Release kinetics of sparingly soluble drugs from ethyl cellulose-walled microcapsules: theophylline microcapsules.

Release rates of theophylline from ethyl cellulose-coated microcapsules were measured as a function of wall thickness and core particle size. The kinetic data conformed with first order release and also the Higuchi matrix model. However, application of the differential rate treatment, hitherto applied only to drug matrix dispersions, showed that release from the microcapsules definitely followed the first order equation. For the purpose of confirming that the release process was membrane-controlled, the experimental rate constants were transformed into effective permeability constants (P1) with the aid of the microcapsule dimensional parameters needed in the relevant equations and compared with the permeability constant (P) of theophylline measured experimentally using planar ethyl cellulose membranes. P1 values decreased linearly to a moderate extent with wall thickness, probably due to decrease in porosity during wall-formation. P1 values of the thicker-walled microcapsules were found to be of the same order as the membrane P value, supporting a release mechanism of membrane control under non-steady state conditions.