Sugibayashi K, Hayashi T, Morimoto Y
Faculty of Pharmaceutical Sciences, Josai University, 1-1 Keyakidai, Sakado, Saitama, Japan.
J Control Release. 1999 Nov 1;62(1-2):201-8. doi: 10.1016/s0168-3659(99)00039-5.
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 EN and its metabolite nicotinic acid (NA) through the skin were obtained after 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 that metabolic saturation from EN to NA takes place in the viable skin at higher EN application. Further permeation studies of EN or NA were then carried out on full-thickness skin or stripped skin with an esterase inhibitor to measure their permeation parameters, such as partition coefficient of EN from the donor solution to the stratum corneum and diffusion coefficients of EN and NA in the stratum corneum and the viable epidermis and dermis. Separately, enzymatic parameters (Michaelis constant K(m) and maximum metabolism rate V(max)) were obtained from the production rate of NA from different concentrations of EN in the skin homogenate. The obtained permeation and enzymatic parameters were then introduced to differential equations showing Fick's second law of diffusion in the stratum corneum and the law with Michaelis-Menten metabolism in the viable epidermis and dermis. The calculated steady-state fluxes of EN and NA by the equations were very close to the obtained data. We then measured the esterase distribution in skin microphotographically using fluorescein-5-isothiocyanate diacetate. A higher enzyme concentration was observed in the epidermal cells and near hair follicles than in the dermis. Simulation studies using the even and the partial enzyme distribution models suggested that no significant difference between the models was observed in the skin permeations of EN and NA, whereas concentration-distance profiles of EN and NA were very different. This finding suggests that the total amount of enzyme in skin which converts EN to NA is a determinant of the metabolic rate of EN in skin. The present approach is a useful tool for analyzing simultaneous transport and metabolism of many drugs, especially those showing Michaelis-Menten type-metabolic saturation in skin.
使用切除的无毛大鼠皮肤进行了烟酸乙酯（EN）的体外渗透研究，并对药物的皮肤同时转运和代谢进行了动力学跟踪。对于所应用的所有EN浓度，在短暂的滞后时间后，EN及其代谢产物烟酸（NA）通过皮肤获得了相当好的稳态通量。这些稳态通量与EN的初始供体浓度不成正比：EN和NA曲线分别为凹形和凸形，这表明在较高的EN应用量下，在有活力的皮肤中发生了从EN到NA的代谢饱和。然后，使用酯酶抑制剂对全层皮肤或剥离皮肤进行了EN或NA的进一步渗透研究，以测量它们的渗透参数，例如EN从供体溶液到角质层的分配系数以及EN和NA在角质层、有活力的表皮和真皮中的扩散系数。另外，从皮肤匀浆中不同浓度EN产生NA的速率获得了酶学参数（米氏常数K(m)和最大代谢速率V(max)）。然后将获得的渗透和酶学参数引入显示角质层中菲克第二扩散定律以及有活力的表皮和真皮中米氏-门坦代谢定律的微分方程中。通过这些方程计算得到的EN和NA的稳态通量与获得的数据非常接近。然后，我们使用异硫氰酸荧光素二乙酸酯通过显微摄影测量了皮肤中的酯酶分布。观察到表皮细胞和毛囊附近的酶浓度高于真皮。使用均匀和部分酶分布模型的模拟研究表明，在EN和NA的皮肤渗透中，模型之间未观察到显著差异，而EN和NA的浓度-距离分布非常不同。这一发现表明，皮肤中将EN转化为NA的酶总量是皮肤中EN代谢速率的决定因素。本方法是分析许多药物同时转运和代谢的有用工具，尤其是那些在皮肤中表现出米氏-门坦型代谢饱和的药物。
