Liquid biopsies are widely used for the study of circulating tumor cells and cell-free DNA, but going forward, there will be a very important addition: exosomes. These cargo-carrying extracellular vesicles contain critical information that has shown tremendous promise for diagnostic, prognostic, and even therapeutic use in a range of disease areas.
Until recently, research into exosomes was a fairly niche area. Now, though, the potential for clinical use has more and more scientists exploring the biology and applications of these vesicles. Indeed, a market report from Grand View Research predicts that the worldwide market value for exosome research and clinical use will be $2.28 billion by 2030, powered by an impressive compound annual growth of 18.8%.1
There is tremendous opportunity here. However, realizing the potential will require a technical leap forward: today’s tools have limitations in isolating, detecting, and characterizing exosomes with high accuracy and reproducibility. Most efforts to study exosomes do not take advantage of the rich information contained within exosome subpopulations. These subpopulations could reveal data about processes happening at different areas of the body, such as disease signatures, which would otherwise fade into the background of a global analysis.
To uncover the richness of information provided by exosomes, high-throughput platforms are needed to characterize exosomes at the single-vesicle level with little or no sample preparation required. This kind of analysis is a challenge because many of the tools used to look at exosomes were actually designed to analyze cells, which are a million times larger than exosomes. Consequently, such platforms can characterize only the exosomes that are large enough to give sufficient signal, and scientists who rely on these tools find that their downstream results are confounded by irrelevant data.
Even with the best current technologies, difficulties in isolating and detecting exosomes remain, but the real challenge lies in characterizing these vesicles. No conventional platform is capable of fully interrogating the biology of exosomes—for example, elucidating key biomarkers they carry—which is now a serious limitation in the ability to understand their true biology and function. Moving exosomes into clinical use cannot happen without overcoming this limitation. It is essential that we work together to develop and validate new technology designed specifically for exosome research, rather than continuing to jury-rig conventional tools for imperfect analysis.
How could exosomes be used?
Exosomes serve as a communications system between cells, carrying messages from one cell to another. All types of cells use exosomes for this purpose. Messages conveyed by exosomes can be dictated by their protein or nucleic acid cargo, or by receptor-ligand interaction. These vesicles carry essential information about their originating cells, and they can be found in biofluids such as blood, saliva, and urine. Their easy accessibility is one of the most compelling reasons for developing exosomes as clinical biomarkers.
Because the molecular cargo of each exosome serves as a snapshot of its cell of origin, scientists believe that these vesicles are quite promising for diagnostic and prognostic use in a range of clinical applications. They could be used to elucidate the activity of their host cells, such as the detection of important biomarkers for a host of disease types. Cancer is an obvious example; exosomes that have been released by tumor cells can be isolated from liquid biopsy samples and analyzed to reveal the presence of cancer in a patient—and perhaps even more actionable information, such as which therapies might be most effective or whether there is a high risk for metastasis. Early studies suggest that exosomes could be clinically useful well beyond cancer, into areas such as cardiology, regenerative medicine, and neurodegenerative diseases.
Besides serving as biomarkers, detecting or monitoring the progression of disease, exosomes could, researchers hope, also act as drug-delivery vehicles. Because exosomes are designed to bring molecular cargo from one cell to another, the idea is that they could be temporarily commandeered and loaded with new cargo—that is, a therapeutic. In theory, this use of exosomes could enable highly targeted delivery of drugs to specific types of cells, sparing all the other kinds of cells from damage.
Why is analysis so challenging?
With all of this potential, it’s no wonder that scientists are so eager to study exosomes. But these extracellular vesicles have proven remarkably resistant to characterization with current technologies. Exosomes are usually between 30–200 nm in diameter, a size that makes them all but invisible to most cell-oriented sorting or analysis platforms, including the old standby of flow cytometry.
The most common methods for targeting exosomes to date typically involve purification followed by bulk analysis techniques to query biomarkers. Purifying exosomes is best performed with density gradient ultracentrifugation, but that is a notoriously laborious and low-throughput approach. Purification can also be done with precipitation, size-exclusion chromatography, or ultrafiltration; these methods have the advantage of higher throughput, but unfortunately, they are also known to introduce biases related to co-purification of other macromolecules or biophysical composition. Any analysis data based on these upstream processes would have to go through deconvolution steps to remove the effects of these biases for more accurate results.
For clinical use, any method that requires a significant amount of manual labor or deconvolution processes is less likely to see widespread adoption. Clearly, new technologies are needed to enable the purification, enrichment, and analysis of exosomes—ideally, with an automated workflow that delivers bias-free results and is scalable throughput.
As we develop new technologies for exosome characterization, there will be a related opportunity to establish new standards for reporting research on traits such as exosome diversity, content, and origin. Such standards will be essential for incorporating exosomes into clinical use. It is encouraging to see standards discussions joined by influential groups such as the International Society of Extracellular Vesicles, the American Society for Exosomes and Microvesicles, publishing operations (including the one responsible for the Journal of Extracellular Vesicles), and the EV-TRACK Consortium.
If we can overcome the current challenges in identifying and characterizing exosomes, it would allow us to take a large step forward both in improving liquid biopsies and in developing nanomedicine more broadly. A significant effort from the research and clinical communities is required, but it could have extraordinary implications if it were to enhance our ability to noninvasively diagnose, treat, and monitor disease.
1. Grand View Research. Exosomes Market Size to Reach $2.28 Billion by 2030 | CAGR: 18.8%. January 2018.