IPCS INCHEM Home


    PACLOBUTRAZOL

    EXPLANATION

         Paclobutrazol is a plant growth regulator. It was reviewed for
    the first time at the present meeting.

    EVALUATION FOR ACCEPTABLE INTAKE

    BIOLOGICAL DATA

    Biochemical aspects

    Absorption, distribution and excretion

    Rats

         A single dose of 5 mg(2MBq)/kg 14C-paclobutrazol (triazole-
    labelled) was administered orally to four male and four female
    Crl:CD(SD)Br rats. By seven days post-dosing, 96-97% of the
    administered dose had been recovered in both sexes, with 58-65% in
    urine and 28-37% in faeces. In urine, 70-75% of the total urinary
    radioactivity was recovered in the first 24 hours, and 95-96% in the
    first 72 hours. Recovery of radioactivity by the faecal route was
    57-74% in the first 48 hours and 96-98% by 120 hours. Measurable
    radioactivity was found in the GI contents and liver of most of the
    animals and in the carcass of one male and the kidney of one female
    (Cresswell et al., 1983).

         In a similar study a single oral dose of 14C-paclobutrazol
    (triazole-labelled) at 250 mg(2MBq)/kg bw was given to four male and
    four female CRI:CD(SD)Br rats. Approximately 98% of the administered
    radioactivity was recovered in urine, faeces and cage washings in both
    sexes during the 168 hours following dosing: 51-54% in urine, 43-45%
    in faeces. Approximately 75% of urinary radioactivity was collected in
    the first 48 hours and about 70-88% of faecal radioactivity was
    collected by 72 hours (mainly 24-72 hours). Radioactivity was detected
    in GI contents and liver of most of the animals and in the carcass of
    one female (Cresswell et al., 1984a).

         A single dose of 10 mg of triazole-labelled 14C-paclobutrazol
    (1.93 MBq/mg)/kg bw was administered by gavage to three male and three
    female Alderlay Park rats. A total of 32-48% of the administered
    radioactivity given to the males was recovered in urine over four days
    and 44-61% in faeces, with 89% of the urinary radioactivity collected
    in the first 24 hours and 92% of faecal radioactivity in the first
    48 hours. In females 48-56% of the radioactivity was recovered in
    urine and 34-41% in faeces with 84% of urinary radioactivity collected
    in the first 24 hours and 82% of faecal radioactivity in the first 48
    hours. Tissue residues were low at 96 hours. The highest residues were
    in liver: 0.08 g equiv./g tissue in males and 0.18 g equiv./g tissue
    in females. Detectable amounts of radioactivity were observed in
    kidney, testes/ovary and fat (Jones et al., 1983).

         As part of the same study one additional male and one additional
    female Alderlay Park rats were given 10 mg (10.6 MBq/mg)
    14C-(triazole-labelled)paclobutrazol/kg bw by savage. At 72 hours
    post-dosing the animals were sacrificed, frozen, and sectioned for
    autoradiogram preparation. The majority of the radioactivity was found
    in the GI tract with small intestine > large intestine > stomach.
    The only organ showing labelling was the liver, which was lightly and
    uniformly labelled (Jones et al., 1983).

         A second autoradiography study in the rat was performed with
    14C-triazole-labelled paclobutrazol at a dose level of 250 mg/kg bw
    given to one male and one female Alpk/AP rat by gavage. A total of 60%
    of the administered dose in the male and 45% in the female was
    recovered during the 48 hours before sacrifice. Very little
    radioactivity was recovered in expired CO2 in either sex.
    Proportions of urinary and faecal excretion were similar in the male
    (about 29% recovered via each route), but urinary excretion
    predominated in the female (36% vs. 7% by the faecal route). The
    autoradiograph indicated that the majority of labelling was in the GI
    tract with similar amounts in small and large intestine and less in
    stomach in the male and similar amounts in stomach and small intestine
    and less in large intestine in the female. Lower amounts of
    radioactivity were observed in the liver and kidney. Liver was
    uniformly labelled. The renal pelvis contained more radioactivity than
    the cortex or medulla. A few foci of radioactivity were seen in
    perirenal fat in both sexes (Jones et al., 1984).

         A single oral dose of 250 mg/kg bw 14C-paclobutrazol
    (triazole-labelled) was administered to 10 female Alpk/AP rats.
    Urinary excretion accounted for 38-70% of the administered
    radioactivity with 85% of this amount recovered in the first 48 hours
    (primarily between 24 and 48 hours). Faecal excretion accounted for
    14-45% of the administered dose. Of this, 93% was recovered between 24
    and 72 hours.

         Administration of 14C-paclobutrazol as a single oral dose of
    250 mg/kg bw to two male and two female bile cannulated rats resulted
    in the recovery of radioactivity in bile of 75% of the dose in males
    and 55% in females. Faecal excretion accounted for 3% of the dose in
    males and 7% in females, while urinary excretion accounted for 20% in
    males and 33% in females. The biliary route is, therefore, of
    considerable importance in the elimination of paclobutrazol in the rat
    (Jones et al., 1986).

         Groups of 3 male rats received daily oral doses of 5 mg
    14C-paclobutrazol (triazole-labelled)/kg bw/day for 3, 7, 14, 21,
    28, 35, 42 days (1 group each), or 49 days (7 groups). The rats were
    sacrificed one day after the final dose, except for the six extra
    groups dosed for 49 days, which were sacrificed 3, 7, 14, 21, 28 and

    35 days after the final dose, respectively. Twenty-four-hour recovery
    of radioactivity after one dose was 70%, with 41% in urine and 29% in
    faeces. After 49 doses, 24-hour recovery was 58%, with 44% in urine
    and 14% in faeces. Tissue levels of radioactivity increased with
    increasing duration of dosing. Highest concentrations were observed in
    liver at all intervals, followed by kidney and blood. Fat levels were
    low or undetectable.

         Tissue levels declined to undetectable levels in fat and blood by
    21 days and in kidney and liver by 28 days. The reduction in
    radioactivity from day 42 levels appeared to be biexponential in liver
    and kidney. The half-lives of each phase of the decline were 1.36 and
    6.69 days for liver and 1.56 and 9.26 days for kidney. Blood levels
    declined rapidly with a half-life of 3.16 days (Greenslade et al.,
    1984).

    Dogs

         14C-paclobutrazol (triazole-labelled) was administered as a
    single oral dose of 5 mg (0.2 HBq)/kg bw to three male and three
    female beagle dogs. Peak blood and plasma levels were observed 0.5-1.5
    hours after dosing. Peak mean plasma levels were 4.453 and 4.105 g
    equiv./ml in males and females, respectively. Peak mean blood levels
    were 2.821 and 2.650 g equiv./ml, respectively. Blood and plasma
    levels declined rapidly to four hours and then more slowly. Blood
    levels were below the limit of detection by 48 hours in both sexes.
    Plasma levels were not detectable by 72 hours in males and 48 hours in
    females. Half-life in plasma was 8.1 hours in males and 7.7 hours in
    females; in blood: 5.3 hours in males and 7.5 hours in females.

         A total of 93% in males and 97% in females of the administered
    radioactivity was recovered in urine, faeces and cage washings in 168
    hours with excretion by the urinary route somewhat higher: 51-57% in
    urine and 33-42% in faeces. Excretion was relatively rapid: 93-95% of
    urinary radioactivity and 76-80% of faecal radioactivity was collected
    in the first 24 hours after dosing.

         Tissue levels were below the limit of detection except in one
    male, where 0.057 g equiv./g tissue was detected in the liver
    (Cresswell et al., 1984b).

    Biotransformation

         The metabolism of paclobutrazol was studied in the rat. Metabolic
    profiles in urine, bile and faeces from Alpk/AP rats given either 5 or
    250 mg 14C-paclobutrazol (triazole-labelled)/kg bw indicated
    extensive metabolism. However, biotransformation was limited to the
    tertiary butyl moiety with no metabolism detected in the triazole or
    halogenated phenyl rings. The two main metabolites in urine, bile and

    faeces were paclobutrazol diol and paclobutrazol acid, which were both
    excreted in conjugated and unconjugated forms. A mechanism of
    biotransformation involving a two-stage oxidation process by way of
    the hepatic mixed function oxidase system is proposed.

         Following oxidation to the diol the fate of the compound
    (excretion or further oxidation to the carboxylic acid) was sex- and
    dose-dependent. At 250 mg/kg bw males excreted about 20% of the dose
    in urine (mainly as the acid) while females excreted about 30% of the
    dose by this route, but mainly as free and conjugated diol (2/3 diol,
    1/3 acid). At 5 mg/kg bw the same pattern was observed in males but
    females excreted a higher percentage of acid than at 250 mg/kg. In
    both sexes most of the dose was excreted in bile in the form of diol
    conjugates which were eliminated in faeces. The authors conclude that
    the males had a greater capacity to oxidize paclobutrazol than the
    females (Jones et al., 1986).

    Toxicological studies

    Special studies on oncogenicity

    Mice

         Groups of 51 weanling Charles River Crl:CD-1(ICR)BR(SPF)
    mice/sex/dose were given diets containing 0, 25, 125 or 750 ppm
    paclobutrazol (92.4% pure) for 104 weeks. Additional groups of 12
    mice/sex/dose were given the same diets for 52 weeks for interim
    examination. Dietary analyses prior to feeding indicated that the
    diets were generally in the range of 90-100% of the nominal
    concentration. There were no treatment related clinical signs of
    toxicity. Mortality was not dose-related. At 76 weeks 75-90% of the
    males and 86-96% of the females were alive. No effects were observed
    on body weights or water intake. Food consumption was slightly higher
    in the 750 ppm group during the first few weeks of the study but the
    observation was not considered to be biologically significant. There
    were no toxicologically significant changes in haematological or
    clinical chemistry parameters at either 52 or 104 weeks with the
    possible exception of reduced triglyceride values in males in the
    750 ppm group at both intervals.

         Liver weights were significantly increased at week 52 in males
    given 750 ppm diet and at week 104 in males and females in this dose
    group. At termination, males at the 750 ppm dose level had increased
    severity of the liver steatosis which was observed in virtually all of
    the males. A similar increase in severity was not observed in females.
    No other lesions appeared to be treatment-related. A number of tumour
    types were observed in all groups. No treatment-relationship was
    indicated by the incidence of any individual rumour type or by the
    overall incidence of tumours. The NOEL in this study was 125 ppm
    (equal to 15 mg/kg bw/day based on food intake and body weight)
    (Shaw, 1986a).

    Rats

         Groups of 50 weanling outbred Crl:CD(SD)BR (Sprague-Dawley
    derived) rats/sex/dose were given diet containing 0, 50, 250 or
    1250 ppm paclobutrazol (92.4% pure) for 104 weeks. Additional groups
    of 10 rats/sex/dose were given the same diets for 52 weeks for interim
    examination. Diet analyses indicated that the concentrations were
    generally 90-100% of the nominal concentration. No effects were noted
    on the clinical condition of the animals or their survival. At 104
    weeks 58-66% of the males and 42-56% of the females were alive.

         Survival did not drop below 50% until after week 100. Cumulative
    body weight gains in females were consistently and significantly lower
    than controls in the 1250 ppm group but males were not affected.
    During the early weeks of the study food and water consumption in the
    1250 ppm females was slightly lower than in controls. Platelet counts
    were lower in the 1250 ppm group throughout the study in males and
    during the second year in females. Considerable variability was
    observed in clinical chemistry values and differences noted could not
    be unequivocably related to treatment. Urine parameters were not
    affected. Liver weights were increased in both males and females in
    the 1250 ppm group at termination but not at week 52. The only
    non-neoplastic histopathological lesion which was treatment related
    was an increased incidence of large hepatocytes which was frequently
    accompanied by steatosis. This lesion was primarily observed in males
    and females at 1250 ppm but was also observed in eight males at
    250 ppm and one control male.

         A number of tumour types were observed but the incidence of
    individual tumour types was generally similar in all groups. The
    number of males with malignant tumours was slightly higher in treated
    groups than controls, especially in the 1250 ppm group. In this group
    the increase was primarily due to the occurrence of gliomas (3 vs. 9
    in controls) and haemolymphoreticular neoplasias of various types
    (5 vs. 1 in controls). In females gliomas were observed only in the
    250 ppm group, suggesting that the glioma incidences are cluster
    observations unrelated to treatment. The NOEL in this study was
    250 ppm (equal to 11 mg/kg bw/day based on food intake and body
    weight) (Shaw, 1986b).

    Special studies on mutagenicity

         Paclobutrazol was negative in four of six mutagenicity studies.
    In the two studies (micronucleus assay in mice and cytogenetic assay
    in rats) in which equivocally positive results were observed, the
    assay was in bone marrow and the positive results were observed at the
    first sampling time in both sexes. Variability observed in the
    negative controls in both studies suggest that these studies should be
    repeated to clarify results (Table 1).

    Special study on reproduction

         Groups of 30 female and 15 male weanling Alpk/AP (Wistar-derived)
    rats were given diet containing 0, 50, 250 or 1250 ppm of
    paclobutrazol (92.4% pure). After 11-12 weeks the animals were mated
    on a 2:1 basis. If mating did not occur within ten days the male was
    replaced by a proven male. The F0 parents were mated twice (F1a
    and F1b) and the F1 parents only once (F2a). Diets were maintained
    throughout all phases of the study.

         In the parental generations, both males and females given
    1250 ppm had lower body weight gains than controls but food intakes
    were lower only in females. In the F0 parents there was a low
    incidence in all groups of thickened eyelids, chromodacryorrhea and
    twisted snout/dental malocclusion. In the F1 parents these
    observations were largely confined to the 250 and 1250 ppm groups but
    were considered elevated only in the 1250 ppm females, Mating
    performance, pregnancy rate, duration of gestation, litter size and
    litter viability were unaffected by treatment. Pup weight gains were
    reduced only at 1250 ppm in F2a pups just prior to weaning. The pups
    showed the same clinical signs as the parents with variable incidences
    of chromodacryorrhea, thickened eyelids and malocclusion/twisted
    snout, which appeared to be elevated only at 1250 ppm. Liver weights
    were increased in the parental females and in both male and female
    pups of all litter generations in the 1250 ppm group only.
    Centrilobular fatty changes in liver were observed in the F0 parent
    females and the Fla and F2a pups of both sexes at 1250 ppm. There were
    no histopathological changes related to the clinical observations. The
    NOAEL for toxic effects in this study was considered to be 250 ppm
    (equivalent to 12.5 mg/kg bw/day) and the NOEL for reproductive
    effects was 1250 ppm paclobutrazol (equivalent to 62.5 mg/kg bw/day)
    (Wickramaratne, 1987).

    Special studies on teratogenicity

    Rats

         Groups of 24 mated female Alpk/AP, SPR (Wistar-derived) rats were
    administered paclobutrazol (92.4% pure) at dose levels of 0, 40, 100
    or 250 mg/kg by/day by gavage on days 615 of gestation. The day a
    positive vaginal smear was obtained was designated gestation day 0.

        Table 1.  Results of mutagenicity on paclobutrazol

                                                                                                             
                                              Concentration of
    Test system          Test organism        paclobutrazol          Results             Reference
                                                                                                             

    Ames test 1/         Salmonella           1.6-5000 g/plate      negative            Callander,
                         typhimurium                                                     1982
                         TA98, TA100,
                         TA1535, TA1537
                         and TA1538

    Mouse lymphoma       Mouse L5178Y         1.0-100 g/ml          negative            McGregor &
    assay 1/             TK +/- cells                                                    Riach, 1983

    Micronucleus         Mouse                0-140 mg/kg            equivocally         Phillips
    assay (bone                               in corn oil            weakly              et al., 1983
    marrow                                                           clastogenic 2/

    Gytogenetic          Rat                  0-300 mg/kg            equivocally         Richardson
    assay (bone                               in corn oil            weakly              et al., 1984
    marrow)                                                          clastogenic 2/

    Unscheduled DNA      Rat                  0-400 mg/kg bw         negative            Trueman, 1986
    synthesis (liver)                         in corn oil

    Dominant lethal      Mouse                0-300 mg/kg bw         negative            Wickramaratne
    test                                      by gavage for                              et al., 1983
                                              5 days
                                                                                                             

    1/   With and without metabolic activation
    2/   Reviewer's conclusion. Authors concluded negative results, noting high variability in negative
         controls in both studies. The studies should have been repeated.
    
         In the 250 mg/kg bw/day group one female died and four were
    killed in moribund condition after 2-5 doses. There were no deaths in
    the other groups. Signs of urinary incontinence were noted at a higher
    frequency in the 250 mg/kg bw/day group. At 250 mg/kg bw/day body
    weight gain, food consumption and food utilization were reduced during
    the dosing period days 6-9. Body weight gain was also reduced at
    100 mg/kg bw. There were no treatment-related effects on numbers of
    implantations, live foetuses, early or late deaths, or on the sex and
    weight of the foetuses. Cleft palate was observed in one foetus (of
    297 examined) at 40 mg/kg bw/day and three foetuses (of 234 examined)
    at 250 mg/kg bw/day. It is stated that in a preliminary study six
    foetuses (of 85 examined) at 240 mg/kg bw had cleft palate. These
    foetuses were all in one litter. There were single foetuses with cleft
    palate at 0 and 80 mg/kg bw in this preliminary study. There was a
    dose-related increase in skeletal abnormalities with impaired
    ossification at all dose levels. These observations were suggestive of
    a foetotoxic effect and a no-effect level could not be established
    (Killick et al., 1983a).

         Groups of 24 mated female Alpk/AP (Wigtar-derived) SPF rats were
    administered paclobutrazol (92.4% pure) by gavage at dose levels of 0,
    2.5, 10, 40 or 100 mg/kg bw/day on days 7-16 of gestation. In this
    study the day a positive vaginal smear was obtained was designated
    gestation day 1.

         There were no deaths and no clinical signs of toxicity. No
    treatment-related effects were noted on maternal body weight, food
    consumption, numbers of implantations, early deaths, late deaths, or
    live foetuses, sex and weight of foetuses, or uterine weights. The
    only major external/visceral defect which appeared treatment-related
    was unilateral hydroureter in eight foetuses (from 4 litters) at
    100 mg/kg bw/day. No foetus had cleft palate. There was a significant
    increase in the incidence of kidney and ureter defects at 40 and
    100 mg/kg bw/day. There was a dose-related increase in severity of
    these lesions as well. Considered with these changes the incidence of
    hydroureter at 100 mg/kg appeared to be a foetotoxic effect rather
    than a teratogenic effect. There were also increased incidences of
    extra (14) ribs and impaired skeletal ossification at 40 and
    100 mg/kg bw/day. The NOEL for foetotoxic effects in this study was
    10 mg/kg bw/day. Considering the incidence of cleft palate in the
    previous study (although it was not observed in this study), it seems
    reasonable to consider 100 mg/kg bw/day as the NOEL for teratogenic
    effects (Killick et al., 1984).

    Rabbits

         Female New Zealand White rabbits were mated with proven males
    from the same supplier. Due to poor mating performance, however,
    artificial insemination was used to supplement the natural matings.
    Groups of 18 mated (or inseminated) females were administered
    paclobutrazol (92.4% pure) at doses of 0, 25, 75 or 125 mg/kg bw/day
    by gavage on days 6-18 of gestation (day of mating designated day 0 of
    gestation).

         Two deaths occurred in each of the control and 125 mg/kg bw/day
    groups. Two females at 75 mg/kg bw/day aborted and were sacrificed.
    The females in the 125 mg/kg bw/day group lost weight on days 6-9 of
    gestation but thereafter showed similar body weight gains as observed
    in the other groups. Food consumption was variable and not clearly
    influenced by treatment. Pregnancy rate was low in this study: 56, 67,
    83 and 50% at 0, 25, 75 and 125 mg/kg bw/day, respectively. Overall
    pregnancy was higher in females mated naturally (84%) than in those
    inseminated artificially (43%). There were no treatment-related
    differences between groups in numbers of corpora lutea, implantations,
    early or late intra-uterine deaths, live foetuses, or dams with
    intra-uterine deaths. Sex ratio of foetuses, uterus weight, total
    litter weight and mean litter weight were not affected by treatment.

         Major defects were observed only in the treated groups (Table 2A)
    but, with the exception of the multiple vertebral defects, were
    consistent with the expected rates of spontaneous defects observed in
    historical controls in this laboratory. There was no indication of a
    treatment relationship for minor defects, skeletal variants or fore or
    hind limb ossification scores (Killick et al., 1983b).

         Because of the poor mating performance and often unsuccessful
    artificial insemination, the numbers of pregnant females in the
    control and 125 mg/kg bw/day were less than that normally considered
    to acceptable for an adequate study. The authors, therefore, did not
    consider the study to be fully adequate and a second study was
    undertaken.

         In the second study all females were artificially inseminated.
    Groups of 18 females were given paclobutrazol (92.4% pure) at dose
    levels of 0, 25, 75 or 125 mg/kg bw/day by gavage on days 7-19 of
    gestation. The day of insemination was designated day 1 of gestation.
    No treatment-related effects were observed on clinical condition.
    During the first days of dosing (days 7-10), the 125 mg/kg bw/day
    group lost weight but there were no effects after this period. Food
    intake was slightly lower in this group. In this study pregnancy rates
    were higher with 83-94% of the females in each group pregnant. Numbers
    of corpora lutea and implantation sites were similar in all groups.

    Post-implantation loss was slightly increased in the 125 mg/kg bw/day
    group as a result of a higher incidence of late intra-uterine deaths.
    However, the incidence of early intra-uterine deaths were lower than
    controls in this group and the number of live foetuses did not suggest
    a treatment-related effect. Sex ratio was similar in all groups. No
    effect was observed on uterus weights. However, mean foetal weight was
    slightly lower in the 125 mg/kg bw/day group and mean litter weights
    slightly lower in both the 75 and 125 mg/kg bw/day groups.

         Major defects were observed in 2/109, 4/129, 3/103 and 5/106
    foetuses at 0, 25, 75 and 125 mg/kg bw/day, respectively. The types of
    defects (Table 2B) were similar to those observed in the previous
    study except that the only vertebral column defect was scoliosis. The
    multiple vertebral defects observed previously were not seen in this
    study. The incidence of severe flexure of forepaws was increased in
    the 125 mg/kg bw/day group compared to current and historical
    controls.

         A few minor defects and variations occurred at slightly increased
    incidences in the 125 mg/kg bw/day group. These were pale spleen,
    partial ossification of the transverse process of the cervical
    vertebra and the presence of 13 ribs (normal length or short
    floating). At 75 mg/kg bw/day, only the incidence of partial
    ossification of the transverse process of the 7th cervical vertebra
    was slightly increased compared to controls. This observation did not
    appear to be clearly indicative of an embryotoxic effect at this dose
    level (Killick et al., 1986).

         Overall, the dose level of 75 mg/kg bw/day appeared to be a NOEL
    for embryotoxic effects. It may be that the NOEL for teratogenic
    effects is 125 mg/kg bw/day but although the incidence of
    malformations at this dose level in the two studies was not
    consistent, a teratogenic effect cannot be ruled out at this time.
    Therefore, it seems reasonable to accept 75 mg/kg bw/day as a
    conservative NOEL for teratogenic effects.

    Acute toxicity

         The acute toxicity of paclobutrazol to several animal species is
    given in Table 3. Signs of toxicity following oral or i.p.
    administration were subdued behaviour, unsteady gait, signs of urinary
    incontinence in rats and mice, and hypothermia. Signs of toxicity
    following dermal administration of a dose of 2000 mg/kg bw were
    urinary incontinence, upward curvature of the spine and desquamation
    of the application site. No effects were observed after dermal
    application of 1000 mg/kg bw.

        Table 2A. Malformations in rabbits in teratology study with paclobutrazol

                                                                                                             

                                                          Number of foetuses (litters)* affected
                                                                                                             

    mg/kg/day                                              0           25            75          125
    No. of foetuses examined                              63           89            97           52
                                                                                                             

    Head defects
      Encephalocoele                                      0            0             0           1(1)
      Anencephaly                                         0            0             0           1(1)a/

    Cardiac defects
      Persistent truncus arteriosus, enlarged aorta,
      common entry for aorta and pulmonary artery         0            0             1(1)        1(1)a/

    Limb defects
      Forepaw flexion, severe                             0            1(1)          0           0
      Multiple (clubbed feet, shortened forelimbs)        0            0             0           1(1)a/

    Vertebral column defects
      Fused vertebra                                      0            0             1(1)        0
      Multiple defects                                    0            1(1)          0           3(2)a/
                                                                                                             

    *    Number in parenthesis is number of litters affected
    a/   Same foetus
            Table 2B. Malformations in rabbits in teratology study with paclobutrazol

                                                                                              

                                                    Number of foetuses (litters)* affected
                                                                                              

    mg/kg/day                                         0          25           75           125
    Number of foetuses examined                     109         129          103          106
                                                                                              

    Head defects
      Encephalocoele, cleft palate,
      gross malformation of skull                   0           0            1(1) b/      0

    Cardiac defects
      Persistent truncus arteriosus,
      enlarged aorta, reduced pulmonary artery      1(1)        4(3) a/      0            0

    Umbilical hernia                                0           0            1(1)         0

    Limb defects
      Forepaws extremely flexed                     1(1)        0            2(2) b/c/    4(3)

    Vertebral column defects
      Scoliosis                                     0           0            0            1(1)
                                                                                              

    *    Number in parenthesis is number of litters affected
    a/   One also had persistent ductus arteriosus
    b/   Same foetus
    c/   One also had malrotation of a hind limb
    
    Short-tens studies

    Dogs

         Groups of one male and one female beagle dogs/dose level
    (16-18 weeks old) were given paclobutrazol (91.9% pure) in gelatin
    capsules at dose levels of 0, 15, 75 and 225 mg/kg bw/day daily before
    feeding for 6 weeks. No treatment-related effects were observed with
    respect to clinical condition, body weights, food consumption,
    haematology, urinalysis, or histopathological observations. There were
    increased plasma alkaline phosphatase activities in the female at
    75 mg/kg bw/day and both sexes at 225 mg/kg bw/day. Liver/body weight
    ratios were increased in the male at 75 mg/kg bw/day and both sexes at
    225 mg/kg bw/day. These changes were probably adaptive rather than
    indicative of toxic effects and 225 mg/kg can be taken as a NOAEL for
    this study (Clapp et al., 1983).

        Table 3.  Results of acute toxicity assays with paclobutrazol a/

                                                                                              
                                                LD50
    Species        Sex          Route           (mg/kg bw)       Reference
                                                                                              

    Mouse          M            oral            490               Barber & Parkinson, 1982
                   F            oral            1219              "               "

    Rat            M            oral            1945              "               "
                   F            oral            1336              "               "

    Guinea pig     M            oral            542               "               "
                   F            oral            400-640           "               "

    Rabbit         M            oral            835               "               "
                   F            oral            937               "               "

    Rat            M & F        dermal          >1000             "               "

                   M & F        dermal          >2000 b/          Pritchard, 1984

    Rabbit         M & F        dermal          >1000             Barber & Parkinson, 1982

    Rat            M            i.p.            160-250           "               "
                   F            i.p.            99                "               "
                                                                                              

    a/   97.0% pure unless otherwise specified
    b/   92.4% pure
    
         Groups of six beagle dogs/sex/dose level (22-29 weeks old) were
    given paclobutrazol (92.4% pure) in gelatin capsules at dose levels of
    0, 15, 75 or 300 mg/kg bw/day daily for one year. One of the control
    dogs and one at 15 mg/kg bw/day received each others' capsules for 16
    days during weeks 8-10. There were no deaths and no signs of toxic
    effects or ophthalmoscopic changes. At 300 mg/kg bw/day body weight
    gains were lower than in controls, particularly in the males. No
    effects were observed on food consumption or haematological or urinary
    parameters. Serum alkaline phosphatase activity was increased in both
    sexes given 75 or 300 mg/kg bw/day. Liver weights were increased at
    these two dose levels and hepatic aminopyrine-N-demethylase activity
    was also increased in both sexes at these dose levels.  Histo-
    pathological lesions of liver were minimal (mild hepatocellular
    swelling in a few males and an increased incidence of slight
    ballooning of hepatocytes in females). The liver effects may have been
    adaptive rather than toxic. While 15 mg/kg bw/day was a clear NOEL in
    this study, 75 mg/kg bw/day can be accepted as a NOAEL (Clapp
    et al., 198a).

    Long-Term studies

         See under "Special studies on oncogenicity".

    COMMENTS

         Paclobutrazol, administered orally to rats and dogs, was rapidly
    absorbed and excreted in urine and faeces. Tissue residues were found
    only in liver and kidney and declined rapidly after single or repeated
    dosing.

         In rats most of the material excreted was in the form of
    metabolites. The metabolic pathway was a two-stage oxidation process
    of the tertiary butyl moiety. Oxidation to the diol 5-(4-chlorophenyl)-
    2,2-dimethyl-4-(1H-1,2,4-triazol-1-yl)pentane-1,3-diol was followed
    either by excretion or further oxidation to 5-(4-chlorophenyl)-
    3-hydroxy-2,2-dimethyl-4-(1H-1,2,4-triazol-1-yl) pentanoic acid. Males
    had a greater capacity to oxidize paclobutrazol than females.

         Paclobutrazol is of slight to moderate acute toxicity to mice,
    rats, guinea pigs and rabbits.

         A one-year study in dogs showed decreased body-weight gains and
    liver effects at 300 mg/kg bw/day. Adaptive liver change was the only
    effect observed at 75 mg/kg bw/day. In a two-year study in rats
    reduced body-weight gains and liver effects were observed at the
    highest dose level tested: 1250 ppm. A two-year study in mice also
    showed liver effects at the highest dose level tested: 750 ppm. There
    was no evidence of oncogenicity in either rats or mice.

         There were four negative (two in vitro, two in vivo) and two
    equivocally positive mutagenicity assays. The equivocal studies were
    the mouse micronucleus test and the in vivo cytogenetics assay in
    rats, both of which assayed bone marrow.

         Paclobutrazol did not affect fertility or reproductive
    performance of rats at doses which produced some maternal toxicity.
    Equivocal teratogenic effects were observed in both rats and rabbits,
    with cleft palate observed at doses of 240-250 mg/kg bw in rats (in
    two studies) and skeletal malformations (multiple vertebral defects
    and forelimb flexure) at a dose of 125 mg/kg bw in rabbits. No-effect
    levels were demonstrated in both species.

    TOXICOLOGICAL EVALUATION

         Mouse:    125 ppm in the diet, equal to 15 mg/kg bw/day
         Rat:      10 mg/kg bw/day (based on foetotoxicity)
         Dog:      75 mg/kg bw/day

    ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN

         0-0.1 mg/kg bw

    STUDIES WHICH WILL PROVIDE INFORMATION VALUABLE FOR THE CONTINUED
    EVALUATION OF THE COMPOUND

         Observations in man.

    REFERENCES

    Barber, J.E. & Parkinson, G.R. 1982. PP333: acute oral, dermal and
    intraperitoneal toxicity. Unpublished report no. CTL/P/748 from ICI
    Central Toxicology Laboratory, Alderley Park, Macclesfield, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Callander, R.D. 1982. PP333  An evaluation in the Salmonella/
    microsome mutagenicity assay. Unpublished report no CTL/P/722 from
    ICI Central Toxicology Laboratoy, Alderley Park, Macclesfield, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Clapp, M.J.L., Kalinowski, A.E., Chart, I.S., Gore, C.W. & Scales, D.
    1983. PP333: 6 weeks oral dosing study in dogs. Unpublished report no
    CTL/P/767 from ICI Central Toxicology Laboratory, Alderley Park,
    Macclesfield, UK. Submitted to WHO by Imperial Chemical
    Industries PLC, UK.

    Clapp, M.J.L., Kalinowski, A.E., Chalmers, D.T., Chart, I.S., Gore,
    C.W., Stonard, M.D. & Godley, M.J. 1984. Paclobutrazol: 1 year oral
    dosing study in dogs. Unpublished report no. CTL/P/958 from ICI
    Central Toxicology Laboratory, Alderley Park, Macclesfield, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Cresswell, D.G., Vickers, J. & Hopkins, R. 1983. (14C)-
    Paclobutrazol: excretion and tissue retention of a single oral
    dose (5 mg/kg) in the rat. Unpublished report no 345672/267 from
    Hazleton Laboratories Europe Ltd., Harrogate, UK. Submitted to WHO by
    Imperial Chemical Industries PLC, UK.

    Cresswell, D.G., Vickers, J. & Hopkins, R. 1984a. (14C)-
    Paclobutrazol: excretion and tissue retention of a single oral
    dose (250 mg/kg) in rat. Unpublished report no 326872/268 from
    Hazleton Laboratories Europe Ltd., Harrogate, UK. Submitted to WHO by
    Imperial Chemical Industries PLC, UK.

    Cresswell, D.G., Ward, J. & Hopkins, R. 1984b. (14C)-Paclobutrazol:
    absorption, excretion and tissue retention of a single oral dose
    (5 mg/kg) in the dog. Unpublished report no. 3494-72/270 from Hazleton
    Laboratories Europe Ltd., Harrogate, UK. Submitted to WHO by Imperial
    Chemical Industries PLC, UK.

    Greenslade, D., Vickers, J. & Hopkins, R. 1984. (14C)-Paclobutrazol:
    bioaccumulation of repeated oral doses (5 mg/kg/day) in the rat.
    Unpublished report no. 3743-72/269 from Hazleton Laboratories Europe
    Ltd., Harrogate, UK. Submitted to WHO by Imperial Chemical Industries
    PLC, UK.

    Jones, B.K., Choo, C.K., Williams, D.M. & Soames, A.R. 1983.
    Paclobutrazol: excretion and tissue retention of a single oral dose
    (10 mg/kg) in the rat. Unpublished report no. CTL/P/870 from ICI
    Central Toxicology Laboratory, Alderley Park, Macclesfield, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Jones, B.K., Williams, D.M., Galvin, J. & Soames, A.R. 1984.
    Paclobutrazol: whole body autoradiography in the rat following a
    single oral dose (250 mg/kg). Unpublished report no. CTL/P/1035 from
    ICI Central Toxicology Laboratory, Alderley Park, Macclesfield, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Jones, B.K., Ladd, R.M. & Galvin, J. 1986. Paclobutrazol:
    biotransformation in the rat. Unpublished report no. CTL/P/1036 from
    ICI Central Toxicology Laboratory, Alderley Park, Macclesfield, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Killick, M.E., Piggott, G.H., Banham, P.B. & Thomas, M.R. 1983a.
    Paclobutrazol: teratogenicity study in the rat. Unpublished report
    no. CTL/P/842 from ICI Central Toxicology Laboratory, Alderley Park,
    Macclesfield, UK. Submitted to WHO by Imperial Chemical Industries
    PLC, UK.

    Killick, M.E., Litchfield, M.H., Banham, P.B. & Thomas, M.R. 1983b.
    Paclobutrazol: teratogenicity study in the rabbit. Unpublished report
    no. CTL/P/861 from ICI Central Toxicology Laboratory, Alderley Park,
    Macclesfield, UK. Submitted to WHO by Imperial Chemical Industries
    PLC, UK.

    Killick, M.E., Piggott, G.H., Banham, P.B. & Thomas, M.R. 1984.
    Paclobutrazol: second teratogenicity study in the rat. Unpublished
    report no. CTL/P/997 from ICI Central Toxicology Laboratory, Alderley
    Park, Macclesfield, UK. Submitted to WHO by Imperial Chemical
    Industries PLC, UK.

    Killick, M.E., Piggott, G.H., Pate, I. & Banham, P.B. 1986.
    Paclobutrazol: second teratogenicity study in the rabbit. Unpublished
    report no. CTL/P/1460 from ICI Central Toxicology Laboratory, Alderley
    Park, Macclesfield, UK. Submitted to WHO by Imperial Chemical
    Industries PLC, UK.

    McGregor, D.B. & Riach, C.G. 1983. PP333: assessment of mutagenic
    potential in the mouse lymphoma mutation assay. Unpublished report
    no. 2529 from Inveresk Research International, Musselburgh, UK.
    Submitted to WHO by Imperial Chemical Industries PLC, UK.

    Phillips, C.E., Richardson, C.R., Hart, D. & Longstaff, E. 1983. An
    evaluation of paclobutrazol in the mouse micronucleus test.
    Unpublished report no. CTL/P/848 from ICI Central Toxicology
    Laboratory, Alderley Park, Macclesfield, UK. Submitted to WHO by
    Imperial Chemical Industries PLC, UK.

    Pritchard, V.K. 1984. Paclobutrazol: acute dermal toxicity.
    Unpublished report no. CTL/P/1173 from ICI Central Toxicology
    Laboratory, Alderley Park, Macclesfield, UK. Submitted to WHO by
    Imperial Chemical Industries PLC, UK.

    Richardson, C.R., Howard, C.A., Longstaff, E., Thomas, M.G., Banham,
    P.B., Beck, S.L. & Godley, M.J. 1884. Paclobutrazol: a cytogenetic
    study in the rat. Unpublished report no. CTL/P/891 from ICI Central
    Toxicology Laboratory, Alderley Park, Macclesfield, UK. Submitted to
    WHO by Imperial Chemical Industries PLC, UK.

    Shaw, D.C. 1986a. Paclobutrazol: 104 week oral (dietary
    administration) combined toxicity and carcinogenicity study in the
    mouse with a 52 week interim kill. Unpublished report no. 5014-72/274
    from Hazleton Laboratories Europe Ltd., Harrogate, UK. Submitted to
    WHO by Imperial Chemical Industries PLC, UK.

    Shaw, D.C. 1986b. Paclobutrazol: 104 week oral (dietary
    administration) combined toxicity and carcinogenicity study in the rat
    with a 52 week interim kill. Unpublished report no. 5055-72/273 from
    Hazleton Laboratories Europe Ltd., Harrogate, UK. Submitted to WHO by
    Imperial Chemical Industries PLC, UK.

    Trueman, R.W. 1986. Paclobutrazol: assessment for the induction of
    unscheduled DNA synthesis in rat hepatocytes in vivo. Unpublished
    report no. CTL/P/1608 from ICI Central Toxicology Laboratory, Alderley
    Park, Macclesfield, UK. Submitted to WHO by Imperial Chemical
    Industries PLC, UK.

    Wickramaratne, G.A., Kinsey, D.L., Banham, P.B. & Thomas, M.R. 1983.
    Paclobutrazol: dominant lethal study in the mouse. Unpublished report
    no. CTL/P/922 from ICI Central Toxicology Laboratory, Alderley Park,
    Macclesfield, UK. Submitted to WHO by Imperial Chemical Industries
    PLC, UK.

    Wickramaratne, G.A. 1987. Paclobutrazol: two generation reproduction
    study in rats. Unpublished report no. CTL/P/1496 from ICI Central
    Toxicology Laboratory, Alderley Park, Macclesfield, UK. Submitted to
    WHO by Imperial Chemical Industries PLC, UK.
    


    See Also:
       Toxicological Abbreviations