Optimizing the individual dosing of paroxetine in major depressive disorder with therapeutic drug monitoring.

Introduction: Previous studies have examined the correlation between paroxetine concentrations and therapeutic efficacy in patients diagnosed with major depressive disorder (MDD), but findings have been contradictory.

Aims: This study aimed to investigate the relationships among plasma concentrations, severity of symptoms, and adverse drug reactions (ADRs) to optimize individual dosing.

Methods: Eighty-seven MDD patients, after completing treatment with paroxetine, were divided into low-concentration (LC, n = 38), medium-concentration (MC, n = 27), and high-concentration (HC, n = 22) groups, based on cutoff value concentrations with the 50% response rate and the laboratory alert level from the 2017 consensus guidelines for therapeutic drug monitoring in neuropsychopharmacology. The severity of depression and anxiety was evaluated using a 17-item Hamilton Depression Scale (HAMD-17) and Hamilton Anxiety Scale (HAMA), respectively. Dosage, plasma concentrations, scale scores, and ADRs were recorded across the three groups at different treatment stages to define the therapeutic reference range.

Results: The 4-week plasma concentration of paroxetine (65.00 ng/mL) could predict the clinical response in MDD patients at 8 weeks. Symptom relief in patients with 4-week paroxetine concentrations ranging from 65.00 to 120.00 ng/mL at 8 weeks was greater than in those with concentrations below 65.00 ng/mL, with no significant difference observed above this range. In addition, more cases of liver injury and weight gain were observed in patients with high paroxetine concentrations.

Conclusion: Our results support that early paroxetine concentration may predict clinical efficacy and the incidence of ADRs, thus improving individual dosing regimens for MDD patients.