**5.3 EVs and chemoresistance**

Among the many challenges limiting the treatment of EOC, resistance to standard chemotherapy regimens exists as a frustrating inevitability in most patients

#### *Extracellular Vesicles and Ovarian Cancer DOI: http://dx.doi.org/10.5772/intechopen.101412*

with advanced disease; and EVs seem to play an integral role in this process. As the first-line regimen for EOC, platinum-based chemotherapy is the most effective treatment for EOC; yet 80% of patients with advanced EOC relapse, most within 2 years [3]. Following recurrence of the cancer, most people develop chemoresistance and succumb to the disease. An EOC that is *platinum-resistant* is defined as disease that recurs or progresses within 6 months of completion of the last treatment with a platinum-based regimen. Once a patient's cancer reaches this state, expectations for disease control change, with low response rates to subsequent chemotherapies and a median survival falling below 12 months [3].

While platinum resistance is a complicated, multifactorial process that still needs further elucidation, EVs may help to better understanding this transformation. EOC EVs function as an intercellular communication system. Interestingly and frighteningly, EVs excreted by platinum-resistant tumor cells are capable of inducing resistance in other tumor cells [29]. While this mechanism is not well understood, once the EVs that mediate this process are better defined, they can become targets for possible therapeutic intervention. Furthermore, by understanding the EV content that conveys chemoresistance between cancer cells, scientists can alter the EVs to send information directly to tumor cells that reverses this resistance, allowing first-line treatments to again become effective.

The cytotoxic effect of platinum-based drugs such as cisplatin relies on the uptake of the chemotherapy into cells followed by DNA binding, leading to the formation of DNA crosslinks and breaks that result in apoptosis [3]. In patients that develop platinum resistance, some of their cancer cells exhibit reduced uptake or increased efflux of platinum agents, a process that EVs may facilitate [1, 3]. Transport proteins that have been implicated in this mechanism of drug resistance such as the lysosomal proteins ATPase copper-transporting alpha and beta have been found in EOC EVs, allowing cancer cells to survive against chemotherapy [22]. Is it possible to negate the effect of these EVs through targeted therapies? By disrupting this EV communication system with antibodies or other novel therapies, researchers can provide hope to these patients by overcoming chemoresistance and making their chemotherapy more effective.

#### **5.4 EVs and immunosuppression**

One important technique that allows EOC to proliferate and spread is the ability to suppress the immune system. By further understanding the elaborate underlying mechanisms through which EOC EVs dampen immunity, researchers will be able to block immune escape by the cancer cells, producing new treatments. By preventing immune suppression within the tumorigenic microenvironment, ovarian cells that were previously protected within this nurturing space would be freshly susceptible to immune cells that could find and eliminate the cancer cells [1].

Given the significant promise for novel treatments for EOC that reactivate the suppressed immune system, studies are already underway that target EOC EVs. One therapy utilizes dendritic cells as a map that directs the immune system toward the cancer. In one study these dendritic cells, known for presenting specific foreign antigens to the immune system for identification and targeting, were exposed to EVs isolated from the ascites of EOC patients [1]. The dendritic cells then presented tumor-specific antigens from the cancer EVs to resting T cells that subsequently differentiated and then killed EOC cells [30]. Dendritic cells may be harvested from a patient with EOC, cultured with isolated EOC EVs, and then reintroduced to the patient as an autologous injection that then directs the patient's own T cells to

eradicate the cancer. This concept elegantly demonstrates the potential for unleashing the immune system on cancer cells using EVs.

Another interesting avenue for treating EOC is through the utilization of immunoglobulins that directly target EVs. The serum of patients with EOC is more immunologically reactive when compared to the serum of healthy patients and patients with benign ovarian disease, indicating a robust immune response against the malignancy. As many studies have proven before, the natural immunoreactivity that the human body mounts against EOC is insufficient because the cancer employs tactics to evade the immune system, a process in which EVs play a significant role [22]. While immune evasion is a hallmark characteristic of EOC, the immune system may be mobilized against the cancer by findings ways to target EVs with immunoglobulins. Researchers can develop antibodies that specifically target EOC EVs, tagging them for the immune system so that they can be destroyed, effectively dismantling the vital EV communication system for the cancer cells and limiting the cancer's ability to grow and spread. While this novel use of EVs is exciting, more research is needed to use this method. Mainly, scientists need to better characterize EVs to develop targets for immunoglobulins. Also, it is difficult for antibodies to target the content within EVs because it is protected by the vesicular walls, so proteins on the vesicle wall may provide a unique target for the antibodies. As scientists better understand the unique protein signatures of EOC EVs, immunity-based therapeutics may provide promising new avenues for treating these patients.

## **5.5 EVs and angiogenesis**

Angiogenesis, a vital component of cancer proliferation and progression, has become an important focus in the care of patients with EOC. Ovarian cancers have previously been recognized for their role in promoting angiogenesis; so, by targeting these specific EVs in combination with other antitumor treatments, more effective regimens may be developed for combating this cancer. In the study GOG 218, Burger et al. conducted a clinical trial in which they incorporated a vascular endothelial growth factor (VEGF) inhibitor into the standard primary chemotherapy regimen for advanced EOC [31]. While patients on the VEGF inhibitor experienced a longer period of progression-free survival, they did not live any longer when compared to those who did not receive the treatment. While the inhibition of VEGF, a family of proteins recognized for stimulating the formation of blood vessels, clearly has some effect on tumor growth, other factors appear to be at play that limit the effectiveness of this therapy. One explanation is that EVs play a role in angiogenesis that circumvents the use of VEGF. Ovarian cancer-derived EVs that contain proteins such as CD147, metastasis-associated protein 1, and activating transcription factor 2 appear to have a key effect on angiogenesis that promotes cancer proliferation [32, 33]. A treatment for EOC could include antibodies or some other novel therapy that targets cancer EVs that carry these proteins that stimulate angiogenesis and then eliminate the ability for the cancer to develop its own blood supply.

Another appealing area of active research is the study of common dietary supplements that may have antiangiogenic properties through the production of antiangiogenic EVs. A promising supplement, Amla extract, derived from the Indian Gooseberry tree, has long been suspected to have cancer preventative properties [34]. One recent study tested the supplement on EOC cells and noted increased expression of EV miR-375 which appears to block the proangiogenic proteins SNAIL1 and IGF1R [34]. With a better understanding of the mechanism of this supplement and many

others, scientists may 1 day provide dietary recommendations that can enhance a patient's standard chemotherapy regimen or even derive a novel pharmacologic treatment that blocks blood vessel formation, helping to better destroy EOC cells [4].
