Influence of pH and plasticizers on drug release from ethylcellulose pseudolatex coated pellets

Six potential plasticizers for an ethylcellulose (EC) pseudolatex coating system (Aquacoat) were evaluated at three levels (25, 30, and 35%) to study the influence of these additives on the release of a model compound, propranolol hydrochloride, from pellets in two different media, dilute HCl and phosphate buffer, pH 7.4. For the majority of the plasticizers, the release rate decreased when larger amounts of plasticizer were incorporated into the coating. However, for the plasticizers dibutyl sebacate and dibutyl adipate, no further reduction in the release rate was observed following dissolution testing in dilute HCl when the level of plasticizer was increased from 30 to 35%. This suggests that the saturation capacity of these plasticizers in the film coating had been exceeded. The media pH was found to influence the dissolution characteristics of the coatings. Faster release rates as well as earlier curve inflection points and T50% values were observed for plasticizers evaluated in phosphate buffer. All plasticizers used were independent of pH. Correlation of the dissolution results with properties of free films indicated that slower release (more complete film formation) is associated with softer and weaker films with greater elongation.