This is a dose-seeking study that will test the safety of increasing doses of Rexin-G, given intravenously, in patients with advanced or metastatic pancreatic cancer who have failed standard chemotherapy. Rexin-G is a tumor-targeted gene therapy vector that contains a "killer" gene that blocks the action of the human cyclin G1 gene. Cyclin G1 is a cell cycle control element that plays an important role in cancer growth. When injected into a vein, the Rexin-GTM vector seeks out and accumulates in cancerous tumors, therefore, increasing the concentration of the drug in the cancerous tumors and not in normal neighbouring organs.