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Description of key information

Key value for chemical safety assessment

Additional information

ETH50 (molecular weight 537 daltons) shows a very low water solubility ( 0.03 µg/L at 21 °C) and a partition coefficient (logPow) > 5.6 indicating a rather high lipid solubility of the molecule. This range of factors suggests a low tendency to penetrate skin. This outcome was demonstrated in both the in vitro (rat and human skin) and in vivo (rat) percutaneous absorption studies indicating only about 0.1 % of dermally applied test item in a cream formulation would pass to the systemic circulation (see below). Additionally, the test item did not reach quantifiable concentrations in rat plasma after 13 weeks of daily gavage doses as high as 1000 mg/kg bw/day.

The kinetics of the small amount of ETH50 absorbed percutaneously, using 14C-test item, indicates an absence of accumulation in tissues. The test item applied to skin remains at or in the stratum corneum and there is a very limited movement to deeper layers of the skin.

The test item has a neglectable vapour pressure as indicated by the relatively high melting point (281.3 °C). Therefore, the exposure to vapor of the test item would not be expected under normal conditions.

ETH50 has been tested for acute oral and dermal toxicity in rats up to a dose level of 2000 mg/kg/day. No signs of systemic toxicity occurred. These animal data support the interpretation that the test item shows a diminished bioavailability at least under acute conditions. In the repeated dose toxicity studies (dermal/ oral), ETH50 did not lead to evident systemic toxicity, indicating an absence of toxicological potential and/or the absence of bioavailability via the given routes of exposure .

The substance ETH50 registered represents the form of pure (bulk) non-nano material.  As such, however, it is not usable for its intended use in cosmetic products. In order to transform it into suitable cosmetic ingredient, ETH50 has to be micronized in a water-mill. As a consequence of the extreme hydrophobicity of substance, surfactants need to be added to introduce it in the water phase. After milling, rheology modifiers prevent the nano-particles from agglomerating and stabilize the resulting preparation. Thus, the substance is marketed in a 50 % mixture together with different components; it is not available as pure material in nanoform. Therefore, the toxicokinetic studies available have been performed with the bulk (non-nano) form of ETH50 (relating to the substance registered) and/or a formulation containing micronized ETH50 (relating to the marketed mixture for the use in cosmetics).

The following relevant experimental data are available for ETH50:

Basic Toxicokinetics

Toxicokinetics (OECD TG 417):

In the chosen key study, the fate of [14C] labeled ETH50 bulk (non-nano form) was investigated in male rats after single oral administration. [14C] test item (particle size: d 0.5 = 6 μm) was administered to male rats at a nominal dose level of 100 mg/kg bw (RCC A89280, 2007). The excretion of radioactivity in urine and feces was measured in daily intervals up to 96 hours after administration. The concentration of radioactivity in blood, plasma, liver, kidneys, muscle, and fat was determined 96 hours after administration. The blood and plasma kinetics after oral administration was investigated. Additionally, the urinary and fecal metabolite pattern was established. After oral administration the test item was very poorly absorbed from the gastro intestinal tract into system circulation. The extent of absorption, calculated based on the urinary excretion, accounted for only 0.73 % of the administered dose. Almost the complete dose was excreted unabsorbed with the feces as unchanged parent, accounting for 97.2 % of dose within 48 hours after administration. The maximum concentration of radioactivity in blood and plasma was achieved 1 hour after administration, accounting for 2.463 and 4.359 μg test item equivalents/g, respectively. This plateau level remained constant until 8 hours post dosing. Thereafter the concentrations in blood and plasma decreased with a half-life (8-48 h) of about 13 hours. The AUC values (0-96 h) were calculated to be 65.2 and 114.3 μg·h/g for blood and plasma, respectively. The tissue residues, 96 hours after administration, were consequently very low. The highest concentration was found in abdominal fat accounting for 1.712 μg test item equivalents/g. All other selected tissues and organs revealed concentrations below 0.110 μg test item equivalents/g. The urinary metabolite pattern investigated revealed 7 metabolite fractions. However, the major fraction (U7) represented only 0.19 % of the dose. All other fractions were below 0.1 % of the dose. The fecal metabolite pattern consisted essentially of unchanged test item. In summary, after oral administration of [14C] test item only a very low amount of radioactivity was absorbed from the gastro intestinal tract and almost the complete dose was excreted unabsorbed as unchanged parent with the feces.

Toxicokinetics (OECD TG 417) - nano sized material:

The fate of [14C] labeled ETH50 (nano form) was investigated in male rats after single oral administration (RCC B23613, 2007).

The test item was administered to male rats at a nominal dose level of 100 mg/kg bw. The mean particle size of the test item was found to be about 86 nm. The excretion of radioactivity in urine and feces was measured in daily intervals up to 96 hours after administration. The concentration of radioactivity in blood, liver, kidneys, muscle, fat and whole carcass was determined 96 hours after administration. The blood kinetics after oral administration was investigated. After oral administration the test item was very poorly absorbed from the gastro intestinal tract into systemic circulation. The extent of absorption, calculated based on the urinary excretion, accounted for only 0.06 % of the administered dose. Almost the complete dose was excreted unabsorbed with the feces accounting for 93.34 % of dose within 48 hours after administration.

The maximum concentration of radioactivity in blood was achieved 1 hour after administration, accounting for 0.360 μg test item equivalents/g. This plateau level remained constant until 8 hours post dosing. Thereafter, the concentration in blood decreased with a terminal half-life of about 31 hours. The AUC value, being an index of bioavailability, was calculated to be 17.9 μg·h/g for blood. The tissue residues, 96 hours after administration, were consequently very low. The highest concentration was found in abdominal fat but like all other selected tissues and organs the concentrations were all below the LOQ.

In summary, after oral administration of ETH50 (nano form), only a very low amount of radioactivity was absorbed from the gastro intestinal tract and almost the complete dose was excreted unabsorbed with the feces.

Dermal absorption

Skin Absorption in vivo:

In the key study, i.e. an in vivo dermal penetration study acc. to OECD 427 and GLP, the dermal absorption of ETH50 (non-nano form), was determined in rats (RCC A22432, 2005). The formulated test item was dermally applied at a nominal dose level of 2 mg/cm² to a skin area of 10 cm². The selected dose level reflects the usual dermal exposure for sun creams. The exposure time of 6 hours, recommended in the OECD guideline, is based on the anticipated exposure period of a consumer. The dermal absorption of the test item during a 6 hour exposure period was determined. Furthermore, the amount remaining in/on the skin at the application site after washing was determined at three additional time points of 24, 48 and 72 hours after application of the test item in order to estimate the depletion of the dose associated with the application site. The association of the remaining test item in/on the skin at the application site was investigated by skin stripping in order to separate the stratum corneum from the epidermis. Urine and feces were collected up to 72 hours after administration. Residue levels in blood, plasma, liver, kidneys, GI tract and carcass were also measured up to 72 hours after application.

During 6 hours exposure to formulated test item, only 0.11 % of the dose was systemically absorbed. The penetration rate was calculated to be 0.3345 μg/cm²·h. The highest blood and plasma concentrations were found at the end of exposure (6 hours), accounting for 0.1272 and 0.2327 ppm test item equivalents, respectively. Thereafter, the concentration in blood and plasma decreased reaching the limit of quantification within 48 hours. The dermally absorbed test item and/or its radiolabeled metabolites were exclusively present in plasma. The highest residues in liver and kidneys were also found 6 hours after start of exposure, accounting for 0.1812 ppm in liver and 0.1051 ppm in kidneys. The depletion kinetics of the residues was very similar to that observed in blood. At the end of exposure period 90-92 % of the dose could be dislodged from the application site. After the washing procedure, 6 hours after start of exposure, 2.1-4.4 % of the dose remained in/on the treated skin area located almost exclusively in/on the stratum corneum. Less than 0.1 % of the dose was found in the lower skin layers (corium and subcutis). During the 3 days after exposure the remaining amount of test item in/on the treated skin after washing lead only to a very low increase of the systemic absorption The amount totally absorbed accounted for 0.15 % of the applied dose, indicating that the remaining amount of the test item in/on the skin was not available for dermal absorption.

The radioactivity removed, 6 hours after administration, was analyzed and shown to be unchanged test item, indicating the test item remained stable during the exposure period. The systemically absorbed test item was slowly excreted with the urine and the feces. Within 72 hours 0.05 % and 0.03 % of the dose were excreted with the urine and feces, respectively. Total recovery was 92-97% of the applied radioactivity in each subgroup.

In summary, ETH 50 (non-nano form) penetrated very slowly and to a very low extent through rat skin after dermal application. The systemic absorption was very low during the 6 hours exposure period. Although the test item penetrated to some extent into the stratum corneum, it did not lead to an significant increase of the systemic absorption during the observation period of 3 days.

Skin Absorption in vitro:

In the chosen in vitro key study for ETH50 (non-nano form) acc. to OECD 428, and GLP, the percutaneous penetration was determined using split-thickness skin membranes from rat and human skin (RCC A00112, 2005). The skin membranes were set up in flow-through diffusion cells, the formulated test item was applied onto the skin membranes and the perfusates collected at defined time intervals. One dose level of 2 mg test item/cm² was used reflecting a concentration of 10 % test item in the final formulation. The formulated [14C] test item was applied onto skin membranes of 200 μm thickness leading to an actual area concentration of 1912 μg/cm². For each species 7 replicates were used. The exposure to the test item was performed under non-occluded conditions over an exposure time of 24 hours. During the exposure period the receptor fluid (6 % (w/v) polyethoxyoleate (PEG 20 oleyl ether) dissolved in physiological saline (0.9 % NaCl w/v)) was collected in hourly intervals between 0-6 hours and thereafter in 2 hours intervals until the end of the experiments. At the end of the experiment the remaining [14C] test item was removed from the skin membranes by rinsing the skin membranes three times with a mild soap solution. The skin membranes were removed from the diffusion cell and consecutively stripped until the stratum corneum was removed from the skin membrane. After application of [14C] test item an average of only 0.12 % of the applied dose penetrated through the rat skin membrane within 24 hours. The mean flux was calculated to be 0.209 μg/cm²/h. At the end of exposure 71 % of the applied dose could be washed off from the skin membranes. After skin membrane rinse 21 % of the dose remained in/on the skin membrane and the major part of this remaining test item was located in the stratum corneum, i.e. 16 % of the applied dose was determined in tape strips. A minor part, i.e. 5 % of the applied dose, was found in the remaining skin membrane after tape stripping.

For human skin membranes the penetration of the test item resembles very closely to that observed in rat skin membrane. Within 24 hours of exposure only 0.10 % of the applied dose penetrated totally through human skin membranes. The mean flux was calculated to be 0.178 μg/cm²/h. Also for human skin membranes 73 % of the applied dose could be washed off 24 hours after start of exposure. In stratum corneum 15 % of the applied dose was found and only 0.18 % in the remaining skin membrane after tape stripping. The amount of test item in lower skin layers was significantly lower in human skin membranes as compared to rat skin membranes. Therefore the total absorption, based on the amount penetrated through the skin membrane (perfusate) and the amount measured in the remaining skin membrane layers after tape stripping, was 5.07 % and 0.28 % of the applied dose for rat and human skin membranes, respectively. Based on the flux values a human/rat ration of 1:1.2 was calculated for dermal penetration. In conclusion, ETH50 (non-nano form) applied to rat and human skin membranes, penetrated at an extremely low rate and to a very limited extent through the skin membranes. The penetration through rat split-thickness skin membranes was slightly higher than through human split-thickness skin membranes. Although, the test item entered the skin membrane after exposure, it did not penetrate through the membrane to a significant extent. The penetration into lower skin layers (below stratum corneum) was more pronounced in rat skin membrane.

In the chosen in vitro key study for ETH50 (nano form) acc. to OECD 428 and GLP, the percutaneous penetration was determined in vitro using split-thickness skin membranes from rat and human skin (RCC B23624, 2007).

The skin membranes were set up in flow-through diffusion cells, the formulated [14C] test item was applied onto the skin membranes and the perfusates collected at defined time intervals. One dose level of 2 mg test item/cm² was used. The mean particle size of the test item was found to be about 86 nm. The formulated [14C] test item was applied onto skin membranes of 200 μm thickness leading to an area concentration of 1971.7 μg/cm². For rat 7 replicates were used and 5 replicates for human. The exposure to the test item was performed under non-occluded conditions over an exposure time of 24 hours. During the exposure period the receptor fluid (6 % (w/v) polyethoxyoleate (PEG 20 oleyl ether) dissolved in physiological saline (0.9% NaCl (w/v)) was collected in hourly intervals between 0-6 hours and thereafter in 2 hours intervals until the end of the experiments. At the end of the experiment the remaining [14C] test item was removed from the skin membranes by rinsing the skin membranes three times with a mild soap solution and after that once with THF. The skin membranes were removed from the diffusion cell and consecutively stripped until the stratum corneum was removed from the skin membrane.

After application of [14C] test item an average of only 0.02 % of the applied dose penetrated through the rat skin membrane within 24 hours. The mean flux was not calculated because most of the values were below LQ, so an estimated flux was calculated by using the values above LQ and setting all values that were below LQ to the corresponding LQ; this estimated flux was about 0.052 μg/cm²/h. At the end of exposure 82.73 % of the applied dose could be washed off from the skin membranes. After skin membrane rinse 14.81 % of the dose remained in/on the skin membrane and the major part of this remaining test item was located in the stratum corneum, i.e. 13.45 % of the applied dose was determined in tape strips. A minor part, i.e. 1.36 % of the applied dose, was found in the remaining skin membrane after tape stripping.

For human skin membranes the penetration of the test item resembles very closely to that observed in rat skin membrane. Within 24 hours of exposure only 0.02 % of the applied dose penetrated totally through human skin membranes. The estimated flux calculated by using LQ values is 0.042 μg/cm²/h. Also for human skin membranes 94.49 % of the applied dose could be washed off 24 hours after start of exposure. In stratum corneum 3.73 % of the applied dose was found and only 0.04 % in the remaining skin membrane after tape stripping.

The amount of test item in lower skin layers was remarkably less in human skin membranes as compared to rat skin membranes. Therefore, the total absorption, based on the amount penetrated through the skin membrane (perfusate) and the amount measured in the remaining skin membrane layers after tape stripping, was 1.38 % and 0.06 % of the applied dose for rat and human skin membranes, respectively. In conclusion, ETH50 (nano-form) applied to rat and human skin membranes, penetrated at an extremely low rate and to a very limited extent through the skin membranes. The penetration through rat split-thickness skin membranes was slightly higher than through human split-thickness skin membranes. Although the test item entered the skin membrane after exposure, it did not penetrate through the membrane to a significant extent. The penetration into lower skin layers (below stratum corneum) was more pronounced in rat skin membrane.

In support, the percutaneous penetration of ETH50 (nano form) was determined in vitro using split-thickness skin membranes from pre-damaged human skin (RCC C08835, 2008).The skin membranes were set up in flow-through diffusion cells, the formulated test item was applied onto the skin membranes and the perfusates were collected at defined time intervals. One dose levels of 2 mg test item/cm² was used. Within 24 hours the portion of radiolabeled test item penetrating through pre-damaged human skin membranes accounted for 0.76 %. An amount of 101.14% of the applied dose could be removed by skin rinse and additionally 3.4 % by tape stripping. In the lower skin layers about 0.05 % of the dose was found. Together with the amount measured in the remaining skin membrane after tape stripping, the amount totally penetrated in/through pre-damaged human skin membrane accounted for 0.81 % of the applied dose. The flux at steady-state conditions was determined to be 0.281 μg/cm²/h through pre-damaged human skin membranes. In conclusion, ETH50 (nano form) penetrated at an extremely low rate and to a very limited extent through pre-damaged human skin membranes.

Overall, the present data on toxicokinetics do not indicate any potential for bioaccumulation of ETH50.