Characterization of Pharmaceutical Cocrystals and Salts by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy

Multicomponent solids such as cocrystals have emerged as a way to control and engineer the stability, solubility, and manufacturability of solid active pharmaceutical ingredients (APIs). Cocrystals are typically formed by solution- or solid-phase reactions of APIs with suitable cocrystal coformers, which are often weak acids. One key structural question about a given multicomponent solid is whether it should be classified as a salt, where the basic API is protonated by the acid, or as a cocrystal, where the API and coformer remain neutral and engage in hydrogen bonding interactions. It has previously been demonstrated that solid-state NMR spectroscopy is a powerful probe of structure in cocrystals and salts of APIs; however, the poor sensitivity of solid-state NMR spectroscopy usually restricts the types of experiments that can be performed. Here, relayed dynamic nuclear polarization (DNP) was applied to reduce solid-state NMR experiment times by 1–2 orders of magnitude for salts and cocrystals of a complex API. The large sensitivity gains from DNP facilitates rapid acquisition of natural isotopic abundance 13 C and 15 N solid-state NMR spectra. Critically, DNP enables double resonance 1 H– 15 N solid-state NMR experiments such as 2D 1 H– 15 N HETCOR, 1 H– 15 N CP-build up, 15 N{ 1 H} J -resolved/attached proton tests, 1 H– 15 N DIPSHIFT, and 1 H– 15 N PRESTO. The latter two experiments allow 1 H– 15 N dipolar coupling constants and H–N bond lengths to be accurately measured, providing an unambiguous assignment of nitrogen protonation state and definitive classification of the multicomponent solids as cocrystals or salts. These types of measurements should also be extremely useful in the context of polymorph discrimination, NMR crystallography structure determination, and for probing hydrogen bonding in a variety of organic materials.