Impact of nicotinamide on human tumour hypoxic fraction measured using the comet assay

Background and purpose: Nicotinamide has been shown to reduce hypoxia in experimental tumours, but there are no data that measure the hypoxic fraction at the time of irradiation in humans. This study investigates whether nicotinamide with radiation can reduce human tumour hypoxia. Materials and methods: Twenty-two patients undergoing palliative radiotherapy for treatment of accessible metastatic tumours were exposed to two doses of radiation (3.5- 8 Gy, median 6 Gy) separated by 1-6 days. Directly on completion of the first dose, two fine needle aspirate biopsies (FNAB) were taken and analyzed for hypoxic fraction using the alkaline comet assay. On the second day of radiation, 13 patients were given 80 mg/kg nicotinamide post-operatively on an empty stomach 2 h before treatment; the remaining nine patients acted as controls. A second comparative pair of aspirates were obtained immediately on completion of the second fraction. Results: Sixteen tumours were suitable for analysis (nine nicotinamide and seven controls). Marked inter-tumour variations in hypoxic fraction were noted (0-67%). Both nicotinamide treated tumours and controls demonstrated a significant increase in the percentage of cells containing heavily damaged DNA following the second dose of radiation (P = 0.01). A significant reduction in mean hypoxic fraction after the second radiation treatment was noted (22 to 13%, P = 0.04). This reduction was predominantly due to the nicotinamide treated group, where mean hypoxic fraction fell from 25 to 11% (P = 0.08) compared to the much smaller change in the radiation only control group, 18 to 15% (P = 0.3). Conclusions: The decrease in hypoxic fraction suggests that nicotinamide can improve tumour hypoxia measured at the time of irradiation. Exposure to the first dose of radiation, or an effect of the first FNAB on microregional tumour blood flow may also contribute.