Reevaluation of solubility of tolbutamide and polymorphic transformation from Form I to unknown crystal form.

Thermodynamic stability order of tolbutamide between polymorphs was evaluated using calorimetry and spectroscopic analysis. The heat of solution (DeltaH) of Forms I-III measurements were carried out in dimethylsulfoxide between 298.2K and 319.2K. It was found that the DeltaH of Forms II and III was increased nearly parallel with a temperature rise. However, change of the DeltaH of Form I with a temperature rise was not in correspondence with that of other forms. Solubility data confirmed the change in DeltaH of Form I around 308.2K. XRD-DSC measurement of Form I detected a polymorphic transformation (Form I(L)-->unknown form) at 311K. Obtained data suggested that the new crystal form (Form I(H)) would exist above 311K, and the order of thermodynamic stability was "Form III<Form I(L)<Form II" below the transition temperature, on the other hand, the order was changed in "Form I(L) and Form I(H)<Form III<Form II" above the transition temperature. The crystal structure of Form I(H) was determined by measurement of PXRD pattern on BL19B2 at SPring-8, and the change in the solid-state NMR spectrum confirmed the transformation of the crystal structure of Form I(L).