Colorectal cancer (CRC) is one of the most commonly diagnosed cancers worldwide, with over 1.9 million new cases in 2020 and 930,000 deaths. It ranks third in incidence in Europe and America, and the global burden is expected to increase by up to 60% by 2030. Alarmingly, incidence is rising among adults aged 20 to 49, particularly in the 40 to 49 age group.
One of the biggest challenges in CRC management is that the disease is often detected only at an advanced stage, when treatment options are limited and prognosis is poor. Most cases develop through an adenoma-to-carcinoma sequence, where benign polyps slowly transform into malignant tumors. Key mutations in genes such as APC, KRAS, BRAF, TP53, and SMAD4 drive this progression over years or even decades, creating a window for early intervention if the right detection tools exist.
Current CRC screening methods - colonoscopy, stool-based tests, and blood-based liquid biopsies - each have significant limitations. Colonoscopy is the gold standard but is invasive, resource-intensive, and requires full bowel preparation and sedation. Non-invasive stool tests like FIT and the multi-target stool DNA test have incomplete sensitivity, particularly for early-stage disease. Blood-based tests using circulating tumor DNA (ctDNA) and circulating microRNAs show promise but lack standardization for routine clinical use.
This review focuses on a relatively new category of cancer biomarkers: exosomes, tiny vesicles shed by cells into body fluids that carry molecular cargo reflecting the health status of the cells that produced them. Exosomes are generating considerable excitement as potential non-invasive diagnostic tools for colorectal cancer.
Exosomes are tiny membrane-enclosed vesicles, between 30 and 150 nanometers in diameter, that are actively released by cells into the surrounding environment. They belong to a broader family of extracellular vesicles (EVs) that also includes larger microvesicles and apoptotic bodies. What makes exosomes distinctive is their biogenesis pathway and the molecular cargo they carry.
Exosome biogenesis begins with endocytosis, where the cell membrane folds inward to create early endosomes. These mature into multivesicular bodies (MVBs), which contain small intraluminal vesicles (ILVs). The cell then has a choice: fuse MVBs with lysosomes for degradation, or fuse them with the plasma membrane to release the ILVs as exosomes into the extracellular space. The ESCRT (Endosomal Sorting Complex Required for Transport) protein machinery oversees the sorting of molecular cargo into these vesicles.
Exosomes have a characteristic lipid bilayer membrane enriched with cholesterol, sphingomyelin, ceramides, and tetraspanin proteins (CD9, CD63, CD81). This membrane encapsulates a diverse cargo including proteins, RNA molecules (particularly microRNAs and long non-coding RNAs), DNA fragments, and lipids. Critically, this cargo reflects the molecular state of the originating cell - making cancer-derived exosomes distinct from those produced by normal cells.
Exosomes can be isolated from virtually any body fluid including blood, urine, cerebrospinal fluid, saliva, and breast milk. This accessibility is a major advantage: obtaining a blood sample is far less invasive than performing a tissue biopsy, and exosomes can be collected repeatedly over time to monitor disease progression or treatment response - a concept called liquid biopsy.
Cancer cells undergo dramatic changes in lipid metabolism, and these changes are reflected in the lipid composition of the exosomes they release. The exosomal lipid bilayer is particularly rich in phosphatidylserine (PS), sphingomyelin (SM), ceramides (Cer), and phosphatidic acid (PA), which contribute not only to membrane structure but also to signaling and intercellular communication.
Research by Bestard-Escalas et al. found that phosphatidylcholine (PC) levels are consistently elevated in exosomes from patients with various colorectal lesions compared to healthy controls. A particularly striking pattern emerged: lipids containing mono-unsaturated fatty acids (like PC 34:1) were reduced, while lipids with di- or poly-unsaturated fatty acids (like PC 38:4) were elevated in cancerous tissue. Based on this lipid remodeling, the researchers proposed the 34:1/38:4 ratio as a potential diagnostic biomarker, demonstrating specificity of 94.4% and a positive predictive value of 96%.
Research by Elmallah et al. further showed that lipid profiles in exosomes differ between primary CRC and metastatic CRC. Sphingomyelin and PC were increased in all CRC exosomes, but specific species like PE 34:2 and ceramide d18:1/24:1 could potentially distinguish patients with metastasis from those with localized disease - information that is critically important for treatment planning.
Exosomal lipids from colon cancer cells also showed a pro-inflammatory profile with elevated omega-6 fatty acids and arachidonic acid, and reduced omega-3 fatty acids, compared to normal colon cells. This imbalance in the omega-6 to omega-3 ratio may contribute to the chronic inflammatory environment that promotes CRC development. Notably, exosomes from adipose tissue can inhibit ferroptosis (iron-dependent cell death) in CRC cells by delivering the protein MTTP, potentially contributing to chemotherapy resistance.
MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression. They are among the most abundant RNA species in exosomes and are protected from degradation by the exosomal membrane, making them stable in blood samples. Multiple studies have identified specific miRNAs that are elevated in the plasma exosomes of colorectal cancer patients compared to healthy individuals.
Landmark research identified a panel of seven serum exosomal miRNAs - let-7a, miR-1229, miR-1246, miR-150, miR-21, miR-223, and miR-23a - that were significantly elevated in CRC patients, including those with early-stage disease. Importantly, levels of these miRNAs dropped significantly after surgical tumor removal, suggesting they are tumor-derived and could be used to monitor treatment response.
Some exosomal miRNAs appear to reflect specific molecular features of the tumor. miR-1246 was found at higher levels in patients with TP53 mutations in their tumors. This miRNA contributes to macrophage reprogramming in the tumor microenvironment, promoting immune evasion. Similarly, miR-223-3p promotes macrophage polarization toward the M2 phenotype, which suppresses anti-tumor immunity and increases cancer cell proliferation and migration.
The stage-dependence of exosomal miRNA profiles is particularly promising. miR-17-5p and miR-92a-3p levels were associated with CRC stage and grade, suggesting they could help determine how advanced a patient's cancer is. The let-7a/SNAP23 axis has also been identified as potentially relevant to monitoring disease progression, as let-7a affects mitochondrial oxidative phosphorylation processes that CRC cells rely on for energy and growth.
Exosomal proteins offer another rich source of colorectal cancer biomarkers. Research by Palmqvist et al. identified 36 proteins elevated in serum exosomes from CRC patients, including fibronectin 1 (FN1), annexin A1 (ANXA1), galectin-binding protein LGALS3BP, and matrix metalloproteinase 9 (MMP9). These proteins are involved in cancer cell invasion, metastasis, and signaling. In contrast, 22 proteins were reduced, including insulin-like growth factor I (IGF1) and heat shock protein HSP90AA1.
Several exosomal proteins have shown particular promise as surgical response markers. Heat shock protein 60 (Hsp60) was found at elevated levels in the exosomes of CRC patients before tumor removal but not afterward, suggesting it could confirm complete tumor resection. Similarly, glypican-1 (GPC1) and the ribonucleoprotein RPPH1 were elevated before surgery and returned to normal levels after tumor removal.
A bioinformatics study identified six exosomal proteins - NHP2, OLFM4, TOP1, SAMP, transgelin (TAGL), and TRIM28 - that differed in expression between CRC tissue and adjacent normal tissue. The combination of CPNE3 (copine protein III) with CEA, the traditional CRC blood marker, showed better diagnostic performance than either marker alone, pointing toward the value of multi-marker panels.
Exosomal proteins also reflect metastatic status. High levels of IRF-2 (interferon regulatory factor 2) in serum exosomes were found in CRC patients with lymph node metastasis, while RPPH1 overexpression correlated with advanced TNM stages and worse prognosis. These proteins may not only indicate that cancer is present but also reveal how far it has spread, information crucial for treatment planning.
Extracting pure exosomes from biological fluids like blood is technically challenging because exosomes are nanoscale, present in low quantities, and co-exist with other particles and proteins. Several isolation methods are used, each with trade-offs between purity, yield, and practicality for clinical use.
Differential ultracentrifugation is the most common method: blood is spun at progressively higher speeds to pellet different types of particles. Exosomes require very high centrifugal forces (100,000g or more). Density gradient centrifugation provides higher purity but lower yield. Size exclusion chromatography separates particles by size with good purity but less efficiency. Immunoaffinity capture uses antibodies against exosome surface proteins to selectively isolate them - the most specific but most expensive approach. Polymer-based precipitation is simpler but less pure.
Once isolated, exosomes must be characterized to confirm their identity and measure their properties. Key techniques include nanoparticle tracking analysis (NTA), which tracks individual particles moving under Brownian motion to determine size and concentration; electron microscopy for direct visualization; flow cytometry for surface protein profiling; Western blot for specific protein detection; RNA sequencing for transcriptomic profiling; and Raman/SERS spectroscopy for label-free molecular fingerprinting.
The lack of standardized protocols across research groups is a major barrier to clinical implementation. A blood sample processed in one laboratory may yield different exosome populations than the same sample processed in another. The International Society for Extracellular Vesicles (ISEV) MISEV2023 guidelines represent the most comprehensive attempt to standardize exosome research, specifying requirements for isolation, characterization, and reporting to improve reproducibility across studies.
The diagnostic potential of exosomes is being accelerated by new technologies. Microfluidic platforms can process small blood volumes rapidly, automating exosome isolation and analysis in a format compatible with clinical laboratories. These miniaturized systems reduce processing time from hours to minutes, and some can perform both isolation and real-time detection in a single integrated device.
Artificial intelligence (AI) and deep learning are transforming how exosomal data are analyzed. Convolutional neural networks and feature fusion transformers have been applied to Raman spectroscopy and SERS spectra of exosomes, enabling accurate early detection of colorectal cancer from spectral signatures that would be impossible for humans to interpret. Platforms like ChatExosome use large language models to provide clinicians with interpretable, evidence-based diagnostic recommendations from complex exosomal data.
A key clinical milestone is the EXONERATE clinical trial (NCT05972421), which evaluated plasma-derived exosomal protein signatures including EpCAM, CD133, and glypican-1 as predictive biomarkers of response to anti-EGFR therapy in patients with RAS wild-type metastatic CRC. The results showed 82% diagnostic accuracy in distinguishing deep treatment responders from non-responders, outperforming ctDNA-based approaches. This demonstrates that exosomes can guide personalized treatment selection, not just cancer detection.
Exosomes in the tumor microenvironment are not passive carriers but active participants in cancer progression. CRC-derived exosomes can activate cancer-associated fibroblasts, polarize macrophages toward a pro-tumor phenotype, promote new blood vessel formation via VEGF delivery, and even help cancer cells resist chemotherapy. This dual role as both diagnostic markers and functional mediators of disease makes exosomes particularly compelling targets for future therapeutic interventions.
This review establishes that exosomal miRNAs, proteins, and lipids all show meaningful differences between colorectal cancer patients and healthy individuals, with several markers correlating with disease stage, metastatic status, and treatment response. Together, these molecular classes represent a rich and complementary biomarker toolkit for non-invasive CRC diagnostics.
Despite this promise, significant obstacles remain. The heterogeneity of exosome populations, even from the same cell type, leads to inconsistent cargo concentrations. Overlapping content between tumor-derived and normal exosomes complicates the identification of cancer-specific markers. Current isolation techniques vary considerably in yield and purity, and non-vesicular contamination (particularly from abundant plasma proteins) remains a concern that can produce false positive signals.
The standardization challenge is particularly acute for exosomal lipids and proteins compared to nucleic acids. Lipids and proteins are more susceptible to degradation during isolation, are present in low abundance, and are affected by many methodological variables. Nucleic acids benefit from amplification technologies (like PCR) that improve detection even from small amounts, giving them a current advantage in clinical development. Multi-omic approaches combining miRNA, protein, and lipid signatures into AI-analyzed panels are likely to outperform single-biomarker approaches.
The path forward requires large-scale multicenter clinical studies with standardized protocols, validation in diverse patient populations, and development of point-of-care diagnostic platforms that can be used outside of specialized research laboratories. As these challenges are addressed, exosome-based liquid biopsy is poised to become a core component of precision oncology, enabling earlier detection, better staging, and real-time treatment monitoring for colorectal cancer.