Background: Abemaciclib is a selective cyclin-dependent kinase 4 & 6 inhibitor, approved for hormone receptor-positive (HR+), HER2- advanced breast cancer (aBC) as monotherapy or in combination with fulvestrant or nonsteroidal aromatase inhibitors. Abemaciclib is also approved in combination with endocrine therapy for the adjuvant treatment of HR+, HER2-, node-positive, high risk early BC (EBC). Despite demonstrated efficacy and overall tolerable safety profile, some pts discontinue abemaciclib without a prior dose reduction, highlighting the need for appropriate symptom management. This analysis aims to provide guidance on management of AEs for pts with aBC or EBC receiving abemaciclib. Methods: Study level data were collected for publications related to MONARCH 1, 2, and 3, and monarchE clinical trials. Endpoints included incidence of any-grade AEs, dose modifications, and discontinuations. Data on monitoring of AEs were also extracted. Results are presented descriptively. Results: Highest any-grade incidence of abemaciclib-associated AEs across trials include diarrhea (82-90%), fatigue (41-65%), nausea (30-64%) and neutropenia (37-50%). Notable AEs leading to dose reduction, omission and discontinuation were: diarrhea (14-21%; 15-24%; 1-5%), neutropenia (8-13%; 16-17%; 1-3%), and fatigue (5%; 5%; 2%) respectively. In the monarchE trial, discontinuations due to AEs were highest in the first month and stabilized after 6 months. AE monitoring and management are presented in Table. Conclusions: Appropriate AE monitoring, symptom management strategies, and patient counseling should be used to manage AEs with a goal to impact early discontinuation of abemaciclib.

^*ALT, AST and serum bilirubin.