4-(octadecylamino)-4-oxoisocrotonic acid

Endpoint summary

Description of key information

Absorption

A systemic bioavailability after oral uptake is considered likely. Based on physico-chemical properties, dermal and inhalation uptake is likely to be low.

Distribution/Accumulation

No distribution to a significant degree within the body is expected. No bio-accumulation potential has been identified.

Metabolism

The main metabolic pathway anticipated is enzymatic hydrolysis of the amide bond in the parent molecule mainly resulting in stearic acid and maleic acid and subsequent oxidation of the hydrolysis products.

Excretion

The parent compound is expected to be excreted mainly via the bile/feces pathway while metabolites will be exhaled as CO2.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

In accordance with Annex VIII, Column 1, Item 8.8 of Regulation (EC) No. 1907/2006 (REACH) and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2017), assessment of the toxicokinetic behaviour of the test substance was conducted to the extent that can be derived from the relevant available information on physico-chemical and toxicological characteristics. There are no studies available evaluating the toxicokinetic properties of the registered substance.

The molecular weight of the registered substance is 367.6 g/mol. The substance is a white solid (powder) and its water solubility and log Pow have been determined to be 5.9 mg/L at 25 °C and 2.53, respectively. The parameters indicate a lipophilic character of the substance. As can be expected from its physical state, the vapor pressure of the registered substance is very low. It was measured to be ≤ 6.8E-4 Pa at 20 °C and ≤ 1.3E-3 Pa at 25 °C (OECD Guideline 104, effusion method: isothermal thermogravimetry).

 

Absorption

Absorption is a function of the potential of 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 of the registered substance is 367.6 g/mol, absorption of the molecules in the gastrointestinal tract is in general anticipated. Absorption after oral administration is also hinted to be favoured when the “Lipinski Rule of Five” (Lipinski et al.; 2001); Ghose et al., 1999) is applied to 4-(octadecylamino)-4-oxoisocrotonic acid since 3 out of the 4 rules are fulfilled.

Moreover, observation of systemic effects in toxicity investigations following exposure by any route is an indication for substance absorption. In order to further elucidate the absorption potential after oral exposure, available data on oral toxicity of 4-(octadecylamino)-4-oxoisocrotonic acid are also considered. The acute toxicity after oral administration has been investigated. Rats were treated with the limit dose of 2000 mg/kg bw. During the study period, no animal died. No clinical signs of toxicity were observed and all animals showed normal body weight gain. Necropsy did not reveal any macroscopic findings. Thus, the oral LD50 value was determined to be > 2000 mg/kg bw.

The available study indicates only a low potential for toxicity after acute oral exposure. No quantitative assumptions can be made regarding the absorption potential based on the experimental data. Nevertheless, based on the theoretical considerations and the experimental evidence, a systemic bioavailability after oral exposure is considered likely.

 

Dermal

It is commonly accepted that smaller molecules are taken up through the skin more easily than bigger ones; 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 to be absorbed (ECHA, 2017). As the molecular weight of 4-(octadecylamino)-4-oxoisocrotonic acid is 367.6 g/mol, a dermal absorption of the molecule cannot be excluded completely. In general, the dermal uptake of substances with high water solubility of > 10 g/L and log Pow < 0 will be low, as those substances may be too hydrophilic to cross the stratum corneum. log Pow values between one and four favor dermal absorption (values between 2 and 3 are optimal), in particular if water solubility is high. In contrast, log Pow values < -1 suggest that a substance is not likely to be sufficiently lipophilic to cross the stratum corneum. Therefore, dermal absorption is likely to be low (ECHA, 2017). As the registered substance has a log Pow value of 2.53 and a water solubility of 5.9 mg/L, dermal uptake cannot be excluded. The limited potential for dermal absorption is also supported by a QSAR calculation of the dermal absorption rate. Calculations with the Episuite 4.1, DERMWIN 2.02 tool yielded a dermal permeability constant Kp of 3.88 cm/h.

If a substance is a skin irritant or corrosive, damage to the skin surface may enhance penetration. Furthermore, if a substance has been identified as skin sensitiser, then some uptake must have occurred previously, although it may only have been a small fraction of the applied dose (ECHA, 2017). The available data on skin irritation / corrosion of the registered substance are, therefore, also considered for assessment of dermal absorption. In suitable in vitro studies with reconstructed three-dimensional human epidermis, the neat substance revealed neither corrosive nor irritating effects. Tissue viabilities in all experiments were about 100% indicating that no irritation effects have been induced. Moreover, in an adequate in vivo skin sensitisation study, the registered substance did not induce local irritation and showed no sensitising effects.

In conclusion, taking into account the calculated permeability constant and available experimental data on skin irritation and skin sensitisation, dermal uptake of 4-(octadecylamino)-4-oxoisocrotonic acid is considered unlikely.

 

Inhalation

Based on its very low vapor pressure of ≤ 6.8E-4 Pa at 20 °C, vapors of the registered substance are unlikely to be available for respiratory absorption. However, in humans, particles with aerodynamic diameters below 100 μm have the potential to be inhaled. Particles with aerodynamic diameters below 50μm may reach the thoracic region and those below 15μm the alveolar region of the respiratory tract (ECHA, 2017). The granulometric analysis of 4-(octadecylamino)-4-oxoisocrotonic acid revealed a D50 value of 9 µm thus indicating that the substance might well be transported into the deeper regions of the lungs in humans. Moreover, moderate log Pow values (between -1 and 4) are favourable for absorption directly across the respiratory tract epithelium by passive diffusion. Since the log Pow of 4-(octadecylamino)-4-oxoisocrotonic acid was determined to be 2.53, diffusion through the respiratory epithelium cannot be excluded.

Overall, considering the physico-chemical parameters of particle size and log Pow, respiratory absorption of the registered substance is assumed to be possible but rather low.

 

Distribution/Accumulation

Distribution of a compound within the body depends on the physico-chemical 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 its extracellular concentration particularly in fatty tissues (ECHA, 2017). Substances with log Pow values of 3 or less would unlikely accumulate in adipose tissues with the repeated intermittent exposure patterns normally encountered in the workplace but may accumulate if exposures are continuous. Once exposure to the substance stops, the substance will gradually eliminate at a rate depending on its half-life (ECHA, 2017; Stryer, 1996).

Since the test substance is lipophilic (log Pow 2.53) and has a water solubility of 5.9 mg/L, the distribution into cells is likely. If the substance is absorbed and available systemically, its intracellular concentration in adipose tissue can be expected to be higher than the extracellular concentration.

 

Metabolism

No metabolism studies are available with the registered substance. Prediction of compound metabolism based on physico-chemical data is difficult. Structure information gives some but no certain clue on reactions occurring in vivo. Therefore, the potential metabolites following enzymatic conversion were predicted using the QSAR OECD toolbox (v4.2; OECD, 2018). This QSAR tool predicts which metabolites may result from enzymatic activity in the liver and by bacteria in the gastrointestinal tract. Moreover, products of acid- and base-catalysed hydrolysis in the gastric juices are also predicted. Although prediction of metabolites formed in the skin is also possible using the OECD toolbox, no products of skin metabolism were predicted for 4-(octadecylamino)-4-oxoisocrotonic acid.

 

Acid- and base-catalysed hydrolysis

4-(octadecylamino)-4-oxoisocrotonic acid is the mono-amide of stearoyl amine with maleic acid. Since amide functions are capable of acid- and base-catalysed hydrolysis, the resulting hydrolysis products stearoyl amine and maleic acid are predicted. However, since amide hydrolysis is in general slower than hydrolysis of esters (Sykes, 1988), it can be expected that formation of stearoyl amine and maleic acid in gastric juice is of minor importance only.

 

Hepatic metabolism

18 hepatic metabolites were predicted for 4-(octadecylamino)-4-oxoisocrotonic acid. The metabolites draw a coherent picture of the metabolic fate of the registered substance. The primary conversion in the liver is the cleavage of the amide bond and, therefore, formation of maleic acid and stearoyl amine. Subsequently, stearoyl amine is oxidised to form stearic acid (i.e., replacement of the amine function by a carboxy function). Stearic acid is then further metabolised by means of β-oxidation, resulting in the formation of fatty acids with varying even-numbered carbon-chain lengths. In addition to stearoyl amine and maleic acid, 11 out of the 18 predicted metabolites are fatty acids of different carbon-chain lengths emphasizing the importance of this metabolic pathway. The remaining 5 metabolites originate from partial oxidative processes, i.e., addition of hydroxyl functions, or from oxidative processes taking place prior to cleavage of the amide bond.

 

Microbial activity in the gastrointestinal tract

Up to 73 metabolites were predicted to result from all kinds of microbiological metabolism for the registered substance. Most of the metabolites were found to be very similar to those predicted for hepatic metabolism. However, a number of small break-down products and metabolites with intact amino functions (mono- as well as di-amines) are also predicted. It is important to note that not all of the theoretically predicted metabolites will occur in nature. The high number of potential metabolites reflects the complexity of microbial degradation processes identified.

 

Fate of metabolites

The main products of the metabolic conversion of 4-(octadecylamino)-4-oxoisocrotonic acid, i.e., stearic acid and maleic acid can be expected to be further metabolised by oxidative processes typical for fatty acids. Fatty acids are degraded by mitochondrial β-oxidation which takes place in most animal tissues and uses an enzyme complex for a series of oxidation and hydration reactions, resulting in the cleavage of acetate groups in the form of acetyl-CoA. The alkyl chain length is reduced by 2 carbon atoms during each β-oxidation cycle. Alternative pathways for oxidation can be found in the liver (ω-oxidation) and the brain (α-oxidation). Each C₂-unit resulting from β-oxidation enters the citric acid cycle as acetyl-CoA, through which it is completely oxidized to CO₂ (CIR, 1987; IOM, 2005; Lehninger, 1998; Stryer, 1996).

 

Excretion

Only limited conclusions on excretion of a compound can be drawn based on physico-chemical data. Due to metabolic changes, the finally excreted compound may have few or none of the physico-chemical properties of the parent compound. In addition, metabolic break-down or conjugation reactions may lead to very different molecular weights of the final products. It is, however, generally accepted that a low molecular weight (< 300 g/mol) and good water solubility favors excretion via urine. On the other hand, low water solubility and a more lipophilic character of a molecule will lead to excretion in the bile/feces (ECHA, 2017).

Due to the fact that 4-(octadecylamino)-4-oxoisocrotonic acid has a limited water solubility and a high log Pow of 2.53, the substance is expected to be excreted mainly via the bile/feces pathway.

 

References

Cosmetic Ingredient Review Expert Panel (CIR) (1987) Final report on the safety assessment of oleic acid, lauric acid, palmitic acid, myristic acid, stearic acid. J. of the Am. Coll. of Toxicol.6(3):321-401.

Ghose et al. (1999). A Knowledge-Based Approach in Designing Combinatorial or Medicinal Chemistry Libraries for Drug Discovery. J. Comb. Chem. 1 (1): 55-68.

ECHA (2017). Guidance on information requirements and chemical safety assessment - Chapter 7c: Endpoint specific guidance; Version 3; June 2017; European Chemicals Agency, Helsinki, Finland

Institute of the National Academies (IOM) (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). The National Academies Press.http://www.nap.edu/openbook.php?record_id=10490&page=R1(last accessed 2017-12-08)

Lehninger, A.L., Nelson, D.L. and Cox, M.M. (1998).Prinzipien der Biochemie. 2. Auflage. Heidelberg Berlin Oxford: Spektrum Akademischer Verlag.

Lipinski et al. (2001). Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv. Drug Del. Rev. 46: 3-26.

OECD (2018). OECD QSAR Toolbox, Version 4.2,http://www.qsartoolbox.org

Stryer, L. (1996). Biochemie. 4. Auflage. Heidelberg Berlin Oxford: Spektrum Akademischer Verlag.

Sykes, P. (1988). Reaktionsmechanismen in der Organischen Chemie, 9. Überarbeitete Auflage, VCH Verlagsgesellschaft mbH, Weinheim, Germany