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Description of key information

The test item decreased the pulmonary microsomal BROD and ERDO activities which are markers for the Cytochrome P450IIB and P450IA family, respectively.

This effect was not observed in the liver microsomes.

The primary route for phenol excretion involves conjugation of the hydroxyl group as an sulfate or glucuronide. Minor metabolic pathways utilize side chain oxidation and/or ring hydroxylation. Urinary excretion of phenyl sulfate and phenyl glucuronide was detected in man after oral ingestion of phenol. (Scientific literature review of phenols in flavor usage, Vol.1, Flvor and Extract Manufacturers Association (FEMA), 1985)

The primary route of metabolism of xylene isomers in human liver is the formation of benzylalcohol. (Human Cytochrome P450 Isoform Specificity in the Regioselective Metabolism of Toluene and o-, m- and p-Xylene, JPET, Vol 278, No. 1, 1995)

Key value for chemical safety assessment

Additional information

For the test item there are no particular studies on adsorption, distribution, metabolism and excretion in vivo available. Thus, the information and data on toxicokinetics is limited.

Only one publication on rats (Day, 1992) is available dealing with pulmonary metabolites of p-xylene (including the test item) to investigate the divergent effects of p-xylene on pulmonary and hepatic metabolism. Further in vivo and in vitro studies on the metabolism of p-xylene which is metabolised to the test item are summarised below for completeness.

Day (1992):

 To investigate hepatic and pulmonary metabolism of the test item rats were given the test item intraperitoneally for 3 days and effects on microsomal cytochrome P450 enzyme activities were investigated 12 hours later. The test item decreased the pulmonary microsomal BROD and ERDO activities which are markers for the Cytochrome P450IIB and P450IA family, respectively. This effect was not observed in the liver microsomes.

Bakke (1970):

 In a study on rats the hydroxylation of p-xylene was studied following oral administration. Phenolic metabolites were quantitatively estimated in hydrolysed urine samples by gas chromatography. The aromatic hydrocarbon was administered at a dose of 100 mg/kg bw. As a result only 1 % of the administered dose of p-xylene was metabolized to 2,5-dimethylphenol.

  

Smith (1982):

 The metabolism of p-Xylene was investigated using isolated perfused rabbit lungs and livers. The metabolites produced from p-Xylene were characterized by HPLC.

The major hepatic metabolite was p-toluric acid (N-p-toluylglycine), with smaller amounts of toluic acid (p-methylbenzoic acid) and p-methylbenzyl alcohol being produced. Rabbit livers did not produce detectable amounts of p-tolualdehyde, 2,5-dimethylphenol or any glucuronide conjugates of p-xylene derivatives. The major pulmonary metabolite of p-xylene was p-methylbenzyl alcohol. The predominance of this metabolite reflects the deficiency of lung tissue in alcohol dehydrogenase.

A qualitative difference between hepatic and pulmonary metabolism of p- Xylene was the formation of small quantities of 2,5-dimethylphenol by lung, but not liver. This phenol is a minor urinary metabolite of p-Xylene in rabbits (Bray et al., 1950) and the data of this study suggest a role for the lung in the formation of this compound in vivo.

   

Sedivek (1976):

 A study with human volunteers was conducted to investigate metabolic changes of xylenes in the human organism Fifteen exposure experiments were carried out, in which four persons each, were simultaneously exposed to a defined concentration of o-, m-, and p-xylene vapors and also to their mixture at a ratio of 1:1:1. The concentrations were in all cases around 0.2 – 0.4 mg /L. The period of exposure amounted to exactly 8 h.

The chief metabolites are toluic acids which are excreted in conjugated form with glycine as so-called toluric (=methylhippuric) acids.

The minor metabolites of xylenes are compounds hydroxylated on the aromatic ring. After exposure to o-xylene the presence of 2,3 and 3,4-xylenol was observed in urine; after exposure to m-xylene the presence of 2,4 xylenol, and finally after exposure to p-xylene the presence of 2,5-xylenol. The excretion of xylenols reaches a maximum as a rule just after termination of exposure.

It was proved by balance calculation that of the total amount of xylene retained in the organism during exposure, more than 95 % is excreted in the form of toluric acid (o-97.1 %; m-99 2 %; p-95 1 %) and only a small part in form of xylenol (o-0.86 %; m-1 98 %; p-0 05 %).

Carlone (1974):

In an in vitro metabolism study with p-Xylene optimum conditions for metabolism in rabbit liver or lung microsomal enzymes have been studied.

The major metabolite of p-xylene in vitro, as determined by t.l.c., of liver and lung microsomal incubation mixtures was p-toluic acid. Phenobarbital pre-treatment of rabbits (multiple doses, intraperitoneal or intravenous) induced the liver microsomal xylene-metabolizing system approximately 3-fold, whereas there was no change in activity of the lung enzyme.

Chlorpromazine pre-treatment, intraperitoneal (but not intravenous), of rabbits stimulated hepatic microsomal xylene-metabolizing activity. Administration of 1,2,3,4 dibenzanthracene or 3-methylcholanthrene to rabbits resulted in decreased xylene metabolism in vitro by liver and lung microsomes.

Gagnaire (2007):

Blood and brain concentrations of o-, m- and p-xylene were determined in young adult male Sprague-Dawley rats following oral gavage administration of 8.47 mmol/kg of each isomer. Levels ofp-xylene in blood and brain (Cmax, AUC) were approx. double, and tissue half-lives slightly shorter, compared to those found for o- and m-xylene. Levels of p-xylene in brain (Cmax, AUC) were around 2 to 3-fold greater than those for blood. The results indicate slight toxicokinetic differences for the three xylene isomers in the rat following acute oral exposure.

Blood and brain concentrations of p-xylene were determined in rats and guinea pigs following acute or sub-acute (10 day) oral gavage administration of 8.47 mmol/kg, or single inhalation exposure to 1800 ppm. The concentration of p-xylene in blood and brain was 3-9 fold greater for rats compared to guinea pigs irrespective of the route and/or duration of exposure. The AUC was 5 -12 fold greater for rats than guinea pigs. The half-life in blood and brain was comparable in both species following acute or sub-acute oral administration, however the half-life in rat blood after acute inhalation exposure was approx. 4 -fold longer than that of the guinea pig. The results demonstrate clear differences in disposition of p-xylene in rats and guinea pigs following oral and inhalation exposure.

Bioaccumulation potential

In rats, the individual xylene isomers are all rapidly absorbed with peak concentrations in blood occurring between 0.5 and 2 hours after oral administration. Peak concentrations in brain coincided with those in blood but were approximately 2.5-3 x greater. The elimination half life from both blood and brain was approximately 2.5-4 hours. Systemic exposure to xylene was lower following repeated oral doses than after a single oral dose indicating induction of metabolising enzymes (Gagnaire et al., 2007). Following exposure of human volunteers by inhalation (0.2 or 0.4mg/L for 4 hours) to xylene isomers either individual or as a mixture, approximately 64% of the inhaled dose was retained; this value was independent of dose or duration of exposure. Following exposure, approximately 5% of the retained dose was eliminated in exhaled air with the remainder excreted as metabolites in urine. The major urinary metabolite was methyl hippuric acid; trace amounts of xylenols were also detected (Sedivec and Flek, 1976). After exposure of volunteers to 200mg xylene/m3for 4h, elimination of unchanged xylene in urine was biphasic with half lives of approximately 1 and 11 hours; only 0.0015% of the absorbed dose was excreted unchanged in urine Janasik et al. (2008). Kawai et al. (1991) and Inoue et al. (1993) determined methyl hippuric acids (MHA) in end-shift urine samples from workers occupationally exposed to xylene, both groups found a significant linear correlation between the time weighted average intensity of exposure and MHA excretion and concluded that this could be used as a marker of exposure.

The metabolism of ethylbenzene has been reviewed in the recent RAR (2008). Absorption via inhalation and the oral route was considered and it was concluded that for risk characterisation purposes inhalation absorption of 65 % was applicable for humans and 45 % for animals. For inhalation via the oral route, 100% oral absorption should be assumed for animals and humans.

Although ethylbenzene is rapidly distributed through the body, there is no evidence of ethylbenzene accumulation in fat or fat-rich tissues (RAR, 2008). There are some species differences in metabolism. Side-chain oxidation leads to major metabolites in humans being e.g. mandelic acid, phenylglyoxylic acid with hippuric acid and benzoic acid being the major metabolites in rats. Ring oxidation is a minor metabolic pathway.  With rapid metabolism, ethylbenzene and its metabolites are eliminated rapidly, mainly as urinary metabolites with minor loss via exhalation and excretion in faeces. Following exposure, excretion is virtually complete within 24 hrs.

Dermal absoprtion:

The permeability of xylene through skin from hairless rats was determined in vitro; when applied occluded, the flux was 0.22mg/cm2/h. The dermal penetration of xylene was 0.224% in 8h (Ahaghotu et al., 2005). The dermal absorption of the individual xylene isomers waspredicted using a model which considers dermal absorption as a two stage process, permeation of the stratum corneum followed by transfer from the stratum corneum to the epidermis. The QSAR for each process was derived by fitting each model equation to experimentally derived values using an iterative non-linear least squares approach. Dermal flux and percent absorption were predicted using physicochemical values using values determined at approximately 25°C. Model predictions for o-, m- and p-xylene isomers were approximately 13.9, 11.8 and 10.9% respectively; the corresponding values for the maximum fluxes were 0.000264, 0.000259 and 0.000254mg/cm2/min (ten Berge, 2009). In human volunteers exposed dermally to m-xylene, skin penetration occurred rapidly with detectable concentrations in blood within minutes of exposure beginning; the dermal flux was approximately 2µg/cm2/min. Unchanged xylene was detected in exhaled air but accounted for only 10-15% of that excreted as methyl hippuric acid in urine (Riihimaki, 1979; Engstrom et al., 1977).Howevermore relevant is a comparison of the aqueous permeability of o-xylene between rats and volunteers in vivo (Thrall and Woodstock 2003). The estimated human and rat aqueous permeability coefficients were found to be 0.005 and 0.058 cm/hr, respectively. The water solubility of xylene is about 200 mg/litre (0.2 mg/cm3). This means that the maximum absorption through human skin is estimated to be 0.005 * 0.2 = 0.001 mg/cm2per hour on the condition that the skin is not damaged. Furthermore, due to intermittent exposure of the skin the major part of xylene will evaporate. According to REACH Guidance appendix 14-1 about 1 mg of xylene will evaporate per minute from the skin. So evaporation from the skin will be much faster than absorption. In order to err on the safe side it is assumed, that 1% of xylene is absorbed from the daily dermal dose caused by intermittent dermal occupational exposure

The dermal absorption of ethylbenzene is comprehensively reviewed in the recent RAR (2008). Liquid ethylbenzene is rapidly absorbed through the skin, whereas absorption is negligible after exposure of humans to ethylbenzene vapour. For human volunteers, who had immersed one hand for up to 2 hrs in aqueous solutions of ethylbenzene (Dutkiewicz and Tyras, 1967 cited in RAR, 2008) dermal absorption was up to 45%. However an uptake value of 4% by skin (Susten et al, 2005) is considered to be more representative for incidental contact.

References: 

BAKKE OM, SCHELINE RR; TOXICOL APPL PHARMACOL 16 (3): 691-700 (1970)

SEDIVEC V, FLEK J; INT ARCH OCCUP ENVIRON HEALTH 37 (3): 205-17 (1976)

Day BJ, Carlson GP; Research Communications in Chemical Pathology and Pharmacology 76 (1): 117-20 (1992)

Smith, B.R. et al., THE JOURNAI. OV PHARMACOLOGY AND EXPCRIMENTAL THERAPEUTICS, Vol 223, No 3, (1982)

Carlone, M.F., XENOBIOTICA, 1974, VOL. 4, NO. 11, 705-715, (1974)

Gagnaire F et al., Toxicology 231, 147 -158, 2007