Complex formulations often pose persistent challenges to nitrosamine research due to their complicated matrices and low limits for nitrosamine impurities. I will continue discussions related to nitrosamine research under this topic, and welcome everyone to leave comments.
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I consistently told my team that we needed to always consider the need for SPE cleanup. I think I also saw an article using super critical CO2. I would always be a little skeptical of that route. It seems to always have its theoretical proponents more than use cases.
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Depending on the structure of the nitrosamine, the well‑known QuEChERS method from pesticide analysis can also work surprisingly well. Essentially, it’s just a liquid–liquid partition between acetonitrile and water after the addition of salts. As a result, many highly polar matrix components are removed. In general, liquid–liquid extraction is a very effective way to reduce matrix load. However, I try to develop methods that avoid the use of “bad” solvents like dichloromethane whenever possible.
Dispersive SPE (for example with C18) has also worked very well for some nitrosamines. Also its easier to perform than normal SPE.
Ultimately, the most effective approach is often to optimize sensitivity. For example by using a different mobile‑phase additive and thereby detecting a more favorable adduct. (M+H / M+Na / M+NH4 / etc.) This allows you to dilute the sample to the point where the matrix no longer has any significant impact. Of course, I’m aware that this is not always possible.
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Why Is Nitrosamine Investigation in Topical Semi-Solid Formulations Challenging?
First, matrix complexity can lead to significant analytical interference.
Topical semi-solid formulations commonly contain various functional excipients, including surfactants, lipid-based matrices, polymeric thickeners, pressure-sensitive adhesives, and penetration enhancers. These components may generate substantial background signals during mass spectrometric analysis, potentially masking trace-level nitrosamine peaks. They may also cause ion suppression, incomplete extraction of target impurities, or reduced method robustness.
Second, the risk of false-positive results should not be overlooked.
When a formulation contains secondary amine-containing active pharmaceutical ingredients and trace levels of nitrite are present in the system, in situ nitrosation may occur during sample preparation, such as pH adjustment, heating, or prolonged standing, leading to the formation of nitrosamine drug substance-related impurities (NDSRIs).
Third, the required detection limits are approaching the technical limits of trace-level analysis.
In accordance with ICH M7(R2) and global regulatory expectations, some NDSRIs have very low acceptable intake limits. When converted into product-specific specification limits, analytical methods are often required to achieve detection capability at the ppb level. This places stringent demands on method specificity, sensitivity, accuracy, and resistance to matrix-related interference.
Today, I would like to discuss with you the research on NDSRIs related to local anesthetic drugs of the “caine” class. Caine drugs are commonly used local anesthetics in surgery, dentistry, dermatology, and pain management, with representative agents including lidocaine, prilocaine, tetracaine, bupivacaine, ropivacaine, mepivacaine, articaine, benzocaine, and others. They act by reversibly blocking voltage-gated sodium channels on neuronal cell membranes, inhibiting Na⁺ influx, thereby interrupting nerve impulse conduction and achieving localized analgesia while the patient remains conscious.
Structurally, caine drugs typically consist of a lipophilic aromatic or heterocyclic ring, an intermediate connecting chain, and a hydrophilic alkylamine group. It is precisely these amine moieties that render them potential precursors to NDSRIs under certain conditions. In other words, the amino group serves both as a critical basis for pharmacological activity and physicochemical properties, and as a potential source of nitrosamine impurity risk. Moreover, due to the specific nature of their indications, caine drugs are often formulated as creams, patches, and other topical dosage forms. This significantly increases the complexity of analytical testing for NDSRIs in these drug products. I would welcome any literature or effective analytical methods that colleagues may wish to share regarding the detection of caine-class drugs.