DNA damage and repair in the bone marrow of rats treated with four chloroethylnitrosoureas.

DNA is considered to be an important target for the antitumor and toxic properties of the chloroethylnitrosoureas. Since the main target for their dose-limiting toxicity and the antileukemic efficacy is believed to be the bone marrow, we have compared the formation and subsequent removal of DNA-DNA interstrand cross-links in the bone marrow of rats which had received a single i.p. injection (100 mumol/kg) of four chloroethylnitrosoureas. The kinetics of cross-link removal was identical for chlorozotocin, which is known to have low chemical carbamoylating activity, to that of 1,3-bis(2-chloroethyl)-1-nitrosourea, a drug with a relatively high carbamoylating capacity. The differential bone marrow toxicity exhibited by these two agents could not, therefore, be explained by a carbamoylation-mediated difference in the rate and extent of DNA-DNA interstrand cross-link removal. The peak level and overall magnitude of cross-links were, however, found to vary considerably with the chemical structure of the analogues. Both 1-(2-chloroethyl)-1-nitroso-3-(methylene-carboxamido)urea and 1-(2-hydroxyethyl)-3-(2-chloroethyl)-3-nitrosourea, were much more effective in inducing interstrand cross-links than 1,3-bis(2-chloroethyl)-1-nitrosourea or chlorozotocin. This differential cross-linking did not, however, parallel the single-dose acute toxicity of these agents but reflected to a greater extent differences in their antileukemic activity. Considering the widely differing biological properties of this class of compounds, the measurement of DNA-DNA interstrand cross-linking in vivo might prove relevant in the evaluation of novel nitrosoureas.