On the dimorphism of prednisolone: The topological pressure-temperature phase diagram involving forms I and II.

ABSTRACT

The dimorphism of the corticosteroid anti-inflammatory drug prednisolone has been investigated by the construction of a topological pressure-temperature phase diagram, using crystallographic and calorimetric data. The system is enantiotropic, because the temperature of the I-II equilibrium under atmospheric conditions (400-463 K) is lower than that of the two melting equilibria (518.7 K for form II and 526.3 K for form I). The slope of the I-II equilibrium in the pressure-temperature phase diagram is negative and relatively steep; therefore, form II, which is the stable form at room temperature, will not easily encounter conditions where form I will become stable even under industrial processing conditions. On the other hand, extreme small amounts of form I have been observed to spontaneously transform into form II in a time interval of about six years at room temperature and it can be concluded that although form I is very persistent under ambient conditions, it does slowly convert into form II. Moreover, the system does not obey the density rule.