On the dimorphism and the pressure-temperature state diagram of gestodene, a steroidal progestogen contraceptive

The pressure-temperature phase diagram of the dimorphism of the contraceptive drug gestodene is constructed using the temperature and enthalpy of fusion of form I (469.5K, 107Jg^-1), and those of the endothermic transition from form II to form I (311K, 8.52Jg^-1). At ordinary pressure, the sign of the enthalpy of this transition indicates that these polymorphs are enantiotropically related and that form II, whose melting temperature is calculated to be about 452K, is the stable form at room temperature. Considering the inequality in the specific volumes of the two polymorphs, it is shown that the two forms remain enantiotropically related on increasing pressure, because the I-II equilibrium and the melting equilibria I-L and II-L diverge as a consequence of the negative slope dP/dT of the solid-solid equilibrium. In addition, it is demonstrated that the heats of dissolution, inferred from solubility measurements, lead to virtually the same value of the heat of transition from II to I as for the differential scanning calorimetry measurements.