title: Clarifying the importance of CYP2C19 and PON1 in the mechanism of clopidogrel bioactivation and in vivo antiplatelet response.
abstract: OBJECTIVE
  It is thought that clopidogrel bioactivation and antiplatelet response are related to cytochrome P450 2C19 (CYP2C19). However, a recent study challenged this notion by proposing CYP2C19 as wholly irrelevant, while identifying paraoxonase-1 (PON1) and its Q192R polymorphism as the major driver of clopidogrel bioactivation and efficacy. The aim of this study was to systematically elucidate the mechanism and relative contribution of PON1 in comparison to CYP2C19 to clopidogrel bioactivation and antiplatelet response.
  
  
  RESULTS
  First, the influence of CYP2C19 and PON1 polymorphisms and plasma paraoxonase activity on clopidogrel active metabolite (H4) levels and antiplatelet response was assessed in a cohort of healthy subjects (n = 21) after administration of a single 75 mg dose of clopidogrel. There was a remarkably good correlation between H4 AUC (0-8 h) and antiplatelet response (r2 = 0.78). Furthermore, CYP2C19 but not PON1 genotype was predictive of H4 levels and antiplatelet response. There was no correlation between plasma paraoxonase activity and H4 levels. Secondly, metabolic profiling of clopidogrel in vitro confirmed the role of CYP2C19 in bioactivating clopidogrel to H4. However, heterologous expression of PON1 in cell-based systems revealed that PON1 cannot generate H4, but mediates the formation of another thiol metabolite, termed Endo. Importantly, Endo plasma levels in humans are nearly 20-fold lower than H4 and was not associated with any antiplatelet response.
  
  
  CONCLUSIONS
  Our results demonstrate that PON1 does not mediate clopidogrel active metabolite formation or antiplatelet action, while CYP2C19 activity and genotype remains a predictor of clopidogrel pharmacokinetics and antiplatelet response.
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine,  Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario,  Canada.
authors.forename: Inna Y
authors.initials: IY
authors.lastname: Gong
authors.forename: Natalie
authors.initials: N
authors.lastname: Crown
authors.forename: Colin M
authors.initials: CM
authors.lastname: Suen
authors.forename: Ute I
authors.initials: UI
authors.lastname: Schwarz
authors.forename: George K
authors.initials: GK
authors.lastname: Dresser
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Knauer
authors.forename: Daisuke
authors.initials: D
authors.lastname: Sugiyama
authors.forename: Marianne K
authors.initials: MK
authors.lastname: DeGorter
authors.forename: Sarah
authors.initials: S
authors.lastname: Woolsey
authors.forename: Rommel G
authors.initials: RG
authors.lastname: Tirona
authors.forename: Richard B
authors.initials: RB
authors.lastname: Kim
mesh_terms: D000293:Adolescent; D000328:Adult; D019540:Area Under Curve; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics*; D043303:Aryldialkylphosphatase / Q000235:genetics*; D001711:Biotransformation / Q000187:drug effects; D000077144:Clopidogrel; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000493:pharmacokinetics / Q000494:pharmacology; D006801:Humans; D008874:Midazolam / Q000493:pharmacokinetics / Q000494:pharmacology; D008875:Middle Aged; D010974:Platelet Aggregation / Q000187:drug effects / Q000235:genetics; D010975:Platelet Aggregation Inhibitors / Q000493:pharmacokinetics / Q000494:pharmacology*; D011110:Polymorphism, Genetic / Q000235:genetics*; D013988:Ticlopidine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000494:pharmacology; D055815:Young Adult
chemical_list: D004791:Enzyme Inhibitors; D010975:Platelet Aggregation Inhibitors; D000077144:Clopidogrel; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D043303:Aryldialkylphosphatase; D013988:Ticlopidine; D008874:Midazolam