In the frame of increasingly stringent quality assessment required by the regulators, the pharmaceutical industry has to face increasingly sophisticated counterfeiting practices. Counterfeits based on deliberate copying of processes or on the infringement of current patents for generic medicines are not straightforward to detect, unless the molecular probe is the active molecule itself. In this context, impurity profiling is limited. A tool based on the determination of intramolecular isotopic profiles has been developed to provide manufacturers of APIs (Active Pharmaceutical Ingredients) with a new solution to meet this double requirement. Stable isotope analyses by NMR gives direct access to site-specific isotope content at natural abundance. In this report, it is shown how both ²H and ¹³C NMR spectrometry can provide complementary and valuable information that could be applied to link APIs to their manufacturing source. Isotopic ¹³C NMR offers additional benefits over ²H NMR in using robust adiabatic polarization transfer methods, leading to a tremendous reduction in experimental time. Two approaches are illustrated. Firstly, the use of ²H and single pulse ¹³C NMR spectra obtained on 20 commercial ibuprofen samples from different origins show that this combined strategy leads to (i) a unique intramolecular isotope identification and (ii) a preliminary classification of the samples according to the synthetic pathways of the main industrial processes. An approach employing polarization transfer methods applied to 11 commercial naproxen samples, for which ²H and single pulse ¹³C NMR spectra are not exploitable and/or are not accessible in reasonable time. The relative and partial intramolecular ¹³C distribution obtained on naproxen by applying this methodology is sufficiently informative to allow the same conclusions as for ibuprofen. The additional benefits that these approaches should bring to API manufacturers are discussed.
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