N,N-dicyclohexylbenzothiazole-2-sulphenamide

Endpoint summary

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

basic toxicokinetics

Type of information:

other: read across approach justification for repeated dose toxicity dermal

Adequacy of study:

other information

Reliability:

other: read across approach repeated dose toxicity dermal

Rationale for reliability incl. deficiencies:

other: read across approach repeated dose toxicity dermal

Principles of method if other than guideline:

other: read across justification

Preliminary studies:

read across approach: repeated dose toxicity dermal

Read across approach

There were no dermal repeated dose data for available the sulfonamide DCBS. To overcome this uncertainty a read across approach were conducted with the structure analogs sulfenamides CBS, TBBS, MBS and MBT. Several toxicological endpoints were compared with each other to reveal a substantiable toxicological profile of DCBS. Several physicochemical and toxicological parameters of these compounds were compared with each other.

All substances have a solid physical state and a powder-like appearance. The vapour pressure of the substances is very low. The water solubitlity differes substantially, reflecting the different functional groups which are linked to the basic structure, the MBT.

The acute and dermal toxcicty is very low, indicated for the sulfenamides by a oral LD50 above 5000 mg/kg and for the hydrolyse above 2000 mg/kg bw and day.

	CBS	DCBS	TBBS	MBS
CAS	95-33-0	4979-32-2	95-31-8	102-77-2
Physical state	solid	solid	solid	solid
Water Solubility	0.32 mg/l at 21 °C and pH 7 (Monsanto, 1980)	1.9 µg/l (CERI, 2001)	0.3 mg/L at RT (Monsanto, 1978)	54.6 mg/L at RT (Monsanto, 1978)
Vapour Pressure	4.53*10-7 hPa at 25 °C (Monsanto, 1980)	< 7.0 x 10-5 PA at 100°C (MITI, 1194)	< 0.0000021 hPa at 25°C
log Pow	ca. 4.93 (Monsanto, 1980).	>4.80 at 25°C	3.9 at RT
Acute toxicity oral (LD50 rat)	5300 mg/kg bw	> 5000 mg/kg bw	> 5000 mg/kg bw	> 5000 mg/kg bw
Acute toxicity dermal (LD50 rabbit)	> 7940 mg/kg bw	> 5000 mg/kg bw	> 7940 mg/kg bw	> 7940 mg/kg bw
Acute toxicity inhalation (LC50)	no data	no data	no data	no valid data
Skin irritation	not irritating	not irritating	not irritating	skin irritating
Eye irritation	not irritating	not irritating	not irritating	eye irritating
Skin sensitization	sensitizer* *human repeated insult test	not sensitizing (guinea pig test)	sensitizer* *human repeated insult test	sensitizer* *human repeated insult test
Repeated dose oral (NOAELrat)	80 mg/kg bw/d (TG 407)	36.9 mg/kg (TG 408)	100 mg/kg bw/d (TG 408)	50 mg/kg bw/d (chronic study)
Repeated dose dermal (NOAEL)	2000 mg/kg bw/d (TG 410)	no data	2000 mg/kg bw/d (TG 410)	2000 mg/kg bw/d (TG 410)
Repeated dose inhalation (NOAEC)	no valid data	no data	no valid data	no valid data
molecular mass	264.4 g/mol (EU risk assessment 2008)	346.59 g/mol (OECD SIDS 2004)	238.39 g/mol(OECD SIDS 2003)	252.36 g/mol

Reference 2

Endpoint:

basic toxicokinetics

Type of information:

other: assumption toxicokinetics

Adequacy of study:

supporting study

Reliability:

other: assumption toxicokinetics

Rationale for reliability incl. deficiencies:

other: assumption: toxicokinetics DCBS, which based on physico-chemical data and acute and repeated dose toxicity data of DCBS

Principles of method if other than guideline:

assumption toxicokinetic of DCBS, which based on physico-chemical data and acute and repeated dose toxicity data of DCBS

GLP compliance:

no

Type:

other:

Results:

assumption: toxicokinetics

There are no study data available for the toxicokinetic of the test substance N, N-dicyclohexyl-2-benzothiazolesulfenamide (DCBS.) However, considering the experiences with DCBS in acute and repeated dose toxicity studies, a characterization of DCBS toxicokinetics can be conducted.

DCBS is a solid powder with a low vapour pressure < 7.0 x 10^-5Pa at 100 °C (OECD SIDS 2004); therefore inhalation exposure to the vapour might be negligible.

DCBS is practically insoluble in water (1.9 µg/l at 25°C). However, the molecular mass of 346.59 g/mol and the n-octanol/water coefficient (log Pow of > 4.80 at 25°C) suggest intestinal absorption subsequent to oral ingestion.

This assumption is confirmed by data from acute oral toxicity studies and repeated dose toxicity studies. However, acute toxicity is low because LD50 values are high (greater than 5000 mg/kg bw, Monsanto 1985); systemic availability was indicated by clinical signs like hypoactive, decreased food consumption, and other abnormalities (nasal discharge, oral discharge, wet rales, ocular discharge, urinary straining, unthrifty coat, soft stool, and eyes closed). In addition, studies with repeated oral dosing of rats indicating bioavailability of the compound by showing dose related toxic effects like a decrease in body weight gain and food consumption (Monsanto 1988, 1989).

The dermal toxicity is very low, indicated by a dermal LD50 greater than 5000 mg/ kg bw (Monsanto 1985). New Zealand White rabbits were dosed with 5000 mg/ kg bw. A 14-day observation period followed application. No mortality was observed. Most animals were free of signs of systemic toxicity, although several occurrences of nasal discharge were seen, primarily in a single animal, and a few occurrences of ocular irritation were noted in another animal. Gross postmortem observations were similar to those seen in control animals in this laboratory or were considered to represent normal physiological variation. Based on these findings the authors concluded that the LD50 is greater than 5000 mg/kg bw (Monsanto 1985).

Description of key information

Experimental Data

There are no study data available for the toxicokinetic of the test substance N, N-dicyclohexyl-2-benzothiazolesulfenamide (DCBS.) However, considering the experiences with DCBS in acute and repeated dose toxicity studies, a characterization of DCBS toxicokinetics can be conducted. DCBS is a solid powder with a low vapour pressure < 7.0 x 10-5 Pa at 100 °C (OECD SIDS 2004); therefore inhalation exposure to the vapour might be negligible. DCBS is practically insoluble in water (1.9 µg/l at 25°C). However, the molecular mass of 346.59 g/mol and the n-octanol/water coefficient (log P of > 4.80 at 25°C) suggest intestinal absorption subsequent to oral ingestion. This assumption is confirmed by data from acute oral toxicity studies and repeated dose toxicity studies. However, acute toxicity is low because LD50 values are high (greater than 5000 mg/kg bw, de Groot 1975, Monsanto 1985); systemic availability was indicated by clinical signs like hypoactive, decreased food consumption, and other abnormalities (nasal discharge, oral discharge, wet rales, ocular discharge, urinary straining, unthrifty coat, soft stool, and eyes closed). In addition, studies with repeated oral dosing of rats indicating bioavailability of the compound by showing dose related toxic effects like a decrease in body weight gain and food consumption (Monsanto 1988, 1989). The dermal toxicity is very low, indicated by a dermal LD50 greater than 5000 mg/ kg bw (Monsanto 1985). New Zealand White rabbits were dosed with 5000 mg/ kg bw. A 14-day observation period followed application. No mortality was observed. Most animals were free of signs of systemic toxicity, although several occurrences of nasal discharge were seen, primarily in a single animal, and a few occurrences of ocular irritation were noted in another animal. Gross postmortem observations were similar to those seen in control animals in this laboratory or were considered to represent normal physiological variation. Based on these findings the authors concluded that the LD50 is greater than 5000 mg/kg bw (Monsanto 1985).

ADME

Absorption - Oral/G.I.

The structure of DCBS contains several functional groups which may lead to electrophilic/nucleophilic attack which could lead to the evolution of ionizable groups. However due to the sulphane bonding between the benzothizole moiety and the other secondary amine moieties in DCBS, there is a stabilizing effect which will diminish the likelihood of these interactions as opposed to for example BT. Therefore, for the parent substance, ionisation is unlikely to affect absorption.

DCBS is likely to be readily absorbed due the molecular weight being below 500 (DCBS MW is 346.6). DCBS has a half-life of 53 hours and therefore is potentially in the G.I. tract for a limited period of time and any toxicity observed may be due to the degradants. However, the absorption and behaviour of the parent compound can still play a significant role in the toxicokinetics of the substance as it will still remains at 50 % or greater for greater than 2 days. Hence, there is time for absorption, distribution, metabolism and possibly excretion prior to full hydrolysis of the substance. The drivers of toxicity will be explored later, and regardless of parent toxicokinetics, the metabolites may still be the drivers.

The particle size distribution of DCBS was determined and the median diameter was 28 µm with the main fraction of 91 % distributing in the range of 10 to 100 µm (Currenta, 2010); and hence the tested DCBS particles with sieved size smaller than 100 µm are mainly dominated by inhalable fraction as defined in EN 481/ISO 7708 (1995). The log P of DCBS will allow passage across the biological membranes of the gastro-intestinal tract but, it is insoluble in water (1.9 mg/l at 25°C, CERI, 2001) which may restrict absorption of the molecule. However, for lipophilic substances micellular solubilisation may be an important mechanism for absorption and the mechanism is of particular importance for highly lipophilic compounds (log P >4), particularly those that are poorly soluble in water (1 mg/l or less) that would otherwise be poorly absorbed (ECHA R.7c, 2017), such as DCBS. The molecular size is not below 200 Da, and as such carriage through the epithelial barrier by the bulk passage of water is less likely. In an acute oral toxicity study using polyethylene glycol or 1 % methocel A15C (Dow) as a vehicle (de Groot, 1975 and Monsanto, 1985 respectively) and in a repeat dose toxicity delivered via the diet (Monsanto 1988) DCBS was readily absorbed. Both studies were performed in the rat. The results of the acute oral toxicity (de Groot, 1975 and Monsanto, 1985) and repeated dose oral toxicity (Monsanto, 1988) studies performed in the rat, suggest that DCBS is absorbed from the gastro-intestinal tract. Systemic effects are observed following exposure in a dose related toxic effect. This supports the aforementioned absorption processes. See Table 8, for all toxicological responses and studies.

Absorption – Respiratory/inhalation

The vapour pressure for DCBS was determined to be < 0.00001 Pa at 100 °C (LAUS, 2010) and decomposes at 300 °C, this shows that DCBS has low volatility when in a solvent and is not readily available as a vapour. Therefore, inhalation exposure to the vapour might be negligible.  The particle size is 91 % in the range of 10-100 µm which means that DCBS does have the ability to be inhaled and potentially reach the thoracic and alveolar regions in humans (ISO 7708:1995, 2017). DCBS may have the potential to be absorbed in the lower respiratory tract. DCBS is poorly soluble (1.9 µg/L) thus this will limit the amount of the substance that can be absorbed directly via dissolution in mucus. Poorly water-soluble dusts depositing in the nasopharyngeal region could be coughed or sneezed out of the body or swallowed (Schlesinger, 1995). It may be that considering the size that the particles reach the tracheo-bronchial region and would mainly be cleared from the lungs by the mucociliary mechanism and swallowed. However, a small amount may be taken up by phagocytosis and transported to the blood via the lymphatic system. A fraction of the particle below 15 µm may reach the alveolar region where they would mainly be engulfed by alveolar macrophages and either translocate particles to the ciliated airways or carry particles into the pulmonary interstitium and lymphoid tissues. Due to the biologically relevant signs observed in both the repeat dose toxicity, acute oral toxicity studies and by the dermal route, no inhalation study was performed (Annex VIII of REACH). However, it is clear from the other category members that some effects are present in all regions and align with the expected absorption based on the categories physical-chemical properties.

Absorption - Dermal

In particulate form DCBS is likely to be absorbed to a lesser degree than when dissolved or co-administered with a vehicle. The molecular weight is between 100 to 500 Da, so based on molecule size only moderate absorption might be expected.  As discussed, though some ionisable groups are present on the parent compound, these are unlikely to ionise due to the stabilising effects of the chemical structure and stearic hinderance. So no slowing of uptake due to this is expected. The water solubility (1.9 µg/L) suggests low absorption by the epidermis with a log P that also suggests limited rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum as high (log P value is between > 4 < 6). Surface tension is not a desired property and the vapour pressure i slow, increasing contact time with the skin should exposure be present. There are no trace elements present in DCBS. Based on the physical-chemical parameters alone, absorption will be low to moderate.

For DCBS only an acute dermal toxicity study is available. There is no dermal repeated dose study available for the DCBS. The acute dermal toxicity of DCBS was evaluated in New Zealand albino rabbits (Monsanto 1985). No mortality occurred. The dermal LD50 for the test substance DCBS was greater than 5000 mg/kg bw. The results of the acute toxicity study sgow DCBS is not acutely toxic via this route. The other category members also show limited absorption and effect during subacute repeated exposures. The category is consistent with its physical-chemical interpretation for absorption and DCBS is expected to behave in the same manner as the category members, in line with the similar physical-chemical attributes and the consistency shown across the group despite some differences in water solubility and log P. The skin sensitisation results of a guinea pig maximisation test (Monsanto, 1985) indicate that there is no sensitisation at 25 % DCBS challenge. However, the substance is classified as there is human data on all other category members showing skin sensitisation. DCBS is likely to exert its skin sensitisation effects via the same mechanism as the other category members where it breaks down to MBT which leads to skin sensitisation effects by reaction with skin protein molecules, via nucleophilic substitution (SN2) to form disulphide bridge between skin proteins and the benzothiazole moiety,  as observed in papers by American Chemistry Council (2003) and Adams (2006). Like the other category members there is likely limited uptake via the dermal route. However, unlike the other group members there is no current data which would support its classification and systemic availability during skin sensitisation testing.

Distribution

The small molecular size of CBS means that wide distribution is possible. The MBS log P value is over P > 0 and therefore it is likely to distribute into cells and the concentration has the potential to be higher intracellularly as opposed to extracellularly, this is particularly in regard to fatty tissues. Based on the existing in vivo data, the distribution of the substance was to many organs these were but not limited to, kidneys, thymus, adrenal, testes and spleen. The results of the oral repeated dose studies performed in the rat shows wide systemic distribution. The studies support that there is wide distribution of DCBS after exposure. 

Metabolism

Results of the oral repeated dose studies in the rat show changes in the liver. This suggests that following ingestion, DCBS is likely predominately metabolized in the liver (first pass metabolism) following oral administration and further metabolism/hydrolysis within the G.I tract prior (enterohepatic circulation) to excretion in the urine or faeces. This is also expected considering the information on the group members and the noted urinary straining and soft stools in the repeat dose studies. Moreover, the results in the reproductive toxicity studies show effects not similar to MBT, but (di)cyclohexylamine.  This suggests that again cleavage at the thiol bond between MBT and its substituent, resulting in MBT and dicylcohexylamine. It is these metabolites that may be responsible for the toxicity, this will be further discussed in ensuing chapters. The expected metabolites are supported by the predictions of rat liver metabolism from the OECD QSAR Toolbox (v4.5).

Excretion

The results of some of the repeated dose oral toxicity studies show changes in the kidneys and liver of rats. This suggests that urinary and faecal excretion may be a significant route for removal of DCBS. It is also possible considering the substances lipophilicity and, conjugation of MBT, that enterohepatic circulation is possible.  Faecal clearance for MBT has been well documented for other category members There is no evidence that DCBS is found at higher concentrations in breast milk than in blood/plasma. DCBS does not excrete into the hair or nails. Mucus from the lungs would be cleared and excreted or swallowed and undergo the same processes as if administered orally.

Potential for accumulation

Based on the information provided from the log P value of ca. 5, it is possible that DCBS may accumulate. However, due to its extensive metabolism and subsequent excretion, the accumulation potential is thought to be low.  Particles are > 1 µm and thus, accumulation in the lungs may be less likely. The log P is > 4 < 6 so it is possible that the parent substance may accumulate in the stratum corneum, however, due to the metabolism and hydrolysis expected, and the fact that the metabolites and hydrolysis products have log P values less than 3, accumulation here is unlikely.

Key value for chemical safety assessment

Bioaccumulation potential:

low bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

100

Absorption rate - inhalation (%):

100

Additional information

A summary of the available study data and desk based toxicokinetic assessment is provided below. This substance is part of a category; please refer to section 13.2 for further information.