Chlorpyrifos

Carcinogenicity

Description of key information

Study Type	Species	Findings	Guideline	Reliability
24 month Feeding Study	Rats	NOEL: 0.1 mg/kg bw/day	EPA OPP 83-1 and 83-2	1
105 weeks Feeding Study	Mouse	Results of this study did not reveal an oncogenic response in mice	no guideline followed	2

Key value for chemical safety assessment

Carcinogenicity: via oral route

Link to relevant study records

Reference

Reference 1

Endpoint:

carcinogenicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

EPA OPP 83-2 (Carcinogenicity)

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: EPA OPP 83-1 (Chronic Toxicity)

Deviations:

no

GLP compliance:

yes

Specific details on test material used for the study:

Lot Number: AGR 214637
Purity: 98.5%

Species:

rat

Strain:

Fischer 344

Details on species / strain selection:

The Fischer-344 rat was selected as the experimental model because it has been used extensively for toxicity-oncogenicity studies, has a well known biological database and a high degree of uniformity, and is available in large numbers from a reputable supplier. In addition, this strain of rat was used in the subchronic study.

Sex:

male/female

Route of administration:

oral: feed

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

Analyses to confirm test material concentrations were performed on samples of premixes, control feed, and all dose-level diets at the start of the study and at approximately 3-month intervals until the end of the study. Overall, the analytically determined dietary concentrations were sufficiently comparable to targeted concentrations for meaningful interpretation of the study.

Duration of treatment / exposure:

24 months

Frequency of treatment:

Daily

Dose / conc.:

0.05 mg/kg bw/day

Dose / conc.:

0.1 mg/kg bw/day

Dose / conc.:

1 mg/kg bw/day

Dose / conc.:

10 mg/kg bw/day

No. of animals per sex per dose:

60 (10 rats/sex/dose level were randomly designated at the start of the study for an interim sacrifice at 12 months. The remaining animals were continued on test until 24 months or they died or were killed moribund)

Control animals:

yes, plain diet

Clinical signs:

effects observed, non-treatment-related

Description (incidence and severity):

There were no clinical signs for individual animals which suggest a relationship to test substance exposure. In addition to the individual clinical observations, a group observation of mild yellowish perineal staining was noted for the top dose female rats for much of the study. 30-40% of the 10 mg/kg bw/day group females had perineal staining from approximately the 6th month until it was no longer distinguishable at about 20 months. A significant amount of perineal staining was not present in high dose males or any other group of animals.

Mortality:

mortality observed, non-treatment-related

Description (incidence):

There were no statistically or biologically significant differences in the mortality pattern between controls and any treated dose groups.

Body weight and weight changes:

effects observed, treatment-related

Description (incidence and severity):

In males, the body weights of rats in the 1 and 10 mg/kg bw/day groups were consistently lower than the control group in a dose-related manner, throughout the study. At the higher dose level, the weight depression was approximately 7-9% throughout the study and was consistently identified as statistically significant. While statistically significant at most measurement intervals, the weight gain depression in the 1 mg/kg bw/day group was less pronounced than in the higher dose level. The group mean body weights of females in the top dose level were slightly but statistically significantly decreased relative to controls from approximately the second month on study through most of the first year. After that time, they were not distinguishable from control values.

Food consumption and compound intake (if feeding study):

no effects observed

Haematological findings:

effects observed, non-treatment-related

Description (incidence and severity):

None of the findings in treated animals were interpreted as direct effects of exposure to test substance.

Clinical biochemistry findings:

effects observed, treatment-related

Description (incidence and severity):

There were a number of values which were statistically identified as different from the control means at various dose levels and sampling periods. However, in male rats the only parameters which were consistently different and appeared to be associated with the treatment regimen were decreases in cholesterol, total protein and globulin in the 10 mg/kg bw/day group at 6, 12, and 18 months. (These differences were not remarkable at 24 months; however, this is not unexpected due to the high variability and intercurrent disease entities in the geriatric population). The decreases in these parameters do not appear to indicate any direct organ toxicity but are probably secondary effects of altered metabolism, possibly as a result of the adrenal changes.
As in males, the cholesterol values for female rats in the top dose were significantly decreased at 6, 12, and 18 months. Globulin values were significantly decreased at 6 and 12 months but not at 18 or 24 months.
All other statistically identified values in males and females for clinical chemistry or electrolytes were considered incidental and due to random variability because they were either not dose-related or were inconsistent over time as to occurrence or direction of change relative to control.

Urinalysis findings:

effects observed, treatment-related

Description (incidence and severity):

Urine specific gravity was consistently elevated in male rats of the top dose group at all sampling periods and was statistically significant at all but the 12-month time period. While this persistent elevation appears to be treatment related, it does not appear to be an indicator of a direct renal effect since there was significantly less renal pathology in the top dose than in the control animals. Female rats in the high dose also had a mild but statistically significant increase in urine specific gravity at the 6- and 12-month sampling periods, but not after 12 months. All other urinary parameters in either males or females appeared to be unaffected by test substance administration.

Organ weight findings including organ / body weight ratios:

effects observed, non-treatment-related

Description (incidence and severity):

At both sacrifice intervals, the body weights of the top dose males were lower than controls, indicative of the decreased weight gain throughout the study in this dose group. The smaller size of the male rats of the 1 mg/kg bw/day group is not apparent in the sample chosen for the 12-month sacrifice. At the terminal sacrifice, the final fasted body weight of the 0.1 mg/kg bw/day group is statistically significantly decreased relative to controls, which probably represents group-to-group variability in the geriatric population. The in-life body weights of this group were comparable to controls throughout the entire study.
The only organ weight changes in males which appear to be effects of test compound administration are the significant increases in absolute and relative adrenal weights of the top dose animals at both sacrifice intervals. The other dose levels were unaffected. The statistically identified increase in relative brain weight at both necropsies and the decreased absolute kidney and liver weights at 24 months in the top dose males were considered a reflection of the lower body weight in this group of animals.
In female rats, there were no biologically significant changes in body or organ weights at the 12-month sacrifice interval. At 24 months; however, the relative and absolute adrenal weights in the top dose animals were significantly increased similar to the male rats but to a lesser degree. The absolute brain weight, but not the relative weight, in female rats of the 10 mg/kg bw/day group was statistically significantly increased at the 24-month sacrifice. With the greater variability at this age, this increase cannot be causally related to test substance administration.

Gross pathological findings:

effects observed, non-treatment-related

Description (incidence and severity):

There was no obvious pattern of changes in any dose group at either sacrifice interval suggestive of a treatment-related effect.

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Description (incidence and severity):

At the 12-month sacrifice, the only histopathologic alteration associated with test substance administration was an excess of fatty vacuolization of the zona fasciculata in the adrenal cortex of male rats given 10 mg/kg bw/day of the test material. This change would appear to account for the increase in adrenal weight noted for this group of male rats at this sacrifice interval. As noted in the data, control male rats of this age normally have some degree of mild vacuolization of the mid-adrenal cortex (recorded as very slight), but females do not. The alteration in top dose males was an exacerbation of this vacuolization without obvious concommitant necrosis or cellular degeneration. There were no histopathologic changes in female rats at any dose level associated with the test material at 12 months.

Histopathological findings: neoplastic:

effects observed, treatment-related

Description (incidence and severity):

Histopathologic examination of male rats of the top dose designated for terminal sacrifice revealed changes in three organ systems that appear to be treatment-related: an increase in slight adrenal cortical vacuolization similar to that seen at the 12-month sacrifice; a decrease in the severity of chronic progressive glomerulonephropathy (chronic renal disease); and a decreased incidence of mild biliary hyperplasia in the liver. In female top dose rats, the only apparent treatment-related effect was a similar decrease in the incidence of biliary hyperplasia. There were no histopathologic changes in any dose level other than the 10 mg/kg bw/day level which were interpreted as effects of test sub stance administration. There were no statistically identified linear trends or pairwise increases in any tumors for either males or females at any dose level. In addition, the total number of primary, benign, or malignant tumors was comparable in all groups.

Other effects:

effects observed, treatment-related

Description (incidence and severity):

Plasma and brain cholinesterase values were depressed

Relevance of carcinogenic effects / potential:

There is no indication of any excess of masses, unusual tumors, or early onset in any treatment group versus respective controls. The type and incidence of cutaneous and subcutaneous "masses" noted were within the normal range of neoplasms for the Fischer-344 rat.

Key result

Dose descriptor:

NOEL

Effect level:

0.1 mg/kg bw/day

Based on:

test mat.

Sex:

male/female

Basis for effect level:

body weight and weight gain

histopathology: neoplastic

histopathology: non-neoplastic

organ weights and organ / body weight ratios

urinalysis

other: Cholinesterase activity

Conclusions:

NOEL (rat): 0.1 mg/kg bw/day
There was, however, no increase in tumors of any type in any organ or tissue at any of the dose levels tested.

Executive summary:

The study was conducted following the guideline, EPA OPP 83-1 and 83-2. Sixty rats/sex/dose level were fed test substance in their diets at concentrations formulated to provide 0 (control), 0.05, 0.1, 1, or 10 mg/kg bw/day. Ten rats/sex/dose level were randomly designated at the start of the study for an interim sacrifice at 12 months. The remaining animals were continued on test until 24 months or they died or were killed moribund. Parameters measured during the study were clinical observations; body weights; feed consumption; hematology; clinical chemistries and electrolytes; plasm, RBC and brain cholinesterase; urinalyses; and gross and histopathologic examination.

The primary effects of administration of 10 mg/kg bw/day to male rats for up to 2 years were a decrease in body weight gain relative to controls, depression of plasma, RBC, and brain cholinesterase, and an increase in the size of the adrenal glands characterized by increased fatty vacuolization of the zona fasciculata. Other effects of the treatment regimen, which may be secondary to these changes, included decreases in serum cholesterol, total protein and globulin, an increase in urine specific gravity, and a decrease in some common geriatric conditions such as chronic renal disease and biliary hyperplasia. The mechanism of these secondary effects is unknown, but they may be the result of changes in adrenal function, metabolic alterations associated with the smaller size of the animals, and/or the results of mild chronic stress.

Altered parameters in females at the top dose were generally similar to those in males but were less pronounced. There was a transient depression of body weight and an increase in adrenal weight at the terminal sacrifice, however, there was no histopathologic lesion associated with this increase in adrenal weight. Plasma and brain cholinesterase values were depressed but not erythrocyte levels. Like males, serum cholesterol and globulins were mildly depressed, and urine specific gravity was elevated, generally coinciding with the periods of weight gain depression, and there was a decreased incidence of biliary hyperplasia. Clinically, there was also mild perineal staining in these female rats throughout much of the study.

At the 1 mg/kg bw/day treatment level, the only effects of treatment in males were depression of body weight gain and a depression of plasma and RBC cholinesterase levels. In females, plasma cholinesterase depression was the only effect at this dose level.

The chronic toxicity noted at the top dose in this study was considered indicative of a sufficient challenge for the assessment of oncogenicity without producing excessive mortality. There was, however, no increase in tumors of any type in any organ or tissue at any of the dose levels tested.

The No-Observed-Effect-Level (NOEL) for all parameters in both males and females for this study was 0.1 mg/kg bw/day.

Reference 2

Endpoint:

carcinogenicity: oral

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

This study was conducted to evaluate the long-term toxicity and oncogenic potential of the test substance which might result from lifetime ingestion in the diet of mice. Groups of CD-1 mice (56/sex/group) were maintained on diets containing 0, 0.5, 5 and 15 ppm of test substance for 105 weeks. Parameters evaluated included behavior, physical condition, mortality, food consumption, body weights, organ weights and tumor incidence including gross and microscopic examination of all mice which died, or were killed in a moribund state or sacrificed at termination of the study.

GLP compliance:

no

Specific details on test material used for the study:

Ref. No.: 1-550-2
Purity: 99.6 ± 0.9%

Species:

mouse

Strain:

CD-1

Sex:

male/female

Route of administration:

oral: feed

Vehicle:

other: diet

Analytical verification of doses or concentrations:

yes

Duration of treatment / exposure:

105 weeks

Frequency of treatment:

Daily

Dose / conc.:

0.5 ppm

Remarks:

0.05 mg/kg/day

Dose / conc.:

5 ppm

Remarks:

0.5 mg/kg/day

Dose / conc.:

15 ppm

Remarks:

1.5 mg/kg/day

No. of animals per sex per dose:

56

Control animals:

yes, plain diet

Clinical signs:

effects observed, non-treatment-related

Description (incidence and severity):

There were no changes in behavior or physical appearance during the course of the study that could be attributed to ingestion of test substance in the diet.

Mortality:

mortality observed, non-treatment-related

Description (incidence):

In neither sex was there a statistically significant difference between the control and treated groups in survival rate.

Body weight and weight changes:

effects observed, non-treatment-related

Description (incidence and severity):

No statistically significant differences were observed between the mean body weights of the control and treated groups of animals during the course of the study. The mean body weight of males ingesting 15 ppm test substance showed a slight decrease at day 420 when compared to the mean values of the control and other treatment groups; at each of the other observation intervals their mean body weight was comparable to other male groups. This isolated observation was considered of no toxicologic significance. The mean body weight of all treated groups of female mice was greater than the control mean beginning with day 84 of the study. All male and female study groups, including controls, showed a slight decrease in mean body weight during the final two months of the study.

Food consumption and compound intake (if feeding study):

effects observed, non-treatment-related

Description (incidence and severity):

Statistically significant decreases in mean daily food consumption were observed on study day 168 in males given 5 ppm and on study days 504 and 588 in males given 0.5 ppm. In female mice, a statistically significant decrease in food consumption occurred on study day 476 in females given 5 ppm and on study day 504 in females given 15 ppm. Food consumption of female mice given 0.5 ppm showed a statistically significant increase over the mean control value on day 28. The food consumption curves show that food consumption by all groups of male and female mice, including controls, showed a decline from 2 to 3 months until 10 to 11 months when it was once again increased. Thereafter all study groups showed generally consistent deviations in mean food consumption. In summary, the few sporadic instances in which there was a statistical decrease or increase in food consumption between the control and various treatment groups showed no consistent trend and were, therefore, considered of no toxicological significance.

Organ weight findings including organ / body weight ratios:

effects observed, non-treatment-related

Description (incidence and severity):

Organ weights were unaffected at any dose level of the test substance. The differences in heart and kidney weights of males of the low dose level were not considered to be of toxicological significance due to the absence of a dose-response relationship and the lack of gross or microscopic lesions unique to this group as compared to other male groups including the controls. Similarly, the statistical difference in the weight of the liver of females of the low and intermediate dose levels were not considered to be of toxicological significance, particularly in the absence of a dose response relationship.

Gross pathological findings:

effects observed, non-treatment-related

Description (incidence and severity):

None of the major and minor lesions or the other gross and microscopic non-neoplastic lesions were considered related to lifetime ingestion of test substance in the diet at any of the dose levels tested.

Histopathological findings: non-neoplastic:

effects observed, non-treatment-related

Description (incidence and severity):

A variety of non-tumor lesions, categorically of a degenerative, inflammatory or proliferative nature was observed in a random fashion in animals of both the treated and control groups. Although the incidence rate of hyperplastic nodules of the liver of males of the intermediate dose level group was increased statistically over that of control males, the increased incidence was considered fortuitous and unrelated to treatment since it was not observed in a dose-response fashion and also showed a lack of correlation with the incidence rate of hyperplastic nodules in other groups of treated and control animals. Thus, all of the non-tumor lesions were considered to be spontaneous and unrelated to treatment with test substance.

Histopathological findings: neoplastic:

effects observed, non-treatment-related

Description (incidence and severity):

A variety of types of tumors, tumor-like lesions and other non-proliferative lesions occurred in a spontaneous or random fashion in animals of both the control and treated groups. None of these lesions were considered to be related to ingestion of test substance in the diet.

Relevance of carcinogenic effects / potential:

The proliferative lesions (tumor and tumor-like lesions) which were observed consisted of a wide variety and occurred, albeit, at a generally low incidence rate, in a random or spontaneous fashion among animals of both the treated and control groups. There were no statistically significant differences observed in the incidence rate of tumors between animals of the treated and control groups except for a statistically significant but non-dose related increase in adenomas of the lung in males of the intermediate dose level group. However, when the incidence rates of adenomas and carcinomas of the lung were combined and analyzed (an acceptable practice due to their biological behavior), there were no statistical differences in tumors of the lung between any of the treated and control groups of animals.
A variety of non-tumor lesions, categorically of a degenerative, inflammatory or proliferative nature was observed in a random fashion in animals of both the treated and control groups. Although the incidence rate of hyperplastic nodules of the liver of males of the intermediate dose level group was increased statistically over that of control males, the increased incidence was considered fortuitous and unrelated to treatment since it was not observed in a dose-response fashion and also showed a lack of correlation with the incidence rate of hyperplastic nodules in other groups of treated and control animals. Thus, all of the non-tumor lesions were considered to be spontaneous and unrelated to treatment with the test substance.

Key result

Remarks on result:

other: Results of this study did not reveal an oncogenic response in mice

Key result

Critical effects observed:

no

Conclusions:

Results of this study did not reveal an oncogenic response in mice

Executive summary:

This study was conducted to evaluate the long-term toxicity and oncogenic potential of test substance which might result from lifetime ingestion of test substance in the diet of mice.

Groups of CD-1 mice (56/sex/group) were maintained on diets containing 0, 0.5, 5 and 15 ppm of test substance (approximately 0.05, 0.50 and 1.50 mg/kg/day) for 105 weeks. Parameters evaluated included behavior, physical condition, mortality, food consumption, body weights, organ weights and tumor incidence including gross and microscopic examination of all mice which died, or were killed in a moribund state or sacrificed at termination of the study.

None of the above parameters appeared to be affected by treatment at any dose level of test substance. A variety of types of tumors, tumor-like lesions and other non-proliferative lesions occurred in a spontaneous or random fashion in animals of both the control and treated groups. None of these lesions were considered to be related to ingestion of test substance in the diet. Thus, results of this study revealed neither evidence of toxicity nor an oncogenic response in mice administered test substance in the diet during the major portion of their natural lifespan.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Quality of whole database:

Rat (dietary) 24-months: Under the conditions of the study, methomyl was not a carcinogen.

Mouse (dietary) 105-week: Under the conditions of the study, methomyl was not a carcinogen.

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 results of chronic feeding studies in rats and mice, it can be concluded that the test substance does not pose a carcinogenic concern for humans. Therefore, the test substance is not classified for carcinogenicity according to EU Classification, Labelling and Packaging of Substances and Mixtures (CLP) Regulation (EC) No. 1272/2008.

Additional information

In a 2-year oncogenicity dietary rat study, 0.05, 0.1, 1, or 10 mg/kg/day chlorpyrifos was provided. There was no increase in tumors of any type in either males or females at any dose level. The No-Observed-Effect-Level (NOEL) for all parameters in both males and females for this study was 0.1 mg/kg/day.  

  

In a 105-week oncogenicity mice study, diets containing 0, 0.5, 5 and 15 ppm of chlorpyrifos (approximately 0.05, 0.50 and 1.50 mg/kg/day) was provided. A variety of types of tumors, tumor-like lesions and other non-proliferative lesions occurred in a spontaneous or random fashion in animals of both the control and treated groups. None of these lesions were considered to be related to ingestion of chlorpyrifos in the diet. Thus, results of this study revealed neither evidence of toxicity nor an oncogenic response in mice administered chlorpyrifos in the diet during the major portion of their natural lifespan.  

  

No evidence of carcinogenicity was observed in long-term studies of toxicity/carcinogenicity with chlorpyrifos in rats and mice