in 2022 ( 286 ).…”