FEBANTEL, FENBENDAZOLE AND OXFENDAZOLE
First draft prepared by
Dr R. Fuchs
Department of Experimental Toxicology and Ecotoxicology
Institute for Medical Research and Occupational Health
Febantel, fenbendazole and oxfendazole were previously evaluated
at the thirty-eighth meeting of the Committee (Annex 1, reference 97).
A temporary ADI of 0-25 µg/kg bw was established for fenbendazole
at that time, based on a NOEL of 5 mg/kg bw/day in a long-term
toxicity/ carcinogenicity study in rats and a safety factor of 200.
Additional information on the mechanism of the observed increased
incidence of liver tumours in female rats at high doses, including the
results of an in vivo DNA binding study, was requested.
A temporary ADI of 0-10 µg/kg bw was established for febantel at
the thirty-eighth meeting, based on a NOEL of 2 mg/kg bw/day in a
2-generation reproductive toxicity study in rats and a safety factor
of 200. Additional information on the genotoxic and carcinogenic
potential of febantel, including the results of an in vivo DNA
binding assay in rats, was requested.
A temporary ADI of 0-4 µg/kg bw was established for oxfendazole
at the thirty-eighth meeting, based on a NOEL of 0.7 mg/kg bw/day in a
carcinogenicity study in rats and a safety factor of 200. Additional
data from genotoxicity and teratogenicity studies were requested,
including the results of an in vivo DNA binding assay in rats.
At its present meeting, the Committee considered the data
available at the thirty-eighth Committee meeting (Annex 1,
reference 97), together with the results of additional genotoxicity
study on fenbendazole, which are summarized in this monograph
2. BIOLOGICAL DATA
2.1 Toxicological studies
Combined long-term toxicity/carcinogenicity studies on febantel
in rats and mice did not reveal increased incidences of any tumour
types (Bomhard & Kaliner, 1985; Bombard & Mager, 1987; Annex 1,
No evidence of a distinct mutagenic potential of febantel could
be derived from a variety of mutagenicity tests. Febantel was positive
in the dominant lethal test in male mice only at the extremely high
dose of 2000 mg/kg bw/day given twice daily at 24 h intervals, but was
negative at the lower test dose of 500 mg/kg bw/day. This level was
far in excess of the therapeutic dose (Machemer & Dycka, 1976; Annex
1, reference 98). As febantel had no point mutagenic effect, a DNA
binding assay was not deemed necessary.
18.104.22.168 Long-term toxicity/carcinogenicity studies
There were no increase in tumour incidence in a 2-year
carcinogenicity study in mice with doses of fenbendazole up to
405 mg/kg bw/day (Goldenthal, 1980; Annex 1, reference 98).
A lifetime toxicity/carcinogenicity study (including an in utero
phase) of fenbendazole in Charles River CD rats used the F1
generation of pups that had been exposed to the same dose levels
in utero. Fenbendazole was fed in the diet at dose levels of 0, 5,
15, 45 or 135 mg/kg bw/day. In utero exposure to 45 mg/kg bw/day and
135 mg/kg bw/day caused severe toxicity in the pups characterized by
decreased body weights, diarrhoea, bloated stomachs, icterus and
alopecia. F1 pups exposed to 135 mg/kg bw/day had 33% decreased body
weights as compared to controls, while pups exposed to 45 mg/kg bw/day
were 15% below controls. There was also increased mortality, 18% of
pups exposed to 135 mg/kg bw/day and 14% of the pups exposed to
45 mg/kg bw/day died by day 21 of age as compared to 6% in the
controls. These data suggest that pups at 45 and 135 mg/kg bw/day had
been dosed above the MTD prior to the start of the lifetime study.
This toxicity and increased neonatal mortality severely limit the
conclusions that can be drawn from this study.
The histopathological changes observed in the liver of treated
animals were reported as hepatocellular hypertrophy, vacuolation and
bile duct proliferation in the 15, 45 and 135 mg/kg bw/day groups,
hepatocellular hyperplasia and biliary cysts in the 45 and 135 mg/kg
bw/day groups, and hepatocellular adenomas and carcinomas in the
135 mg/kg bw/day group. A group of independent pathologists reviewed
the histopathologic findings and concluded that fenbendazole treatment
did not result in an increased incidence of hepatocellular neoplasms
(Goldenthal, 1981; Lewis, 1982; Brown, 1982; Annex 1, reference 98).
22.214.171.124 Special studies on genotoxicity
The results of additional genotoxicity studies on fenbendazole
are given in Table 1.
Table 1. Results of genotoxicity studies on fenbendazole
Test system Test object Concentration Results Reference
Forward mutation Mouse up to 100 µg/ml Negative Den Boer &
assay1 lymphoma Horn, 1986
1 Both with and without rat liver S9.
Fenbendazole was assayed in L5178Y TK+/- mouse lymphoma cells
using doses ranging from 2.5 to 10 µg/ml without activation and from
4.0 to 10 µg/ml with metabolic activation. The observed toxicity
ranged from low to moderate. The substance showed steep toxicity so
that treatment in the range of 10 to 20% relative growth was not
possible. All of the treatment, both with and without metabolic
activation, induced mutation frequencies that were below the minimum
criterion of mutagenesis. The authors concluded that fenbendazole was
not mutagenic in the mouse lymphoma assay (Den Boer & Horn, 1986).
However, the test concentrations used in this study were considerably
lower than those used in the previous study when concentrations up to
62 µg/ml were tested (Cifone & Myhr, 1983).
In the opinion of independent experts, a DNA binding study would
not result in further clarification of the clastogenic effects of
fenbendazole in vitro on V79 cells, but would merely show whether
there might be an additional potential for point mutations (letter
from professor C. Schlatter, Institute for Toxicology, Zurich,
Switzerland, to Dr Müller, Hoechst AG, 1991; submitted to WHO by
Hoechst AG, Frankfurt, Germany).
Genotoxicity studies on all essential genetic endpoints gave
negative results. Clastogenic effects of benzimidazoles in vitro are
related to the inhibition of tubulin formation. In vivo assays for
binding to DNA in liver following the oral administration of
fenbendazole have not been performed.
In an expert review on the toxicology of fenbendazole, the
authors concluded that it showed no mutagenic, genotoxic or
carcinogenic potential and that sufficient data were available to
establish an ADI for fenbendazole residues in animal-derived food
(Bolt & Gansevendt, 1991).
Oxfendazole has been tested only in a bacterial mutation assay
that gave negative results (Mourot, 1990; Annex 1, reference 98).
The doses used in the oral rat carcinogenicity study were
selected on the basis of results of a 3-month oral dosing study in
rats. Based on increased mortality at 600 mg/kg (equal to 48 mg/kg
bw/day for males and 50 mg/kg bw/day for females), and elevated ALP
levels at 200 mg/kg (equal to 17 mg/kg bw/day for males and 18 mg/kg
bw/day for females), a level of 100 mg/kg (equal to 7.4 mg/kg bw/day
for males and 7.8 mg/kg bw/day for females) was determined to be the
maximum tolerated dose that could be administered over a 2-year
lifespan of rats without compromising survival (these doses have been
corrected and do not correspond exactly to those given in Food
Additives Series 29 (Annex 1, reference 98). In addition, in the oral
carcinogenicity study in rats, histopathologic findings of
hepatocellular vacuolation in the liver of rats fed 30 mg/kg of feed
(equal to 2.0 mg/kg bw/day for males and 2.4 mg/kg bw/day for
females), and 100 mg/kg of feed (equal to 6.6 mg/kg bw/day for males
and 7.8 mg/kg bw/day for females) further suggested that there was
sufficient systemic exposure to oxfendazole to show that it had been
tested adequately. There was no carcinogenic effect of oxfendazole at
any dose level (De Pass & Bidlack, 1987; Annex 1, reference 98).
The thirty-eighth Committee (Annex 1, reference 97) requested a
teratogenicity study in rabbits using oxfendazole at sufficiently high
doses to explore its teratogenic potential. This study was not
Febantel and fenbendazole have been tested in a range of
genotoxicity assays, while oxfendazole has been tested only in an Ames
test in which four strains of Salmonella typhimurium were used. The
compounds consistently gave negative results in the Ames test, a test
for DNA repair and in vivo cytogenetic assays. Febantel,
fenbendazole and a metabolite (2-amino-5-phenylsulfinyl-2-
benzimidazole) were weakly positive in the mouse lymphoma forward
mutation assay in the presence of a metabolic activation system.
Febantel, when tested at a sufficiently high dose to reduce fertility,
was found to induce dominant lethal mutations in mice.
The present Committee noted that available data on the mode of
action of benzimidazoles as inhibitors of the polymerization of
tubulin to microtubulin, with consequent disruption of mitosis,
together with the results of genotoxicity tests, do not provide any
evidence of a direct interaction of these compounds with DNA. It
therefore concluded that the positive results in some genotoxicity
assays are likely to have been caused by an indirect mechanism, and
that further studies on the binding of febantel, fenbendazole and
oxfendazole to DNA are not required.
The present Committee re-evaluated the long-term toxicity/
carcinogenicity studies on all three compounds that were reviewed
at the thirty-eighth meeting, and reached the conclusions given
Febantel. Combined long-term toxicity/carcinogenicity studies
on febantel in rats and mice did not reveal increased incidences of
any types of tumour. At its thirty-eighth meeting (Annex 1, reference
97), the Committee was of the opinion that higher doses could have
been used in the carcinogenicity study in rats. However, in the group
of rats given febantel at 500 mg/kg in the diet (equal to 40 mg/kg
bw/day), decreases in body-weight gain, slight anaemia (in females
only), increased liver weight, hepatocellular enlargement and
vacuolization, and significant increases in serum ALP activity were
observed. These changes were not seen at the lower doses or in the
control groups. The present Committee considered that these changes
indicated that the dose levels used were sufficient for the
carcinogenic potential of febantel to be assessed, and concluded that
there was no evidence that it was carcinogenic in mice or rats.
Fenbendazole. In a lifetime study with an initial in utero
phase, rats were dosed with fenbendazole at doses of 0, 5, 15, 45 or
135 mg of fenbendazole/kg bw/day. There were no increases in tumour
incidence at 5, 15 or 45 mg/kg bw/day. At 135 mg/kg bw/day only a
marginal increase in the incidence of liver tumours was observed. At
the beginning of the lifetime study in rats, the mean body weights of
the highest-dose group (84 g for males and 79 g for females) were more
than 40% lower than those of the controls (142 g for males and 123 g
for females). Furthermore, the rats in the highest-dose group showed
signs of toxicity at that time, including alopecia, icterus,
diarrhoea, and lethargy. The Committee considered that the results for
this group could not be used for the assessment of the carcinogenicity
of fenbendazole because the maximum tolerated dose had been exceeded
in utero. However, proliferative liver lesions in the group
receiving 45 mg/kg bw/day, described as hepatocellular hyperplasia and
biliary cysts, which were not observed at lower doses, indicated that
this dose level was high enough to permit an assessment of the
carcinogenic potential of fenbendazole. On the basis of this study in
rats and the negative results in the carcinogenicity study in mice
reviewed at the thirty-eighth meeting (Annex 1, reference 97), the
Committee concluded that there was no evidence that fenbendazole
possessed carcinogenic potential.
Oxfendazole. The highest dose used in the rat carcinogenicity
study was selected in the light of the results of a 3-month oral
toxicity study in rats. It was based on increased mortality at a dose
level of 600 mg/kg in the diet (equal to 48 mg/kg bw/day for males and
50 mg/kg bw/day for females) and elevated ALP levels at a dose level
of 200 mg/kg in the diet (equal to 17 mg/kg bw/day for males and
18 mg/kg bw/day for females). A dose level of 100 mg/kg in the diet
(equal to 7.4 mg/kg bw/day for males and 7.8 mg/kg bw/day for females)
was selected as the highest dose in the carcinogenicity study. No
carcinogenic effects were seen at any dose level in this study. The
present Committee considered that histopathological findings of
hepatocellular vacuolation in the liver of rats fed more than 2 mg/kg
bw/day provided evidence of sufficient systemic exposure to
oxfendazole, and concluded that the study was adequate for assessing
the carcinogenic potential of the compound. On the basis of this study
and the negative results in the carcinogenicity study in mice reviewed
at the thirty-eighth meeting (Annex 1, reference 97), the present
Committee concluded that there was no evidence that oxfendazole
possessed carcinogenic potential.
At its thirty-eighth meeting (Annex 1, reference 97), the
Committee had requested an additional teratogenicity study in rabbits
using oxfendazole. This was not provided at the present meeting.
Instead, the sponsor suggested that the available reproductive data in
sheep were sufficient. The Committee concluded, however, that the
study in sheep did not adequately explore the teratogenic potential of
oxfendazole, because it was designed to assess the safety of the drug
in the target species, not to study its teratogenicity.
The available data from reproductive and teratogenicity studies
with each of the three drugs suggest that oxfendazole is the most
potent in producing embryotoxic effects. The data on fenbendazole
suggest that the rabbit is the species most sensitive to the
embryotoxic and teratogenic effects of these three compounds. The
Committee therefore decided that an adequate teratogenicity study in
rabbits with oxfendazole was still required.
The Committee noted that, on the basis of the available
toxicological data, the NOELs identified at the thirty-eighth meeting
and a safety factor of 100, ADIs of 0-20 and 0-50 µg/kg bw could have
been established for febantel and fenbendazole, respectively. However,
it also noted that febantel, fenbendazole and oxfendazole have common
metabolic routes, and that the residue data demonstrate that
oxfendazole is the major residue in food-producing animals following
administration of these three compounds. Therefore, the Committee
assigned a group temporary ADI of 0-4 µg/kg bw to febantel,
fenbendazole and oxfendazole based on the NOEL of 0.7 mg/kg bw/day for
oxfendazole identified at the thirty-eighth meeting and a safety
factor of 200 (Annex 1, reference 97). The results of a teratogenicity
study in rabbits, in which oxfendazole is administered at sufficiently
high doses for its teratogenic potential to be adequately explored,
are required by the Committee for evaluation in 1998.
Bolt HM & Gansevendt B (1991). Evaluation of the toxicology of
fenbendazole. Unpublished report from the Institut fur
Arbeitsphysiologie, Universitat Dortmund, Dortmund, Germany. Submitted
to WHO, by Hoechst AC, Frankfurt, Germany.
Bomhard, E & Kaliner G (1985). BAY Vh 5757 chronic toxicity and
carcinogenicity studies in mice. 21-month feeding study. Unpublished
report No. 13961 from Bayer AG. Submitted to WHO by Bayer AG,
Bomhard E & Mager H (1987). BAY Vh 5757 combined chronic toxicity and
carcinogenicity studies in Wistar rats. Unpublished report No. 15844
from Bayer AG. Submitted to WHO by Bayer AG, Leverkusen, Germany.
Brown WR (1982). Lifetime oral toxicity study of fenbendazole in rats,
histopathology - liver. Research pathology Serices Inc.,
New Britain, PA, USA. Unpublished report. Submitted to WHO by Hoechst
AG, Frankfurt am Main, Germany.
Cifone MA & Myhr BC (1983). Mutagenicity evaluation of BAY L5156 in
the mouse lymphoma forward mutation assay. Unpublished project
No. 10989 from Litton Bionetics Inc. Kensington, MD, USA. Submitted to
WHO by Bayer AG, Leverkusen, Germany
De Pass LR & Bidlack DE (1987) Carcinogenicity study in rats with
RS-8858 (oxfendazole) mixed in the feed. Unpublished report
No. 53-R-83-8858-PO-CA from Syntex Inc. Palo Alto, CA, USA. Submitted
to WHO by Syntex Inc. Palo Alto, CA, USA.
Den Boer WC & Horn AJW (986) Mutagenic evaluation of S711881 in the
L 5178 Y TK +/- mouse lymphoma forward mutation assay. Unpublished
report from Hazleton Biotechnologies, Veenendaal, Netherlands
No. 1003E. Submitted to WHO by Hoechst AG, Frankfurt, Germany.
Goldenthal EJ (1980) 24-month oral carcinogenicity study in mice.
Unpublished report, International Research and Development
Corporation, Mattawan, NJ, USA. Submitted to WHO by Hoechst AG,
Frankfurt am Main, Germany.
Goldenthal EI (1981) Lifetime oral toxicity study in rats. Unpublished
report. International Research and Development Corporation, Mattawan,
MI, USA. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.
Lewis JC (1982) Review and conclusions of the histopathology data of
"Lifetime oral toxicity study of fenbendazole in rats". Unpublished
report No. 102E from Hoechst AG, Frankfurt, Germany. Submitted to WHO
by Hoechst AG.
Machemer L & Dycka G (1976) BAY Vh 5757 dominant lethal test on male
mice to test for mutagenic effects. Unpublished report No. 6031 from
Bayer A.G. Submitted to WHO by Bayer AG Leverkusen, Germany.
Mourot D (1990) Oxfendazole Ames test. Unpublished report from
Laboratoire des médicaments vétérinaires, La Haute-Manche-Jarene,
Fougčres. Submitted to WHO by Centre National d'études vétérinaires et
alimentaires, Javene, Fougčres, France.