Differential radioprotection of normal tissues by hydrophilic chemical protectors

Analogous to certain radiosensitizers which are too hydrophilic to enter tumor cells, certain radioprotectors, because of their hydrophilicity, may also be hindered from entering tumor cells and thus protect only normal tissues. In testing this hypothesis, we utilized thin layer chromatography as convenient means to measure radioprotector hydrophilicity. Dose reduction factors (DRF's) for hematopoietic radioprotection were determined in BALB/c mice given half maximum tolerated doses (MTD/2) of 11 radioprotectors 30 min prior to graded doses of gamma rays. DRF's for tumor protection were determined in MCa-11 tumor-bearing mice using a regrowth delay assay. Differential radioprotection was found to be significantly correlated (r = 0.86) with hydrophilicity. Thus, radioprotector hydrophilicity appears to be a significant factor in the differential radioprotection observed and should be useful in designing or selecting better differential radioprotectors.