Glucuronidation of olanzapine by cDNA-expressed human UDP-glucuronosyltransferases and human liver microsomes

Olanzapine is a widely used, newer antipsychotic agent, which is metabolized by various pathways: hydroxylation and N-demethylation by cytochrome P450, N-oxidation by flavin monooxygenase and direct glucuronidation. In vivo studies have pointed towards the latter pathway as being of major importance. Accordingly, the glucuronidation reaction was studied in vitro using cDNA-expressed human UDP-glucuronosyltransferase (UGT) enzymes and a pooled human liver microsomal preparation (HLM). Glucuronidated olanzapine was determined by HPLC after acid or enzymatic hydrolysis. The following UGT-isoenzymes were screened for their ability to glucuronidate olanzapine: 1A1, 1A3, 1A4, 1A6, 1A9, 2B7 and 2B15. Only UGT1A4 was able to glucuronidate olanzapine obeying saturation kinetics. The K(m) value was 227 micromol/l (SE 43), i.e. of the same order of magnitude as for other psychotropic drugs, and the V(max) value was 2370 pmol/(min mg) (SE 170). Glucuronidation was also mediated by the HLM preparation, but a saturation level was not reached. The olanzapine glucuronidation reaction was inhibited by several drugs known as substrates for UGT1A4, e.g. amitriptyline, trifluoperazine and lamotrigine. Thus, competition for glucuronidation by UGT1A4 represents a possibility for drug-drug interactions in subjects receiving several of these psychotropic drugs at the same time. Whether such possible interactions are of any clinical importance may await further studies in patients.