Release kinetics of indomethacin from pressure sensitive adhesive matrices

Release properties of indomethacin (IDM) from pressure sensitive adhesive (PSA) matrices consisting of styrene-isoprene-styrene block co-polymer to pH 7.4 phosphate buffer solution were investigated and kinetically analyzed. The amount of IDM released was increased with increasing initial concentration of the drug in the PSA matrix. When the drug began to be saturated in the matrix beyond its solubility, no further increase in the released amount was observed. These results could be explained by there being little contribution of the crystalline drug in the PSA matrices to the release. On the other hand, decrease in the thickness of the PSA matrix caused marked increase in the release rate of IDM. The reason might be rapid direct release of the crystalline drug from the PSA surface to the buffer solution, and this was confirmed by scanning electron microscopic observation of the surface. Since these results could not be fully analyzed by T. Higuchi's or W.I. Higuchi's equation, a simple compartment model was designed. This model was adequate to describe all release profiles of IDM from the PSA matrices. Obtained kinetic parameters were consistent with the release mechanism described above.