Simultaneous transport and metabolism of ethyl nicotinate in hairless rat skin after its topical application: the effect of enzyme distribution in skin

An in vitro permeation study of ethyl nicotinate (EN) was carried out using excised hairless rat skin, and simultaneous skin transport and metabolism of the drug were kinetically followed. Fairly good steady-state fluxes of E N and its metabolite nicotinic acid (NA) through the skin were obtained aft er a short lag time for all the concentrations of EN applied. These steady- state fluxes were not proportional to the initial donor concentration of EN : EN and NA curves were concave and convex, respectively, which suggests th at metabolic saturation from EN to NA takes place in the viable skin at hig her EN application. Further permeation studies of EN or NA were then carrie d out on full-thickness skin or stripped skin with an esterase inhibitor to measure their permeation parameters, such as partition coefficient of EN f rom the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis. Sepa rately, enzymatic parameters (Michaelis constant K-m and maximum metabolism rate V-max) were obtained from the production rate of NA from different co ncentrations of EN in the skin homogenate. The obtained permeation and enzy matic parameters were then introduced to differential equations showing Fic k's second law of diffusion in the stratum corneum and the law with Michael is-Menten metabolism in the viable epidermis and dermis. The calculated ste ady-state fluxes of EN and NA by the equations were very close to the obtai ned data. We then measured the esterase distribution in skin microphotograp hically using fluorescein-5-isothiocyanate diacetate. A higher enzyme conce ntration was observed in the epidermal cells and near hair follicles than i n the dermis. Simulation studies using the even and the partial enzyme dist ribution models suggested that no significant difference between the models was observed in the skin permeations of EN and NA, whereas concentration-d istance profiles of EN and NA were very different. This finding suggests th at the total amount of enzyme in skin which converts EN to NA is a determin ant of the metabolic rate of EN in skin. The present approach is a useful t ool for analyzing simultaneous transport and metabolism of many drugs, espe cially those showing Michaelis-Menten type-metabolic saturation in skin. (C ) 1999 Elsevier Science B.V. All rights reserved.