Abstract PD4-02: Advantageous polypharmacology of abemaciclib revealed by omics profiling of CDK4/6 inhibitors

Abstract Small molecule inhibitors of cyclin-dependent kinases (CDK) 4/6 have created new opportunities for the treatment of advanced hormone-receptor positive (HR+) breast cancer and show promise in other malignancies. Three CDK4/6 inhibitors, palbociclib (PD0332991; Ibrance), ribociclib (LEE011; Kisqali), and abemaciclib (LY2835219), which have been approved or are in late stage trials, are reported to be broadly similar although recent data suggest that abemaciclib has distinct single-agent activity in patients and a unique adverse effects profile. Differences in pharmacokinetics and relative potency for CDK4 versus CDK6 are postulated to account for these differences. In this paper, we use molecular and functional profiling by mRNA sequencing, mass spectrometry-based proteomics, and GR-based dose-response assays to obtain complementary views of the mechanisms of action of CDK4/6 inhibitors. We show that abemaciclib, but not ribociclib or palbociclib, is a potent inhibitor of kinases other than CDK4/6, including CDK1/Cyclin B, which appears to cause arrest in the G2 phase of the cell cycle, and CDK2/Cyclin E/A, which is implicated in resistance to palbociclib. Whereas ribociclib and palbociclib induce cytostasis, and cells adapt to these drugs within 2-3 days of exposure, abemaciclib induces cell death and durably blocks cell proliferation. Abemaciclib is active even in retinoblastoma protein (pRb)-deficient cells in which CDK4/6 inhibition by palbociclib or ribociclib is completely ineffective. The degree of polypharmacology of small molecule drugs is increasingly viewed as an important consideration in their design, with implications for efficacy, toxicity, and acquired resistance. In the case of CDK4/6 inhibitors, we propose that abemaciclib polypharmacology elicits unique molecular responses that are likely to be therapeutically advantageous. More generally, we propose that multi-omic approaches are required to fully elucidate the spectrum of targets relevant to drug action in tumor cells. We expect such understanding to assist in stratifying patient populations and ordering sequential therapies when resistance arises. Citation Format: Hafner M, Mills CE, Subramanian K, Chen C, Boswell SA, Everley RA, Juric D, Sorger PK. Advantageous polypharmacology of abemaciclib revealed by omics profiling of CDK4/6 inhibitors [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD4-02.