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CANCER ASSESSMENT DOCUMENT

EVALUATION OF THE CARCINOGENIC POTENTIAL OF

Aminocyclopyrachlor 

PC Code 288008

Final

November 9, 2011

CANCER ASSESSMENT REVIEW COMMITTEE

HEALTH EFFECTS DIVISION

OFFICE OF PESTICIDE PROGRAMS

AMINOCYCLOPYRACHLOR CANCER ASSESSMENT DOCUMENT FINAL

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TABLE OF CONTENTS

Executive Summary.........................................................................................................................5

I. Introduction .................................................................................................................................7

II. Background Information ............................................................................................................7

III. Evaluation of Carcinogenicity Studies......................................................................................8

1. Combined Chronic Toxicity/Carcinogenicity Study in Rats...........................................8

2. Carcinogenicity Study in Mice ......................................................................................16

IV. Toxicology..............................................................................................................................16

1. Metabolism ....................................................................................................................16

2. Mutagenicity ..................................................................................................................17

3. Structure Activity Relationship......................................................................................19

4. Subchronic and Chronic Toxicity..................................................................................19

5. Mode of Action Studies.........................................................................19

V. Committee’s Assessment of the Weight-of-the Evidence .......................................................20

VI. Classification of Carcinogenic Potential ................................................................................21

VII. Quantification of Carcinogenicity.........................................................................................21

VIII. Bibliography..........................................................................................................................21
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EXECUTIVE SUMMARY

On August 17, 2011, the Cancer Assessment Review Committee (CARC) of the Health Effects 

Division of the Office of Pesticide Programs met to evaluate the carcinogenic potential of

aminocyclopyrachlor. 

Jessica Ryman of Risk Assessment Branch IV presented the chronic toxicity/ carcinogenicity 

study in rats (MRID 48333607) and the carcinogenicity study in mice (MRID 48333606). 

Aminocyclopyrachlor technical was administered to Crl:CD(SD) rats (70/sex/dose) at dietary 

dose levels of 0, 600, 2000, 6000, or 18000 ppm (0/0, 27/29, 97/100, 279/309, 892/957 mg/kg 

bw/day in males/females) for 2 years to assess carcinogenicity. A satellite group of animals 

(10/sex/dose) was sacrificed at one year to assess chronic toxicity. Aminocyclopyrachlor was 

administered to young adult male and female Crlj:CDl(ICR) mice (60/sex/group) in diets that 

contained 0, 300, 1000, 3000, or 7000 ppm DPX-MAT28 for approximately 18 months. This 

corresponded to doses of approximately 0/0, 39/50, 133/171, 393/527, and 876/1190 mg/kg 

bw/day in males/females. Mutagenicity data and structure activity relationship data were also 

discussed.

The Committee considered the following for a weight-of-evidence determination of the 

carcinogenic potential of aminocyclopyrachlor.

Carcinogenicity

 Rat

• Brain Tumors: The CARC concluded that the brain tumors (glial cell tumors and granular 

cell tumors) observed in male rats were spontaneous lesions and were not considered to be 

treatment-related. This was based on the following weight-of-evidence considerations: 

1) Although there were significant trends and pair-wise comparisons at the high dose for 

astrocytomas and glial cell tumors combined by the Peto analysis, the incidences were 

within the historical control range. For granular cell tumors, there was a significant trend 

only, but no significant pair-wise comparison of any dose group with the controls. In 

addition, the Fishers Exact test showed trends only and no pair-wise comparisons of any 

dose level with the controls for combined glial tumors and for granular cell tumors. 2) No 

supporting treatment-related pre-neoplastic lesions were observed in the brain (or any other 

tissue). (Chemicals known to cause brain tumors also are commonly tumorigenic in other 

tissues, J. Pletcher, Consulting Pathologist, Personal Communication); 3) The tumors 

occurred at the limit dose; no other tumors were seen; 4) No treatment-related brain

neoplastic lesions were observed in female rats. 5) No evidence of genotoxicity was 

observed in in vitro or in vivo genotoxicity studies. 6) Brain levels of radioactivity after 

administration of aminocyclopyrachlor at times corresponding to peak plasma concentration 

were very low (<0.01%) and were undetectable after 72 hours.; 7) the Pathology Peer 

Review also concluded that the brain tumors were spontaneous tumors and not treatment-
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related (MRID 48333607). 

• Adequacy of Dosing: Dosing was considered adequate, and not excessive, for assessing 

carcinogenicity since the high dose approached the limit dose in both sexes (892/957 mg/kg 

bw/day in males/females). The high dose also caused mild decreases on body weight 

(decrease up to 12%) and body weight gain (decrease up to 11%) in both sexes during the 

study, but had resolved by the conclusion of the study.

Mouse

• There were no treatment-related increases in tumor incidences in either sex. 

• Adequacy of Dosing: Dosing was considered adequate since the high dose in this study 

approached the limit dose (876/1190 mg/kg/day in males/females). No adverse effects 

(including changes in body weight) were observed at this dose level. 

Mutagenicity: There is no concern for mutagenicity.

Structure Activity Relationship: There is no sufficient basis for SAR concern.

Classification and Quantification of Carcinogenic Potential

In accordance with the EPA's Final Guidelines for Carcinogen Risk Assessment (March, 2005), 

the CARC classified aminocyclopyrachlor as “Not Likely to be Carcinogenic to Humans.” 

There were no treatment-related tumors seen in male or female rats or mice at doses that were 

adequate to assess carcinogenicity. There is also no concern for mutagenicity.

Quantification of carcinogenicity is not required.
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I. INTRODUCTION

On August 17, 2011, the Cancer Assessment Review Committee (CARC) of the Health Effects 

Division of the Office of Pesticide Programs met to evaluate the carcinogenic potential of

aminocyclopyrachlor. 

II. BACKGROUND INFORMATION

Aminocyclopyrachlor is a pyrimidine carboxylic acid herbicide that was registered for non-food 

use in 2010. It is now being registered for the first food use on rangeland, pastures, and

Conservation Reserve Program (CRP) acres. There are three forms of aminocyclopyrachlor: 

the parent acid (PC Code 288008), the potassium salt (PC Code 288010), and the methyl ester 

(aminocyclopyrachlor-methyl, PC Code 288009). All three forms are components of both 

manufacturing and end-use products. Aminocyclopyrachlor is considered the active ingredient

due to rapid hydrolysis of aminocyclopyrachlor-methyl to aminocyclopyrachlor by cellular 

esterases. The pesticidal mode of action of aminocyclopyrachlor is dysregulation of gene 

products involved in auxin receptor activity, which interferes with normal shoot and root 

development to stop the growth of weeds. 

The only tumors observed in chronic toxicity/carcinogenicity studies in two species were brain 

tumors in male rats. The registrant contends that these tumors are not treatment-related, based on 

a consensus opinion following peer review by six board-certified pathologists. 

NH

Cl OH

NH

Cl CH

NH

Cl

Aminocyclopyrachlor 

(a.i.)

Aminocyclopyrachlor-methyl

(methyl ester) 

Aminocyclopyrachlor, K salt

(Parent acid)
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III. EVALUATION OF CARCINOGENICITY STUDIES

1. Combined Chronic Toxicity/Carcinogenicity Study in Rats 

Moon, Kyoung. (2010). DPX-MAT28 Technical: Carcinogenicity 2-Year Feeding Study in Rats. 

Korea Institute of Toxicology, Daejeon, Republic of Korea. Laboratory Study Number: 

IG07280, November 24, 2010, MRID 48333607. Unpublished.

A. Experimental Design

Aminocyclopyrachlor technical (DPX-MAT28, Lot/batch# J-868-001-3, Purity 88.3-90.5%) was 

administered to Crl:CD(SD) rats (70/sex/dose) at dietary dose levels of 0, 600, 2000, 6000, or 

18000 ppm (0/0, 27/29, 97/100, 279/309, 892/957 mg/kg bw/day in males/females) for 2 years to 

assess carcinogenicity. A satellite group of animals (10/sex/dose) was sacrificed at one year to 

assess chronic toxicity. 

B. Statistical Analysis by HED 

Brain tumors were observed only in male rats. No tumors were observed in female rats or in 

mice of either sex. Statistical analyses were run on male rat mortality and brain tumor data. 

C. Discussion of Survival Data

There was no statistically significant trend in mortality with increasing doses of 

aminocyclopyrachlor in male rats; however, there was a statistically significant negative pairwise comparison of the 600 ppm dose group with the controls at p < 0.05 (Table 1) (TXR No. 

0055738).
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Table 1. Aminocyclopyrachlor – Crl:CD(SD) Rat Study (MRID 48333607)

Male Mortality Rates+ and Cox or Generalized K/W Test Results

Weeks

Dose (ppm) 1-26 27-52 52i 53-78 79-105f Total

0 1/80 1/79 10/78 12/68 29/56 43/70

(61)

600 0/80 0/80 10/80 14/70 17/56 31/70

(44)*n

2000 3/80 6/77 10/71 8/61 28/53 45/70

(64)

6000 0/80 4/80 10/76 10/66 33/56 47/70

(67)

18,000 0/80 3/80 10/77 6/67 25/61 34/70

(49) +

Number of animals that died during interval/Number of animals alive at the beginning of the 

interval.

i

Interim sacrifice at week 52. f

Final sacrifice at weeks 104-105. n

=negative pair-wise comparison with control

( )Percent.

Note: Time intervals were selected for display purposes only.

Significance of trend denoted at control.

Significance of pair-wise comparison with control denoted at dose level.

If *, then p < 0.05. If **, then p < 0.01.
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D. Neoplastic lesions

Male rats had statistically significant trends at p < 0.01, and significant pair-wise 

comparisons of the 18,000 ppm dose group with the controls at p < 0.05, for (malignant) brain 

astrocytomas and combined astrocytoma, glioma and oligodendrogliomas. There were 

statistically significant trends in (malignant) brain gliomas at p < 0.05 and in brain granular cell 

tumors at p < 0.01. It should be noted that the first brain glioma was observed at week 50 and 

that only week 52 interim sacrifice animals in the control and 18,000 ppm dose groups were 

examined. All interim sacrifice animals are considered to be “at risk” since the interim sacrifice 

occurred after week 50, therefore, all interim sacrifice animals should have received full 

histopathological evaluations. Potentially more animals could have had tumors which would 

have been diagnosed if these animals had been examined. The statistical analyses of the tumors 

in the male rats were based upon Peto’s Prevalence Test (Tables 2a and 2b) (TXR No. 0055738).

Since the effect on survival was not dose-related, the statistical analyses of the brain 

tumors in male rats were also run using the Fisher’s Exact Test and Exact Test for Trend. For 

combined glial cell tumors and the granular cell tumors, there was a statistically significant trend 

only, but no significant pair-wise comparison at any dose (Tables 3a and 3b).
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Table 2a. Aminocyclopyrachlor – Crl:CD(SD) Rat Study (MRID 48333607)

Male Brain Tumor Rates+ and

Peto’s Prevalence Test Results

Dose (ppm)

0 600 2000 6000 18,000

Astrocytomas

(%)

p =

0/64

(0)

0.00130**

0/66

(0)

-

0/60

(0)

-

1/65

(2)

0.16290

3a/65

(5)

0.03760*

Gliomas

(%)

p =

0/78

(0)

0.02052*

0/70

(0)

-

0/62

(0)

-

0/66

(0)

-

1b/78

(1)

0.12883

Oligodendrogliomas

(%)

p =

0/27

(0)

0.48486

0/39

(0)

-

0/25

(0)

-

1c/22

(5)

0.13397

0/36

(0)

-

Combined

(%)

p =

0/78

(0)

0.00058**

0/70

(0)

-

0/62

(0)

-

2/66

(3)

0.07203

4/78

(5)

0.01696*

+Number of tumor bearing animals/Number of animals examined, excluding those that died before observation of 

the first tumor.

a

First astrocytoma observed at week 63, dose 18,000 ppm.

b

First glioma observed at week 50, dose 18,000 ppm.

c

First oligodendroglioma observed at week 105 in an interim sacrifice animal, dose 6000 ppm.

Note: Significance of trend denoted at control.

Significance of pair-wise comparison with control denoted at dose level.

If *, then p < 0.05. If **, then p < 0.01.

Historical control data from the performing lab (taken from p. 42 of the Study Report)

• Malignant Astrocytoma: Incidence†: 26/2146 (1.21%), Range (per study): 0-4.9%

• Malignant Gliomaa

: Incidence†: 3/2146 (0.14%), Range (per study) 0-1.92%

• Malignant Oligodendroma: Incidence†: 3/2146 (0.14%), Range 0-2.0%

†2146 male brains examined in 30 carcinogenicity studies 

a

including “undifferentiated glioma” 
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Table 2b. Aminocyclopyrachlor – Crl:CD(SD) Rat Study (MRID 48333607)

Male Brain Tumor Rates+ and

Peto’s Prevalence Test Results

Dose (ppm)

0 600 2000 6000 18,000

Granular Cell Tumors

(%)

p =

0/35

(0)

0.00190**

0/42

(0)

-

0/35

(0)

-

0/34

(0)

-

2a/40

(5)

0.05452

+Number of tumor bearing animals/Number of animals examined, excluding those that died 

before observation of the first tumor.

a

First granular cell tumor observed at week 98, dose 18,000 ppm.

Note: Significance of trend denoted at control.

Significance of pair-wise comparison with control denoted at dose level.

If *, then p < 0.05. If **, then p < 0.01.

Historical control data

• Granular cell tumors: Incidence: 8/2146 (0.37%), Range: 0-2.0%
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Table 3a. Aminocyclopyrachlor – Crl:CD(SD) Rat Study (MRID 48333607)

Male Combined Brain Tumor Rates+

(Astrocytomas, Gliomas and Oligodendrogliomas) and

Fisher’s Exact Test and Exact Test for Trend Test Results

Dose (ppm)

0 600 2000 6000 18,000

Combined

(%)

p =

0/78

(0)

0.00349**

0/70

(0)

1.00000

0/62

(0)

1.00000

2/66

(3)

0.20833

4a/78

(5)

0.06009

+Number of tumor bearing animals/Number of animals examined, excluding those that died 

before week 53.

a

First tumor observed at week 50, dose 18,000 ppm.

Table 3b. Aminocyclopyrachlor – Crl:CD(SD) Rat Study (MRID 48333607)

Male Brain Tumor Rates+ and

Fisher’s Exact Test and Exact Test for Trend Test Results

Dose (ppm)

0 600 2000 6000 18,000

Granular Cell Tumors

(%)

p =

0/68

(0)

0.03927*

0/70

(0)

1.00000

0/61

(0)

1.00000

0/66

(0)

1.00000

2a/66

(3)

0.24071

+Number of tumor bearing animals/Number of animals examined, excluding those that died 

before observation of the first tumor.

a

First granular cell tumor observed at week 98, dose 18,000 ppm.

Note: Significance of trend denoted at control.

Significance of pair-wise comparison with control denoted at dose level.

If *, then p < 0.05. If **, then p < 0.01.
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Pathology Peer Review 

The registrant contends that the brain tumors observed in male rats were not treatment-related. 

This conclusion was reached after a Pathology Peer Review Group organized by Experimental 

Pathology Laboratories, Inc. (Sterling, VA USA) was convened in order to review the brain 

histopathology and interpret the findings. (Note: this group is referred to in the Study Report as 

a Pathology Working Group (PWG). Although this Group followed the Agency’s guidance for 

conducting a PWG in 83-6, the Group was convened before the Study Report was written and 

not afterward. As such, the Group’s expert opinion is incorporated into the Study Report and is 

not a separate document. To avoid confusion, this group is referred to herein as a Pathology Peer 

Review Group). This Group was composed of 5 independent consulting pathologists with 

expertise in rodent toxicity and carcinogenicity studies, and with specific expertise in 

neuropathology. A sixth pathologist with similar background served as the chairperson and 

author of the final report. All of the brain histopathology slides were independently reviewed by 

one of the Group pathologists prior to the Pathology Peer Review Group meeting to confirm the 

findings of the Study pathologist. During the Group’s meeting, the brain slides with neoplastic 

findings were coded and reviewed by each panel member. Following this independent review, 

the panel members reached consensus on the diagnosis for each slide and conducted a weight-ofevidence evaluation to determine if any histopathological findings were spontaneous or 

associated with test substance administration. 

The Pathology Peer Review Group concluded that the astrocytomas in the high dose males were 

not treatment related. The basis for this conclusion was: 

• No evidence of non-neoplastic glial cell proliferation caused by the test substance in male 

or female rats at any dose. 

• No decrease in tumor latency in the high dose groups compared to other treatment groups 

with tumors. 

• The age of onset of astrocytomas (Days 393-728) were consistent with age-related 

neoplasms. 

• No treatment-related brain neoplasms were observed in female rats. 

• No preferential accumulation of aminocyclopyrachlor in the brain as compared to other 

tissues. 

• No evidence of genotoxicity was observed in genotoxicity assays. 

The full Report of the Pathology Peer Review Group is available in MRID 48833607.

E. Non-neoplastic lesions 

All non-neoplastic lesions in the 1-year interim sacrifice animals and in the 2 year main study 

group animals were consistent with background lesions that are typical of rats of this age and 

strain. 
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F. Adequacy of Dosing for Assessment of Carcinogenicity

Dosing was adequate, and not excessive, for assessing carcinogenicity since the high dose 

approached the limit dose in both sexes (892/957 mg/kg bw/day in males/females). The high 

dose also caused mild decreases on body weight (decrease up to 12%) and body weight gain

(decrease up to 11%) in both sexes during the study that had resolved by the conclusion of the 

study (see Table 4 below). 

Table 4 –Body weights (BW) 

g±SD 0 600 ppm 2000 ppm 6000 ppm 18,000 ppm

MALES initial BW

216.3 ±12.82

(N=80)

216.2 ±12.27

(N=80)

216.3 ±11.34

(N=80)

210.4 ±10.40

(N=80)

214.2 ±10.70

(N=80)

Day 189

656.3 ±63.25 

(N=79)

664.0 ± 72.90 

(N=80)

664.3 ±75.47 

(N=79)

647.8 ±81.24 

(N=80)

616.8 ±56.79**

(↓ 6%)

(N=80)

Day 385

813.2±81.11

(N=67)

816.1±86.22

(N=69)

819.7±108.73

(N=61)

820.2±113.91

(N=66)

751.5±81.69**

(N=66)

(↓ 8%)

Day 539 

860.0±115.31

(N=57)

877.0±114.69

(N=59)

870.6±116.75

(N=54)

884.5±155.21

(N=59)

801.9±124.85**

(N=63)

(↓ 7%)

Final BW

877.3±139.43

(N=27)

879.3±152.83

(N=39)

856.2±170.70

(N=27)

840.9±165.12

(N=25)

788.5±163.19

(N=36)

FEMALES Initial BW 

158.7±9.65

(N=80)

158.6±8.74

(N=80)

157.1±8.94

(N=80)

155.0±8.39*

(N=80)

(↓2%)

154.2±9.90**

(N=80)

(↓3%)

Day 189

345.8±40.75

(N=80)

349.5±34.08

(N=80)

349.6±31.11

(N=79)

346.2±32.50

(N=80)

324.2±28.47**

(N=79)

(↓6%)

Day 385

441.9±80.88

(N=66)

438.3±68.10

(N=69)

441.7±68.40

(N=68)

445.6±53.91

(N=69)

402.7±54.09**

(N=66)

(↓9%)

Day 539 

505.1±111.38

(N=59)

500.2±98.55

(N=52)

503.6±98.90

(N=54)

500.8±73.09

(N=54)

442.5±72.41**

(N=56)

(↓12%)

Final BW

535.2±153.47

(N=27)

538.7±82.23

(N=27)

533.3±121.84

(N=25)

518.8±86.13

(N=26)

487.3±85.64

(N=23)

*Significant differences from control at p<0.01

**Significant differences from control at p<0.01
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2. Carcinogenicity Study in Mice

Jung-Im Huh, Ph.D. (2010). DPX-MAT28 Technical: Carcinogenicity 18-Month Feeding Study 

in Mice. Korea Institute of Toxicology, Republic of Korea. Laboratory Study Number: IG07280, 

October 15, 2010 MRID 48333606. Unpublished. 

A. Experimental Design

Aminocyclopyrachlor (Batch/Lot# J-868-001-3, Purity 88.3-90.5%) was administered to young 

adult male and female Crlj:CDl(ICR) mice (60/sex/group) in diets that contained 0, 300, 1000, 

3000, or 7000 ppm DPX-MAT28 for approximately 18 months. This corresponded to doses of 

approximately 0/0, 39/50, 133/171, 393/527, and 876/1190 mg/kg bw/day in males/females. 

B. Discussion of Survival and Tumor Data

The performing laboratory did not find any treatment-related changes in survival in either sex. 

There were also no treatment-related increases in tumor incidence in either sex. Therefore, no 

in-house statistical analyses were performed. 

C. Non-neoplastic lesions 

All non-neoplastic lesions were consistent with background lesions that are typical of mice of 

this age and strain. 

D. Adequacy of Dosing for Assessment of Carcinogenicity

The high dose in this study approached the limit dose (876/1190 mkd in males/females). No 

adverse effects (including changes in body weight) were observed at this dose level. Dosing was 

considered adequate and not excessive. 

IV. TOXICOLOGY

1. Metabolism

Pharmacokinetic studies in rats show that both aminocyclopyrachlor and aminocyclopyrachlormethyl are rapidly absorbed, with peak concentrations occurring in plasma and red blood cells 

(RBCs) at 0.3-1.0 hours for aminocyclopyrachlor and at 0.3-0.6 hours after dosing for 

aminocyclopyrachlor-methyl. Aminocyclopyrachlor-methyl was rapidly metabolized to 

aminocyclopyrachlor (within 30 minutes). No other metabolites were identified for either 

compound. The areas under the curve (AUCs) for both compounds scaled with dose, indicating 

that metabolic processes were not saturated. The (plasma) elimination half-life of 

aminocyclopyrachlor of 5.6-5.7 hours was shorter than that for aminocyclopyrachlor-methyl at 
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8.7-13.3 hours. The majority of the administered dose of both compounds was excreted within 

24 h, although elimination differed slightly. For aminocyclopyrachlor, 35.8-53.71% was 

eliminated in the urine and 31.4-48.0% was eliminated in the feces. For aminocyclopyrachlormethyl, the majority was recovered in the urine 78.80-78.82%, with a smaller amount eliminated 

in the feces (1.72-4.57%). Neither aminocyclopyrachlor nor aminocyclopyrachlor-methyl was 

eliminated as expired air. No sex differences in absorption, distribution, metabolism, or 

excretion were observed for either aminocyclopyrachlor or aminocyclopyrachlor-methyl. 

Repeated-dose metabolism studies with aminocyclopyrachlor and aminocyclopyrachlor-methyl 

showed that radioactivity was found primarily in the GI tract and GI contents, with brain levels 

of radioactivity at less than 0.01% at the peak time after administration. Brain levels of 

radioactivity were undetectable after 72 hours. 

2. Mutagenicity

Genotoxicity studies were conducted using aminocyclopyrachlor or aminocyclopyrahclor-methyl 

as the test material (Table 5). For aminocyclopyrachlor, there was no evidence of induced 

mutant colonies over background in the presence or absence of S9 activation in in vitro bacterial 

or mammalian gene mutation assays. There was also no evidence of chromosomal aberrations 

induced over background in the presence or absence of S9 activation in vitro. In vivo, there were 

no increases in the frequency of micronucleated polychromatic erythrocytes in bone marrow. 

For aminocyclopyrachlor-methyl, in vitro bacterial gene mutation studies showed no evidence 

of mutant colonies over background in the presence or absence of S9 activation. 

Together, these studies indicate that aminocyclopyrahclor-methyl and aminocyclpyrahclor are 

not genotoxic. 
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Table 5-Genotoxicity Studies

Aminocyclopyrachlor

Gene Mutation

870.5100 

In vitro Bacterial Gene Mutation

47560019 (2007)

Acceptable/guideline

0, 50, 150, 500, 1500, or 5000 

μg/plate (+/-S9)

There was no evidence of induced 

mutant colonies over background in 

the presence or absence of S9-

activation.

Gene Mutation

870.5300 

In vitro Mammalian Cells Gene 

Mutation (Chinese Hamster Ovary 

Cells) 

47560020 (2007)

Acceptable/guideline

0, 750, 1000, 1500, 1750, or 2150 

μg/mL (+/-S9)

There was no evidence of induced 

mutant colonies over background in 

the presence or absence of S9-

activation.

Cytogenetics 

870.5375 

In vitro Mammalian Cytogenetics

Chromosomal Aberration Assayhuman peripheral blood 

lymphocytes

47560021 (2007)

Acceptable/guideline

0, 267, 534, 1068, or 2136 μg/mL 

(+/-S9)

There was no evidence of 

chromosome aberrations induced 

over background in the presence or 

absence of S9-activation.

Cytogenetics-other

870.5395 In Vivo Mammalian 

Cytogenetics - Erythrocyte 

Micronucleus-mouse

47560022 (2007)

0, 500, 1000, or 2000 mg/kg (limit 

dose)

No significant increase in the 

frequency of micronucleated 

polychromatic erythrocytes in bone 

marrow after any treatment time.

Aminocyclopyrachlor-methyl

Gene Mutation

870.5100 

In vitro Bacterial Gene Mutation

47560033 (2005)

Acceptable/Non-guideline

0, 100, 333, 1000, 3333, or 5000 

μg/plate (+/-S9)

There was no evidence of induced 

mutant colonies over background in 

the presence or absence of S9-

activation.
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3. Structure-Activity Relationship

Aminocyclopyrachlor has three structural alerts/features (monocyclic heterocyclic aromatic 

amine, halogenated aromatic, nucleoside analog) of potential carcinogenic activity; however, the 

predictive capability of these features tends to be limited or marginal. The concern is further 

mitigated by the presence of a carboxylic acid moiety (which can promote rapid excretion), the 

ring position of chlorine (not ortho to ring nitrogen and therefore not a good leaving group), and 

the bulky cyclopropane substituent. This is supported by the submitted experimental data 

showing rapid excretion of the chemical and lack of genotoxicity. Overall, there is no sufficient 

basis for SAR concern.

4. Subchronic and Chronic Toxicity

Toxic effects of aminocyclopyrachlor and aminocyclopyrachlor-methyl via the oral route of 

exposure (in the diet) in subchronic (90-Day) rodent studies were limited to decreases in body 

weights (≤10%), body weight gains, food consumption, and food efficiency in both sexes when 

these compounds were administered to rats at the limit dose (1000 mg/kg/day) with no adverse 

effects observed in mice. Similar results were observed in chronic toxicity/carcinogenicity 

studies in rats and mice conducted near the limit dose. Only mild effects were observed on body 

weight in rats, which were limited to sporadic decreases in body weight of up to 12% that 

recovered by the end of the study. No adverse effects were observed in mice. 

5. Mode of Action Studies

No mode of action studies were submitted.
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V. COMMITTEE’S ASSESSMENT OF THE WEIGHT-OF-THE EVIDENCE

The Committee considered the following for a weight-of-evidence determination of the 

carcinogenic potential of aminocyclopyrachlor.

1. Carcinogenicity

Rat

• Brain Tumors: The CARC concluded that the brain tumors (glial cell tumors and granular 

cell tumors) observed in male rats were spontaneous lesions and were not considered to be 

treatment-related. This was based on the following weight-of-evidence considerations: 

1) Although there were significant trends and pair-wise comparisons at the high dose for 

astrocytomas and glial cell tumors combined by the Peto analysis, the incidences were 

within the historical control range. For granular cell tumors, there was a significant trend 

only, but no significant pair-wise comparison of any dose group with the controls. In 

addition, the Fishers Exact test showed trends only and no pair-wise comparisons of any 

dose level with the controls for combined glial tumors and for granular cell tumors. 2) No 

supporting treatment-related pre-neoplastic lesions were observed in the brain (or any other 

tissue). (Chemicals known to cause brain tumors also are commonly tumorigenic in other 

tissues, J. Pletcher, Consulting Pathologist, Personal Communication); 3) The tumors 

occurred at the limit dose; no other tumors were seen; 4) No treatment-related brain

neoplastic lesions were observed in female rats. 5) No evidence of genotoxicity was 

observed in in vitro or in vivo genotoxicity studies. 6) Brain levels of radioactivity after 

administration of aminocyclopyrachlor at times corresponding to peak plasma concentration 

were very low (<0.01%) and were undetectable after 72 hours.; 7) the Pathology Peer 

Review also concluded that the brain tumors were spontaneous tumors and not treatmentrelated (MRID 48333607). 

• Adequacy of Dosing: Dosing was considered adequate, and not excessive, for assessing 

carcinogenicity since the high dose approached the limit dose in both sexes (892/957 mg/kg 

bw/day in males/females). The high dose also caused mild decreases on body weight 

(decrease up to 12%) and body weight gain (decrease up to 11%) in both sexes during the 

study, but had resolved by the conclusion of the study.

Mouse

• There were no treatment-related increases in tumor incidences in either sex. 

• Adequacy of Dosing: Dosing was considered adequate, and not excessive, since the 

high dose in this study approached the limit dose (876/1190 mg/kg/day in 

males/females). No adverse effects (including changes in body weight) were observed at 

this dose level. 
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2. Mutagenicity: There is no concern for mutagenicity.

3. SAR: There is no sufficient basis for SAR concern.

VI. CLASSIFICATION OF CARCINOGENIC POTENTIAL

In accordance with the EPA's Final Guidelines for Carcinogen Risk Assessment (March, 2005), 

the CARC classified aminocyclopyrachlor as “Not Likely to be Carcinogenic to Humans.” 

There were no treatment-related tumors seen in male or female rats or mice at doses that were 

adequate to assess carcinogenicity and there was no concern for mutagenicity. 

VII. QUANTIFICATION OF CARCINOGENICITY

Quantification of carcinogenicity is not required.

VII. BIBLOGRAPHY

47560019 Wagner, V.; VanDyke, M. (2008) DPX-MAT28 Technical: Bacterial Reverse 

Mutation Assay. Project Number: AC02HJ/503/BTL, 17293, H/27937. Unpublished study 

prepared by Bioreliance and Dupont Haskell Laboratory. 

47560020 Clarke, J. (2007) DPX-MAT28 Technical: In vitro Mammalian Cell Gene Mutation 

Test (CHO/HGPRT Assay). Project Number: AC02HJ/782/BTL, 17293, 515. Unpublished 

study prepared by Bioreliance and Dupont Haskell Laboratory. 

47560021 Gudi, R.; Rao, M. (2007) DPX-MAT28 Technical: In vitro Mammalian 

Chromosome Aberration Test in Human Peripheral Blood Lymphocytes. Project Number: 

AC02HJ/341/BTL, DUPONT/22709, 17293. Unpublished study prepared by Bioreliance. 

47560022 Krsmanovic, L.; Huston, T. (2007) DPX-MAT28 Technical: Mouse Bone Marrow 

Erythrocyte Micronucleus Test: Final Report. Project Number: AC02HJ/123/BTL, 17293, 572. 

Unpublished study prepared by Dupont Haskell Laboratory and Bioreliance. 

47560033 Ford, L. (2005) IN-KJM44: Bacterial Reverse Mutation Test. Project Number: 

DUPONT/17971, 15744, 502. Unpublished study prepared by E.I. du Pont de Nemours and 

Company. 33 p.

48333606 Jung-Im Huh, Ph.D. (2010). DPX-MAT28 Technical: Carcinogenicity 18-Month 

Feeding Study in Mice. Korea Institute of Toxicology, Republic of Korea. Laboratory Study 

Number: IG07280, October 15, 2010 Unpublished.
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48333607 Moon, Kyoung. (2010). DPX-MAT28 Technical: Carcinogenicity 2-Year Feeding 

Study in Rats. Korea Institute of Toxicology, Daejeon, Republic of Korea. Laboratory Study 

Number: IG07280, November 24, 2010, MRID Unpublished.

Brunsman, L. 2011. Aminocyclopyrachlor Qualitative Risk Assessment Based On Crl:CD(SD) 

Rat Dietary Study. August 3, 2011. TXR No. 0055738.