2-[4-[[2-(cyanoimino)hexahydro-4,6-dioxo-5-pyrimidyl]azo]phenyl]-6-methylbenzothiazole-7-sulphonic acid, compound with 2,2',2''-nitrilotris[ethanol] (1:1)

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

1. Toxicokinetic analysis

 

The test item is a red orange powder at room temperature with a molecular weight of 632.7 g/mol. A log Pow of -5 was estimated from the solubility of the substance in octanol and water.

The vapour pressure could not be determined as the substance is a solid which decomposes at > 210 °C without melting.

The test substance is a mono-constituent with a purity of ca. 80%

 

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 200 g/L - 300 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 -5, a limited oral absorption seems to be likely.

 

The yellow discoloured urine noted in the OECD 422 study with an analogous substance (read-across source substance Similar Substance 01) in all high-dose male and female F0 animals proofs that the substance is partially absorbed. This is supported by the observation of slightly brownish discoloured urine in one of the acute oral studies.

The discoloured faeces observed in the OECD 422 study with an analogous substance (read-across source substance Similar Substance 01) and in one of the acute oral toxicity studies with the registered substance itself 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 substance decomposes at >210°C the vapour pressure could not be determined. Therefore, it is unlikely that the substance will be available as a vapour. The substance is marketed as a non-solid preparation. Therefore the particle size distribution was not determined.

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 too hydrophilic (water solubility far above 10,000 mg/L (200 g/L - 300 g/L) and log P value of -5) to cross the lipid rich environment of the stratum corneum.

This is supported by the in vivo skin irritation and skin sensitisation study, 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 -5.

 

1.3 Metabolism

 

In the Ames test with the registered substance and a HPRT assay with an analogous substance (read-across source substance Similar Substance 01) no remarkable differences in regard to genotoxicity and cytotoxicity were seen in the presence or absence of metabolic activation systems. The results indicate that neither genotoxic nor toxic metabolites were formed in those test systems.

In the in vitro micronucleus test, the test substance induced the formation of micronuclei in human TK6 cells in the absence and the presence of metabolic activation. Toxicity was noted only in the presence, but not in the absence of metabolic activation system (S9). This points towards the formation of a toxic metabolite in the presence of S9 in the in vitro MN test.

 

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 substance. 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 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 in 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 in the OECD 422 study with an analogous substance (read-across source substance Similar Substamce 01) and in one of the acute oral toxicity studies with the registered substance itself confirms the urinary excretion of the substance.

In the OECD 422 study with an analogous substance (read-across source substance Similar Substance 01) and in one of the oral toxicity studies with the registered substance itself also discoloured faeces were 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 with an analogous substance (read-across source substance Similar Substance 01) and in one of the acute oral studies with the registered substance itself. Bioaccumulation of the target and source substance after continuous exposure is not to be expected.

Due to its high hydrophilicity it is assumed that the target and source substance are 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 with an analogous substance (read-across source substance Similar Substance 01) and in one of the acute oral toxicity studies. 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)