Determination of the interactions between an antidepressant amphiphilic drug and human serum albumin

Abstract

Interactions of the amphiphilic antidepressant drug clomipramine hydrochloride with the blood plasma protein human serum albumin (HSA) were investigated to gain an understanding on the complexation mechanism of drug-protein molecules. The colloidal solution was considered as a binary system in which buffer and drug molecules were regarded as the solvent for the HSA–clomipramine complex. To elucidate the interactions involved in the drug-protein complex formation by changing the net electrical charge of the biopolymer, the protein-antidepressant interactions in aqueous buffered solutions of pHs 3.0 and 5.5 (isoelectric point of HSA = 4.9) were investigated using conductivity, ζ-potential and static and dynamic light scattering techniques. An increase of the critical micelle concentration of the antidepressant was detected as a consequence of extensive binding to the protein. From ζ-potential measurements, the adsorption Gibbs energies of the drugs onto the protein were derived using the different theoretical models. Measurements of the number of adsorbed drug molecules and hydrodynamic radii of HSA-antidepressant complexes as a function of drug concentration have shown a gradual increase of size typical of a saturation rather than a denaturation process of the protein. A larger drug adsorption at pH 5.5 than at pH 3.0 was also observed as a consequence of a more important specific binding at the former pH. The interaction potential between the HSA–clomipramine complexes and their stability were determined from the dependence of the diffusion coefficients with protein concentration by application of the DLVO colloidal stability theory. The results indicate a decreasing stability of the colloidal dispersion of the drug-protein complexes with increase in the concentration of added drug.