by Christopher J. Thompson, Ph.D., Vice President of Commercial Development, BrightSpec, Inc.
For decades, researchers have relied on analytical techniques like mass spectrometry (MS) and gas chromatography (GC) to navigate the complexities of drug formulation, identify impurities, and ensure product quality. Yet, as these traditional methods grapple with inherent limitations—particularly in the analysis of isomeric compounds, chiral molecules, and volatile organic solvents—the industry finds itself at a crossroads. Molecular rotational resonance (MRR) spectroscopy is a revolutionary technology poised to turn the tide in drug development. By confronting the challenges that have long plagued MS and GC, MRR is not just an alternative—it’s a bold new frontier that redefines what’s possible in pharmaceutical analysis.
When gold standards rust
As the demands of modern drug development evolve, the limitations of MS- and GC-based techniques have become increasingly apparent. MS, for all its strengths, struggles to differentiate between isomeric compounds—molecules that share the same molecular formula but differ in structural arrangement. This often results in low confidence in the final structure, making it challenging to accurately identify and quantify individual components, particularly in complex mixtures where isomeric and chiral compounds coexist.
Although effective for many applications, GC relies on separating compounds based on their volatility and polarity. This process demands extensive method development and meticulous optimization—time-consuming and resource-intensive tasks. Furthermore, the reliance on costly consumables only adds to the complexity and expense of GC analysis.
A particularly daunting challenge for researchers is the analysis of chiral compounds—molecules that exist as non-superimposable mirror images, each with potentially different biological activities. As such, determining the enantiomeric purity of these molecules is crucial, yet traditional methods like chiral GC or MS often require specialized columns or reagents, further escalating costs and time requirements. Even with these resources, the sensitivity and resolution needed to quantify enantiomeric excess accurately may still fall short.
Overcoming challenges with MRR spectroscopy
MRR spectroscopy provides an additional tool to address limitations that have long held MS and GC back. By capturing the rotational transitions of molecules in the gas phase, MRR delivers a one-of-a-kind spectral fingerprint that reveals the intricate details of a molecule's three-dimensional structure with unparalleled precision. This cutting-edge technique outperforms traditional methods in analyzing isomeric mixtures, chiral compounds, and volatile organic solvents, positioning MRR as the go-to powerhouse for pharmaceutical research. By combining this absolute spectra fingerprint with the contextual information of chromatography and mass spectra, MRR can provide a more comprehensive view of complex mixtures.
Mastering isomers with MRR
Until now, identifying and quantifying isomeric compounds without complex separation procedures has been challenging. MRR spectroscopy changes the game by offering distinct rotational spectra for each isomer, enabling researchers to easily distinguish even the most closely related structures.1 This capability is particularly valuable in the pharmaceutical industry, where the presence of isomeric impurities can profoundly impact drug safety and efficacy.
For example, a recent breakthrough study uses MRR spectroscopy to analyze regioisomeric impurities in the synthesis of an HIV integrase inhibitor.1,2 MRR’s high resolution and pinpoint selectivity identified and quantified these impurities quickly, providing vital insights that streamlined the synthetic process. This is the true power of MRR—a tool that turns complexity into clarity and redefines what’s possible in drug development.
Chiral analysis and enantiomeric purity
A distinct advantage over traditional methods that rely on chiral columns or derivatization reagents is MRR’s ability to directly analyze chiral molecules in their natural form. Chiral tagging, as it’s called, is a powerful technique that simply attaches a small and well-characterized chiral molecule (the “tag”) to the analyte molecule without changing its structure. MRR spectroscopy is then able to differentiate between the "right-handed" (R) or "left-handed" (S) enantiomers based on their unique rotational spectra. This approach simplifies the analysis and dramatically enhances sensitivity, enabling precise determination of enantiomeric excess (EE) without the need for reference standards.3 Accurately and rapidly measuring EE ensures the correct enantiomeric purity for a safer and more effective drug product. This is essential for meeting regulatory standards and optimizing the therapeutic outcomes of chiral drugs.
A recent study showcases MRR's effectiveness in determining the EE of pantolactone, a key chiral intermediate in the synthesis of pantothenic acid (Vitamin B5).4 The results demonstrate quantitative agreement with traditional chiral GC methods but with a marked reduction in analysis time and sample consumption. Additionally, MRR's high precision was corroborated in studies analyzing chiral pharmaceuticals, where it consistently delivered quick and reliable results.
Simplifying volatile solvent analysis
Traditional methods for solvent analysis, such as headspace GC, often involve complex sample preparation and may not always achieve the necessary sensitivity for certain solvents (e.g., Class 2 Mixture C solvents). Thus, development timelines can face weeks of delays and the unnecessary use of valuable resources. MRR spectroscopy, though, can directly analyze solvents without the need for separation or consumables. The technique provides equivalent or better sensitivity than GC, with the added benefit of faster analysis times.
Volatile organic solvents in final drug products can pose safety risks. MRR spectroscopy effectively analyzes residual solvents in pharmaceutical products, meeting the stringent requirements of the U.S. Pharmacopeia Chapter <467> for Class 2 solvents.The innovative approach addresses longstanding industry challenges, providing a rapid, accurate, and highly selective method for identifying and quantifying residual solvents without the extensive method development required by traditional GC.
Future perspectives
Beyond its applications in isomer and chiral analysis, MRR spectroscopy has been recognized as a potential high-impact solution in process analytical technology (PAT) and reaction monitoring. MRR's ability to provide real-time, high-resolution data on reaction mixtures makes it an attractive option for monitoring complex chemical processes. Unlike traditional PAT techniques, which often require offline analysis and extensive sample preparation, MRR has the potential to be integrated directly into the production line, providing continuous feedback on reaction progress and product quality.5,6 Future applications could include broader reaction classes and solvent compatibility.
For instance, MRR was used to monitor the asymmetric hydrogenation of artemisinic acid, a key step in synthesizing the antimalarial drug artemisinin.7 The technique allowed for the real-time quantitation of reaction products and impurities, optimizing reaction conditions and reducing the time required for process development.
As the pharmaceutical industry accelerates toward more complex and demanding drug development challenges, more accurate, faster, and cost-effective analytical techniques are becoming increasingly important. Although invaluable, mass spectrometry and gas chromatography are not without their challenges, particularly in analyzing isomeric and chiral compounds. Molecular rotational resonance spectroscopy transcends the need for complex separations and consumable-intensive processes, streamlining workflows and cutting down analysis times dramatically. The high redundancy of MRR’s spectral data ensures an extraordinary level of certainty, transforming what was once a daunting analytical challenge into a straightforward, binary decision.
As MRR continues to prove its versatility and power—from isomer and chiral analysis to real-time process monitoring—this technology is not merely an enhancement but a fundamental shift in how pharmaceutical analysis can be conducted.
References
- Neill JL, Evangelisti L, Pate BH. Analysis of isomeric mixtures by molecular rotational resonance spectroscopy. Anal Sci Adv. 2023.
- Neill JL, Sheridan P, Holder AM, et al. Rapid quantification of isomeric and dehalogenated impurities in pharmaceutical raw materials using MRR spectroscopy. Anal Chem. 2020;92(17):11961-11967.
- Craig SM, Marshall MD, Peeters JW, et al. Rapid Enantiomeric Excess Measurements of Enantioisotopomers by Molecular Rotational Resonance Spectroscopy. J Am Chem Soc. 2024;146(2):815-826.
- López M, Murakami T, McCarthy MC, McMahon RJ. Chiral analysis of pantolactone with molecular rotational resonance spectroscopy. Anal Chem. 2022;94(1):457-464.
- The Sleeping Giant Awakes: Defining a New Era of MRR Spectroscopy. Technology Networks. Published 2023. Accessed August 14, 2024. technologynetworks.com/analysis/articles/the-sleeping-giant-awakes-defining-a-new-era-of-mrr-spectroscopy-385291
- Byars AA, Kompally KR, Mechnick E, et al. An automated, highly selective reaction monitoring instrument using molecular rotational resonance spectroscopy. Precis Chem. 2024;2(1):57-62.
- Neill JL, Yang Y, Muckle MT, et al. Online stereochemical process monitoring by molecular rotational resonance spectroscopy. Org Process Res Dev. 2019;23(5):1046-1051.
About the author: Dr. Christopher Thompson earned his Ph.D. in Physical Chemistry from the University of Massachusetts Amherst. Over the next 17 years, he held various scientific and customer-facing roles at Bruker Daltonics, culminating in his position as Global Business Development Manager for the FT-ICR business unit. In late 2020, Dr. Thompson joined BrightSpec, a burgeoning start-up, where he now serves as Vice President of Commercial Development, spearheading the commercialization efforts of MRR technology.