FENBUTATIN OXIDE
First draft prepared by M.A. Clevenger
Office of Pesticide Programs
US Environmental Protection Agency
Washington DC USA
EXPLANATION
Fenbutatin oxide was previously evaluated by the JMPR in 1977
(Annex I, reference 28) when an ADI of 0-0.03 mg/kg bw was
allocated. The data reviewed in 1977 consisted of studies on
pharmacokinetics, short-term toxicity in mice, rats, and dogs, long-
term toxicity in mice and rats, reproductive toxicity in rats,
neurotoxicity in rats, developmental toxicity in rabbits, and
genotoxicity. The present review evaluates studies conducted since
the 1977 review. Relevant portions of the previous monograph have
been incorporated into this toxicological monograph.
EVALUATION FOR ACCEPTABLE DAILY INTAKE
BIOLOGICAL DATA
Biochemical aspects
Absorption, distribution, and excretion
Rats
Groups of 2 male and 2 female rats were fed an average of 19.5
ppm 119Sn-labelled fenbutatin oxide (with or without a 3 day
withdrawal period before killing following 3 or 6 days exposure) for
1, 3 or 6 days to study the absorption, distribution and excretion
of fenbutatin. Most of the administered 119Sn was rapidly excreted
in the faeces following multiple dosing (80-90% of the administered
119Sn). Essentially no 119Sn was excreted via the urine. The GI
tract contained approximately 30, 15 and 5% of the administered
119Sn in animals sacrificed after 1, 3 or 6 days of dosing,
respectively. The highest tissue concentrations of 119Sn were
found in liver (0.015-0.09 mg/kg), kidneys (0.015-0.055 mg/kg) and
fat (< 0.01-0.065 mg/kg) . No 119Sn was detected in brain or bone
tissue (Loeffler, 1972).
The pharmacokinetics of fenbutatin oxide following oral
administration was studied using groups (5/sex) of Crl:CD BR rats.
One group received a single low dose of 10 mg 119mSn-labelled
fenbutatin oxide/kg bw. A second group received a single high dose
of 500 mg/kg bw of radiolabelled compound. A third group received
multiple doses (14 daily doses) of unlabelled compound followed by a
single dose of radiolabelled compound. Urine and faeces were
collected over 7 days at which time rats were sacrificed and tissues
collected.
Over a 5-7 day period, 83%-100% of the administered dose was
excreted in the faeces with less than 1% of the radiolabel detected
in the urine. Excretion was somewhat slower following the single
high dose compared to the low-dose exposure. For all dosage
regimens, excretion was almost complete within 72 h. The excretion
pattern was similar for all three dosage regimens and for males and
females. Tissue residues were low 5-7 days after treatment. Highest
radioactivity was found in liver, kidney, and heart. Concentrations
ranged from 0.2 to 3 ppm following the low dose (single or repeated)
and high dose, respectively. The elimination half-life was estimated
to be 24 h following the low dose (single or repeated) and 40 h
following the high dose (Reynolds, 1989).
Biotransformation
Rats
Analysis of the faeces from rats dosed with 119Sn-labelled
fenbutatin oxide indicated that ß,ß-dimethylphenethylstannoic acid
and 1,3-dihydroxy-1,1,3,3-tetrakis(2-methyl-2-phenylpropyl)-
distannoxane are the major metabolites (Loeffler, 1972)
The metabolism of fenbutatin oxide was studied in rats by
analyzing faeces obtained in the pharmacokinetics study described
above (Reynolds, 1989). Metabolites were identified by co-
chromatography with reference compounds using TLC. The majority (86-
96%) of the extracted radiolabel co-chromatographed with unchanged
fenbutatin oxide. Two minor metabolites were found in faecal
extracts, one of which was unidentified. The other metabolite was
tentatively identified as 1,3-dihydroxy-1,1,3,3-tetrakis(2-methyl-2-
phenylpropyl)-distannoxane (IN-CG200). Traces of IN-CG200 were
detected as an impurity of the test material prior to dosing
(Reynolds, 1989).
Effects on enzymes and other biochemical parameters
In starch gel electrophoretic studies on sera from rats fed
either 0 or 600 ppm fenbutatin oxide in the diet for 6 months (2
buffer systems) control and test animal sera were identical. The
effect of 1-phenylanatine or heat was similar on both sera.
Comparisons with intestinal, bone, liver and kidney alkaline
phosphatase indicated that the serum alkaline phosphatase was
similar to intestinal alkaline phosphatase. Further studies on rat
sera derived from animals fed 0 or 600 ppm fenbutatin oxide in the
diet for one year, utilizing heat inactivation, selective substrates
and inhibitors, confirmed the absence of any difference between
control and test animal sera, as well as confirming the similarity
of the serum alkaline phosphatase to intestinal alkaline phosphatase
(Pickering, 1973a).
In a further series of studies, the elevation of serum alkaline
phosphatase observed in rats after administration of 300 ppm
fenbutatin oxide in the diet for 90 to 120 days was shown to be
completely reversible following one month's withdrawal from
exposure. Limited data indicate that the reversal of elevated serum
alkaline phosphatase in the test rats may be completed within 7 days
of withdrawal of the fenbutatin oxide (Pickering, 1973b).
Fenbutatin oxide and other tri-substituted tin compounds were
shown to inhibit uptake and stimulate release of 5-hydroxytryptamine
(5-HT) in rat platelets in vitro (at 10 œM) and to a lesser extent
in platelets isolated after intraperitoneal injection (2.5 mg/kg
bw). Interference with membrane ATP was suggested as the site of
inhibition of 5-HT uptake by organotins (Johnson & Knowles, 1983).
Fenbutatin oxide (10 œM) did not affect aggregation of rat
platelets in vitro (Knowles & Johnson, 1986).
Toxicological studies
Acute toxicity studies
Acute toxicity studies are summarized in Table 1. Fenbutatin
oxide has low toxicity by the oral route but is highly toxic by the
inhalation route. Inhalation exposure produced necrosis of
bronchiolar epithelium, lung congestion and edema, and lesions in
renal tubule epithelium (Parker, 1981).
Table 1 Acute toxicity of fenbutatin oxide
LD50 LC50
Species Strain Sex Route mg/kg bw mg/l Reference
Rat Crl:CD BR M oral 44001 Sarver (1988)
Crl:CD BR F inhalation 0.0721,2 Valentine (1987)
(4 hr exp)
Sprague-Dawley M&F inhalation 0.231 Parker (1981)
(4 hr exp) 0.143
CFE M&F i.p. 334 Cassidy (1978)
1 Purity of technical fenbutatin oxide > 98%.
2 LD50 for males could not be calculated. Lowest achievable concentration of
0.046 mg/l produced 1/5 deaths.
3 Concentration corrected for particles in respirable range (<4.5 œm).
4 Purity of test material unspecified.
Short-term toxicity studies
Mice
Groups of 5-10 young or mature male Swiss-Webster mice were
administered fenbutatin oxide (purity unspecified) at 137, 411, or
1230 ppm (260, 780, or 2340 œeq/kg diet), equal to 21, 62 or 190
mg/kg bw/day, for 7 days. Mice were sacrificed 4 or 7 days after
treatment. In young mice (13-14 g bw), a dose level of 137 ppm had
no effect on body, brain, heart, liver, or spleen weight 7 days
after treatment. In mature mice (27-29 g bw), a dose level of 411
ppm had no effect on body-weight, haemoglobin, haematocrit, or
erythrocyte count 4 days after treatment. Spleen weight was reduced
20% and WBC count was reduced 11% compared to control values. At
1230 ppm, body-weight, spleen weight, and WBC count were decreased
and haemoglobin, haematocrit, and erythrocyte count were increased.
A number of other triorganotin compounds were more potent than
fenbutatin oxide producing effects on body-weight, spleen weight,
and blood composition at lower dose levels. The NOAEL was 137 ppm
(equal to 21 mg/kg bw/day) based on slight effects on the spleen and
WBC count at 411 ppm. (Ishaaya et al., 1976).
Rats
Five groups of 5 male and 5 female rats were fed 0, 30, 100,
300 or 1000 ppm fenbutatin oxide in the diet for 5 weeks. Body-
weight was significantly reduced, as was food intake at 300 and 1000
ppm. Reductions in absolute brain, heart, liver, spleen and kidney
weights were noted in both sexes at 1000 ppm. Spleen weight was
reduced in males at 300 ppm and brain, liver, spleen and kidney
weights were reduced at 300 ppm in females. Serum alkaline
phosphatase was increased in both sexes and SGPT levels were
elevated in females at 1000 ppm (Simpson, 1972a).
Three groups of five male rats were used in a paired feeding
study, one group being a normal control, the second being fed 600
ppm and the third pair-fed at the food intake levels of the 600 ppm
group. The study ran for 5 weeks. Results (daily body-weights)
indicated that the reduction in body-weight at 600 ppm can be
totally accounted for by reduced food intake (Simpson, 1972b).
Groups of 6 male and 6 female rats were fed 50, 100, 300 or 600
npm fenbutatin oxide (97%) in the diet for 3 months. Concurrent
controls comprised 12 male and 12 female rats. Health and behaviour
were comparable in all groups. Body-weight gain was reduced in males
at 600 ppm and in females at 300 and 600 ppm. Food intake was
reduced in males at 600 ppm and in females during the first 3 or 4
weeks of the study at 300 and 600 ppm. Absolute organ weights of
liver and kidney were reduced in both sexes, heart in females, and
brain and spleen in males at 600 ppm. At 300 ppm, liver, spleen and
kidney absolute weights were reduced in males. Organ/body-weight
ratios were elevated for brain in males at 600 ppm and in females at
300 and 600 ppm. Heart and liver to body-weight ratios were elevated
in females at 600 ppm and the heart to body-weight ratio was
increased at 300 ppm. The testes to body-weight ratio was elevated
in males at 300 and 600 ppm. Haematology parameters were comparable
in all groups. Clinical chemistry determinations were comparable for
total protein, SGOT and electrolytes (K+, Na+ and Cl-). BUN
and SAP in males were elevated at 300 and 600 ppm in males, as was
SGPT at 300 ppm. In females, BUN was elevated at 100 ppm and above.
Serum protein fractions were comparable in all groups. No compound-
related gross or histopathological effects were noted (Simpson &
Thorpe, 1973).
Groups of rats, 20/sex/control group or 10/sex/test group, were
fed 0, 3, 10, 30, 100 or 300 ppm of 99% 1,3-dihydroxy-1,1,3,3-
tetrakis(2-methyl-2-phenylpropyl)-distannoxane (a metabolite of
fenbutatin oxide) for 90 days. Health and behaviour were comparable
in all groups. Body-weights of 300 ppm males were significantly
decreased throughout the study. This was also true for the females
in the 30, 100 and 300 ppm groups, but the suppression was not
significant. Slight reductions in haemoglobin, erythrocyte count and
packed cell volume occurred in the 300 ppm males. No changes were
noted in any of the other groups of either sex. The other clinical
values of animals killed after 13 weeks showed no dose-related
differences when compared to the control values. Gross and
microscopic examination of a wide range of tissues from all control,
100 and 300 ppm animals revealed no consistent changes associated
with exposure to the metabolite. Organ weights of males at 13 weeks
did not differ from control weights, while the organ body-weight
ratios were increased for brain, heart, liver, kidneys and testes in
this group and for the liver in the 100 ppm male group. Brain,
spleen and kidney weights were reduced in the 300 ppm females
(Simpson & Dix, 1972).
Rabbits
Groups of 5-10 male and 5-10 female New Zeeland white rabbits
received dermal applications (6 h/day) of technical fenbutatin oxide
(98% purity) at doses of 0.05, 0.5, or 5 mg/kg bw for 21 consecutive
days. Control groups received dermal applications of the vehicle,
dimethyl phthalate. Doses of 0.5 and 5 mg/kg bw produced mild to
severe skin irritation, which was not completely reversed after a
14-day recovery period. Slight irritation observed sporadically in
control and 0.05 mg/kg bw groups was attributed to the wrapping
technique and the vehicle. No systemic toxicity was observed. The
NOAEL for systemic toxicity was greater than 5 mg/kg bw (Brock,
1988).
Dogs
Five groups of 2 male and 2 female beagle dogs were orally
dosed (gelatin capsules) with 0, 10, 30, 100 or 300 mg fenbutatin
oxide/kg bw/day for 5 weeks. Slight vomiting and diarrhoea occurred
at 100 and 300 mg/kg bw/day which was associated with decreased
body-weight gain. At 30 mg/kg bw/day and above, serum alkaline
phosphatase was elevated. No pathological changes were observed
(Simpson, 1972a).
Groups of dogs (8/sex/control group and 4/sex/test group) were
administered, by capsule, 0, 2.5, 5, 15, 30 or 60 mg/kg bw/day 97%
fenbutatin oxide for 2 years. Blood samples were taken every six
weeks to measure BUN, glucose, plasma protein, sodium, potassium,
chloride, SGPT, SGOT and alkaline phosphatase. Every 12 weeks
haemoglobin, packed cell volume, RBC, WBC, differential WBC counts,
prothrombin and kaolin-cephalin coagulation times were determined
Urine analysis was carried out every 3 months. Emesis and diarrhoea
occurred in most of the dosed dogs. These effects continued in some
dogs in the 30 and 60 mg/kg bw/day groups throughout the study,
although at a reduced frequency towards the end of the study. Lower
dosage groups showed none of these effects after the first year.
Convulsions occurred in 5 dogs, predominantly female, during the
second year (control, one female; 2.5 mg/kg bw/day, one female; 15
mg/kg bw/day, one female and one male; and 30 mg/kg bw/day, one
female). None of the 60 mg/kg bw/day animals exhibited this effect.
EEGs with simultaneous single lead monitoring of ECGs in one female
each in the 0, 2.5 and 30 mg/kg bw/day groups revealed a normal EEG
for the control dog, while the other 2 dogs showed normal waking
rhythms but abnormal activity during light sleep. The investigators
contended that the lack of a dose-response relationship suggests a
diagnosis of spontaneous epilepsy not related to exposure to
fenbutatin oxide. General health and behaviour of all other dogs
were similar to control dogs. Significant reductions in rate of
weight gains occurred in both sexes in the 60 mg/kg bw/day group.
Males receiving 30 mg/kg bw/day exhibited no significant rate of
gain. This effect was not evident in females of this group.
Haematological and clinical chemistry and urine data showed no
differences between control and test groups. Necropsies revealed no
gross changes. Microscopic examination, including frozen sections of
liver and kidney stained for lipid, revealed no compound-related
pathological changes in a wide range of tissues. Organ weights were
not affected by exposure to the test compound. The NOAEL was 15
mg/kg bw/day (Granville & Dix, 1973).
Long-term toxicity/carcinogenicity studies
Mice
Groups of 48 male and 48 female Carworth Farm No. 1 strain mice
were fed 50, 100, 300 or 600 ppm fenbutatin oxide in the diet for 18
months. A control group comprised 96 males and 96 females. All
animals were examined grossly at death. Microscopic examination was
undertaken on all animals dying during the study and on all 600 ppm
mice, on 10 mice/sex at 50, 100 and 300 ppm, and on 45 controls/sex
at 18 months. Survival rate was comparable in all groups and
exceeded 75%. There was a high incidence of tumours in all groups.
However there was no indication of compound or dose-related tumour
induction (Granville et al., 1973).
Rats
Groups of rats (114/sex/control group and 72/sex/test group)
were fed 0, 50, 100, 300 or 600 ppm 97% fenbutatin oxide for two
years. Rats were killed at 3 and 12 months (12/sex/control group and
6/sex/test group) and at 6 months (24/sex/control group and
12/sex/test group). Organ weights of brain, heart, liver, spleen,
kidney and testes were recorded and a wide variety of tissues
examined from all rats dying or killed at interim periods and from
all rats from the control, 300, and 600 ppm groups. Tissues were
examined from 10 animals/sex in the 50 and 100 ppm groups. Blood
determinations were made at each kill period for haemo-globin, PCV,
RBC, WBC and WBC differential counts; prothrombin and kaolin-
cephalin coagulation times; BUN; total protein; potassium, sodium
and chloride, SGPT, SGOT and alkaline phosphatase. Serum proteins
were fractionated electrophoretically and estimates made of albumin
and alpha, ß and gamma-globulin concentrations.
Fenbutatin oxide rendered the diet unpalatable to both sexes
causing, in the earIy months of the study, significant reductions in
food consumption and body-weight gains. Males were more affected
than females. For the remainder of the study all groups exhibited
similar rates of gain to the control group except both sexes
receiving 300 and 600 ppm, which exhibited reduced body weiphts
throughout the study. BUN levels were increased at the higher dose
levels for the first 6 months but remained within normal limits for
the remainder of the study. Serum alkaline phosphatase activity was
increased in both sexes at 300 and 600 ppm throughout the study, in
the 100 ppm males at 6 months, and in the 100 ppm females at 3
months and 2 years. All other haematological and clinical chemistry
values remained within normal limits at all times. Survival and
behaviour were not affected by administration of the compound. No
compound-related tissue lesions nor any increase in tumour
incidences were found. Organ weights revealed no compound-related
differences except kidney-weight reduction in all treated males and
the 600 ppm females at 2 years. No specific compound-related lesions
were seen in the kidney of any treated group. Absolute and relative
testes weights, unaccompained by hypertrophy, were noted in the 300
and 600 ppm groups at 2 years. The NOAEL was 50 ppm, equivalent to
2.5 mg/kg bw/day (Simpson et al., 1973).
Reproduction studies
Rats
In a standard 3 generation, 2 litter/generation reproduction
study, groups of 10 male and 20 female rats were fed 0, 50, 100 or
300 ppm fenbutatin oxide (98% purity). Rats of the F0 generation
commenced on diet and were mated at 100 days of age, and litters
were culled to 10 pups/litter on post-partum day 5. Necropsy was
performed on 10 male and 10 female F3b weanlings from the 0 and
300 ppm group, and on 5 weanlings/sex from the 50 and 100 ppm
groups. In the 300 ppm group, parents and pups were smaller,
hyperactive and irritable; mean litter size was slightly reduced in
the F1b litters, and in the two F3 generations survival to
weaning was reduced at 300 ppm and the testicular organ/body-weight
ratio was reduced in the F3b weanlings. Necropsies of F2b adults
did not show any compound-related changes. No consistent effects
were observed at 50 or 100 ppm with respect to fertility, gestation,
viability, or lactation indices, litter size, litter weight, or
necropsy of F3b weanlings (Hine et al., 1973).
Technical fenbutatin oxide (99.4%) was administered
continuously to Crl:CD BR rats over two generations at 0, 40, 75,
250, or 500 ppm in the diet, (equal to 2.8, 5.2, 17 or 38 mg/kg
bw/day for males and 3.2, 6.0, 20 or 44 mg/kg bw/day for females).
The first parental (F0) animals (30/sex) were treated beginning 72
days prior to mating through weaning of the F1 offspring. Selected
F1 offspring (30/sex) were treated beginning 105 days before
mating through weaning of the F2 offspring. During the pre-mating
period, body-weight, body-weight gain, and food consumption were
reduced in F0 and F1 males and females receiving 500 ppm and
females receiving 250 ppm. Body-weights of F0 and F1 females
were also reduced during lactation at 500 ppm. Reproductive
performance of F0 and F1 rats was unaffected. During lactation,
mean body-weights of F1 and F2 offspring were reduced at 250 and
500 ppm. The NOAEL was 75 ppm, equal to 6.0 mg/kg bw/day, based on
reduced body-weight of parental animals and offspring at 250 and 500
ppm (Bentley, 1990).
Special studies on embryo/fetotoxicity
Rats
The developmental toxicity of fenbutatin oxide was studied in
Wistar rats. Fenbutatin oxide (98.7% purity) was administered by
oral gavage to groups of 27 mated rats at doses of 0, 15, 30, or 60
mg/kg bw/day. Controls received the vehicle, aqueous
carboxymethylcelluose. Rats were treated on days 6-15 of gestation
and sacrificed on day 20. The incidence of diarrhoea was increased
at the high dose of 60 mg/kg bw/day. Body-weight was reduced (4-8%)
at 30 and 60 mg/kg bw/day during the treatment period. Body-weight
of the high-dose group remained lower than controls post-exposure
(day 20). The only other finding was a slight increase in pre-
implantation loss at 60 mg/kg bw/day (1.1 per litter) compared to
controls (0.6 per litter). Because treatment began around the time
of implantation, the increase in pre-implantation loss may or may
not have been related to treatment. The NOAEL for maternal toxicity
was 15 mg/kg bw/day, based on reduced weight gain at 30 mg/kg
bw/day. The NOAEL for embryo-fetal toxicity was 30 mg/kg bw/day
based on increased pre-implantation loss at 60 mg/kg bw/day (Dix,
1981a).
Rabbits
Three groups of 15 pregnant rabbits were dosed by gelatin
capsule, on days 6-18 inclusive of gestation (mating day = day 0),
with 3 or 10 mg fenbutatin oxide/kg bw/day, or with 37.5 mg
thalidomide/kg bw/day. Twenty-six rabbits were used as capsule-only
treated controls. Rabbits were killed on day 28 of pregnancy. A
repeat experiment differed only in that 20 pregnant females/dose
level and 30 controls were used, autopsy being on day 29. Fetuses
were incubated for 24 h following removal from the uterus, after
which approximately 1/3 were examined for visceral abnormalities,
and 2/3 for skeletal defects. Prior to clearing for skeletal
examination, viscera from these animals were also examined for
defects. Although some maternal deaths occurred, they were random
between groups. Mean live litter size, resorption rate, fetal loss,
survival of incubated fetuses and incidence of anomalies were not
affected by fenbutatin oxide at either dose level. Crown-rump length
and fetal weight were comparable to controls except at 3 mg/kg
bw/day, in the repeat experiment, where a significant increase was
noted. The positive control group behaved as expected (Dix & Wilson,
1973).
The developmental toxicity of fenbutatin oxide was studied in
New Zeeland white rabbits. Fenbutatin oxide (98.7% purity) was
administered orally in capsules to groups of 18-23 artificially
inseminated rabbits at doses of 0, 1, 5, or 10 mg/kg bw/day. Rabbits
were treated on days 6-18 of gestation and sacrificed on day 29.
Maternal toxicity was observed at 5 and 10 mg/kg bw/day. The high
dose was extremely toxic producing anorexia and substantial weight
loss during and after the treatment period. The group receiving 5
mg/kg bw/day also experienced anorexia. Anorexia may have been the
result of gastric irritation. One-third of the does treated with 5
or 10 mg/kg bw groups showed lesions in the gastric mucosa at
necropsy. Twelve of 20 does (60%) receiving 10 mg/kg bw/day aborted
compared to 3/11 (27%) controls. Another 5/20 (25%) at the high dose
had litters with no live fetuses compared to 2/11 (18%) controls. At
5 mg/kg bw/day, 2/17 (12%) aborted and 4/17 (24%) had litters with
no live offspring. Mean post-implantation loss per litter was 1.0,
1.2, 2.8, and 3.2 for the 0, 1, 5, and 10 mg/kg bw/day groups,
respectively. Litter weights were reduced at the high dose. The
NOAEL for maternal toxicity and embryo-fetal toxicity was 1 mg/kg
bw/day based on clinical signs and gastric lesions in does and
increased post-implantation loss at 5 mg/kg bw/day (Dix, 1981b).
Special studies on genotoxicity
The results of genotoxicity studies are summarized in Table 2.
Fenbutatin oxide tested negative in assays for point mutation in
bacteria and mammalian cells, chromosome aberrations in vitro and
in vivo, and unscheduled DNA synthesis.
Special study on male reproduction
Rabbits
The effect of a single high dose of fenbutatin oxide on the
reproductive system of male rabbits was investigated. The study was
conducted as a follow-up to a report of a reduced spermatogenesis
and increased multinucleate spermatid formation in an LD50 study
using doses of 1500-3000 mg/kg bw and no control group (Simpson,
1972). Male New Zeeland white rabbits received a single oral dose of
100, 500, or 1500 mg/kg bw of fenbutatin oxide (98.3% purity). The
control group consisted of pair fed groups and an unpaired group
(i.e. fed ad libitum). Rabbits were sacrificed and examined 21
days after treatment. Five of 8 rabbits receiving 1500 mg/kg bw and
one each from the 100 and 500 mg/kg bw groups died. Two controls
paired with 1500 mg/kg bw rabbits died due to dosing errors.
Fenbutatin oxide-treated groups became emaciated due to markedly
reduced food intake and weight gain. Body-weight of paired controls
were similarly reduced except the group receiving 500 mg/kg bw had
lower weights than its paired control. Fenbutatin oxide treatment
produced lesions of the gastric mucosa including ulceration,
erosion, and haemorrhage. Histological examination of reproductive
tissues and testes weight did not reveal any significant differences
between fenbutatin oxide groups and paired and unpaired controls.
This study using a control group did not show male reproductive
toxicity in rabbits following acutely toxic doses of fenbutatin
oxide (Dix, 1981c).
Special studies on neurotoxicity
Rats
Following preliminary studies to determine suitable dose levels
and to confirm the induction of brain edema by triethyltin bromide,
a comparative edema assay of five organotin compounds (including
fenbutatin oxide, and its metabolite) for edema formation in the
central nervous system of rats was undertaken. Groups of 15 male
rats were intubated with corn oil (10 mg/kg bw, 40 mg/kg bw
triethyltin bromide, 1000 mg/kg bw fenbutatatin oxide, 100 mg/kg bw
1,3-dihydroxy-1,1,3,3-tetrakis (2-methyl-2-phenylpropyl)-
distannoxane and sizeable doses of two other tin compounds. The
triethyltin bromide group showed increased brain water content after
48 h. Seven of ten brains examined at 24 and 48 h showed spongeosis
of white matter in the cerebellum and sometimes of the corpus
callosum and pons. No histological changes were observed in 4 rats
killed after 6 h. Fenbutatin oxide (24 and 48 h kills), and the
metabolite (6, 24 and 48 h kills) did not show either increased
water content or abnormal brain histology, although rats on both
compounds showed signs of intoxication (Samuels & Dix, 1972).
Dogs
Fenbutatin oxide was reported to have no effect on the EEG
pattern in dogs treated with oral doses of 30 mg/kg bw/day for 14
days. Groups of 2 male and 2 female beagle dogs were used. Each dog
served as its own control. EEG was performed prior to treatment,
during treatment, and 7 and 14 days after treatment. During the
treatment period, the dogs exhibited occasional soft/loose faeces
and tremors and experienced weight loss and reduced food intake.
These symptoms resolved post-exposure. Later treatment of the same
dogs with nikethamide (up to 1 g intravenously), an analeptic agent,
produced paroxysmal activity (Greenough, 1991).
Observations in humans
No data available.
Table 2. Results of genotoxicity assays on fenbutatin oxide
Conc. of
Test System Test object fenbutatin oxide Purity Results Reference
Ames test (1) S. typhimurium 5-300 µg/plate dissolved 98% Negative Arce (1987)
TA98, TA97, TA100, in acetone
TA1535
S. typhimurium ? ? Negative Moriya et al. (1983)
TA98, TA100, TA1535,
TA1537, TA1538
E. coli mutation E. coli, WP2 hcr ? ? Negative Moriya et al. (1983)
assay (1)
V79/HGPRT mutation V79 Chinese hamster lung cells 0.3-27 µg/ml suspended 98.8% Negative Davis (1988)
assay (1) in DMSO
CHO/HGPRT mutation Chinese hamster ovary cells 0.025-7.5 µg/ml dissolved 99.4% Negative Stahl (1988)
assay (1) (CHO-K1-BH4) in acetone
Unscheduled DNA Male rat (Crl:CD BR) primary 0.001-50 µg/ml dissolved 99.4% Negative Bentley (1988)
synthesis culture hepatocytes in acetone
In vitro Human lymphocytes, healthy male and 0.7-5 µg/ml dissolved 99.4% Negative Vlachos (1988a)
cytogenetics (1) female volunteers in acetone
In vivo cytogenetics Male and female Crl:(ICR)BR mice, 500-5000 mg/kg bw orally X 1 99.4% Negative Vlachos (1988b)
bone marrow
(1) Both with and without metabolic activation.
COMMENTS
Fenbutatin oxide is poorly absorbed from the gastrointestinal
tract. Most (more than 90%) was excreted unchanged in faeces. Less
than 1% was excreted in urine.
Fenbutatin oxide has low acute oral toxicity. The World Health
Organization has classified fenbutatin oxide as unlikely to present
acute hazard in normal use. It is highly irritating to the skin,
lungs, and gastrointestinal tract. By the oral route, bolus
administration was particularly irritating. Oral administration to
dogs resulted in diarrhoea and vomiting. Following gavage
administration, rabbits exhibited anorexia and developed gastric
mucosal lesions. An increase in SAP in a two-year study in rats may
also have been related to gastrointestinal tract injury.
A seven-day feeding study in mice gave some indication that
fenbutatin oxide possessed less immunotoxicity potential than other
organotin compounds. However, the data were inadequate to evaluate
immunotoxicity.
In a two-generation reproduction study in rats using dietary
concentrations of 0, 40, 75, 250, or 500 ppm, the NOAEL was 75 ppm,
equal to 6.0 mg/kg bw/day, based on reduced weight of adults and
offspring at 250 ppm. Reproductive performance was unaffected.
In a teratology study in rats at doses of 0, 15, 30, or 60
mg/kg bw/day, the NOAEL for maternal toxicity was 15 mg/kg bw/day
based on reduced body-weight gain at 30 mg/kg bw/day. The NOAEL for
embryo-fetal toxicity was 30 mg/kg bw/day based on an increase in
pre-implantation loss at 60 mg/kg bw/day. In a study in rabbits at
doses of 0, 1, 5, or 10 mg/kg bw/day, the NOAEL for maternal and
embryofetal toxicity was 1 mg/kg bw/day based on clinical signs of
toxicity and gastric lesions in does and an increase in post-
implantation loss at 5 mg/kg bw/day. No teratogenic effects were
found in rats or rabbits.
After reviewing the available genotoxicity data the Meeting
concluded that fenbutatin oxide was not genotoxic.
The 1977 Joint Meeting reviewed two-year studies in rats and
dogs in which NOAELS of 2.5 mg/kg bw/day and 15 mg/kg bw/day,
respectively, were observed. A multi-generation reproduction study
in rats, in which the NOAEL was 100 ppm, equivalent to 5 mg/kg
bw/day, was also reviewed at that time.
The ADI of 0-0.03 mg/kg bw previously allocated in 1977 (which
was based on the NOAEL of 2.5 mg/kg bw/day (50 ppm) observed in a
two-year dietary study in rats in which an increase in SAP was
observed at higher doses) was retained. A lower NOAEL of 1 mg/kg
bw/day from a teratology study in rabbits, in which GIT irritation
was observed, was considered less reflective of human exposure
because of the high sensitivity of the GIT of the rabbit and the
particular physiological characteristics of this species. Therefore,
this study was not used as the basis of the ADI.
TOXICOLOGICAL EVALUATION
Level causing no toxicological effect
Rat: 50 ppm, equivalent to 2.5 mg/kg bw/day (two-year
study reviewed by JMPR in 1977)
75 ppm, equal to 6.0 mg/kg bw/day (two-generation
reproduction study)
15 mg/kg bw/day (teratology study, maternal toxicity)
Rabbit: 1 mg/kg bw/day (teratology study, maternal and
embryofetal toxicity)
Dog: 15 mg/kg bw/day (two-year study reviewed by JMPR in
1977).
Estimate of acceptable daily intake for humans
0-0.03 mg/kg bw
Studies which will provide information valuable in the continued
evaluation of the compound
1. Adequate information on the immunotoxic potential of
fenbutatin oxide.
2. Observations in humans.
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See Also:
Toxicological Abbreviations
Fenbutatin oxide (Pesticide residues in food: 1977 evaluations)
Fenbutatin oxide (Pesticide residues in food: 1979 evaluations)