Influence of pH and phospholipid species on release of acetaminophen from tablets containing phospholipids

The release of acetaminophen (AAP) from tablets containing phospholipids was examined using hydrogenated soybean phospholipid (HSL) and its main components, phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI), although the PI was not well purified (PI rich). Tablets compressed with 400 kgf had about 9% porosity and 2-4 kgf hardness. The release patterns of AAP from the tablets were fitted to Higuchi's square root of time law. The release rate was influenced by the pH of the medium, though solubility of AAP did not change with pH. PC tablets showed faster release at pH of less than 3 than that at pH of above 3, whereas PI rich and HSL tablets showed faster release at pH of above 3 than that at pH of less than 3. The release rate from PE tablets was little affected by pH. A linear relationship exists between the release rate of AAP and the rate of water absorption by the tablet. The ionization state of the phospholipids changes with the pH of the medium, and affects the hydration characteristics. The fully ionized state, at pH of less than 3 in the case of PC and above 3 in the case of PI is most effective on hydration. PE does not fully ionize because of intermolecular hydrogen bonding.