Lack of effect of continuous glycyrrhizin administration on the pharmacokinetics of the P-glycoprotein substrate talinolol in healthy volunteers

Purpose To investigate the effects of repeated glycyrrhizin ingestion on the oral pharmacokinetics of talinolol, a probe drug for P-glycoprotein (P-gp) activity in humans. Methods Fourteen healthy adult male subjects were enrolled in a two-phase randomized crossover-design study. In each phase the volunteers received placebo or compound glycyrrhizin tablets (75 mg glycyrrhizin three times daily) for 6 days. On the seventh day, a single oral dose of 100 mg talinolol was administered, and blood samples were obtained to determine plasma talinolol concentrations, measured in plasma by high-performance liquid chromatography with an ultraviolet detector. Non-compartmental analysis was used to characterize talinolol plasma concentration-time profiles. All pharmacokinetics parameters were calculated using DAS ver. 2.1 software, and statistical analyses were performed with SPSS ver. 13.0 software. Analysis of variance was used to check the difference of the means of the pharmacokinetic parameters between the two treatments at a significance level of 0.05. Results All treatments were well tolerated during the study period. The geometric mean +/- standard deviation of the AUC(0-infinity) for talinolol treated by glycyrrhizin and talinolol treated by placebo was 2,218.3 +/- 724.3 and 1,988.2 +/- 649.2 ng.h/mL, respectively. The 90 % confidence intervals for the ratio of adjusted geometric means (glycyrrhizin: placebo) for AUC(0-infinity) and C-max fell wholly within the interval [80, 125]. Six days of glycyrrhizin treatment resulted in no significant alterations in the pharmacokinetic parameters (AUC(0-infinity), AUC(0-24), C-max, t(max), t(1/2)) for talinolol. Conclusions Continuous glycyrrhizin administration had no induction effect on the expression of P-gp in our trial. Further research is needed to study the direct inhibition effect of glycyrrhizin on the function of P-gp with the simultaneous administration of both glycyrrhizin and P-gp substrate.