Hexasodium 4,4'-[vinylenebis[(3-sulphonato-4,1-phenylene)imino[6-morpholino-1,3,5-triazine-4,2-diyl]imino]]bis[5-hydroxy-6-(phenylazo)naphthalene-2,7-disulphonate]

Endpoint summary

Additional information

1.Toxicokinetic analysis of 4,4'-[1,2-ethenediylbis[(3-sulfo-4,1-phenylene)imino[6-(4-morpholinyl)-1,3,5-triazine-4,2-diyl]imino]]bis[5-hydroxy-6-(2-phenyldiazenyl)-2,7-naphthalenedisulfonic acid] hexasodium salt (CAS 17791-81-0)

The test substance is a black/brown powder at room temperature with a molecular weight of 1673.437 g/mol and a density of 1374 kg/cm3 at 20°C. The test substance has no melting temperature between -20°C and decomposition of the test item, starting at about 270°. Due to this melting point, the vapour pressure cannot be determined with current methods but is considered to be very low. The substance is easily soluble in water as indicated by the measured water solubility value of 120 g/L at 20°C. The experimentally determined partition coefficient between octanol and water (log Kow) value of -6.5 is very low.

1.1.Absorption:

Oral route:

Following oral administration, the likelihood of systemic absorption through the walls of the intestinal tract depends on several physicochemical substance properties. In order to obtain a conclusive judgement of a substance’s potential to be able to reach the systemic circulation, important physicochemical factors such as molecular weight, water solubility and the log Kow need to be considered. According to ECHA Guidance Document R.7c, the smaller the molecule the more easily it through the walls of the gastrointestinal tract (GI). Furthermore, ionisable groups limited passive absorption across biological membranes. Since the molecular weight of the test substance is 1673.437 g/mol and ionisable groups (e.g. sulphonate, hydroxyl groups) are present within the structure of the molecule, an uptake of the test substance into the systemic circulation via gastro-intestinal (GI) tract is limited. However, small amounts of such substances may be transported into epithelial cells by pinocytosis or persorption (passage through gaps in membranes left when the tips of villi are sloughed off). In addition, the very low log Kow indicates that the substance is very hydrophilic and the high water solubility facilitates the absorption of such a substance due to its ability to dissolve into the GI fluids and hence make contact with the mucosal surface. Thus, passive diffusion of the substance is possible due to its hydrophilic nature, but is limited by the rate at which the substance partitions out of the gastrointestinal fluid. With regard to the toxicological data, no signs of systemic toxicity were observed in the oral acute toxicity key study. Referring to the OECD 422 study, there are indications that the test substance is poorly absorbed after oral exposure. The kidney displayed red discoloration in the highest test group of 4 out of 10 males and all females. In addition, histopathological effects were found in the spleen at the highest dose group in males. The results also show that high concentrations are required to resorb the substance after oral exposure.

Inhalation route:

The test substance has a low volatility potential due to its very low vapour pressure. Thus, inhalation as a vapour to a high extend is unlikely. However, absorption via inhalation is possible as absorption following ingestion did also occur. Due to the high water solubility, dusts would readily dissolve into the mucus lining the respiratory tract. Uptake of high amounts directly across the respiratory tract epithelium is not expected as the substance exhibits a high molecular weight and high hydrophilicity.

Dermal route:

To assess the potential of a substance to cross the skin, basic physicochemical properties of the substance, i.e. molecular weight and lipophilicity (log Kow), should be taken into account. In general, dermal absorption of the test substance is anticipated to be moderate to high if water solubility is between 100 – 10,000 mg/L. However, test substances with log Kow values < -1 indicate poor lipophilicity and thus skin penetration is unlikely. Based on the high molecular weight (1673.437 g/mol) of the test substance and its very low log Kow (-6.5) dermal absorption is likely to be low. This assumption is supported by the toxicological data achieved by an acute dermal toxicity (read across) and a skin sensitization study. Both studies did not reveal that relevant amounts were absorbed into the systemic circulation as no systemic effects were observed. In addition, the test substance is not considered to be a skin irritant and thus does not enhance skin penetration.

1.2.Distribution

As mentioned above, the physicochemical properties and toxicological data revealed that small amounts of the test substance can become systemically available following oral exposure. Once absorbed, the distribution of the test substance via blood stream can be assumed. Due to its high water solubility and very low Kow value distribution in fatty issues is unlikely. In general, the transport efficiency to body tissues is limited by the rate at which the test substances cross cell membranes. For instance, access of highly water soluble substances to the central nervous system (CNS) or testes is likely to be restricted by the blood-brain and blood-testes barriers (Rozman and Klaasen, 1996). The spleen was identified as potential target organ in an OECD 422 study.

1.3.Metabolism

Biotransformation of a substance aimed to increase the hydrophilicity of lipophilic substances by Phase I (functionalization) and Phase II (conjugation) enzymes. Due to the high water solubility of the test substance, metabolic conversion is unlikely. In addition, based on the results of the Ames test, HPRT test and MNT in vitro it can be assumed that the test substance is not enzymatically activated (toxified) during the metabolism as the parent compound showed no higher toxicity compared to the metabolic activated substance.

1.4.Excretion

Excretion can occur via the urine especially for small (below 300 g/mol) and water-soluble substance and/or via biliary excretion predominately for larger molecules. Based on the high water solubility and the high molecular weight of the test substance, excretion might occur via urine and/or feaces. The assumption that test substance is excreted via feaces is supported by the fact that discoloured feaces were observed in the OECD 422 study.

2.Summary

Based on the physicochemical properties particularly water solubility and log Kow, absorption via the gastrointestinal tract is likely but limited by the high molecular weight of the test substance. Uptake of relevant amounts following dermal exposure most likely does not occur due to its high molecular weight and its very low log Kow. Based on its low vapour pressure it is unlikely that the test substance will become systemically available after inhalation. After absorption, the test substance will circulate within the blood stream. Bioaccumulation is not to be expected based on the high water solubility and low partitioning coefficient. Most likely, enzymatic activation does not occur as the parent compound was not more toxic than the metabolic activated test substance. Fast excretion via urine and/or faeces is assumed due to the high water solubility and the molecular weight of the test substance. The observation of coloured feaces during the OECD 422 study confirmed excretion of the test substance via feaces and in addition the assumption that the test substance is poorly absorbed.

3. References

ECHA (2014), Guidance on information requirements and chemical safety assessment, Chapter R.7c: Endpoint specific guidance.

Rozman K.K., Klaassen C.D. (1996) Absorption, Distribution, and Excretion of Toxicants. In Klaassen C.D. (ed.) Cassarett and Doull's Toxicology: The Basic Science of Poisons. McGraw-Hill, New York.