MONARCH 1, A Phase II Study of Abemaciclib, a CDK4 and CDK6 Inhibitor, as a Single Agent, in Patients with Refractory HR⁺/HER2^- Metastatic Breast Cancer.

Abstract

<b>Purpose:</b> The phase II MONARCH 1 study was designed to evaluate the single-agent activity and adverse event (AE) profile of abemaciclib, a selective inhibitor of CDK4 and CDK6, in women with refractory hormone receptor-positive (HR⁺), HER2^- metastatic breast cancer (MBC).<b>Experimental Design:</b> MONARCH 1 was a phase II single-arm open-label study. Women with HR⁺/HER2^- MBC who had progressed on or after prior endocrine therapy and had 1 or 2 chemotherapy regimens in the metastatic setting were eligible. Abemaciclib 200 mg was administered orally on a continuous schedule every 12 hours until disease progression or unacceptable toxicity. The primary objective of MONARCH 1 was investigator-assessed objective response rate (ORR). Other endpoints included clinical benefit rate, progression-free survival (PFS), and overall survival (OS).<b>Results:</b> Patients (<i>n</i> = 132) had a median of 3 (range, 1-8) lines of prior systemic therapy in the metastatic setting, 90.2% had visceral disease, and 50.8% had ≥3 metastatic sites. At the 12-month final analysis, the primary objective of confirmed objective response rate was 19.7% (95% CI, 13.3-27.5; 15% not excluded); clinical benefit rate (CR+PR+SD≥6 months) was 42.4%, median progression-free survival was 6.0 months, and median overall survival was 17.7 months. The most common treatment-emergent AEs of any grade were diarrhea, fatigue, and nausea; discontinuations due to AEs were infrequent (7.6%).<b>Conclusions:</b> In this poor-prognosis, heavily pretreated population with refractory HR⁺/HER2^- metastatic breast cancer, continuous dosing of single-agent abemaciciclib was well tolerated and exhibited promising clinical activity. <i>Clin Cancer Res; 23(17); 5218-24. ©2017 AACR</i>.