Protection of committed murine haemopoietic progenitors against BCNU toxicity does not predict protection of primitive, multipotent spleen colony-forming cells - implications for chemoprotective gene therapy

The effect of expression of an O-6-benzylguanine (O-6-beG)resistant mutant (hATPA/GA) of human O-6-alkylguanine-DNA alkyltransferase (ATase) on the in vivo toxicity and clastogenicity of the anti-tumour agent N,N'-bis(2-chlor oethyl)-N-nitrosourea (BCNU) to murine bone marrow has been investigated. W hen compared with control animals, the bipotent granulocyte-macrophage colo ny-forming (GM-CFC) progenitor population of the hATPA/GA transduced mice w ere somewhat more resistant to BCNU (1.4-fold, P = 0.047) and this effect w as more significant in the presence of the ATase inactivator O-6-beG (3.5-f old, P = 0.001). The polychromatic erythrocytes were also significantly pro tected against BCNU-induced clastogenicity both in the presence (P < 0.001) and absence of O-6-beG (P < 0.05). The primitive, multipotent spleen colon y-forming cells (CFU-S) in these animals also showed moderate (1.6-fold, P= 0.034) protection in the absence of O-6-beG but in the presence of the ina ctivator they remained as sensitive to BCNU toxicity as those in the contro l animals (P= 0.133). This result contrasts with previous findings demonstr ating significant hATPA/GA-mediated, O-6-beG-resistant protection against t he toxicity and clastogenicity of a number of O-6-alkylating agents, includ ing temozolomide, fotemustine and chlorozotocin. The possibility that our s trategy for protective gene therapy may be highly agent and cell-type speci fic is unexpected and has possible implications for clinical trials of this approach using BCNU or related agents.