The possible role of electron-transfer complexes in the antitumour action of amsacrine analogues.

Amsacrine is a DNA intercalating agent which is active against a number of tumours in mice and is used for the treatment of leukaemia in humans. In its DNA-bound form, amsacrine efficiently quenches the fluorescence of ethidium. Fluorescence lifetime studies demonstrate two populations of DNA-bound ethidium. The first, whose fluorescence lifetime is constant at approx. 3 ns and whose proportion increases with increasing amsacrine binding ratio, may comprise molecules bound in close proximity to amsacrine. The second, whose fluorescence lifetime is longer and variable (10-24 ns) and whose proportion decreases with increasing amsacrine binding ratio, may comprise molecules three or more base-pairs away from ethidium. Studies with a number of derivatives of 9-anilinoacridine containing different anilino substituents suggest that the observed wide variation in quenching capacity is correlated with the magnitude of the substituent dipole moment in a particular direction. Consideration of the geometry of the DNA-binding complex indicates that the negative pole of a dipole established in the anilino ring is directed towards a positively charged site on the ethidium molecule. Quenching of ethidium fluorescence may therefore occur where an electron-transfer complex has formed between ethidium and amsacrine molecules. To ascertain whether electron-transfer complex formation is biologically important in the amsacrine series, ethidium quenching has been quantitated and compared with activity against a transplantable neoplasm in mice, the Lewis lung carcinoma. Compounds which strongly quench ethidium fluorescence are in general highly active antitumour agents. The results are discussed in terms of a model where amsacrine has both a DNA-binding and a protein-binding domain, the latter possibly interacting by formation of an electron-transfer complex. The most likely protein-binding domain is on the enzyme topoisomerase II, the target for its cytotoxic activity.