Effects of radiomodifiers on high and low LET responses in vivo

The radioprotector WR-2721 and the radiosensitizer misonidazole have been studied for their influence on the response of mouse skin and tumours when irradiated with either single or fractionated doses of 240 kV X-rays or 3 MeV neutrons. The modification of radiosensitivity to neutrons was significantly less than that for X-rays, both for tumours and skin. When the two drugs were given in combination before single neutron doses, competitive interaction was observed.

Radioprotection of two mouse tumors by WR-2721 in single and fractionated treatments

The radioprotective effect of WR-2721 has been studied in two murine tumors using single doses or five daily fractions. Single dose irradiations of the SA FA resulted in a highly variable radio-protective response. In one experiment large protection factors (1.2-2.5) were obtained, with the greatest protection at low X ray doses. In later experiments with the same tumor, there was little or no radioprotection. The Ca MT was significantly protected against single dose irradiation with both 250 mg/kg and 400 mg/kg of the drug. In a five fraction schedule the extent of radioprotection for CA MT was greater than with single X ray doses for the same drug dose per fraction. Tumor protection factors from the present work and from the literature are compared with published protection factors for normal tissues. Significant tumor radioprotection is seen in most studies. The data indicate more variability in the extent of tumor protection for a given drug dose than is seen in normal tissues. Tumor protection is often greatest at low X ray doses which may be a result of preferential protection of the better-oxygenated tumor cells.

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.

Interaction of radiosensitizers and WR-2721. I. Modification of skin radioprotection

We have studied the radiomodifying action in mouse skin of WR-2721 and misonidazole (MISO) when used alone or in combination. The radioprotection with WR-2721 was drug-dose dependent and highly influenced by the O2 concentration at the time of irradiation. Significant sensitizaton was observed with MISO, especially in air-breathing mice. The combination of WR-2721 and MISO produced a radiation response intermediate between the resistant and sensitive responses to either drug alone. The precise degree of sensitivity was dependent on the relative doses of protector and sensitizer. We have also studied the interaction of both drugs in terms of drug-induced lethality, which showed a clear toxic interaction. The WR-2721 LD50 was reduced by a factor of 1.4 with only 200 mg/kg of MISO. We conclude that the combination of WR-2721 and MISO shows an interaction in terms of drug toxicity and radiation response, such that the radioprotection of skin is reduced or even abolished with low doses of MISO.

Experimental radiotherapy with WR-2721 and misonidazole

The data from a large series of experiments on mouse skin and tumors are summarized. Radioprotection with WR-2721 has been observed in both tumors and normal tissues. The protection factors are generally, but not always, higher for skin than for tumors. The protection observed in skin is greater for mice irradiated breathing air than for those irradiated in oxygen. It is postulated that the different protection factors observed in different normal tissues and tumors may reflect differences in tissue oxygenation levels. The combination of misonidazole and WR-2721 has been studied in terms of the modification of radiosensitivity and also as the modification of lethal toxicity. An interaction has been observed in all aspects. The toxicity of WR-2721 increases in the presence of misonidazole. The WR-2721 radioprotection of both skin and tumors decreases if the sensitizer is added. Likewise the radiosensitization with misonidazole is diminished when WR-2721 is present. These results indicate an interaction at the site of radiation injury and they also demonstrate that WR-2721 can adequately penetrate into hypoxic tumor cells.

Radioprotection of mouse skin by WR-2721: the critical influence of oxygen tension

The epidermal clone assay has been used to study the radioprotective effect of WR-2721 on mouse skin under different conditions of oxygenation and under anoxia. The skin has shown a progressive decrease in sensitivity as the inspired gas was changed from 100% oxygen towards 0% oxygen. Compared with mice breathing 100% oxygen, those breathing air are partially protected. The inspired oxygen concentration to given half the full oxygen effect is 10--12%. The radioprotection observed with 400 mg/kg WR-2721 is markedly dependent on the ambient oxygen concentration. The protection factor is 1.1 or less in mice breathing 5%, 1% of 0% oxygen. Protection is maximal (1.95) in air and in 50% oxygen and diminishes to 1.6 at higher oxygen tensions.

Radioprotection of mouse skin by WR-2721 in single and fractionated treatments

The radioprotective action of WR-2721 on mouse skin has been studied using single doses, two and five fractions. Significant radioprotection was observed with 200-500 mg/kg, in animals breathing air or oxygen at the time of irradiation. The degree of radioprotection increased with increasing dose, and was significantly greater in air than in oxygen, especially at low drug doses. Cumulative drug toxicity limited the fractionated treatments to lower drug doses, but the protection observed was similar to that seen with single doses at the same drug levels. The protection factors observed are lower than many published values for skin, and we conclude that a single protection factor cannot yet be stated for any dose of WR-2721. Local tissue oxygenation may account for some of the discrepancies in different studies.