The lag times delayed by the hydration of various thicknesses of films were studied in an attempt to deliver drugs to various sites in the gastrointestinal (GI) tract. In a theoretical simulation, it was found that the lag time could be controlled by varying the thickness of the coated polymer, which was equivalent to the amount of polymer coated assuming that the density of the dry polymer was constant. A higher ratio of drug solubility relative to the dosing amount promoted rapid release of drug after the lag period. Diltiazem hydrochloride was selected as a model drug with high and pH-independent water solubility. In vitro dissolution in the pH change medium demonstrated that the release pattern of diltiazem hydrochloride from pellets coated with various thicknesses of Eudragit RS film was kept unchanged. Drug release exhibited a lag period followed afterwards by instantaneous release. The relationship between the lag time and the square of the amount of polymer coated, as well as that between the release rate at steady state and the inverse of the amount of polymer coated, appeared to be linear as predicted.
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