Something in the air–Unveiling the role of atmospheric nitrogen oxides in nitrosamine formation from amine-containing APIs: Mechanistic insights and risk assessment

Atmospheric nitrogen oxides (NO x ) have emerged as a potential contributor to nitrosamine formation in active pharmaceutical ingredients (APIs) containing vulnerable amines. This study investigates the extent and mechanisms of NO x -mediated nitrosation under controlled conditions and evaluates its relevance to real-world scenarios during API manufacturing, storage and analysis. Five structurally diverse APIs were exposed to NO, NO₂, and their mixtures at concentrations representative of urban air and forcing conditions. Nitrosamine formation was confirmed for all free base APIs, whereas corresponding salts exhibited significantly lower reactivity. Overall conversion was poor, with only ∼3% of available NO x consumed, even under high humidity and elevated NOx levels. Mechanistic analysis suggests two pathways: solution-phase nitrosation in residual moisture and gas-solid surface reactions, with NO/NO₂ mixtures forming N₂O₃ as the most potent nitrosating species. Ambient air experiments revealed that small, open samples are most susceptible, while bulk storage poses minimal risk due to NO x depletion and limited replenishment. These findings highlight the importance of considering NO x exposure during analytical handling and certain manufacturing steps involving forced airflow, while supporting a low risk for typical API storage.