Gut microbial metabolism of immunosuppressive drugs: from metagenomic associations to functional enzyme biomarkers

PURPOSE OF REVIEW: Inter-individual variability in immunosuppressant exposure remains a major challenge in transplantation. Pharmacogenomics and host-related factors contribute to this variability, with increasing evidence suggesting that the gut microbiome is also an important determinant of drug metabolism. This review summarizes recent advances in microbiome-mediated metabolism of immunosuppressive drugs like tacrolimus and mycophenolate and highlights emerging functional approaches to identify clinically relevant microbial enzymes. RECENT FINDINGS: Microbial β-glucuronidases contribute to the enterohepatic recirculation of mycophenolic acid by deconjugating mycophenolic acid glucuronide, influencing systemic exposure and gastrointestinal toxicity. In parallel, gut bacteria such as Faecalibacterium prausnitzii can directly metabolize tacrolimus into less active metabolites, potentially contributing to variability in drug exposure and dose requirements. Recent studies further demonstrate that metaproteomic and enzyme activity-based approaches provide greater functional resolution than metagenomics alone for identifying microbiome-associated drug metabolism pathways. SUMMARY: Current evidence supports a significant role for the gut microbiome in immunosuppressant pharmacokinetics and functional microbial enzymes represent promising biomarkers of immunosuppressant disposition. Moving beyond gene-level associations toward the functional characterization of microbial enzymes through the integration of metaproteomics, metabolomics, and enzyme activity assays with pharmacokinetic modeling may facilitate development of microbiome-based biomarkers and microbiome-guided precision dosing strategies in transplantation. Future integration of microbiome-derived functional data into therapeutic drug monitoring and pharmacokinetic models may improve individualized immunosuppressive therapy and transplant outcomes.