Metformin is an oral antidiabetic drug with a putative antineoplastic effect in breast cancer; however, its efficacy in clinical trials has been mixed. Triple negative breast cancer (TNBC) is an aggressive disease with poor outcomes for which new treatment strategies are needed. We aimed to define the pharmacological effect (synergistic, additive or antagonistic effect) of combination treatment with conventional chemotherapy agents (paclitaxel or carboplatin) and metformin in vitro.

We carried out crystal violet cytotoxicity assays in primary breast cell lines derived from patients from our institution's breast cancer clinic (Luminal B cell lines: MBCD25 and MBCDF-D5; triple negative cell lines: MBCD11 and MBCDF-B4; epithelial phenotype: MBCD25 and MBCD11; mesenchymal phenotype: MBCDF-D5 and MBCDF-B4) and previously characterized triple negative breast cancer cell cultures (BRCA1WT: MDA-MB-231 and BRCA1-/-: HCC-1937) to estimate the EC50 for each cell line, drug and drug combination (paclitaxel + metformin and carboplatin + metformin). We used Chou and Talalay's method to estimate combination indices for each drug combination and cell line in Compusyn for Windows. Finally, we performed Western blot analysis to determine expression of Bcl-2, Bcl-xL, Bax, and Caspase 3 in each treatment condition.

Sensitivity to each drug differs among the different cell lines. Sensitivity to paclitaxel is similar among all primary cell cultures and lowest in BRCAWT TBNC cell lines (EC50: MBCD25 0.46 µg/mL, MBCD11 0.26 µg/mL, MBCDF-D5 0.29 µg/mL, MBCDF-B4 0.24 µg/mL, MDA-MB-231 0.1 µg/mL, HCC-1937 0.77 µg/mL). Sensitivity to carboplatin is lowest amongst primary breast cultures with an epithelial phenotype, intermediate for cultures with a mesenchymal phenotype and in the BRCA1-/- TNBC cell line and highest in BRCAWT TNBC (EC50: MBCD25 36.84 µM, MBCD11 15.24 µM, MBCDF-D5 198.1 µM, MBCDF-B4 125.6 µM, MDA-MB-231 244 µM, HCC-1937 139.5 µM). Plotted combination indices show that cotreatment of paclitaxel with metformin is synergistic in all cell lines except MBCDF-B4. Cotreatment of carboplatin with metformin is synergistic in primary cell lines with an epithelial phenotype and in TNBC, especially in BRCA deficient cells. Expression of antiapoptotic proteins such as Bcl-2 correlated with higher drug resistance, but not with synergy or antagonism in combination with metformin.

Combination treatment of primary breast cancer cultures and TNBC cell lines with metformin and carboplatin or paclitaxel has a variable pharmacological effect. TNBC cell lines were intrinsically drug resistant and metformin was effective in abrogating drug resistance. The high expression of antiapoptotic proteins of the Bcl-2 family in drug-resistant cell lines is remarkable due to the effectiveness of BH3 mimetics in hematologic malignancies and warrants further investigation in breast cancer.

Citation Format: Ossian Longoria, José Esparza López, Eucario León-Rodríguez, María de Jesús Ibarra-Sánchez. Synergistic cytotoxicity of metformin cotreatment with paclitaxel or carboplatin in triple negative breast cancer cell lines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1416.