Ionic partition diagrams of ionisable drugs: pH-lipophilicity profiles, transfer mechanisms and charge effects on solvation

Abstract

The transfer of a series of weak acids, bases and ampholytes has been studied by cyclic voltammetry at the water ∣ 1,2-dichloroethane interface in order to determine their lipophilicity. Physicochemical parameters such as the standard Gibbs energy of transfer and the standard partition coefficients of the ionised forms I of these compounds (log P 0′ I ) were measured as a function of aqueous pH, and the dissociation constants in the organic phase were deduced from these data. The results obtained are presented in the form of ionic partition diagrams which define the domains of predominance of all the species present in both phases. This representation affords both a precise description of the mechanisms governing the transfer from one phase to the other, and a reliable assessment of pH lipophilicity profiles. The various partition coefficients are compared to evaluate qualitatively the lipophilicity of each species, and the differences between the partition coefficients of the charged and the neutral form of a compound are interpreted in terms of their chemical structure, showing an increase in organic phase affinity for ions with intramolecular charge stabilisation. This study demonstrates also that both charged and neutral species can penetrate into the organic phase and shows the importance of the passive transfer of organic ions for drug disposition and pharmacokinetics.