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Toxicological information

Carcinogenicity

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Description of key information

The key chronic study was conducted by NTP (1986).  The study comprises the oral gavage administration of mixed xylenes to rats (0, 250, or 500 mg/kg/day) and mice (0, 500 or 1000 mg/kg/day) for 5 days/week for 103 weeks.  There was no evidence of carcinogenicity.
No studies are available regarding cancer in animals exposed via inhalation to mixed xylene or the individual xylene isomers.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records
Reference
Endpoint:
carcinogenicity: oral
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: GLP status not known, near guideline study, published in peer reviewed literature, limitations in design and/or reporting but otherwise adequate for assessment.
Qualifier:
equivalent or similar to
Guideline:
EU Method B.32 (Carcinogenicity Test)
Principles of method if other than guideline:
Mixed xylene was administered by oral gavage to groups of 50 male and 50 female F344/N rats at doses of 0, 250 or 500 mg/kg bw/day for 103 weeks. Animals were observed for survival, clinical signs and body weight gain and subject to a full necropsy with tissue histopathology at termination.

GLP compliance:
not specified
Species:
rat
Strain:
other: F344/N
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Charles River Breeding Laboratories, Kingston, NY, USA
- Age at study initiation: 7 weeks
- Housing: 5 per sex /cage in Polycarbonate cages
- Diet: NIH 07 Rat and Mouse Ration (Zeigler Bros., Inc., Gardners, PA, USA); available ad libitum
- Water: ad libitum
- Acclimation period: 19 days

ENVIRONMENTAL CONDITIONS
- Temperature: 23° ± 1°C
- Humidity: 40 - 60%
- Air changes: 15 air changes/hr
- Photoperiod: 12 hr/d light; 12 hr/d dark

IN-LIFE DATES: From: 30 June 1980 To: 2 July 1982
Route of administration:
oral: gavage
Vehicle:
corn oil
Details on exposure:
Oral (gavage): 0, 250 or 500 mg/kg xylenes (mixed) in corn oil; 4 mL/kg

Preparation: Weighed portions of xylenes (mixed) were placed in a graduated cylinder and mixed with corn oil to achieve the proper volume. The mixtures were shaken vigorously for 10 seconds.

Maximum Storage Time: 2 wks

Storage Conditions: Approximately 24ºC, 46% humidity under fluorescent light.
Analytical verification of doses or concentrations:
yes
Details on analytical verification of doses or concentrations:
The concentrations of xylenes in corn oil was analysed by gas chromatography with flame ionization detection following extraction with methanol.
During the 2-year studies, the dose preparations were analyzed once every 2 months, with concentrations varying from 94.6% to 106.9% (within 10% target concentrations).
Duration of treatment / exposure:
5 days per week for 103 weeks.
Frequency of treatment:
Once daily (5 days / week).
Remarks:
Doses / Concentrations:
0, 250 or 500 mg/kg
Basis:
nominal conc.
No. of animals per sex per dose:
50 male / 50 female per group
Control animals:
yes, concurrent vehicle
Details on study design:
Dose selection rationale: Based on weight gain depression at 1,000 mg/kg in both sexes in the 14-day studies and in males in the 13-week studies and on the clinical signs in the 14-day studies, doses selected for rats for the 2-year studies were 0, 250, and 500 mg/kg xylenes (mixed) in corn oil by gavage, administered 5 days per week.
Observations and examinations performed and frequency:
CAGE SIDE OBSERVATIONS: Yes
- All animals were observed twice daily

DETAILED CLINICAL OBSERVATIONS: Yes
- Clinical signs were recorded once per day for 16 months and then once per month.

BODY WEIGHT: Yes
- Body weights were recorded weekly for 12 weeks and monthly thereafter

OPHTHALMOSCOPIC EXAMINATION: No data

HAEMATOLOGY: No data

CLINICAL CHEMISTRY: No data

URINALYSIS: No data

NEUROBEHAVIOURAL EXAMINATION: No data

Data were recorded in the NTP Carcinogenesis Bioassay Data System. The data elements included descriptive information on the chemicals, animals, experimental design, survival, body weight, and individual pathologic results.
Sacrifice and pathology:
Necropsy and histopathological examination performed on all animals, where possible. During necropsy, all organs and tissues were examined for grossly visible lesions. Tissues were preserved in 10% neutral buffered formalin, embedded in paraffin, sectioned, and stained with haematoxylin and eosin. The following tissues were examined: gross lesions and tissue masses, mandibular lymph nodes, salivary gland, femur, including marrow, thyroid gland, parathyroids, small intestine, colon, liver, prostate / testis or ovaries / uterus, heart, oesophagus, stomach, brain, thymus, trachea, pancreas, spleen, skin, lungs and mainstem bronchi, kidneys, adrenal glands, urinary bladder, pituitary gland, eyes (if grossly abnormal), and mammary gland.
Statistics:
Survival Analyses: Kaplan and Meier (1958); Cox (1972) and Tarone (1975). All reported P values for the survival analysis are two-sided. Calculation of Incidence for neoplastic and non-neoplastic lesions. Analysis of Tumour Incidence: Mantel and Haenszel (1959). Continuity-corrected tests were used in the analysis of tumour incidence, and reported P values are one-sided. Life Table Analyses-- Mantel-Haenszel (1959) method used to obtain an overall P value. Life table method of Cox (1972) and of Tarone (1975). The underlying variable considered by this analysis is time to death due to tumour. Incidental Tumour Analyses-- (Haseman, 1984) Unadjusted Analyses--Primarily, survival-adjusted methods are used to evaluate tumour incidence. The Fisher exact test for pairwise comparisons and the Cochran-Armitage linear trend test (Armitage, 1971; Gart et al., 1979).
Clinical signs:
effects observed, treatment-related
Mortality:
mortality observed, treatment-related
Body weight and weight changes:
effects observed, treatment-related
Histopathological findings: non-neoplastic:
no effects observed
Histopathological findings: neoplastic:
no effects observed
Details on results:
Mortality - Although the mortality was dose related in male rats (final survival: vehicle control 36/50, low dose 26/50, high dose 20/50), many of the early deaths in the dosed males were gavage related. Survival of the high dose males was significantly lower than that of the vehicle control after week 103.

Bodyweight - Bodyweights of high dose male rats were 5%-8% lower than those of the vehicle controls after week 59.

Tumour findings - There were no significant changes in the incidences of neoplastic or non-neoplastic lesions which were considered to be related to the administration of xylenes (mixed).

Testis findings - Although the overall incidences of interstitial cell tumours were comparable in male rat groups (vehicle control, 43/50; low dose,38/50; high dose, 41/49), survival-adjusted analyses indicated an increased incidence in the high dose group relative to vehicle controls. This apparent effect was due primarily to animals dying between weeks 62 and 92, for which the incidence of interstitial cell tumours was 13/13 for the high dose group compared with 4/9 for vehicle controls. Tumour incidences were comparable during the other time intervals. It is doubtful that this marginal effect is compound related.

Haematopoietic System and Pituitary Gland - Dose-related decreases in the incidences of mononuclear cell leukaemia (vehicle control,22/50; low dose, 18/50; high dose, 11/50) and pituitary gland adenoma or carcinoma (combined) (vehicle control, 24/49; low dose, 22/50; high dose, 12/45) were observed in male rats. However, these differences were due primarily to decreased survival of the high dose group relative to that of the vehicle controls.
Sex:
male/female
Basis for effect level:
other: no evidence of carcinogenicity of xylenes (mixed) for male or female F344/N rats given 250 or 500 mg/kg
Remarks on result:
other: Effect type: carcinogenicity (migrated information)
Conclusions:
There was no evidence of treatment-related carcinogenicity following gavage administration of mixed xylenes to male and female F344/N rats at doses of 0, 250 or 500 mg/kg body weight/day for up to 103 weeks.
Executive summary:

The carcinogenicity of mixed xylene was investigated in male and female F344/N rats following oral (gavage) administration at doses of 0, 250 or 500 mg/kg bw/day for 103 weeks. Animals were observed for survival, clinical signs and body weight gain and subject to a full necropsy with tissue histopathology at sacrifice. There was no evidence of treatment-related carcinogenicity in either sex under these conditions.

Endpoint conclusion
Endpoint conclusion:
no adverse effect observed
Dose descriptor:
NOAEL
500 mg/kg bw/day
Study duration:
chronic
Species:
rat
Quality of whole database:
Studies conducted in rats and mice demonstrate no evidence of carcinogenicity.

Carcinogenicity: via inhalation route

Endpoint conclusion
Endpoint conclusion:
no study available

Carcinogenicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

Mixed xylene (CAS 1330-20-7) comprises individual xylene isomers (m-xylene, o-xylene, p-xylene) and <10% ethyl benzene. The following information is available to characterise the carcinogenic potential of this substance.

Non-human information

No animal studies are available on the carcinogenic effects of mixed xylene or the individual xylene isomers following dermal or inhalation exposure.

The carcinogenicity of mixed xylene (17% ethyl benzene) following oral exposure has been evaluated in chronic studies with rats and mice; however, no animal studies are available on the carcinogenic effects of the individual xylene isomers following oral exposure. Results of the chronic oral studies with mixed xylene have been negative (NTP, 1986), with no increase in tumour incidence compared with the control animals. Treatment involved administration of 0, 250, or 500 mg/kg/day doses of mixed xylene in corn oil by gavage 5 days/week for 103 weeks to groups of F344/N rats, 50 animals per group. B6C3F1 mice were treated in a similar manner but given 0, 500 or 1000 mg/kg/day of mixed xylenes in corn oil by gavage. A large number of gavage-related deaths were a confounding factor. This study did not comprehensively examine systemic effects but it did include a complete histopathological examination of all tissues as well as determination of body weight gain. Based on histopathology of all organ systems, a NOAEL of 500 mg/kg/day was observed for rats and a NOAEL of 1000 mg/kg/day was observed for mice. In conclusion there was no evidence of carcinogenicity of mixed xylenes following oral administration.

Equivocal results reported by Maltoni et al (1983, 1985) following exposure of rats to xylenes are viewed to be unreliable (IPCS, 1997) as analysis was conducted by combining all tumours; this is an unacceptable basis for analysis particularly in aged animals. In addition, no data were provided to allow an analysis on an individual tumour-type basis.

The carcinogenicity of ethyl benzene following inhalation exposure has been evaluated in chronic studies with F344 rats and B6C3F1 mice (NTP, 1999). Increases in tumor rates in male (kidney and testis) and in female rats (kidney), in male mice (lung) and in female mice (liver) were reported following exposure to 750 ppm (3255 mg/m3) ethyl benzene. The RAR (2008) concluded that genotoxicity was not the responsible mode in the initiation of the reported tumors and that other non-genotoxic mechanisms may be involved. Furthermore, the rapporteur concluded that there is sufficient evidence to conclude that the kidney tumors in the male and female rats are associated with the rat strain-specific high incidence of chronic progressive nephropathy (CPN) which is not found in humans. The testis, liver and lung tumours were types that occur at high or very high spontaneous rates in the F344 rat and B6C3F1 mice strains used. The rapporteur concluded that ethyl benzene may enhance tumour development in genetically disposed animals or reduce the latency periods in tumour development. There are currently no detailed mechanisms explaining the increase in tumour rates some species and strain specificity is postulated (RAR, 2008). Consequently the toxicological significance and relevance to human health of these findings is uncertain and it is considered that ethylbenzene does not pose a carcinogenic risk for humans.

Human information

There is no data indicating any convincing evidence of an increased risk of cancer as a consequence of exposure to xylenes. IARC (1999) has placed xylene in Group 3: "The agent is not classifiable as to its carcinogenicity to humans". The animal data indicates that xylenes would not be carcinogenic or genotoxic in humans.


Justification for selection of carcinogenicity via oral route endpoint:
Results of chronic oral studies with mixed xylenes have been negative, with no increase in tumour incidence in treated rats given up to 500 mg/kg bw/d for 103 weeks or in mice following chronic oral treatments of up to 1000 mg mixed xylenes/kg bw/d (NTP, 1986).

Justification for classification or non-classification

No classification of mixed xylenes streams for carcinogenicity is warranted under DPD or GHS/CLP.