Acetaminophen in the guinea pig: metabolite identification in blood, urine, and bile

The biotransformation of single acute oral doses of acetaminophen (100 mg/kg body weight) in adult male guinea pigs was studied by collecting serial blood, urine, and bile samples post-treatment and identifying and quantitating the concentrations of parent drug and excretory products by high performance liquid chromatography. The plasma half-life (beta t/2) (mean +/- SD) of acetaminophen was 1.87 +/- 0.30 h, while that of the only metabolite detected in plasma, the glucuronide, was 2.41 +/- 0.64 h. In 24-h urine samples, the predominant product was the glucuronide (90%) with a small amount of the sulphate conjugate (7.0%) and approximately 3.0% acetaminophen. In bile, the glucuronide was the major metabolite detected initially but, with time, this product decreased concomitantly with an increase in the cysteinyl conjugate. No sulphate was detected in bile but two unidentified metabolites were detected, having distinct column retention times and comprising approximately 6-10% of the total excretory products. The results demonstrated that glucuronidation is a high capacity biotransformation pathway for acetaminophen in this species, only small amounts of other conjugated products being detectable under usual circumstances.