Efficacy of abemaciclib (abema) after palbociclib (palbo) in patients (pts) with metastatic breast cancer (MBC).
e12521 Background: CDK 4/6 inhibitors (Abema and Palbo) have shown efficacy in patients with HR+/HER2- MBC. Abema is distinct from other CDK 4/6 inhibitors and was approved after the approval of Palbo. There is limited data on sequential use or cross resistance. The aim of this study was to analyze the response to Abema based therapy in MBC pts with prior exposure to Palbo. Methods: We queried our EMR database for pts who received Abema from 9/2017-9/2018 after having received prior Palbo. Under IRB approved protocol we retrospectively collected demographics, tumor characteristics, disease status, toxicities and survival. Results: 23 female pts were identified. Four patients discontinued Palbo due to toxicities: two of them remained on Abema for 8 months and the other 2 discontinued Abema due to toxicities. The remaining 19 pts (median age 57 years, range 39-76) received a mean of 5.6 prior therapies (range (1-11) including Palbo in combination with endocrine therapy (fulvestrant 9 in pts and Aromatase inhibitors (AI) in 10 pts) at the time of Abema based therapy. 73.6% had visceral involvement including brain metastasis in 26%. The median progression-free survival (PFS) of Palbo in combination with endocrine therapy was 8 months (range 2.3-14.3). Abema was given in combination with endocrine therapy in 15 pts (9 pts fulvestrant and 6 pts AI) and 4 pts received it as a single agent. The median PFS of Abema based therapy was 7.0 months (range 1.8-12.1). Four pts (21%) had a longer PFS on Abema compared to prior Palbo PFS (table). Abema was discontinued due to toxicities in 15.7%. No partial or complete response was observed but 33% had stable disease. Conclusions: Abema showed response in a significant number of pts previously exposed to Palbo and heavily treated with multiple lines of chemotherapy for widespread metastatic disease. Prospective studies are warranted to better assess the response to Abema after exposure to CDK4/6 inhibitors. [Table: see text]