Preparation, characterization, and controlled release from coprecipitates of fluorescein and magnesium hydroxide

Magnesium hydroxide was precipitated as a lyophobic sol in the presence of various concentrations of fluorescein sodium (3'6'-dihydroxyspiro[isobenzofuran-1(3H),9'-[9H]xanthen++ +]-3-one, disodium salt) ranging in molar equivalents between 0.1 and 2 times that of the hydroxide. Coprecipitates were washed and dried, and release of the dye and magnesium was determined (pH 7.4, 37 degrees C) from rotating disks. Release rates varied depending upon fluorescein content. The rate of dye release was retarded by less than or equal to 10(4) times that of fluorescein sodium alone, implying the existence of some form of solid association between the components of the coprecipitates. The presence of the dye in certain concentrations reduced magnesium hydroxide dissolution rates by a factor of three. Fluorescein dissolution rates, when expressed as percent release, passed through a minimum (coincident with the dye-induced reduction in Mg(OH)2 dissolution). Adsorption experiments revealed evidence for multiaffinity binding of fluorescein at the surface of freshly precipitated Mg(OH)2. Magnesium, fluorescein, and water contents of the coprecipitates were characterized by atomic absorption and UV spectroscopy and by thermogravimetric analysis. Fluorescein content increased in direct proportion to its initial concentration in solution. Controlled, but variable release of this easily assayed dye is possible by employing precipitates with different fluorescein contents.