Advances in NMR‐Based Characterization of Drug Substances, Impurities, Degradants, and Drug Metabolites

Advances in NMR-Based Characterization of Drug Substances, Impurities, Degradants, and Drug Metabolites. Methods in Drug Discovery and Discovering Lead Molecules, by Josep Saurí, Mikhail Reibarkh, R. Thomas Williamson, Gary E. Martin. Burger’s Medicinal Chemistry and Drug Discovery, 8th Edition. 27 April 2021. https://doi.org/10.1002/0471266949.bmc302

In the approximately 70 years since the first reports of experiments that led to what is now known as nuclear magnetic resonance or NMR spectroscopy, the technique has become the cornerstone of molecular structure characterization. From the isolation of strychnine to the confirmation of its complex structure in a total synthesis reported by Woodward consumed a century and a half. In contrast, using a modern 600 MHz NMR instrument equipped with a 1.7 mm MicroCryoProbe™, that task can now be accomplished in 24 hours, including the acquisition of ¹⁵N chemical shift information on a few milligrams of sample. Such is the magnitude of the advances that have been made in the discipline. This article is not intended to make NMR spectroscopists out of a practicing medicinal chemistry; rather, the article is intended to acquaint the reader with recent advances in NMR techniques that are making it possible to successfully and unequivocally characterize ever increasingly more complex structures being synthesized in the laboratory as potential drug candidates or isolated from nature. Our goal was to augment the NMR knowledge of the medicinal chemist, better equipping him or her to more effectively interact with the NMR spectroscopy staff at the chemist’s facility. The article begins with a discussion of new experimental methods that allow the acquisition of broadband decoupled ¹H NMR spectra that resemble an ¹H-decoupled ¹³C spectrum, with every proton resonance reduced to a singlet. Pure-shift heteronuclear shift correlation methods are described next, followed by new long-range heteronuclear chemical shift correlation experiments and then hyphenated techniques to identify components of complex, overlapped proton homonuclear spin systems. Techniques are also described for the unequivocal identification of vicinal neighbor carbon resonances that can be invaluable when dealing with complex impurities or degradation products. Covariance NMR data processing methods that allow 2D NMR experiments to be treated as large numerical matrices that can be manipulated using matrix algebraic operations are briefly discussed. Finally, anisotropic NMR methods are considered, which afford investigators the means of orthogonally verifying chemical structure constitution and configuration without investigator bias that often can lead to erroneously assigned structures. The latter eventuality, of course, can slow development timelines or could potentially have devastating patent protection consequences and regulatory implications.