1054Background: Resistance mechanisms to CDK 4/6 inhibition are not well defined. Outcome data on hormone receptor positive (HR+) metastatic breast cancer patients (MBCP) treated with palbociclib (PA) after treatment with everolimus (EV) are lacking. The PALOMA 3 trial (P3) showing benefit of PA plus fulvestrant (FU) compared to FU in HR+ MBCP after progression on endocrine therapy excluded women previously treated with EV. The aim of our study was to investigate the outcomes of HR+ MBCP with prior EV treatment on PA based therapy. Methods: This is a retrospective, single institute review of HR+, HER 2 nonamplified MBCP from Jan 2014 - Nov 2016 treated with PA after treatment with EV. Women who received EV for < 1 month or PA < 14 days were excluded. Progression free survival (PFS) was defined as the time from the initiation of PA to the date of progression as determined by treating physician based on radiological, biochemical and/or clinical criteria. Response rates were determined based on available rad...