Reduction of tumor hypoxia and inhibition of DNA repair by nicotinamide after irradiation of SCCVII murine tumors and normal tissues.

The alkaline comet assay was used to measure both tumor hypoxic fraction and DNA strand break rejoining kinetics in individual cells from tumors and tissues of C3H/HeN mice exposed to ionizing radiation and nicotinamide. The percentage of hypoxic cells in SCCVII marine squamous cell carcinoma decreased from 18.4 to 4.4% in mice injected with a clinically relevant dose of 200 mg/kg nicotinamide 30 min before irradiation. At higher doses (500 and 800 mg/kg), nicotinamide also increased the half-time of strand break rejoining in tumor, thymus, spleen, bone marrow, and testis from 10-20 min to 40-80 min. Cells from the brain rejoined radiation-induced breaks 3-5 times more slowly than did cells from other tissues and showed no additional delay after nicotinamide. Cells with extensive numbers of strand breaks appeared 24 h after treatment with nicotinamide and radiation and 48 h after treatment with radiation alone. For most tissues, damage was more consistent with necrosis than with apoptosis. The percentage of heavily damaged cells was dependent on tissue type, time after irradiation, radiation dose, nicotinamide dose, and sequence of administration. In SCCVII tumors of air-breathing mice, nicotinamide enhanced radiation-induced cell killing primarily in cells close to the vasculature, but in tumors clamped before irradiation, 500 mg/kg nicotinamide did not increase cell kill. It appears that in addition to promoting tumor reoxygenation, nicotinamide inhibits DNA strand break rejoining in tumors and most normal tissues and promotes the earlier appearance of radiation-damaged cells, perhaps through inhibition of poly(ADP-ribose) polymerase.