Oral hydrophilic matrices having non uniform drug distribution for zero-order release: A literature review

Oral hydrophilic matrices for prolonged release mostly show a decrease in the rate of drug release over time, owing to the increasing length of the diffusional path and progressive reduction of the area at the interface between glassy and rubbery matrix. In addition, burst effect may also occur due to the fraction of drug present on the surface of the system, which is released when the external polymer particles are not fully swollen yet. Different strategies have been attempted in order to address these issues and, ideally, to reach zero-order release. The approaches proposed are based on geometric modulation of the release area, control of the swelling behavior or initial non-uniform distribution of the active ingredient throughout the polymer matrix. The present article offers an extensive analysis of the various methods described in the literature for reaching zero-order release leveraging non-uniform distribution of the drug in hydrophilic polymeric systems. In this respect, special attention is given to the design of the main delivery platforms reviewed, their manufacturing, in vitro release profiles and analytical techniques for assessing drug concentration patterns within the solid units.