Contributed Commentary by George Daaboul
December 18, 2018 | Exosome research has been a relatively niche field for some years now, but today there is growing enthusiasm among a much broader community about the diagnostic, prognostic, and therapeutic potential of these vesicles. Following on the heels of circulating tumor cells and cell-free DNA, exosomes represent the next great opportunity for liquid biopsy applications.
Financial trends also support the tremendous promise of this area. The global market value for exosome research and clinical use is expected to reach $2.28 billion by 2030, according to a report from Grand View Research, with compound annual growth of 18.8%.
Making the most of this opportunity hinges on having the right technology for isolation, detection, and characterization of exosomes — and unfortunately, the tools used today to interrogate these nanovesicles are not up to the task. Many scientists are using technology designed to sort or analyze cells, but exosomes are up to 1 million times smaller than cells. Too often, detection platforms mistake other particulate for exosomes, resulting in researchers’ inability to understand the true nature of exosome biology and function. In addition, no conventional analytical tool is able to fully characterize exosome populations to link biomarkers to the biological question at hand.
It is imperative that we overcome these technical limitations by dedicating the necessary resources to the development and validation of new technology. Only then will we be able to advance nanomedicine and realize the extraordinary potential of exosomes.
Exosome Uses
All cell types use exosomes as a communication system between cells. Messages to recipient cells are facilitated via exosomes through receptor-ligand interaction or by delivering their protein and nucleic acid content. Circulating exosomes contain a collection of messages from their original host cells. They are found in biofluids including blood, urine, saliva, and other easily accessible samples, making them ideal for clinical use.
An exosome’s molecular content is akin to a fingerprint of its original cell. The potential to characterize this cellular activity via exosomes has been rapidly recognized as a compelling candidate for both diagnostic and prognostic applications. Proteins or nucleic acids encapsulated in exosomes can be queried for biomarkers of various types of disease or other biological information. For example, exosomes shed by cancer cells can be detected in liquid biopsy samples, where they would not only indicate the presence of cancer, but also potentially aid in informing drug response and metastatic risk. Already, scientists believe that exosomes will be useful in cancer, cardiology, neurodegenerative diseases, and regenerative medicine.
Beyond that, many academic and commercial drug development groups are engineering exosomes to be used as drug-delivery vehicles because exosomes naturally circulate in biological fluids and transport molecular cargo from one cell to another. If successful, this approach could allow for careful targeting of therapeutics to particular cell types — such as sending cancer-killing agents directly to tumors, without jeopardizing healthy normal cells nearby.
Technical Hurdles
Despite their appeal, exosomes have proven quite difficult to work with. This is primarily due to their small size. With a diameter typically smaller than 100 nanometers, these nanovesicles are 1 million times smaller than cells by volume. The conventional tools used for cellular analysis, such as flow cytometry, are not able to easily characterize these tiny vesicles.
Scientists currently purify exosomes and then apply bulk analysis techniques to analyze biomarker content. The gold standard purification method is density gradient ultracentrifugation, which is low-throughput and laborious. Other methods that are higher throughput and more accessible — such as size exclusion chromatography, ultra-filtration, and precipitation — introduce biases due to biophysical composition or co-purification of other macromolecules. Therefore, downstream analytical results need to take into account the biases of the purification and enrichment method used to generate the results. When developing clinical applications, the scalability of sample processing methods must be taken into account because changing sample processing during translation might yield different results due to enriching for different populations of exosomes.
Improved technologies are needed to specifically detect and characterize exosomes directly from a sample. This will allow unbiased characterization of biofluids for disease-specific biomarkers that can later be enriched to improve assay performance.
What’s Next
Developing better exosome characterization technologies and standards is an iterative process. As we develop better technologies that will allow more detailed characterization of single vesicles, we will have a better understanding of exosome composition and diversity, which in turn will inform how we should develop standards. Given the dynamic state of the field, many organizations and researchers are working toward standards for reporting exosome research. Groups such as the International Society of Extracellular Vesicles, the American Society for Exosomes and Microvesicles, EV-TRACK, and the Journal of Extracellular Vesicles are all helping to make progress in standardizing exosome research.
Exosomes offer an extraordinary opportunity to improve our understanding of human biology and to change how we diagnose, monitor, and treat disease. In order to fully appreciate the biological value of exosomes, we must come together as a research community and establish the techniques and standards necessary.
George Daaboul, PhD, is Chief Scientific Officer of NanoView Biosciences. He can be reached at gdaaboul@nanoviewbio.com.