Fusel oil

Endpoint summary

Description of key information

Based on the absorption characteristics of the main constituents, Fusel oil is supposed to be absorbed rapidly and almost completely. Following absorption Fusel oil is supposed to be distributed well within the body, predominantly within the water compartment. Based on the excretion of the main constituents, Fusel oil is supposed to be excreted unchanged or metabolised. In addition, metabolites may be included in the intrinsic biochemical pathways. No accumulation of any constituents is supposed.

Additional information

Fusel oil is a UVCB substance comprising a complex mixture of alcohols, aldehydes, esters and other substances. The constituents and their concentration ranges are known. Fusel oil contains 4 main constituents being above ≥ 10%. In total, the 4 main constituents account for ≥ 80% of all constituents. In order to fulfil the standard information requirements set out in Annex IX in accordance with Annex XI, 1.5, of Regulation (EC) No 1907/2006, read-across from surrogate substances was conducted.

In accordance with Article 13 (1) of Regulation (EC) No 1907/2006, "information on intrinsic properties of substances may be generated by means other than tests, provided that the conditions set out in Annex XI are met.” In particular for human toxicity, information shall be generated whenever possible by means other than vertebrate animal tests, which includes the use of information from surrogate substances (grouping or read-across).

The physicochemical, toxicological and ecotoxicological properties of the main constituents of Fusel oil determine, to a great extent, the physicochemical, toxicological and ecotoxicological properties of Fusel oil itself. Therefore, having regard to the general rules for grouping of substances and read-across approach laid down in Annex XI, 1.5, of Regulation (EC) No 1907/2006, a read-across is appropriate as their physicochemical, toxicological and ecotoxicological properties are likely to be similar. A detailed justification for use of read-across is given in chapter 13 of the technical dossier.

In accordance with Annex VIII, Column 1, Item 8.8 of Regulation (EC) 1907/2006 and with Guidance on information requirements and chemical safety assessment Chapter R.7c: Endpoint specific guidance (ECHA, 2012), assessment of the toxicokinetic behaviour of Fusel oil was conducted to the extent that can be derived from the relevant available information on physicochemical and toxicological characteristics in combination with available data evaluating the toxicokinetic properties of the individual constituents of Fusel oil.  

Absorption

For 2-methylbutan-1-ol (CAS No. 137-32-6) complete absorption was observed 1 h after intraperitoneal administration (Haggard et al., 1945). Primary amyl alcohol, containing 1-pentyl alcohol and 2-methylbutan-1-ol is readily absorbed through the lungs (OECD, 2006) and is eliminated after metabolism postulated to occur by alcohol and aldehyde dehydrogenases. Continuous exposure of rats to 2000 ppm primary amyl alcohol (approximately 65% 1-pentyl alcohol, 35% 2-methylbutan-1-ol) over a 90-minute interval resulted in peak blood levels of 451 μM 1-pentyl alcohol and 217 μM 2-methylbutan-1-ol at 20 minutes after start of exposure.  

Intraperitoneal application of 1000 mg/kg bw 3-methylbutan-1-ol (CAS No. 123-51-3), applied as 4 consecutive treatments with 250 mg/kg bw in 15 minute-intervals, led to complete absorption of the test substance within 1 h after administration (Haggard et al., 1945). After intraperitoneal application of 100 mL/kg bw 3-methylbutan-1-ol (CAS No. 123-51-3), applied as 4 consecutive treatments with 25 mL/kg bw at 15 minute-intervals, 3-methylbutan-1-ol was detected for 2 h after dosing. No 3 -methylbutyraldehyde was observed. When 3-methylbutan-1-ol was given in combination with 20% ethanol in water, 3-methylbutan-1-ol could be detected for 10 h after dosing. Furthermore, 3-methylbutyraldehyde could be observed for 8 h.

A single oral administration of 1600 mg/kg bw 2-methylpropan-1-ol (CAS No. 78-83-1) to male rabbits resulted in a maximum concentration in the blood of 0.8 g/L (after 1 h) (Saito, 1975). 2-methylpropan-1-ol could no longer be detected in the blood 6 h after administration. Blood pH levels dropped to 7.2-7.3 from the 30-minute time point until 4 hours post-dosing. Changes in blood pH were considered due to depressed respiratory activity and not due to the production of metabolites (e.g. isobutyric acid). Urinary levels of isobutyraldehyde were 0.12 mg/mL while isobutyric acid was present in trace amounts.

Respiratory bioavailability studies conducted with 2-methylpropan-1-ol have correlated airborne 2-methylpropan-1-ol levels with internal blood levels of 2-methylpropan-1-ol and isobutyric acid (OECD, 2004a). Inhalation of 2000 ppm (6060 mg/m³ ) 2-methylpropan-1-ol in a closed chamber resulted in 2-methylpropan-1-ol levels up to 278 μM and isobutyric acid levels up to 93 μM. Blood levels of 2-methylpropan-1-ol decreased to 155 μM by 90 minutes and isobutyric acid levels were not detectable.

In a physiologically based pharmacokinetic (PBPK) model predictions are that for men exposed to ethanol (CAS No. 64-17-5), at 0.942 and 1.88 mg/L for 8 h and for the lower breathing rate in men exposed to 9.42 mg/L, the liver is able to metabolize ethanol at the rate it enters the body. However, for the higher breathing rate in men exposed to 9.42 mg/L and for men exposed to 37.6 or 63.6 mg/L the rate of ethanol delivery via breathing exceeds metabolic capacity and ethanol blood levels consequently rise for the duration of the exposure. Men exposed to 20 mg/L ethanol for 4 h also showed a continued accumulation during exposure at the higher breathing rate but little or no accumulation at the lower breathing rate (summarized in OECD, 2004b).

Other work has shown a similarly good correlation between inhalation exposure and blood alcohol concentrations. A group of human volunteers (24 in one experiment, 16 in the second) were exposed to ethanol vapour concentrations up to 3610 mg/m³ and resultant blood ethanol concentrations (BEC) measured of between 0.00066 and 0.0056 mg/cm³. Regression analysis of the data shows that BEC = exposure (ppm) × 0.0029 (with a 7% error for 95% confidence) (Seeber, 1994). Around 60% of inhaled ethanol vapour is absorbed (Lester, 1951; Kruhoffer, 1983).

Dermal absorption

Based on QSAR calculations of the main constituents of Fusel oil and their physico-chemical properties a low dermal absorption can be expected for Fusel oil. 2-methylbutan-1-ol (CAS No. 137-32-6), 3-methylbutan-1-ol (CA 123-51-3), 2-methylpropan-1-ol (CAS No. 78-83-1), and ethanol (CAS No. 64-17-5) may be too hydrophilic to cross the lipid rich environment of the stratum corneum based on their water solubility > 10000 mg/L and the log P value below 0 (ECHA, 2012). Further details are given in the analogue justification report attached to chapter 13 of the technical dossier.

Available data from in vitro and in vivo studies with ethanol show that ethanol has a low potential for dermal absorption. Ethanol (64-17-5) penetration through pig's skin in vitro was greater in occluded cells than in non-occluded cells (2.19 mg/cm² and 0.10 mg/cm² in 24 h, respectively) (Pendlington et al., 2001). At the maximum flux under occlusion, the amount of ethanol penetrating from a 1m² area of skin would give a blood alcohol level of about 40 mg/L in a 70 kg man. In a comparative human use study, none of the blood samples taken from sixteen human volunteers exhibited a detectable level of alcohol (Pendlington et al., 2001). Ethanol has a very low octanol : water partition coefficient and this is seen as contributing to the poor dermal uptake of ethanol in intact human skin. This study suggests that a systemic dose of ethanol is likely to be very low after the use of formulations delivering ethanol to the skin.

Distribution and accumulation

A single dose of 2000 mg/kg bw of aqueous 3-methylbutan-1-ol (CAS No. 123-51-3) was given to fasted Wistar WAG rats (Gaillard and Derache, 1965). Blood and urine were collected for up to 8 h for measurement of test material levels. No substance was found in urine. 3-methylbutan-1-ol was detected in blood at 15 min (7 mg/100 mL), increasing to peak concentrations of 17 mg/100 mL at 1 h and declining to 3 mg/100 mL 4 h after administration.

Similarly, ethanol (CAS No. 64-17-5), 2-methylbutan-1-ol (CAS No. 137-32-6), and 2-methylpropan-1-ol (CAS No. 78-83-1) are supposed to be distributed well within the water compartments of the body and no accumulation is supposed.

Metabolism

In vitro experiments have demonstrated additional oxidation of 3-methylbutan-1-ol (CAS No. 123-51-3) and 2-methylbutan-1-ol (CAS No. 137-32-6) by rat liver microsomes via CYP P450 enzymes, and glucuronidation. Only at very high concentrations (300-400 mmol/L) ethanol was a competitive inhibitor of the glucuronidation of e. g. 3-methyl-butan-1-ol. By comparison, the competitive inhibiting effect of ethanol on oxidation of 2-methylbutan-1-ol could already be seen at very low ethanol concentrations of 5-10 mmol/L (Iwersen and Schmoldt, 1995).

In a further study the metabolism of alcohols by the isolated perfused rat liver was studied using livers from female Wistar rats. Material was added directly to the perfusate. The time-dependent decline in the concentration of iso-amyl alcohol in the liver perfusate followed first-order kinetics when the perfusate concentration of the alcohol was below 1 mmol (Auty and Branch, 1976).

Oxidative metabolism of different alcohols by purified human alcohol dehydrogenases, class I-III, was examined in vitro (Ehrig et al., 1988). At substrate concentrations between 10 and 100 µM of 3-methyl-1-butanol, the formation of 3-methylbutyraldehyde was mainly catalysed by class I ADH (Km 32 µM). At substrate concentrations between 10 and 100 µM of 2-methyl-1-butanol, the formation of 2-methylbutyraldehyde was mainly catalysed by class I ADH (Km 30 µM).

In an effort to understand the elimination kinetics of aliphatic alcohols found in alcoholic beverages, research was conducted with human subjects (Rüdell et al., 1983). Test subjects consumed 2-methylpropan-1-ol (CAS No. 78-83-1) in an ethanol/water vehicle over a two hour time period. Blood and urine samples were collected prior to consumption, at the end of the two-hour consumption period, at one, two, eight (urine only), and nine hours after the end of the exposure period. Isobutanol and its metabolites isobutyraldehyde and isobutyric acid were observed in the blood at the end of the consumption period. The addition of ethanol to the test beverage altered the rate of isobutanol metabolism by enzyme inhibition, but had no influence on the metabolite spectrum.

Following any route of ethanol (CAS No. 64-17-5) intake resulting in an elevated blood ethanol level (BEL), metabolism proceeds in three basic steps. First, ethanol is oxidized within the cytosol of hepatocytes to acetaldehyde; second, acetaldehyde is rapidly converted to acetate, mainly in the mitochondria; and third, acetate produced in the liver is released into the blood and is oxidized by peripheral tissues to acetic acid and ultimately carbon dioxide, and water. The rapid conversion of the intermediate aldehyde means that concentrations are usually very low. The main pathway for ethanol metabolism proceeds via alcohol dehydrogenase. However, other pathways for ethanol oxidation have been described including a microsomal ethanol-oxidizing system located in the endoplasmic reticulum and a catalase system located in the peroxisomes. The rate of hepatic metabolism of ethanol is concentration independent except at very low or very high concentrations. Blood ethanol in humans decreases more rapidly at concentrations over 300 mg/dL than at concentrations below this level, possibly due to oxidation by the microsomal ethanol oxidizing system. The maximum rate of metabolism is 100 - 125 mg/kg bw/hour, although tolerant individuals may have higher metabolic rates (up to 175 mg/kg/hour) due to enzyme induction. Adults metabolize 7 - 10 g ethanol/h reducing blood ethanol concentrations at a rate of 15 - 20 mg/100 mL/h. Ethanol is metabolized more rapidly in chronic alcohol abusers (up to 40 mg/100 mL/h) and in children (up to 28 mg/100 mL/h) (cited in OECD, 2004b).

Excretion

Toxicokinetics of 2-methylbutan-1-ol (CAS No. 137-32-6) was investigated rats (Haggard et al., 1945). Intraperitoneal administration of the test substance led to complete absorption within 1 h after administration. 4 h after administration no substance was detectable in tissues. 5 h after administration 0.22% and 0.86% of the test substance were observed in urine and expired air, respectively. Oral application of 25 mmol 2-methylbutan-1-ol /rabbit (corresponding to approx. 735 mg/kg bw) led to excretion of 10% of the administered substance into urine as glucuronides within 24 h after administration of the test substance. The urine did not contain aldehydes or ketones (Kamil et al., 1953).

As for 2-methylbutan-1-ol the toxicokinetics of 3-methylbutan-1-ol (CAS No. 123-51-3) were observed using the same experimental design. Complete absorption of the test substance occurred within 1 h after administration. After 4 h no substance was detectable in tissues. 5 h after administration 0.97% and 0.27% of the test substance were observed in urine and expired air, respectively (Haggard et al., 1945). Oral application of 25 mmol amyl alcohol/rabbit (corresponding to approx. 735 mg/kg bw) led to excretion of 9% of the test substance into urine as glucuronides within 24 h after administration. The urine did not contain aldehydes or ketones (Kamil et al., 1953).

In the study from Rüdell et al. (1983) the elimination kinetics of aliphatic alcohols found in alcoholic beverages was observed with human subjects (more details in section “Metabolism”). Urinary concentrations of 2-methylpropan-1-ol (CAS No. 78-83-1; isobutanol) peaked at the one-hour postexposure time point. Urinary levels of the main metabolite isobutyric acid peaked at the end of the two-hour exposure period. Urinary levels of isobutyraldehyde, a minor metabolite, peaked at the eight hour post-exposure time point. Thus, 2-methylpropan-1-ol is excreted unmetabolized or metabolized via urine.

Ethanol is eliminated from the body mainly by metabolism in the liver and to a low extent by urinary excretion and pulmonary exhalation. Other tissues such as kidney, stomach and intestines oxidize ethanol to a small extent. Excretion occurs unchanged via kidneys and lungs at 5 – 10 % of an absorbed dose (Conibear, 1988). Such a rate of elimination is supported by the data from Jones and the PBPK modeling work described above (OECD, 2004b; Jones 1993).

 

Conclusion

Based on the absorption characteristics of the main constituents, Fusel oil is supposed to be absorbed rapidly and almost completely. Following absorption Fusel oil is supposed to be distributed well within the body, predominantly within the water compartment. Based on the excretion of the main constituents, Fusel oil is supposed to be excreted unchanged or metabolised. In addition, metabolites may be included in the intrinsic biochemical pathways. No accumulation of any constituents is supposed.

References

ECHA (2012) Guidance on information requirements and chemical safety assessment – Chapter 7c: Endpoint specific guidance. European Chemicals Agency, Helsinki

OECD (2006) SIDS Initial assessment report for SIAM 22. Primary amyl alcohol (mixed isomers). CAS No: 71-41-0; 137-32-6. UNEP Publications

OECD (2004a) SIDS Initial assessment report for SIAM 19. Isobutanol. CAS No: 78-83-1. UNEP Publications

OECD (2004b) SIDS Initial assessment report for SIAM 19. Ethanol. CAS No: 64-17-5. UNEP Publications