The association of early toxicity and outcomes for patients treated with abemaciclib.

1053Background: Abemaciclib is a CDK4 & 6 inhibitor dosed on a continuous schedule and has demonstrated efficacy with an acceptable safety profile in patients (pts) with hormone receptor-positive, human epidermal growth factor receptor 2-negative advanced breast cancer as monotherapy and in combination with endocrine therapy in the MONARCH 1, 2, and 3 trials. Early onset, low grade diarrhea was the most common toxicity and was typically manageable and reversible with antidiarrheal medication and/or dose reduction. Neutropenia was the most frequent grade 3/4 toxicity in the abemaciclib arms, typically occurring in the first 2 cycles. Here we provide an assessment of the association between these early toxicities, dose adjustment, and progression-free survival (PFS). Methods: Enrollment criteria, study designs, and endpoints were previously reported (Dickler et al. 2017; Sledge et al. 2017; Goetz et al. 2017). To examine the impact of dose reductions on efficacy, a time-dependent covariate analysis of dose ...