A dynamic mathematical model for transdermal drug delivery is developed on the basis of the bi-layer skin/twocompartment body model. The effects of metabolism reaction in the viable skin, the drug binding and reservoir function in the stratum corneum, and the solubility and diffusivity of the drug in the skin on the permeation ratetime profile are extensively simulated. The effects of the pharmacokinetic parameters on the plasma concentration profile are also analyzed. The present model is useful not only for analyzing the rate of skin permeation but also for predicting the plasma concentration after transdermal drug delivery.

Release of D&C Yellow No. 10 and anhydrous theophylline have been determined from a thermosoftening, hydrophilic matrix, Gelucire 50/13, incorporating a water-soluble additive, polyethylene glycol 4000. As additive level increased, release also increased. The effect of mixtures of Gelucire 50/13 (G50/13) and Gelucire 50/02 (G50/02) on release was also investigated as a function of temperature and pH. As the level of G50/02 increased, release decreased and became predominantly diffusional. As temperature was increased, release changed from diffusion to a mixed model of both diffusion and erosion. At basic pH, release from these composite systems became more erosional in character, possibly reflecting partial hydrolysis of the ester-linked matrices. Diffusion coefficients and apparent diffusion coefficients were calculated in G50/02 and G50/13 matrices, respectively, and were in agreement with published data.

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