Linalool

Endpoint summary

Description of key information

Absorption

The rapid absorption of linalool was shown in a study in rats upon oral bolus administration even at relatively high doses with the Cmax being observed already after 40 min (Shi et al., 2016, Parke, 1974a).

 

Distribution

Quantification of linalool concentrations in blood (Shi et al.), plasma, liver, kidney, brain and fat (Nöldner, 2011) after single exposure in rats showed distribution of the substance into these tissues (increasing concentrations from plasma to brain, to liver to kidney to fat). In the same experiment, measurable linalool concentrations were also detected in these tissues after administration of linalyl acetate and silexan (mixture of linalool and linalyl acetate), indicating that linalyl acetate is metabolized to linalool.

In a inhalation study, mice that were exposed to 5 mg linalool/L by inhalation showed blood concentrations of 7 -9 ng/mL immediately after termination of exposure. (Jirovetz, 1991).

 

Metabolism

Metabolism of linalyl acetate to linalool was investigated in several in vitro studies. It was found that linalyl acetate fastly hydrolyses in gastric juice (half-lives between <5 and 7.3 minutes) (OECD SIDS, 2002, Bickers, 2003, Marnett, 2014). Linalyl acetate was also hydrolysed in pancreatic fluids, but with higher half-lifes (52.2 or 52.8 minutes) (OECD SIDS, 2002). Hydrolysis products were found to be linalool and acetic acid (ester hydrolysis) (OECD SIDS, 2002). Linalool, to some extent, was ring-closed to yield alpha-terpineol (Bickers, 2003, Marnett, 2014). Another in vitro study investigated phase II metabolism after incubation of linalool with isolated liver microsomes of rats ans guinea pigs. It was shown that linalool is conjugated with glucuronic acid to the respective glucuronid derivative. The activity of the phase II enzyme UDP-glucuronosyltransferase towards linalool was enhanced by induction with phenobarbital in microsomes of both Wistar rats and guinea pigs. Induction with 3-methylcholanthrene in Wistar rats did not result in an enhancement of activity (Boutin, 1985).

In vivo, linalyl acetate was also rapidly metabolized which resulted in high plasma and tissue levels of linalool without a clear tendency for accumulation after repeated application (Nöldner, 2011 + Nöldner, 2013). Metabolisation pathways of linalool were investigated in different in vivo studies. Phase I metabolism of linalool was investigated after oral exposure to rats and showed that the metabolites 8-hydroxy-linalool and 8-carboxy-linalool were formed, indicating that 8-hydroxylation (allylic hydroxylation) is the major phase I pathway (Chadha, 1984). This was supported by another in vivo study in rats, showing that activity of cytochrome P450 enzymes was increased upon repeated dose treatment of rats with linalool (Parke, 1974b).

The same study also showed that the activity of glucuronyltransferases was increased, indicating that glucuronidation is the main phase II metabolisation mechanism for linalool. It was furthermore concluded that linalool is mainly metabolized in the liver (Parke, 1974b).

After intraperitoneal administration, it was shown that biliary conjugates and non-polar ether-extractable metabolites are formed (glucuronide- and sulfate conjugates) (Parke, 1974a).

In vitro investigations of linalool metabolism by human skin enzymes showed that linalool is metabolised by CYP2C19 and CYP2D6 to (R/S)-furanoid-linalool oxide, (R/S)-pyranoid-linalool oxide and (cis/trans)-8-hydroxylinalool. A linear relationship was observed between the formation of metabolites and incubation time, enzyme and linalool concentration (Meesters, 2007).

 

Excretion

In vivo, Linalool was rapidly eliminated from plasma with a half-life of approximately 45 min (Shi et al., 2016). 97% of linalool was eliminated from tissues after 72h after a single oral dose of 500 mg/kg bw in rats (Parke, 1974a). Linalool could not be detected in plasma after 240 min (Shi et al., 2016). Intraperitoneal administration showed that 97% of glucuronide- and sulfate conjugates were excreted within 72 h, with the majority (ca. 80%) after 36 h and 95% after 48 h. 60% of administered radioactivity occured in urine, 15% in faeces and 25% in expired air (Parke, 1974a).

Given the fast elimination from plasma and tissues and given that glucuronidation and sulfatation were shown as the main phase II metabolisation mechanisms, it can be concluded that linalool does not accumulate and is excreted fastly in form of sulfate and glucuronide conjugates through urine, feces and air.

 

Dermal absorption

Two in vitro studies similar to OECD guideline 428 were performed to determine dermal absorption of linalool. In the first study, occlusion of the test system resulted in a higher absorption rate after 24 hours (3.0% after open application versus 12.7% after occlusion) after incubation with 201 µg/cm² of linalool (Green, 2007). In the second study, linalool was applied at 500 mg/0.65 cm² to human skin and the amount of linalool in the stratum corneum and epidermis and dermis was determined after 1, 2, and 4 h. Under the conditions of this in vitro penetration study in human skin, 0.17% of the applied dose could be recovered in epidermis and dermis (Cal and Sznitowska, 2003). An in vivo dermal absorption study confirms the limited dermal absorption of linalool. In this study the dermal absorption of linalool from lavender oil was studied in one male human subject. The applied dose was about 7 mg linalool. Results showed that linalool is present in plasma peaks approximately after 20 min (ca. 120 ng/mL). Plasma levels returned to its background levels after 90 min indicating rapid elimination from plasma (Jäger et al., 1991, please refer to section 7.10.5).

 

Conclusion

If linalyl acetate is orally taken up, it is rapidly hydrolysed to linalool and acetic acid in gastric juice. Linalool, to some extent, is ring-closed to yield alpha-terpineol. Linalool is fastly absorbed in the GI tract, distributed in different tissues and metabolised in the liver. In the liver, it is metabolised predominantly by cytochrome P 450 enzymes and is than conjugated with glucuronic acid or sulfate to the respective glucuronide- and sulfate conjugates. Linalool conjugates are rapidly excreted through urine, feces and air.

The dermal absorption of linalool is limited. Here, too, linalool absorbed into plasma is rapidly degraded.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

Background

The substance linalool is widely used as flavour and fragrance ingredient. Considerable information on toxicokinetic behaviour, metabolism and dermal absorption is available. Toxicokinetic studies focus on metabolism of linalool; four studies addressing this subject are available in the REACH dossier. A study concerning the absorption, distribution and excretion of linalool via the oral route is also available, as well as a study on the absorption of linalool after inhalation. Dermal absorption of linalool was studied in one in vivo and two in vitro studies.

 

Studies on toxicokinetics in the dossier

The oral absorption of linalool in rats was rapid after an oral dose of 500 mg/kg bw linalool (radiolabelled). Within 2 days after treatment, radioactivity was excreted via urine (approx. 60%), expired air (approx. 23%), and faeces (approx. 15%) indicating that at least 85% of the applied dose was absorbed. A separate experiment showed that considerable enterohepatic circulation is possible, and that bilary conjugates and non-polar ether extractable metabolites are formed and excreted via faeces. Overall, this may indicate that radioactivity was completely absorbed. Three percent of the dose was distributed in tissues after 72 h of dosing; linalool was detected in liver (0.5%), gut (0.6%), skin (0.8%) and skeletal muscle (1.2%). Other organs including kidneys contained insignificant residual radioactivity. Metabolites detected in urine and bile indicated that linalool is largely excreted in the form of glucuronic acid conjugates. (Parke et al., 1974a)

 

After exposure to linalool by inhalation, linalool could be detected in blood of exposed mice (7-9 ng/mL serum). (Jirovetz et al., 1991) Other reports showed that after inhalation of linalool at 3.21 mg/L for 30, 60, and 90 min, plasma levels were around 1, 2.5, and 3 ng/mL plasma. (Buchbauer et al., 1991)

 

To determine dermal absorption, two in vitro studies were performed that were both similar to guideline OECD 428. In the first study, human skin was exposed to radiolabelled linalool for 24 hours using an in vitro diffusion cell test system. Prepared human skin membranes were exposed to 40.2 mg/mL (201 µg/cm^2, 1.2 cm^2 cell) radiolabelled linalool (in 70/30% ethanol/water) for 24 hours under open or occluded conditions. The amount of test substance left on the skin (by wipe), in the donor chamber, stratum corneum (tape strips), epidermis, filter paper and receptor chamber were determined by liquid scintillation counting. One control skin membrane was used (unoccluded) where only the vehicle was added. Evaporation loss was also determined in an additional experiment. A reference substance (benzoic acid) was used to test the validity of the test sytem. Linalool was found to penetrate through skin and after 24 hours, 3.0% of the applied dose was recovered in receptor fluid in the open test system, versus 12.7% in the receptor fluid in the occluded test system. It was found that about 97% of the applied linalool dose evaporated within 24 hours, resulting in a very low total recovery in the open test system. Benzoic acid was found to penetrate rapidly through skin, total recovery was 89.6%. The test system is therefore shown to be valid. Under the conditions of this test, linalool was found to penetrate through human skin. Occlusion of the test system resulted in a higher absorption rate after 24 hours (3.0% versus 12.7%). However, a large portion (97%) of the applied dose evaporated within 24 hours (Green, 2007).

 

A second in vitro dermal absorption study studied the absorption of linalool in human skin after 4 hours of (occluded) exposure. Linalool easily penetrated into the skin, however it was not detected in the acceptor fluid. The dermal absorption of linalool was determined to be 0.17%. (Cal and Sznitowska, 2003)

 

An in vivo dermal absorption study confirms the limited dermal absorption of linalool. In this study the dermal absorption of linalool from lavender oil was studied in one male human subject. The applied dose was about 7 mg linalool. Results showed that linalool present in plasma peaks approximately after 20 min (ca. 120 ng/mL). Plasma levels return to almost background after 90 min indicating rapid elimination from plasma. (Jäger et al., 1991, please refer to section 7.10.5)

 

In four studies, metabolism of linalool was studied, mainly by investigating enzyme induction upon exposure with linalool. Increased activity of cytochrome P450, cytochrome b5, biphenyl-4-hydroxylase and 4-methylumbelliferone glucuronyltransferase were observed in rats that were exposed to linalool for 64 days at a dose level of 500 mg/kg bw/day (Parke et al., 1974b).

 

In another in vivo study in which rats (600 mg/kg bw/d) were exposed to linalool for six days, increased P450-activity was observed up to three days of dosing, whereas activities had returned to normal after six days of dosing. In the same study, the metabolites 8-hydroxy linalool and 8-carboxy linalool were identified in urine (after acidic hydrolysis) after 20 days of exposure, indicating that 8-hydroxylation seems to be the major Phase I metabolism pathway. In addition, other minor metabolites could not be characterised. (Chadha and Madhava Madyastha, 1984)

 

The metabolism of linalool by human skin P450 enzymes CYP2C19 and CYP2D6 was studied in another in vitro study. Linalool was metabolized to (R/S)-furanoid-linalool oxide, (R/S)-pyranoid-linalool oxide and (cis/trans)-8-hydroxylinalool, dependent on time, linalool, and enzyme concentration (Meesters et al., 2007).

 

Phase II metabolism was also studied using an in vitro experiment. It was shown that when the phase II enzyme UDP-glucuronosyltransferase (UDPGT) was induced by phenobarbital, UDPGT specific activity towards linalool was higher than when UDPGT was induced by 3-methylcholanthrene. These experiments indicate that linalool is conjugated with glucuronic acid. (Boutin, J.A. et al., 1985)

 

Toxicokinetic behaviour of linalool in rats showed rapid absorption from the gastrointestinal tract after an oral dose of 300 mg/kg using 0.5% CMC as vehicle. Tmax was reached after 40 min with a Cmax of 1915.45 ng/mL. The half-life was estimated to be 44.72 minutes and the AUC 76003.4 ng/min/mL. (Shi et al., 2016)

 

It can be concluded that linalool is rapidly absorbed after oral administration (at least 85%). After 72h, 97% of the administered radioactivity were excreted. 3% of the dose was detected in tissues (liver, gut, skin and skeletal muscle) 72 h after dosing. Linalool was excreted mainly via urine (60%), exhaled air (23%) and faeces (15%) and is subject to enterohepatic recirculation. Both phase I and phase II enzymes (mainly glucuronidation) are responsible for the metabolism of linalool, 8-hydroxylation seems to be the major Phase I pathway. The dermal absorption of linalool is low: 0.17% after 4 hours exposure (occluded). Experiments show that most of the dermally applied linalool evaporates from skin.

 

Information from public literature

The most reliable references from literature were included in the dossier and summarized in the paragraph above. Additionally, an OECD SIDS assessment is available in which the toxicokinetics of linalool is discussed. In the document the rapid oral absorption of linalool is confirmed, as well as the excretion in urine, faeces, and via the expired air. The conclusion of the OECD SIDS assessment is that the relatively rapid overall excretion of linalool and its metabolites suggests no long-term hazard.

Bickers et al. (2003) and Marnett et al. (2014) both report the results of in vitro metabolism studies of linalool and linalyl acetate, the acetic acid ester of linalool. Linalyl acetate is quickly hydrolysed to linalool and acetic acid in gastric juice with a half-life of less than 5 minutes. In addition, Linalyl acetate is also hydrolyzed in homogenates of rat intestinal mucosa, liver, and intestinal fluid. Thus, it can be expected that Linalyl acetate will be hydrolyzed pre-systemically to linalool and acetate. The pre-systemic metabolism of linalyl acetate is supported by experimental data in rats. Nölder et al., 2011 administered linalyl acetate at a single dose of 37 mg/kg bw to rats. Likewise, linalool was given at approximately 30 mg/kg bw to rats. The bioavailability and organ distribution of linalool and linalyl acetate were investigated. After administration of linalool, the peak concentration in plasma was 33 ng linalool/mL, 218 ng/g in liver, 541 ng/g in kidney, 1140 ng/g in fat and 43 ng/g in brain tissue. After administration of linalyl acetate, linalyl aceate was only found in brain and fat tissue. However, the linalool concentrations were 10 ng linalool/mL plasma, 274 ng/g in liver, 255 ng/g in kidney, 244 ng/g in fat, and 0 ng/g in brain. Overall, the available data nicely show that Linalylaceate is rapidly cleaved into linalool and acetate which are afterwards systemically available.

 

Information from other studies

Physical/chemical parameters such as log Kow, water solubility, vapour pressure, and molecular weight, as well as parameters like hydrolysis can provide useful information regarding the behaviour of a substance in the body. Linalool has a log Kow of 2.9 and a water solubility of 1.56 g/l. The vapour pressure is 0.273 hPa and the molecular weight is 154.24. No information on hydrolysis is available.

The moderate log Kow and good water solubility, as well as the relatively low molecular weight would favour oral, respiratory and dermal absorption. This confirms the findings found for oral absorption, as well as the presence of systemic effects in a 28-day repeated dose toxicity study. However, dermal absorption is low according to the available data. This can be explained by the high vapour pressure of linalool. This finding is confirmed by the available data on dermal absorption. The results of a 90-day dermal toxicity study partly confirm this, effects were mostly local on the skin (skin irritation), however, decreases in body weight, and increases in liver and kidney weight were also observed in the highest dose tested, indicating that some absorption may occur esp. on irritated skin.

 

Conclusions

In conclusion, linalool is rapidly absorbed via oral exposure and is excreted mainly via urine, expired air, and faeces. Excretion is as well rapid. Overall, the data indicate no potential for bioaccumulation. The data indicate further, that linalool is metabolized extensively to harmless metabolites metabolites. Dermal absorption is low. Linalool topically applied to human skin evaporates to a high extent due to its vapour pressure.

 

References

Bickers, D., Calow, P., Greim, H., Hanifin J.M., Rogers A.E., Saurat J.H., Sipes I.G., Smith R.L., Tagami H. (2003) A toxicologic and dermatologic assessment of linalool and related esters when used as fragrance ingredients, Food Chem Toxicol 21: 919 -942.

 

Boutin, J.A., Thomassin, J., Siest, G., Cartier, A. (1985). Heterogeneity of hepatic microsomal UDP-glucuronosyltransferase activities. Conjugation of phenolic and monoterpenoid aglycones in control and induced rats and guinea pigs. Biochemical Pharmacology, 34, 113: 2235-2249

 

Cal., K., Sznitowska, M. (2003). Cutaneous absorption and elimination of three acyclic terpenes. Journal of Controlled Release, 92: 369-376

 

Chadha, A., Madhava Madyastha, K. (1984). Metabolism of geraniol and linalool in the rat and effects on liver and lung microsomal enzymes. Xenobiotica, 1984, 14, 5: 365-374

 

Green, D.M. (2007). In vitro human skin penetration of fragrance material linalool under both in-use and occluded conditions from an ethanol/water vehicle. Research Institute for Fragrance Materials (RIFM), report nr. R03/13a/05

 

Jäger, W., Buchbauer, G., Jirovetz, L., Fritzer, M. (1991). Percutaneous absorption of lavender oil from a massage oil. Journal of the Society of Cosmetic Chemists, 43: 49-54

 

Jirovetz, L., Jäger, W., Burchbauer, G., Nikiforev, A., Raverdino, V. (1991). Investigations of animal blood samples after fragrance drug inhalation by gas chromatography/mass spectrometry with chemical ionization and selected ion monitoring. Biological Mass Spectrometry, 20: 801-803

 

Marnett L.J., Cohen S.M., Fukushima S., Gooderham N.J., Hecht S.S., Rietjens I.M.C.M., Smith R.L., Adams T.B., Bastaki M., Harman C.L., McGowen M.M., Taylor S.V. (2014) GRASr2 Evaluation of Aliphatic Acyclic and Alicyclic Terpenoid Tertiary Alcohols and Structurally Related Substances Used as Flavoring Ingredients, Journal of Food Science Vol. 79, Nr. 4, doi: 10.1111/1750-3841.12407

 

Meesters, R.J.W., Duisken, M., Hollender, J. (2007). Study on the cutochrome P450-mediated oxidative metabolism of the terpene alcohol linalool: Indication of biological epoxidation. Xenobiotica, 37 (6):604-617

 

Nölder M., Germer S., Koch E. (2011) Pharmacokinetics of linalool and linalyl acetate, the two main constituents of silexan, an essential oil from Lavandula angustifolia flowers, in rats, Planta Med 2011; 77 - PM44, DOI: 10.1055/s-0031-1282802, congress abstract

 

OECD SIDS (2002). Linalool. UNEP Publications.

 

Parke, D.V., Quddusur Rahman, K.H.M., Walker, R. (1974a). The absorption, distribution and excretion of linalool in the rat. Biochemical Society Transactions 2: 612-615

 

Parke, D.V., Quddusur Rahman, K.H.M., Walker, R. (1974b). Effect of linalool on hepatic drug-metabolizing enzymes in the rat. Biochemical Society Transactions 2: 615-618.

 

Shi. F., Zhao, Y., Firempong C.K., Xu, X. (2016) Preparation, characterization and pharmacokinetic studies of linalool-loaded nanostructured lipid carriers, Pharmaceutical Biology 54:10, 2320-2328, DOI: 10.3109/13880209.2016.1155630.