Fractionated radiation therapy in combination with adenoviral delivery of the cytosine deaminase gene and 5-fluorocytosine enhances cytotoxic and antitumor effects in human colorectal and cholangiocarcinoma models

Radiosensitization of human gastrointestinal tumors by 5-fluorouracil (5-FU ) has been studied in vitro and clinically in human cancer therapy trials. The bacterial enzyme cytosine deaminase (CD) converts the nontoxic prodrug 5-fluorocyfosine (5-FC) into 5-FU. Human colon cancer cells stably expressi ng CD have been shown by other investigators to be sensitized to radiation following treatment with 5-FC. We previously used an adenoviral vector unde r control of the cytomegalovirus promoter (AdCMVCD) encoding the CD gene in combination with 5-FC and a single fraction of radiation exposure to enhan ce cytotoxicity to human cholangiocarcinoma cells in vitro and in vivo. The purpose of this study was to determine whether AdCMVCD infection and 5-FC with multiple fraction low-dose radiotherapy results in enhanced cytotoxici ty, in the present study, we utilized AdCMVCD and 5-FC with single fraction radiotherapy to demonstrate enhanced cytotoxicity to WiDr human colon carc inoma cells in vitro. Additionally, we tested this gene therapy/prodrug tre atment strategy employing a fractionated radiation dosing schema in animal models of WiDr colon carcinoma and SK-ChA-1 cholangiocarcinoma. A prolonged WiDr tumor regrowth delay was obtained with AdCMVCD infection in combinati on with systemic delivery of 5-FC and fractionated external beam radiation therapy compared with control animals treated without radiation, without 5- FC, or without AdCMVCD. The results of treatment with AdCMVCD + 5-FC + radi ation therapy to cholangiocarcinoma xenog-rafts were equivalent to those ob tained with systemic 5-FU administration + radiation. Thus, the use of AdCM VCD can be effectively combined with clinically relevant 5-FC and radiation administration schemes to achieve enhanced tumor cell killing and increase d control of established tumors of human gastrointestinal malignancies.