Local Anesthetics: Acid-base Behaviour and Inclusion with Cyclodextrins

Abstract

Protonation of local anesthetics, LA, and their inclusion complexes with cyclodextrins, CD, are the central issue of this review. The physicochemical properties of LA, such as acid-base character, are crucial to understand their behavior because their capacity to reach and to block sodium channels and act as anesthetics, depends on their protonation state. Consequently, different techniques that are used to determine acid-base equilibrium constants, together with the data published are reviewed. Potentiometry and spectrophometry, are by far the most used techniques. Although these are classical methods for this kind of determinations, the calibration procedure and the approximations used are not always described properly. Besides, the extent of complexation with cyclodextrins varies greatly with the protonation state of the involved molecules, an interesting fact in the administration of LA and in the determination of their inclusion constants. A great variety of experimental techniques have been used to achieve this goal, in fact, there is not a universal method and different properties that change with the inclusion process can be employed, but the system characteristics are of crucial importance in their choice. Several methods are discussed together with the values of constants gathered from the literature. Keywords: Local anesthetic, Cyclodextrin, Inclusion complex, Acid-base equilibrium constant, Spectrophotometry, Fluorescence, Potentiometry, Solubility.