Comparative mutagenicity of halogenated pyridines in the Salmonella typhimurium/mammalian microsome test

Weight of evidence approach using a TK gene mutation assay with human TK6 cells for follow-up of positive results in Ames tests: a collaborative study by MMS/JEMS

BACKGROUND

Conflicting results between bacterial mutagenicity tests (the Ames test) and mammalian carcinogenicity tests might be due to species differences in metabolism, genome structure, and DNA repair systems. Mutagenicity assays using human cells are thought to be an advantage as follow-up studies for positive results in Ames tests. In this collaborative study, a thymidine kinase gene mutation study (TK6 assay) using human lymphoblastoid TK6 cells, established in OECD TG490, was used to examine 10 chemicals that have conflicting results in mutagenicity studies (a positive Ames test and a negative result in rodent carcinogenicity studies).

RESULTS

Two of 10 test substances were negative in the overall judgment (20% effective as a follow-up test). Three of these eight positive substances were negative after the short-term treatment and positive after the 24 h treatment, despite identical treatment conditions without S9. A toxicoproteomic analysis of TK6 cells treated with 4-nitroanthranilic acid was thus used to aid the interpretation of the test results. This analysis using differentially expressed proteins after the 24 h treatment indicated that in vitro specific oxidative stress is involved in false positive response in the TK6 assay.

CONCLUSIONS

The usefulness of the TK6 assay, by current methods that have not been combined with new technologies such as proteomics, was found to be limited as a follow-up test, although it still may help to reduce some false positive results (20%) in Ames tests. Thus, the combination analysis with toxicoproteomics may be useful for interpreting false positive results raised by 24 h specific reactions in the assay, resulting in the more reduction (> 20%) of false positives in Ames test.

Meeting report, ICH M7 relevant workshop: use of (Q)SAR systems and expert judgment

Use of Quantitative Structure-Activity Relationships ((Q)SAR) prediction tools has been increasing since the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH) M7 guideline was issued in June 2014. The Japanese Environmental Mutagen Society and the Bacterial Mutagenicity Study Group took the initiative of the workshop on (Q)SAR in 2016 to discuss using (Q)SAR to predict mutagenicity. The aim of the workshop was to form a common understanding on the current use of (Q)SAR tools in industry and for regulatory purposes and on the process of expert judgment. This report summarizes the general session that reviewed the use of (Q)SAR tools and the case study session that discussed expert judgment.

2-Hydroxypyridine-N-oxide (HOPO): Equivocal in the ames assay

2-Hydroxypyridine-N-oxide (HOPO) is a useful coupling reagent for synthesis of active pharmaceutical ingredients. It has been reported to be weakly mutagenic in the Ames assay (Ding W et al. []: J Chromatogr A 1386:47-52). According to the ICH M7 guidance (2014) regarding control of mutagenic impurities to limit potential carcinogenic risk, mutagens require control in drug substances such that exposure not exceeds the threshold of toxicological concern. Given the weak response observed in the Ames assay and the lack of any obvious structural features that could confer DNA reactivity we were interested to determine if the results were reproducible and investigate the role of potentially confounding experimental parameters. Specifically, Ames tests were conducted to assess the influence of compound purity, solvent choice, dose spacing, toxicity, type of S9 (aroclor vs phenobarbital/β-napthoflavone), and lot variability on the frequency of HOPO induced revertant colonies. Initial extensive testing using one lot of HOPO produced no evidence of mutagenic potential in the Ames assays. Subsequent studies with four additional lots produced conflicting results, with an ∼2.0-fold increase in revertant colonies observed. Given the rigor of the current investigation, lack of reproducibility between lots, and the weak increase in revertants, it is concluded that HOPO is equivocal in the bacterial reverse mutation assay. It is highly unlikely that HOPO poses a mutagenic risk in vivo; therefore, when it is used as a reagent in pharmaceutical synthesis, it should not be regarded as a mutagenic impurity, but rather a normal process related impurity. Environ. Mol. Mutagen. 59:312-321, 2018. © 2018 Wiley Periodicals, Inc.

Scientific opinion on flavouring group evaluation 77, revision 3 (FGE.77Rev3): consideration of pyridine, pyrrole and quinoline derivatives evaluated by JECFA (63rd meeting) structurally related to pyridine, pyrrole, indole and quinoline derivatives evaluated by EFSA in FGE.24Rev2

The Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids of the EFSA was requested to consider evaluations of flavouring substances assessed since 2000 by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) and to decide whether further evaluation is necessary, as laid down in Commission Regulation (EC) No 1565/2000. The present consideration concerns a group of 22 pyridine, pyrrole and quinoline derivatives evaluated by JECFA (63rd meeting). The revision of this consideration is made since additional genotoxicity data have become available for 6-methylquinoline [FL-no: 14.042]. The genotoxicity data available rule out the concern with respect to genotoxicity and accordingly the substance is evaluated through the Procedure. For all 22 substances [FL-no: 13.134, 14.001, 14.004, 14.007, 14.030, 14.038, 14.039, 14.041, 14.042, 14.045, 14.046, 14.047, 14.058, 14.059, 14.060, 14.061, 14.065, 14.066, 14.068, 14.071, 14.072 and 14.164] considered in this Flavouring Group Evaluation (FGE), the Panel agrees with the JECFA conclusion, 'No safety concern at estimated levels of intake as flavouring substances' based on the Maximised Survey-derived Daily Intake (MSDI) approach. Besides the safety assessment of these flavouring substances, the specifications for the materials of commerce have also been evaluated, and the information is considered adequate for all the substances. For the following substances [FL-no: 13.134, 14.001, 14.030, 14.041, 14.042, 14.058, 14.072], the Industry has submitted use levels for normal and maximum use. For the remaining 15 substances, use levels are needed to calculate the modified Theoretical Added Maximum Daily Intakes (mTAMDIs) in order to identify those flavouring substances that need more refined exposure assessment and to finalise the evaluation.

Photodegradation of 2-chloropyridine in aqueous solution: Reaction pathways and genotoxicity of intermediate products

2-Chloropyridine, an important precursor of the chemical industry is also a persistent water pollutant. The genotoxicity of photolytically treated 2-chloropyridine aqueous solution to human lymphocytes initially increases and fluctuates during treatment finally reaching the control values after prolonged treatment. Intermediate products formed were identified; a kinetic scheme for their formation is presented. To identify the source of genotoxicity variations and the potential in vitro effects on human lymphocytes of the partially photo-treated aqueous solution, the genotoxicity of four (the only) commercially available intermediates, i.e., 1H-pyrrole-2-carboxaldehyde, 6-chloro-2-pyridinecarboxylic acid, 2,3-dichloropyridine and 2-pyridinecarbonitrile was measured; the obtained results were used for the reasoning on the variation of the solution genotoxic (including clastogenic as well as aneugenic) events and cytotoxic activity. It was found that 1H-pyrrole-2-carboxaldehyde and 6-chloro-2-pyridinecarboxylic acid were highly genotoxic even at the very low concentration measured here. Thus, they likely had a significant contribution to the photolytically treated solution genotoxicity. 2,3-Dichloropyridine was found to be genotoxic but only at concentrations higher than the ones measured in this work. Thus, it was not likely to have contributed to the solution genotoxicity. Finally, at the concentrations measured in this work 2-pyridinecarbonitrile was found to be only cytotoxic.

Induction of micronuclei by 2-hydroxypyridine in water and elimination of solution genotoxicity by UVC (254 nm) photolysis

2-Hydroxypyridine (2-HPY) is a major first-stage product formed upon the photolytic destruction of 2-halogenated pyridines. Genotoxicity of 2-HPY in water was studied as a function of concentration. Aqueous solutions of 2-HPY were irradiated by ultraviolet (UV) at 254 nm. 2-HPY concentration, solution total organic carbon (TOC) concentration and solution genotoxicity were measured as a function of treatment time and their profile as a function of time is presented in this work. 2-HPY was found to be genotoxic at all concentrations in the range of 5-400 μg ml(-1). 2-HPY mineralises completely upon prolonged UV irradiation. All untreated and irradiated solution samples, taken at different photo-treatment times, were tested in cultured human lymphocytes applying the cytokinesis block micronucleus (CBMN) assay. The genotoxicity of the solution was reduced near to the control level after prolonged UV irradiation.

Genotoxicity study of photolytically treated 2-chloropyridine aqueous solutions

2-Chloropyridine (2-CPY) has been identified as a trace organic chemical in process streams, wastewater and even drinking water. Furthermore, it appears to be formed as a secondary pollutant during the decomposition of specific insecticides. As reported in our previous work, 2-CPY was readily removed and slowly mineralised when subjected to ultraviolet (UV) irradiation at 254 nm. Moreover, 2-CPY was found to be genotoxic at 100 microg ml(-1) but it was not genotoxic at or below 50 microg ml(-1). In this work 2-CPY aqueous solutions were treated by means of UV irradiation at 254 nm. 2-CPY mineralisation history under different conditions is shown. 2-CPY was found to mineralise completely upon prolonged irradiation. Identified products of 2-CPY photolytic decomposition are presented. Solution genotoxicity was tested as a function of treatment time. Aqueous solution samples, taken at different photo-treatment times were tested in cultured human lymphocytes applying the cytokinesis block micronucleus (CBMN) assay. It was found that the solution was genotoxic even when 2-CPY had been practically removed. This shows that photo-treatment of 2-CPY produces genotoxic products. Upon prolonged irradiation solution genotoxicity values approached the control value.

Evaluation of new 2,2′-dimethyl-5,5′-dipropoxybenzidine- and 3,3′-dipropoxybenzidine-based direct dye analogs for mutagenic activity by use of the Salmonella/mammalian mutagenicity assay

As part of a continuing study aimed at establishing structure-activity relationships and heuristic principles useful for the design of non-genotoxic azo dyes, a series of new direct dyes based on two non-mutagenic benzidine analogs, 2,2'-dimethyl-5,5'-dipropoxybenzidine and 3,3'-dipropoxybenzidine, were evaluated for mutagenic activity in Salmonella typhimurium strains TA98 and TA100. These strains are widely used for mutagenicity screening and have been shown to detect the mutagenic activity of benzidine analogs. While some toxicity was seen with some dyes at high doses, all of the dyes examined were judged non-mutagenic with and without metabolic activation in the standard Salmonella plate-incorporation assay. The results in the standard test are consistent with the properties of the diamines themselves. However, only one of the dyes was non-mutagenic when a reductive-metabolism pre-incubation assay was used. The results of this study suggest that although benzidine analogs are potential replacements for benzidine, there is a need to understand which mutagenic products are produced when reductive metabolism is present. There is also a need to know whether or not metal complexes of these dyes are mutagenic. Such information will allow the development of new non-mutagenic azo dyes.

Identification and quantification of pesticides, industrial chemicals, and organobromine compounds of medium to high polarity in the North Sea

Solid-phase extraction of 20 L seawater samples enabled the enrichment and determination of a wide array of organic substances, including compounds of medium to high polarity, in the pg/L-range. A number of contaminants was detected and quantified throughout the North Sea, among them the pesticides dichlobenil (2,6-dichlorobenzonitrile), metolachlor and terbuthylazine as well as the industrial chemicals dichloropyridines (DCPy, 4 isomers) and nitrobenzene. Concentrations attained values up to 1.4 ng/L for dichlobenil, 0.83 ng/L for terbuthylazine, 0.61 ng/L for metolachlor, 0.13 ng/L for 2,6-DCPy, 4.37 ng/L for nitrobenzene and 1-8 ng/L for tris(chloropropyl)phosphates (TCPP). A number of North Sea water samples was screened for non-target compounds, revealing the presence of further contaminants, e.g., lindane and TCPP, as well as several biogenic and/or anthropogenic organobromine substances, among which bromoindols, -phenols and -anisoles were identified.

Toxicity and anaerobic biodegradability of pyridine and its derivatives under sulfidogenic conditions

Attempts were made to correlate the chemical structure of pyridine and 15 pyridine derivatives with both their biodegradability by estuarine sediment microorganisms under anaerobic conditions and also with their toxicity to the marine bacterium Vibrio fischeri Beijerinck 1889 by using the Microtox bacterial assay. Among monosubstituted pyridines, comparisons of different substituents at positions C-2, C-3, or C-4 atom of the pyridine ring showed that isomers of carboxylpyridine (COOHPYR), hydroxypyridine (OHPYR), and cyanopyridine (CNPYR) were more susceptible to biotransformation than isomers of chloropyridine (ClPYR) and methylpyridine (CH3PYR) in anoxic estuarine sediment slurries under sulfidogenic conditions. Isomers with the functional group at the C-2 or C-3 atom of the pyridine ring were biotransformed faster than those with the same functional group at C-4. The only exception was 4-ClPYR, which was biotransformed within 130 days, while 2- and 3-ClPYR continued to persist in the anoxic sediment slurries. Median effect concentrations (EC50) of pyridine and pyridine derivatives were in the range of 0.027 to 49.1 mmol/L. Pyridine derivatives with -CN and -OH functional groups tended to be less toxic, while pyridine derivatives with -CH3, -Cl, and -COOH functional groups tended to be more toxic. Isomers with the substituent at C-2 were less toxic than the C-3 or C-4 isomers. There was no clear correlation between the pseudo-first-order rate constants for the microbial transformation of pyridine and its derivatives and their toxicity to the marine bacterium.

Genotoxicity of three pyridine compounds to L5178Y mouse lymphoma cells

The L5178Y mouse lymphoma assay was used to examine the potential mutagenicity of three halogenated pyridine compounds. Position effects of the halogen moiety and the role of metabolic activation were analyzed based on induced mutant frequency, gross chromosome aberrations, and micronuclei. Without activation, 2-chloropyridine, 3-chloropyridine, and 2-chloro-5-trifluoromethylpyridine produced a small increase in mutant frequency; only the 2-chloropyridine activity was significantly increased with activation. All three compounds were also clastogenic as demonstrated by increases in chromosome aberrations and micronuclei (except for 2-chloro-5-trifluoromethylpyridine which did not induce micronuclei either with or without activation).

The effect of alkoxy substituents on the mutagenicity of some aminoazobenzene dyes and their reductive-cleavage products

15 aminoazobenzene dyes and 7 of their reductive-cleavage products were examined in the Salmonella/microsome assay with strains TA98, TA100, TA1535, TA1537 and TA1538. Dyes tested included 5 derivatives of 4-aminoazobenzene with different alkoxy substituents (-OCH3, -OCH2CH3, -OCH2CH2 CH3, -OCH2CH2CH2CH3 or -OCH2CH2OH) in the 8-position as well as the corresponding derivatives of 4-[(4-aminophenyl)azo]-N,N-diethylaniline and 4-[(4-aminophenyl)azo]-N,N-bis(2-hydroxyethyl)aniline. In general, as the size of the substituent ortho to the primary amino group of the dyes was increased, the mutagenicity decreased. A similar trend was observed for the reductive-cleavage products. The results from the latter aspect of this study suggest that the mutagenicity of aminoazobenzene dyes can not be accounted for solely from the properties of their reductive-cleavage products.

The effect of alkoxy substituents on the mutagenicity of some phenylenediamine-based disazo dyes

16 phenylenediamine-based disazo dyes were examined in the Salmonella/mammalian microsome assay with strains TA98, TA100 and TA1538. All of the dyes contain an alkoxy group ortho to one of the azo linkages. Increasing the size of this alkoxy substituent from 1 to 4 carbons led to a decrease in mutagenic activity in certain instances while no change was noted in other cases. Comparison of the mutagenicity of the disazo dyes with their potential reductive-cleavage products suggests that (1) the reductive-cleavage products are not solely responsible for the mutagenicity of the disazo dyes, and (2) significant reductive-cleavage of the disazo dyes is not taking place in the standard Salmonella assay.

Reduction potentials of imine-substituted, biologically active pyridines: possible relation to activity

Cyclic voltammetry data were obtained for a number of biologically active compounds which incorporate imine substitution on the pyridine nucleus. The reductions in acid (iminium ion formation) were for the most part reversible, and in the range of -0.5 to -0.7V. The toxic effect of these drugs is thought to be caused by the generation of reactive oxygen radicals that arise via charge transfer, or by disruption of electron transport chains.