Targeted therapies, involving new class drug molecules have shown inhibitory
activities against signaling pathways that are responsible for triggering various
carcinomas. Tyrosine kinases form an important class of enzymes of these signaling
pathways and can be a useful target to develop effective therapeutic agents as tyrosine
kinase inhibitors to treat various cancers. Some of the FDA-approved drug molecules,
known as tyrosine kinase inhibitors are vandetanib (VDB), lapatinib (LAP), gefitinib
(GEF) and sunitinib (SU), which are currently being used for the treatment of medullary
thyroid, breast, lung and renal cancers, respectively. Interaction mechanisms of VDB,
LAP, GEF and SU binding to human serum albumin (HSA), the major transport protein
in the human blood circulation were explored using various spectroscopic techniques
such as fluorescence, absorption and circular dichroism (CD) along with in silico
studies. Quenching of the protein fluorescence upon addition of these ligands was
characterized as the static quenching, which confirmed the complex formation between
the ligand and the protein. Such complex formation was also affirmed by absorption
spectral results. Moderate binding affinity for these interactions was evident from the
binding constant (Ka) values, obtained at 298 K, which had fallen in the range of 104–
105 M–1 except VDB–HSA interaction, which showed relatively weaker binding
affinity. Thermodynamic data for the binding equilibria predicted involvement of
hydrophobic and van der Waals interactions along with hydrogen bonds in stabilizing
drug–HSA complexes, which was also supported by molecular docking results. The far-
UV and the near-UV CD spectra showed changes in the secondary and the tertiary
structures, respectively, of HSA upon ligand binding. Three-dimensional fluorescence spectral results also indicated ligand-induced microenvironmental perturbations around
protein fluorophores. Binding of these ligands to HSA offered significant protection to
the protein against thermal denaturation. Competitive site-marker displacement results
as well as molecular docking analyses revealed preferred binding location of these drug
molecules primarily at site I for VDB and SU while at site III for LAP and GEF, located
in subdomains IIA and IB, respectively, of HSA. The influence of a few common ions
on the binding reaction between the ligand and HSA was also noticed
