The gut microbiome and drug-resistant epilepsy: Microbiome-antiseizure medication interactions and implications for pharmacoresistance.

Drug-resistant epilepsy (DRE) affects approximately one-third of patients with epilepsy and represents a major unmet clinical need. While traditional hypotheses of pharmacoresistance have focused on alterations in drug targets, efflux transporter overexpression, and intrinsic disease severity, the gut microbiome has recently emerged as a potentially modifiable factor that may function as a systems-level modifier of these established mechanisms rather than a standalone pathway. The gut microbiome harbors a vast repertoire of drug-metabolizing enzymes capable of directly biotransforming orally administered antiseizure medications (ASMs)-including valproic acid, lamotrigine, carbamazepine, and oxcarbazepine-thereby altering their pharmacokinetics, bioavailability, and therapeutic efficacy. Additionally, microbial metabolites modulate host cytochrome P450 enzymes, nuclear receptors, and efflux transporters such as P-glycoprotein, while bacterial β-glucuronidases influence the enterohepatic recirculation of glucuronidated ASMs. Conversely, chronic ASM exposure reshapes the gut microbial ecosystem, creating a self-perpetuating cycle of dysbiosis and pharmacoresistance. This narrative review synthesizes current evidence on microbiome-ASM interactions in DRE, proposes a concrete experimental pipeline for characterizing ASM-specific microbial biotransformation, and outlines a framework for integrating physiologically based pharmacokinetic modeling with microbiome data. We discuss clinical implications for epileptologists-including the role of therapeutic drug monitoring in detecting microbiome-mediated pharmacokinetic variability, the concept of microbiome-neutral ASM selection, and earlier deployment of the ketogenic diet as a microbiome-targeted intervention. We highlight the translational potential of pharmacomicrobiomics-the study of how microbiome variation influences drug disposition and response-and identify critical knowledge gaps that warrant future investigation. PLAIN LANGUAGE SUMMARY: About one in three people with epilepsy continue to have seizures despite treatment. This review summarizes growing evidence that the gut microbiome-the community of bacteria living in the intestines-can influence how seizure medications work by altering their absorption, metabolism, and clearance. The medications themselves can reshape the microbiome in return, creating a cycle that may sustain treatment failure. Understanding this gut-drug relationship may open new paths to personalized epilepsy care through diet, probiotics, and microbiome-guided prescribing.