Abstract 1014: Characterization of breast cancer cells resistant to letrozole and palbociclib combination therapy and the potential of dinaciclib to treat such resistance

Treatment of estrogen receptor positive (ER+) breast cancer have included single-agent endocrine therapy, such as with aromatase inhibitors (AIs) and, more recently, AIs in combination with the cyclin-dependent kinase 4/6 inhibitor palbociclib. While combination therapy is effective, drug resistance is a well-known phenomenon and breast cancer cells can acquire resistance to aromatase inhibitor and palbociclib individually. In anticipation of patient who may become resistant to combination therapy with aromatase inhibitor (letrozole) and palbociclib, this current study has 1) developed MCF7Ca cells resistant to both letrozole and palbociclib (MCF7Calet+palb); 2) characterized them in comparison to drug naive (MCF7Ca) and letrozole alone-resistant (LTLTCa) cells; and 3) explored potential treatments. Characterization of MCF7Calet+palb cells by phase contrast microscopy, mammosphere assays, and invasion assays, indicate that they have more basal/mesenchymal morphology, produce more mammospheres (an indicator of cancer stem cells; 150±10 vs. 80±10 in MCF7Ca and 123± 8 in LTLTCa cells), and are more invasive (1200±50 cells vs. 791±15 in MCF7Ca cells). The more aggressive phenotype of MC7Calet+palb correlated with changes in protein expression. Compared to the other cell lines, MCF7Calet+palb expressed significantly lower fold expression levels of cell cycle cyclin D1 (0.2±0.1), phosphorylated CDK9 (0.5±0.1), and phosphoRb and total Rb (0.4±0.2), but higher levels of phosphoCDK1 and CDK2 (1.4±0.1 and 3.1±0.6). Regarding growth pathway proteins, MCF7Calet+palb comparably had increased HER2 and phosphorylated Akt (6.2±1.3 and 3.3±1.2, resp.) as LTLTCa vs. MCF7Ca cells, but had even more reduced levels of ERα (0.04 vs. 0.1±0.4 in LTLTCa and 1±0.01 in MCF7Ca). Cancer stem cell marker breast cancer resistance protein (BCRP) was increased further in MCF7Calet+palb cells (2.11-fold). Based on these results, the effect of dinaciclib, a broader range CDK inhibitor, was studied. In MTT assays, dinaciclib was shown to be effective in inhibiting cell viability in all three cell types. Dinaclicib’s IC50s were 1.4 nM (MC7Ca) 9.5 nM (LTLTCa) and 12.4 nM in MCF7Calet+palb cells, while palbociclib IC50s were 1.5 μM, 11.2 μM, and 118 μM, respectively. Dinaciclib also significantly reduced mammosphere formation 0.01-fold and cell invasion/migration by 0.3-fold vs. untreated. Lastly, preliminary exome sequencing data suggested highly mutated FGFRL1, ARL11, and FCGBP genes, among others, when MCF7Calet+palb cells were compared to MCF7Ca cells. Overall, these results suggest that acquired resistance to combination therapy with letrozole and palbociclib induces genetic, molecular, and phenotypic changes in breast cancer cells, and that dinaciclib may be an effective treatment to either treat and/or prevent this resistance. Citation Format: Armina Kazi, Emily Shukdinas, Saranya Chumsri, Gauri J. Sabnis. Characterization of breast cancer cells resistant to letrozole and palbociclib combination therapy and the potential of dinaciclib to treat such resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1014.