Background: CDK4/6i, including palbociclib and ribociclib (R), have demonstrated remarkable benefit in progression free survival (PFS) in patients (pts) with HR+, HER2- MBC with anti-estrogen therapy. Switching between anti-estrogen therapies at disease progression is standard of care in the treatment of HR+ MBC. We evaluate the strategy of switching anti-estrogen therapy to fulvestrant (F) and maintaining CDK4/6 inhibition with R in pts with HR+, HER2- MBC who have progressed on an AI + CDK4/6i. Methods: Trial Design Phase II, multi-center, randomized, double-blind, placebo-controlled trial to evaluate F +/- R in pts with HR+, HER2- MBC who have previously progressed on any AI + CDK4/6i: Screened at 2 different scenarios: Scenario 1: Before receiving any CDK4/6i Scenario 2: Time of progression of disease (POD) while being treated with an AI + CDK4/6i Intervention At randomization, pts assigned 1:1 to either a) F + R or b) F + placebo, with treatment given in 4-week cycles. Major Eligibility Criteria 1, Metastatic BC, 2. HR+ HER2-, 3. Measurable or unmeasurable disease Specific Aims Primary: PFS. Secondary: Objective response rate, clinical benefit rate, overall survival, and duration of response. Biomarker assessment: amplification of cyclin D1 and cyclin E, phosphoRb and TK1 expression, Rb1 and p16 loss, and ctDNA for ESR1 and PIK3CA mutations. Statistical Methods Assuming a median PFS of 3.8 months with F alone, we predict that F + R will lead to a median PFS of at least 6.5 months. A one-sided log-rank test with a sample size of N = 120 and alpha = 0.025, achieves 80% power to detect a difference in PFS of 2.7. With a 10% dropout, n = 132. Clinical trial registry number NCT02632045.