Antidepressant-like components of Hypericum perforatum extracts:: An overview of their pharmacokinetics and metabolism

Extracts of Hypericum perforatum are becoming increasingly popular for the treatment of mild to moderate depression, despite the lack of consensus on their efficacy. Although the mechanism(s) of this action are still debated, several components, including the naphthodianthrones hypericin and pseudohypericin, the acylphloroglucitiol hyperforin and some flavonols, are believed to play major roles in the antidepressant-like effects. Some of these also increase the expression of the P-glycoprotein transporter and others the expression of cytochrome P450 enzymes, possibly contributing to the interactions involving the extracts and conventional drugs. However, few pharmacokinetic studies of naphthodianthrones and hyperforin have appeared and none has yet evaluated the exposure to unchanged quercetin and its glycosides after intake of extracts. There are no formal pharmacokinetic studies in special populations. Bioavailability appears low, giving variable steady-state plasma concentrations, whose prediction may be complicated by non-linearity for hypericin and hyperforin. Data on tissue distribution are scarce, and it appears that hypericin and hyperforin do not reach the central nervous system in appreciable concentrations in animals. Clearance is low-intermediate, with little or no unchanged compounds excreted with urine. Although some potentially active conjugated metabolites have been identified for quercetin and its glycosides after intake of authentic compounds or flavonol-rich foods, these too have been characterised little with regard to their pharmacokinetics and central activities. Thus, further pharmacokinetic and pharmacodynamic studies of the main components and their metabolites are urgently needed to clarify the role of each constituent and provide more rational and safe regimens for people preferring "natural" drugs.