Molecular basis of indomethacin-human serum albumin interaction

Detailed Experimental and In Silico Investigation of Indomethacin Binding with Human Serum Albumin Considering Primary and Secondary Binding Sites

The interaction of indomethacin with human serum albumin (HSA) has been studied here considering the primary and secondary binding sites. The Stern–Volmer plots were linear in the lower concentration range of indomethacin while a downward curvature was observed in the higher concentration range, suggesting the presence of more than one binding site for indomethacin inside HSA due to which the microenvironment of the fluorophore changed slightly and some of its fraction was not accessible to the quencher. The Stern–Volmer quenching constants (KSV) for the primary and secondary sites were calculated from the two linear portions of the Stern–Volmer plots. There was around a two-fold decrease in the quenching constants for the low-affinity site as compared to the primary binding site. The interaction takes place via a static quenching mechanism and the KSV decreases at both primary and secondary sites upon increasing the temperature. The binding constants were also evaluated, which show strong binding at the primary site and fair binding at the secondary site. The binding was thermodynamically favorable with the liberation of heat and the ordering of the system. In principle, hydrogen bonding and Van der Waals forces were involved in the binding at the primary site while the low-affinity site interacted through hydrophobic forces only. The competitive binding was also evaluated using warfarin, ibuprofen, hemin, and a warfarin + hemin combination as site markers. The binding profile remained unchanged in the presence of ibuprofen, whereas it decreased in the presence of both warfarin and hemin with a straight line in the Stern–Volmer plots. The reduction in the binding was at a maximum when both warfarin and hemin were present simultaneously with the downward curvature in the Stern–Volmer plots at higher concentrations of indomethacin. The secondary structure of HSA also changes slightly in the presence of higher concentrations of indomethacin. Molecular dynamics simulations were performed at the primary and secondary binding sites of HSA which are drug site 1 (located in the subdomain IIA of the protein) and the hemin binding site (located in subdomain IB), respectively. From the results obtained from molecular docking and MD simulation, the indomethacin molecule showed more binding affinity towards drug site 1 followed by the other two sites.

Comparative Studies of Interactions between Fluorodihydroquinazolin Derivatives and Human Serum Albumin with Fluorescence Spectroscopy

In the present study, 3-(fluorobenzylideneamino)-6-chloro-1-(3,3-dimethylbutanoyl)-phenyl-2,3-dihydroquinazolin-4(1H)-one (FDQL) derivatives have been designed and synthesized to study the interaction between fluorine substituted dihydroquinazoline derivatives with human serum albumin (HSA) using fluorescence, circular dichroism and Fourier transform infrared spectroscopy. The results indicated that the FDQL could bind to HSA, induce conformation and the secondary structure changes of HSA, and quench the intrinsic fluorescence of HSA through a static quenching mechanism. The thermodynamic parameters, ΔH, ΔS, and ΔG, calculated at different temperatures, revealed that the binding was through spontaneous and hydrophobic forces and thus played major roles in the association. Based on the number of binding sites, it was considered that one molecule of FDQL could bind to a single site of HSA. Site marker competition experiments indicated that the reactive site of HSA to FDQL mainly located in site II (subdomain IIIA). The substitution by fluorine in the benzene ring could increase the interactions between FDQL and HSA to some extent in the proper temperature range through hydrophobic effect, and the substitution at meta-position enhanced the affinity greater than that at para- and ortho-positions.

Interaction of Non-Steroidal Anti-Inflammatory Drugs, Etoricoxib and Parecoxib Sodium, with Human Serum Albumin Studied by Fluorescence Spectroscopy

The mechanism of interaction of the non-steroidal anti-inflammatory drugs, etoricoxib and parecoxib sodium, with human serum albumin (HSA) was studied using fluorescence spectroscopy. There was only one class of binding site with association constants of the order of 10(4). Thermodynamic parameters suggest that van der Waals and hydrogen bonding interactions in the case of etoricoxib, and electrostatic and hydrogen bonding interactions in the case of parecoxib sodium, are predominantly involved in the binding. Studies in the presence of the hydrophobic probe, 1-anilinonaphthalene-8-sulfonate (ANS), showed that hydrophobic interactions are not involved in the binding of these drugs to HSA. Displacement studies using the site-specific probe, dansylsarcosine piperidinium salt (DSS), showed that the drugs are bound at site II on the HSA molecule. However, etoricoxib and parecoxib sodium are bound at different regions within site II. Increase of pH and the presence of salt caused significant changes in the association constants and the concentration of free pharmacologically active drug. Stern-Volmer analysis of the binding data indicated that the tryptophan residues of albumin are not fully accessible to anionic parecoxib sodium and a predominantly static quenching mechanism is operative in each case.

Predicting plasma protein binding of drugs: a new approach

In spite of the large amount of plasma protein binding data for drugs, it is not obvious and there is no clear consensus among different disciplines how to deal with this parameter in multidimensional lead optimization strategies. In this work, we have made a comprehensive study on the importance of plasma protein binding and the influencing factors in order to get new insights for this molecular property. Our analysis of the distribution of percentage plasma protein binding among therapeutic drugs showed that no general rules for protein binding can be derived, except for the class of chemotherapeutics, where a clear trend towards lower binding could be observed. For the majority of indication areas, however, empirical rules are missing. We present here an extensive list of multiply determined primary association constants for binding to human serum albumin (HSA) for 138 compounds from the literature. Correlating these binding constants with the percentage fraction of protein bound showed that the percentage data above 90%, corresponding to a binding constant below 6 microM, are of insufficient accuracy. Furthermore, it could be demonstrated that the lipophilicity of drugs, traditionally felt to dominate binding to HSA, is not the only relevant descriptor. Here, we report a generic model for the prediction of drug association constants to HSA, which uses a pharmacophoric similarity concept and partial least square analysis (PLS) to construct a quantitative structure-activity relationship. It is able to single out the submicromolar to nanomolar binders, i.e. to differentiate between 99.0 and 99.99% plasma protein binding. Depending on the system, this can be important in medicinal chemistry programs and may together with other computed physicochemical and ADME properties assist in the prioritization of synthetic strategies.

The role of electrostatic interactions in human serum albumin binding and stabilization by halothane

Electrostatic interactions have been proposed as a potentially important force for anesthetics and protein binding but have not yet been tested directly. In the present study, we used wild-type human serum albumin (HSA) and specific site-directed mutants as a native protein model to investigate the role of electrostatic interactions in halothane binding. Structural geometry analysis of the HSA-halothane complex predicted an absence of significant electrostatic interactions, and direct binding (tryptophan fluorescence and zonal elution chromatography) and stability experiments (hydrogen exchange) confirmed that loss of charge in the binding sites, by charged to uncharged mutations and by changing ionic strength of the buffer, generally increased both regional (tryptophan region) and global halothane/HSA affinity. The results indicate that electrostatic interactions (full charges) either do not contribute or diminish halothane binding to HSA, leaving only the more general hydrophobic and van der Waals forces as the major contributors to the binding interaction.