Background: Selective serotonin reuptake inhibitors (SSRIs) exert substantial variability in effectiveness in patients with major depressive disorder (MDD), with up to 50-60% not achieving adequate response. Elucidating pharmacokinetic factors that explain this variability is important to increase treatment effectiveness.
Objectives: To examine potential modification of the relationship between paroxetine serum concentration (PSC) and serotonin transporter (SERT)-occupancy by single nucleotide polymorphisms (SNPs) of the ABCB1 gene, coding for the P-glycoprotein (P-gp) pump, in MDD patients. To investigate the relationship between ABCB1 SNPs and clinical response.
Methods: Patients had MDD and received paroxetine 20 mg/day. We measured PSC after 6 weeks. We quantified SERT-occupancy with SPECT imaging (n = 38) and measured 17-item Hamilton Depression Rating Scale (HDRS17)-scores at baseline and after 6 weeks (n = 81). We genotyped ABCB1 at rs1045642 [3435C>T], rs1128503 [1236C>T], rs2032582 [2677G>T/A] and rs2235040 [2505G>A]. For our primary aim, we modeled mean SERT-occupancy in an Emax nonlinear regression model with PSC and assessed whether the model improved by genetic subgrouping. For our secondary aim, we used multivariate linear regression analysis.
Results: The rs1128503 and rs2032582 SNPs modified the relationship between PSC and SERT-occupancy in both our intention-to-treat and sensitivity analyses at the carriership level. However, we could not detect significant differences in clinical response between any of the genetic subgroups.
Conclusion: Pharmacokinetic influences of the ABCB1 rs1128503 and rs2032582 represent a potentially relevant pharmacogenetic mechanism to consider when evaluating paroxetine efficacy. Future studies are needed to support the role of ABCB1 genotyping for individualizing SSRI pharmacotherapy.