Abstract 4785: NMS-E668, a highly potent orally available RET inhibitor with selectivity towards VEGFR2 and demonstrated antitumor efficacy in multiple RET-driven cancer models

RET chromosomal rearrangements initially identified in a subset of papillary thyroid cancers, as well as gain-of-function point mutations present in ca. 70% of medullary thyroid carcinomas, are well established as oncogenic events that induce constitutive RET activation. More recently, recurring activating RET gene rearrangements have also been found to be expressed in 1-2% of lung adenocarcinomas and subsets of other solid tumors, including colorectal and salivary gland carcinomas. RET kinase is therefore a validated actionable therapeutic target in multiple tumor types, and several small-molecule inhibitors targeting RET are being explored in clinical settings. A common feature of most advanced agents is their lack of selectivity and in particular their potent cross-reactivity against VEGFR2, a receptor tyrosine kinase whose inhibition has been described to be associated with dose-limiting cardiovascular toxicity. Indeed, the high homology between the two kinases renders identification of ATP competitive compounds that selectively inhibit RET over VEGFR2 a highly challenging task. Here we describe the preclinical activity of NMS-E668, a potent and selective ATP-competitive RET inhibitor characterized by favorable activity, selectivity and ADME profiles. NMS-E668 has low nM potency on RET and excellent biochemical selectivity when tested against a kinome panel, notably including circa 30-fold selectivity over VEGFR2. Importantly, 30-fold selectivity of NMS-E668 for RET vs. VEGFR2 was confirmed at the cellular level using NIH-3T3 cells engineered to express activated forms of the two receptors. NMS-E668 potently and selectively inhibited the proliferation of RET-dependent tumor cells, including TT medullary carcinoma cells harboring a RET C634W activating point mutation and LC-2/ad lung carcinoma cells bearing the oncogenic fusion protein CCDC6-RET. NMS-E668 also potently inhibited IL3-independent growth of Ba/F3 cells expressing KIF5B-RET, the RET rearrangement most commonly found in lung adenocarcinomas. Importantly, the proliferation of circa 100 non-RET-dependent tumor cell lines was not significantly affected by NMS-E668, confirming again its selectivity. Tested in vivo against the TT xenograft tumor model NMS-E668 displayed an excellent tumor growth inhibition with complete tumor regression achieved in all animals treated at the higher dose and with confirmed ex vivo target modulation. Good activity was also observed in additional RET-dependent model