What is the clinical significance and potential use of exosomes (extracellular vesicles) in medical diagnosis and treatment?

Exosomes: Clinical Significance and Current Applications

Exosomes are not ready for routine clinical use in diagnosis or treatment due to critical technical limitations, lack of standardization, and absence of definitive clinical guidelines, though they show promise as biomarkers in cardiovascular disease and are undergoing early-phase therapeutic trials in oncology and immunology. 1, 2

Current Clinical Status

Diagnostic Applications

Exosomes function as a "liquid biopsy" containing proteins, lipids, coding and non-coding RNAs, and DNA from their cells of origin, making them attractive for biomarker discovery. 1

Cardiovascular Disease Biomarkers

• The number of procoagulant extracellular vesicles (EVs) is elevated in peripheral blood of patients with acute coronary syndrome (ACS) and chronic ischemic heart disease. 1
• Endothelium-derived microvesicles can discriminate between patients with stable angina, first-time myocardial infarction, and recurring myocardial infarction. 1
• Exosomal miR-208a content correlates with troponin-I levels after cardiac surgery with superior sensitivity compared to whole-blood measurements. 1
• Microvesicles containing miR-126 and miR-199a predict cardiovascular events in patients with stable coronary artery disease, while freely circulating miRNA does not. 1

Metabolic Disease Markers

• Elevated levels of certain miRNAs in exosomes are associated with metabolic syndrome and diabetes, though practical clinical relevance remains unestablished. 1
• EV-associated cystatin C content shows marginal elevation (9% difference) in metabolic syndrome patients with cardiovascular disease. 1

Therapeutic Applications

Established Clinical Trials

The first Phase I exosome trial used autologous dendritic cell-derived exosomes loaded with tumor antigens for non-small cell lung cancer, demonstrating feasibility of large-scale production and safety of administration, though it did not induce detectable effector T cell responses. 1

• A Phase II trial (NCT01159288) showed positive effects on natural killer cells in some cancer patients. 1
• Phase I studies are exploring MSC-derived exosomes for Type 1 diabetes mellitus (NCT02138331). 1
• The first human interventional study (2017) successfully used MSC exosomes to treat steroid-refractory graft-versus-host disease. 3

Cardiovascular Therapeutic Potential

• MSC-derived exosomes demonstrate cardioprotective, pro-angiogenic, anti-inflammatory, and immunomodulatory effects without requiring cell engraftment. 3
• In porcine ischemia-reperfusion models, cardiosphere-derived cell exosomes reduced infarct size when delivered intramyocardially but not intracoronary. 3

Critical Limitations Preventing Clinical Implementation

Technical Barriers

Current technical limitations for EV isolation do not allow definitive guidelines for the use of EVs as biomarkers. 1

The European Society of Cardiology identifies these specific deficiencies: 1

• Lack of standardized pre-analytical and isolation procedures
• Absence of gold standard for processing, characterization, and purity assessment
• Unknown influence of comorbidities, co-medications, and confounding factors
• Lack of disease specificity and tissue-specific markers

Pharmacokinetic Concerns

Unmodified exosomes are rapidly cleared by the reticuloendothelial system in liver and spleen with a half-life of approximately 2-4 minutes after intravenous injection. 1, 3

Safety Concerns

High-dose intravenous administration poses risks, as concentrations exceeding 400 μg caused rapid asphyxiation in murine models. 1, 2, 3

• Surface molecules including phospholipids and proteins determine pharmacokinetics and cellular uptake patterns. 3
• Intramyocardial delivery proves more effective than intracoronary administration for cardiac applications in large animal models. 3

Methodological Requirements

Adequate isolation techniques are of utmost importance if exosomal miRNAs or proteins are to be assessed as biomarkers, as miRNAs are transferred by protein complexes, HDL, or EVs, and protein contamination is difficult to control. 1

The European Society of Cardiology mandates: 1

• Pilot observations from small cohorts must be validated in larger patient datasets
• Reproducibility of isolation procedures and normal range levels in healthy populations must be reported
• Evidence of additional value over current gold-standard biomarkers must be provided

Common Pitfalls and Caveats

Isolation and Characterization

• Methods for EV isolation need improvement before clinical utilization of EV-based diagnostics is possible, despite detectable amounts of relatively clean EVs being isolatable with size exclusion chromatography. 1
• The exosomal proteo-transcriptome may vary from donor cells, creating uncertainty about how accurately exosomes mirror their cellular source. 3

Mechanistic Studies

Mechanistic experiments must include "sham" EV-depleted control groups and dose-response curves to relate mechanism of action to EV concentration. 1

• Because hundreds of different bioactive molecular species exist in any EV preparation, it is difficult to ascribe function to a specific EV component. 1
• Network effects of multiple miRNAs and other mediators should be considered rather than attributing all effects to a single component. 1

Confounding Factors

Comorbidities and medications alter myocardial phenotype and response to ischemia/reperfusion, potentially interfering with diagnostic opportunities using microvesicles. 1

• Patients on statin treatment have significantly lower numbers of circulating microvesicles carrying markers of activated cells at similar plasma cholesterol levels. 1

Alternative Established Therapies

For skin rejuvenation applications specifically: 2

• Autologous platelet concentrates (PRP/PRF) demonstrate favorable outcomes for skin texture, tone, elasticity, and fine lines with good tolerability and established safety profiles
• PRF shows superiority over PRP for canthal smoothness and wrinkles at 3 months
• Three treatment sessions spaced 21 days apart with maintenance every 6 months represent standardized protocols