Predicting DNA-Reactivity of N‑Nitrosamines: A Quantum Chemical Approach

N-Nitrosamines (NAs) are a class of
reactive organic
chemicals that humans may be exposed to from environmental sources,
food but also impurities in pharmaceutical preparations. Some NAs
were identified as DNA-reactive mutagens and many of those have been
classified as probable human carcinogens. Beyond high-potency mutagenic
carcinogens that need to be strictly controlled, NAs of low potency
need to be considered for risk assessment as well. NA impurities and
nitrosylated products of active pharmaceutical ingredients (APIs)
often arise from production processes or degradation. Most NAs require
metabolic activation to ultimately become carcinogens, and their activation
can be appropriately described by first-principles computational chemistry
approaches. To this end, we treat NA-induced DNA alkylation as a series
of subsequent association and dissociation reaction steps that can
be calculated stringently by density functional theory (DFT), including
α-hydroxylation, proton transfer, hydroxyl elimination, direct
SN2/SNAr DNA alkylation, competing hydrolysis
and SN1 reactions. Both toxification and detoxification
reactions are considered. The activation reactions are modeled by
DFT at a high level of theory with an appropriate solvent model to
compute Gibbs free energies of the reactions (thermodynamical effects)
and activation barriers (kinetic effects). We study congeneric series
of aliphatic and cyclic NAs to identify trends. Overall, this work
reveals detailed insight into mechanisms of activation for NAs, suggesting
that individual steric and electronic factors have directing and rate-determining
influence on the formation of carbenium ions as the ultimate pro-mutagens
and thus carcinogens. Therefore, an individual risk assessment of
NAs is suggested, as exemplified for the complex API-like 4-(N-nitroso-N-methyl)aminoantipyrine which is considered
as low-potency NA by in silico prediction.