In vitro adsorption-desorption of fluphenazine dihydrochloride and promethazine hydrochloride by microcrystalline cellulose.

Fluphenazine dihydrochloride and promethazine hydrochloride were adsorbed in vitro from suspensions of the tableting excipient, microcrystalline cellulose. Studies were undertaken to determine how this adsorption phenomenon was affected by the type of phenothiazine derivative, the type of microcrystalline cellulose, and pH and ionic strength adjustment. The smaller the microcrystalline cellulose particle size, the more drug was adsorbed. Changes in the pH, the ionic strength, and the valency of the cation used to adjust the ionic strength all had a major effect on the extent of adsorption. The adsorption process was rapidly and completely reversed in vitro at gastric pH and ionic strength values.