Outcome of palbociclib based therapy in hormone receptor positive metastatic breast cancer patients after treatment with everolimus

1054 Background: 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 radiological data. Clinical benefit was defined as a complete response (CR), partial response (PR) or stable disease of at least 24 weeks. Results: 23 patients with mean age 67 years (42 to 81) were identified. 95% were postmenopausal, 81% had ECOG performance status 0 or 1, 83% had visceral metastases, 95% had > 2 lines of prior endocrine therapy (ET), 82% shown prior sensitivity to ET, 82% received prior chemotherapy, of which 84% were in metastatic setting. Kaplan Meier estimate showed median PFS of 2.9 months (95% CI 2.0-4.2); median PFS of P3 PA cohort was 9.5 months (95% CI 9.2-11.0). Fisher’s exact test comparing study cohort with P3 PA cohort showed statistically significant differences in objective response (CR or PR) rates of 0/23 (0%) vs. 66/347 (19%, p = 0.02) & clinical benefit ratio of 4/23 (17.4%) vs. 231/347 (66.5%, p = 0.00). Conclusions: Outcomes with PA in HR+ EV treated MBCP were worse when compared to the P3 PA cohort data. Treatment with EV may lead to resistance to CDK inhibition. Though limited by size, our data suggests that use of PA after EV is associated with low response & clinical benefit rates. Further studies are necessary to confirm the findings to determine sequencing of targeted therapies.