**8. Extracellular vesicles and clinical therapy**

Since EVs reflect the physiological and pathological states of the parent cell, and control the energy production machinery of tumour cells, developing EVs as therapeutic strategy and drug delivery system is a promising clinical therapeutic strategy. In cancer, tumour-derived EVs have been identified in various types of body fluids of cancer patients and reflect the characteristics of the tumour cells [46, 62, 64]. Once internalised, alter the metabolism of recipient cells. Thus, EVs can act as biomarkers in disease prognosis, diagnosis, and treatment. Studies have shown the value of EV-derived proteins and miRNAs as prognostic and diagnostic markers in different types of cancer [44, 57, 64, 68].

Tumour-derived EVs have been shown to play vital roles in the resistance of tumour cells to anti-cancer therapy such as chemotherapy and radiotherapy [58, 62, 69]. This may be due to EVs' ability to mediate the transfer of miRNA, lncRNA and proteins associated with drug resistance to recipient cells. Proteins such as transient receptor potential channel 5 (TrpC5) and annexin-6 as well as miRNAs such as miR-310a and miR-17-92 family are highly expressed or upregulated in EVs released from patients with a poor response to chemotherapy and/ or radiotherapy [40, 56, 57, 60]. Chemotherapy and radiation affect the function of EVs of target cells. Irradiated and drug-treated cells released EVs that confer a drug-resistant phenotype and reduce sensitivity of recipient cells to the chemotherapy/radiotherapy [57, 58].

However, EVs-derived biomolecules are also used as drug targets for cancer treatment. For instance, miRNAs found in EVs promote glycolysis of CAFs and are involved in pre-metastatic niche formation [53, 54, 64]. As a result, miRNA inhibitors have been used to target and reverse this effect. Fas ligand (FasL) found in EVs of activated T-cells also induce cancer metastasis upon interaction between cancer cells and FasL positive EVs [44, 54]. To ameliorate this effect, several studies have focused on using GW4869 to inhibit the secretion and release of EVs from cells with promising results [40, 54]. Thus, GW4869 might serve as a useful therapeutic strategy to inhibit communication different cells within the TME.

The cargo of EVs can also be useful as a drug delivery system in cancer treatment as EVs deliver bioactive molecules through the plasma membrane barriers with low cytotoxicity. In recent years, various molecules such as miRNAs, siRNAs and therapeutic molecules are incorporated into EVs to cross the blood-brain barrier to treat different types of tumours including brain tumours more efficiently [53, 64]. EVs have also been used to deliver chemotherapeutic drugs such as cisplatin and paclitaxel to increase concentration of these drugs in specific cells or organs [40, 53, 64]. Red blood cells-derived EVs have also been used to deliver drugs in liver cancer treatment through a macrophage-dependent manner [53, 54]. However, it is important to explore the process of cargo selection in the formation of EVs to focus the treatment strategy on specific molecules transported by EVs

from tumour cells or other cells within the TME. There are still discrepancies and difficulties surrounding the methods of isolation and purification of EVs from multiple body fluids [39, 42]. Thus, developing standard methods to isolate EVs may provide the gateway to further explore the possibility of targeting bioactive molecules in EVs and using EVs as a delivery system to carry therapeutic drugs to cells within the TME for cancer treatment.
