Benzoic acid, 4-hydroxy-, C18-22-alkyl esters

Endpoint summary

Additional information

Basic toxicokinetics

There are no in vivo studies available in which the toxicokinetic behaviour of Benzoic acid, 4-hydroxy-, C18-22-alkyl esters (CAS 201305-16-0) 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, 2012), assessment of the toxicokinetic behaviour of the substance 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, 2012) and taking into account further available information on structurally related substances.

The substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is a UVCB substance consisting of esters of 4-hydroxybenzoic acid with long-chain (C18-C22) alcohols.

The substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is a waxy solid at room temperature and atmospheric pressure with a molecular weight range of 390.60-446.71 g/mol, drop point of 65-71 °C at ca. 1013 hPa (Frerichs, 2011), water solubility < 0.05 mg/L at 20 °C and pH=7-7.2 (Frischmann, 2011), estimated log Pow > 10 (Hopp, 2011) and estimated vapour pressure < 0.0001 Pa at 20°C (Nagel, 2011).

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, 2012).

Oral

In general, molecular weights below 500 g/mol and log Pow values between -1 and 4 are favourable for absorption via the gastrointestinal (GI) tract, provided that the substance is sufficiently water soluble (> 1 mg/L). Lipophilic compounds may be taken up by micellar solubilisation by bile salts, but this mechanism may be of particular importance for highly lipophilic compounds (log Pow > 4), in particular for those that are poorly soluble in water (≤ 1 mg/L) which would otherwise be poorly absorbed (Aungst and Chen, 1986; ECHA, 2012).

The physicochemical characteristics (high log Pow and low water solubility) of the substance and the molecular weight are in a range suggestive of low to moderate absorption from the gastrointestinal tract subsequent to oral ingestion. The rate of absorption may be further decreased by the fact that the substance is still a solid at body temperature. The available data on acute oral toxicity for structurally related esters provide limited evidence for absorption, since mortality or indication of systemic toxicity occurred (if at all) only at very high dose levels (Brown, 1979; Sado, 1973; Matthews et al., 1956). As indicated above, being a highly lipophilic and poorly water soluble substance, some degree of absorption may occur by micellar solubilisation.

The potential of a substance to be absorbed in the (GI) tract may be influenced by chemical changes taking place in GI fluids as a result of metabolism by GI flora, by enzymes released into the GI tract or by hydrolysis. These changes will alter the physicochemical characteristics of the substance and hence predictions based upon the physico-chemical characteristics of the parent substance may no longer apply (ECHA, 2012).

The in vitro hydrolysis of Benzoic acid, 4-hydroxy-, C18-22-alkyl esters was investigated in gastric and intestinal fluid simulants in a study conducted following the test method described in EFSA Note for Guidance for Food Contact Materials (Oßberger, 2013). The test material (dissolved in acetonitrile) was added to gastric and intestinal fluid simulants at final concentrations of 38.4 and 29.37 ppm, respectively, and incubated at 37 °C. Samples were taken after 0, 1, 2 and 4 h and appropriately treated for analysis by gas chromatography coupled with a mass spectrometer (GC/MS). For each sampling time point, the concentrations of the parent esters (test material components) Octadecyl-4-hydroxybenzoate, Icosyl-4-hydroxybenzoate, Docosyl-4-hydroxybenzoate and Tetracosyl-4-hydroxybenzoate as well as the hydrolysis product 4-Hydroxybenzoic acid were quantitatively determined.

A time-dependent decrease in parent ester concentrations was observed in both digestive fluid simulants, the decrease occurring more rapidly in gastric than in intestinal fluid simulant. Hydrolysis of the test material, expressed as the relative (overall) ester concentration after 1, 2 and 4 h incubation compared to the 0 h samples, was 22.8, 41.0 and 66.1% in gastric fluid simulant, and 9.0, 9.5 and 15.8% in intestinal fluid simulant, respectively.

The test material is thus anticipated to undergo rapid hydrolysis in the gastrointestinal tract after oral exposure, hydrolysis taking place more rapidly in the stomach than in the intestine.

The hydrolysis products 4-hydroxybenzoic acid and long-chain fatty alcohols are anticipated to be readily absorbed from the GI tract. Studies in dogs and humans indicate that 4-hydroxybenzoic acid is efficiently absorbed and rapidly metabolised (Jones et al., 1956). In general, monomeric long-chain alcohols, in particular cetyl (C16) and stearyl (C18) alcohols, are readily absorbed from the gastrointestinal tract (CIR, 1985, 1988). However, it appears that the rate of absorption is dependent on the chain length and/or species. Thus, whereas cetyl alcohol is well-absorbed in rats (63-96%), it is less well-absorbed in poultry (26%) (CIR, 1988). The absorption rate of stearyl alcohol in rats (ca. 57%) appears to be less than that for cetyl alcohol (CIR, 1985).

The oral absorption of the structurally related methyl-, ethyl-, propyl- and butyl esters of 4-hydroxybenzoic acid (parabens) has been studied in several studies and species including rabbits (Tasukamoto and Terade, 1960, 1962, 1964), dogs (Jones et al., 1956), rats (Derache and Gourdon, 1963), cats (Phillips et al., 1978) and humans (Jones et al., 1956; Sabalitschka and Neufeld-Crzelliter, 1954). Overall, the studies consistently show that these substances are readily absorbed in the gastrointestinal tract and rapidly metabolised, the first step being hydrolysis of the ester bond in particular in the liver and kidneys (Jones et al, 1956).

Taken together, the available information indicates that the oral absorption of the parent substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is low to moderate based on molecular weight and physicochemical properties. The substance is anticipated to undergo ester hydrolysis in the GI tract, primarily in the stomach and to a lesser extent in the intestine. The resulting hydrolysis products 4-hydroxybenzoic acid and long-chain fatty alcohols are readily absorbed and rapidly metabolised. A fraction of substance may be absorbed unchanged by micellar solubilisation and be hydrolysed by endogeneous esterases, e.g. in the liver.

Inhalation

The substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is a waxy solid with a very low vapour pressure 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, 2012). Lipophilic compounds with a log Pow > 4, that are poorly soluble in water like Benzoic acid, 4-hydroxy-, C18-22-alkyl esters can be taken up by micellar solubilisation.

Overall, a systemic bioavailability of Benzoic acid, 4-hydroxy-, C18-22-alkyl esters in humans is considered possible after inhalation of aerosols with aerodynamic diameters below 15 μm.

Dermal

The dermal uptake is lower for solids than for liquids, since solids need to dissolve into the surface moisture of the skin before uptake can begin. Molecular weights below 100 g/mol favour dermal uptake, while for those above 500 g/mol the molecule may be too large. Dermal uptake is anticipated to be low, if the water solubility is < 1 mg/L. Log Pow values above 6 reduce the uptake into the stratum corneum and decrease the rate of transfer from the stratum corneum to the epidermis, thus limiting dermal absorption (ECHA, 2012).

The physicochemical characteristics (log Pow > 10 and water solubility < 0.05 mg/L) of the substance and the molecular weight (390.60-446.71 g/mol) are in a range suggestive of low absorption through the skin.

The dermal permeability coefficient (Kp) can be calculated from log Pow and molecular weight (MW) applying the following equation described in US EPA (2004):

log(Kp) = -2.80 + 0.66 log Pow – 0.0056 MW

The Kp is thus estimated to be 19.88-40.98 cm/h. Considering the water solubility (0.00005 mg/cm³), the dermal flux is estimated to be ca. 9.94E-04 to 2.05E-03 mg/cm²/h.

If a substance shows skin irritating or corrosive properties, damage to the skin surface may enhance penetration. If the substance has been identified as a skin sensitizer then some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2012).

Based on read-across from the available experimental data for structurally related substances, the substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is considered to be not skin irritating and not skin sensitising. The data thus suggest a low level of dermal uptake.

In conclusion, based on molecular weight and physicochemical and toxicological properties, the dermal absorption rate of Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is anticipated to be low.

The available information on the derma absorption potential of structurally related (short-chain) esters of 4-hydroxybenzoic acid (parabens) indicates that these substances (i.e. methyl- and butylparaben) readily penetrate the stratum corneum. Hydrolysis of parabens occurs in the skin by means of carboxylesterases present in the keratinocytes. Thus, much of the substance is dermally absorbed and systemically bioavailable as the hydrolysis products (i.e. 4-hydroxybenzoic acid and the corresponding alcohol) rather than as the unchanged ester (Soni et al., 2005; CIR, 2008).

In conclusion, based on molecular weight and physicochemical and toxicological properties, the dermal absorption rate of Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is anticipated to be low. Hydrolysis of the parent compound into 4-hydroxybenzoic acid and the corresponding long-chain alcohols may occur by means of carboxylesterase activity in the keratinocytes. While the dermal absorption of 4-hydroxybenzoic acid is anticipated to be high, the absorption rate and bioavailability of long-chain alcohols is expected to be lower than that of short-chain alcohols.

Distribution and accumulation

Distribution of a compound within the body depends on the physicochemical properties of the substance; especially the molecular weight, the lipophilic character and the water solubility. 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, 2012).

As discussed under oral absorption, the substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is anticipated to undergo enzymatic hydrolysis in the gastrointestinal tract prior to absorption. A fraction of material may be absorbed unchanged by micellar solubilisation and is likely to be hydrolysed by endogeneous esterases. Therefore, assessment of distribution and accumulation of the hydrolysis products is considered more relevant than that of the parent compound.

Studies investigating the toxicokinetic behaviour of 4-hydroxybenzoic acid and short-chain parabens consistently show that 4-hydroxybenzoic acid is rapidly metabolised (conjugated) and its conjugates excreted via the urine (Soni et al., 2005; CIR, 2008). It follows, that absorbed 4-hydroxybenzoic acid is systemically distributed through blood circulation and high concentrations of unchanged and conjugated compound are anticipated in the liver and kidneys. In dogs given free 4-hydroxybenzoic acid and its methyl-, ethyl-, propyl- and butyl esters by intravenous injection, high concentrations of 4-hydroyxbenzoic acid (and/or esters and metabolites) were found in plasma, liver and kidney, and appreciable concentrations of free acid and esters were found in the brain, spleen and pancreas (Jones et al., 1956).

Data available from chronic administration studies in dogs indicate that 4-hydroxybenzoic acid and parabens do not accumulate in the body, the rate of urinary excretion reaching ≥ 96% of the administered dose per day (Jones et al., 1956; Matthews et al., 1956).

As described under Metabolism, alkyl alcohols are enzymatically oxidised to the corresponding carboxyl acids, a reaction taking place after absorption. Thus, oxidation of absorbed C18-C22 alcohols results in the corresponding fatty acids. Fatty acids are distributed all over the organism and can be absorbed by different tissues. Proposed uptake mechanisms range from passive diffusion to facilitated diffusion or a combination of both (Abumrad et al., 1984; Harris et al., 1980). Fatty acids can be stored as triglycerides (98% occurring in adipose tissue depots), be incorporated into cell membranes or they are oxidised via the β-oxidation and tricarboxylic acid cycle pathways of catabolism (Masoro, 1977; CIR 1987, 2001; IOM, 2005). Thus, stored fatty acids underlie a continuous turnover as they are permanently metabolised and excreted. Bioaccumulation of fatty acids takes place, if their intake exceeds the caloric requirements of the organism.

In conclusion, the substance Benzoic acid, 4-hydroxy-, C18-22-alkyl esters is anticipated to undergo ester hydrolysis prior to or following absorption, thus unlikely being available for systemic distribution and accumulation, e.g. in adipose tissue. The available information indicate that the hydrolysis product 4-hydroxybenzoic acid is readily distributed via blood circulation, rapidly metabolised and excreted mainly in the urine without evidence for accumulation even after chronic exposure. Long-chain fatty alcohols resulting from ester hydrolysis are anticipated to be enzymatically oxidised to the corresponding fatty acids and metabolised accordingly.

Metabolism

4-hydroxybenzoic acid and short-chain parabens

Several studies with free short-chain parabens conducted in different animal species including humans consistently show that parabens are hydrolysed to 4-hydroxybenzoic acid and conjugated with glycine, glucuronic acid and/or sulphate by means of the corresponding Phase II metabolism enzymes (Soni et al., 2005; CIR, 2008). Both the free and conjugated forms are sufficiently water soluble to be rapidly excreted in the urine.

Long-chain alcohols

In general, the metabolism of primary alkyl alcohols involves rapid oxidation to the corresponding transient intermediate aldehydes and further to the corresponding carboxylic acids. Thus, long-chain alcohols are oxidised to long-chain fatty acids. The reactions are catalysed by endogenous alcohol and aldehyde dehydrogenases. The alcohol oxidation rate increases with increasing chain length. Cytosolic NAD+/NADH-dependent aldehyde dehydrogenase is the most active enzyme for aldehyde oxidation to carboxylic acid, and its activity increases with increasing relative molecular mass of the substrate. Alcohol oxidation to the corresponding carboxylic acid occurs already following absorption from the intestine to the lymph. This has been experimentally shown in thoracic duct cannulated rats given radiolabelled cetyl or stearyl alcohol, in which the absorbed radioactive material appeared in the lymph. Of this, a high proportion was found in lymph triglycerides and phospholipids, thus indicating rapid oxidation to carboxylic acid, esterification with glycerol and lymphatic transport in chylomicrons (CIR, 1985, 1988; Lehninger, 1998; Stryer, 1996; WHO, 1999).

Aliphatic saturated carboxylic acids are primarily metabolised in the fatty acid β-oxidation pathway and the tricarboxylic acid cycle. The β-oxidation of fatty acids takes place in most vertebrate tissues using an enzyme complex for a series of oxidation and hydration reactions which result in the cleavage of acetate groups as acetyl CoA. The alkyl chain length is reduced by 2 carbon atoms in each β-oxidation cycle, thus releasing acetic acid. Another carboxyl group remains on the shortened alkyl chain for further β-oxidation.

Excretion

As indicated above, 4-hydroxybenzoic acid and its conjugates are rapidly and extensively excreted in the urine. Excretion in the bile occurs at a much lesser extent (Kiwada et al., 1979; CIR, 2008).

Since the hydrolysis products C18-C22 alcohols are anticipated to be oxidised to the corresponding fatty acids, the excretion routes of this substance class are considered. In general, fatty acids are entirely catabolised by oxidative processes resulting in carbon dioxide and water as the principal excretion products. Small amounts of ketone bodies arising from the oxidation of fatty acids are excreted via the urine (IOM, 2005).

 

A detailed reference list is provided in the technical dossier (see IUCLID, section 13) and within the CSR.