PHARMACEUTICALS
Case Study: Bromhexine
Tertiary amines may undergo nitrosation through a nitrosative cleavage( or dealkylation) mechanism. 3 In the case of unsymmetrical tertiary amines, up to three distinct products can theoretically be formed. However, dealkylation requires at least one α-hydrogen adjacent to the amine nitrogen, proceeding via an iminium intermediate.
As an example, a respiratory agent named bromhexine contains an electron-rich benzylic amine that facilitates nitrosative dealkylation. 4 The presence of an ortho-primary amino group promotes this transformation by strongly donating electron density into the aromatic system. Resonance stabilisation of the benzylic carbocation( or quinone methide iminium ion) lowers the energetic barrier for C – N bond cleavage and facilitates the elimination of N-nitrosocyclohexylmethylamine( NCMA, Figure 3). 5
While the bromine substituents are inductively electron-withdrawing, their ability to donate electron density through resonance contributes to the overall electronic environment and influences the reactivity of the amino group. Available kinetic data indicate that nitrosative dealkylation of tertiary aliphatic amines occurs two to three orders of magnitude slower than the nitrosation of secondary amines, reducing but not eliminating the risk of nitrosamine formation.
( a) H
R, NA
I
R
R = Tert-alkyl or aryl
( b) H H
�NA
R '
R ' = Alkyl or aryl
R './
I
N ½
N +?'
Diazonium
Dealkylative Nitrosation
dealkylative Nitrosation
Dehydration
◄
►
►
R \ NH
Nitrosation
j Nitrosation HN�------- ►�
I
R '
OH NI �
N
I
R '
Fragmentation
◄
- MeCHO
Certain structural features, including dialkyl aromatic amines, may facilitate more rapid dealkylation or direct nitrosamine release from nitrosoammonium intermediates. Such compounds may constitute a higher-risk class because only one equivalent of nitrite is required, similar to secondary amines.
Regulatory guidance
Based on the latest revision by the European Medicines Agency( EMA), 244 nitrosamines derivatives have been identified and reported with their maximum allowable intakes( MAIs). 6 Regulatory assessments have identified these impurities, each with very low allowable daily intake limits, reflecting their mutagenic and carcinogenic risk.
Under ICH M7( R1) 7, nitrosamines are classified as Class 1 mutagenic carcinogens, with acceptable intakes typically between 26.6 and 96 ng / day for a single impurity. When multiple nitrosamines or multiple drug products contribute to patient exposure, limits must be assessed cumulatively and in relation to the maximum daily dose to ensure overall exposure remains acceptable.
Managing nitrosamine risk requires a proactive and predictive sciencebased approach combining detailed risk assessment with confirmatory analytical corroboration rather than relying on reactive measures. 8
R 0 \ // N-N
Not a Coe nitrosamine
0 // H
N, NA
I
R '
CoC nitrosamine
Biological activation
Figure 3- Tertiary amine nitrosation & biological activation of nitrosamines: Generation of nitrosamine that is not part of CoC( a) & which could belong to CoC( b)
The ICH M 7 guideline provides a riskbased framework to identify, assess and control potentially mutagenic impurities arising from API synthesis. It relies on understanding how impurities may form, evaluating their mutagenic risk using experimental or in silico methods and determining their presence in the final product through analytical data or knowledge of process removal.
Nevertheless, regulators emphasise that theoretical justifications must be supported by strong experimental data. Both US FDA and EMA guidance caution against reliance on calculations alone, particularly when amines or nitrosating agents may be present at or near stoichiometric levels, even at very low concentrations. As a result, confirmatory analytical testing on representative validation or commercial-scale batches is strongly encouraged, even when the predicted risk appears minimal.
Since 2023, global authorities have reinforced harmonised, riskbased frameworks that mandate formal risk assessments, confirmatory testing where risks are identified, and reporting to regulators. 8 The FDA, EMA, EDQM, USP, Health Canada, ANVISA, China’ s NMPA and other agencies have issued extensive guidance on acceptable intake limits, analytical controls, and lifecycle risk mitigation, reflecting evolving toxicological approaches such as the Carcinogenic Potency Categorisation Approach( CPCA) and enhanced Ames testing.
Because nitrosamines are regulated at extremely low levels, their quantification depends on highly sensitive mass spectrometry-based analytical methods rather than conventional techniques. In response to past detection failures, regulators and pharmacopoeias have established and validated advanced LCMS and GCMS methodologies, driving global adoption of robust testing strategies.
Overall, regulatory consensus underscores that nitrosamine control is a continuous obligation, integrating toxicology, analytical science, process chemistry and quality systems to protect patient safety throughout development and commercial supply.
SEP / OCT 2026 SPECCHEMONLINE. COM
27