Extracellular expression of cytosine deaminase results in increased 5-FU production for enhanced enzyme/prodrug therapy.

Background: The cytosine deaminase/5-fluorocytosine (CD/5-FC), strategy for cancer gene therapy shows considerable promise in experimental models but, because CD is a cytosolic enzyme, intracellular production of 5-fluorouracil (5-FU) causes the demise of the transduced cells before cytotoxic concentrations of 5-FU can be achieved within the extracellular milieu. Materials and Methods: A soluble secreted form of CD was constructed and evaluated compared to intracellular CD in vitro and in vivo. Results: The secreted form of CD temporarily spared transduced cells and enhanced accumulation of extracellular 5-FU. Cytosolic CD produced rapid inhibition of thymidylate synthase and cell death before significant extracellular concentrations of 5-FU developed. Finally, tumors expressing the secreted form of CD had an improved response to 5-FC treatment compared to tumors expressing intracellular CD. Conclusion: Further evaluation of extracellular expression of CD for enzyme/prodrug therapy may provide improvements in this commonly studied gene therapy strategy.