Measurements of caffeine and plasma metabolite/caffeine ratios as a test for hepatic drug-oxidizing capacity in goats.

The aim of this investigation was to determine the pharmacokinetics and demethylation of caffeine (CF) and the metabolite/CF ratios that correlated best with CF clearance, which were used to evaluate hepatic drug-oxidizing capacity of CF after a single intravenous dose (5 mg/kg) in hair goats (n = 9). Pharmacokinetic parameters of CF and its metabolites, theobromine (TB), paraxanthine (PX) and theophylline (TP), were calculated. The plasma metabolic ratios TB/CF, PX/CF, TP/CF and TB+PX+TP/CF were determined at 6, 8 and 10 h after CF administration to evaluate their hepatic drug-oxidizing capacity. The plasma concentration-time data of CF were fit to a two-compartment model in all animals. The clearance of CF was 0.08 ± 0.02 L/h/kg, and the volume of distribution was 0.91 ± 0.16 L/kg. The demethylation fractions of CF to TB, PX and TP were 0.24, 0.37 and 0.39, respectively. Correlations between the metabolic ratios and CF clearance were quite high, except for the PX/CF ratio, particularly at 6 h (r = 0.650-0.750, P < 0.01, 0.05) and 10 h (r = 0.650-0.767, P < 0.01, 0.05). Plasma metabolite/CF ratios, except for the PX/CF ratio, may be useful as an alternative to measurements of CF clearance for the determination of the hepatic drug-oxidizing capacity in goats.