Estrogen receptor-positive (HR+) breast cancer is the most common form of breast cancer that is accountable for the majority of breast cancer mortality. Currently, the cyclin dependent kinase inhibitor palbociclib in combination with endocrine therapy represents the new standard first and second line therapy for patients with metastatic HR+ breast cancer. While palbociclib has been shown to significantly improved progression free survival in combination with aromatase inhibitor (AI) and fulvestrant, resistance will inevitably occur in patients with metastatic breast cancer. However, the mechanisms of resistance to the combination of palbociclib and AI remain largely unknown. It is also unclear whether resistance mechanisms would be the same if palbociclib was given as first line treatment in combination with letrozole vs. if palbociclib was given as second line treatment after letrozole resistance had already occurred. To address this, previously well established and well characterizied letrozole-sensitive MCF7Ca cells and letrozole-resistant LTLTCa cells were subjected to continuous, long term treatment with increasing doses of letrozole and palbociclib until resistance to both drugs was achieved (MCF7Calet+palb and LTLTCapalb, respectively). Preliminary MTT cell viability assays indicate palbociclib IC50s of 750 nM in MCF7Ca and LTLTCa cells vs. 21 uM in palbociclib and letrozole-resistant cells. First line and second line palbociclib resistance correlated with changes in morphology, protein expression, and cancer stem cell characteristics. Under phase contrast microscopy, first line and second line palbociclib-resistant MCF7Ca and LTLTCa cells were larger in size, more irregular in shape, and tended not to grow in epithelial cell-like groups compared to palbociclib-sensitive MCF7Ca and letrozole cells, with second-line palbociclib resistant cells exhibiting these characteristics the most. Western blot showed that ER protein expression in ER+/HER2- MCF7Ca cells decreased with first line palbociclib and letrozole resistance, and that both ER and HER2 protein expression were decreased with second line palbociclib resistance in ERlow/HER2+ LTLTCa cells. Lastly, mammosphere assays demonstrated increasing percentage of cancer stem cells with letrozole resistance alone (4 per 1000 cells plated MCF7Ca vs. 58 per 1000 planted LTLTCa) and with palbociclib and palbociclib resistance (317 per 1000 cells plated MCF7Ca and 202 per 1000 cells plated LTLTCa cells). Overall, these results indicate that 1) resistance to palbociclib, whether as first line or second line treatment has significant effects on breast cancer cells that may be relevant to patient diagnosis and treatment.

Citation Format: Armina A. Kazi, Antony Sare, Saranya Chumsri, Angela Brodie. Mechanisms of resistance to palbociclib and aromatase inhibitors in hormone receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2053. doi:10.1158/1538-7445.AM2017-2053