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Toxicological information

Genetic toxicity: in vitro

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Administrative data

Endpoint:
in vitro gene mutation study in bacteria
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2020
Report date:
2020

Materials and methods

Test guideline
Qualifier:
according to guideline
Guideline:
OECD Guideline 471 (Bacterial Reverse Mutation Assay)
GLP compliance:
yes (incl. QA statement)
Type of assay:
bacterial reverse mutation assay

Test material

Constituent 1
Reference substance name:
RDX
IUPAC Name:
RDX
Test material form:
solid: particulate/powder
Remarks:
migrated information: powder
Details on test material:
No data available.
Specific details on test material used for the study:
Hexogen: batch n°A007, Purity > 99.9%
Dissolved in DMSO at 51.8 mg/L

Method

Species / strainopen allclose all
Species / strain / cell type:
S. typhimurium TA 1535, TA 1537, TA 98 and TA 100
Details on mammalian cell type (if applicable):
CELLS USED
Salmonella typhimurium
All of the Salmonella strains are histidine dependent by virtue of a mutation through the histidine operon and are derived from S. typhimurium strain LT2 through mutations in the histidine locus.
Strains Genotype Type of mutations indicated
TA1537 his C 3076; rfa-; uvrB-: frame shift mutations
TA98 his D 3052; rfa-; uvrB-;R-factor
TA1535 his G 46; rfa-; uvrB-: base-pair substitutions
TA100 his G 46; rfa-; uvrB-;R-factor

MEDIA USED
- Top agar was prepared using 0.6% Bacto agar (lot number 9105946 expiry date 01/2024) and 0.5% sodium chloride with 5 mL of 1.0 mM histidine and 1.0 mM biotin or 1.0 mM tryptophan solution added to each 100 mL of top agar. Vogel-Bonner Minimal agar plates were purchased from SGL Ltd (lot numbers 54834 expiry date 07/2020 and 54898 expiry date 07/2020).
Additional strain / cell type characteristics:
other:
Species / strain / cell type:
E. coli WP2 uvr A
Details on mammalian cell type (if applicable):
CELLS USED
Escherichia coli
Strain Genotype Type of mutations indicated
WP2uvrA trp-; uvrA-: base-pair substitution

MEDIA USED
- Top agar was prepared using 0.6% Bacto agar (lot number 9105946 expiry date 01/2024) and 0.5% sodium chloride with 5 mL of 1.0 mM histidine and 1.0 mM biotin or 1.0 mM tryptophan solution added to each 100 mL of top agar. Vogel-Bonner Minimal agar plates were purchased from SGL Ltd (lot numbers 54834 expiry date 07/2020 and 54898 expiry date 07/2020).
Metabolic activation:
with and without
Metabolic activation system:
The S9-mix was prepared before use using sterilized co-factors and maintained on ice for the duration of the test.

S9 5.0 mL
1.65 M KCl/0.4 M MgCl2 1.0 mL
0.1 M Glucose-6-phosphate 2.5 mL
0.1 M NADP 2.0 mL
0.2 M Sodium phosphate buffer (pH 7.4) 25.0 mL
Sterile distilled water 14.5 mL

A 0.5 mL aliquot of S9-mix and 2 mL of molten, trace histidine or tryptophan supplemented, top agar were overlaid onto a sterile Vogel-Bonner Minimal agar plate in order to assess the sterility of the S9-mix. This procedure was repeated, in triplicate, on the day of each experiment.
Test concentrations with justification for top dose:
Experiment 1: The test item was tested using the following method. The maximum concentration was 5000 μg/plate (the OECD TG 471 maximum recommended dose level). Eight concentrations of the test item (1.5, 5, 15, 50, 150, 500, 1500 and 5000 μg/plate) were assayed in triplicate against each tester strain, using the direct plate incorporation method.
Experiment 2: The dose range used for Experiment 2 was determined by the results of Experiment 1 and was 1.5, 5, 15, 50, 150, 500, 1500 and 5000 μg/plate.
Eight test item concentrations were selected in Experiment 2 in order to ensure the study achieved at least four non-toxic dose levels as required by the test guideline, and were selected based on the urgency of testing, the lack of cytotoxicity noted in Experiment 1, and the potential for a change in the cytotoxicity of the test item following the change in test methodology from plate incorporation to pre-incubation.
Vehicle / solvent:
DMSO
Controls
Untreated negative controls:
yes
Negative solvent / vehicle controls:
yes
True negative controls:
no
Positive controls:
yes
Positive control substance:
4-nitroquinoline-N-oxide
9-aminoacridine
N-ethyl-N-nitro-N-nitrosoguanidine
benzo(a)pyrene
other: 2-aminoanthracene
Details on test system and experimental conditions:
All of the plates were incubated at 37 ± 3 °C for between 48 and 72 hours and scored for the presence of revertant colonies using an automated colony counting system. The plates were viewed microscopically for evidence of thinning of the background bacterial lawn (toxicity). Manual counts were performed for TA1537 dosed in the absence of metabolic activation at 5000 μg/plate because of test item precipitation.
Evaluation criteria:
There are several criteria for determining a positive result. Any, one, or all of the following can be used to determine the overall result of the study:
1. A dose-related increase in mutant frequency over the dose range tested (De Serres and Shelby, 1979).
2. A reproducible increase at one or more concentrations.
3. Biological relevance against in-house historical control ranges.
4. A fold increase greater than two times the concurrent solvent control for TA100, TA98 and WP2uvrA or a three-fold increase for TA1535 and TA1537 (especially if accompanied by an out-of-historical range response (Cariello and Piegorsch, 1996)).
5. Statistical analysis of data as determined by UKEMS (Mahon et al., 1989).

A test item is considered non-mutagenic (negative) in the test system if the above criteria are not met.
Although most experiments give clear positive or negative results, in some instances the data generated prohibit making a definite judgment about test item activity. Results of this type are reported as equivocal.
Statistics:
Statistical significance was confirmed by using Dunnett’s Regression Analysis (* = p < 0.05) for those values that indicate statistically significant increases in the frequency of revertant colonies compared to the concurrent solvent control. Values that are statistically significant but are within the in-house historical vehicle/untreated control range are not flagged for statistical significance in the data tables.

Results and discussion

Test resultsopen allclose all
Key result
Species / strain:
S. typhimurium TA 1535
Metabolic activation:
with and without
Genotoxicity:
negative
Cytotoxicity / choice of top concentrations:
no cytotoxicity, but tested up to precipitating concentrations
Vehicle controls validity:
valid
Untreated negative controls validity:
valid
True negative controls validity:
not applicable
Positive controls validity:
valid
Key result
Species / strain:
S. typhimurium TA 1537
Metabolic activation:
with and without
Genotoxicity:
negative
Cytotoxicity / choice of top concentrations:
no cytotoxicity, but tested up to precipitating concentrations
Vehicle controls validity:
valid
Untreated negative controls validity:
valid
True negative controls validity:
not applicable
Positive controls validity:
valid
Key result
Species / strain:
S. typhimurium TA 98
Metabolic activation:
with and without
Genotoxicity:
negative
Cytotoxicity / choice of top concentrations:
no cytotoxicity, but tested up to precipitating concentrations
Vehicle controls validity:
valid
Untreated negative controls validity:
valid
True negative controls validity:
not applicable
Positive controls validity:
valid
Key result
Species / strain:
S. typhimurium TA 100
Metabolic activation:
with and without
Genotoxicity:
negative
Cytotoxicity / choice of top concentrations:
no cytotoxicity, but tested up to precipitating concentrations
Vehicle controls validity:
valid
Untreated negative controls validity:
valid
True negative controls validity:
not applicable
Positive controls validity:
valid
Key result
Species / strain:
E. coli WP2 uvr A
Metabolic activation:
with and without
Genotoxicity:
negative
Cytotoxicity / choice of top concentrations:
no cytotoxicity, but tested up to precipitating concentrations
Vehicle controls validity:
valid
Untreated negative controls validity:
valid
True negative controls validity:
not applicable
Positive controls validity:
valid
Additional information on results:
Experiment 1 (plate incorporation)

The maximum dose level of the test item in the first experiment was selected as the OECD TG 471 recommended dose level of 5000 μg/plate.
There was no visible reduction in the growth of the bacterial background lawn at any dose level, either in the presence or absence of metabolic activation (S9-mix).
A test item precipitate (globular in appearance) was noted at 5000 μg/plate in both the presence and absence of metabolic activation (S9-mix). This observation did not prevent the scoring of revertant colonies.
There were no significant increases in the frequency of revertant colonies recorded for any of the bacterial strains, with any dose of the test item, either with or without metabolic activation (S9-mix).

Experiment 2 (pre-incubation)

The maximum dose level of the test item in the second experiment was the same as for Experiment 1 (5000 μg/plate).
There was no visible reduction in the growth of the bacterial background lawn at any dose level, either in the presence or absence of metabolic activation (S9-mix).
A test item precipitate (white/globular in appearance) was noted at 5000 μg/plate in both the presence and absence of metabolic activation (S9-mix). This observation did not prevent the scoring of revertant colonies.
No biologically relevant increases in the frequency of revertant colonies were recorded for any of the bacterial strains, with any dose of the test item, either with or without metabolic activation (S9-mix). Two statistically significant values were noted (TA98 at 5000 μg/plate in the absence of metabolic activation (S9-mix) and TA100 at 1500 μg/plate in the presence of metabolic activation (S9-mix), however as the maximum fold increase was only 1.7 times the concurrent vehicle controls and the mean colony count was within the in-house historical vehicle/untreated control range for the relevant strains the responses were considered of no biological relevance. Therefore, these values have not been highlighted in Table 4 and Table 5 as they did not meet the required criteria for a positive response.

Applicant's summary and conclusion

Conclusions:
In this Reverse Mutation Assay ‘Ames Test’ using strains of Salmonella typhimurium and Escherichia coli (OECD TG 471) the test item RDX did not induce an increase in the frequency of revertant colonies that met the criteria for a positive result, either with or without metabolic activation (S9-mix). Under the conditions of this test RDX was considered to be non-mutagenic.
Executive summary:

The test method was designed to be compatible with the guidelines for bacterial mutagenicity testing published by the OECD Guidelines for Testing of Chemicals No. 471 "Bacterial Reverse Mutation Test".Salmonella typhimuriumstrains TA1535, TA1537, TA98 and TA100 andEscherichia colistrain WP2uvrAwere treated with the test item using both the Ames plate incorporation and pre-incubation methods at eight dose levels, in triplicate, both with and without the addition of a rat liver homogenate metabolizing system (10% liver S9 in standard co-factors). Because of the explosivity of the test item as such, it was provided to the laboratory already diluted in dimethyl sulphoxide (50 g/L), which was the vehicle used for the assay. The dose range for the first experiments (plate incorporation method) was 1.5 to 5000 μg/plate, as recommended by the guideline. The same range was used for the second experiment (pre-incubation method). The vehicle (dimethyl sulphoxide) control plates gave counts of revertant colonies within the normal range. All of the positive control chemicals used in the test induced marked increases in the frequency of revertant colonies, both with and without metabolic activation. Thus, the sensitivity of the assay and the efficacy of the S9-mix were validated. Similarly in the two experiments, there was no visible reduction in the growth of the bacterial background lawn at any dose level, either in the presence or absence of metabolic activation (S9-mix). A test item precipitate (globular in appearance) was noted at 5000 μg/plate in both the presence and absence of metabolic activation (S9-mix) in Experiments 1 and 2. This observation did not prevent the scoring of revertant colonies. There were no significant increases in the frequency of revertant colonies recorded for any of the bacterial strains, with any dose of the test item, either with or without metabolic activation (S9-mix) in Experiment 1 (plate incorporation method).

No biologically relevant increases in the frequency of revertant colonies were recorded for any of the bacterial strains, with any dose of the test item, either with or without metabolic activation (S9-mix) in Experiment 2 (pre-incubation method).Two statistically significant values were noted (TA98 at 5000 μg/plate in the absence of metabolic activation (S9-mix) and TA100 at 1500 μg/plate in the presence of metabolic activation (S9-mix), however as the maximum fold increase was only 1.7 times the concurrent vehicle controls and the mean colony count was within the in-house historical vehicle/untreated control range for the relevant strains the responses were considered of no biological relevance. Therefore, these values have not been highlighted in Table 4 and Table 5 as they did not meet the required criteria for a positive response.

In this Reverse Mutation Assay ‘Ames Test’ using strains ofSalmonella typhimuriumandEscherichia coli(OECD TG 471) the test item RDX did not induce an increase in the frequency of revertant colonies that met the criteria for a positive result, either with or without metabolic activation (S9-mix). Under the conditions of this test RDX was considered to be non-mutagenic.