Insights into the mechanisms of chitosan–anionic polymers-based matrix tablets for extended drug release

[Display omitted] The aim of this study was to investigate drug release mechanisms from physical mixtures of chitosan–anionic polymers-based matrix tablets and to obtain a comprehensive understanding about release characteristics. Six types of anionic polymers (i.e., Eudragit® L100, sodium alginate, carrageenan, carboxymethylcellulose sodium, carbomer and xanthan gum) and two model drugs (i.e., theophylline and metoprolol succinate) with varied solubility were chosen. Texture analyzer, differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were applied to better understand drug release mechanisms. In vitro release experiments were conducted in a pH-changing medium to simulate the physiological condition of the gastrointestinal tract. Interestingly, a common phenomenon was observed in all the CS–anionic polymers-based matrix tablets investigated here, that is, the inner layer of the swollen tablets was coated by CS–anionic polymer polyelectrolyte complexes (PEC)-based film formed by self-assembly. Formation of the in situ self-assembled film was further confirmed by texture analysis, DSC, and FTIR. It was further identified that properties of the film were influenced by the characteristics of anionic polymers and the physiological conditions of the gastrointestinal tract. Moreover, this novel structure could alter swelling and erosion-based release mechanisms of the tablets. In addition, drug release characteristics from CS–anionic polymer systems depended on the properties of anionic polymers and the drug solubility. In conclusion, our studies may broaden current views on cationic polymer–anionic polymer-based oral matrix tablets for extended release.