Interactions between phytochemicals from traditional Chinese medicines and human cytochrome P450 enzymes

Traditional Chinese medicine (TCM) formulas with fixed combinations rely on "sovereign, minister, assistant and guide" and fuzzy mathematical quantitative law, leading to greater challenges for the identification of active ingredients. Transformation and metabolic studies involving the Phase I drug-metabolizing enzyme cytochrome P450 (CYP) might potentially solve some of these challenges. The pharmacological effects can not be attributed to one active ingredient in TCMs, but integrated effects resulting from the combined actions of multiple ingredients. However, it is only after long-term administration that most ingredients exert their actions, which can result in prolonged exposure to herbs in vivo. Therefore, interactions between herbal compounds and CYPs appear to be inevitable. Yet unlike Western drugs, experimental determination of the absorption and disposition properties is not commonly carried out for TCMs. Moreover, the use of TCM as injections is an innovation aimed to improve efficiency in extensive clinical use in Mainland China. Therefore, in recent years, cases of adverse drug reactions (ADR) mainly concerning allergic reactions involving TCMs such as ShenMai injection and QingKaiLing injection have been reported, which have attracted attention with regard to the legal responsibilities for TCM approval. The lack of information on the ADME characteristics, especially the metabolic stability and interaction potential between CYPs and herbs, increases ADR occurrence due to TCMs. In this article, we review the most common herbs used in TCM prescriptions and fixed combinations of their usable frequency, and summarize the current understanding of the ability of phytochemical ingredients to act as substrates, inhibitors or inducers of human CYP enzymes, through which the key role of CYP enzymes on the herb disposition and toxicity is highlighted. The potential interaction between herbal phytochemicals and CYP enzymes dominates the target exposure, which further helps to elucidate the herbal pharmacological basis, assess the individual toxic risk of herbal remedies and gain mechanistic insight into herb-drug interactions (HDIs).