Drug release from diffusion pellets coated with the aqueous ethyl cellulose dispersion Aquacoat ECD-30 and 20% dibutyl sebacate as plasticizer: partition mechanism and pore diffusion.

The release of the hydrophilic etofylline and the lipophilic propyphenazone (octanol/water partition coefficient PC = 0.35 and 119, respectively) from diffusion pellets coated with the aqueous ethyl cellulose dispersion Aquacoat ECD-30 and 20% dibutyl sebacate (DBS) as plasticizer is investigated as a function of pH. The relatively slow release is not constant, due to the broad distribution of different release rates within the pellet population and the non-linearity of the release of each diffusion pellet itself. The release proceeds according to a partition mechanism at a pH < 6. The partition mechanism is not influenced by the osmotic pressure difference between the release medium and the saturated solution within the diffusion pellets. The diffusion coefficients of different drugs in the plasticized coating are in the range 1 to 5 x 10(-8) cm2/s. At a of pH > 6 an additional hydrophilic pathway without partition exists if the diffusion pellets did not have any contact with an acidic medium. This is due to the strongly increased water uptake of more than 20% by the coatings as a consequence of the dissociation of carboxyl groups in the ethyl cellulose.