A Phase Ib Study Of Abemaciclib With Therapies For Metastatic Breast Cancer.

522 Background: Abemaciclib, an inhibitor of cyclin dependent kinases CDK4 and CDK6, demonstrated safety and clinical activity as a single agent for hormone receptor positive (HR+) metastatic breast cancer (MBC) irrespective of HER2 amplification (Tolaney, SABCS 2014). This study (NCT02057133) evaluates safety, pharmacokinetics (PK), and antitumor activity of abemaciclib combined with endocrine or HER2-targeted therapies for MBC. Methods: Patients (pts) in 6 cohorts received abemaciclib 150-200 mg every 12 hours (Q12H) with letrozole 2.5 mg/d (Part A), anastrozole 1 mg/d (Part B), tamoxifen 20 mg/d (Part C), exemestane 25 mg/d (Part D), exemestane 25 mg/d + everolimus 5 mg/d (Part E), or trastuzumab 6-8 mg/kg every 21 days (Part F). Pts in Parts A-E had HR+, HER2- MBC and in Part F had HER2+ MBC. Eligibility included measurable disease or nonmeasurable bone disease by RECIST v1.1, ECOG performance status ≤ 1, no prior chemotherapy for metastatic disease (Parts A-E), and ≥ 1 chemotherapy regimen for metast...