Irinotecan binds to the internal cavity of beta-lactoglobulin: A multi-spectroscopic and computational investigation.

The binding of irinotecan, a potent anti cancer drug, to bovine beta-lactoglobulin (β-LG) was investigated by various spectroscopic techniques including fluorimetry, circular dichroism (CD), UV-vis, and Fourier transform infrared (FT-IR), in 10mM phosphate buffer, pH 7.75, in combination with a molecular docking study. Analysis of the fluorescence quenching data showed that combined static and dynamic quenching occurs, with the predominant contribution of the static mode. Molecular docking results were in full agreement with the results obtained from thermodynamic analysis of the fluorescence data indicating the existence of one binding site for irinotecan in β-LG structure and revealed the hydrophobic nature of the interaction between irinotecan and the protein. The binding distance between β-LG and irinotecan, r, was estimated to be 5.74nm based on the Förster's theory of non-radiative energy transfer. The obtained results of near-UV CD and FT-IR experiments suggested the occurrence of partial compactness of the protein structure upon irinotecan binding. Based on the experimental data and the possible binding mode revealed by molecular docking study, we concluded that irinotecan binds to the hydrophobic calyx of β-LG with induction of some alterations in the secondary and tertiary structure of the protein.