Microenvironmental pH and microviscosity inside pH-controlled matrix tablets: An EPR imaging study

Incorporation of pH modifiers is a commonly used strategy to enhance the dissolution rate of weakly basic drugs from sustained release solid dosage forms. Electron paramagnetic resonance imaging (EPRI) was applied to spatially monitor pHM and the rotational correlation time (τR), a parameter which is closely related to the surrounding microviscosity inside HPMC (hydroxypropylmethylcellulose) matrix tablets. Fumaric, citric, and succinic acid were employed as pH modifiers. 4-(methylamino)-2-ethyl-5,5-dimethyl-4-pyridine-2-yl-2,5-dihydro-1H-imidazole-1-oxyl (MEP) was used as spin label. Fumaric and citric acid reduced the pHM to equal extents in the initial phase. With the progress of hydration, the more soluble citric acid diffused out from the tablet resulting in an increase in pHM, originating at the outer layers. In contrast, fumaric acid maintained a constantly reduced pHM inside the entire tablet. Due to its lower acidic strength, succinic acid did not reduce the pHM as effectively as the other pH modifiers used. The more water-soluble acids stimulated the water penetration into the matrix system, thereby rapidly decreasing τR. Once the matrix tablets were hydrated, the included pH modifiers influenced τR insignificantly. EPRI, a novel approach for monitoring pHM and τR non-invasively and spatially resolved, was used successfully for the optimization of an pH-controlled formulation.