Drug binding to human erythrocytes in the process of ionic drug-induced hemolysis. Flow microcalorimetric approaches.

Erythrocyte hemolysis induced by cationic phenothiazine derivatives and anionic non-steroidal anti-inflammatory drugs was compared, by flow microcalorimetry, with respect to thermodynamic characteristics for drug binding to intact human erythrocytes. Phenothiazines having high hemolytic activities bound strongly to erythrocyte cells, inducing an immediate hemolytic action characterized by an endothermic heat effect prior to saturating available binding sites. The thermodynamic observable delta H and delta S fell within the ranges of -119 to -65.1 kJ/mol and -308 to -128 J/mol/K, respectively, for these cationic species. There was a linear relationship between the hemolytic activity and the degree of exothermicity of delta H which was enhanced significantly by the presence of a halogen atom(s) at the C-2 position of the phenothiazine nucleus in the order of H less than Cl less than CF3. Anti-inflammatory drugs, however, bound to quite different sites in the erythrocytes with lower affinities and higher capacities than cationic drugs. The latter was characterized by small negative delta H (-17.3 to -7.1 kJ/mol) and positive delta S (10 to 41 J/mol/K). In the calorimetric profiles observed during hemolysis by anionic drugs, two stages were seen: the first, an exothermic process, arising from drug binding to the erythrocytes; the second, an endothermic process, corresponding to the heat of dilution of hemoglobin released from erythrocytes. Hemolysis occurred after the binding sites on the erythrocytes were saturated with drugs. Our data suggest that the binding activities of ionic drugs, such as the amounts of the bound drug and their binding energies to erythrocytes, contribute to the hemolysis.