Teflubenzuron

Carcinogenicity

Description of key information

mice: not carcinogenic, LOAEL (systemic toxicity) 2.1 mg/kg bw/d

rats: not carcinogenic, NOAEL (systemic toxicity) 4.8 mg/kg bw/d

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

2.1 mg/kg bw/day

Study duration:

chronic

Species:

mouse

Quality of whole database:

OECD Guideline and GLP carcinogenicity studies available for both rats and mice.

System:

hepatobiliary

Organ:

liver

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

Based on the criteria listed in Regulation (EC) 1272/2008 (CLP) for carcinogenicity, the increased incidence of hepatocellular adenoma in male mice can be considered "limited evidence of carcinogenicity", however taking into consideration the additional factors as per Regulation (EC) 1272/2008 Annex I, section 3.6.2.2.6, the effects were observed in one species and one sex only, the lesions observed were solely benign neoplasms without increased progression to malignancy and without changes in latency. Only the liver was affected and also only in the context of chronic heptatotoxicity. Therefore, the classification criteria are not considered fulfilled and no classification for carcinogenicity is warranted.

Additional information

The potential of the test substance to cause carcinogenic effects was investigated in both mice and rats.

 

Mice:

In a 78-week oncogenicity study according to OECD Guideline 451 and GLP, NMRI mice were administered the test substance in feed at doses of 0, 15, 75 or 375 ppm. At 52 weeks, 10 animals per sex and dose were sacrificed for interim results while another 50 animals per sex and dose were treated for 78 weeks prior to analysis.

Throughout the course of the treatment, no substance-related mortality, clinical signs or changes in food consumption were observed. Hematological analysis revealed no substance-related findings. The body weight gains in high dose males were slightly reduced compared to the controls, all other animals showed similar body weight gains as the control animals. Clinical biochemistry investigations showed moderately increased aspartate-aminotransferase (ASAT) activity for male mice of group 4 at 52 and 78 weeks of treatment. A trend for increased ASAT activity was noted for male mice of group 3 at 78 weeks and for the female mice of group 4 at 52 and 78 weeks of treatment. Alanine-aminotransferase (ALAT) activity was markedly increased for male mice and slightly increased for female mice of group 4 at 52 and 78 weeks of treatment. A trend for increased ALAT activity was noted for male mice of group 3 at 52 and 78 weeks of treatment. Lactate dehydrogenase (LDH) activity was slightly increased for male mice of group 4 at 52 weeks of treatment and for both sexes of group 4 at 78 weeks of treatment. Alkaline phosphatase (ALP) activity was slightly increased for male mice of group 4 at 52 weeks of treatment. A trend for increased ALP activity was noted for male mice of group 4 at 78 weeks of treatment. Further, markedly increased ornithine carbamyl-transferase (OCT) activity was found for male mice of group 4 at 52 and 78 weeks of treatment. A trend for increased OCT activity was noted for the female mice of group 4 at 52 and 78 weeks of treatment. In agreement with the hepatic enzyme activity changes observed, moderate to marked liver enlargement was noted for both sexes of group 4 (375 ppm) after 52 as well as 78 weeks of treatment. Moderately increased liver to body weight ratios were also noted for group 3 (75 ppm) males after 78 weeks of treatment. Gross pathological examination upon sacrifice revealed an increased incidence of hepatic nodules and foci in males of group 4 (375 ppm) after 78 weeks of treatment. Histopathologically, this was reported as nodular hepatic hyperplasia in high and mid-dose males. In treated males, various combinations of the following hepatic changes were observed in a dose related manner: in mid- and high dose groups: centrilobular or diffuse hepatocellular hypertrophy, centrilobular to disseminated single cell necrosis, diffuse Kupffer cell proliferation, disseminated phagocytic cell foci, patchy centrilobular to bridging glycogen storage and multifocal aggregates of lipofuscin-storing macrophages. In males of the low dose group, only the incidence of hepatocellular hypertrophy, single cell necrosis and phagocytic cell foci was increased when compared to the control group. There was no difference in the severity of single cell necrosis between control and low dose group. The increased incidence of single cell necrosis in hepatocytes is interpreted as adverse effect, especially since this finding was already observed at the 52 week interim kill. In the absence of historical control data, this finding in the low dose has to be considered substance-related and adverse.

In females, an increased incidence of all of the above-mentioned changes was encountered in the high dose, whereas in the mid dose only an increased incidence of single cell necrosis and phagocytic cell foci was encountered. In the low dose, only an increased incidence of single cell necrosis was noted when compared with the control group.

 

Interpretation non-neoplastic liver effects in mice:

Effects observed in low-dose animals were limited to histopathological changes, namely increased incidences of hepatocellular hypertrophy, single cell necrosis and slightly elevated incidences of phagocytic cell foci. Additionally, nodular hyperplasia was observed. Taking into consideration the criteria mentioned in Thoolen et al. 2010, which is considered the current gold standard for evaluation of histopathological liver damage, the nodular hyperplasia is most likely a regenerative nodular hyperplasia, which is considered non-neoplastic. The hepatocellular hypertrophy observed was not accompanied by liver enzyme changes, however single cell necrosis was observed in the histopathological evaluation. The increased incidence of single cell necrosis in hepatocytes is interpreted as adverse effect, especially since this finding was already observed at the 52 week interim kill. In the absence of historical control data, this finding in the low dose has to be considered substance-related and adverse.Liver enzyme changes were detected at the mid and high dose levels. Based on the described findings, no NOAEL for systemic toxicity was identified and the dose level of 15 ppm was established as LOAEL under the conditions of this study, corresponding to 2.1 mg/kg bw/d in males and 3.1 mg/kg bw/d in females.

 

Interpretation of neoplastic lesions in mice:

An increased incidence of hepatocellular adenomas was observed in male mice of groups 3 and 4.

The independent expert statement re-evaluated the slides and commented on biological relevance and statistical significance of the observed liver lesions. "Group 1 already showed a relatively “high” incidence of animals bearing hepatocellular (hyperplastic) nodules 2/60, hepatocellular adenomas 8/60, and hepatocellular carcinomas 2/60. In Group 2, there was no statistically significant increase in the incidence of hepatocellular adenomas and/or hepatocellular carcinomas. In Group 3, there was a slight but statistically not significant increase in the incidence of hepatocellular adenomas (13/60 vs. 8/60). Hepatocellular carcinomas developed in Group 3 with a similar frequency as observed in the controls. In Group 4 (375 ppm [...]), hepatocellular nodules were observed in 12 animals against 2 animals seen in the controls, showing a statistically significant Chi-square value of 8.0863 (probability <0.01).

However. the incidence of hepatocellular adenomas and hepatocellular carcinomas did not increase statistically in Group 4 above the incidence of those two hepatocellular types of tumors observed in the controls. Consequently, this analysis of the observed nodular liver lesions indicates that chronic administration of [the test substance] did not affect the incidence of either hepatocellular adenomas or hepatocellular carcinomas either alone or in combination. The only effect of treatment has been manifested in the increase in the incidence of the non-neoplastic hepatocellular nodules observed in animals exposed to 375 ppm of [the test substance]."

Therefore, it was concluded that the test substance did not manifest either tumorigenic effect (enhancement of hepatocellular adenomas) or carcinogenic effect (enhancement of hepatocellular carcinomas) in mice under the conditions tested.

 

 

 

Rats:

The potential carcinogenic properties of the test substance were tested in two independent but complimentary studies in rats. In the first study according to OECD Guideline 453 and GLP, Wistar rats were treated with 0, 20, 100 or 500 ppm test substance in feed. For the interim sacrifice, 10 animals per sex, dose and timepoint were sacrificed and analyzed after 53 weeks or 107 weeks of treatment while another 50 males and 50 females were treated for a total of 120 weeks. In the second study according to EPA OPPTS 83-5 guideline and GLP, Wistar rats were treated with 0, 2500 or 10000 ppm test substance in feed. Again, 10 animals per sex and dose were analyzed at the chronic toxicity time point while 50 per sex and dose were analyzed after 111 weeks for carcinogenicity.

Neither of the studies showed and substance-related mortality or clinical signs, no effects on food consumption were observed. Similarly, no findings were reported for hematological analysis, ophthalmological examinations or urinalysis. Body weight gains were slightly decreased in females treated with 2500 or 10000 ppm test substance. The bodyweight gains were considered to be similar for all groups of males. While clinical chemistry changes were very slight and in part also only temporary in the first study with doses up to 500 ppm, liver enzyme activity changes were more consistent in the second study. Aspartate aminotransferase (ASAT) activity was slightly increased for males of 2500 and 10000 ppm groups at 26, 52, 104 and 111 weeks of treatment. Similar trends were apparent for females of the 2500 ppm group at 52, 104 and 111 weeks of treatment, and for females of the 10000 ppm group at 111 week time intervals, but statistical significance was attained for group 3 females at 111 weeks, only. Alanine aminotransferase (ALAT) activity was slightly increased for treated males at 26, 52 and 111 weeks, but the changes were significant only for the 2500 ppm males at 78 weeks. The increased transaminase activities reflect hepatocellular enzyme leakage and suggest marginal hepatotoxicity starting at 2500 ppm. In accordance with the clinical biochemistry findings, the relative liver weights of males treated with 500 ppm were slightly increased after 120 weeks of treatment. These findings were of greater magnitude in males treated with 10000 ppm test substance; absolute and relative liver and kidney weights were moderately increased relative to male controls. Histologically, the incidences of hepatocellular hypertrophy as well as fatty changes in the liver increased in animals treated with 2500 ppm or 10000 ppm in a dose-dependent manner. Based on the effects observed in both studies, the NOAEL for chronic toxicity in rats was determined to be 100 ppm, equivalent to ca. 4.8 mg/kg bw/d under the conditions of these studies.

 

Interpretation of neoplastic lesions in rats:

A statistically significant increase in the incidence of mesenteric lymph node hemangiomas was noted for males treated with 500 ppm, relative to male controls. The historical control data for this type of tumor indicate that the incidence rate in the concurrent control group was lower than in the historical control group and that the incidence rate in the high dose group was within the historical control range. Using the historical "time to tumor discovery" data for male controls, there was no evidence to suggest that the test substance caused an earlier development of benign mesenteric lymph node hemangiomas. Thus, the increased incidences of mesenteric lymph node hemangiomas in the high dose group were considered not biologically relevant. This is supported by the findings of the second study, in which the animals were exposed up to 10000 ppm test substance. An incidence of 12% was found in controls versus 16% in the 10000 ppm group. Overall, no dose-response for mesenteric lymph node hemangiomas was observed since the 17% incidence at 500 ppm stand versus the 13.6% incidence at 2500 ppm and 16% incidence at 10000 ppm.

Pancreatic exocrine carcinomas, which are occasionally noted in aged untreated rats were diagnosed in one female of the 100 ppm group and two male rats of the 500 ppm group. Based on the low number of affected rats, it cannot be concluded from the first study alone that the increased incidence of pancreatic exocrine carcinomas in males of the 500 ppm group is a treatment-related effect. The incidences for this lesion in the second study were 0%, 4.4% and 0% at 0, 2500 and 10000 ppm, respectively. Therefore, this finding is considered not related to test substance administration.

The occurrence of adenocarcinomas of the uterus in the 10000 ppm group at an incidence of 10% compared to no tumors in the concurrent control females of the second study was not considered to be treatment related in view of a control incidence of 8% noted in the first chronic toxicity/oncogenicity study.

It can therefore be concluded that the test substance is not carcinogenic in rats under the conditions of these studies, including at the highest dose administered of 10000 ppm, equivalent to 487.3 and 615.2 mg/kg bw/day for males and females, respectively.