In vitro and in vivo enhancement of ganciclovir-mediated bystander cytotoxicity with gemcitabine.

To improve bystander cell killing with HSV-TK/GCV, we have utilized dFdCyd to reduce endogenous dGTP, which competes with GCVTP for incorporation into DNA. In this study we demonstrate the ability of dFdCyd to enhance GCV-mediated bystander cytotoxicity in cultured SW620 human colon carcinoma cells as well as in a murine xenograft model. In vitro, dFdCyd reduced cellular dGTP levels and produced a fourfold increase in the GCVTP:dGTP ratio. This elevated GCVTP:dGTP ratio resulted in a twofold increase in GCVMP incorporation into DNA in bystander cells cocultured with HSV-TK-expressing cells. The combination of GCV and dFdCyd was determined to be synergistic by isobologram analysis of bystander cytotoxicity. Tumors in mice treated with GCV and dFdCyd exhibited a significant growth delay requiring 40 days to obtain approximately 10 times their initial size compared to tumors in PBS- or single-drug-treated animals, which grew rapidly, increasing to a similar size in just 19 to 24 days. In addition, complete tumor regression was observed only in animals treated with both drugs. Furthermore, dFdCyd alone or in combination with GCV produced no evidence of toxicity or significant weight loss. These data suggest that dFdCyd may improve the clinical efficacy of HSV-TK/GCV gene therapies.