Solid-State Characterization of Three Polymorphs of an Orally Available Analog of Diethylenetriaminepentaacetic Acid

The present work investigated the physical and thermal characteristics of three polymorphic forms (namely, PF1, PF2, and PF3) of a diethyl ester analog of diethylenetriaminepentaacetic acid (C2E2) produced under varying conditions. The identity of each form of C2E2 was confirmed by 1H-NMR, 13C-NMR, and mass spectroscopy. The different polymorphic forms exhibited solubilities ranging from 40 to 150 mg/mL. Powder X-ray diffraction (PXRD) and electron microscopy confirmed that all three forms were crystalline, two of which being scaly, and the third being well-formed. Infrared and Raman spectroscopy revealed differences in the C = O bonding region while differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) showed widely different melting points with only one thermal event for each compound. The comparison of the melting points and heats of fusion show that the PF1 is monotropically related to both PF2 and PF3, while PF2 and PF3 are enantropically related. Our finding indicates that PF3 is the thermodynamically stable polymorph and will be used for in vitro and in vivo experiments.