Recently, coated pellets have gained attention of the pharmaceutical industry as they represent a relatively easy way leading to controlled drug release. Pellets of appropriate properties containing approx. 40 % of diclofenac sodium were prepared by roto-agglomeration. For the coating, two different aqueous dispersions (Surelease® and Eudragit®RS 30 D) were selected. Generally, the drug release rate slowed down as the coating load increased from 10 to 22 %. However, big differences between diclofenac sodium release from pellets coated with Surelease® and Eudragit®RS30D at equivalent coating loads were observed. Although Eudragit®RS 30 D provided membranes successfully controlling drug release over an extended period of 24 hours, the coating process with Surelease® led to a film of a very poor quality. Faster release from ethyl cellulose coated pellets could be explained in terms of a higher solubility of diclofenac sodium in alkaline aqueous dispersion of ethyl cellulose and its migration into the coat during the coating process and/or the unsatisfactory curing of the ethyl cellulose film. Therefore possible interactions between the coating and the drug should be always considered as suggested in this study.

        Key words: coated pellets – diclofenac sodium – Surelease® – Eudragit® RS 30 D