Received for publication April 13, 2009.
Revised May 6, 2009.
Accepted for publication May 6, 2009.

GLYCEROLYSIS OF ACYL GLUCURONIDES AS AN ARTIFACT OF IN VITRO DRUG METABOLISM INCUBATIONS

Abstract

During an investigation of the in vitro glucuronidation of benoxaprofen by human liver S-9 fraction, an unusual drug-related entity possessing a protonated molecular ion that was 74 mass units greater than parent drug was observed. It was identified as the glycerol ester of benoxaprofen. Formation of this entity required inclusion of UDPGA in the incubation suggesting the formation of benoxaprofen acyl glucuronide followed by transesterification with the glycerol present in the incubation due to its presence as a stabilizer for liver subcellular fractions. Formation occurred during the sample work up procedure while the samples were subjected to evaporation in vacuo, which does not remove glycerol. Conversion of purified benoxaprofen acyl glucuronide to the glycerol ester was demonstrated in glycerol at 37°C. Other drugs that are converted to acyl glucuronides in vitro (diclofenac, mefenamic acid, tolmetin, and naproxen) were also shown to form corresponding glycerol esters when incubated with human liver S-9 fraction and UDPGA. The potential formation of glycerol esters of carboxylic acid drugs undergoing acyl glucuronidation in vitro represents an experimental artifact to which drug metabolism scientists should be aware.

Key words: glucuronidation, metabolite identification