Applications of the ion-pair concept to hydrophilic substances with special emphasis on peptides

Previous investigations have shown that ionic drugs with high aqueous solubilities can be lipophilized by ion-pair formation, with appropriate counter-ions. This type of association may prove promising for several biopharmaceutical, analytical and technological applications. This review examines the ion-pair concept with special emphasis on its application to peptides. The conditions for ion-pair formation of different molecules, as well as their transfer to organic solvents, are described. The use of ion-pairs to increase the permeation of various therapeutic agents, including peptidic drugs, is also discussed. Recent uses of ion-pairs in micro- and nanoencapsulation of peptides are also commented upon.