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

Abstract

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,NЈ-bis(2-chloroethyl)-Nnitrosourea (BCNU) to murine bone marrow has been investigated. When compared with control animals, the bipotent granulocyte-macrophage colony-forming (GM-CFC) progenitor population of the hATPA/GA transduced mice were somewhat more resistant to BCNU (1.4-fold, P = 0.047) and this effect was more significant in the presence of the ATase inactivator O 6 -beG (3.5-fold, P = 0.001). The polychromatic erythrocytes were also significantly protected against BCNU-induced clastogenicity both in the presence (P Ͻ 0.001) and absence of O 6 -beG (P Ͻ 0.05). The primitive, multipotent spleen colony-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 inactivator they remained as sensitive to BCNU toxicity as those in the control animals (P = 0.133). This result contrasts with previous findings demonstrating significant hATPA/GAmediated, O 6 -beG-resistant protection against the toxicity and clastogenicity of a number of O 6 -alkylating agents, including temozolomide, fotemustine and chlorozotocin. The possibility that our strategy for protective gene therapy may be highly agent and cell-type specific is unexpected and has possible implications for clinical trials of this approach using BCNU or related agents.

Phase I study of fotemustine in pediatric patients with refractory brain tumors

“…To circumvent this lack of specificity, others have investigated the strategy of selectively enhancing the repair capacity of bone marrow for ATase, via ex vivo transfer and in vivo expression of a mutant version of ATase in early hemapoietic progenitor cells 40–43. The transduction of this mutant version of the enzyme in mice hematologic progenitor cells has been achieved and these cells are refractory to ATase inactivation 44, 45. Another strategy to diminish the DLT of nitrosoureas (fotemustine) is to use chemoprotective agents such as amifostine, which was developed originally as a radioprotector against nuclear warfare 46.…”