The Release Mechanism of an Oral Controlled-Release Delivery System for Indomethacin

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Abstract
This study was carried out to fully characterize the release kinetics of an oral controlled-release tablet formulation of indomethacin with xanthan gum. Matrix swelling, matrix erosion, and drug diffusion studies were performed to elucidate the operative release mechanisms of a tablet compressed from a ternary mixture of indomethacin-xanthan gum-lactose. Drug release tests were performed according to the USP paddle method in phosphate buffer pH 7.4, concurrently with the dissolution of the gum. Mean dissolution time (MDT) of the drug was calculated from the release profile and it was used as a parameter to evaluate the influence of (a) polymer content in the dosage form, (b) ionic strength of the medium, and (c) the rotation speed of the paddle on the release characteristics of the drug. There is a linear relationship between MDT and the inverse of polymer content. Within the range of ionic strength of the gastrointestinal tract, the salt concentration of the dissolution medium has a negative (inhibitory) effect on release rate of the drug and on matrix swelling. A positive (enhancing) influence of the salt concentration on drug diffusion in the hydrated matrices was noted. The polymer dissolution follows almost immediately the dissolution of the drug. A linear relationship between MDT and the inverse of paddle rotation speed has been observed. Swelling-controlled erosional process is the operative mechanism for indomethacin release from xanthan gum matrices.

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