Delivering the Gut to the Brain: Drug Delivery Strategies for Microbiota-Derived Therapeutics in Depression

Major depressive disorder remains a leading cause of disability worldwide, and the limited efficacy and delayed onset of conventional antidepressants have intensified interest in the microbiota–gut–brain axis as a source of therapeutic targets. Microbiota-associated candidates—short-chain fatty acids, bile acid and tryptophan metabolites, and neuroactive amines—show mood-relevant activity, yet almost none has reached the clinic. This review reframes that gap as a delivery problem. Rather than treating entry into the central nervous system as a universal requirement, we distinguish strategies intended for local intestinal, peripheral systemic, and direct central action, and we argue that delivery is a major but not exclusive translational bottleneck. We outline the barriers these agents face—upper gastrointestinal loss, poor colonic targeting, rapid metabolite turnover, first-pass exposure, and the blood–brain barrier—and synthesize delivery strategies across two fronts. Colon-targeted systems are technically established but have been validated for non-depression indications, whereas brain-directed approaches—bacterial extracellular vesicles, detoxified membrane-coated carriers, receptor-mediated transcytosis, and intranasal routes—reach the brain mainly in selected preclinical models. We foreground a paradox: microbial extracellular vesicles are at once one of the better-documented bio-derived routes for brain exposure in preclinical studies and prominent drivers of neuroinflammation, which defines a risk–opportunity continuum. We close with a route-specific validation roadmap encompassing quantitative exposure, target engagement, chronic efficacy, and safety.