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Toxicological information

Carcinogenicity

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Description of key information

Several mouse skin-painting studies were performed (OECD 453 equivalent; OECD 451 equivalent; non-guideline) in order to test the carcinogenic potential of residual aromatic extracts. Results from investigations into RAEs are presented and provide evidence that RAEs have variable carcinogenic potential.  

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion
Endpoint conclusion:
no study available

Carcinogenicity: via inhalation route

Endpoint conclusion
Endpoint conclusion:
no study available

Carcinogenicity: via dermal route

Endpoint conclusion
Study duration:
chronic
Species:
mouse
Quality of whole database:
Several dermal skin painting studies are available which demonstrate that RAEs have variable carcinogenic potential which can be predicted based on results from modified Ames assays.

Additional information

Mouse skin painting studies were conducted on RAEs to evaluate their carcinogenic potential. Results demonstrate that RAEs could be either carcinogenic or non-carcinogenic to mouse skin following dermal application. 

 

A two year dermal carcinogenicity study was conducted on two RAE samples, MRD-96--601 and MRD-96-657.Groups of 50 male C3H/HeNCrlBR mice were administered a volume of 37.5 µL of Residual Aromatic Extracts (RAE) twice a week for 104 weeks (ExxonMobil, 2005). Three other groups of 50 male C3H/HeNCrlBR mice served as the control group, with one group serving as a positive control (2% MRD-86-882 -catalytically cracked clarified oil in MRD-96 -701 -100 LP solvent neutral), the second group serving as a carrier (acetone) control, and the third group serving as a negative control (MRD-96-701). Unscheduled mortality was reported in animals treated with the test materials as well as the negative control group. Clinical signs were not seen during the investigation. A difference in dermal irritation was noted with MRD-601 dose animals presenting no dermal sores while some animals treated with MRD-657 presented with dermal sores. Substance MRD-601 was determined to be non-carcinogenic and substance MRD-657 had an incidence of carcinogenicity situated within the range of background incidence. 

 

In a dermal carcinogenicity assay, groups of 50 C3H/HeJ mice (sex not specified) were treated with 12 different refined petroleum streams (RPS) via dermal application of which 5 were RAEs (Mobil, 2001). The backs of animals were clipped free of hair twice a week and 50 µL of test materials was administered to the clipped skin twice a week for 78 weeks. A similar group of 50 animals served as negative and positive control. All animals were observed for overt signs of toxicity, mortality or appearance of tumours twice a day on week days and once a day during weekends. Animals were weighed weekly once during the first month, following which they were weighed once every two months. At study termination all surviving animals were sacrificed and necropsied. In addition, all treated skin with and without tumours, the sternum and rib, liver thymus, lungs, gall bladder, and kidneys were harvested and preserved for detailed histopathological analysis.  

 

No significant mortality or morbidity was noted in animals that exhibited no tumours. The positive control group responded as expected with a 98% tumour incidence comprised mainly of papillomas or advanced tumours. Group 2, the negative control group, did not exhibit any tumours. Of animals treated with test material number 9 24% exhibited tumours. One animal exhibited an early papilloma which progressed quickly to an advanced tumour. The remaining animals did not develop papillomas until late in the study. Animals treated with test material number. 5 exhibited only one tumour and these appeared later in the study as well and were considered equivalent to the background incidence and were not considered evidence of carcinogenic potential. Animals treated with test material numbers 3, 7, and 11 did not exhibit any tumours following dermal application. In conclusion among the 5 RAEs that were tested in this study, only 1 exhibited tumourigenic activity (in addition to the positive control). The remaining RAEs did not demonstrate any activity in the assay above the expected background level.

 

In a carcinogenicity study, a Residual Aromatic Extract (RAE) was dermally administered to 50 male C3H mice at a dose level of 25 mg, twice weekly, for 80 to 104 weeks (Kane et al., 1994). Development of horny lesions were observed and diagnosed as a papilloma or an "advanced tumour". If a horny lesion developed and reached 1-3 mm, and persisted for one week, it was diagnosed as a papilloma. If the lesion continued to grow, replaced surrounding tissue and became ulcerated or necrotic, it was diagnosed as an "advanced tumour". For advanced tumours, the animal was sacrificed and the tumour confirmed histologically. If the papilloma regressed the application of the test material recommenced. The ratio of mice developing tumours was expressed as a percentage of Final Effective Number (FEN). RAE-treated animals had percent FEN lower than 11.5%. A LOEL and NOAEL were not calculated.

In a two year skin painting cancer assay that showed a weak carcinogenic effect, groups of 50 female CF1 strain mice were treated with 0.1 mL neat test materials (1157 and 1158) on the clipped dorsal skin three times per week for 2 years (BP, 1991). An additional group of 5 mice were allocated to each treatment group and were sacrificed after 22 or 52 weeks. Appearance and development or regression of superficial tissue was recorded on a weekly basis during study duration. Weekly observations were made to determine the onset of lesions that were subsequently diagnosed as tumours. A separate group (number not specified) of animals were treated with N1 oil as the positive control, while another group (number not specified) of animals served as the untreated control.

 

Exposure to 1157 and 1158 caused minor skin irritation. No toxic effects attributable to the two test materials were noted in animals. There was no change in body weight gain and none of the treated animals died. Changes noted following necropsy were considered to be normal in the strain of mice used in the study. Histopathological examination of various tissue masses indicated no treatment related tumours at other sites besides the skin in any of the treated groups. Animals treated with 1157 exhibited minor skin irritation (acanthosis) at the treatment site. Six of the 50 treated mice had eight tumours of epidermal origin. Six out of the eight were benign tumours (papillomas or keratoacanthomas), while two were malignant (squamous cell carcinomas). The mean latency period for the onset of these tumours was 78 weeks. Mice treated with 1158 had minimal skin irritation at the treatment site. Three of the 50 treated mice exhibited tumours of epidermal origin, of which two were benign (papilloma and keratoacanthoma), while one tumour was malignant (squamous cell carcinoma). The mean latency period for the onset of these tumours was 82 weeks. Additionally, a single sarcoma was also observed in the mouse bearing the malignant epidermal tumour. However, the authors were not certain that this related to the test material. Animals treated with the positive control, N1, exhibited tumours as expected, while untreated animals showed no incidence of cutaneous tumours. The study authors reported that though the number of malignant tumours was small in animals treated with 1157, these tumours along with the benign tumours (6 in all) should be regarded as significantly higher compared to the control group. Based on this result, the authors suggested that 1157 is weakly tumourogenic and may induce skin tumours in humans. While the number of tumours exhibited by 1158 was lower compared to 1157, if viewed in a regulatory context, as a material consisting of a high (60%) proportion of 1157, 1158 should be considered weakly tumourogenic as well.

 

Additional supporting information on the carcinogenicity of RAEs (Blackburn 1996, Shell Research, 1991) is presented in the dossier.

Justification for classification or non-classification

RAEs exhibited mixed results in carcinogenicity testing with some materials demonstrating no carcinogenic potential and others a low response preceded by a long latency period. In order to discriminate between RAEs with and without carcinogenic potential a predictive test was identified. The modified Ames test was developed to be an effective screening tool for petroleum products which had low mutagenic activity. Investigating the correlation between results from the modified Ames Assay and dermal carcinogenicity studies revealed that a mutagenicity index (MI) of 0.4 identified the difference in carcinogenic potential; those with an MI less than 0.4 were non carcinogenic while those with MI greater than or equal to 0.4 could be considered carcinogenic. 

RAEs with an MI less than 0.4 are not classified as carcinogenic according to CLP Regulation, (EC)1272/2008.

RAEs with an MI greater than or equal to 0.4, cause concern for man owing to possible carcinogenic effects but in respect of which the available information is not adequate for making a satisfactory assessment. There is some evidence from appropriate animal studies, but this is insufficient to place the substance in category 1. Therefore RAEs with an MI greater than or equal to 0.4 are classified as category 2, H351 according to CLP Regulation, (EC)1272/2008.

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