Optimization of Topical Therapy: Partitioning of Drugs into Stratum Corneum

To optimize a topical formulation for therapeutic effect generally implies that the flux of drug into the skin be maximized. This requirement means that the product of drug concentration in the vehicle (C_v) and drug partition coefficient (PC) between stratum corneum (SC) and vehicle be as large as possible. While C_v is a formulation variable which can be easily manipulated up to the drug's saturation solubility, PC is a parameter that is difficult to predict a priori. However, there is no question that an ability to evaluate PC would greatly facilitate the efficient screening of drugs and formulations. We have measured the SC/water and SC/isopropylmyristate (a model lipophilic vehicle) PCs of seven drugs: acitretin, progesterone, testosterone, diazepam, estradiol, hydrocortisone, and caffeine. SC/ water PCs were determined as a function of the following variables: (i) initial drug concentration in the vehicle, (ii) length of equilibrium, (iii) SC source and preparation technique, and (iv) SC delipidization. The data obtained were reproducible and physicochemically consistent, and they show that useful partitioning information from both aqueous and nonaqueous vehicles can be obtained with the biological tissue of greatest relevance. The SC/water PCs of the steroids were in reasonable agreement with previous measurements. A facile approach to an integral determinant of formulation optimization is suggested, therefore, by these observations.