Abstract 5813: Thymoquinone influences the anticancer properties of paclitaxel and gemcitabine against breast cancer cells

The use of anti-cancer adjuvant therapy is rationalized by potentiating the efficacy, and/or protecting from major side effects of chemotherapeutics. Thymoquinone is naturally occurring compound with cumulative evidences of anticancer properties. In this study, we assessed the role of thymoquinone as a potential adjuvant therapeutic in human breast cancer cell lines. Sulfa-Rhodamine-B assay was used to evaluate the effect of thymoquinone on the cytotoxic profile of paclitaxel and gemcitabine in breast cancer cell lines (MCF-7 and T47D). After 72 h of exposure, thymoquinone showed cytotoxic effects against MCF-7 and T47D with IC50's of 64.93±14 µM and 165±2 µM respectively. Combining thymoquinone with paclitaxel increased its IC50 from 0.28±0.1 to 0.3±0.004 µM in MCF-7 cells, with combination index indicative of antagonism (CI-value = 1.53). Similarly, the combination of thymoquinone with paclitaxel in T47D resulted in increasing its IC50 by approximately 1.6 folds (0.09±8 to 0.14±0.02 µM); and the CI-value was 1.66 indicating an antagonistic effect. On the other hand, the IC50 of gemcitabine was significantly reduced after the combination with thymoquinone in both cell lines; MCF-7, T47D, by 15 folds (0.9±0.1 to 0.058±12 µM) and 6 folds (14.3±2 to 2.3±0.2 µM), respectively. In this combination the CI- values were indicative of strong synergism in both cell lines; MCF-7, T47D (0.15 and 0.30 respectively). Further investigation showed that thymoquinone did not affect the cell cycle distribution, however, paclitaxel showed a significant accumulation of cells at G2/M phase in both cell lines after 24, 48h of exposure. On the other hand, gemcitabine caused significant anti-proliferative effect reflected by increasing cell population in S-phase with reciprocal decrease in G2/M cells in both cell lines. The combination of thymoquinone with paclitaxel in T47D didn't enhance paclitaxel-induced G2/M arrest, however it caused significant increase in cell death attributed to this arrest from 38.1% to 69.5%; and from 44.5% to 60.4% after 24 and 48 h of exposure, respectively. Moreover, the combination of thymoquinone with gemcitabine increased S-phase arrest induced by gemcitabine from 44.3% to 49.9%. Accordingly, thymoquinone increased total cell death induced by gemcitabine against T47D from 23.1% to 49.3% and from 15.1 to 64.5 after 24 h and 48 h of exposure, respectively. Using annexin-V/FITC differential staining, thymoquinone significantly increased the percent o