Aniline

Endpoint summary

Additional information

Aniline is rapidly and well absorbed in animals after oral, dermal and inhalation exposure. The extent of absorption after oral intake of a single dose amounts for up to 96% for rats (Bus et al., 1978; McCarthy et al., 1985; Kao et al., 1978). In mice, rabbits and dogs the extent of oral absorption was reported to be lower than for rodents, i. e. 72%, 70%, 50%, respectively. (McCarty et al., 1985; Kao et al., 1978; Parke, 1960). Dermal absorption of topically applied aniline in humans was estimated to amount up to 24% (Baranowska-Dutkiewicz, 1982).

After absorption aniline is widely distributed in the body. The highest concentration is found in the blood, especially erythorcytes followed by plasma, the spleen, kidneys, lung, heart, brain and fat (Bus and Sun, 1979; Bus et al., 1978; McCarthy et al, 1985; Khan et al., 1995). Accumulation in the spleen of rats was observed after repeated oral administration of aniline for 10 days (Bus and Sun, 1979; Khan et al., 1995). In rats, half life for aniline clearance from the blood ranged from 16 minutes to 3.5 hours depending on the dose applied (Harrison and Jollow, 1987). Aniline is able to cross the placental barrier in rats, with similar blood concentrations and half life (1.5 h) for blood clearance in fetal and maternal blood plasma (Maickel and Snodgrass, 1973).

The metabolism of aniline is similar in human and animals and urinary excretion is the main route for elimination. In several studies it has been shown that aniline is metabolized prior to elimination. In rodents and non-rodents most of the orally applied aniline is recovered in the urine within 24 hours after application (Bus et al., 1978; McCarthy et al., 1985; Kao et al., 1978, Parke, 1960) whereas excretion via the feces and expired air accounts for only less 2% and 0.2%, respectively (Kao et al. 1978; McCarthy et al., 1985). Urinary excretion of unchanged aniline in rodents was observed to a maximum of 5% (McCarthy et al., 1985).

Aniline is metabolized in the liver by N-acetylation and N-hydroxylation as well as hydroxylation of the aromatic ring (Kao et al., 1978; McCarthy et al, 1987). The main pathway is N-acetylation to acetanilide by N-acetyltransferases followed by cytochrome P-450 dependent p-hydroxylation to N-acetyl-p-aminophenol which is mainly excreted in urine as sulphate in rats and as glucuronide in other species. The amount of p-aminophenol relative to o-aminophenol conjugates in the urine was 8:1 in rats and 1:6 in mice. In rats, sulphate conjugation dominates however, becomes saturated at higher doses (250 mg/kg) leading to increased excretion of unconjugated metabolites and glucuronides. In mice and non-rodent species urinary excretion of glucuronides prevails. After a single application of aniline in rodents, 70 - 96% of the dose is excreted in the urine within 24 hours (Kao et al., 1978; Bus et al., 1978; McCarthy et al, 1987). Whereas rapid and complete urinary excretion of acetanilides and aminophenols has been reported, N-hydroxylated metabolites (phenolhydroxylamine) or its conjugates cannot be found in the urine (Kao et al., 1978).

The N-acetylation pathway is an important route by which aniline is detoxified, whereas cytochrome P-450 dependent N-hydroxylation leading to phenolhydroxylamine is the principal route by which aniline produces toxic effects. Phenylhydroxylamine has been shown to be the metabolite responsible for aniline induced methemoglobin (MetHb) formation (Harrison and Jollow, 1987). In the liver, N-hydroxylated aniline is rapidly reduced back to its parent compound however; some phenolhydroxylamine can be taken up from erythrocytes. Once within the erythrocyte, phenolhydroxylamine is rapidly oxidized to nitrosobenzene in the presence of oxyhemoglobin with concurrent formation of MetHb. To a lesser extent than the liver, erythrocytes have the capacity to reduce nitrosobenzene back to phenolhydroxylamine and aniline, however in the presence of oxyhemoglobin reoxidation to nitrosobenzene predominates resulting in redox cycling. Aminophenols may also initiate redox cycling however their formation and their potency to induce MetHb in vivo have been shown to be much lower. Without metabolic activation aniline did not induce MetHb in cultured erythrocytes (Harrison and Jollow, 1987).

MetHb formation in rats was not different after single exposure to airborne aniline for 8 or 12 hours. Half live of MetHb was determined to be 75 min. No carry-over of methemoglobinemia was observed between repeated exposures to aniline at vapor concentrations of 100 ppm for 8 hours (Kim and Carlson, 1986). In Beagle dogs exposed head-only to aniline vapor in a concentration of 174 mg/m3 for 4 hours (total exposure: 15 mg/kg bw) the MetHb concentration was about 5%. In the same species a single oral dose of 15 mg aniline/kg bw lead to a MetHb level of 26%. The 5-fold lower potency of MetHb formation by inhalation could be related to hepatic first-pass metabolism or could be caused by a reduced retention (i.e. 20%) of the inhaled aniline vapor. Maximum MetHb concentration was observed in less than one hour after cessation of inhalation exposure and three hours after oral administration and returned to normal within one day after administration independent of the exposure route. The half live of MetHb in dogs after inhalation of aniline was 100 min (Pauluhn, 2002).

Based on available toxicokinetic data in experimental animals and studies on human volunteers on MetHb formation and urinary p-aminophenol excretion it is assumed that aniline is well absorbed following inhalation, oral and dermal exposure in humans.

Rapid MetHb formation as a response to oral aniline exposure was observed in human volunteers who received a single bolus dose. Maximum levels of MetHb were detected at 2 hours after exposure, but normal levels at 3 hours. No statistically significant increase in MetHb formation (up to 1.8%) was observed after ingestion of 15 mg aniline. After a single bolus dose of 35 mg aniline an increase in MetHb concentration of 3.7% was observed (Jenkins et al., 1972). Half life of aniline in man was reported to be 3.5 hours (Piotrowski, 1972).

The uptake of airborne aniline (5 -30 mg/m3) via the respiratory tract (2 -11 mg/h) and skin of unclothed volunteers (3 -11 mg/h) was similar. Clothing (working overall) reduced the dermal uptake form airborne aniline by approximately 40%. A pulmonary retention of nearly 90% was reported. The absorption rate of aniline vapor through the skin ranges from 0.2-0.4 μg/cm2/h and is approximately 1000 times lower than absorption of topically applied liquid aniline (0.2-0.7 mg/cm2/h). The absorption rate of skin aniline vapor through the skin increased of approximately 30% with an increase in skin temperature (25 to 30°C) and humidity in the air (35 to 75%). With moistened skin absorption rate was reported to be 3.8 mg/cm2/h (Piotrowski, 1957; Dutkiewicz, 1961; Dutkiewicz and Piotrowski, 1961). Dermal absorption of topically applied aniline for 30 min was 2.5 mg/cm2/h. Maximum urinary excretion rate of p-aminophenol was between 4 and 6 hours from the beginning of the exposure (Baranowska-Dutkiewicz, 1982).

Polymorphism in human N-acetyltransferase divides the human population in two groups, i. e. those with a high enzyme activity (fast acetylateor) and those with a low enzyme activity (slow acetylators). Particularly affected from aniline-induced hematotoxicity are about 50% of Europeans which have a genetically caused lower acitivity of N-acetyltransferase meaning that the reaction of aniline to acetanilide is retarded in favor to formation of phenylhydroxylamine. As a result, MetHb levels are higher in slow acetylators (1.0 - 1.5%) than in fast acetylators (0.7 – 1.2 %) after occupational exposure to airborne aniline below the OEL in Germany (Lewalter and Korallus, 1985).