Engineering Small Extracellular Vesicles for Colon-Targeted Delivery: Microenvironment-Responsive Design, Therapeutic Mechanisms, and Clinical Translation.

Small extracellular vesicles (sEVs) are endogenous lipid-bilayer nanovesicles that combine cargo protection, biological membrane compatibility, and modifiable surface recognition. For colon-targeted delivery, their value depends on engineering them to withstand gastric acidity and proteolysis, negotiate the mucus barrier, and respond to colon-specific cues, including the distal gastrointestinal pH profile, microbiota-derived enzymes, inflammatory reactive oxygen species, and lesion-associated receptors such as integrins, CD44, and folate receptors. This review critically compares sEVs with conventional colon-targeted drug delivery systems and summarizes sEV biogenesis, cargo sorting, isolation, characterization, exogenous and endogenous loading, surface engineering, and passive, active, and stimuli-responsive targeting strategies. Therapeutic applications are evaluated in inflammatory bowel disease and colorectal cancer, with attention to epithelial barrier restoration, immune regulation, macrophage polarization, apoptosis, tumor-microenvironment remodeling, and emerging intestinal indications such as irritable bowel syndrome. Recent clinical trials, Good Manufacturing Practice requirements, potency and release specifications, safety assessment, and regulatory uncertainties are also discussed. A disease- and route-oriented decision framework is proposed to connect cargo properties, administration route, target-cell biology, manufacturing feasibility, and clinically meaningful endpoints. Although engineered sEVs remain investigational and no therapeutic product has achieved full regulatory approval, integration of microfluidic manufacturing, organ-on-chip validation, artificial intelligence-assisted design, and image-guided precision delivery may improve reproducibility and translational readiness.