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Link to relevant study record(s)

Reference
Endpoint:
basic toxicokinetics in vivo
Type of information:
other: Literature review
Adequacy of study:
supporting study
Study period:
Review published in 2007
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
secondary literature
Objective of study:
toxicokinetics
Qualifier:
no guideline followed
Principles of method if other than guideline:
Review of the available toxicokinetic literature for hexyl salicylate and related salicylic acid esters
GLP compliance:
no
Details on test animals and environmental conditions:
No further data
Metabolites identified:
yes

The 17 salicylate substances assessed in the RIFM review indicate consistent metabolism by hydrolysis to form salicylic acid and the alcohol of the corresponding side chain. This pattern of metabolism is consistent with information on other esters which are hydrolysedin vivoby carboxylesterases or esterases, especially the A-esterases.

In vivo metabolic data are available for methyl salicylate and one human metabolism study is available on phenyl salicylate. Carboxylesterases show extensive tissue distribution with respect to hydrolysis of methyl salicylate. In vitr ostudies demonstrate greatest activity in the liver, but also extensive activity in the intestines, kidney, pancreas and spleen. Both the liver and intestines can contribute to the pre-systemic hydrolysis of salicylates.

Oral consumption of 0.42 mL methyl salicylate by human volunteers resulted in the rapid appearance of salicylic acid in the plasma. At 15 and 90 minutes post administration, salicylic acid concentrations were 2-4 times higher in plasma than the parent methyl salicylate. The hydrolysis of methyl salicylate was also demonstrated following oral administration to dogs at a dose level of 300 mg/kg bw; metabolism was almost complete within 1 hour of administration. Gavage dosing of rats with methyl salicylate (300 mg/kg bw) resulted in the appearance of free salicylate in plasma and tissues within 20 minutes.  Salicylic acid was also found in the plasma of pregnant rats exposed dermally to 2000 mg/kg bw/d methyl salicylate.

Results from a study in a single human volunteer show that ingestion phenyl salicylate resulted in a rapid increase in free urinary phenol concentration, indicating rapid hydrolysis.

In vitro metabolism studies using mouse skin absorption models have shown variable results with respect to the degree of hydrolysis, from <5% for methyl salicylate to 25-30% for ethyl salicylate and total absorption of 100% of butyl salicylate. In an in vitro guinea pig skin preparation, 38% of the absorbed methyl salicylate was metabolized to salicylic acid in non-viable skin. In viable skin, 57% of methyl salicylate metabolised to 21% salicyluric acid and 36% salicylic acid.

Metabolism of salicylic acid

Based on numerous metabolic studies in both humans and experimental animals, salicylic acid undergoes metabolism primarily in the liver. At low, non-toxic doses, approximately 80% of salicylic acid is further metabolised in the liver via conjugation with glycine and subsequent formation of salicyluric acid. Salicylic acid also undergoes glucuronide conjugation. The metabolism of salicylic acid is characterized by first order kinetics at low doses and zero order kinetics at doses that saturate conjugation capacity. A small amount of salicylic acid is oxidized to gentisic acid, a product that in turn may be subject to glucuronide conjugation.

The activity of salicylic acid metabolic pathways (i.e., extensive glycine and/or glucuronide conjugation followed by partial degradation of the conjugates) is evidenced by the finding of glucuronide, glycine, or sulphate conjugates as the major urinary metabolites of several alkyl-and alkoxy-benzyl derivatives. These compounds are close structural analogues of the salicylates, in rats, rabbits, dogs, and humans. The consistency of the degradation pathway is such that it can be assumed for hexyl salicylate to follow a similar path.

 

Metabolism of hexanol

For salicylates, following hydrolysis to salicylic acid, the resulting side chain could be expected to be further metabolised. In the case of the alcohol formed following hydrolysis (i.e. hexanol), further metabolism would result in the formation of the corresponding aldehydes and acids, with eventual degradation to carbon dioxide by the fatty acid pathway and the tricarboxylic acid cycle.

Conclusions:
Data from structurally-related salicyclic acid esters indicate rapid metabolism by hydrolysis to liberate free salicylic acid. In the case of hexyl salicylate, metabolism will produce the initial metabolites salicylic acid and hexanol.
Executive summary:

Data from structurally-related salicyclic acid esters indicate rapid metabolism by hydrolysis to liberate free salicylic acid. In the case of hexyl salicylate, metabolism will produce the initial metabolites salicylic acid and hexanol.

Description of key information

Toxicokinetics

The 17 salicylate substances assessed in the RIFM review indicate consistent metabolism by hydrolysis to form salicylic acid and the alcohol of the corresponding side chain. This pattern of metabolism is consistent with information on other esters which are hydrolysedin vivoby carboxylesterases or esterases, especially the A-esterases. In vivo metabolic data are available for methyl salicylate and one human metabolism study is available on phenyl salicylate. Carboxylesterases show extensive tissue distribution with respect to hydrolysis of methyl salicylate. In vitro studies demonstrate greatest activity in the liver, but also extensive activity in the intestines, kidney, pancreas and spleen. Both the liver and intestines can contribute to the pre-systemic hydrolysis of salicylates. Oral consumption of 0.42 mL methyl salicylate by human volunteers resulted in the rapid appearance of salicylic acid in the plasma. At 15 and 90 minutes post administration, salicylic acid concentrations were 2-4 times higher in plasma than the parent methyl salicylate. The hydrolysis of methyl salicylate was also demonstrated following oral administration to dogs at a dose level of 300 mg/kg bw; metabolism was almost complete within 1 hour of administration. Gavage dosing of rats with methyl salicylate (300 mg/kg bw) resulted in the appearance of free salicylate in plasma and tissues within 20 minutes.  Salicylic acid was also found in the plasma of pregnant rats exposed dermally to 2000 mg/kg bw/d methyl salicylate.

Results from a study in a single human volunteer show that ingestion phenyl salicylate resulted in a rapid increase in free urinary phenol concentration, indicating rapid hydrolysis.

In vitro metabolism studies using mouse skin absorption models have shown variable results with respect to the degree of hydrolysis, from <5% for methyl salicylate to 25-30% for ethyl salicylate and total absorption of 100% of butyl salicylate. In an in vitro guinea pig skin preparation, 38% of the absorbed methyl salicylate was metabolized to salicylic acid in non-viable skin. In viable skin, 57% of methyl salicylate metabolised to 21% salicyluric acid and 36% salicylic acid.

Based on numerous metabolic studies in both humans and experimental animals, salicylic acid undergoes metabolism primarily in the liver. At low, non-toxic doses, approximately 80% of salicylic acid is further metabolised in the liver via conjugation with glycine and subsequent formation of salicyluric acid. Salicylic acid also undergoes glucuronide conjugation. The metabolism of salicylic acid is characterized by first order kinetics at low doses and zero order kinetics at doses that saturate conjugation capacity. A small amount of salicylic acid is oxidized to gentisic acid, a product that in turn may be subject to glucuronide conjugation.

The activity of salicylic acid metabolic pathways (i.e., extensive glycine and/or glucuronide conjugation followed by partial degradation of the conjugates) is evidenced by the finding of glucuronide, glycine, or sulphate conjugates as the major urinary metabolites of several alkyl-and alkoxy-benzyl derivatives. These compounds are close structural analogues of the salicylates, in rats, rabbits, dogs, and humans. The consistency of the degradation pathway is such that it can be assumed for hexyl salicylate to follow a similar path.

 

For salicylates, following hydrolysis to salicylic acid, the resulting side chain could be expected to be further metabolised. In the case of the alcohol formed following hydrolysis (i.e. hexanol), further metabolism would result in the formation of the corresponding aldehydes and acids, with eventual degradation to carbon dioxide by the fatty acid pathway and the tricarboxylic acid cycle.

Dermal absorption

A number of estimates of the dermal absorption of hexyl salicylate are available, using read-across, skin membranes in vitro or mathematical modelling.

In an in vitro study in human skin using the structural analogues isoamyl salicylate and hexyl salicylate, Jimbo (1983) calculated dermal absorption values of 0.008% and 0.031% respectively.  The reliability of this study is, however, limited by the low temperature used, the use of non-viable skin, the fact that amount within the epidermis was not determined and the low solubility of the salicylates in the saline receptor fluid. The RIFM expert panel reviewed a structurally related group of fragrance materials including various salicylates for information including dermal absorption. Based on this review, a dermal absorption value of 0.005% is proposed for hexyl salicylate (Belsito, 2007). Watkinson et al. (1992) describe the application of a mathematical model to estimate the extent of dermal absorption of substances used in sunscreen formulations. Based on the mathematical modelling described in this paper, the dermal absorption of hexyl salicylate when used as an ingredient in sunscreen formulations is calculated to be relatively low. Dermal absorption values of 0.00003% (at 2 hours), 0.0006% (at 6 hours) and 0.005% (at 12 hours) are calculated, based on application of 40 ug/cm2 to a skin area of 1.4 m2. A more recent and more reliable study (Maas, 2016) investigated the dermal absorption of hexyl salicylate in a standard study in human skin in vitro, coupled with an assessment of the extent of metabolism by viable human skin. This study demonstrates the extensive dermal metabolism of hexyl salicylate to salicylic acid by human skin esterases. For the purposes of the risk assessment, a conservative assumption is that all of the hexyl salicylate present in the skin (i.e. all of the radioactivity not removed by washing at the end of the exposure period) is potentially metabolised and absorbed a salicylic acid. Dermal absorption values of 0.8%, 7.8% and 2.7% are therefore calculated for hexyl salicylate concentrations of 100%, 20% and 0.1% respectively.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
100
Absorption rate - dermal (%):
7.8
Absorption rate - inhalation (%):
100

Additional information