Bis(16-methylheptadecyl) malate

Endpoint summary

Additional information

Basic toxicokinetics

There were no studies available in which the toxicokinetic behaviour of Diisooctadecyl malate (CAS 67763-18-2) has been investigated.

Therefore, in accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) No 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2014c), assessment of the toxicokinetic behaviour of the substance Diisooctadecyl malate (CAS 67763-18-2) 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, 2014c) and taking into account further available information on structurally similar substances and hydrolysis products.

The substance Diisooctadecyl malate (CAS 67763-18-2) is an organic liquid with a molecular weight of ca. 639.06 g/mol. The measured water solubility was 9.8 µg/L at 20 °C (Schwarzkopf, 2015). The log Pow was calculated to be >10 for the main constituents (Birkhofer, 2014, KOWWIN calculation, refer to IUCLID section 4.7).

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, 2014c).

Oral

The smaller the molecule, the more easily it will be taken up. In general, molecular weights below 500 are favourable for oral absorption (ECHA, 2014c). As the molecular weight of Diisooctadecyl malate is 639.06 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 Diisooctadecyl malate with a log Pow >10 and a water solubility of 9.8 µg/L.

In an acute oral toxicity study with Diisooctadecyl malate (CAS 67763-18-2) performed equivalent or similar to OECD TG 401 (limit test), a LD50 value of >5000 mg/kg bw was derived. In another acute oral toxicity study with the read across substance Di C12-13 Alkyl Malate (CAS 149144-85-4) performed according to OECD TG 401 (limit test), a LD50 value of >5000 mg/kg bw was derived.

Dicarboxylic acid esters are expected to have the same metabolic fate as aliphatic acid esters. Aliphatic acid esters are rapidly hydrolysed to the corresponding alcohol and fatty acid by esterases (Fukami and Yokoi, 2012; Lehninger, 1970). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different locations in the organism: after oral ingestion, diesters undergo stepwise 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, all dicarboxylic esters used within this analogue approach undergo stepwise hydrolysis of the ester bonds by gastrointestinal enzymes (Lehninger, 1970; Mattson and Volpenhein, 1972). The esterases catalysing the reaction are present in most tissues and organs, with particularly high concentrations in the GI tract and the liver (Fukami and Yokoi, 2012). The respective alcohol as well as the dicarboxylic acid is formed. The physico-chemical characteristics of the cleavage 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, 2012). However, also for both cleavage products, it is anticipated that they are absorbed in the gastro-intestinal tract. The alcohol components cover a wide range of carbon chain length (C6-C17, MW: 130.23 g/mol for 2-ethylhexanol to 270.49 g/mol for isostearyl alcohol). In case of alcohols with long carbon chains (e.g. C17) and thus relatively low water solubility absorption will take place by micellar solubilisation (Ramirez et al., 2001). Small and water soluble cleavage products will dissolve into the gastrointestinal fluids (ECHA, 2012). Furthermore, cleavage products with high water solubility like malic acid do not have the potential to accumulate in adipose tissue due to their low log Pow and are thus widely distributed within the body and rapidly eliminated via renal excretion.

Overall, a systemic bioavailability of Diisooctadecyl malate and/or the respective hydrolysis products in humans is considered possible but limited after oral uptake of the substance due to its high molecular weight.

Dermal

The smaller the molecule, the more easily it may be taken up. In general, a molecular weight below 100 favours dermal absorption, above 500 the molecule may be too large (ECHA, 2014c). As the molecular weight of Diisooctadecyl malate is ca. 639.06 g/mol, dermal absorption of the molecule is not likely.

If the substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration (ECHA, 2014c). As Diisooctadecyl malate is not skin irritating in humans, enhanced penetration of the substance due to local skin damage can be excluded.

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, 2014c). With a log Pow >10 and a water solubility <1 mg/L, dermal uptake of Diisooctadecyl malate is likely to be low.

In an acute dermal toxicity study with the read across substance Di C12-13 Alkyl Malate (CAS 149144-85-4) performed according to EU Method B.3, a LD50 value of >2000 mg/kg bw was derived. In addition, a subacute repeated dose toxicity study via the dermal route was performed according to EU Method B.9 with Di C12-13 Alkyl Malate (CAS 149144-85-4) resulting in a NOAEL greater than 1000 mg/kg bw/day.

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).

Inhalation

Diisooctadecyl malate has a predicted vapour pressure below 0.0001 Pa (Szymoszek, 2015, QSAR calculation, SPARC v4.6.), thus being of 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, 2014c). Lipophilic compounds with a log Pow >4, that are poorly soluble in water (1 mg/L or less) like Diisooctadecyl malate can be taken up by micellar solubilisation.

Overall, a systemic bioavailability of Diisooctadecyl malate in humans is considered to be low.

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 Diisooctadecyl malate may have the potential to accumulate in adipose tissue (ECHA, 2014c).

Absorption is a prerequisite for accumulation within the body. Due to its MW and high log Pow, absorption is expected to be minimal for BisDiisooctadecyl malate, therefore accumulation is not favoured as well. In case of esterase-catalysed hydrolysis, the hydrolysis products isostearyl alcohol and malic acid are produced. Both isostearyl alcohol and malic acid 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. Therefore, tissue accumulation is not expected.

The cleavage product of Bis(2-ethylhexyl) adipate (CAS 103-23-1), 2-ethylhexanol is known to be metabolized and excreted well, thus no accumulation is expected (Deisinger, 1994). The second hydrolysis product, the fatty acid, 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 metabolized and excreted. Bioaccumulation of fatty acids only takes place, if their intake exceeds the caloric requirements of the organism. The available information indicates that dimeric fatty acids are poorly absorbed and that the absorbed fraction follows the same pattern of metabolism and excretion as the monomeric acids. Thus, no significant bioaccumulation in adipose tissue is expected.

Overall, the available information indicates that no significant bioaccumulation of the parent substance in adipose tissue is anticipated.

Distribution

Distribution within the body through the circulatory system depends 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, 2014c).

Distribution of the parent substance is not expected as only very limited absorption will occur. Only the potential hydrolysis products of BisDiisooctadecyl malate might be distributed within the body.

 

Metabolism

Dicarboxylic acid esters are expected to be rapidly hydrolysed to the corresponding alcohol and fatty acid by esterases (Fukami and Yokoi, 2012; Lehninger, 1970). Alcohol metabolism proceeds by oxidation to the corresponding carboxylic acids, followed by a stepwise elimination of C2-units in the mitochondrial β-oxidation process (OECD SIDS, 2006). Depending on the carbon chain length dicarboxylic acids are either predominantly excreted unchanged via urine or metabolised via peroxisomal and mitochondrial β-oxidation (Passi et al., 1983). The products of β-oxidation of odd-chain dicarboxylic acids are acetyl-CoA and malonyl-CoA, which cannot be oxidized further, are used in lipogenesis. Moreover 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).

This hypothesis is supported by experimental toxicokinetics data from the structurally analogue substance Bis(2-ethylhexyl) adipate (DEHA, CAS 103-23-1). In this study the elimination, distribution and metabolism of DEHA were assessed in rats according to a protocol similar to OECD Guideline 417 (Takahashi et al., 1981). Adipic acid was found as main metabolite in urine in a short time and its excretion reached 20-30% of the administered dose within 6 h. In blood it was found at 1% and in liver at 2-3%; mono-(2-ethylhexyl) adipate (MEHA) was the second metabolite found, but to a very low extent. Thus, cleavage of parent substance was shown in vivo within 6 hours into adipic acid (20-30% of administered dose in urine, 1% of administered dose in blood, 2-3% of administered dose in liver) and MEHA to a lower extent. From these results, it is clear that orally ingested DEHA is rapidly hydrolyzed to MEHA and adipic acid which is the main intermediate metabolite. In addition, DEHA was hydrolyzed to MEHA and adipic acid in vitro by tissue preparations from liver, pancreas and small intestine. When testing MEHA, the monoester was more rapidly hydrolyzed to adipic acid than DEHA by these preparations, and the intestinal preparation was the most active one among them (Takahashi et al., 1981).

Overall, the part of Diisooctadecyl malate that has become systemically available, may be hydrolysed and the hydrolysis products are metabolized by beta oxidation and/or glucuronidation. However, due to its high molecular weight, absorption of the parent substance is likely to be limited and thus, no extensive metabolism is expected but rather direct elimination.

 

Excretion

The main route of excretion of Diisooctadecyl malate is expected to be excretion of unabsorbed substance with the faeces. The second route of excretion is expected to be by expired air as CO2 after metabolic degradation (beta-oxidation). The potential hydrolysis products might also be excreted via the urine, unchanged or metabolised and exhaled (Deisinger, 1994; Hsieh and Perkins, 1976).

The metabolism of 2-ethylhexanol was studied in rats (Deisinger, 1994). Upon oral application, 2-ethylhexanol was absorbed effectively by the gastrointestinal tract. The major portion of the dose was excreted within 24 h, primarily in the urine. Smaller amounts of the dose were excreted in the faeces primarily within 24 h. A mean of 11% of the dose was recovered as 14CO2, but only a fraction of a percent of the dose was recovered from the breath as [14C] volatile organics. Upon dermal application, only a small portion of the dose was absorbed, which was eliminated primarily in the urine, with smaller amounts eliminated in the faeces, and as 14CO2, in the breath.

Moreover, some information on the possible hydrolysis product of Diisooctadecyl malate, malic acid, is available. Male Albino Wistar rats were administered radio–labelled malic acid in an aqueous solution by gavage or by intraperitoneal injection. Most of the radioactivity was excreted with exhaled air as carbon dioxide, after 24 hours 91.6 % and 83.4 % of orally and intraperitoneally administered substance. After oral administration 3.1 % were found in the urine and 0.6 % in the faeces, after intraperitoneal administration 8.8 % were found in the urine and 1.4 % in the faeces. Malic acid is an intermediate in the physiologically tricarboxylic acid (Krebs) cycle and will be further oxidised to oxaloacetic acid and therefore, plays an essential role in carbohydrate metabolism (International Journal of Toxicology, 1991).