Exploring the binding interaction of potent anticancer drug topotecan with human serum albumin: spectroscopic, calorimetric and fibrillation study

This manuscript describes the interaction of the topoisomerase I inhibitor anticancer drug topotecan with human serum albumin using microcalorimetry, circular dichroism, and atomic force microscopy imaging techniques. Conformational change in albumin was ascertained from circular dichroism and synchronous fluorescence study that revealed a small but definitive partial unfolding of the protein structure upon drug binding. Isothermal titration calorimetry study indicated a favorable exothermic interaction with a binding affinity of the order of similar to 10(5)M(-1) at 293.15K. A 1:1 binding stoichiometry was established. The binding reaction was largely enthalpy dominated with negative standard molar Gibbs energy change. Ionic strength variation study revealed that non-polyelectrolytic forces played dominant role in the interaction and remained almost invariant at all salt concentrations. Upon complex formation, the stabilization of the protein structure against thermal denaturation occurred. Atomic force microscopy study enabled imaging of fibrils of the protein and its complex with topotecan.