Radiation sensitivity change of hemopoietic cells induced by hydroxyurea.

Hemopoietic tissues of mice contain cells capable of proliferation and differentiation to form endogenous spleen colonies (Endo-CFU-S) in irradiated animals. Hydroxyurea (HU) given before irradiation of C57BL mice for endogenous CFU assay increased the number of surviving CFU-S twofold. Either HU or HSATT given after the initial stimulus abolished this rise. In vivo and in vitro 'suicide' assays of spleen cells indicated that cells were stimulated by the HU or the initial stimulus to enter into DNA synthesis shortly after stimulation. The resting CFU cell population appeared to be positioned close to DNA-S phase and responded to the stimulus by entry into S phase. Whole body irradiation at intervals after HU revealed a changing radiation sensitivity of the endogenous CFU-S consistent with drug-induced progression with decreased radiation sensitivity of the S phase cohort of cells.