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    FENBENDAZOLE

    First Draft Prepared by
    Dr. William C. Keller,
    Food and Drug Administration,
    Rockville, Maryland USA

    1.  EXPLANATION

          Fenbendazole is a light brownish-gray odourless, tasteless
    crystalline powder which is insoluble in water, but highly soluble
    in DMSO. It is a broad spectrum veterinary anthelmintic used in
    canines, equines, ruminants and swine. Fenbendazole has not been
    previously evaluated by the Joint FAO/WHO Expert Committee on Food
    Additives.

    2.  BIOLOGICAL DATA

    2.1  Biochemical aspects

    2.1.1  Absorption, distribution, and excretion

          Studies were performed to obtain basic information on the
    pharmacokinetics of 14C-fenbendazole after single oral doses in
    dogs, rats, rabbits, and sheep. The doses used were 5 mg/kg b.w. for
    sheep and 10 mg/kg b.w. for the other species. The material tested
    was administered as an aqueous suspension in 2% starch mucilage. All
    data are based on radioactivity, with metabolism not taken into
    account.

          Absorption was slow, but more rapid in monogastrics. The
    highest concentrations measured in blood were 0.9 g/ml in rats 5 to
    7 hours post-administration, 0.9 g/ml in rabbits 8 hours
    post-administration, 0.4 g/ml in dogs 24 hours post-administration
    and 0.32 g/ml in sheep 2-3 days post-administration. Elimination
    t from blood was 6 hours in rats, 13 hours in rabbits, 15 hours
    in dogs, and one day in sheep. In all animals except rabbits,
    elimination occurred >90% in faeces, with <7% in urine. In
    rabbits, excretion was 75% in faeces and 21% urine. At three days
    post-administration 98% elimination in dogs, 92% elimination in
    rabbits, and 99% elimination in rats had occurred. Hepatic
    distribution at 7 days was highest (2.7 ppm) in sheep and lowest
    (0.06 ppm) in rats (Kellner & Christ, 1973).

          Investigations into the pharmacokinetics of 14C-fenbendazole
    after intravenous and oral administration were carried out in the
    rat, rabbit, dog, sheep, and pig (oral only). All species received
    5 mg/kg b.w. fenbendazole as a 2.5% aqueous suspension in 2% starch
    mucilage. All data are based on radioactivity, with metabolism not
    taken into account.

          The decrease in blood levels after i.v. administration occurred
    with a t of 10 and 13 hours in the dog, >7 hours in the rat, 12
    and 18 hours in the rabbit, and 13 hours in sheep. The decrease in
    blood levels after oral administration occurred with a t of 15 
    3 hours in the dog, 6  1 hours in the rat, 13  4 hours in the
    rabbit, 9.4  1.1 hours in the pig and 28  4 hours in sheep. The
    t for monogastrics was similar for oral and i.v. administration,
    but for sheep the post-iv elimination was much faster due to lack of
    a rumen effect on absorption. Elimination occurred mainly in the
    faeces and was nearly complete after one week. Intestinal absorption
    was about 70% in the rabbit, 25-50% in the rat, >33% in the pig,
    25% in sheep and >20% in the dog. Elimination via the urine during
    7 days following i.v. administration was significant: rabbit 38 -
    40%, rat 23  5.7%, dog 29 - 38% and sheep 34%. Of this about 90% in

    the rat, 69-77% in the dog, 55% in the rabbit, and 45% in the sheep
    was eliminated within 24 hours. Urinary elimination in the pig and
    sheep following oral administration was 31-36% and 8-11%,
    respectively. The largest component of the dose was eliminated in
    the faeces by all species. The distribution pattern for fenbendazole
    6 - 8 hours after an oral dose in the rabbit, dog, and rat showed
    the highest levels in the liver (Villner et al., 1974).

    2.1.2  Biotransformation

          Sheep and rabbits received 10 mg/kg b.w. 14C-fenbendazole
    orally, and faeces and urine were evaluated. About 10-20% of
    excretion occurred in urine with the remainder being in faeces.
    About 50-75% of the urinary extract occurred as the p-OH metabolite
    (Klpffer, 1973).

          A single dose of fenbendazole was administered orally to rats,
    rabbits, and dogs, and serum levels of parent compound and of three
    metabolites (oxfendazole, oxfendazole sulfone, and p-OH oxfendazole)
    were determined post-administration at various times. Rats received
    500 mg/kg b.w., while rabbits and dogs received 25 mg/kg b.w. The
    detection limit was 0.05 ppm for parent drug and the NH2
    metabolite (which was not distinguishable from parent), and 0.01 ppm
    for the other metabolites. Blood levels of parent drug were
    detectable after 24 hours in all species and in the dog and rat at
    48 hours (rabbit blood levels were not evaluated at 48 hours).
    Levels of SO and SO2 metabolites were also found, while
    practically no traces of the p-OH metabolite were found (Dwel &
    Uihlein, 1980).

          The disposition and metabolism of fenbendazole were studied
     in vivo in cattle, chickens, sheep, goats, rabbits, and  in vitro
    in hepatic preparations from cattle, sheep, goats, rabbits, rats,
    chickens, ducks, turkeys, and catfish. The major urinary metabolite
    when fenbendazole was administered either i.v. or orally (5 mg/kg
    b.w.) was p-OH fenbendazole. The sulfoxide and sulfone appeared in
    plasma but were recovered in only trace amounts in urine or faeces,
    and the amine was a minor metabolite appearing only occasionally in
    plasma (Short et al., 1988).

          A general metabolism scheme may be found on page 23. The ready
    interconversion of oxfendazole and fenbendazole is notable. Also
    important, in considering the benzimidazoles as a group, is the
    entry of febantel to the oxfendazole-fenbendazole pool through
    either conversion of febantel to fenbendazole directly or the
    conversion of febantel through the sulfoxide to oxfendazole.

    2.2  Toxicological studies

    2.2.1   Table 1: Acute toxicity studies

                                                                                      

    Species     Sex         Route      LD50              Reference
                                       (mg/kg b.w.)
                                                                                  

    Mouse       M&F         oral       > 10 000          Scholz & Schultes,
                                                         1973a

    Mouse       M&F         s.c.       > 3 200           Scholz & Schultes,
                                                         1973a

    Mouse       M&F         i.p.       > 3 200           Scholz & Schultes,
                                                         1973a

    Rat         M&F         oral       > 10 000          Scholz & Schultes,
                                                         1973b

    Rat         M&F         s.c.       > 2 000           Kramer & Schultes,
                                                         1973a

    Rat         M&F         i.p.       > 1 250           Kramer & Schultes,
                                                         1973b

    Rabbit      M&F         oral       > 5 000           Scholz & Schultes,
                                                         1973c

    Dog         M&F         oral       > 500             Scholz & Schultes,
                                                         1973d

    Swine       M&F         oral       > 5 000           Duwel, 1974

    Sheep       F           oral       > 500             Wilkins, 1973
                                                                                  
    

          The oral LD50 of p-OH fenbendazole was >10 000 mg/kg b.w. in
    mice and rats (Kramer & Schultes, 1974c,d)

    2.2.2   Short-term studies

    2.2.2.1   Mice

          Fenbendazole was submitted to a general pharmacologic screening
    procedure in mice over a dose range of 100 to 300 mg/kg b.w. The
    following observations were reported: Increased motor activity
    induced by administering the CNS stimulant methamphetamine was not
    changed by pretreatment with fenbendazole, pretreatment of mice with
    fenbendazole had no influence on hexobarbitone anaesthesia as
    measured by sleep time, fenbendazole treatment was found to have no
    effect on pain perception using the radiant heat method test,
    maximal electroshock convulsions in mice were not affected by
    fenbendazole, and fenbendazole had no apparent anticonvulsant
    activity against pentamethyletetrazole (Vogel & Alpermann, 1973).

    2.2.2.2   Rats

          Fenbendazole was submitted to a general pharmacologic screening
    procedure in rats over a dose range of 100 to 300 mg/kg b.w. The
    following observations were reported: Fenbendazole was determined
    not to have anti-inflammatory activity in the rat paw oedema test
    using carrageenan as an irritant, there was no change in body
    temperature when fenbendazole was administered to yeast-fevered
    rats, and fenbendazole was negative for diuretic activity (Vogel &
    Alpermann, 1973).

          Fenbendazole was administered via stomach tube to groups of 10
    male and 10 female immature Wistar rats at the rate of 0, 25, 250,
    and 2500 mg/kg b.w./day for 30 days. Food consumption was determined
    continuously and body weight twice weekly. Rats were observed daily
    and clinically examined weekly. Haematology, clinical chemistry and
    urinalysis were determined prior to treatment initiation and at
    sacrifice. Half the animals were sacrificed one day after treatment
    ceased and the remainder were sacrificed 8 days after treatment
    ceased. The rats received a complete gross necropsy, organ weights
    were determined, and a standard array of tissues was examined
    histopathologically following sacrifice. No treatment-related
    findings were reported (Kramer & Schultes, 1973c).

          Fenbendazole was administered via stomach tube to groups of 15
    male and 15 female immature Wistar rats at the rate of 0, 25, 200,
    and 1600 mg/kg b.w./day for 90 days. At treatment day 61 doses for
    five male and female rats were increased to 2500 mg/kg b.w./day.
    Food intake was determined continuously and body weight twice
    weekly. Rats were observed daily and clinically examined weekly.
    Haematology, clinical chemistry and urinalysis were determined prior
    to treatment initiation, at the sixth week of treatment and at

    sacrifice. Ten animals were sacrificed 1 day after treatment ceased
    and the remainder were sacrificed 7 days after treatment ceased. The
    rats received a complete gross necropsy, organ weights were
    determined, and a standard array of tissues was examined
    histopathologically following sacrifice. Two rats from the 1600 and
    5 rats from the 2500 mg/kg b.w./day groups were observed to have
    tremors after the 84th treatment. No other treatment-related
    findings were reported (Kramer & Schultes, 1974a).

          A 15-week oral toxicity study in Charles River CD rats was
    performed to evaluate the toxicity of fenbendazole in rats.
    Fenbendazole was provided in the feed to groups of 50 rats/sex
    derived from the F1a generation of a 3-generation reproductive
    toxicology study at levels to yield a dose of 160, 400, or
    1000 mg/kg b.w./day. Animals were observed daily and weights and
    food consumption recorded weekly. At 12 weeks clinical chemistry,
    haematology, urinalysis, and ophthalmic examinations were conducted.
    Rats which died during the study and rats from the 1000 mg/kg
    b.w./day and control groups were necropsied.

          The treated male groups all gained less weight than controls
    but there was a reverse relationship to dose. The treated groups
    also weighed less than controls at study initiation: the 1000 mg/kg
    b.w./day male mean weight was 75% of controls and the female mean
    weight was 73% of controls. No other treatment-related observations
    were reported. It should be noted [see Goldenthal 1979b] that this
    study was initiated as a 2-year chronic study that was apparently
    terminated due to toxicity (Goldenthal, 1979a).

    2.2.2.3  Dogs

          Fenbendazole was administered to 6-week old puppies to
    determine the safety of the 22.2% granules or 10% suspension
    formulations. Groups of 3 beagle pups/sex received doses of 0, 50,
    or 250 mg/kg b.w./day granules or 250 mg/kg b.w./day suspension
    provided in gelatin capsules for 6 days. Clinical signs,
    haematology, and clinical chemistry were evaluated. No
    treatment-related effects were reported (Mehring, 1982).

          Fenbendazole was administered in gelatin capsules to groups of
    2 male and 2 female beagles for 30 consecutive days at the rate of
    0, 25, 80, or 250 mg/kg/day. The dogs were 7 to 34 months old at
    initiation of the experiment. Food consumption was determined daily
    and weights were determined weekly. Dogs were observed daily and
    examined weekly for treatment-related signs. Haematology, clinical
    chemistry, and urinalysis were performed prior to treatment
    initiation and at sacrifice. Following sacrifice, dogs received a
    gross necropsy and organ weights were determined. A standard
    selection of organs was evaluated histopathologically.
    Treatment-related lesions reported include lymph follicle
    proliferation in the region of the stomach pyloric glands at

    250 mg/kg b.w./day and low-grade diffuse cellular centrilobular
    fatty degeneration of the liver in males at 80 and 250 mg/kg
    b.w./day. The NOEL was 25 mg/kg b.w./day (Kramer & Schultes, 1973d).

          Fenbendazole was administered in gelatin capsules to 4 groups
    of 3 male and 3 female beagle dogs for 90 consecutive days at the
    rate of 0, 20, 50, or 125 mg/kg b.w./day. The dogs were 8 to 48
    months old at initiation of the experiment. Food consumption was
    determined daily and weights were determined weekly. Dogs were
    observed daily and examined weekly for treatment-related signs.
    Haematology, clinical chemistry, and urinalysis were performed prior
    to treatment initiation, after 30 days of treatment, and at
    sacrifice. Following sacrifice, dogs received a gross necropsy and
    organ weights were determined. A standard selection of organs was
    evaluated histopathologically. The gastric mucosa was reported to
    contain lymph follicles and lymphocytic infiltration in both treated
    and control dogs. No treatment-related findings were reported
    (Kramer & Schultes, 1974b).

          The toxicity of fenbendazole was evaluated in beagle dogs by
    administering fenbendazole in gelatin capsules to 4 groups of 4 male
    (5.5 to 9.7 kg) and 4 female (4.8 to 7.3 kg) dogs at levels of 0,
    20, 50, and 125 mg/kg b.w./day for 6 months. Dogs were housed
    individually in metal metabolism cages. Food consumption and weights
    were determined weekly. Dogs were observed daily and examined weekly
    for treatment-related signs. Reflexes were examined prior to
    treatment and at 1, 3 and 6 months of treatment. Ophthalmoscopic
    examinations, haematology, clinical chemistry, and urinalysis were
    performed prior to treatment and at 3 and 6 months. Following
    sacrifice dogs received a gross necropsy and organ weights were
    determined. A standard selection of organs from the control and
    125 mg/kg b.w./day treatment groups were evaluated
    histopathologically. In addition, tissues from dogs in the 20 and
    50 mg/kg b.w./day groups in which gross lesions were observed or in
    which treatment-related lesions were observed in the 125 mg/kg group
    were also evaluated histopathologically.

          A treatment-related nodular appearance of the gastric mucosa
    was observed at gross necropsy in all treated groups but not
    controls. Formation of lymphoid nodules in the gastric mucosa
    corresponding to this gross lesion was observed on histopathologic
    evaluation in all treatment groups. Focal encephalomalacia,
    satellitosis and neuronophagia were observed in the cerebra of two
    dogs at the 125 mg/kg b.w./day level and slight perivascular
    inflammation and gliosis were reported in the cerebrum of another
    animal at the 125 mg/kg b.w./day level. Since effects were observed
    at the lowest dose of 20 mg/kg b.w./day, there was no NOEL (Wazeter
    & Goldenthal, 1977).

          The toxicity of fenbendazole was evaluated in beagle dogs by
    administering fenbendazole in gelatin capsules to 5 groups of 6 male
    (5.9 to 14.1 kg) and 6 female (5.4 to 11.6 kg) dogs at levels of 0,
    4, 8, 12, and 20 mg/kg b.w. daily for 6 months. Dogs were housed
    individually in metal metabolism cages. Food consumption and weights
    were determined weekly. Dogs were observed daily and examined weekly
    for treatment-related signs. Reflexes were examined prior to
    treatment and at 1, 3 and 6 months of treatment. Ophthalmoscopic
    examinations, haematology, clinical chemistry, and urinalysis were
    performed prior to treatment and at 3, and 6 months. After 6 months
    of treatment 4 dogs of each sex/group were sacrificed while the
    remaining 2 dogs of each sex/group were sacrificed 3 weeks after
    treatment had ceased. At sacrifice, each dog received a complete
    gross necropsy and adrenals, brain, heart, kidneys, liver, spleen,
    pituitary, testes, ovaries, and thyroid were weighed. A standard
    selection of organs from the control and 20 mg/kg b.w./day treatment
    groups were evaluated histopathologically. In addition, tissues from
    dogs in the 4, 8, and 12 mg/kg b.w./day groups in which gross
    lesions were observed, and mesenteric lymph nodes, stomach, colon,
    and testes were also evaluated histopathologically.

          An increased number of treated dogs had lymphocytic foci in the
    lamina propria of the gastric mucosa compared to control dogs. The
    extent of this effect appeared to be dose-related. Very slight to
    moderate hyperplasia of mesenteric lymph nodes was also increased in
    treated groups. The investigators believed the increased incidence
    in treated groups may indicate a treatment-related local irritant
    effect. Mesenteric lymph nodes from dogs sacrificed 3 weeks after
    cessation of treatment were deemed to have hyperplasia and
    congestion (Goldenthal, 1978).

          An independent histopathologic review of stomach mucosal tissue
    and mesenteric lymph nodes from this and the previous study was
    performed. This review concluded that lymphoid hyperplasia of the
    gastric mucosa was associated with fenbendazole treatment. The
    reviewers were not convinced that this effect was a direct irritant
    effect or that the gastric effect was associated with lymph node
    hyperplasia. They further stated that any nonspecific irritant could
    produce the lymph node changes (Dua, 1981).

          Histopathologic changes reported in the two previous studies
    occurred in the control as well as the treated dogs. The incidence
    occurring at the high doses suggests a treatment-related effect,
    however, the incidence of changes occurring in the low-dose group
    appears to be within the range of biological variation.
    Interpretation of these changes would have been facilitated by
    availability of historical control data. Results from this study are
    consistent with a NOEL of 4 mg/kg/day. This conclusion is supported
    by the following study.

          A further 14-week study using 3 beagles/sex/dose was done to
    assess the oral toxicity of fenbendazole, particularly effects on
    stomach mucosal lymphoid follicles and mesenteric lymph nodes. Dogs
    received doses of 0, 1, 2, 5, or 10 mg/kg b.w./day provided in
    gelatin capsules. Food consumption was determined daily and weights
    were determined weekly. Dogs were observed daily and examined weekly
    for treatment-related signs. Haematology, clinical chemistry, and
    urinalysis were performed prior to treatment initiation, and at week
    13. Following sacrifice, dogs received a gross necropsy and liver,
    kidney, and spleen weights were determined. Organs evaluated
    histopathologically included liver, kidney, spleen, mesenteric lymph
    nodes, samples of the stomach regions and tissues with gross
    lesions. No treatment-related findings were reported (Doerr &
    Carmines, 1983).

    2.2.2.4   Swine

          The toxicity of fenbendazole was evaluated by dosing 8 growing
    pigs via gavage with 2000 mg fenbendazole/kg b.w./day for 14 days.
    The animals were observed daily, and blood and urine samples were
    collected on alternate days including baseline values. The pigs were
    sacrificed 10 days after the last dose and subjected to complete
    gross and histopathologic examination. Four pigs developed pneumonia
    during the treatment. Treatment-related leukopenia developed on day
    6 of dosing but returned to normal on day 18, 4 days after treatment
    ceased. Both the segmented neutrophils and lymphocytes were
    affected. Sorbitol dehydrogenase values were significantly increased
    from treatment day 4, but returned to baseline on day 20, 6 days
    after treatment ceased. No treatment-related gross or
    histopathologic lesions were reported (Hayes et al., 1983a).

          The toxicity of fenbendazole was evaluated in groups of 5
    female pigs in which fenbendazole was provided in feed at the rate
    of 0, 25, 75, or 125 mg/kg b.w./day for 5 days. They were observed
    daily. Blood and urine samples were collected on days -5, -2, 0, 3,
    7, 10 and 15. The pigs were sacrificed 10 days after the last dose
    and subjected to complete gross and histopathologic examination.
    Leukopenia developed on day 3 for the 75 and 125 mg/kg b.w./day
    groups and after treatment ceased, on day 7 in the 25 mg/kg b.w./day
    group, but all groups had returned to normal on day 15, 10 days
    after treatment ceased. The segmented neutrophils and lymphocytes
    were affected. In groups receiving 75 and 125 mg/kg b.w./day,
    sorbitol dehydrogenase values were significantly increased from
    treatment day 3, but returned to baseline on day 10, 5 days after
    treatment ceased. No treatment-related gross or histopathologic
    lesions were reported (Hayes et al., 1983b).

          Thirty sows were divided into groups receiving 0, 3, 9, 15 and
    25 mg fenbendazole/kg b.w./day in feed for 3 days. The animals were
    maintained for an additional 10 days after treatment ceased. Blood
    and urine were collected on alternate days including during a 10-day
    pretreatment phase. Clinical chemistry, haematology, urinalysis and
    gross and histopathologic observations revealed no treatment-related
    effects. The NOEL was 25 mg/kg b.w./day (Booze & Oehme, 1983).

    2.2.3  Long-term/carcinogenicity studies

    2.2.3.1   Mice

          The carcinogenicity of fenbendazole was evaluated in Charles
    River CD-1 mice. In a 2-year study, 480 six-week old mice were
    divided into 4 groups of 60 male and 60 female mice which received
    fenbendazole in the diet at levels targeted to provide doses of 0,
    45, 135, and 405 mg/kg b.w./day. The test material was mixed with
    the diet by adding it to 500 g of food and blending, then mixing
    this premix with the mouse diet in a blender. The fenbendazole
    concentration was based on the most recent body weight and food
    consumption data, and fresh diets were prepared weekly. Samples of
    the diet containing the test substance were taken at periodic
    intervals during the study. Mice were observed daily for general
    physical appearance, behaviour, and toxic signs, and observations
    recorded weekly except moribundity/mortality which were recorded
    daily. Individual body weights and food consumption were recorded
    weekly. After 24 months of fenbendazole administration the surviving
    mice were sacrificed and complete necropsies performed. All animals
    that died during the course of the study were also necropsied.
    Histopathologic evaluation was performed on a standard array of
    tissues from the control and high-dose group and all tissues where
    lesions were observed at necropsy in all dose groups.

          Survival in treated groups was somewhat reduced when compared
    with controls: Control: - 55% M and 60% F; 45 mg/kg b.w./day - 43% M
    and 37% F; 135 mg/kg b.w./day - 47% M and 42% F; and 405 mg/kg
    b.w./day - 37% M and 43% F. Sporadic body weight gain differences
    between treatment groups and controls occurred at various times
    during the study, but no apparent treatment-related alteration was
    observed. Food consumption was comparable for all groups. The
    incidence of inflammatory lesions and proliferative lesions was
    unrelated to fenbendazole treatment. Total numbers of benign and
    malignant neoplasms for the treated group and control were similar.
    No compound effect was evident histopathologically. Based on these
    results the investigators concluded that no fenbendazole-related
    effects were observed in any treatment group during the study
    (Goldenthal, 1980b).

    2.2.3.2  Rats

          The chronic toxicity, including carcinogenicity, of
    fenbendazole was evaluated in Charles River CD rats. In a lifetime
    study including an  in utero phase fenbendazole was provided in the
    diet at dose levels of 0, 5, 15, 45, and 135 mg/kg b.w./day. Groups
    of 50 male and 50 female rats, derived from a three-generation
    reproduction study performed using the same dose-levels (as the
    F1a rats) were used in the study. Surviving male rats were
    terminated at week 123 and surviving females were terminated at week
    125. Febendazole was mixed with the diet. The fenbendazole
    concentration was based on the most recent body weight and food
    consumption data and fresh diets were prepared weekly. On day 0 and
    on day 7 of weeks 1, 14, 52, 104, and 122 samples of the diet
    containing the test substance were taken.

          Rats were observed daily for general physical appearance,
    behaviour, and toxic signs and observations recorded weekly except
    moribundity/mortality which were recorded daily. Individual body
    weights and food consumption were recorded weekly. Opthalmoscopic
    examination was performed at 3, 6, 12, 18, and 24 months and at
    termination. Haematology, clinical chemistry, and urinalysis were
    performed on samples obtained at 3, 6, 12, 18, and 24 months from 10
    rats/sex/dose (selected randomly). The surviving rats were
    sacrificed and complete necropsies performed. The following organs
    were weighed: adrenals, brain, heart, kidneys, liver, spleen,
    testes, and ovaries. All animals that died during the course of the
    study were necropsied but no organ weights were taken.
    Histopathologic evaluation of a standard array of tissues from the
    control and high-dose group, and liver and mesenteric lymph nodes,
    and all tissues in which lesions were observed at necropsy in the
    other dose groups was performed.

          Treatment-related physical signs reported included diarrhoea
    and red material in faeces (45 mg/kg b.w./day and 135 mg/kg
    b.w./day) and reddish-brown urine (15, 45, and 135 mg/kg b.w./day).
    Mortality was not statistically different from controls for any
    treatment group. At 80 weeks, survival for all groups exceeded 80%,
    except high dose males with 72% survival. At terminal sacrifice,
    survival was: Controls: - 34% M and 36% F; 5 mg/kg b.w./day - 42% M
    and 28% F; 15 mg/kg b.w./day - 46% M and 38% F; 45 mg/kg b.w./day -
    44% M and 36% F; and 135 mg/kg b.w./day - 24% M and 24% F. Body
    weights at terminal sacrifice were significantly lower for the 45
    and 135 mg/kg b.w./day groups compared with controls. This is
    reflective of the weight differences at the beginning of the study.
    However, weight gains were also reduced for the 45 and 135 mg/kg
    b.w./day groups versus controls. Food consumption was comparable for
    all groups. Sporadic significant differences occurred in the
    haematologic, clinical chemistry and urinalysis parameters. Of
    these, only the alkaline phosphatase in the 15, 45 and 135 mg/kg
    b.w./day groups and SGOT in the 135 mg/kg b.w./day group were

    consistently elevated in a manner suggesting biological
    significance. The following were noted at gross necropsy:
    enlargement or cyst formation in lymph nodes of rats from the 45 and
    135 mg/kg b.w./day groups, liver mass and/or nodule formation in
    rats of the 135 mg/kg b.w./day group, cyst formation in the liver of
    females in the 135 mg/kg b.w./day group, and testicular masses among
    males at the 135 mg/kg b.w./day dose-level.

          Treatment-related histopathologic findings reported included:
    sinus ectasia and reactive hyperplasia of the mesenteric lymph nodes
    in all but the low dose level; centrilobular hepatocellular
    hypertrophy, focal hepatocellular hyperplasia, hepatocellular
    cytoplasmic vacuolation, focal bile duct proliferation, and biliary
    cyst formation in the 45 and 135 mg/kg b.w./day dose levels, nodular
    hepatocellular hyperplasia in female rats of the 45 and 135 mg/kg
    b.w./day dose levels, and testicular interstitial cell adenomas in
    the 135 mg/kg b.w./day male rats. Based on these findings the
    authors concluded the no effect level for this study was 5 mg/kg
    b.w./day (Goldenthal, 1980c).

          Subsequent to the above report the liver histopathology slides
    from the study were evaluated by an independent pathologist. Slides
    from all study animals were evaluated. The following
    fenbendazole-related changes were reported for the treatment groups:
    hepatocellular hypertrophy, vacuolation and bile duct proliferation
    in the 15, 45, and 135 mg/kg b.w./day groups, hepatocellular
    hyperplasia and biliary cysts in the 45 and 135 mg/kg b.w./day
    groups, and hepatocellular adenomas and carcinomas in the 135 mg/kg
    b.w./day group. A low incidence of hepatic tumours was noted in this
    study including in controls. No treatment-related changes were
    reported for the 5 mg/kg b.w./day group (Brown, 1982).

          Some differences in criteria and terminology were noted between
    the original and review pathologists. Additionally the review
    pathologist reviewed only the liver slides. Therefore a consensus
    report of hepatic lesions was generated as shown in Table 2:

        Table 2: Results of consensus report on hepatic lesions
                                                                                               

                                  Dose (mg/kg b.w./d Fenbendazole)

    Liver lesions                  0       5       15        45         135       Historical
                                                                        (%)       control
                                                                                               

    Periportal            M        0       2       12*       22*        28*
    hypertrophy           F        1       1        8*       25*        21*

    Centrilobular         M        0       0        1         2         13*
    hypertrophy           F        0       0        1         0          5

    Diffuse               M        0       0        1         6*         6*
    hypertrophy           F        0       0        0         3          4

    Focal vacuolation     M        1       2        1         8*         8*
                          F        4       1        3         0          2

    Periportal            M        5       7       12         9          6
    vacuolation           F        4       8        9        14*        14*

    Bile duct             M        8       7       12        11          7        3-52
    proliferation         F        7       8        7        21*        26*       1.6-27

    Biliary cysts         M        1       1        0         6          8*
                          F        1       3        1        12*        29*

    Cholangiosclerosis    M        3       2        3         0          0
                          F        1       3        1        12*        29*

    Nodular/focal         M        4       6        7        11         13*       0-15
    hyperplasia           F       11       8        3        14         19        0-18

    Neoplastic nodule     M        0       2        1         1          3        0-5.7
                          F        2       0        2         0          3        0-5.7

    Adenomas              M        0       0        0         1          1        0-3.3
                          F        1       0        0         0          0        0-2.9

    Carcinomas            M        1       1        3         0          2        0-5
                          F        0       0        0         1          3        0-0.8
                                                                                               

    *     Statistically significant compared to concurrent controls.
          Note: Group sizes were 50 apart from 49 in high dose male group.
    
          Historical controls from 10 studies were provided, which had a
    combined incidence of 1 hepatic carcinoma in 980 female rats while
    the present study contained 1 hepatic carcinoma in the 45 mg/kg
    b.w./day group and 3 hepatic carcinomas in the 135 mg/kg b.w./day
    group females (Muser & McClain, 1982).

          Subsequent to this a pathology working group (PWG) was convened
    to evaluate the liver histopathology slides from the fenbendazole
    chronic rat study. The PWG comprised a chair and 5 additional
    independent pathologists. Prior to the PWG the chair reviewed all
    liver slides in a blind fashion and issued a report. For the PWG
    review the original pathologist's (OP) and the consensus diagnoses
    between the original pathologist and review pathologist (RP) when
    appropriate had to be matched with the PWG chair's diagnoses. All
    hepatocellular neoplasms, all slides showing nodular hyperplasia,
    nodular hypertrophy, or neoplastic nodule diagnosed by the OP or RP
    and focal hyperplasia diagnosed by the chair, and all slides showing
    biliary cyst/cholangioma were reviewed by the PWG and reported as
    shown in Table 3:

        Table 3. Results of PWG report
                                                                                               

                                         Treatment Group (male/female)
                                       Cont        5          15          45          135
                                                                                               

    Nonneoplastic changes

    Focal hyperplasia                   3/6        4/4         1/3         1/9         5/16

    Foci of cellular                   22/24      28/26       29/23       43/29       43/44
    alteration

    Neoplastic changes

    Hepatocellular adenoma              0/1        0/0         0/0         1/1         2/2

    Hepatocellular carcinoma            1/0        1/0         3/0         0/1         3/2

    Combined neoplasms                  1/0        1/0         3/0         1/1         5/4

    Hepatocellular hypertrophy          0/2        2/6        17/20       38/40       42/48

    Cholangioma                         1/0        0/1         0/0         1/3         2/10

    Biliary cysts                       0/0        1/1         1/0         2/6         3/12
                                                                                               

    Note:  Group sizes as in previous table.
    
          The conclusions of the PWG were:

    1.    Lifetime treatment of CD rats with fenbendazole in the diet did
    not result in a significant compound-related increase in
    hepatocellular neoplasms. Differences in incidences among groups
    were considered to reflect normal biological variation.

    2.    Lifetime treatment of CD rats with fenbendazole in the diet was
    associated with a significant increase in hepatocellular foci of
    cellular alteration at dose levels of 45 and 135 mg/kg b.w./day in
    males and 135 mg/kg b.w./day in females. There was also an increase
    in hepatocellular focal hyperplasia in females at 135 mg/kg
    b.w./day. Foci of cellular alteration and focal hyperplasia were
    considered to be toxic lesions that were not associated with
    induction of hepatocellular neoplasms in this study.

    3.    Lifetime treatment of CD rats with fenbendazole in the diet was
    associated with a compound-related increase in hepatocellular
    hypertrophy at dose levels of 15, 45, and 135 mg/kg b.w./day. It is
    interpreted as a common adaptive response to toxicity unrelated to
    the formation of hepatic neoplasms.

    4.    Lifetime treatment of CD rats with fenbendazole in the diet was
    associated with a compound-related increase in "cholangiomas" in
    females at the 135 mg/kg b.w./day dose level. The incidence of
    biliary cysts was also increased in females at both 45 and 135 mg/kg
    b.w./day. Biliary cysts and "cholangiomas" were slightly increased
    in males at 135 mg/kg b.w./day. The weight of all of the evidence
    permitted the consensus opinion of the PWG that the
    "cholangioma"/biliary cysts observed in this study represent a toxic
    proliferative lesion probably initiated  in utero by administration
    of excessively high doses of fenbendazole.

          There was a reduction in the incidence of hepatocellular
    carcinoma in female rats in the 135 mg/kg b.w./day group (2 vs 3)
    reported by the PWG compared with the previous consensus report,
    although this incidence is still high when compared to historical
    controls (Sauer, 1986).

    2.2.4  Reproduction studies

    2.2.4.1  Rats

          The potential reproductive effects of fenbendazole were studied
    in a 3-generation reproduction study in Charles River CD rats in
    which fenbendazole was administered in the diet to provide doses of
    0, 160, 400, and 1000 mg/kg b.w./day. Eighty male and 160 female

    rats (weighing 63 to 114 g) were evenly distributed among the
    treatment groups. Except during mating the rats were individually
    housed in wire-mesh cages. After 70 days of treatment, at
    approximately 100 days of age, the F0 parental rats were housed 2
    females/male within the same treatment group for 15 days to produce
    the F1 generation. The females were examined daily and presence of
    sperm or vaginal plug was designated day 0 of pregnancy. Females
    were separated and allowed to deliver, with this date designated day
    0 of lactation. The pups were counted, sexed and weighed at
    designated intervals during lactation. At 21 days pups from the
    F1a litters were selected to comprise a 2-year oral toxicity study
    in rats. After weaning, the F0 parental rats were reduced to 10
    males and 20 females per group and after a 10-day rest period, the
    surviving F0 parental rats were mated a second time to produce the
    F1b litters. The F1b litters were raised and evaluated in the
    same manner as the F1a litters. After weaning the F1b litters
    were allowed to remain together for 1 week and then 10 males and 20
    females were selected to comprise the F1 generation. After
    weaning, any F1b pups not selected for the next generation were
    discarded. Five male and 5 female F0 rats per group were
    sacrificed and necropsied and the remainder discarded.

          Due to signs of toxicity observed at 15 weeks in the F1a
    generation rats being used in a 2-year study, the reproductive
    toxicology study was terminated after 30 weeks. The F1 rats were
    examined externally, sacrificed and discarded. A reduction in weight
    gains was consistently seen in treated groups. In general this
    effect was more severe in the higher dose groups. Other parameters
    were less consistently affected (Goldenthal, 1979b).

          The potential reproductive effects of fenbendazole were studied
    in a second 3-generation reproduction study in Charles River CD
    rats. Fenbendazole was administered in the diet at levels designed
    to provide doses of 0, 5, 15, 45, and 135 mg/kg b.w./day. Fresh
    diets containing appropriate fenbendazole concentrations were
    prepared weekly throughout the study. Diet samples were collected at
    study initiation, 3 months, 1 year, and at study termination.
    Three-hundred rats were initially distributed among the five
    treatment groups to provide 20 males and 40 females per group. Rats
    were individually housed except during mating and lactation.

          Rats were maintained on their respective diets throughout the
    duration of each generation. After 70 days of treatment, at about
    100 days of age, the F0 parental rats were mated (2 females/male)
    to produce the F1a litters. Rats were maintained together for 15
    days and females were checked daily for sperm. This finding was
    designated day 0 of pregnancy and females were then placed in
    separate cages during gestation. The day of delivery was designated
    lactation day 0, and litter size, live and dead pups were

    determined. Dams and pups were observed daily and litters were
    evaluated on days 1, 4, 7, 14, and 21. Due to decreased body
    weights, pups were allowed to remain with dams for an additional
    week before weaning. The F1a pups were then utilized for a
    life-time carcinogenicity study. After weaning the F0 females and
    males were reduced to 20 and 10 animals per treatment group, rested
    for 10 days and then mated a second time to produce the F1b
    litters.

          An identical procedure to that used to produce the F1a
    litters was used for the F1b litters, except that females were
    housed with different males within the treatment group. After
    weaning the F1b pups were maintained together for 1 week, and then
    10 males and 20 females were randomly selected from each group to
    become the F1 parents. The remaining F1b pups were examined and
    sacrificed. Following weaning 5 male and 5 female F0 parents were
    sacrificed and received a complete gross necropsy including
    determination of organ weights. The following organs from the
    control and 135 mg/kg groups were also evaluated
    histopathologically: thyroid, heart, lung, stomach, kidneys, spleen,
    adrenals, urinary bladder, and gonads. Livers from all dose groups
    were evaluated. At approximately 100 days of age, the F1 parental
    rats were mated to produce the F2a litters. The F2a litters were
    handled in the same manner as the F1a litters, except that at
    weaning they were examined for abnormalities, sacrificed and
    discarded rather than being used for a chronic study. The F2b
    litter and subsequent F2 parental rats and F3a and F3b litters
    were produced and evaluated in the same manner. The rats were
    observed daily and examined weekly and weights and feed consumption
    were recorded on a weekly basis. Observations for the reproductive
    aspects of this study included male and female fertility, length of
    gestation period, litter size and weight at various stages of
    lactation, and pup weight at weaning. All pups dying during
    lactation were examined by necropsy or skeletal staining.

          A summary of observations reported for the 45 and 135 mg/kg
    b.w./day parental rats at various times throughout the study
    includes: soft stool with diarrhoea and red discharge, reddening and
    yellowish staining of the anal-genital region and emaciation,
    reduced weight gains and food consumption, and slight to moderate
    histopathologic hepatic changes including hepatocellular
    hypertrophy, biliary hyperplasia, and lymphoid cell infiltration.
    These changes were more severe for the 135 mg/kg b.w./day group.
    Alterations in the 5 and 15 mg/kg b.w./day groups were marginal or
    inconsistent when compared to controls. A summary of observations on
    reproductive or pup effects of fenbendazole reported for the 135 and
    45 mg/kg b.w./day groups at various times throughout the study
    includes: reduced fertility indices, survival indices, pup weight,
    and lactational growth, as well as diarrhoea, yellow color, reduced

    activity, bloated stomach, and alopecia. These effects were also
    more pronounced in the high-dose group. Alterations in the 5 and 15
    mg/kg b.w./day groups were marginal or inconsistent when compared to
    controls. Based on these results the investigators concluded that
    the NOEL for this study was 15 mg/kg b.w./day for maternal and
    reproductive toxicity (Goldenthal, 1980a).

    2.2.5  Special studies on embryotoxicity and/or teratogenicity

    2.2.5.1  Rats

          The potential embryotoxicity of fenbendazole was evaluated in
    Wistar rats. Sexually mature virgin female Wistar rats were mated
    with fertile males, to provide 4 groups of 20 pregnant animals. Dams
    were evaluated for presence of sperm and the day on which sperm was
    detected was designated gestation day one. Dams in these groups
    received doses of 0, 25, 250, or 2500 mg/kg b.w./day fenbendazole
    via stomach tube in a 2% starch mucilage vehicle at the rate of
    10 ml/kg b.w./day on gestation days 7-16. Rats were observed daily,
    and weighed weekly. Doses were based on the most recent body weight.
    Food intake was monitored continuously. All dams were sacrificed on
    gestation day 21 and fetuses were delivered by caesarean section.
    Each dam received a gross necropsy and organs were weighed. The
    uterus was opened and number and placement of live and dead fetuses
    and resorptions were determined. The fetuses were examined
    externally for abnormalities; about 50% were then processed for
    Alizarin red staining to evaluate skeletal abnormalities and the
    remainder were processed with Bouin's solution for evaluation of
    soft tissue abnormalities.

          No treatment-related effects were noted in any of the maternal
    or fetal parameters. One litter in the high-dose group was comprised
    entirely of abnormal pups. Abnormalities included shortened, twisted
    tails, fused vertebral centrae, diaphragmatic hernia, and
    hydrocephalus. The investigators judged this single litter to be an
    anomalous finding as no other treatment group or litter within the
    high-dose treatment group demonstrated any treatment-related effect.
    Based on results of this study the investigators concluded the NOEL
    to be 2500 mg/kg b.w./day (Kramer & Baeder, 1973).

          Fenbendazole was administered orally to Sprague-Dawley rats on
    days 8 to 15 of gestation. There was no evidence of embryotoxic or
    teratogenic effects at either of the doses used - 60 and 120 mg/kg
    b.w./day. Similar results were obtained with the 6-hydroxy
    derivative; and the sulfone metabolite was also without embryotoxic
    or teratogenic effect at the highest dose used (66 mg/kg b.w./day).
    The sulfoxide (i.e., oxfendazole) at a dose level of 16 mg/kg
    b.w./day caused nearly 80% embryolethality and an increase in the
    number of external malformations. At a dose of 2 mg/kg b.w./day
    there was 100% embryolethality (Delatour and Lapras, 1979).

          Febantel and several of its metabolites were tested for
    possible embryotoxicity. The compounds were given to rats, by
    gavage, during days 8 to 15 of pregnancy, up to the maximum
    tolerated dose. Fenbendazole showed no adverse effect at the highest
    dose used - 66 mg/kg b.w./day, but administration of febantel and
    its two sulfoxide metabolites (the second being oxfendazole)
    produced teratogenic effects (increased incidences of external and
    skeletal abnormalities) and embryolethality with no-observed-effect
    levels of 22, 23 and 10 mg/kg b.w./day respectively (Delatour
    et al., 1981a).

    2.2.5.2  Rabbits

          The potential embryotoxicity of fenbendazole was studied by
    administering fenbendazole to pregnant 5-7 month old yellow silver
    rabbits. Eleven to 14 females were employed per dose. Rabbits were
    mated twice (at 6-hour intervals) with fertile males. After mating,
    rabbits were maintained in metal cages with metal gratings. The
    pregnant rabbits were divided into 4 groups of 10 and administered
    doses of 0, 10, 25, and 63 mg/kg b.w./day on gestation days 7-19.
    The test material was administered as suspension in 2% starch
    mucilage with a stomach tube at the rate of 5 ml/kg b.w. The rabbits
    were observed daily, and weighed weekly. Food intake was monitored
    continuously. Does were sacrificed on gestation day 29, received a
    complete gross necropsy, and fetuses were delivered by caesarean
    section. The uterus was evaluated for resorptions and fetuses were
    examined grossly for abnormalities. The fetuses were subsequently
    maintained in an incubator for 24 hours. About 50% of the fetuses
    were then processed in Bouin's solution for soft tissue examination,
    and 50% using Alizarin red stain for skeletal examination.

          One doe in the 63 mg/kg b.w./day group aborted on gestation day
    27, while 2 does in this group and one in the 25 mg/kg b.w./day
    group were found to have resorbed their litters. An increase in
    skeletal anomalies (13th rib) and delayed ossification of cranial
    bones occurred in the 63 mg/kg b.w./day group. Based on these
    findings the investigators concluded the NOEL for this study was
    25 mg/kg b.w./day (Scholz & Baeder, 1973).

    2.2.5.3  Dogs

          Two groups of 12 bitches received oral doses of 100 mg/kg
    b.w./day fenbendazole in capsules on gestation days 14-22 or 22-30.
    A similar control group received empty capsules. Each group
    contained approximately half nulliparous and half multiparous
    bitches. Bitches were observed daily through weaning of pups at 6
    weeks. A necropsy was performed on all stillborn pups and pups dying
    prior to 42 days. About half the bitches in each group produced
    litters. No treatment-related findings were reported (Mehring,
    1981).

    2.2.5.4  Swine

          Eight groups of 10 sows received 3 mg fenbendazole/kg in feed
    daily for 3 days during week 1, 2, 3, 4, 7, 10, 13, or 14 of
    gestation. Ten sows were treated prior to breeding and an equal
    number of controls were used. Fenbendazole had no adverse effect on
    sows or litters (Evans, 1980).

          A total of 104 sows were treated orally with 500 mg/kg b.w./day
    (?) fenbendazole once or occasionally multiple times between
    gestation days 8 and 33. The sows were allowed to deliver their
    litters. The gestation period, live and dead piglets, and
    deformities were determined. Live pigs were radiographed for
    skeletal abnormalities. No maternal effects or fetal effects were
    reported (Tiefenbach, 1984; Baeder 1988).

    2.2.5.5  Sheep

          In a series of studies pregnant ewes were treated with oral
    doses of fenbendazole in the following manner:

           a)  A group of 15 ewes received 15 mg/kg b.w. four times
               during gestation. The exact days of gestation were
               variable.

           b)  A group of 10 ewes were given a single dose of 50 mg/kg
               b.w. 96 hours following servicing.

           c)  A group of 19 ewes received 15 mg/kg b.w. every 4 weeks
               during gestation for a total of 7 doses.

          Each experiment showed no effects on lambing and no apparent
    abnormalities in the offspring (Wilkins, 1973).

    2.2.5.6  Cattle

          A group of 27 cows were given oral doses of 50 mg/kg b.w.
    fenbendazole on days 12 and 21 of gestation, then at 3 week
    intervals until the fifth month, and subsequently at 2-monthly
    intervals. Another group of 35 cattle was given 20 mg/kg b.w.
    fenbendazole on days 9, 10, 19, 30, 69, 99, 129, 159, and 189 of
    gestation. In both experiments calving progressed normally and there
    were no apparent abnormalities in the offspring (Muser & Lapras,
    1979).

    2.2.5.7  Horses

          Pregnant mares were given oral doses of fenbendazole at 10 or
    25 mg/kg b.w./day in the last trimester of pregnancy. Other mares
    were given single oral doses of 5 mg/kg b.w. within 1 to 7 weeks of
    foaling. In neither case were there apparent effects on foals (Paul
    & Muser, 1981).

    2.2.6  Special studies on testicular function

    2.2.6.1  Sheep

          A group of 4 rams were given a single oral dose of 50 mg/kg
    b.w. fenbendazole. Another group of 5 rams were dosed orally with
    15 mg/kg b.w. monthly for 4 months. Semen quality was unaffected
    either during or following treatment (Wilkins, 1973).

    2.2.6.2  Horses

          Effects on testicular function in stallions were examined in a
    study using a single oral dose of 20 mg/kg b.w. fenbendazole. The
    animals were castrated 4, 12, 26, 60, or 72 h after dosing. No
    effects were seen on seminal volume, spermatozoa counts and
    morphology, testicular size and weight or serum testosterone levels
    (Squires et al., 1978).

    2.2.7  Special studies on genotoxicity

        Table 4. Results of genotoxicity studies
                                                                                               

    Test              Test object          Dose or            Results        Reference
    system                                 Concentration
                                                                                               

    Ames test1        S.typhimurium        1-2500 g          Negative       Mourot, 1990
                      TA97, 98, 100,
                      102

    Ames test1        S.typhimurium        1-5000 mg          Negative       Mazza et al.,
                      TA98, 100,                                             1981
                      1535, 1537,
                      1538

    Ames test1        S.typhimurium        1-10000            Negative       Rabenold &
                      TA1435, 1537         g/plate                          Brusick, 1982

    Mitotic           HeLa cells           1 mg/ml            Positive       Puenter, 1978
    index

    Forward           Mouse                up to 62.25        Weakly         Cifone &
    mutation1         lymphoma             g/ml              positive2      Myrh, 1983a
    assay

    DNA               Primary rat          0.5-100            Negative       Myrh &
    repair1           hepatocytes          g/ml                             Brusick, 1982a

    Micronucleus      Mouse RBCs           3000 mg/kg         Negative       Horstmann et
    test                                                                     al., 1986

    Cytogenetics      Chinese              1000-4000          Negative       Muller et al.,
    assay             hamster              mg/kg                             1986
                      marrow bone
                                                                                               

    1.   Both with and without rat liver S9.
    2.   Positive in the presence, but not in the absence of metabolic activation.
    
    2.3  Observations in humans

          Five healthy male subjects were given oral doses of 300 mg
    fenbendazole with breakfast, and 6 healthy male subjects were given
    600 mg fenbendazole 12 hours after their last meals. Serum
    concentrations were monitored. The following parameters were
    evaluated: Blood pressure, pulse rate, symptom list and self-rating
    scale, and clinical chemistry values. Serum values were detected in
    2/5 subjects receiving fenbendazole with a meal and 0/6 subjects
    receiving fenbendazole without food. No relevant changes were
    established in the subjects (Rupp & Hajdu, 1974).

    3.  COMMENTS

          Comprehensive toxicological data on fenbendazole were provided,
    including the results of studies on its kinetics, metabolism, short-
    and long-term toxicity, carcinogenicity, genotoxicity, reproductive
    toxicity, embryotoxicity, and teratogenicity.

          The rate of absorption following oral administration was slow,
    but more rapid in monogastric animals. The extent of absorption was
    25-50% in rats, less than 20% in dogs, 70% in rabbits, 25% in sheep,
    and more than 33% in pigs. Elimination was greater than 90% within 3
    days, with the majority in the faeces. Fenbendazole was metabolized
    to oxfendazole (the sulfoxide), oxfendazole sulfone, and amine
    metabolites, which were detectable in plasma. The major urinary
    metabolite was 4-hydroxy-fenbendazole, with traces of oxfendazole
    and oxfendazole sulfone. The metabolic pathway was similar in rats,
    rabbits, dogs, sheep, cattle, goats, and chickens.

          In a 24-month study in mice in which fenbendazole was given in
    the diet, there were sporadic body-weight differences between
    treated and control groups but no meaningful relationship with drug
    treatment. Survival was reduced in the treated groups, but only at
    the highest dose of 405 mg/kg b.w./day could it be attributed to
    fenbendazole administration. There were no increases in tumour
    incidence. The NOEL was 135 mg/kg b.w./day.

          Rats born during a multigeneration study received fenbendazole
    in the diet for 123 weeks at doses of 5, 15, 45, and 135 mg/kg
    b.w./day. Diarrhoea was observed at 45 and 135 mg/kg b.w./day;
    weight gain was reduced at these doses and in females at 5 mg/kg
    b.w./day. Lymph nodes were affected at all but the low dose of
    5 mg/kg b.w./day, showing enlargement or cyst formation, sinus
    dilatation and reactive hyperplasia. The incidence of testicular
    interstitial-cell adenomas was increased in males at the dose level
    of 135 mg/kg b.w./day. The major target organ was the liver, which
    was affected at and above 15 mg/kg b.w./day. The following
    alterations were noted: increased serum alkaline phosphatase
    activity, hepatocellular hypertrophy and hyperplasia, bile duct
    proliferation and biliary cysts, and cytoplasmic vacuolation. There
    was a slight increase in the incidence of hepatocellular carcinomas
    in females at 135 mg/kg b.w./day.

          The Committee noted that the liver histopathology slides from
    the chronic toxicity study in rats had been assessed three times
    and, although there were some differences in criteria and
    terminology, a consensus had been reached. The incidence of
    hepatocellular carcinomas was very low, and there was no
    statistically significant increase as compared with controls.
    Nevertheless, given the extremely low incidence in concurrent and

    historical controls, tumours found in females receiving the highest
    dose of fenbendazole may have been related to treatment. It was
    noted that the small increase in carcinomas was observed against a
    background of statistically significant focal hyperplasia. The NOEL
    in this study was found to be 5 mg/kg b.w./day, based on
    pathological changes in the liver and lesions in the lymph nodes.

          In a series of studies in dogs, fenbendazole was administered
    in capsules for periods of 6 days to 6 months. The major toxic
    effect was lymphoid hyperplasia in the gastric mucosa and mesenteric
    lymph nodes, resulting in an overall NOEL of 4 mg/kg b.w./day. This
    effect was considered by the Committee less important than the
    changes seen in the liver of rats.

          A three-generation reproduction study was conducted in rats
    given fenbendazole in the diet at doses of 5, 15, 45, and 135 mg/kg
    b.w./day. Toxic effects in adult animals including diarrhoea,
    reduced weight gain, and pathological changes in the liver, were
    observed at and above 45 mg/kg b.w./day. At these doses there were
    also reductions in fertility, survival, and growth of neonates
    during lactation. The NOEL was 15 mg/kg b.w./day.

          Fenbendazole was tested for embryotoxicity and teratogenicity
    in rats and rabbits dosed by gavage. Embryotoxicity was not seen in
    either species, while fetotoxicity in the form of an increased
    frequency of occurrence of 13th ribs and delayed ossification of
    cranial bones occurred in rabbits given a dose of 63 mg/kg b.w./day.
    The NOEL were 2500 mg/kg b.w./day in rats and 25 mg/kg b.w./day in
    rabbits.

          In dogs, pigs, sheep, and cattle, the oral administration of
    fenbendazole at various times during the gestation period did not
    result in treatment-related effects in the offspring.

          Fenbendazole did not produce mutations in bacteria or
    chromosomal aberrations in two different  in vivo tests. It
    increased the mitotic index of HeLa cells  in vitro, an effect that
    could be related to the ability of benzimidazoles to interfere with
    tubulin polymerization and thus inhibit spindle formation.

    4.  EVALUATION

          The most significant toxicological findings with fenbendazole
    were in the rat liver. Since fenbendazole appears to be
    nongenotoxic, the Committee considered that a threshold would exist
    for these effects. Thus a NOEL was based on the absence of
    histopathological changes in the liver at 5 mg/kg b.w./day in the
    long-term toxicity/carcinogenicity study in rats.

          A temporary ADI of 0-25 g/kg b.w. was established based on the
    NOEL of 5 mg/kg b.w./day and the application of a safety factor of
    200.

          Even though a temporary ADI was established, it was not used
    for recommending MRLs. Before the toxicological issues relating to
    this compound can be resolved, additional information is required to
    explain the mechanism of the observed increased incidence of tumours
    in female rats at high doses, including the results of a study of
     in vivo DNA binding in the rat liver following oral administration
    of fenbendazole (see Summary section on the benzimadozoles).

    5.  REFERENCES

    BAEDER (1988) Teratogenicity study of fenbendazole in sows (German
    porker) when administered orally in the feed. Hoechst-Roussel
    unpublished report. Submitted to WHO by Hoechst AG, Frankfurt am
    Main, Germany.

    BOOZE, T.F. & OEHME, F.W. (1983) Safety evaluation of fenbendazole
    in swine.  Amer J. Vet. Res., 44, 1117-19.

    BROWN, W.R. (1982) Lifetime oral toxicity study of fenbendazole in
    rats histopathology - liver. Research Pathology Services Inc., New
    Britain, Penna. USA unpublished report. Submitted to WHO by Hoechst
    AG, Frankfurt am Main, Germany.

    CIFONE M.A. & MYHR B.C. (1983) Mutagenicity evaluation of BAY L 5156
    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.

    DELATOUR, P. & LAPRAS, M. (1979) Comparative embryotoxicity of
    fenbendazole and its metabolites.  Collection Med. Leg. Toxicol.
     Mdicale, 111, 143.

    DELATOUR. P., DANDON, M., GARNIER, F. & BENOIT, E. (1982a)
    Metabolism - embryotoxicity relationship of febantel in the rat and
    sheep.  Ann. Rech. Vet., 13, 163-170.

    DELATOUR, P. & PARISH, R. (1986) Benzimidazole anthelmintics and
    related compounds: Toxicity and evaluation of residues. In: Rico, A.
    (ed) Drug Residues in Animals, Academic Press, Inc. Orlando,
    Florida, pp. 175-204.

    DOERR, B., & CARMINES, E. (1983) Final report on 14-week oral
    toxicity study of fenbendazole in dogs. Hoechst-Roussel unpublished
    report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    DUA, P. (1981) Pathology review of microslides of stomach and lymph
    node sections of dogs. FDA report to Hoechst-Roussel
    Pharmaceuticals, Somerville, NJ, USA.

    DWEL (1974) Report on acute oral toxicity test on pigs.
    Hoeschst-Roussel Unpublished report. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    DWEL & UIHLEIN (1980) Determination of the serum levels of
    fenbendazole and its metabolites in rats, rabbits, and dogs after
    oral administration of fenbendazole, Hoechst-Roussel Unpublished
    report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    EVANS, L.E. (1980) Reproduction, teratogenicity, and fertility study
    in boars treated with fenbendazole. Hoechst-Roussel unpublished
    report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    GOLDENTHAL, E.I. (1978) Six-month oral toxicity study in dogs.
    Unpublished report, International Research and Development
    Corporation, Mattawan, MI, USA. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    GOLDENTHAL, E.I. (1979a) Fifteen-week 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.

    GOLDENTHAL, E.I. (1979b) Three-generation reproduction study in
    rats. Unpublished report, International Research and Development
    Corporation, Mattawan, MI, USA. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    GOLDENTHAL, E.I. (1980a) Three-generation reproduction study in
    rats. Unpublished report, International Research and Development
    Corporation, Mattawan, MI, USA. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    GOLDENTHAL, E.I. (1980b) 24-month oral carcinogenicity study in
    mice. Unpublished report, International Research and Development
    Corporation, Mattawan, MI, USA. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    GOLDENTHAL, E.I. (1980c) 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.

    HAYES, R.H., OEHME, F.W. & LEIPOLD, H. (1983a) Toxicity
    investigation of fenbendazole, an anthelmintic of swine.  Amer J.
     Vet. Res., 44, 1108-11.

    HAYES, R.H., OEHME, F.W. & LEIPOLD, H. (1983b) Safety of
    fenbendazole in swine.  Amer J. Vet. Res., 44, 1112-16.

    HORSTMANN, SCHTZ, MAYER & LANGER (1986) An oral mutagenicity study
    of fenbendazole in mice. Hoechst-Roussel unpublished report.
    Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    KELLNER & CHRIST (1973) Pharmacokinetic studies in sheep, dogs,
    rabbits, and rats after administration of HOE 881, Hoechst-Roussel
    unpublished report. Submitted to WHO by Hoechst AG, Frankfur am
    Main, Germany.

    KLPFFER, G. (1973) Tests on the metabolism of S71 1881 in sheep,
    rabbits, and cattle, Hoechst-Roussel unpublished report. Submitted
    to WHO by Hoechst AG, Frankfurt am Main, Germany.

    KRAMER & BAEDER (1973) A teratogenicity test of HOE 881 with oral
    administration in Wistar rats. Hoechst-Roussel unpublished report.
    Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1973a) Report on an acute subcutaneous safety
    evaluation of HOE 881 in rats. Hoechst-Roussel unpublished report.
    Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1973b) Report on an acute intraperitoneal safety
    evaluation of HOE 881 in rats. Hoechst-Roussel unpublished report.
    Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1973c) A subchronic safety evaluation following
    repeated oral administration (30 days) of HOE 881 in rats.
    Hoechst-Roussel unpublished report. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1973d) A subchronic safety evaluation following
    repeated oral administration (30 days) of HOE 881 in dogs.
    Hoechst-Roussel unpublished report. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1974a) A subchronic safety evaluation (90 days)
    of HOE 881 in rats. Hoechst-Roussel unpublished report. Submitted to
    WHO by Hoechst AG, Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1974b) A subchronic tolerance trial (90 days)
    with HOE 881 in dogs. Hoechst-Roussel unpublished report. Submitted
    to WHO by Hoechst AG, Frankfurt am Main, Germany.

    KRAMER & SCHULTES (1974c) Report on an acute oral test for the
    tolerability of S 732542 in mice. Unpublished report No. 116E/112D
    from Hoechst AG. Submitted to WHO by Hoechst AG, Frankfurt am Main,
    Germany.

    KRAMER & SCHULTES (1974d) Report on an acute oral test for the
    tolerability of S 732542 in rats. Unpublished report No. 115E/111D
    from Hoechst AG. Submitted to WHO by Hoechst AG, Frankfurt am Main,
    Germany.

    MAZZA, G., GALIZZI, A., DeCARLI, L. (1981) Microbiologic
    determination of the mutagenic activity of the compound.
    Hoechst-Roussel unpublished report. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    MEHRING, J.S. (1981) Safety study in pregnant bitches with Panacur
    (fenbendazole) granules 22.2%. Laboratory Research Enterprises Inc,
    Kalamazoo, MI, USA unpublished Report. Submitted to WHO by Hoechst
    AG, Frankfurt am Main, Germany.

    MEHRING, J.S. (1982) Panacur (fenbendazole) granules 22.2% and
    suspension 10% safety studies in puppies. Laboratory Research
    Enterprises Inc, Kalamazoo, MI, USA unpublished Report. Submitted to
    WHO by Hoechst AG, Frankfurt am Main, Germany.

    MOUROU, D. (1990) Fenbendazole Ames test. Submitted to WHO by Centre
    National D'Etudes Vtrinaires et Alimentaires, Fougres, France.

    MULLER, JUNG, & WEIGAND (1986) Cytogenetic test on bone marrow cells
    of the Chinese hamster -  in vivo chromosomal analysis,
    Hoechst-Roussel unpublished report. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    MUSER, R.K. & LAPRAS, M. (1979) Safety of fenbendazole use in
    cattle.  Mod. Vet. Pract., 65, 371.

    MUSER, R. & McCLAIN, J. (1982) Fenbendazole suspension 10% - cattle.
    Hoechst-Roussel unpublished report. Submitted to WHO by Hoechst AG,
    Frankfurt am Main, Germany.

    MYRH, B.C. & BRUSICK, D.J. (1982) Evaluation of BAY L 5156 in the
    primary rat hepatocyte unscheduled DNA synthesis assay. Unpublished
    project No. 20991 from Litton Bionetics Inc. Submitted to WHO by
    Bayer AG, Leverkusen, Germany.

    PAUL, J.W. & MUSER, R.K. (1981) Use of fenbendazole in horses.  Mod.
     Vet. Pract., 62, 557.

    PUENTER, J. (1978) Comparative studies on the effect of the
    anthelmintics fenbendazole and mebendazole on mammalian cell
    cultures. Hoechst-Roussel unpublished report. Submitted to WHO by
    Hoechst AG, Frankfurt am Main, Germany.

    RABENOLD, C. & BRUSICK, D.J. (1982) Mutagenicity evaluation of BAY L
    5156 Batch 810030 in the Ames  Salmonella/microsome plate test.
    Unpublished project No. 20988 from Litton Bionetics Inc. Submitted
    to WHO by Bayer AG, Leverkusen, Germany.

    RUPP, W. & HAJDU, P. (1974) Investigations into the pharmacokinetics
    and tolerability of HOE 881 in healthy subjects, Hoechst-Roussel
    unpublished report. Submitted to WHO by Hoechst AG, Frankfurt am
    Main, Germany.

    SAUER, R.M. (1986) Pathology working group report on fenbendazole in
    CD-1 rats, Pathco Inc., Gaithersburg, MD, USA, unpublished report.
    Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    SCHOLZ & BAEDER (1973) A teratogenicity test of HOE 881 with oral
    administration in yellow-silver rabbits. Hoechst-Roussel unpublished
    report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    SCHOLZ & SCHULTES (1973a) Report on an acute oral safety evaluation
    of the anthelmintic HOE 881 in mice. Hoechst-Roussel unpublished
    report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    SCHOLZ & SCHULTES (1973b) Report on an acute oral safety evaluation
    of HOE 881 in rats. Hoechst-Roussel unpublished report. Submitted to
    WHO by Hoechst AG, Frankfurt am Main, Germany.

    SCHOLZ & SCHULTES (1973c) Report on an exploratory acute oral safety
    evaluation of the anthelmintic HOE 881 in rabbits. Hoechst-Roussel
    unpublished report. Submitted to WHO by Hoechst AG, Frankfurt am
    Main, Germany.

    SCHOLZ & SCHULTES (1973d) Report on an acute oral safety evaluation
    of HOE 881 in dogs. Hoechst-Roussel unpublished report. Submitted to
    WHO by Hoechst AG, Frankfurt am Main, Germany.

    SHORT, C.R., BARKER, S.A., FLORY, W. (1988) Comparative drug
    metabolism and disposition in minor species.  Vet. Hum.
     Toxicol., 30 (supp 1), 2-8.

    SQUIRES E.L., AMACUR, R.P., PICKETT, B.W., BERNDTSON, W.E.,
    SHIDELER, R.K. & VOSS, J.L. (1978) Effect of fenbendazole on
    reproductive function in stallions.  Theriogenology, 9, 447-455.

    TIEFENBACH, B. (1984) Report on teratogenicity testing of 500 mg
    fenbendazole per kg body weight in sows (German heavy porkers) when
    administered orally in the feed. Hoechst-Roussel unpublished report.
    Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    VILLNER, CHRIST & RUPP (1974) Investigations into pharmacokinetics
    after the intravenous and oral administration of HOE 881-14C in
    rats, rabbits, dogs, sheep, and pigs, Hoechst-Roussel unpublished
    report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany.

    VOGEL & ALPERMANN (1973) Summary of pharmacology HOE 881 dose range
    of 100 to 300 mg/kg, Hoechst-Roussel unpublished report. Submitted
    to WHO by Hoechst AG, Frankfurt am Main, Germany.

    WAZETER, F.X. & GOLDENTHAL, E.I. (1977) Six-month oral toxicity
    study in dogs. International Research and Development Corporation
    unpublished report. Submitted to WHO by Hoechst AG, Frankfurt am
    Main, Germany.

    WILKINS, C.A. (1973) Prufungen von Panacur - Teratological trial.
    Unpublished report from Hoechst Research Institute, Malelane,
    Republic of South Africa. Submitted to WHO by Hoechst AG, Frankfurt
    am Main, Germany.


    See Also:
       Toxicological Abbreviations
       FENBENDAZOLE (JECFA Evaluation)