Study On The Metabolism And Stereoselective Pharmacokinetics Of Ondansetron

Ondansetron, 1, 2, 3, 9-tetrahydro-9-methyl-3-[(2-methy-1H-imidazol-1-yl)methyl] -4H-carbazol-4-one, is a 5-hydroxytryptamine type 3 (5-HT₃) receptor antagonist used in the treatment of chemotherapy- and radiotherapy-induced nausea and emesios. In previous studies on the metabolism and pharmacokinetics of ondansetron, it is extensively metabolized in the liver through the cytochrome P450 system. Ondansetron undergoes metabolic pathways including hydroxylation of benzene ring followed by glucuronide or sulphate conjugation and N-demethylation.1. ObjectiveThe thesis aimed to isolate and identify metabolites of ondansetron, to investigate ondansetron metabolism in microbial model and in mammals, to study the stereoselective pharmacokinetics of ondansetron, and to gain further understanding stereoselective pharmacokinetics mechanism of ondansetron.2. MethodMetabolites of ondansetron were isolated from the biotransformed culture sample of ondansetron by Mucor circinelloides AS 3.3421 and urine, feces and bile sample of rat after multi-dose oral administration of ondansetron and identified by NMR and ESI-MS. Metabolites of ondansetron in the microbial model and in mammals were investigated by LC/MSⁿ method. The stereoselective pharmacokinetics of ondansetron was studied by assays for determination of ondansetron, its metabolites by LC/MS/MS, the stereoselective pharmacokinetics mechanism was studied by ondansetron enantiomers chiral separated and determination of ondansetron metabolites by LC/MS/MS.3. ResultsA total of 11 metabolites of ondansetron were isolated from the biotransformed culture sample of ondansetron by Mucor circinelloides AS 3.3421 and urine, feces and bile sample of rat. They were 8 phase I metabolites, 3 phase II (2 glucuronides, 1 sulphate conjugations) metabolites, which could be used as reference substances in the studies on metabolism and stereoselective pharmacokinetics of ondansetron. Using multi-stage ion trap mass spectrometric analysis, the characteristic fragment ions of ondansetron and its metabolites were obtained.Metabolites of ondansetron in the microbial model of and in mammals (rat and human) were investigated. A total of 26 metabolites were found and identified by comparisons of their chromatographic behaviors and multi-stage mass spectra to those of ondansetron and 11 isolated standards. These metabolites included 1-hydroxy-ondansetron (M1, M2), conjugated metabolite of M1 with glucuronic acid (M20); 2-hydroxy-ondansetron (M13); 5-hydroxy-ondansetron (M12), conjugated metabolite of M12 with glucuronic acid (M19); 6-hydroxy-ondansetron (M6), conjugated metabolites of M6 with glucuronic acid (M18) and with sulphuric acid (M24); 7-hydroxy-ondansetron (M5), conjugated metabolites of M5 with glueuronic acid (M10) and with sulphurie acid (M23); 8-hydroxy-ondansetron (M7), conjugated metabolites of M7 with glucuronic acid (M17) and with sulphuric acid (M11); N-demethyl-ondansetron (MS); M1 and M2 N-demethylation (M15, M16); M6 N-demethylation (M14), conjugated metabolite of M14 with glucuronic acid (M22); M5 N-demethylation (M3), conjugated metabolites of M3 with glucuronie acid (M9) and with sulphuric acid (M25); M7 N-demethylation (M4), conjugated metabolites of M4 with glucuronic acid (M21) and with sulphuric acid (M26). The metabolic pathways of ondansetron in the microbial model and in mammals were oxidation of N-demethylation, 1-hydroxylation, 5-hydroxylation, 6-hydroxylation, 7-hydroxylation, 8-hydroxylation, combination of N-demethylation and hydroxylation, hydroxylation followed by glucuronide or sulpate conjugation. One new metabolic pathway (hydroxylation on 1-position) was found in this study, and the new pathway was mediated by CYP1A2.The stereoselective pharmacokinetics showed that R-ondansetron cleared slower than S-ondansetron in rat. The stereoselective pharmacokinetics of ondansetron enantiomer was caused by stereoselecive metabolism of cytochrome P450 enzymes: CYP1A2, 2C9, 2C19, 2D6 and 3A4.4. DiscussionMore metabolites of ondansetron in mammals were detected than microbial model, and no metabolites of phaseâ…¡was detected in microbial model. There should be stereoselective pharmacokinetics of ondansetron enantiomers in human, because the stereoselecive metabolism was found in human liver microsomes.