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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
carcinogenicity: oral
Type of information:
experimental study
Adequacy of study:
key study
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.
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
other: F344/N
Details on test animals or test system and environmental conditions:
- 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

- 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
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:
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).
Doses / Concentrations:
0, 250 or 500 mg/kg
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:
- All animals were observed twice daily

- Clinical signs were recorded once per day for 16 months and then once per month.

- Body weights were recorded weekly for 12 weeks and monthly thereafter






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.
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 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.
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
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:
500 mg/kg bw/day
Study duration:
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

Justification for classification or non-classification

No classification of xylenes for carcinogenicity is warranted under CLP.

Additional information

The carcinogenicity of mixed xylenes and the xylene isomers was reviewed and reported by ATSDR (2007).

Non-human Information

The carcinogenicity of mixed xylene 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) are viewed to be unreliable (IPCS, 1997) as analysis was conducted by combining all tumours; 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.

No studies are available regarding cancer in animals exposed via inhalation or dermal routes to mixed xylene or xylene isomers.

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 xylenes 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).