CDK 4/6 inhibitors and stereotactic radiation in the management of hormone receptor positive breast cancer brain metastases.

PURPOSE: Cyclin-dependent kinase (CDK) 4/6 inhibitors are becoming increasingly utilized in the setting of advanced, hormone receptor (HR+) positive breast cancer. Pre-clinical data suggests a potential synergy between radiation therapy (RT) and CDK4/6 inhibitors. We assessed clinical outcomes of patients treated at our institution with the use of CDK4/6 inhibitors and stereotactic radiation in the management of HR+ breast brain metastases.
METHODS: A retrospective analysis of patients who received stereotactic radiotherapy for HR+ brain metastases within 6 months of CDK4/6 inhibitor administration was performed. The primary endpoint was neurotoxicity during or after stereotactic radiation. Secondary endpoints were local brain control, distant brain control, and overall survival (OS).
RESULTS: A total of 42 lesions treated with stereotactic radiation in 15 patients were identified. Patients received either palbociclib (n = 10; 67%) or abemaciclib (n = 5; 33%). RT was delivered concurrently, before, or after CDK4/6 inhibitors in 18 (43%), 9 (21%), and 15 (36%) lesions, respectively. Median follow-up following stereotactic radiation was 9 months. Two lesions (5%) developed radionecrosis, both of which received four prior RT courses to the affected lesion prior to onset of radionecrosis and subsequently managed with steroids and bevacizumab. Six- and 12-month local control of treated lesions was 88% and 88%, while 6- and 12-month distant brain control was 61% and 39%, respectively. Median OS was 36.7 months from the date of brain metastases diagnosis.
CONCLUSIONS: Stereotactic radiation to breast brain metastases was well tolerated alongside CDK4/6 inhibitors. Compared to historical data, brain metastases control rates are similar whereas survival appears prolonged.