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Skin sensitisation

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Administrative data

Endpoint:
skin sensitisation: in chemico
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2017-06-02 to 2017-06-30
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2017
Report date:
2017

Materials and methods

Test guideline
Qualifier:
according to guideline
Guideline:
OECD Guideline 442C (In Chemico Skin Sensitisation: Direct Peptide Reactivity Assay (DPRA))
Version / remarks:
adopted 4 February 2015
GLP compliance:
yes (incl. QA statement)
Type of study:
direct peptide reactivity assay (DPRA)

Test material

Constituent 1
Chemical structure
Reference substance name:
2-({[3-(isocyanatomethyl)-3,5,5-trimethylcyclohexyl]carbamoyl}oxy)ethyl prop-2-enoate
EC Number:
815-462-6
Cas Number:
124451-79-2
Molecular formula:
C17H26N2O5
IUPAC Name:
2-({[3-(isocyanatomethyl)-3,5,5-trimethylcyclohexyl]carbamoyl}oxy)ethyl prop-2-enoate
Test material form:
liquid
Details on test material:
2-({[3-(isocyanatomethyl)-3, 5, 5-trimethylcyclohexyl]carbamoyl}oxy)ethyl prop-2-enoate of Evonik Degussa GmbH Batch SB 4663 of 24 November 2016

In chemico test system

Details on the study design:
Preparation of the cysteine or lysine-containing peptides
- Stock solutions of cysteine (Ac-RFAACAA-COOH) and lysine (Ac-RFAAKAA-COOH) containing synthetic peptides of purity higher than 95% were freshly prepared just before their incubation with the test item.
- The final concentration of the cysteine peptide was 0.669 mM in pH 7.5 phosphate buffer, whereas the final concentration of the lysine peptide was 0.667 mM in pH 10.2 ammonium acetate buffer.

Preparation of the test item
- Solubility of the test item in an appropriate solvent was assessed before performing the assay.
- 101.3 mg test item were dissolved in 3 mL acetonitrile (CAS no. 75-05-8) immediately before testing to prepare a 100 mM solution.
- The test item solution was then tested as such without any further dilution by incubating at 1:10 and 1:50 ratio with the cysteine and lysine peptides, respectively.

Positive control, reference controls and coelution control
- Cinnamic aldehyde (CAS no. 14371-10-9) was used as positive control (PC) at a concentration of 100 mM in acetonitrile.
- In addition reference controls (i.e. samples containing only the peptide and added acetonitrile) were also included in the HPLC run sequence and these were used to verify the HPLC system suitability prior to the analysis (reference controls A), the stability of the reference controls over time (reference control B) and to verify that the solvent used to dissolve the test item does not impact the percent peptide depletion (reference control C).
- The appropriate reference control for the test item was used to calculate the percent peptide depletion for the test item.
- In addition a coelution control constituted by the test item alone for the test item analysed was included in the run sequence to detect possible coelution of the test item with either the lysine or the cysteine peptide.

Incubation of the test item with the cysteine and lysine peptide solutions
- Cysteine and lysine peptide solutions were incubated in glass autosampler vials with the test item at 1:10 and 1:50 ratio, respectively.
- The reaction solution was left in the dark at 25 ± 2.5°C for 24 ± 2 hours before running the HPLC analysis.
- The test item assay was analyzed in triplicate for both peptides.
- Samples were visually inspected prior to HPLC analysis.
- If a precipitate would be observed immediately upon addition of the test item solution to the peptide solution, due to low aqueous solubility of the test item, in this case one cannot be sure how much test item remained in the solution to react with the peptide.
- Therefore, in such a case, a positive result could still be used, but a negative result is uncertain and would be interpreted with due care.
- No precipitate or phase separation was observed.

Preparation of the HPLC standard calibration curve
- A standard calibration curve was generated for both the cysteine and the lysine peptides.
- Peptide standards were prepared in a solution of 20% acetonitrile : buffer using 100 mM sodium phosphate buffer (pH 7.5) for the cysteine peptide and 100 mM ammonium acetate buffer (pH 10.2) for the lysine peptide.
- Using serial dilution standards of the peptide stock solution (0.669 mM of cysteine peptide in sodium phosphate or 0.667 mM lysine peptide in ammonium acetate), 6 calibration standards were prepared to cover the range from 0.534 to 0.0167 mM.
- A blank of the dilution buffer was also included in the standard calibration curve.
- Suitable calibration curves should have an r2 > 0.99.
- If a test item promotes the oxidation of the cysteine peptide, the peak of the dimerised cysteine peptide would have been visually monitored.
- If dimerisation appears to have occurred, this would be noted as percent peptide depletion would be over-estimated leading to false positive predictions and/or assignment to a higher reactivity class.
- HPLC analysis for the cysteine and lysine peptides were performed on one day.
- All test item solutions were freshly prepared for both assays on one day.
- The analysis was timed to assure that the injection of the first sample (reference control C) starts 22 to 26 hours after the test item was mixed with the peptide solution.
- The HPLC run sequences were set up in order to keep the HPLC analysis time to less than 30 hours.

Results and discussion

Positive control results:
Cinnamic aldehyde was used as positive control at a concentration of 100 mM in acetonitrile. Treatment with the positive control item revealed a cysteine and lysine peptide depletion of 76.77% cysteine and 54.49% lysine. These values are within the required range of 60.8% and 100% for the cysteine peptide and between 40.2% and 69.0% for the lysine peptide. Therefore, the study can be regarded as valid.

In vitro / in chemico

Resultsopen allclose all
Key result
Run / experiment:
other: incubating at 1:10 and 1:50 ratio with the cysteine peptides
Parameter:
other: cysteine peptide depletion
Value:
100
Vehicle controls validity:
valid
Negative controls validity:
valid
Positive controls validity:
valid
Key result
Run / experiment:
other: incubating at 1:10 and 1:50 ratio with the lysine peptides
Parameter:
other: lysine peptide depletion
Value:
92.82
Vehicle controls validity:
valid
Negative controls validity:
valid
Positive controls validity:
valid
Other effects / acceptance of results:
No precipitate in the reaction mixture at the end of the incubation time and no coelution were observed.

Data evaluation
The concentrations of cysteine or lysine peptide were photometrically determined at 220 nm in each sample by measuring the peak area (area under the curve, AUC) of the appropriate peaks and by calculating the concentration of peptide using the linear calibration curve derived from the standards.
The percent peptide depletion was determined in each sample by measuring the peak area and dividing it by the mean peak area of the relevant reference control C according to the formula described below.

% peptide depletion= 1-[( Peptide peak area in replicate injection / Mean peptide peak area in reference controls C)] x 100


Acceptance criteria
The following criteria must be met for a run to be considered valid:
a) The standard calibration curve should have an r2 > 0.99.
b) The mean percent peptide depletion value of the three replicates for the positive control cinnamic aldehyde should be between 60.8% and 100% for the cysteine peptide and between 40.2% and 69.0% for the lysine peptide and the maximum standard deviation (SD) for the positive control replicates should be < 14.9% for the percent cysteine depletion and < 11.6% for the percent lysine depletion.
c) The mean peptide concentration of reference controls A should be 0.50 ± 0.05 mM and the coefficient of variation (CV) of peptide peak areas for the nine reference controls B and C in acetonitrile should be <15.0%.
If one or more of these criteria is not met, the run would have been repeated.

The following criteria must be met for a test item’s results to be considered valid:
a) The maximum standard deviation for the test item replicates should be < 14.9% for the percent cysteine depletion and < 11.6% for the percent lysine depletion.
b) The mean peptide concentration of the three reference controls C in the appropriate solvent should be 0.50 ± 0.05 mM.
If these criteria were not met, the data would have been rejected and the run have been repeated for that specific test item.


Prediction model see table "any other information on results"


Any other information on results incl. tables

Prediction model

The mean percent cysteine and percent lysine depletion value was calculated for each test item. Negative depletion was considered as “0” when calculating the mean. By using the cysteine 1:10/lysine 1:50 prediction model shown in Table below, the threshold of 6.38% average peptide depletion was used to support the discrimination between skin sensitisers and non-sensitisers in the framework of an Integrated Approach to Testing and Assessment (IATA). Application of the prediction model for assigning a test item to a reactivity class (i.e. low, moderate and high reactivity) may perhaps prove useful to inform potency assessment within the framework of an IATA.

Cysteine 1:10/lysine 1:50 prediction model

Mean of cysteine and lysine % depletion

Reactivity Class

DPRA Prediction

0% <mean % depletion6.38%

No or minimal reactivity

Negative

6.38%<mean % depletion22.62%

Low reactivity

Positive

22.62%<mean % depletion42.47%

Moderate reactivity

42.47%<mean % depletion100%

High reactivity

No co-elution was observed.

There might be other cases where the overlap in retention time between the test item and either of the peptides is incomplete. In such cases percent peptide depletion values would be estimated and used in the cysteine 1:10/lysine 1:50 prediction model, however assignment of the test item to a reactivity class would not be made with accuracy.

A single HPLC analysis for both the cysteine and the lysine peptide would be sufficient for a test item when the result is unequivocal. However, in cases of results close to the threshold used to discriminate between positive and negative results (i.e. borderline results), additional testing may be necessary. If situations where the mean percent depletion falls in the range of 3% to 10% for the cysteine 1:10/lysine 1:50 prediction model or the cysteine percent depletion falls in the range of 9% to 17% for the cysteine 1:10 prediction model, a second run would be considered, as well as a third one in case of discordant results between the first two runs.

 

Applicant's summary and conclusion

Conclusions:
Test item-treated samples revealed a mean cysteine and lysine peptide depletion of 96.41% and, hence, classified positive as sensitiser (high reactivity class) in the Direct Peptide Reactivity Assay (DPRA).
Executive summary:

The purpose of this study was to determine the sensitising potential of test item in a Direct Peptide Reactivity Assay (DPRA). The study was performed according to OECD guideline 442C. The DPRA is an in chemico method which proposes to address the molecular initiating event of the skin sensitisation AOP (Adverse Outcome Pathway), namely protein reactivity, by quantifying the reactivity of test chemicals towards model synthetic peptides containing either lysine or cysteine. Cysteine and lysine percent peptide depletion values are then used to categorise a substance in one of four classes of reactivity for supporting the discrimination between skin sensitisers and non-sensitisers. Relative peptide concentration was measured by high-performance liquid chromatography (HPLC) with gradient elution and UV detection at 220 nm.

The test item was dissolved at a concentration of 100 mM in acetonitrile. Three reference controls containing only 0.5 mM cysteine or lysine peptide solution and the solvent acetonitrile were also included in the HPLC run sequence and were used to verify the HPLC system suitability prior to analysis (reference controls A), the stability of the reference controls over time (reference control B) and to verify that the solvent used to dissolve the test item does not impact the percent peptide depletion (reference control C). The appropriate reference control for the item was used to calculate the percent peptide depletion for the test item. Each sample was tested in triplicate.

The test item-treated samples revealed a cysteine peptide depletion of 100% and lysine peptide depletion of 92.82% (mean peptide depletion of 96.41%) and, hence, the test item has to be classified positive as sensitiser (high reactivity class) in the Direct Peptide Reactivity Assay (DPRA).

Cinnamic aldehyde was used as positive control at a concentration of 100 mM in acetonitrile. Treatment with the positive control item revealed a cysteine and lysine peptide depletion of 76.77% cysteine and 54.49% lysine. These values are within the required range of 60.8% and 100% for the cysteine peptide and between 40.2% and 69.0% for the lysine peptide. The maximum standard deviation (SD) for the positive control replicates were < 14.9% for the percent cysteine depletion and < 11.6% for the percent lysine depletion. Therefore, the study can be regarded as valid.

No precipitate in the reaction mixture at the end of the incubation time and no coelution were observed.

The linearity of standard calibration curve was r2 = 0.9997 for cysteine peptide and 1.0000 for lysine peptide. Hence the requirement of r2 > 0.99 was met.

The mean peptide concentration of reference control A was 0.495 mM and, hence well within the accepted range of 0.50 ± 0.05 mM and the coefficient of variation (CV) of peptide peak areas for the nine reference controls B and C in acetonitrile was <15.0%.

All acceptance criteria of validity were fulfilled in this test.

 

Conclusion

The test item revealed a mean cysteine and lysine peptide depletion of 96.41% and, hence, the test item has to be classified positive as sensitiser (high reactivity class) in the Direct Peptide Reactivity Assay (DPRA).