1,4-bis[(2-ethyl-6-methylphenyl)amino]anthraquinone

Endpoint summary

Description of key information

Taking into account physicochemical parameter and experimental data on all individual category members and the entire category as a whole, it is assumed that members of this category are not absorbed to a toxicologically significant level by all routes of exposure, oral, dermal and inhalation.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

General toxicokinetic considerations for systemic toxicity

 

Based on the available physicochemical data a qualitative toxicokinetic (TK) assumption is derived which is further specified concerning the experimental toxicological data of the considered endpoints.

 

The TK phase begins with exposure and results in a certain concentration of the ultimate toxicant at the target site (tissue dose). This concentration is dependent on the absorption, distribution, metabolism, and excretion (ADME) of the substance. ADME describes the uptake of a substance into the body and its lifecycle within the body, including excretion.

 

Absorption: how, how much, and how fast the substance enters the body.

 

Distribution: reversible transfer of substances between various parts of the organism, i.e. body fluids or tissues.

 

Metabolism: the enzymatic or non-enzymatic transformation of the substance of interest into a structurally different chemical (metabolite).

 

Excretion: the physical loss of the parent substance and/or its metabolite(s); the principal routes of excretion are via the urine, bile (faeces), and exhaled air.

 

Metabolism and excretion are the two components of elimination, which describe the loss of substance by the organism, either by physical departure or by chemical transformation.

 

Absorption:

The most imported physicochemical data (molecular weight, water solubility and logPow) determining the absorption of the six anthraquinone dyes combined for the scenario 6 Read-Across are all in the same range. The molecular weight of the six category members is between 418 and 535 g/mol. The water solubility is very low, with values below 0.02 mg/L, limit of detection (range < 0.02 to 0.0009 mg/mL). The log Kow of all category members is very high > 8 (range 8.2 to 10.7, experimentally determined).

 

Each of the considered parameter alone indicates a low bioavailability. Taken all parameters together a toxicological relevant absorption is not assumed.

  

Distribution:

In general, the smaller the molecule, the wider the distribution. Based on the molecular weights of the group members, the distribution of the group members is limited. This assumption is supported by the very low water solubility and high log Kow which are not favourable for active transport or passive diffusion through body membranes. Overall, it can be assumed that due to the low water solubility and high log Kow no absorption and no wide distribution within the body is assumed for all anthraquinone dyes.

  

Metabolism:

There are no experimental data available. In a QSAR evaluation (OECD Toolbox V 4.5) the predicted main phase I metabolic pathways are the oxidation of the alkyl side-chains and/or oxidation of the aromatic ring systems including an di-imine formation. A reduction of the benzoquinone to the hydroxybenzoquinone might also be possible; see Appendix 2 for further details on possible metabolites of Reinblau RLW as an example for the anthraquinone dyes.

 

However, metabolism is considered as not relevant, because no absorption is predicted. In case of a negligible absorption, no hydrolysis products are expected in toxicologically significant doses.

 

Excretion:

It is assumed that after oral ingestion the anthraquinone dyes are not absorbed and excreted via the faeces.

 

Overall, based on the physicochemical parameters of the anthraquinone dyes no relevant absorption, distribution or metabolism is expected and the anthraquinone dyes are assumed to be excreted via the faeces without absorption.

 

At the current time, there are no toxicokinetic studies available, but the toxicokinetic assumption is supported by the available toxicological studies which demonstrate all anthraquinone dyes are not toxic after acute dosing, the absence of coloration of internal organs in studies with repeated exposure to anthraquinone dyes and no coloration of the urine, the absence of adverse effects in all studies with repeated exposure and the absence of mutagenic potential in in vitro genotoxicity studies in the absence and presence of liver S9 metabolic activation mix.

 

The ECHA Guidance on Information Requirements and Chemical Safety Assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017) is followed to justify the assumed negligible absorption of all category members after oral, inhalation and dermal exposure. To support this appraisal, a proof-of-concept toxicokinetic study is planned within the dose range finding stage of an OECD 422 Combined repeated dose toxicity with reproduction/developmental toxicity screening study on Solvent Violet 36. At defined timepoints following final dosing, tail vein samples will be drawn; samples of appropriate organs for determination of ADME profile will be taken (e.g. blood, liver, kidney, brain and stomach) at subsequent TK animal necropsy. Following extraction with lipophilic solvent, analysis by LC/MS method for the test item (non-GLP, including pre-defined Limit Of Quantification) will be undertaken. It is expected that a positive result will be obtained from sites of local exposure (stomach), but not from other organs sampled.

 

Assessment of potential oral absorption

 

Taking into account predicted and experimental data on all individual category members and the entire category as a whole, it is assumed that members of this category are not orally absorbed and consequently no systemic toxicity is expected after oral dosing.

 

 

This deduction is in accordance to the ECHA Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance, Version 3.0, November 2017, Table R.7.12-1 (Interpretation of data regarding oral/GI absorption).

The absence of systemic toxicological toxicity is based on the physicochemical nature of the group members.

 

For oral/GI absorption generally the smaller the molecule the more easily it may be taken up. Molecular weights below 500 g/mol are favourable for absorption; molecular weights above 1000 g/mol do not favour absorption. The molecular weight of the six category members is between 418 and 535 g/mol. Based on the molecular weight, a moderate absorption is predicted, however additional parameters have to be considered.

 

For oral/GI absorption the water solubility is an important parameter: Water-soluble substances will readily dissolve into the gastrointestinal fluids. Absorption of very hydrophilic substances by passive diffusion may be limited by the rate at which the substance partitions out of the gastrointestinal fluid. However, if the molecular weight is low (less than 200 g/mol) the substance may pass through aqueous pores or be carried through the epithelial barrier by the bulk passage of water.

 

In case of the six category members, the water solubility is extremely poor in the low µg/L region and the molecular weight of all category members is clearly above 200 g/mol.

Therefore, the absorption by active or passive diffusion out of the GI tract is very limited and no or a toxicological not relevant very low absorption of the test substances is predicted.

 

Another important parameter for oral/GI absorption is the log Kow: Moderate log Kow values (between -1 and 4) are favourable for absorption by passive diffusion. Any lipophilic compound may be taken up by micellular solubilisation, but this mechanism may be 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.  The log Kow of all category members is >8 (range 8.2 and 10.7). Therefore, passive diffusion of the category members is unlikely. 

 

All category members are dyes. If absorption occurred, colouring of internal organs or urine would be the consequence, unless metabolism occurred. This observation was not reported in any study available for any anthraquinone dye. The absence of oral absorption and consequently no systemic toxicity is supported by all experimental in vivo data summarized further below.

 

Overall, based on the physicochemical parameters of the anthraquinone dyes no relevant absorption, distribution and metabolism is expected after oral exposure and the anthraquinone dyes are assumed to be excreted via the faeces without absorption.

 

Assessment of potential absorption after inhalation

 

Taking into account all available data, it is assumed that members of this category are not absorbed after inhalation and consequently no systemic toxicity is expected after exposure via inhalation.

 

This deduction is in accordance to the ECHA Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance, Version 3.0, June 2017, Table R.7.12-1 (Interpretation of data regarding respiratory absorption).

 

This assumption is based on the physicochemical nature of the group members. For respiratory absorption, the vapour pressure is a reliable predictor whether a substance may be available for inhalation as a vapour. As a general guide, highly volatile substances are those with a vapour pressure greater than 25∙ 10³ Pa (or a boiling point below 50°C). Substances with low volatility have a vapour pressure of less than 0.5 ∙ 10³ Pa (or a boiling point above 150 °C). The vapour pressure of the category members are in the range of 10^-11 to 10^-17 Pa. These extremely low vapour pressure values indicate that no toxicologically relevant vapour concentrations of any group member is assumed in the air.

 

The water solubility plays a minor role for non-volatile substances due to the extremely low exposure concentration. In general, vapours of very hydrophilic substances may be retained within the mucus. Low water solubility, like small particle size enhances penetration to the lower respiratory tract. For absorption of deposited material similar criteria as for GI absorption apply.

 

All category members are solids at room temperature. An important parameter for respiratory absorption of mist and/or dust is the log Kow: Moderate log Kow values (between -1 and 4) are favourable for absorption by passive diffusion. Any lipophilic compound may be taken up by micellular solubilisation, but this mechanism may be of particular importance for highly lipophilic compounds (log Kow >4), particularly those that are poorly soluble in water (1 mg/l or less) that would otherwise be poorly absorbed. The log Kow of all category members is >8 (range 8.2 and 10.7) and therefore passive diffusion of the category members is unlikely.

 

Valid inhalation toxicity studies are not available; invalid acute and repeated dose inhalation toxicity studies which were disregarded due to methodological deficiencies and insufficient reporting are summarized in Appendix 3.

 

Overall, based on the extremely low vapour pressure no toxicological relevant anthraquinone dye vapour concentration is assumed in the air and taking into account the low water solubility and high Log Kow toxicologically relevant absorption of mist or dust particles is not assumed.

 

Assessment of potential dermal absorption

 

Taking into account predicted and experimental data on all individual category members and the entire category as a whole, it is assumed that members of this category are not dermally absorbed and consequently no systemic toxicity is expected after dermal exposure.

 

This deduction is in accordance to the ECHA Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance, Version 3.0, June 2017, Table R.7.12-1 (Interpretation of data regarding dermal absorption).

 

The absence of systemic toxicological toxicity is based on the physicochemical nature of the group members. All category members are solids at room temperature. Generally, liquids and substances in solution are taken up more readily than dry particulates. Dry particulates will have to dissolve into the surface moisture of the skin before uptake can begin. The physical state of the category members does not support absorption.

 

For dermal absorption, a molecular weight less than 100 g/mol favour dermal uptake. Above 500 g/mol the molecule may be too large. The molecular weight of the six category members is between 418 and 535 g/mol and, consequently not favourable for absorption via the skin.

 

Additional parameters should be considered, e.g. the substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis. Therefore, if the water solubility is below 1 mg/l, dermal uptake is likely to be low. In case of the six category members water solubility is very low, with values below 0.02 mg/L (range < 0.02 to 0.0009 mg/mL). Therefore, the partition of the group members into the epidermis is restricted and the anthraquinone dyes are unlikely to penetrate through the skin.

 

Another important parameter for dermal absorption is the log Kow: For substances with log Kow values < 0, poor lipophilicity will limit penetration into the stratum corneum and hence limit dermal absorption. Values < –1 suggest that a substance is not likely to be sufficiently lipophilic to cross the stratum corneum, therefore dermal absorption is likely to be low. Log P values between 1 and 4 favour dermal absorption (values between 2 and 3 are optimal) particularly if water solubility is high. Above 4, the rate of penetration may be limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. Above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin and uptake into the stratum corneum itself may be slow. The log Kow of all category members is > 8 (range 8.2 and 10.7). Therefore, a slow and limited absorption is predicted.

 

In the acute dermal toxicity studies, no signs of toxicity were reported with LD₅₀ values > 2000 mg/kg bw up to > 5000 mg/kg bw. In addition, all category members are not irritating to skin or eyes.

 

Altogether, based on the physical state, molecular weight, the water solubility, and log Kow a toxicological relevant dermal absorption is not assumed. The experimental data of the acute dermal toxicity studies further support that toxicological relevant absorption of the category members through the skin is unlikely.

 

Overall conclusion on toxicokinetic:

Taking into account physicochemical parameter and experimental data on all individual category members and the entire category as a whole, it is assumed that members of this category are not absorbed to a toxicologically significant level by all routes of exposure, oral, dermal and inhalation.

To further strengthen this hypothesis, some quantitative toxicokinetic analysis will be included in the range-finder of the OECD TG 422 study that will be started on Solvent Violet 36. The dossiers will be updated as soon as the results become available.