The effects of thickness and hardness of the coating film on the drug release rate of theophylline granules coated with chitosan-sodium tripolyphosphate complex.

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Abstract
The drug release from theophylline granules coated with chitosan-sodium tripolyphosphate complex followed zero-order kinetics after an induction period, which depended on the swelling of the granules during the dissolution. The swelling was greater in the acidic medium than in the alkaline one, resulting in an increase of the drug release rate. A linear correlation between the drug release rate and the reciprocal of coating film thickness was found. By hardening the coating film with glutaraldehyde, the swelling of the granules was reduced and the drug release rate was decreased. Pre-drying of the granules prior to the hardening was fairly effective in reducing the drug release rate. The coating film became seamless after treatment with glutaraldehde. Even when the dissolution solvent was exchanged from disintegration test solution No. 1 (pH 1.2) to No. 2 (pH 6.8), the drug release rate changed little.