Reaction mass of 2-[4-[(Hexahydro-2,4,6-trioxo-5-pyrimidyl) azo]phenyl]-6-methylbenzothiazole-7-sulphonic acid, compound with 2,2',2''-nitrilotris[ethanol] (1:1) and Lithium 2-[4-[(hexahydro-2,4,6-trioxopyrimidin-5-yl)azo]phenyl]-6-methylbenzothiazole-7-sulphonate

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

1. Toxicokinetic analysis

The test item is a dark orange powder at room temperature with a molecular weight of 465.4 g/mol - 514.3 g/mol. The log Pow estimated based on individual solubilities (water solubility:150 g/L and 202 g/L; octanol solubility 25 mg/L) was calculated to be -4 at 20 °C. The vapour pressure could not be determined as the melting point is above 300°C.

 

The test item is a multi-constituent containing CAS 65036-46-6 and CAS 35342-16-6 with the typical concentration of about 50% each.

 

1.1 Absorption

Oral route

Subsequent to oral exposure the substance will dissolve/dissociate in the gastrointestinal fluids since it is a highly water soluble salt (water solubility of 150 g/L - 202 g/L). Absorption will depend on the pH in the gastrointestinal tract, since the absorption of the ionic form is restricted whereas the absorption of the non-ionic form is favoured.

Generally, oral absorption is favoured for substances with a molecular weight below 500 g/mol and with a log Pow value between -1 and 4. As the molecular weight of the dissociated dye is below 500 g/mol, but the log Pow is -4, a limited oral absorption seems to be likely.  

 

The yellow discoloured urine noted in the OECD 422 study in all high-dose male and female F0 animals proofs that the substance is partially absorbed. The discoloured faeces observed in the same study and the acute oral toxicity study are no clear indication that oral absorption had occurred, since it might have been caused by the excretion of non-absorbed substance.

 

Overall oral absorption does occur, but only to a limited extent.

 

Inhalation route

Due to the fact that the melting point is above 300°C the vapour pressure could not be determined. Therefore, it is unlikely that the substance will be available as a vapour. In humans, particles with aerodynamic diameters < 100 µm have the potential to be inspired, < 50 µm may reach thoracic region and < 15 µm may reach the alveolar region.

The particle size distribution of D10=9 µm, D50=126 µm and D90=465 µm indicates that a significant fraction of airborne particle may be inspired, whereas only a small fraction can potentially reach the alveolar region.

As the substance is very hydrophilic (negative log Pow and high water solubility) and has a molecular weight above 200 (which does not favour absorption through aqueous pores) inhaled particles are likely to be dissolved and retrained in the mucus and transported out of the respiratory tract.

 

Overall, absorption via the inhalation route is assumed to be low.

 

Dermal route

Dermal absorption will be very low as the substance is a dry particulate and too hydrophilic (water solubility far above 10,000 mg/L (150 g/L and 202 g/L) and log P value of -4) to cross the lipid rich environment of the stratum corneum.

This is supported by the in vitro skin irritation and sensitisation study (LLNA) in rabbit and mouse, respectively, where no indication of systemic availability of the substance (i.e. systemic toxicity or discoloured urine) has been reported.

 

1.2 Distribution

It is assumed that due to the very high water solubility the absorbed substance is not widely distributed since the substance cannot easily diffuse across membranes. Moreover, due to the molecular weight of > 200 distribution through aqueous channels and pores is restricted.

An accumulative potential in adipose tissue can be excluded due to the very low log P value of -4.

 

1.3 Metabolism

The genotoxicity studies (Ames test, HPRT assay, in vitro Micronucleus test) indicate no remarkable differences in regard to genotoxicity and cytotoxicity in the presence or absence of metabolic activation systems. The results demonstrate that neither genotoxic nor toxic metabolites were formed in those test systems.

 

Generally, it is likely that common protein interaction such as cytochrome P450 oxidases interaction during Phase I metabolism introduce a reactive or polar group in the test item. Those might be further processed into polar compounds during the metabolism in Phase II.

However, as the substance is already very hydrophilic a metabolism by Phase I and II enzymes leading to even more hydrophilic metabolites does not seem to be likely.

 

This assumption is supported by result obtained from the in vivo rat metabolism simulator contained in the QSAR toolbox v.4.2. The simulator predicted - besides the unchanged parent compound - only the metabolites formed by the cleavage of the azo bond, but no further enzymatic Phase I or II reactions.

   

1.4 Excretion

The major routes of excretion for substances with systemic circulation are urine and faeces.

The substance characteristics favourable for urinary excretion are low molecular weight (below 300 g/mol by the rat), good water solubility and ionization at the pH of urine. The parent substance fulfils the latter two characteristics, but has molecular weight above 300 g/mol which favours biliary excretion in the rat. The metabolites which might be formed by cleavage of the azo bond are expected to be eliminated in the urine as their molecular weight is below 300 g/mol and they are hydrophilic.

 

The discoloured urine (yellow) observed in all high-dose male and female F0 animals of the OECD 422 study confirms the urinary excretion of the substance.

In this study and the acute oral toxicity study also discoloured faeces was noted. It is assumed that this is mainly related to the excretion of non-absorbed material and to a lesser extent to the biliary excretion of absorbed substance.

 

2. Summary and conclusion

 

Based on the physicochemical characteristics, particularly high water solubility and low octanol-water partition coefficient, oral absorption is expected to be higher than absorption by the inhalation and dermal route. Particularly, dermal absorption is assumed to be very low.

Absorption via oral route and distribution in the systemic circulation is evidenced by the observation of discoloured urine in the OECD 422 study.

Bioaccumulation of the substance after continuous exposure is not to be expected.

Due to its high hydrophilicity it is assumed that the substance is not extensively metabolised. A cleavage of the azo bond has been predicted by the in vivo rat metabolism simulator of the QSAR toolbox.

Excretion in the urine has been observed in the OECD 422 study. Biliary excretion of the parent compound might also occur in the rat as the molecular weight is above 300 g/mol.

3. References

 

[1] ECHA (2017), Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance, Version 3.0, June 2017

[2] QSAR Toolbox:http://www.qsartoolbox.org/(Version 4.2)