Some quantitative evaluation of first-pass metabolism of salicylamide in rabbit and rat

The first-pass metabolism of salicylamide (SAM) was studied in rabbits and rats paying main attention to the amounts of the glucuronide (SAMG) and sulfate (SAMS) formed. The roles of the intestine and liver for SAMG and SAMS formation during the first-pass were also examined quantitatively. Further, an equation was developed to estimate the hepatic first-pass metabolism based on the experiments of intravenous and oral administration and intestinal perfusion experiment. Significant species differences were found in these two animals. The extent of the first-pass metabolism was larger in rabbits (ca95%) than in rats (ca60%). Of the 95% first-pass metabolism in rabbits, about 30% is due to the intestine whereas the remaining 65% is due to the liver. Of the 60% in rats, about 20 and 40% are estimated to be due to the intestinal and hepatic first-pass metabolism, respectively. In rabbits SAMG and SAMS are formed together in nearly equal amounts in the liver and at the ratio 4 : 1 in the intestine. In rats, on the other hand, SAMG is exclusively formed in the intestine and SAMS in the liver. In this context, the equation described here was applied to the data of Gugler et al. (J. Pharmacol. Exp. Ther., 195, 416-423 (1975)) who carried out a study on the first-pass metabolism of SAM in dogs, and consequently, reported that glucuronidation occurs primarily in the liver whereas sulfation occurs primarily in the intestine in dogs.