Binding of topotecan to chromatin: Insights into cooperative binding and comparison with DNA

Topotecan (TPT) is an anticancer drug widely used in cancer therapy. Although the interaction of TPT with DNA is a subject of few reports, no work has been reported on the binding affinity of TPT to DNA-histone complex in chromatin structure. In the present study we have focused on the effect of TPT on chromatin employing various types of spectroscopy and equilibrium dialysis techniques. The results showed that TPT quenched with chromatin chromophores and decreased fluorescence emission intensity corresponding to aromatic residues of histone proteins. The UV absorbance at 260 and 210 nm in decreased in a dose dependent manner. Upon binding of the drug, ellipticity at 222 nm in the circular dichroism profile became more positive implying reduction of α-helix content of histones. The binding is positive cooperative with association constant (Ka) of 2.65 × 102 M−1 and 1.11 × 102 M−1 for chromatin and DNA respectively indicating higher affinity of TPT to chromatin compared to DNA. From the results it is concluded that in the cell nucleus, TPT, as a potent anticancer drug, exerts its biological action through binding to chromatin and in this process not only DNA but also histone proteins play a fundamental role.