NITROFURAL
(NITROFURAZONE)
First draft prepared by
Dr K.N. Woodward
Veterinary Medicine Directorate
Ministry of Agriculture, Fisheries and Food
Weybridge, Surrey, United Kingdom
Nitrofural also known as
nitrofurazone, is a broad-spectrum
bactericidal drug. It also possesses some
antiprotozoal activity. It
is used both therapeutically and
prophylactically in a number of
food-producing species including pigs, sheep,
goats, cattle,
chickens and turkeys (Orphahell, 1991). The
chemical structure is
shown below in Figure 1.
Nitrofural had not been previously
evaluated by the Joint
FAO/WHO Expert Committee on Food Additives.
2.1.1.1
Rats
Nitrofural appeared to be well
absorbed in the rat after oral
administration of 100 mg/kg bw. At 4 h
after administration plasma
levels had reached 4.5 mg/l. No other time
points were investigated.
Around 5% and 0.5% of the dose were
excreted in the 24 h urine and
faeces, respectively. As acknowledged by
the authors, the values are
likely to be underestimates as
nitrofurazone was shown in this study
to be rapidly metabolized after incubation
with rat liver slices.
There was no evidence of deposition in any
organ (Paul et al.,
1960).
2.1.1.2 Cattle
A single oral dose of 14 mg/kg bw
nitrofural administered to
five pre-ruminant calves resulted in
maximum plasma concentrations
of 3.5 mg/litre at 3 h after
administration. The final elimination
half-life was 5 h and around 2% of the
administered dose was
recovered in the urine (Nouws et al., 1987).
As described in paragraph 2.1.1.1,
nitrofural is rapidly
metabolized by rat liver slices.
Nitrofurans, including nitrofural,
undergo metabolic reduction at the nitro
group to generate reactive
species which can covalently bind to
cellular macromolecules
(Polnaszek et al., 1984; Kutcher & McCalla, 1984;
McCalla 1979;
McCalla et al., 1975). Data obtained from studies
with isolated
perfused rat liver suggested that
conjugation to glutathione
followed by biliary excretion was
important; only 0.27% of the
administered perfusate was excreted
unchanged (Sorrentino et al.,
1987). Conjugation with amino acids such as
methionine may also
occur (Hoener, 1988).
Ring opening of nitrofurans is known
to occur (Swaminathan &
Lower, 1978) but there are insufficient
data to demonstrate this
with nitrofurazone.
In chickens nitrofural increased
gamma-aminobutyric acid and
glutamate concentrations in the brain at
oral doses of 25 and 50
mg/kg bw for 5 days (Ali, 1988).
Nitrofural, given orally at doses
of 10 and 20 mg/kg bw for 7 days, induced
thiamine deficiency in
chickens (Ali 1983).
Nitrofural was found to inhibit the
uptake of oxygen by various
rat tissues (Paul et al., 1952). Nitrofural inhibited
respiration
in isolated mouse liver mitochondria
suggesting that these may be
potential sites for toxicity (Lim et al., 1986).
Nitrofural and other nitrofurans are
relatively potent
inhibitors of MAO in mammalian and avian
species (Hoogenboom, 1991).
As with other nitrofurans, nitrofural
can affect the endocrine
system resulting in alterations of hormone
levels. In laying
turkeys, nitrofural decreased luteinizing
hormone and prolactin in
plasma (Ali et al., 1987). Similar effects were noted
in young
male turkeys (Ali et al., 1988).
Nitrofural has been shown to inhibit
utilization of
progesterone as a steroid percursor by the
adrenal in rats. In this
study it was shown that nitrofurans,
including nitrofural, block
11-hydroxylation of deoxycorti-costerone to
corticosterone.
Sprague-Dawley rats (1 year old) given
nitrofurans for 325 days
underwent constant oestrous. The authors
concluded that nitrofurans
were antagonists of normal adrenal function
and believed that this
could lead to endocrine stress (King &
Currie, 1972).
The LD50 values for
nitrofural are shown in Table 1. Signs of
toxicity included roughened coats,
hyperexcitability, tremors,
reduced activity, weakness, convulsion, and
respiratory collapse
(Krantz & Evans 1945; Anderson, 1983).
Table 1. Acute toxicity of nitrofural
Species/strain Sex Route LD50 Reference
(mg/kg bw)
Rat (unspecified) M
oral 590 Krantz & Evans,
1945
Rat (Donryu) M&F oral 590 Miyaji, 1971
Rat (Wistar) M oral 800 Anderson, 1983
Mouse ? oral 380 Krantz & Evans,
(unspecified) 1945
Mouse (ICR Jcl) M&F oral 590 Miyaji, 1971
Mouse ? i.p. 300 Smith et al.,
(unspecified) 1963
2.2.2.1
Mice
Groups of 5 male and 5 female B6C3F1
mice were given
diets containing 0, 630, 1250, 2500, 5000
or 10 000 ppm nitrofural,
equivalent to 0, 94.5, 188, 375, 750 or
1500 mg/kg bw/day for 14
days.
All mice given the three highest
dietary levels died during the
study, as did 3/5 given 188 mg/kg bw/day.
Animals in all the groups
except females given 94.5 mg/kg bw/day lost
weight and all dosed
animals displayed rough coats and
convulsive seizures (National
Toxicology Program (NTP), 1988).
Groups of 10 male and 10 female B6C3F1
mice were given
diets containing 0, 70, 150, 310, 620 or
1250 ppm nitrofural,
equivalent to 0, 10, 23, 47, 93 or 188
mg/kg bw/day for 13 weeks.
High mortality was noted at the
highest dietary level (6/10
males and 9/10 females). Mortality was
relatively high at the
penultimate dietary level (3/10 males and
5/10 females). Elevated
relative liver weights were noted in mice
given the two highest
dietary levels. Animals given the two
highest dietary levels also
displayed hyperexcitability and convulsive seizures,
and males in
these groups had a very high incidence
(80-90%) of testicular
hypoplasia (NTP, 1988).
2.2.2.2
Rats
Groups of 10 male and 10 female F344/N
rats were given diets
containing 0, 630, 1250, 2500, 5000 or 10
000 ppm nitrofural,
equivalent to 0, 63, 125, 250, 500 or 1000
mg/kg bw/day, for 14
days.
All rats given 500 or 1000 mg/kg
bw/day died. Feed consumption
was decreased at all levels in excess of
the lowest level. Signs of
toxicity included lethargy and roughened
coats. Seizures occurred in
animals given 250 mg/kg bw/day and above.
Histopathologic
examination revealed aspermatogenesis in
dosed males (NTP, 1988).
Groups of 10 male and 10 female F344/N
rats were given diets
containing 0, 150, 310, 620, 1200 or 2500
ppm nitrofural, equivalent
to 0, 15, 31, 62, 120 or 250 mg/kg bw/day,
for 13 weeks.
All rats survived until the end of the
study but final body
weights of animals given 62, 120 or 250
mg/kg bw/day were 11%, 32%
and 55% lower, respectively, than control
values. Relative liver
weights for treated rats were significantly
higher than control
animals. Atrophy constricture of the
hind-quarters occurred in males
and females given the highest dietary
level.
Moderate to severe degeneration of the
seminiferous epithelium
of the testes was noted in males given the
4 highest doses. The
no-effect-level was 15 mg/kg bw/day. Males
and females given the two
highest dietary levels had osteoporosis in
the metaphyseal region.
(NTP, 1988).
In a study designed to investigate
effects on the testes,
45-day-old male Donryu rats were given a
diet containing 0.2%
nitrofural (equal to 300 mg/kg bw/day) for 9
days. Three to 6 rats
given nitrofural were sacrificed every day
after feeding began.
A decrease in testis weight occurred
by day 5 but
histologically, a decrease in spermatozoa
and spermatids occurred at
day 1, with disappearance by days 3 and 4.
Here, the stratified
appearance of the normal testis had
disappeared and only basal
spermatogonia and Sertoli cells remained.
Slight oedema and
exudation occurred in interstitial tissue
but Leydig cells appeared
normal (Miyaji et al., 1964).
2.2.3.1
Mice
Groups of 50 male and 50 female B6C3F1
mice were given
diets containing 0, 150 or 310 ppm
nitrofural equal to 0, 14 or 29
mg/kg bw/day for 2 years.
There were no effects on body weights
and survival was only
slightly affected at the highest dose in
males and females (27/50
and 35/50 for males and females,
respectively) compared with
controls (39/50 for each sex).
Females given nitrofural were found to
have increased
incidences of ovarian atrophy (44/50 and
38/50 in low- and high-dose
groups respectively) compared with controls
(7/47) and tubular cell
hyperplasia (23/50 and 21/50 in low- and
high-dose groups) when
compared with controls (1/47). However,
there were no effects on the
testes in males.
There were increased incidences of
ovarian granulosa cell
tumours in low- (4/50) and high-dose (9/50)
females when compared
with control values (1/47). Similarly,
there were increased
incidences of benign mixed tumours of the
ovary in low- (17/50) and
high-dose (20/50) females compared with
controls (0/47). There were
no elevated incidences of any tumour type
in male mice (NTP, 1988;
Kari et al., 1989).
In a study of transplacental
carcinogenesis, a group of 20
pregnant ICR/Jcl mice was given 3
subcutaneous injections at
75 mg/kg bw nitrofural on days 13, 15 and
17 of gestation. The
incidence of tumours in offspring was not
increased when compared
with untreated controls when they were
examined 32 weeks after
birth.
In the same study, newborn mice (61)
were given a subcutaneous
injection at 75 mg/kg bw 12 h after birth
and on days 7, 14 and 21.
The offspring were examined at 32 weeks
after birth when an
increased incidence of pulmonary papillary
adenomas was noted
(Nomura et al., 1984).
2.2.3.2
Rats
A group of 20 female Holtzman rats was
fed a diet containing
1000 ppm nitrofural (150 mg/kg bw/day) for
36 weeks. A similar group
of 30 females was fed the same level for
44.5 weeks. Animals were
killed 15-19 weeks after dosing was
completed. At termination, there
was an elevated incidence of mammary
tumours in rats fed
nitrofural-containing diets when compared
with controls (Morris
et al., 1969; Morris, 1965).
Groups of 35 female Holtzman rats were
fed diets containing
0, 500 or 1000 ppm nitrofural (0, 75 or 150
mg/kg bw/day) for 45
weeks, and then a basal diet for a further
8 weeks. Rats consuming
nitrofural at 75 mg/kg bw/day nitrofural
had no increased tumour
incidence but in those given 150 mg/kg
bw/day the numbers of
tumour-bearing rats were increased. At week
53, the numbers with
detectable mammary tumours were
significantly increased in both the
75 (10/35) and 150 (12/35) mg/kg bw/day
groups, when compared with
untreated controls (2/35). None of the
reported tumours were
malignant (Siedler & Searfoss, 1966).
Groups of 20 male and 20 female CFE
rats were fed diets
containing nitrofural to give a daily
intake of 50-55 mg/kg bw/day
for 45 weeks. At the end of this period the
animals were returned to
the basal diets for a further 7 weeks.
There was no increased
incidence of any tumours in the male rats.
A significant increase in
the incidence of benign mammary tumours was
noted at week 52 (12/20)
compared with untreated controls (0/20)
(Siedler & Searfoss, 1967).
Groups of 50 male and 50 female F344/N
rats were fed diets
containing 0, 310 or 620 ppm nitrofural
equal to 0, 11 or 24 mg/kg
bw/day for 2 years. There were reductions
in the rate of weight gain
and final body weights were reduced in
males and females given
dietary nitrofural. Mortality was increased
in males given the high
dietary level (30/50; 60%) compared with
controls (17/50; 34%).
The major non-neoplastic finding was
degeneration of joint
articular cartilages in males and females
in several areas of the
body, notably, the vertebral and knee
joints. A no-effect-level was
not identifiable. Testicular degeneration
occurred in 49/50 low
dietary level and 47/50 high dietary level
males compared with 12/50
in controls.
There was an increased incidence of
mammary fibroadenomas in
both low and high dietary level females
(36/50; 72% in both groups)
when compared with controls (8/49; 16%) but
the incidence of
adenocarcinomas was not elevated. In males,
the incidence of
mesotheliomas in the tunica vaginalis was
increased but not in a
dose-related manner (0/50, 7/50 and 2/50 in
the control, low and
high dietary level groups). There was also
an increased incidence of
sebaceous adenoma of the skin of male rats
but only at the high
dietary level (0/50, 0/50 or 5/50 for
controls, low and high dietary
levels, respectively). A slightly increased
incidence of carcinoma
of the preputial gland was seen in males
(1/50, 8/50 and 5/50 in the
control, low and high dietary groups,
respectively). The incidence
of
testicular tumours was reduced in dosed males (NTP, 1988; Kari
et al., 1989).
It was concluded by the authors that
the data from male rats
provided only equivocal evidence of
carcinogenicity. The increases
in incidences of the various tumours were
small and not
dose-related.
The induction of ovarian tumours noted
in mice treated with
chemicals is relatively rare. Such an
effect had been noted with
only 8 chemicals from the NTP studies among
which were nitrofural
itself, 1,3-butadiene, 4-vinylcyclohexene,
nitrofurantoin, benzene
and delta-9-tetrahydrocannabinol. The
tumours were mainly in mice
but rats too were also affected (Maronpot,
1987).
Nitrofural is unusual in that of 143
chemicals which had been
tested by the NTP by 1989, only 5,
including nitrofural, produced
only benign neoplastic lesions (Huff et al., 1989). Many other
nitrofurans tested have been shown to be
carcinogenic in rodents and
many of these have produced malignant
tumours (Cohen & Bryan, 1973;
Cohen, 1978; Cohen et al., 1973).
Nitrofural and other nitrofurans
inhibited the
hepatocarcinogenesis by 4-(dimethylamino)
azobenzene in rats (Akao
et al., 1971). It also inhibited the
growth of fibrosarcomas in
mice and demonstrated antineoplastic
properties against seminomas
and carcinoma of the prostate in humans
(Friedgood & Green, 1950;
Friedgood & Ripstein, 1951).
The International Agency for Research
on Cancer (IARC) has
concluded that there was limited evidence
for the carcinogenicity of
nitrofurazone in experimental animals and
inadequate evidence for
its carcinogenicity in humans. In its overall
evaluation, IARC
placed nitrofural in Group 3 (not
classifiable as to its
carcinogenicity) (IARC, 1990).
No information available.
2.2.5.1
Mice
A group of 16 pregnant ICR/Jcl mice was
given subcutaneous
injections at 100 mg/kg bw nitrofural on
days 9, 10 and 11 of
gestation. A group of 6 pregnant mice was
given a single
subcutaneous injection at 300 mg/kg bw
nitrofural on day 10 of
gestation. Controls were given water only
on days 9, 10 and 11 of
gestation.
The 100 mg/kg bw regime did not
produce an elevated incidence
of malformations in 185 fetuses examined.
However, the 300 mg/kg bw
dose on day 10 of gestation led to an
overall incidence of
malformations of 21% compared with 0.3% in
controls. These were
mainly tail anomalies, oligodactyly, and
leg defects (unspecified)
(Nomura et al., 1984).
In a prescreening developmental test,
female CD-1 mice were
given nitrofural (100 mg/kg bw/day) by
gavage on days 6-13 of
gestation. Nitrofural caused a decrease in
the numbers of viable
litters (28/35 compared with 45/45 in
controls) and slight
reductions in birth weights. By the
authors' criteria, these effects
constituted positive results (Hardin et al., 1987).
In a preliminary teratogenicity study,
groups of 6-7 CD-1 mice
were given diets containing 0, 100, 500,
1000, 2000 or 3000 ppm
nitrofural, equivalent to 0, 15, 75, 150,
300 or 450 mg/kg bw/day,
on days 6-17 of gestation.
Dams given the two highest dietary
levels died during the
study. There were no differences in food
consumption in the other
groups and no effects on weight gain. There
were no differences in
implantation site and resorption parameters
except in those given 75
mg/kg bw/day where an increased incidence
of resorptions was noted
(Price et al., 1985).
In a second preliminary teratology
study, groups of 12 pregnant
CD-1 mice were given diets containing 0,
75, 250, 500 or 750 ppm
nitrofural equivalent to 0, 11, 38, 75 or
112 mg/kg bw/day, on days
6-17 of gestation. Approximately 8%
mortality occurred at the
highest dietary level and hyperactivity,
tremor, paralysis and
convulsions were seen at the two highest
levels. There were
dose-related decreases in food consumption
and body weights. Gravid
uterine weights decreased with increasing
nitrofural intake. There
were no effects on implants/dam but the
numbers of resorptions per
litter increased with dose. There were no
increases in the
incidences of malformations (Price et al., 1985).
In the main teratogenicity study,
groups of 26 pregnant CD-1
mice were given diets containing 0, 38, 75,
250 or 500 ppm
nitrofural, equivalent to 0, 6, 11, 38 or
75 mg/kg bw/day on days
6-15 of gestation. There were no maternal
deaths and no effects on
food and water consumption. Gravid uterine
weights were unaffected
by compound intake and there were no
effects on reproductive
parameters except for an increased
incidence of late fetal deaths at
marginally maternally toxic doses. The NOEL
was 11 mg/kg bw/day
(Price et al., 1985).
2.2.5.2
Rabbits
Groups of 22-27 New Zeeland white
rabbits were given gavage
doses of 0, 5, 10, 15 or 20 mg/kg bw/day
nitrofural in corn oil on
days 6-19 of gestation.
At sacrifice, the uterine contents of
121 pregnant animals were
examined. There was no evidence of any
teratogenic effect nor
materno- or embryo/feto-toxicity at doses
of 5-15 mg/kg bw/day. At
20 mg/kg bw/day, two dams died, there was reduced
weight gain,
relative and absolute maternal liver
weights were increased and an
increased incidence of fetal
malformations/litter was observed,
which was suggestive of fetotoxicity.
Hence, effects on the
developing fetus occurred only at
maternally toxic levels
(Price et al., 1987). The NOEL was 15 mg/kg
bw/day.
Nitrofural has been tested in a range
of studies (Table 2). In
general, in vitro studies of forward mutation in
bacteria and
mammalian cells, sister chromatid exchange,
DNA damage and tests for
clastogenicity have proved positive while in vivo studies,
including those for cytogenetic effects and
the micronucleus test,
have proved negative.
In studies where the metabolic activation
was incorporated into
the agar layer, the mutagenic activities of
a number of nitrofurans
(and nitroimidazoles) were reduced,
suggesting that the metabolites
of reductive metabolism may be less mutagenic
than those formed
after oxidative metabolism (Skeggs et al., 1984). Metabolic
reduction is believed to be the major route
of biotransformation of
nitrofurans and nitroimidazoles and the
metabolites so formed may
account for the radiosensitizing properties
of these agents (Olive,
1979, 1980, 1978b).
The results obtained with nitrofural
are similar to those noted
with a wide range of nitrofurans, and in
particular, the propensity
of these compounds for activity in in vitro systems is evident
(Klemencic & Wang, 1978). The mechanism
of mutagenicity is unclear,
as is the route of activation, although
nitroreduction appears to be
an early step (McCalla, 1981, 1983;
Matsuoka et al., 1979; McCalla
et al., 1970, 1971). Nitrofural and other
nitrofurans are
cytotoxic to hepatocytes in vitro (Hoogenboom et al., 1991), but
as the tumours caused by nitrofural are
largely in endocrine organs,
this is unlikely to be relevant to
carcinogenesis by this substance.
Nitrofural is a skin sensitizer in
humans and there have been
reports of contact dermatitis following
exposure to the drug (Bajaj
& Gupta, 1986; Laubstein and Niedergesass,
1970; Lo et al., 1990).
When used as an experimental
antineoplastic agent in humans
with prostatic or testicular tumours,
testicular degeneration
occurred similar to that noted in
experimental animals (Friedgood &
Ripstein, 1951). The doses used were
unclear in the original text.
In the Collaborative Perinatal Project
conducted in the USA by
the National Institute of Neurological and
Communicative Diseases
and Stroke in 1958-1965, only 0.5% (234/50282)
of mother-child pairs
had been exposed to nitrofural during
months 1-4 of gestation. There
was no increased risk of malformations in
this group (Heinonem
et al., 1977).
Table 2. Results of genotoxicity assays
on nitrofurazone
Test system Test object Concentration Results Reference
Ames test1 S. typhimurium 0-75 µg/ml Positive Baars et al.,
TA1538
1980
Ames test1 S. typhimurium 0-5 µg/plate Positive Goodman et al.,
TA98, TA100
1977
Ames test1 S. typhimurium 0-100 µg/plate Positive NTP, 1988
TA100, TA1535,
TA1537, TA98
Ames test1 S. typhimurium 1-20 µg/plate Positive Ni et al., 1987
TA98, 98NR, 98/
1.8-DNP6
Ames test1 S. typhimurium 0.1-10 µg/plate Positive Yahagi et al.,
TA1535, TA100
1976
Ames test1 S. typhimurium 0-250 nmole/plate Positive Gajewska et al.,
TA97, TA102
1990
Ames test1 S. typhimurium 0.1-100 µg/plate Positive Chandler &
TA98 Parenti,
1982
Forward mutation S. typhimurium 0-0.5 µg/ml Positive Karube et al.,
TA100 electrode 1982
Forward mutation1 E. coli 0-75 µg/ml Positive Baars et al.,
343/113/(R-9
1980
Table 2. cont'd
Test system Test object Concentration Results Reference
Forward mutation1 E. coli WP2 0-240 µM Weak Bryant & McCalla,
Positive 1980
Forward mutation1 E. coli K12 0-10 mm Positive Chessin et al.,
1978
Forward mutation1 E. coli W2
10-300 µM Positive Lu et al., 1979
WP2 uvrA 0-250 µM Positive
Forward mutation2 E. coli K12 not specified Positive Maier et al.,
343/13
1979
Forward mutation1 E. coli WP2 30-120 µM Positive McCalla &
uvrA-
Voutsinos, 1974
Forward mutation1 E. coli B and S unspecified Positive Zampieri &
strains
Greenberg, 1974
Fluctuation test1 S. typhimurium 0-1 ng/ml Positive Green et al., 1977
TA1535, TA100
Positive
E. coli WP2 uvrA 0-100 ng/ml
Forward mutation1 A. nidulans 0-1000 µg/plate Negative Bignami et al.,
1982
Forward mutation1 N. crassa 0-200 µg/ml Positive Ong, 1977
74-OR60-29A
SOS Chromotest E. coli PQ37 0.01-01 ng/sample Positive Gajewska et al.,
1990
Table 2. cont'd
Test system Test object Concentration Results Reference
DNA damage Mouse L cells 500 µM Positive Olive & McCalla,
1977
DNA damage Mouse L929 cells )0-500 µM Positive Olive & McCalla,
Hamster BHK21 ) 1975
cells )
Human KB cells )
DNA damage Swiss mice 0.1% in diet Positive Olive, 1978a
in vitro (150 mg/kg
bw/day)
Unscheduled DNA Rat thymocytes unspecified Positive Tempel, 1980
synthesis
Unscheduled DNA Rat and mouse 1.1 x 10-3-1.1 mg/ml Negative
Mori et al., 1987
synthesis hepatocytes
Sister chromatid Chinese hamster 0-15 µg/ml Positive NTP, 1988
exchange ovary cells
Forward mutation1 Mouse lymphoma 0-300 µg/ml Positive NTP, 1988
L5178Y cells 0-400 µg/ml
Forward mutation1 Chinese hamster 100 µg/ml Positive Olive, 1981
V79 cells
Forward mutation1 Chinese hamster 0-200 µg/ml Negative Phillips, 1983
ovary - HGPRT
Forward mutation1 Chinese hamster 0-200 µg/ml Negative Anderson &
ovary - HGPRT Phillips, 1985
Table 2. cont'd
Test system Test object Concentration Results Reference
Sex-linked D. melanogaster 5 mM Negative Kramers, 1982
recessive lethal
mutation
In vitro Chinese hamster 0-200 µg/ml Positive Anderson &
cytogenetics bone marrow
Phillips, 1985
In vitro Chinese hamster 0-600 µg/ml Positive NTP, 1988
cytogenetics ovary
In vitro Chinese hamster 0.1 mg/ml Positive3 Matsuoaka et al.,
cytogenetics cells (male
1979
In vitro Human lymphocytes not specified Positive Tonomura &
cytogenetics
Sasaki, 1978
In vivo Chinese hamster 0-400 mg/kg bw Negative Anderson &
cytogenetics bone marrow Phillips, 1985
In vivo Wistar rat bone 0-400 mg/kg bw or Negative Anderson, 1983
cytogenetics marrow 1-150 mg/kg
bw/day for 5 days
Micronucleus test Sprague-Dawley rat )0-60 mg/kg bw; Negative Goodman et al.,
)half dose 30 h and
1977
Long Evans rat )half dose 6 h,
)prior
to sacrifice
1 With and without hepatic metabolic
activation.
2 With and without testicular metabolic
activation.
3 In presence but not absence of metabolic
activation.
The results of acute and short-term
toxicity studies,
carcinogenicity studies, and teratogenicity
studies were considered
by the Committee, in addition to several
genotoxicity studies. Some
limited pharmacokinetic data were also
available which demonstrated
good absorption of the drug in rats and
cattle, but no data were
available on the extent of
biotransformation nor on the identities
of any metabolites formed. A comparison
with other nitrofurans
suggests that nitrofural probably undergoes
extensive
biotransformation.
In acute oral toxicity studies,
nitrofural was slightly toxic
to mice (LD50 = 300-600 mg/kg
bw) and rats (LD50 = 600-800 mg/kg
bw).
Testicular hypoplasia was noted in
rats and mice given dietary
nitrofural for 13 weeks. The NOEL for this
effect was 15 mg/kg
bw/day in a 13-week study in rats.
Nitrofural was tested for its ability
to induce forward
mutations in bacteria, yeast, and mammalian
cells in vitro. In the
majority of these studies it produced
positive results. Positive
results were also noted in tests for DNA
damage in bacteria and in
mammalian cells in vitro, in in vitro studies for clastogenic
effects in Chinese hamster ovary or bone
marrow cells, and in human
lymphocytes. However, in vivo studies of clastogenic potential
in
Chinese hamster or rat bone marrow were
negative. The results
suggest that nitrofural is genotoxic in vitro but not in vivo.
Carcinogenicity studies with
nitrofural were conducted in rats
and mice. In a 2-year dietary study in B6C3F1
mice, the
animals were given nitrofural equal to
daily oral doses of 14 or
29 mg/kg bw/day. In female mice, nitrofural
induced a high incidence
of ovarian atrophy and of ovarian
tubular-cell hyperplasia in both
dose groups when compared with controls.
There were also increased
incidences of ovarian granulosa-cell and
benign mixed tumours in
low-and high-dose groups. There were no
elevated incidences of any
tumour type in male mice.
In a 2-year carcinogenicity study in
rats, testicular
degeneration was one of the major
non-neoplastic findings. There
were also elevated incidences of
degeneration of the joint articular
cartilages at several sites in both males
and females. No-effect
levels could not be identified for these
findings. There was an
increased incidence of mammary
fibroadenomas in females given the
low and high doses of 11 and 24 mg/kg
bw/day.
Teratogenicity studies were conducted
in mice and rabbits. In
mice, the rate of late fetal death was
increased in treated animals
at maternally toxic doses; the NOEL was 11
mg/kg bw/day. Pregnant
rabbits were given gavage doses of up to 20
mg/kg bw/day. At the
highest dose, there was an elevated
incidence of minor fetal
malformations per litter but this dose
produced maternal toxicity.
The NOEL was 15 mg/kg bw/day. The data
suggest that nitrofural is
not a direct teratogen but produces
fetotoxic effects at maternally
toxic doses in mice and rabbits.
Although nitrofural was tumorigenic in
rats and mice, the
tumours it produced were benign and were
restricted to endocrine
organs and the mammary gland. Moreover, the
mutagenicity studies
suggest that nitrofural is genotoxic in vitro but not in vivo.
Taken together, the data suggest that
nitrofural is a secondary
carcinogen producing its effects in
endocrine-responsive organs by a
mechanism that remains to be elucidated. In
rats, nitrofural has
been shown to disrupt the utilization of
progesterone and blocks the
synthesis of cortisone from deoxycortisone.
Effects on
steroidogenesis may therefore be involved
in the process of tumour
formation. The closely related compound
nitrofurantoin also produced
benign ovarian tumours in the same strain
of mouse, which were
thought to be secondary to drug-induced
ovarian atrophy. This type
of effect cannot be excluded in the
biogenesis of the ovarian
tumours produced by nitrofural. However,
the Committee recognized
that no-effect levels for these effects
could not be identified. The
Committee noted that nitrofural had been
reviewed by the
International Agency for Research on
Cancer, which concluded that
there was limited evidence for the
carcinogenicity of nitrofural in
animals but inadequate evidence for humans.
The Committee concluded that it could
not establish an ADI for
nitrofural because no-effect levels had not
been established for the
tumorigenic effects. It noted that,
although a no-effect level
(15 mg/kg bw/day) had been established in
rats for testicular
hypoplasia in a 13-week study, a slightly
lower dose (11 mg/kg
bw/day) resulted in a high incidence of
testicular degeneration in
the 2-year carcinogenicity study. There was
no NOEL for this effect
in the 2-year study and, moreover, no study
on reproductive
performance was available. The degenerative
changes in the joints of
rats were seen in both males and females
but a no-effect level could
not be established.
Before reviewing nitrofural again the
Committee would wish to
see:
1.
Further data from long-term studies in rats which would
allow the identification of
no-effect levels for the
effects on joint articular
cartilages and for testicular
degeneration.
2.
Data to support the view that tumour formation in rodents
following nitrofural
administration has an endocrine
origin. If a mechanism were
demonstrated, no-effect levels
for suitable end-points would
need to be identified.
3.
Additional data on the identity, quantity and biological
characteristics of nitrofural
metabolites.
nitofurazon adalah,nitrofural adalah
