Gene therapy of metastatic colon carcinoma: Regression of multiple hepaticmetastases by adenoviral expression of bacterial cytosine deaminase

Colon carcinoma accounts for 20% of deaths due to malignancies in the Weste rn world. Once metastases occur, therapeutic options are limited, with an a pproximate 5-year survival of only 5%. To investigate the potential of new gene therapeutic approaches, a hepatic micrometastasis model of colon carci noma in BALB/c mice was established. Inoculation of syngeneic MCA26 colon c arcinoma cells into the spleens of 18- to 20-week-old mice resulted in the formation of multiple hepatic metastases. Selective transduction of develop ing hepatic metastases was demonstrated using a P-galactosidase-expressing recombinant adenovirus. Cytosine deaminase (CD) can metabolize 5-fluorocyto sine into the chemotherapeutic reagent 5-fluorouracil (5FU). The antitumora l potential of this suicide gene therapy approach was explored by systemic application of a recombinant replication-deficient adenovirus encoding for the bacterial CD gene under the control of the cytomegalovirus promoter (Ad .CMV-CD). Injection into the rail vein of tumor-bearing mice resulted in de layed tumor growth with significant reduction in hepatic metastases. The po tential of this experimental approach for possible future clinical applicat ions was evaluated by investigating adenoviral transduction efficiency, 5FU sensitivity, and 5-fluorocytosine-dependent Ad.CMV-CD toxicity in a variet y of human colon cancer cell lines. Although the murine cell lines MCA26 an d CC36 were highly sensitive to 5FU, the human colon cancer cell lines show ed a 1-100 times higher resistance to 5FU. Specific Ad.CMV-CD toxicity corr elates with 5FU toxicity. Transduction efficiency in human colon carcinoma cell lines was shown to be 10-1700 times higher compared with murine cell l ines, thus compensating for 5FU resistance. In conclusion, suicide gene the rapy using CD may be promising as an adjuvant treatment regimen for hepatic micrometastases of human colon carcinoma.