Salmonella typhimurium TA100 and TA1535 and E. coli WP2 uvrA are highly sensitive to detect the mutagenicity of short Alkyl-N-Nitrosamines in the Bacterial Reverse Mutation Test

HESI GTTC ring trial: Concordance between Ames and rodent carcinogenicity outcomes for N-nitrosamines (NAs) with rat and hamster metabolic conditions

A multi-sector study (i.e., Ring Trial) was designed to improve the in vitro detection of N-nitrosamine (NA)-associated mutagenicity by optimizing the bacterial reverse mutation (i.e., Ames) assay protocol and testing various conditions on the sensitivity and specificity for the prediction of rodent carcinogenicity. A total of 29 NAs and 3 N-nitroso drug-like compounds from different structural classes and carcinogenicity outcomes were tested (two independent laboratories per compound) across 5 bacterial strains using a 30-min pre-incubation protocol. To evaluate the impact of different metabolic activating systems (MASs), testing conditions included the use of 10 or 30 % liver S9 fractions prepared from rats or hamsters pretreated with inducers of enzymatic activity. Results indicate that E. coli and Salmonella typhimurium strains detecting single base pair mutations, coupled with MASs containing 30 % hamster S9s were the most sensitive (90 %) for identifying NAs that are rodent carcinogens. Regarding MAS combinations, the highest sensitivity was 30 % rat and 30 % hamster (93 %), but has low specificity (45 %), with good laboratory agreement for the Ames calls (91 %). DMSO and water were considered suitable solvents, except for small-molecular weight alkyl NAs. These results will support harmonized Ames testing of NAs, giving high confidence for a negative result.

Resolution of historically discordant Ames test negative/rodent carcinogenicity positive N-nitrosamines using a sensitive, OECD-aligned design

The in vitro bacterial reverse mutation (Ames) test is crucial for evaluating the mutagenicity of pharmaceutical impurities. For N-nitrosamines (NAs) historical data indicated that for certain members of this chemical class, the outcomes of the Ames test did not correlate with their associated rodent carcinogenicity outcomes. This has resulted in negative outcomes in an OECD (Organization for Economic Cooperation and Development)-aligned Ames test alone (standard or enhanced) no longer being considered sufficient by regulatory authorities to assess potential carcinogenic risk of NAs if present as impurities in drug products. Consequently, extensive follow-up in vivo testing can be required to characterize the potential mutagenicity and genotoxic carcinogenicity of NA impurities (i.e. beyond that defined in the ICH M7 guideline for non-NA impurities). We previously demonstrated that the mutagenicity of alkyl-nitrosamines can be detected by the appropriately designed, OECD-aligned Ames test and identified those conditions that contributed most to assay sensitivity. This OECD-aligned Ames test design was used to assess seven NAs, i.e. (methyl(neopentyl)nitrosamine, N-methyl-N-nitroso-2-propanamine, N-nitrosodiisopropylamine, bis(2-methoxyethyl)nitrosoamine, N-nitroso-N-methyl-4-fluoroaniline, dinitrosoethambutol, (R,R)- and mononitrosocaffeidine) that were reported to be negative in historical Ames tests but positive in rodent carcinogenicity studies. All seven of the NAs were demonstrated to be mutagenic in the OECD-aligned Ames test and therefore these compounds should no longer be considered as discordant (false negatives) with respect to the correlation of the Ames test and rodent carcinogenicity. These results confirm the sensitivity of the OECD-aligned Ames test for the detection of NA mutagenicity and provides further support of its pivotal placement within the ICH M7 framework for the assessment of mutagenic impurities in pharmaceuticals to limit potential carcinogenic risk. In addition, we present data for 1-cyclopentyl-4-nitrosopiperazine, that indicates it could serve as a suitable positive control to provide further confidence in the sensitivity of the Ames test for the NA chemical class.

An Enhanced Metabolization Protocol for In Vitro Genotoxicity Assessment of N-Nitrosamines in Mammalian Cells

N-Nitrosamines (NAs) are probable human carcinogens and were detected as impurities in pharmaceuticals, which led to a concern for human health. NAs require metabolic activation before they become mutagenic, and not all NAs are mutagenic since their reactivity is related to their structure. While some NAs are potent mutagens in vivo, in vitro metabolization with exogenous S9 liver extract is generally less efficient. While an enhanced bacterial mutagenicity protocol was recently developed, which uses increased concentrations of S9 liver extracts, there presently is not an improved metabolization protocol suitable for mammalian cell genotoxicity assays. Therefore, we optimized a hamster S9 liver extract-based protocol for in vitro NA metabolization and assessed the genotoxic potential of various NAs using ToxTracker. With this enhanced metabolization protocol (EMP), the genotoxic potency of N-nitrosodimethylamine (NDMA) increased approximately 200-fold compared with the standard S9 liver extract-based exposure protocol in ToxTracker. The EMP was further validated with seven additional mutagenic NAs to which humans are commonly exposed: N-nitrosodiethylamine (NDEA), N-nitrosodiethanolamine (NDELA), N-nitrosodibutylamine (NDBA), N-nitrosofluoxetine (NF), 1-nitrosopyrrolidine (NPYR), N-nitrosomorpholine (NMOR), and 1-cyclopentyl-4-nitrosopiperazine (CPNP), and two non-mutagenic NAs: N-nitrosobupropion (NBuPRO) and N-nitrosoproline (NPRO). Genotoxicity could be confirmed for six NAs using the EMP, demonstrating that mammalian cells and the new approach methodology (NAM) ToxTracker may have potential when investigating NA-related genotoxicity.

Ames mutagenicity of 15 aryl, benzyl, and aliphatic ring N-nitrosamines

The Ames mutagenicity test is an effective means of screening compounds for their carcinogenic potential. Here, we conducted Ames tests on 15 aryl, benzyl, and aliphatic ring N-nitrosamines. Then, by using two indicators of mutagenicity strength calculated from the Ames test results, namely, maximum specific activity (MSA; number of revertant colonies) and maximum fold increase (MFI; relative ratio of increased colonies), we examined the relationship between Ames mutagenicity strength and Carcinogenic Potency Categorization Approach (CPCA) potency category, which is a structure-activity-relationship-based prediction of the carcinogenic potency of nitrosamines. Eleven of the test compounds were Ames positive and four were negative. Of the 11 positive compounds, three were categorized as strong positive (MSA ≥1000), five as medium positive (100 ≤ MSA <1000), and three as weak positive (MSA <100). The compounds with an aliphatic ring showed a negative relationship between mutagenicity strength (i.e., MSA or MFI) and carcinogenic potential (i.e., CPCA category), whereas, the alpha-methyl aryl N-nitrosamines did not. Overall, MSA and MFI were found to be detailed indicators of the carcinogenic potency of the N-nitrosamines and can potentially be used to support CPCA categorization.

Selection of solvent and positive control concentration for enhanced Ames test conditions for N-nitrosamine compounds

The Ames test is a widely used bacterial mutagenicity assay to evaluate the potential of chemical compounds to induce mutations. In recent years, there has been growing concern regarding the presence of N-nitrosamines in pharmaceuticals, food, and other consumer products. N-Nitrosamines are probable mutagens and carcinogens. To address the reduced sensitivity of the standard Ames test for N-nitrosamines, particularly N-nitrosodimethylamine, the European Medicines Agency (EMA) and United States Food and Drug Administration (FDA) have recently published recommendations for enhanced Ames test (EAT) conditions. However, there is a lack of clear guidance on the selection of N-nitrosamine positive control concentrations, particularly for 1-cyclopentyl-4-nitrosopiperazine, and the amount of solvent to be used in the EAT. This study aims to address the current gap in concentration and volume specifications by providing a comprehensive guide to set up enhanced Ames test conditions specifically for N-nitrosamine compounds using appropriate amounts of solvent, new solvents, and strain-specific positive control concentrations.

Optimizing the detection of N-nitrosamine mutagenicity in the Ames test

Accurately determining the mutagenicity of small-molecule N-nitrosamine drug impurities and nitrosamine drug substance-related impurities (NDSRIs) is critical to identifying mutagenic and cancer hazards. In the current study we have evaluated several approaches for enhancing assay sensitivity for evaluating the mutagenicity of N-nitrosamines in the bacterial reverse mutagenicity (Ames) test. Preincubation assays were conducted using five activation conditions: no exogenous metabolic activation and metabolic activation mixes employing both 10% and 30% liver S9 from hamsters and rats pretreated with inducers of enzymatic activity. In addition, preincubations were conducted for both 60 min and 30 min. These test variables were evaluated by testing 12 small-molecule N-nitrosamines and 17 NDSRIs for mutagenicity in Salmonella typhimurium tester strains TA98, TA100, TA1535, and TA1537, and Escherichia coli strain WP2 uvrA (pKM101). Eighteen of the 29 N-nitrosamine test substances tested positive under one or more of the testing conditions and all 18 positives could be detected by using tester strains TA1535 and WP2 uvrA (pKM101), preincubations of 30 min, and S9 mixes containing 30% hamster liver S9. In general, the conditions under which NDSRIs were mutagenic were similar to those found for small-molecule N-nitrosamines.

Bioengineering of glucan coated silver nanoparticles as dynamic biomedical compound; in vitro and in vivo studies

The use of silver nanoparticles (AgNPs) gaining importance for the treatment of microbial infections and are in great demand due to their efficient broad antibacterial action but there is only one problem that silver nanoparticles can cause tissue damage. Therefore, the present study evaluated antimicrobial potential and intricacy of glucan coated silver nanoparticles in comparison with free silver nanoparticles. In this study, glucan coated silver nanoparticles (Glucan-AgNPs) by using Pleurotus spps. were characterized for their antimicrobial, minimum inhibitory concentration (MIC), biofilm inhibition, mutagenicity potential, hemolytic activities and histological examination through in vitro and in vivo analysis. The liver, kidney, intestine, and skin tissues were examined to gauge the adverse effects of the treatment method's toxicity by silver deposition. The results of this study have shown that mushroom's glucan extracted from Pleurotus spps. are excellent reducing agent and due to their best capping ability they reduce the toxicity of AgNPs and enhance their antimicrobial activities. The highest zone of inhibition was observed by Glucan-AgNPs from P. ostreatus (24 mm) against S. aureus while least zone of inhibition was resulted from Glucan-AgNPs from P. sapidus (14 mm) against B. subtilis. The results for biofilm inhibition showed excellent biofilm inhibition ability of Glucan-AgNPs. In results, maximum inhibition 95.2 % was observed by Glucan-AgNPs from P. ostreatus against S. aureus, while minimum inhibition 79.2 % by Glucan-AgNPs of P. sapidus against E. coli. Furthermore, Glucan-AgNPs treated mice showed no deposition and damage in the organs. Glucan-AgNPs has a higher efficacy in treating microbial infection.

Non-clinical investigations about cytotoxic and anti-platelet activities of gamma-terpinene

Gamma-terpinene (γ-TPN) is a cyclohexane monoterpene isolated from plant essential oils, such as tea tree (Melaleuca alternifolia), oregano (Origanum vulgare), rosemary (Rosmarinus officinalis L.), thyme (Thymus vulgaris Marchand), and eucalyptus (Eucalyptus sp.). Terpenes are widely studied molecules pharmacologically active on the cardiovascular system, hemostasis, and antioxidant actions. Herein, it was investigated the cytotoxic and antiplatelet activity of γ-TPN using different non-clinical laboratory models. For in silico evaluation, the PreADMET, SwissADME, and SwissTargetPrediction softwares were used. Molecular docking was performed using the AutoDockVina and BIOVIA Discovery Studio databases. The cytotoxicity of γ-TPN was analyzed by the MTT assay upon normal murine endothelial SVEC4-10 and fibroblast L-929 cells. Platelet aggregation was evaluated with platelet-rich (PRP) and platelet-poor (PPP) plasma from spontaneously hypertensive rats (SHR), in addition to SVEC4-10 cells pre-incubated with γ-TPN (50, 100, and 200 µM) for 24 h. SHR animals were pre-treated by gavage with γ-TPN for 7 days and divided into four groups (negative control, 25, 50, and 100 mg/kg). Blood samples were collected to measure nitrite using the Griess reagent. Gamma-TPN proved to be quite lipid-soluble (Log P = +4.50), with a qualified profile of similarity to the drug, good bioavailability, and adequate pharmacokinetics. It exhibited affinity mainly for the P2Y12 receptor (6.450 ± 0.232 Kcal/mol), moderate cytotoxicity for L-929 (CC50 = 333.3 µM) and SVEC 4-10 (CC50 = 366.7 µM) cells. The presence of γ-TPN in SVEC 4-10 cells was also able to reduce platelet aggregation by 51.57 and 44.20% at lower concentrations (50 and 100 µM, respectively). Then, γ-TPN has good affinity with purinergic receptors and an effect on the reversal of platelet aggregation and oxidative stress, being promising and safe for therapeutic targets and subsequent studies on the control of thromboembolic diseases.

Estimation of acceptable daily intake values based on modeling and in vivo mutagenicity of NDSRIs of fluoxetine, duloxetine and atomoxetine

Nitrosamine drug substance related impurities or NDSRIs can be formed if an active pharmaceutical ingredient (API) has an intrinsic secondary amine that can undergo nitrosation. This is a concern as 1) nitrosamines are potentially highly potent carcinogens, 2) secondary amines in API are common, and 3) NDSRIs that might form from such secondary amines will be of unknown carcinogenic potency. Approaches for evaluating NDSRIs include read across, quantum mechanical modeling of reactivity, in vitro mutation data, and transgenic in vivo mutation data. These approaches were used here to assess NDSRIs that could potentially form from the drugs fluoxetine, duloxetine and atomoxetine. Based on a read across informed by modeling of physicochemical properties and mechanistic activation from quantum mechanical modeling, NDSRIs of fluoxetine, duloxetine, and atomoxetine were 10-100-fold less potent compared with highly potent nitrosamines such as NDMA or NDEA. While the NDSRIs were all confirmed to be mutagenic in vitro (Ames assay) and in vivo (TGR) studies, the latter data indicated that the potency of the mutation response was ≥4400 ng/day for all compounds-an order of magnitude higher than published regulatory limits for these NDSRIs. The approaches described herein can be used qualitatively to better categorize NDSRIs with respect to potency and inform whether they are in the ICH M7 (R2) designated Cohort of Concern.

Metabolic activation of short-chain alkyl N-nitrosamines using Aroclor 1254 or phenobarbital/beta-naphthoflavone-induced rat or hamster S9 - A comparative analysis

N-nitrosamines, a very heterogeneous class of chemicals, may enter humans in small amounts through various sources and are produced endogenously, too. Some are known to be mutagenic carcinogens and have recently been detected as impurities in several marketed pharmaceuticals. Despite their known mutagenic properties, the suitability of the bacterial reverse mutation (Ames) assay and in particular the use of induced rat liver S9 to detect their mutagenic potential, is often discussed. Recently, it could be demonstrated that induced rat liver S9 is capable of metabolizing small alkyl nitrosamines to exert their mutagenic potential (Bringezu & Simon, 2022). In this project, the mutagenic potential of nitrosamines in vitro under different S9 conditions applying the preincubation protocol and OECD 471-compliant standard Ames test recommendations was investigated. These conditions included various amounts of S9 fraction from hamster and rat, uninduced or induced with Aroclor 1254 or Phenobarbital/beta-Naphthoflavone (PB/NF). The findings indicated that in addition to induced S9, uninduced hamster S9 also demonstrated effectiveness. Moreover, both rat and hamster S9 fractions exhibited suitable responses in terms of mutation frequencies. Increasing the S9 content did not increase the sensitivity of the Ames test. However, above 20% S9, reduced mutation frequency was observed in the higher concentration range suggesting cytotoxicity to the bacteria. Thus, limiting the S9 content to 10% provides reliable results and relates to a lower number of animals required for S9 production which is in concordance with the 3R principles (reduce, refine, replace) for animal testing. In addition, results obtained show that uninduced and induced hamster S9 are similarly effective, doubting the requirement of pretreating animals with enzyme inducers. Further investigations to compare mutagenicity data and rat and hamster S9 proteome analyses are ongoing.

Ames test study designs for nitrosamine mutagenicity testing: qualitative and quantitative analysis of key assay parameters

The robust control of genotoxic N-nitrosamine (NA) impurities is an important safety consideration for the pharmaceutical industry, especially considering recent drug product withdrawals. NAs belong to the 'cohort of concern' list of genotoxic impurities (ICH M7) because of the mutagenic and carcinogenic potency of this chemical class. In addition, regulatory concerns exist regarding the capacity of the Ames test to predict the carcinogenic potential of NAs because of historically discordant results. The reasons postulated to explain these discordant data generally point to aspects of Ames test study design. These include vehicle solvent choice, liver S9 species, bacterial strain, compound concentration, and use of pre-incubation versus plate incorporation methods. Many of these concerns have their roots in historical data generated prior to the harmonization of Ames test guidelines. Therefore, we investigated various Ames test assay parameters and used qualitative analysis and quantitative benchmark dose modelling to identify which combinations provided the most sensitive conditions in terms of mutagenic potency. Two alkyl-nitrosamines, N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) were studied. NDMA and NDEA mutagenicity was readily detected in the Ames test and key assay parameters were identified that contributed to assay sensitivity rankings. The pre-incubation method (30-min incubation), appropriate vehicle (water or methanol), and hamster-induced liver S9, alongside Salmonella typhimurium strains TA100 and TA1535 and Escherichia coli strain WP2uvrA(pKM101) provide the most sensitive combination of assay parameters in terms of NDMA and NDEA mutagenic potency in the Ames test. Using these parameters and further quantitative benchmark dose modelling, we show that N-nitrosomethylethylamine (NMEA) is positive in Ames test and therefore should no longer be considered a historically discordant NA. The results presented herein define a sensitive Ames test design that can be deployed for the assessment of NAs to support robust impurity qualifications.

Antimutagenic activity of different forms of Bifidobacterium lactis probiotic against aflatoxin B1 by Ames method

The health risks caused by aflatoxins, as one of the most important contaminants of human food and feed and the main cause of cancer, especially hepatocellular carcinoma (HCC) were investigated. The aim of the study was to assess the antimutagenic effects of Bifidobacterium lactis (B. lactis) probiotic against aflatoxin B1 (AFB1). The study was conducted with 27 treatments and three replications. The independent variables were aflatoxin concentrations at three levels of 5, 15, and 25 ng/g and probiotic content in three forms of cellular sedimentation (CS), cell-free supernatant (CFS), and cell suspension. The antimutagenic activity of B. lactis against AFB1 was measured. The lowest score of antimutagenic activity of B. lactis was observed in bacterial cellular sediment treatment at 107 CFU/g and 25 ng/g of AFB1 (20.8 ± 3.80%) and the highest score was achieved with cell suspension at 109 CFU/g and 5 ng/g of AFB1 (74.9 ± 7.11%). In addition, the lack of mutagenicity of probiotics was confirmed. Therefore, probiotics not only alleviate aflatoxin in food matrices and benefit the consumer, but also notably decrease mutagenicity of AFB1.

Mechanisms of Nitrosamine Mutagenicity and Their Relationship to Rodent Carcinogenic Potency

A thorough literature review was undertaken to understand how the pathways of N-nitrosamine transformation relate to mutagenic potential and carcinogenic potency in rodents. Empirical and computational evidence indicates that a common radical intermediate is created by CYP-mediated hydrogen abstraction at the α-carbon; it is responsible for both activation, leading to the formation of DNA-reactive diazonium species, and deactivation by denitrosation. There are competing sites of CYP metabolism (e.g., β-carbon), and other reactive species can form following initial bioactivation, although these alternative pathways tend to decrease rather than enhance carcinogenic potency. The activation pathway, oxidative dealkylation, is a common reaction in drug metabolism and evidence indicates that the carbonyl byproduct, e.g., formaldehyde, does not contribute to the toxic properties of N-nitrosamines. Nitric oxide (NO), a side product of denitrosation, can similarly be discounted as an enhancer of N-nitrosamine toxicity based on carcinogenicity data for substances that act as NO-donors. However, not all N-nitrosamines are potent rodent carcinogens. In a significant number of cases, there is a potency overlap with non-N-nitrosamine carcinogens that are not in the Cohort of Concern (CoC; high-potency rodent carcinogens comprising aflatoxin-like-, N-nitroso-, and alkyl-azoxy compounds), while other N-nitrosamines are devoid of carcinogenic potential. In this context, mutagenicity is a useful surrogate for carcinogenicity, as proposed in the ICH M7 (R2) (2023) guidance. Thus, in the safety assessment and control of N-nitrosamines in medicines, it is important to understand those complementary attributes of mechanisms of mutagenicity and structure-activity relationships that translate to elevated potency versus those which are associated with a reduction in, or absence of, carcinogenic potency.

Comparison of the transgenic rodent mutation assay, error corrected next generation duplex sequencing, and the alkaline comet assay to detect dose-related mutations following exposure to N-nitrosodiethylamine

N-Nitrosodiethylamine (NDEA), a well-studied N-nitrosamine, was tested in rats to compare the dose-response relationship of three genotoxicity endpoints. Mutant / mutation frequencies were determined using the transgenic rodent (TGR) gene mutation assay and error corrected next generation sequencing (ecNGS) (i.e., duplex sequencing (DS)), and genetic damage was detected by the alkaline comet assay. Big Blue® (cII Locus) animals (n = 6 per dose group) were administered doses of 0.001, 0.01, 0.1, 1, 3 mg/kg/day NDEA by oral gavage. Samples were collected for cII mutation and DS analyses following 28-days of exposure and 3 days recovery. In a separate study, male Sprague-Dawley (SD) rats (n = 6 per dose group) were administered the same doses by oral gavage for two consecutive days and then samples collected for the alkaline comet assay. A dose-related increase in mutant / mutation frequencies of the liver but not duodenum was observed using the TGR assay and DS with DS resulting in a slightly more sensitive response, with a lower benchmark dose (BMD). In addition, a dose-related increase in percent tail DNA was observed in the liver using the alkaline comet assay. Therefore, DS and comet assays showed good utility for hazard identification and dose-response analysis of a representative N-nitrosamine comparable to the TGR gene mutation assay.

A deep dive into historical Ames study data for N-nitrosamine compounds

Mutagenicity data is a core component of the safety assessment data required by regulatory agencies for acceptance of new drug compounds, with the OECD-471 bacterial reverse mutation (Ames) assay most widely used as a primary screen to assess drug impurities for potential mutagenic risk. N-Nitrosamines are highly potent mutagenic carcinogens in rodent bioassays and their recent detection as impurities in pharmaceutical products has sparked increased interest in their safety assessment. Previous literature reports indicated that the Ames test might not be sensitive enough to detect the mutagenic potential of N-nitrosamines in order to accurately predict a risk of carcinogenicity. To explore this hypothesis, public Ames and rodent carcinogenicity data pertaining to the N-nitrosamine class of compounds was collated for analysis. Here we present how variations to the OECD 471-compliant Ames test, including strain, metabolic activation, solvent type and pre-incubation/plate incorporation methods, may impact the predictive performance for carcinogenicity. An understanding of optimal conditions for testing of N-nitrosamines may improve both the accuracy and confidence in the ability of the Ames test to identify potential carcinogens.

Strategies for Assessing Acceptable Intakes for Novel N-Nitrosamines Derived from Active Pharmaceutical Ingredients

The detection of N-nitrosamines, derived from solvents and reagents and, on occasion, the active pharmaceutical ingredient (API) at higher than acceptable levels in drug products, has led regulators to request a detailed review for their presence in all medicinal products. In the absence of rodent carcinogenicity data for novel N-nitrosamines derived from amine-containing APIs, a conservative class limit of 18 ng/day (based on the most carcinogenic N-nitrosamines) or the derivation of acceptable intakes (AIs) using structurally related surrogates with robust rodent carcinogenicity data is recommended. The guidance has implications for the pharmaceutical industry given the vast number of marketed amine-containing drugs. In this perspective, the rate-limiting step in N-nitrosamine carcinogenicity, involving cytochrome P450-mediated α-carbon hydroxylation to yield DNA-reactive diazonium or carbonium ion intermediates, is discussed with reference to the selection of read-across analogs to derive AIs. Risk-mitigation strategies for managing putative N-nitrosamines in the preclinical discovery setting are also presented.

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 S_N2/S_NAr DNA alkylation, competing hydrolysis and S_N1 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.