Ion-transfer voltammetry of local anesthetics at an organic solvent/water interface and pharmacological activity vs. ion partition coefficient relationship.

The ion-transfer reaction of local anesthetics at an organic solvent/water interface has been studied using cyclic voltammetry (CV) with a stationary nitrobenzene (NB)/water (W) interface. Procaine and seven other local anesthetics gave reversible or quasi-reversible voltammograms at the NB/W interface in the pH range between 0.9 and 9.6. These drugs are present in aqueous solution in either neutral or ionic form, or both forms. The half-wave potential, as determined by the midpoint potential in CV, vs. pH curves, were determined and analyzed to determine the partition coefficients of both neutral and ionic forms of the drugs between NB and W. The partition coefficients of the ionic forms were derived from their formal potential of transfer at an NB/W interface. The dissociation constants of ionic forms of the drugs in NB were also deduced. A high correlation between the pharmacological activity and the partition coefficient of the ionic form of amide-linked local anesthetics has been shown.