Synergistic cytotoxic effects of tamoxifen and black cohosh on MCF-7 and MDA-MB-231 human breast cancer cells: an in vitro studyThis article is one of a selection of papers published in this special issue (part 2 of 2) on the Safety and Efficacy of Natural Health Products.

Breast cancer cell cultures were exposed to different concentrations of black cohosh, estradiol (E2), and tamoxifen to examine the effect on cell proliferation; cytotoxicity was assessed by using sulforhodamine B (SRB) dye solution. E2 (10−10–10−8 mol/L) markedly stimulated the proliferation of MCF-7 cells (p < 0.01). Tamoxifen stimulated MCF-7 cell proliferation at 10−6 mol/L and 10−5 mol/L (p < 0.005) but inhibited in a dose-dependent fashion the proliferative effect of E2 (p < 0.001). Black cohosh alone did not show any stimulatory effect, but exhibited a cytotoxic effect, which was significant at 103 μg/mL (p < 0.001). Adding black cohosh at 100–103 μg/mL to E2 at 10−9 mol/L also resulted in a dose-dependent inhibition of E2 proliferative effect. Interestingly, the combination of black cohosh (100–103 μg/mL) with increasing tamoxifen concentrations further inhibited MCF-7 cell growth. On MDA-MB-231 cells, neither E2 nor tamoxifen displayed any detectable effect. However, black cohosh inhibited MDA-MB-231 cell proliferation at 103 μg/mL (p < 0.05), and this inhibitory effect was enhanced by increasing tamoxifen concentrations. This study reveals a cytotoxic effect of black cohosh on both estrogen-sensitive and estrogen-insensitive breast cancer cells and a synergism with tamoxifen for inhibition of cancerous cell growth.