Pharmacokinetics and Pharmacogenomics of Ribociclib in Race-based Cohorts

Patients will receive ribociclib 600 mg oral (PO) daily for 21 consecutive days of 28-day cycles and endocrine therapy with either letrozole or fulvestrant. Fulvestrant 500 mg intramuscular (IM) will be administered on days 1, 15, 29 of cycle one, and then monthly. Letrozole 2.5 mg oral daily continuously. If premenopausal women have not undergone bilateral salpingo- oophorectomy, they will receive a luteinizing hormone-releasing hormone (LHRH) agonist (e.g., goserelin leuprolide) for ovarian suppression in combination with fulvestrant or letrozole and ribociclib.