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 µg equiv./g tissue in males and 0.18 µg 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 µg equiv./ml in males and females, respectively. Peak mean blood levels were 2.821 and 2.650 µg 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 µg 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 µg/plate negative Callander, typhimurium 1982 TA98, TA100, TA1535, TA1537 and TA1538 Mouse lymphoma Mouse L5178Y 1.0-100 µg/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