Evaluation of reversible polymorphic phase transitions by thermal analysis

Crystalline states of 1,2-dihydro-6-neopentyl-2-oxonicotinic acid, an investigational antidiabetic drug, were evaluated by thermal analyses. Two polymorphs were detected for the drug, Form I (m.p. 193 °C) and Form II (m.p. 196 °C). Interconversion of the polymorphs upon cyclic solid-melt transitions provided confirmation of the crystal forms. Solidification of the melt was observed to occur either at 162 or 182°C with the formation of Form I or Form II crystals, respectively. Form I underwent partial conversion to Form II upon heating at 10°C min−1 when nucleating crystals of Form II were present in the sample. Differential scanning calorimetry (DSC) thermograms were recorded for different lots of the drug, solventrecrystallized samples, and a series of known mixtures of Form I and II polymorphs. The study illustrates the usefulness of cyclic heat-cool studies to characterize polymorphic crystal forms of drugs.