In vitro metabolism of 6-mercaptopurine by human liver cytosol

1. The aim of this study was to investigate 6-mercaptopurine (6MP) metaboli sm by human liver cytosol in vitro. 2. Cytosol was prepared from seven human livers (A-G). A single cytosol (C) was used to optimize incubation conditions. 3. Cytosols A-G were incubated with 6MP at 2, 10 and 500 mu M for two fixed times (5 and 48 h). Parent drug, thiopurine and thionucleotide metabolites were quantitated by high performance liquid chromatography at all time poi nts. 4. At 5 and 48 h the 2 mu M and 10 mu M 6MP incubations contained both 6MP and its initial nucleotide metabolite, thioinosine 5'-monophosphate (TIMP). In addition, the 10 mu M 6MP 48 h incubates contained small amounts of 6-t hioguanine (6TG, median 0.12 mu M). At 500 mu M 6MP all seven liver incubat es produced a range of metabolites. At 48 h these included thiouric acid, 8 -hydroxy-6-mercaptopurine and 6-methylmercaptopurine (median 31, 19.5 and 8 .8 mu M respectively), with TIMP, 6TG, thioxanthine and thioxanthine nucleo tide at median concentrations of 61, 0.79, 2.11 and 0.80 mu M respectively. Thioguanine nucleotides, major metabolites measured in vivo, were not dete cted. 5. These results indicate that the human liver 6MP metabolic profile is dep endent upon drug concentration.