N-cyclohexylbenzothiazole-2-sulfenamide

Endpoint summary

Additional information

In the up-to-date assessment, results from the Japanese Ministry of Environment are used mostly as key studies for aquatic toxicity, because they are more sensitive and reliable than the results from either Monsanto or Bayer AG. The most sensitive acute toxicity is to aquatic algae (Selenastrum capricornutum) tested according to OECD TG 201 "Alga, Growth Inhibition Test". After 72 hours of exposure, an ECr50 of 0.15mg/L and a NOEC of 0.0084 mg/L were obtained. A long-term study is available for aquatic invertebrates (Dapnia magna) tested according to OECD TG 211 "Daphnia magna Reproduction test". After 21 days of exposure, an EC50 of 0.12 mg/L and a NOEC of 0.058 mg/L were obtained. For the assessment of microorganisms in biological treatment plants, an EC50 value of > 10000 mg/l after 3 hours exposure at a pH 7.2 - 8.1 and temperature of 20.6 - 22.8 °C was observed.

 

It was concluded in chapter 5.1 that dissolved CBS can be rapidly degraded by hydrolysis and photolysis, the hydrolysis half-life was determined to be 13.4 h at 20 °C. Toxicity on Daphnia (acute and chronic) and on Fish (acute and prolonged) was determined under semi-static or flow-through conditions, respectively (MOE Japan, 1997). 

 

For hydrolysable substances used in aquatic ecotox tests, REACH Guidance Document R7b (2017), p. 86 states: :
“Where degradation is rapid (e.g. half-life < 1 hour), the available test data will frequently define the hazard of the degradation products since it will be these that have been tested. These data may be used to classify the parent substance in the normal way.
Where degradation is slower (e.g. half-life > 3 days), it may be possible to test the parent substance and thus generate hazard data in the normal manner using a suitable renewal regime. The subsequent degradation may then be considered in determining whether an acute or chronic hazard class should apply.
Where degradation rates fall between these two, testing of either parent and/or degradates should be considered on a case-by-case basis. “

CBS hydrolyses with a DT50 of 13.4 h thus falling in the category where either parent or degradation product could be tested. In the following considerations are discussed which kind of study is recommended:

1. In the case of the sulphenamide category, a study with the degradation product benzothiazole-2-thiol (MBT) is already available. This study (FELS test) has been performed in accordance similar to an accepted international guideline (OECD 210, FELS test) and has been evaluated as Klimisch 2.

2. There are acute and chronic ecotox studies with CBS and the transformation products MBT, BT and cyclohexylamine available. The results are presented in the table below (effect values are based on measured concentrations):

 

	CBS (mg/L)	MBT (mg/L)	BT (mg/L)	Cyclohexylamine (mg/L)
Daphnia 48h-EC 50	0.79	0.71	19 (MOE, 1997)	36.3 (MITI, 1997)
Daphnia 21d-NOEC	0.058	0.08	1.5 (MOE, 1997)	1.6 (MITI, 1997)
Fish 96h LC 50	2.1	0.73	39 (MOE, 1997)	33 (MITI, 1997)
Fish 14d-LC50	0.74	0.67 (8d-LC50)	28 (MOE, 1997)	19 (MITI, 1997
Fish 89d NOEC	intended to use MBT	0.048	n/a	n/a
Algae 72h-EC 50 72-h-NOEC	0.15 0.0084	0.5 0.066	>46 (MOE, 1997) 8.5 (MOE, 1997)	29.3 (MITI, 1997) 10.3 (MITI, 1997)

 

Whereas for algae, CBS appears to be more toxic than MBT, the effect values for CBS and MBT in acute daphnia and fish exhibit very comparable. This can also be seen in the chronic tests in daphnia. These results suggest that CBS and its degradation product MBT have a similar toxicological profile and no relevant differences in a chronic test for CBS would be expected. The other transformation products BT and  cyclohexylamine are less toxic for all observed endpoints. Moreover, the effectvalues all lie well above the water solubility of CBS (0.32 mg/L, Monsanto 1980)

3. MBT was identified as the hydrolysis product in studies for the category members TBBS and DCBS. In a study for CBS (Monsanto 1984) however, Benzothiazole (BT) was designated as the hydrolysis product. Newly processed careful evaluation of this study lead to some doubts on correct identification of the hydrolysis product. For this reason, a new hydrolysis study including elucidation of the degradation products has been conducted (Currenta 2022). It was found that with pH 7 and at 20 °C CBS transforms completely to cyclohexylamine, MBT and BT. The latter occur in a ratio of 55% to 45% respectively.

As a conclusion the existing chronic fish test for MBT is sufficient to explain the toxicity of CBS as it is found as one of the main hydrolysis product and serves as worst case for this endpoint.

With respect to the overall aquatic toxicity of CBS, it is found that the transformations products show either a comparable (MBT) or much lower toxicity (cyclohexylamine, BT) than the registered substance. Moreover, the effectvalues of the latter all lie well above the water solubility of CBS (0.32 mg/L, Monsanto 1980). For the hazard assassment the toxicity to algae of the parent substance is identified as the lowest effect value relevant for PNEC derivation. Please refer to the Read Across Justification for further information.

 

Static acute tests: If the present quality criteria are strictly applied, the static acute tests, where no monitoring of the test concentrations occurred, should be regarded as not valid. The main degradation products appearing during the time frame of acute tests are benzothiazole (BT) and benzothiazolone (BTon). BT and BTon are more than one order of magnitude less toxic than CBS. Thus the results of the static tests with nominal concentrations can be considered as the upper limit for the actual effect value.