VDEPT - AN ENZYME PRODRUG GENE-THERAPY APPROACH FOR THE TREATMENT OF METASTATIC COLORECTAL-CANCER

Colorectal carcinoma (CRC) remains a significant medical challenge wit h an expected 350000 new cases per year. Although the primary cancer c an be successfully controlled by surgical resection, metastatic diseas e to the liver is the most common demise of the CRC patient. New innov ative approaches must be developed for the treatment of CRC hepatic me tastasis if the overall 2- and 5-year survival rates and quality of li fe assessments are to improved. We now describe an innovative gene the rapy approach for the treatment of metastatic CRC, an approach called VDEPT. In this approach, an artificial chimeric gene is created which consists of two components: (1) the transcriptional regulatory sequenc e (TRS) of the human carcinoembryonic antigen gene (CEA); and (2) the protein coding domain of the nonmammlian cytosine deaminase gene (CD). This artificial gene will express CD only in cells which naturally ex press CEA. Expression of CD in CEA-positive cells is, by itself, nonto xic. However, CD can convert the nontoxic prodrug, 5-fluorocytosine (5 -FCyt), to the toxic anabolite, 5-fluorouracil (5-FUra). Hence, the to xic compound, 5-FUra, will be selectively produced in cells which expr ess CD. Since expression of CD is restricted to CEA-positive cells, 5- FUra will be selectively produced in CEA-positive cells. Hence, tumor- specific expression of CD permits the tumor-specific production of 5-F Ura at high concentrations for extended periods of time directly at th e tumor site. The artificial, chimeric gene can be delivered to CEA-po sitive tumors via a replication-defective retroviral vector. Chimeric genes composed of the human CEA promoter and the coding sequence of CD were created and engineered into a retroviral gene delivery vector. T hese chimeric genes selectively expressed CD in CEA-positive cells whi ch resulted in the selective conversion of 5-FCyt to 5-FUra in the CEA -positive tumor cells. Human tumor xenografts demonstrated that expres sion of CD in solid tumors can generate complete cures if only 4% of t he solid tumor cell mass expressed this enzyme. In vivo gene transfer has indicated that retroviral vectors can delivery and express CD chim eric genes in liver tumors at this 4% level.