Teflubenzuron

Endpoint summary

Description of key information

- mouse: LOAEL 2.1 mg/kg bw/d (oncogenicity study)

- rat: NOAEL 4.8 mg/kg bw/d (combined chronic toxicity and carcinogenicity study)

- dog: NOAEL 17.3 mg/kg bw/d (chronic toxicity study)

- monkey: NOAEL 250 mg/kg bw/d (14 -22 day dose range-finding study)

Key value for chemical safety assessment

Toxic effect type:

dose-dependent

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

2.1 mg/kg bw/day

Study duration:

chronic

Species:

mouse

System:

hepatobiliary

Organ:

liver

Repeated dose toxicity: inhalation - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: inhalation - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Mode of Action Analysis / Human Relevance Framework

The liver is clearly the target organ in all species investigated. Among the different species, mice are most sensitive to substance-induced liver effects and the mouse oncogenicity study resulted in a LOAEL of 15 ppm corresponding to 2.1 mg/kg bw/d.

Rats showed similar effects, albeit at higher doses, the NOAEL derived from a combined chronic toxicity and carcinogenicity study in rats was 4.8 mg/kg bw/d.

In the beagle dog, the lowest NOAEL derived was 17.3 mg/kg bw/d in a chronic feeding study, demonstrating an even lower sensitivity to test substance-related effects. Finally, a dose-range finding study in monkeys was also available which showed very minor effects following administration of the test substance via gavage at dose levels of 1250 mg/kg bw/d and in which the NOEL was set at 250 mg/kg bw/d following 17 day exposure.

While the short exposure in the monkey is not comparable to the longer exposure in the other species, the data indicate that rodents and mice in particular are more sensitive to test substance induced hepatotoxicity than dogs or monkeys. Therefore, it is feasible to consider the effects observed in mice a worst-case for risk assessment, since it can be anticipated that humans will be less sensitive, as extrapolated from the existing monkey data.

 

Additional information

Several studies on repeated dose toxicity of the test substance via the oral route are available. The substance has been tested in rat, mouse, dog and monkey in subacute, subchronic and chronic studies over a large range of concentrations. In all species, the target organ was the liver as indicated by increases in different liver enzymes (ALT, AST, OCT) as well as liver weight increases in some of the studies.

 

 

Rat:

In rats, the substance was tested in three 28-day studies via dietary administration. Two of the studies were dose-range finding studies with doses between 100 and 15000 ppm. Starting at 5000 ppm, increased levels of AST and ALT were observed, however no increases in liver organ weights correlated with these findings. The data of both studies is congruent in terms of type and severity of effects. However, due to lack of histopathological examination, no NOAEL was set in these studies. Additionally, the dose-response and plasma levels of the test substance were investigated in a dietary 28-day study with dose ranges of 500-32000 ppm. It found that the plasma levels of the test substance increased from 500 to 1000 ppm, but reached a plateau at 1000-2000 ppm and did not increase further even at higher dose levels.

 

In a 90-day dietary study in Wistar rats, conducted according to EPA OPPTS 82-1 and GLP, 10 males and 10 females each were administered doses of 0, 100, 1000 or 10000 ppm test substance via the diet. Additionally, 5 males and females each were employed as recovery animals for control and high dose group. Substance-related findings at the end of the administration period were increased liver weight in high dose females and increased testes weights in high dose males. Both effects were reversible at the end of the recovery period. Additionally, increased levels of AST, ALT, ALP and OCT were found in males but not in females at the end of treatment, but not after recovery. No histological changes were identified in the liver. Based on the data from other studies, the effects on testes weights were considered incidental as they were not observed in any other study. The liver was identified as target organ and the NOAEL was set at 1000 ppm (corresponding to 81.5 mg/kg bw/d in males and 94.0 mg/kg bw/d in females).

 

Additionally, two complementary combined chronic toxicity and carcinogenicity studies were conducted in rats via the dietary route according to or similar to OECD guideline 453 and under GLP.

For the first study, Wistar rats were administered 0, 20, 100 or 500 ppm test substance in the diet. The animals were treated for 53 weeks (10 per sex and dose), 107 weeks (10 per sex and dose) or 120 weeks (50 per sex and dose). In the second study, Wistar rats were administered 0, 2500 or 10000 ppm test substance in the diet. The animals were treated for 104 weeks (10 per sex and dose) or 111 weeks (50 per sex and dose). In both studies, systemic exposure to the test substance was monitored by determination of plasma levels of test substance in addition to the guideline-required parameters.

Slight effects on body weights were observed for females in the 2500 and 10000 ppm groups.

First indications of clinical chemistry changes were observed at 500 ppm; males showed slightly increased activities of AST and ALT as well as intermittent increased OCT activity. This trend was slightly stronger at higher doses, with increased AST activity for males and females at 2500 and 5000 ppm. ALT activity was also significantly increased in males at 2500 and 5000 ppm, whereas it was slightly but not significantly increased in females. At 10000 ppm, the absolute (but not relative) liver weights in males but not in females were increased. Concomitantly, the incidence of hepatocellular hypertrophy was increased and also fatty changes in the liver were found more frequently in animals treated with 2500 or 10000 ppm of test substance.

Based on the increased liver enzymes at 500 ppm as well as a slight but statistically significant increase in relative liver weights in males at this dose, the combined data from both chronic dietary studies allow for determination of a NOAEL at 100 ppm (corresponding to 4.8 mg/kg bw/d in males and 5.9 mg/kg bw/d in females).

 

 

Mouse:

In a 14-day dose-range finding study, C57BL/6J mice (5 per sex and dose) were administered the test substance at doses of 0, 1000, 3000 or 6000 ppm in the diet. All doses showed the liver as target organ with absolute and relative liver weight increases in males starting in the 1000 ppm group. At higher dose levels, female animals showed the same effects and a dose-dependent increase in severity was observed. Additionally, ALT and AP were significantly increased in all dose levels, together with other clinical chemistry parameters indicting hepatotoxic effects of the test substance. No NOAEL was identified in this study.

 

In a 90-day dietary study comparable to OECD guideline 408 and conducted under GLP, ICR mice (12 per sex and dose) were administered the test substance at levels of 0, 100, 1000 or 10000 ppm in feed. In congruence with the effects observed in the described dose-range finding study, liver weights of both males and females were significantly increased starting at 1000 ppm. Additionally, males of the 10000 ppm group showed significantly elevated levels of ALT (GPT) and Glucose, whereas a trend towards increased AP was visible, however not statistically significant. High dose females were more strongly affected in clinical biochemistry parameters with significantly increased levels of AP, Glucose, ALT (GPT) and total cholesterol. Gross pathology examination resulted in increased incidences of enlarged and darkened liver in high and mid dose animals, confirmed by histopathology with increased incidences of liver centrilobular swelling in high and mid dose as well as increased incidence of centrilobular hepatocellular fatty change in mid dose animals.

The results of the study have shown that the liver is the target organ in both male and female mice. A NOAEL was established at 100 ppm (equivalent to 11.85 and 13.7 mg/kg bw/d for males and females respectively).

 

Additionally, data from an oncogenicity study in NMRI mice is available. While the study has been conducted according to OECD guideline 451, these data can nevertheless be used for assessment of repeated dose toxicity since several key parameters, such as clinical chemistry and histopathology, were thoroughly investigated. In the study, 10 animals per sex and dose were used for a 52 week termination and 50 per sex and dose were used for a 78 week termination time point. The animals were administered the test substance in diet at concentrations of 0, 15, 75, or 375 ppm.

While no substance related mortality or clinical signs were observed, the body weights of high dose males were slightly reduced when compared to the controls, by approximately 9% at 52 weeks and 5% at 78 weeks. Both males and females of the high dose group as well as males of the mid dose group showed increased activity of AST. Similarly, males and females of the high dose group showed marked and slight increases in activity of ALT, respectively. In males of the mid dose, a trend towards increased ALT activity was noted. Lactate dehydrogenase activity was slightly increased in both sexes as 78 weeks of treatment while OCT activity was markedly increased in high dose males only; high dose females showed only a trend towards increased OCT activity. Absolute and relative liver weights were increased in both males and females of the high dose group while in the mid dose group only relative liver weights were significantly increased in males.

Histopathology showed treatment-related effects in all treated animals, albeit to varying degrees. Animals in low dose group (15 ppm) showed increased incidences of diffuse and/or centrilobular hepatocellular hypertrophy as well as increased incidences of single cell necrosis and slightly increased phagocytic cell foci as well as lipofuscin accumulation. These findings increase in incidence and severity with increasing dose levels. In addition, animals in the mid dose (75 ppm) group showed nodular hyperplasia, patchy glycogen storage and Kupffer cell proliferation, while in animals of the high dose group (375 ppm) increased incidences of bile duct proliferation were reported in addition.

Therefore, it can be concluded that also in a long-term study in mice, the liver is clearly the target organ. 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. 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 observed in mid- and high dose animals is most likely a regenerative nodular hyperplasia, which is considered non-neoplastic. The hepatocellular hypertrophy observed was not accompanied by liver enzyme changes in the low dose, however single cell necrosis was observed in the histopathological evaluation. The increased incidence of single cell necrosis in hepatocytes of low dose animals 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.

 

 

Dog:

In a 28-day dietary dose range-finder, beagle dogs (1 per sex and dose) were administered the test substance in feed at doses of 0, 100, 1000 or 10000 ppm. No treatment-related findings were observed in any of the parameters investigated (including clinical biochemistry with liver enzymes).

 

In a subsequent 90-day dietary study according to OECD guideline 409 and GLP, beagle dogs (4 per sex and dose) were administered the test substance in feed at concentrations of 0, 100, 1000, and 10000 ppm. Substance-related findings were restricted to clinical biochemistry and necropsy findings. Two animals of the high dose group showed increased levels of AST, ALT, AP and OCT, however the peak increases were observed at weeks 4 (both animals) and 8 (one animal). Additionally, slightly increased absolute liver weights (significant in males) and relative liver weights (significant in females) were observed. Congruent with the increased liver enzymes, histopathological findings in two high dose animals morphologically similar to chronic active hepatitis were reported. The hepatopathy was characterized by moderate piecemeal, bridging and single cell necrosis as well as portal, inter- and intralobular infiltration of granulocytes, lymphocytes and plasma cells. This pathology was moderate in one of the animals and slight in the other. In addition to the two high dose animals, a slight variant of this finding was also observed in a low dose animal. In the absence of findings in the mid dose as well as a lack of clinical chemistry changes, the relevance of this finding was unclear.

Therefore, a second 90-day dietary study in beagle dogs was conducted at dose levels of 30 and 100 ppm, but otherwise similar to the previous study to determine the relevance of liver histopathology findings in the 100 ppm dose. No substance-related findings were observed, therefore demonstrating that the slight hepatopathy found in one of the 100 ppm animals in the previous study was an incidental finding and would not be considered related to treatment.

Based on the data of both studies combined, liver was the target organ for the test substance in beagle dogs following dietary administration. The NOAEL for 90 day dietary administration in beagle dogs was set at 1000 ppm (corresponding to 33.7 mg/kg bw/d in males and 42.8 mg/kg bw/d in females).

 

In a chronic dietary study according to EPA OPP 83-1 and GLP, beagle dogs (4 per sex and dose) were administered the test substance for 52 weeks at dietary dose levels of 0, 30, 100 or 500 ppm. Slightly increased liver weights were found in males of the high dose group, which were statistically significant. However, in the absence of histopathological correlates and clinical biochemistry changes these findings are not considered adverse. Therefore, the NOAEL for chronic dietary administration to beagle dogs was set at 500 ppm (corresponding to 17.3 mg/kg bw/d in males and 17.98 mg/kg bw/d in females).

 

 

Monkey:

Cynomolgus monkeys (1 per sex and dose) were administered the test substance as gavage in 1% CMC. The animals were treated for either 22 days at 50 mg/kg bw/d or 17 days at 250 mg/kg bw/d or 1250 mg/kg bw/d for 14 days. No substance-related findings were reported for treatment up to 250 mg/kg bw/d. Animals treated for 14 days with the test substance at 1250 mg/kg bw/d showed increased glucose levels in the clinical chemistry parameters, which were considered a treatment-related finding. Therefore, the NOAEL in this study was set at 250 mg/kg bw/d.

 

 

Justification for classification or non-classification

Based on the data available and considering the criteria laid down in regulation (EC) 1272/2008 (CLP), no classification for specific organ toxicity following single or repeated exposure with the test substance is warranted.