NTP Study Reports
Abstract for TR-448 - Isobutyl Nitrite
TR-448
Toxicology and Carcinogenesis Studies of
Isobutyl Nitrite (CAS No. 542-56-3) in F344 Rats and
B6C3F1 Mice (Inhalation Studies)
Chemical Formula: C4H9NO2 | - | 3D Structure* |
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Isobutyl nitrite is used to a
limited extent as an intermediate in the syntheses of aliphatic
nitrites. It is also an ingredient of various incenses or room
odorizers and is used as a euphoric. The chemical has also been
used as a jet propellant and in the preparation of fuels. Isobutyl
nitrite was nominated by the Consumer Product Safety Commission
to the NTP for toxicology and carcinogenicity studies because
of its possible contribution to the high incidence of Kaposi's
sarcoma among male homosexual acquired immune deficiency syndrome
patients and because of the lack of available data on the potential
carcinogenicity of isobutyl nitrite. Male and female F344/N rats
and B6C3F1 mice were exposed to isobutyl nitrite (purity of 93%
or greater) by inhalation for 16 days, 13 weeks, or 2 years. Genetic
toxicology studies were conducted in Salmonella typhimurium ,
cultured Chinese hamster ovary cells, Drosophila melanogaster,
and mouse peripheral blood.
16-DAY STUDY IN RATS
Groups of five male and five female
F344/N rats were exposed to 0, 100, 200, 400, 600, or 800 ppm
(approximately 420, 840, 1,700, 2,500, or 3,300 mg/m3)
isobutyl nitrite by inhalation for 6 hours per day, 5 days per
week for a total of 12 exposures during a 16-day period. All males
and females exposed to 600 or 800 ppm and one 400 ppm female died
on the first day of the study. Final mean body weights and mean
body weight gains of 400 ppm males and females were significantly
lower than those of the controls. Clinical findings observed in
400 ppm males and females included ocular discharge, lethargy,
hunched posture, and rough coats. Absolute and relative lung weights
of all exposed groups of males and of 200 and 400 ppm females
were less than those of the controls. Chemical-related hyperplasia
of the bronchial epithelium was observed in 200 and 400 ppm males
and females and hyperplasia of the nasal turbinate epithelium
was observed in rats exposed to 400 ppm or less. Hemosiderin pigmentation
was observed in the spleen of 200 and 400 ppm males and females
and bone marrow hematopoietic hyperplasia was observed in rats
exposed to 400 ppm or less.
16-DAY STUDY IN MICE
Groups of five male and five female
B6C3F1 mice were exposed to 0, 100, 200, 400, 600, or 800 ppm
(approximately 420, 840, 1,700, 2,500, or 3,300 mg/m3)
isobutyl nitrite by inhalation for 6 hours per day, 5 days per
week for a total of 12 exposures during a 16-day period. Three
males and four females exposed to 800 ppm died before the end
of the study. Final mean body weights and mean body weight gains
of 600 and 800 ppm males and females were significantly lower
than those of the controls. Mice exposed to 400 ppm or greater
were lethargic and exhibited hunched posture and rough coats.
Absolute and relative lung weights of 600 and 800 ppm males and
the relative lung weight of 600 ppm females were significantly
greater than those of the controls. Chemical-related hyperplasia
of the bronchiolar epithelium was observed in all exposed groups
of males and females. Lymphocytic atrophy of the spleen and thymus
was observed in males and females exposed to 400 ppm or greater.
13-WEEK STUDY IN RATS
Groups of 10 male and 10 female
F344/N rats were exposed to 0, 10, 25, 75, 150, or 300 ppm (approximately
42, 105, 315, 630, or 1,260 mg/m3) isobutyl nitrite
by inhalation for 6 hours per day, 5 days per week for 13 weeks.
All rats survived to the end of the study. Final mean body weights
and mean body weight gains of 300 ppm males and females were significantly
lower than those of the controls, as was the mean body weight
gain of 150 ppm females. Clinical findings observed during the
study included ruffled fur in 300 ppm males and females, hypoactivity
in 300 ppm males, and hyperactivity in 150 and 300 ppm females.
A very mild chemical-related methemoglobinemia and anemia occurred
in male and female rats in the 75, 150, and 300 ppm groups. Hematopoietic
hyperplasia occurred in the bone marrow of all exposed groups
of males and females and was considered to be a secondary response
to the anemia and methemoglobinemia. There was minimal hemosiderin
pigment accumulation in the spleens of males and females exposed
to 75 ppm or greater, mild to moderate epithelial cell hyperplasia
of the nasal mucosa was observed in 300 ppm males and females,
and minimal hyperplasia occurred in 150 ppm males and females.
Hyperplasia of the bronchial epithelium was observed in 300 ppm
males and females.
13-WEEK STUDY IN MICE
Groups of 10 male and 10 female
B6C3F1 mice were exposed to 0, 10, 25, 75, 150, or 300 ppm (approximately
42, 105, 315, 630, or 1,260 mg/m3) isobutyl nitrite
by inhalation for 6 hours per day, 5 days per week for 13 weeks.
There were no chemical-related deaths. Final mean body weights
and mean body weight gains of 150 and 300 ppm females were significantly
less than those of the controls. Final mean body weights and mean
body weight gains of exposed groups of males were similar to those
of the controls. There were no chemical-related clinical findings.
A very mild chemical-related methemoglobinemia occurred in male
and female mice in the 150 and 300 ppm groups. A very mild anemia
occurred in the 300 ppm groups. In the lung, increased incidences
of mild to moderate hyperplasia of the bronchiolar epithelium
occurred in males and females exposed to 300 ppm. Minimal hyperplasia
occurred in males exposed to 75 ppm or greater and in females
exposed to 150 ppm. Minimal epithelial cell hyperplasia of the
nasal mucosa was observed in 300 ppm males. Increased hematopoiesis
of the spleen, secondary to the hematotoxicity, occurred in males
exposed to 75 ppm or greater and in females exposed to 150 or
300 ppm. Increased hemosiderosis of the spleen occurred in males
exposed to 300 ppm and in females exposed to 75 ppm or greater.
2-YEAR STUDY IN RATS
Based on the low final mean body
weights, anemia, and the mild to moderate nasal mucosal lesions
and the hyperplastic bronchial lesions observed in 300 ppm males
and females, isobutyl nitrite exposure concentrations selected
for the 2-year inhalation study in rats were 37.5, 75, and 150
ppm.
Groups of 56 male and 56 female
rats were exposed to 0, 37.5, 75, or 150 ppm (equivalent to 0,
158, 315, or 630 mg/m3) isobutyl nitrite by inhalation
for 6 hours per day, 5 days per week, for 103 weeks. Ten male
and 10 female rats from each group were evaluated at 15 months
for clinical pathology and histopathology.
Survival, Body Weights,
Clinical Findings, Hematology, and Clinical Chemistry
Survival rates of exposed groups
of rats were greater than those of the controls, and the survival
rates of 75 and 150 ppm males were significantly greater than
that of the control. Mean body weights of 150 ppm males and females
were 3% to 11% lower than those of the controls throughout the
course of the study. There were no clinical findings considered
to be related to isobutyl nitrite exposure. A very mild methemoglobinemia
and anemia occurred in male and female rats exposed to 75 or 150
ppm.
Pathology Findings
Incidences of alveolar/bronchiolar
adenoma and alveolar/bronchiolar adenoma or carcinoma (combined)
occurred with significant positive trends in exposed males and
females, and the incidences of these neoplasms in 75 ppm males
and in 150 ppm males and females were significantly greater than
those in the controls. The incidence of alveolar/bronchiolar carcinoma
was significantly greater in 150 ppm male rats than that in the
controls. The incidences of alveolar epithelial hyperplasia were
also increased in 75 and 150 ppm males and in all exposed groups
of females. The incidences of mononuclear cell leukemia in exposed
groups of males and females were significantly less than those
in the controls.
2-YEAR STUDY IN MICE
Based on the low final mean body
weight of 300 ppm females and the mild to moderate bronchiolar
hyperplasia observed in 300 ppm males and females, isobutyl nitrite
exposure concentrations selected for the 2-year inhalation study
in mice were 37.5, 75, and 150 ppm.
Groups of 60 male and 60 female
mice were exposed to 0, 37.5, 75, or 150 ppm (equivalent to 0,
158, 315, or 630 mg/m3) isobutyl nitrite by inhalation
for 6 hours per day, 5 days per week, for 103 weeks. As many as
10 male and 10 female mice from each group were evaluated at 15
months for clinical pathology and histopathology.
Survival, Body Weights, Clinical Findings, and Hematology and Clinical Chemistry
Survival rates of exposed groups
of males were similar to those of the controls. Survival rates
of exposed groups of females were greater than those of the controls,
and the survival rate in 37.5 ppm females was significantly greater
than that of the controls. Mean body weights of exposed groups
of males and of 37.5 and 75 ppm females were similar to those
of the controls throughout the study. Mean body weights of 150
ppm females were lower than those of the controls from week 20
until the end of the study. There were no biologically significant
clinical findings noted in the 2-year study in mice. A very mild
methemoglobinemia and anemia occurred in male and female mice
exposed to 75 or 150 ppm.
Pathology Findings
Incidences of alveolar/bronchiolar
adenoma and alveolar/bronchiolar adenoma or carcinoma (combined)
occurred with significant positive trends in exposed males and
females, and the incidences of these neoplasms were significantly
greater than those in the controls in 75 ppm males and in 150
ppm males and females. Incidences of alveolar epithelial hyperplasia
were significantly increased in 75 and 150 ppm male and female
mice. Thyroid gland follicular cell adenoma occurred with a significant
positive trend in male mice; the incidences of thyroid gland follicular
cell hyperplasia were
increased in all exposed groups of males, and the incidences in
males exposed to 37.5 or 150 ppm were significantly greater than
those in the controls. Incidences of serous exudate and olfactory
epithelium atrophy in the nose of 150 ppm females were significantly
greater than those in the controls. Incidences of minimal to mild
hemosiderin pigment in the spleen of 75 and 150 ppm male mice
were significantly greater than those in the controls.
GENETIC TOXICOLOGY
Isobutyl nitrite was found to
be mutagenic in vitro and in vivo. It induced base-pair
substitution mutations in Salmonella typhimurim strains
TA100 and TA1535 and sister chromatid exchanges and chromosomal
aberrations in cultured Chinese hamster ovary cells. Positive
responses in the S. typhimurium tests required S9 activation,
but isobutyl nitrite induced chromosomal effects in cultured Chinese
hamster ovary cells with and without S9. In vivo, no induction
of sex-linked recessive lethal mutations was noted in the germ
cells of male Drosophila melanogaster exposed to isobutyl
nitrite via feeding or injection. However, significant increases
in micronucleated normochromatic erythrocytes were observed in
the peripheral blood of male and female mice treated with isobutyl
nitrite for 90 days by inhalation.
CONCLUSIONS
Under the conditions of these
2-year inhalation studies, there was clear evidence of carcinogenic
activity of isobutyl nitrite in male and female F344/N rats
based on the increased incidences of alveolar/bronchiolar adenoma
and alveolar/bronchiolar adenoma or carcinoma (combined). There
was some evidence of carcinogenic activity of isobutyl
nitrite in male and female B6C3F1 mice based on the increased
incidences of alveolar/bronchiolar adenoma and alveolar/bronchiolar
adenoma or carcinoma (combined) in males and females. The increased
incidence of thyroid gland follicular cell adenoma in male mice
may have been related to isobutyl nitrite exposure.
Exposure of rats and mice to isobutyl
nitrite by inhalation for 2 years resulted in increased incidences
of alveolar epithelial hyperplasia (male and female rats and mice),
thyroid gland follicular cell hyperplasia and splenic hemosiderin
pigmentation (male mice), and serous exudate and atrophy of the
olfactory epithelium of the nose (female mice).
Exposure of rats to isobutyl nitrite by inhalation for 2 years resulted in decreased incidences of mononuclear cell leukemia in males and females.
Report Date: July 1996
Pathology Tables, Survival and Growth Curves from NTP 2-year Studies
Target Organs & Incidences from 2-year Studies
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