Pharmacokinetic Analysis of in Vivo Metabolism of Amino Acid or Dipeptide Conjugates of Salicylic Acid in Rabbit Intestinal Microorganisms

We analyzed the pharmacokinetics of salicylic acid (SA)–amino acid (alanine, glutamic acid, methionine, and tyrosine) or SA–dipeptide (glycylglycine) conjugates in rabbits, by using a model that takes into account the metabolism of prodrug to SA by intestinal microorganisms and, also, by model-independent analysis. The blood concentration profiles of these prodrugs and released SA following intracecal and oral administration to rabbits were obtained previously (Nakamura et al., J. Pharm. Pharmacol., 44, 295–299, 1992; Chem. Pharm. Bull., 40, 2164–2168, 1992; Int. J. Pharm., 87, 59–66, 1992; J. Pharm. Pharmacol., 44, 713–716, 1992). First, the overall in vivo behavior was evaluated by statistical moment analysis. Next, the blood concentration profiles of prodrug and SA following intracecal and oral administration were simultaneously fitted to the above model. In general, good agreement was observed between fitted lines and experimental data for every prodrug, suggesting the validity of this model. The obtained parameters characterized the difference in the rate of metabolism and absorption among the prodrugs. Lower absorbability and enhanced hydrolysis rate of the prodrug lead to prolonged blood concentration of SA.