**3. Integrins in ovarian cancer treatment**

The current standard of care for ovarian cancer involves surgical debulking followed by treatment with multiple cycles of platinum- and taxane-based chemotherapy [3]. While this treatment regimen is often effective initially, the rapid development of resistance to these drugs is one of the main challenges in the treatment of ovarian cancer [126]. This has led researchers to seek new treatment strategies, such as targeting cell surface receptors that are overexpressed in cancer and tumor endothelial cells [127, 128]. Since research has shown integrins play an important role in vascular development and mediate the adhesion of disseminated cancer cells [28, 129–133], targeting integrins could be a rational treatment approach in ovarian cancer.

One integrin expressed in proliferating vascular endothelial cells, and some tumor cells, is the αvβ3 integrin [134]. In an *in ovo* study from 1994, blocking αvβ3 integrin led to the disruption of angiogenesis on a chick chorioallantoic membrane (CAM) and the regression of human melanoma tumors grown on the CAM through the induction of apoptosis in associated angiogenic vascular cells [135]. More recently, the cancer-promoting role of αvβ3 integrin was demonstrated *in vitro* in a panel of cancer cell lines, including in ovarian cancer cells [136–141]. These efforts led to preclinical *in vivo* studies using the humanized monoclonal antibody, etaracizumab, to inhibit angiogenesis by blocking αvβ3 integrin. The efficacy of etaracizumab in ovarian cancer was explored by Landen *et al.*, who generated orthotopic mouse models of ovarian cancer using three chemotherapy sensitive ovarian cancer cell lines: HeyA8, SKOV3ip1, and A2780ip2 (the "ip" cell lines were generated by injecting parental lines into the peritoneum of a mouse, then harvesting, isolating, and re-culturing the tumor cells) [142]. The authors reported that, after injection of etaracizumab, tumor size was significantly reduced in SKOV3ip1 and HeyA8 models, but not in A2780ip2 models. The underlying reason for this may be poor αvβ3 integrin expression in A2780ip2 cells, which was confirmed after flow cytometry and Western Blot analysis. Interestingly, when etaracizumab was combined with paclitaxel, A2780ip2 tumors were reduced in size by 72.8% compared to paclitaxel alone. These findings suggest that while etaracizumab alone did not reduce the size of A2780ip2 tumors, etaracizumab in combination with paclitaxel led to a synergistic reduction in A2780ip2 tumor size. The same synergism was not observed in HeyA8 tumors even though it was effective as a monotherapy, which the authors suggest may be due to the varied roles of the Akt pathway in the three cell lines. Proliferation in HeyA8 cells is driven, in significant part, by the MEK/ERK pathway and not the Akt pathway, while the other two cell lines have constitutive activation of Akt, potentially explaining the observed discrepancies in the efficacy of tumor reduction.

A follow-up study from the same research group assessed the efficacy of combining etaracizumab with the clinically approved VEGF receptor antibody, bevacizumab [143]. Taxane-sensitive (SKOV3ip1 and HeyA8), and -resistant (SKOV3TRip2) tumors were treated with single-agent therapies or with a cocktail of the two antibodies. Additionally, the individual antibodies, or the cocktail, were tested in combination with paclitaxel. In the SKOV3ip1 model, both individual agents as well as the etaracizumab-bevacizumab cocktail reduced tumor size, with the cocktail proving more effective than single agents alone. Furthermore, paclitaxel efficacy was increased in combination with bevacizumab or the cocktail, but not with etaracizumab, in the SKOV3ip1 model. In SKOV3TRip2 cells, bevacizumab or etaracizumab individually sensitized cells to paclitaxel. In HeyA8 cells, while bevacizumab alone significantly reduced tumor weight, neither etaracizumab alone, nor in combination with bevacizumab or paclitaxel, led to significant tumor size reduction, consistent with the findings reported above. Despite the literature supporting the anti-tumor activity of αvβ3 inhibition, there is also evidence that αvβ3 expression in ovarian cancer cells may inhibit tumor progression and reduce metastasis [144, 145], warranting further investigation into the value of targeting this integrin pair for ovarian cancer treatment.

Another drug that has been evaluated in preclinical and clinical studies for integrin-targeted treatment of ovarian cancer is the humanized α5β1 antibody volociximab. As previously mentioned, α5 and β1 integrins have been implicated in ovarian cancer cell adhesion and migration [28, 146]; however, α5β1 integrin is also associated with endothelial cell proliferation and survival [147, 148]. Kim *et al.* [131] blocked α5β1 integrins in human tumors grown on CAMs and found that that α5β1 regulates angiogenesis through the same pathway as αvβ3 integrin. Blocking α5β1 integrin using volociximab also proved successful in a cynomolgus monkey model of choroidal neovascularization [147], leading to a phase I clinical trial assessing volociximab in 21 patients with pathologically confirmed solid malignancies in 2008 [149]. After demonstrating safety in phase I trials, volociximab was tested in a single-arm, multiinstitutional, phase II study. 14 patients with platinum-resistant, advanced stage epithelial ovarian cancer or primary peritoneal cancer received weekly intravenous volociximab at a dose of 15 mg/kg until progression or unacceptable toxicity. Among the patients whose responses were evaluable, only one patient remained in a stable condition, while the disease of the other 13 patients progressed. Although volociximab did not progress clinically for the treatment of ovarian cancer, the inhibition of neovascularization using the anti-VEGF receptor bevacizumab was approved in 2018 for the treatment of women with advanced (stage III or IV) ovarian cancer in combination with chemotherapy following initial surgical resection [150–152]. Unlike bevacizumab, α1β5 and αvβ3 integrins can block multiple growth factor pathways and cause apoptosis of proliferating endothelial cells, thus targeting angiogenesis from multiple routes. Although this strategy seems promising, integrin inhibitor drugs have not been recognized clinically because of inconsistent results and insufficient clinical activity.

Targeting integrins for selective drug delivery is another strategy of interest in the context of ovarian cancer treatment. The arginine-glycine-aspartic acid (RGD) tripeptide motif is found in many ECM proteins including collagen, fibronectin, and vitronectin. Since this motif is recognized by many integrins, chemotherapy agents can be coupled with RGD to deliver them selectively to ovarian cancer cells that overexpress certain integrins. This was shown by Pilkington-Miksa *et al.* who

*Integrins in Ovarian Cancer: Survival Pathways, Malignant Ascites and Targeted Photochemistry DOI: http://dx.doi.org/10.5772/intechopen.106725*

### **Figure 4.**

*Integrins as therapeutic targets in ovarian cancer. Integrin-targeted drugs, or anti-integrin antibodies, can be directly toxic to tumor cells that overexpress integrins (left) or can inhibit tumor vasculature (right), thereby decreasing tumor size.*

synthesized an αvβ3 integrin-binding RGD-paclitaxel conjugate that was more effective than unconjugated paclitaxel at decreasing tumor volume in a xenograft ovarian cancer model [153]. RGD-modified liposomes containing paclitaxel (RGD-SSL-PXT) have also been synthesized and tested in *in vitro* and *in vivo* ovarian cancer cell models [154]. Zhao *et al.* reported that the intracellular uptake of RGD-SSL-PXTs by SKOV3 cells was more than 6-fold higher, relative to non-targeted liposomes, and the tumor inhibition efficacy of RGD-SSL-PXTs was superior to both paclitaxel and non-targeted liposomes containing paclitaxel. In summary, there are a variety of ways that integrins can be targeted to reduce tumor burden in ovarian cancer. Thus far, as demonstrated in **Figure 4**, researchers have explored integrin inhibition in the context of vascular development as well as the selective delivery of RGD-modified cancer therapeutics; however, further research is needed to fully understand the potential value of targeting integrins in ovarian cancer treatment.
