Swelling studies of gelatin. II: Effect of additives

Interactions between gelatin and six cationic, anionic, and nonionic drugs or excipients were investigated through their effects on initial swelling rate and equilibrium swelling of gelatin. Short rectangular strips of Type B gelatin containing the additives were immersed in buffer solutions of pH 7.0 at 20 degrees C. Their weight gain due to uptake of buffer solution and their weight loss due to leaching of the additive and of gelatin were determined as a function of time. During preparation of the strips, methyprylon and dicloxacillin sodium crystallized, while octoxynol 9 separated as small droplets in the gelatin matrix. Up to 7% of gelatin leached into the buffer solution during 96 h of immersion from strips of plain gelatin and strips containing five additives. The sixth additive, cetylpyridinium chloride, tripled the amount of gelatin leached while most of this additive remained in the gelatin strip. The other five additives were largely or completely extracted by the buffer solution. Potassium chloride underwent the fastest leaching, being completely dissolved within the first half hour. Octoxynol 9 was extracted most slowly because the swelling solution formed a viscous liquid crystalline phase inside the gelatin. Swelling followed second-order kinetics. Initial swelling rates and equilibrium swelling were calculated with a linearized function. Cetylpyridinium chloride, dodecylammonium chloride, and methyprylon reduced the initial swelling rate of gelatin while dicloxacillin sodium increased it. Octoxynol 9 and potassium chloride left it unchanged. Cetylpyridinium chloride and dodecylammonium chloride reduced the equilibrium swelling of gelatin substantially. Dicloxacillin sodium and octoxynol 9 increased it substantially, while potassium chloride and methyprylon increased it slightly. The extensive interaction of the cetylpyridinium ion with gelatin may result in reduced bioavailability.