Overcoming acquired doxorubicin resistance of ovarian carcinoma cells by verapamil‑mediated promotion of DNA damage‑driven cytotoxicity

The efficacy of anticancer therapeutics is limited by acquired drug resistance of tumor cells. The present study aimed to characterize and overcome resistance mechanisms to the anthracycline derivative doxorubicin (Doxo). To this end, comparative analyses of Doxo‑induced stress responses of parental A2780 ovarian carcinoma cells and Doxo‑resistant A2780ADR variants were performed. A2780ADR cells revealed cross‑resistance to multiple compounds, including anticancer drugs [cisplatin (CisPt) and etoposide (Eto)] and DNA repair/DNA damage response (DDR) inhibitors (olaparib, niraparib, entinostat, prexasertib and rabusertib). A2780ADR cells formed markedly fewer DNA double‑strand breaks (DSB) following Doxo exposure compared with parental A2780 cells, resulting in a mitigated DDR, reduced proliferation inhibition and attenuated apoptosis. Potential resistance mechanisms identified to contribute to Doxo resistance of A2780ADR cells include increased Doxo efflux due to increased multi‑drug resistance gene 1 (MDR1) expression and reduced topoisomerase IIα protein expression. Substantial resensitization of A2780ADR cells to Doxo was achieved by both the RAC1 GTPase inhibitor EHT1864, the histone deacetylase inhibitor entinostat (EST) and, most effectively, the calcium channel blocker verapamil (Ver). Notably, Ver‑mediated sensitization also pertains to Eto and CisPt. The synergistic effect of Ver in combination with Doxo, which is reflected by low combination index (CI<0.8), probably involves inhibition of MDR1‑mediated drug export, increased intracellular steady state levels of Doxo and elevated DSB formation, eventually promoting pro‑toxic mechanisms of the DDR. However, combination treatment with Doxo and Ver also increased the cytotoxic response of non‑malignant murine cardiomyocytes, murine embryonic stem cells and human induced pluripotent stem cells. Taken together, the present study suggested inhibition of MDR1‑mediated Doxo efflux by Ver a useful approach to overcome acquired drug resistance of A2780ADR cells by stimulating DDR‑related cytotoxicity, yet at the price of a potentially increased risk of normal tissue toxicity.