In vitro-in vivo correlation in man of a topically applied local anesthetic agent using numerical convolution and deconvolution.

The aim of this study was to evaluate the relevance of the in vitro permeation method used at our laboratory in predicting in vivo dermal and transdermal absorption. Two different emulsions, a submicron oil-in-water (o/w) emulsion and a semisolid water-in-oil (w/o) emulsion, containing a model compound were investigated. The in vitro permeation rate of the compound from these emulsions was measured using static diffusion cells with human skin as membrane. The emulsions were allowed to remain in contact with the skin in the donor chamber for 15, 60, and 240 min. The study was monitored for 240 min and the steady state flux was calculated. The systemic concentration of the compound was measured in vivo as a function of time after dermal application to healthy volunteers with 15 and 60 min of application. A short-lasting i.v. infusion study in healthy volunteers was used to simulate the i.v. bolus dose. Numerical convolution was used to predict the in vivo plasma concentration of the compound while the in vivo absorption rate of the compound was estimated using numerical deconvolution. To establish correlation, the predicted in vivo flux was compared with the corresponding observed in vitro parameter after adjusting for the lag time. No major differences were seen in the systemic plasma levels between the two emulsions, which is in close agreement with the steady state flux measured in vitro. A linear correlation representing a point-to-point relationship was established for each of the investigated formulations and application times. The longer application time was predicted more accurately for both emulsions.

Physicochemical interaction of local anesthetics with lipid model systems--correlation with in vitro permeation and in vivo efficacy.

In dermal/transdermal drug administration stratum corneum (SC) is often the rate-limiting step. Furthermore, the intercellular lipid domain of SC is nowadays widely accepted as the major contributor to the skin barrier. The current work investigates whether the difference in the level of topical efficacy of local anesthetic compounds correlates with the type of interaction between the drug and the intercellular lipids of SC. Therefore, local anesthetics of varying topical efficacy were evaluated with respect to their effect on the morphology of various model lipid systems using small and wide angle X-ray diffraction (SWAXD) and differential scanning calorimetry (DSC). The model lipids used were glyceryl monooleate, sphingomyelin and lipids isolated from human SC. Furthermore, partitioning into isolated human SC as well as permeation through isolated human SC and human tape-stripped skin were investigated in vitro. The results indicate that local anesthetics may act as their own permeation enhancers by increasing the degree of hydrocarbon chain fluidity of the intercellular lipids. Eventually these interactions may induce non-lamellar reversed types of liquid crystalline structures locally in SC, which further facilitate the drug mobility. The large difference in topical efficacy of the investigated local anesthetics could not be explained simply by looking at their effect on the phase behavior of lipid model systems. Despite the similarities in physicochemical properties of these substances, the in vitro skin permeability differed markedly (AD>EMLA>lidocaine>prilocaine>sameridine). Thus, it was concluded that sufficient drug permeability over SC is essential to obtain local anesthesia by blocking the superficial nociceptors.