INVESTIGATING THE EFFECT OF ENANTIOMERIC RATIO AND PREPARATION METHOD ON CRYSTALLINE FORMS OF PROLINE USING SOLID-STATE NMR SPECTROSCOPY

Abstract

Chiral molecules are prevalent among currently marketed pharmaceutical products, many of which are solid formulations. The solid-state form of a drug can have a dramatic effect on its solubility, dissolution rate (hence bioavailability), physical stability, and interaction with excipients; therefore, understanding the solid forms that exist for a drug molecule is critical to ensure product performance and safety. Analysis of solid systems typically requires the application of several analytical techniques, one or two of which may be particularly helpful. In this thesis work, solid-state NMR spectroscopy (SSNMR) was found to be a particularly powerful method for characterizing proline enantiomers in the solid state. Using SSNMR, we evaluated the differences in crystal forms of proline that resulted from changes in enantiomeric ratio and crystallization conditions. Various ratios of D- and L-proline (0-50% L-proline with 100-50% D-proline) were crystallized from aqueous solution and by lyophilization, spray drying, and cryogrinding. These methods produced multiple crystalline forms, including previously unreported polymorphs and chiral defects, in which the L-proline molecules were kinetically trapped in the D-proline crystal lattice. This thesis work has significant implications for the development of pharmaceutical solids. Whereas typical solid-form screening assays involve crystallizing from various solvents, we used lyophilization, spray drying, and cryogrinding. These nontraditional crystallization methods produced high-energy solids (e.g., metastable polymorphs, defects, and amorphous material) and an unreported more thermodynamically stable co-crystal polymorph. Additionally, the presence of L-proline enantiomeric impurity altered the crystallization process of D-proline, therein affecting the crystal product. "Chiral doping" is potentially a valuable addition to current solid-form screening processes, as it may produce forms that would not be observed during normal screening methods. SSNMR was highly suited for investigating proline enantiomers in the solid state. Different solid forms, including crystal defects and amorphous material, possessed different peaks in 13C CP-MAS NMR spectra. Also, isotopic labeling, combined with spectral subtraction, allowed for identification and relative quantitation of solid forms within a wide range of enantiomeric ratios. Phase separation among these forms was confirmed by 1H T1 relaxation measurements, and 2D-SSNMR experiments demonstrated the potential to provide in-depth structural information.