RELATIONSHIP BETWEEN LETHAL EFFECTS AND TOPOISOMERASE II-MEDIATED DOUBLE-STRANDED DNA BREAKS PRODUCED BY ANTHRACYCLINES WITH DIFFERENT SEQUENCE SPECIFICITY

The role of the site selectivity of topoisomerase II poisoning in the cytotoxic activity of anthracyclines has not been established. In this article, we have thus studied the levels and persistence of double-st randed DNA breaks (DSB) along with the cytotoxic activity in human leu kemic HL60 cells of seven anthracyclines, including doxorubicin, dauno rubicin, and idarubicin, as well as sugar-modified analogues character ized by an altered sequence specificity. Epimerization at the 3' posit ion of the sugar moiety markedly affected the biological activity; ind eed, a dramatic reduction of drug effects was evident for ino-3'-epi-h ydroxy-4'-deoxy-4'-amino-daunorubicin. The studied analogues could be gathered into three groups based on the DSB/cytotoxicity ratio. At equ itoxic concentrations: (a) parent drugs and mino-3'-epi-hydroxy-4'-deo xy-4'-amino-daunorubicin endowed with the same sequence specificity st imulated low DSB levels; (b) 3'-epi-daunorubicin and 3'-deamino-4'-deo xy-4'-epi-amino-idarubicin, which have a different sequence specificit y, and teniposide (a structurally unrelated poison) stimulated higher amounts of DSB; and (c) demethoxy-3'-deamino-3'-hydroxy-4'-epi-doxorub icin stimulated the highest DSB levels. For the last agent, a faster r ate of cleavage resealing, which is consistent with a reduced DNA bind ing affinity, could account for the increased DSB/cytotoxicity ratio c ompared with parent drugs. However, for other analogues, the observed differences in DSB persistence/resealing could not completely explain the different DSB/cytotoxicity ratios. The results thus suggest that t he cytotoxic potency of anthracyclines may be the result of an interpl ay of the level, the persistence, and the genomic localization of topo isomerase II-mediated DNA cleavage.