Topoisomerase II mediated DNA lesions induced by acridine-4-carboxamide and 2-(4-pyridyl)quinoline-8-carboxamide.

Acridine-4-carboxamide and 2-(4-pyridyl)quinoline-8-carboxamide represent a new generation of antitumor intercalators related to amsacrine (m-AMSA), a classic topoisomerase II-targeted drug. We examined the ability of these tricyclic carboxamides to induce DNA lesions that reflect the stabilization of topoisomerase II cleavage complexes. DNA-protein cross-links (DPC) and DNA double-strand breaks (DSB) were assessed in mouse fibrosarcoma cells (line 935.1). DPC were rapidly formed and readily reversible. A bell-shape concentration dependence suggested a self-inhibition of DPC at higher drug levels. In isolated nuclei, DPC formation by 2-(4-pyridyl)quinoline-8-carboxamide required ATP and was inhibited by novobiocin, a topoisomerase II inhibitor. Acridine-4-carboxamide and 2-(4-pyridyl)quinoline-8-carboxamide were also potent inducers of DSB. In contrast to DPC, however, DNA breaks continued to increase with drug concentration. These DSB were masked (presumably by non-covalently associated proteins) when analyzed by nucleoid sedimentation. Thus, while both DPC and DSB seemed to be topoisomerase mediated, at least some DSB appeared to lack the enzyme bound covalently. DNA lesions by tricyclic carboxamides occurred, in general, at drug concentrations comparable to those needed to inhibit cell survival. Also, the tricyclic carboxamides inhibited the catalytic activity of isolated topoisomerase II. The results indicate that tricyclic carboxamides interfere with the action of topoisomerase II. However, the mechanisms of enzyme inhibition by these drugs differ from the classical trapping of topoisomerase in covalent cleavage complex m-AMSA.