Abstract 4414: Inhibition of CDK2 overcomes primary and acquired resistance to CDK4/6 inhibitors

Abstract The cyclin dependent kinase (CDK) family of proteins is associated with cell cycle progression and transcriptional regulation. Abnormalities in the cell cycle, including modifications to the function of CDKs, their regulators (cyclins), or their natural inhibitors, are frequently associated with the formation and growth of tumors. While recent advances in treatments using CDK inhibition have focused on targeting CDK4/6, with regulatory approvals of palbociclib, ribociclib, and abemaciclib, these compounds only target tumors sensitive to CDK4/6 inhibitors. We are focused on developing a novel, potent, and selective inhibitor of CDK2 to treat patients whose tumors are insensitive to CDK4/6 inhibition, either by primary resistance or acquired resistance by prior treatment with a CDK4/6 inhibitor. CDK2, a serine-threonine kinase, is a member of the CDK family that binds two regulatory cyclins, E and A. CDK2 when complexed to cyclin E is involved in the G1 to S-phase transition. However, when CDK2 is complexed with cyclin A, the cell will progress through the S to M-phase. Overexpression of cyclin E has been described in a subset of triple negative breast cancer (TNBC), tumors with acquired resistance to CDK4/6 inhibition such as ER+ Her2- breast cancer, as well as ovarian, lung, and other tumor types. We have utilized medicinal chemistry and structure activity relationship (SAR) modeling, starting from our proprietary scaffold, to generate a series of small molecule CDK2 inhibitors with drug-like properties. These compounds were initially screened for potency and selectivity using an array of biochemical and in-vitro assays. We identified multiple small molecule inhibitors with sub-nanomolar biochemical IC50s for CDK2 when complexed with either cyclin E or cyclin A and evaluated their activity in normal cell lines, a breast cancer cell line resistant to CDK4/6 inhibition, TNBC, and other tumor cell lines. We examined cell cycle analysis, EdU incorporation, cell proliferation, caspase activation, and western blot analysis of genes associated with the downstream targets of CDK2. These CDK2 inhibitors potently arrest cells in the G2/M-phase and inhibit proliferation in a manner dependent on time and dose, with a corresponding decrease in phosphorylated Rb. Our lead compound was also evaluated in mouse xenograft studies for efficacy, investigating tumor growth inhibition in a model with cyclin E overexpression and a model with an acquired resistance to CDK4/6 inhibition. These potent CDK2 inhibitors demonstrate a potentially promising method of treating tumors with primary or acquired resistance to CDK4/6 inhibitors. Citation Format: Claire R. Hall, John E. Bisi, Jay C. Strum. Inhibition of CDK2 overcomes primary and acquired resistance to CDK4/6 inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4414.