Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester

Endpoint summary

Link to relevant study record(s)

Reference

Endpoint:

dermal absorption in vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study report with acceptable restrictions (no data on analytical purity of unlabelled test substance, non-GLP, limited documentation of methods and results)

Reason / purpose for cross-reference:

reference to same study

Principles of method if other than guideline:

An in vivo percutaneous absorption study was carried out with the substance using both control rats and rats treated topically with 2000 mg/kg bw/day for thirteen weeks. The percutaneous absorption of the substance in rats was measured over 4 days following a single topical administration of 14C-labelled test substance (2000 mg/kg bw).

GLP compliance:

no

Radiolabelling:

yes

Species:

rat

Strain:

Sprague-Dawley

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Details on the strain: Rat/crl COBS CD[SD] BR/Charles River, Lakeview, New Jersey
- Source: Charles River, Lakeview, New Jersey
- Age at study initiation: 49 days
- Weight at study initiation: males: 161.9 - 166.8 g; females: 149.2 - 150.5 g
- Housing: individually, in suspended, stainless steel cages, with wire mesh bottoms and fronts
- Diet: Purina Certified Lab Chow #5002 in pellet form, ad libitum
- Water: tap water, delivered by an automatic watering system, ad libitum

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 21
- Humidity (%): 50
- Photoperiod (hrs dark / hrs light): 12/12

Type of coverage:

occlusive

Vehicle:

unchanged (no vehicle)

Duration of exposure:

1. Pretreated rats: 13 weeks topical exposure (unlabelled test substance)
Treatment: 4 days (14C-labelled test substance)
2. Untreated rats: control rats from 13-week dermal study
Treatment: 4 days (14C-labelled test substance)

Doses:

2000 mg/kg bw

No. of animals per group:

5 animals/sex/dose

Control animals:

no

Details on study design:

APPLICATION OF DOSE:
A plastic ring with an inside area of 1.2 cm2 was attached securely to the back of each rat with adhesives. A single dose of the test substance was applied to completely cover the skin inside the ring (58 µL). The amount of test substance used was equivalent to the high dose used in the thirteen week study (on a µg/cm2 skin surface basis).

SITE PROTECTION / USE OF RESTRAINERS FOR PREVENTING INGESTION: yes
To prevent the rats from removing the test substance, a wire mesh cover was attached to the ring. Each rat was fitted with a carboard "Elizabethan" collar and placed in a separate metabolism cage for once-daily collection of urine and feces.

SAMPLE COLLECTION
- Collection of urine and faeces: once daily
- Terminal procedure: blood, tissues and organs were removed from the rats after sacrifice for determination of levels of radioactivity, as measured by liquid scintillation counting.
- Analysis of organs: liver, kidney. stomach, bladder, small intestine, and large intestine

Key result

Dose:

2000 mg/kg bw

Parameter:

percentage

Absorption:

10.6 - 11.6 %

Remarks on result:

other: 4 days

Remarks:

rats previously untreated

Key result

Dose:

2000 mg/kg bw

Parameter:

percentage

Absorption:

9.1 - 10.8 %

Remarks on result:

other: 4 days

Remarks:

rats pre-treated for 13 weeks at 2000 mg/kg bw/day with the test substance

Table 1: Percentage of the applied 14C-labelled test substance recovered in the body

 

	Male	Female
Control rats	11.6% (SD = 3.2)	10.6% (SD = 5.3)
Pre-treated rats	10.8% (SD = 1.6)	9.1% (SD = 0.6)

No significant differences were observed in comparing the percutaneous absorption of the test substance in male and female rats. Furthermore, no significant differences were observed in comparing the percutaneous absorption of the substance in control and pretreated rats. Thus, it appears that repeated treatment of the skin with the test substance at 2000 mg/kg bw/day for thirteen weeks had no effect on the percutaneous absorption of the substance in male and female rats.

 

The absorbed substance was observed to eliminate very slowly from the animals following diffusion through the stratum corneum. After four days, 52-63% of the substance absorbed by control rats and 81-87% absorbed by treated rats was still retained in the body. These results suggest that thirteen weeks of dermal treatment with the substance had an effect on the elimination of absorbed test substance from the animals.

Table 2: In vivo percutaneous absorption of 14C-labelled test substance (2000 mg/kg bw) in control rats

 

		Summed & applied dose recovered (mean & SD of 4 rats)
Sex	Days	Urine	Faeces	Tissues	Total
Male	1	0.19 (0.21)	0.02 (0.01)	-	0.21 (0.22)
	2	0.87 (0.75)	0.08 (0.12)	-	0.95 (0.82)
	3	2.68 (1.65)	0.36 (0.31)	-	3.04 (1.88)
	4	3.52 (1.76)	0.70 (0.25)	7.35 (1.50)	11.57 (3.18)
Female	1	0.04 (0.07)	0.04 (0.04)	-	0.08 (0.12)
	2	0.31 (0.47)	0.08 (0.08)	-	0.39 (0.55)
	3	3.75 (4.21)	0.25 (0.24)	-	4.00 (4.14)
	4	4.73 (4.61)	0.42 (0.23)	5.49 (1.00)	10.64 (5.33)

 

 

Table 3: In vivo percutaneous absorption of 14C-labelled test substance (2000 mg/kg bw) in pre-treated rats

 

		Summed & applied dose recovered (mean & SD of 4 rats)
Sex	Days	Urine	Faeces	Tissues	Total
Male	1	0.05 (0.07)	0.06 (0.03)	-	0.11 (0.08)
	2	0.27 (0.22)	0.14 (0.07)	-	0.41 (0.25)
	3	0.56 (0.40)	0.37 (0.16)	-	0.92 (0.42)
	4	0.73 (0.43)	0.64 (0.27)	9.42 (1.64)	10.79 (1.56)
Female	1	0.13 (0.13)	0.06 (0.03)	-	0.18 (0.13)
	2	0.70 (0.68)	0.14 (0.07)	-	0.85 (0.73)
	3	1.02 (0.86)	0.26 (0.13)	-	1.28 (0.98)
	4	1.34 (0.90)	0.35 (0.15)	7.43 (1.29)	9.13 (0.55)

 

Description of key information

There were no studies available in which the toxicokinetic behaviour of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester (EC 947-912-3) has been investigated. Therefore, assessment of the toxicokinetic behaviour is conducted to the extent that can be derived from the relevant available information (substance specific data on physico-chemical and toxicological properties, information on structurally similar substances and hydrolysis products). A systemic bioavailability of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester in humans is considered possible but limited after oral uptake of the substance due to the high molecular weight and low water solubility. The oral absorption of the hydrolysis products is considered to be high. The dermal absorption potential is predicted to be low to moderate. Under normal use and handling conditions, availability for respiratory absorption of the substance in form of vapours, gases, or mists is considered negligible. However, the substance may be available for respiratory absorption in the lung after inhalation of aerosols, if the substance is sprayed. No significant bioaccumulation of the parent substance Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester or its hydrolysis products in adipose tissue is anticipated. Distribution of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is not expected as only very limited absorption will occur. The hydrolysis products, however, are expected to be distributed widely within the body. The part of Fatty acids, C18-unsaturated,1,6 Hexanediol Diester that has become systemically available may be hydrolysed and the hydrolysis products are metabolized by β-oxidation and/or glucuronidation. The main route of excretion of the unabsorbed fraction of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is expected to be via the feces. The second route of excretion is expected to be by expired air as CO₂after metabolic degradation (β-oxidation). The potential hydrolysis products might also be excreted via the urine, unchanged or metabolized, and exhaled.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

There were no studies available in which the toxicokinetic behaviour of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester (EC 947-912-3) has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) No. 1907/2006 and with the Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017), assessment of the toxicokinetic behaviour of the substance Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is conducted to the extent that can be derived from the relevant available information. This comprises a qualitative assessment of the available substance specific data on physico-chemical and toxicological properties according to Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017) and taking into account further available information on structurally similar substances and hydrolysis products.

The substance Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is an organic yellow liquid with a molecular weight range of 643 to 647 g/mol and a vapour pressure of < 0.0001 Pa at 20°C. The measured water solubility is < 0.57 µg/L at 20°C and the log Pow > 10.

Absorption

Absorption is a function of the potential for a substance to diffuse across biological membranes. The most useful parameters providing information on this potential are the molecular weight, the octanol/water partition coefficient (log Pow) value and the water solubility. The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2017).

Oral

The smaller the molecule, the more easily it will be taken up. In general, molecular weights below 500 g/mol are favourable for oral absorption (ECHA, 2017). As the molecular weight range of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is 643 to 647 g/mol, absorption of the molecule in the gastrointestinal tract is considered to be low.

If absorption occurs, the favourable mechanism will be absorption by micellar solubilisation, as this mechanism is of importance for highly lipophilic substances (log Pow > 4), which are poorly soluble in water (1 mg/L or less); like Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester with a log Pow > 10 and a water solubility of < 0.57 µg/L.

Esters of diols and carboxylic acids are expected to have the same metabolic fate as aliphatic monoesters of alcohol and carboxylic acid. Esters of carboxylic acid and alcohols are hydrolysed to the corresponding alcohol and fatty acid by esterases (Fukami and Yokoi, 2012; Lehninger, 1998). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the organism. After oral ingestion, esters undergo enzymatic hydrolysis in the gastro-intestinal fluids. In contrast, substances which are absorbed through the pulmonary alveolar membrane or through the skin enter the systemic circulation directly before entering the liver where hydrolysis takes place. After oral ingestion, the esters undergo stepwise hydrolysis of the ester bonds by gastro-intestinal enzymes (Lehninger, 1998; Mattson and Volpenhein, 1972). The esterases catalysing the reaction are present in most tissues and organs, with particularly high concentrations in the gastro-intestinal tract and the liver (Fukami and Yokoi, 2012). The respective alcohol as well as the carboxylic acid are formed. The physico-chemical characteristics of the hydrolysis products (e.g. physical form, water solubility, molecular weight, log Pow, vapour pressure, etc.) are likely to be different from those of the parent substance, and hence the predictions based upon the physico-chemical characteristics of the parent substance do no longer apply (ECHA, 2017). For the hydrolysis products, it is anticipated that they are absorbed in the gastro-intestinal tract. Free fatty acids and alcohols are readily absorbed by the intestinal mucosa. Within the epithelial cells, fatty acids are (re-)esterified with glycerol to triglycerides. In general, short-chain or unsaturated fatty acids are more readily absorbed than long-chain, saturated fatty acids. As for fatty acids, the rate of absorption of alcohols is likely to increase with decreasing chain length (Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1962, 1964; OECD SIDS, 2006; Sieber, 1974). The absorption of alcohols with long carbon chains (e.g. C20) and thus relatively low water solubility will take place by micellar solubilisation (Ramirez et al., 2001). Small and water soluble hydrolysis products, like 1,6-Hexanediol, will dissolve into the gastro-intestinal fluids (ECHA, 2017).

Overall, a systemic bioavailability of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester in humans is considered possible but limited after oral uptake of the substance due to the high molecular weight and low water solubility. The oral absorption of the hydrolysis products is considered to be high.

Dermal

The smaller the molecule, the more easily it may be taken up. In general, a molecular weight below 100 g/mol favours dermal absorption, above 500 g/mol the molecule may be too large (ECHA, 2017). In contrast to the oral uptake, substances which are absorbed through the skin enter the systemic circulation directly before entering the liver where hydrolysis takes place. As the molecular weight of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester ranges from 643 to 647 g/mol, dermal absorption of the molecule is likely to be limited.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration. In addition, if the substance has been identified as a skin sensitiser then some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2017). No skin irritation potential was observed in an in vitro skin irritation study with Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester (Dako, 2019a, key). As QSAR predictions for Fatty acids, C18 -unsaturated, 1,6 Hexanediol Diester (Dr. Knoell Consult, 2017) revealed no alerts for protein binding indicative of skin sensitising properties and based on a weight of evidence approach on all available data for Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester and its source substances, it is concluded that Fatty acids, C18 -unsaturated, 1,6 Hexanediol Diester does not have a skin sensitising potential.

For substances with a log Pow above 4, the rate of dermal penetration is limited by the rate of transfer between the stratum corneum and the epidermis, but uptake into the stratum corneum will be high. For substances with a log Pow above 6, the rate of transfer between the stratum corneum and the epidermis will be slow and will limit absorption across the skin, and the uptake into the stratum corneum itself is also slow. The substance must be sufficiently soluble in water to partition from the stratum corneum into the epidermis (ECHA, 2017). Thus, with a log Pow > 10 and a water solubility < 0.57 µg/L, dermal uptake of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is likely to be low.

In support of this, data available for several fatty acids indicate that the skin penetration both in vivo (rat) and in vitro (rats and human) decreases with increasing chain length. Thus, after 24 h exposure about 0.14% and 0.04% of C16 and C18 soap solutions are absorbed through human epidermis applied in vitro at 217.95 µg C16/cm² and 230.77 µg C18/cm². At 22.27 µg C16/cm² and 24.53 µg C18/cm², about 0.3% of both C16 and C18 soap solutions is absorbed through rat skin after 6 h exposure in vivo (Howes, 1975). In addition, there is an in vivo percutaneous absorption study, carried out with the source substance Ditridecyl adipate (CAS 16958-92-2) using both control rats and rats treated topically with 2000 mg/kg bw/day for thirteen weeks. The percutaneous absorption of the substance in the untreated control rats was 10.6 - 11.6% and 9.1 - 10.8% in the pretreated animals. Thus, showing no differences between the control and the pretreated rats and no differences in comparing the percutaneous absorption of the test substance in male and female rats. The absorbed substance was, however, observed to eliminate very slowly from the animals following diffusion through the stratum corneum. After four days, 52 - 63% of the substance absorbed by control rats and 81 - 87% absorbed by treated rats was still retained in the body. These results suggest that thirteen weeks of dermal treatment with the substance had an effect on the elimination of test substance from the animals. As explained in the analogue justification, this result is considered to be valid also for the target substance. (Exxon, 1986a, key).

Overall, based on the available information and using a worst-case approach, the dermal absorption potential of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is predicted to be low to moderate (Key, Eccon, 1986).

Inhalation

Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester has a vapour pressure below 0.0001 Pa at 20 °C, i.e. a low volatility. Therefore, under normal use and handling conditions, inhalation exposure and thus availability for respiratory absorption of the substance in the form of vapours, gases, or mists is considered negligible. However, the substance may be available for respiratory absorption in the lung after inhalation of aerosols, if the substance is sprayed. In humans, particles with aerodynamic diameters below 100 μm have the potential to be inhaled. Particles with aerodynamic diameters below 50 μm may reach the thoracic region and those below 15 μm the alveolar region of the respiratory tract (ECHA, 2017). Lipophilic compounds with a log Pow > 4, that are poorly soluble in water (1 mg/L or less) like Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester can then be taken up by micellar solubilisation.

Accumulation

Highly lipophilic substances tend in general to concentrate in adipose tissue, and depending on the conditions of exposure may accumulate. Although there is no direct correlation between the lipophilicity of a substance and its biological half-life, it is generally the case that substances with high log Pow values have long biological half-lives. The high log Pow of > 10 implies that Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester may have the potential to accumulate in adipose tissue (ECHA, 2017).

Absorption is a prerequisite for accumulation within the body. Due to its molecular weight and high log Pow, absorption is expected to be minimal for Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester, therefore accumulation is not favoured as well. In case of esterase-catalysed hydrolysis, the hydrolysis products (Fatty acids C18 unsaturated) and 1,6 Hexandiol are produced. All hydrolysis products are expected to feed into the physiological process of fatty acid oxidation and tricarboxylic acid (Krebs) cycle, respectively and subsequent metabolic pathways, finally leading to expiration as CO2. Fatty acids can be stored as triglycerides in adipose tissue depots or be incorporated into cell membranes. At the same time, fatty acids are also required as a source of energy. Thus, stored fatty acids underlie a continuous turnover as they are permanently metabolised and excreted. Bioaccumulation of fatty acids only takes place, if their intake exceeds the caloric requirements of the organism. Thus, no significant bioaccumulation in adipose tissue is expected. In addition, the alcohol component is metabolised by β-oxidation and/or glucuronidatio to form more water-soluble molecules, which can be excreted via the urine (WHO, 1999; see also paragraph ‘Metabolism’ below).

Overall, the available information indicates that no significant bioaccumulation of the parent substance Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester or its hydrolysis products in adipose tissue is anticipated.

 

Distribution

Distribution within the body through the circulatory system depends primarily on the molecular weight, the lipophilic character and water solubility of a substance. In general, the smaller the molecule, the wider is the distribution. If the molecule is lipophilic, it is likely to distribute into cells and the intracellular concentration may be higher than extracellular concentration, particularly in fatty tissues (ECHA, 2017).

Distribution of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is not expected as only very limited absorption will occur. The hydrolysis products of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester are expected to be distributed widely within the body.

Metabolism

Esters of diols and carboxylic acids are expected to be hydrolysed to the corresponding alcohols and fatty acids by esterases (Fukami and Yokoi, 2012; Lehninger, 1970). Alcohol metabolism proceeds by oxidation to the corresponding carboxylic acids (i.e. hexanedioic acid), followed by a stepwise elimination of C2-units in the mitochondrial β-oxidation process (OECD SIDS, 2006). In addition, the alcohol component may also be conjugated to form a more water-soluble molecule and excreted via the urine (WHO, 1999). In an alternative pathway, the alcohol may be conjugated with e.g. glutathione and excreted directly, bypassing further metabolism steps. Depending on the carbon chain length dicarboxylic acids are either predominantly excreted unchanged via urine or metabolised via peroxisomal and mitochondrial β-oxidation like the monocarboxylic acids (Passi et al., 1983). Even-chain dicarboxylic acids produce acetyl-CoA and succinyl-CoA, which is a gluconeogenesis precursor (Grego and Mingrone, 1994). Further oxidation of the C2-untis (acetyl-CoA) via the citric acid cycle leads to the formation of H2O and CO2 (Lehninger, 1970; Stryer, 1994). In addition glucuronidation of only partially hydrolysed monoesters has been observed (Elcombe, 1986).

Overall, the part of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester that has become systemically available may be hydrolysed and the hydrolysis products are metabolised by β-oxidation and/or glucuronidation.

Excretion

The main route of excretion of Fatty acids, C18-unsaturated, 1,6 Hexanediol Diester is expected to be excretion of the unabsorbed fraction via the feces. The second route of excretion is expected to be by expired air as CO2 after metabolic degradation (β-oxidation). The potential hydrolysis products might also be excreted via the urine, unchanged or metabolised, and exhaled (Deisinger, 1994; Hsieh and Perkins, 1976).

In an in vivo percutaneous absorption study, carried out with the source substance Ditridecyl adipate (CAS 16958-92-2) using both control rats and rats treated topically with 2000 mg/kg bw/day for thirteen weeks. (Exxon, 1986a, key), the absorbed substance was observed to eliminate very slowly from the animals following diffusion through the stratum corneum. After four days, 52 - 63% of the substance absorbed by control rats and 81 - 87% absorbed by treated rats was still retained in the body. These results suggest that thirteen weeks of dermal treatment with the substance had an effect on the elimination of test substance from the animals which may be the result of one or a combination of the following factors: isotope dilution due to the presence of unlabelled test substance from previous dosings in the 13-week dermal study, saturation or alteration of the metabolic pathway of the substance in rats and / or a decrease in the excretion rate of the substance and/or its metabolites in the rats.

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