New Melting Data of the Two Polymorphs of Prednisolone

Prednisolone is known to exist in two anhydrous solid polymorphic forms. The substance is known to degrade upon melting, resulting in erroneous melting data, as shown by the widely scattered results reported in the literature. In this article, thermal analyses carried out at different scan rates show that the onset temperature and the enthalpy value of the signal increase with the scan rate and reach plateau values for high scan rates. Owing to flash scanning calorimetry, the plateau value for the temperature has been identified as the "true" temperature of melting of both polymorphs. This consistent set of new thermodynamic data on the two solid forms leads to the conclusion that both forms are unambiguously enantiotropes of each other. The solid-solid transition has been observed experimentally for the first time and has been confirmed by calculation.