RADIOSENSITIZATION OF MOUSE SKIN BY OXYGEN AND DEPLETION OF GLUTATHIONE

Purpose: To determine the oxygen enhancement ratio (OER) and shape of the oxygen sensitization curve of mouse foot skin, the extent to which glutathione (GSH) depletion radiosensitized skin, and the dependence of such sensitization on the ambient oxygen tension. Methods and Mater ials: The feet of WHT mice were irradiated with single doses of 240 kV p x-rays while mice were exposed to carbogen or gases with oxygen/nitr ogen mixtures containing 8-100% O-2. The anoxic response was obtained by occluding the blood supply to the leg of anesthetized mice with a t ourniquet, surrounding the foot with nitrogen, and allowing the mice t o breathe 10% O-2. Further experiments were performed to assess the ef ficacy of this method to obtain an anoxic response. Radiosensitivity o f skin was assessed using the acute skin-reaction assay. Glutathione l evels were modified using two schedules of DL-buthionine sulphoximine (BSO) and diethylmaleate (DEM), which were considered to produce exten sive and intermediate levels of GSH depletion in the skin of the foot during irradiation. Results: Carbogen caused the greatest radiosensiti zation of skin, with a reproducible enhancement of 2.2 relative to the anoxic response. The OER of 2.2 is lower than other reports for mouse skin. This may indicate that the extremes of oxygenation were not pro duced, although there was no direct evidence for this. When skin radio sensitivity was plotted against the logarithm of the oxygen tension in the ambient gas, a sigmoid curve with a K value of 17-21% O-2 in the ambient gas was obtained. Depletion of GSH caused minimal radiosensiti zation when skin was irradiated under anoxic or well-oxygenated condit ions. Radiosensitization by GSH depletion was maximal at intermediate oxygen tensions of 10-21% O-2 in the ambient gas. Increasing the exten t of GSH depletion led to increasing radiosensitization, with sensitiz ation enhancement ratios of 1.2 and 1.1, respectively, for extensive a nd intermediate levels of GSH depletion. In mice exposed to 100% O-2, a significant component of skin radiosensitivity was due to diffusion of oxygen directly through the skin. Pentobarbitone anesthesia radiose nsitized skin in mice exposed to 100% O-2 by a factor of 1.2, but did not further sensitize skin in mice exposed to carbogen. Conclusions: G lutathione levels and the local oxygen tension at the time of irradiat ion were important determinants of mouse foot skin radiosensitivity. T he extent to which GSH levels altered the radiosensitivity of skin was critically dependent on the local oxygen tension. These results have significant implications for potential clinical application of GSH dep letion.