Adenoviral Gene Transfer of the Human Inducible Nitric Oxide Synthase Gene Enhances the Radiation Response of Human Colorectal Cancer Associated with Alterations in Tumor Vascularity

Nitric oxide is a potent radiosensitizer of tumors, but its use clinically is limited by serious side effects when administered systemically. We have demonstrated previously that gene transfer of the inducible nitric oxide synthase gene (iNOS) into colorectal cancer cells enhances radiation-induced apoptosis in vitro. The objectives of this study were to further characterize the effects of iNOS gene transfer on the radiosensitivity of human colorectal cancer cells in vitro and tumors grown in athymic nude mice. Adenoviral gene transfer of iNOS (AdiNOS) into human colorectal cancer cell lines (HCT-116 and SNU-1040 cells) significantly enhanced the effects of radiation with sensitizing enhancement ratios (0.1) of 1.65 and 1.6, respectively. The radiation enhancement induced by iNOS was associated with increased iNOS expression and nitric oxide production and prevented by L-NIO, an enzymatic inhibitor of iNOS. AdiNOS treatment of HCT-116 tumors combined with radiation (2 Gy x three fractions) led to a 3.4-fold greater (P < 0.005) tumor growth delay compared with radiation (RT) alone. AdiNOS plus RT also caused significant (P < 0.01) tumor regression with 63% of tumors regressing compared with only 6% of tumors treated with RT. AdiNOS plus RT significantly (P < 0.001) increased the percentage of apoptotic cells (22 ± 4%) compared with either tumors treated with control vector plus RT (9 ± 1%), AdiNOS alone (9 ± 3%), or no treatment (2 ± 1%). These radiosensitizing effects of AdiNOS occurred at low infection efficiency (4% of tumor infected), indicating a significant bystander effect.