Oncolytic viruses (OVs) have been engineered or selected for cancer cell-specific infection however, we have found that following intravenous administration of vesicular stomatitis virus (VSV), tumor cell killing rapidly extends far beyond the initial sites of infection. We show here for the first time that VSV directly infects and destroys tumor vasculature in vivo but leaves normal vasculature intact. Three-dimensional (3D) reconstruction of infected tumors revealed that the majority of the tumor mass lacks significant blood flow in contrast to uninfected tumors, which exhibit relatively uniform perfusion. VSV replication in tumor neovasculature and spread within the tumor mass, initiates an inflammatory reaction including a neutrophil-dependent initiation of microclots within tumor blood vessels. Within 6 hours of intravenous administration of VSV and continuing for at least 24 hours, we observed the initiation of blood clots within the tumor vasculature whereas normal vasculature remained clot free. Blocking blood clot formation with thrombin inhibitors prevented tumor vascular collapse. Our results demonstrate that the therapeutic activity of an OV can go far beyond simple infection and lysis of malignant cells.

671 Background: Pexa-Vec is a vaccinia virus engineered to express granulocyte-macrophage colony stimulating factor (GM-CSF), thereby stimulating anti-tumor immunity, direct oncolysis, and tumor vascular disruption. ( Nat Rev Cancer 2009). Pexa-Vec was shown to replicate in metastatic tumors following intratumoral (IT) or intravenous (IV) administration ( Lancet Oncol 2008; Nature 2011). Methods: RCC patients failing at least 1 prior VEGF/R-targeted therapy received five weekly IV Pexa-Vec infusions. Starting at Week 6, patients with disease control or otherwise clinically benefitting from treatment could continue to receive IV infusions every 3 weeks. The primary study objective was radiographic response based on modified Response Evaluation Criteria (RECIST) 1.0. Secondary objectives included disease control rate, progression free survival and safety. Results: All seventeen patients enrolled received the initial 5 weekly Pexa-Vec infusions. Twelve patients received at least one additional infusion (median Pexa-Vec infusions = 8; range 5-12). The treatment regimen was well-tolerated. Transient influenza-like illness (100%), asthenia (47%), anemia (29%) and nausea (29%) were the most common adverse events. All patients were evaluable radiographically at Week 6. The RECIST disease control rate was 76% at Week 6 including 1 complete response. Conclusions: Pexa-Vec was well-tolerated and associated with one complete RECIST response and 76% disease control at Week 6 in patients with advanced RCC. Further trials of Pexa-Vec in RCC patients are warranted.