DIMETHIPIN EXPLANATION Dimethipin is a harvest-aid dessicant or moisture-reduction chemical used on oilseeds, potatoes, and tomatoes. It was evaluated for the first time by the present Meeting. IDENTITY CHEMICAL NAME: 2,3-Dihydro-5,6-dimethyl-l,4-dithiin 1,1,4,4-tetraoxde SYNONYMS: Harvade(R), UB1-N252 STRUCTURAL FORMULA:OTHER INFORMATION ON IDENTITY AND PROPERTIES MOLECULAR WEIGHT: 210 STATE: A white crystalline solid with mild odour MELTING RANGE: 162-167°C SOLUBILITY (gram solute/100 grams solvent at 25°C): Distilled water 0.3; dimethyl formamide 32.4; benzene 1.96; xylene 0.57; methanol 0.05; chloroform 7.92; ethylene dichloride 7.59 STABILITY: Stable for at least 2 years at +30°C and -30°C OCTANOL/WATER PARTITION COEFFICIENT: 1.0 at 1 X concentration 2.0 at 5 X concentration DENSITY: 0.47 g/ml at 20°C Specifications of technical material Technical dimethipin has a purity of about 98%. Depending on the starting material used in the manufacturing process, as many as 4 impurities greater than 0.1% could be present. The impurities include: (i) 1,4-dithiin, 2,3-dihydro-5,6-dimethyl-l,1,4-trioxide; (ii) 1,4-dithiin, 5-ethyl-2,3-dihydro-l,1,4,4-tetraoxide; (iii) 1,4-dithiane, 2-methyl-3-methylene-l,1,4,4-tetraoxide; and (iv) 1,3-dithiolane, 2-ethyl-2-methyl-l,1,3,3-tetraoxide. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOLOGICAL DATA Biochemical aspects Absorption, distribution, elimination, and biotransformation Rat In rats (3 males and 2 females) given a single oral dose of approximately 3.8 mg/kg b.w. of 14C-dimethipin (labelled in the 2,3-position of the dithiin ring; 96% radiochemically pure) in distilled water, an average of about 89% of the administered radioactivity was excreted in 48 hours via the urine and faeces. (One female rat, identically treated, eliminated only about 42% of the given dose via these routes over the same period. Reason for the low 14C recovery was said to be unclear, as tissue and blood levels in this animal were similar to those in the other treated animals.) In general, faecal elimination slightly exceeded urinary excretion. Less than 0.1% of the administered 14C was detected in the expired air. At sacrifice 96 hours after treatment, mean total-residue levels in the tissues analyzed (excluding the blood) amounted to about 1% of the given dose. Residue concentrations were highest in lung, heart, liver, and kidney, and lowest in the gastrointestinal tract, brain, muscle, and fat. Blood-residue levels ranged from 2 to 7.7% of the administered 14C dose. No significant sex difference was apparent in rate, route of elimination, or tissue-residue levels of the compound (Caplan & Merricks, 1978). Analysis of pooled urine and faceal samples from the above-treated rats (excluding the female rat with unusually-low excretary rate) indicated the presence of about 5% of the 14C recovered in the urine or in the faeces (corresponding, respectively, to 2% and 1% of the administered dose) as the unchanged parent compound. The other readioactive components in the urine and the faeces were highly polar (Smilo et al., 1978). Goat Two goats, 1 of them with a cannulated bile duct, were fed 14C-dimethipin (98% radiochemically pure) at 500 ppm in the diet for 3 days. Samples of urine, liver, bile, and kidney were collected for characterization and identification of metabolites. (No details of experimental conditions were provided. Data were presented apparently on only one of the treated animals.) Figure 1 presents the proposed metabolic pathways for dimethipin in the goat. Invariably, in urine, bile, liver and kidney, metabolites III and IV were identified together with the unchanged parent compound. The latter accounted for 2-8% of the 14C recovered in each case. Some alcohol metabolites of dimethipin, viz. metabolites I, II, and V, were identified only in the urine and bile. Most of the radioactive components in urine, bile, liver, and kidney were of a highly polar nature, and were present as conjugates such as glucuronide, cysteine, and acetylcysteine conjugates (McManus, 1984). Proposed metabolic pathways for dimethipin in the goat
TOXICOLOGICAL STUDIES Special study on reproduction Rat Groups of 15 male and 25 female Charles-River CD(SD)BR rats, 5 weeks old, were fed diets containing technical dimethipin (99.7% pure) at 0, 50, 200, or 800 ppm for 105 days prior to mating (1 male and 2 females; "sibling and half-sibling mating avoided") to initiate a 2-generation (2 litters/generation) reproduction study (day 0 of gestation = day when positive vaginal smear or copulatory plug detected). Weanlings of the second litter (F1b) were selected to become parents of the next generation and mated after being placed on the test diets for 125 days. In each generation, the second mating trial took place at least 14 days after weaning at 21 days of age of the first litters, i.e. F1a and F2a. In the parental generations, mortality (the incidence of which was not dose-related) occured only in the females. No compound-related behavioural abnormalities were apparent. F0 and F1b female adults at 800 ppm weighed less than the concurrent controls during the premating period, throughout gestation, and practically throughout the lactation periods. Food consumption was depressed at 800 ppm in the pre-mating period in F0 females from weeks 6-10, in F1b females from weeks 6-17, and in F1b males at weeks 1, 4, and 9. Fertility in males ("a demonstrated ability to impregnate at least 1 female"), mating index (% females mated), gestation index (% mated females with viable litters), the number of days required by females to mate, and the duration of gestation period in treated groups were all comparable to control values. With respect to the progeny generations, the mean number of pups per litter born alive, survival of pups to days 4, 7, 14, and 21, sex ratio, and behaviour of pups were not adversely affected. Pup weight was reduced at 800 ppm in F1a and F1b litters on days 7, 4, and 21. Pup weights on day 21 were decreased in F1a litters at 200 ppm and in F2a litters at 800 ppm. Gross external examination of all pups, including those found dead, revealed only one abnormal pup in the study, which was a stillbirth in a F1a litter at 200 ppm. Findings from gross pathological examination of all parental animals sacrificed after weaning (F0) or 30 days after weaning (F1b) of the second litters (i.e. after 32 weeks and 39 weeks of dietary feeding, respectively) and weanlings from each progeny generation were not significantly different between control and treated groups. Organ-weight determinations on all F1b adults and on 5 male and 5 female weanlings per group of F1b and F2b litters showed increased organ/body-weight ratios of the liver at both 200 and 800 ppm, and of the kidney and brain at 800 ppm in F1b adult females. The organ/body-weight ratio of the liver was, however, depressed at 50 ppm in F1b adult females. Microscopic evaluation of a wide range of tissues, including the liver and kidney, from all F1b adults plus 5 male and 5 female weanlings per group from F1b and F2b litters, and of gross lesions and gonads from F0 adults, indicated no significant changes attributable to treatment. The study showed 200 ppm as a no-effect level for reproduction (The finding of a decrease in pup weight on day 21 at 200 ppm in F1a litters was unlikely to be compound-induced, as it occurred only in a single generation and was not recurrent in nature) (Kehoe & MacKenzie, 1982). Special studies on teratogenicity Rat Groups of sexually-mature mated female rats (BLU:(SD)BR) were intubated with technical dimethipin (97.5% purity) as a suspension in corn oil at 0, 80, 400, or 800 mg/kg b.w./day from day 6 through day 15 of gestation (Day 0 = day vaginal plug observed). An additional group of mated female rats, treated with 250 mg/kg b.w./day of acetylsalicylic acid (aspirin), was used as the positive control. Due to "excessive deaths" of dams at both 400 and 800 mg/kg b.w./day within 8 days of initiation of treatment, these dosage groups were terminated and were not investigated further. Two new dosage groups, 30 and 160 mg/kg b.w./day, were then added to the test 2 weeks after the study began, with the original dosage group of 80 mg/kg b.w./day being maintained as the intermediate level (no concurrent control groups were included for the 2 new dosage groups). The dams were sacrificed on day 20 of gestation and the foetuses were removed by Caesarean section for gross external, visceral, and skeletal examination. At dosage levels up to 160 mg/kg b.w./day, no compound-related mortality or clinical signs were observed. Growth rate of dams during gestation was comparable in all groups. The total number of dams per group that were pregnant and alive on day 20 ranged from 20 to 22. The mean number of implantation sites or live foetuses, foetal weight, and sex ratio were unaffected. An increase in the mean number of resorptions per dam, without a concomitant rise in the incidence of pregnant dams with resorption(s), was noted at 160 mg/kg b.w. There were no significant differences between control and treated groups in skeletal or visceral malformations of foetuses. The positive control group exhibited a number of foetal abnormalities such as encephalomenigocele and gastroschisis. The study demonstrated that the compound was non-teratogenic, non-fetotoxic, and non-toxic to the dams at levels as high as 160 mg/kg b.w. (Knickerboker et al., 1977). Rabbit Groups of 16 sexually-mature female Dutch Belted rabbits, artificially inseminated, were intubated with technical dimethipin (98.3% purity) as a suspension in 0.5% carboxymethyl cellulose at 0, 7.5, 20, or 40 mg/kg b.w./day (at a constant volume of 1 ml/kg b.w.) on days 6-27 of gestation (day 0 of gestation = the day of insemination). The does were sacrificed on day 28 of pregnancy, and the uterine contents were examined. Foetuses (including those aborted or dead) were subjected to examination for gross, skeletal, and visceral abnormalities. No deaths were noted. A slight increase, as compared to concurrent controls, "in the number of females with a reduction in the amount of faeces observed beneath the cage" was reportedly seen at both 20 and 40 mg/kg b.w. at various intervals during gestation. Food consumption data were not available. Does at 40 mg/kg b.w. showed an actual weight loss between days 6 and 12, and maternal weight gain was depressed in a dose-response pattern in all treated groups between days 6 and 28. Fertility rate ranged from approximately 88 to 94% in control and treated groups. One doe each at 0, 20, and 40 mg/kg b.w. "aborted" on day 28. Seven non-viable foetuses were found in the one doe each at 0 and 20 mg/kg b.w. that "aborted". The doe at 40 mg/kg b.w. which "aborted" had 3 late resorptions. At terminal sacrifice, gross pathological findings in treated does were comparable to those in the contols. No significant differences between controls and treated groups were found in the mean number of corpora lutea, implantations, early or late resorptions, viable or non-viable foetuses, or foetal weight. A non-dose-related increase in post-implantation loss due mainly to a non-dose-dependent increase in early resorptions was noted in all treated groups. The values of this particular parameter in the treated groups, however, were within the range of historical control values submitted. The sex ratio (M/F) of foetuses was elevated at 40 mg/kg b.w., with the mean number of female foetuses being reduced. An increase in incidence of foetuses (as well as litters containing foetuses) with 27 presacral vertebrae and with scoliosis (with or without associated rib anomalies) was observed at 40 mg/kg b.w., as compared to concurrent control or historical control incidences presented in the report from a total of 951 foetuses from 149 litters (from an unspecified number of studies with Dutch Belted rabbits over an unspecified period of time). There was no apparent dose- or compound-related increase in frequency of foetal soft-tissue abnormalities. Although the possibility exists that the increase in incidence of the particular skeletal anomalies seen at 40 mg/kg b.w. might be related to maternal toxicity, prudence dictates that 20 mg/kg b.w. be considered as a clear-cut teratogenic no-effect level (McMeekin et al., 1981). Special studies on mutagenicity Dimethipin was without mutagenic activity in a number of assays with micro-organisms and mammalian cells, except with the mouse lymphoma forward-mutation assay in the presence of metabolic activation (Table 1). Special studies on eye and skin irritation In an eye irritation study with New Zealand White rabbits, dimethipin (presumably technical grade of unspecified purity) was found to be extremely irritating to the eye (Baker et al., 1976). Dimethipin (presumably technical grade of unspecified purity) was shown to be slightly irritating to the skin of New Zealand White rabbits in a primary skin-irritation test (Baker et al., 1976). Special study on skin sensitization Results of a dermal-sensitization study in male guinea-pigs (Hartley strain) indicated technical dimethipin (purity not given) to be a weak skin sensitizer (Madison, 1983). Table 1. Results of mutagenicity assays on dimethipin* Test Organisms/ Dosage levels Results References tissues tested In vitro Reverse S. typhimurium 1-1000 Negative Jagannath & mutation strains TA1538, µg/plate (1) Brusick, TA1537, TA1535, 1978, 1981 TA98, & TAlO0 S. cerevisiae 1-1000 Negative Jagannath & strain D4 µg/plate (1) Brusick, 1978 Mitotic non- S. cerevisiae 1-2000 Negative Bootman & Lodge, dysjunction, strain D6 µg/plate (1) 1982 recombination, & mutation Mitotic gene S. cerevisiae 125-2000 Negative Forster et al., conversion strain D4 µg/ml (1) 1984a Chromosome Chinese hamster 5-50 Negative Sorg et al., 1983 aberration ovary cells µg/ml (1) Sister chromatid Chinese hamster 1.56-25 Negative Galloway & Brusick, exchange ovary cells µg/ml(3) (1) 1981 3.1-200 µg/ml(4) Mouse lymphoma L5178Y TK+/- 1.56-75 (2) Myhr & Brusick, forward mouse lymphoma µg/ml(3) 1981 mutation cells 12.5-200 µg/ml(4) Table 1. (Con't) Test Organisms/ Dosage levels Results References tissues tested In vivo Micronucleus Mouse 2 successive Negative Forster et al., daily oral (5) 1984b doses at 22,73.3, or 220 mg/ kg b.w./day (males) or at 30, 100, or 300 mg/ kg b.w./ day (females) * Technical dimethipin (> 98% purity) was used in all the above studies. (1) With or without metabolic activation. (2) Negative without metabolic activation but positive with metabolic activation at 75 µg/ml and above. (3) Non-activated. (4) Activated. (5) 1/5 Males and 5/5 females at the top dosage level died after the first dose. Acute toxicity Table 2. Acute toxicity of dimethipin* LD50 Species Sex Route (mg/kg b.w.) Reference Mouse M oral 440 Shapiro, 1977a F 600 Rat M&F oral 1180 Varner & Matthews, 1977 M i.p. 235 Shapiro, 1977b F 236 M&F inhalation LC50 Babish, 1977 (1h exposure) > 20 mg/l Rabbit M&F dermal > 12,000 Reagen & Becci, (24h exposure) 1982 * Technical dimethipin of > 97.5% purity was used in the oral studies. In both mice and rats, death occurred 24 to 48 hours after treatment. The principal toxic effects in rats were a decrease in body tone and slowed respiration. Data were not provided on mice with respect to toxic signs (Shapiro, 1977a; Varner & Matthews, 1977). Dog Groups of 24-hour fasted crossbred dogs were given single oral doses of technical dimethipin (98.3% purity) in gelatin capsules at 0 (4 males), 464 (4 females), 600 (3 males and 1 female), 1000 (2 males and 2 females), or 1560 mg/kg b.w. (1 male and 3 females). All 4 animals at 1560 mg/kg b.w., 1 male and 2 females at 1000 mg/kg b.w., and 1 male and 1 female at 600 mg/kg b.w. died 8-28 hours post-dosing. Emesis, observed in all animals of all treated groups, usually within 1 hour of dosing, was recurrent in many of the affected animals, generally during the next 2 to 4 hours. Other frequently-noted toxic signs included general "depression" and inappetance, salivation, tremor, and loose blood-stained stools. Survivors recovered from the toxic signs 2 to 10 days after treatment. The major finding at necropsy was marked irritation to the stomach and the intestine at and above 600 mg/kg b.w. The author calculated the oral LD50 of the compound in dogs to be 690 mg/kg b.w. (Strong, 1981). Short-term studies Rat Male and female Charles-River rats (15 per sex per group, 28 days old) were fed dietary levels of technical dimethipin (> 99.2% purity) at 0, 100, 300, or 1000 ppm for 95 days. There was no treatment- related mortality or abnormal behaviour. Food consumption was slightly depressed in females at 1000 ppm throughout the study. Body weight was unaffected. Haematology, blood chemistry, and urinalysis, determined in 10 males and 10 females per control and top-dosage group after 45 and 85 days of the study, indicated no significant compound-related effects. At termination, organ/body-weight ratios of liver and kidney were elevated in females at 1000 ppm. Significant differences were not observed between control and treated groups in gross pathological changes. Histopathological evaluation of a variety of tissues, including liver and kidney, from 10 males and 10 females per control and top-dosage group failed to show any lesions attributable to inclusion of the compound in the diet. The no-effect level appeared to be 300 ppm (Marias et al., 1976). Dog Groups of 4 male and 4 female purebred Beagle dogs (about 6 months old) were given technical dimethipin (> 99.2% purity) in their diet at 0, 100, 300, or 1000 ppm for 90 days. No mortality occurred. Compound-related effects were not apparent on behaviour, body weight, or food consumption during the study or on blood chemistry or haematology conducted after 42 and 85 days of dietary feeding. Urinalysis determined at the same 2 intervals indicated an increase (not dose-related) in incidence of females of all treated groups with moderate to large amounts of "crystals" in their urinary sediments after 85 days. Other urinalysis parameters were not affected (that the particular urinary finding in females of all treated groups was not likely to be compound-related was supported by the absence of effect on any of the urinalysis parameters monitored in the 1-year feeding study in dogs, summarized below). At terminal sacrifice, organ weights and gross pathology were not altered by treatment. Microscopic examination of a large number of tissues, including the testis, from each animal in the study revealed esophagus lesions characterized by "focal mucosal vesicles containing a few acute inflammatory cells" in 1/8 animals at 300 ppm and 3/8 animals at 1000 ppm, but in none of the concurrent control or of the lowest-dosage (100 ppm) groups. Based on the data, 100 ppm could be considered as the no-effect level (Burtner et al., 1976). Male and female purebred Beagle dogs (6 males and 6 females per group; about 7.5 months old), individually caged, were given technical dimethipin (99.7% purity) in their diet at 0, 300, 1000, or 3000 ppm for one year. One male and three females at 3000 ppm died or were sacrificed in extremis between weeks 13 and 52. "Thinness", a major clinical sign, was seen frequently in most animals at 3000 ppm and infrequently in one animal at 1000 ppm. Other signs such as dehydration and paleness of the gums were also noted infrequently at 3000 ppm. Actual weight loss or growth depression and a decrease in food consumption were evident in both sexes at 3000 ppm throughout, or practically throughout, the study. Females at both 300 and 1000 ppm exhibited a marginal, but not consistently dose-dependent, growth reduction (= 10%) between weeks 12 and 48. Animals of the top-dosage group exhibited abnormalities in the T-wave in the electrocardiograms taken at termination and, upon ophthalmoscopic examination, increased incidence of conjunctival discharge and inflammation as well as corneal irregularities and roughening at week 27, but not at week 52. Findings at physical examination described as "severe to slight thinness and irregular or erratic heart beat" were noted almost exclusively in animals at 3000 ppm throughout the study. Monthly determinations of haematology and blood chemistry revealed deviations from control values, mainly at 3000 ppm (both sexes), in many parameters including decreased haematocrits, increased platelet counts, and depressed values of total protein, albumin, globulin, calcium, BUN, and creatinine at most sampling intervals. Animals at 1000 ppm displayed decreased values of BUN (both sexes) and creatinine (males) at many of the sampling intervals. As compared to concurrent controls, males of all treated groups seemingly showed a slight but consistent and generally dose-related decrease in erythrocyte counts and haemoglobin levels. However, these findings were unlikely to be treatment-associated, because values of these 2 haematological parameters for males even at 3000 ppm were within the normal ranges given both for control Beagles in the published literature (Bushby, 1970) and those reportedly recorded for control Beagle dogs maintained in the testing laboratory. Urinalysis, including microscopy of urinary sediments, conducted bi-monthly gave no significant findings related to treatment. At termination, gross pathological findings in the treated groups were not significantly different from those in the controls. There was an increase in organ/body-weight ratio of kidneys at both 1000 and 3000 ppm (both sexes), of liver in males at 3000 ppm and in females at 1000 ppm and above, of brain at 3000 ppm (both sexes), and of testes at 3000 ppm. Histopathological evaluation of an extensive number of tissues from each animal showed the occurrence of testicular degeneration in 0/6, 2/6, 1/6, and 3/6 males at 0, 300, 1000, and 3000 ppm respectively. Severe and diffuse testicular degeneration was seen in one affected male each at 300 ppm and 3000 ppm, while generally mild and focal degeneration of the testes was present in the other affected males. Admittedly, the incidence and severity of testicular degeneration did not follow a dose-response relationship. However, on account of the complete absence of the lesion from the concurrent controls, and from a total of 7 similar 1-year studies conducted at the testing laboratory comprising over 20 control male dogs (McGee, 1983), the possibility of the testicular lesion being related to the compound could not be completely ruled out. Other microscopic findings likely to be attributable to treatment included the presence in the top-dosage group of hypocellularity of the bone marrow, lesions in the gastrointestinal tract (gastritis, edema, ulceration) and heart (haemorrhage), and thymus atrophy, as well as an increased incidence of lesions in the kidney (nephritis), liver (centrilobular degeneration), lymph node (lymphadenitis), and spleen (hyperplasia) at both 1000 and 3000 ppm. The study did not permit the setting of a no-effect level, mainly because of uncertainty concerning the occurrence of testicular degeneration, which occurred at all dose levels (Benson, 1981). Long-term studies Mouse Male and female CD-1 mice (50 animals per sex per group, about 50 days old), housed 5 per sex per cage, were fed dietary levels of technical dimethipin (97-98% purity) at 0, 80, 400, or 2000 ppm for 78 weeks to evaluate the chronic toxicity and carcinogenic potential of the compound. (Animals fed dimethipin were kept in the same room for about 35 weeks as animals in a companion study receiving a highly photodegradable compound identified only with a code name. It was stated, but unsubstantiated with data, that, based on results of diet analyses, dimethipin was stable in the diet for 7 days and that the mixture of dimethipin and basal diet was satisfactorily homogeneous.) All animals in the study, viz., those sacrificed in extremis or that died during the study, and those sacrificed terminally, were subjected to gross pathological examination and to microscopic evaluation of a wide range of tissues, including the brain. Mortality was not influenced by treatment. Between 58 and 78% of the males and 76 and 86% of the females of the control and treated groups were still alive at the conclusion of the study. A slight (< 10%) and non-dose-related decrease in body-weight gain was noted in males at 400 ppm and above during the first 13 weeks. Food consumption was not affected in any consistent dose-related pattern. There were no significant differences between control and treated groups in clinical signs and incidence of palpable nodules or tissue masses. Haematology determined on 5 males and 5 females per group at 3 intervals of the study revealed a significant increase in males at 2000 ppm of haematocrit levels at week 13 and of haemotocrit, haemoglobin, and erythrocyte values at week 78. Terminal haematocrit levels were elevated in females at both 400 and 2000 ppm. Females of all treated groups showed a statistically-significant increase, albeit not strictly dose-dependent, of erythrocyte counts at week 78. Blood- chemistry and urinalysis parameters were not studied. Gross pathological alterations and weights of selected organs were not significantly different between control and treated groups. Based on tabulated data on "individual histopathology findings" (no detailed histopathological data with morphological descriptions of lesions on individual animals were available), compound-induced non- neoplastic changes were not evident. The only notable neoplastic finding appeared to be an elevated incidence of pulmonary (aveolar/bronchiolar) tumours in males at 2000 ppm, as shown in Table 3. Table 3 Number of males with lung tumours/number of males with the lung examined microscopicallya Pulmonary tumour Control 80 ppm 400 ppm 2000 ppm Historical controlb (alveolar/bronchiolar) Total Range adenocarcinomac 1/49 3/49 3/50 7/50 6/168 0/13(0)- (2) (6.1) (6) (14)d (3.6) 2/24(8.3) adenoma 5/49 3/49 3/50 5/50 15/168 1/24(4.2)- (10.2)e (6.1) (6) (10)f (8.9) 2/13(15.4) adenocarcinoma 6/49 6/49 6/50 12/50 21/168 7/61(11.5)or or adenoma (12.2) (12.2) (12) (24) (12.5) 2/13(15.4) a Figure in parenthesis indicates incidence in %. b Data presented in the submitted report on a total of 168 CD-1 control male mice from five 78-week studies over an unspecified timeframe. c Multiple adenocarcinomas noted in 1 control and 2 animals at 2000 ppm. d 1/7, ... e 1/5, or ... f 2/5 of the lung-tumour bearers died or were sacrificed at an unspecified time during the study. All other bearers of pulmonary tumours were terminal survivors. When compared to concurrent-control or historical-control incidences from a total of 5 studies, the incidence of lung adenocarcinoma, but not of adenoma alone, was significantly increased in males at 2000 ppm (P < 0.05, Fisher exact probability). However, comparison with the maximum historical-control incidence with respect to lung adenocarcinoma at 2000 ppm was not significant (P > 0.05). Time-to-tumour (adenocarcinoma) or the multiplicity of tumours was not modified by treatment. The combined incidence of lung adenocarcinoma and adenoma was significantly elevated when compared to the historical-control incidence from a total of 5 studies, but not when compared with concurrent-control or maximum historical-control incidences. There was no dose-related increase in the incidence of benign and/or malignant lung tumours in the females. Other than lung tumours, the incidence, location, and type of tumours in treated groups were comparable to controls. About 30% of the male and 40% of the female concurrent controls were found to have tumours. Lymphoma (in males), lung tumours (in both sexes), and hepatocellular carcinoma (in males) were the most frequently-observed spontaneous tumours. It should be noted that the animals were about 50 days old at the initiation of the study. This could compromise the sensitivity of the test as a carcinogenicity study. The study demonstrated that 80 ppm, equal to 12.3 mg/kg b.w./day, is a virtual no-effect level, based on the monitored criteria other than tumours. The lung-tumour data are unclear (Serota et al., 1981a). Rat Groups of 50 male and 50 female rats (Sprague-Dawley CD strain, about 40 days old), individually caged, were fed technical dimethipin (97-98% purity) in their diet at 0, 40, 200, or 1000 ppm for 104 weeks to assess the chronic toxicity and carcinogenic potential of the compound. (The control group served as common controls for this study and for a companion study on a chemical identified only by a code name. Whether treated animals from the two studies were kept in the same room was not specified.) All animals sacrified in moribund condition or dying during the study, and all survivors sacrificed terminally (during weeks 105 and 106), were subjected to necropsy and histopathological examination of a variety of tissues, including the brain plus any "unusual" lesions. Sections of spinal cord and "head" from 10 male and 10 female terminal survivors per group were also evaluated microscopically. Survival, not adversely affected by treatment, appeared to be better in the top-dosage group than in the concurrent control or in lower-dosage groups. At the end of 104 weeks, with the exception of males at 200 ppm having a survival rate on only 44%, between 50 and 72% of the males and females of all groups, including the control, were still alive. Clincial signs related to treatment were not evident. There were no dose- or compound-related effects on food consumption or incidences of palpable nodules, tissue masses, or wart- like lesions. A slight but consistent growth depression (< 5% in males and < 10% in females) was seen at 1000 ppm between weeks 43 and 95 in males and between weeks 51 through 87 in females. Haematology and blood chemistry conducted on 5 males and 5 females per group at 5 intervals over the course of the study indicated that females of all treated groups displayed an increase of total-protein values at week 13 only, and a decrease of platelet counts at week 104, the only sampling interval for this particular parameter. Other deviations from controls were also observed in certain haematological and blood-chemistry values, but these were essentially confined to the top-dosage group. Significant differences were not apparent between control and treated groups in urinalysis parameters monitored on 5 males and 5 females per group at the same intervals as the blood studies. Gross pathological changes in treated animals were not significantly different from those in the controls. Organ-weight determinations showed an increase (non-dose-related in males and dose- related in females) of the organ/body-weight ratio of the liver in males of all treated groups and in females at both 200 and 1000 ppm. In addition, absolute weight and organ/body-weight ratios of the adrenal gland were decreased in females of all treated groups. Histopathologically, "focally-dilated bile ducts containing basophilic homogeneous material" occurred in 1 male and 1 female in the control group, 2 males and 3 females at 40 ppm, 5 males and 9 females at 200 ppm, and 33 males and 18 females at 1000 ppm. This finding was likely to be related to inclusion of the compound in the diet. Microscopic changes in other tissues, including the adrenal gland, were comparable in treated animals to those in the controls. An interesting but non- dose- or compound-related microscopic finding was that 9-27% of the males in the control and treated groups showed lactation and/or galactocoele. Whether this might be connected with the observation of an increased incidence of mammary fibroadenoma in males of the top- dosage group, as pointed out later, was not certain. An evaluation of the tumour data revealed that the only noteworthy findings were those tabulated in Table 4. The data in Table 4 seem to indicate increased incidences of astrocytoma in males and hepatocellular carcinoma in females at both 200 and 1000 ppm and of mammary fibroadenoma in males at 1000 ppm. The incidence of none of these tumours was, however, significantly different from the concurrent controls (P > 0.05, Fisher exact probability). This result was confirmed for hepatocellular carcinoma when compared with historical control incidences. However, with respect to astrocytoma in males, the incidence was significantly increased at both 200 and 1000 ppm (P< 0.05) when compared with historical-control incidence from a total of 7 studies. Comparison with maximum historical-control incidence was nevertheless not significant (P > 0.05), even at 1000 ppm. It might be noted that one additional glioma was reportedly found (by a consultant to the company) in the control group upon microscopic evaluation of 3 additional brain sections from each animal of the male control and treated groups. In addition, no pre-neoplastic lesions (gliosis) were seen in the original or additional brain slides (Squire, 1984). The latency period of astrocytoma was apparently not reduced by treatment. Overall, no significant differences between control and treated groups were noted in the incidence of animals with tumours (benign and/or malignant), benign tumours, malignant tumours, or multiple primary tumours. Over 80% of the males and 90% of the females of the concurrent control group were tumour bearers, with pituitary adenoma and adrenal tumours in both sexes and mammary tumours in females being the most prevalent tumours. The study indicated that 40 ppm, the lowest tested-level, was a minimum-effect level on parameters other than tumours (Serota et al., 1981b). EVALUATION COMMENTS In rats, the compound is readily absorbed, metabolized, and eliminated via the urine and faeces. It is degraded primarily to polar metabolites, with less than 5% of a single oral dose being recovered as unchanged dimethipin in the excreta. In goats, dimethipin is extensively degraded, also primarily to polar metabolites. Biotransformation of the compound includes hydrolysis, oxidation, decarboxylation, a ring opening, and conjugation. Dimethipin has an oral LC50 value of about 500 mg/kg b.w. in mice and 1200 mg/kg b.w. in rats. A 2-generation (2 litters/generation) reproduction study in rats, as well as teratology studies in rats and rabbits, were negative. Practically all of the mutagenic studies available were negative. The only positive mutagenic response was seen with the mouse lymphoma forward mutation assay and only in the presence of metabolic activation. A 1-year feeding study in dogs failed to show a no-effect level, mainly because of the uncertainty concerning the presence of testicular degeneration even at 300 ppm, the lowest tested level. However, taking into consideration the lack of a dose-response relationship regarding incidence and severity of the testicular lesion, the absence of effect of the compound on the testes of rats in No. of animals with the tumour/No. of animals with the tissue examined microscopically Location *Historical Control & type of tumour Control 40 ppm 200 ppm 1000 ppm Total Range Male Female Male Female Male Female Male Female Male Female Male Female Brain Astrocytoma 1a/-50(2) 1/49(2) 0/49(0) 2/50(4) 3b/-50(6) 0/50(0) 5c/50(10) 0/50(0) 1/279 0/266 0/43-1/21 0/21-0/37 ** 2/50(4) (0.4) (0) (0)-(4.8) (0) Mammary gland *** Fibroadenoma 0/43(0) N.P. 0/31(0) N.P. 0/33(0) N.P. 3/45(6.7) N.P. N.A. N.A. N.A. N.A. Liver Hepatocellular cardnoma 2/50(4) 1/50(2) 1/50(2) 1/50(2) 5/50(10) 4/50(8) 2/50(4) 4/50(8) 13/305 8/285 0/35-3/34 0/32-2/31 (4.3) (2.8) (0)-(8.8) (0)-(6.5) Neoplastic nodule **** 2/50 6/50 2/50 5/50 5/50 5/50 2/50 5/50 N.A. N.A. N.A. N.A. Figure in parenthesis indicates incidence in % a astrocytoma present in one animal found dead at 20 weeks. This incidence (1/50 or 2%) was used as a basis of comparison in statistical analyses. b all 3 brain tumours found in terminal survivors. c the time of detection of the brain tumour ranging irom 71 to 101 weeks with the mean being 83.4 weeks. N.P. data not presented due to the complete absence of a dose-response relationship re incidence N.A. not available (Con't) * Data included in the submitted report from a total of seven 104-week studies In Sprague-Dawley rats over an unspecified time-frame. ** The histopathological examination of 3 additional brain section from each male of control and all treated groups reportedly yielded one additional glioma which was found in one control terminal survivor. It was also indicated that no preneoplastic lesions (gliosis) were found in the original and the additional brain slides (Squire, 1984) *** The historical control incidence of mammary fibroadenoma in an unspecified number of male Sprague-Dawley rats maintained in the testing laboratory was stated to range from 0 to 3% (Serota 1985). **** Not considered as tumour per se but included for information only. the 2-generation reproduction study, and the similarly-negative effect on the testes of mice and rats in the long-term studies, plus the virtual absence of other toxic effects at the lowest-tested level, it was concluded that a level somewhat below 300 ppm could reasonably be taken as a no-effect level for this species. In this connection, the Meeting agreed that 100 ppm, the no-effect level established in an available 90-day dog-feeding study, be accepted as a 1-year no-effect level for this species. In the long-term mouse and rat studies, there was seemingly an increase in the incidence of lung adenocarcinoma in male mice of the top-dosage group (2000 ppm) and of astrocytoma in male rats of both the intermediate- and top-dosage groups (i.e. 200 ppm and 1000 ppm). Although the evidence for a causal relationship between these particular tumour findings and treatment did not appear to be convincing, such a possibility could not be excluded at this time. Before a more definitive opinion on the tumourigenic/carcinogenic potential of the compound can be expressed, additional information, as indicated under "Further work or information required", is needed on those mouse and rat studies from which data on historical control incidences of certain tumours (specifically lung tumours in mice and astrocytoma and liver tumours in rats) were obtained. In view of the concerns with respect to increased incidence of lung adenocarcinoma in male mice and of astrocytoma in male rats of the long-term studies, a temporary ADI was allocated. TOXICOLOGICAL EVALUATION LEVEL CAUSING NO TOXICOLOGICAL EFFECT Mouse: 80 ppm in the diet, equal to 12.3 mg/kg b.w. Dog: 100 ppm in the diet, equivalent to 2.5 mg/kg b.w. ESTIMATE OF TEMPORARY ACCEPTABLE DAILY INTAKE FOR MAN 0-0.003 mg/kg b.w. FURTHER WORK OR INFORMATION REQUIRED (by 1987) 1. Information on those mouse and rat studies from which data on historical control incidences of certain tumours, viz. lung tumours in CD-1 mice and astrocytoma and liver tumours in Sprague-Dawley rats, were obtained and presented in the submitted reports. (Information for each of the studies should include date of each study, age of animals at initiation, mortality rate of animals, and experimental conditions such as diet, number of animals per cage, etc.) 2. Further pharmacokinetic and metabolic studies in rats and/or a non-rodent mammalian species using appropriate multiple-dosage levels. 3. Acute oral toxicity studies on major plant metabolites which are not found in animals if these are liable to occur at substantial residues. DESIRED 1. Results of diet analysis for the 18-month mouse and 2-year rat feeding studies. 2. Studies on any potential oestrogenic or other hormonal effect of dimethipin, including the determination of the affinity of the compound for steroid receptors. 3. Further studies in dogs to assess effects, if any, of dimethipin on the testis. 4. Observations in man. REFERENCES Babish, J.G. Harvade(R) (N252) Tech. C-8-1162-00. Acute inhalation (1977) study in rats. Unpublished report from Food and Drug Research Laboratories, Inc., submitted to WHO by Uniroyal Inc., USA. Baker, R.C., Mastri, C.W., Kinoshita, F.K., & Keplinger, M.L. Acute (1976) irritation tests with a sample coded N252-C10406, Lot No. BL7668 in albino rabbits. Unpublished report (IBT No. 8530- 08861) from Industrial Bio-Test Laboratories, Inc., USA, submitted to WHO by Uniroyal Inc., USA (validated by the Canadian Health Protection Branch). Benson, B.W. 1-Year dietary toxicity study in dogs with N252. (1981) Unpublished report from International Research and Development Corp., USA, submitted to WHO by Uniroyal Inc., USA. Bootman, J. & Lodge, D.C. ARS7728 (technical grade Harvade (R): (1982) Assessment of its ability to induce genetic damage in Saccharomyces cerevisiae. Unpublished report from Life Science Research, England, submitted to WHO by Uniroyal Inc., USA. Burtner, B.R., Kennedy, G.L., Kinoshita, F.K, & Keplinger, M.L. 90-Day (1976) sub-acute oral toxicity study with UBI-N252 technical in dogs. Unpublished report (IBT No. 611-08063) from Industrial Bio-Test Laboratories, Inc., USA, submitted to WHO by Uniroyal Inc., USA (validated by the Canadian Health Protection Branch). Bushby, S.R.M. "Hematological studies during toxicity tests." In (1970) Methods in Toxicology, edited by Paget, G.E. Blackwell Scientific Publications, Oxford and Edinburgh. Caplan, J. & Merricks, D.L. 14C-Harvade(R) radiocarbon study in rats. (1978) Unpublished report from Biospherics Inc., USA, submitted to WHO by Uniroyal Inc., USA. Forster, R., Edwards, N., & Nunziata, A. Mitotic gene conversion in (1984a) Saccharomyces cerevisiae D4. Test substance: Dimethipin tech. 131001-M-00284. Final report. Unpublished report from Life Science Research, Roma Toxicology Centre, Italy, submitted to WHO by Uniroyal Inc., USA. Forster, R., Mosesso, P., & Nunziata, A. Micronucleus test. Test (1984b) substance: Dimithipin tech. Final report. Unpublished report from Life Science Research, Roma Toxicology Centre, Italy, submitted to WHO by Uniroyal Inc., USA. Galloway, S.M. & Brusick, D.J. Mutagenicity evaluation of Harvade(R) (1981) (N252) 98% D-11401 in the sister chromatid exchange assay with Chinese hamster ovary (CHO) cells. Final Report. Unpublished report from Litton Bionetics, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Jagannath, D.R. & Brusick, D.J. Mutagenicity evaluation of N-252, (1978) technical lot D10406, BL8998 CC0005 in the Ames Salmonella/microsome plate test. Final report. Unpublished report from Litton Bionetics, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Jagannath, D.R. & Brusick, D.J. Mutagenicity evaluation of Harvade(R) (19981) (N252) 98% D-11401 in the Ames Salmonella/microsome plate test. Final report. Unpublished report from Litton Bionetics, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Kehoe, D.F. & MacKenzie, K.M. Final report. Two-generation rat (1982) reproduction study with N252. Unpublished report from Hazleton Raltech Inc., USA, submitted to WHO by Uniroyal Inc., USA. Knickerbocker, M., Re, T.A., & Babish, J.G. Teratologic evaluation of (1977) N252 (Harvade(R)) technical in Sprague-Dawley rats. Unpublished report from Food and Drug Research Laboratories, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Madison, W.A. Technical grade Harvade(R). Dermal sensitization in (1983) guinea pigs (modified closed patch technique). Unpublished report from Hazleton Raltech, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Marias, A.J., Kennedy, G.L., Kinoshita, F.K., & Keplinger, M.L. 90-Day (1976) sub-acute oral toxicity study with UBI-N252 technical in albino rats. Unpublished report (IBT No. 622-08070) from Industrial Bio-Test Laboratories, Inc., USA, submitted to WHO by Uniroyal Inc., USA (validated by the Canadian Health Protection Branch). McGee, D.H. Unpublished letter from International Research Development (1983) Corp., USA, to Uniroyal Chemical Co., submitted to WHO by Uniroyal Inc., USA. McManus, J.P. Metabolism of Harvade(R) in goats. Unpublished (1984) report from Uniroyal Chemical Co., submitted to WHO by Uniroyal Inc., USA. McMeekin, S.O., Schardein, J.L., & Blair, M. N252 (Harvade(R) (1981) technical). Teratology study in rabbits. Unpublished report from International Research and Development Corp., USA, submitted to WHO by Uniroyal Inc., USA. Myhr, B.C. & Brusick, D.J. Mutagenicity evaluation of Harvade(R) (N252) (1981) in the mouse lymphoma forward mutation assay. Revised final report. Unpublished report from Litton Bionetics, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Reagan, E.L. & Becci, P.J. Acute dermal toxicity study (LD50) in (1982) albino rabbits of Harvade(R) (N252). Unpublished report from Food and Drug Research Laboratories Inc., USA, submitted to WHO by Uniroyal Inc., USA. Serota, D.G., Alsaker, R.D., & Banas, D. 18-Month toxicity and (1981a) oncogenicity study in mice. N252 technical. Final report. Unpublished report from Hazleton Laboratories America, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Serota, D.G., Alsake, R.D., Dawkins, K.K., & Kunalzins, W. 104-Week (1981b) chronic toxicity study in rats. N252 (Harvade(R) technical) final report. Unpublished report from Hazleton Laboratories America, Inc., USA, submitted to WHO by Uniroyal Inc., USA. Serota, D.G. Unpublished letter from Hazleton Laboratories America, (1985) Inc. to Uniroyal Inc., USA, submitted to WHO by Uniroyal Inc., USA. Shapiro, R. Untitled and unpublished report from Product Safety Labs, (1977a) USA, submitted to WHO by Uniroyal Inc., USA. Shapiro, R. Untitled and unpublished report from Product Safety Labs, (1977b) USA, submitted to WHO by Uniroyal Inc., USA. Smilo, A.R., Fuller, G.B., Tortora, N.J., Curtiss, K.. & Cardona, R.A. (1978) Characteriztion of excretory metabolites from a 14C- Harvade(R) rat balance study. Unpublished report from Uniroyal Chemical, submitted to WHO by Uniroyal Inc., USA. Sorg, R.M., Naismith, R.W., & Matthews, R.J. CHO metaphase analysis (1983) in vitro chromosome aberration analysis in Chinese hamster ovary cells (CHO). PH320-UN-001-83 Harvade(R). Unpublished report from Pharmakon Research International Inc., USA, submitted to WHO by Uniroyal Inc., USA. Squire, R.A. Unpublished letter to Uniroyal Chemical, submitted to WHO (1984) by Uniroyal Inc., USA. Strong, M.B. Studies on the acute oral toxicity of N252 (Harvade(R)) in (1981) crossbred dogs. Unpublished report from Ciba-Geigy Australia Ltd., submitted to WHO by Uniroyal Inc,, USA, Varner, L.L. & Matthews, R.J. Acute oral LD50 in rats. Uni-N-252 (1977) C.N.D.-9801 Lot No. BL-6731. Revised report. Unpublished report from Pharmakon Laboratories, USA, submitted to WHO by Uniroyal Inc., USA.
See Also: Toxicological Abbreviations Dimethipin (Pesticide residues in food: 1988 evaluations Part II Toxicology) Dimethipin (JMPR Evaluations 1999 Part II Toxicological)