Background: Abemaciclib is a selective inhibitor of CDK4 and CDK6 kinase activity. It is approved for patients with hormone receptor (HR) positive, human epidermal growth factor receptor 2 (HER2) negative, advanced or metastatic breast cancer (MBC) previously treated: in combination with fulvestrant for patients with disease progression following endocrine therapy (MONARCH 2) and as monotherapy for patients with disease progression after endocrine therapy and chemotherapy for MBC (MONARCH 1). The patients in these trials were CDK 4/6 inhibitor-naïve. It has not yet been studied in patients who previously received a CDK 4/6 inhibitor. Methods: We performed a chart review of patients with HR positive, HER2-negative MBC treated at Rush University Medical Center who progressed on palbociclib, either with an aromatase inhibitor (AI) or fulvestrant, and were subsequently treated with abemaciclib with or without fulvestrant. We documented patient demographics, prior treatment, and response to abemaciclib therapy. Results: 21 female patients, mean age 57.8 (+/- 13.2y), were included. Patients had received 1-5 prior lines of endocrine therapy and 0 – 4 prior lines of chemotherapy for MBC. All patients received prior palbociclib: 14 patients with an AI, 6 patients with fulvestrant, and 1 patient received palbociclib with an AI and then with fulvestrant. Of the 21 patients, 17 were treated with abemaciclib monotherapy and 4 received abemaciclib with fulvestrant. SD was seen in 19% of patients (4/21) and 62% had PD (13/21). The CBR was 29% (6/21) and all of these patients received abemaciclib monotherapy. Due to expected toxicities of the drug (diarrhea, neutropenia, and thrombocytopenia), 19% (4/21) of patients discontinued treatment. 4 patients continued abemaciclib monotherapy for greater than 8.3M. 3 patients were on treatment for less than 35 days; 2 stopped due to expected toxicities and one had progression of disease on physical exam. Median treatment duration was 3.1M. Conclusions: This retrospective chart review of 21 patients demonstrates that abemaciclib has limited activity as a single agent in patients previously treated with palbociclib.