4.2.1. Molecular Targets Implicated in Cell Growth/Migration

One of the most targeted gene products in OC is EGFR, as it promotes cell growth, apoptosis inhibition, cell invasion, and neovascularization (Table 2). This gene product is also particularly attractive as it represents a targetable surface antigen for an siRNA delivery system [64] (see Section 4.1). The expression of the *Metastasis associated in colon cancer 1* (*MACC1*) is often upregulated in many cancers. Its targeting by short hairpin shRNA in OC cells resulted in the downregulation of cell migration and proliferation [77]. *Metastasis-associated gene 1* (*MTA1*) is another gene product relevant for invasion and metastasis in OC. Notably, its siRNA-mediated down-regulation not only reduced cell migration, it also promoted cell anoikis [78]. This is a form of apoptosis occurring in cells detached from the extracellular matrix. Anoikis resistance is relevant for the survival of OC cells in ascites. The *Wilms tumor gene* (*WT1*) is overexpressed in OC but not in normal ovarian tissue; thus, it is of particular interest in terms of the specificity of siRNA effects (limited to OC cells). *WT1* targeting resulted in OC cell arrest in the G0–G1 phase of the cell cycle and promoted cell apoptosis [79]. Rac1 is a well characterized member of the Rho family that interacts with effector molecules and regulates cytoskeleton organization and membrane trafficking [80]. Rac-1 is implicated in OC cell survival, tumor angiogenesis, resistance to therapeutics, and contribution to extraperitoneal dissemination [81]. Deregulation of *Polo-like kinase 1 (Plk1)* was shown to be responsible for mitotic defects by affecting cell cycle checkpoints, thus resulting in aneuploidy and tumorigenesis [82]. Overexpression of *Plk1* has been observed in many cancerous tissues, including OC [83], and was shown to correlate with tumor stage and grade and poor patient prognosis. *Notch1, Claudin3 (CLDN3), Nin one binding protein (NOB1p)*, and *Cyclooxigenase-2 (COX-2)* (reviewed in [61]), represent other suitable targets related to OC growth and migration.

In addition to the above-discussed siRNA targets, there are others, such as *E2 promoter binding factor 1* (*E2F1*) and *peptidylprolyl cis–trans isomerase, NIMA-interacting 1* (*PIN1*), which we believe might merit the attention of future experimentation. *E2F1* belongs to a family of transcription factors with the ability to activate or repress the expression of genes promoting cell proliferation. *E2F1* in particular mostly triggers the transcription of pro-proliferative genes, thus favoring cell growth. In OC, its level directly correlates to unfavorable disease-free and overall survival, particularly in HGSOC (reviewed in Reference [84]). Moreover, recent evidence connects *E2F1* over-expression with reduced drug sensitivity [85]. Together, these findings make *E2F1* an attractive novel molecular target in OC. However, as *E2F1* also plays a relevant role in non-tumor cells, the use of an OC-targeted delivery system would be desirable. Among *E2F1* transcription targets, there is *PIN1* [86,87]. It accelerates the conversion of cis and trans isomers, thus inducing conformational changes able to regulate the functions of its substrates, such as *p53, p73, p27Kip1, p21waf1*/*cip1*, and *cyclin D1*, all proteins involved in the control of cell proliferation. The net result is the activation of oncogenes and inactivation of tumor suppressor genes in cancer cells. In OC (HGSOC), *PIN1* is overexpressed and when knocked down or chemically inhibited, OC cell death is induced [88,89]. The fact that *E2F1* regulates *PIN1* transcription and that both genes are involved in OC make this molecular circuit an interesting target for novel therapeutic approaches in OC.

#### 4.2.2. *Molecular Targets Implicated in Angiogenesis*

Cancer neo-angiogenesis is necessary to support the increased demand for oxygen and nutrients of the growing tumor cells. Thus, the targeting of one or more of the elements of the angiogenic pathway has the potential to negatively regulate tumor cell growth. In several tumors, including OC, different components of the angiogenic pathway have been targeted with different therapeutic molecules [61]. Among these, the vascular endothelial growth factors (VEGFs) and their tyrosine kinase receptors (VEGFR-1/Ftl-1, VEGFR-2, VEGFR-3/Ftl-4) have often been considered (Table 3). Some studies have also considered the targeting of *plexin domain containing 1 (PLXDC1*), previously known as TEM7, which is overexpressed in endothelial cells of all four tumor types [90]. *PLXDC1* promotes endothelial cell migration and invasion.

#### 4.2.3. Molecular Targets Implicated in Drug Resistance

About 80% of OC patients, who respond to the first-line PTX chemotherapy relapse sooner or later [22]. Thus, the development of drug resistance in OC is a major problem. To try to circumvent this aspect, gene products responsible for multidrug resistance have been considered as possible targets for novel therapeutic approaches (Table 4). *Multidrug resistance gene 1 (MDR1)* is a membrane-bound P-glycoprotein overexpressed in chemo-resistant OC cells [91]. MDR1 favors drug efflux from the cells, thus resulting in a shortening of drug permanence into the cell. This, in turn, leads to the reduced therapeutic effects of the drug. Another gene product related to drug resistance is *survivin (SVV)*. As member of the *Inhibitor of apoptosis protein (IAP)* family [92], *SVV* induces chemotherapy and radiotherapy resistance in OC [93] through the inhibition of apoptosis. As this protein is overexpressed in many tumors compared to their normal tissue counterparts, it represents an attractive target for siRNA tumor-specific effects. Another target able to induce chemo-resistance is the *Focal adhesion kinase (FAK)*. This is a non-receptor tyrosine kinase of which the overexpression is present in 68% of epithelial OCs [94]. Moreover, *FAK* levels significantly correlate with shorter overall patient survival. Not only is *FAK* involved in the regulation of OC cell migration, invasion, adhesion, proliferation, and survival, it has recently been discovered that it promotes PTX resistance in OC as well as in other cancers [94]. Drug resistance is triggered via the promotion of *MDR-1* activity. Finally, *B-cell lymphoma 2 (BCL-2)* is the founding member of the *BCL-2* family of apoptosis regulatory proteins. In particular, *BCL-2* has anti-apoptotic function, and, recently, its involvement in the induction of chemo-resistance in OC has been described. Indeed, its targeting resulted in enhanced cisplatin-induced apoptosis in OC spheroids [95].
