Binding of an anticancer drug, axitinib to human serum albumin: Fluorescence quenching and molecular docking study

Binding characteristics of a promising anticancer drug, axitinib (AXT) to human serum albumin (HSA), the major transport protein in human blood circulation, were studied using fluorescence, UV-vis absorption and circular dichroism (CD) spectroscopy as well as molecular docking analysis. A gradual decrease in the Stern-Volmer quenching constant with increasing temperature revealed the static mode of the protein fluorescence quenching upon AXT addition, thus confirmed AXT-HSA complex formation. This was also confirmed from alteration in the UV-vis spectrum of HSA upon AXT addition. Fluorescence quenching titration results demonstrated moderately strong binding affinity between AXT and HSA based on the binding constant value (1.08±0.06×10(5)M(-1)), obtained in 10mM sodium phosphate buffer, pH7.4 at 25°C. The sign and magnitude of the enthalpy change (∆H=-8.38kJmol(-1)) as well as the entropy change (∆S=+68.21Jmol(-1)K(-1)) clearly suggested involvement of both hydrophobic interactions and hydrogen bonding in AXT-HSA complex formation. These results were well supported by molecular docking results. Three-dimensional fluorescence spectral results indicated significant microenvironmental changes around Trp and Tyr residues of HSA upon complexation with AXT. AXT binding to the protein produced significant alterations in both secondary and tertiary structures of HSA, as revealed from the far-UV and the near-UV CD spectral results. Competitive drug displacement results obtained with phenylbutazone (site I marker), ketoprofen (site II marker) and hemin (site III marker) along with molecular docking results suggested Sudlow's site I, located in subdomain IIA of HSA, as the preferred binding site of AXT.