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Description of key information

Short description of key information on bioaccumulation potential result: 
Distribution and metabolism of 1,2,4-trimethylbenzene (pseudocumene) was analysed in the rat and 2,3,6-trimethylphenol was identified as metabolite in both studies (Huo et al., 1989 and Swiercz et al., 2001).

Key value for chemical safety assessment

Additional information

There are no studies available in which the toxicokinetik properties of 2,3,6 -trimethylphenol were investigated. There are two studies in which the distribution and metabolism of 1,2,4 - trimethylbenzene (pseudocumene, CAS No. 95-63-6) was analysed and 2,3,6 -trimethylphenol (CAS No. 2416 -94 -6) was identified as metabolite.

Using the Software Lhasa Meteor, the predicted metabolism of 2,3,6 trimethylphenol was compared with two structural analogues (Smithers REACH Services. 2,3,6 trimethylphenol & two structural analogues for REACH-Biomet Review Report 2010- see section 13 of the IUCLID 5.2. dossier for registration under REACH).

The predicted metabolic pathways for the substances appear broadly similar based on similar metabolic reactions when stopping at phase 2 and considering probable and plausible transformations. Most reactions are anticipated to occur at the aromatic alcohol site. However, the additional methyl groups may be the site of hydroxylation with subsequent oxidation of the alcohol groups. Post oxidation, these sites would then be subject to various phase 2 reactions dependent upon the respective species. Phase 2 metabolism/conjugation reactions typically lead to a water-soluble product, which can then be excreted in the bile or urine. When continuing after phase 2 metabolism, similar reactions areevident for the three substances. Overall, the presence and position of the methyl groups appear to yield negligible influence on the predicted pathways with regard to the reaction types and the sites of reactions when continuing past phase 2.

In the study of Huo et al., 1989 distribution and metabolisms of 1,2,4 - trimethylbenzene (pseudocumene, CAS No. 95-63-6) were analysed.

For distribution 4 groups of 3 male hooded-Wistar rats (approximately 200 g), which were fasted overnight before receiving an oral dose of 14C-124TMB (0.8mmol=0.49 µCi/kg) in olive oil (48 mg/ml) (Huo et al 1999). The animals were then transferred to separate metabolism cages and groups were killed after 3, 6, 12 and 24 hours. Samples of blood, major organs, muscle and adipose tissue were removed and weighed.

14C-124TMB was rapidly and widely distributed throughout the body with the highest levels in adipose tissue. No other preferential uptake of 14C-124TMB by any of the organs or tissues examined was evident. Tissue levels declined rapidly within 24h after dosage, with more than 99% of the administered radioactivity recovered in the urine during this period.

For the study of metabolism (Huo et al 1999), urine extracts were prepared for analysis of free and total (conjugated) metabolites. Recoveries from urine of rats dosed with 14C-124TMB showed that this method recovered 87.7% (range 86-90%, n=3) of the radioactivity. Urine or urine hydrolysates were analysed by gas-liquid chromatography-mass spectrometry (GLC-MS). A complex mixture of isomeric trimethylphenols, dimethylbenzyl alcohols, dimethylbenzoic acids and dimethylhippuric acids excreted in the urine accounted for more than 81% of the administered dose. The major metabolites were 3,4-dimethylhippuric acid (30.2 %dose), 2,4 -dimethylbenzyl alcohol (12.7% dose, primarily as sulphate and glucuronide conjugates) and 2,5-dimethylbenzyl alcohol (11.7% dose, primarily as sulphate and glucuronide conjugates).The influence of steric factors on oxidation at aromatic carbon adjacent to methyl substituent site appears to be minimal given that the proportion of the phenolic metabolites, including 2,3,6-trimethylphenol with 4.0 % are formed in approximately equal proportions.

A further metabolism study was performed using of the inhalation route (Swierczet al., 2001). Thereby, male Wistar rats were exposed to pseudocumene (CAS No. 95 -93 -6) vapours at the concentration of 25, 100 or 250 ppm in the dynamic inhalation chambers for 6 h. Urine samples were collected from the exposed animals and metabolites were analysed by gas chromatography with a mass spectrometry detector.

Five metabolites were identified and measured: 3,4-dimethylbenzoic acid (3,4-DMBA), 2,4-dimethylbenzoic acid (2,4-DMBA), 2,5-dimethylbenzoic acid (2,5-DMBA), 2,3,5-trimethylphenol (2,3,5-TMPH) and 2,3,6-trimethylphenol (2,3,6-TMPH). The metabolites were measured in rat urine without or after hydrolysis.

Moderate or low correlation was found between the intensity of exposure and the concentration of trimethylphenol isomers (2,3,5-TMPH, r= 0 .60 ; 2,3,6-TMPH, r = 0.33). Mean concentration of trimethylphenol isomers (S.D.) in the urine of rats exposed to pseudocumene at 25, 100 or 250 ppm was: 4.5 (1.8), 10.0 (2.6) and 10.5 (5.0) mg/l for 2,3,5-TMPH and 2,3,6-TMPH 3.5 (1.2), 5.8 (2.9) and 5.8 (1.5) mg/1 for 2,3,6 -TMPH.

 

 

 

Discussion on bioaccumulation potential result:

There are no studies available in which the basic toxicokinetik properties of 2,3,6 -trimethylphenol were investigated. There are two studies in which the distribution and metabolism of 1,2,4 - trimethylbenzene (pseudocumene, CAS No. 95-63-6) was analysed and 2,3,6-trimethylphenol (CAS No. 2416 -94 -6) was identified as metabolite.

In the study of Huo et al., 1989 distribution and metabolisms of 1,2,4 - trimethylbenzene (pseudocumene, CAS No. 95-63-6) were analysed.

For distribution4 groups of 3 male hooded-Wistar rats (approximately 200 g), which were fasted overnight before receiving an oral dose of 14C-124TMB (0.8mmol=0.49 µCi/kg) in olive oil (48 mg/ml) (Huo et al 1999). The animals were then transferred to separate metabolism cages and groups were killed after 3, 6, 12 and 24 hours. Samples of blood, major organs, muscle and adipose tissue were removed and weighed.

14C-124TMB was rapidly and widely distributed throughout the body with the highest levels in adipose tissue. No other preferential uptake of 14C-124TMB by any of the organs or tissues examined was evident. Tissue levels declined rapidly within 24h after dosage, with more than 99% of the administered radioactivity recovered in the urine during this period.

For the study of metabolism (Huo et al 1999), urine extracts were prepared for analysis of free and total (conjugated) metabolites. Recoveries from urine of rats dosed with 14C-124TMB showed that this method recovered 87.7% (range 86-90%, n=3) of the radioactivity. Urine or urine hydrolysates were analysed by gas-liquid chromatography-mass spectrometry (GLC-MS). A complex mixture of isomeric trimethylphenols, dimethylbenzyl alcohols, dimethylbenzoic acids and dimethylhippuric acids excreted in the urine accounted for more than 81% of the administered dose. The major metabolites were 3,4-dimethylhippuric acid (30.2 %dose), 2,4 -dimethylbenzyl alcohol (12.7% dose, primarily as sulphate and glucuronide conjugates) and 2,5-dimethylbenzyl alcohol (11.7% dose, primarily as sulphate and glucuronide conjugates).The influence of steric factors on oxidation at aromatic carbon adjacent to methyl substituent site appears to be minimal given that the proportion of the phenolic metabolites, including 2,3,6-trimethylphenol with 4.0 % are formed in approximately equal proportions.

A further metabolism study was performed using of the inhalation route (Swierczet al., 2001). Thereby, male Wistar rats were exposed to pseudocumene (1,2,4 - trimethylbenzene, CAS No. 95 -93 -6) vapours at the concentration of 25, 100 or 250 ppm in the dynamic inhalation chambers for 6 h. Urine samples were collected from the exposed animals and metabolites were analysed by gas chromatography with a mass spectrometry detector.

Five metabolites were identified and measured: 3,4-dimethylbenzoic acid (3,4-DMBA), 2,4-dimethylbenzoic acid (2,4-DMBA), 2,5-dimethylbenzoic acid (2,5-DMBA), 2,3,5-trimethylphenol (2,3,5-TMPH) and 2,3,6 -trimethylphenol (2,3,6-TMPH). The metabolites were measured in rat urine without or after hydrolysis.

Moderate or low correlation was found between the intensity of exposure and the concentration of trimethylphenol isomers (2,3,5-TMPH, r= 0 .60 ; 2,3,6-TMPH, r = 0.33). Mean concentration of trimethylphenol isomers (S.D.) in the urine of rats exposed to pseudocumene at 25, 100 or 250 ppm was: 4.5 (1.8), 10.0 (2.6) and 10.5 (5.0) mg/l for 2,3,5-TMPH and 2,3,6-TMPH 3.5 (1.2), 5.8 (2.9) and 5.8 (1.5) mg/1 for 2,3,6 -TMPH.