Engineering Modifications of Mesenchymal Stem Cell-Derived Extracellular Vesicles to Enhance Circulation, Evade Systemic Clearance, and Improve Brain Targeting Following Intravenous Administration in Experimental Neuroinflammatory Disorders: A Systematic Review

Neuroinflammatory disorders, including ischemic stroke, traumatic brain injury, Alzheimer's disease, Parkinson's disease, multiple sclerosis, spinal cord injury, and diabetic cognitive impairment, are among the leading causes of long-term neurological disability and mortality worldwide. Although these disorders differ in their underlying etiologies, they share common pathological mechanisms characterized by persistent neuroinflammation, activation of resident microglia and infiltrating immune cells, oxidative stress, neuronal apoptosis, and disruption of the blood-brain barrier (BBB). These pathological processes contribute to progressive neuronal damage and functional impairment, highlighting the need for therapeutic strategies capable of effectively modulating the neuroinflammatory microenvironment. Mesenchymal stem cell-derived extracellular vesicles (MSC-EVs), including exosomes and other small extracellular vesicles, have emerged as promising cell-free therapeutic agents because they carry diverse bioactive molecules, such as proteins, lipids, messenger RNAs, microRNAs, and other regulatory cargo, that can suppress inflammation, promote neuroprotection, stimulate angiogenesis, and enhance tissue repair. Compared with whole-cell therapies, MSC-EVs offer several advantages, including lower immunogenicity, reduced risk of tumor formation, easier storage and handling, and improved safety profiles. Consequently, MSC-EVs are increasingly being investigated as potential therapeutics for a wide range of neurological diseases. Despite these advantages, the clinical translation of intravenously administered MSC-EVs remains limited by several biological barriers. After systemic administration, a substantial proportion of EVs are rapidly cleared from the circulation through uptake by macrophages of the mononuclear phagocyte system (MPS), leading to predominant accumulation in the liver, spleen, lungs, and kidneys. Furthermore, the blood-brain barrier significantly restricts the delivery of circulating EVs to injured brain tissue. As a result, only a small fraction of administered EVs reaches the central nervous system, thereby limiting their therapeutic efficacy. To overcome these challenges, numerous engineering strategies have recently been developed to improve the pharmacokinetic properties, biodistribution, and brain-targeting capabilities of MSC-derived EVs. These approaches include genetic engineering of parental MSCs to express targeting proteins; surface functionalization with peptides, antibodies, aptamers, or receptor-binding ligands; chemical modifications such as PEGylation and click chemistry; biomimetic membrane coating using immune or blood cell membranes; incorporation of magnetic nanoparticles for externally guided targeting; and the development of multifunctional hybrid nanovesicles. Collectively, these engineering approaches aim to prolong systemic circulation, evade immune-mediated clearance, enhance BBB penetration, increase accumulation within diseased brain regions, and ultimately improve therapeutic outcomes following intravenous administration. Although an increasing number of experimental studies have investigated engineered MSC-EVs in animal models of neuroinflammatory diseases, the available evidence remains fragmented across different disease models, engineering platforms, targeting strategies, and outcome measures. To date, no comprehensive systematic review has specifically synthesized the evidence regarding engineering modifications designed to optimize the intravenous delivery of MSC-derived extracellular vesicles for neuroinflammatory disorders. The primary objective of this systematic review is to systematically identify, critically appraise, and synthesize preclinical evidence evaluating engineering modifications of MSC-derived extracellular vesicles that enhance systemic circulation, reduce clearance by the mononuclear phagocyte system, improve brain targeting, and increase therapeutic efficacy following intravenous administration in experimental neuroinflammatory disorders. Specifically, this review will examine the various engineering techniques employed, compare their mechanisms of action, evaluate their effects on pharmacokinetics and biodistribution, assess improvements in BBB penetration and brain accumulation, and summarize their impact on neurological, behavioral, histological, and molecular outcomes. Eligible studies will include controlled in vivo animal experiments investigating engineered MSC-derived extracellular vesicles administered intravenously in experimental models of neuroinflammatory disorders. Electronic searches will be conducted in PubMed/MEDLINE, Scopus, Embase, and Web of Science without publication year restrictions. Two independent reviewers will perform study selection, data extraction, and methodological quality assessment using the SYRCLE Risk of Bias tool for animal intervention studies. Data synthesis will primarily consist of a structured narrative review, while quantitative meta-analysis will be considered where sufficient methodological and statistical homogeneity exists among included studies. The review will summarize evidence regarding engineering strategies such as surface ligand modification, genetic engineering, chemical conjugation, biomimetic membrane coating, nanoparticle-assisted targeting, and hybrid extracellular vesicle technologies. Outcomes of interest will include circulation half-life, systemic clearance, organ biodistribution, BBB penetration, brain accumulation, neurobehavioral performance, neuroinflammatory markers, microglial polarization, neuronal survival, apoptosis, oxidative stress, blood-brain barrier integrity, and safety profiles. The expected outcome of this project is a comprehensive synthesis of current preclinical evidence identifying the most promising engineering approaches for enhancing the intravenous delivery of MSC-derived extracellular vesicles to the brain. The review is expected to clarify the relative advantages and limitations of existing engineering technologies, identify methodological gaps and sources of heterogeneity within the literature, and provide evidence-based recommendations for future preclinical and translational research. Ultimately, this work aims to support the development of more effective extracellular vesicle-based therapeutics capable of overcoming biological delivery barriers and improving treatment outcomes for patients with neuroinflammatory disorders.