Evidence for the direct interaction of reduced metronidazole derivatives with DNA bases

The electrochemical behaviour of the bioreductive redox active nitroimidazole drug metronidazole has been examined in the presence and absence of the DNA bases using three electrochemical techniques, all of which indicate the capacity for interaction between reduced products and DNA bases. The 4-electron metronidazole (RNO2) metronidazole-hydroxylamine (RNHOH) couple in an aqueous medium shows a positive shift in reduction potential upon addition of thymine, adenine and guanine, but a negative shift for cytosine. Interpretation of these results for an irreversible process is, however, inconclusive. In dimethylformamide/H2O the presence of DNA base on the one-electron addition product, the nitro radical anion, was examined by cyclic voltammetry. All except guanine resulted in interaction with the metronidazole nitro radical anion (RNO2-), as measured by the decrease in the return-to-forward peak current ratio, in the following order of increasing reactivity: cytosine, adenine and thymine (at a metronidazole: base ratio of 1:1). The increase in the stability of the radical anion by increasing the pH of the dimethylformamide/H2O medium resulted in a decreased reaction with thymine.