Abstract 1205: Rubrofusarin and toralactone sensitize resistant MCF-7adr cell line to paclitaxel via inhibiting P-glycoprotein efflux activity
Authors: Salwa D. Alqahtani, Hanan A. Assiri, Fahad A. Al-Abbasi, Ali M. El-Halawany, Ahmed M. Al-Abd
Journal: Cancer Research (2017)

Abstract The use of anti-cancer adjuvant therapy is rationalized by potentiating the efficacy and/or protecting from major side effects of chemotherapeutics. Paclitaxel is an effective spindle toxin anticancer agent used for the treatment of breast cancer. Rubrofusarin and toralactone are naturally occurring structurally related naphthopyrones with potent antioxidant and hence chemoprotective activity. The purpose of this study is to investigate the influence of rubrofusarin and toralactone on the cytotoxic profile of paclitaxel (PTX) against chemo-resistant breast cancer cell line (MCF-7adr) compared to parent MCF-7 cells. MCF-7adr was prepared by continuous culturing of MCF-7 cells in media containing subcytotoxic concentration of doxorubicin (IC10%) for two months duration. Toralactone and rubrofusarin showed moderate cytotoxic effects against MCF-7 cells with IC50’s of 32.4±4.2 µM and 13.9±6.1 µM, respectively. Interestingly, toralactone and rubrofusarin showed stronger cytotoxicity against MCF-7adr cells with IC50’s of 7.3±0.7 µM and 7.8±0.6 µM, respectively. PTX treatment was opposed by significant resistance from MCF-7adr cells compared to parent MCF-7 cells having IC50’s of 4.9±0.9 µM and 57.7±4.5 nM, respectively. Combination of torolactone or rubrofusarin with PTX did not significantly improve its cytotoxicity against MCF-7; IC50’s were 24.2±2.2 nM and 69.7±6.6 nM, respectively compared to 57.7±4.5 nM for PTX alone. Surprisingly, combination with either torolactone or rubrofusarin abolished the resistance of MCF-7adr to PTX resulting in IC50’s of 3.4±0.2 nM and 4.9±0.5 6 nM, respectively compared to 4.9±0.9 µM for PTX alone. Using flowcytometric analysis, rubrofusarin and toralactone increased median intracellular fluorescence signal count of rhodamine dye (non-cytotoxic P-gp substrate) within MCF-7adr from 1794.7±21.5 to 2003.3±12.5 and 2113±10.5 cellular events, respectively. Using purified membrane bound humanized recombinant P-gp molecules; it was found that both rubrofusarin and toralactone might be inhibiting P-gp efflux activity via dual interaction with both P-gp ATPase enzyme subunit and P-gp active binding site. In both MCF-7 and MCF-7adr cells, PTX exerted significant cell cycle block at G2/M-phase with significant increase in the pre-G dead cell fraction from 3.4±0.9% to 23.6±2.1% and from 1.4±0.6% to 17.2±3.2%, respectively. Neither rubrofusarin nor toralactone exerted any significant influence for PTX induced cell cycle interruption in both normal (MCF-7) and resistant (MCF-7adr) cell lines. In conclusion, rubrofusarin and toralactone can enhance the cytotoxic profile of P-gp substrates, such as PTX, via inhibiting P-gp efflux activity and enhancing the intracellular entrapment of P-gp substrates. Citation Format: Salwa D. Alqahtani, Hanan A. Assiri, Fahad A. Al-Abbasi, Ali M. El-Halawany, Ahmed M. Al-Abd. Rubrofusarin and toralactone sensitize resistant MCF-7adr cell line to paclitaxel via inhibiting P-glycoprotein efflux activity [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 1205. doi:10.1158/1538-7445.AM2017-1205