The current cancer therapeutic modalities have encountered tremendous challenges in treating hypoxic cancer cells due to poor drug delivery for effective chemotherapy or low level of tumor oxygenation for effective radiation therapy. Bacteria can penetrate into hypoxic tumor regions and deliver therapeutic proteins, therefore treating cancer cells that usually escape from conventional cancer therapies. Recently, the attenuated Salmonella typhimurium strain was introduced with >10,000 virulence attenuation and ~1,000 tumor tropism in a variety of animal tumor models. The bacteria can be easily manipulated to introduce therapeutic genes and proteins for cancer treatment. However, in a human melanoma trial, the bacterial strain showed poor tumor tissue attraction, indicating that the bacteria injected i.v. in the cancer patients might be eliminated by human immune response. To resolve this issue, we hypothesize that genetic engineering of the attenuated Salmonella strain with therapeutic protein expressions may improve bacterial infiltration in tumor tissues by overcoming the host immune response. This was tested experimentally in Lewis lung carcinoma (LLC), a murine tumor model that eliminated the parent strain in immunocompetent C57BL/6J mice. Initially, an anti-cancer protein recombinant methioninase was successfully expressed in the Salmonella strain. The attenuated Salmonella typhimurium expressing methioninase not only infiltrated the LLC tumors in C57BL/6J mice but also induced massive tumor tissue destruction. On the other hand, the original Salmonella strain, which was not able to survive in the LLC tumors in the C57BL/6J mice, could successfully infiltrate the LLC tumors implanted in the immunodeficient CB-17 SCID mice. Therefore, the original Salmonella strain was likely eliminated by the host immune response in LLC on C57BL/6J mice. Similar results were observed with Salmonella expressing TNFalpha or a combination of Methioninase and TNFalpha. Tumor cell apoptosis and macrophage activation were revealed by TUNEL and immunochemical staining. Using magnetic resonance diffusion-weighted imaging (DWI) technique, the rapid tumor tissue destruction induced by the bacteria was monitored longitudinally in vivo in a period of two weeks. Importantly, the new bacteria were observed to migrate to distant tumor sites and, therefore, may be used to treat cancer metastasis. In conclusion, the Salmonella strains expressing methioninase and/or TNFalpha have overcome the host immune barrier and infiltrated LLC tumor tissues that usually eliminated the parent salmonella strain in C57BL/6J mice. The rapid tumor tissue destruction can be monitored in vivo by DWI. Thus, attenuated bacteria expressing protein drugs may be designed to overcome human immune barriers for effective human cancer treatment.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA