The primary objectives are to assess longitudinal changes in allele frequency of ESR1 mutation in plasma in patients treated with Fulvestrant plus palbociclib compared to tamoxifen plus palbociclib.~Patients with ER+ breast cancer who had 1 to 3 prior lines of endocrine therapy and up to one line of chemotherapy for MBC, excluding fulvestrant and tamoxifen, will be randomized in a 1:1 ratio to receive fulvestrant 500mg IM Q28 days with one extra dose on Day15 of the first cycle (as a loading dose) plus palbociclib 125mg/day PO on a 21 days on/7 days off schedule or tamoxifen 20mg PO daily plus palbociclib 125mg/day PO on a 21 days on/7 days off schedule.
To assess longitudinal changes in allele frequency of ESR1 mutation in plasma in patients treated with Fulvestrant plus palbociclib compared to tamoxifen plus palbociclib