Skin sensitization by misonidazole: a demonstration of uniform mild hypoxia

Oxygen-dependent protection of radiation lung damage in mice by WR 2721

The modification of early and late radiation damage to the mouse lung by oxygen and WR 2721 has been studied by measurement of breathing rate, lethality, pleural fluid and hydroxyproline content. Protection by hypoxia and sensitization by hyperoxia of early radiation pneumonitis were demonstrated. There was a tendency for the protective effect of WR 2721 to decrease as the breathed oxygen concentration was raised above normal levels. WR 2721 protection of the late damage was higher (PF = 1.6-1.65) than was seen for early pneumonitis (PF = 1.3-1.35) when either breathing rate or lethality were used. Protection factors (PF) gained from measurements of pleural fluid at a year after treatment were similar to those for other endpoints of late damage (PF = 1.7). In contrast, the measurement of fibrosis through determination of lung hydroxyproline at 1 year gave a somewhat lower protection factor for WR 2721. In the same experiments the degree of epilation on the dorsal thorax was scored at 6 weeks. One hundred per cent oxygen gave enhancement (dose enhancement factor (DEF) = 1.2), 9 per cent oxygen reduced damage (DEF less than 0.7) and WR 2721 gave PF values in excess of 1.4 at all oxygen concentrations used. This showed that the radiation response of hair follicles was more sensitive to WR 2721 or to changes of oxygen than the lung. The results presented indicate a competitive interaction between WR 2721 and oxygen for the same injury site causing a shift in the oxygen K curve to higher oxygen concentrations. The validity of applying functional or survival measurements to assess the extent of pulmonary fibrosis is discussed.

The oxygen dependence of protection by aminothiols: implications for normal tissues and solid tumors

This paper reviews the role of oxygen in the protection of both normal tissues and tumors in vivo by WR-2721. Although the presence of hypoxic cells in tumors is well accepted, data are presented that suggest there is a range of oxygen concentrations and thus OERs in normal tissues in vivo, too, and that some may, in fact, be radiobiologically hypoxic. Thus, the range of protection factors in normal tissues (which appears unrelated to tissue drug levels), as well as tumors can perhaps be explained by the range of OERs in these same tissues. The question of whether protection decreases with radiation dose per fraction is related to the distribution of oxygen in both normal tissues and tumors, i.e., whether oxygen is homogeneously or heterogeneously distributed among the cells. It is hypothesized that sulphydryl compounds may equalize the OER differential between tumors and normal tissues and thus remove the natural advantage (radioresistance) of the tumor when treated with radiation. Thus, no loss of therapeutic benefit would occur when sulphydryl compounds were given with radiation if the normal tissue were better oxygenated than the tumor.

The response of mouse skin to hyperthermia combined with fast neutrons or X-rays

The effects of hyperthermia combined with fast neutrons (mean energy approximately 7.5 MeV) or X-rays (250 kVp) were studied in the skin of the mouse ear and foot. Hyperthermia was achieved by immersion in water at temperatures of 41.5-43.0 degrees C for 1 hour. The heat treatments used caused no observable tissue injury other than transient erythema but they enhanced the response to both neutrons and X-rays. The enhancement of neutron damage increased as the heating temperature was increased, as is well known for X-rays. When heat was given after irradiation the thermal enhancement ratio (t.e.r.) for neutrons was similar to that for X-rays. When heat was given before irradiation the neutron t.e.r. was less than that for X-rays. Consequently, the relative biological effectiveness of fast neutrons compared with X-rays was not altered by giving heat after irradiation but it was reduced by giving heat before irradiation.

Modification of the radiation response of the mouse kidney by misonidazole and WR-2721

The radiation response of the mouse kidney has been assayed after a range of X ray doses given with or without the nitroimidazole misonidazole or the aminothiol WR-2721. Sensitization and protection of the kidney were investigated by comparing the X ray dose needed to achieve a particular level of injury in the presence or absence of the drug. Two functional assays and kidney weight at sacrifice were used to obtain dose response curves. Urine output and 51Chromium EDTA excretion were used as functional assays at 25 and 49 weeks after irradiation. They demonstrated no radio-sensitization by misonidazole with 1, 2 or 5 fractions of X rays. Significant radioprotection was seen when 400 mg kg 1 WR-2721 was given before single X ray doses (PF = 1.34). Similar radioprotection was observed when renal weight at 1 year after irradiation was used as the third assay of damage. These results confirm that the kidney responds as a well-oxygenated normal tissue with only a small protection being afforded against radiation injury by WR-2721.

Interaction of misonidazole and WR-2721--II. Modification of tumour radiosensitization

Two types of mouse tumour have been used to study the radiomodifying actions of Misonidazole (MISO) and WR-2721 when used alone and in combination with each other. Single dose studies were performed in both of the tumours and fractionated studies were performed on the anaplastic carcinoma, CA MT. Radioprotection with WR-2721 was seen in both tumours, being most marked at low X-ray doses. The protection was more obvious and the sensitization by MISO less in the fractionated experiment. The combination of MISO and WR-2721 gave an intermediate response compared with either drug used alone, resulting in some sensitization with single doses and an overall protection with repeated small doses. An interactive toxicity of the 2 drugs was also observed, suggesting an additive effect when assessed in terms of lethality. These studies indicate that the effects of both MISO and WR-2721 are dependent upon the oxygen status of the cells in the tumour, and that MISO can act in an oxygen-mimetic manner to modify the radioprotection observed with WR-2721.