Abstract
The transition temperature and the heat of transition between three polymorphic forms of phenobarbital (form I, form II, and hydrate) were determined by various methods. Bioavailability of these three polymorphic forms was also evaluated by the in vivo absorption test. The transition temperature and the heat of transition were determined by the solubility measurement, the dissolution rate measurement, and differential scanning calorimetry. Crystalline energy differences were determined from the values of the heat of solution measured. The transition temperature and the heat of transition between form I and form II, and between hydrate and form II were determined to be 80.9°and 0.23 kcal/mol, and 49.1°and 1.52 kcal/mol, respectively, by the solubility measurement. These values agreed approximately with those obtained by other methods. On the other hand, the crystalline energy differences were in good agreement with those of the heat of transition by the solubility measurement. In the in vivo absorption test the plasma concentration versus time curves after oral administration of the three polymorphic forms of phenobarbital in rabbits were obtained. These results indicated that each polymorphic form was similar and these polymorphic states of phenobarbital do not affect its bioavailability.
