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Description of key information

Hydrocarbons, C9, aromatics are typically metabolized by side chain oxidation to alcohol and carboxylic acid derivatives. These metabolites can be glucuronidated and excreted in the urine or further metabolized before being excreted. The majority of the metabolites are excreted in the urine and to a lower extent, in the faeces. Excretion is rapid with the majority of the elimination occurring within the first 24 hours of exposure. As a result of the lack of systemic toxicity and the ability of the material to be rapidly excreted, bioaccumulation of the test substance in the tissues is not likely to occur.

Key value for chemical safety assessment

Additional information

Animal Data: Tissue levels of 1,2,4-trimethylbenzene following exposure to 100 ppm for 12 hours per day for 3 days are summarized in Table 1.

Table11. Distribution of 1,2,4-trimethylbenzene in rat tissues (mean value from four animals) following exposure to 100 ppm of the substances 12 h daily for 3 days. Values in parentheses are from animals that had a 12-h recovery period after the last exposure (10). 

 

Tissue

Concentration (µmol/kg)

Blood

17.1

Brain

36.5

Liver

35.4

Kidney

103.6

Fat

1070 (120)

Following inhalation exposure in rats, the fat: blood partition coefficient of trimethylbenzenes is around 63 (10, 13). The data in Table 1 is consistent with this partition coefficient given the selective redistribution of 1,2,4-trimethylbenzene to adipose tissue. Twelve hours following the cessation of exposure, adipose tissue levels of 1,2,4-trimethylbenzene were decreased by a factor of approximately 9, demonstrating rapid mobilization of this substance from body adipose tissue. Thus normally, long-term accumulation of this material in fat does not occur. Trimethylbenzene does not selectively redistribute to other body tissues other than adipose tissues.

Oral dosing of rats with 14C-1,2,4-trimethylbenzene was associated with a rapid and wide tissue distribution of radioactivity throughout the body, with selective and preferential re-distribution to adipose tissue (14). There was no preferential uptake of radioactivity into any other organ or tissue (14).

Summary: Collectively, these data demonstrate that during inhalation exposure, Hydrocarbons, C9, aromatics undergo substantial partitioning into adipose tissues. Following cessation of exposure, the level of Hydrocarbons, C9, aromatics in body fats rapidly declines. Thus, the Hydrocarbons, C9, aromatics are unlikely to bioaccumulate in the body. Selective partitioning of the Hydrocarbons, C9, aromatics into the non-adipose tissues is unlikely. No data is available regarding distribution following dermal absorption. However, distribution following this route of exposure is likely to resemble the pattern occurring with inhalation exposure.

METABOLIC TRANSFORMATION

Animal Data: In rats, urinary metabolites of 1,2,4-trimethylbenzene consist of a complex mixture of isomeric triphenols, the sulphate, glucuronide and mercapturic conjugates of dimethylbenzyl alcohols, dimethylbenzoic acids and dimethylhippuric acids (14). The major metabolites are 3,4-dimethylhippuric acid (30.2% of the dose), sulphate and glucuronide conjugates of 2,4-dimethylbenzyl alcohol (12.7% of the dose), and sulphate and glucuronide conjugates of 2,5-dimethylbenzyl alcohol (11.7% of the dose) (14).

 In rats, approximately 78% of an oral dose of 1,3,5-trimethylbenzene is excreted as 3,5-dimethylhippuric acid; an additional 7.6 and 8.2% were excreted as glucuronic and sulphuric acid conjugates (21). The corresponding values for the glycine, glucuronic and sulphuric acid conjugates of 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene were 43.2, 6.6, and 12.9% and 17.3, 19.4, and 19.9%, respectively (21). In rabbits, the major urinary metabolites of 1,2,4-trimethylbenzene following oral dosing are 2,4-dimethylbenzoic acid and 3,4-dimethylhippuric acid (22).

Biotransformation of 1,2,4-trimethylbenzene to dimethylbenzoic acids in the rat follows the Lineweaver-Burk equation with Km ranging from 7-28 mg/l and Vmax ranging from 23-96 mg/h/kg depending on the particular species of dimethylbenzoic acid formed (7). Biotransformation to 3,4-dimethylbenzoic acid is favoured, given its Km of 28 mg/l and Vmax of 96 mg/h/kg. Notably, in rats, the biotransformation of trimethylbenzenes is inhibited by pre-treatment with ethanol and enhanced by ethyl acetate (8, 23). In rats, 1,3,5-trimethylbenzene is an inducer of cytochrome p450, cytochrome b5, aminopyrine N-demethylase, aryl hydrocarbon hydroxylase, aniline hydroxylase, and NADPH-cytochrome c reductase in the rodent liver (24). In the kidney, 1,3,5-trimethylbenzene induces cytochrome P-450 and cytochrome b5 (24). Thus, trimethylbenzenes are inducers of their own metabolism in the rat. These data are consistent with the low propensity for bioaccumulation of trimethylbenzenes in mammals.

ELIMINATION AND EXCRETION

Animal Data: Following oral administration of 14C-1,2,4-trimethylbenzene, > 99% of the administered radioactivity was excreted in urine within 24 hours (14). The predominant urinary species being the 3,4-dimethylhippuric acid metabolite (14). Likewise, urine is the major route of excretion of trimethylbenzene metabolites in rats following inhalation exposure. As in humans, there is a strong correlation between the level of trimethylbenzene inhalation exposure and the concentration of metabolites in urine.

Human information

DISTRIBUTION

Human data: 1,3,5-trimethylbenzene has a very large volume of distribution (30 L/kg), implying wide tissue distribution and substantial partitioning in the tissues (2). One hour post exposure blood levels of 1,3,5-trimethylbenzene in human volunteers exposed to 25 ppm for 4 hours were similar to the steady-state level that occurred during the exposure period (2, 3). These data, combined with the very high oil: air partition coefficients of trimethylbenzenes (9620-11300) imply substantial redistribution of inhaled trimethylbenzenes to fatty tissues (2, 3, 6)

METABOLIC TRANSFORMATION

Human data: The major metabolites of trimethylbenzenes most commonly identified in urine are the dimethylbenzoic acid and dimethylhippuric acid derivatives of the parent molecule (1-3, 5, 15-20). Both the dimethylbenzoic and hippuric acid metabolites of the trimethylbenzenes are commonly used for biomonitoring of human exposures.

ELIMINATION AND EXCRETION

Human data: The identified routes of excretion of the trimethylbenzenes following inhalation exposure in humans include: (1) exhalation of the unchanged volatile parent substance (3); (2) urinary excretion of the unchanged volatile parent substance (25); and (3) urinary excretion of metabolites (1-3, 5, 15-20). Post-exposure exhalation of unmetabolized trimethylbenzenes accounts for 20-37% of the absorbed amount (2). Urinary excretion of unmetabolized trimethylbenzenes is low (= 0.002%) (2). Overall urinary excretion of metabolites (predominantly 3,4-dimethylhippuric acid) of 1,2,4-trimethylbenzene in the first 24 hours post-exposure accounts for 22% of the inhaled dose (18). Urinary excretion of unconjugated dimethylbenzoic acid metabolites accounts for only small percent of the inhaled dose of trimethylbenzenes (18). The bulk of the absorbed dose of trimethylbenzenes is metabolized and excreted in urine, predominantly as their dimethylhippuric acids or conjugated (sulphated or glucuronidated) dimethylbenzoic acid derivatives (1-3, 5, 15-20). The urinary excretion of dimethylhippuric acids is well-correlated with exposure to trimethylbenzenes and has been commonly used for biomonitoring purposes.

The initial blood clearance of trimethybenzenes in man is 0.6-1 l/hr/kg (2). However, trimethylbenzenes have longer terminal half-lives in blood (T½ 78-120 hr) due to their extensive partitioning into adipose tissues (2). The kinetics of urinary elimination of unmetabolized 1,3,5-trimethylbenzene follows a biphasic pattern with a T½ for Phase I of 0.45-0.88 hr and a T½ for Phase II of 6.7-19.2 hr.25 Elimination of unmetabolized 1,3,5-trimethylbenzene in exhaled air is biphasic with an initial T½ of 1 hr (3). Urinary elimination of dimethylbenzoic acids following inhalation exposure to 1,3,5-trimethylbenzene is biphasic with a T½ for Phase I of 13 hr and a T½ for Phase II of 60 hr (3). Notably, co-exposure to white spirits interferes with the metabolic elimination of 1,2,4-trimethylbenzene (5, 16).

Summary and discussion of toxicokinetics

Hydrocarbons, C9, aromatic are readily absorbed when inhaled or ingested. C9 Aromatic fluids are poorly absorbed dermally with an estimated overall percutaneous absorption rate of approximately 2ug/cm2/hr. Bioaccumulation of C9 Aromatic fluids is not expected.

METABOLIC TRANSFORMATION

Summary: Collectively these data demonstrate that Hydrocarbons, C9, aromatics may undergo several different Phase I dealkylation, hydroxylation and oxidation reactions which may or may not be followed by Phase II conjugation to glycine, sulphation or glucuronidation. However, the major predominant biotransformation pathway is typical of that of the alkylbenzenes and consists of: (1) oxidation of one of the alkyl groups to an alcohol moiety; (2) oxidation of the hydroxyl group to a carboxylic acid; (3) the carboxylic acid is then conjugated with glycine to form a hippuric acid. The minor metabolites can be expected to consist of a complex mixture of isomeric triphenols, the sulphate and glucuronide conjugates of dimethylbenzyl alcohols, dimethylbenzoic acids and dimethylhippuric acids. Consistent with the low propensity for bioaccumulation of the Hydrocarbons, C9, aromatics, these substances are likely to be significant inducers of their own metabolism.

ELIMINATION AND EXCRETION

Summary: Collectively these data demonstrate that the predominant route of excretion of Hydrocarbons, C9, aromatics following inhalation exposure involves either exhalation of the unmetabolized parent compound, or urinary excretion of its metabolites. When oral administration occurs, there is little exhalation of unmetabolized Hydrocarbons, C9, aromatics, presumably due to the first pass effect in the liver. Under these circumstances, urinary excretion of metabolites is the dominant route of excretion.

References

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