TARGETING THE RhoGEF BETA-PIX TO ENHANCE THE ACTIVITY OF BEVACIZUMAB IN GLIOBLASTOMA: A NANOPARTICLE MEDIATED GENE SILENCING APPROACH

Glioblastoma (GBM), a highly invasive brain malignancy, remains an incurable disease. Angiogenesis, the formation of new vasculature, is a defining feature of this disease. Targeting GBM angiogenesis with Bevacizumab (Bev) is associated with improved progression free survival, but may also enhance tumour invasion into the surrounding parenchyma (Norden et al., 2008) and is not curative. Rho GTPases and their activators, guanine nucleotide exchange factors (GEFs), play central roles in the invasive process (4). Herein, we sought to identify and target GEFs of importance in mediating GBM invasion with a view to improving Bev response. We report a novel mechanism by which GBM tumours invade and proliferate via overexpression of the GEF beta-PIX gene which was shown to be increased at the invasive edge in 74% of GBM tumours assessed (n=19), compared with tumour core (Hoelzinger et al., 2005). We have further demonstrated that siRNA-mediated knockdown of beta-PIX in GBM patient-derived xenograft cell cultures and cell lines resulted in decreased cell invasion in 3D, cell proliferation and survival assays in vitro. An in vivo pilot study whereby beta-Pix knockdown was achieved using commercially available alphaV-beta3 integrin targeting nanoparticles (InVivoPlex Aparna Bio Corp), suggested that treatment with beta-PIX siRNA nanoparticles in combination with Bev could improve survival compared with Bev- alone in tumour-bearing animals. To further develop this strategy, we have recently designed and characterized a proprietary novel biodegradable, RGD-targeting and cholesterol-stabilized polyplex system for siRNA delivery in GBM. This novel nanoparticle system supports efficient gene silencing, and demonstrates a low toxicity profile in vitro and in vivo. We are currently performing advanced pre-clinical efficacy studies employing a clinically relevant GBM rodent resection model (Sweeney et al., 2014), to determine if nanoparticle mediated beta-PIX gene silencing will improve survival outcomes when combined with Bevacizumb and delivered in the adjuvant setting.