The differential response of resistant and sensitive strains of Escherichia coli to the cytotoxic effects of 5-aziridino-2,4-dinitrobenzamide (CB 1954)

Abstract

The antineoplastic agent 5-aziridino-2,4-dinitrobenzamide (CB 1954) has an exceptionally high therapeutic index against the Walker rat carcinoma 256 but no effect on other experimental tumours. Escherichia coli B/r is much more resistant to cytotoxic alkylating agents than E. coli B s−1, and these strains were therefore used to determine the basis of the differential cytotoxicity of CB 1954. There is a semilogarithmic dose-survival relationship in both strains, B/r being some 26 times more resitant than B s−1. Studies using 3H-labelled CB 1954 showed that although the drug binds to the same extent to DNA in both strains, at the D 37 for B/r there are 560 alkylations per average genome compared with only 16 alkylations per genome in B s−1. These figures agree closely with those obtained with the bifunctional sulphur mustard but are 2–3 orders of magnitude lower than those found with monofunctional agents. The extent of reaction with RNA and protein in both strains is approximately the same, RNA binding more than DNA, and protein less, on a molar basis. Consideration of the relative target sizes of functional RNA and protein molecules in relation to binding at the D 37 excludes them as possible targets for inactivation. DNA synthesis and cell division in B/r resume at the control rate after a short, dose-depending lag following drug treatment, whereas in B s−1 DNA synthesis and cell division are permanently blocked. E. coli B/r has a greater capacity to excise CB 1954 residues from its DNA than B s−1 following treatment of both strains at the D 37 for B/r. The evidence suggests that, in these bacteria, CB 1954 exerts its cytotoxic effect by alkylation of DNA and subsequent hindrance of DNA replication and cell division. The resistant strain probably owes this resistance to its greater ability to remove that proportion of the alkylating residues sufficient to allow resumption of DNA replication and cell division.