title: In vitro proguanil activation to cycloguanil is mediated by CYP2C19 and CYP3A4 in adult Chinese liver microsomes.
abstract: OBJECTIVE
  To identify the cytochrome P450 isoforms involved in proguanil (PG) activation to cycloguanil (CG) in Chinese liver microsomes.
  
  
  METHODS
  The kinetics of the CG formation from PG was determined in the liver microsomes of 6 Chinese subjects. Selective chemical inhibitors to various cytochrome P450 isoforms were employed to conduct inhibition experiments. The relationship between the CG formation and S-mephenytoin 4'-hydroxylation was analyzed.
  
  
  RESULTS
  The kinetic behaviors of CG formation were described well by a single-enzyme Michaelis-Menten equation in five livers. The apparent Km and Vmax were (82 +/- 47) mumol.L-1 and (8 +/- 6) pmol.min-1.mg-1 protein, respectively. However, the remaining one displayed a two-enzyme kinetic behavior. Inhibition experiments showed that troleandomycin (100 mumol.L-1) and diethyldithiocarbamate (100 mumol.L-1), as potent CYP3A4 and CYP2E1 inhibitors, respectively, reduced the formation rate of CG by 81.1% and 47.23%, while quinidine (10 mumol.L-1), furafylline (20 mumol.L-1), and sulfaphenazole (10 mumol.L-1), which were inhibitors towards CYP2D6, 1A2 and 2C9/10, respectively, did not display significant inhibition. At a low PG concentration of 5 mumol.L-1, the CG formation correlated well with S-mephenytoin 4'-hydroxylation (r = 0.805, P < 0.05). Nevertheless, when a high substrate concentration (500 mumol.L-1) was used, the correlation coefficient decreased (r = 0.581, P < 0.05).
  
  
  CONCLUSIONS
  The present study indicates that CYP3A4 and CYP2C19 are involved in PG activation to CG in adult Chinese liver microsomes. CYP2C19 played an important role in the clearance of PG at a substrate concentration close to in vivo therapeutic concentrations, while CYP3A4 gradually made a dominant contribution with the increase of PG concentration.
authors.affiliation: Pharmacogenetics Research Institute, Hunan Medical University, Changsha 410078, China.
authors.forename: A H
authors.initials: AH
authors.lastname: Lu
authors.forename: Y
authors.initials: Y
authors.lastname: Shu
authors.forename: S L
authors.initials: SL
authors.lastname: Huang
authors.forename: W
authors.initials: W
authors.lastname: Wang
authors.forename: D S
authors.initials: DS
authors.lastname: Ou-Yang
authors.forename: H H
authors.initials: HH
authors.lastname: Zhou
mesh_terms: D000328:Adult; D000962:Antimalarials / Q000493:pharmacokinetics*; D001189:Aryl Hydrocarbon Hydroxylases; D044466:Asian People; D001711:Biotransformation; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000378:metabolism*; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000502:physiology; D002727:Proguanil / Q000493:pharmacokinetics*; D014227:Triazines / Q000378:metabolism; D014217:Troleandomycin / Q000494:pharmacology
chemical_list: D000962:Antimalarials; D014227:Triazines; C026009:cycloguanil; D003577:Cytochrome P-450 Enzyme System; D014217:Troleandomycin; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; C104464:CYP3A protein, human; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D002727:Proguanil