Blends of aqueous polymer dispersions used for pellet coating: importance of the particle size.

Blends of aqueous dispersions of a water-insoluble and an enteric polymer, namely ethyl cellulose:hydroxypropyl methylcellulose acetate succinate (EC:HPMCAS) and ethyl cellulose:methacrylic acid ethyl acrylate copolymer (EC:Eudragit L), were used as coating materials to control theophylline release from matrix pellets. Varying the polymer blend ratio, broad ranges of drug release patterns were obtained at low as well as at high pH. Interestingly, the resulting release profiles were rather similar for both types of blends in 0.1 M HCl, whereas significant differences were observed in phosphate buffer pH 7.4. Surprisingly, drug release at high pH was much slower for EC:HPMCAS blends compared to EC:Eudragit L blends, although HPMCAS leached out more rapidly (and to a higher extent) from the film coatings than Eudragit L. To explain these phenomena and to better understand the underlying drug release mechanisms, thin polymeric films of identical composition as the pellet coatings were prepared and physicochemically characterized before and upon exposure to the release media. Importantly, the polymer particle size was identified to be a very crucial formulation parameter, determining the resulting film coating structure and properties. The Eudragit L particles are much smaller than the HPMCAS particles (nano- vs. micrometer size range) and, thus, more effectively hinder the formation of a continuous and mechanically stable EC network. Consequently, the EC structures remaining after enteric polymer leaching at high pH are mechanically much weaker in the case of Eudragit L. Upon exposure to phosphate buffer, water-filled cracks are formed, through which the drug rapidly diffuses out. In contrast, the EC structures remaining upon HPMCAS leaching are mechanically stronger and drug release is controlled by diffusion through the polymeric remnants.