Solution-mediated phase transformation of salts during dissolution: investigation using haloperidol as a model drug

Soluble salts can undergo solution-mediated phase transformation to a lower solubility form due to pH gradients in the gastrointestinal tract. Therefore, dissolution rate rather than solubility may be the best predictor of bioavailability for such compounds. The purpose of this project was to examine the kinetics of the conversion of a basic compound, haloperidol, and its salt forms using a flow-through dissolution apparatus and rotating disk method in neutral conditions. The effects of buffer concentration, salt form, dissolution apparatus, and hydrodynamics were examined. Raman microscopy was used to characterize solids after dissolution. Haloperidol mesylate and haloperidol chloride showed a decrease in dissolution rate with time in the dissolution media. Haloperidol mesylate and haloperidol chloride dissolution rates also decreased with increasing buffer capacity. Raman microscopy confirmed phase conversion from the salt forms to the free base form in phosphate buffer. Hydrodynamics did not affect the time course of the solution-mediated phase transformation of salt forms. Dissolution and precipitation appear to be a function of pH close to the surface of the dissolving solid. In situations where equilibrium solubility of salts cannot be assessed experimentally, dissolution experiments are useful for examining the extent and duration of the dissolution rate enhancement.