Comparative pharmacokinetics and metabolisms of caffeine in sheep breeds

The purpose of this study was to investigate the effect of breed on the pharmacokinetics and metabolism of caffeine (CF) and the hepatic metabolic capacity in sheep. CF was administered as a single intravenous dose of 5 mg/kg b.w. in Morkaraman (MK), Akkaraman (AK) and Anatolia Merino (AM) sheep breeds. The plasma levels of CF and its primary metabolites, theobromine (TB), paraxanthine (PX) and theophylline (TP), were measured using high-performance liquid chromatography. Pharmacokinetic parameters of CF and its metabolites were calculated. Plasma TB+PX+TP/CF metabolic ratio was determined as an alternative to CF clearance for the determination of hepatic metabolic capacity. In the three breeds, all kinetic parameters of CF differed significantly (P<0.05) except for volume of distribution. Elimination of CF was slow in the MK (ClT; 0.03 ± 0.01 l hr/kg, t1/2λz; 15.74 ± 7.35 hr) and AM (ClT; 0.05 ± 0.02 l hr/kg, t1/2λz; 9.68 ± 5.21 hr) breeds when compared with the AK breed (ClT; 0.08 ± 0.01 l hr/kg, t1/2λz; 6.84 ± 0.79 hr). There was significant correlation (r2=0.904, P<0.01) between CF clearance and the plasma TB+PX+TP/CF ratio calculated at 7 hr after CF administration. The plasma TB+PX+TP/CF ratios were statistically different (P<0.05) among the breeds (MK, 0.155 ± 0.062; AK, 0.468 ± 0.107; AM, 0.254 ± 0.099). These results suggest that the pattern of drug biotransformation should be consistently tested for all breeds within species. Further studies are needed to determine the biochemical and molecular events underlying such an effect.