Melatonin's protection against DNA damage by the iron(III)-adriamycin complex

Adriamycin is a first line cancer treatment drug for breast cancer and many soft tissue carcinomas. Adriamycin is a highly effective anti-cancer drug. It has a fused ring system that is planar, hydrophobic and electron rich. These features allow intercalation into DNA which, along with its topoisomerase inhibition, results in its anti-cancer effectiveness. Despite its success, its use is limited by a dose-dependent cardiotoxicity.; Adriamycin forms tight complexes with iron in varying iron:drug ratios. The chelated complex [Fe+3-Adriamycin (1:2)] oxidatively cleaves DNA via the generation of reactive oxygen species (ROS). Enzymes associated specifically with heart mitochondria increase ROS formation explaining why adriamycin is selectively cardiotoxic. Pretreatment of mice with the neural hormone melatonin eliminates the cardiotoxic effectiveness of adriamycin therapy without reducing the drug's antitumor effect (Wahab et al. 2000, Tumori 86: 157–162). This has recently been verified in human cancer patients (Lissoni et al. 1999, European Journal of Cancer 35: (12) 1688–1692). Melatonin is soluble in both lipid and aqueous environments and has no known side-effects. This study analyzes the mechanistic features of DNA damage by Fe+3 -adriamycin and how melatonin ameliorates this damage.; An Fe+3-adriamycin (1:2) complex + glutathione cleaves DNA; this oxidative DNA cleavage does not occur with adriamycin +/− glutathione or with FeADR in the absence of glutathione. Melatonin reduces this oxidative DNA cleavage by 67%. Using a supercoiled-to-nicked-circular-conversion assay in conjunction with spectroscopic analyses give results revealing: (1) Fe +3-adriamycin (1:2) complex + glutathione forms a reactive intermediate. (2) Melatonin protects the DNA from cleavage by this reactive intermediate. (3) Enzymatic assays show that H₂O₂ is the primary ROS formed with downstream production of the hydroxy radical via the Fenton reaction. (4) Fe+3-adriamycin (1:2) intercalates into DNA to the same degree as uncomplexed adriamycin. (5) Melatonin also binds to DNA but not by intercalation. These experiments indicate that the cardioprotective effect of melatonin in adriamycin therapy may stem from melatonin's interaction with a reactive intermediate from Fe+3-adriamycin (1:2) complex + glutathione and/or a direct interaction of melatonin with DNA.