Abstract P5-19-13: Clinical activity of abemaciclib, an oral cell cycle inhibitor, in metastatic breast cancer
Authors: Tolaney Sara M , Rosen Lee S , Beeram Muralidhar , Goldman Jonathan W , Gandhi Leena , Tolcher Anthony W , Papadopoulos Kyriakos P , Rasco Drew W , Myrand Scott P , Kulanthaivel Palaniappan , Andrews Joan M , Frenzel Martin , Cronier Damien M , Chan Edward M , Flaherty Keith T , Wen Patrick Y , Shapiro Geoffrey I , Patnaik Amita 
Published: May 2015

Background: Abemaciclib, a small molecule inhibitor with selectivity against cyclin-dependent kinases 4 and 6 (CDK4/6), induces G1 arrest in Rb-proficient human breast cancers. In an early phase clinical trial, the safety and antitumor activity of abemaciclib (LY2835219) were evaluated in 2 cohorts of patients with metastatic breast cancer (mBC). One cohort evaluated single-agent abemaciclib in an unselected population of patients with mBC [Part D], while the combination of abemaciclib plus fulvestrant was evaluated in patients with hormone receptor positive (HR+) mBC [Part G]. We previously reported early results for these 2 cohorts of patients with mBC treated with either single-agent abemaciclib or the combination of abemaciclib plus fulvestrant (Patnaik et al, ASCO 2014). In the single-agent cohort, 47 patients with previously treated mBC were enrolled (36 HR+). All patients with &gt;30% tumor reduction had HR+ mBC (13 of 36 patients). In this group of 13 patients with HR+ mBC, 9 patients had confirmed response for an objective response rate of 25%, and 4 patients had unconfirmed response. This study was ongoing with 14 of 36 HR+ mBC patients on treatment at time of analysis (range 238-471 days). Patients continuing on single-agent abemaciclib included 4 patients with unconfirmed response and 6 patients with confirmed response. For the combination of abemaciclib plus fulvestrant, 18 patients with HR+ mBC enrolled and 13 patients (72%) were still on treatment (range 31-143 days) at the time of analysis.

Methods: In the single-agent cohort, patients with mBC were treated with abemaciclib at 150 or 200mg orally every 12 hours on a continuous schedule. In the combination cohort, patients with HR+ mBC (n=18) were treated with the combination of abemaciclib plus fulvestrant. Patients received abemaciclib at 200mg orally every 12 hours on a continuous schedule. Patients also received fulvestrant at 500mg intramuscularly every month. NCI CTCAE v4.0 was used to grade adverse events (AEs) and RECIST v1.1 was used to assess tumor response.

Results: In the single-agent cohort, patients began enrolling in May 2012 with the last patient enrolled in March 2013. Patients had a median of 7 prior systemic therapies and 81% of patients had ≥2 metastatic sites. In the combination cohort, patients began enrolling in September 2013 with the last patient enrolled in January 2014. Patients in the combination cohort had a median of 4 prior systemic therapies and 67% of patients had ≥2 metastatic sites.

An updated analysis will be presented for objective response rate, duration of treatment and clinical benefit rate and will include an additional 6 months of information for both the single-agent and combination cohorts. New analyses will include time to response, duration of response, change in tumor size over time, and characteristics of responders. In addition, safety data will include longer term follow-up through approximately September 2014.

Conclusions: Abemaciclib is an oral cell cycle inhibitor that demonstrates single-agent activity against mBC, especially for HR+ disease. Based on its safety and efficacy profile, abemaciclib warrants further clinical investigation in confirmatory studies, both as a single agent and in combination with endocrine therapy.

Citation Format: Sara M Tolaney, Lee S Rosen, Muralidhar Beeram, Jonathan W Goldman, Leena Gandhi, Anthony W Tolcher, Kyriakos P Papadopoulos, Drew W Rasco, Scott P Myrand, Palaniappan Kulanthaivel, Joan M Andrews, Martin Frenzel, Damien M Cronier, Edward M Chan, Keith T Flaherty, Patrick Y Wen, Geoffrey I Shapiro, Amita Patnaik. Clinical activity of abemaciclib, an oral cell cycle inhibitor, in metastatic breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-19-13.