Mass spectral identification of probenecid metabolites in rat bile

Abstract

The metabolism of probenecid (p-(di-n-propylsulfamyl)benzoic acid) by the liver has been studied in biliary cannulated rats with ligated renal pedicles. The unchanged drug, N-depropylated probenecid, and three glucuronides of metabolites of this drug readily appeared in the bile. When rats were given a single 50 mg/kg dose of [¹⁴C] probenecid, 96% of the administered radioactivity was excreted in the bile of renal-ligated rats in 8 hr. Approximately 64% of the drug in this system appeared as metabolites. Four major metabolites were identified by their gas-liquid chromatograms and mass spectra. They were the N-dealkylated drug, an ester glucuronide of a side chain oxidized metabolite (metabolite B) and two ether glucuronides of side-chain hydroxylated drug (metabolites A and C). None of the previously reported acyl glucuronide of probenecid was found in rat bile and none of the metabolites included in this paper have been reported before. In the homozygotous Gunn rat, reported to be deficient in glucuronidating enzymes, the same biliary metabolites as appeared in Sprague-Dawley rats, were found. It is proposed that one may be able to very rapidly assess the importance of drug metabolism for a new agent if it is administered to renal-ligated animals, collected in bile, and then subjected to GLC-MS procedures. The overall technique appears to have several advantages over the usual methods of collecting and assaying urine for drugs and their metabolites.