FDA and US Pharmacopeia Explore Expanding Use of NMR for Drug Quality Testing

Nuclear magnetic resonance (NMR) spectroscopy continues to be used in a variety of applications, including drug quality testing and drug identification. The Food and Drug Administration (FDA) has been exploring the use of this technique, along with mass spectrometry (MS), in connection with staff at the United States Pharmacopeia (USP). Spectroscopy spoke with FDA chemist, David Keire, on this topic.

Why are the use of NMR spectroscopy and MS being explored by the Food and Drug Administration (FDA) as candidates for the next generation of United States Pharmacopeia (USP) monograph assays?

Modern pharmaceutical products are often complex drugs derived from a biotechnology process rather than small-molecule drugs manufactured by organic synthesis. Because the size and complexity of pharmaceuticals has increased, the tests used to ensure their quality must necessarily provide more information on the multiple characteristics relevant to drug identity and purity to sufficiently describe their analytical attributes. Thus the FDA has explored the used of orthogonal information-rich assays (for example, MS and NMR) that measure multiple attributes in one experiment as possible USP monograph tests for complex drugs. A quick literature search shows that MS and NMR methods are in common use for the analysis of pharmaceuticals and that these approaches can provide “fingerprint” patterns for identification of complex drugs in an easy manner. In addition, NMR and MS are proven techniques for identifying and measuring the level of impurities in formulated drug products. Thus these technologies are strong candidates for inclusion in a modernized USP monograph for complex drugs.

What are some of the specific drug characteristics that can be elucidated by NMR that cannot be clarified with high performance liquid chromatography (HPLC) or other methods?

Analysis of drugs by HPLC with common detectors lacks specificity, as peaks in a chromatogram may contain co-eluted species that are structural isomers, closely related to the drug of interest or even completely different species that are eluted under the same conditions. In addition, sample matrix effects and different response factors for each analyte can complicate HPLC analyses. Finally, the sensitivity of the HPLC elution profiles to variability in buffer composition, column lifecycle, and equilibration time are well known.

By contrast, an NMR spectrum can give a specific, immediate (within one scan) read-out on drug identity without separation via chemical shifts and intensities of signals from the drug molecule. In addition, the samples are prepared with pure solvents and there are no HPLC column-life effects or hour-long equilibration times needed to acquire NMR spectra. Furthermore, because NMR has exquisite sensitivity to molecular structure, the spectra provide distinct patterns of signals for the drug and isomers or closely related compounds (for example, degradation products or impurities) in the sample. For quantification, the NMR experiment can be run in a way so that the area under each peak is directly proportional to the concentration of the NMR active nucleus that gave rise to the signal (that is, no individual response factors or standards are necessary). The limitations of NMR are the amount of sample needed for analysis relative to other techniques, the cost of the instrumentation, and the limitation on the size of the molecule that can be effectively analyzed (~