Pharmacogenetic Implications for Antidepressant Therapy in Major Depression: A Systematic Review Covering 2019-2024

Background/Objectives: Major depressive disorder (MDD), including late-onset forms, is a prevalent and disabling condition. Despite multiple pharmacological treatment options, over half of patients fail to achieve full remission. This systematic review aims to assess current evidence on the influence of pharmacogenetic factors on antidepressant response and safety, with a focus on patients with major and late-life depression. Methods: We conducted a systematic review following PRISMA guidelines (PROSPERO: CRD42020212345). Studies published in the past five years involving adult patients with MDD or late-onset depression and pharmacogenetic data were included. Results: From 793 abstracts screened, 29 studies with 39,975 participants were included. CYP2C19 and CYP2D6 were the most frequently analyzed genes (41% and 17% of studies, respectively). Poor metabolizers for CYP2C19 showed higher plasma levels of SSRIs, leading to increased adverse effects. In contrast, ultrarapid metabolizers had significantly lower response rates. Variants in SLC6A4 and other genes (e.g., HTR2A, ABCB1) were also associated with treatment outcomes. Combinatorial pharmacogenetic testing showed superior predictive value compared to single-gene approaches. Conclusions: Genetic variants in CYP2C19, CYP2D6, and SLC6A4 may affect the efficacy and tolerability of antidepressant therapy. Integrating this information into clinical practice may allow more personalized prescribing and improved outcomes.