In vitro metabolic interactions between black cohosh (Cimicifuga racemosa) and tamoxifen via inhibition of cytochromes P450 2D6 and 3A4

Abstract

1. Women who experience hot flashes as a side effect of tamoxifen (TAM) therapy often try botanical remedies such as black cohosh to alleviate these symptoms. Since pharmacological activity of TAM is dependent on the metabolic conversion into active metabolites by the action of cytochromes P450 2D6 (CYP2D6) and 3A4, the objective of this study was to evaluate whether black cohosh extracts can inhibit formation of active TAM metabolites and possibly reduce its clinical efficacy.

2. At 50 μg/mL, a 75% ethanolic extract of black cohosh inhibited formation of 4-hydroxy- TAM by 66.3%, N-desmethyl TAM by 74.6% and α-hydroxy TAM by 80.3%. In addition, using midazolam and dextromethorphan as probe substrates, this extract inhibited CYP3A4 and CYP2D6 with IC₅₀ values of 16.5 and 50.1 μg/mL, respectively.

3. Eight triterpene glycosides were identified as competitive CYP3A4 inhibitors with IC₅₀ values ranging from 2.3–5.1 µM, while the alkaloids protopine and allocryptopine were identified as competitive CYP2D6 inhibitors with K_i values of 78 and 122 nM, respectively.

4. The results of this study suggests that co-administration of black cohosh with TAM might interfere with the clinical efficacy of this drug. However, additional clinical studies are needed to determine the clinical significance of these in vitro results.

Keywords::

• Triterpenes
• alkaloids
• P450 inhibition
• drug−herb interaction

Acknowledgement

This work was supported by grant P50AT00155 from the Office of Dietary Supplements, the National Institute of General Medical Sciences, the Office for Research on Women’s Health and the National Center for Complementary and Alternative Medicine.

Declaration of interest

The authors declare no conflict of interest.