Investigation of drug release from pellets coated with different shellac types

Even though most commercially available shellac types meet the specifications of the pharmacopoeias, their physicochemical properties and thus drug release may vary considerably. So far a comparison of drug release from dosage forms coated with different shellac types has not been made.

Drug release from pellets coated with different shellac types was investigated and the data correlated to the physicochemical properties of shellac.

Theophylline pellets were coated with three different commercially available shellac types of Indian and Thai origin. The minimum coating level (CL) to achieve gastric resistance was determined for each shellac type. The drug release characteristics from the different formulations were correlated with the physicochemical properties of the shellac types such as pK(a), acid value, and intrinsic dissolution rate.

Gastric resistance was achieved at comparatively low CLs for all investigated shellac types. At pH 7.4 all investigated formulations showed complete drug release within 45 minutes. Drug release at pH 6.8 was prolonged and occurred by swelling and drug diffusion through the coating layer. However, the required minimum CL and drug release profiles especially at pH 6.8 varied considerably. Of the investigated shellac types, the Thai shellac stands out providing both gastric resistance at low CLs and fast drug release at high pH 6.8.

Although a prediction of the release characteristic could not be made from the pK(a), the intrinsic dissolution rate turned out to be a good indicator for the drug release behavior.