Radioprotection of normal tissues against gamma rays and cyclotron neutrons with WR-2721: LD₅₀ studies and ³⁵S-WR-2721 biodistribution

The ability of WR-2721 to protect mice against two modes of death following whole-body radiation with ¹³⁷Cs γ rays or d(22)+Be neutrons was examined. For single fractions, 400 mg/kg WR-2721 was administered prior to irradiation. In two-fraction exposures, the dose was 275 mg/kg given prior to each fraction. Dose modification factors (DMFs) were calculated as ratios of LD₅₀ values. For single fractions of γ rays, the DMF was 1.74 for the LD(50/7) end point and for LD(50/30), the DMF for single fractions was 2.25. For two fractions 3 hr apart, it was 1.88. For single fractions of cyclotron neutrons, the DMF was 1.32 for LD(50/7). Measured with the LD(50/30) end point, the DMF for single neutrons doses was 1.41 and for two fractions, 1.19. Substantial radioprotection of bone marrow and intestinal epithelium against cyclotron neutrons was seen in these investigations. Biodistribution studies were done following ip injection of ³⁵S-labeled WR-2721 into C3H mice bearing RIF-1 tumors. Blood levels peaked at 10 min after injection and declined thereafter. Most normal tissues achieved maximum levels of ³⁵S at 30 to 60 min postinjection and high concentrations were retained in most tissues for up to 2 hr. Assuming that all ³⁵S is in parent compound or dephosphorylated radioprotective metabolites, the concentration of protector (milligram per gram tissue) in various organs at 30 min postinjection ranked as follows: kidney > submandibular gland ≥ liver = lung > gut > heart ≥ blood > skin > tumor > brain. High levels of ³⁵S were achieved and retention times were long in certain normal tissues which respond at early or late times postirradiation and may be dose limiting in radiotherapy: kidney, liver, salivary gland, and lung. These combined observations suggest that there is potential for protecting dose-limiting, late-responding normal tissue in the radiotherapy of human cancer with both neutrons and conventional radiotherapy.