Combinatorial effects of thymoquinone on the anti-cancer activity of doxorubicin

Abstract

Purpose: Doxorubicin is a mainstay of cancer chemotherapy despite its clinical limitations that arise from its cardiotoxicity and the high incidence of multi-drug resistance. Recent studies revealed a protective effect of thymoquinone, a non-toxic constituent of the essential oil of Nigella sativa, against doxorubicin-induced cardiotoxicity. We now investigated the influence of thymoquinone on various other effects exerted by doxorubicin in human cancer cells.

Methods: Doxorubicin, thymoquinone and equimolar mixtures of both were tested for cytotoxicity on human cells of HL-60 leukaemia, 518A2 melanoma, HT-29 colon, KB-V1 cervix, and MCF-7 breast carcinomas as well as multi-drug-resistant variants thereof and on non-malignant human fibroblasts (HF). Apoptosis induction was analysed via DNA fragmentation, activity studies of the caspases-3, -8 and -9, determination of changes in the mitochondrial membrane potential and in the ratio of the mRNA expressions of pro- and anti-apoptotic proteins bax and bcl-2. The generation of reactive oxygen species (ROS) was assessed by the NBT assay.

Results: Thymoquinone improved the anti-cancer properties of doxorubicin in a cell line-specific manner. We found a significant rise of the growth inhibition by doxorubicin in HL-60 and multi-drug-resistant MCF-7/TOPO cells when thymoquinone had been added. The mode of action of both drugs and of their mixture was mainly apoptotic. In HL-60 cells, the drug mixture caused an additional concentration maximum of effector caspase-3 not observed for either of the pure drugs. The impact of the drug mixture on the mitochondria of HL-60 cells was also greater than those of the individual quinones alone. In addition, the drug mixture led to a higher concentration of reactive oxygen species in HL-60 cells.

Conclusions: In summary, thymoquinone is a booster for the anti-cancer effect of doxorubicin in certain cancer cell lines. Distinct improvements on efficacy, selectivity, and even breaches of multi-drug resistance were observed for equimolar mixtures of doxorubicin and thymoquinone.