In vitro Adsorption Studies of Cimetidine

The adsorption of cimetidine on selected pharmaceuticals including kaolin, activated charcoal, talc, and nonsystemic antacids was determined at pH 5.0 and 25°. The Langmuir and Freundlich adsorption isotherms showed that cimetidine adsorption was significant with activated charcoal, kaolin, talc, and magnesium trisilicate and was virtually nonexistent with magnesium hydroxide and aluminum hydroxide. Equations expressing the Freundlich and Langmuir adsorption isotherms were evaluated for each adsorbent. The approximate amounts of cimetidine adsorbed per gram of adsorbent were 25.6, 0.402, 0.291, and 0.343 mg for charcoal, kaolin, talc, and magnesium trisilicate, respectively. These in vitro studies indicate that some cimetidine may be lost when it is administered concomitantly with pharmaceutical adsorbents.