(S)-(4,5-dihydro-5-thioxo-1,3,4-thiadiazol-2-yl) benzenecarbothioate

Toxicity to aquatic algae and cyanobacteria

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
There is data available on the hydrolysis behaviour of the registered substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate (CAS 51988-14-8) in unbuffered water at room temperature, hence posing the same conditions as tests on aquatic toxicity. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products, as expected, by hydrolytical cleavage of the thioester group. Based on this, it can be concluded that the registered substance hydrolyzes very rapidly in water, i.e. the half-life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Hence, the substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with the hydrolysis products 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0). According to the “Read-Across Assessment Framework” (RAAF) (ECHA 2017) the analogue approach is followed. The basis for this analogue approach is the “(Bio)transformation to common compound(s)” (scenario 1). “This scenario covers the analogue approach for which the read-across hypothesis is based on (bio) transformation to common compound(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst-case approach” (ECHA 2017). The target substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate (CAS 51988-14-8) is rapidly hydrolysed to 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0). Therefore, the proposed source substances for the target substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate are its final hydrolysis products DMTD and benzoic acid.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Target chemical: Benzenecarbothioic acid, S-(4,5-dihydro-5-thioxo-1,3,4-thiadiazol-2-yl) ester, CAS 51988-14-8, SMILES Code O=C(Sc(nnc1S)s1)c(cccc2)c2
Source chemical 1: Benzoic acid, CAS 65-85-0, SMILES Code O=C(O)c(cccc1)c1
Source chemical 2: 1,3,4-Thiadiazolidine-2,5-dithione, CAS 1072-71-5, SMILES Code N(NC(=S)S1)C1=S
All substances do not contain impurities to an extent which is expected to alter the outcome of the experimental results or read-across approach.

3. ANALOGUE APPROACH JUSTIFICATION
5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate was shown to hydrolyze rapidly in unbuffered water at room temperature, which simulates rather closely the conditions in aquatic toxicity tests. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products, as expected, by hydrolytical cleavage of the thioester group. No further assessment of the hydrolysis of benzoic acid and DMTD has been conducted because these substances are known to be hydrolytically stable. There are no non-common compounds, which have to be taken into account in this read-across approach.
The performed approach is further supported by the OECD Guidance Document on Aqueous-Phase Aquatic Toxicity Testing of Difficult Test Chemicals (Series on Testing and Assessment No. 23, 8 Feb 2019 - ENV/JM/MONO(2000)6/REV1). In this document under point 82 it is stated that “If the test chemical is likely to be unstable, a decision to test the parent test chemical and/or its degradation products, if identified, should be based on a consideration of its half-life under test and real-world conditions. The following decision criteria are suggested only as a guide for static and static-renewal exposure systems with test solution renewal intervals of 24 hours:
• Half-life >3 days: test parent chemical;
• Half-life < 3 days and > 1 hour: consider on a case-by-case basis, and include possible testing of degradation products;
• Half-life < 1 h: test degradation products.”

Having a half-life of 1.91 h the registered substance belongs to the category of ‘< 3 days and > 1 hour’. Thus, the testing strategy has to be decided on a case-by-case basis. With 1.91 h the half-life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Also, the value is close to the threshold of 1 h. Therefore, the registrant concluded that the substance already qualifies to hydrolyze rapidly, and testing of hydrolysis products is suitable.
Further, with regard to the available data on aquatic toxicity, for better comparison the estimated values are regarded first, it can be shown that the presence of the parent compound (less than 50% of the initial quantity after 2 h) has no impact on the results of the aquatic toxicity tests, at least none which may underestimated the actual hazard. Besides, it can be assumed that the target substance already degrades to a certain extend during preparation of the hypothetical test solutions, even if precautions as cited in ENV/JM/MONO(2000)6 are taken. As can be seen in the section 4 - Data matrix, the EC50 or LC50 values vary over a broad range. However, the lowest values were estimated for the hydrolysis product DMTD, so this substance can be regarded as worst-case for the target substance and it is most relevant for a possible classification, not the parent compound. Although no actual test data on the parent compound is available, available test data on the hydrolysis products indicate that DMTD is the more toxic one, which supports the above-mentioned conclusion, as estimated data reveal the same trend in magnitudes.
Hence, the registered substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0).

4. DATA MATRIX
The following information was estimated via US EPA EpiSuite estimation tool or taken from actual data sources:

Property CAS 51988-14-8 (target) CAS 65-85-0 (source 1) CAS 1072-71-5 (source 2)
Water solubility (EpiSuite) 395.1 mg/L 2493 mg/L 2.638e+005 mg/L
logPow (EpiSuite) 2.05 1.87 -0.6349
Biodegradability (EpiSuite) not readily biodegradable readily biodegradable not readily biodegradable
Acute toxicity fish, 96h LC50 (EpiSuite) 31.672 mg/l (Esters) 1300.781 mg/l (Neutral Organics-acid) 12.705 mg/l (Hydrazines)
8.588 mg/l (Thiols and Mercaptans) 0.624 mg/l (Thiocarbamate, Di(Substit))
187.325 mg/l (Neutral Organic SAR) 28692.277 mg/l (Neutral Organic SAR)
Acute toxicity daphnids, 48h LC50 (EpiSuite) 65.467 mg/l (Esters) 730.075mg/l (Neutral Organics-acid) 109.112 mg/l (Hydrazines)
1.073 mg/l (Thiols and Mercaptans) 1.754 mg/l (Thiocarbamate, Di(Substit))
106.884 mg/l (Neutral Organic SAR) 12773.367 mg/l (Neutral Organic SAR)
Acute toxicity algae, 96h EC50 (EpiSuite) 27.631 mg/l (Esters) 518.374 mg/l (Neutral Organics-acid) 2.622 mg/l (Hydrazines)
0.748 mg/l (Thiols and Mercaptans) 0.190 mg/l (Thiocarbamate, Di(Substit))
81.240 mg/l (Neutral Organic SAR) 3479.678 mg/l (Neutral Organic SAR)
Acute toxicity daphnids, 48h EC50 (test data) 860 mg/l 5.95 mg/L
Acute toxicity algae (test data) Inhibition starts at 1630 mg/l (96 hr) (pH = 7)
EC50 (14d) = >10 mg/l EC50 (72h) > 160 mg/L

Reason / purpose for cross-reference:

read-across source

Key result

Duration:

14 d

Dose descriptor:

EC50

Remarks:

in A. inaequalis

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Key result

Duration:

14 d

Dose descriptor:

EC50

Remarks:

A. cylindrica

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Key result

Duration:

14 d

Dose descriptor:

EC50

Remarks:

A. variabilis

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Key result

Duration:

14 d

Dose descriptor:

EC50

Remarks:

C. pyrenoidosa

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Key result

Duration:

14 d

Dose descriptor:

EC50

Remarks:

S. quadricauda

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Duration:

14 d

Dose descriptor:

EC50

Remarks:

A. inaequalis

Effect conc.:

9 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

biomass

Remarks on result:

other: SD = 1.0 ppm

Duration:

14 d

Dose descriptor:

EC50

Remarks:

A. cylindrica

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

biomass

Duration:

14 d

Dose descriptor:

EC50

Remarks:

A. variabilis

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

biomass

Duration:

14 d

Dose descriptor:

EC50

Remarks:

C. pyrenoidosa

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

biomass

Duration:

14 d

Dose descriptor:

EC50

Effect conc.:

> 10 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

biomass

Validity criteria fulfilled:

not specified

Conclusions:

Testing was sufficiently documented and performed similar to OECD 201 in 5 algae, however, exposure to the test item was prolonged to 12-14. Thus, the results were so obtained via a scientifically reasonable method and there is no doubt that the obtained results are not reliable. Further, due to the broad variety of species and longer exposure duration, the risk of underestimating the actual hazard of benzoic acid is nearly not given. Under the conditions used for the test, the EC50 (12-14d) of benzoic acid was determined to be >10 mg/l.
Based on these results, benzoic acid does not need to be classified as acute toxic to the aquatic environment, however, the necessity for classification as aquatic chronic according to Regulation 1272/2008, can not be derived from the given data. The same applies for the target substance 5-Mercapto-1,3,4-thiadiazol-2-thiol-benzoate.

Executive summary:

In a 12-14 day toxicity study, the cultures of cyanobacteria (blue-green algae) Anabaena inaequalis, A. cylindrica, and A. variabilis, green algae Chlorella pyrenoidosa and Scenedesmus quadricauda were exposed to benzoic acid at nominal concentrations of 0 to 10ppm under static conditions similar to OECD 201.  The EC50 values based on growth rate were >10 mg/l.

This toxicity study is classified as acceptable and satisfies the guideline requirements for an algae toxicity study.

Reference 2

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
There is data available on the hydrolysis behaviour of the registered substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate (CAS 51988-14-8) in unbuffered water at room temperature, hence posing the same conditions as tests on aquatic toxicity. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products, as expected, by hydrolytical cleavage of the thioester group. Based on this, it can be concluded that the registered substance hydrolyzes very rapidly in water, i.e. the half-life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Hence, the substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with the hydrolysis products 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0). According to the “Read-Across Assessment Framework” (RAAF) (ECHA 2017) the analogue approach is followed. The basis for this analogue approach is the “(Bio)transformation to common compound(s)” (scenario 1). “This scenario covers the analogue approach for which the read-across hypothesis is based on (bio) transformation to common compound(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst-case approach” (ECHA 2017). The target substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate (CAS 51988-14-8) is rapidly hydrolysed to 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0). Therefore, the proposed source substances for the target substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate are its final hydrolysis products DMTD and benzoic acid.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Target chemical: Benzenecarbothioic acid, S-(4,5-dihydro-5-thioxo-1,3,4-thiadiazol-2-yl) ester, CAS 51988-14-8, SMILES Code O=C(Sc(nnc1S)s1)c(cccc2)c2
Source chemical 1: Benzoic acid, CAS 65-85-0, SMILES Code O=C(O)c(cccc1)c1
Source chemical 2: 1,3,4-Thiadiazolidine-2,5-dithione, CAS 1072-71-5, SMILES Code N(NC(=S)S1)C1=S
All substances do not contain impurities to an extent which is expected to alter the outcome of the experimental results or read-across approach.

3. ANALOGUE APPROACH JUSTIFICATION
5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate was shown to hydrolyze rapidly in unbuffered water at room temperature, which simulates rather closely the conditions in aquatic toxicity tests. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products, as expected, by hydrolytical cleavage of the thioester group. No further assessment of the hydrolysis of benzoic acid and DMTD has been conducted because these substances are known to be hydrolytically stable. There are no non-common compounds, which have to be taken into account in this read-across approach.
The performed approach is further supported by the OECD Guidance Document on Aqueous-Phase Aquatic Toxicity Testing of Difficult Test Chemicals (Series on Testing and Assessment No. 23, 8 Feb 2019 - ENV/JM/MONO(2000)6/REV1). In this document under point 82 it is stated that “If the test chemical is likely to be unstable, a decision to test the parent test chemical and/or its degradation products, if identified, should be based on a consideration of its half-life under test and real-world conditions. The following decision criteria are suggested only as a guide for static and static-renewal exposure systems with test solution renewal intervals of 24 hours:
• Half-life >3 days: test parent chemical;
• Half-life < 3 days and > 1 hour: consider on a case-by-case basis, and include possible testing of degradation products;
• Half-life < 1 h: test degradation products.”

Having a half-life of 1.91 h the registered substance belongs to the category of ‘< 3 days and > 1 hour’. Thus, the testing strategy has to be decided on a case-by-case basis. With 1.91 h the half-life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Also, the value is close to the threshold of 1 h. Therefore, the registrant concluded that the substance already qualifies to hydrolyze rapidly, and testing of hydrolysis products is suitable.
Further, with regard to the available data on aquatic toxicity, for better comparison the estimated values are regarded first, it can be shown that the presence of the parent compound (less than 50% of the initial quantity after 2 h) has no impact on the results of the aquatic toxicity tests, at least none which may underestimated the actual hazard. Besides, it can be assumed that the target substance already degrades to a certain extend during preparation of the hypothetical test solutions, even if precautions as cited in ENV/JM/MONO(2000)6 are taken. As can be seen in the section 4 - Data matrix, the EC50 or LC50 values vary over a broad range. However, the lowest values were estimated for the hydrolysis product DMTD, so this substance can be regarded as worst-case for the target substance and it is most relevant for a possible classification, not the parent compound. Although no actual test data on the parent compound is available, available test data on the hydrolysis products indicate that DMTD is the more toxic one, which supports the above-mentioned conclusion, as estimated data reveal the same trend in magnitudes.
Hence, the registered substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0).

4. DATA MATRIX
The following information was estimated via US EPA EpiSuite estimation tool or taken from actual data sources:

Property CAS 51988-14-8 (target) CAS 65-85-0 (source 1) CAS 1072-71-5 (source 2)
Water solubility (EpiSuite) 395.1 mg/L 2493 mg/L 2.638e+005 mg/L
logPow (EpiSuite) 2.05 1.87 -0.6349
Biodegradability (EpiSuite) not readily biodegradable readily biodegradable not readily biodegradable
Acute toxicity fish, 96h LC50 (EpiSuite) 31.672 mg/l (Esters) 1300.781 mg/l (Neutral Organics-acid) 12.705 mg/l (Hydrazines)
8.588 mg/l (Thiols and Mercaptans) 0.624 mg/l (Thiocarbamate, Di(Substit))
187.325 mg/l (Neutral Organic SAR) 28692.277 mg/l (Neutral Organic SAR)
Acute toxicity daphnids, 48h LC50 (EpiSuite) 65.467 mg/l (Esters) 730.075mg/l (Neutral Organics-acid) 109.112 mg/l (Hydrazines)
1.073 mg/l (Thiols and Mercaptans) 1.754 mg/l (Thiocarbamate, Di(Substit))
106.884 mg/l (Neutral Organic SAR) 12773.367 mg/l (Neutral Organic SAR)
Acute toxicity algae, 96h EC50 (EpiSuite) 27.631 mg/l (Esters) 518.374 mg/l (Neutral Organics-acid) 2.622 mg/l (Hydrazines)
0.748 mg/l (Thiols and Mercaptans) 0.190 mg/l (Thiocarbamate, Di(Substit))
81.240 mg/l (Neutral Organic SAR) 3479.678 mg/l (Neutral Organic SAR)
Acute toxicity daphnids, 48h EC50 (test data) 860 mg/l 5.95 mg/L
Acute toxicity algae (test data) Inhibition starts at 1630 mg/l (96 hr) (pH = 7)
EC50 (14d) = >10 mg/l EC50 (72h) > 160 mg/L

Reason / purpose for cross-reference:

read-across source

Key result

Duration:

96 h

Dose descriptor:

LOEC

Effect conc.:

1 630 mg/L

Nominal / measured:

not specified

Conc. based on:

not specified

Basis for effect:

growth rate

Remarks on result:

other:

Remarks:

Inhibition starts at 1630 mg/l (96 hr) (pH = 7)

Validity criteria fulfilled:

not applicable

Conclusions:

Various data is available on Benzoic acid and benzoates, most relevant here also sodium benzoate, as it can be expected that both benzoic acid and sodium benzoate will be present ionized in water and hence are identical anions. Data is taken from a peer-reviewed data collection / Assessment report, indicating that the results are sufficiently reliable for hazard assessment. In conclusion, from the data (fish, daphnia, algae, bacteria) it is obvious that neutralization of the pH greatly reduces (up to one order of magnitude) the acute toxicity of benzoic acid. This is also supported by the lower toxicity observed with the sodium benzoate. Under environmental relevant conditions therefore the acute toxicity of benzoic acid, sodium benzoate and potassium benzoate for all four trophic levels is > 100 mg/l. Hence, benzoic acid does not need to be classified as toxic to the environment, neither acute nor chronic. The same applies for the target substance 5-Mercapto-1,3,4-thiadiazol-2-thiol-benzoate.

Executive summary:

Various data is available on Benzoic acid and benzoates. Data is taken from a peer-reviewed data collection / Assessment report, indicating that the results are sufficiently reliable for hazard assessment. The relevant findings are:

 

Chemical	Species	Protocol	Result
Benzoic acid	Scenedesmus quadricauda	other	EC50 (3 h) = 75 mg/l
	Scenedesmus quadricauda	Cell multiplication inhibition test; static	Inhibition starts at 1630 mg/l (96 hr) (pH = 7)
	Chlorella pyrenoidosa	other	EC50 (3 h) = 60 mg/l
	Anabaena variabilis	other	EC50 (14d) = >10 mg/l
Sodium benzoate	Green algae	ECOSAR	EC50 (96 h) = 478 mg/l
Benzyl alcohol	Chlorella pyrenoidosa	other	EC50 (3 h) = 95 mg/l
	Haematococcus pluvialis	other	EC50 (4 h) = 2600 mg/l
	Scenedesmus quadricauda	Cell multiplication inhibition test	Inhibition starts at 640 mg/l (96 h)

 

Benzoic acid does not need to be classified as toxic to the environment, neither acute nor chronic.

Reference 3

Endpoint:

toxicity to aquatic algae and cyanobacteria

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
There is data available on the hydrolysis behaviour of the registered substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate (CAS 51988-14-8) in unbuffered water at room temperature, hence posing the same conditions as tests on aquatic toxicity. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products, as expected, by hydrolytical cleavage of the thioester group. Based on this, it can be concluded that the registered substance hydrolyzes very rapidly in water, i.e. the half-life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Hence, the substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with the hydrolysis products 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0). According to the “Read-Across Assessment Framework” (RAAF) (ECHA 2017) the analogue approach is followed. The basis for this analogue approach is the “(Bio)transformation to common compound(s)” (scenario 1). “This scenario covers the analogue approach for which the read-across hypothesis is based on (bio) transformation to common compound(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst-case approach” (ECHA 2017). The target substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate (CAS 51988-14-8) is rapidly hydrolysed to 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0). Therefore, the proposed source substances for the target substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate are its final hydrolysis products DMTD and benzoic acid.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Target chemical: Benzenecarbothioic acid, S-(4,5-dihydro-5-thioxo-1,3,4-thiadiazol-2-yl) ester, CAS 51988-14-8, SMILES Code O=C(Sc(nnc1S)s1)c(cccc2)c2
Source chemical 1: Benzoic acid, CAS 65-85-0, SMILES Code O=C(O)c(cccc1)c1
Source chemical 2: 1,3,4-Thiadiazolidine-2,5-dithione, CAS 1072-71-5, SMILES Code N(NC(=S)S1)C1=S
All substances do not contain impurities to an extent which is expected to alter the outcome of the experimental results or read-across approach.

3. ANALOGUE APPROACH JUSTIFICATION
5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate was shown to hydrolyze rapidly in unbuffered water at room temperature, which simulates rather closely the conditions in aquatic toxicity tests. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products, as expected, by hydrolytical cleavage of the thioester group. No further assessment of the hydrolysis of benzoic acid and DMTD has been conducted because these substances are known to be hydrolytically stable. There are no non-common compounds, which have to be taken into account in this read-across approach.
The performed approach is further supported by the OECD Guidance Document on Aqueous-Phase Aquatic Toxicity Testing of Difficult Test Chemicals (Series on Testing and Assessment No. 23, 8 Feb 2019 - ENV/JM/MONO(2000)6/REV1). In this document under point 82 it is stated that “If the test chemical is likely to be unstable, a decision to test the parent test chemical and/or its degradation products, if identified, should be based on a consideration of its half-life under test and real-world conditions. The following decision criteria are suggested only as a guide for static and static-renewal exposure systems with test solution renewal intervals of 24 hours:
• Half-life >3 days: test parent chemical;
• Half-life < 3 days and > 1 hour: consider on a case-by-case basis, and include possible testing of degradation products;
• Half-life < 1 h: test degradation products.”

Having a half-life of 1.91 h the registered substance belongs to the category of ‘< 3 days and > 1 hour’. Thus, the testing strategy has to be decided on a case-by-case basis. With 1.91 h the half-life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Also, the value is close to the threshold of 1 h. Therefore, the registrant concluded that the substance already qualifies to hydrolyze rapidly, and testing of hydrolysis products is suitable.
Further, with regard to the available data on aquatic toxicity, for better comparison the estimated values are regarded first, it can be shown that the presence of the parent compound (less than 50% of the initial quantity after 2 h) has no impact on the results of the aquatic toxicity tests, at least none which may underestimated the actual hazard. Besides, it can be assumed that the target substance already degrades to a certain extend during preparation of the hypothetical test solutions, even if precautions as cited in ENV/JM/MONO(2000)6 are taken. As can be seen in the section 4 - Data matrix, the EC50 or LC50 values vary over a broad range. However, the lowest values were estimated for the hydrolysis product DMTD, so this substance can be regarded as worst-case for the target substance and it is most relevant for a possible classification, not the parent compound. Although no actual test data on the parent compound is available, available test data on the hydrolysis products indicate that DMTD is the more toxic one, which supports the above-mentioned conclusion, as estimated data reveal the same trend in magnitudes.
Hence, the registered substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0).

4. DATA MATRIX
The following information was estimated via US EPA EpiSuite estimation tool or taken from actual data sources:

Property CAS 51988-14-8 (target) CAS 65-85-0 (source 1) CAS 1072-71-5 (source 2)
Water solubility (EpiSuite) 395.1 mg/L 2493 mg/L 2.638e+005 mg/L
logPow (EpiSuite) 2.05 1.87 -0.6349
Biodegradability (EpiSuite) not readily biodegradable readily biodegradable not readily biodegradable
Acute toxicity fish, 96h LC50 (EpiSuite) 31.672 mg/l (Esters) 1300.781 mg/l (Neutral Organics-acid) 12.705 mg/l (Hydrazines)
8.588 mg/l (Thiols and Mercaptans) 0.624 mg/l (Thiocarbamate, Di(Substit))
187.325 mg/l (Neutral Organic SAR) 28692.277 mg/l (Neutral Organic SAR)
Acute toxicity daphnids, 48h LC50 (EpiSuite) 65.467 mg/l (Esters) 730.075mg/l (Neutral Organics-acid) 109.112 mg/l (Hydrazines)
1.073 mg/l (Thiols and Mercaptans) 1.754 mg/l (Thiocarbamate, Di(Substit))
106.884 mg/l (Neutral Organic SAR) 12773.367 mg/l (Neutral Organic SAR)
Acute toxicity algae, 96h EC50 (EpiSuite) 27.631 mg/l (Esters) 518.374 mg/l (Neutral Organics-acid) 2.622 mg/l (Hydrazines)
0.748 mg/l (Thiols and Mercaptans) 0.190 mg/l (Thiocarbamate, Di(Substit))
81.240 mg/l (Neutral Organic SAR) 3479.678 mg/l (Neutral Organic SAR)
Acute toxicity daphnids, 48h EC50 (test data) 860 mg/l 5.95 mg/L
Acute toxicity algae (test data) Inhibition starts at 1630 mg/l (96 hr) (pH = 7)
EC50 (14d) = >10 mg/l EC50 (72h) > 160 mg/L

Reason / purpose for cross-reference:

read-across source

Key result

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

55.1 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: yield

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

EC20

Effect conc.:

27.37 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: yield

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

EC10

Effect conc.:

18.99 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: yield

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

EC50

Effect conc.:

> 160 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

EC20

Effect conc.:

82.35 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

EC10

Effect conc.:

32.86 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

growth rate

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

LOEC

Effect conc.:

40 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other: both growth rate and yield

Remarks on result:

other: Results should be based on the nominal concentrations applied

Duration:

72 h

Dose descriptor:

NOEC

Effect conc.:

20 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

other:

Remarks:

both growth rate and yield

Remarks on result:

other:

Remarks:

Results should be based on the nominal concentrations applied

Details on results:

- Exponential growth in the control (for algal test): yes
- Observation of abnormalities (for algal test): none stated
- Unusual cell shape: none stated
- Colour differences: none stated
- Flocculation: not stated
- Aggregation of algal cells: not stated
- Any stimulation of growth found in any treatment: For the parameter yield an activation of 8.2 and 5.7% was determined at the two lowest main test concentrations of 10 and 20 mg/L compared to the control, respectively. For the parameter growth rate an activation of 1.5 and 1.0% was determined at the two lowest main test concentrations of 10 and 20 mg/L compared to the control, respectively.
- Any observations (e.g. precipitation) that might cause a difference between measured and nominal values: none stated
- Effect concentrations exceeding solubility of substance in test medium: no

Results with reference substance (positive control):

not required

Validity criteria fulfilled:

yes

Conclusions:

The study was performed according to OECD TG 201 with no relevant deviations. The results were so obtained via a scientifically reasonable method. The validity criteria for the study as given by the OECD Guideline 201 were all met and the study is therefore considered to be valid. Hence, there is no doubt that the obtained results are not reliable: The toxic effect was investigated by determination of the inhibition of the growth rate of the algae and the yield during the exposure period of 72 hours. The test was performed using five concentrations in the range of 10 to 160 mg/L.
As a conclusion of the analytical part of this study, it can be stated that the concentrations of 2,5-Dimercapto-1,3,4-thiadiazole (“DMTD”) remained sufficiently stable during incubation of 72 hr: after 72 hr of incubation recoveries of 99.0 to 101.5 % of the initial values measured at time t0 were obtained in the treatments after 72 hr. Based on this, the reported effect concentrations (EC values) should refer to nominal concentrations of the test item.

The following EC-values (72h; growth rate; nominal concentrations) were determined accordingly:
LOEC 40 mg/L
NOEC 20 mg/L
EC50 > 160 mg/L
Based on these results, the test item does not need to be classified as acute or chronic toxic to the aquatic environment. The same applies for the target substance 5-Mercapto-1,3,4-thiadiazol-2-thiol-benzoate.

Executive summary:

The toxicity of 2,5-Dimercapto-1,3,4-thiadiazole (“DMTD”) towards algae was tested according to OECD-Guideline No. 201, in the Version dated 23-March-2006, under GLP. The toxic effect was investigated by determination of the inhibition of the growth rate of the algae and the yield during the exposure period of 72 hours. The test was performed using five concentrations in the range of 10 to 160 mg/L. The following EC-values were obtained:

On Basis of the Nominal Concentrations [mg test item/ L]

Yield (0 - 72hr)                                                            EC10          18.99           95%-CL    lower          18.14                            upper          19.87                             EC20          27.37            95%-CL    lower          26.17                            upper          28.65                             EC50          55.11            95%-CL    lower          52.04                            upper          58.25                   Yield  LOEC          40.00                           NOEC          20.00 Section-by-section growth rate (0-72hr)                            EC10         n.d. (>160)            95%-CL    lower         n.d. (>160)                            upper         n.d. (>160)                            EC20         n.d. (>160)           95%-CL    lower         n.d. (>160)                            upper         n.d. (>160)

Section-by-section growth rate (0-72hr)                             EC50          n.d. (>160)            95%-CL    lower           n.d. (>160)                            upper           n.d. (>160)                           LOEC           n.d. (>160)                           NOEC           n.d. (>160) Growth rate (0 - 72 hr)                                                EC10          32.86            95%-CL    lower          16.65                            upper          64.85                             EC20          82.35            95%-CL    lower          39.56                            upper        175.72                             EC50          n.d. (>160)            95%-CL    lower           n.d. (>160)                           upper           n.d. (>160)      Growth rate  LOEC          40.00                           NOEC          20.00 n.d.: not determined due to mathematical reasons or inappropriate data

As a conclusion of the analytical part of this study, it can be stated that the concentrations of 2,5-Dimercapto-1,3,4-thiadiazole (“DMTD”) remained sufficiently stable during incubation of 72hr:after 72 hr of incubation recoveries of 99.0 to 101.5 % of the initial values measured at time t₀ were obtained in the treatments after 72 hr. Based on this, the reported effect concentrations (EC values) should refer to nominal concentrations of the test item.

The validity criteria for the study as given by the OECD Guideline 201 were all met and the study is therefore considered to be valid.

Description of key information

Read across to DMTD: OECD 201, GLP compliance, Desmodesmus subspicatus (previous name: Scenedesmus subspicatus), static, ErC50 (72h, growth rate): > 160 mg/L, NOEC(72h) 20 mg/L, EC10(72h, growth rate): 32.86 mg/L 

Read across to Benzoic acid: OECD 201, cyanobacteria (blue-green algae) Anabaena inaequalis, A. cylindrica, and A. variabilis, green algae Chlorella pyrenoidosa and Scenedesmus quadricauda, static, EC50 (12-14d): > 10 mg/L

Read across to Benzoate: Scenedesmus quadricauda, LOEC (96h, growth rate): 1630 mg/L

Key value for chemical safety assessment

EC10 or NOEC for freshwater algae:

20 mg/L

Additional information

There is data available on the hydrolysis behaviour of the registered substance 5-Mercapto-1,3,4-thiadiazole-2-thiol-benzoate in unbuffered water at room temperature, hence posing the same conditions as test on aquatic toxicity. The hydrolytic degradation of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate can be described as a pseudo-first order kinetic and the half-life of 5-mercapto-1,3,4-thiadiazole-2-thiol-benzoate was determined to be 1.91 h. 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0) were identified as degradation products as expected by hydrolytical cleavage of the thioester group. It can be concluded that the substance hydrolyzes very rapidly in water, i.e. the half life is max. 1/25 of the test duration when put into aqueous test systems, e.g. daphnia or algae acute toxicity tests. Hence, the substance as such does not need to be tested in those systems, it is sufficient for hazard assessment to use data gained with 1,3,4-Dimercapto-thiadiazol (DMTD, CAS 1072-71-5) and benzoic acid (CAS 65-85-0).

Toxicity to aquatic algae and cyanobacteria for DMTD

The study was performed according to OECD TG 201 with no relevant deviations. The results were so obtained via a scientifically reasonable method. The validity criteria for the study as given by the OECD Guideline 201 were all met and the study is therefore considered to be valid. Hence, there is no doubt that the obtained results are not reliable: The toxic effect was investigated by determination of the inhibition of the growth rate of the algae and the yield during the exposure period of 72 hours. The test was performed using five concentrations in the range of 10 to 160 mg/L. As a conclusion of the analytical part of this study, it can be stated that the concentrations of 2,5-Dimercapto-1,3,4-thiadiazole (“DMTD”) remained sufficiently stable during incubation of 72 hr: after 72 hr of incubation recoveries of 99.0 to 101.5 % of the initial values measured at time t0 were obtained in the treatments after 72 hr. Based on this, the reported effect concentrations (EC values) should refer to nominal concentrations of the test item.

The following EC-values (72h; growth rate; nominal concentrations) were determined accordingly:
LOEC 40 mg/L
NOEC 20 mg/L
EC10 32.86 mg/L 
EC50 > 160 mg/L

Toxicity to aquatic algae and cyanobacteria for Benzoic acid

In a 12-14 day toxicity study, the cultures of cyanobacteria (blue-green algae) Anabaena inaequalis, A. cylindrica, and A. variabilis, green algae Chlorella pyrenoidosa and Scenedesmus quadricauda were exposed to benzoic acid at nominal concentrations of 0 to 10ppm under static conditions similar to OECD 201.  

Toxicity to aquatic algae cyanobacteria for Benzoates

Various data is available on Benzoic acid and benzoates, most relevant here also sodium benzoate, as it can be expected that both benzoic acid and sodium benzoate will be present ionized in water and hence are identical anions. Data is taken from a peer-reviewed data collection / Assessment report, indicating that the results are sufficiently reliable for hazard assessment. In conclusion, from the data (fish, daphnia, algae, bacteria) it is obvious that neutralization of the pH greatly reduces (up to one order of magnitude) the acute toxicity of benzoic acid. This is also supported by the lower toxicity observed with the sodium benzoate. Under environmental relevant conditions therefore the acute toxicity of benzoic acid, sodium benzoate and potassium benzoate for all four trophic levels is > 100 mg/L.