title: Inhibition of CYP2D6 by quinidine and its effects on the metabolism of cilostazol.
abstract: OBJECTIVE
  In vitro results are inconclusive as to whether cilostazol is metabolised by cytochrome P450 isoenzyme 2D6 (CYP2D6). The goals of this study were (1) to assure the dose of quinidine and timing relative to cilostazol used in this study were adequate to cause inhibition of CYP2D6, (2) to evaluate carryover effects of quinidine administration, and (3) to evaluate the effect of CYP2D6 deficiency and administration of quinidine (a CYP2D6 inhibitor) on the pharmacokinetics of a single 100 mg oral dose of cilostazol.
  
  
  METHODS
  This study was conducted as a single-centre, open-label, randomised sequence, 2-period, crossover pharmacokinetic trial. Water alone (treatment without quinidine) or two 200 mg oral doses of quinidine sulfate with water were administered 25 hours and 1 hour prior to a single 100 mg dose of cilostazol in period 1. Study participants were crossed over to opposite treatment in period 2. Metoprolol 25 mg, used as a positive control, was administered 1 hour after quinidine sulfate with water or using water alone to assess the magnitude of CYP2D6 inhibition by quinidine.
  
  
  METHODS
  22 healthy nonsmoking Caucasian (14 male and 8 female) volunteers participated in the study.
  
  
  METHODS
  Serial blood and urine samples were collected at predose and after cilostazol administration to characterise cilostazol and its metabolite pharmacokinetics. Additional plasma samples were taken to assess the pharmacokinetics of quinidine. Urine samples were collected to measure metoprolol and hydroxymetoprolol.
  
  
  RESULTS
  Administration of metoprolol with quinidine caused a significant (p < 0.001) decrease in the urinary 4-hydroxymetoprolol/metoprolol ratio compared with administration of metoprolol alone (42-fold decrease, 0.065 vs 2.707). Hence, quinidine effectively converted extensive metabolisers of CYP2D6 to poor metabolisers of CYP2D6. The 21-day washout period was adequate to have complete recovery from quinidine inhibition of CYP2D6. The analysis of variance demonstrated that the mean maximum plasma concentration (Cmax) for cilostazol, both adjusted and unadjusted for the free fraction, was higher in the control group than in the quinidine group (p = 0.023). However, the time to Cmax (p = 0.669), the area under the plasma concentration-time curve from time zero to infinity (AUC infinity; p = 0.133), and the apparent oral clearance (p = 0.135) were unchanged. The geometric mean ratios (90% confidence interval) comparing with quinidine (test) and without quinidine (reference) coadministration for Cmax and AUC infinity are 0.86 (0.77, 0.95) and 0.92 (0.84, 1.00), respectively. Similar patterns were observed for OPC-13015 and OPC-13213 with regard to Cmax, area under the plasma concentration-time curve from time zero to the last measurable concentration at time t, and AUC infinity (where determinable). The slight decrease in the systemic availability of cilostazol and its metabolites was thought to be a result of the increased gastrointestinal motility secondary to quinidine.
  
  
  CONCLUSIONS
  Administration of quinidine sulfate 200 mg profoundly inhibited CYP2D6-mediated metabolism. The effects of quinidine inhibition of CYP2D6 metabolism were completely reversible during the 21-day washout period. Coadministration of quinidine with cilostazol had no substantial effect on cilostazol or its metabolites (OPC-13015 and OPC-13213). Hence, CYP2D6 does not have a significant contribution in the metabolic elimination of cilostazol.
authors.affiliation: Department of Clinical Pharmacokinetics/Pharmacodynamics & Metabolism, Otsuka America Pharmaceutical, Inc., Rockville, MD 20850, USA. steveb@mocr.oapi.com
authors.forename: S L
authors.initials: SL
authors.lastname: Bramer
authors.forename: A
authors.initials: A
authors.lastname: Suri
mesh_terms: D000284:Administration, Oral; D000319:Adrenergic beta-Antagonists / Q000378:metabolism / Q000652:urine; D000328:Adult; D000704:Analysis of Variance; D019540:Area Under Curve; D000077407:Cilostazol; D018592:Cross-Over Studies; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000493:pharmacokinetics / Q000494:pharmacology*; D005260:Female; D006207:Half-Life; D006801:Humans; D008297:Male; D008657:Metabolic Clearance Rate; D008790:Metoprolol / Q000378:metabolism / Q000652:urine; D008875:Middle Aged; D011802:Quinidine / Q000493:pharmacokinetics / Q000494:pharmacology*; D013777:Tetrazoles / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics; D014665:Vasodilator Agents / Q000008:administration & dosage / Q000378:metabolism* / Q000493:pharmacokinetics
chemical_list: D000319:Adrenergic beta-Antagonists; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D004791:Enzyme Inhibitors; D013777:Tetrazoles; D014665:Vasodilator Agents; D008790:Metoprolol; D011802:Quinidine; D000077407:Cilostazol