title: Effects of drug interactions on biotransformation and antiplatelet effect of clopidogrel in vitro.
abstract: OBJECTIVE
  The conversion of clopidogrel to its active metabolite, R-130964, is a two-step cytochrome P450 (CYP)-dependent process. The current investigations were performed to characterize in vitro the effects of different CYP inhibitors on the biotransformation and on the antiplatelet effect of clopidogrel.
  
  
  METHODS
  Clopidogrel biotransformation was studied using human liver microsomes (HLM) or specific CYPs and platelet aggregation using human platelets activated with ADP.
  
  
  RESULTS
  Experiments using HLM or specific CYPs (3A4, 2C19) revealed that at clopidogrel concentrations >10 microM, CYP3A4 was primarily responsible for clopidogrel biotransformation. At a clopidogrel concentration of 40 microM, ketoconazole showed the strongest inhibitory effect on clopidogrel biotransformation and clopidogrel-associated inhibition of platelet aggregation with IC(50) values of 0.03 +/- 0.07 microM and 0.55 +/- 0.06 microM respectively. Clarithromycin, another CYP3A4 inhibitor, impaired clopidogrel biotransformation and antiplatelet activity almost as effectively as ketoconazole. The CYP3A4 substrates atorvastatin and simvastatin both inhibited clopidogrel biotransformation and antiplatelet activity, less potently than ketoconazole. In contrast, pravastatin showed no inhibitory effect. As clopidogrel itself inhibited CYP2C19 at concentrations >10 microM, the CYP2C19 inhibitor lansozprazole affected clopidogrel biotransformation only at clopidogrel concentrations < or =10 microM. The carboxylate metabolite of clopidogrel was not a CYP substrate and did not affect platelet aggregation.
  
  
  CONCLUSIONS
  At clopidogrel concentrations >10 microM, CYP3A4 is mainly responsible for clopidogrel biotransformation, whereas CYP2C19 contributes only at clopidogrel concentrations < or =10 microM. CYP2C19 inhibition by clopidogrel at concentrations >10 microM may explain the conflicting results between in vitro and in vivo investigations regarding drug interactions with clopidogrel.
authors.affiliation: Division of Clinical Pharmacology & Toxicology, Department of Biomedicine and Swiss Center of Applied Human Toxicology (SCAHT), University and University Hospital of Basel, Switzerland.
authors.forename: Anja
authors.initials: A
authors.lastname: Zahno
authors.forename: Karin
authors.initials: K
authors.lastname: Brecht
authors.forename: Michael
authors.initials: M
authors.lastname: Bodmer
authors.forename: Daniel
authors.initials: D
authors.lastname: Bur
authors.forename: Dimitrios A
authors.initials: DA
authors.lastname: Tsakiris
authors.forename: Stephan
authors.initials: S
authors.lastname: Krähenbühl
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors; D000069059:Atorvastatin; D017291:Clarithromycin / Q000493:pharmacokinetics / Q000494:pharmacology; D000077144:Clopidogrel; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; D065692:Cytochrome P-450 CYP3A Inhibitors; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006538:Heptanoic Acids / Q000493:pharmacokinetics / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007654:Ketoconazole / Q000493:pharmacokinetics / Q000494:pharmacology; D008862:Microsomes, Liver / Q000378:metabolism*; D008958:Models, Molecular; D010974:Platelet Aggregation / Q000187:drug effects; D010975:Platelet Aggregation Inhibitors / Q000493:pharmacokinetics* / Q000494:pharmacology*; D010976:Platelet Count; D011758:Pyrroles / Q000493:pharmacokinetics / Q000494:pharmacology; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors; D019821:Simvastatin / Q000493:pharmacokinetics / Q000494:pharmacology; D013379:Substrate Specificity; D013988:Ticlopidine / Q000031:analogs & derivatives* / Q000493:pharmacokinetics / Q000494:pharmacology
chemical_list: D065692:Cytochrome P-450 CYP3A Inhibitors; D004791:Enzyme Inhibitors; D006538:Heptanoic Acids; D010975:Platelet Aggregation Inhibitors; D011758:Pyrroles; D011994:Recombinant Proteins; D000069059:Atorvastatin; D000077144:Clopidogrel; D019821:Simvastatin; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D017291:Clarithromycin; D013988:Ticlopidine; D007654:Ketoconazole