Abstract P5-08-09: Synergistic cytotoxicity of digoxin and 5-fluorouracil in doxorubicin-resistant breast cancer cell lines

Abstract

Abstract BACKGROUND: Metastatic breast cancer is the leading cause of cancer death for women in the US. Cytotoxic chemotherapy remains one of the most important systemic treatment options for breast cancer patients. To enhance the tumor response to chemotherapy, attention has been focused on agents that reverse multidrug resistance (MDR) and increase the sensitivity of tumor cells to chemical drugs. Although many reversal drugs have been identified in vitro, their clinical application has been limited due to their own toxicity. It was reported that 5 years after mastectomy, the recurrence rate of breast cancer among patients on cardiac glycoside therapy, namely digoxin or digitoxin, was almost 10 times lower as compared with those who were not on cardiac glycoside therapy. Digoxin has also been shown to inhibit hypoxia-inducible factors (HIF) and block lung metastasis in a breast cancer model. Interestingly, digoxin was demonstrated to stimulate cell death in various cell lines including breast cancer. Therefore the aim of this study was to examine the reversal effect of combined digoxin and 5-fluorouracil (5-FU) on MDR resistance in human breast cell lines MCF-7 and MDA-MB-231 in vitro, as well as its mechanism of action. METHODS: Wild-type and doxorubicin-resistant ER-positive (MCF-7 and MCF-7/DoxR) and triple negative (MDA-MB-231 and MDA-231/DoxR) breast cancer lines were used for this study. We have previously shown that both doxorubicin-resistant cell lines are also cross-resistant to 5-FU. Cell viability of both resistant cancer cell lines to 5-FU, digoxin, and both 5-FU with digoxin were compared to control at 96 hours of incubation both in normoxia and hypoxia using MTT assay. Western blot was used to quantify the level of HIF-1α and p-glycoprotein (P-gP). RESULTS: In both doxorubicin-resistant cell lines, sub-IC50 concentration of digoxin together with 5-FU significantly decreased the cell viabilities at 96 hours compared to single agent digoxin and 5-FU. Under both normoxic and hypoxic conditions, the same synergistic cytotoxic effects were demonstrated. Western blot revealed that HIF-1α and P-gP were decreased in both doxorubicin-resistant breast cancer cells treated with the digoxin and 5-FU combination compared to single agent digoxin or 5-FU. CONCLUSION: The combination of digoxin and 5-FU demonstrates a synergistic cytotoxic effect in doxorubicin-resistant breast cancer cell lines that is maintained under hypoxic conditions. This is, at least in part, via the inhibitory effects on both HIF-1α and P-gP. The combination of digoxin and 5-FU could be an effective clinical treatment strategy to overcome MDR in breast cancer. A phase II clinical trial is ongoing to test this hypothesis in patients with doxorubicin-resistant metastatic breast cancer. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-08-09.