Both adriamycin and daunomycin chelate Fe3+, forming complexes with similar spectroscopic properties but very different redox chemistry. Anaerobic Fe3+-adriamycin and Fe3+-daunomycin complexes of identical stoichiometry give rise to identical optical absorbance and EPR spectra with maxima at 600 nm and signals at g = 4.2 and g = 2.01. In anaerobic preparations, however, the 600 nm absorption band and the EPR signals of the Fe3+-adriamycin complexes decrease as Fe3+ is reduced to Fe2+, with the appearance of an adriamycin free-radical signal at g = 2.0035, while the spectra of Fe3+-daunomycin complexes remain unchanged, with no free-radical signal appearing. Polarographic measurements demonstrate that the Fe3+-adriamycin complexes consume O2 while the Fe3+-daunomycin complexes do not. Measurements in the presence of catalase and superoxide dismutase suggest that 75% of the O2 consumed by Fe3+-adriamycin is reduced to H2O2 or .O2-. Spin-trapping experiments demonstrate that the Fe3+-adriamycin complexes generate .OH, while the daunomycin complexes do not. Qunatitation of .OH generation by Fe3+-adriamycin demonstrates that the initital rate of .OH generation approaches the rate of total O2 consumption. DNA cleavage studies show that only the Fe3+-adriamycin complexes cleave DNA in the absence of exogenous added H2O2. This DNA cleavage can be blocked by catalasa or .OH radical scavengers. These results indicate that the side-chain hydroxyl group is essential for Fe3+ reduction, subsequent O2 reduction, as well as for the generation of the drug free radical, since adriamycin and daunomycin are structurally identical except for this hydroxyl. The difference in redox chemistry of the iron complexes of adriamycin and daunomycin is the only known mechanism which can explain the difference in anti-tumor potency and cardiotoxicity of these two structurally similar drugs.
ASJC Scopus subject areas
- Molecular Biology