The interaction of selected phytochemicals, HIV drugs, and commercial-source herbal teas and capsules with human cytochrome P450 3A4 and P-glycoprotein

As the popularity of natural health product (NHP) use increases, unfortunately, so does the frequency of cases reporting suspected adverse interactions. Adverse reactions associated with concomitant NHP-therapeutic use may result from competing interactions at the level of the key xenobiotic-biotransforming phase I enzyme cytochrome P450 3A4 (CYP3A4) or the membrane-bound ATP-dependent protein pump P-glycoprotein (P-gp). In this study, selected NHPs of interest to the HIV+ community were assessed for their ability to affect these two important mechanisms of drug disposition. Briefly, commercial-source teas and powdered extracts in capsule formulations were examined for their ability to inhibit CYP3A4-mediated metabolism of the coadministered reference substrate dibenzyl-fluorescein, and their ability to stimulate P-gp ATPase activity. Among the herbal capsules, it was found that aqueous extracts of two different goldenseal (Hydrastis canadensis L.) products were the most inhibitory of CYP3A4-mediated metabolism among all NHPs tested (IC50: 3.03 and 3.23 mg/mL). Goldenseal and milk thistle [Silybum marianum (L.) Gaertn.] teas were found to stimulate P-gp ATPase to a greater degree than the reference positive control verapamil (20 μM). As well, 70% ethanol extracts of one goldenseal product (designated NRP 121) and aqueous extracts of another (designated NRP 17) had the highest relative P-gp ATPase activity overall. Milk thistle and goldenseal products were further analyzed for levels of their marker constituents by HPLC; silibinin and berberine in the experimental design were found to be at biologically relevant concentrations. High-throughput in vitro studies such as the CYP3A4 inhibition and P-pg ATPase screening assays may help to determine which NHPs interact with drug disposition mechanisms, thus ensuring that the occurrence of adverse events due to competing interactions is minimized. They are useful for selecting specific NHPs, NHP formulations, phytochemicals, or drugs to be advanced to more detailed in vivo experimental designs. Although the extrapolation of the current in vitro findings to clinical effects may well be considered speculative, the overall in vitro data should still be viewed as a signal of potential in vivo interactions. © 2008 Informa Healthcare USA, Inc.