Abstract 2830: The major human metabolites of abemaciclib are inhibitors of CDK4 and CDK6

Abemaciclib (LY2835219) is an ATP-competitive inhibitor of cyclin dependent kinases 4 and 6 (CDK4 and CDK6) which is currently undergoing clinical evaluation for the treatment of breast and lung cancers. A radiolabeled disposition study following a single 150-mg oral dose of [14C]LY2835219 in healthy subjects indicated that in plasma, in addition to parent drug, the presence of 5 metabolites denoted as M1, M2, M18, M20 and M22. Abemaciclib (34%), M20 (26%), M2 (13%), and M18 (5%) constituted the majority of the plasma exposure. This study investigated the in vitro biological activity of these human circulating metabolites, with the exception of the trace metabolite, M1, and compared their potencies with the parent drug abemaciclib. Specifically non-small cell lung cancer (NSCLC) cells, colorectal cancer (CRC) cells and breast cancer cell lines were evaluated for growth inhibition, cell cycle inhibition and biomarker expression following treatment with abemaciclib and the metabolites. The metabolites were also profiled and compared to abemaciclib for inhibition of CDK4, CDK6, CDK1, and CDK9 in cell-free biochemical kinase assays. The IC50 values for the inhibition of CDK4 and CDK6, for metabolites M2, M18, and M20, but not M22, were between 1 and 3 nM and were nearly equivalent in potency to abemaciclib. Likewise, metabolites M2, M20, and M18 inhibited cell growth and cell cycle progression in a concentration-dependent manner that was consistent with the inhibition of CDK4 and CDK6 since these outcomes correlated with the concentration-dependent inhibition of various biomarkers such a phospho-serine 780-Rb (pRb), topoisomerase II-alpha (Topo IIα), and phospho-serine 10-histone H3 (pHH3). In this regard, metabolites M2 and M20 showed potencies nearly identical with abemaciclib in the cancer cell lines evaluated, whereas depending on the endpoint measured, the potency of M18 was approximately 3-20-fold lower than abemaciclib. M22 showed the least potency for growth inhibition and little or no inhibition of biomarker expression or cell cycle progression at concentrations below 2 μM. Although the cell-free kinase assays showed that like abemaciclib, M2, M18, and M20 had potential to inhibit CDK9, no measurable inhibition of CDK9 by any of these compounds was observed in cancer cells, indicating that the primary targets driving cell cycle inhibition for these metabolites in cancer cells were CDK4 and CDK6 and not CDK9. Studies with abemaciclib, M2 or M20 in breast cancer cells showed that all 3 compounds induced senescence in addition to growth inhibition following 6-8 days of treatment at 200 and 500 nM. In total the results indicated that the major human metabolites of abemaciclib, M2 and M20, are effective inhibitors of CDK4 and CDK6 that are remarkably similar to abemaciclib in regards to their effects in cancer cells on growth, senescence, and other phenotypic responses. Citation Format: Teresa Burke, Raquel Torres, Ann McNulty, Jack Dempsey, Stanley Kolis, Palaniappan Kulanthaivel, Richard Beckmann. The major human metabolites of abemaciclib are inhibitors of CDK4 and CDK6. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2830.