title: Stereoselective metabolism of rabeprazole-thioether to rabeprazole by human liver microsomes.
abstract: OBJECTIVE
  Rabeprazole is metabolized mainly non-enzymatically to rabeprazole-thioether. This in vitro study was designed to clarify the stereoselective oxidation mechanism and to identify the enzyme(s) involved in the metabolic breakdown of rabeprazole-thioether to rabeprazole.
  
  
  METHODS
  Rabeprazole-thioether was incubated with human liver microsomes and several recombinant cytochrome P450 (CYP) enzymes (CYPs 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4). High-performance liquid chromatography was used for identification and quantification of each rabeprazole enantiomer.
  
  
  RESULTS
  The K(m ) and V(max ) values for the formation of (R)-rabeprazole from rabeprazole-thioether in human liver microsomes were 6.6 microM and 92 pmol/min/mg protein, respectively, whereas those for the formation of (S)-rabeprazole were 5.1 microM and 21 pmol/min/mg protein, respectively. CYP3A4 was found to be the major enzyme responsible for(R)- and (S)-rabeprazole formation from rabeprazole-thioether. The intrinsic clearance (V(max ) /K(m )) for the oxidation by CYP3A4 of (R)-rabeprazole was 3.5-fold higher than that for the (S)-enantiomer (81 nl/min/pmol of P450 vs. 23 nl/min/pmol of P450). On the other hand, CYP2C19 and CYP2D6 were the main enzymes catalyzing the formation of desmethylrabeprazole-thioether from rabeprazole-thioether. The mean K(m ) and V(max ) values of desmethylrabeprazole-thioether formation for CYP2C19 were 5.1 microM and 600 pmol/min/nmol of P450, respectively, whereas those for CYP2D6 were 15.1 microM and 736 pmol/min/nmol of P450, respectively.
  
  
  CONCLUSIONS
  Rabeprazole is reduced mainly non-enzymatically to rabeprazole-thioether, which is further stereoselectively re-oxidized by CYP3A4 mainly to (R)-rabeprazole. The difference in the enantioselective disposition of rabeprazole is determined by stereoselectivity in CYP3A4-mediated metabolic conversion from rabeprazole-thioether to rabeprazole.
authors.affiliation: Department of Pharmacy, Akita University Hospital, 1-1-1 Hondo, Akita, 010-8543, Japan.
authors.forename: Masatomo
authors.initials: M
authors.lastname: Miura
authors.forename: Shigeru
authors.initials: S
authors.lastname: Satoh
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Tada
authors.forename: Tomonori
authors.initials: T
authors.lastname: Habuchi
authors.forename: Toshio
authors.initials: T
authors.lastname: Suzuki
mesh_terms: D053799:2-Pyridinylmethylsulfinylbenzimidazoles / Q000378:metabolism*; D019540:Area Under Curve; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000201:enzymology*; D054328:Proton Pump Inhibitors; D064750:Rabeprazole; D013237:Stereoisomerism; D013440:Sulfides / Q000378:metabolism*
chemical_list: D053799:2-Pyridinylmethylsulfinylbenzimidazoles; D054328:Proton Pump Inhibitors; D013440:Sulfides; C523099:rabeprazole-thioether; D064750:Rabeprazole; D003577:Cytochrome P-450 Enzyme System