Analysis of the effect of hydroxyurea on stem cell (CFU-s) kinetics.

Hydroxyurea induces profound changes in the pluripotential haemopoietic stem cell (CFU-s) kinetics. The main feature of these changes is a synchronous entry of resting G0 CFU-s into the cell cycle. The analysis of the passage of the CFU-s cohort through the cell cycle has been largely based on the examination of the fraction of CFU-s which synthesize DNA in the S phase of the cell cycle. This analysis has, however, been hampered by the fact that both the sensitivity of the S phase CFU-s to hydroxyurea and their sensitivity in the [3H]thymidine suicide technique vary as the cells pass through the S phase. Methods which overcome these difficulties have been used in the experiments presented in this paper. It was demonstrated that hydroxyurea kills only about 80% of the S phase CFU-s. The sensitivity to hydroxyurea gradually decreases as the cells approach the middle part of the S phase and increases again as the cells enter the late portions of the S phase. The degree of CFU-s synchrony at the point of entry into and exit from, the S phase has been established. Mathematical analysis of the available data suggests that CFU-s pass through the S phase with a mean transit time of 4.79 hr (standard deviation, 1.45 hr). Hydroxyurea, administered in vivo, blocks CFU-s in the late G1 phase. The duration of this G1-S block, induced by a dose of 1000 mg of hydroxyurea per kg body weight, is approximately 2 hr. The CFU-s in the middle of the S phase, which survive hydroxyurea administration, are also blocked in their passage through the S phase. These cells, however, seem to finish the S phase with a delay of approximately 2 hr.