Single-strand DNA breaks induced by chromophore-modified anthracyclines in P388 leukemia cells

Single-strand DNA breaks induced by chromophore-modified anthracyclines related to doxorubicin (including 11-deoxydaunorubicin, 4-demethoxydaunorubicin, 4-demethoxy-11-deoxy-4'-epi-daunorubicin, 4-demethyl-6-O-methyldoxorubicin) in cultured P388 leukemia cells were determined by the filter alkaline elution method. The tested analogues differed markedly in their cytotoxic potency. In the range of cytotoxic concentrations, 11-deoxydaunorubicin produced single-strand DNA break frequency of the same order of magnitude as that produced by doxorubicin, while other derivatives caused much more marked damage on DNA than doxorubicin. Since DNA breaks were found to be protein associated, the type of DNA damage produced by all tested derivatives presumably resulted by action of DNA topoisomerases II, as proposed for doxorubicin and other intercalating agents. Although the "potent" (with respect to DNA damage) derivatives, except 4-demethyl-6-O-methyldoxorubicin, showed an increased cellular drug accumulation as compared to doxorubicin, this did not account for the marked differences in ability to damage DNA. 4-Demethyl-6-O-methyldoxorubicin was the most efficient derivative, producing DNA breaks in a lower range of cellular drug content. A striking biphasic dose-response curve was observed for the 4-demethoxy derivatives, suggesting a complex mechanism of interaction among drug, DNA, and enzyme. A lack of correlation was noted among DNA binding affinity, induction of strand breaks, and cytotoxic activity of these chromophore-modified derivatives. From these observations, it is suggested that multiple actions of anthracyclines at the DNA level are responsible for their cytotoxic activity, which is not simply related to inhibition of a specific DNA-dependent enzyme and/or function.