Metastatic colon carcinoma is second only to lung cancer as a cause of death from malignancy in the United States, accounting for 60,000 fatalities a year. Eighty percent of the patients who died of colon cancer have metastases in the liver, and half of them have only liver metastases at the time of death. Current treatments include surgery and chemotherapy, but the mean survival time of the patients is only 37 months. We propose to explore the potential of developing a new treatment modality for metastatic colon carcinoma by somatic gene therapy, using a strategy of combing """"""""suicide gene"""""""" with """"""""cancer vaccine"""""""" approaches. It is hypothesized that there are two principles in tumor rejection in this model: (1) tumor cells killed in vivo by the suicide gene will undergo phagocytosis by antigen presenting cells which process and present the tumor antigen(s) to the T cells; (2) this process, as well as the subsequent T-cell response, will be enhanced in the presence of local cytokine expression. The suicide gene of choice is that of Herpes Simplex Virus thymidine kinase (HSV-tk), which confers gancilovir sensitivity to dividing cells. A metastatic colon carcinoma model was established by implantation of MCA-26 cells in the livers of syngeneic mice, and in vivo administration of recombinant adenoviral vectors containing the HSV-tk and mouse interleukin 2 (mIL2) genes was indeed more efficacious than using the tk or mIL2 vector alone in causing hepatic tumor regression. Furthermore, mice treated with the tk + mIL2 vectors also developed a strong systemic cellular immune response that provided complete protection against a distal site challenge of parental tumor cells. While the preliminary results are very encouraging, the cellular immunity waned with time and the treated animals eventually succumbed to hepatic tumor relapse or distal organ metastasis in survival studies. Two strategies will be investigated to achieve long term cure in this metastatic model; (1) to test the hypothesis that an enhanced and more effective cellular immunity induced by additional cytokines may abolish all viable tumor cells in the treated animals, several recombinant adenoviral vectors containing additional cytokine genes known to induce cellular immunity against colon carcinoma will be constructed and tested in combination with the HSV-tk vector and/or the mIL2 vector; (2) to test the hypothesis that the maintenance of long term anti-tumoral immunity may require prolonged cytokine gene expression and presentation of tumor antigen(s) to the host immune system, irradiated tumor cells expressing various cytokines will be used as vaccines on the treated animals. If successful, the results may be used as a scientific foundation for future gene therapy of metastatic colon carcinoma in man.
