An evaluation of 6 chromosomal mutagens in the AS52/XPRT mutation assay utilizing suspension culture and soft agar cloning

Evaluation of the Suitability of Mammalian In Vitro Assays to Assess the Genotoxic Potential of Food Contact Materials

Background: Non-targeted screening of food contact materials (FCM) for non-intentionally added substances (NIAS) reveals a great number of unknown and unidentified substances present at low concentrations. In the absence of toxicological data, the application of the threshold of toxicological concern (TTC) or of EU Regulation 10/2011 requires methods able to fulfill safety threshold criteria. In this review, mammalian in vitro genotoxicity assays are analyzed for their ability to detect DNA-damaging substances at limits of biological detection (LOBD) corresponding to the appropriate safety thresholds. Results: The ability of the assays to detect genotoxic effects varies greatly between substance classes. Especially for direct-acting mutagens, the assays lacked the ability to detect most DNA reactive substances below the threshold of 10 ppb, making them unsuitable to pick up potential genotoxicants present in FCM migrates. However, suitability for the detection of chromosomal damage or investigation of other modes of action makes them a complementary tool as part of a standard test battery aimed at giving additional information to ensure safety. Conclusion: improvements are necessary to comply with regulatory thresholds to consider mammalian genotoxicity in vitro assays to assess FCM safety.

Value and limitation of in vitro bioassays to support the application of the threshold of toxicological concern to prioritise unidentified chemicals in food contact materials

Some of the chemicals in materials used for packaging food may leak into the food, resulting in human exposure. These include so-called Non-intentionally Added Substances (NIAS), many of them being unidentified and toxicologically uncharacterized. This raises the question of how to address their safety. An approach consisting of identification and toxicologically testing all of them appears neither feasible nor necessary. Instead, it has been proposed to use the threshold of toxicological concern (TTC) Cramer class III to prioritise unknown NIAS on which further safety investigations should focus. Use of the Cramer class III TTC for this purpose would be appropriate if amongst others sufficient evidence were available that the unknown chemicals were not acetylcholinesterase inhibitors or direct DNA-reactive mutagens. While knowledge of the material and analytical chemistry may efficiently address the first concern, the second could not be addressed in this way. An alternative would be use of a bioassay capable of detecting DNA-reactive mutagens at very low levels. No fully satisfactory bioassay was identified. The Ames test appeared the most suitable since it specifically detects DNA-reactive mutagens and the limit of biological detection of highly potent genotoxic carcinogens is low. It is proposed that for a specific migrate, the evidence for absence of mutagenicity based on the Ames test, together with analytical chemistry and information on packaging manufacture could allow application of the Cramer class III TTC to prioritise unknown NIAS. Recommendations, as well as research proposals, have been developed on sample preparation and bioassay improvement with the ultimate aim of improving limits of biological detection of mutagens. Although research is still necessary, the proposed approach should bring significant benefits over the current practices used for safety evaluation of food contact materials.

A review of the genotoxic, mutagenic, and carcinogenic potentials of several lower acrylates

Lower alkyl acrylate monomers include methyl-, ethyl-, n-butyl-, and 2-ethylhexyl acrylate. These acrylates are used in the manufacture of acrylic polymers and copolymers for plastics, food packaging, adhesives, and cosmetic formulations. Although there is limited potential for human environmental exposure, occupational exposure can occur via inhalation and dermal contact. Recently, new genotoxicity data have been generated, along with in silico and in vitro read-cross analyses, for these acrylates. The availability of high-throughput screening (HTS) data through the ToxCast™/Tox21 databases allows for consideration of computational toxicology and organization of these data according to the ten key characteristics of carcinogens. Therefore, we conducted a comprehensive review to evaluate the mechanistic, toxicokinetic, animal, and human data, including HTS data, for characterizing the potential carcinogenicity, mutagenicity, and genotoxicity of these acrylates. Toxicokinetic data demonstrate that these acrylates are metabolized rapidly by carboxylesterase hydrolysis and conjugation with glutathione. HTS data demonstrated an overall lack of bioactivity in cancer-related pathways. Overall, the genotoxicity and mutagenicity data support a cytotoxic, non-genotoxic mechanism for these acrylates. Cancer bioassay studies conducted by the oral, dermal, and inhalation routes in animal models with these acrylates did not show any increase in tumor incidence, with two exceptions. At high doses, and secondary to chronic site-of-contact irritation and corrosion, rodent forestomach tumors were induced by oral gavage dosing with ethyl acrylate, and skin tumors were observed following chronic dermal dosing with 2-ethylhexyl acrylate in C3H/HeJ inbred mice (a strain with deficiencies in wound healing), but not in the outbred NMRI strain. For both dermal and forestomach cancers, tumorigenesis is secondary to high doses and long-term tissue damage, shown to be reversible. With evidence that these chemicals are not genotoxic, and that they cause forestomach and dermal tumors through chronic irritation and regenerative proliferation mechanisms, these acrylates are unlikely to pose a human cancer hazard.

Genotoxic Activities of Aniline and its Metabolites and Their Relationship to the Carcinogenicity of Aniline in the Spleen of Rats

Aniline (in the form of its hydrochloride) has been shown to induce a rather rare spectrum of tumors in the spleen of Fischer 344 rats. The dose levels necessary for this carcinogenic activity were in a range where also massive effects on the blood and non-neoplastic splenotoxicity as a consequence of methemoglobinemia were to be observed. This review aimed at clarifying if aniline itself or one of its metabolites has a genotoxic potential which would explain the occurrence of the spleen tumors in rats as a result of a primary genetic activity. The database for aniline and its metabolites is extremely heterogeneous. With validated assays it ranges from a few limited Ames tests (o- and m-hydroxyacetanilide, phenylhydroxylamine, nitrosobenzene) to a broad range of studies covering all genetic endpoints partly with several studies of the same or different test systems (aniline, p-aminophenol, p-hydroxyacetanilide). This makes a direct comparison rather difficult. In addition, a varying number of results with as yet not validated systems are available for aniline and its metabolites. Most results, especially those with validated and well performed/documented studies, did not indicate a potential of aniline to induce gene mutations. In five different mouse lymphoma tests, where colony sizing was performed only in one test, aniline was positive. If this indicates a peculiar feature of a point mutagenic potential or does represent a part of the clastogenic activity for which there is evidence in vitro as well as in vivo remains to be investigated. There is little evidence for a DNA damaging potential of aniline. The clastogenic activity in vivo is confined to dose levels, which are close to lethality essentially due to hematotoxic effects. The quantitatively most important metabolites for experimental animals as well as for humans (p-aminophenol, p-hydroxyacetanilide) seem to have a potential for inducing chromosomal damage in vitro and, at relatively high dose levels, also in vivo. This could be the explanation for the clastogenic effects that have been observed after high doses/concentrations with aniline. They do not induce gene mutations and there is little evidence for a DNA damaging potential. None of these metabolites revealed a splenotoxic potential comparable to that of aniline in studies with repeated or long-term administration to rats. The genotoxicity database on those metabolites with a demonstrated and marked splenotoxic potential, i.e. phenylhydroxylamine, nitrosobenzene, is unfortunately very limited and does not allow to exclude with certainty primary genotoxic events in the development of spleen tumors. But quite a number of considerations by analogy from other investigations support the conclusion that the effects in the spleen do not develop on a primary genotoxic basis. The weight of evidences suggests that the carcinogenic effects in the spleen of rats are the endstage of a chronic high-dose damage of the blood leading to a massive overload of the spleen with iron, which causes chronic oxidative stress. This conclusion, based essentially on pathomorphological observations, and analogy considerations thereof by previous authors, is herewith reconfirmed under consideration of the more recently reported studies on the genotoxicity of aniline and its metabolites, on biochemical measurements indicating oxidative stress, and on the metabolism of aniline. It is concluded that there is no relationship between the damage to the chromosomes at high, toxic doses of aniline and its major metabolites p-aminophenol/p-hydroxyacetanilide and the aniline-induced spleen tumors in the rat.

Final report on the safety assessment of Acrylates Copolymer and 33 related cosmetic ingredients

Ingredients in the Acrylates Copolymer group all contain the monomers acrylic acid or methacrylic acid or one of their salts or esters. These ingredients are considered similar in that they are uniformly produced in chemical reactions that leave very little residual monomer. Although residual acrylic acid may be as high as 1500 ppm, typical levels are 10 to 1000 ppm. There is sufficient odor if residual monomers are present to cause producers to keep levels as low as possible. These ingredients function in cosmetics as binders, film formers, hair fixatives, suspending agents, viscosity-increasing agents, and emulsion stabilizers. Concentrations may be as high as 25% if used as a binder, film former, or fixative; or as low as 0.5% if used as a viscosity-increasing agent, suspending agent, or emulsion stabilizer. These very large polymers exhibit little toxicity. In rabbits and guinea pigs, Acrylates Copolymer did produce irritation, but no evidence of sensitization was found. The principle concern regarding the use of these polymer ingredients is the presence of toxic residual monomers. In particular, although 2-ethylhexyl acrylate was not genotoxic, it was carcinogenic when applied at a concentration of 21% to the skin of C3H mice. Lower concentrations (2.5%) and stop-dose studies at high concentrations (43%) were not carcinogenic. 2-Ethylhexyl acrylate was not carcinogenic in studies using NMRI mice. Whether an increase in carcinogenesis was seen or not, there was evidence of severe dermal irritation in these 2-ethylhexyl acrylate studies. Another concern regarding residual monomers was inhalation toxicity. Although the acrylic acid monomer is a nasal irritant, exposure to the monomer from use of these polymers in cosmetic formulations would always be less than the established occupational exposure limits for nasal irritation. Although there appears to be a huge variation in the mix of monomers used in the synthesis of these polymers, they are similar in that the polymers, except for dermal irritation, are not significantly toxic, and residual monomer levels are kept as low as possible. Although the monomers may be toxic, the levels that would be found in cosmetic formulations are not considered to present a safety risk. Accordingly, these Acrylate Copolymers are considered safe for use in cosmetic formulations when formulated to avoid irritation.

Concurrent detection of gene mutations and chromosome aberrations induced by five chemicals in a CHL/IU cell line incorporating a gpt shuttle vector

We previously established a transgenic Chinese hamster CHL/IU cell line, designated as KN63, for concurrent analysis of gene mutations and chromosome aberrations. The KN63 cell line contains copies of a shuttle vector with the Escherichia coli gpt gene as a mutational target in its chromosome. To evaluate the sensitivity of the cell line to various types of mutagens, methyl methanesulfonate (MMS), N-ethyl-N-nitrosourea (ENU), mitomycin C (MMC), vincristine sulfate (VIN) and C.I. basic red 9 hydrochloride (CIB) were assayed. KN63 cells were treated with each test chemical and gene mutations were detected in the gpt gene of the shuttle vector rescued from the KN63 cell genome into an E. coli host. Chromosome aberrations were concurrently evaluated by conventional metaphase analysis. MMS, ENU and MMC induced both gene mutations and structural chromosome aberrations in KN63 cells, with more efficient induction of the latter. VIN, a well-known aneugen, produced only numerical changes to chromosomes, while CIB was negative for both types of alteration. KN63 cells were as sensitive to MMS, ENU, MMC and VIN as Chinese hamster cell lines such as CHL, CHO and V79 cells. The characteristics of test chemicals indicated by this system should be useful for understanding endpoints in chemical mutagenesis.

A comparison of the soft agar and microtitre methodologies for the L5178Y tk +/- mouse lymphoma assay

The L5178Y tk +/- mouse lymphoma assay (MLA) has been validated as a sensitive and specific test system for the detection of mutagens/clastogens. There are currently two methodologies for performing the MLA: the original soft agar procedure and the newer microtitre procedure. While both procedures are considered acceptable, a limited amount of comparative information exists for the two methods. In this report the two methods were compared with regard to: (1) spontaneous and induced mutant frequencies; (2) cloning efficiencies; and (3) colony size distributions for mutants. In addition, small and large mutant colonies from microtitre wells were rechallenged for trifluorothymidine (TFT) resistance. In a majority of the cases, cloning efficiency values were higher for the microtitre as were the spontaneous and induced mutation frequency (MF) values. Nevertheless, when responses were compared according to mutation index (fold increase over background MF) the results from the two systems were often similar. More spontaneous small colonies were observed in the microtitre assay. While colony size distribution for induced mutant colonies was compound specific, generally, more small colonies were counted in microtitre. All mutant clones that were rechallenged with TFT demonstrated resistance. Aside from the differences mentioned above, both the microtitre and the soft agar procedures appear equally capable of identifying mutagenic agents.

An evaluation of the twofold rule for assessing a positive response in the L5178Y TK+/- mouse lymphoma assay

The L5178Y tk+/- mouse lymphoma assay (MLA) has been in use for more than 15 years as a tool for evaluating the mutagenic potential of various agents. As with other genetic toxicology test systems, one criterion for a positive response has been the requirement of at least a 2-fold increase in mutant frequency (MF) as compared to the respective MF of the solvent controls. More recently, an actual specific increase in MF has been proposed as a criterion for determining a positive response in the MLA; however, this may not be appropriate for laboratories with a low, yet stable, background MF. The twofold rule criterion was evaluated in our laboratory with 66 compounds. The mutagenic status of these compounds was previously determined in other test systems and at one or more laboratories, including Lilly Research Laboratories. The results of this evaluation demonstrate that the twofold rule is an effective method for identifying mutagenic agents in the MLA at LRL where a lower, yet acceptable, background mutation frequency is the norm. A small number of compounds (6) yielded results discordant with the literature; however, these compounds have been previously found to be either difficult to detect in genotoxic assays or to show specific sensitivity in the MLA.

Genotoxic effects of p-aminophenol in Chinese hamster ovary and mouse lymphoma cells: results of a multiple endpoint test

p-Aminophenol (PAP), a metabolite of aniline and acetaminophen, has been reported to be mutagenic in the L5178Y mouse lymphoma assay, but not in the CHO/HGPRT assay. In the present study, the effects of PAP in these two cells lines were examined to determine if the difference in activity is related to an intrinsic difference in the cell lines. CHO and L5178Y tk +/- mouse lymphoma cells were treated with PAP for 4 hr and assayed for 3 genetic endpoints: gene mutation at the HGPRT or TK locus, respectively; chromosomal aberrations at approximately 20 hr after initiation of treatment; and single-strand DNA breaks as detected by the single cell electrophoresis assay immediately after treatment. All treatments were conducted in the absence of S9. There was a dose-related, significant increase in TFT-resistant mouse lymphoma cells at dose levels that reduced survival to < or = 50% of concurrent controls. In CHO cells, however, there was no increase in thioguanine-resistant cells at dose levels that reduced cell survival to < 20%. These results are consistent with published reports on PAP. While the CHO cells were slightly more resistant to the toxic effects of PAP, the dose levels used in the two cell lines did not differ by more than 2-fold. At equivalent survival levels, PAP induced a significant (up to 20% aberrant cells) number of aberrations, primarily complex rearrangements, in both cell lines. In the single cell electrophoresis assay, there was a reproducible dose-related increase in cells with single-strand DNA breaks with both the L5178Y cells and the CHO cells. The induction of single-strand breaks and chromosome aberrations by PAP suggests that, mechanistically, PAP produces similar genetic damage in both CHO and L5178Y cell lines, but intrinsic differences between assay systems are responsible for the divergent gene mutation results.