Nicotinamide and Carbogen: Relationship between pO₂ and Radiosensitivity in Three Tumour Lines

Abstract

The effects of carbogen breathing, nicotinamide injection and their combination on tumour radiosensitivity were correlated with changes in tumour O₂ tension to determine the relationship between radiosensitivity and measured pO₂. The radiosensitivity (in vivo—in vitro colony assay) and O₂ tension (computerized pO₂ histograph KIMOC 6650) of two human xenografted tumours (HRT18 and Na11+) and one murine tumour (EMT6) were measured under similar experimental conditions. A single dose of radiation was delivered (8 Gy for HRT18 and Na11+, 12 Gy for EMT6). Carbogen breathing, nicotinamide injection, and their combination all significantly increased radiosensitivity in the three cell lines (p < 0·05); the most efficient treatment was carbogen plus nicotinamide. The radiosensitization was optimum for EMT6 and Na11+. Mean and median pO₂ increased with all three treatments, except for carbogen breathing in EMT6. Carbogen breathing had little effect on the proportion of low pO₂ values, but induced pO₂ values > 30 mmHg in all three tumour lines. Nicotinamide decreased the proportion of low pO₂ values. This effect is larger with the combination carbogen plus nicotinamide. Almost all pO₂ values < 2·5 mmHg were eliminated for HRT18 and EMT6. The relationship between radiosensitization and pO₂ was significant when pO₂ was expressed as the percentage of values below 7, 8, 9 and 10 mmHg for the three lines.