Development of shellac-coated sustained release pellet formulations

Shellac is an important coating material for food products. Since the introduction of aqueous ammoniacal solutions it also regained importance for pharmaceutical applications. Because of the comparatively high dissolution pH of this material, further additives are required if shellac is used as enteric coating material. However, this dissolution behaviour of shellac may be of interest for sustained release or colon targeting applications. In the present study different subcoats containing calcium chloride, citric acid or Eudragit(®) E, respectively, were applied to immediate release theophylline pellets which were subsequently coated with shellac. Drug release from the resulting pellet formulations was measured. The mechanism of interaction between the modifying subcoat ingredients and the shellac coating was investigated using FT-IR spectroscopy. All formulations with modifying subcoat prolonged drug release. Whereas the effect of calcium chloride was a result of ionic interactions with shellac, the effect of citric acid was a reduction of the degree of dissociation of shellac. The influence of Eudragit(®) E can be explained by the solubility characteristics of this basic polymer. The application of modifying subcoats is an easy and effective means to achieve sustained release from shellac-coated dosage forms. The choice of a suitable substance and the adjustment of its concentration allow tailor made sustained release profiles.