Partial permselective coating adds an osmotic contribution to drug release from swellable matrixes

A swellable matrix tablet is described which is partially coated with cellulose acetate (CA) to obtain a film having the shape of a cup, whose permeability to water and solutes was altered by mixing increasing amounts of poly(ethylene glycol) 400 (PEG). The drug-release mechanism from such systems was assessed by carrying out drug-release experiments both in water and saline solutions. Drug permeability through the polymeric cup and SEM analysis on the films were also performed. It was found that the systems exhibited drug-release kinetics very close to linearity. The mechanisms governing drug release were (i) drug diffusion through the uncoated gel layer, (ii) drug transport through the gel layer due to the osmotic pressure difference, and (iii) drug diffusion through the cup pores. The relative importance of each contribution depended on the amount of PEG in the film. The systems with a cup containing 1%, 13%, and 33% PEG w/w behaved in part as osmotic systems, whereas the system having a permeable cup behaved as a hybrid reservoir system. These modifications of the coating permeability introduce a further possibility of modulating drug-release kinetics and lead to a reduced dependence of swellable matrix tablet release on environmental conditions.