Herb-drug interactions: focus on metabolic enzymes and transporters

As the uses of herbal medicines from traditional natural products are increased, the need for pharmacokinetic studies and relevant data are also increased for safe pharmacotherapy. The market entry for the traditional herbal medicine is easier compared with that for synthetic drugs because of a lower regulatory barrier. Thus, the exact mechanisms for the absorption, distribution, metabolism and excretion of active components in herbal medicines and the potential herb-drug interactions are not always fully understood. Recently, there has been an increasing interest in pharmacokinetic studies of herbal remedies and relevant data of commonly used herbal remedies are accumulating in this field. In this review, the effects of nine botanicals (ginkgo, green tea, grapes, licorice, saw palmetto, garlic milk thistle, ginseng and St. John's wort) on metabolic enzymes and transporters affecting absorption and disposition of herbal products are summarized. The source of samples (extracts and individual components), the species (human and animal) and in vivo and in vitro systems were separately reviewed for a better understanding of herb-drug interactions.