A THERAPEUTIC BENEFIT FROM COMBINING NORMOBARIC CARBOGEN OR OXYGEN WITH NICOTINAMIDE IN FRACTIONATED X-RAY TREATMENTS

The ability of normobaric oxygen and carbogen (95% O2 + 5% CO2) combined with nicotinamide to enhance the radiosensitivity of two rodent adenocarcinomas and of mouse skin and kidneys, using a 10 fraction radiation schedule, was compared with the effect of radiation in air with and without the drug. Tumour response was assayed using local control and regrowth delay, and compared with acute skin reactions, decreased renal Cr-51-EDTA clearance and reduction in haematocrit. Nicotinamide increased the radiation sensitivity of CaNT tumours under all three different oxygen concentrations tested (21, 95 and 100% oxygen). The effect was statistically significant for oxygen and carbogen but not for air; the combination of nicotinamide with carbogen gave the greatest increase in tumour radiosensitivity. Relative to treatments in air without the drug, the enhancement ratios (ER) at the TCD50 level were 1.17, 1.65 and 1.83 for CaNT tumours irradiated in air, oxygen or carbogen and injected with nicotinamide 1 h before each fraction. The ER in CaRH tumours irradiated in carbogen plus the drug was 1.83, which was greater, but statistically not significantly different, to that seen with carbogen alone (ER = 1.68). In skin, relative to air without the drug, the increase in radiosensitivity by nicotinamide was greater in oxygen and carbogen than in air (1.29, 1.36 and 1.08, respectively). The ERs for both assays of renal damage were similar and lower than those in skin: less-than-or-equal-than 1.07, less-than-or-equal-to 1.13 and less-than-or-equal-to 1.16 for irradiations done in air, oxygen and carbogen plus nicotinamide, relative to air alone. A comparison of these results in the tumours and normal tissues showed that a significant therapeutic benefit was obtained with normobaric oxygen and carbogen combined with nicotinamide. This benefit is greater than observed with other radiosensitizers tested so far. Toxic side effects of the treatment are unlikely in a clinical situation, since prolonged administration of nicotinamide is well tolerated in man. The combination of normobaric carbogen with nicotinamide could be an effective method of enhancing tumour radiosensitivity in clinical radiotherapy where hypoxia limits the outcome of treatment.