Preformulation study of moisture effect on the physical stability of pyridoxal hydrochloride

During a preformulation programme the effect of atmospheric moisture on pyridoxal HCI (PL) was investigated. A complete moisture sorption isotherm was derived for PL at 25 o C. In addition, PL samples were stored at 11 and 75% relative humidity (RH) for 2 weeks. Changes in the samples were detected by differential scanning calorimetety (DSC), infrared spectroscopy (IR) and scanning electron microscopy (SEM). DSC and IR studies provided strong evidence of two polymorphic forms of PL. In addition, DSC thermograms of the samples stored at 75% RH showed changes indicative of a partial polymorphic transformation; further, these changes in crystal habit were observed in the SEM study. The moisture sorption isotherm shows an abrupt decrease in moisture content in the region of 57-68% RH. This may correspond to the formation of the less hygroscopic, stable polymorph. PL is physically unstable at elevated levels of moisture, however, exposure to certain levels of RH may enhance its stability due to transformation of polymorphs.