title: In vitro inhibition of the cytochrome P450 (CYP450) system by the antiplatelet drug ticlopidine: potent effect on CYP2C19 and CYP2D6.
abstract: OBJECTIVE
  To examine the potency of ticlopidine (TCL) as an inhibitor of cytochrome P450s (CYP450s) in vitro using human liver microsomes (HLMs) and recombinant human CYP450s.
  
  
  METHODS
  Isoform-specific substrate probes of CYP1A2, 2C19, 2C9, 2D6, 2E1 and 3A4 were incubated in HLMs or recombinant CYPs with or without TCL. Preliminary data were generated to simulate an appropriate range of substrate and inhibitor concentrations to construct Dixon plots. In order to estimate accurately inhibition constants (Ki values) of TCL and determine the type of inhibition, data from experiments with three different HLMs for each isoform were fitted to relevant nonlinear regression enzyme inhibition models by WinNonlin.
  
  
  RESULTS
  TCL was a potent, competitive inhibitor of CYP2C19 (Ki = 1.2 +/- 0.5 microM) and of CYP2D6 (Ki = 3.4 +/- 0.3 microM). These Ki values fell within the therapeutic steady-state plasma concentrations of TCL (1-3 microM). TCL was also a moderate inhibitor of CYP1A2 (Ki = 49 +/- 19 microM) and a weak inhibitor of CYP2C9 (Ki > 75 microM), but its effect on the activities of CYP2E1 (Ki = 584 +/- 48 microM) and CYP3A (> 1000 microM) was marginal.
  
  
  CONCLUSIONS
  TCL appears to be a broad-spectrum inhibitor of the CYP isoforms, but clinically significant adverse drug interactions are most likely with drugs that are substrates of CYP2C19 or CYP2D6.
authors.affiliation: Division of Clinical Pharmacology, Departments of Medicine and Pharmacology, Georgetown University Medical Center, Washington DC 20007, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Ko
authors.forename: Z
authors.initials: Z
authors.lastname: Desta
authors.forename: N V
authors.initials: NV
authors.lastname: Soukhova
authors.forename: T
authors.initials: T
authors.lastname: Tracy
authors.forename: D A
authors.initials: DA
authors.lastname: Flockhart
mesh_terms: D000927:Anticonvulsants / Q000378:metabolism; D000996:Antitussive Agents / Q000378:metabolism; D001189:Aryl Hydrocarbon Hydroxylases; D002753:Chlorzoxazone / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D065731:Cytochrome P-450 CYP2C19; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D003915:Dextromethorphan / Q000378:metabolism; D006801:Humans; D007004:Hypoglycemic Agents / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008617:Mephenytoin / Q000378:metabolism; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors*; D009125:Muscle Relaxants, Central / Q000378:metabolism; D010615:Phenacetin / Q000378:metabolism; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology*; D011994:Recombinant Proteins; D013988:Ticlopidine / Q000494:pharmacology*; D014044:Tolbutamide / Q000378:metabolism
chemical_list: D000927:Anticonvulsants; D000996:Antitussive Agents; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D065607:Cytochrome P-450 Enzyme Inhibitors; D007004:Hypoglycemic Agents; D009125:Muscle Relaxants, Central; D010975:Platelet Aggregation Inhibitors; D011994:Recombinant Proteins; D003915:Dextromethorphan; D014044:Tolbutamide; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D010615:Phenacetin; D002753:Chlorzoxazone; D013988:Ticlopidine; D008617:Mephenytoin