Enhanced post-irradiation recovery of the haemopoietic system in animals pretreated with a variety of cytotoxic agents.

It is known that pretreatment of mice with bacterial endotoxin and certain stathmokinetic agents between 1 and 3 days prior to exposure to ioninzing radiation reduce radiation lethality. In this communication it is shown that pretreatment with cytosine arabinoside, methotrexate, nortestosterone and chlorambucil reduces radiation (1000 rad) induced lethality. This reduction can be ascribed to enhanced regeneration of the haemopoietic system in pretreated animals and not to increased survival of colony-forming cells (CFU) in these animals. Regeneration of CFUs was underway within 24 hr after 900 rad in the pretreated mice but did not start until day 3 in mice treated with gamma radiation only. Two agents, namely radiation itself (either 75 or 150 rad) and busulphan (10 mg/kg) did not reduce the lethal effects of subsequent gamma irradiation nor enhance the regeneration of CFUs, even though radiation, like the protective cytosine arabinoside, induces early CFUs proliferation. The administration of nucleoside precursors of DNA enhanced regrowth of haemopoietic stem cells to an extent comparable with that of the most effective pretreatment, cytosine arabinoside. It is postulated that drugs like cytosine arabinoside operate by causing cell death, providing a source of DNA that con enhance the regrowth of surviving stem cells in the bone marrow.