Is quinine a suitable probe to assess the hepatic drug-metabolizing enzyme CYP3A4?

Aims To evaluate the antimalarial agent quinine as a potential in vivo probe for hepatic cytochrome P450 (CYP) 3A4 activity. Methods Ten healthy adult volunteers received, by randomized crossover design, either a single oral dose of quinine sulphate (600 mg) alone, or quinine sulphate (600 mg) plus the CYP3A4 inhibitor troleandomycin (TAO; 500 mg every 8 h). Plasma and urine samples were collected before quinine administration, and up to 48 h thereafter, then analysed by h.p.l.c. for both quinine and its CYP3A4-generated metabolite, 3-hydroxyquinine. During both phases, the erythromycin breath test (ERMBT) was administered at specific times to assess hepatic CYP3A4 activity. Results Compared with control, TAO treatment significantly decreased the mean time-averaged ERMBT result by 77% (95% CI, 68, 85%), the mean apparent oral clearance of quinine (CL/F ) by 45% (95% CI, 39, 52%), and the mean apparent formation clearance of 3-hydroxyquinine (CL3-OH ) by 81% (95% CI, 76, 87%). There was no correlation between the TAO-mediated percent decrease in the time-averaged ERMBT result and the percent decrease in CL/F or in CL3-OH . When TAO and control treatments were analysed separately, there were no significant correlations between the time-averaged ERMBT result and CL/F , CL3-OH , or single plasma quinine concentration at 12, 24, and 48 h. Conclusions Quinine may be a useful probe to detect inhibition of liver CYP3A4 activity within an individual. Further studies are needed to determine whether it can provide a quantitative measure of CYP3A4 activity suitable for intersubject comparison.