A new study has reported that biochanin A reduces the potency of tamoxifen treatment in female rats. Biochanin A is an isoflavone (a type of phytoestrogen) found in red clover; it is also present in soybeans, chickpeas, peanuts, mung beans, alfalfa sprouts and other legumes. To conduct the study, the authors administered biochanin A orally to female rats who were also given tamoxifen orally. A control group of rats received tamoxifen only.

Tamoxifen is a substrate (the substance acted upon by an enzyme) of cytochrome 3A (CYP3A) and P-glycoprotein (P-gp). Given that biochanin A is a known inhibitor of CYP 3A and P-gp, biochanin A was expected to increase the bioavailability of tamoxifen. Instead, animals pretreated with biochanin A were found to have significantly decreased overall and peak plasma concentrations of tamoxifen compared to control animals. As a result, the relative bioavailability of tamoxifen in animals co-administered with biochanin A was remarkably reduced compared with the control group. The authors conclude that if the results are further confirmed by clinical trials, tamoxifen dosages should be adjusted to avoid potential drug interaction when tamoxifen is used clinically in combination with biochanin A and biochanin A-containing dietary supplements.

Comments regarding the study

Biochanin A is found in supplements such as red clover and soybean extract. Isoflavones, including genistein and daidzein, among others, are the most important ingredients in red clover (in fact, biochanin A is a derivative of genistein). Red clover is used by some women to treat menopausal symptoms. The authors make it clear that they were surprised by the study results; they expected to see a benefit from adding biochanin A to tamoxifen treatment but instead found that it reduced tamoxifen's potency. Based on the study results, biochanin A and red clover supplements should not be used during tamoxifen treatment.

Please see our article on tamoxifen for more information on how to optimize treatment with tamoxifen and reduce side effects.