Crystal Structures of Tetramorphic Forms of Donepezil and Energy/Temperature Phase Diagram via Direct Heat Capacity Measurements

Abstract

Donepezil is used for the palliative treatment of mild to moderate dementia of the Alzheimer’s disease. Donepezil crystallized as four solvent-free polymorphs including forms I, II, C, and F which differ in molecular conformations and packing. Conformational difference comes from the torsion of the 2, 3-dihydroinden-1-one moiety and methyl-benzyl ring with respect to piperidyl ring. Similar melting temperatures and heats of fusion were observed for four solvent-free polymorphic forms and made polymorph selection by solvent crystallization method poor. The relative thermodynamic stability relationships of each polymorph with respect to the amorphous form were determined using direct heat capacity (Cp) measurement and then used to evaluate the relative thermodynamic stability between polymorphs. Form F was the stable form over the temperature ranges we studied. Other than form F, form C was the stable form among three polymorphic forms including I, II, and C, below 53 °C, and is enantiotropically related to both forms I and II. Form II is the stable form above 53 °C and monotropically related to form I. The thermodynamic relationships between polymorphs were further confirmed by measuring the solubility over temperatures ranging from 35 to 60 °C in ethanol. Slurry conversion in ethanol, isopropyl alcohol, and cyclohexane was conducted to provide a guideline to obtain pure and desired polymorphic forms. The establishment of the thermodynamic relationships among four polymorphic forms greatly facilitated polymorph selections of donepezil.