Netropsin and bis-netropsin analogs as inhibitors of the catalytic activity of mammalian DNA topoisomerase II and topoisomerase cleavable complexes

This study examined the ability of netropsin and related minor groove binders to interfere with the actions of DNA topoisomerases II and I. We evaluated a series of netropsin dimers linked with flexible aliphatic chains of different lengths. These agents are potentially able to occupy longer stretches of DNA than the parental drug as a result of bidentate binding. Both netropsin and its dimers were found: (i) to inhibit the catalytic activity of isolated Topoisomerase II and (ii) to interfere with the stabilization of the cleavable complexes of Topoisomerase II and I in nuclei. Dimers with linkers consisting of 0-4 and 6-9 methylene groups (n) were far more inhibitory than netropsin against isolated Enzyme and in the nuclear system. The compound with n = 5 was less active than netropsin in both assays while the dimer with n = 10 inhibited only the isolated Enzyme. The comparison of dimers with fixed linker length (n = 2) but varying number of N-methylpyrrole residues (from 1 to 3) revealed that the inhibitory properties were enhanced with increasing number of N-methylpyrrole units. For dimers with varying linker length, drug ability to inhibit catalytic activity of isolated Topoisomerase II was positively correlated with calf thymus DNA association constants. In contrast, no such correlation existed in nuclei. However, the inhibitory effects in the nuclear system were correlated with the association constants for poly(dAdT). The results indicate that bidentate binding can significantly enhance anti-topoisomerase activity of netropsin related dimeric minor groove binders. However, other factors such as the length of the linker, the number of pyrrole moieties and the nature of the target (isolated Enzyme/DNA versus chromatin in nuclei) also contribute to these activities.