Altered radiosensitivity of hematopoietic stem cells by vincristine pretreatment: superoxide dismutase activity as a possible mechanism.

The effect of vincristine (VCR) on hematopoietic stem cell and progenitor compartments and its ability to induce transient periods of radioresistance was investigated so that we could ascertain the drug-radiation intertreatment interval affording optimal radioprotection and determine if its ability to induce increased levels of superoxide dismutase (SOD) is a potential mechanism for this radioprotection. Measurement of marrow stem cell and progenitor compartments demonstrated that these subsets displayed differential sensitivity to VCR and that this sensitivity appeared to be proportional to how "primitive" the subset was. Treatment with VCR prior to irradiation was observed to enhance significantly both 8- and 12-day spleen colony-forming unit recovery with maximal radioprotection occurring for a drug-radiation interval of 12-48 hours. Monitoring of copper-zinc SOD levels demonstrated an increase in activity following VCR that was localized in a fraction of the bone marrow enriched for stem cells and progenitors. The temporal pattern of this increase, however, did not correlate with the drug-radiation schedules affording optimal radioprotection, which indicates that other factors appear to be operative in this radioprotection as well.