Summary of Toxicology Studies With Dithiopyr
Dennis P. WARD
Toxicology Department, The Agricultural Grop, A Unit of Monsanto Company(Received February 20, 1993)
This report summarizes the results of laboratory toxicology studies conducted with
dithiopyr. All studies were conducted in compliance with Good Laboratory Practicestandards and study protocols followed internationally recognized pesticide testing
guidelines. For all tests conducted with dithiopyr, technical grade material was used.
DESCRIPTION OF THE TEST CHEMICAL
Dithiopyr is a new herbicide developed by Monsanto Company for the control ofannual grasses and selected broadleaf weeds in established turf and rice paddies.
Dithiopyr has the following physical and chemical properties :Common name : Dithiopyr
Synonyms: MON 7200, MON 15100
Chemical name : S,S-Dimethylester-2-(difluoromethyl)-4-(2-methylpropyl)6-(trifluoromethyl)-3,5-pyridinedicarbothioic acid
Chemical structure :H3C

O

CH(CH3)2
O
CH3
S

F3C

CF3

S

Molecular formula : C15H16F5NO2S2
Molecular weight : 401.4
Physical state : Pale yellow crystalline solid
Melting point : 65˚C
Vapor pressure: 4 x 10-6 mmHg (25˚C)
Stability : Stable to heat, light, acid and alkali
Octanol/water partition coefficient : 5.625 x l04
Water solubility : 1.38 mg/l (20˚C)
ACUTE TOXICITY STUDIES
1. Dithiopyr Technical Material
The results of acute toxicity studies with dithiopyr are displayed in Table 1. Inthese tests, no distinctive signs of toxicity were observed following exposure via thedifferent routes.

2. Tests with Dimension® Turf Herbicide
The results of acute toxicity studies with the formulated product, Dimension Turf
Herbicide, are displayed in Table 2. Dimension is an organic solvent-based liquid
formulation containing approximately 13% dithiopyr and is registered for use in the
United States.
3. Tests with Dictran® EC Herbicide
The results of acute toxicity studies with the formulated product, Dictran EC
Herbicide, are displayed in Table 3. Dictran EC is an organic solvent-based liquid
formulation containing approximately 33% dithiopyr and is registered for use inJapan.
IRRITATION STUDIES
1. Primary Dermal Irritation
Topical semi-occlusive exposure at 2 sites on the shaved backs of 6 New Zealandwhite (NZW) albino rabbits to 0.5 g of dithiopyr for 4 hr resulted in very slighterythema. All evidence of dermal irritation was gone by 72 hr. The primary irritationindex was calculated to be 0.2 on a scale of 8.0.
(Bio/dynamics Inc., 1988)

Table 1 Results of acute toxicity tests with dithiopyr.
Species, strain

RouteRat, S-D

Oralc)

Mouse, CD-1

Oralc)

Rat, S-D

Dermal (24-hr)c)

Rat, F-344

Inhalation (4-hr)c)

a)

Sex

Vehiclea),b)Median lethal dose

M
F
M
F
M
F
M
F

1 % Tween 80
1 % Tween 80
1 % Tween 80
1 % Tween 80
DMSO
DMSO

> 5000 mg/kg> 5000 mg/kg> 5000 mg/kg> 5000 mg/kg>5000 mg/kg> 5000 mg/kg> 5.98 mg/l
> 5.98 mg/l

Oral dosing solutions were prepared as aqueous suspensions containing 1%
Tween 80.
b)
DMSO : dimethylsulfoxide.
c)
The Institute of Environmental Toxicology, 1988.

Table 2 Results of acute toxicity tests with Dimension®a) turf herbicide.
Species, strain

RouteRat, S-D

Oralb)

Rabbit, NZW

Dermal (24-hr)b)Rat, S-D

Inhalation (4-hr)c)

Sex

VehicleMedian lethal dose

M
F
M
F
M
F

-

4100 mg/kg
3000 mg/kg> 5000 mg/kg> 5000 mg/kg
11 mg/l
8.9 mg/la)Dimension is a registered trademark of the Monsanto Company.
Bio/dynamics Inc., 1987.
c)
Bio/dynamics Inc., 1989.
b)

Table 3 Results of acute toxicity tests with Dictran®a) EC herbicide.
Species, strain

RouteRat, S-D

Oralb)

Mouse, CD-1

Oralb)

Rabbit, NZW

Dermal (24-hr)b)Rat, S-D

Inhalation (4-hr)c)

Sex

VehicleMedian lethal dose

M
F
M
F
M
F
M
F

-

3397 mg/kg
4129 mg/kg
1996 mg/kg
1672 mg/kg> 5000 mg/kg> 5000 mg/kg> 4.3 mg/ld)> 4.3 mg/1d)a)Dictran is a registered trademark of Monsanto Japan Ltd. and Dainippon Ink &Chemicals, Inc.
b)
Bio/dynamics Inc., 1989.
c)
Monsanto Environmental Health Laboratory, 1989.
d)Represents maximum attainable atmospheric concentration.

2. Primary Ocular Irritation
Mild conjunctival irritation was observed up to 24 hr following instillation of 64
mg of dithiopyr into the conjunctival sac of 6 NZW albino rabbits. No iridial or
corneal involvement was apparent, All evidence of conjunctival irritation was goneby. 48 hr.
(Bio/dynamics Inc., 1988)
DERMAL SENSITIZATION STUDIESThe potential of dithiopyr to induce delayed contact hypersensitivity was evaluatedin a modified Buehler assay.1, 2) Induction comprised topical occlusive exposure of 5
male and 5 female Hartley albino guinea pigs to undiluted dithiopyr (moistened with

saline) for 6 hr, once per week for 3 weeks. Treated and naive guinea pigs (5 per sex)
were challenged 14 days after the last induction dose with undiluted dithiopyr (moistenedwith saline). Dithiopyr exhibited no evidence of dermal sensitization potential.
(Bio/dynamics Inc., 1988)
SUBCHRONIC STUDIES
1. Four-week Feeding Study in Rats
Dithiopyr was administered via the diet to groups of 6 male and 6 female Fischer-344(F-344) rats for 4 weeks at concentrations of 0, 300, 1000, 3000, 10,000 and 30,000
ppm. Decreased body weight and food consumption, as well as clinical signs of
toxicity were observed at the higher dose levels. Liver enlargement and discoloration,
thymic atrophy, emaciation, and increased liver, kidney and thyroid/parathyroid weights
were noted on gross postmortem examination. Due to an elevation in liver and kidneyweights at the low dose level, the no observable effect level (NOEL) is considered tobe less than 300 ppm.
(The Institute of Environmental Toxicology, 1986)
2. Thirteen-week Feeding Study in Rats
Dithiopyr was administered via the diet to groups of 12 male and 12 female F-344
rats for 13 weeks at concentrations of 0, 10, 100, 1000 and 5000 ppm. Decreased
body weight gain, decreased feed efficiency and mild anemia were observed at thehigher dose level. Post-mortem examination revealed primarily cholestasis and renal
disease. Elevated plasma alkaline phosphate (AP), γ-glutamyl trans-peptidase (γ-GT),urea nitrogen, total bilirubin, and cholesterol levels, along with liver enlargement anddiscoloration, hepatocellular swelling and bile duct proliferation, were indicative ofan obstruction in bile flow. Evidence of renal toxicity included : increased organ
weight, organ discoloration, increased urine protein and specific gravity, and anincreased incidence or severity of focal tubular atrophy. Other findings included :multiple organ weight effects, thyroid follicular hypertrophy, adrenal cortical
hypertrophy, and pulmonary foam cell aggregation. The subchronic NOEL in rats is
considered to be 10 ppm.
(The Institute of Environmental Toxicology, 1988)
3. Four-week Feeding Study in Mice
Dithiopyr was administered via the diet to groups of 6 male and 6 female CD-1
mice for 4 weeks at concentrations of 0, 300, 1000, 3000, 10,000 and 30,000 ppm.
Mortality, body weight depression, decreased food consumption, and clinical signs of
toxicity were observed at the higher dose levels. Liver enlargement and discoloration,
adrenal enlargement, and atrophy of the thymus, spleen, seminal vesicles, ovaries anduterus were noted on gross post-mortem examination. Due to an elevation in liver
weight in females at the low dose level, the NOEL is considered to be less than 300
ppm.
(The Institute of Environmental Toxicology, 1987)

4. Thirteen-week Feeding Study in Mice
Dithiopyr was administered via the diet to groups of 12 male and 12 female CD-1
mice for 13 weeks at concentrations of 0, 10, 100, 1000 and 5000 ppm. Mortality,
decreased body weight gain, reduced feed efficiency, and mild anemia were observedat the higher dose level. By study termination many high dose animals appeared
jaundiced, emaciated and had distended abdomens. Postmortem examination revealed
primarily liver and kidney toxicity. Elevated plasma AP, alanine aminotransferase(ALT), aspartate aminotransferase (AST), urea nitrogen and cholesterol levels wereindicative of cholestasis and liver toxicity. These biochemical changes wereaccompanied by a significant increase in liver weight, hepatocellular swelling,vacuolation and necrosis, dissociation of hepatocellular cords, and bile duct
proliferation. Increased kidney weights, decreased urine protein and specific gravity,
and acidophilia of proximal tubular cells were indicative of mild renal toxicity. Otherfindings included : multiple organ weight effects, adrenal cortical hypertrophy andovarian atrophy. The subchronic NOEL in mice is considered to be 10 ppm.
(The Institute of Environmental Toxicology, 1989)
5. Two-week Oral Study in Dogs
Dithiopyr was administered orally via gelatin capsule to 1 male and 1 female
beagle dog for 2 weeks at dose levels of 0, 10, 30, 60, 100 and 200 mg/kg/day.
Emesis and decreased body weight were observed at the higher dose levels. Biochemicaland histopathological evidence of liver toxicity, manifest as increased AP, AST, and
γ-GT levels in plasma, along with hepatocellular swelling, pigment deposition, andfocal necrosis, were apparent on postmortem examination. The NOEL is considered
to be 10 mg/kg/day.
(The Institute of Environmental Toxicology, 1988)
6. Thirteen-week Oral Study in Dogs
Dithiopyr was administered orally via gelatin capsule to groups of 6 male and 6
female beagle dogs for 13 weeks at dose levels of 0, 1, 10 and 30 mg/kg/day.
Postmortem examination revealed liver toxicity and moderate to severe cholestasis at
the higher dose levels. Specific findings included : increased liver weights, increased
AP, AST and γ-GT levels in plasma, along with liver swelling, distended gallbladders
with sandy stones, pigment deposition in bile canaliculi and Kupffer cells, andmononuclear cell infiltration of the liver. The subchronic NOEL in dogs is considered
to be 1 mg/kg/day.
(The Institute of Environmental Toxicology, 1988)
7. Three-week Dermal Study in Rats
Dithiopyr was applied under semi-occlusive dressing to the intact skin of 5 maleand 5 female Sprague-Dawley (S-D) rats at dose levels of 0, 50, 500 and 1000

mg/kg/day for 6 hr/day, 5 days/week for 3 weeks. Mild transient dermal irritation wasobserved. Increased liver weight at the high dose level was the only indication of asystemic effect. The NOEL is considered to be 500 mg/kg/day.
(Bio/dynamics Inc., 1989)
CHRONIC TOXICITY AND ONCOGENICITY STUDIES
1. Twenty-four-month Feeding Study in Rats
In a combined chronic toxicity and oncogenicity study, dithiopyr was administered
via the diet to groups of 90 male and 90 female F-344 rats for 104 weeks at concentrationsof 0, 3, 10, 100 and 300 ppm. Animal appearance, behavior, survival, body weightgain and food consumption were comparable in treated and control groups. Postmortem
examination provided evidence of liver and kidney toxicity. There were no statisticallysignificant or biologically significant increases in neoplastic lesions.
Increased AP, AST, ALT, urea nitrogen and cholesterol levels in plasma wereindicative of mild liver toxicity and cholestasis. These biochemical changes wereaccompanied by a significant increase in liver weight, focal hepatocellular necrosis,
spongiosis hepatis and bile duct proliferation. Renal effects included : increased
urinary protein and an increased severity of glomerulonephropathy. The chronic NOELin rats is considered to be 10 ppm (equivalent to a daily intake of 0.36 mg/kg
bodyweight in males and 0.43 mg/kg b.w. in females) .
(The Institute of Environmental Toxicology, 1989)
2. Eighteen-month Feeding Study in MiceIn an oncogenicity study, dithiopyr was administered via the diet to groups of 70
male and 70 female CD-1 mice for 78 weeks at concentrations of 0, 3, 30 and 300
ppm. Animal appearance, behavior, survival, body weight gain and food consumption
were comparable in treated and control groups. Postmortem examination revealed
primarily liver toxicity. There were no statistically significant or biologically significant
increases in neoplastic lesions.
Liver enlargement and discoloration, hepatocellular swelling, bile duct proliferation,and pigment deposition in hepatocytes, Kupffer cells and the bile canaliculi wereindicative of mild liver toxicity and cholestasis. Other findings included : increased
adrenal weights, adrenal cortical swelling, spleen enlargement and increased splenic
extramedullary hematopoiesis. The chronic NOEL in mice is considered to be 3 ppm(equivalent to a daily intake of 0.31 mg/kg b.w. in males and 0.37 mg/kg b.w. infemales).
(The Institute of Environmental Toxicology, 1989)
3. Twelve-month Oral Study in Dogs
Dithiopyr was administered orally via gelatin capsule to groups of 6 male and 6
female beagle dogs for 52 weeks at dose levels of 0, 0.5, 5 and 25 mg/kg/day. Otherthan an increased frequency of vomiting early in the study, there were no clinicallyovert signs of toxicity observed. Animal survival, body weight gain and foodconsumption were comparable in the treated and control groups. Postmortem
examination revealed liver toxicity and cholestasis. Specific findings included :increased AP levels in plasma, liver enlargement and discoloration, the presence of
black sandy materials in the gallbladder, hepatocellular necrosis and fibrosis, pseudobile duct formation, bile duct proliferation, and increased mucoidal secretion in the
gallbladder. Brown pigment deposition was also found in the bile canaliculi, Kupffer
cells and the kidneys. The chronic NOEL in dogs is considered to be 0.5 mg/kg/day.
(The Institute of Environmental Toxicology, 1989)
REPRODUCTION AND DEVELOPMENTAL TOXICITY STUDIES
1. Two-generation Reproduction Study in Rats
Dithiopyr was administered via the diet to groups of 24 male and 24 female S-D
rats over 2 consecutive generations at concentrations of 0, 25, 250 and 2500 ppm.
Parental animal appearance, behavior and survival were comparable in the treatedand control groups. Parental body weight gain during the premating growth phases
was depressed by 7-10% at the high dose level ; food consumption was slightly
depressed in high-dose females. Dithiopyr administration had no effect on animal
mating, fertility, length of gestation, parturition, litter size, sex ratio or pup viability.
The appearance and behavior of off-spring during the lactation period was comparable
in the treated and control groups. Throughout the lactation period, body weights of Fl
and F2 generation pups and weanlings were 6-21% lower than control at the highdose level.
Postmortem examination of parental animals revealed primarily liver toxicity,
consisting of organ enlargement and discoloration, hepatocellular swelling and necrosis,
and bile stasis. Other findings included : increased kidney weight, focal renal tubular
atrophy, thyroid follicular hypertrophy, and adrenal cortical hypertrophy. Postmortem
examination of the offspring revealed liver enlargement and the appearance of whitespots at the periphery of the liver lobes. These findings were accompanied byhistopathological evidence of hepatocellular swelling and localized areas ofhepatocellular necrosis, fibrosis and mineralization. The white liver spots are considereda reversible lesion due to their declining incidence with increasing animal age andtheir complete absence in adults. The NOELS for systemic and reproductive toxicity
in rats are considered to be 25 and 2500 ppm, respectively.
(The Institute of Environmental Toxicology, 1989)
2. Developmental Toxicity Study in Rats
Dithiopyr was administered daily via gastric intubation in an aqueous suspension
to groups of 24 mated female S-D rats on days 6 through 15 of gestation at doselevels of 0, 30, 300 and 1000 mg/kg/day. Maternal toxicity in the form of a slight
reduction in food consumption during the first few days of dosing was observed at
the high dose level. Dithiopyr administration had no effect on the incidence of viable
fetuses, resorptions, postimplantation loss, fetal sex ratio or fetal body weight. There
were no significant differences between treated and control group incidences of fetal
malformations or developmental variations. The NOELS for maternal anddevelopmental toxicity in the rat are considered to be 300 and 1000 mg/kg/day,
respectively.
(The Institute of Environmental Toxicology, 1987)
3. Developmental Toxicity Study in Rabbits
Dithiopyr was administered daily via gastric intubation in an aqueous suspension
to groups of 18 mated female NZW rabbits on days 7 through 19 of gestation at doselevels of 0, 150, 500 and 1000 mg/kg/day. Maternal toxicity in the form of a slight
reduction in body weight during the first few days of dosing was observed at the highdose level. Dithiopyr administration had no effect on the incidence of viable fetuses,
resorptions, postimplantation loss, fetal sex ratio or fetal body weight. There were no
significant differences between treated and control group incidences of fetal
malformations or developmental variations. The NOELS for maternal anddevelopmental toxicity in the rabbit are considered to be 500 and 1000 mg/kg/day,
respectively.
(Hazleton Laboratories America, Inc., 1989)
GENETIC TOXICITY STUDIES
1. Gene Mutation Assays
The potential of dithiopyr to induce reverse gene mutations was evaluated inAmes' plate incorporation assays employing the histidine auxotrophic Salmonella
typhimurium strains TA98, TA100, TA1535, TA1537 and TA1538 in both the presence
and absence of an Arochlor 1254-induced rat liver metabolic activation system.
Dithiopyr was tested to its limit of solubility at dose levels ranging from 0.01 to 5.0
mg/plate. No mutagenic effect was observed.
(SRI International, 1987)
The potential of dithiopyr to induce reverse gene mutations was also evaluated in
the tryptophan auxotrophic Escherichia coli strain WP2uvrA in both the presence andabsence of an Arochlor 1254-induced rat liver metabolic activation system. Dithiopyr
was tested at dose levels ranging from 0.05 to 5.0 mg/plate. No mutagenic effect wasobserved.
(The Institute of Environmental Toxicology, 1986)
The potential of dithiopyr to induce gene mutations was evaluated at the hypoxanthine
guanine phosphoribosyl transferase (HGPRT) gene locus of cultured Chinese hamster
ovary cells in both the presence and absence of an Arochlor 1254-induced rat liver
metabolic activation system. Dithiopyr was tested at concentrations ranging from 10
to 100 µg/ml in the presence of metabolic activation and at concentrations ranging
from 10 to 300 µg/ml in the absence of metabolic activation. No mutagenic effectwas observed.

(Monsanto Environmental Health Laboratory, 1987)

2. Chromosomal Damage Assay
The potential of dithiopyr to produce chromosome damage was evaluated in vitro
using Chinese hamster lung cells in both the presence and absence of an Arochlor
1254-induced rat liver metabolic activation system. Dithiopyr was tested atconcentrations ranging from 0.01 to 1.0 mM. No mutagenic effect was observed.
(The Institute of Environmental Toxicology, 1986)
3. DNA Damage Assays
The potential of dithiopyr to damage DNA was evaluated in vitro in a rat hepatocyte
DNA repair test. Cultured hepatocytes from F-344 rats were exposed to dithiopyr atconcentrations ranging from 0.1 to 1000 µg/ml. DNA damage was measured asunscheduled DNA synthesis. No genetic damage was observed.
(SRI International, 1987)
The potential to induce DNA damage was also evaluated in vitro in a bacterial
recombination assay using Bacillus subtilis strains H17 (rec+ ) and M45 (rec-) both in
the presence and absence of an Arochlor 1254-induced rat liver metabolic activation
system. Dithiopyr was tested at dose levels ranging from 0.2 to 20 mg/disc. No
genetic damage was observed.
(The Institute of Environmental Toxicology, 1986)
PHARMACOLOGY STUDIES
Acute intraperitoneal injections of dithiopyr in 3 male and 3 female ICR mice atdose levels of 0, 19.5, 78, 313, 1250 and 5000 mg/kg resulted in animal deaths at thehigh dose level and weak effects on motor activity, muscle tone and autonomic
nervous system function at levels of 313 mg/kg and above. The pharmacological
NOEL in mice is considered to be 78 mg/kg. Acute oral administration of dithiopyr
to 3 male albino rabbits at dose levels of 0, 313, 1250 and 5000 mg/kg resulted in no
behavioral or neurological effects.
Acute oral administration of dithiopyr to 3 anesthetized male albino rabbits at doselevel of 0, 1250 and 5000 mg/kg resulted in no measurable effects on respiration or
cardiovascular function. The pharmacological NOEL in rabbits is considered to be
5000 mg/kg.
(The Institute of Environmental Toxicology, 1988)
CONCLUSION
A comprehensive toxicology database has been developed to support the use ofdithiopyr products worldwide. The results of laboratory animal tests indicate that
dithiopyr displays low mammalian toxicity following acute oral, dermal or inhalation
exposure. Dithiopyr has low dermal and ocular irritation potential and does not produce
allergic skin reactions. Subchronic and chronic exposure produces primarily liver andkidney toxicity, effects for which threshold exposure levels have been established.
Dithiopyr is not genotoxic or oncogenic and does not interfere with normal reproduction
and development.
Human expdsure to dithiopyr is expected to be very low. Field application rates are
low, typically in the range of 0.1 to 0.5 kg a.i./ha. Residues of dithiopyr are notdetectable in rice (unpublished Monsanto data from residue trials in rice). Dermal
pharmacokinetics studies with rhesus monkeys have shown that only 0.08% of a
topically-applied dose of 14C-dithiopyr is systemically absorbed.3) Human biologicalmonitoring studies have shown that systemic exposure to dithiopyr resulting fromfield application averages 1.01 x 10-4 mg dithiopyr/kg b.w./kg a.i. applied.3)
Granular formulations of dithiopyr for rice paddy use and an emulsifiable concentrate(EC) formulation for turf use were registered in Japan in April of 1991. A withholding
limit for pesticide registration was established at 0.1 ppm for rice.* EC formulationsof dithiopyr for turf use were registered in the United States in June of 1991. Whenused in accordance with label directions, these products will not adversely affecthuman health.
* Withholding hmits are estabhshed by the Environment Agency for food crop
registrations of pesticides in Japan and serve the same purpose as maximum residue
hmits.
REFERENCES
1) E. Buehler: Arch. Dermatol. 91, 171 (1965)
2) H. Ritz & E. Buehler: "Current Concepts in Cutaneous Toxicity," ed. by V. Drill& P. Lazar Academic Press, New York, pp. 25-40, 1980
3) J. Cowell, C. Lottman & M. Manning: Arch. Environ. Contam. Toxicol. 21, 195(1991)

モンサント社アグリカルチュラル・グループ毒性部
デニス・P・ウォード
ジチオピルは，モンサント社が開発した一年生イネ科雑草および一部の広葉雑草を対象とする芝地および水田用除草剤である．動物実験の結果，本剤の哺乳動物に対する急性経
口，経皮，吸入毒性はいずれも軽微で，眼および皮膚一次刺激性は低く，皮膚感作性も認められなかった．亜急性および慢性毒性試験の結果，主として肝および腎に検体投与による影響が認められたが，これらの影響には閾値が存在した．また本剤には，遺伝毒性および催腫瘍性も認められず，正常な繁殖や発生過程を阻害することもなかった．実使用条件
における使用薬量が低く，皮膚浸透性が低いことっさらに食品中の残留が検出限界以下であることから，ジチオピルの人間への暴露の可能性は極めて低いものと考えられる．

Contacts
Toxicology Department, The Agricultural Group, MonsantoCompany, 800, N.
Lindbergh Blvd., St. Louis, M063167, U.S.A.
Registration, Planning Development Department, Agroscience Division, MonsantoJapan Limited., 520 Kokusai Bldg., 3-1-1, Marunouchi, Chiyoda-ku, Tokyo 100,Japan

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