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Description of key information

Key value for chemical safety assessment

Skin sensitisation

Link to relevant study records

Referenceopen allclose all

Endpoint:
skin sensitisation: in vitro
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 442D (In Vitro Skin Sensitisation: ARE-Nrf2 Luciferase Test Method)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Type of study:
activation of keratinocytes
Key result
Remarks on result:
no indication of skin sensitisation

ARE-Nrf2 Luciferase Test (KeratinoSens™) – Results summary

Study Number: SQ46WL

Test Date: 13 August 2020

SPS 5290 Stage 3 Results

Test item conc. (µM)

0.98

1.95

3.91

7.81

15.63

31.25

62.5

125

250

500

1000

2000

Mean fold induction

0.78

0.81

3.76

1.02

1.02

0.92

1.08

1.19

1.63

2.21

3.40

9.24

Statistically significant

N/A

N/A

Yes

N/A

N/A

N/A

N/A

N/A

Yes

Yes

Yes

Yes

Viability (%)

115.80

105.40

108.22

99.99

99.27

101.07

93.85

90.68

104.03

93.78

91.76

38.41

Imax

9.24

 

EC1.5(µM)

214.03

IC30(µM)

1407.83

IC50(µM)

1782.70

Determination criteria for the skin sensitisation potential of the test item

Result

Is the Imax≥1.5 fold and statistically significant

Yes

Is the cellular viability >70% at the EC1.5determining concentration

Yes

Is the EC1.5value <1000 µM

Yes

Is there a dose-response increase for luciferase induction

Yes

KeratinoSens™ prediction

Positive

 

 

Positive Control (cinnamic aldehyde) Results

Positive control conc. (µM)

4

8

16

32

64

Mean fold induction

1.15

4.79

2.66

2.79

7.67

Statistically significant

N/A

Yes

Yes

Yes

Yes

Viability (%)

90.60

104.18

85.48

95.66

44.90

Imax

7.67

 

EC1.5(µM)

4.39

IC30(µM)

N/A

IC50(µM)

N/A

 

Test Acceptance Criteria

Result

Luciferase activity induction obtained with the positive control statistically significant above the threshold of 1.5 in at least one of the test concentrations

Yes

Pass

Average induction of positive control at 64 µM between 2 – 8

Yes (7.67)

Pass

EC1.5of positive control within two standard deviations of the historical mean (‑2.36 to 28.67)

Yes (4.39)

Pass

CV% of blank values < 20%

No (32.3%)

Fail

Interpretation of results:
study cannot be used for classification
Conclusions:
After 48h exposure of cells with 12 concentrations of the test item, Luciferase measurements and MTT viability testing were performed.
The test item was classified as Positive according to the KeratinoSens prediction model.
Endpoint:
skin sensitisation, other
Type of information:
(Q)SAR
Adequacy of study:
weight of evidence
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
DEREK (skin sensitisation)
1 Substance
1.1 CAS number 1429755-57-6
1.2 EC number 814-560-6
1.3 Chemical name
IUPAC E-N'-{2-cyano-4-[3-(2-hydroxy-1,1-dimethylethyl)-thioureido]-phenyl}-N,N-dimethylformamidine
Other
Other
1.4 Structural formula

1.5 Structure codes
SMILES N#Cc1cc(ccc1(N=CN(C)C))NC(NC(C)(C)CO)=S
InChI 1S/C15H21N5OS/c1-15(2,9-21)19-14(22)18-12-5-6-13(11(7-12)8-16)17-10-20(3)4/h5-7,10,21H,9H2,1-4H3,(H2,18,19,22)/b17-10+
Other
Stereochemical features Not applicable

2 General Information
2.1 Date of QPRF 30 September 2020
2.2 Author and contact details Covance Laboratories Limited, London Road, Shardlow, Derbyshire, DE72 2GD

3 Prediction
3.1 Endpoint (OECD Principle 1)
Endpoint Skin Sensitisation
Dependent variable Not applicable
3.2 Algorithm (OECD Principle 2)
Model or submodel name Skin sensitisation in mammal
Model version DEREK Nexus 6.0.1, Nexus: 2.2.1.
Knowledge Base: Derek KB 2018 1.1, Version 434 from 23/11/2017
Reference to QMRF The QMRF with the identifier Q13-34-36-315 is available from the JRC QMRF inventory (http://qsardb.jrc.it/qmrf/).
Predicted values (model result) Alert matched: 836 Thiourea
Alert matched: 432 Thiol or thiol exchange agent
Alert matched: 432 Thiol or thiol exchange agent
Predicted values (comments) Skin sensitisation in mammal is at least Equivocal (There is an equal weight of evidence for and against the proposition)
Input for prediction Smiles
Calculated descriptor values Descriptor Value
LogP 0.69
LogKp -4.18

3.3 Applicability domain (OECD Principle 3)
Domains Alert: 432 Thiol or thiol exchange agent
Alert Description image
Match with query compound:



Structural analogues Examples structure provided for the alert are:
2-mercaptobenzothiazole which has been reported to be strong sensitiser in the GPMT.

3-mercaptopropane-1,2-diol reported as a skin sensitiser in GPMT

4,4'-dithiodimorpholine reported as a skin sensitiser in GPMT

morpholinyl-mercaptobenzothiazole reported as a skin sensitiser in GPMT

Domains Alert:836 Thiourea
Alert Description image

Match with query compound


Structural analogues Examples structure provided for the alert are:
diphenylthiourea reported as a skin sensitiser in GPMT

N,N'-diethylthiourea reported as a skin sensitiser in GPMT

Consideration on structural analogues Alert: 432 Thiol or thiol exchange agent and Alert:836 Thiourea. Results of test data are in concordance with predicted results
3.4 The uncertainty of the prediction (OECD principle 4)
Skin sensitisation in mammal is at least EQUIVOCAL, i.e. There is an equal weight of evidence for and against the proposition.
3.5 The chemical and biological mechanisms according to the model underpinning the predicted result (OECD principle 5)
Alert: 432 Thiol or thiol exchange agent.
This alert describes the skin sensitisation of aromatic amines and their N-protonated forms according to the toxicophore shown.

In order to elicit a sensitisation response aromatic amines require transformation to a species capable of reacting with a skin protein nucleophilic group. Two key mechanisms through which this could be achieved have been postulated, both of which may have a role to play.

Mechanism 1:
By analogy with the generally accepted mechanism for Ames test mutagenicity, it has been suggested that N-hydroxylation occurs, possibly mediated by cytochrome P450 enzymes [Westphal et al]. Subsequent O-esterification is thought to involve two principal processes; acetylation mediated by N-acetyltransferase, or sulphation mediated by sulphotransferase. Non-enzymatic cleavage can then give rise to a reactive nitrenium ion. The bioactivation of aromatic amines to the hydroxylamine has been confirmed within human keratinocytes [Reilly et al], while O-acetylation has been demonstrated in rodent skin [Kawakubo et al]. Sulphotransferases have been detected in human and rodent skin, although O-sulphation of the hydroxylamine has not been confirmed [Smith and Hotchkiss].

Mechanism 2:
In an alternative proposal, the protein-reactive species is a nitroso compound, formed by non-enzymatic autoxidation of a hydroxylamine [Naisbitt et al 2001]. Nitroso compounds have been shown to be highly reactive with thiol groups (glutathione), but unreactive towards amine groups (lysine, aniline) [Naisbitt et al 1996].

Alert:836 Thiourea
This alert covers the skin sensitisation potential of thioureas. While thiourea itself gave negative results in the GPMT [ECHA 1981], positive results have been reported for several substituted derivatives, including diethylthiourea (N,N'-diethylthiourea, DETU), diphenylthiourea (DPTU), dibutylthiourea (DBTU) and dilaurylthiourea (DLTU) [Nakamura et al, Momma et al]. Ethylene thiourea (ETU) has also produced a weakly positive response in the GPMT [Matsushita et al]. Despite activity in the GPMT, DETU, DBTU and DPTU gave negative results when tested in the LLNA [Momma et al, Samuelsson et al]. These compounds are believed to exhibit low skin penetration and as such the GPMT may be a more sensitive test than the LLNA since the former test uses intradermal, rather than topical, application. This is supported by positive results reported for DBTU, DPTU and DLTU in the sensitive LLNA, a variant of the LLNA which employs intradermal delivery [Ikarashi et al]. Human patch tests have identified several thioureas, including thiourea itself [Geier and Fuchs], DETU [Kroft and van der Valk, Liippo et al 2011], DBTU and DPTU, as potential human contact sensitisers [Liippo et al 2010].

It has been suggested that these compounds act as prohaptens rather than reacting with skin proteins directly. Metabolic oxidation of the thionocarbonyl group of thioureas may yield sulphenic (S-OH), sulphinic (-SO2H) or sulphonic acids (-SO3H) [Bergstrom et al, Kalgutkar et al]. These electrophilic metabolites are capable of reacting with nucleophilic groups in proteins. This mechanism is supported by in vitro trapping experiments using a skin-like P450 cocktail of the most common enzymes present in skin [Samuelsson et al]. Alternatively, thioureas may decompose to give electrophilic isothiocyanate compounds, although studies suggest that this is less important than metabolic activation [Samuelsson et al].

The scope of the alert is limited to thioureas where these compounds can undergo the mechanism discussed above. Both alkyl and aryl substituents are permitted based on the activity of compounds such as DETU and DPTU. Thioureas eliciting skin sensitisation in their thiol tautomeric form are covered elsewhere in the knowledge base.

4 Adequacy (Optional)
4.1 Regulatory purpose Skin sensitisation endpoint for assessing the skin sensitisation potential with in vitro and in silico methods according to ECHA Guidance, Chapter R.7a, 2016.

4.2 Approach for regulatory interpretation of the model result
Result is directly applicable since no conversion of the result is required.

4.3 Outcome Skin sensitisation in mammal is at least EQUIVOCAL. The predictive performance of this alert is considered to be high (for details, see software printout).

4.4 Conclusion The prediction is considered reliable and the result will used together with other predictions in an argument for a weight of evidence conclusion.
Qualifier:
no guideline followed
Principles of method if other than guideline:
- Principle of test: QSAR for skin sensitisation
- Short description of test conditions: n/a
- Parameters analysed / observed: QSAR for skin sensitisation
Remarks on result:
positive indication of skin sensitisation

DEREK (skin sensitisation) 

 

1

Substance

 

 

 

 

1.1

CAS number

 

1429755-57-6

 

 

1.2

EC number

 

814-560-6

 

 

1.3

Chemical name

 

 

 

 

 

 

IUPAC

E-N'-{2-cyano-4-[3-(2-hydroxy-1,1-dimethylethyl)-thioureido]-phenyl}-N,N-dimethylformamidine

 

 

 

 

Other

 

 

 

 

 

Other

 

 

 

1.4

Structural formula

 

 

 

 

 

 

 

 

 

1.5

Structure codes

 

 

 

 

 

 

SMILES

N#Cc1cc(ccc1(N=CN(C)C))NC(NC(C)(C)CO)=S

 

 

 

 

InChI

1S/C15H21N5OS/c1-15(2,9-21)19-14(22)18-12-5-6-13(11(7-12)8-16)17-10-20(3)4/h5-7,10,21H,9H2,1-4H3,(H2,18,19,22)/b17-10+

 

 

 

 

Other

 

 

 

 

 

Stereochemical features

Not applicable

 

 

 

2

General Information

 

 

 

 

2.1

Date of QPRF

 

30 September 2020

 

 

2.2

Author and contact details

Covance Laboratories Limited, London Road, Shardlow, Derbyshire, DE72 2GD

 

 

 

3

Prediction

 

 

 

 

3.1

Endpoint (OECD Principle 1)

 

 

 

 

 

Endpoint

Skin Sensitisation

 

 

 

 

Dependent variable

Not applicable

 

 

3.2

Algorithm (OECD Principle 2)

 

 

 

 

 

Model or submodel name

Skin sensitisation in mammal

 

 

 

 

Model version

DEREK Nexus 6.0.1, Nexus: 2.2.1.
Knowledge Base: Derek KB 2018 1.1, Version 434 from 23/11/2017

 

 

 

 

Reference to QMRF

The QMRF with the identifier Q13-34-36-315 is available from the JRC QMRF inventory (http://qsardb.jrc.it/qmrf/).

 

 

 

 

Predicted values (model result)

Alert matched: 836 Thiourea

Alert matched: 432 Thiol or thiol exchange agent

Alert matched: 432 Thiol or thiol exchange agent

 

 

 

 

Predicted values (comments)

Skin sensitisation in mammal is at least Equivocal (There is an equal weight of evidence for and against the proposition)

 

 

 

 

Input for prediction

Smiles

 

 

 

 

Calculated descriptor values

Descriptor

Value

LogP

0.69

LogKp

-4.18

 

 

3.3

Applicability domain (OECD Principle 3)

 

 

 

 

Domains

Alert: 432 Thiol or thiol exchange agent

Alert Description image

Match with query compound:

     

 

 

 

 

 

 

 

 

 

Structural analogues

Examples structure provided for the alert are:

 2-mercaptobenzothiazole which has been reported to be strong sensitiser in the GPMT.

3-mercaptopropane-1,2-diol reported as a skin sensitiser in GPMT

4,4'-dithiodimorpholine reported as a skin sensitiser in GPMT

morpholinyl-mercaptobenzothiazole reported as a skin sensitiser in GPMT

 

 

 

 

Domains

Alert:836 Thiourea

Alert Description image

Match with query compound

 

 

 

 

 

Structural analogues

Examples structure provided for the alert are:

diphenylthioureareported as a skin sensitiser in GPMT

N,N'-diethylthioureareported as a skin sensitiser in GPMT

 

 

 

 

Consideration on structural analogues

Alert: 432 Thiol or thiol exchange agent and Alert:836 Thiourea. Results of test data are in concordance with predicted results

 

 

3.4

The uncertainty of the prediction (OECD principle 4)

 

 

 

 

 

Skin sensitisation in mammal is at least EQUIVOCAL, i.e. There is an equal weight of evidence for and against the proposition.

 

 

3.5

The chemical and biological mechanisms according to the model underpinning the predicted result (OECD principle 5)

 

 

 

 

 

Alert: 432 Thiol or thiol exchange agent.

This alert describes the skin sensitisation of aromatic amines and their N-protonated forms according to the toxicophore shown.

 

In order to elicit a sensitisation response aromatic amines require transformation to a species capable of reacting with a skin protein nucleophilic group. Two key mechanisms through which this could be achieved have been postulated, both of which may have a role to play.

 

Mechanism 1:

By analogy with the generally accepted mechanism for Ames test mutagenicity, it has been suggested that N-hydroxylation occurs, possibly mediated by cytochrome P450 enzymes [Westphal et al]. Subsequent O-esterification is thought to involve two principal processes; acetylation mediated by N-acetyltransferase, or sulphation mediated by sulphotransferase. Non-enzymatic cleavage can then give rise to a reactive nitrenium ion. The bioactivation of aromatic amines to the hydroxylamine has been confirmed within human keratinocytes [Reilly et al], while O-acetylation has been demonstrated in rodent skin [Kawakubo et al]. Sulphotransferases have been detected in human and rodent skin, although O-sulphation of the hydroxylamine has not been confirmed [Smith and Hotchkiss].

 

Mechanism 2:

In an alternative proposal, the protein-reactive species is a nitroso compound, formed by non-enzymatic autoxidation of a hydroxylamine [Naisbitt et al 2001]. Nitroso compounds have been shown to be highly reactive with thiol groups (glutathione), but unreactive towards amine groups (lysine, aniline) [Naisbitt et al 1996].

 

Alert:836 Thiourea

This alert covers the skin sensitisation potential of thioureas. While thiourea itself gave negative results in the GPMT [ECHA 1981], positive results have been reported for several substituted derivatives, including diethylthiourea (N,N'-diethylthiourea, DETU), diphenylthiourea (DPTU), dibutylthiourea (DBTU) and dilaurylthiourea (DLTU) [Nakamura et al, Momma et al]. Ethylene thiourea (ETU) has also produced a weakly positive response in the GPMT [Matsushita et al]. Despite activity in the GPMT, DETU, DBTU and DPTU gave negative results when tested in the LLNA [Momma et al, Samuelsson et al]. These compounds are believed to exhibit low skin penetration and as such the GPMT may be a more sensitive test than the LLNA since the former test uses intradermal, rather than topical, application. This is supported by positive results reported for DBTU, DPTU and DLTU in the sensitive LLNA, a variant of the LLNA which employs intradermal delivery [Ikarashi et al]. Human patch tests have identified several thioureas, including thiourea itself [Geier and Fuchs], DETU [Kroft and van der Valk, Liippo et al 2011], DBTU and DPTU, as potential human contact sensitisers [Liippo et al 2010].

 

It has been suggested that these compounds act as prohaptens rather than reacting with skin proteins directly. Metabolic oxidation of the thionocarbonyl group of thioureas may yield sulphenic (S-OH), sulphinic (-SO2H) or sulphonic acids (-SO3H) [Bergstrom et al, Kalgutkar et al]. These electrophilic metabolites are capable of reacting with nucleophilic groups in proteins. This mechanism is supported by in vitro trapping experiments using a skin-like P450 cocktail of the most common enzymes present in skin [Samuelsson et al]. Alternatively, thioureas may decompose to give electrophilic isothiocyanate compounds, although studies suggest that this is less important than metabolic activation [Samuelsson et al].

 

The scope of the alert is limited to thioureas where these compounds can undergo the mechanism discussed above. Both alkyl and aryl substituents are permitted based on the activity of compounds such as DETU and DPTU. Thioureas eliciting skin sensitisation in their thiol tautomeric form are covered elsewhere in the knowledge base.

 

 

 

4

Adequacy (Optional)

 

 

 

 

4.1

Regulatory purpose

Skin sensitisation endpoint for assessing the skin sensitisation potential with in vitro and in silico methods according to ECHA Guidance, Chapter R.7a, 2016.

 

 

 

 

 

 

 

4.2

Approach for regulatory interpretation of the model result

 

 

 

 

Result is directly applicable since no conversion of the result is required.

 

 

 

 

 

 

 

4.3

Outcome

Skin sensitisation in mammal is at least EQUIVOCAL. The predictive performance of this alert is considered to be high (for details, see software printout).

 

 

 

 

 

 

 

4.4

Conclusion

The prediction is considered reliable and the result will used together with other predictions in an argument for a weight of evidence conclusion.

Interpretation of results:
Category 1 (skin sensitising) based on GHS criteria
Conclusions:
Skin sensitisation in mammal is at least EQUIVOCAL. The predictive performance of this alert is considered to be high.
The prediction is considered reliable and the result will used together with other predictions in an argument for a weight of evidence conclusion.
Endpoint:
skin sensitisation, other
Type of information:
(Q)SAR
Adequacy of study:
weight of evidence
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
DEREK (EC3, potency)
1 Substance
1.1 CAS number 1429755-57-6
1.2 EC number 814-560-6
1.3 Chemical name
IUPAC E-N'-{2-cyano-4-[3-(2-hydroxy-1,1-dimethylethyl)-thioureido]-phenyl}-N,N-dimethylformamidine
Other
Other
1.4 Structural formula

1.5 Structure codes
SMILES N#Cc1cc(ccc1(N=CN(C)C))NC(NC(C)(C)CO)=S
InChI 1S/C15H21N5OS/c1-15(2,9-21)19-14(22)18-12-5-6-13(11(7-12)8-16)17-10-20(3)4/h5-7,10,21H,9H2,1-4H3,(H2,18,19,22)/b17-10+
Other
Stereochemical features Not applicable

2 General Information General Information
2.1 Date of QPRF 30 September 2020
2.2 Author and contact details Covance Laboratories Limited, London Road, Shardlow, Derbyshire, DE72 2GD

3 Prediction
3.1 Endpoint (OECD Principle 1)
Endpoint EC3
Dependent variable Not applicable
3.2 Algorithm (OECD Principle 2)
Model or submodel name Derek EC3 Model
Model version 1.2.0
Reference to QMRF The QMRF for skin sensitisation alert with the identifier Q13-34-36-315 is available from the JRC QMRF inventory (http://qsardb.jrc.it/qmrf/). No QMRF for the EC3 model available. Further details can be obtained from https://www.lhasalimited.org/products/EC3-predictions-for-skin-sensitisation.htm
Predicted values (model result) 3.0% (moderate sensitiser)
Predicted values (comments) Based on structures triggering 432 Thiol or thiol exchange agent
Input for prediction Smiles
Calculated descriptor values Alert and fingerprints used for selecting analogues
3.3 Applicability domain (OECD Principle 3)
Domains Tautomer 2
Based on structures triggering 432 Thiol or thiol exchange agent 11/17 compounds used in calculation.
Structural analogues i. LLNA EC3 % Median: 2.9% (moderate sensitiser), Similarity: 26%
CAS 2495-37-6 Saflufenacil, CAS: 372137-35-4
ii. LLNA EC3 % Median: 0.8% (strong sensitiser) Similarity: 26%
CAS 140-11-4 Budesonide, CAS: 51333-22-3
iii. LLNA EC3 % Median: 4.3% (strong sensitiser), Similarity: 35%

iv. LLNA EC3 % Median: 15% (weak sensitiser), Similarity: 33%



Domains Tautomer 3
Based on structures triggering 432 Thiol or thiol exchange agent 10/17 compounds used in calculation.
Structural analogues i. LLNA EC3 % Median: 0.80% (stong sensitiser), Similarity: 24%
CAS 2495-37-6 Saflufenacil, CAS: 372137-35-4
ii. LLNA EC3 % Median: 2.9% (moderate sensitiser) Similarity: 22%
CAS 140-11-4 Budesonide, CAS: 51333-22-3
iii. LLNA EC3 % Median: 4.3% (strong sensitiser), Similarity: 35%

iv. LLNA EC3 % Median: 15% (weak sensitiser), Similarity: 33%



Consideration on structural analogues Alert: 432 Thiol or thiol exchange agent. (tautomer 2) Results of test data have high concordance with predicted result. There mean structural similarity is 22 % and is therefore considered low.
Alert: 432 Thiol or thiol exchange agent. (tautomer 3) Results of test data have high concordance with predicted result. There mean structural similarity is 20 % and is therefore considered low.
3.4 The uncertainty of the prediction (OECD principle 4)
DEREK assessment: Skin sensitisation in mammal is at least Equivocal, i.e. There is an equal weight of evidence for and against the proposition.
3.5 The chemical and biological mechanisms according to the model underpinning the predicted result (OECD principle 5)
Alert: 432 Thiol or thiol exchange agent.
This alert describes the skin sensitisation of aromatic amines and their N-protonated forms according to the toxicophore shown.

In order to elicit a sensitisation response aromatic amines require transformation to a species capable of reacting with a skin protein nucleophilic group. Two key mechanisms through which this could be achieved have been postulated, both of which may have a role to play.

Mechanism 1:
By analogy with the generally accepted mechanism for Ames test mutagenicity, it has been suggested that N-hydroxylation occurs, possibly mediated by cytochrome P450 enzymes [Westphal et al]. Subsequent O-esterification is thought to involve two principal processes; acetylation mediated by N-acetyltransferase, or sulphation mediated by sulphotransferase. Non-enzymatic cleavage can then give rise to a reactive nitrenium ion. The bioactivation of aromatic amines to the hydroxylamine has been confirmed within human keratinocytes [Reilly et al], while O-acetylation has been demonstrated in rodent skin [Kawakubo et al]. Sulphotransferases have been detected in human and rodent skin, although O-sulphation of the hydroxylamine has not been confirmed [Smith and Hotchkiss].

Mechanism 2:
In an alternative proposal, the protein-reactive species is a nitroso compound, formed by non-enzymatic autoxidation of a hydroxylamine [Naisbitt et al 2001]. Nitroso compounds have been shown to be highly reactive with thiol groups (glutathione), but unreactive towards amine groups (lysine, aniline) [Naisbitt et al 1996].

4 Adequacy (Optional)
4.1 Regulatory purpose Skin sensitisation endpoint for assessing the skin sensitisation potency with in silico methods according to ECHA Guidance, Chapter R.7a, 2016.

4.2 Approach for regulatory interpretation of the model result
Result is directly applicable since no conversion of the result is required.

4.3 Outcome Using the result identified from the target compound, the conclusion is an EC3 value of 3.0%.

4.4 Conclusion The prediction is considered reliable and will be used together with predictions from other models in a weight of evidence conclusion.
Qualifier:
no guideline followed
Principles of method if other than guideline:
- Principle of test: QSAR for skin sensitisation
- Short description of test conditions: n/a
- Parameters analysed / observed: QSAR for skin sensitisation
Remarks on result:
positive indication of skin sensitisation


DEREK (EC3, potency) 

1

Substance

 

 

 

1.1

CAS number

 

1429755-57-6

 

1.2

EC number

 

814-560-6

 

1.3

Chemical name

 

 

 

 

 

IUPAC

E-N'-{2-cyano-4-[3-(2-hydroxy-1,1-dimethylethyl)-thioureido]-phenyl}-N,N-dimethylformamidine

 

 

 

Other

 

 

 

 

Other

 

 

1.4

Structural formula

 

 

 

 

 

 

 

1.5

Structure codes

 

 

 

 

 

SMILES

N#Cc1cc(ccc1(N=CN(C)C))NC(NC(C)(C)CO)=S

 

 

 

InChI

1S/C15H21N5OS/c1-15(2,9-21)19-14(22)18-12-5-6-13(11(7-12)8-16)17-10-20(3)4/h5-7,10,21H,9H2,1-4H3,(H2,18,19,22)/b17-10+

 

 

 

Other

 

 

 

 

Stereochemical features

Not applicable

 

2

General Information

General Information

 

 

2.1

Date of QPRF

 

30 September 2020

 

2.2

Author and contact details

Covance Laboratories Limited, London Road, Shardlow, Derbyshire, DE72 2GD

 

3

Prediction

 

 

 

3.1

Endpoint (OECD Principle 1)

 

 

 

 

Endpoint

EC3

 

 

 

Dependent variable

Not applicable

 

3.2

Algorithm (OECD Principle 2)

 

 

 

 

Model or submodel name

Derek EC3 Model

 

 

 

Model version

1.2.0

 

 

 

Reference to QMRF

The QMRF for skin sensitisation alert with the identifier Q13-34-36-315 is available from the JRC QMRF inventory (http://qsardb.jrc.it/qmrf/). No QMRF for the EC3 model available. Further details can be obtained from https://www.lhasalimited.org/products/EC3-predictions-for-skin-sensitisation.htm

 

 

 

Predicted values (model result)

3.0% (moderate sensitiser)

 

 

 

Predicted values (comments)

Based on structures triggering 432 Thiol or thiol exchange agent

 

 

 

Input for prediction

Smiles

 

 

 

Calculated descriptor values

Alert and fingerprints used for selecting analogues

 

3.3

Applicability domain (OECD Principle 3)

 

 

 

Domains

Tautomer 2

Based on structures triggering 432 Thiol or thiol exchange agent 11/17 compounds used in calculation.

 

 

 

Structural analogues

i.

LLNA EC3 % Median: 2.9% (moderate sensitiser), Similarity: 26%

CAS 2495-37-6

Saflufenacil, CAS: 372137-35-4

ii.

LLNA EC3 % Median: 0.8% (strong sensitiser) Similarity: 26%

CAS 140-11-4

Budesonide, CAS: 51333-22-3

iii.

LLNA EC3 % Median: 4.3% (strong sensitiser), Similarity: 35%

 

 

iv.

LLNA EC3 % Median: 15% (weak sensitiser), Similarity: 33%

 

 

 

 

 

 

 

 

 

Domains

Tautomer 3

Based on structures triggering 432 Thiol or thiol exchange agent 10/17 compounds used in calculation.

 

 

 

Structural analogues

i.

LLNA EC3 % Median: 0.80% (stong sensitiser), Similarity: 24%

CAS 2495-37-6

Saflufenacil, CAS: 372137-35-4

ii.

LLNA EC3 % Median: 2.9% (moderate sensitiser) Similarity: 22%

CAS 140-11-4

Budesonide, CAS: 51333-22-3

iii.

LLNA EC3 % Median: 4.3% (strong sensitiser), Similarity: 35%

 

 

iv.

LLNA EC3 % Median: 15% (weak sensitiser), Similarity: 33%

 

 

 

 

 

 

 

 

 

Consideration on structural analogues

Alert: 432 Thiol or thiol exchange agent. (tautomer 2) Results of test data have high concordance with predicted result. There mean structural similarity is 22 % and is therefore considered low.

Alert: 432 Thiol or thiol exchange agent. (tautomer 3) Results of test data have high concordance with predicted result. There mean structural similarity is 20 % and is therefore considered low.

 

3.4

The uncertainty of the prediction (OECD principle 4)

 

 

 

 

DEREK assessment: Skin sensitisation in mammal is at least Equivocal, i.e. There is an equal weight of evidence for and against the proposition.

 

3.5

The chemical and biological mechanisms according to the model underpinning the predicted result (OECD principle 5)

 

 

 

 

Alert: 432 Thiol or thiol exchange agent.

This alert describes the skin sensitisation of aromatic amines and their N-protonated forms according to the toxicophore shown.

 

In order to elicit a sensitisation response aromatic amines require transformation to a species capable of reacting with a skin protein nucleophilic group. Two key mechanisms through which this could be achieved have been postulated, both of which may have a role to play.

 

Mechanism 1:

By analogy with the generally accepted mechanism for Ames test mutagenicity, it has been suggested that N-hydroxylation occurs, possibly mediated by cytochrome P450 enzymes [Westphal et al]. Subsequent O-esterification is thought to involve two principal processes; acetylation mediated by N-acetyltransferase, or sulphation mediated by sulphotransferase. Non-enzymatic cleavage can then give rise to a reactive nitrenium ion. The bioactivation of aromatic amines to the hydroxylamine has been confirmed within human keratinocytes [Reilly et al], while O-acetylation has been demonstrated in rodent skin [Kawakubo et al]. Sulphotransferases have been detected in human and rodent skin, although O-sulphation of the hydroxylamine has not been confirmed [Smith and Hotchkiss].

 

Mechanism 2:

In an alternative proposal, the protein-reactive species is a nitroso compound, formed by non-enzymatic autoxidation of a hydroxylamine [Naisbitt et al 2001]. Nitroso compounds have been shown to be highly reactive with thiol groups (glutathione), but unreactive towards amine groups (lysine, aniline) [Naisbitt et al 1996].

 

4

Adequacy (Optional)

 

 

 

4.1

Regulatory purpose

Skin sensitisation endpoint for assessing the skin sensitisation potency with in silico methods according to ECHA Guidance, Chapter R.7a, 2016.

 

 

 

 

 

4.2

Approach for regulatory interpretation of the model result

 

 

 

Result is directly applicable since no conversion of the result is required.

 

 

 

 

 

4.3

Outcome

Using the result identified from the target compound, the conclusion is an EC3 value of 3.0%.

 

 

 

 

 

4.4

Conclusion

The prediction is considered reliable and will be used together with predictions from other models in a weight of evidence conclusion.

Interpretation of results:
Category 1B (indication of skin sensitising potential) based on GHS criteria
Conclusions:
Using the result identified from the target compound, the conclusion is an EC3 value of 3.0%.
The prediction is considered reliable and will be used together with predictions from other models in a weight of evidence conclusion.
Endpoint:
skin sensitisation, other
Type of information:
(Q)SAR
Adequacy of study:
weight of evidence
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
1. SOFTWARE
QSAR Toolbox 4.4

2. MODEL (incl. version number)
QSAR Toolbox 4.4
Database version: 4.4
TPRF v4.4

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
N#Cc1cc(ccc1(N=CN(C)C))NC(NC(C)(C)CO)=S

4. SCIENTIFIC VALIDITY OF THE (Q)SAR MODEL
[Explain how the model fulfils the OECD principles for (Q)SAR model validation. Consider attaching the QMRF or providing a link]
see attached justificaiton

5. APPLICABILITY DOMAIN
[Explain how the substance falls within the applicability domain of the model]
see attached justification

6. ADEQUACY OF THE RESULT
[Explain how the prediction fits the purpose of classification and labelling and/or risk assessment]
Profiling with OECD Toolbox indicates that while no protein binding alert for skin sensitisation was triggered for the compound itself, Times metabolism simulator suggest SPS 5290 stage 3 to act as a pro hapten via Schiff base formation with carbonyl compounds. There is uncertainty in the model due to many category members and neighbors used for the read across being non sensitisers.
Qualifier:
no guideline followed
Principles of method if other than guideline:
- Principle of test: QSAR for skin sensitisation
- Short description of test conditions: n/a
- Parameters analysed / observed: QSAR for skin sensitisation
Remarks on result:
positive indication of skin sensitisation
Interpretation of results:
Category 1 (skin sensitising) based on GHS criteria
Conclusions:
Profiling with OECD Toolbox indicates that while no protein binding alert for skin sensitisation was triggered for the compound itself, Times metabolism simulator suggest SPS 5290 stage 3 to act as a pro hapten via Schiff base formation with carbonyl compounds. There is uncertainty in the model due to many category members and neighbors used for the read across being non sensitisers.
Endpoint:
skin sensitisation, other
Type of information:
(Q)SAR
Adequacy of study:
weight of evidence
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
1. SOFTWARE
QSAR Toolbox 4.4

2. MODEL (incl. version number)
QSAR Toolbox 4.4
Database version: 4.4
TPRF v4.4

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
N#Cc1cc(ccc1(N=CN(C)C))NC(NC(C)(C)CO)=S

4. SCIENTIFIC VALIDITY OF THE (Q)SAR MODEL
[Explain how the model fulfils the OECD principles for (Q)SAR model validation. Consider attaching the QMRF or providing a link]
see attached justificaiton

5. APPLICABILITY DOMAIN
[Explain how the substance falls within the applicability domain of the model]
see attached justification

6. ADEQUACY OF THE RESULT
[Explain how the prediction fits the purpose of classification and labelling and/or risk assessment]
The positive/negative prediction with toolbox concluded the substance to be sensitising. Using the same methodology to reach this conclusion, a prediction of EC3 was also carried out. The predicted EC3 suggests the compound to be a weak sensitiser. However, it is noteworthy that two of the 5 nearest compounds to the target structure are strong and moderate sensitisers, suggesting some uncertainty in the prediction.
Qualifier:
no guideline followed
Principles of method if other than guideline:
- Principle of test: QSAR for skin sensitisation
- Short description of test conditions: n/a
- Parameters analysed / observed: QSAR for skin sensitisation
Remarks on result:
positive indication of skin sensitisation
Interpretation of results:
Category 1B (indication of skin sensitising potential) based on GHS criteria
Conclusions:
Profiling with OECD Toolbox indicates that while no protein binding alert for skin sensitisation was triggered for the compound itself, Times metabolism simulator suggest SPS 5290 stage 3 to act as a pro hapten via Schiff base formation with carbonyl compounds. There is uncertainty in the model due to many category members and neighbors used for the read across being non sensitisers. Further to this, using the same methodology it was possible to derive a predicted EC3 value for the compound, suggesting it to be a weak sensitizer, though this prediction is similarly mared by the nearest neighbors, some of which were strong and moderate sensitisers, suggesting some uncertainty.
Endpoint conclusion
Endpoint conclusion:
adverse effect observed (sensitising)

Respiratory sensitisation

Endpoint conclusion
Endpoint conclusion:
no study available

Justification for classification or non-classification

Introduction

The prediction of the skin sensitising potential ofSPS 5290 stage 3was performed with BIOVIA Discovery Studio (TOPKAT) 2020, VEGA NIC 1.1.5 (CAESAR, IRFMN/JRC), OECD QSAR Toolbox 4.5, Toxtree 3.1.0 and DEREK Nexus 6.0.1. The TOPKAT model for skin sensitisation was extended by including 70 compounds from the Covance database. The model statistics are reported in section Information to Extended TOPKAT Model.

The prediction results for SPS 5290 stage 3 are detailed in the (Q)SAR prediction reporting formats (QPRFs). Reliability is assigned to each prediction by expert judgement considering similar structures in the data set, prediction statistics, the applicability domain and alerts, where applicable. For details, it is referred to the QPRFs. Toxtree is based on classification trees and does not provide such information and thus no reliability is assigned. All original model results are presented in the software printout section.

Appraisal of (Q)SAR Modelling

TOPKAT predicted SPS 5290 stage 3 not to be sensitising however considerations on structural similar compounds in the training set and prediction statistics indicate no confidence in the prediction.

CAESAR predicted SPS 5290 stage 3 to be sensitising. The query compound is however out of the applicability domain of the model and the prediction not considered reliable.

The query compound was predicted to be sensitising by the IRFMN/JRC model. The query compound is however out of the applicability domain of the model and the prediction not considered reliable.

Table1    Prediction Results

Model

Prediction result

Reliability

TOPKAT

Not sensitising

Not reliable

CAESAR (VEGA)

Sensitising

Not reliable

IRFMN/JRC (VEGA)

Sensitising

Not reliable

OECD Toolbox

Sensitising

Low

OECD Toolbox (EC3)

29.6 % (weak senitiser)

Low

DEREK

Skin sensitisation in mammal is PLAUSIBLE
Alert matched: Alert: 432 Thiol or thiol exchange agent, Alert: 836 Thiourea

Not applicable

DEREK (EC3)

3.0% (moderate sensitiser)

Not applicable

Toxtree

Alert for Acyl Transfer agent

Not applicable

 

Profiling with OECD Toolbox indicates that while no protein binding alert for skin sensitisation was triggered for the compound itself,Times metabolism simulator suggest SPS 5290 stage 3 to act as a pro hapten via Schiff base formation with carbonyl compounds.There is uncertainty in the model due to many category members and neighbors used for the read across being non sensitisers. Further to this, using the same methodology it was possible to derive a predicted EC3 value for the compound, suggesting it to be a weak sensitizer, though this prediction is similarly mared by the nearest neighbors, some of which were strong and moderate sensitisers, suggesting some uncertainty.

Derek predicted SPS 5290 stage 3 to be sensitizing. Derek identified two alerts based on the structural features of the substance, namely the substance can tautomerise to contain a thiol or thiourea.The mechanism of skin sensitisation by thiols (R2 = H) is proposed to be nucleophilic attack of the thiol on disulphide bonds in the skin proteins to form a new disulphide bridge.Thiol compounds are proposed to act as prohaptens rather than reacting with skin proteins directly.Derek was also able to predict an EC3 value based on the thiol alert, predicting the compound to be a moderate sensitizer with the two nearest compounds being moderate and strong sensitisers, thus a similar uncertainty to that of the toolbox prediction presents itself.

Conclusion

With the models being used generally considered to be either of low reliability, or possessing some degree of uncertainty, while there is an argument for a weight of evidence, the discussed reliability of the predictions leaves low confidence in the conclusions. The prediction is therefore considered unreliable and further in vitro / in chemico testing was recommended.
A further OECD 442D study (SQ46WL) was performed which displayed positive results, lending some confidence to the predictions and overall weight of evidence. A further OECD 442E study is ongoing for further confirmation.