Phase Ib/II Trial of Abemaciclib and Elacestrant in Patients With Brain Metastasis Due to HR+/Her2- Breast Cancer

This is a multi-institutional, Phase Ib/II study in patients with HR+/Her2- breast cancer metastatic to the brain. Patients may have received up to two prior lines of systemic chemotherapy for locally advanced or metastatic disease. There will be no limit on prior use of endocrine therapy including aromatase inhibitors, tamoxifen and fulvestrant, given a documented clinical benefit of elacestrant in this setting. Single agent abemaciclib at a dose of 150 mg twice a day and elacestrant at a dose of 400mg / day will be administered to all patients, with allowances for up to 2 dose reductions due to treatment related toxicities. Patients will be evaluated for treatment emergent adverse events (AEs) during study participation, and toxicity will be assessed according to the National Cancer Institute (NCI) Common Terminology Criteria for Adverse Events, Version 5.0. After accrual of first 10 patients, the accrual will be halted and safety analysis (phase IB) will be performed. If response to the combination therapy will be documented in 2 or less patients accrual will be stopped. If at least 3 patients will have a documented response to therapy, accrual will continue to complete Stage 2