Investigation of multiple dose citalopram on the pharmacokinetics and pharmacodynamics of racemic warfarin

Aims An open, controlled, randomized, crossover study was conducted in healthy males to assess the possible occurrence of a pharmacokinetic/pharmacodynamic interaction between warfarin and the selective serotonin re-uptake inhibitor citalopram. Methods Twelve subjects received a single 25 mg dose of racemic warfarin either alone or on Day 15 of a 21-day oral dosing regimen of 40 mg citalopram daily. Blood samples for pharmacokinetic analysis were obtained over a 168 h period after warfarin dosing. The degree of anticoagulation was assessed by the prothrombin time. Results Citalopram produced no change in the pharmacokinetics of (R)- and (S)-warfarin, indicating that citalopram does not alter the metabolism of warfarin mediated via CYP1A2, CYP3A4 and CYP2C9. Citalopram coadministration resulted in a statistically significant increase in the maximum prothrombin time (Rmax; by 1.6±3.0 s) and the area under the prothrombin time-time curve (AUCPT; by 5.0±5.7%). The 90% confidence intervals for Rmax and AUCPT ratios (citalopram+warfarin/warfarin alone) were 1.01–1.10 and 1.03–1.07, respectively. Conclusions The small increase in prothrombin time observed in this study with coadministration of citalopram and warfarin is not considered to be of importance in the clinical setting.