**1. Introduction**

Cancer is described as one of the challenging diseases globally, which accounts for 19 million newly diagnosed cases and over 10 million deaths annually making it the leading cause of death [1]. The burden of cancer incidence and mortality is rapidly growing worldwide [1]. Cancer development in humans is a multistep process, which involves various genetic or epigenetic changes and results in the malignant transformation of the normal cells [2]. Recently the use of proteomics, genomics and bioinformatic techniques has unraveled the intricate interplay of numerous cellular genes and regulatory genetic elements that account for the cancerous phenotypes. Higher mortalities associated with cancers are as a result of the absence of very reliable cancer biomarkers, which could be used to diagnose early cancers, predict prognostics and treatment response as well as detection of biomarkers for drug resistance [3]. The unavailability of good biomarkers is a major hindrance for cancer treatment. Cancer biomarkers are not only important for diagnostic purposes but can also be of great prognostic value. With the identification of the right biomarker the cancer progression and effect of chemotherapeutic drugs can be evaluated in great detail [4]. Again, the presence of resistance to therapy, disease relapse, and individual differences continue to reduce the survival chances of cancer patients and makes the disease

impossible to cure [5]. It is predicted that therapeutic response assessment, especially treatment response prediction, is valuable to guide treatment strategy determinations and provide responsive therapy for better survival [6]. The identification of reliable cancer biomarkers in the management of cancers may play a crucial role in reducing cancer-related mortality.

Cancer biomarkers are biological molecules that suggest the presence of cancer in a patient. They are either produced by the cancer cells or by other non-cancer cells in response to cancer [7]. Cancer biomarkers may be used to identify the presence of cancer and also help determine its stage, subtype, and whether they will respond to therapy [8]. Cancer biomarkers identified from serum are the most desirable form of the biomarkers that can be used for regular personalized screening, diagnosis, establishing prognosis, monitoring treatment, and detecting relapse. Cancer biomarkers can be classified into three main categories: prognostic biomarkers, which allow prediction of the disease course and survival chances; predictive biomarkers; to assess if a patient benefits from a certain treatment; and pharmacodynamic biomarkers, which are used in the clinics to guide personalized drug dosing and response assessment. In recent years, a group of biological molecules receiving research attention due to their potential utility as circulating biomarkers for cancer are the extracellular vesicles.

Extracellular vesicles (EVs) are small, lipid-bound particles containing nucleic acid and protein cargo which are excreted from cells under a variety of normal and pathological conditions [9]. Recent studies indicated that cancer-associated EVs play pivotal roles in constructing favorable microenvironments for cancer cells. They are therefore considered as new and promising biomarkers for many cancer types. EVs secreted from a variety of cancer types, including pancreatic cancer, ovarian cancer, prostate cancer, breast cancer, colorectal cancer, glioblastoma multiforme (GBM) are reported to contain cancer-associated protein markers [10]. The EVs play important roles in the regulation of intercellular communication and cell microenvironment homeostasis and again as important biomarkers of various cancers. As EVs are increasingly revealed to play important roles in cancer development and to carry specific information related to cancer state. In cancer research, growing evidence indicates that EVs possess the ability to promote tumor growth, metastasis, and angiogenesis [11] mediate tumor immune responses [12]; and stimulate chemotherapeutic resistance, Identification and modification of cancer cell-derived extracellular vesicles may allow for the development of novel diagnostic, preventive and therapeutic approaches in cancers. This chapter summarizes the functions of EV's in cancers, their potential as biomarkers and therapeutic targets. It further emphasizes the roles of EV's in cancer prognosis, treatment response and drug resistance.
