4-chloroaniline

Carcinogenicity

Additional information

The main effect observed after long term exposure to p–chloroaniline is an increase in tumors of the spleen in rats (fibrosarcomas and osteosarcomas) at a high dose of 18 mg/kg. These type of tumours is typical for aniline and related chemicals and is related to the hematotoxic and methemoglobin-forming activity. Repeated exposure to p-chloroaniline leads to cyanosis and methemoglobinemia, and secondary effects occur in blood, liver, spleen, and kidneys and is accompanied by extramedullary hematopoiesis as evidence of a regenerative anemia. Therefore an indirect mechanism is assumed for the tumorigenic activity of p-chloroaniline instead of a genotoxic mode of action.

For the assessment of the human relevance of the spleen tumours observed in rats the following should be considered:

There are physiological differences between rats and humans (information taken from the MAK value documentation for aniline; more details can be found there (http://onlinelibrary.wiley.com/doi/10.1002/3527600418.mb6253e0026b/pdf):

1) Unlike in rats, in which about 5% of erythropoiesis takes place in the spleen throughout the animals lifetime, hematopoiesis stops after birth in the human spleen (as long as the bone marrow function is not impaired)

2) Whereas haematopoiesis takes place in rat spleens via the proliferation of local erythroblasts, in human spleens it takes place via the immigration of erythroid precursors from the blood and subsequent proliferation in the splenic sinus.

3) Under hypoxic conditions, extramedullary haematopoiesis is initially activated in rat spleens, whereas no haematopoiesis is detected in human spleens even if there are anaemic conditions, such as sickle-cell anaemia and thalassaemia.

4) As a result of severe haemolytic anaemia, rat spleens can take on up to 80% of erythropoiesis.

Splenic tumours are extremely rare in humans and splenic tumours have not been reported for human conditions accompanied by methaemoglobinaemia (congenital methaemoglobinaemia), haemolysis (sickle cell anaemia and thalassaemia) or the increased incorporation of iron (haemochromatosis).

Background information on the mechanism for the tumour induction related to methemoglobin forming agents can be found in the MAK value documentation for aniline (http://onlinelibrary.wiley.com/doi/10.1002/3527600418.mb6253e0026b/pdf) as well as in the publications by Nyska et al., Association of liver hemangiosarcoma and secondary iron overload in B6C3F1 mice – the National Toxicology Program experience, Toxicologic Pathology 32, 222-228, 2004 and Cohen et al., Hemansiosarcoma in rodents: Mode-of-Action evaluation and human relevance, Toxicological Sciences 111, 4-18, 2009.

Regarding pheochromocytoma it is known that such tumours occur frequently especially in male rats following a variety of conditions, including hypoxia and the occurrence under such toxic conditions in animal experiments is to be assessed as a secondary effect (Greim et al., Chemically induced pheochromocytomas in rats: mechanisms and relevance for human risk assessment. Critical Reviews in Toxicology 39, 695-718, 2009).

Overall, whereas there is clear evidence of a tumourigenic activity in the (male) rat it can be assumed that the tumour response is related to a secondary mechanisms as a result of the hematotoxic activity. Based in the species differences related to the respective secondary spleen toxicity the spleen tumours observed in the rat after treatment with o-chloroaniline at hematotoxic doses cannot be directly translated to the human situation. Also for the pheochromocytoma a link to hypoxia is to be expected and therefore assessed as a secondary effect. Whereas there is some evidence of a tumourigenic activity in the (male) mouse again there is evidence that the effects observed are secondary to the hematotoxic activity. For both species the effect level for the hematotoxic activity is well below the effect level related to increased tumour incidences.