Clinical assessment of CYP2D6‐mediated herb–drug interactions in humans: Effects of milk thistle, black cohosh, goldenseal, kava kava, St. John's wort, andEchinacea

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Abstract
Cytochrome P450 2D6 (CYP2D6), an important CYP isoform with regard to drug–drug interactions, accounts for the metabolism of ∼︁30% of all medications. To date, few studies have assessed the effects of botanical supplementation on human CYP2D6 activityin vivo. Six botanical extracts were evaluated in three separate studies (two extractsperstudy), each incorporating 16 healthy volunteers (eight females). Subjects were randomized to receive a standardized botanical extract for 14 days on separate occasions. A 30‐day washout period was interposed between each supplementation phase. In study 1, subjects received milk thistle (Silybum marianum) and black cohosh (Cimicifuga racemosa). In study 2, kava kava (Piper methysticum) and goldenseal (Hydrastis canadensis) extracts were administered, and in study 3 subjects received St. John's wort (Hypericum perforatum) andEchinacea(Echinacea purpurea). The CYP2D6 substrate, debrisoquine (5 mg), was administered before and at the end of supplementation. Pre‐ and post‐supplementation phenotypic trait measurements were determined for CYP2D6 using 8‐h debrisoquine urinary recovery ratios (DURR). Comparisons of pre‐ and post‐supplementation DURR revealed significant inhibition (∼︁50%) of CYP2D6 activity for goldenseal, but not for the other extracts. Accordingly, adverse herb–drug interactions may result with concomitant ingestion of goldenseal supplements and drugs that are CYP2D6 substrates.