Extracellular vesicles (EVs) are membrane-enclosed particles that contain molecular content that is excreted from most cells and they are able to modulate downstream targets following uptake ^[1][2][1,2]. Isolated molecular content within EVs may provide insight into the interior state of a cell [1]. The lipid membrane of EVs protects the molecular content from enzymatic degradation, making these organelles promising potential diagnostic and drug delivery tools ^[3][4][3,4]. The role of EVs in normal and pathophysiological cell interactions is being extensively researched ^{[5][6][7][8][9]}[5,6,7,8,9]. For example, while EVs have been identified as playing a role in normal cell-to-cell communication, when homeostasis is altered in a system (such as an increase in oxidative stress), EV content may change and alter downstream targets ^[2][8][10][2,8,10]. The role of EVs in normal and pathophysiological processes warrants continued research to determine the potential utilization of EVs in translational research, since EVs can be isolated from body fluids, providing biochemical insights into patients. However, the role of EVs and the molecular content of EVs following cancer therapy have not been fully elucidated, in particular, those EVs with increased levels of oxidative damage following cancer therapy.

Advancements in diagnostic methods, screening technology, and cancer treatments have led to a steady decline in the cancer death rate since the 1990s [11]. Specifically, a 2018 review of pediatric oncology patients showed that the death rates for both children (ages 0 to 14) and adolescents (15 to 19) had declined by more than half since 1975 (from 4.9 to 2.0 in children and from 5.9 to 2.9 in adolescents) [11]. Ironically, as cancer therapy becomes more effective, more patients are surviving cancer and living longer, but they are often living with one or more of the unintended consequences caused by therapy. A prominent consequence observed in pediatric cancer survivors is cancer therapy-induced cognitive impairment [12]. While the underlying pathogenesis of cognitive impairment is complex, a growing body of evidence implicates oxidative damage, which results from an imbalance in the reduction-oxidation (redox) regulatory system where the amount of oxidants exceeds the capacity of the antioxidant system to remove the excessive amount of reactive oxygen species (ROS), an important mechanism contributing to damage in the brain microenvironment [13]. In particular, the role of 4-hydroxy-2-nonenal (HNE, a highly reactive end-product of lipid peroxidation) in neurodegenerative disorders is well-established and HNE has been shown to be elevated in a number of different diseases ^{[14][15][16][17][18]}[14,15,16,17,18].

The consequences of cancer therapy on pediatric and adolescent patients, who are still developing neurocognitive capabilities and who have the potential to live long and productive lives, is critically important to address ^[19][20][19,20]. It has been demonstrated that oxidative stress may be a contributing factor to the off-target tissue effects in patients receiving cancer therapy, since at least 50% of current chemotherapies are associated with increased ROS production, and radiation therapy utilizes free radicals to exert its therapeutic effects ^[21][22][21,22]. In addition to the direct impact of oxidative stress in contributing to off-target tissue side effects (particularly, to the brain ^[23][24][23,24]), pro-inflammatory cytokines such as TNF-α have also been demonstrated to contribute to the negative consequences of oxidative stress by increasing ROS production and mitochondrial dysfunction ^[25][26][27][25,26,27].

2. Extracellular Vesicles: Biogenesis and Characterization

EVs are membrane-enclosed particles that contain molecular components specific to their cell of origin and circulate freely throughout the body ^[28][29]. Originally believed to be a mechanism for the cell to dispose of unwanted molecular products that could not be degraded by other methods, EVs are now known to be conduits of cell-to-cell communication. Nucleic acids (e.g., mRNA and miRNA) are often among their contents, and they have the ability to alter the phenotype of their target cell following internalization ^[29][30]. The classification of EVs encompasses a highly heterogeneous group of extracellular particles. The two major groups of EVs are based primarily on their size but also on their biogenesis, contents, and the physiological roles that they play. Even though it remains a challenge to separate the two distinct groups during application ^[30][31], it is important to highlight the two main groups that form the generic term “extracellular vesicles”. Exosomes constitute the first category of EVs. They typically range from 30 to 150 nm in diameter and are derived from the endosomal membrane budding inward, forming intraluminal vesicles (ILVs) during multivesicular endosome (MVE) maturation [1]. These liposomal particles are then secreted from the cell upon the fusion of MVE with the cell surface lipid bilayer. Due to their MVE origin (also known as multivesicular bodies, MVBs), they contain markers that are associated with the endosomal pathway [2]. Exosomes are secreted by various cell types in the body and may play a role in cell-to-cell communication and elimination of undesirable products within the cell, and are part of the normal and pathophysiological processes in the body [2]. Microvesicles are the second category of EVs. These molecules are larger in diameter (100 to 1000 nm) than exosomes and have a different route of biogenesis. Microvesicles are formed by budding of the plasma membrane and may possess markers such as integrins and P-selectin ^[1][2][1,2]. The outward budding of the plasma membrane due to apoptosis that creates an apoptotic body has long been known; however, the study of microvesicles budding from healthy cells is a more recent area of interest. The current understanding of microvesicle biogenesis is that flippases, floppases, scramblases, and calpain rearrange the composition of the phospholipid bilayer, allowing for physical bending of the membrane, and also allowing for microvesicle formation to occur more efficiently [1]. Figure 1 provides a schematic of the differences in biogenesis between the two different types of EVs.