Pharmacogenetic-Guided Antidepressant Prescribing in Adolescents (PGx-GAP): Study Protocol for a Randomized Controlled Trial

<b>Background</b>: Treating depression and anxiety in adolescents can be challenging due to interindividual variability in medication response. With current trial-and-error prescribing practices, adolescents may undergo multiple medication changes over months or years before an effective and tolerated drug and dose are identified. Pharmacogenomic (PGx) testing can identify interindividual differences in drug metabolism, and evidence supporting PGx-guided prescribing in adults with mental disorders is growing. However, its impact on pediatric psychotropic prescribing remains underexplored. <b>Methods</b>: This is a protocol for a parallel-arm, multicentre, randomized controlled trial. Canadian adolescents aged 12-17 years who are initiating or switching a selective serotonin reuptake inhibitor (SSRI) for depression and/or an anxiety disorder under physician care are eligible. A total of 452 participants will be randomized 1:1 to PGx-guided SSRI prescribing (experimental) or SSRI prescribing based on current practice guidelines (control). Participants, caregivers, prescribing clinicians, outcome assessors, and investigators will be blinded to treatment allocation. Dual primary outcomes are symptom remission at 12 weeks, measured with the Quick Inventory of Depressive Symptomatology-Adolescent (QIDS-A17-SR) and the Screen for Child Anxiety Related Disorders (SCARED). Secondary outcomes, assessed at 4, 8, and 12 weeks, include participant- and physician-rated changes in depressive and anxiety symptoms, role functioning, health-related quality of life, health care utilization, cost-effectiveness, side-effect burden, medication burden, and adherence. Multiple testing will be addressed using the Hochberg method, and a parallel gated analysis will account for non-actionable genotypes. Secondary analysis will estimate minimal clinically important differences for symptom and role-functioning change with PGx-guided therapy. <b>Discussion</b>: At the time of writing, 36 participants have consented and been randomized to an intervention. This trial will evaluate whether PGx-guided prescribing improves symptom remission in adolescents treated with SSRIs. If efficacious, results should be interpreted with existing pediatric pharmacokinetic, observational, and adult trial data to inform PGx use in managing pediatric anxiety and depressive disorders.