INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
WORLD HEALTH ORGANIZATION
TOXICOLOGICAL EVALUATION OF CERTAIN
VETERINARY DRUG RESIDUES IN FOOD
WHO FOOD ADDITIVES SERIES 41
Prepared by:
The 50th meeting of the Joint FAO/WHO Expert
Committee on Food Additives (JECFA)
World Health Organization, Geneva 1998
FEBANTEL, FENBENDAZOLE, AND OXFENDAZOLE (addendum)
First draft prepared by
Dr R. Fuchs
Department of Experimental Toxicology and Ecotoxicology
Institute for Medical Research and Occupational Health
Zagreb, Croatia
1. Explanation
2. Biological data
2.1 Developmental toxicity
2.2 Tumour promotion
3. Comments
4. Evaluation
5. References
1. EXPLANATION
Febantel, fenbendazole, and oxfendazole were evaluated at the
thirty-eighth and forty-fifth meetings of the Committee (Annex 1,
references 97 and 119).
Fenbendazole and oxfendazole are benzimidazoles and are
metabolically interconvertible in vivo. Febantel is a prodrug that
can be converted in vivo by cyclization to fenbendazole or after
oxidation at the sulfur atom and subsequent cyclization to
oxfendazole.
At the forty-fifth meeting, the Committee established a temporary
group ADI of 0-4 µg/kg bw for febantel, fenbendazole, and oxfendazole,
on the basis of a NOEL for oxfendazole of 0.7 mg/kg bw per day in a
two-year study in rats, applying a safety factor of 200 because the
ADI was temporary. The Committee requested the results of a study of
teratogenicity in rabbits in which oxfendazole was administrated at
sufficiently high doses for its teratogenic potential to be adequately
explored.
2. BIOLOGICAL DATA
2.1 Developmental toxicity
In a pilot study of the developmental toxicity of oxfendazole
(99.3% pure) in 0.5% carboxymethylcellulose, groups of eight pregnant
New Zealand white rabbits were given doses of 0, 1, 5, 12, or 25 mg/kg
bw per day by gavage. None of the doses (including the maximal dose)
affected body weights, and no adverse clinical signs of toxicity were
seen. Feed consumption and absolute and relative liver weights were
not changed in comparison with controls. In the second phase of the
study, groups of five does were given oxfendazole at 0 or 25 mg/kg bw
per day by gavage on days 7-19 day of gestation. No maternal toxicity
and no changes in body weight, body-weight gain, or feed consumption
were seen. In phase III, the maximal tolerated dose for pregnant
rabbits was determined. Groups of eight does were given 0, 100, or 200
mg/kg bw per day on days 7-19 of gestation, and groups of four animals
were given 500 or 1000 mg/kg bw per day on days 8-19 days of
gestation. Treatment-related early resorptions, decreased litter size
and fetal weight, fetal malformations, and changes in ossification
were reported at doses of 100 and 200 mg/kg bw per day. The fetal
malformations included biologically relevant differences in the
interrelated average numbers of ossification sites per fetus in the
thoracic vertebrae (increased), lumbar vertebrae (decreased), and ribs
(increased) as compared with controls. The doses of 200, 500, and
1000 mg/kg bw per day resulted in mortality and/or abortion of
litters. None of the fetuses at 500 or 1000 mg/kg bw per day survived
to day 29 of gestation. On the basis of these results, doses of 0, 10,
30, and 45 mg/kg bw per day of oxfendazole were used in the definitive
study of developmental toxicity in rabbits. The highest dose used was
ninefold higher than the therapeutic dose of oxfendazole used in
cattle, sheep, and horses: a single dose of 5 mg/kg bw, which can be
repeated every four to six weeks (Hoberman, 1997).
In the definitive study, oxfendazole was given to groups of 20
pregnant rabbits on days 7-19 of presumed gestation, at doses of 0,
10, 30, or 45 mg/kg bw per day. All animals survived the treatment,
and no abortions or premature deliveries occurred during the study. No
effects were seen on the average body weights of does, body-weight
gain, gravid uterine weight, or absolute (g/day) or relative (g/kg per
day) feed consumption. No maternal or developmental toxicity occurred.
It did not affect the number of fetuses, their viability, or the sex
ratios and did not cause malformations or variations (Hoberman, 1996).
2.2 Tumour promotion
Induction of the cytochrome P450 isoenzymes CYP1A1/2, 2B1/2, and
4A1 is associated with promotion of liver tumours, and rat liver
tumour promoters inhibit gap-junction intracellular communication, as
evidenced by a decrease in the amount of the protein connexin 32. The
two-stage liver carcinogenesis model in rats was used to examine
whether oxfendazole promotes liver tumours, and the activity of P450
isozymes, gap-junction intracellular communication, and the presence
of foci of the placental form of glutathione S-transferase
(GST-P+) were measured. Five groups of 10-15 four-week-old Fischer
344 rats were initiated with a single intraperitoneal injection of 100
mg/kg bw N-nitrosodiethylamine (NDEA), and one week later they
received a diet containing oxfendazole at 0, 10, 100, 250, or 500 ppm
for eight weeks. Five animals from the NDEA-initiated groups treated
with 0 or 250 ppm oxfendazole were killed after one week of receiving
the diet, and the remaining animals were killed at the end of the
treatment period.
No significant difference in body-weight gain was seen between
treated and control groups. After one week of treatment with 250 ppm
oxfendazole, initiated animals had significantly increased relative
liver weights. At the end of treatment with oxfendazole, liver weights
were increased in animals treated with NDEA and 250 or 500 ppm
oxfendazole or with 100, 250, or 500 ppm oxfendazole alone. Treatment
with NDEA and 250 ppm oxfendazole for one week did not cause
histological changes, but treatment with 100 ppm or more of
oxfendazole for eight weeks caused hepatocellular hypertrophy and
fatty degeneration, irrespective of pretreatment with NDEA. In animals
given NDEA plus 500 ppm oxfendazole, single hepatocytes sometimes
showed necrosis, and a marked increase in smooth endoplasmic
reticulum, with the appearance of lipid droplets in the cytoplasm, was
seen by elecron microscopy. The liver P450 isozymes CYP1A1/2, 2B1/2,
and 4A1 were markedly induced in animals initiated with NDEA and
treated with 250 or 500 ppm oxfendazole for one or eight weeks;
induction of CYP2E1 and 3A2 was not remarkable in these groups.
Significant induction of CYP1A1/2 was also seen in NDEA-initiated
animals treated with 10 or 100 ppm oxfendazole for eight weeks, and
the CYP2B1/2 and 4A1 isozymes were induced in those given NDEA plus
100 ppm oxfendazole.
The number of connexin 32-positive spots per hepatocyte was
significantly decreased in the animals receiving NDEA plus 250 ppm
oxfendazole after one week as compared with controls, and the numbers
and areas of spots were significantly reduced in a dose-dependent
manner in the rats treated with oxfendazole. In all NDEA-initiated
animals, GST+ focal lesions, consisting of single hepatocytes or
mini-foci of hepatocytes strongly positive for the GST-P marker
enzyme, were observed. The number of GST-P+ single cells was
significantly increased in the NDEA-initiated animals treated with 250
or 500 ppm oxfendazole as compared with those given NDEA alone;
however, NDEA-initiated animals treated with 100 ppm oxfendazole had
fewer GST-P+ single cells than those that received only NDEA. No
positive foci were found in uninitiated groups with no oxfendazole
administration (Mitsumori et al., 1997). The results strongly
suggest that oxfendazole promotes liver tumours.
3. COMMENTS
The Committee considered the results of studies of the
developmental toxicity of oxfendazole in rabbits, which were conducted
to appropriate standards for study protocol and conduct. In a
preliminary study in three phases, doses up to 1000 mg/kg bw per day
were given by gavage to pregnant rabbits during the critical days of
gestation. Doses of 100 mg/kg bw per day and higher produced fetal
malformations. On the basis of these results, the compound was
administered by gavage to pregnant rabbits on days 7-19 of gestation
at doses of 0, 10, 30, or 45 mg/kg bw per day. No maternal toxicity,
embryotoxicity, or effects on fetal morphology were observed.
The Committee considered a published study of
initiation-promotion with oxfendazole in rats, in which the activity
of P450 enzymes, gap-junctional intercellular communication, and
expression of the placental form of glutathione S-transferase were
examined. Although some enzymatic and immunohistochemical changes were
seen that suggested that oxfendazole might have tumour-promoting
potential in rats, the results of the studies of genotoxicity and
carcinogenicity evaluated at the thirty-eighth and forty-fifth
meetings presented sufficient evidence that oxfendazole is not
carcinogenic.
4. EVALUATION
The Committee established a group ADI of 0-7 µg/kg bw for
febantel, fenbendazole, and oxfendazole on the basis of a NOEL of 0.7
mg/kg bw per day in a two-year study on oxfendazole in rats (evaluated
at the thirty-eighth and forty-fifth meetings of the Committee) and
applying a safety factor of 100. The Committee noted that this ADI
provides a safety factor of 1000 for teratogenic effects in sheep,
which was considered at the thirty-eighth meeting to be the most
sensitive species for the teratogenic effects of oxfendazole.
5. REFERENCES
Hoberman, A.M. (1996) An oral developmental toxicity (embryo-fetal
toxicity/teratogenicity) definitive study with oxfendazole in rabbits.
Unpublished study No. 101-026 from Argus Research Laboratories, Inc.,
Horsham, Pennsylvania, USA. Submitted to WHO by Hoechst Roussel Vet,
Wiesbaden, Germany.
Hoberman, A.M. (1997) An oral development toxicity (embryo-fetal
toxicity/teratogenicity) pilot study with oxfendazole in rabbits.
Unpublished study No. 101-026 from Argus Research Laboratories, Inc.,
Horsham, Pennsylvania, USA. Submitted to WHO by Hoechst Roussel Vet,
Wiesbaden, Germany.
Mitsumori, K., Onodera, H., Shoda, T., Uneyama, C., Imazawa, T.,
Takegawa, K., Yasuhara, K., Watanabe, T. & Takahashi, M. (1997) Liver
tumour-promoting effects of oxfendazole in rats. Food Chem.
Toxicol., 35, 799-806.
See Also:
Toxicological Abbreviations