Preservation techniques of stem cells extracellular vesicles: a gate for manufacturing of clinical grade therapeutic extracellular vesicles and long-term clinical trials

Extracellular vesicles (EVs) are nanosized vesicles released by different cells and have been separated from most of the body fluids. These vesicles play a central role in cell-to-cell communications as carry a distinct cargo including proteins, RNA species, DNAs, and lipids that are meant to be shipped and exchanged between cells at both systemic and paracrine levels. They serve in regulating normal physiological processes. EVs released from stem cells exert similar therapeutic effect to their originating cells. Clinical application of EVs requires the preparation of sufficient and viable active therapeutic EVs as well as implementing suitable methods for long-term preservation to expedite both their clinical and commercial uses. Cryopreservation is the most common method used to preserve decomposable biomaterials. However, cryopreservation causes cryoinjury to cells which therefore necessitate the use of cryoprotectants. Two types of cryoprotectants exist: penetrating and non-penetrating. In freeze drying, the watery content is sublimed from the product after it is frozen. This drying process is pertinent to thermo-liable substances and those unstable in aqueous solutions for prolonged storage periods. In spray drying technique, the solution containing EVs is firstly atomized, then droplets are rapidly converted into a dry powder using heated gas. Even with the exposure to high temperatures of the drying gas, spray drying is considered suitable for heat-sensitive materials. EVs are considered a promising cell-free therapy, but the lack of proper preservation limits its benefits. Preservation of EVs will initiate a vast amount of clinical trials on different species and different clinical problems.