Abstract

The concurrent use of herbal medicinals with prescription and over-the-counter drugs carries a risk for unanticipated adverse drug-botanical pharmacokinetic interactions, particularly as a result of cytochrome P450 (P450) inhibition. Extracts of goldenseal (Hydrastis canadensis) containing approximately equal concentrations (∼17 mM) of two methylenedioxyphenyl alkaloids, berberine and hydrastine, inhibited with increasing potency (CYP2C9) diclofenac 4′-hydroxylation, (CYP2D6) bufuralol 1′-hydroxylation, and (CYP3A4) testosterone 6β-hydroxylation activities in human hepatic microsomes. The inhibition of testosterone 6β-hydroxylation activity was noncompetitive with an apparent K_i of 0.11% extract. Of the methylenedioxyphenyl alkaloids, berberine (IC₅₀ = 45 μM) was the more inhibitory toward bufuralol 1′-hydroxylation and hydrastine (IC₅₀ ∼350 μM for both isomers), toward diclofenac 4′-hydroxylation. For testosterone 6β-hydroxylation, berberine was the least inhibitory component (IC₅₀ ∼400 μM). Hydrastine inhibited testosterone 6β-hydroxylation with IC₅₀ values for the (+)- and (-)-isomers of 25 and 30 μM, respectively. For (-)-hydrastine, an apparent K_i value of 18 μM without preincubation and an NADPH-dependent mechanism-based inhibition with a kinactivation of 0.23 min^-1 and a K_I of ∼110 μM were determined. Cytochrome P450 metabolic-intermediate (MI) complex formation could be demonstrated for both hydrastine isomers. With expressed P450 isoforms, hydrastine formed a P450 MI complex with CYP2C9, CYP2D6, and CYP3A4. Coexpression of cytochrome b₅ with the P450 isoforms enhanced the rate but not the extent of P450 MI complex formation.