Summary of Toxicity Studies on Imazapyr
Technical Department, Cyanamid (Japan) Ltd.
(Received July 15, 1997 ; Accepted August 20, 1997)
DESCRIPTION OF THE TEST COMPOUND
Imazapyr is a nonselective herbicide developed by American Cyanamid Company.
Through a number of field trials for the biological activity, it has been confirmed that
imazapyr isopropylamine has been effective for perennial and annual weeds in non-crop
field. The product was registered in 1984 in the U.S.A.
In Japan, we started the field trials with the aqueous solution of 25% imazapyr
isopropylamine salt (AS) <tradename : ARSENAL*> as a herbicide for non-crop use
in 1982 and registered it in 1987. Field trials with a l% AS <tradename : CHOPPER*>
were also started for weed growth suppression for railway and road uses with theencouraged performance for perennial weeds. We registered it in 1993.
This article provides a toxicological feature of imazapyr obtained from toxicologicalstudies with the imazapyr technical grade, its isopropylamine salt technical grade(49% water solution) and the formulation of aqueous solution (AS) containing 25%
imazapyr isopropylamine salt.
The chemical structure and physical properties of imazapyr isopropylamine salt
are given below :Common name : Imazapyr isopropylamine salt
Code name : AC252,925, CL252,925
Product name : ARSENAL, CHOPPER
Chemical name : Isopropylammonium=(RS)-2-(4-isopropyl-4-methyl-5-oxo-2imidazolin-2-yl) nicotinate (IUPAC)
Structural formula :H3C O

COOH H2NCH(CH3)2

H
N
N N

H3C

CH3

Molecular formula : C13H15N3O3 C 3H9N
Molecular weight : 320.4
Appearance : Yellowish green to green liquid as water solution
Melting point : 128-130˚C
Solubility (g/l solvent) : Water > 600, methanol > 500, isopropanol approx. 10,
xylene < 5, cyclohexane < 5Partition coefficiant (n-octanol/water) : 1.3 as the acid tech.
Stability : No decomposition at 37˚C for 9 months, 45˚C for 3 months

Table 1
Test
substance
Imazapyrisopropylamine
technical(49.3%)

Imazapyr technical

Imazapyrisopropylamine
25% AS

Animal Administration
LD50
speciesroute(mg/kg)Male
FemaleIntraperitoneal Male
FemaleSubcutaneous Male
FemaleDermalMale
Female
Mouse
Oral
Male
FemaleIntraperitoneal Male
FemaleSubcutaneous Male
FemaleTesing facility,
reporting year

Rat

Oral

> 10,000> 10,000
4,200
3,700
> 5,000> 5,000> 2,000> 2,000> 10,000> 10,000
3,450
3,000
> 5,000> 5,000

Medical Scientific
Research, Laboratory,
1983

Rat

Oral

American CyanamidCompany, 1983

RabbitDermalRat

InhalationMale> 5,000
Female>5,000Male> 2,000
Female> 2,000Male> 1.3 mg/l
Female > 1.3 mg/l
(analytical)

Rat

Oral

Mouse

Oral

RabbitDermalRat

Inhalation

Male
FemaleMale
FemaleMale
FemaleMale
Female> 5,000> 5,000> 5,000>5,000> 2,148>2,148>0.2 mg/l>0.2 mg/l(analytical)

Food and Drug ResearchLaboratories, 1983
American CyanamidCompany, 1983
American CyanamidCompany, 1986
American CyanamidCompany, 1983
Food and Drug ResearchLaboratories, 1983

ACUTE TOXICITY STUDIES
Results of acute toxicity studies with imazapyr technical, imazapyr isopropylamine
technical, and imazapyr isopropylamine 25% AS formulation at different routes ofadministration are shown Table l.
After oral administration with imazapyr technical, no signs of toxicity were observedin rats. Decreased activity was observed in both rats and mice after oral administration
with imazapyr isopropylamine technical and also with imazapyr isopropylamine 25%
AS formulation.
By dermal administration, no toxic signs were observed.
After intraperitoneal administration of the imazapyr isopropylamine technical,
decreased activity, blepharoptosls, ataxia, sedation, cyanosis, convulsion, and decreased
body weight were observed in both rats and mice. After subcutaneous administrationof imazapyr isopropylamine technical, decreased activity, blepharoptosls, and sedation
were observed in mice, but only decreased activity was observed in rats.

In inhalation study with imazapyl technical, slight amount of nasal discharge wasobserved on the first day. No toxic signs were observed In the study with imazapyr
isopropylamine 25% AS.
IRRITATION STUDIES
1. Primary Eye Irritation Studies in Rabbits1 ) Nine male New Zealand White rabbits were dosed with 100 mg of imazapyr
technical instilled into the conjunctival sac of the right eye. The eyes of three of thenine rabbits were flushed with 200 ml of tap water after being exposed to the test
material for approximately 20 sec (20-sec group). After 24 hr of exposure, the treatedeyes of the remaining six animals were rinsed with tap water (24 hr group) andexamined for irritation using fluorescein. The eyes were then scored at 24, 48 and 72
hr, and at 4 and 7 days post-dosing according to the "Draize Method."
In the 20-sec group, mild to moderate irritation responses were observed in the
conjunctiva within 24 hr ; however they had disappeared by 72 hr. In the 24-hr group,
mild to moderate irritation responses were observed in the cornea within 24 hr, which
disappeared by 72 hr; and in the conjunctiva within 24 hr, which disappeared by 7
days.
Based upon the results, the product was considered to be irritating to the rabbit eyewith complete recovery within 7 days of exposure.
(American Cyanamid Company, 1983)
2) Nine male New Zealand White rabbits were dosed with 0.1 ml of imazapyr
isopropylamine 25% AS instilled into the conjunctive sac of the right eye. The eyesof three of the nine rabbits were flushed with 200 ml of tap water after being exposedto the test material for approximately 20 sec (20-sec group). After 24 hr of exposure,the treated eyes of the remaining six animals were rinsed with tap water (24-hrgroup) and examined for irritation using fluorescein. The eyes were then scored at 24,
48, 72 hr, and 4 and 7 days post-dosing according to the "Draize method."
In the 20-sec group, mild to moderate irritation responses were observed in the
conjunctiva within 24 hr ; however they had disappeared after 72 hr. In the 24-hrgroup, mild to moderate irritation responses were observed in the cornea within 24 hr(2 of 6 animals), which disappeared by 7 days ; and in the conjunctiva within 24 hr (6of 6 animals), which disappeared by 7 days.
Based upon the results, the product was considered to be irritating to the rabbit eyewith complete recovery within 7 days of exposure.
(American Cyanamid Company, 1983)
2. Primary Skin Irritation Studies in Rabbits
1) Six male New Zealand White rabbits were dosed dermally with an aqueous
paste of imazapyr technical that was applied to an intact site and an abraded site on
the same animal. The test material was left in contact with the skin for 24 hr. Siteswere scored for irritation after the 24 hr contact period and again at 72 hr postdosing
according to the "Draize Method."
Slight erythema was observed on the abraded site (2 of 6 animals) at 24 hr post-dosing.
No other reactions were observed.
Based upon the results, the product was considered to be mildly irritating to rabbit
skin.
(American Cyanamid Company, 1983)
2) Six male New Zealand White rabbits were dosed dermally with an aqueous
paste of imazapyr isopropylamine 25% AS that was applied to an intact site and an
abraded site on the same animal. The test material was left in contact with the skinfor 24 hr. Sites were scored for irritation after the 24-hr contact period and again at
72 hr post-dosing according to "Draize method."
Slight erythema was observed on the intact site and abraded site at 24 and 72 hr
post-dosing. Slight edema was observed on the abraded site at 24 hr post-dosing.
Based upon the results, the product was considered to be mildly irritating to rabbit
skin.
(American Cyanamid Company, 1983)
3. Dermal Sensitization Study in Guinea Pigs
The study was performed according to the Buehler's Method.
Male Hartley guinea pigs were dermally dosed with imazapyr isopropylamine 25%
AS to an intact site on the back of each animal for 6 hr. These inductive applications
were administered once per week for three consecutive weeks. Following these three
applications, test animals were rested for 14 days and were then given a challengedose of imazapyr isopropylamine 25% AS to an intact site on the right flank of eachanimal. The site was examined 24 and 48 hr postdosing.
No skin reactions were observed after challenge doses. It was concluded that
imazapyr isopropylamine 25% AS was not a skin sensitizer in the male Hartley
guinea pig under the conditions of this study. (Toxicology Pathology Services, 1983)
SUBACUTE TOXICITY
1. 21-Day Dermal Toxicity Study in Rabbits
Imazapyr technical was applied to the closed-cripped intact or abraded skin on theback of ten male and ten female New Zealand White rabbits at dose levels of 0, 100,
200 and 400 mg/kg/day for five days each week over a three week period. The testsite was covered with a gauze patch, which was moistened with saline, and occluded
for 6 hr per day.
There were no consistent or distinct adverse treatment-related effects indicative ofsystemic toxicity in evaluation of body weights, food consumption, hematology,
serum chemistry, clinical observations, necropsy observations, and histopathology
including skin reactions. Microscopic evaluation of all tissues from control and highdose rabbits and all remarkable tissues from low and middle dose rabbits did notreveal any consistent or distinct effects related to topical treatment with imazapyr

technical.

(Toxicology Pathology Services, 1983)

SUBCHRONIC TOXICITY
1. 13- Week Feeding Toxicity Study in Rats
CD rats received imazapyr isopropylamine in the diet at levels of 0, 1000, 5000
and l0,000 ppm for 13 consecutive weeks.
All rats survived the dosing period. Weight gains and food efficiency of male rats
were depressed at 10,000 ppm in 7th week and 8th week, respectively. No test
substance-related changes were observed in food intake, water intake, urinalysis,hematological or biochemical parameters, or organ weights (absolute and relative).
There were no gross or microscopic changes attributed to ingestion of imazapyr
isopropylamine.
It was considered that the maximum no-effect level of imazapyr isopropylamine in
the rat diet for this study was 5000 ppm (325 mg/kg/day in males and 370 mg/kg/day
in females).
(Medical Scientific Research Laboratory, 1983)
TERATOGENICITY STUDIES
1. Teratogenicity Study in Rats
Imazapyr technical was administered orally, via gavage, to groups of 22-24 female
CD rats during gestation days 6-15 at dose levels of 0, 100, 300, or 1000 mg/kg/day.
All females survived to terminal sacrifice. Maternal body weights and body weightgains for treated females were similar to the control group. Salivation was observed
at 1000 mg/kg group. No gross pathologic changes were noted which were consideredrelated to maternal exposure to imazapyr technical. Reproductive data for treateddams were similar to the control group. Mean fetal body weight and crown-rumplength data for fetuses from treated dams were comparable to the control group.
Examination of fetal external, skeletal and visceral development revealed no aberrant
structural changes which appeared related to maternal exposure to imazapyr technical.
In conclusion, imazapyr technical was not teratogenic to CD rats under these test
conditions.
(Toxigenics, 1983)
2. Teratogenicity Study in Rabbits
Imazapyr technical was administered orally, via gavage, to groups of 16-18 female
New Zealand White rabbits during gestation days 6-18 at dose levels of 0, 25, 100 or
400 mg/kg/day. The dose levels were set up based upon a preliminary test. In the test,
imazapyr technical was administered orally at dose levels of 0, 250, 500, 1000 or2000 mg/ka/day to five rabbits during gestation days 6-18. Mortality occurred in the
preliminary test (2 of 5 animals at 250 mg/kg/day, 4 of 5 animals at 1000 mg/kg/day,and 5 of 5 animals at 2000 mg/kg/day died) ; resulting in the choice of 400 mg/kg/day
as the highest dose for the definitive study.
In the definitive study, maternal body weight data for treated rabbits were similarto the control group and no antemortem observations were noted which could becorrelated with exposure to imazapyr technical. Two control and two high doseanimals died during the dosing period ; gross pathologic findings were limited topulmonary change. Maternal reproductive data, and fetal body weight and crown-rumplength data for the treated groups were comparable to the control group.
There were no consistent, deleterious findings noted during the evaluations of the
fetal internal, skeletal, and internal head development which were considered related
to exposure to imazapyr technical.
In conclusion, imazapyr technical was not teratogenic to New Zealand White
rabbits under these test conditions.
(Toxigenic, 1983)
MUTACENICITY STUDIES
1. DNA Repair Test
DNA repair was determined in the in vitro rec-assay using Bacillus subtilis strains
H-17 (Rec+) and M-45 (Rec-). Imazapyr isopropylamine dissolved in water, wastested at dose levels of 0.27 to 26.6 mg/disk.
Negative results were obtained in all assays, indicating that the test substance did
not cause DNA damage.
(Japan Food Research Laboratories, 1984)
2. Reverse Mutation Assay
Imazapyr isopropylamine was tested in the Bacterial/Microsome (Ames) assay atconcentrations up to 5000 µg per plate using Salmonella typhimurium strains TA98,
TA100, TA1535, TA1537 and TA1538, and Escherichia coli strain WP2uvrA withand without metabolic activation.
Negative results were obtained in all assays, indicating that the test substance did
not cause genetic damage.
(Food and Drug Safety Center, Hitano Research Institute, 1982)
3. Chromosomal Aberration Assay
Chinese hamster ovary cells were exposed to 5000, 1700, 500, 170 and 50 µg/ml
of imazapyr technical for either 2 hr in the presence of metabolic activation followed
by a 1, 6 or 10 hr expression period or 3, 8 or 12 hr in the absence of metabolic
activation.
There was no significant increase in the frequency of chromosomal aberrations atany dose level tested, at any sampling time in the presence or absence of metabolic
activation. Under the conditions of this test, imazapyr technical was not clastogenic
at any dose level.
(Hazleton Laboratories America, 1984)
GENERAL PHARMACOLOGY STUDIES
Male mice or male rabbits were orally administered imazapyr isopropylamine atlevels of 1000, 3000, and l0,000 mg/kg to define the effect on gross behavior, centralnervous system, and digestive system. Male rabbits or male rats were administeredintravenously imazapyr isopropylamine at 100, 300, and 1000 mg/kg to define theaffect on respiratory and circulatory system, and levels of 300, 1000, 3000 mg/kg forthe effect on skeletal muscle.
Imazapyr isopropylamine produced a stimulant effect on gross behavior and increased
the sleeping time induced by hexobarbital at high dose in mice, slightly increased
muscle contractility in rats, depressed gross behavior at high dose in rabbits, slightly
changed respiratory rate, blood pressure, and heart rate in rabbits, and increased thevolume of urine at high dose in both mice and rabbits. No effect on the digestivesystem was observed.
(Medical Scientific Research Laboratory, 1992)
CONCLUSION
A number of toxicological studies with imazapyr were conducted to define thetoxicological profile.
The studies show that imazapyr is extremely low acute toxicity or subchronic
toxicity, no mutagenic or teratogenic potential. Irritant effect in eyes and slight irritanteffect on skin are noted, but no dermal sensitization is observed.
Imazapyr is a product with a wide safety margin for humans according to the use
recommendations established.
要約

イマザピルの毒性試験の概要
日本サイアナミッド株式会社技術開発部
イマザピルはアメリンカン・サイアナミッド社が開発した多年生及び一年生雑草を対象とする非農耕地用の非選択性除草剤である．またそのユニークな作用特性により1%液剤
は多年生雑草を対象とした抑草剤として開発に成功した．イマザピルは各種毒性試験の結果，きわめて安全性の高い薬剤であることが示された．急性毒性は非常に弱く普通物に相当した．変異原性，催奇形性は認められず，顕著な薬理作用も認められなかった．眼に対する刺激性，皮膚に対する弱い刺激性はみられたが，皮膚感作性は陰性であった．ラットを用いた亜急性毒性試験では，高用量群（10,000 ppm）の雄ラットで一時的に体重増加抑制，食餌効率の低下がみられたのみで非常に毒性が低かった．以上より，イマザピルは，その使用方法，使用上の注意事項を厳守すればきわめて安全性の高い薬剤であると考えられる．

Contact
Technical Department, Cyanamid Japan Ltd.
1-4-30 Roppongi Minato-ku Tokyo106 Japan
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