title: The antitussive effect of dextromethorphan in relation to CYP2D6 activity.
abstract: OBJECTIVE
  To test the hypothesis that inhibition of cytochrome P450 2D6 (CYP2D6) by quinidine increases the antitussive effect of dextromethorphan (DEX) in an induced cough model.
  
  
  METHODS
  Twenty-two healthy extensive metaboliser phenotypes for CYP2D6 were studied according to a double-blind, randomised cross-over design after administration of: (1) Placebo antitussive preceded at 1 h by placebo inhibitor; (2) 30 mg oral DEX preceded at 1 h by placebo inhibitor (DEX30); (3) 60 mg oral DEX preceded at 1 h by placebo inhibitor (DEX60); (4) 30 mg oral DEX preceded at 1 h by 50 mg oral quinidine sulphate (QDEX30). Cough frequency following inhalation of 10% citric acid was measured at baseline and at intervals up to 12 h. Plasma concentrations of DEX and its metabolites were measured up to 96 h by h.p.l.c.
  
  
  RESULTS
  Inhibition of CYP2D6 by quinidine caused a significant increase in the mean ratio of DEX to dextrorphan (DEX:DOR) plasma AUC(96) (0.04 vs 1.81, P<0.001). The mean (+/-s.d.) decrements in cough frequency below baseline over 12 h (AUEC) were: 8% (11), 17% (14.5), 25% (16.2) and 25% (16.9) for placebo, DEX30, DEX60 and QDEX30 treatments, respectively. Statistically significant differences in antitussive effect were detected for the contrasts between DEX60/placebo (P<0.001; 95% CI of difference +80, +327) and QDEX30/placebo (P<0.001, +88, +336), but not for DEX30/placebo, DEX30/DEX60 or DEX30/QDEX30 (P=0.071, -7, +241; P=0.254, -37, +211; P=0.187, -29, +219, respectively).
  
  
  CONCLUSIONS
  A significant antitussive effect was demonstrated after 60 mg dextromethorphan and 30 mg dextromethorphan preceded by 50 mg quinidine using an induced cough model. However, although the study was powered to detect a 10% difference in cough response, the observed differences for other contrasts were less than 10%, such that it was possible only to imply a dose effect (30 vs 60 mg) in the antitussive activity of DEX and enhancement of this effect by CYP2D6 inhibition.
authors.affiliation: Pulmonary Medicine, Division of Clinical Sciences, University of Sheffield, Northern General Hospital, Sheffield, S5 7AU, UK.
authors.forename: R
authors.initials: R
authors.lastname: Abdul Manap
authors.forename: C E
authors.initials: CE
authors.lastname: Wright
authors.forename: A
authors.initials: A
authors.lastname: Gregory
authors.forename: A
authors.initials: A
authors.lastname: Rostami-Hodjegan
authors.forename: S T
authors.initials: ST
authors.lastname: Meller
authors.forename: G R
authors.initials: GR
authors.lastname: Kelm
authors.forename: M S
authors.initials: MS
authors.lastname: Lennard
authors.forename: G T
authors.initials: GT
authors.lastname: Tucker
authors.forename: A H
authors.initials: AH
authors.lastname: Morice
mesh_terms: D000293:Adolescent; D000328:Adult; D000996:Antitussive Agents / Q000493:pharmacokinetics / Q000627:therapeutic use*; D003371:Cough / Q000188:drug therapy*; D018592:Cross-Over Studies; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism*; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D003915:Dextromethorphan / Q000493:pharmacokinetics / Q000627:therapeutic use*; D004311:Double-Blind Method; D004347:Drug Interactions; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011802:Quinidine / Q000494:pharmacology; D016896:Treatment Outcome
chemical_list: D000996:Antitussive Agents; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D004791:Enzyme Inhibitors; D003915:Dextromethorphan; D019389:Cytochrome P-450 CYP2D6; D011802:Quinidine