Microcalorimetric Study for the Binding of Ionic Drugs to Human Erythrocytes and the Ghost Membranes

Abstract

Abstract The binding of phenothiazine derivatives (as cationic drugs) and non-steroidal anti-inflammatory drugs (as anionic drugs) to human erythrocytes and ghost membranes has been compared with respect to their thermodynamic characteristics, by flow microcalorimetry at pH 7.4 and 37°C. From enthalpy-entropy correlation, it was shown that anionic and cationic drugs are bound to different binding sites on the membranes. Phenothiazines bind to a single common site of the erythrocyte membranes with relatively high binding affinities (K = 104-105 M−1). The binding is entropy-driven and characterized by a small negative enthalpy (ΔH) and a positive entropy change (ΔS), reflecting hydrophobic interactions. However, the binding reaction for the intact erythrocytes shows large negative values for both AH and AS. The values of K for the membranes and ΔH for the intact erythrocytes increased with the increase of the hydrophobic character of the substituent group at the C-2 position of the phenothiazine nucleus (H < Cl < CF3). The results indicate that phenothiazines bind and/or penetrate to the inner membranes of the erythrocytes and react with intracellular components such as haemoglobin, while anti-inflammatory drugs bind to the surface protein on the membranes with a lower affinity (K = 103 M−1) than phenothiazines, reflecting the small negative ΔH and positive ΔS for the interaction with intact erythrocytes.