title: Effect of citalopram on plasma levels of oral theophylline.
abstract: BACKGROUND
  Citalopram and theophylline may be prescribed together to treat patients with depression and asthmatic disease. Because theophylline has a low therapeutic index, small changes in plasma levels may result in therapeutic failure or adverse effects. Both citalopram and theophylline are metabolized by cytochrome P450 (CYP) isozymes. Theophylline is metabolized by CYP1A2; however, the extent to which citalopram interacts with this isozyme in vivo is not known.
  
  
  OBJECTIVE
  This study was conducted to investigate whether citalopram alters plasma levels of oral theophylline.
  
  
  METHODS
  In an open-label, multiple-dose study, healthy nonsmoking volunteers 18 to 45 years of age were administered a single oral dose of theophylline (300 mg) on day 1. Beginning on day 3, citalopram 40 mg was administered daily through day 24 to achieve steady-state plasma levels. On day 23 a single oral dose of theophylline 300 mg was coadministered with citalopram 40 mg. Fasting plasma levels of theophylline were measured on day 1 (in the absence of citalopram) and on day 23 (in the presence of steady-state plasma concentrations of citalopram) periodically for 36 hours.
  
  
  RESULTS
  Thirteen subjects (8 men and 5 women) participated; all completed the study. One subject was not included in the pharmacokinetic calculations. Citalopram treatment had no effect on the pharmacokinetic characteristics of theophylline.
  
  
  CONCLUSIONS
  Citalopram dosing to steady state did not inhibit or induce the metabolism of theophylline in this population of healthy volunteers. Dose adjustment of theophylline thus may not be necessary in patients receiving concurrent therapy with citalopram.
authors.affiliation: H. Lundbeck A/S, Valby-Copenhagen, Denmark. sem@lundbeck.com
authors.forename: S E
authors.initials: SE
authors.lastname: Møller
authors.forename: F
authors.initials: F
authors.lastname: Larsen
authors.forename: M
authors.initials: M
authors.lastname: Pitsiu
authors.forename: P E
authors.initials: PE
authors.lastname: Rolan
mesh_terms: D000284:Administration, Oral; D000293:Adolescent; D000328:Adult; D019540:Area Under Curve; D015283:Citalopram / Q000494:pharmacology*; D005260:Female; D006207:Half-Life; D006801:Humans; D008297:Male; D008875:Middle Aged; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology*; D013806:Theophylline / Q000008:administration & dosage / Q000009:adverse effects / Q000097:blood* / Q000493:pharmacokinetics
chemical_list: D015283:Citalopram; D017367:Selective Serotonin Reuptake Inhibitors; D013806:Theophylline