Comparison of the protective effects of three phosphorothioate radioprotectors in the RIF-1 tumor

Three aminoalkyl phosphorothioates, WR-2721, WR-3689, and WR-77913, were compared as radioprotectors of RIF-1 tumors irradiated in vivo and assayed for cell survival in vitro. The protector doses were 50% of the acute drug LD₅₀. The radiation dose modifying factors for the three drugs were nearly equal, ranging from 1.5 to 1.7 at surviving fractions of 0.1 and 0.05. Using biodistribution data obtained with ³⁵S labeled drugs, the uptake in tumors was calculated as micromoles drug per gram of tumor. On this basis, tumor levels of WR-77913 were 4.5-fold those of WR-2721, and WR-3689 uptake was 2.7-fold greater than uptake of WR-2721. Thus, on molar basis, WR-2721 appears to be the most effective protector, but all three phosphorothioates protect this tumor moderately well. In diffusible substance autoradiographs of ³H WR-3689 labeled tumors, label was generally distributed over cells with no evidence of preferential localization over nuclei.