Enhancing hemopoietic drug resistance: A rationale for reconsidering the clinical use of mitozolomide

Retroviral gene transfer was used to achieve expression in mouse bone marrow of a mutant form of the DNA repair protein O-6-alkylguanine-DNA alkyltransferase (hATPA/GA), which exhibits resistance to inactivation by O-6-benzylguanine (O-6-beG). After reconstitution of mice with transduced bone marrow, similar to 50% of the bipotent granulocyte-macrophage colony-forming cell (GM-CFC) and multipotent spleen colony-forming unit (CFU-S) hemopoietic populations showed expression of the transgene; this expression was associated with resistance to either mitozolomide or to a combination of O-6-beG and mitozolomide, relative to mock-transduced controls. Thus, at a dose of mitozolomide in vivo that allowed only 70% and 62% survival of mock-transduced GM-CFC and CFU-S, respectively, the hATPA/GA CFC were totally resistant to the same dose of milozolomide (P < .05 and .001, respectively). In the presence of O-6-beG, the toxicity of milozolomide was greatly potentiated. Only 24% and 18%, respectively, of mock-transduced GM-CFC and CFU-S survived combination treatment, whereas 45% (P < .05) and 37% (P < .01) of GM-CFC and CFU-S, respectively, from hATPA/GA mice survived the same combination of doses. Furthermore as a result of trans gene expression, the number of micronucleated polychromatic erythrocytes induced by mitozolomide was significantly reduced (P < .05) by 10% relative to mock-transduced controls, indicating the potential of this approach to reduce the frequency of mutation associated with chemotherapy exposure. The protection against the toxic and clastogenic effects of mitozolomide in both primitive and more mature hemopoietic cells suggests that the severe myelosuppression that halted further clinical investigation of this drug could be substantially ameliorated by the exogenous expression of O-6-alkylguanine-DNA alkyltransferase. Therefore, these data raise the prospect for the reinvestigation of mitozolomide and other proscribed drugs in the context of genetically protected hemopoiesis.