Stabilization of misoprostol with hydroxypropyl methylcellulose (HPMC) against degradation by water

The stability of misoprostol oil is significantly improved in a hydroxypropyl methylcellulose (HPMC) dispersion (1:100). In order to assess the effect of water on misoprostol stability, the rate of misoprostol degradation was investigated in the misoprostol/HPMC dispersion at 55 degrees C, along with the water sorption isotherm, under seven different relative humidity (RH) conditions ranging from 0 to 81%. The results indicated that the first-order rate constants of misoprostol degradation increased in a concave-up fashion as the water content of the dispersion increased. Below 30% relative humidity (approximately 2% water), the first-order rate constants of misoprostol degradation were found to be minimum. The results of the stability study were interpreted in terms of the changing structure of HPMC as it related to the mobility of water and misoprostol within the HPMC dispersion.