Aliphatic amine N-oxides of DNA binding agents as bioreductive drugs

The DNA binding and cytotoxicity of four intercalating agents, namely bis-alkylamino (-N(CH2)2N(CH3)2) substituted anthraquinone, anthrapyrazole and anthracene, and mono (N(CH2)2N(CH3)2) acridinone, have been compared with their respective aliphatic amine N-oxides -N(CH2)2N+(O-)(CH3)2. The results show that, unlike the intercalators, the N-oxides do not bind to DNA. Molecular modelling illustrates that the delta + nature of the intercalator alkylamino side chains in the protonated form allows for an attractive electrostatic interaction with phosphates of the DNA backbone, whereas the delta- partial charge on the N-oxide makes such an interaction not permissible; indeed, the electrostatic interaction with the DNA phosphates will be repulsive. The N-oxides show little or no cytotoxicity against V79 cells at concentrations equimolar to the IC90 (concentration that inhibits 90% of cell proliferation) of the respective intercalators. However, the cytotoxicity of anthrapyrazole N-oxide against hypoxic V79 cells in the presence of an activating system of S9 liver fraction was enhanced significantly. The results indicate that N-oxides of DNA-affinic agents have potential as bioreductive prodrugs, since they possess low aerobic toxicity but under hypoxic conditions can be metabolised to a potent cytotoxic species presumed to be a DNA-binding tertiary amine.