Influence of structural modifications at the 3' and 4' positions of doxorubicin on the drug ability to trap topoisomerase II and to overcome multidrug resistance.

To better define the role of the amino sugar in the pharmacological and biochemical properties of anthracyclines related to doxorubicin and daunorubicin, we have investigated the effects of various substituents at the 39- and 49-positions of the drug on cytotoxic activity and ability to stimulate DNA cleavage mediated by DNA topoisomerase II. The study shows that the nature of the substituent at the 39-position but not the 49-position is critical for drug ability to form cleavable complexes. The amino group at the 39-position is not essential for cytotoxic and topoisomerase II-targeting activities, because it can be replaced by a hydroxyl group without reduction of activity. However, the presence of bulky substituents at this position (i.e., morpholinyl derivatives) totally inhibited the effects on the enzyme, thus supporting previous observations indicating that the cytotoxic potencies of these particular derivatives are not related to topoisomerase II inhibition. This conclusion is also supported by the observation that 39-morpholinyl and 39-methoxymorpholinyl derivatives are able to overcome atypical (i.e., topoisomerase II-mediated) multidrug resistance. Because a bulky substituent at the 49-position did not reduce the ability to stimulate DNA cleavage, these results support a critical role of the 39-position in the drug interaction with topoisomerase II in the ternary complex. An analysis of patterns of cross-resistance to the studied derivatives in resistant human tumor cell lines expressing different resistance mechanisms indicated that chemical modifications at the 39-position of the sugar may have a relevant influence on the ability of the drugs to overcome specific mechanisms of resistance.