Linalyl acetate

Endpoint summary

Additional information

Linalyl acetate (synonyms: 1,6-Octadien-3-ol, 3,7-dimethyl-, acetate; 1,5-dimethyl-1-vinylhex-4-en-1-yl acetate) has a molecular weight of 196.29 g/mol and is a colorless liquid, which has a low to moderate solubility in water (30 mg/l at 20 °C). It has a low vapor pressure of <1 hPa (20°C) and a log Po/w of 3.9.

When considering the vapour pressure of linalyl acetate, limited adsorption of the substance via the inhalative route is to be expected. Furthermore, in a standardized inhalation hazard test with a saturated vapour atmosphere, no significant substance loss was recorded at 20°C, indicating that the enrichment of the air by the volatile parts of the test compound was negligable (BASF 1969). In line, 0/12 rats each died after 8 h exposure and no symptoms were observed at room temperature.

Evidence for systemic availability of linalyl acetate via the oral route can be derived from toxikokinetic data and a repeated dose toxicity study in rats, conducted with the metabolically related compound linalool. In a subacute study, rats received 160, 400 or 1000 mg/kg bw/d of the tested material (containing 72.9 % linalool in essential oil) via gavage during 28 days (HLA 642-460), resulting in changes in clinico-chemical parameters and macroscopic/ histopathologic changes in liver and kidneys.

Labelled linalool (14C) was administered orally (gavage, 500 mg/kg bw) and intraperitoneally (20 mg/animal) to male Wistar rats (Parke 1974). Linalool was rapidly and completely absorbed in rats after oral administration. The majority was excreted via urine (60% of administered radioactivity), expired air (23%) and faeces (15%) 72 hours after application, whereas only 3% of the dose were detected in tissues, i.e. brain, lung, liver, heart, spleen, gastro-intestinal tract, kidney, skin and skeletal muscle. Intraperitoneal administration with a subsequent collection of bile revealed, that approx. 25% of the dosed linalool is excreted via the bile probably by formation of respective glucuronide and sulfate conjugates. Transfer of the bile from a linalool treated animal into the duodenum of an untreated animal further indicated the occurrence of enterohepatic circulation.

In a subchronic dermal study, linalool was administered dermally to rats over a 91 day period at doses of 250, 1000 and 4000 mg/kg bw/day (T&O 79-201), resulting in increases in mortality and changes of liver and kidney weights. Considering molecular weight and Po/w of linalyl acetate, a certain rate of dermal penetration is to be expected.

Taken together, rapid and complete oral absorption and indications for bioavailablility after topical application are given for linalool and consequently, systemic availability of linalyl acetate is to be expected via these exposure routes.

 

Concerning metabolisation, the present data indicate an extensive ester hydrolysis of linalyl acetate in intestinal fluids, which results in linalool and acetic acid as metabolites. In acidic artificial gastric juice, linalyl acetate is found to be rapidly hydrolyzed (t_1/2< 5 min) to yield linalool which rearranges into and the ring closed form alpha-terpineol (data available from secondary source, Bickers 2003). In contrast, linalyl acetate is slowly (t_1/2= 121 min) hydrolyzed to a mixture of linalool and the ring closed form in neutral gastric juice. Linalyl acetate also hydrolyzed in homogenates of rat intestinal mucosa, blood, and liver, but at rates much slower than in acidic gastric juice (rate constant for hydrolysis k=0.01-0.0055 min^-1vs. >5 min^-1in gastric juice). Based on these observations, it is concluded, that linalyl acetate hydrolyzes in gastric juice to yield linalool which to some extent is ring-closed to yield alpha-terpineol.

In another study, available from a secondary source, linalyl acetate is hydrolysed in gastric juice with half-life values of 3.6 or 7.3 minutes (dependent on the calculation method used). In pancreatic fluids, higher half-life values were determined, i.e. 52.2 or 52.8 minutes (OECD SIDS 2002). As described above, hydrolysis occurs more rapidly at the low pH of gastric fluids. This is supported by the findings of a hydrolysis study of linalyl acetate in buffer solutions at pH 4, 7 and 9, showing a reduction after 2.4 hours incubation of the relative test substance concentration by 98%, 97% or 96%, respectively. Therefore, it is expected that linalool and acetic acid are the substance that will enter the systemic circulation after oral uptake of linalyl acetate. The carboxylic acid formed by hydrolysis of the linalyl acetate is a normal constituent of the body and is known to be easily and rapidly metabolized. In publications with limited documentation (available as abstracts), single or multiple (14 day) administration of lavender oil (linalool and linalyl acetate represent the main constituents, i.e. 70 -80% ) resulted in evident maximum levels of linalool, whereas linalyl acetate levels were not detectable or present in much lower concentrations in blood plasma or different organs/tissues (liver, kidneys, brain and fat tissue; Nölder 2013). These data indicate a quick and efficient metabolization of linalyl acetate to linalool in vivo.

In an review by the RIFM expert panel, glucuronic acid conjugation and excretion of linalool was described to be the primary route (RIFM Expert panel 2008). Allylic oxidation after repeated dosing, leading to 8-hydroxylinalool and 8-carboxylinalool, was described, whereas reduction metabolites such as dihydro- and tetrahydrolinalool have been identified in the urine either free or in the conjugated form after single dosing. Linalool undergoes partial ring closure to yield mainly alpha-terpineol and minor amounts of the terpenoid primary alcohols, geraniol and nerol. In acidic (pH 1.8) artificial gastric juice and in neutral media (pH 7.5), linalool is rapidly rearranged to yield alpha-terpineol and small amounts of geraniol and nerol. Both linalool and alpha-terpineol may then be either conjugated and excreted or oxidized to more polar excretable metabolites.

Studies on genotoxicity (Ames-Test, gene mutation in mammalian cells in-vitro, micronucleus assay in-vivo) of linalyl acetate or linalool were negative, i.e. there is no indication of a reactivity of linalyl acetate or its metabolites under the test conditions.

 

Taking into account the log Po/w, and the considerations on the metabolism, accumulation of linalyl acetate is considered to be unlikely.