Category Archives: Pharmaceuticals

DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients #DNPNMR

Zhao, L., et al., DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients. Magn. Reson. Chem., 2017. 0(0).

https://www.ncbi.nlm.nih.gov/pubmed/29193278

Solid-state NMR spectroscopy has become a valuable tool for the characterization of both pure and formulated active pharmaceutical ingredients (APIs). However, NMR generally suffers from poor sensitivity that often restricts NMR experiments to nuclei with favorable properties, concentrated samples, and acquisition of one-dimensional (1D) NMR spectra. Here, we review how dynamic nuclear polarization (DNP) can be applied to routinely enhance the sensitivity of solid-state NMR experiments by one to two orders of magnitude for both pure and formulated APIs. Sample preparation protocols for relayed DNP experiments and experiments on directly doped APIs are detailed. Numerical spin diffusion models illustrate the dependence of relayed DNP enhancements on the relaxation properties and particle size of the solids and can be used for particle size determination when the other factors are known. We then describe the advanced solid-state NMR experiments that have been enabled by DNP and how they provide unique insight into the molecular and macroscopic structure of APIs. For example, with large sensitivity gains provided by DNP, natural isotopic abundance, (13) C-(13) C double-quantum single-quantum homonuclear correlation NMR spectra of pure APIs can be routinely acquired. DNP also enables solid-state NMR experiments with unreceptive quadrupolar nuclei such as (2) H, (14) N, and (35) Cl that are commonly found in APIs. Applications of DNP-enhanced solid-state NMR spectroscopy for the molecular level characterization of low API load formulations such as commercial tablets and amorphous solid dispersions are described. Future perspectives for DNP-enhanced solid-state NMR experiments on APIs are briefly discussed.

In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR #DNPNMR

Ni, Q.Z., et al., In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR. The Journal of Physical Chemistry B, 2017. 121(34): p. 8132-8141.

http://dx.doi.org/10.1021/acs.jpcb.7b07213

A principal advantage of magic angle spinning (MAS) NMR spectroscopy lies in its ability to determine molecular structure in a noninvasive and quantitative manner. Accordingly, MAS should be widely applicable to studies of the structure of active pharmaceutical ingredients (API) and formulations. However, the low sensitivity encountered in spectroscopy of natural abundance APIs present at low concentration has limited the success of MAS experiments. Dynamic nuclear polarization (DNP) enhances NMR sensitivity and can be used to circumvent this problem provided that suitable paramagnetic polarizing agent can be incorporated into the system without altering the integrity of solid dosages. Here, we demonstrate that DNP polarizing agents can be added in situ during the preparation of amorphous solid dispersions (ASDs) via spray drying and hot-melt extrusion so that ASDs can be examined during drug development. Specifically, the dependence of DNP enhancement on sample composition, radical concentration, relaxation properties of the API and excipients, types of polarizing agents and proton density, has been thoroughly investigated. Optimal enhancement values are obtained from ASDs containing 1% w/w radical concentration. Both polarizing agents TOTAPOL and AMUPol provided reasonable enhancements. Partial deuteration of the excipient produced 3× higher enhancement values. With these parameters, an ASD containing posaconazole and vinyl acetate yields a 32-fold enhancement which presumably results in a reduction of NMR measurement time by ∼1000. This boost in signal intensity enables the full assignment of the natural abundance pharmaceutical formulation through multidimensional correlation experiments.

Static DNP-NMR Spectroscopy to Characterize Active Pharmaceutical Ingredients #DNPNMR

Dynamic Nuclear Polarization in general is no new method, but the focus of modern applications has initially been on bio-macromolecules under magic-angle-spinning (MAS) conditions.

One application that came out-of-the-blue was using DNP-NMR spectroscopy to study surface materials by DNP-NMR spectroscopy (for example Lafon et al., 2011) opening up a complete new research area within material science that traditionally struggled with very low signal-to-noise (S/N) ratios.

Even the application of DNP-NMR spectroscopy to study small molecules was not immediately evident, but as demonstrated in Rossini et al, 2012 DNP offers the possibility to record 13C correlation spectra of unlabeled molecules such as glucose in just 16 hours. Without DNP this experiment would require months of spectrometer time.

The majority of the DNP-NMR experiments that have been reported in recent years use gyrotron-based DNP-NMR systems and MAS-DNP probes operating at about 100 K. Alternatively, there is a small group of researchers that use DNP systems based on a solid-state microwave source. These systems have are typically limited by their output power, which ranges between >80 mW at 263 GHz (400 MHz 1H NMR) to < 200 mW at 197 GHz (300 MHz 1H NMR). At lower frequencies the output power increases and > 500 mW can be reached for systems operating at 95 GHz. A comprehensive overview of low-power DNP-NMR systems can be found in Siaw et al., 2016.

Because of the limited output power, DNP experiments are performed at temperatures < 20 K, which requires cooling with liquid helium (very common for example in EPR experiments) and can be cost-effective when using a cryostat (e.g. at 10 K the consumption is about 0.5 l/hr). Furthermore, with the increasing popularity of cryogen-free systems some cryostats don’t require any liquid cryogens anymore for cooling. The main advantage is the reduced cost since a solid-state source based DNP-NMR system typically comes at a 10th of the cost of a gyrotron-based system.

At first sight it seems as if the applications of static DNP are very limited. However, when I was at ENC this year I listened to a talk by David A. Hirsh entitled “35Cl Dynamic Nuclear Polarization Solid-State NMR of Active Pharmaceutical Ingredients”. David is a graduate student in the group of Rob Schurko, University of Windsor and gave a very nice talk on using DNP-NMR spectroscopy to characterize Active Pharmaceutical Ingredients (API) using 35Cl solid-state NMR spectroscopy. Since 35Cl is a quadrupole nucleus the corresponding NMR spectra are typically very broad. MAS does only have a small effect, mainly on the center transition, and traditionally wide-line spectra of static solids are recorded.

To overcome sensitivity issues, the group has developed pulse sequences such as WURST-CPMG or BRAIN-CP to rapidly record broad 35Cl patterns even at moderate magnetic field strengths (e.g. 9.4 T, 400 MHz 1H NMR). However, recording a single spectrum often requires several hours of signal averaging to achieve a sufficiently high signal-to-noise (S/N) ratio. With the aid of DNP these acquisition times can be dramatically reduced to just minutes. In his talk at ENC David described using a grytron-based DNP-NMR system, equipped with a MAS-DNP probe head in his experiments. Polarizing the sample is done while the rotor is spinning, but the rotor is stopped prior to recording the wide-line NMR spectrum. 

This experiment seems to be ideally suited for a low-power DNP-NMR system for static solids, using a cryostat for sample cooling. This would greatly simplify the experiment because starting and stopping the rotor is not required anymore. Because the experiment is performed at much lower temperatures, there will be an additional boost in sensitivity and multi-dimensional correlation experiments should be possible, experiments that are close to impossible to perform without the aid of DNP.

In recent years the NMR community has witnessed the transition of DNP-NMR spectroscopy from an exotic method with a limited number of applications to a method with more and more applications. High-field DNP-NMR spectroscopy either based on a gyrotron or using a low-power solid-state source is still a very young method with many possibilities and I’m very excited to see what other applications lie in the future. I am however convinced that DNP-NMR spectroscopy will find their way into many more labs in the future and that the method will become an integral part of the NMR toolbox.

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