Strategies for the design of hydrophilic matrix tablets with controlled microenvironmental pH

Incorporation of weak acids as pH modifiers enhances the release of weakly basic drugs in higher pH environments by reducing the microenvironmental pH (pHM). The objectives of this study were: (a) to investigate the relationship between pHM, drug release, and pH modifier release and (b) to achieve simultaneous release of the drug and the pH modifier over the entire dissolution time (6 h, phosphate buffer, pH 6.8). Using dipyridamole as a model drug, we investigated drug and acid release and determined the average pHM potentiometrically using tablet cryosections. The first approach was based on incorporating different concentrations of pH modifiers in conventional matrix tablets based on hydroxypropylmethylcellulose. Owing to its high acidic strength and low aqueous solubility, fumaric acid resulted in simultaneous release and maintained a constant acidic pHM. Secondly, press-coated matrix tablets, comprising an acidic reservoir, were found to be a valuable approach for retarding the diffusion of more water-soluble acids. Using the power law expression (Mt/M∞ = ktn) it became evident that the inclusion of acids increased drug release. Higher acid concentrations tended to decrease n standing for the slope, whereas the release constant k increased. Furthermore, the medial check term parameters depended on the type of pH modifier used.