Propanoic acid, 2-hydroxy-, C12-13-branched alkyl esters

Endpoint summary

Additional information

Basic toxicokinetics

There are no experimental studies available in which the toxicokinetic behaviour of Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters has been assessed.

In accordance with Annex VIII, Column 1, Item 8.8.1, of Regulation (EC) 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2008), 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 physicochemical and toxicological properties according to the relevant Guidance (ECHA, 2008) and taking into account available information on the analogue substances from which data was used for read-across to cover data gaps.

The substance Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters (UVCB) is a mono- and dilactate ester of branched alcohols with C12 and C13 chain lengths and has a molecular weight ranging from 258.40 – 330.46 g/mol. The substance is a colourless liquid (typical odour) at 20 °C with a melting point of < -45 °C at atmospheric pressure (Cassani, 2011), water solubility of 4 - 5 mg/L at 25 °C and pH 7 (Andriollo, 2011), determined log Pow of 5.7 at 25 °C (Andriollo, 2011) and determined vapour pressure of < 5 Pa at 20 °C (Cassani, 2011).

Absorption

Absorption is a function of the potential for a substance to diffuse across biological membranes. The most useful parameters to provide information on this potential are the molecular weight, octanol/water coefficient (log Pow) value and water solubility (ECHA, 2008). The log Pow value provides information on the relative solubility of the substance in water and lipids (ECHA, 2008). 

Oral

In general, molecular weights below 500 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, 2008).

The physicochemical characteristics (log Pow and water solubility) of the substance and the molecular weight are in a range suggestive of absorption from the gastrointestinal tract subsequent to oral ingestion.

The available acute oral toxicity data on Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters resulted in an LD50 value > 5000 mg/kg bw and no systemic effects (Biffi, 1992). Likewise, data on the acute oral toxicity of the source substances Alcohols, C12-13-branched and linear (CAS 740817-83-8) and Lactic acid (CAS 50-21-5), which are the expected hydrolysis products, showed both a LD50 value > 2000 mg/kg bw (Prinsen, 1998; Smyth, 1941).

This indicates that Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters has a low potential for toxicity, although no assumptions can be made regarding the absorption potential based on the experimental data.

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, 2008).

The available data from a enzymatic hydrolysis test with Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters indicate that the ester bond of the parent substance is rapidly cleaved in the intestine in the presence of pancreatic enzymes. A rapid hydrolysis of the ester bond was demonstrated in an in-vitro hydrolysis study, conducted with gastric juice- and intestinal juice simulant, and leading to the hydrolysis products Alcohols, C12-13-branched and linear and Lactic acid. The overall results show a decrease of the Propanoic acid, 2-hydroxy-, C12-13-branched alkyl esters of about 11 to 23% within 1 hour, and about 36 to 66 % within 4 hours in the gastric medium for (di)lactate esters. In the pancreatic digestion a decrease of the Propanoic acid, 2-hydroxy-, C12-13-branched alkyl esters of about 75 % to ≥ 95 % within 1 hour was seen, and about 80 % to ≥ 95 % within 4 hours for (di)lactate esters. The mass balance of this reaction displays good congruence of expected and measured values for the lactate esters (Ossberger and Ulrich, 2013). The substance Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters is therefore anticipated to be rapidly hydrolysed to lactic acid and branched alcohols with C12 and C13 chain length, respectively.

Free long-chain aliphatic alcohols are readily absorbed by the intestinal mucosa. Usually, they are subsequently oxidised to the corresponding (fatty) acids. 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. In rats given a single dose of radiolabelled octadecanol via duodenal cannula, 56.6 ± 14% of the administered material was absorbed within 24 h. As for fatty acids, the rate of absorption is likely to increase with decreasing chain length (Greenberger et al., 1966; IOM, 2005; Mattson and Volpenhein, 1962, 1964; OECD, 2006; Sieber, 1974).

Lactic acid occurs endogenously as a component of various physiological pathways, and is thus anticipated to be readily absorbed and metabolised (Lehninger, 1993).

In conclusion, based on the available information, the physicochemical properties and molecular weight of Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters suggest oral absorption. However, it was demonstrated that the substance undergoes rapid enzymatic hydrolysis in the gastrointestinal tract and the absorption of the ester hydrolysis products is also considered relevant. The absorption rate of the hydrolysis products is considered to be high.

Dermal

The dermal uptake of liquids and substances in solution is higher than that of dry particulates, since dry particulates need to dissolve into the surface moisture of the skin before uptake can begin. Molecular weights below 100 favour dermal uptake, while for those above 500 the molecule may be too large. Dermal uptake is anticipated to be low, if the water solubility is < 1 mg/L; low to moderate if it is between 1-100 mg/L; and moderate to high if it is between 100-10000 mg/L. Dermal uptake of substances with a water solubility > 10000 mg/L (and log Pow < 0) will be low, as the substance may be too hydrophilic to cross the stratum corneum. Log Pow values in the range of 1 to 4 (values between 2 and 3 are optimal) are favourable for dermal absorption, in particular if water solubility is high. For substances with a log Pow 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. 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, 2008).

The physicochemical properties (log Pow > 4 and water solubility between 4 - 5 mg/mL) of the substance and the molecular weight (258.40 - 272.42 g/mol) are in a range suggestive moderate absorption through the skin.

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 sensitiser then some uptake must have occurred although it may only have been a small fraction of the applied dose (ECHA, 2008).

The experimental animal and human data on Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters shows that no significant skin irritation (Biffi, 1992; Celleno, 1992) and skin sensitisation (Biffi, 1993) occurred, which excludes enhanced penetration of the substance due to local skin damage.

Overall, based on the available information, the dermal absorption potential of Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters is predicted to be moderate. With regard to the hazard assessment of the substance, absorption via the dermal route is assumed to be 100% compared to the oral route in a worst case approach.

Inhalation

As the vapour pressure of Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters is low (< 5 Pa at 20 °C), the volatility is also low. Therefore, the potential for exposure and subsequent absorption via inhalation during normal use and handling is considered to be negligible.

Due to the limited information available, absorption via inhalation is in general assumed to be 200% compared with the oral route in a worst case approach.

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, 2008).

The substance Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters will mainly be absorbed in the form of the hydrolysis products. The fraction of ester absorbed unchanged will undergo enzymatic hydrolysis by ubiquitous esterases, primarily in the liver (Fukami and Yokoi, 2012). Consequently, the hydrolysis products are the most relevant components to assess. Both hydrolysis products (branched alcohols with C12 and C13 chain length and lactic acid) are expected to be distributed widely in the body.

After being absorbed, free long-chain aliphatic alcohols are readily absorbed by the intestinal mucosa and transported via the lymphatic system. Usually, they are subsequently oxidised to the corresponding (fatty) acids. Fatty acids are (re-)esterified along with other fatty acids into triglycerides and released in chylomicrons. Fatty acids of carbon chain length ≤ 12 may be transported as the free acid bonded to albumin directly to the liver via the portal vein, instead of being re-esterified. Chylomicrons are transported in the lymph to the thoracic duct and eventually to the venous system. Upon contact with the capillaries, enzymatic hydrolysis of chylomicron triacylglycerol fatty acids by lipoprotein lipase takes place. Most of the resulting fatty acids are taken up by adipose tissue and re-esterified into triglycerides for storage. Triacylglycerol fatty acids are likewise taken up by muscle and oxidised for energy or they are released into the systemic circulation and returned to the liver (IOM, 2005; Johnson, 1990; Johnson, 2001; Lehninger, 1993; Stryer, 1996). Experimental data supporting this hypothesis can be found in the public literature: Twenty-four hours after intraduodenal administration of a single dose of radiolabelled octadecanol to rats, the percent absorbed radioactivity in the lymph was 56.6 ± 14. Thereof, more than half (52-73%) was found in the triglyceride fraction, 6-13% as phospholipids, 2-3% as cholesterol esters and 4-10% as unchanged octadecanol. Almost the entire radioactivity recovered in the lymph was localized in the chylomicron fraction. Thus, the alcohol is oxidised to the corresponding fatty acid and esterified in the intestine as described above (Sieber, 1974).

Lactic acid is a small molecule with high water solubility, thus being easily distributed in the body (ECHA, 2008). Absorption occurs by active transport and, to a minor extent, by diffusion via cell membranes in the anionic form lactate (Phypers and Pierce, 2006).

Taken together, the hydrolysis products of Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters are anticipated to distribute systemically. Long-chain alcohols are rapidly converted into the corresponding fatty acids by oxidation and distributed in form of triglycerides, which can be used as energy source or stored in adipose tissue. Stored fatty acids underlie a continuous turnover as they are permanently metabolised for energy and excreted as CO2. Bioaccumulation of fatty acids takes place, if their intake exceeds the caloric requirements of the organism.

Metabolism

The metabolism of Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters initially occurs via enzymatic hydrolysis of the ester resulting in branched alcohols with C12 and C13 chain length and Lactic acid, respectively.

This assumption has been verified in a hydrolysis test with Propanoic acid, 2-hydroxy-, C12-13-branched-alkyl esters. Pancreatic digestion resulted in a decrease of the (di)lactate esters of about 75 to ≥ 95% within 1 h and 80 to ≥ 95% within 4 h, respectively. Thus, the available data provide evidence that most of the parent substance is rapidly cleaved in the intestine in the presence of pancreatic enzymes.

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). Depending on the route of exposure, esterase-catalysed hydrolysis takes place at different places in the body. After oral ingestion, esters of alcohols and carboxylic acids undergo enzymatic hydrolysis already in the gastrointestinal tract. In contrast, substances which are absorbed through the pulmonary alveolar membrane or through the skin may enter the systemic circulation directly before entering the liver where hydrolysis will generally take place.

The branched C12 and C13 alcohols will mainly be metabolised to the corresponding carboxylic (fatty) acids via the aldehyde as a transient intermediate (Lehninger, 1993). The stepwise process starts with the oxidation of the alcohol by alcohol dehydrogenase to the corresponding aldehyde, where the rate of oxidation increases with increased chain-length. Subsequently, the aldehyde is oxidised to carboxylic (fatty) acid, catalysed by aldehyde dehydrogenase. Both the alcohol and the aldehyde may also be conjugated with e.g. glutathione and excreted directly, by passing further metabolism steps (WHO, 1999). A major metabolic pathway for linear and branched fatty acids is the beta-oxidation for energy generation. In this multi-step process, the fatty acids are at first esterified into acyl-CoA derivatives and subsequently transported into cells and mitochondria by specific transport systems. In the next step, the acyl-CoA derivatives are broken down into acetyl-CoA molecules by sequential removal of 2-carbon units from the aliphatic acyl-CoA molecule. Further oxidation via the citric acid cycle leads to the formation of H2O and CO2 (Lehninger, 1993). Branched-chain acids can be metabolised via the same beta-oxidation pathway as linear, depending on the steric position of the branch, but at lower rates (WHO, 1999). The alpha-oxidation pathway is a major metabolic pathway for branched-chain fatty acids where a methyl substituent at the beta-position blocks certain steps in the beta-oxidation (Mukherji, 2003). Generally, a single carbon unit is cleaved off the branched acid in an additional step before the removal of 2-carbon units continues. Alternative pathways for long-chain fatty acids include the omega-oxidation at high dose levels (WHO, 1999).

The naturally occurring metabolite L-lactate is produced by most mammalian cells during various physiological pathways (Lehninger, 1993). L-lactate is almost completely metabolised during gluconeogenesis and citric acid cycle, and most of metabolism takes place in the liver within the periportal hepatocytes (Phypers and Pierce, 2006; Lehninger, 1993). The metabolism of L[14C]lactic acid was investigated in rabbits, showing that circulating lactic acid was depleted and renewed at a rapid rate, with a turnover time of approximately 30 min. The majority of the lactate was oxidised to carbon dioxide, whereas only a small amount of the lactate was accounted for glucose or glycogen, or by the oxidation of them (Anderson, 1998). Mitochondria-rich tissues, such as skeletal and cardiac myocytes and proximal tubule cells remove the rest of the lactate by converting it to pyruvate by lactate dehydrogenase (Phypers and Pierce, 2006). Pyruvate is key intermediate involved in various metabolic pathways, such as glycolysis and gluconeogenesis (Lehninger, 1993). In the process of aerobic respiration, pyruvate is converted into acetyl-CoA via oxidative decarboxylation by the enzyme pyruvate dehydrogenase. Acetyl CoA is the starting point of the citric acid cycle for energy production and plays a major role in fatty acid metabolism and further biochemical reactions (Lehninger, 1993).

Excretion

The metabolites of branched C12 and C13 alcohols may be conjugated to glucuronides or sulphates, which subsequently can be excreted via urine or bile or cleaved in the gut with the possibility of reabsorption (entero-hepatic circulation) (WHO, 1998).

Lactic acid is almost completely metabolised either during gluconeogenesis or the citrus acid cycle for energy generation, resulting in the formation of CO2, which is excreted via exhalation (Lehninger 1993). Only less than 5% of lactate is excreted via urine (Phypers and Pierce, 2006).

 

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