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

Retroviral gene transfer was used to achieve expression in mouse bone marro w of a mutant form of the DNA repair protein O-6-alkylguanine-DNA alkyltran sferase (hATPA/GA), which exhibits resistance to inactivation by O-6-benzyl guanine (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 po pulations showed expression of the transgene; this expression was associate d 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 mit ozolomide 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 pre sence of O-6-beG, the toxicity of milozolomide was greatly potentiated. Onl y 24% and 18%, respectively, of mock-transduced GM-CFC and CFU-S survived c ombination 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 d oses. Furthermore as a result of trans gene expression, the number of micro nucleated polychromatic erythrocytes induced by mitozolomide was significan tly reduced (P < .05) by 10% relative to mock-transduced controls, indicati ng the potential of this approach to reduce the frequency of mutation assoc iated with chemotherapy exposure. The protection against the toxic and clas togenic effects of mitozolomide in both primitive and more mature hemopoiet ic cells suggests that the severe myelosuppression that halted further clin ical investigation of this drug could be substantially ameliorated by the e xogenous expression of O-6-alkylguanine-DNA alkyltransferase. Therefore, th ese data raise the prospect for the reinvestigation of mitozolomide and oth er proscribed drugs in the context of genetically protected hemopoiesis.