Ionization behaviour and tautomerism-dependent lipophilicity of pyridine-2(1H)-one cardiotonic agents

The acid-base properties of pyridine-2(1H)-one derivatives, analogues of the cardiotonic agent milrinone, were studied by capillary zone electrophoresis (CZE). Electrophoretic mobility and pH data were fitted to equilibrium expressions and apparent dissociation constants (pKa) calculated by non-linear regression. Compared with the ultraviolet (UV) spectrophotometric method and potentiometric titrations, the CZE technique showed advantages, such as rapidity and applicability to compounds that are sparingly soluble in water. Based on the pKa values, intramolecular electronic interactions were assessed. The lipophilicity of a number of derivatives was also examined, by determining their n-octanol/water distribution coefficients over a wide pH range, and found to be significantly affected by 2-pyridone/2-hydroxypyridine tautomerism. As revealed by a comparison between experimental and calculated log P values, electron withdrawing substituents, especially at the C(6) position of 2-pyridone, favour the less polar hydroxypyridine tautomers both in water and octanol. Our results indicate that the positive inotropism of milrinone-related compounds could be explained taking ionization and tautomerism into account.