Benzyl cinnamate

Endpoint summary

Description of key information

Absorption

Dermal absorption

One study is available that examines the speed at which the test substance is absorbed through the skin of mice (Meyer, 1959). In this study, the test item is applied together with 0.25% eserin to the shaved abdominal skin of the test animals. The test item hence functions as the vehicle. Eserin is used as an indicator of absorption because of its measurable effects on the striated muscles. The lag time between the application of the eserin and the observable periodical irritation of the jaw muscles is taken as a measure for the absorption velocity of the test item. When the test substance was used as the vehicle, no apparent effects on the jaw muscles were observed. Hence, this test suggests that the test item is not absorbed easily through the skin.

Additional to this experimental finding, some preliminary predictions can be made regarding dermal uptake of a substance based on its physico-chemical properties.

In order for a substance to cross the stratum corneum, the substance should be sufficiently lipophilic (= sufficient solubility in fat). On the other hand, partitioning from the stratum corneum into the epidermis requires sufficient hydrophilicity (= sufficient solubility in water). Hence, the likelyhood of dermal absorption is determined by the substance's log Kow and water solubility values.

According to Nielsen et al. (2010), dermal absorption is likely for a substance if it has the following properties:

- a vapour pressure < 100 Pa

- a log Kow between 1 and 4

- a water solubility in the range of 1 - 100 mg/L (moderate absorption), or 100 - 10000 mg/L (high absorption).

The test substance has a vapour pressure of 7 Pa, a log Kow of 4.18 and a water solubility of 3.6 mg/L. Based on these physico-chemical properties, the dermal absorption of the substance can be anticipated to be quite low, as the substance is rather lipophilic, which is reflected in its relatively high log Kow value and relatively low water solubility. This is in line with the experimental findings by Meyer.

Oral absorption

No studies are available that address the absorption of the test substance via the oral route. Nevertheless, also in this case some preliminary predictions can be made from its physico-chemical properties.

Oral absorption can occur along the entire length of the gastro-intestinal tract. However, it should be kept in mind that substances can undergo chemical changes in the gastro-intestinal fluids prior to absorption. This is of particular relevance for the test substance, as further explained below in the section on metabolism. The substance will undergo an enzymatic hydrolysis of its ester moiety under the influence of gastro-intestinal esterase enzymes. This hydrolysis process gives rise to the formation of benzyl alcohol and cinnamic acid, both of which can subsequently be absorbed from the gastro-intestinal tract.

Inhalation absorption

No studies are available that address the absorption of the test substance via the inhalation route. Nevertheless, also in this case some preliminary predictions can be made from its physico-chemical properties.

The main physico-chemical parameters determining the extent to which a liquid substance may be absorbed by the inhalation route are the vapour pressure, the log Kow and the water solubility.

Nielsen et al. (2010) gives the following criteria:

- Highly volatile substances are considered those substances that have a vapour pressure exceeding 25000 Pa (or boiling point < 50°C), whereas a vapour pressure < 500 Pa (or boiling point > 150C°) indicates a low volatility.

 - Log Kow > 0 indicates the potential for direct absorption across the respiratory tract epithelum. Substances with a log Kow value between 0 and 4 are sufficiently lipophilic to allow crossing the alveolar and capillary membranes, and hence are likely to be absorbed as well.

 - Sufficient water solubility increases the potential for inhalation absorption. Nevertheless, very hydrophilic substances may be retained within the mucus, and transported out of the respiratory tract by clearance mechanisms.

 

Based on the physico-chemical properties of the test item (Vapour pressure 7 Pa, log Kow 4.18 and water solubility 3.6 mg/L), it can be concluded that absorption of the substance following exposure via the inhalation route is unlikely. The substance has a low volatility, limiting exposure via the inhalation route. Furthermore, the uptake of the substance via this route is limited by its lipophilicity.

 

Metabolism and Excretion

As mentioned above, the metabolism of the test substance is determined by the substance's tendency to undergo enzymatic hydrolysis, giving rise to the formation of benzyl alcohol and cinnamic acid.

Primary metabolic step: enzymatic hydrolysis of the test substance

One in vitro study is available in which the test item was incubated with a preparation of pancreatin in 0.5M phosphate buffer. Pancreatin is a digestive juice excreted by the pancreas. It contains a variety of enzymes, including esterases: a class of enzymes that is capable of the hydrolytic cleavage of ester groups. After 2 hours of incubation of the substance with pancreatin, the extent of hydrolysis of the test item was examined by means of gas-liquid chromatography. The test substance showed 80% hydrolysis under the conditions of the experiment, indicating that the ester hydrolyses rapidly by pancreatin.

The primary metabolites formed upon the ezymatic hydrolysis of the substance are the corresponding benzyl alcohol and cinnamic acid. The further metabolism and excretion of both primary metabolites will be further discussed below.

Absorption, metabolism and excretion of benzyl alcohol

The metabolic fate of benzyl alcohol has been addressed by the Joint Expert Committee on Food Additives in the framework of the safety evaluation of certain food additives and contaminants by the World Health Organisation (WHO, 2002). Benzyl alcohol was evaluated as part of a group of 37 benzyl derivatives, which furthermore included benzaldehyde, benzoic acid and related esters, acetals et cetera.

Benzyl derivatives have been shown to be absorbed rapidly through the gut. Thereafter, benzyl alcohol is quickly oxidized in two steps, first to benzaldehyde and subsequently to benzoic aicd. Benzoic acid is excreted primarily as its glycine conjugate (hippurate) in the urine. At high doses, when the formation of the glycine conjugate is limited due to glycine depletion following saturation of the metabolic pathway, free benzoic acid may sequester acetyl coenzyme A or be excreted unchanged or as the glucuronic acid conjugate.

It is to be noted that benzoic acid is also an endogenous substance in humans. Endogenous benzoic acid is formed through the phenylalanine – tyrosine pathway.

Absorption, metabolism and excretion of cinnamic acid

The metabolic fate of cinnamic acid has also been addressed by the Joint Expert Committee on Food Additives (WHO, 2001). Cinnamic acid has been shown to be rapidly absorbed from the gut. Cinnamic acid, like most carboxylic acids, is subsequently converted to its acyl coenzyme A ester. The thus formed cinnamoyl CoA can undergo conjugation with glycine, or can be first oxidized to yield benzoyl CoA by beta-oxidation. Benzoyl CoA can then in turn be conjugated with glycine to the corresponding hippurate, or can be hydrolyzed to the free benzoic acid; of which the further metabolic fate has been discussed above.

Overall, the substance will be absorbed via the oral route mainly, but will not bioaccumulate in the organism. Instead, it is expected to be metabolized and excreted quickly.

References:

Nielsen E, Ostergaard G, Larsen JC (2010). Toxicological Risk Assessment of Chemicals; A Practical Guide. Published by: Informa Healthcare. ISBN-13: 9780849372650.

WHO (2001). WHO Technical Report Series 901 – Evaluation of certain food additives and contaminants. 55^thReport of the Joint FAO/WHO Expert Committee on Food Additivies. WHO, Geneva, 2001. International Programme on Chemical Safety.

WHO (2002). WHO Technical Report Series 909 – Evaluation of certain food additives and contaminants. 57^thReport of the Joint FAO/WHO Expert Committee on Food Additivies. WHO, Geneva, 2002. International Programme on Chemical Safety.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Absorption

No studies are available that address the absorption of the test substance via the oral or the inhalation route. Few data from an older dermal study are available on absorption velocity.
Nevertheless, some preliminary predictions can be made from its physico-chemical properties.

Oral absorption

Oral absorption can occur along the entire length of the gastro-intestinal tract. However, it should be kept in mind that substances can undergo chemical changes in the gastro-intestinal fluids prior to absorption. This is of particular relevance for benzyl cinnamate, as further explained below in the section on metabolism.
The test substance will undergo an enzymatic hydrolysis of its ester moiety under the influence of gastro-intestinal esterase enzymes. This hydrolysis process gives rise to the formation of cinnamic acid and benzyl alcohol.

Cinnamic acid and benzyl derivates have been shown to be rapidly absorbed from the gastro-intestinal tract.

Dermal absorption

A number of physico-chemical factors determine the extent to which a substance may be absorbed by the dermal route. In order for a substance to cross the stratum corneum, the substance should be sufficiently lipophilic (= sufficient solubility in fat). On the other hand, partitioning from the stratum corneum into the epidermis requires sufficient hydrophilicity (= sufficient solubility in water). Hence, the likelyhood of dermal absorption is determined by the substance's log Kow and water solubility values.

According to Nielsen et al. (2010), dermal absorption is likely for a substance if it has the following properties:

- a vapour pressure < 100 Pa

- a log Kow between 1 and 4

- a water solubility in the range of 1 - 100 mg/L (moderate absorption), or 100 - 10000 mg/L (high absorption).

The test substance has a vapour pressure of 7 Pa, a log Kow of 4.18 and a water solubility of 3.6 mg/L. Based on these physicochemical properties, it can be concluded that the dermal absorption of the substance can be considered to be quite low, as the substance is rather lipophilic, which is reflected in its relatively high log Kow value and relatively low water solubility. This is in line with the experimental findings by Meyer.

In a older study the dermal absorption velocity of the test substance by applying the test item together with 0.25% eserin (indicator of resporption because of its measurable effects on the striated muscles) to abdominal skin of mice for 2 hours was observed. The lag time between the application till the observable periodical irritation of the jaw muscles is taken as a measure of the absorption velocity of the test item. The test substance benzyl cinnamate was not found to be resorbed in a measurable quantity in this test system.

Inhalation absorption

The main physico-chemical parameters determining the extent to which a liquid substance may be absorbed by the inhalation route are the vapour pressure, the log Kow and the water solubility.

Nielsen et al. (2010) gives the following criteria:

- Highly volatile substances are considered those substances that have a vapour pressure exceeding 25000 Pa (or boiling point < 50°C), whereas a vapour pressure < 500 Pa (or boiling point > 150C°) indicates a low volatility.

- Log Kow > 0 indicates the potential for direct absorption across the respiratory tract epithelium.

Substances with a log Kow value between 0 and 4 are sufficiently lipophilic to allow crossing the alveolar and capillary membranes, and hence are likely to be absorbed as well.

- A sufficient water solubility increases the potential for inhalation absorption. Nevertheless, very hydrophilic substances may be retained within the mucus, and transported out of the respiratory tract by clearance mechanisms.

Based on the physico-chemical properties of the test item (Log Kow 4.18 and water solubility 3.6 mg/L), it can be concluded that absorption of the substance following exposure via the inhalation route is unlikely. However, the very low vapour pressure (7 Pa) and high boiling point (358°C) will to an important extent mitigate the likelihood of exposure via the inhalation route.

Metabolism and Excretion

As mentioned above, the metabolism of test substance is determined by the substance's tendency to undergo enzymatic hydrolysis, giving rise to the formation of cinnamic acid and benzyl alcohol in the gastrointestinal tract and further metabolization after absorption mostly in the liver.

Primary metabolic step: enzymatic hydrolysis of the test item

In a preliminary investigation on metabolism using an in vitro hydrolysis procedure designed to establish the relative rates of enzymic hydrolysis of the ester. The test substance was incubated with a preparation of pancreatin in 0.5 M phosphate buffer. Incubations were made under arbitrary experimental conditions and the extent of hydrolysis of the ester by pancreatin was determined after 2 h by gas-liquid chromatography. The test substance showed 80% hydrolysis under the conditions of the experiment, indicating that the ester hydrolyses rapidly by pancreatin. The author presumes that esters hydrolysing rapidly in vitro, will also be degraded rapidly in the intact animal.

Further metabolism and excretion of the primary metabolites

The primary metabolites formed upon the ezymatic hydrolysis of the substance are the carboxylic acid cinnamic acid and its corresponding alcohol, benzyl alcohol.

In animals, most carboxylic acids, such as cinnamic acid, are converted to acyl coenzyme A esters. Cinnamoyl coenzyme A undergoes either glycine conjugation catalysed by a glycine N-acyl transferase or beta-oxidation, eventually leading to the formation of benzoyl coenzyme A. This is in turn either conjugated with glycine, yielding hippuric acid, or hydrolysed to yield free benzoic acid, which is then excreted.

Benzyl derivatives have been shown to be absorbed rapidly through the gut, metabolized primarily in the liver, and excreted in the urine.

Once absorbed, benzyl derivatives are oxidized and excreted primarily as the glycine conjugate of benzoic acid (hippurate). At high doses, the formation of the glycine conjugate is limited; when glycine is depleted, free benzoic acid may sequester acetyl coenzyme A or be excreted unchanged or as the glucuronic acid conjugate. Aromatic ring substitution is anticipated to have little influence on the principal pathway of metabolism.

References

Nielsen E, Ostergaard G, Larsen JC (2010). Toxicological Risk Assessment of Chemicals; A Practical

Guide. Published by: Informa Healthcare. ISBN-13: 9780849372650.

WHO Food Additives Series 46: Cinnamyl Alcohol and Related Substances. http://www.inchem.org/documents/jecfa/jecmono/v46je07.htm. Accessed online on Nov. 3, 2017.

WHO Food Additives Series 48: Safety Evaluation of Certain Food Additives and Contaminants; Benzyl Derivatives. http://www.inchem.org/documents/jecfa/jecmono/v48je14.htm. Accessed online on Nov. 3, 2017.