Transport of micelle-solubilized steroids across microporous membranes

The effect of solubilization by micelles on the transport of steroids across microporous membranes has been studied theoretically and experimentally. Our theoretical model requires the following parameters: micelle and drug diffusion coefficients in free solution, the distribution coefficient of the drug between the bulk and micellar phases, and micelle and membrane pore radii. The steroids used were hydrocortisone, testosterone, and progesterone. Since the model accounts for the flux of free drug as well as micelle-solubilized drug, the distribution coefficient of the drug between micelles and the aqueous phase had to be determined by solubilization experiments for each of the steroids. Membrane pore diameters, as determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), ranged from approximately 500 to 4000 A. Steroid diffusion coefficients were calculated from membrane diffusion experiments. Quasi-elastic light scattering was used to find the free-solution diffusion coefficients and hydrodynamic radii of the micelles. With these experimentally determined parameters, the model is shown to be capable of predicting the rate of transport of micelle-solubilized drugs through microporous membranes. The application of our model to the design of controlled-release devices is also discussed.