Correlation of the radiosensitization potency afforded by nitroacridine intercalators with their electron scavenging efficiency in DNA.

Nitracrine (1-NC) and its nitro-positional analogues are electron-affinic DNA intercalating compounds that act as hypoxic cell radiosensitizers in cell culture, but are too rapidly metabolized to provide radiosensitization in vivo. We have explored the electron-trapping efficiencies of nitroacridines to see if such compounds can scavenge radicals formed in irradiated DNA and if the efficiency of such trapping is related to their radiosensitization properties. We have shown that a correlation does indeed exist as 1-NC, the most potent radiosensitizer, scavenges approximately 45% of the migrating electrons (formed by attachment of eaq- to the DNA) compared with approximately 4% for 4-NC, the least efficient radiosensitizer. The quenching of the delayed fluorescence from acridine orange bound to DNA was also used to probe intracellular uptake and association with DNA. The results are in accord with earlier measurements of average uptake and a suggestion that intercalators which form DNA complexes with short residence times will be preferable to highly bound agents as radiosensitizers. Since 1-NC possesses the smallest association constant with DNA of the four compounds, it is suggested that its high radiosensitization potency may well be related to it being able to interact with more radical sites on the DNA than the other analogues, in addition to its capture of migrating electrons in DNA.