Dissolution rates of high energy sulfathiazole--povidone coprecipitates II: characterization of form of drug controlling its dissolution rate via solubility studies.

Solubility studies were made to characterize the form of sulfathiazole controlling the rate of dissolution exhibited in previously reported dissolution rate studies of sulfathiazole coprecipitated with povidone. The aqueous solubility of the high energy form of sulfathiazole obtained using sulfathiazole--povidone coprecipitates was determined in the presence of polymer in solution. Its aqueous solubility in the absence of polymer was determined by extrapolation. The solubility value was much greater than either the supercooled melt or the crystalline forms of sulfathiazole. Stabilization of these sulfathiazole solutions supersaturated with respect to the more stable crystalline form was achieved by the addition of sufficient polymer to the solution to prevent nucleation of the crystalline form. The concentration of polymer required to prevent nucleation of the crystalline forms was much higher than the previously reported concentrations required to inhibit crystal growth of sulfathiazole. The ratio of the solubility value obtained for the coprecipitated sulfathiazole as compared with its crystalline form I was in agreement with the ratio of their dissolution rates obtained the plateau regions of the dissolution rate experiments reported previously. The extrapolated aqueous solubility values in the absence of povidone were obtained as a function of temperature and were utilized to obtain thermodynamic parameters. The difference in the heat of solution of the two forms of sulfathiazole from the slope of the van't Hoff plots of the extrapolated solubility values was 1618 cal/mole, in excellent agreement with the literature value. The free energy, enthalpy, and entropy at 27 degrees for the coprecipitated drug relative to its crystalline form I were 1125 cal/mole, 8439 cal/mole, and 24 eu, respectively, indicating the high degree of molecular randomness and lack of structure in these high energy systems. These results provide strong evidence for the presence of an amorphous state of sulfathiazole as the controlling phase of both the solubility and dissolution rate experiments involving the high energy form of sulfathiazole obtained by coprecipitation with povidone.