| Abstract: | The release of norethindrone from a poly(ortho ester) derived from 3,9-bis(methylene)2,4,8,10-tetraoxaspiro[5,5] undecane and 1,6-hexanediol was studied. Thin circular devices containing incorporated drug and a water soluble salt, such as sodium carbonate, sodium sulfate, or sodium chloride, were placed in water buffered at pH 7.4 at 37°C, and release of drug was followed spectrophotometrically. It was found that rate of drug release was close to zero order for long periods of time and that release of the drug from the hydrophobic matrix occurred as a consequence of osmotic imbibing of water driven by the water soluble salt. Thus, a swelling front moves at a slow and constant rate through the solid device and drug release takes place by rapid diffusion of the drug from the swelling front. Polymer erosion lags behind drug release, and no erosion takes place with basic salts such as sodium carbonate when mechanical properties of the matrix are adequate to accommodate osmotic swelling. |
