Paroxetine serum concentrations in depressed patients and response to treatment

Introduction: There is no established relationship between the serum concentration of selective serotonin reuptake inhibitors (SSRIs) and clinical response in depressed patients. Methods: We analyzed paroxetine concentrations in serum of 46 depressed patients during treatment with a fixed dosage of 40 mg paroxetine. Results: After 5 weeks 29 patients responded to treatment, while 17 did not. Analysis of variance with repeated measures (ANOVA-rm) revealed a significant effect of "response" with responders having lower serum concentrations throughout the treatment period, when compared to non-responders. After 2, 3, and 4 weeks of treatment, we could define an upper threshold of paroxetine serum concentrations (week 1 : 22.7 ng/mL; week 2:43 ng/mL; week 3:53.4 ng/mL; week 4:39.1 ng/mL) above which response to treatment was unlikely. Conclusion: We conclude that - in contrast to other pharmacological approaches high rather than low drug serum concentrations may be associated with non-response in paroxetine treatment of depressed patients.