Preparation and evaluation of Eudragit gels. II: In vitro release of salicylic acid, sodium salicylate, and ketoprofen from Eudragit L and S organogels.

The in vitro dissolution characteristic of salicylic acid, sodium salicylate, and ketoprofen from Eudragit L and S organogels was investigated by the rotation disk method. The dissolution pattern of salicylic acid and erosion of Eudragit L polymer from the organogels followed apparent zero-order kinetics, providing strong evidence for a surface erosion mechanism and negligible diffusional release of salicylic acid. On the other hand, the dissolution of salicylic acid from Eudragit S organogels was a linear function of the square root of time. The apparent dissolution rate of salicylic acid from Eudragit S organogels increased with increasing temperature from 32 to 42 degrees C and agitation rate from 50 to 200 rpm. A linear relationship was obtained between the logarithm of apparent dissolution rate constants and the reciprocal of absolute temperatures. The activation energy for release of salicylic acid from Eudragit S organogels was in the range of 2.99 to 5.57 kcal/mol. From various experimental results, it was concluded that the release process of salicylic acid from Eudragit S organogels was diffusion controlled through the organogels matrix.