Phosphate buffer interferes dissolution of prazosin hydrochloride in compendial dissolution testing

The purpose of this study was to elucidate the lack of supersaturation behavior in the dissolution profile of prazosin hydrochloride (PRZ-HCl) in the compendial dissolution test. The equilibrium solubility was measured by a shake-flask method. Dissolution tests were performed by a compendial paddle method with a phosphate buffer solution (pH 6.8, 50 mM phosphate). The solid form of the residual particles was identified by Raman spectroscopy. In the pH range below 6.5, the equilibrium solubility in phosphate buffer was lower than that in the unbuffered solutions (pH adjusted by HCl and NaOH). Raman spectra showed that the residual solid was a phosphate salt of PRZ. In the pH range above 6.5, the pH-solubility profiles in the phosphate buffer solutions and the unbuffered solutions were the same. The residual solid was a PRZ freebase (PRZ-FB). In the dissolution test, PRZ-HCl particles first changed to a phosphate salt within 5 min, then gradually changed to PRZ-FB after several hours. Since the intestinal fluid is buffered by the bicarbonate system in vivo, the dissolution behavior in vivo may not be properly evaluated using a phosphate buffer solution. For drugs with a low phosphate solubility product, it is necessary to consider this aspect.