19F Solid-state NMR characterization of pharmaceutical solids

Probing Chemical Equilibrium in Frozen Sodium Phosphate Buffer Solution by 31P Solid-State NMR

Phosphate buffers are crucial for cryopreservative stability in pharmaceuticals, food processing, biomedical sciences, and biology. However, their freeze concentrates lack quantitative characterization, especially regarding the physicochemical properties of phosphate salt species in equilibrium at subzero temperatures. This study employs 31P solid-state NMR (ssNMR) to analyze frozen sodium phosphate (NaP) solutions, providing insights into phase composition, ionic strength, and pH. For the first time, we have directly quantified phosphate species in frozen NaP buffer, including crystallized disodium phosphate dodecahydrate (Na2HPO4·12H2O) content and the concentrations of H2PO4- and HPO42- in the freeze concentrate. This enabled the calculation of the pH as well as the ionic strength in the freeze concentrate. Trehalose effectively mitigated pH shifts in buffer solutions by preventing the selective crystallization of salt, a spectroscopic phenomenon not previously observed experimentally.

Cyclodextrin Metal-Organic Frameworks as a Drug Delivery System for Selected Active Pharmaceutical Ingredients

The cyclodextrin-based metal-organic frameworks (CD MOFs) are a suitable molecular platform for drug delivery systems of various active pharmaceutical ingredients (APIs). The low toxicity and cost-efficient synthesis make CD MOFs an attractive host for the encapsulation of APIs. In this study, we created a model system based on γCD-K MOFs with widely used drugs carmofur (HCFU), 5-fluorouracil (5-FU), and salicylic acid (HBA) to study host-guest encapsulation methods using different crystallization protocols. The host-guest complexes of API:CD MOF in an in-depth study were investigated by liquid chromatography-mass spectrometry (LC-MS), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and 19F- and 13C-detected solid-state NMR spectroscopy (ssNMR). These techniques confirmed the structure and interaction sites within the encapsulation product in the host-guest complex. We also evaluated the toxicity and biocompatibility of the API:CD MOF complex using in vitro and in vivo methods. The cytotoxicity, hepatotoxicity, and neurotoxicity were established with cell lines of fibroblasts (BJ), human liver cell line (HepG2), and human oligodendrocytic cells (MO3.13). Then, Danio rerio was used as an in vivo experimental model of ecotoxicity. The results showed the choice of γCD-K-5 as the most protective and safe option for drug encapsulation to decrease its toxicity level against normal cells.

Solid-State NMR Spectroscopy to Probe State and Phase Transitions in Frozen Solutions

Freezing is commonly encountered during the processing and storage of biomacromolecule products. Therefore, understanding the phase and state transitions in pharmaceutical frozen solutions is crucial for the rational development of biopharmaceuticals. Solid-state nuclear magnetic resonance spectroscopy (ssNMR) was used to analyze solutions containing sodium phosphate buffer, histidine, and trehalose. Upon freezing, crystallization of disodium phosphate hydrogen dodecahydrate (Na2HPO4·12H2O, DPDH) and histidine was identified using 31P and 13C ssNMR, respectively, and confirmed by synchrotron X-ray diffractometry (SXRD). Using histidine as a molecular probe and based on the chemical shifts of atoms of interest, the pH of the freeze concentrate was measured. The unfrozen water content in freeze concentrates was quantified by 1H single pulse experiments. 13C-insensitive nuclei enhancement by polarization transfer (INEPT) and cross-polarization (CP) experiments were used as orthogonal tools to characterize the solutes in a "mobile" and a more "solid-like" state in the freeze-concentrated solutions, respectively. The above analyses were applied to a commercial monoclonal antibody (mAb) formulation of dupilumab. This work further establishes ssNMR spectroscopy as a highly capable biophysical tool to investigate the attributes of biopharmaceuticals and thereby provide insights into process optimization and formulation development.

Pushing the limit of MQMAS for low-γ quadrupolar nuclei in pharmaceutical hydrochlorides

Solid-state NMR of quadrupolar nuclei such as ³⁵Cl has become a useful tool to characterize polymorphism in pharmaceutical hydrochlorides. The two-dimensional multiple-quantum magic-angle spinning (MQMAS) experiment can achieve isotropic resolution, and separate quadrupolar line shapes for samples with multiple sites but the pulse sequence efficiency is often low, limiting applications due to the intrinsically low NMR signals and rf field from the low gyromagnetic ratios γ. The use of cosine low-power MQMAS pulse sequences and high magnetic fields is presented to push the limit of MQMAS for insensitive low-γ quadrupolar nuclei. The improved efficiency and fields up to 35.2 T enable the acquisition of MQMAS spectra for pharmaceutical samples with multiple ³⁵Cl sites, large quadrupolar couplings and/or in diluted dosage forms.

Efficient analysis of pharmaceutical drug substances and products using a solid-state NMR CryoProbe

Solid-state nuclear magnetic resonance (ssNMR) is a high-resolution and versatile spectroscopic tool for characterizing pharmaceutical solids. However, the inherent low sensitivity of NMR remains a significant challenge in the analysis of natural abundance drug substances and products. Here, we report, for the first time, the application of a CPMAS CryoProbe™ to improve the sensitivity of ¹³C and ¹⁵N detection by approximately 5 to 6 times for solid-state analysis of a commercial pharmaceutical drug posaconazole (POSA). The sensitivity enhancement enables two-dimensional (2D) ¹³C-¹³C and ¹H-¹⁵N correlation experiments, which are otherwise time-prohibitive using regular MAS probes, for resonance assignment and structural elucidation. These polarization transfer and correlation experiments reveal drug-drug and drug-polymer interactions in amorphous POSA and its amorphous solid dispersion formulation. Our results demonstrated that the CPMAS CryoProbe™ can be widely applied for routine pharmaceutical analysis and advanced structural investigations with significantly enhanced efficiency and throughput.

Ultrafast Magic Angle Spinning Solid-State NMR Spectroscopy: Advances in Methodology and Applications

Solid-state NMR spectroscopy is one of the most commonly used techniques to study the atomic-resolution structure and dynamics of various chemical, biological, material, and pharmaceutical systems spanning multiple forms, including crystalline, liquid crystalline, fibrous, and amorphous states. Despite the unique advantages of solid-state NMR spectroscopy, its poor spectral resolution and sensitivity have severely limited the scope of this technique. Fortunately, the recent developments in probe technology that mechanically rotate the sample fast (100 kHz and above) to obtain "solution-like" NMR spectra of solids with higher resolution and sensitivity have opened numerous avenues for the development of novel NMR techniques and their applications to study a plethora of solids including globular and membrane-associated proteins, self-assembled protein aggregates such as amyloid fibers, RNA, viral assemblies, polymorphic pharmaceuticals, metal-organic framework, bone materials, and inorganic materials. While the ultrafast-MAS continues to be developed, the minute sample quantity and radio frequency requirements, shorter recycle delays enabling fast data acquisition, the feasibility of employing proton detection, enhancement in proton spectral resolution and polarization transfer efficiency, and high sensitivity per unit sample are some of the remarkable benefits of the ultrafast-MAS technology as demonstrated by the reported studies in the literature. Although the very low sample volume and very high RF power could be limitations for some of the systems, the advantages have spurred solid-state NMR investigation into increasingly complex biological and material systems. As ultrafast-MAS NMR techniques are increasingly used in multidisciplinary research areas, further development of instrumentation, probes, and advanced methods are pursued in parallel to overcome the limitations and challenges for widespread applications. This review article is focused on providing timely comprehensive coverage of the major developments on instrumentation, theory, techniques, applications, limitations, and future scope of ultrafast-MAS technology.

Ultrafast magic angle spinning NMR characterization of pharmaceutical solid polymorphism: A posaconazole example

Protons represent the most NMR-sensitive nucleus in pharmaceutical compounds. Therefore, proton-detected solid-state NMR techniques under fast magic angle spinning are among the few solutions to overcome the challenge of low sensitivity to analyze natural abundant drug substances and products. In this study, we report the structural characterization of crystal polymorphs of a commercial drug molecule, posaconazole, with a relatively large molecular weight of 700.8 g·mol^-1 and at the natural abundance. The enhanced sensitivity and resolution at 100 kHz MAS enables the exploration of the distinct intermolecular packing in posaconazole forms I, III, and γ. These results demonstrate that proton-detected homo- and heteronuclear correlation methods can probe the structural details of pharmaceutical polymorphism.

19F fast MAS (60-111 kHz) dipolar and scalar based correlation spectroscopy of organic molecules and pharmaceutical formulations

¹⁹F magic angle spinning (MAS) NMR spectroscopy is a powerful tool for characterization of fluorinated solids. The recent development of ¹⁹F MAS NMR probes, operating at spinning frequencies of 60-111 kHz, enabled analysis of systems spanning from organic molecules to pharmaceutical formulations to biological assemblies, with unprecedented resolution. Herein, we systematically evaluate the benefits of high MAS frequencies (60-111 kHz) for 1D and 2D ¹⁹F-detected experiments in two pharmaceuticals, the antimalarial drug mefloquine and a formulation of the cholesterol-lowering drug atorvastatin calcium. We demonstrate that ¹H decoupling is essential and that scalar-based, heteronuclear single quantum coherence (HSQC) and heteronuclear multiple quantum coherence (HMQC) correlation experiments become feasible and efficient at the MAS frequency of 100 kHz. This study opens doors for the applications of high frequency ¹⁹F MAS NMR to a wide range of problems in chemistry and biology.

New Development in Understanding Drug-Polymer Interactions in Pharmaceutical Amorphous Solid Dispersions from Solid-State Nuclear Magnetic Resonance

Pharmaceutical amorphous solid dispersions (ASDs) represent a widely used technology to increase the bioavailability of active pharmaceutical ingredients (APIs). ASDs are based on an amorphous API dispersed in a polymer, and their stability is driven by the presence of strong intermolecular interactions between these two species (e.g., hydrogen bond, electrostatic interactions, etc.). The understanding of these interactions at the atomic level is therefore crucial, and solid-state nuclear magnetic resonance (NMR) has demonstrated itself as a very powerful technique for probing API-polymer interactions. Other reviews have also reported exciting approaches to study the structures and dynamic properties of ASDs and largely focused on the study of API-polymer miscibility and on the identification of API-polymer interactions. Considering the increased use of NMR in the field, the aim of this Review is to specifically highlight recent experimental strategies used to identify API-polymer interactions and report promising recent examples using one-dimensional (1D) and two-dimensional (2D) experiments by exploiting the following emerging approaches of very-high magnetic field and ultrafast magic angle spinning (MAS). A range of different ASDs spanning APIs and polymers with varied structural motifs is targeted to illustrate new ways to understand the mechanism of stability of ASDs to enable the design of new dispersions.