Beyond enrichment: pharmacogenetic heterogeneity in treatment-resistant depression

Genetic variation has been proposed as a potential contributor to antidepressant nonresponse, but its role in treatment-resistant depression (TRD) remains unclear. This study used pharmacogenetics (PGx) to characterize genetic variation in TRD and determine whether actionable PGx variation and drug-gene interaction (DGI) mismatch were associated with antidepressant nonresponse and TRD burden.

This observational study included 158 individuals with TRD recruited from outpatient clinics in Western Australia. Genotype and genotype-predicted phenotypes for CYP2B6, CYP2C19, and CYP2D6 were derived from commercial PGx testing and compared with ethnicity-matched reference populations from ClinPGx. Antidepressant-specific DGIs were classified as actionable or nonactionable according to Clinical Pharmacogenetics Implementation Consortium guidelines, and unsupervised clustering was used to identify clusters based on these actionability profiles. Analyses were performed to determine if actionable PGx variation, cluster membership, or PGx mismatch was associated with TRD burden (number of failed antidepressant trials).

PGx variation in the TRD cohort was consistent with population expectations, with no evidence of enrichment for actionable PGx variants. Clustering identified six clusters with distinct and gene-specific patterns of PGx variation independent of demographic and clinical characteristics. However, neither PGx mismatch nor cluster membership were associated with TRD burden.

These findings suggest that actionable PGx phenotypes are neither enriched in TRD nor associated with greater TRD severity. Rather, the results indicate that TRD does not represent a single, unified PGx-predicted 'poor pharmacological responder' phenotype but instead reflects a biologically heterogeneous collection of distinct PGx profiles.