Intrahepatic mutagenesis assay: a sensitive method for detecting N-nitrosomorpholine and in vivo nitrosation of morpholine

Toxicokinetic and mass balance of morpholine in rats

Morpholine (MOR) has a broad spectrum of use and represents high risk of human exposure. Ingested MOR can undergo endogenous N-nitrosation in the presence of nitrosating agents forming N-nitrosomorpholine (NMOR), classified as possible human carcinogen by the International Agency for Research on Cancer.In this study, we evaluated the MOR toxicokinetics in six groups of male Sprague-Dawley rats orally exposed to 14C-radiolabelled MOR and NaNO2. The major urinary metabolite of MOR, N-nitrosohydroxyethylglycine (NHEG), was measured through HPLC as an index of endogenous N-nitrosation. Mass balance and toxicokinetic profile of MOR were determined by measuring radioactivity in blood/plasma and excreta.MOR reached maximum blood concentration 30 minutes after administration. Elimination rate was rapid (70% in 8h). Most of the radioactivity was excreted in the urine (80.9 ± 0.5%) and unchanged 14C-MOR was the main compound excreted in the urine (84% of the dose recovered). 5.8% of MOR is not absorbed and/or was not recovered.Endogenous nitrosation of MOR was demonstrated by the detection of NHEG. The maximum conversion rate found was 13.3 ± 1.2% and seems to be impacted by the MOR/NaNO2 ratio.These results help refining our knowledge of the endogenous production of NMOR, a possible human carcinogen.

Re-evaluation of potassium nitrite (E 249) and sodium nitrite (E 250) as food additives

The Panel on Food Additives and Nutrient Sources added to Food (ANS) provided a scientific opinion re-evaluating the safety of potassium nitrite (E 249) and sodium nitrite (E 250) when used as food additives. The ADIs established by the SCF (1997) and by JECFA (2002) for nitrite were 0-0.06 and 0-0.07 mg/kg bw per day, respectively. The available information did not indicate in vivo genotoxic potential for sodium and potassium nitrite. Overall, an ADI for nitrite per se could be derived from the available repeated dose toxicity studies in animals, also considering the negative carcinogenicity results. The Panel concluded that an increased methaemoglobin level, observed in human and animals, was a relevant effect for the derivation of the ADI. The Panel, using a BMD approach, derived an ADI of 0.07 mg nitrite ion/kg bw per day. The exposure to nitrite resulting from its use as food additive did not exceed this ADI for the general population, except for a slight exceedance in children at the highest percentile. The Panel assessed the endogenous formation of nitrosamines from nitrites based on the theoretical calculation of the NDMA produced upon ingestion of nitrites at the ADI and estimated a MoE > 10,000. The Panel estimated the MoE to exogenous nitrosamines in meat products to be < 10,000 in all age groups at high level exposure. Based on the results of a systematic review, it was not possible to clearly discern nitrosamines produced from the nitrite added at the authorised levels, from those found in the food matrix without addition of external nitrite. In epidemiological studies there was some evidence to link (i) dietary nitrite and gastric cancers and (ii) the combination of nitrite plus nitrate from processed meat and colorectal cancers. There was evidence to link preformed NDMA and colorectal cancers.

6 Final Report on the Safety Assessment of Morpholine

Morpholine is used in cosmetic products as a surfactant and emulsifier at concentrations up to 5%.

Morpholine is metabolized in guinea pigs but was not significantly metabolized in rats, dogs, or rabbits. Dermal LD50s in rabbits ranged between 0.3 and 1.2 g/kg. The oral LD50s in rats were between 1.1 and 1.6 g/kg; in guinea pigs the oral LD50 was 0.9 g/kg.

In studies of acute and short-term dermal toxicity Morpholine as an undiluted and unneutralized solution or as a diluted and unneutralized solution applied daily to the skin of guinea pigs and rabbits, respectively, caused the deaths of the test animals within 2 weeks. In both cases, the skin was necrotic.

Unneutralized solutions of Morpholine caused severe corneal necrosis, but upon neutralization Morpholine was not injurious to rabbit eyes.

In short-term inhalation studies (in rats) with varying concentrations of Morpholine, the effects observed included irritation of the mucous membranes and an increased respiratory rate. Chronic inhalation studies of Morpholine in rats and guinea pigs reported changes in the nervous system activity and arterial and peripheral blood pressure. At high concentrations Morpholine produced swelling of the alveolar cells and atrophy of lymphoid elements in the spleen. At lower concentrations a decrease in the size of the lymph nodules in the spleen was noted.

Morpholine was a weak positive mutagen in L5178 mouse lymphoma assay, in BALB/3T3 malignant cell transformation and fibroblast transformation assays, and in sister chromatid exchange assays, but was negative in the Ames test with and without metabolic activation. At nontoxic doses Morpholine did not increase the rate of DNA repair in rat hepatocytes. Results of other mutagenic assays varied according to the system used.

Nitrosation of Morpholine produces N-nitrosomorpholine, which has been mutagenic in a variety of test systems. Simultaneous exposure of laboratory animals to Morpholine and nitrites has caused a number of different cancers. A carcinogenic response was produced in rats in a long-term feeding study of Morpholine in which nitrites were present in the diet.

In humans, Morpholine is absorbed and is considered to be a skin and eye irritant, as well as a skin sensitizer. A formulation containing 1% Morpholine indicated that the ingredient was neither an irritant nor sensitizer.

Morpholine is not considered to be an animal carcinogen. It reacts easily with nitrosating agents, resulting in the formation of N-nitrosomorpholine. Under conditions of use, it is highly unlikely that Morpholine is totally free of carcinogenic nitrosoamines. Without quantitative data regarding the formation of N-nitrosomorpholine under conditions of use, it cannot be concluded that Morpholine is safe for use in cosmetic products.

Detection of in vivo genotoxicity of endogenously formed N-nitroso compounds and suppression by ascorbic acid, teas and fruit juices

The genotoxicity of endogenously formed N-nitrosamines from secondary amines and sodium nitrite (NaNO(2)) was evaluated in multiple organs of mice, using comet assay. Groups of four male mice were orally given dimethylamine, proline, and morpholine simultaneously with NaNO(2). The stomach, colon, liver, kidney, urinary bladder, lung, brain, and bone marrow were sampled 3 and 24 h after these compounds had been ingested. Although secondary amines and the NaNO(2) tested did not yield DNA damage in any of the organs tested, DNA damage was observed mainly in the liver following simultaneous oral ingestion of these compounds. The administration within a 60 min interval also yielded hepatic DNA damage. It is considered that DNA damage induced in mouse organs with the coexistence of amines and nitrite in the acidic stomach is due to endogenously formed nitrosamines. Ascorbic acid reduced the liver DNA damage induced by morpholine and NaNO(2). Reductions in hepatic genotoxicity of endogenously formed N-nitrosomorpholine by tea polyphenols, such as catechins and theaflavins, and fresh apple, grape, and orange juices were more effective than was by ascorbic acid. In contrast with the antimutagenicity of ascorbic acid in the liver, ascorbic acid yielded stomach DNA damage in the presence of NaNO(2) (in the presence and absence of morpholine). Even if ascorbic acid acts as an antimutagen in the liver, nitric oxide (NO) formed from the reduction of NaNO(2) by ascorbic acid damaged stomach DNA.

Evaluation of Escherichia coli K-12 343/113 and derived strains for microbial mutagenicity assays

Several characteristics of the E. coli K-12 mutagenicity tester strains 343/113 and 343/120 have been investigated for their effects on induced mutagenesis using the arg56 and nad113 genes, and resistance to valine. We found, as have earlier authors, that the nad113 marker is relatively specific for detecting frameshift-inducing mutagens and relatively insensitive to agents that cause point mutations. In contrast, both the Arg and Val markers are primarily specific for reversion (or mutation) induction by point mutagens. In all cases tested, the Arg and Val markers respond to mutagens in a qualitatively similar manner. We have enhanced the sensitivity of this tester system to a wide variety of mutagens by permeabilization of the tester cell population using Tris-EDTA treatment. This treatment prior to mutagen exposure enables the detection of mutagenicity of several compounds that are weakly mutagenic or nonmutagenic in untreated cells. We have also increased the mutagenicity of some chemicals by preincubating with rat-liver S9 at pH values other than 7.4. For diethylnitrosamine, for instance, maximal induction occurred at pH 6.5, and for benzo[a]pyrene, maximal induction was at pH 6.8.

The unique role of rodents in the detection of possible human carcinogens and mutagens

Some of the probable reasons underlying the observation that not all chemicals shown to be genotoxic in vitro are capable of eliciting tumours in rodents or humans are discussed using appropriate examples. It is suggested that a substantial proportion of the resources currently available for conducting rodent carcinogenicity bioassays should be employed in the short-term evaluation in vivo of some of the many hundreds of chemicals recently defined as genotoxic in vitro, rather than in the protracted evaluation of a few chemicals, often of unknown activity in vitro, for carcinogenicity. A decision tree approach to the evaluation of chemicals for human mutagenic/carcinogenic potential is presented which is at variance with the construction and philosophy of many of the current legislative guidelines. The immediate need for the adoption of one of the available short-term in vivo liver assays, and/or the development of a short-term in vivo rodent assay capable of concomitantly monitoring different genetic end-points in a range of organs or tissues is emphasized.

Studies on the mutagenic activity of ascorbic acid in vitro and in vivo

In vitro data are presented to show that ascorbic acid does not have intrinsic mutagenicity towards strain TA100 of S. typhimurium if deionized water is used to prepare the incubation medium. The addition of Cu2+ ions to the bacterial medium that contains ascorbic acid, or the use of tap water and ascorbic acid alone, causes a mutagenic and cytotoxic response that is blocked by EDTA. Additional in vitro data demonstrate that hydrogen peroxide is mutagenic to S. typhimurium strain TA100 and it is suggested that ascorbic acid may be mutagenic and cytotoxic through the generation of hydrogen peroxide. In vivo studies using a sensitive intrahepatic host-mediated mutagenicity assay indicate that ascorbic acid is not genotoxic in guinea pigs even when the dietary intake of vitamin C is above the level required for tissue saturation (5000 mg/kg body weight/day).

The intrasanguineous host mediated assay procedure using Saccharomyces cerevisiae: comparison with two other metabolic activation systems

3 metabolic activation systems--organ homogenates, perfused liver, and the intrasanguineous host mediated assay (IHMA)--were compared in their abilities to activate demethylnitrosamine (DMN) and induce gene conversion in Saccharomyces cerevisiae D4. Both rats and mice were used for the organ homogenates and IHMA studies. All activation systems were able to activate DMN; where the different organs were compared, liver was more active than kidney, followed by lung. The IHMA was the most sensitive of the systems examined.

An evaluation of the host-mediated assay and body fluid analysis. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The methodologies and status of the Host-Mediated Assay were reviewed using the published literature available up to June 1980. The Working Group reviewed 274 documents, including abstracts, research articles, review articles, and publicly available contracts and grant final reports. From this group, abstracts and reviews were rejected from critical evaluation. 77 documents were accepted and reviewed by the Working Group and the test results summarized. These selected documents yielded 208 chemicals that were evaluated in th host-mediated assay. Of these chemicals, 133 were mutagenic in this assay with one or more indicators. 76 chemicals, several of which are not considered to be carcinogenic, were not detected by any of the indicators. Of the 208 chemicals, 125 had been tested in carcinogenicity assay in rodents. 90, or 71%, of the carcinogens were detected as mutagens in the Host-Mediated Assay. In several cases, those carcinogens not detected may have been negative because of improper selection of the indicator. The Working Group concluded that the Host-Mediated Assay is an important test in mutagenicity/carcinogenicity research and that, by proper selection of protocols and indicators, valuable information can be gained that otherwise would be overlooked strict, in vitro assays.

Bacterial systems for carcinogenicity testing

During the past 30 years, bacterial test systems have been extensively refined in their ability to detect not only mutagenic agents but, in many cases, carcinogenic ones as well. Since many carcinogens are known to be activated within the mammalian body, major improvements in bacterial test systems were made when representative parts of mammalian metabolism were included as part of the test protocol. Presently, systems of great simplicity and convenience are available for the efficient detection of gene mutations, lysogenic induction of prophages, and differential DNA repair. These qualities render bacterial systems potentially useful in distinguishing between carcinogens and non-carcinogens, in characterizing induced mutation spectra, and possibly in quantifying mutagenic potency that may be used to predict tumor-initiating potency. Sensitive strains of Salmonella typhimurium. Escherichia coli and Bacillus subtilis with altered DNA-repair capacities have been constructed which accurately identify many carcinogens. Comparative studies have shown that techniques using these strains can be standardized to some extent and that the majority of carcinogens are active in all adequately sensitive genetic systems. Because of this redundancy, it may be sufficient to employ only one standardized set of tester strains and methodology. However, serveral classes of known carcinogens are undetected or underestimated when assayed in standard testing procedures. Some of these chemicals can be efficiently recognized as mutagens upon varying the methodology, the genetic endpoint, or the mammalian activation system. Thus, to modify and adjust the experimental protocol to the particular type of chemical under study and to calibrate the system with appropriate carcinogenic and non-carcinogenic reference compounds is advisable. It is noteworthy that chemical carcinogens which probably act by non-genotoxic mechanisms thus far remain undetected in bacterial tests. Newly developed systems which measure specific types of genetic events, such as transpositions of DNA segments and derepression of genes, presently are being tested for their ability to detect such carcinogens. A final matter of growing concern is the increasing number of environmental chemicals that are found to be mutagenic in bacteria but for which information about carcinogenic activity in vivo is insufficient. The possible use of bacteria for quantifying mutagenic potency and extrapolating this information to tumor-initiating potency can be envisaged in three ways: (i) direct extrapolation from standard in vitro tests, (ii) indirect extrapolation making use of an in vitro/in vivo comparison of induced effects (the parallelogram method) as devised by Sobels [138] on the basis of identical dose (to DNA), and (iii) host-mediated assays to assess mutagenic potency of carcinogens in selected organs of mammals...

A mutagenicity methodology for assessing the formation of N-dimethylnitrosamine in vivo

The mutagenicity test methodology in vitro has been extensively used during recent years in the identification of potential carcinogenic agents. Mutagenic analyses have been applied to the study of chemical reaction products for the demonstration of the formation of mutagenic agents. Recent studies have indicated that secondary and tertiary amines, when reacted with nitrite in acidic conditions, yield N-nitroso compounds, including the potent carcinogen N-dimethylnitrosamine (NDMA). This finding raises the problem of risk evaluation of several food components of human diets for the presence of potential carcinogenic compounds. By combining a mutagenicity test procedure with yeast cells inoculated into the blood system of mice and incubated in the liver for various times (minutes or hours) we have devised a model methodology which allows the detection of the formation of N-dimethylnitrosamine (NDMA) at a level lower than 1 mg/kg. The methodology has been examined for its use in the study of activators of the nitrosation, such as thiocyanate, and of inhibitors of the nitrosation, such as ascorbic acid and tannic acid. Other food components of the human diet, such as red wine, have also been investigated by this methodology.

Methods for analysis of the mutagenicity of indirect mutagens/carcinogens in eukaryotic cells

The review discusses the variety of methods for activation of indirect mutagens/carcinogens and testing them in cell cultures, especially in mammalian cell cultures. After the necessity for including metabolizing components in mutagenicity tests has been pointed out, the enzymes that transform foreign compounds metabolically, and the factors influencing them, are described. In the main section the various methods of activating indirect mutagens/carcinogens are presented. The methods of including in vivo metabolism in mutagenicity tests are: Analysis of cells from organisms contaminated with a chemical (III.l.a); body fluid-mediated mutagenesis (III.l.b); host-mediated assay (III.l.c). The following activation systems are suitable for including in vitro metabolism of test compounds in mutagenicity tests: Liver and lung perfusion (III.2.a.alpha); organ slices and homogenates (III.2.a.beta); subcellular fractions (III.2.a.gamma); cultivated cells (cell-mediated mutagenesis) (III.2.b); nonenzymatic activation systems (III.2.c). Finally the main factors that influence the metabolism of test substances are summarized. Two factors illustrate the mutagenicity tests with regard to the metabolism of mammalian livers and the methods of performing mutagenicity tests in man.