Molecular Specificity of 5-Androstenediol as a Systemic Radioprotectant in Mice

We compared in vivo radioprotective efficacy of 5-androstenediol (5-AED) to that of ten other steroids: 17alpha-androstenediol, dehydroepiandrosterone, 5-androstenetriol (AET), 4-androstenedione (AND), testosterone, estradiol, fluasterone, 16alpha-bromoepiandrosterone, 16alpha-fluoro-androst-5-en-17alpha-ol (alpha-fluorohydrin, AFH), and 16alpha-fluoro-androst-5-en-17beta-ol (beta-fluorohydrin). Steroids were administered 24 or 48 hr before, or 1 hr after, whole-body gamma-irradiation. Two days after irradiation at 3 Gy, blood elements were counted. In addition, after irradiation at 9-12.5 Gy, survival was recorded for 30 days. The results showed radioprotective efficacy was specific for 5-AED. One other steroid, AFH, demonstrated appreciable survival effects but was less efficacious than 5-AED. AND and AET produced slight enhancement of survival in some experiments. This is the first demonstration that the prophylactic window for survival enhancement by 1 subcutaneous (s.c.) injection of 5-AED is as long as 48 hr in mice. Moreover, the results indicate that 1 s.c. injection of 5-AED 1 hr after irradiation is much less effective than 1 injection 24-48 hr before irradiation. Comparing the molecular features of steroids with radioprotective efficacy leads to the following conclusions: 1) these effects are due to interaction with specific receptors, since s.c. injection of extremely similar molecules with the same physicochemical properties as 5-AED were not radioprotective; 2) the 17-hydroxyl group is essential; 3) this group must be in the beta configuration in the absence of nearby side groups; 4) a halogen atom at 16 changes the 17-hydroxyl specificity to alpha; 5) the 3beta-hydroxyl group is not essential; 6) addition of a 7beta-hydroxyl group is deleterious; and 7) the effects are not due to activation of sex steroid receptors.