title: Effect of metabolic blockade on the psychoactive effects of dextromethorphan.
abstract: OBJECTIVE
  Variation in the activity of cytochrome P450 2D6 (CYP2D6) affects the pharmacokinetics and effectiveness of dextromethorphan (DM), because it controls the production of dextrorphan, an active metabolite, with higher affinity for the NMDA receptor than the parent compound. This study examined whether pharmacological inhibition of CYP2D6 activity with quinidine would mimic the genetic mutation and thus also alter the psychoactive effects of DM.
  
  
  METHODS
  In a single-blind, within-subjects study, eight healthy volunteers (all homozygous for the wild type allele for CYP2D6) received placebo and varying doses of DM, both with and without quinidine pre-treatment. Pharmacokinetic and pharmacodynamic measures were assessed at baseline and every hour post-drug for 6 h.
  
  
  RESULTS
  Compared to the no quinidine condition, quinidine pre-treatment decreased the area under the dose-response curve on subjective measures of positively reinforcing effects (e.g., euphoria, p < 0.04; drug liking, p < 0.05), and was significantly greater for measures of dysphoria (e.g., unpleasantness, p < 0.02). These changes corresponded to increased DM and decreased dextrorphan plasma concentrations.
  
  
  CONCLUSIONS
  Compared to DM alone, quinidine pre-treatment inhibited DM metabolism and changed its subjective effects, demonstrating that the psychoactive properties of DM are a function of drug metabolism. These results demonstrate the relationship between CYP2D6 activity, plasma drug levels, and psychoactive drug effects, and have implications for both the abuse liability and therapeutic utility of DM.
authors.affiliation: Centre for Addiction and Mental Health, Nicotine Dependence Clinic, Toronto, Ontario, Canada. laurie_zawertailo@camh.net
authors.forename: Laurie A
authors.initials: LA
authors.lastname: Zawertailo
authors.forename: Rachel F
authors.initials: RF
authors.lastname: Tyndale
authors.forename: U
authors.initials: U
authors.lastname: Busto
authors.forename: Edward M
authors.initials: EM
authors.lastname: Sellers
mesh_terms: D000293:Adolescent; D000328:Adult; D014150:Antipsychotic Agents / Q000493:pharmacokinetics*; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D003915:Dextromethorphan / Q000008:administration & dosage* / Q000378:metabolism* / Q000493:pharmacokinetics / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D004791:Enzyme Inhibitors / Q000008:administration & dosage; D005260:Female; D006801:Humans; D008297:Male; D008657:Metabolic Clearance Rate / Q000187:drug effects; D008875:Middle Aged; D010147:Pain Measurement; D011802:Quinidine / Q000008:administration & dosage; D016037:Single-Blind Method; D055815:Young Adult
chemical_list: D014150:Antipsychotic Agents; D065690:Cytochrome P-450 CYP2D6 Inhibitors; D004791:Enzyme Inhibitors; D003915:Dextromethorphan; D011802:Quinidine