REGIONAL PRO-DRUG GENE-THERAPY - INTRAVENOUS ADMINISTRATION OF AN ADENOVIRAL VECTOR EXPRESSING THE ESCHERICHIA-COLI CYTOSINE DEAMINASE GENEAND SYSTEMIC ADMINISTRATION OF 5-FLUOROCYTOSINE SUPPRESSES GROWTH OF HEPATIC METASTASIS OF COLON-CARCINOMA

Direct administration of an adenoviral vector expressing the cytosine deaminase gene (AdCMV.CD) to tumors of colon carcinoma cells, with con comitant systemic administration of 5-fluorocytosine (5FC), results in focal production of 5-fluorouracil (5FU) and suppression of tumor gro wth. Based on the demonstration that in vivo adenovirus-mediated gene transfer to intrahepatic tumors is relatively inefficient compared wit h in vivo gene transfer to hepatocytes, we developed a 'regional' prod rug strategy using in vivo Ad-mediated CD gene transfer to normal live r, permitting hepatocytes to convert 5FC into 5FU to treat local metas tasis effectively in a 'trans' fashion, To show that hepatocytes can g enerate and export sufficient 5FU lo achieve this goal, primary rat he patocytes were exposed to AdCMV. CD and 5FC. Evaluation of the superna tants by spectrophotometry and by HPLC demonstrated significant conver sion of 5FC into 5FU. When supernatants of hepatocytes exposed to AdCM V.CD and 5FC were transferred to cultures of CT26 mouse colon carcinom a cells, the CT26 Viability was reduced by 80%. To show that this regi onal AdCMV.CD/5FC prodrug strategy can suppress tumor growth in vivo, a model of metastatic colon carcinoma was established by injecting CT2 6 cells into the left lobe of the liver of syngeneic Balb/c mice. The next day, AdCMV.CD was transferred to hepatocytes by intravenous admin istration, and 5FC treatment was started the following day. Evaluation of tumor growth after 15 days showed marked suppression of tumor grow th in AdCMV.CD- and 5FC-treated animals compared to control groups (P < 0.007). We conclude that primary hepatocytes are capable of converti ng 5FC into 5FU and exporting sufficient amounts of 5FU to the local m ilieu to suppress the growth of liver metastases of colon carcinoma ce lls.