The in vitro release of medroxyprogesterone acetate from a silicone rubber matrix was studied. A nonlinear dependence of release rate upon medroxyprogesterone acetate concentration within the matrix was found. Based upon a model system, equations were derived to explain this behavior and to include other parameters which may influence the release rate. Since the model, in part, is dependent upon a receding medroxyprogesterone acetate layer within the matrix, a photograph depicting depletion zones as a function of time is presented. In contrast to the T. Higuchi model for drug release, this model includes the boundary diffusion layer. Comparison of the two models suggested that when the boundary layer was considered, a better fit of experimental data to theory was found. The applicability of the model to an in vivo system is discussed. This study has suggested that the partition coefficient, diffusion coefficients, medroxyprogesterone acetate concentration within the polymer, and agitation conditions play important roles in the release process.
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