Exosomes in Toxicology: Emerging Biomarkers and Mechanistic Mediators of Toxicant‐Induced Injury

This focused critical review evaluates exosomes and exosome-enriched extracellular vesicle fractions as mechanistically informative biomarkers and potential mediators of toxicant-induced injury in experimental and regulatory toxicology. Rather than treating exosomes only as diagnostic carriers, this review distinguishes vesicle-associated biomarker signals from vesicle-mediated mechanisms of toxicity and evaluates how reported findings support association, temporal biomarker value, functional transfer, or stronger claims of causality. Evidence from hepatotoxicity, nephrotoxicity, neurotoxicity, and environmental toxicology indicates that vesicle-associated miRNAs, proteins, lipids, and metabolites may reflect key toxicological processes, including oxidative stress, inflammation, mitochondrial dysfunction, immune activation, tissue remodeling, and metabolic disruption. Among the toxicological applications reviewed, hepatotoxicity and nephrotoxicity currently provide the strongest evidence for translational evaluation because vesicle cargo can be linked to organ-enriched injury markers and accessible biofluids. Environmental exposure studies further support the feasibility of extracellular vesicle-associated miRNAs as exposure-responsive biomarkers, although much of this evidence remains associative and requires stronger dose-response and longitudinal validation. This review also emphasizes that altered vesicle cargo does not automatically establish that exosomes actively drive toxicity. Mediator claims require evidence of functional cargo transfer, recipient-cell response, vesicle inhibition or depletion, rescue experiments, or in vivo validation. Methodologically, toxicological interpretation is limited by uncertainty in distinguishing bona fide exosomes from small extracellular vesicles or exosome-enriched fractions, as well as by isolation-dependent co-isolates, inconsistent normalization, and limited cross-study comparability. From a translational perspective, the most realistic near-term applications are early biomarker discovery and longitudinal monitoring. However, broader regulatory implementation will require standardized vesicle definitions, reproducible analytical workflows, dose-response validation, and direct comparison with established toxicological endpoints.