DNA adducts and liver DNA replication in rats during chronic exposure to N-nitrosodimethylamine (NDMA) and their relationships to the dose-dependence of NDMA hepatocarcinogenesis

Adverse tumor events induced by ranitidine: an analysis based on the FAERS database

BACKGROUND

Ranitidine induced tumor adverse events remains a contradictory clinical question, due to the limited evidence of tumor risk associated with ranitidine in the real world. The purpose of this study was to evaluate the association of ranitidine with all types of tumors through the FAERS database and to provide a reference for clinical use.

RESEARCH DESIGN AND METHODS

Cancer cases associated with ranitidine in the FAERS database from the first quarter of 2004 to the fourth quarter of 2023 were extracted to analyze demographic characteristics, and a disproportion analysis was performed.

RESULT

A total of 662,998 ranitidine-related cancer cases were screened, and the 50-59 and 60-69 groups accounted for the largest proportion. In PT signal detection, ranitidine was associated with 98 PT, including penal cancer stage II, gastric cancer stage II, et al. In terms of outcome events, adverse events were higher in men (20.65%) than in women (18.47%).

CONCLUSIONS

Ranitidine may induce various tumor-related adverse reactions, especially in long-term users and elderly patients. For these patients, tumor screening should be strengthened, and long-term use of ranitidine should be avoided. Since this study cannot prove causality, further evidence is needed for prospective studies with a larger sample size.

Effects of hindgut acidosis on production, metabolism, and inflammatory biomarkers in previously immune-activated lactating dairy cows

Previous stressors and systemic inflammation may increase the intestine's susceptibility to hindgut acidosis (HGA). Therefore, our experimental objectives were to evaluate the effects of isolated HGA on metabolism, production, and inflammation in simultaneously immune-activated lactating cows. Twelve rumen-cannulated Holstein cows (118 ± 41 d in milk; 1.7 ± 0.8 parity) were enrolled in a study with 3 experimental periods (P). Baseline data were collected during P1 (5 d). On d 1 of P2 (2 d), all cows received an i.v. lipopolysaccharide (LPS) bolus (0.2 µg/kg of body weight; BW). During P3 (4 d), cows were randomly assigned to 1 of 2 abomasal infusion treatments: (1) control (LPS-CON; 6 L of H₂O/d; n = 6) or (2) starch infused (LPS-ST; 4 kg of corn starch + 6 L of H₂O/d; n = 6). Treatments were allocated into 4 equal doses (1.5 L of H₂O or 1 kg of starch and 1.5 L of H₂O, respectively) and administered at 0000, 0600, 1200, and 1800 h daily. Additionally, both treatments received i.v. LPS on d 1 and 3 of P3 (0.8 and 1.6 µg/kg of BW, respectively) to maintain an inflamed state. Effects of treatment, time, and their interaction were assessed. Repeated LPS administration initiated and maintained an immune-activated state, as indicated by increased circulating white blood cells (WBC), serum amyloid A (SAA), and LPS-binding protein (LBP) during P2 and P3 (29%, 3-fold, and 50% relative to P1, respectively) for both abomasal infusion treatments. Regardless of abomasal treatment, milk yield and dry matter intake were decreased throughout P2 and P3 but with lesser severity following each LPS challenge (54, 44, and 37%, and 49, 42, and 40% relative to baseline on d 1 of P2, d 1 and d 3 of P3, respectively). As expected, starch infusions markedly decreased fecal pH (5.56 at nadir vs. 6.57 during P1) and increased P3 fecal starch relative to LPS-CON (23.7 vs. 2.4% of dry matter). Neither LPS nor starch infusions altered circulating glucose, insulin, nonesterified fatty acids, or β-hydroxybutyrate, although LPS-ST cows had decreased blood urea nitrogen throughout P3 (16% relative to LPS-CON). Despite the striking reduction in fecal pH, HGA had no additional effect on circulating WBC, SAA, or LBP. Thus, in previously immune-activated dairy cows, HGA did not augment the inflammatory state, as indicated by a lack of perturbations in production, metabolism, and inflammatory biomarkers.

Identification of key genes in hepatocellular carcinoma associated with exposure to TCDD and α-endosulfan by WGCNA

2,3,7,8-tet-rachlorodibenzo-p-dioxin (TCDD) and α-endosulfan are two typical persistent organic pollutants (POPs), both of which accumulate in the liver and have potential carcinogenic hepatic effects. The underlying molecular mechanisms of pathogenesis of hepatocellular carcinoma (HCC) remain elusive when exposure to POPs. The aim of this study is to explore the key genes involved in HCC when exposure to TCDD and α-endosulfan by weighted gene co-expression network analysis (WGCNA). First, we performed co-expressed analysis on HCC and normal condition, based on WGCNA. In results, seven co-expressed modules were identified from 56 human liver samples, and the brown module correlated with five stages of HCC. Subsequently, we predicted that human five liver diseases were associated with exposure to TCDD and/or α-endosulfan by Nextbio analysis. Functional enrichment analysis showed that the brown module enriched in oxidation-reduction process, DNA replication, oxidoreductase activity and aging, which were the same as the results when exposure to the mixture of TCDD and α-endosulfan. Lastly, based on the protein-protein interaction network, we identified three novel genes including HK2, EXO1 and PFKP as key genes in HCC associated with exposure to TCDD and α-endosulfan mixture. In addition, survival analysis of key genes in Kaplan-Meier plotter demonstrated that aberrant expression levels of all the three key genes were associated with poor prognosis of HCC. Finally, Western blot analysis confirmed that protein expression levels of PFKP and HK2 in the three exposed groups were significantly elevated, while EXO1 were significantly upregulated when exposure to TCDD and α-endosulfan mixture in HepaRG cells. This study provides a new perspective to the understanding of the genetic mechanism of HCC when exposure to POPs.

Use of the benchmark-dose (BMD) approach to derive occupational exposure limits (OELs) for genotoxic carcinogens: N-nitrosamines

N-Nitrosamines are potent carcinogens and considered non-threshold carcinogens in various regulatory domains. However, recent data indicate the existence of a threshold for genotoxicity, which can be adequately demonstrated. This aspect has a critical impact on selecting the methodology that is applied to derive occupational exposure limits (OELs). OELs are used to protect workers potentially exposed to various chemicals by supporting the selection of appropriate control measures and ultimately reducing the risk of occupational cancer. Occupational exposures to nitrosamines occur during manufacturing processes, mainly in the rubber and chemical industry. The present study derives OELs for inhaled N-nitrosamines, employing the benchmark dose (BMD) approach if data are adequate and read-across for nitrosamines without adequate data. Additionally, benchmark dose lower confidence limit (BMDL) is preferred and more suitable point-of-departure (PoD) to calculate human health guidance values, including OEL. The lowest OEL (0.2 μg/m³ ) was derived for nitrosodiethylamine (NDEA), and nitrosopiperidine (NPIP) (OEL = 0.2 μg/m³ ), followed by nitrosopyrrolidine (NPYR) (0.4 μg/m³ ), nitrosodimethylamine (NDMA), nitrosodimethylamine (NMEA), and nitrosodipropylamine (NDPA) (0.5 μg/m³ ), nitrosomorpholine (NMOR) (OEL = 1 μg/m³ ), and nitrosodibutylamine (NDBA) (OEL = 2.5 μg/m³ ). Limits based on "non-threshold" TD50 slope calculation were within a 10-fold range. These proposed OELs do not consider skin absorption of nitrosamines, which is also a possible route of entry into the body, nor oral or other environmental sources. Furthermore, we recommend setting a limit for total nitrosamines based on the occupational exposure scenario and potency of components.

Effects of hindgut acidosis on production, metabolism, and inflammatory biomarkers in feed-restricted lactating dairy cows

Study objectives were to evaluate the effects of hindgut acidosis (HGA) on production, metabolism, and inflammation in feed-restricted (FR) dairy cows. Twelve rumen-cannulated cows were enrolled in a study with 3 experimental periods (P). During P1 (5 d), baseline data were collected. During P2 (2 d), all cows were FR to 40% of their baseline feed intake. During P3 (4 d), cows remained FR and were assigned to 1 of 2 abomasal infusion treatments: (1) control (FR-CON; 6 L of H₂O/d; n = 6) or (2) starch (FR-ST; 4 kg of corn starch + 6 L of H₂O/d; n = 6). Respective treatments were partitioned into 4 equal doses (1 kg of corn starch/infusion) and were abomasally infused daily at 0000, 0600, 1200, and 1800 h. All 3 P were analyzed independently and the effects of treatment, time, and treatment × time were assessed using PROC MIXED, and P1 and P2 data were analyzed using the treatments cows were destined to be assigned to during P3. Hallmark production and metabolic responses to feed restriction were observed in both treatments, including decreased milk yield (39%) and energy-corrected milk (32%), circulating glucose (12%), insulin (71%), and increased circulating nonesterified fatty acids (3.2-fold) throughout both P2 and P3, relative to P1. However, despite a marked reduction in fecal pH (0.96 units), the aforementioned metrics were unaltered by HGA. During P3, starch infusions increased circulating β-hydroxybutyrate, with the most pronounced increase occurring on d 2 (81% relative to FR-CON). Further, feed restriction decreased blood urea nitrogen during P2 (17% relative to P1) in both treatments, and this was exacerbated by starch infusions during P3 (31% decrease relative to FR-CON). In contrast to our hypothesis, neither feed restriction nor HGA increased circulating acute-phase proteins (serum amyloid A and lipopolysaccharide binding protein) relative to P1 or FR-CON, respectively. Thus, despite marked reductions in fecal pH, prior feed restriction did not appear to increase the susceptibility to HGA.

A Bioanalytical Method for Quantification of N-nitrosodimethylamine (NDMA) in Human Plasma and Urine with Different Meals and following Administration of Ranitidine

Control of N-nitrosoamine impurities is important for ensuring the safety of drug products. Findings of nitrosamine impurities in some drug products led FDA to develop new guidance providing recommendations for manufacturers towards prevention and detection of nitrosamine impurities in pharmaceutical products. One of these products, ranitidine, also had a published in vivo study, which has since been retracted by its authors, suggesting a potential for in vivo conversion of ranitidine to the probable human carcinogen, N-nitrosodimethylamine (NDMA). FDA subsequently initiated a randomized, double-blind, placebo-controlled, crossover clinical investigation to assess the potential for in vivo conversion of ranitidine to NDMA with different meals. A bioanalytical method toward characterization of NDMA formation was needed as previously published methods did not address potential NDMA formation after biofluid collection. Therefore, a bioanalytical method was developed and validated as per FDA's Bioanalytical Method Validation guidance. An appropriate surrogate matrix for calibration standards and quality control sample preparation for both liquid matrices (human plasma and urine) was optimized to minimize the artifacts of assay measurements and monitor basal NDMA levels. Interconversion potential of ranitidine to NDMA was monitored during method validation by incorporating the appropriate quality control samples. The validated methods for NDMA were linear from 15.6 pg/mL to 2000 pg/mL. Low sample volumes (2 mL for urine and 1 mL for plasma) made this method suitable for clinical study samples and helped to evaluate the influence of ranitidine administration and meal types on urinary excretion of NDMA in human subjects.

N-nitrosodimethylamine-Contaminated Valsartan and Risk of Cancer: A Nationwide Study of 1.4 Million Valsartan Users

Background Since July 2018, numerous lots of valsartan have been found to be contaminated with N-nitrosodimethylamine (NDMA). We aimed to assess the association between exposure to valsartan products contaminated with NDMA and the risk of cancer. Methods and Results This study was based on data from the Système National des Données de Santé, which is a national database that includes all French residents' health-related expenses. The target population was consumers of valsartan between January 1, 2013 and December 31, 2017, aged between 40 and 80 years old. The association of exposure to contaminated valsartan with the occurrence of any malignancy and cancer by location was evaluated by fitting Cox proportional hazards models weighted by the inverse probability of treatment. A total of 1.4 million subjects without any history of cancer were included. A total of 986 126 and 670 388 patients were exposed to NDMA-contaminated and uncontaminated valsartan, respectively. The use of the NDMA-contaminated valsartan did not increase the overall risk of cancer (adjusted hazard ratio [aHR], 0.99 [95% CI, 0.98-1.0]). However, exposed patients had a higher risk of liver cancer (aHR, 1.12 [95% CI, 1.04-1.22]) and melanoma (aHR, 1.10 [95% CI, 1.03-1.18]). We estimated a mean of 3.7 and 5.8 extra cases per year per 100 000 person-years of liver cancer and melanoma, respectively. Conclusions Our study was the largest to date to examine cancer risks associated with exposure to NDMA-contaminated valsartan. Our findings suggest a slight increased risk of liver cancer and melanoma in patients exposed to NDMA in regularly taken medications.

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.

Genotoxicity evaluation of a valsartan-related complex N-nitroso-impurity

Recently, the formation of genotoxic and carcinogenic N-nitrosamines impurities during drug manufacturing of tetrazole-containing angiotensin-II blockers has been described. However, drug-related (complex) nitrosamines may also be generated under certain conditions, i.e., through nitrosation of vulnerable amines in drug substances in the presence of nitrite. An investigation of valsartan drug substance showed that a complex API-related N-nitrosamine chemically designated as (S)-2-(((2'-(1H-tetrazol-5-yl)-[1,1'-biphenyl]-4-yl)methyl)(nitroso)amino)-3-methylbutanoic acid (named 181-14) may be generated. 181-14 was shown to be devoid of a mutagenic potential in the Non-GLP Ames test. According to ICH M7 (R1) (2018), impurities that are not mutagenic in the Ames test would be considered Class 5 impurities and limited according to ICH Q3A (R2) and B (R2) (2006) guidelines. However, certain regulatory authorities raised the concern that the Ames test may not be sufficiently sensitive to detect a mutagenic potential of nitrosamines and requested a confirmatory in vivo study using a transgenic animal genotoxicity model. Our data show that 181-14 was not mutagenic in the transgenic gene mutation assay in Muta^TMMice. The data support the conclusion that the Ames test is an adequate and sensitive test system to assess a mutagenic potential of nitrosamines.

Pharmacoepidemiological Research on N-Nitrosodimethylamine-Contaminated Ranitidine Use and Long-Term Cancer Risk: A Population-Based Longitudinal Cohort Study

N-Nitrosodimethylamine (NDMA), a carcinogenic chemical, has recently been identified in ranitidine. We conducted a population-based study to explore ranitidine use and cancer emergence over time. Using the Taiwan National Health Insurance Research Database, a population-based cohort study was conducted. A total of 55,110 eligible patients who received ranitidine between January 2000 and December 2018 were enrolled in the treated cohort. We conducted a 1:1 propensity-score-matching procedure to match the ranitidine-treated group with the ranitidine-untreated group and famotidine controls for a longitudinal study. The association of ranitidine exposure with cancer outcomes was assessed. A multivariable Cox regression analysis that compared cancer risk with the untreated groups revealed that ranitidine increased the risk of liver (hazard ratio (HR): 1.22, 95% confidence interval (CI): 1.09-1.36, p < 0.001), lung (HR: 1.17, CI: 1.05-1.31, p = 0.005), gastric (HR: 1.26, CI: 1.05-1.52, p = 0.012), and pancreatic cancers (HR 1.35, CI: 1.03-1.77, p = 0.030). Our real-world observational study strongly supports the pathogenic role of NDMA contamination, given that long-term ranitidine use is associated with a higher likelihood of liver cancer development in ranitidine users compared with the control groups of non-ranitidine users treated with famotidine or proton-pump inhibitors.

The genotoxic potential of mixed nitrosamines in drinking water involves oxidative stress and Nrf2 activation

Nitrosamine by-products in drinking water are designated as probable human carcinogens by the IARC, but the health effects of simultaneous exposure to multiple nitrosamines in drinking water remain unknown. Genotoxicity assays were used to assess the effects of both individual and mixed nitrosamines in finished drinking water produced by a large water treatment plant in Shanghai, China. Cytotoxicity and genotoxicity were measured at 1, 10-, 100- and 1000-fold actual concentrations by the Ames test, Comet assay, γ-H2AX assay, and the cytokinesis-block micronuclei assay; oxidative stress and the Nrf2 pathway were also assessed. Nitrosamines detected in drinking water included NDMA (36.45 ng/L), NDPA (44.68 ng/L), and NEMA (37.27 ng/L). Treatment with a mixture of the three nitrosamines at 1000-fold actual drinking-water concentration induced a doubling of revertants in Salmonella typhimurium strain TA100, DNA and chromosome damage in HepG2 cells, while 1-1000-fold concentrations of compounds applied singly lacked these effects. Treatment with 100- and 1000-fold concentrations increased ROS, GSH, and MDA and decreased SOD activity. Thus, nitrosamine mixtures showed greater genotoxic potential than that of the individual compounds. N-Acetylcysteine protected against the nitrosamine-induced chromosome damage, and Nrf2 pathway activation suggested that oxidative stress played pivotal roles in the genotoxic property of the nitrosamine mixtures.

Cytotoxic and genotoxic profiles of benzo[a]pyrene and N-nitrosodimethylamine demonstrated using DNA repair deficient DT40 cells with metabolic activation

Benzo[a]pyrene and N-nitrosodimethylamine are major genotoxic compounds present in cigarette smoke, food and oil. To examine the type(s) of DNA damage induced by these compounds, we used a panel of DNA-repair-pathway-deficient mutants generated from chicken DT40 cells and achieved metabolic activation of the test compounds by including rat liver S9 mix. Consistent with expections, benzo[a]pyrene and N-nitrosodimethylamine require metabolicactivation to become genotoxic. The REV3(-/-) mutant cell line exhibited the highest sensitivity, in terms of increased cytotoxicity, to the both compounds after metabolic activation consistent with the known ability of these two compounds to induce DNA adducts. Strikingly, we found that the RAD54(-/-)/KU70(-/-) cell line, a mutant defective in the repair of double-strand breaks, is sensitive to benzo[a]pyrene, suggesting that this compound also induces strand breaks in these cells. In this study we combined a previously employed method, metabolic activation by S9 mix, with the use of a DNA-repair mutant panel, thereby broadening the range of compounds that can be screened for potential genotoxicity.

Drinking water as a proportion of total human exposure to volatile N-nitrosamines

Some volatile N-nitrosamines, primarily N-nitrosodimethylamine (NDMA), are recognized as products of drinking water treatment at ng/L levels and as known carcinogens. The U.S. EPA has identified the N-nitrosamines as contaminants being considered for regulation as a group under the Safe Drinking Water Act. Nitrosamines are common dietary components, and a major database (over 18,000 drinking water samples) has recently been created under the Unregulated Contaminant Monitoring Rule. A Monte Carlo modeling analysis in 2007 found that drinking water contributed less than 2.8% of ingested NDMA and less than 0.02% of total NDMA exposure when estimated endogenous formation was considered. Our analysis, based upon human blood concentrations, indicates that endogenous NDMA production is larger than expected. The blood-based estimates are within the range that would be calculated from estimates based on daily urinary NDMA excretion and an estimate based on methylated guanine in DNA of lymphocytes from human volunteers. Our analysis of ingested NDMA from food and water based on Monte Carlo modeling with more complete data input shows that drinking water contributes a mean proportion of the lifetime average daily NDMA dose ranging from between 0.0002% and 0.001% for surface water systems using free chlorine or between 0.001% and 0.01% for surface water systems using chloramines. The proportions of average daily dose are higher for infants (zero to six months) than other age cohorts, with the highest mean up to 0.09% (upper 95th percentile of 0.3%).

Enhancement of oxyanion and diatrizoate reduction kinetics using selected azo dyes on Pd-based catalysts

Azo dyes are widespread pollutants and potential cocontaminants for nitrate; we evaluated their effect on catalytic reduction of a suite of oxyanions, diatrizoate, and N-nitrosodimethylamine (NDMA). The azo dye methyl orange significantly enhanced (less than or equal to a factor of 5.24) the catalytic reduction kinetics of nitrate, nitrite, bromate, perchlorate, chlorate, and diatrizoate with several different Pd-based catalysts; NDMA reduction was not enhanced. Nitrate was selected as a probe contaminant, and a variety of azo dyes (methyl orange, methyl red, fast yellow AB, metanil yellow, acid orange 7, congo red, eriochrome black T, acid red 27, acid yellow 11, and acid yellow 17) were evaluated for their ability to enhance reduction. Hydrogenation energies of azo dyes were calculated using density functional theory and a volcano relationship between hydrogenation energies and reduction rate enhancement was observed. A kinetic model based on Brønsted-Evans-Polanyi (BEP) theory matched the volcano relationship and suggests sorbed azo dyes enhance reduction kinetics through hydrogen atom shuttling between reduced azo dyes (i.e., hydrazo dyes) and oxyanions or diatrizoate. This is the first research that has identified this synergetic effect, and it has implications for designing more efficient catalysts and reducing Pd costs in water treatment systems.

Construction of an N-nitroso database for assessing dietary intake

Dietary N-nitroso compounds are carcinogens synthesized during food processing from two main classes of precursors, oxides of nitrogen and amines or amides. Quantification of the dietary intake of N-nitroso compounds is significant to human cancers, including those of the stomach and upper gastro-intestinal tract, colon, and brain. Previous studies investigating these cancers primarily used proxy estimates of N-nitroso intake and not a full and complete database. In this report, we describe the development of a database to be used in conjunction with a food frequency questionnaire (FFQ) or twenty-four hour dietary records. Published analytical data for N-nitroso compounds were compiled and evaluated for inclusion in the database. The final database consisted of 23 different N-nitroso compounds for 500 foods from 39 different food subgroups. Next, database foods were matched to foods in a standard FFQ by imputation, or calculated value, or assumed zero. Using the FFQ modified with N-nitroso values, we evaluated the ability to compute N-nitroso intakes for a sample of healthy control subjects of cancer epidemiological studies. N-nitroso content of food items ranged from <0.01μg/100 g. to 142 μg/100 g and the richest sources were sausage, smoked meats, bacon, and luncheon meats. The database is useful to quantify N-nitroso intake for observational and epidemiological studies.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Toward a molecular equivalent dose: use of the medaka model in comparative risk assessment

Recent changes in the risk assessment landscape underscore the need to be able to compare the results of toxicity and dose-response testing between a growing list of animal models and, quite possibly, an array of in vitro screening assays. How do we compare test results for a given compound between vastly different species? For example, what dose level in the ambient water of a small fish model would be equivalent to 10 ppm of a given compound in the rat's drinking water? Where do we begin? To initially address these questions, and in order to compare dose-response tests in a standard rodent model with a fish model, we used the concept of molecular dose. Assays that quantify types of DNA damage that are directly relevant to carcinogenesis integrate the factors such as chemical exposure, uptake, distribution, metabolism, etc. that tend to vary so widely between different phyletic levels. We performed parallel exposures in F344 rats and Japanese medaka (Oryzias latipes) to the alkylating hepatocarcinogen, dimethylnitrosamine (DMN). In both models, we measured the DNA adducts 8-hydroxyguanine, N(7)-methylguanine and O(6)-methylguanine in the liver; mutation frequency using lambda cII transgenic medaka and lambda cII transgenic (Big Blue(R)) rats; and early morphological changes in the livers of both models using histopathology and immunohistochemistry. Pulse dose levels in fish were 0, 10, 25, 50, or 100 ppm DMN in the ambient water for 14 days. Since rats are reported to be especially sensitive to DMN, they received 0, 0.1, 1, 5, 10, or 25 ppm DMN in the drinking water for the same time period. While liver DNA adduct concentrations were similar in magnitude, mutant frequencies in the DMN-exposed medaka were up to 20 times higher than in the Big Blue rats. Future work with other compounds will generate a more complete picture of comparative dose response between different phyletic levels and will help guide risk assessors using "alternative" models.

Metal-catalyzed reduction of N-nitrosodimethylamine with hydrogen in water

There is considerable need for the rapid destruction of N-nitrosodimethylamine (NDMA) in water because current alternative treatment methods are relatively inefficient. Powdered metal catalysts in conjunction with hydrogen gas showed notable potential for rapid destruction of N-nitrosodimethylamine (NDMA) in water. Palladium, copper-enhanced palladium, and nickel catalysts showed significant efficacy for NDMA reduction, with observed half-lives on the order of hours using 10 mg L(-1) catalyst metal. Other catalysts were screened because of their well-documented efficacy for reduction of halogenated hydrocarbons, including zerovalent iron, nickel-enhanced iron, nickel, and manganese. Starting with 100 microg L(-1) NDMA, a level observed at multiple field sites, pseudo-first-order kinetics were observed for all catalysts tested. No reaction intermediates were observed in any experiment; the amine group of NDMA was cleaved and reduced to dimethylamine with carbon balance in excess of 97%. Reductive catalysis may prove an efficient technology for mitigating the health risk posed by NDMA; this study provides the foundation for mechanistic and longevity research.

O6-Methylguanine-DNA Methyltransferase Leu84Phe and Ile143Val Polymorphisms and Risk of Colorectal Cancer in the Nurses’ Health Study and Physicians’ Health Study (United States)

OBJECTIVE

O6-methylguanine-DNA methyltransferase (MGMT) removes mutagenic adducts from the O6-position of guanine in DNA. Unrepaired O6-methylguanines result in G:C to A:T transitions in mutated K-ras and p53 in colorectal tumors. Two non-synonymous MGMT coding region variants, Leu84Phe and Ile143Val, lie in close proximity to the reactive 145Cys residue and to a conserved estrogen receptor interacting helix.

METHODS

We assessed the association between the MGMT Leu84Phe and Ile143Val polymorphisms and risk of colorectal cancer in two nested case-control studies: one each in the Nurses' Health Study (NHS) and the Physicians' Health Study (PHS) cohorts.

RESULTS

Among 197 female cases and 2,500 controls from the NHS, the variant 143Val allele was significantly associated with reduced risk of colorectal cancer [odds ratio (OR) = 0.52, 95% confidence interval (CI) 0.33-0.80]. In women, statistically significant gene-environment interactions were found between the Leu84Phe polymorphism and alcohol intake (P = 0.03), BMI (P = 0.04) and postmenopausal hormone use (P = 0.03). The Leu84Phe and Ile143Val polymorphisms were not significantly associated with risk of colorectal cancer among 271 male cases and 451 controls from the PHS.

CONCLUSIONS

Our results suggest that the common Leu84Phe and Ile143Val polymorphisms in MGMT influence risk of colorectal cancer in women possibly through modulating estrogen receptor-dependent transcriptional activation, which has previously been shown to occur in response to DNA alkylation damage.

Photocatalytic degradation of N-nitrosodimethylamine: mechanism, product distribution, and TiO2 surface modification

The photocatalytic degradation (PCD) reaction of N-nitrosodimethylamine (NDMA) in water was investigated using pure and surface-modified TiO2. The PCD products of NDMA were methylamine (MA), dimethylamine (DMA), nitrite, nitrate, and ammonium, and their distribution could be changed by modifying the surface of TiO2. The PCD reaction of NDMA seems to be initiated mostly by OH radicals, not valence band holes, because the addition of excess oxalates (hole scavengers) only moderately retarded the PCD rate. The presence of oxalate, however, enabled a new reductive transformation path in which the CO2-* radicals generated from the oxalate converted NDMA into DMA. In acidic suspensions of pure TiO2, the formation of MA was highly favored over DMA and NH3, whereas all degradation products (MA, DMA, and NH3) were generated at comparable concentrations at basic pH. It is suggested that there are three parallel paths depending on the position of the initial attack of OH radical on NDMA and the product distribution is closely related with which path is favored under a specific condition. DMA production is related to the OH radical attack on the nitrosyl nitrogen. Platinum deposition, silica loading, Nafion coating, and surface fluorination were tested to investigate the effects of TiO2 surface modification on the product distribution. The surface platinization of TiO2 had little effect on the PCD reaction of NDMA under air-equilibrated conditions but accelerated the PCD reaction under deaerated conditions. An enhanced PCD reaction of NDMA was achieved with the silica-loaded TiO2 and Nafion-coated TiO2, both of which favored the formation of DMA over MA. The PCD of NDMA on surface-fluorinated TiO2 was also highly enhanced but favored the formation of MA over the formation of DMA.