Abstract 1733: XL495 is a potent, selective, and orally bioavailable inhibitor of PKMYT1

Background: Protein kinase, membrane-associated tyrosine/threonine-1 (PKMYT1) phosphorylates cyclin-dependent kinase 1 (CDK1) at threonine-14 (Thr14), inhibiting the catalytic activity of the CDK1/cyclin B kinase complex, which blocks cells in G2 from entering M phase. Disruption of this PKMYT1-mediated G2/M checkpoint in cancer cells allows mitotic events to occur while DNA synthesis is still ongoing, resulting in catastrophic levels of DNA damage and cell death. We report preclinical data for XL495, a potent and selective orally bioavailable small molecule inhibitor of PKMYT1. Methods: XL495 selectivity was assessed using a KinomeProfiler™ assay. Viability across hundreds of cancer lines was assessed using the PRISM screen. Viability of cell lines treated with a combination dose response matrix was measured using CellTiterGlo®. Antitumor activity of XL495 orally administered as a single agent was evaluated in a colorectal cancer and a breast cancer cell line-derived xenograft (CDX) model. Antitumor activity of XL495 in combination with irinotecan or gemcitabine was evaluated in a colorectal cancer CDX model. Phosphorylation of CDK1 at Thr14 and phosphorylation of KAP1 at Ser824 from tumors was measured by AlphaLISA™. Drug-like properties were assessed and pharmacokinetic (PK) parameters were determined following administration of XL495 in rat, dog, and monkey. Results: XL495 is a potent, selective inhibitor of PKMYT1 in biochemical and cellular assays. Cancer cell lines with CCNE1 amplification were more sensitive to XL495 than other models. XL495 demonstrated dose-dependent antitumor activity, inhibition of phosphorylation of CDK1 at Thr14, and induction of phosphorylation of KAP1 at Ser824 in CDX tumors. When XL495 was dosed in combination with gemcitabine or irinotecan in the CDX model, tumor regression was achieved. XL495 showed favorable drug-like properties including high solubility and moderate permeability in Caco-2 cells. XL495 exhibited low clearance (CL) in rat and monkey and moderate CL in dog. Bioavailability was high in rat and moderate in dog and monkey. Preliminary PK assessments predict low CL and a long half-life for XL495 in humans. Conclusions: XL495 is potentially a best-in-class PKMYT1 inhibitor. PK assessments in conjunction with preclinical antitumor activity suggest that once-daily oral administration of XL495 may achieve clinical activity alone or in combination with DNA-damaging agents. XL495 is currently under investigation in a phase 1 first-in-human study in patients with advanced solid tumors (NCT06630247). Citation Format: Dana Gwinn, Jimmy Lai, Alice Rico, Christopher Kochansky, Nina M. Muñoz, Ragadeepthi Tunduguru, Aditya Kumar, Pragalath Sundararajan, Akhil Choppa, Donna Fong, Mi Chen, Amy Hildebrand, Jeremy Murray, Christopher Szeto, Matthew G. Rees, Melissa M. Ronan, Jennifer A. Roth, Wei Xu, Paul Gillespie, Paige Mahaney, Dana T. Aftab, Lisa D. Belmont, Jae H. Chang. XL495 is a potent, selective, and orally bioavailable inhibitor of PKMYT1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1733.