The Effect of pH and Ionic Strength on the Release of Quinine Adsorbed on to an Insoluble Sodium Polyphosphate

Abstract

The preparation and evaluation of a potential prolonged-release drug delivery system with the model drug quinine HCl adsorbed onto an insoluble sodium polyphosphate (Maddrell's phosphate type II) is described. The delivery system was prepared by equilibration of the drug with a suspension of the polyphosphate and then compressing the dried adsorbed complex into disks. It was shown that the extraction of the drug from the loose powder was enhanced by increasing the sodium ion concentration and by reducing the pH. The effect of sodium ion concentration upon release of the drug from compressed dish depended upon the pH of the dissolution fluid. At low pH, which slowly dissolved the disks, the zero-order release of quinine was reduced as the ionic strength of the dissolution medium was increased. At near-neutral pH, the release of quinine was first-order at sodium concentrations greater than 0.025 M and was zero-order at sodium concentrations lower than 0.025 M. The release was promoted by an increase in the ionic strength.