Proxyphylline release kinetics from symmetrical three-layer silicone rubber matrices: Effect of different excipients in the outer rate-controlling layers

In this work we present results on the release kinetics of a water-soluble model drug (proxyphylline) from symmetrical three-layer ABA matrices made from silicone rubber. The ABA matrices, consisting of an inner drug-containing layer, loaded at concentration near the percolation threshold, and two drug-free outer layers, were studied with respect to (a) the varying permeability properties of the outer layers achieved by the incorporation of either poly(ethylene glycol), a hydrophilic polymeric excipient, or NaCl, an inorganic salt with high osmotic action, and (b) the relative thickness of the inner to outer layers. In all cases, substantial uniformity of release rate was achieved and the initial burst effect, characterizing the corresponding single-layer system, was suppressed. The results are interpreted on the basis of the permeation mechanisms operating in each individual inner and outer layer. Thus, we have demonstrated that by judicious choice of the composition and relative thickness parameters the release rate can be adjusted to any desired level. Finally, the case of the NaCl excipient, released parallel to proxyphylline, serves as an example of a dual-drug releasing system.