Differential actions of aclarubicin and doxorubicin: the role of topoisomerase I.

Aclarubicin and doxorubicin are DNA binding anthracycline antibiotics of related chemical structure but differing cytotoxic action. Although doxorubicin mediates its cytotoxicity by poisoning the enzyme topoisomerase II, aclarubicin has been hypothesized to inhibit the catalytic action of topoisomerase II. We show here that aclarubicin, in contrast to doxorubicin, is highly effective in inhibiting the action of topoisomerase I. Aclarubicin not only inhibits this enzyme in a cell-free assay but also markedly inhibits DNA-protein cross-linking in H460 human lung adenocarcinoma cells as measured by the K(+)-SDS precipitation technique. It also displaces topoisomerase I from DNA as measured by Western blotting. Aclarubicin reverses the cytotoxicity of both amsacrine and camptothecin in clonogenic survival assays, consistent with the hypothesis that it is a dual topoisomerase I/II inhibitor. We suggest that the self-inhibition of topoisomerase I in short-term assays may mask the underlying activity of aclarubicin as a topoisomerase I poison. In short-term (1-H) drug exposure assays, aclarubicin kills both exponential and plateau phase cells by a non-cell cycle-selective mechanism apparently not involving G2 phase arrest. This may be a consequence of simultaneous inhibition of topoisomerases I and II.