Characterization of the metabolism of fenretinide by human liver microsomes, cytochrome P450 enzymes and UDP-glucuronosyltransferases

BACKGROUND AND PURPOSE Fenretinide (4-HPR) is a retinoic acid analogue, currently used in clinical trials in oncology. Metabolism of 4-HPR is of particular interest due to production of the active metabolite 4'-oxo 4-HPR and the clinical challenge of obtaining consistent 4-HPR plasma concentrations in patients. Here, we assessed the enzymes involved in various 4-HPR metabolic pathways. EXPERIMENTAL APPROACH Enzymes involved in 4-HPR metabolism were characterized using human liver microsomes (HLM), supersomes over-expressing individual human cytochrome P450s (CYPs), uridine 5'-diphospho-glucoronosyl transferases (UGTs) and CYP2C8 variants expressed in Escherichia coli. Samples were analysed by high-performance liquid chromatography and liquid chromatography/mass spectrometry assays and kinetic parameters for metabolite formation determined. Incubations were also carried out with inhibitors of CYPs and methylation enzymes. KEY RESULTS HLM were found to predominantly produce 4'-oxo 4-HPR, with an additional polar metabolite, 4'-hydroxy 4-HPR (4'-OH 4-HPR), produced by individual CYPs. CYPs 2C8, 3A4 and 3A5 were found to metabolize 4-HPR, with metabolite formation prevented by inhibitors of CYP3A4 and CYP2C8. Differences in metabolism to 4'-OH 4-HPR were observed with 2C8 variants, CYP2C8*4 exhibited a significantly lower V(max) value compared with *1. Conversely, a significantly higher V(max) value for CYP2C8*4 versus *1 was observed in terms of 4'-oxo formation. In terms of 4-HPR glucuronidation, UGTs 1A1, 1A3 and 1A6 produced the 4-HPR glucuronide metabolite. CONCLUSIONS AND IMPLICATIONS The enzymes involved in 4-HPR metabolism have been characterized. The CYP2C8 isoform was found to have a significant effect on oxidative metabolism and may be of clinical relevance.