University of Ioannina, PC 45110, Greece
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Basic Biomedical Sciences

"Liquid Biopsy: exosomes. Their role in the confrontation of cancer"

Abstract

Nowadays, the majority of the biomarkers that are being used, help into monitoring and coming into decision for cancer treatment, based on the analysis of primary region or metastasis. Although, cancer is continiously evolving in molecular level, leading to difficulties as for monitoring of its progression. The solution of this problem is given by the method of liquid biopsy.

Despite of the isolation of circulating tumour cells, the capture of circulating tumour DNA and exosomes, comes to the surface dynamically. During the next decade the use of the previously reported biomarkers will lead to the application of the real time precision medicine. The application of liquid biopsy and more specifically the analysis of exosomes is based on the analysis of subcellular vesicles and their ingredients. Compared with other vesicles, such as apoptotic bodies and microvesicles, exosomes are more homogenic as their size and appearance, thus they are easily detected by electronic microscope. Another advantage of exosomes is that are tend to be found in almost every biological fluids and are steady in the circulation. So that with the right methods they can be isolated and used as diagnostic tools. The expression of their surface markers takes place in that, as they make the exosomes stand out of other vesicles. Last but not least, the content of nucleid acids is important as they show the condition of mutations in the primary cell.

Exosomes seems to be extremely important biomarkers, because of their contents and especially mRNA that induces angiogenesis and metastases. Isolating exosomes from biological fluids provides the possibility of analysis of these exact vesicles and thus the detection of mutations, splice variants and gene fusions, as well as gene expression profiling. In comparison with ctDNA f ragments, of which only two copies are actually present in cancer cell, mRNA that originates from the overexpressed gene could have thousands of copies per cell and be present into circulation in high concentrations. In conclusion, analysis of exosomal mRNA might have advantages, especially in patients with limited amounts of detectable ctDNA.  One of the most important advantages of the analysis of exosomes, against the analysis of CTCs or ctDNA, is that inside those small vesicles are being hidden all the important informations that are needed for a study. According to recent studies inside of exosomes are being detected high amounts of ctDNA.

Although, there are many limitations concerning the transportation of the results in vivo. Most of the studies use cell cultures and the exosomes result from the elaboration of them. This cell culture is homogenic in comparison with exosomes that are being heterogenous, as a consequence of their production from different cell types inside the organism, leading to the misunderstanding of the results. Studies that analyse patients’ serum are extremely rare, because there are not markers that allow the safe identification of tumour-originate exosomes, except melanoma. Nevertheless, are being reported as ideal biomarkers of cancer diagnosis. Also, they could be useful as potential vaccines against cancer or drug transfer system in cancer therapy. Targeting tumour-originate exosomes or inhibiting their release, might lead to an important therapeutic approach.

Many of the newly candidate drugs, such as proteins and nucleid acids, are extremely unstable inside of the organism, thus a challenge is created for a succesful approach. Although, exosomes are imitating psysiological drug transfer systems, allowing the transfer of these biological molecules. Due to their small size and composition, they could avoid phagocytosis or their disruption by macrophages and also they could circulate for a long period. Unlike other nanovesicles systems, such as liposomes, exosomes are capable of the avoidance of lysosomes and tranfer substances into the cytoplasm. One of their most important advantage is their capability to penetrate the blood brain barrier. Their localization and detection in vivo are urgent for their understanding, concerning their impact on target organs. Until now there is not an isolation technique with high clearance. The isolation methods are giving low amount of exosomes and their large production scale for trials and drug approval is too expensive. Its possible in the future exosomes use into practice to demand the creation of hybrid vesicles with possible side effects. For the design of such systems, clinical effectiveness has to be studied in detail, as well safety parameters. Also, methods and combinatorial substances for the active target of molecules must be studied for the exosomes to be operational.

Even if the biology of exosomes is known, they are full of heterogeneous components that could create immunogenic results, based on the nature of donor cell. The role of exosomes in tumour evolution is a challenge. One of many approaches is the design of exosomes imitators, that could avoid any disadvantage, such as adverse immune reactions.    

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