UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C.  20460

OFFICE OF 

PREVENTION, PESTICIDES

AND TOXIC SUBSTANCES

Date: March 20, 2008

MEMORANDUM

SUBJECT:	Tributyltin Compounds – Revised Toxicology Chapter in Support
of Issuance of the Reregistration Eligibility Decision (RED) for
Tributyltin Oxide, Tributyltin Maleate and Tributyltin Benzoate.  PC
Code(s):  083001, 083118, 083106.  CAS Registry Number(s): 56-35-9,
4027-18-3, 4342-36-3.  DP#: D

FROM:	Michelle Centra, Pharmacologist

		Risk Assessment and Science Support Branch

Antimicrobials  Division (7510P)

THRU:	Siroos Mostaghimi, Ph.D., Acting Team Leader

		Norm Cook, Branch Chief

		Risk Assessment and Science Support Branch (RASSB)

		Antimicrobials Division (7510P)

TO:		Talia Lindheimer, Risk Assessor

		Risk Assessment and Science Support Branch (RASSB)

	Antimicrobials Division (7510P)

		Jill Bloom, Chemical Review Manager

		Reregistration Branch II

		Special Review and Reregistration Division (7508P)

		Diane Isbell, Team Leader

		Mark Hartman, Branch Chief

		Regulatory Management Branch II

		Antimicrobials Division (7510P)

Attached is the toxicology chapter in support of issuance of the
reregistration eligibility decision (RED) for the Tributyltin Compounds:
Tributyltin Oxide, Tributyltin Maleate and Tributyltin Benzoate.  



REVISED TOXICOLOGY CHAPTER

FOR

TRIBUTYLTIN-CONTAINING CHEMICALS

Tributyltin Oxide (PC Code  083001)

Tributyltin Maleate (PC Code  083118)

Tributyltin Benzoate (PC Code 083106)

Reregistration Case Number: 2620

March 30, 2008

Michelle Centra, Pharmacologist

US EPA/OPP/Antimicrobials Division

Risk Analysis and Science Support Branch

1.0	INTRODUCTION

Use Patterns

Tributyltin containing compounds are used primarily as bacteriocides,
microbiocides, fungicides, algaecides, slimicides, virucides, miticides
and insecticides.  For issuance of the reregistration eligibility
decision, the three chemicals included under the general tributyltin
classification are: tributyltin oxide (PC Code 083001), tributyltin
benzoate (PC Code 083106), and tributyltin maleate (PC Code 083118). 
Examples of some of the primary uses sites in which TBTO containing
products are found include: agricultural premises and equipment;
oilfield/petrochemical injection water systems; industrial recirculating
water cooling systems; animal kennels, medical premises; material
preservation (textiles, metalworking fluids, plastics, construction
materials, etc); and non-pressure treated wood preservation.

	Hazard Characterization

Tin compounds, organic and inorganic, have been studied in a variety of
animals but primarily in rodents following the oral route of ingestion. 
There are limited data on the effects of tins in humans and that
primarily comes from reports of industrial and individual accidental
exposures.  The ATSDR report (2003) on tins also states that of the
various effects described after tin exposure in animal studies,
hematological signs of anemia and gastrointestinal distension appear to
be best identified as tin-related.  Gastrointestinal effects described
in humans following ingestion of tin compounds were nausea, vomiting and
diarrhea.  In experimental animals, as reported by CDC (1991), dietary
exposure to tributyltin oxide has resulted in weight loss,
immunosuppression and microcytic anemia.  CDC’s epidemiological notes
(1991) reports that tributyltin oxide dermal exposure in humans produces
irritant effects, including erythemia, follicular inflammation and
pruritus and has been identified as a potent non-allergenic dermal
irritant.   Industrial and residential exposure to vapors and fumes of
organotin compounds cause eye and throat irritation.  

There is some evidence that many of the tributyltin compounds cause
similar toxic effects.  In 1990, the World Health Organization (WHO,
Environmental Health Criteria 119) reported that tributyltin compounds
are skin, eye and respiratory irritants. Neither tributyltin oxide or
tributyltin maleate cause allergy in dermal guinea-pig sensitization
studies.  Generally, liver, hematological and immune system effects have
been observed in short- and long-term animal toxicity tests.  

Published literature for tributyltin oxide identifies immunotoxicity as
the chemical’s primary toxicological action in laboratory mammals:
tributyltin oxide, and related compounds, dibutyltin and dioctyltin, are
reported immunotoxic and thymolytic agents.  Later reviews by EPA/IRIS
(1997) and the WHO (1999) identify a large body of information
demonstrating the critical effect for tributyltin oxide as depression of
thymus-dependent immunological responses.  Immunological effects are in
evidence in short- and long-term studies in rats and mice where exposure
to tributyltin oxide has been linked to decreased immunoglobulin levels,
decreased thymus weight, various inflammation responses. 

Reproductive and developmental toxicity due to tributyltin oxide and/or
tributyltin benzoate occurred at or near the exposure that also caused
maternal toxicity and was generally characterized by decreased body
weight or body weight gains. However, studies do exist that show
pre-weanling rat pups to be more sensitive to the immunologic effects of
tributyltin oxide than adult rats in their thymus-dependent immunity. 

Some organotin compounds are known to have neurotoxicity effects. 
Triethytin and trimethyltin cause neuronal edema and necrosis of the
central nervous system.  A few studies that were investigating other
effects also showed some minor toxicities of tributyltin on the nervous
system (reduced brain weight, decreased motor activity, lympho- and
hepatobiliary toxicity).  However, oral exposure to tributyltin oxide in
these studies did not cause severe neurological signs or result in
morphological or histopathological changes in brain tissue.  Based on
the evidence from available studies conducted with tributyltin oxide,
there is no suggestion that neurotoxicity is a likely critical or
co-critical effect. 

It is unclear if tributyltin oxide is a carcinogen in rats, however, it
is not a carcinogen in mice and does not appear to be genotoxic.  EPA
has assigned tributyltin oxide to category D (U.S., 1987) or to the
“cannot be determined” category for carcinogenicity (U.S. EPA, 1996)
based on high spontaneous incidences of tumors in Wistar rats, incidence
variability in the treated groups and absence of a dose-effect
relationship.  Therefore the significance of the increases in benign
pituitary tumors, pheichromocytomas and parathyroid tumors at the
highest tested doses in rats remains unclear.    

2.0	BRIDGING OF TRIBUTYLTIN OXIDE, MALEATE AND BENZOATE TOXICITY
DATABASES

In 2005, the OPP’s Health Effects Division evaluated the existing
tributyltin toxicity database to support a risk management decision for
a  proposed new non-food use registration of tributyltin maleate (TBTM)
as a miticide for the treatment of finished carpet, rug backings and
fibers. The existing toxicity database for tributyltin maleate consists
of an acute oral toxicity study and a dermal sensitization study
conducted with the technical grade active ingredient.  This limited
database was considered inadequate to support the new registration and
an evaluation of the toxicity database for the related chemical,
tributyltin oxide, was conducted for the purposes of data bridging.  It
was determined that tributyltin oxide and tributyltin maleate may be
considered toxicologically equivalent, with the provision that specific
studies be submitted to the Agency for bridging of the databases.  These
data included: an immunotoxicity study in mice or rats, a 90-day
neurotoxicity study in rats,   a developmental toxicity study in rats
and a dermal absorption study in rats.  As for bridging of the
tributyltin oxide and tributyltin benzoate toxicity databases, the
registrant proposed using data from the TBT Consortium on
bis(tributyltin)oxide (TBTO) to support TBTB use based on the higher tin
content of TBTO and the consequent greater toxicity, thus making TBTO a
Worst Case chemical (SRRD/GCSB Transmittal Sheet for TBT-containing
chemicals data requirements/data gaps, dated 3/21/90).  This request
would apply to Subdivision Testing Guidelines 81-1, 81-2, 82-2, 83-3a,
84-2a, 84-2b and 84-4.  In this case, the Agency determined that the
registrant can use tributyltin oxide data to support registration of
tributyltin benzoate-containing products if they supply bridging
studies: a dermal absorption study and a 90-day oral study (with
immunological and neurological toxicity data) for each of the technical
grade active ingredients.  To date, none of the study data gaps
identified for tributyltin maleate or tributyltin benzoate have been
submitted to the Agency.

The Antimicrobials Division’s Toxicity Endpoint Selection Committee
(ADTC) met on January 17, 2008 to evaluate the available toxicology data
for the tributyltin-containing compounds; tributyltin oxide, tributyltin
maleate and tributyltin benzoate.  At this time, the issue of bridging
the available toxicity data for these three organotins was re-evaluated.
 The Committee determined that the oxide, maleate and benzoate forms of
tributyltin are considered toxicologically equivalent based on similar
structure and physical chemical properties (e.g., dissociation
constants).  However, to fully assess whether exposure to these
compounds will result in similar toxicities, Tier 1 toxicity testing is
required for each chemical.  These tests include acute toxicity (oral,
dermal, inhalation, eye and dermal irritation and skin sensitization),
subchronic (oral) toxicity, prenatal developmental toxicity and the full
battery of mutagenicity tests.  The outstanding data requirements for
tributyltin oxide, tributyltin maleate and tributyltin benzoate are
identified in tables 1 and 2.    

Table 1.  Toxicological Data Requirements for Non-food Uses of
Tributyltin Compounds

Test	Technical Grade Active Ingredient

	MRID	Required	Satisfied

Tributyltin Oxide

870.1100	Acute Oral Toxicity    	00085004, 92172013, 00085003, 92172004
Yes	Yes

870.1200	Acute Dermal Toxicity	---	Yes	No

870.1300	Acute Inhalation	---	Yes 	No

870.2400	Acute Eye Irritation	---	Yes 	No

870.2500	Acute Dermal Irritation	---	Yes	No

870.2600	Skin Sensitization	00104789, 92172014	Yes	Yes

870.3100	90-Day (oral) Subchronic - Rodent 	41127001	No	No

870.3150	90-Day (oral) Subchronic - Non-rodent	41131001	No	No

870.3200	21-Day (dermal) Subchronic - Rodent	---	No	---

870.3250   90-Day (dermal) Subchronic - Rodent	---	Yes	No

870.3465   90-Day (inhalation) Subchronic - Rodent	---	No	---

870.3700	Prenatal Developmental - Rodent	00137158, 92172016 	Yes	Yes

870.3700	Prenatal Developmental - Non-rodent	40141901, 92172006	Yes	Yes

870.3800	Reproduction & Fertility Effects - Rodent	41693801	Yes	Yes

870.4100   Chronic - Rodent	40623201	Yes	Yes*

870.4100   Chronic - Non-rodent	42549801	Yes	No

870.4300   Combined Chronic-                      
Toxicity/Carcinogenicity - Rat	40623201	Yes	Yes

870.4200   Carcinogenicity - Mouse	42265001	Yes	Yes

870.5100	Bacterial Reverse Mutation Test                                
    	42170001	Yes	Yes

870.5375	In Vitro Chromosome Aberration Test – Human Lymphocytes
40253005	Yes	Yes

870.5450 to 870.5915 Mutagenicity Tests (1 Study)	---	Yes	No

870.6200   90-day neurotoxicity - Hen	---	No	---

870.6200   90-day neurotoxicity - Mammal	---	No	---

Metabolism	01246480, 40253002	Yes	Yes

870.7600   Dermal penetration	40050003	Yes	Yes**

Tributyltin Maleate

870.1100	Acute Oral Toxicity	43851201	Yes	Yes

870.1200	Acute Dermal Toxicity	---	Yes	No

870.1300	Acute Inhalation	---	Yes	No

870.2400	Acute Eye Irritation	---	Yes	No

870.2500	Acute Dermal Irritation	---	Yes	No

870.2600	Skin Sensitization	44142303	Yes	Yes

870.3250	90-Day (dermal) Subchronic -Rodent	---	Yes	No

870.3700	Prenatal Developmental - Rodent	---	Yes	No

870.5100 to 870.5915 Mutagenicity Testing (3 studies)	---	Yes	No

Tributyltin Benzoate

870.1100	Acute Oral Toxicity	42415801	Yes	Yes

870.1200	Acute Dermal Toxicity	42415802	Yes	Yes

870.1300	Acute Inhalation	---	Yes	No

870.2400	Acute Eye Irritation	---	Yes	No

870.2500	Acute Dermal Irritation	42415803	Yes	Yes

870.2600	Skin Sensitization	---	Yes	No

870.3200	21-Day (dermal) Subchronic - Rodent	43177201	Yes	No

870.3250	90-Day (dermal) Subchronic - Rodent	---	Yes	No

870.3700	Prenatal Developmental - Rodent	42903101	Yes	Yes

870.5300  In vitro Mammalian Cell Gene Mutation Test	42412501	Yes	Yes

870.5395  Mammalian Erythrocyte Micronucleus Test	42412502, 42966201	Yes
Yes

870.5550  Unscheduled DNA Syntheses	42412503, 42412504, 42966203,
42966201	Yes	No

*Chronic toxicity study data requirement in rodents is satisfied by the
combined chronic toxicity/carcinogenicity study conducted in rodents.

**Study does not satisfy a guideline requirement, however, it provided
useful information that is adequate for risk assessment purposes. 

 

Table 2.  Generic Data Gaps for Tributyltin Compounds 

Guideline Number	Study Type

                                 Tributyltin Oxide

870.1200	Acute Dermal Toxicity

870.1300	Acute Inhalation Toxicity

870.2400	Acute Eye Irritation

870.2500	Acute Dermal Irritation

870.3250	90-Day (dermal) Subchronic - Rodents

870.5450 

to 

870.5915	Mutagenicity Test (1 Study)

                                   Tributyltin Maleate

870.1200	Acute Dermal Toxicity

870.1300	Acute Inhalation Toxicity

870.2400	Acute Eye Irritation

870.2500	Acute Dermal Irritation

870.3250	90-Day (dermal) Subchronic -Rodents

870.3700	Prenatal Developmental - Rats

870.5100 

to 

870.5915	Mutagenicity Tests (3 studies)

                                    Tributyltin Benzoate

870.1300	Acute Inhalation Toxicity

870.2400	Acute Eye Irritation

870.2600	Skin Sensitization

870.3250	90-Day (dermal) Subchronic - Rats

870.5450

to 

870.5915	Mutagenicity Test (1 study)

 

4.0 	HAZARD ASSESSMENT

4.1	Acute Toxicity

Adequacy of database for Acute Toxicity: The acute toxicity database for
the tributyltin compounds is considered incomplete; the battery of acute
toxicity studies required for labeling purposes has not been submitted
by the registrants for each of the tributyltin compounds (tributyltin
oxide, tributyltin maleate and tributyltin benzoate) grouped in this
hazard assessment.  The available data does show that exposure to
tributyltin oxide and tributyltin maleate can result in severe oral and
dermal toxicities (Toxicity Category II). However, tributyltin oxide and
tributyltin maleate are not dermal sensitizers.  Dermal and oral
exposures to tributyltin benzoate can potentially cause moderate
(Toxicity category III) to severe (Toxicity category II) toxicities,
respectively. 

Table 3. Acute Toxicity Profile for Technical (95.0- 97.5 % a.i.)
Tributyltin Compounds

Guideline Number	Study Type	MRID Number	Results	Toxicity Category

Tributyltin Oxide

870.1100

(§ 81-1)	Acute Oral – Rat 

	00085004,

92172013	LD50 =180 mg/kg (males)

LD50 =150 mg/kg (females)

LD50 =170 mg/kg (combined)	II

870.1100

(§ 81-1)	Acute Oral – Rat

	00085003,

92172004	LD50 =193 mg/kg (males)

LD50 =123 mg/kg (females)

LD50 =160 mg/kg (combined)	II

870.2600

(§ 81-6)	Skin Sensitization –Guinea pigs

	00104789, 92172014	Non sensitizer	Not applicable

Tributyltin Maleate

870.1100

(§ 81-1)	Acute Oral – Rat 

	43851201	LD50 = 224.7 mg/kg	II

870.2600

(§ 81-6)	Skin Sensitization – Guinea pigs

	44142303	Non sensitizer. Not sensitizing; minimal irritation in
response to induction, but no increase in response to challenge dose	Not
applicable

Tributyltin Benzoate

870.1100

(§ 81-1)	Acute Oral – rat

Purity 

	42415801	LD50 =115 mg/kg (males)

LD50 =115 mg/kg (females)

LD50 =115 mg/kg (combined)	II

870.1200

(§ 81-2)	Acute Dermal – rat

Purity 

	42415802	LD50  > 2000 mg/kg (combined)	III

870.2500

(§ 81-5)	Primary Dermal Irritation – rabbit

Purity 

	42415803	Severe Irritation	I



4.2 	Subchronic Toxicity

Adequacy of database for Subchronic Toxicity: Limited data exists for
determining the subchronic toxicity of tributyltin oxide, tributyltin
maleate and tributyltin benzoate. The database for subchronic toxicity
of these organotins is considered incomplete: the available studies are
either non-guideline range-finding studies or they are not adequate for
regulatory purposes.

870.3100 	Subchronic (90-Day Oral Range-Finding ) - Mice

In a 90-day oral toxicity study (MRID 41127001), bis (tri-n-butyltin)
oxide (TBTO; 97.1% a.i., Lot # KYRDO-064M) was administered to 10 CD-1®
mice/sex/dose in the diet at dose levels of 0, 4, 20, 80, or 200 ppm
(equivalent to 0.7, 3.8, 15.1, or 36.9 mg/kg/day in males; 1.0, 4.9,
17.9, or 46.9 mg/kg /day in females) for 13 weeks.

≥80 ppm TBTO had reduced mean food consumption.  The predominant
clinical sign of toxicity was redness, swelling, and scabs on the ears. 
These effects were observed in ≥80 ppm males and ≥20 ppm females. 
Ear inflammation was likely a dermal response to exposure to the test
article in the food while eating.  

Hematological measurements showed decreased mean hemoglobin
concentration, percent hematocrit, and erythrocyte counts ≥80 ppm
males and/or females.  Mean platelet count was elevated in 200 ppm males
and ≥80 ppm females.  Mean total leukocyte count was increased in
≥80 ppm males.  Clinical chemistry results showed increased mean
alkaline phosphatase levels in 200 ppm males and females.  Increased
total protein and blood urea nitrogen were observed in females treated
with ≥ 80 ppm TBTO.  Increased organ weights (adrenal, liver, spleen)
were observed in both sexes following treatment with ≥ 80 ppm TBTO. 
Reduced testes weights were also noted in ≥ 80 ppm males.  

liver, bile duct, gallbladder and spleen of animals treated with ≥80
ppm TBTO in the diet for 90 days, although mild treatment-related
effects were noted at lower doses.  

This range-finding oral toxicity study is classified as
Unacceptable/Non-guideline and does not satisfy the guideline
requirement for a 90-day oral toxicity study (OPPTS 870.3100; OECD 408)
in rodents.  The major deficiency is the inconclusive stability of the
diet formulations.  Without conclusive stability confirmation of the
diet formulations, the reported doses received by the mice in this
feeding study are not reliable.  Therefore, this study is not adequate
for regulatory purposes.

870.3200 	Subchronic (21-Day Dermal Range-finding) Toxicity – Dogs

In a non-guideline, range-finding oral toxicity study (MRID 41131001), 2
Beagle dogs/sex/dose were exposed daily via gavage (1 mL/kg) to
Tributyltin oxide (>96.1% a.i., Batch No.: 2288) in arachis oil for up
to 19 weeks.  The animals (Groups 1-4) were dosed at 0, 0.1, 0.5, or 2.5
mg/kg during Weeks 1-5; 0, 0.2, 1.0, or 5.0 mg/kg during Weeks 6-10; and
0, 10, 1.0, or 5.0 mg/kg during Weeks 11-19.

Two male dogs from the 5 mg/kg/day group received misapplications into
the lungs at the end of Week 8 and were sacrificed 5 days later.  During
Weeks 12 and 14, respectively, two male dogs from the 10 mg/kg/day group
also received misapplications into the lungs and were sacrificed later
that same day.  

i) decreased relative albumin (↓ 8%, p<0.01, Week 18); (iii) increased
relative total α-globulins (↑ 15-16%, NS, Weeks 9, 13, and 18); (iv)
increased relative total β-globulins (↑ 4-14%, NS, Weeks 9, 13, and
18); (v) decreased A/G ratio (decr 14-18%, p<0.05, Weeks 13 and 18); and
(vi) marked increases in absolute (↑ 34%) and relative (to body, incr
26%) liver weights.

At 2.5 mg/kg/day, the following effects were noted: (i) increased ALP
(↑ 64-102%, p<0.05 or NS, Weeks 1, 4, and 5); (ii) decreased relative
albumin (↓ 6%, p<0.05, Week 4); (iii) decreased A/G ratio (decr 15%,
p<0.05, Week 4); (iv) decreased hematocrit (↓ 9%, p<0.05, Week 4); and
(v) decreased hemoglobin (↓ 13%, p<0.01, Week 4).  No clear-cut
evidence of the decreases in hematocrit and hemoglobin were observed
during Weeks 9, 13, or 18; however, it was noted that evaluation was
made difficult by the loss of animals (2 males each) in Groups 2 and 4.

  Additionally, the following treatment-related effects in clinical
chemistry and hematology parameters were noted: (i) decreased relative
albumin (↓ 15-16%, p<0.01, Weeks 9, 13, and 18); (ii) increased
relative total α-globulins (↑ 32-50%, NS or p<0.01, Weeks 9, 13, and
18); (iii) increased relative total β-globulins (↑ 18-20%, NS or
p<0.05, Weeks 9, 13, and 18); (iv) decreased A/G ratio (↓ 31-33%,
p<0.01, Weeks 9, 13, and 18); (v) increased glutamic oxaloacetic
transaminase (GOT, ↑ 70%, NS, Week 18); (vi) increased glutamic
pyruvic transaminase (GPT, ↑ 53%, NS, Week 18); (vii) decreased
glucose (↓ 16%, p<0.05, Weeks 13 and 19); (viii) increased leucocyte
counts (↑ 56-104%, p<0.05, Weeks 9 and 18); (ix) increased neutrophil
counts (↑ 55-184%, p<0.05, Weeks 9 and 18); and (x) marked increases
in absolute (↑ 28%) and relative (↑ 51%) liver weights.

↓ 30%), and water consumption (↓ 26%).  Additionally, the following
hematology and clinical chemistry findings were noted at this dose: (i)
slight increase in leucocyte count (↑ 45%, p<0.05, Week 13); (ii)
increased fibrinogen (↑ 76-87%, p<0.05, Weeks 13 and 18); (iii)
increased ALP (↑ 51-170%, NS, Weeks 13 and 18); (iv) decreased
relative albumin (↓ 14-15%, p<0.01, Weeks 13 and 18); (v) increased
relative total α-globulins (↑ 41-47%, p<0.01, Weeks 13 and 18); (vi)
increased relative total β-globulins (↑ 12-16%, NS, Weeks 13 and 18);
(vii) decreased A/G ratio (↓ 29-33%, p<0.01, Weeks 13 and 18); (viii)
increased GPT (↑ 339%, NS, Week 18); (ix) decreased calcium (↓ 11%,
p<0.05, Week 18); and (x) increased granulopoiesis index (↑ 92%,
p<0.01, Week 19).  The increased granulopoiesis index indicates a
decrease in mature granulocytopoietic cells and is in accordance with
the decrease in neutrophils noted in the peripheral blood of these
animals.   

At 10 mg/kg/day, marked increases in absolute (↑ 22%) and relative
(↑ 74%) liver weights were noted.  Absolute and relative thymus
weights were decreased by 69-78% compared to controls, which
corresponded with the gross finding (small thymus size) noted at this
dose.  The decreases noted in absolute (↓ 55% each) and relative (↓
38-39%) iliac and mesenteric lymph node weights indicated an effect on
the lymphatic tissue.  It was stated that this is a known effect of TBTO
from short-term feeding studies in rats (Schweinfurth and Günzel,
1987).  The following treatment-related histopathological effects (#
affected/2 treated vs. 0 controls) were noted in the two females that
survived to scheduled sacrifice: (i) thymus, marked to complete
thymocyte depletion (2); (ii) spleen, acute hyperemia (2) and reduction
of lymphocytes involving the periarteriolic lymphocytic sheets (PALS),
pyknotic lymphocytes, and the follicles (1-2); (iii) iliac lymph node,
reduction of lymphocytes involving the follicle (2) and inconspicuous
germinal centers (2); (iv) mesenteric lymph node, reduction of
lymphocytes involving the follicle (2) and medulla (2), and
inconspicuous germinal centers (1); (v) liver, Cytoplasmic vacuolization
(2) and presence of giant mitochondria (2); and (vi) ovary, inhibition
of follicle maturation (2).  

↑ 28-53%) and relative (↑ 29-76%) spleen weight were within the
historical control range; however, these findings were considered to be
related to treatment as increased blood content (acute hyperemia) was
observed histologically in all treated groups.

The following findings were considered to be of equivocal toxicity: (i)
slight increases (p<0.05) in specific gravity (↑ 1-2%) at 5 and 10
mg/kg/day during week 18; (ii) increased sedimentation rate after 24
hours (↑ 227%) in the 10 mg/kg/day animals at Week 18 (value was not
statistically significant, but the value of one animal was outside the
historical range); and (iii) minimal to moderate increase of mucous in
the gall bladder noted in 1 animal each at 1.0 mg/kg/day and above (vs.
0 controls), which corresponded with the gross observation of granulated
surface in two animals.  

This oral range-finding study is classified as Acceptable/Non-guideline.
 Although this study does not satisfy the guideline requirement for a
subchronic toxicity study [(OPPTS 870.3200] in non-rodents, it contains
useful information that supports the critical effect of tributyltin
oxide observed in published literature, specifically, immunotoxicity of
the thymus.

4.3 	Prenatal Developmental Toxicity 

Adequacy of database for Prenatal Developmental Toxicity: The database
for prenatal developmental toxicity consists of three acceptable
studies; two conducted with tributyltin oxide (rat and rabbit) and one
conducted with tributyltin benzoate (rabbits).  This database is
considered complete for tributyltin oxide and tributyltin benzoate,
however, a prenatal toxicity study conducted in the rat with tributyltin
maleate remains an outstanding data requirement. 

Prenatal Developmental (Gavage) Toxicity – Rat

In a developmental toxicity study (MRID 00137158), tributyltin oxide
(TBTO, 96.9% a.i., Lot No.VNR00-605K) in Mazola® corn oil was
administered via gastric intubation (gavage) to pregnant female CD®
(Sprague-Dawley derived) rats at doses of 5, 9 and 18 mg/kg/day from
gestation days 6 through 19, inclusive.  Initially the dose levels were
set at 6, 12 and 24 mg/kg/day for the low, mid and high dose groups,
respectively; however, analysis of weekly samples of dosing solutions
prepared by Bio/dynamics and used for the first three weeks of treatment
yielded concentrations less than anticipated.  Thereafter, dose levels
were changed to 5, 9, and 18 mg/kg/day for the low mid and high dose
groups, respectively to reflect the actual dose levels administered to
the animals. Females were sacrificed on Day 20 of gestation and
recovered fetuses were evaluated for external, soft tissue and skeletal
malformations.  The parameters that were evaluated included maternal
mortality, pregnancy rates, in-life observations, body weight change
data, uterine implantation data (i.e., number of implantations,
resorptions, fetuses) and gross postmortem examination data.  Fetuses
were weighed, sexed and evaluated for anomalies. Ossification variation
data were recorded during the fetal skeletal evaluations.  

At the 9 mg/kg dose level, one female died and a second female was
killed in moribund condition with the former being attributed to
dosing-related injury. Mean weight change for the 9 mg/kg/day females,
during the 6-19 day gestation period was lower compared to the control,
but was not statistically significant. Staining of fur in the
ano-genital area and the incidence of fetuses with at least one
ossification variation were significantly increased in the mid-dose
females.  No treatment effect was evident in uterine implantation data,
fetal sex distribution data or fetal weight data. There were no
mortalities in the 18 mg/kg/day dose group. Weight gain during the
gestation period was lower than controls and there was increased
incidence of fur staining in the ano-genital area.  There was an
increase in the mean number of resorption sites, increase in the
percentage of resorptions to implants, with a slightly reduced litter
size in the high dose group.  There was a decrease in mean fetal weight.
No maternal toxicity was evident at the 5 mg/kg dose level.  The
Maternal Toxicity NOAEL was 5 mg/kg/day.  However, the definitive
Maternal Toxicity LOAEL could not be identified due to the lack of
toxicity in maternal animals.

At the 9 mg/kg/day dose level, the types and incidences of ossification
variations observed in this group were generally similar to the control
group; however, the incidence of fetuses with rudimentary structures was
notably increased.  No developmental toxicity was noted in mid-dose
fetuses.  There was an increase in the mean number of resorption sites,
increase in the percentage of resorptions to implants, with a slightly
reduced litter size in the high dose group.  There was a decrease in
mean fetal weight. The incidence of high-dose fetuses with ossification
was increased and fetuses exhibited an increase in the incidence of
rudimentary structures (small discrete ossification(s) adjacent to the
last thoracic or first lumbar vertebral transverse process (es)),
asymmetrical sternebrae, 14th rib pair and cervical ossifications. The
incidence of malformed fetuses was also increased (6.4%), the most
prominent malformation being cleft palate that was observed in 17
fetuses.  The incidence of fetuses with rudimentary structures was
notably increased at all dose levels (litter incidence data were not
provided).  The Developmental Toxicity NOAEL is less than 5 mg/kg/day
(not established).  The Developmental Toxicity LOAEL is equal to or less
than 5 mg/kg/day based on increased incidences of ossification
variations. 

This study is classified as Unacceptable-Guideline/Upgradable.  It does
not satisfy the guideline requirement for a prenatal developmental
toxicity study (OPPTS 870.3700; OPP §83-3, OECD 414) in rats.

870.3700 	Prenatal Developmental (Gavage) Toxicity – Rat

In a developmental study (MRID 42903101), groups of Crl: CD® (SD) BR
VAF/Plus strain rats received the test substance tributyltin benzoate
(Lot No: 1446-6, purity 97.1%) via intra-gastric intubation at doses of
0, 1.0, 4.5 and 20.0 mg/kg/day from 6 through 15 of pregnancy,
inclusive. On Day 20, the females were killed and subjected to post
mortem examination litter values were determined and fetuses examined
for visceral and skeletal abnormalities.

One female in the 4.5 mg/kg/day group totally resorbed her litter at an
early stage of pregnancy having shown signs of impaired respiration,
hunched posture and piloerection. Fetal examination revealed an
increased incidence of extracervical ribs and one fetus with a double
outlet of the right ventricle (and an interventricular septal defect), a
finding consistent with those observed at the higher dosage.   Maternal
toxicity was seen at the 20 mg/kg/day group, clinically manifested as
initial body weight loss, reduced food and increased water consumption.
At an early stage of pregnancy, four females were observed to have
completely resorbed their litters. Among the females that retained live
fetuses till day 20 of pregnancy, mean fetal weight and consequently
litter weights were lower than those seen in the control group.
Increased incidences of fetuses and litters with visceral and skeletal
malformations were observed. These included cardiovascular and eye
defects as well as disturbance of axial development of the skeletal
system. The percentage of fetuses with unossified sternebrae and
asymmetric/bipartite sternebrae was also found to have increased in the
20 mg/kg/day dose group.   At 1.0 mg/kg/day, there were no adverse
effects of treatment on the parent female.  The Maternal Toxicity NOAEL
is 1.0 mg/kg/day and the Maternal Toxicity LOAEL is 4.5 mg/kg/day based
on increased incidences of post-dose salivation, wet coat, and impaired
respiration. 

There was an increase in the number of fetuses and litters with extra
cervical ribs and one fetus with the unusual finding of a double outlet
of the right ventricle (and an interventricular septal defect), findings
consistent with those at 20 mg/kg/day (HDT). In the high dose group,
mean fetal and litter weights were lower than those seen in the control
group. Increased incidences of fetuses and litters with visceral and
skeletal malformations were observed. These included cardiovascular and
eye defects as well as disturbance of axial development of the skeletal
system. The percentage of fetuses with unossified sternebrae and
asymmetric/bipartite sternebrae was also found to have increased in the
20 mg/kg/day dose group.  At 1.0 mg/kg/day, there were no signs of
developmental toxicity.  The Developmental Toxicity NOAEL is 1.0
mg/kg/day and the Developmental Toxicity LOAEL is 4.5 mg/kg/day, based
on increased resorption, a dose related increased in incidence of double
outlet of right ventricle (and an intraventricular septal defect) along
with increased incidence of extracervical ribs at this dosage.

This study is classified as Acceptable/Guideline and satisfies the
guideline requirements for a developmental toxicity study (OPPTS
870.3700; OPP §83-3, OECD 414) in rats. 

Prenatal Developmental (Gavage) Toxicity – Rabbit

In a developmental toxicity study (MRID 40141901), TBTO (>95 % a.i., Lot
No. KY-RDO-064-M) was administered to three groups of 20 inseminated New
Zealand White female rabbits via oral gavage at doses of 0.2, 1.0 and
2.5 mg/kg/day from gestation days (gd) 6 through 18, inclusive. The
control group was dosed with a comparable regimen of 0.5 mL/kg of
Mazola® corn oil.  Throughout gestation, all females were observed
twice daily for appearance and behavior, and body weights were recorded
at appropriate intervals. On gestation day 29, all surviving females
were sacrificed for a scheduled Cesarean section. The uterus and ovaries
were excised and the trimmed uterus and contents were weighed. All
fetuses were weighed, sexed and examined for external, skeletal and
visceral anomalies and developmental variations.

No compound related deaths were observed in any dose group. A marked
increase in the incidence of abortion was observed in the 2.5 mg/kg/day
group when compared with the control group and historical data. None of
the clinical findings in the 0.2 and 1.0 mg/kg/day treatment groups were
suggestive of treatment-related effects. A statistically significant (p
< 0.05) mean body weight loss was observed in the 2.5 mg/kg/day group
from gestation days 6-18. This represented the only treatment-related
effect on maternal body weight gain in the study.

Intrauterine survival and growth of the fetuses were not affected by
oral administration of TBTO in the 0.2 and 1.0 mg/kg/day groups. A
slight but statistically insignificant decrease in mean fetal weights
was noted at the 2.5 mg/kg/day dose level. When compared with the
control group, there were no observable differences in the types and
frequency of fetal malformations and developmental variations indicative
of a response to treatment at all levels tested. Post mortem examination
of the dams did not reveal any consistent changes which could be
considered treatment-related.  The Maternal Toxicity NOAEL is 1.0
mg/kg/day and the Maternal Toxicity LOAEL is 2.5 mg/kg/day, based on
increased incidence of abortion and decreased mean maternal body weight
gain.

A slight but statistically insignificant decrease in mean fetal weights
was noted at the 2.5 mg/kg/day dose level. When compared with the
control group, there were no observable differences in the types and
frequency of developmental toxicity observations indicative of a
response to treatment at all levels tested.  The Developmental Toxicity
NOAEL is equal to or greater than 2.5 mg/kg/day and the Developmental
Toxicity LOAEL is greater than 2.5 mg/kg/day (not established) due to
the absence of any developmental toxicity at the highest dose tested. 

This study is classified as Acceptable/Guideline and satisfies the
guideline requirements for a prenatal developmental toxicity study
(OPPTS 870.3700; OPP §83-3, OECD 414) in rabbits. 

4.4 	Reproductive Toxicity

Adequacy of database for Reproductive Toxicity: The database for
reproductive toxicity of Tributyltin oxide is considered complete and
adequate for regulatory purposes.  

870.3800 	Reproduction and Fertility Effects- Rat

In a two-generation reproduction toxicity study (MRID 41693801),
Tributyltin oxide (97.1%; Lot # KRYDO – 064M) was administered in the
diet to 30 Sprague Dawley rats/sex/dose group at dietary levels of 0,
0.5, 5, or 50 ppm for two successive generations.  The P generation
animals were fed the test diets for approximately 10 weeks prior to
mating to produce the F1 litters.  All litters were weaned on PND 21,
and one F1 weanling/sex/litter was randomly selected to be a parent of
the next generation, following the same procedures described for the
first generation, with the exception that the pre-mating period was 15
weeks for the F1 parents.  

dence was low (≤2 rats per sex/dose group/generation) and unrelated to
dose.  At 50 ppm, anogenital staining was observed in 9/30 P females at
Week 16.

les, absolute (NS) and relative (p≤0.05) weights of the iliac node
were decreased at 5 and 50 ppm.  Additionally at 50 ppm, discolored
mesenteric lymph node was observed in the F1 males (3/30 treated vs 1/30
controls) and females (5/30 treated vs 0/30 controls).  The incidences
of pigments/erythrocytes in the reticuloendothelial cells in the
mesenteric lymph nodes were higher at this dose in the P males (79%
treated vs 66% controls), P females (77% vs 31%), F1 males (80% vs 60%),
and F1 females (80% vs 78%).  Incidences of reticuloendothelial
hyperplasia were increased in the P males (45% treated vs 38% controls),
P females (60% vs 41%), F1 males (77% vs 56%), and F1 females (76% vs
61%).  Additionally in the P generation, increased incidences of
erythrocytes in the sinuses of the mesenteric lymph nodes were noted in
the males (72% treated vs 38% controls) and females (50% vs 28%).  In
the prostate, increased incidence of interstitial lymphocytes were
observed at 50 ppm in the P generation (11/30 treated vs 5/30 controls)
and F1 generation (8/30 treated vs 2/30 controls).  However, because
there was no functional impairment on the reproductive performance in
either generation and there were no other microscopic findings in the
reproductive organs, this finding is considered toxicologically
unimportant.  The LOAEL for parental toxicity is 50 ppm (equivalent to
mg/kg/day 3.47/3.93 in males/females) based on: anogenital staining in
the P dams; decreased body weights in the F1 males and females during
pre-mating and continuing in the F1 males during the mating and
post-mating periods; and decreased absolute and relative thymus weights
in the F1 males.  The parental NOAEL is 5 ppm (equivalent to 0.33/0.39
mg/kg/day in males/females).

There were no adverse effects of treatment on gestation duration, the
numbers of implantations or pups born, the sex ratio, or on the live
birth, viability, lactation, or litter survival indices.  There were no
effects of treatment on the number of days until mating (pre-coital
interval), or on the mating, pregnancy, or fertility indices in either
generation.  

The LOAEL for reproductive toxicity was not observed (greater than 50
ppm).  The reproductive NOAEL is 50 ppm (equivalent to 3.47/3.93
mg/kg/day in males/females). 

Pup body weights were decreased (p≤0.05) in the F1 litters on PND 14
and 21 (↓14-17%) and in the F2 litters on PND 7, 14, and 21
(↓14-20%).  These decreases occurred in the latter part of the
lactation period and increased in magnitude with time.  There were no
treatment-related macroscopic findings, and no organs or tissues from
the pups were examined microscopically.  The LOAEL for offspring
toxicity is 50 ppm (equivalent to 3.47/3.93 mg/kg/day in males/females)
based on decreased pup body weights in both generations.  The NOAEL is 5
ppm (equivalent to 0.33/0.39 mg/kg/day in males/females).

This study is classified as Acceptable/Guideline and satisfies the
guideline requirement for a two-generation reproduction study [OPPTS
870.3800; OECD 416] in the rat.

4.5 	Chronic Toxicity 

870.4100 	Chronic Toxicity

Adequacy of database for Chronic Toxicity: The database for chronic
toxicity of   tributyltin oxide is considered complete and adequate for
regulatory purposes.

In a chronic oral toxicity study in dogs (MRID 42549801), bis
(tri-n-butyltin) oxide (TBTO; 95.9-97.1% a.i.; Batch #s 0830 and 5425)
in arachis (peanut) oil was administered by daily oral gavage (dose
volume 1.0 mL/kg) to four beagle dogs/sex/dose group daily for at least
52 weeks at doses of 0, 0.2, 1.0, or 5.0 mg/kg/day.

Tin was found in the pooled urine samples of all dose groups, including
controls, beginning on Week 1.  Urinary tin levels continued to increase
in the treated groups over the course of treatment; however, levels were
not determined in the controls at Weeks 2, 5, 12, or 26.  At Week 52,
tin levels in the 1.0 and 5.0 mg/kg/day groups had increased
approximately 10-fold over Week 1 levels, while levels in the 0.2
mg/kg/day group had increased approximately 4-fold. However, tin levels
in the controls had increased approximately 2.5-fold during treatment. 
This finding strongly suggested that the controls had been exposed to
the test compound repeatedly over the course of treatment.  It is the
opinion of the reviewers that establishment of a LOAEL and NOAEL are not
possible due to this major deficiency.

No adverse, treatment-related effects were observed on nervous system
functions, ophthalmoscopic examinations, cardiovascular function, or
urinalysis parameters.

4 females.  In the females, decreased (p≤0.05) immunoglobulin G
(↓24-32%; NS at 1.0 mg/kg/day at Weeks 26 and 52) and a (↓59-72%)
were observed at Weeks 13, 26, and 52.  Decreased (p≤0.05)
immunoglobulin A (↓38-68%) was noted in the males at Weeks 13 (NS), 26
(NS at 1.0 mg/kg/day), and 52.  Additionally at this dose, thymus
diminished in size was observed in one male, ALP was increased
(p≤0.05) by 147% in two males, and decreased spermiogenesis and
tubular degeneration was noted in the testis of 1/4 males.

 three females; and γ-glutamyl transferase was increased in all males
and females, all at multiple occasions.  Absolute and relative (to body)
liver weights were increased by 18-63% in the males and by 40-117% in
the females.  Gross pathological changes were confined in all but one
case to animals killed in extremis.  Multifocal areas of whitish
discoloration was noted in the liver of 2/4 males (one at study
termination) and 2/4 females, and all lobes of the liver enlarged was
observed in 1/4 females, all compared to 0 controls.  The following
lesions in the liver were observed at 5.0 mg/kg/day vs. 0 controls:  (i)
fatty change, predominantly mid-zonal in 3/4 males and females; (ii)
ballooning of the hepatocytes, mid-zonal and/or foci of single cells in
3/4 males and 4/4 females; (iii) focal hepatocellular necrosis in 1/4
males and females; (iv) pigment deposition (lipofuscin) in single
degenerated cells in 3/4 males and 4/4 females; and (v) local sinusoidal
fibrosis in 1/4 females.  Decreased spermiogenesis and tubular
degeneration of the testis, and atrophy of the epididymis were both
observed in 2/4 males

 were also observed at 5.0 mg/kg/day.  In the females, decreased
(p≤0.05) immunoglobulin G (↓28-38%) and a (↓71-83%) were observed
at Weeks 13, 26, and 52.  Decreased (p≤0.05) immunoglobulin A
(↓59-72%) was noted in the males at Weeks 13 (NS), 26, and 52. 
Absolute and relative spleen (↓ 14-76%) and thymus (↓ 47-64%)
weights were decreased.  Spleen diminished in size was noted in 2/4
males and 3/4 females, and thymus diminished in size was noted in 2/4
males and 3/4 females, both compared to 0 controls.  The following
lesions were observed in 0 controls except where noted.  Marked to
severe involution of the thymus was observed in 4/4 males and females. 
Atrophy of the spleen was observed in 2/4 males and 3/4 females. 
Atrophy of the lymphatic tissue of the spleen was noted in the males at
a similar incidence to controls (both 1/4); however, the severity was
increased in the treated dogs (average severity 3.0 vs. 1.0 in
controls).  Atrophy of the lymphatic tissue of the spleen was also noted
in 4/4 females.  Atrophy of the lymphatic tissue in the lymph nodes was
observed in the mesenteric nodes in 4/4 males and females and in the
iliac lymph nodes in 2/4 males and 3/4 females.  Atrophy of the
Peyer’s patches in the ileum was noted in 4/4 males and females. 
Atrophy of the bone marrow was noted in 2/4 males and 3/4 females.

Multiple microscopic finding indicative of immunotoxicity (detailed
above) were noted at 1.0 mg/kg/day, particularly in the males.  At 5.0
mg/kg/day, increased severity of immunotoxicity and indications of
systemic toxicity (including mortality, clinical signs of toxicity,
overall body weight losses, decreased body weights and food and water
consumption, increased blood sedimentation rate, decreased nucleated
bone marrow cellularity, increased hepatic enzymes, increased liver
weight, decreased spleen and thymus weights, and gross pathological
findings in the liver, spleen, and thymus) were observed.

This study is classified as Unacceptable/Guideline (Not upgradeable) and
does not satisfy the guideline requirements for a chronic oral toxicity
study [OPPTS 870.4100] in dogs.  The analyses of the dosing solutions
were inadequate, and the presence of tin in the urine of the control
dogs suggests exposure of the control group to the test compound.

4.6 	Carcinogenicity

Adequacy of database for Carcinogenicity: The database for
carcinogenecity of tributyltin oxide is considered complete and adequate
for regulatory purposes.  There are no carcinogenicity studies submitted
for tributyltin maleate or tributyltin benzoate.  

870.4200 	Carcinogenicity

In a carcinogenicity study (MRID 42265001), bis(tri-n-butyltin) oxide
(97.1% pure, lot KYRDO-064M) was administered to 50 CD-1 mice sex/dose
in diet at dose levels of 0, 5, 25, or 50 ppm (equivalent to 0.7, 3.7,
and 7.7 mg/kg/day for low-, mid-, and high-dose males, respectively, and
0.9, 4.8, and 9.2 mg/kg/day for low-, mid-, and high-dose females,
respectively) for 18 months. 

Toxicity was observed at the low dose, with signs including decreased
survivorship in males and increased body weight gain in females.  At 25
and 50 ppm, signs of toxicity included decreased survivorship, increased
body weights and body weight gain, decreased food consumption (high-dose
females only), increased absolute and relative liver weight (high-dose
females only), and an increased incidence of severe renal amyloidosis. 
The LOAEL is 0.7 mg/kg/day for males and 0.9 mg/kg/day for females based
on increased mortality and increased body weight gain.  The NOAEL is not
established (less than 5 ppm; 0.7/0.9 mg/kg/day for males/females).

Under the conditions of this study, there were no treatment-related
increases in tumor incidence in treated animals when compared to
controls.  Dosing is considered adequate based on toxic effects (i.e.,
decreased survivorship, increased body weight and body weight gain,
decreased food consumption, increased absolute and relative liver
weight, and an increased incidence of severe renal amyloidosis)
observed.  

This carcinogenicity study is classified as Acceptable/Guideline and
satisfies the guideline requirement for a carcinogenicity study [OPPTS
870.4200; OECD 451] in mice. 

870.4300 	Combined Chronic Toxicity/Carcinogenicity

In a combined chronic toxicity/carcinogenicity study (MRID 40623201), 50
Wistar rats/sex/dose were exposed to bis (tributyltin) oxide (95.3%
a.i.; Lot No.: CH 356) for up to 106 weeks in the diet at concentrations
of 0, 0.5, 5, or 50 ppm (approximately equivalent to 0, 0.025, 0.25, and
2.5 mg/kg bw/day based on a conversion factor of 1 ppm = 0.05 mg/kg
bw/day).  Additionally, 10 rats/sex/dose were treated similarly for up
to 52 weeks.  Only 8-10 rats/sex/dose were examined for non-neoplastic
lesions and usually only the control and 50 ppm groups were examined
microscopically.

No adverse, treatment-related effect was observed on food consumption.  

At 50 ppm, systemic toxicity was observed.  Mortality was increased in
both sexes and began to be apparent after Week 94.  Survival was
approximately 40% treated vs 70% controls in males and 52% treated vs
74% controls in females.  Increased incidences of the following clinical
signs were observed: emaciation, ataxia, and crusty nostrils in both
sexes; and depression in males.  Additionally, posterior paresis was
increased in the males, although this effect was not clearly related to
dose.  Decreased body weights were observed in males during Weeks 67-95
and females during Weeks 83-95.  Terminal (Week 106) body weights were
decreased in both sexes. Body weight gains were decreased during the
interval of Weeks 51-95 by 314% in males (a weight loss) and by 56% in
females.  Overall body weight gains (Weeks 0-106) were decreased by 16%
in males and 12% in females.  Increased water consumption was observed
in males generally from Weeks 24-92.  

At 50 ppm, toxicity was also noted in the kidney, thyroid, adrenal
gland, and pituitary gland as discussed below.

The clearest effect of the compound in this study was nephrotoxicity. 
Decreased serum creatinine levels were observed in both sexes during
Months 12 and 24, and increased blood urea level was noted in the
females on Months 3 and 24.  In the females, the following differences
were observed during urinalysis:  (i) increased urinary volume
throughout the study; (ii) increased creatinine clearance at Month 3;
and (iii) decreased osmolality throughout the study.  At Week 52,
increased incidences of hydronephrosis were observed in both sexes.  At
Week 106, increased absolute and relative to body kidney weights were
observed in both sexes.  At Week 106 in the kidney, increased incidences
greenish pigments and granular surface in both sexes were observed.  At
Week 106, increased incidence and severity of slight to marked renal
vacuolation/ pigmentation was noted in the males (not clearly
dose-related) and females (dose-related).  

At Months 12 and 24, decreases were noted in free thyroxin in males and
free thyroxin/thyroxin in both sexes.  Decreased absolute and relative
thyroid gland weights were noted in males at Week 52, but were similar
to controls at Week 106. Decreased absolute and relative thyroid weights
were observed in females at Week 106.  Decreased epithelial height in
the thyroid was noted at Weeks 52 and 106 in both sexes, but was not
clearly dose-related in males at Week 106.  

At Week 52, increased absolute adrenal gland weights were observed in
both sexes and increased relative to body adrenal weights were noted in
females.  However, these weights were similar to controls at Week 106. 
Enlarged adrenals were noted in both sexes at Week 106.

Luteinizing hormone was decreased in the females at Months 12 and 24. 
At Week 52, increased incidences of pituitary gland cysts were observed
in females.  At Week 106, increased absolute and relative to body
pituitary weights were observed in both sexes.  An increased incidence
of gross pituitary hemorrhagic tumors was observed in males at Week 106.
 

Although depression of thymus-dependent immunological response is a
known effect of the test compound, there was limited evidence of this
effect in this study.  At 50 ppm, decreased lymphocytes were noted in
both sexes throughout the study, and statistically significant decreases
were observed in females at Month 12 (↓ 20%) and in both sexes at
Month 24 (↓ 27-28%). Serum IgG levels were decreased in the 50 ppm
females throughout the study (↓ 25-38%).  No effect was noted on the
thymus.  Without further corroborating evidence of an immunosuppressant
effect, these findings were not considered adverse.  Serum IgM levels
were increased throughout treatment in both sexes.

Other organs may have been affected at 50 ppm, but the evidence was
unclear.  Increased incidences of atrophy/calcification of the testes,
atrophy of accessory male glands, and hypertrophy/inflammation of the
accessory male glands were observed.  Histological data did not
corroborate a treatment-related effect.  A concurrently submitted
reproductive toxicity study (MRID 41693801) did not corroborate organ
toxicity at doses up to 50 ppm.  Tumor-like lesions were noted in the
uterus in 50 ppm females.  However, histopathological analysis did not
corroborate organ toxicity or a neoplastic effect.

At 5 ppm, some indicators of organ toxicity were noted that became more
evident at 50 ppm; however, these effects were not considered adverse
due to the type of abnormality, the lack of magnitude of difference from
control, and the general lack of sufficient findings to be considered
clearly adverse to the organ system/animal.  The following findings were
noted at 5 ppm: (i) increased water consumption in males generally from
Weeks 1-88 (↑ 7-29%); (ii) increased urinary volume in the females at
Month 24 (↑ 45%); (iii) increased incidences of greenish pigments in
kidneys (24% treated vs 10% controls) and granular surface on kidneys
(48% treated vs 34% controls) in males; and (iv) decreased free
thyroxin/thyroxin in females (↓ 12%).

The LOAEL is 50 ppm (approximately equivalent to 2.5 mg/kg bw/day),
based on increased mortality, systemic toxicity (ataxia, emaciation and
decreased body weight/body weight gain in males and females) and organ
toxicity (kidney, thyroid, adrenal glands, and pituitary).  The NOAEL is
5 ppm (approximately equivalent to 0.25 mg/kg bw/day).

At 50 ppm, increased (p≤0.01) incidences of the following tumors were
observed (% in treated vs controls) at Week 106: (i) anterior pituitary
tumor in males (86% vs 68%) and females (70% vs 44%); (ii) combined
malignant and benign pheochromocytomas in males (66% vs 32%) and females
(68% vs 6%); and (iii) parathyroid adenoma in males (12% vs 0%).  The
incidence of malignant pheochromocytomas was also increased in the males
(12% vs 6%) and females (8% vs 0%).  An increased (not statistically
significant) incidence of granular tumors in the brain was observed in
the 50 ppm males (6% vs 0%).  Evidence from immunohistochemistry
suggested that the pituitary tumors were prolactinomas; these tumors
were often fatal.  The incidences of other neoplasias in the treated
groups were similar to controls.  The conclusion documented by the EPA
in support of summary information on the integrated risk information
system was that: “Although the data on tumor occurrence in this study
are questionable, the tumors in these endocrine organs are of unknown
biological significance for a human health risk assessment.  The results
are also inconclusive because of the increased mortality at the high
dose and because dose spacing reduces the statistical power.”

At the doses tested, there was a treatment-related increase in tumor
incidence in the pituitary, adrenal gland, and parathyroid when compared
to concurrent controls.  However, these results are considered
inconclusive, and the tumors in these endocrine glands are of unknown
biological significance for human health risk assessment.  Dosing was
considered adequate based on toxicity observed systemically and toxicity
in the kidney, thyroid, adrenal glands, and pituitaries.

This study is classified as Acceptable/Guideline and satisfies the
guideline requirements for an oral chronic/carcinogenicity study [OPPTS
870.4300, OECD 453] in rats. 

Mutagenicity

Adequacy of database for Mutagenicity: The tributyltin-containing
chemical database for mutagenicity is considered incomplete (Table 4). 
Acceptable data submitted to the Agency shows that tributyltin oxide did
not induce positive responses in a reverse gene mutation Salmonella
typhimurium assay in the presence or absence of S9 metabolic activation
and was not mutagenic in an in vitro human lymphocyte chromosome
aberration test.  Similarly, tributyltin benzoate was non-mutagenic in
an in vitro CHO/HGPRT gene mutation assay and an in vivo cytogenetics
micronucleus assay in mice.  An Unscheduled DNA Synthesis assay in
cultured rat hepatocytes conducted with tributyltin benzoate was
classified unacceptable and this study does not satisfy the guideline
requirement for a mutagenicity study.    

Table 4.  Summary of Mutagenicity Studies for Technical (97.1 –
103.0%a.i.) Tributyltin Compounds

Guideline No./

Study Type	MRID Number/

Citation	Dosing and Animal Information	Results

870.5100

Bacterial Reverse Mutation Test

	MRID 42170001

Lang, R. (1986) Evaluation in the Ames Salmonella/Microsome Mutagenicity
Test: ZK 21.955: Lab project Number: TX6106. Unpublished study prepared
by Schering Ag. 29 p.

Tributyltin Oxide

	Salmonella typhimurium TA 1535, TA 100, TA1537, TA 1538, and TA 98

Purity: 97.1 ± 3.4 % and 103 ± 5.6% 

0.0001, 0.00025, 0.0005, 0.001, 0.0025, 0.005, 0.01, 0.02 and 0.03
µl/plate (without metabolic activation)

0, 0.001, 0.0025, 0.005, 0.01, 0.02, 0.03, 0.04 and 0.05, 0.08 or 0.15
µl/plate (with metabolic activation)	Negative

No evidence of mutagenic activity in the presence or absence of
metabolic activation (S9 from rat liver) in strains of S. typhimurium.

Acceptable/Guideline

870.5375

In vitro Chromosome Aberration Test (Rat)	MRID 40253005

Brunneman, A. (1986) Evaluation of the Clastogenic Potential in the
Human Lymphocyte Test: ZK 21.955: Lab. Proj. ID IC 4/86. Un- published
study prepared by Schering AG. 23 p.

Tributyltin Oxide

	human lymphocyte

Purity 97.1 ± 3.4% and 

103.0 ± 5.6%

0.0025, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5 and 1.0 µg/ml (without
activation)

0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 2.5 and 5.0 µg/ml with
metabolic activation	Negative

There was no evidence of chromosome aberration induced when compared
with the negative control. However, at higher concentrations reduction
in the mitotic index was observed.

Acceptable/Guideline

870.5300

In vitro Mammalian Cell Gene Mutation Test 	MRID 42412501

Bakke, J. (1991) Evaluation of Cotin 310 in the CHO/HGPRT Gene Mutation
Assay: Final Report: Lab Project Number: LSC-2112-200: 2112-G200-91.
Unpublished study prepared by SRI International. 22 p.

Tributyltin Benzoate

	Chinese Hamster ovary - K1 cells

Purity: 97.1 ± 3.4 % and 103 ± 5.6 % 

0.143, 0.179, 0.224, 0.28 and 0.35 µg/ml (without activation)

0.33, 0.41, 0.51, 0.64, 0.80 µg/ml with activation	Negative

There was no evidence of a concentration-related, positive response that
induced mutant colonies.

Acceptable/Guideline

870.5395

Mammalian Erythrocyte Micronucleus Test	MRID 42412502

O'Loughlin, K. (1991) Bone Marrow Erythrocyte Micronucleus Assay of
Cotin 310 in Swiss Webster Mice: Lab Project Number: 2112-C100-91.
Unpublished study prepared by SRI International. 41 p.

MRID 42966201

Mahoney, D. (1992) Analysis of Tributylin Benzoate: Product Chemistry:
Chemical Purity:

Lab Project Number: 239-39. Unpublished study prepared by Huls America
Inc. 9 p.

Tributyltin Benzoate	Swiss Webster Mice (five males and  five females)

Purity 97.1 ± 3.4% and 

103.0 ± 5.6%

0, 25, 50, 100, 200, or 400 mg/kg/day (Preliminary/Range finding assay)

0, 50, 100 or 200 mg/kg/day (Micronucleus assay)	Negative

There was not a significant increase in the frequency of micronucleated
polychromatic erythrocytes in bone marrow after any treatment time.

Acceptable/Guideline

870.5550

Unscheduled DNA synthesis (Rat)

	MRID 42412503

Bakke, J. (1990) Evaluation of the Potential of Cotin 310 to Induce
Unscheduled DNA Synthesis in the In vitro Hepatocyte DNA Repair Assay
Using the Male F-344 Rat: Final Report: Lab Project Number: LSC-1482:
1482-VO1-90. Unpublished study prepared by SRI International. 21 p.

MRID 42412504, 42966203 (Supplemental Amendment)

Bakke, J. (1992) Evaluation of the Potential of Cotin 310 to Induce
Unscheduled DNA Synthesis in the In vitro Hepatocyte DNA Repair Assay
Using the Male F-344 Rat: Final Report, Amendment One: Lab Project
Number: LSC 1482: 1482-VO1-90. Unpublished study prepared by SRI
International. 7 p.

MRID 42966201

Mahoney, D. (1992) Analysis of Tributylin Benzoate: Product Chemistry:
Chemical Purity: Lab Project Number: 239-39. Unpublished study prepared
by Huls America Inc. 9 p.

Tributyltin Benzoate	Rat Hepatocyte cells

Purity 97.1 ± 3.4% and 

103.0 ± 5.6%

First UDS assay:

1, 5, 10, 25, 50, 100, 250, 500, 750 and 1000 µg/ml (did not satisfy
the minimum required concentrations for evaluating Unscheduled DNA
synthesis)

Second and third assay: 0.005, 0.01, 0.05, 0.1, 0.5, 1.0, 2.5, 5, 10 and
25 µg/ml were tested. Mechanical failure of the incubator prevented
completion of these experiments.

Fourth (Preliminary) and Fifth (Repeat) assay:

0.005, 0.01, 0.05, 0.1, 0.5, 1.0, 2.5, 5, 10 and 25 µg/ml were tested. 

 	No conclusion can be drawn for the mutagenic potential of Cotin 310
(tributyltin benzoate) when tested iin an in vitro rat hepatocyte DNA
repair assay.

Unacceptable/Guideline



4.8	Neurotoxicity

There was no evidence of neurotoxicity in any of the mammalian toxicity
studies submitted to the Agency for tributyltin oxide, tributyltin
maleate or tributyltin benzoate. The World Health Organization (WHO) has
also evaluated the potential neurotoxicity of various organotin
compounds in published literature studies and their assessment cited
below (excerpt from CICAD 14, Tributyltin Oxide, WHO, Geneva, 1999):

“Triethyltin and trimethyltin compounds have been shown to cause
severe neurotoxicity (for a summary, see Boyer, 1989). Triethyltin
causes interstitial oedema throughout the white matter in the spinal
cord and various regions of the brain; less marked damage occurs in the
peripheral nervous system. Trimethyltin also causes severe and permanent
damage to the central nervous system. In this case, however, the effect
is neuronal necrosis, rather than oedema. TBTO, in contrast, causes no
severe neurological signs or morphological or histopathological changes
in brain tissue. In a 4-week study, rats fed a dietary concentration of
320 mg/kg (equivalent to 30 mg/kg body weight per day) exhibited ptosis
or enophthalmia and slight ataxia Krajnc et al., 1984). One chronic
study in dogs (Schuh, 1992) also gave a slight suggestion of
neurotoxicity (atactic gait and apathy). As noted above, however, this
study is significantly flawed. Crofton et al. (1989) measured brain
weight and motor activity in developmental studies. There was some
suggestion of neurotoxicity (based on decreased brain weight in pups) at
exposures in excess of 10 mg/kg body weight per day, but no reported
effects at 5 mg/kg body weight per day. Organotin compounds, including
tributyltin, have recently been shown to induce apoptosis in
immortalized neuronal cell lines (Thompson et al., 1996) and in
pheochromocytoma PC12 cells (Viviani et al., 1995). Although TBTO
induces apoptosis in neural cells in vitro, it does not cause
neurotoxicity in whole animals. Although the potential for neurotoxicity
has not been completely investigated with focused studies, there is no
suggestion that neurotoxicity is likely a critical or co-critical
effect.”

4.9 	Metabolism and Pharmacokinetics

Adequacy of database for Metabolism and Pharmacokinetics: The existing
toxicity database for tributyltin oxide contains two metabolism studies;
one conducted in rats and the second conducted in mice.  Most of the
113Sn-labeled tributyltin oxide administered to rats was eliminated via
the bile and feces, with a very small moiety of tributyltin oxide
eliminated renally.  Metabolites were not identified in this study,
however, the literature on biotransformation of tributyltin oxide
suggest that this compound may by dealkylated to the di- and mono-tin
moieties in addition to degradation to inorganic tin.  In mice, high
levels of 14C- tributyltin oxide were identified in the feces,
suggesting this as the major route of excretion.  Fat and Lung tissue
exhibited high retention of tributyltin oxide.  Although both studies do
not satisfy the guideline requirement for a metabolism study [OPPTS
870.7485], they contain useful information that is adequate for
regulatory purposes.

According to the World Heath Organization (CICAD 14, Tributyltin Oxide,
WHO, Geneva, 1999), “Little definitive information is available on the
pharmacokinetics of tributyltin oxide (TBTO).  TBTO is absorbed from the
gut (20-50%, depending on the vehicle) and via the skin of mammals
(approximately 10%).  Other data suggest absorption in the 1-5% range
via the skin.  TBTO can be transferred across the blood-brain barrier
and from the placenta to the fetus.  Following 14 days of oral
administration, steady-state levels in tissue are reached after 3-4
weeks.  Absorbed material is rapidly and widely distributed among
tissues (principally the liver and kidney).  Metabolism in mammals is
rapid; metabolites are detectable in the blood within 3 h of TBTO
administration.  The principal metabolite appears to be the
hydroxylbutyl compound, which is unstable and rapidly splits to form the
dibutyl derivative and butanol.  In vitro studies, it has been shown
that TBTO is a substrate for mixed function oxidases, but these enzymes
are inhibited by high concentrations of TBTO.  The rate of TBTO loss
differs with different tissues.  TBTO and its metabolites are eliminated
principally via the bile.  The calculated half-time of elimination of
TBTO residues in mice is 29 days.”

870.7485	Metabolism and Pharmacokinetics - Rats

In a metabolism study (MRID 40253002), 113Sn-labelled tributyltin oxide
(>99%a.i., batch ES 1873-3) was administered to 3 or 6 female Wistar-Han
(SPF) rats/dose methods. An intravenous (1 ethanol: 1 water) or gavage
(peanut oil) single dose was administered at levels of 1 mg/kg or 25
mg/kg 113Sn-tributyltin oxide, respectively.  Blood was collected from 3
rats/dose method at 5, 15, 30, and 60 min and 2, 4, 6, 8, 12, 18, 24,
36, 48, and 72 hours after dosing.  Plasma and whole blood were analyzed
for radioactivity and metabolites.  In an excretion study where 3-6
rats/dose method were kept in metabolism cages, excreta were collected
at 24, 48, and 72 hours post dosing, and were analyzed for radioactivity
and metabolites.  The whole carcass and gastrointestinal tracts of 3
rats/dose method were analyzed separately 48 hours after dosing for
radioactivity; the gastrointestinal tracts of 3 rats/dose method were
analyzed for radioactivity 72 after dosing. 

Plasma.  In the intravenous dose group at five minutes post injection,
853±138 ng/mL of 113Sn-TBTO equivalent was found in total plasma and
784±145 ng/mL of the parent compound equaling 92% was found.  After 10
minutes, the first of three peaks in concentration were observed, with
the other peaks at 4.9 hours and 19.7 hours. By 48 hours, the
concentration was down to 4.3±0.8 ng/mL and below the limit of
detection by 72 hours.  In the gavage dose group, the time point of
measurement of the peak plasma level was 24 hours, with the
concentration of that peak being 0.54±0.07 µg/mL (0.066% of
dose/plasma volume).  The absorption of the test substance in plasma had
a half-life of 9.5 hours.  The maximum active ingredient concentration
of total reactivity was measured 8 hours post administration, with a
value of 156 ng/mL, and levels were below detection by 72 hours post
application.

Blood.  Overall, higher concentrations of 113Sn-TBTO were found in the
blood than the plasma.  The concentrations in the blood were on average
2-3 times higher, with the largest difference around 24 hours post
administration for the intravenous test group and 2 hours post
administration for the gavage test group. 

Urine and Feces.  In the intravenous test group, the half-life of the
test substance in urine was approximately 8 hours.  Up to 24 hours post
administration, 9% of the dose had been eliminated via the urine and 31%
with feces.  By the end of the study (72 hours post administration), 14%
of the dose was found in the urine and 52% in feces.  The ratio of
elimination of urine and feces was about 1:4, with the balance of TBTO
and equivalents eliminated by both routes reaching 66% of the dose by 72
hours, when the plasma values were below detection.  In the gavage test
group, up to 24 hours post administration, 4% of the dose had been
eliminated via urine and 1% with feces.  By 72 hours post
administration, 11% of the dose was found in the urine and 62% of the
dose in feces.  The ratio of elimination of urine and feces was about
1:6, with the balance of the TBTO and equivalents eliminated by both
routes reaching 72% of the dose.  The elimination values did not fit a
first order kinetics curve, so a half-life could not be determined.  

Remaining Body.  At 48 hours post administration, an average of 27% of
the dose equivalents had been localized in the rat’s bodies (excluding
the GI tract) via intravenous administration and 14% of the dose
equivalents via gavage administration.  Similarly, 3% of the dose
equivalents were found in the GI tract via intravenous administration
and 57% via gavage administration.  These numbers were decreased to 9%
and 4%, for intravenous and gavage administration, respectively, by 72
hours post administration. 

Metabolite(s).  The concentration of the parent compound and metabolites
in plasma and urine were determined.  The HPLC analysis showed that 5
extractable metabolites were observed along with the parent; intravenous
administration had 2 metabolites besides the parent compound whereas
gavage administration had 5 metabolites.  Extraction efficiency in the
plasma and urine (pH 8) was 90%.  

 

This metabolism study is classified as Acceptable/Non-guideline. 
Although it and does not satisfy the guideline requirement for a
metabolism study [OPPTS 870.7485, OECD 417] in rats, this study contains
useful information that is adequate for regulatory purposes.

870.7485 	Metabolism and Pharmacokinetics – Mice

In a metabolism study (MRID 01246480), [14C] bis (tri-n-butyltin) oxide
(TBTO) was administered to 5 female COBS albino mice/dose/time period in
drinking water at nominal dose levels of 0, 0.51, 3.75, or 18.5 ppm.  In
one experiment, mice were administered 14C-TBTO in drinking water for up
to thirty days; five animals/dose were sacrificed after each time point:
5, 10, 15, 20, 25, or 30 days.  In a second experiment, animals were
administered 14C-TBTO in drinking water for 31 days.  At the end of the
test period, 5 animals/dose were sacrificed, while the remaining mice
were placed on non-14C-TBTO water for an additional 15 days before
sacrifice.  Feces and urine were collected on a daily basis from the
mice in Experiment 2, and organs were excised from all animals for
analysis.

Concentrations of 14C-TBTO were greatest in the kidneys, fat, liver, and
spleen. Concentrations were found to be proportional to administered
dose except in the kidney.  Fifteen days following treatment, a marked
decrease in 14C-TBTO levels was observed in the liver and kidney, while
the fat and lung tissue exhibited relatively high retention.  Feces were
found to possess high levels of 14C-TBTO, suggesting this is the major
route of excretion.  

This metabolism study is classified as Acceptable/Non-guideline. 
Although this study does not satisfy the guideline requirement for a
metabolism study [OPPTS 870.7485, OECD 417] in rodents, when reviewed
together with the metabolism study conducted in rats (MRID  40253002),
it provides information that identifies the major route of excretion for
tributyltin oxide as the feces.

5.0 	Dermal Penetration

There is no acceptable guideline dermal absorption study available in
the tributyltin toxicity database. The existing dermal absorption study
conducted in baboons with tributyltin oxide was classified unacceptable
due to less than 50% recovery of the administered dose.  In addition,
there are no adequate dermal toxicity studies.  A submitted dermal
range-finding study conducted in rats with tributyltin benzoate does not
fulfill the guideline requirement; however, it does serve to establish a
maximum tolerable dose (250 mg/kg/day) for a guideline dermal toxicity
study.  Typically, a default dermal adsorption factor of 100 % would be
appropriate for risk assessment purposes in the absence of data;
however, the Agency has estimated that 10-15% of the labeled compound
reaches the systemic circulation.  Therefore, a 15% dermal absorption
for route-to-route extrapolation is appropriate for assessing the risk
to humans from dermal exposure of tributyltin oxide.

6.0 	Immunotoxicity 

Non-Guideline	18-Month Immunotoxicity (oral) Study - Rat	

Published Literature: Vos, J.G., A. DeKlerk, E.I. Krajnc, V. Van
Loveren, and J. Rozing. 1990. Immunotoxicity of bis(tri-n-butyltin)oxide
in the rat: Effects on thymus- dependent immunity and on nonspecific
resistance following long-term exposure in young versus aged rats.
Toxicol. Appl. Pharmacol. 105: 144-155 (Excerpt from EPA/IRIS Summary of
Tributyltin Oxide, 1997 follows):

 “Subchronic and chronic immunotoxicity studies were conducted in
which weanling SPF-derived Riv:TOX Wistar rats were fed
bis(tri-n-butyltin)oxide [tributyltin oxide (TBTO), purity 95.3%] in
concentrations of 0, 0.5, 5.0 or 50 ppm. Male rats (females not tested)
were evaluated following exposure to TBTO for up to 18 months (Vos et
al., 1990; Krajnc et al., 1987). The authors reported the 5 ppm dietary
concentration to be equivalent to a dose of 0.25 mg/kg-day, indicating
that estimated test doses were 0.025, 0.25 and 2.5 mg/kg-day. Body
weight, absolute thymus weight and absolute spleen weight were measured
in groups of 18, 12 and 12 rats, respectively, following exposure for
4.5 months. Immunologic function studies for specific and nonspecific
resistance were performed in 9-12 rats/group after 4-6 or 15-17 months
of exposure. Antigen-specific functional assays evaluated IgM and IgG
responses to sheep red blood cells (immunized after 16 months), IgM and
IgG responses to ovalbumin and delayed-type hypersensitivity (24-, 48-
and 72- hour) responses to ovalbumin and mycobacterium tuberculosis
(immunized after 6 or 15 months exposure), and resistance to oral
infection by Trichinella spiralis larvae (infected after 5.5 or 16.5
months). Nonspecific resistance was assessed by splenic clearance of
i.v. injected Listeria monocytogenes bacteria (after 5 or 17 months
exposure), and natural cell-mediated cytotoxicity of spleen cells (after
4.5 or 16 months exposure) and peritoneal cells (after 4.5 months
exposure only) using a 4-hour 51Cr-release assay with YAC-lymphoma
target cells. Nonspecific endpoints included the numbers of viable
nucleated thymus and spleen cells and responses of thymus and spleen
cells to T-cell and/or B-cell mitogens (phytohemagglutinin, concanavalin
A, pokeweed mitogen and/or E. coli lipopolysaccharide) after exposure
for 4.5 months (thymus and spleen) or 16 months (spleen only) and
numbers of viable nucleated mesenteric lymph node cells with cell
surface marker analysis (after 6 and 18 months exposure; low-dose group
not tested in this assay). 

No significant effects were observed in the IgM or IgG responses to
sheep red blood cells, the IgM or IgG responses to Trichinella spiralis,
the IgM or IgG responses to ovalbumin or the delayed-type
hypersensitivity responses to ovalbumin and mycobacterium tuberculosis. 

Thymus weight was significantly reduced in the high-dose group (17%
lower than controls, p < 0.05), although the response of thymocytes to
T-cell mitogens was unaltered. No significant alterations in spleen
weight, response of spleen cells to T- and B-cell mitogens or body
weight were found at any dose. Statistically significant changes
occurred in the percentage of mesenteric lymph node T-lymphocytes in the
high-dose group (20% lower than controls after 18 months exposure) and
B-lymphocytes in the mid-dose group (60% higher than controls after 18
months) and in the high-dose group (48% higher than controls after 18
months); however, the absolute number of T- lymphocytes and
B-lymphocytes per lymph node were not altered significantly. The
low-dose group was not tested with these assays. The B-cell increase was
an increase in the percent of B-cells, but the interpretation of these
data is equivocal because they are counter-intuitive when viewed in
context with the other effects, especially the IgE titers. 

In vivo clearance of injected L. monocytogenes was impaired in rats
exposed to the high dose for 17 months, as shown by the approximately
seven- fold increased number of viable bacteria per spleen, indicating
that macrophage function was reduced. Resistance to infection by T.
spiralis was suppressed in rats exposed to the mid or high dose, as
shown by significantly reduced serum IgE titers (50 and 47% lower than
controls after 16.5 months exposure), increased numbers of larvae in
muscle 42 days after infection (56 and 306% higher than controls after
16.5 months), and moderately reduced inflammatory reaction around cysts
in parasitized musculature (qualitative assessment only). 

There was no significant reduction in the activity of natural killer
cells isolated from the peritoneum following exposure of weanling or
aged (1-year old) rats to TBTO for 4.5 months. Also, there was no
significant reduction in the activity of natural killer cells isolated
from the spleen following exposure of weanling rats for 4.5 months. In
contrast, the activity of natural killer cells isolated from the spleen
was suppressed when weanling rats were exposed to all doses of TBTO for
16 months (31, 25 and 36% lower than controls, respectively, at an
effector to target cell ratio of 100, and 32, 18 and 30% lower,
respectively, at an effector to target cell ratio of 50). Based on these
data, the effect did not progress significantly with dose. The authors
considered these data equivocal in this experiment. Because there was no
clear treatment-related effect, EPA will not use the suppression of
natural killer cell activity from this study to estimate the reference
dose. 

Essentially identical results on the immune system were observed
following 4.5 or 16.5 months of exposure. Based on the depression of IgE
titers and the increase in T. spiralis larvae in muscle following 16.5
months of exposure, the LOAEL for immunotoxicity is 0.25 mg/kg-day (5
ppm diet). The NOAEL is 0.025 mg/kg-day (0.5 ppm diet).”  [Krajnc et
al., 1987; Vos et al., 1990]	

Additional Published Literature for Immunotoxicity (Excerpt from CICAD
14, Tributyltin Oxide, WHO, Geneva, 1999):

“A large number of well-conducted studies have shown that TBTO causes
depression of immune functions dependent on the thymus.”  The chronic
study conducted by Vos et al. (1990) shows effects on thymus dependent
immune responses at a dose lower than that at which any other toxic
effects have been observed. This study also establishes that weanling
animals are more sensitive than adults to the effects of TBTO. For
example, following subchronic exposure, the LOAEL in weanling rats was
0.25 mg/kg body weight per day, whereas the LOAEL in aged rats was 2.5
mg/kg body weight per day. The NOAELs were 0.025 and 0.25 mg/kg body
weight per day, respectively. Data from Buckiova et al. (1992) and
Smialowicz et al. (1989) also show that exposure of mice in utero and
exposure of rat pups prior to weaning cause effects at exposures lower
than those required for the same effects in adult animals.

Some recent studies suggest that the mechanism of the immunotoxic
effects is related to induction of apoptosis (programmed cell death)
within the thymus. Raffray & Cohen (1991) demonstrated that thymocytes
in culture showed cellular changes consistent with apoptosis at
concentrations of TBTO that did not affect cell viability. Raffray et
al. (1993) showed that these effects occur independently of a
requirement for protein synthesis and do not require fully conserved
energetics (i.e., the effects occur despite depression of ATP levels to
less than 20% of control values). Raffray & Cohen (1993) demonstrated a
correlation between reduction of thymus weight in animals given a single
oral dose of TBTO and evidence of apoptosis (increased DNA
fragmentation) in thymic cell isolates (principally thymocytes) isolated
from the animals during the period of thymic involution. These workers
also showed that dibutyltin, the major metabolite of tributyltin, is
less effective in inducing apoptosis in vitro, suggesting that the in
vivo toxicity is directly attributable to tributyltin.

A study comparing immunotoxic effects in preweanlings and adult rats
shows that some responses of the developing immune system are more
sensitive to TBTO (Smialowicz et al., 1989). Adult (9 weeks old) male
Fischer rats or pre-weanling (3–24 days old) rats were dosed by oral
gavage 3 times per week for a total of 10 doses. The adults were dosed
with 5, 10, or 20 mg/kg body weight per dose; the pre-weanlings were
dosed with 2.5, 5, or 10 mg/kg body weight per dose. Reductions in
mitogen responses were observed in adults at 10 and 20 mg/kg body weight
and in preweanlings at 5 and 10 mg/kg body weight. The mixed lymphocyte
reaction was suppressed in adults at 20 mg/kg body weight and in
pre-weanlings at 10 mg/kg body weight. Finally, natural killer cell
activity was suppressed only in pre-weanlings at 10 mg/kg body weight.
In this study, the lowest LOAEL is 5 mg/kg body weight per day, and the
lowest NOAEL is 2.5 mg/kg body weight per day.

Pregnant ICR mice were treated with TBTO in Tween 80: ethanol: saline
(1:2:97) by gavage at 0.1 mg/kg body weight per day on gestation days
4–17 or 11–17 (Buckiova et al., 1992). Humoral and cell-mediated
immune responses in offspring were assessed 4 and 8 weeks after birth.
At 0.1 mg/kg body weight per day, the only dose tested, effects in the
offspring included suppressed primary antibody responses to sheep red
blood cells, ovalbumin, and lipopolysaccharide and increased number of
leukocytes. Suppressed delayed type hypersensitivity to sheep red blood
cells and unspecified alterations in polyclonal proliferative responses
of thymocytes and splenocytes were also observed. The significance of
the LOAEL (0.1 mg/kg body weight per day), however, is unclear, because
a full publication of the results is not available.”

7.0 	Classification of Carcinogenic potential

There are no data in humans concerning development of cancer following
exposure to tributyltin oxide (TBTO) and other organotin chemicals. 
Cancer bioassays following oral exposure have been conducted in rats and
mice.  In the carcinogenicity study conducted in rats, increases in the
incidence of benign pituitary tumors, pheochromocytomas, and parathyroid
tumors were observed at the highest dose levels tested.  However, the
significance of these tumors which normally occur in this strain of rat
with variable incidence, is unclear.  Furthermore, the carcinogenicity
study in mice showed no increase in tumors at any site or dose tested. 
There are no structure-activity relationships suggesting that TBTO might
be a carcinogen.  Based on high spontaneous incidences of tumors in
Wistar rats, incidence variability in the treated groups, absence of a
dose-effect relationship and no evidence of genotoxicity in the battery
of mutagenicity tests, EPA has assigned TBTO to category D (U.S., 1987)
or to the “cannot be determined” category for carcinogenicity (U.S.
EPA, 1996).

8.0 	FQPA (Special Sensitivity) Considerations

Although the labeled uses for the tributyltin-containing chemicals have
no direct or indirect food exposures and no established food tolerances,
there remains the potential for special sensitivity to children from
residential exposure to these chemicals.  A Special Sensitivity factor
can be applied to a selected dose if there is evidence of increased
susceptibility to children from non-dietary exposures to pesticides.  

The developmental and prenatal database for tributyltin oxide and
tributyltin benzoate is considered complete and adequate for regulatory
purpose, whereas a prenatal developmental toxicity study conducted in
rats is required for tributyltin maleate to satisfy the bridging
requirement.  Results of the submitted studies show no evidence of pre-
or postnatal special sensitivity to the fetuses and offspring of rats or
rabbits; one developmental toxicity study in the rat conducted with
tributyltin oxide, one developmental toxicity study in the rat conducted
with tributyltin benzoate, and one reproduction toxicity study in rats
conducted with tributyltin oxide.

In contrast, there is some evidence in several published literature
studies that a child might be more sensitive to the toxic effects of
tributyltin oxide.  According to the EPA/IRIS, Toxicological Review of
Tributyltin Oxide (July, 1997), tributyltin oxide causes depression of
immune functions dependent on the thymus, particularly in young animals.
 This is a critical effect that occurs at doses lower than those causing
other toxicities.  “For example, Smialowicz et al. (1989) showed that
immunotoxic effects were observed when weanling rats were dosed for 4.5
or 16.5 months.  A companion study (Vos, et al., 1990) showed that these
effects were absent or occurred at a higher dose when adult rats (1 year
old) were dosed for 5 months.”  Based on the numerous published
literature studies on the immunotoxicity of tributyltin oxide,
application of this factor would provide adequate protection to the most
sensitive population, children. Therefore, a special children’s
sensitivity factor of 10x was applied to the tributyltin oxide
dose/endpoint (BMD10 = 0.03 mg/kg/day) selected for all residential
exposure scenarios. 

9.0	Endocrine Disruptor Effects  XE "IV. Risk Management,
Reregistration, and Tolerance Reassessment Decision:C. Regulatory
Position: 1. Food Quality Protection Act Findings: d. Endocrine
Disruptor Effects"  

EPA is required under the FFDCA, as amended by FQPA, to develop a
screening program to determine whether certain substances (including all
pesticide active and other ingredients) “may have an effect in humans
that is similar to an effect produced by a naturally occurring estrogen,
or other endocrine effects as the Administrator may designate.” 
Following recommendations of its Endocrine Disruptor Screening and
Testing Advisory Committee (EDSTAC), EPA determined that there was a
scientific basis for including, as part of the program, the androgen and
thyroid hormone systems, in addition to the estrogen hormone system. 
EPA also adopted EDSTAC’s recommendation that EPA include evaluations
of potential effects in wildlife.  For pesticides, EPA will use FIFRA
and, to the extent that effects in wildlife may help determine whether a
substance may have an effect in humans, FFDCA authority to require the
wildlife evaluations.  As the science develops and resources allow,
screening of additional hormone systems may be added to the Endocrine
Disruptor Screening Program (EDSP).  When the appropriate screening
and/or testing protocols being considered under the EDSP have been
developed, tributyltin oxide, tributyltin maleate and tributyltin
benzoate may be subject to additional screening and/or testing to better
characterize effects related to endocrine disruption.

  

10.0 	Toxicity Endpoint Selection 

Table 5: Summary of Toxicology Endpoint Selection tc \l1 "     VIII.
SUMMARY OF TOXICOLOGY ENDPOINT SELECTION  for Tributyltin Compounds

  SEQ CHAPTER \h \r 1 Exposure

Scenario	Dose Used in Risk Assessment

(mg/kg/day) 	Special Sensitivity*, UF, Target MOE, 

for Risk Assessment	Study and Toxicological Effects

Dietary Risk Assessments

Acute Dietary

(females 13-49 and general population)	No appropriate endpoints were
identified in the oral toxicity studies that represent a single dose
effect for the general population and females 13-49.  In addition, the
current use patterns for the tributyltin-containing chemicals do not
indicate the potential for direct or indirect dietary exposures. 
Therefore, an acute dietary risk assessment is not required. 

Chronic Dietary

(all populations)	BMD10 = 0.03 mg/kg/day based on immunosupression
(established by EPA/IRIS and used to estimate the oral RfD). 

	Special Sensitivity = 10

UF = 100 (10x inter-species extrapolation, 10x intra-species variation)

 

Chronic RfD (cRfD) =  0.00003 mg/kg/day

Although the current use patterns for the tributyltin-containing
chemicals do not indicate the potential for chronic dietary exposures,
this endpoint is selected for future reference.  A chronic dietary risk
assessment is not required at this time. 	Open Literature Study

Vos et al., (1990) Immunotoxicity of bis (tri-n-butyltin) oxide in the
rat: Effects on thymus-dependent immunity and on nonspecific resistance
following long-term exposure in young vs aged rats.  Toxicol. Appl.
Pharmacol.  105:144-155.

 

NOAEL = 0.025 mg/kg/day

LOAEL = 0.25 mg/kg/day based on immunotoxicity (depression of IgE titers
and increase in T. spiralis larvae in muscle) following 4 months and
16.5 months of exposure to Tributyl Tin Oxide.  (Review by EPA/IRIS,
1997). 

Non-Dietary Risk Assessments

Incidental Oral Short-Term 

(1-30 days) and Intermediate-Term  (1- 6 months) 

	BMD10 = 0.03 mg/kg/day based on immunosupression (established by
EPA/IRIS and used to estimate the oral RfD). 

	Special Sensitivity = 10

UF = 100 (10x inter-species extrapolation, 10x intra-species variation)

Target MOE res. = 1000	Open Literature Study

Vos et al., (1990) Immunotoxicity of bis (tri-n-butyltin) oxide in the
rat: Effects on thymus-dependent immunity and on nonspecific resistance
following long-term exposure in young vs aged rats.  Toxicol. Appl.
Pharmacol.  105:144-155.

 

NOAEL = 0.025 mg/kg/day

LOAEL = 0.25 mg/kg/day based on immunotoxicity (depression of IgE titers
and increase in T. spiralis larvae in muscle) following 4 months and
16.5 months of exposure to Tributyl Tin Oxide.  (Review by EPA/IRIS,
1997). 

Dermal

(all durations)	BMD10 = 0.03 mg/kg/day based on immunosupression
(established by EPA/IRIS and used to estimate the oral RfD). 

	Special Sensitivity = 10

UF = 100 (10x inter-species extrapolation, 10x intra-species variation)

Target MOE occ. = 100

Target MOE res. = 1000	Open Literature Study

Vos et al., (1990) Immunotoxicity of bis (tri-n-butyltin) oxide in the
rat: Effects on thymus-dependent immunity and on nonspecific resistance
following long-term exposure in young vs aged rats.  Toxicol. Appl.
Pharmacol.  105:144-155.

 

NOAEL = 0.025 mg/kg/day

LOAEL = 0.25 mg/kg/day based on immunotoxicity (depression of IgE titers
and increase in T. spiralis larvae in muscle) following 4 months and
16.5 months of exposure to Tributyl Tin Oxide.  (Review by EPA/IRIS,
1997). 

Inhalation

(all durations)	BMD10 = 0.03 mg/kg/day based on immunosupression
(established by EPA/IRIS and used to estimate the oral RfD). 

	Special Sensitivity = 10

UF = 100 (10x inter-species extrapolation, 10x intra-species variation)

Target MOE occ. = 100

Target MOE res. = 1000	Open Literature Study

Vos et al., (1990) Immunotoxicity of bis (tri-n-butyltin) oxide in the
rat: Effects on thymus-dependent immunity and on nonspecific resistance
following long-term exposure in young vs aged rats.  Toxicol. Appl.
Pharmacol.  105:144-155.

 

NOAEL = 0.025 mg/kg/day

LOAEL = 0.25 mg/kg/day based on immunotoxicity (depression of IgE titers
and increase in T. spiralis larvae in muscle) following 4 months and
16.5 months of exposure to Tributyl Tin Oxide.  (Review by EPA/IRIS,
1997). 

Dermal Absorption	Although there is no guideline dermal toxicity study
(range-finding study only) and no acceptable dermal absorption study
(75% recovery of 113Sn-tributyltin oxide), a 15% dermal absorption
factor for tributyltin oxide has been used (EPA/HED) for route-to-route
extrapolation.   

Carcinogenicity	EPA has assigned tributyltin oxide to category D (U.S.,
1987) or to the “cannot be determined” category for carcinogenicity
(U.S. EPA, 1996) based on high spontaneous incidences of tumors in
Wistar rats, incidence variability in the treated groups and absence of
a dose-effect relationship.

UF = uncertainty factor, NOAEL = no observed adverse effect level, LOAEL
= lowest observed adverse effect level, RfD = reference dose, MOE =
margin of exposure, NA = Not Applicable

*The Special Sensitivity factor is applied to a selected dose if there
is evidence of increased susceptibility to children from non-dietary
exposures to pesticides.  Several published literature studies show that
tributyltin oxide causes depression of immune functions dependent on the
thymus, particularly in young animals.  This is a critical effect that
occurs at doses lower than those causing other toxicities.  Therefore,
application of this factor provides adequate protection to the most
sensitive population, children.

11. 	Toxicity Profile Tables

Acute Toxicity Profile Table – (See Section 4, Acute Toxicity, Table
3)

Subchronic, Chronic and other Toxicity Profiles Table (Table 6)



Table 6.  Toxicity Profile for Tributyltin Oxide, Tributyltin Maleate
and Tributyltin Benzoate



Guideline No./

Study Type	

MRID No./

Citation	

Dosing and Animal Information	

Results

Range-finding Toxicity 

Non-guideline

Range-finding Study in Mice	MRID 41127001

Daly, I. (1989) A three month oral range-finding toxicity study in mice
with bis (tri-n-butyltin) oxide (TBTO). Bio/dynamics, Inc. (East
Millstone, New Jersey). Unpublished.

	CD-1® mice (male and female)

Purity 97.1 % a.i.

0, 4, 20, 80, or 200 ppm (equivalent to 0.7, 3.8, 15.1, or 36.9 mg/kg
bw/day in males; 1.0, 4.9, 17.9, or 46.9 mg/kg bw/day in females) for 13
weeks (diet).	NOAEL = 4 ppm (equivalent to 0.7 mg/kg bw/day in males;
1.0 mg/kg bw/day in females).

LOAEL = 20 ppm (equivalent to 3.8 mg/kg bw/day in males; 4.9 mg/kg
bw/day in females) based on mild hepatotoxicity and dermal irritation in
the ear observed in male mice. The  

Unacceptable/Non-guideline

Non-guideline

Range-finding Study in Dogs	MRID 41131001

Schweinfurth, H. (1987) Tributyltin oxide: Systemic toxicity study in
Beagle dogs with daily oral (intragastric) administration over a total
of 18-19 weeks.  Schering AG, Berlin, Germany.  Laboratory Project ID:
TX 87.054, August 5, 1987. Unpublished.

	Beagle dogs 

Purity >96.1% a.i.

0, 0.1, 0.5, or 2.5 mg/kg during Weeks 1-5; 0, 0.2, 1.0, or 5.0 mg/kg
during Weeks 6-10; and 0, 10, 1.0, or 5.0 mg/kg during Weeks 11-19.

	Exposure to Tributyltin oxide 

caused elevated alkaline 

phosphatase and slight changes 

in serum protein composition 

at 1.0 mg/kg/day after 13-14 

weeks of exposure.  While 

further biochemical changes 

and suspected effects on red 

and white blood cells were 

observed at 2.5 mg/kg/day 

after 5 weeks of exposure and 

at 5.0 mg/kg/day after 13-14 

weeks of exposure, there was 

no evidence of organ 

toxicity.  At 10 mg/kg/day, 

general toxic effects leading to 

weight loss and lymphotoxic 

effects, vacuolization of the 

liver cells and inhibition of 

follicle maturation in the 

ovaries (secondary effect) were 

observed.  A dose of 10 

mg/kg/day was considered to 

be too toxic of a dose for the 1-

year oral dog study; therefore, 

dose levels of 0.2, 1.0, and 5.0 

mg/kg/day were recommended 

for the 1-year study.

Unacceptable/Non-guideline

21-Day Dermal Toxicity 

870.3200 

21-Day Dermal Toxicity Study in Rats	MRID 43177201

Naas, Dennis J. (1990) Three week dermal range-finding study in rats
with Tributyltin Benzoate.  WIL Research Laboratories, Inc. (Ashland,
Ohio).  Laboratory study number WIL-159010, December 18, 1990.
Unpublished.	Dorsal skin of Crl:CD®BR rats

Purity not provided

0, 250, 500, 1000, or 2000 mg/kg/day for 6 hours/day

	The maximum tolerable dose for a subsequent 90-day toxicity study was
determined to be 250 mg/kg/day.

Unacceptable/Non-guideline

Prenatal Developmental Toxicity

870.3700 

Prenatal Developmental Toxicity Study 	MRID 00137158

Schroeder, R.; Hogan, G. (1981) A Teratology Study in Rats with
Tributyltin Oxide: Project No. 80-2497A. Final rept. (Unpublished study
received Jan 16, 1984 under 5204-1; prepared by Bio/dynamics, Inc.,
submitted by M & T Chemicals Inc., Rahway, NJ; CDL:252178-A).

MRID 92172016

Schroeder, R. (1992) M&T Chemicals, Inc. Phase 3 Reformat of MRID
00137158. A Teratology Study in Rats with Tributyltin Oxide: Project No.
80-2497A. Prepared by Bio/dynamics, Inc. 83 p.  	Female pregnant CD®
(Sprague-Dawley derived) rats

Purity not reported

0, 5, 9, 18 mg/kg/day (via gastric intubation (gavage)	Maternal Toxicity

NOAEL = 5 mg/kg/day 

LOAEL = could not be identified due to the lack of data.

Developmental Toxicity

NOAEL is less than 5 mg/kg/day LOAEL is equal to or less than 5
mg/kg/day based on increased incidences of ossification variations. 

Unacceptable/Upgradeable

870.3700 

Prenatal Developmental Toxicity Study	MRID 40141901

Nemec, M. (1987) (Tributyltin Oxide) - A Teratology Study in Rabbits
with TBTO: Laboratory Project ID: WIL-B0002. Unpublished study prepared
by Wil Research Laboratories, Inc. 210 p.

MRID 92172006

Stevens, A. (1990) M&T Chemicals, Inc. Phase 3 Summary of MRID 40141901.
A Teratology Study in Rabbits with TBTO: Project No. WIL-B0002. Prepared
by Wil Research Laboratories, Inc. 7 p.	New Zealand White female rabbits


Purity >95%

0, 0.2, 1.0, 2.5 mg/kg/day (oral gavage)	Maternal Toxicity

NOAEL = 1.0 mg/kg/day 

LOAEL = 2.5 mg/kg/day, based on increased incidence of abortion and
decreased mean maternal body weight gain.

Developmental Toxicity

NOAEL =  is equal to or greater than 2.5 mg/kg/day 

LOAEL=  is greater than 2.5 mg/kg/day (could not be established)  

Acceptable/Guideline

870.3700 

Prenatal Developmental Toxicity Study	MRID 42903101

Bryson, A. (1993) Tributyltin Benzoate: A Study of the Effect on
Pregnancy of the Rat: Lab Project Number: NDX/41: NDX 41/921129.
Unpublished study prepared by Huntingdon Research Centre Ltd. 106 p.

	Cr1: CD® (SD) BR VAF/Plus strain rats

Purity 97.1% a.i.

0, 1.0, 4.5 and 20.0 mg/kg/day (intragastric intubation)	Maternal
Toxicity

NOAEL = 1.0 mg/kg/day 

LOAEL = 4.5 mg/kg/day based on increased incidences of post-dose
salivation, wet coat, and impaired respiration. 

Developmental Toxicity 

NOAEL = 1.0 mg/kg/day 

LOAEL = 4.5 mg/kg/day, based on increased resorption, a dose related
increased in incidence of double outlet of right ventricle (and an
intraventricular septal defect) along with increased incidence of
extracervical ribs at this dosage.

Acceptable/Guideline

Reproduction and Fertility Rates

870.3800

Reproduction and Fertility effects 	MRID 41693801

Schroeder, R.E. (1990) A two generation reproductions study in rats with
bis (tri-n-butyltin) oxide.  Bio/dynamics, Inc., East Millstone, NJ. 
Laboratory Project No.:  88-3261, October 22, 1990.  Unpublished

	Sprague Dawley rats

Purity 97.1%

0, 0.5, 5, or 50 ppm for two successive generations (diet)	Parental
Toxicity

NOAEL = 5 ppm (equivalent to 0.33/0.39 mg/kg/day in males/females).

LOAEL = 50 ppm (equivalent to mg/kg/day 3.47/3.93 in males/females)
based on anogenital staining in the P dams; decreased body weights in
the F1 males and females during pre-mating and continuing in the F1
males during the mating and post mating periods; and decreased absolute
and relative thymus weights in the F1 males. 

Reproductive Toxicity

NOAEL = 50 ppm (equivalent to 3.47/3.93 mg/kg/day in males/females)

LOAEL = greater than 50 ppm (not established)

Offspring Toxicity

NOAEL = 5 ppm (equivalent to 0.33/0.39 mg/kg/day in males/females).

LOAEL = 50 ppm (equivalent to 3.47/3.93 mg/kg/day in males/females)
based on decreased pup body weights in both generations 

Acceptable/Guideline

Chronic Toxicity 

870.4100

Chronic Toxicity	MRID 42549801

Schuh, W. (1992) Bis (tri-n-butyltin) oxide (TBTO; ZK 21.955):  12-month
chronic oral toxicity study in beagle dogs.  Schering AG, Berlin,
Germany.  Laboratory Project ID:  Study Number TX 85.330, September 4,
1992. Unpublished.

	Beagle dogs

Purity 95.9-97.1% a.i.

0, 0.2, 1.0, or 5.0 mg/kg/day for 52 weeks (oral gavage)	Multiple
microscopic finding indicative of immunotoxicity (detailed above) were
noted at 1.0 mg/kg/day, particularly in the males.  At 5.0 mg/kg/day,
increased severity of immunotoxicity and indications of systemic
toxicity (including mortality, clinical signs of toxicity, overall body
weight losses, decreased body weights and food and water consumption,
increased blood sedimentation rate, decreased nucleated bone marrow
cellularity, increased hepatic enzymes, increased liver weight,
decreased spleen and thymus weights, and gross pathological findings in
the liver, spleen, and thymus) were observed.

Unacceptable/Guideline

(not upgradeable)

Carcinogenicity

870.4200

Carcinogenicity	MRID 42265001

Daly, I. (1992) An eighteen month oncogenicity feeding study in mice
with bis (tri-n-butyltin) oxide (TBTO). Bio/dynamic, Inc. Project No.
87-3131, March 27, 1992. Unpublished

	CD-1 mice 

Purity 97.1% pure

0, 5, 25, or 50 ppm (equivalent to 0.7, 3.7, and 7.7 mg/kg/day for low-,
mid-, and high-dose males, respectively, and 0.9, 4.8, and 9.2 mg/kg/day
for low-, mid-, and high-dose females, respectively) for 18 months
(diet)	LOAEL = 0.7 mg/kg/day (males) 

LOAEL = 0.9 mg/kg/day (females), based on decreased survivorship and
increased body weight gain.

Acceptable/Guideline



Combined Chronic Toxicity/Carcinogenicity

870.4300

Combined chronic toxicity/Carcinogenicity	MRID 40623201

Wester, P.W.; Krajnc, E.I.; et al. (1988) Bis (tributyltin) oxide –
Two year feeding study in rats with bis (tri-n-butyltin) oxide.  RIVM,
Bilthoven, The Netherlands.  Laboratory Report No.: 658112 002, February
25, 1988.  

Unpublished.	Wistar rats

Purity 95.3% a.i.

0, 0.5, 5, or 50 ppm (approximately equivalent to 0, 0.025, 0.25, and
2.5 mg/kg bw/day (diet)	NOAEL = 5 ppm (approximately equivalent to 0.25
mg/kg bw/day).

LOAEL = 50 ppm (approximately equivalent to 2.5 mg/kg bw/day) based on
increased mortality, systemic toxicity (ataxia, emaciation and decreased
body weight/body weight gain in males and females) and organ toxicity
(kidney, thyroid, adrenal glands, and pituitary. 

Acceptable/Guideline 



Metabolism and Pharmacokinetics

870.7485

Metabolism and Pharmacokinetics	MRID 01246480

Evans, W., D. Smith, and N. Cardarelli (date of study not provided)
Accumulation and excretion of [1-14C] bis (tri-n-butyltin) oxide in
mice. Environmental Management Laboratory, The University of Akron
(Akron, Ohio). Unpublished.	Female COBS albino mice

Purity not reported

0, 0.51, 3.75, or 18.5 ppm	Concentrations of 14C-TBTO 

were greatest in the kidneys, 

fat, liver, and spleen. 

Concentrations were found to 

be proportional to administered 

dose except in the kidney. 

Tissue accumulations were 

concentration dependent. The 

quantities absorbed were 

rapidly cleared when ingestion 

ceased. 

Acceptable/Non-guideline

870.7485

Metabolism and Pharmacokinetics	MRID 40253002

Humpel, M. (1986) Toxicokinetics of 113Sn-labelled tributyltin oxide
after intravenous (1 mg/kg) and intragastric (25 mg/kg) administration
to rats:  bioavailability, excretion, and biotransformation.  Schering
AG, Berlin, Germany.  Laboratory report numbers IC-Kl 3, KI 84 061, and
KI 84 062.  January 23, 1986.  

Unpublished. 	Female Wistar-Han (SPF) rats

Purity (>99%a.i)

1 mg/kg or 25 mg/kg	Absorption was very delayed after gavage
administration, with a half-life of 9.5 hours.  The intravenously 

administered 113Sn-labeled 

TBTO is rapidly distributed, 

with only 3% of the dose 

remaining in plasma volume 

after 5 minutes post 

administration.  Based on the 

high volume of distribution 

and the binding to the cellular 

constituents of the blood, authors 

conclude that TBTO has a

pronounced affinity for tissues.

Acceptable/Non-guideline

Dermal Penetration

870.7600 

Dermal Penetration	MRID 40050003

Hümel, M. (1985) Series 85-2: Percutaneous absorption of tributyltin
oxide (TBTO) through intact skin of baboon. Scherling AG, Berlin 65,
Germany. Laboratory Project ID IC-KI 2, June 12, 1985. Unpublished	Male
baboon

Purity not reported

Dose volume 0.5 mL/25 cm2 skin	The total recovery of radioactively
labeled 113Sn-Tributyltin oxide was less than 50%, perhaps due to the
incomplete removal of unabsorbed TBTO on the skin surface as a result of
extensive spreading from the site of application. However, reviewer
generated data indicates that the total recovery was approximately 75%. 

Unacceptable/Non-guideline

Published Literature

Non Guideline

Literature Study	Vos et al., (1990) Immunotoxicity of
bis(tri-n-butyltin)oxide in the rat: Effects on thymus-dependent
immunity and on nonspecific resistance following long-term exposure in
young vs aged rats.  Toxicol. Appl. Pharmacol.  105:144-155.	Wistar Rats

Purity: 95.3% active ingredient

Exposure to TBTO for up to 18 months 

Dietary concentrations:  0.025, 0.25 and 2.5 mg/kg-day.	NOAEL = 0.025
mg/kg/day

LOAEL = 0.25 mg/kg/day based on immunotoxicity (depression of IgE titers
and increase in T. spiralis larvae in muscle) following 4 months and
16.5 months of exposure to Tributyl Tin Oxide.  (Review by EPA/IRIS,
1997).



REFERENCES 

MRID					CITATION

  00085003 	Auletta, C.S., Hodge, P.S., Minczeski, N. et al. (1981).
Acute Oral Toxicity Study in Rats: Project No. 6656-81. (Unpublished
study received Oct 15, 1981 under 5204-1; prepared by Bio/ dynamics,
Inc., submitted by M & T Chemicals, Inc., Rahway, N.J.; CDL:246070-A).

00085004 	Auletta, C.S., Hodge, P.S., Minczeski, N. et al. (1981). Acute
Oral Toxicity Study in Rats: Project No. 6657-81. (Unpublished study
received Oct 15, 1981 under 5204-1; prepared by Bio/ dynamics, Inc.,
submitted by M & T Chemicals, Inc., Rahway, N.J.; CDL:246070-B).

00104789 	Bruan, W., Rinehart, W. (1979). A Dermal Sensitization Study
in Guinea Pigs: Compound TBTO: Project No. 5056-78. (Unpublished study
received Mar 14, 1979 under 5204-1; prepared by Bio/dynamics, Inc.,
submitted by M & T Chemicals, Inc., Rahway, NJ; CDL: 237804-A).

01246480 	Evans, W., D. Smith, and N. Cardarelli (date of study not
provided) Accumulation and excretion of [1-14C] bis(tri-n-butyltin)
oxide in mice. Environmental Management Laboratory, The University of
Akron (Akron, Ohio). Unpublished.

00137158 	Schroeder, R., Hogan, G. (1981). A Teratology Study in Rats
with Tributyltin Oxide: Project No. 80-2497A. Final rept. (Unpublished
study received Jan 16, 1984 under 5204-1; prepared by Bio/dynamics,
Inc., submitted by M & T Chemicals Inc., Rahway, NJ; CDL:252178-A).

 

 

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 (Tributyltin Oxide) - A Teratology Study in Rabbits with TBTO:
Laboratory Project ID: WIL-B0002. Unpublished study prepared by Wil
Research Laboratories, Inc. 210 p.

40253002 	Humpel, M. (1986). Toxicokinetics of 113Sn-labelled
tributyltin oxide after intravenous (1 mg/kg) and intragastric (25
mg/kg) administration to rats:  bioavailability, excretion, and
biotransformation.  Schering AG, Berlin, Germany.  Laboratory report
numbers IC-Kl 3, KI 84 061, and KI 84 062.  January 23, 1986.
Unpublished. 

40253005 	Brunneman, A. (1986). Evaluation of the Clastogenic Potential
in the Human Lymphocyte Test: ZK 21.955: Lab. Proj. ID IC 4/86. Un-
published study prepared by Schering AG. 23 p.

40623201 	Wester, P.W., Krajnc, E.I. et al. (1988). Bis(tributyltin)
oxide – Two year feeding study in rats with bis(tri-n-butyltin) oxide.
 RIVM, Bilthoven, The Netherlands.  Laboratory Report No.: 658112 002,
February 25, 1988. Unpublished.

41127001 	Daly, I. (1989). A three month oral range-finding toxicity
study in mice with bis (tri-n-butyltin) oxide (TBTO). Bio/dynamics, Inc.
(East Millstone, New Jersey). Unpublished.

41131001 	Schweinfurth, H. (1987). Tributyltin oxide: Systemic toxicity
study in Beagle dogs with daily oral (intragastric) administration over
a total of 18-19 weeks.  Schering AG, Berlin, Germany.  Laboratory
Project ID: TX 87.054, August 5, 1987. Unpublished.

41693801 	Schroeder, R.E. (1990). A two generation reproductions study
in rats with bis (tri-n-butyltin) oxide.  Bio/dynamics, Inc., East
Millstone, NJ.  Laboratory Project No.:  88-3261, October 22, 1990.
Unpublished.

42170001 	Lang, R. (1986). Evaluation in the Ames Salmonella/Microsome
Mutagenicity Test: ZK 21.955: Lab project Number: TX6106. Unpublished
study prepared by Schering Ag. 29 p.

42265001 	Daly, I. (1992). An eighteen month oncogenicity feeding study
in mice with bis(tri-n-butyltin) oxide (TBTO). Bio/dynamic, Inc. Project
No. 87-3131, March 27, 1992. Unpublished.

42412501 	Bakke, J. (1991). Evaluation of Cotin 310 in the CHO/HGPRT
Gene Mutation Assay: Final Report: Lab Project Number: LSC-2112-200:
2112-G200-91. Unpublished study prepared by SRI International. 22 p.

42412502 	O'Loughlin, K. (1991). Bone Marrow Erythrocyte Micronucleus
Assay of Cotin 310 in Swiss Webster Mice: Lab Project Number:
2112-C100-91. Unpublished study prepared by SRI International. 41 p.

42412503 	Bakke, J. (1990). Evaluation of the Potential of Cotin 310 to
Induce Unscheduled DNA Synthesis in the In vitro Hepatocyte DNA Repair
Assay Using the Male F-344 Rat: Final Report: Lab Project Number:
LSC-1482: 1482-VO1-90. Unpublished study prepared by SRI International.
21 p.

42412504  	Bakke, J. (1992). Evaluation of the Potential of Cotin 310 to
Induce Unscheduled DNA Synthesis in the In vitro Hepatocyte DNA Repair
Assay Using the Male F-344 Rat: Final Report, Amendment One: Lab Project
Number: LSC 1482: 1482-VO1-90. Unpublished study prepared by SRI
International. 7 p. (Supplemental copy to MRID 42412503).

42415801 	Allan, S. (1992). Acute Oral Toxicity to Rats of Tributyltin
Benzoate: Lab Project Number: 920351D/NDX 25/AC. Unpublished study
prepared by Huntingdon Research Centre, Ltd. 24 p.

42415802 	Allan, S. (1992). Acute Dermal Toxicity to Rats of Tributyltin
Benzoate: Lab Project Number: 920319D/NDX 26/AC. Unpublished study
prepared by Huntingdon Research Centre, Ltd. 20 p.

42415803 	Liggett, M. (1992). Skin Irritation to Rabbit of Tributyltin
Benzoate: Lab Project Number: 920395D/NDX 27/SE. Unpublished study
prepared by Huntingdon Research Centre, Ltd. 14 p.

42549801 	Schuh, W. (1992). Bis (tri-n-butyltin) oxide (TBTO; ZK
21.955):  12-month chronic oral toxicity study in beagle dogs.  Schering
AG, Berlin, Germany.  Laboratory Project ID:  Study Number TX 85.330,
September 4, 1992. Unpublished.

42903101 	Bryson, A. (1993). Tributyltin Benzoate: A Study of the Effect
on Pregnancy of the Rat: Lab Project Number: NDX/41: NDX 41/921129.
Unpublished study prepared by Huntingdon Research Centre Ltd. 106 p.

42966201 	Mahoney, D. (1992). Analysis of Tributylin Benzoate: Product
Chemistry: Chemical Purity: Lab Project Number: 239-39. Unpublished
study prepared by Huls America Inc. 9 p.

42966201 	Mahoney, D. (1992). Analysis of Tributylin Benzoate: Product
Chemistry: Chemical Purity: Lab Project Number: 239-39. Unpublished
study prepared by Huls America Inc. 9 p.

42966203 	Bakke, J. (1992). Evaluation of the Potential of Cotin 310 to
Induce Unscheduled DNA Synthesis in the In vitro Hepatocyte DNA Repair
Assay Using the Male F-344 Rat: Modification of Final Report Amendment
One: Lab Project Number: LSC-1482: 1482-V01-90. Unpublished study
prepared by SRI International. 7 p. (Duplicate copy of the MRID
42412504).

43177201 	Naas, Dennis J. (1990). Three week dermal range-finding study
in rats with Tributyltin Benzoate.  WIL Research Laboratories, Inc.
(Ashland, Ohio). Laboratory study number WIL-159010, December 18, 1990.
Unpublished.

43851201 	Pucaj, K. (1994). Acute Oral Toxicity Study of Tri-n-Butyl Tin
Maleate (TBT-M) in Rats: Lab Project Number: 86816. Unpublished study
prepared by Nucro-Technics, Inc. 79 p.

44142303 	Yau, L. (1996). Phase 4 Summary: Ultra-Fresh DM-50: Dermal
Sensitization: Tri-N-Butyltin Maleate: Laboratory Project Number: 28752:
206666: TRA/966-1. Unpublished study prepared by Nucro-Technics, Inc. 29
p.

92172004 	Stevens, A. (1990). M&T Chemicals, Inc. Phase 3 Summary of
MRID 00085003. Acute Oral Toxicity Report: Grignard Sourced TBTO:
Project No. 6656-81. Prepared by Bio/dynamics. 7 p. (Supplemental copy
to MRID 00085003).

92172006 	Stevens, A. (1990). M&T Chemicals, Inc. Phase 3 Summary of
MRID 40141901. A Teratology Study in Rabbits with TBTO: Project No.
WIL-B0002. Prepared by Wil Research Laboratories, Inc. 7 p.

92172013 	Stevens, A. (1990). M&T Chemicals, Inc. Phase 3 Summary of
MRID 00085004. Acute Oral Toxicity Study: Alkyl Aluminum Sourced TBTO:
Project No. 6657-81. Prepared by Bio/dynamics, Inc. 7 p. (Supplemental
copy to MRID 00085004).

92172014 	Stevens, A. (1990). M&T Chemicals, Inc. Phase 3 Summary of
MRID 00104789. A Dermal Sensitization Study in Guinea Pigs: Compound
TBTO: Project No. 5056-78. Prepared by Bio/dynamics. 6 p. (Supplemental
copy to MRID 00104789). 

92172016 	Schroeder, R. (1992). M&T Chemicals, Inc. Phase 3 Reformat of
MRID 00137158. A Teratology Study in Rats with Tributyltin Oxide:
Project No. 80-2497A. Prepared by Bio/dynamics, Inc. 83 p.

PUBLISHED LITERATURE

Agency for Toxic Substances and Disease Registry (ATSDR). (2003)
Toxicological profile for tin and compounds (Draft for Public Comment). 
Atlanta, GA: U.S. Department of Health and Human Services, Public Health
Service.

Baroncelli, S., Karrer, D., Turllazzi, P. G. (1990). Embryotoxic
evaluation of bis (tri-n-butylin) oxide (TBTO) in mice. Toxicology
letters, 50: 257-262.

Boyer, I.J. (1989). Toxicity of dibutyltin, tributyltin and other
organotin compounds to humans and to experimental animals. Toxicology
55:253-298.

Bressa, G., Hinton, R.H., Price, S.C., et al. (1991). Immunotoxicity of
tri-n-butyltin oxide (TBTO) and tri-n-butyltin chloride (TBTC) in the
rat. Journal of applied toxicology, 11: 397-402.

Buckiova, D., Dostal, M., Hofmannova, V. (1992) Embryotoxicity of
organotins [abstract]. Reproductive toxicology, 6:178–179.

Centers for Disease Control (CDC). (1991). Acute effect of indoor
exposure to paint containing bis(tributyltin) oxide-Wisconsin, 1991.
MMWR Morb Mortal Wkly Rep. 1991, May 3; 40(17):280-1. [MMWR. Morbidity
and mortality weekly report]

Crofton, K.M., Dean, K.F., Boneck, V.M., et al. (1989). Prenatal or post
natal exposure to bis(tri-n-butyltin) oxide in the rat: postnatal
evaluation of teratology and behavior. Toxicology and applied
pharmacology, 97:113-123.

Karrer, D., Baroncelli, S. and Turillazzi, P.G. (1995). Oral
bis(tri-n-butyltin) oxide in pregnant mice. II. Alterations in
hematological parameters. J. Toxicol. Environ. Health 46: 369-377.

Karrer, D., Baroncelli, S., Ciaralli, L. et al. (1992). Effect of
subchronic bis (tri-n-butyltin) oxide (TBTO) oral administration on
hematological parmeters. Journal of toxicology and environmental health,
46: 369-377.

Krajnc, E.I., Vos, J.G., Wester, PW, Loeber, J.G., van der, Heijden,
C.A., (1987) Toxicity of bis(tri-n-butyltin)oxide (TBTO) in rats.
Unpublished report submitted to the Office of Toxic Substances, US
Environmental Protection Agency, with cover letter dated 18 May 1987
(Document Control No. FYI-OTS-0687-0550 Sequence A).

Kranjc, E.I., Wester, P.W., Loeber, J.G. et al.(1984). Toxicity of bis
(tri-n-butyltin) oxide (TBTO) in rats. I. Short-term effects on general
parameters and on the endocrine and lymphoid systems. Toxicology and
applied pharmacology, 75: 363-386.

Luebke, R.W., Chen, D.H., Dietert, R. et al. (2006). Immunotoxicology
Workgroup. The comparative immunotoxicity of five selected compounds
following developmental or adult exposure. J Toxicol Environ Health B
Crit Rev. 2006 Jan-Feb; 9(1):1-26. [Journal of toxicology and
environmental health. Part B, Critical reviews]

Raffray, M., Cohen, G.M. (1991). Bis(tri- n-butyltin) oxide induces
programmed cell death (apoptosis) in immature rat thymocytes. Archives
of toxicology, 65: 135-139.

Raffray, M., Cohen, G.M. (1993) Thymocyte apoptosis as a mechanism for
tributyltin-induced thymic atrophy in vivo. Archives of toxicology,
67:231–236.

Simalowicz, R.J., Riddle, M.M., Rogers, R.R. et al.(1989).
Immunotoxicity of tributyltin oxide in rats exposed as adults of
pre-weanlings. Toxicology, 57: 97-111.

Snoeji, N.J., Penninks, A.H. and Seinen, W. (1987). Biological Activity
of Organotin compounds - An overview. Environ. Res. 44:335-353.

Schuh, W. (1992) One year chronic feeding study in beagle dogs.
Unpublished report prepared by Schering AG Laboratories for Elf Atochem
North America, Inc., Aceto Chemicals, and Schering Berlin Polymers (MRID
No. 425498).

Thompson, T.A., Lewis, J.M., Dejneka, N.S. et al. (1996) Induction of
apoptosis by organotin compounds in vitro: neuronal protection with
antisense oligonucleotides directed against stannin. Journal of
pharmacology and experimental therapeutics, 276:1201–1214.

Viviani, B., Ross, A.D., Chow, S.C., Nicotera, P. (1995) Organotin
compounds induce calcium overload and apoptosis in PC12 cells.
Neurotoxicology, 16:19–26.

Vos, J.S., De Klerk, A., Kranjc, E.I. et al. (1990). Immunotoxicity of
bis (tri-n-butyltin) oxide in the rat. Effects on thymus dependent
immunity and on nonspecific resistance following long-term exposure in
young versus aged rats. Toxicology and Applied Pharmacology,
105:144-155.

Wester, P.W., Kranjc, E.I., Van Leeuwen, F.X.R. et al. (1990). Chronic
toxicity and carcinogenicity of bis (tri- n-butyltin) oxide (TBTO) in
the rat. Food and Chemical toxicology, 28: 179-196.

WHO working group. (1990). Tributyltin compounds. Environmental Health
Criteria , 116. 273 p.

World Health Organization. Tributyltin oxide. Concise International
Chemical Assessment Document (CICAD) Vol:14 (1999) 24p.

SUPPORTING AGENCY DOCUMENTATION

IRIS (1997) Toxicological Review: Tributyltin oxide (CAS No. 56-35-9):
In support of summary information on the Integrated Risk Information
System (IRIS). U.S. Environmental Protection Agency, July 1997.

Tributyltin maleate (TBTM): Non-Dietary Residential Exposure/Risk
Considerations for Proposed Registration of Untra-Fresh® DM-50 as a New
Indoor Use Pattern of TBTM.  PC Code: 083118.  DP Barcode D291315
(Memorandum: P. Deschamp to G. LaRocca and M. Swindell, March 31, 2005).


Tributyltin Compounds - Report of the Antimicrobials Division Toxicity
Endpoint Selection Committee (ADTC) for Tributyltin Oxide, Tributyltin
Maleate and Tributyltin Benzoate.  PC Code(s):  PC Code(s):  083001,
083106, 083118.  CAS Registry Number(s): 56-35-9, 4342-36-3, 14275-57-1
(Memorandum: M. Centra to T. Lindheimer and J. Bloom, February 4, 2008).

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