Herbal medicines have been widely used for the past millennia. Traditional Chinese medicine (TCM) is a major modality in Chinese medical care and has garnered global attention owing to its pharmacological effects and multi-targeted actions. The increased incidence of sequential or concurrent use of herbs and drugs in patients forces us to consider herb-drug interactions (HDIs) in this modern era. One of the main causes of HDIs is modulation of drug metabolism, in which cytochrome P450 (CYP), UDP-glucuronosyltransferase (UGT), and transporters play primary roles. In this review, we focus on in vivo studies of HDIs, particularly in the treatment of cardiovascular disease (CVD), which is currently the leading cause of disease-related mortality worldwide. A total of 55 HDIs are summarized, and their potential underlying mechanisms are examined. The pharmacokinetic (PK) and pharmacodynamic (PD) effects of three single herbs (Danshen, Ginseng, and Ginkgo) and four compound prescriptions (Shenmai injection, Shengmai-San, Shu-Jing-Hwo-Shiee-Tang, and Wu-Chu-Yu-Tang) are discussed. Due to the complex compositions and PK/PD profiles of TCMs, the determinants of significant HDIs have been listed to further define the pros and cons of HDIs in medical care.

Hepatic clearance has been widely studied for over 50 yr. Many models have been developed using either theoretical or empirical tests to predict drug metabolism. The well-stirred, parallel-tube, and dispersion metabolic models have been extensively discussed. However, to our knowledge, these models cannot fully describe all relevant scenarios in hepatic clearance. We addressed this issue using the isolated perfused rat liver technique with minor modifications. Diazepam was selected to illustrate different levels of drug plasma-protein binding by changing the added concentration of human serum albumin. The free fractions of diazepam at different albumin concentrations were assayed by rapid equilibrium dialysis. The experimental data provide new insights concerning an accepted formula used to describe hepatic clearance. Regarding drug concentrations passing through the liver, the driving force concentration (C_H,ss) in terms of C_in (influx in the liver) or C_out (efflux from the liver) needs to be carefully considered when determining drug hepatic and intrinsic clearances. The newly established model, termed the modified well-stirred model, which was derived from the original formula, successfully estimated hepatic drug metabolism. Using the modified well-stirred model, a theoretical driving force concentration of diazepam passing through the liver was evaluated. The model was further used to assess the predictability of in vitro to in vivo extrapolation. This study was not intended to refute the existing models, but rather to augment them using experimental data. The results stress the importance of proper calculation of dose when the drug clearance deviates from the prediction of the well-stirred model.