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1. Toxicokinetic analysis of [mu-[[3,3'-[(1-oxido-1,2-diazenediyl)bis[[2-(hydroxy-kappa-O)- 4,1-phenylene]-2,1-diazenediyl-kappa-N1]]bis[4-(hydroxy-kappa-O)-2,7-naphthalenedisulfonato]](8-)]]dicopper, tetra sodium ammonium salt (CAS 1713250-52-2) 

The test substance is a black/green fine powder at room temperature with a molecular weight range of 1080 – 1100 g/mol and a density of 1.813 g/cm3 at 20°C. The test substance has no melting temperature between -20°C and begin of decomposition not higher than 230°C. The substance is easily soluble in water as indicated by the measured water solubility value of 136 g/L at 20°C. The experimentally determined partition coefficient between octanol and water (log Kow) value of -5.3 and the vapour pressure less than 0.000001 hPa are very low. The soil adsorption coefficient (log Koc) was calculated to be 10.514 at 20°C based on the free acid.

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 in the range of 1080 – 1100 g/mol and ionisable groups (e.g. sulphonate) 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 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, rats were orally exposed with the test item in polyethylene glycol resulting in a LD50 value of 2900 mg/kg bw. Systemic toxicity such as sedation, dyspnea, ruffled fur, diarrhea, curved and lateral body positions were observed indicating that the substance was absorbed following oral intake. 

Inhalation route

The test substance has a low volatility potential due to its very low vapour pressure of 0.000001 hPa. 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 (1080 – 1100 g/mol) of the test substance and its very low log Kow (-5.3) dermal absorption is likely to be low. This assumption is supported by the toxicological data achieved by an acute dermal toxicity 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

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). A repeated dose study exists, which was performed with the test substance analogue analogue substance 1. In this study, liver, kidney and spleen were identified as potential target organ.

1.3. Metabolism

Two bacterial reverse mutations assays (Ames test) showed that the test substances induced frameshift mutations in the absence of a metabolic system. The effect was reduced but not abolished in the presence of a metabolic activation system. In addition, the test substance was not mutagenic to mammalian cells in vitro and did not show any genotoxic effect in vivo. These results indicated that metabolism of the test substance are unlikely. However, studies with the test substance analogue analogue substance 1 indicated an increased liver weight in the 28-day study which hint metabolism of the test substance. Furthermore, breaking of azo-bounds within the test substance analogue may result in the formation of benzene or naphthalene compounds. This compounds might be transformed by Phase I enzymes and Phase II enzyme to enhance the hydrophilicity and to facilitate the elimination. In the first step, the functionalization, the fractions might be hydroxylated which is catalyzed by Phase I enzymes. In the second step, the conjugation, the hydroxylated compound could be glucuronised by the glucuronosyltransferase.

1.4. Excretion

Excretion can occur via the urine especially for small (below 300 /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. 

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. According to the results of the test substance analogue, Phase I functionalization by hydroxylation and subsequently Phase II conjugation reactions (glucuronidation) may occur. Fast excretion via urine and/or faeces is assumed due to the high water solubility and the molecular weight of the test substance. 

3. References

ECHA (2008), 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.