Caffeine-derived N-nitroso compounds. III: Mutagenicity in S. typhimurium and in vitro induction of DNA single-strand breaks in rat hepatocytes by mononitrosocaffeidine and dinitrosocaffeidine

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.

Salt tea consumption and esophageal cancer: a possible role of alkaline beverages in esophageal carcinogenesis

Salt tea is the most commonly used beverage in Kashmir, India, where esophageal squamous cell carcinoma (ESCC) is the most common cancer. Salt tea is brewed in a unique way in Kashmir, usually with addition of sodium bicarbonate, which makes salt tea alkaline. As little information about the association between salt tea drinking and ESCC was available, we conducted a large-scale case-control study to investigate this association in Kashmir. We recruited 703 histologically confirmed cases of ESCC and 1664 controls individually matched to cases for age, sex, and district of residence. Conditional logistic regression models were used to calculate odds ratios (ORs) and 95% confidence intervals (95% CIs). Participants who consumed >1,250 ml day(-1) showed an increased risk of ESCC (OR = 2.60, 95% CIs = 1.68-4.02). Samovar (a special vessel for the beverage preparation) users (OR = 1.77, 95% CIs 1.25-2.50) and those who ate cereal paste with salt tea (OR = 2.14, 95% CIs = 1.55-2.94) or added bicarbonate sodium to salt tea (OR = 2.12, 95% CIs = 1.33-3.39) were at higher risk of ESCC than others. When analysis was limited to alkaline tea drinkers only, those who both consumed cereal paste with salt tea and used samovar vessel were at the highest risk (OR = 4.58, 95% CIs = 2.04-10.28). This study shows significant associations of salt tea drinking and some related habits with ESCC risk.

Genotoxic and carcinogenic risk to humans of drug-nitrite interaction products

The large majority of N-nitroso compounds (NOC) have been found to produce genotoxic effects and to cause tumor development in laboratory animals; four NOC have been classified by the International Agency for Research on Cancer (IARC) as probably and another 15 as possibly carcinogenic to humans. A considerable fraction of drugs are theoretically nitrosatable due to the presence of amine, amide or other groups which by reacting with nitrite in the gastric environment, or even in other sites, can give rise to the formation of NOC, and in some cases other reactive species. This review provides a synthesis of information on the chemistry of NOC formation, the carcinogenic activity of NOC in animals and humans and the inhibitors of nitrosation reactions. It contains information on the drugs which have been tested for the formation of NOC by reaction with nitrite and the genotoxic-carcinogenic effects of their nitrosation products. In an extensive search we have found that 182 drugs, representing a wide variety of chemical structures and therapeutic activities, were examined in various experimental conditions for their ability to react with nitrite, and 173 (95%) of them were found to form NOC or other reactive species. Moreover, 136 drugs were examined in short-term genotoxicity tests and/or in long-term carcinogenesis assays, either in combination with nitrite or using their nitrosation product, in order to establish whether they produce genotoxic and carcinogenic effects; 112 (82.4%) of them have been found to give at least one positive response. The problem of endogenous drug nitrosation is largely unrecognized. Only a small fraction of theoretically nitrosatable drugs have been examined for the possible formation of genotoxic-carcinogenic NOC, guidelines for genotoxicity testing of pharmaceuticals do not indicate the need of performing the appropriate tests, and patients are not informed that the drug-nitrite interaction and the consequent risk can be reduced to a large extent by consuming the nitrosatable drug with ascorbic acid.

Induction of DNA damage by risk factors of colon cancer in human colon cells derived from biopsies

In order to increase the understanding of the factors responsible for causing human colon cancer, a technique was developed to detect genotoxic effects of chemicals in human colon cells. Risk factors suspected to be associated with the aetiology of human colon cancer were subsequently investigated: the method is based on the measurement of DNA damage in primary cells freshly isolated from human colon biopsies with the single cell microgel ectrophoresis technique ('Comet Assay'). 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3-methyl-3H-imidazo[4,5f]quinoline (IQ), N-methyl-N-nitro-N-nitrosoguanidine (MNNG), dinitrosocaffeidine (DNC) lithocholic acid (LCA), hydrogen peroxide (H2O2) and benzo[a]pyrene (B[a]P) were investigated for their genotoxic and cytotoxic effects following 30 min incubation with colon cells of human, and for comparative purposes also of the rat colon. The nitrosamides (MNNG, DNC) were very genotoxic in human colon cells. MNNG was more genotoxic in human than in rat colon cells. In contrast, the rat colon carcinogens PhIP and IQ were not genotoxic in human colon cells. PhIP did induce DNA damage in rat colon cells, which correlates to its capacity of inducing tumors in this animal tissue. LCA was toxic (rat > human) and concomitantly caused DNA damage in higher concentrations. The widespread contaminant B[a]P was not genotoxic in colon cells of either species using this system. H2O2 was found to be a potent genotoxic agent to both rat and human colon cells (human > rat). In summary, those compounds chosen as representatives of endogenously formed risk factors (MNNG, H2O2, LCA) have a higher toxic and/or genotoxic potency in human colon tissue than in rat colon. They are also more effective in this system than the contaminants tested so far (B[a]P, PhIP, IQ). The newly developed technique is rapid and yields relevant results. It is a novel and useful approach to assess different chemical compounds for genotoxic activities in tumour target tissues of the human.

Caffeine-derived N-nitroso compounds. IV: Kinetics of mononitrosocaffeidine demethylation by rat liver microsomes

The study describes the kinetics of demethylation of mononitrosocaffeidine (MNC), a new asymmetric N-nitrosamine derived from caffeine. The demethylation of its precursor compound caffeidine was also studied. The results presented here suggest (a) that liver microsomes from fasted rats preferentially demethylate the N-methylnitrosamine group in MNC indicating the demethylation by cytochrome P450IIE1, (b) demethylation of MNC shows two apparent Km values, one of 117-166 microM responsible for the demethylation at the N-methylnitrosamino group of MNC, and the other Km of 1.84-2.26 mM for the remaining N-demethylations, (c) in contrast, caffeidine is a low affinity substrate for microsomal demethylation as indicated by a high Km of 14.3-16.3 mM, and (d) the demethylation at amino-N amino-N, and N-1 in both these compounds are mainly catalysed by P450 enzymes induced by Aroclor 1245 in rats.

Detection of genotoxic effects in human gastric and nasal mucosa cells isolated from biopsy samples

To assess genotoxic burdens from chemicals, it is necessary to relate observations in experimental animals to humans. The success of this extrapolation would be increased by including data on chemical activities in human tissues. Therefore, we have developed techniques to assess DNA damage in human gastric and nasal mucosa (GM, NM) cells. Biopsy samples were obtained during gastroscopy from macroscopically healthy tissue of the stomach or from healthy nasal epithelia during surgery. The specimens were incubated for 30-45 min at 37 degrees C with a digestive solution. We obtained 1.5-8 x 10(6) GM cells and 5-10 x 10(5) NM cells per donor, both with viabilities of 80-95%. The cells were incubated in vitro for 1 hr at 37 degrees C with the test compounds added in their appropriate solvents. In GM cells, we studied N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), sodium dichromate (Na2Cr2O7), nickel sulphate (NiSO4), cadmium sulphate (CdSO4), and lindane. In NM cells, lindane was investigated. Each compound was assessed for DNA damaging activity in cells of at least three different human donor samples using the microgel single cell assay. Similar studies were performed with GM and NM cells obtained from Sprague-Dawley rats. We have found human GM cells to be more sensitive to the genotoxic activity of MNNG than rat GM cells (low effective concentration [LEC] = 0.16 and 0.625 micrograms/ml for human and rat, respectively). Human cells were also more sensitive to the cytotoxic/genotoxic activity of NiSO4 (LEC = 5 and 19 mumoles/ml for human and rat, respectively). CdSO4 was genotoxic in human GM cells (LEC = 0.03-0.125 mumoles/ml), whereas no dose-related genotoxicity was observed in rat GM at concentrations up to 0.5 mumoles/ml. In contrast, approximately equal responses regarding genotoxicity and cytotoxicity were observed in rat and human GM for Na2Cr2O7 (0.25-1 mumoles/ml). Lindane, however, was genotoxic in three out of four rat GM but not in human GM cells (0.5-1 mumoles/ml), whereas it was active in both rat and human NM cells. Together with other recently published in vivo findings, our results with lindane can be interpreted according to a parallelogram approach. In view of possible human exposure situations and the sensitivities of the two target tissues from both species, the data imply that lindane will pose a health risk to humans by inhalation but not by ingestion.