CARBADOX 1. EXPLANATION Carbadox is a growth-promoting and antibacterial drug added to swine feed at the rate of 50 ppm. In most areas of the world it is used in animals up to 4 months of age with a 4 week withdrawal period prior to slaughter for human consumption. Carbadox has not been evaluated previously by the Joint FAO/WHO Expert Committee on Food Additives. The structure of carbadox is shown in Figure 1.2. BIOLOGICAL DATA 2.1 Biochemical aspects 2.1.1 Absorption, distribution, and excretion The elimination of carbadox was studied in rats, swine and monkeys. Pigs received 3.5 mg/kg of 14C-carbadox after several of weeks of receiving feed containing 50 g/ton of unlabelled carbadox, while rats and monkeys received a single dose of 5 mg/kg 14C-carbadox. Urine and faeces were collected and assayed for radioactivity. The urinary metabolites were evaluated qualitatively using TLC. Nearly all (13/15) of the metabolites present in swine urine are found in rat and monkey urine. One of the other two was a glycine conjugate of quinoxaline-2-carboxylic acid. All species excreted more than 50% of the dose in urine (swine: 74%, monkey: 61%, rat: 54%) during the 72 hour collection period. The following activities were reported for faeces during the 72 hour period: swine, 17%, monkey, 8-10%, and rat, 29%. Total excretion appeared to be in the 70-90% range over the 72 hour period. The author concluded that the distribution of radioactivity was similar in the three species (von Wittenau, 1969). 2.1.2 Biotransformation Seven-week-old swine received feed containing 50 g/ton of unlabelled carbadox for several weeks, then received a single oral dose of either 3.5 mg/kg or 0.8 mg/kg of 14C-carbadox labelled in the phenyl ring. Peak radioactivity was observed in plasma approximately 3 hours after dosing. The following were identified in plasma at 5-8 hours post dose: carbadox (13%), desoxycarbadox (9-19%), carbadoxaldehyde (13%), and quinoxaline-2-carboxylic acid (19%) (all expressed in terms of total plasma radioactivity). The presence of carbadoxaldehyde in stomach contents was confirmed. Carbadox was rapidly eliminated. Approximately 2/3 of the dose was eliminated in the urine and the remainder in the faeces (total of approximately 90%) within 48-72 hours. Radioactivity equivalent to approximately 0.1 ppm carbadox was found to be retained in the liver at 14 days post dose. Attempts to identify this residual radioactivity were not successful. The only metabolite identified in liver after 24 hours was the major urine-eliminated metabolite, quinoxaline-2-carboxylic acid (Figdor et al., 1969). In a second study seven-week-old pigs received unlabelled carbadox in feed at the rate of 50 g/ton for several weeks followed by a single oral dose of 14C-carbadox labelled in the carbonyl position via stomach tube. The following evaluations were made: expired air was evaluated for 14CO2, labelled material in liver and urine was evaluated to determine if it was methyl carbazate-related, and plasma and urine were evaluated for free hydrazine. Maximum plasma concentrations of radiolabelled material occurred at approximately 3 hours. While early plasma concentrations were similar to those found for ring-labelled carbadox, concentrations at 24 hours remained somewhat higher. Approximately 50% of the radiolabelled material in the plasma at 3 hours was identified as carbadox, while methyl carbazate was estimated to be 30%. The major route of radiolabel excretion was urinary. However, less than half the total amount recovered with ring-labelled carbadox was recovered from carbonyl- labelled carbadox (37% vs 88%). The reason for this is the apparently high conversion of the radiolabel to CO2 (verified in the rat as up to 36%). Radiolabelled material equivalent to 0.1 - 0.34 ppm carbadox was present in liver at 5 days. The authors concluded that some of this material was incorporated CO2 (potentially 25%). The pig receiving 7 mg/kg was found to have eliminated 7% of the dose as free hydrazine in the urine at 24 hours, while pigs receiving lesser doses were not found to have any identifiable hydrazine in their urine (Figdor, 1969). The reported studies support the metabolism scheme illustrated in Figure 2. Extensive studies have been performed not only on carbadox, but also on its metabolites. Studies are summarized separately on each substance in this monograph. 2.1.3 Effects on enzymes and other biochemical parameters The persistence of carbadox-induced adrenal lesions was studied in swine. Groups of 13 animals each received 0, 25, 50, 100, or 200 ppm carbadox in feed for 10 weeks. Five and 11 weeks after withdrawal 2 pigs from each group were necropsied and adrenals evaluated histologically. At five weeks the pigs in the 25 and 50 ppm groups showed recovery of adrenal lesions. At 11 weeks pigs in the 100 and 200 ppm groups had not completely recovered. Plasma aldosterone levels were monitored at 1, 2, 4, 6, and 8 weeks post treatment. At one week aldosterone levels were depressed in the 100 and 200 ppm groups vs controls. All other aldosterone measurements of treated groups were similar to control values (van der Molen et al., 1989a). The effect of carbadox on the renin-angiotensin system was examined in swine. Five groups of 13 five-week-old weaned pigs received feed containing 0, 50, 100, 150, and 200 ppm carbadox to provide target doses of 0, 2, 4, 6, and 8 mg/kg body weight for 10 weeks. By nine weeks the plasma renin levels of all treated groups were significantly higher than controls. At 5 and 10 weeks, renal immunoreactive renin from two to three pigs was evaluated. All pigs demonstrated an increase in immunoreactive renin. It was concluded that carbadox induces activation of the renin-angiotensin system, secondary to suppression of mineralocorticoid secretion (van der Molen et al., 1989b).
The in vitro effect of carbadox on adrenal aldosterone production was evaluated in porcine adrenal slices obtained from 3-5 week old pigs. The adrenal slices were maintained in Krebs solution and exposed to 1-40 µg carbadox/ml. Aldosterone was estimated using RIA at hourly intervals. A dose-dependent decrease in aldosterone production occurred with a reduction of 25-35% at 40 µg/ml (Spierenburg et al; 1988). 2.2. Toxicological Studies 2.2.1 Acute toxicity studies The results of acute toxicity studies with carbadox are shown in Table 1. Table 1: Carbadox acute toxicity data LD50 Species Sex Route (mg/kg b.w.) Reference Mouse M oral 2810 Pfizer, undated a F oral >2810 i.p. 1050 Rat M oral 850 i.p. 810 A study was done to examine the emetic effect of carbadox on monkeys and dogs. Three animals/group received a single dose of carbadox orally via capsule. Doses were 25, 10, 5, and 1 mg/kg b.w./day. In the monkey 1/3 exhibited emesis in the 2 higher dose groups, while in dogs all dose groups had emesis. 2.2.2 Short-term studies 2.2.2.1 Rats Twenty Charles River C-D weanling rats were divided and dosed for 30 days as follows: 2-3 males and females at 50 mg/kg b.w./day and 100 mg/kg b.w./day, and 10 males as untreated controls. Carbadox was administered in the diet. Weight and food consumption were observed weekly and drug levels adjusted as necessary. CBC at 30 days, urinary electrolytes at 1, 2, 3, 14, and 30 days, cholesterol at 30 days and urinalysis at 14 and 30 days were evaluated. Rats received a gross necropsy at the end of the study and tissues were evaluated microscopically. A dose-dependent decrease in weight gain and food- consumption were noted. The authors concluded that all other parameters were normal (Pfizer, 1963). 2.2.2.2 Dogs The short-term toxicity of carbadox was studied by dosing dogs 6 days/week for three weeks. Beagle dogs (1/sex/dose) were initially dosed with 25 or 50 mg/kg b.w./day carbadox via oral capsule. These doses were later reduced to 10 and then 15 mg/kg b.w./day due to emesis. A variety of parameters were evaluated including clinical chemistry, gross necropsy, and microscopic tissue examination at the termination of the study. The dogs lost weight and had elevated SGPT's. It was concluded that carbadox is emetic and possibly hepatotoxic in the dog (Stebbins, 1964). 2.2.3 Long-term/carcinogenicity studies 2.2.3.1 Rats The chronic toxicity and potential tumorigenicity of carbadox were studied in rats. One hundred twenty Charles River C-D rats were divided into 6 groups (10/sex/dose) and received carbadox in the diet at rates providing doses of 100, 50, 25, 10, 5, and 0 mg carbadox/kg b.w./day for 26 months. Feed concentration was adjusted weekly. Clinical signs, body weights, and food consumption were recorded weekly. Haematology and urinalysis were evaluated in 5 rats/sex/dose at 3, 6, 12, 18, and 25 months. Rats were sacrificed at 14 and 112 weeks and received gross necropsies. Tissues were examined histopathologically. Survival was reduced for animals in the high-dose groups. The following observations were reported for animals up to and including the interim sacrifice: 100 mg/kg b.w./day (10 rats/sex necropsied) - Decreased weight gain and food consumption, reduced haemoglobin, RBC's and neutropenia. Microscopic changes reported include pulmonary hemorrhage and edema, adrenalcortical hemorrhage and degeneration, splenic haemosiderin, and thymic atrophy. No animals survived beyond the 14 week sacrifice. 50 mg/kg b.w./day (10 rats/sex necropsied) - Decreased weight gain and food consumption. Microscopic changes reported include pulmonary hemorrhage and edema, adrenalcortical hemorrhage, necrosis, and degeneration, splenic haemosiderin and renocorticomedullary fatty metamorphosis. No animals survived beyond the 14 week sacrifice. 25 mg/kg b.w./day (3 rats/sex necropsied) - Reduced weight gain, slight adrenal cortical atrophy, degeneration/necrosis, and renal tubular fatty change. 5 and 10 mg/kg b.w./day (3 rats/sex necropsied) - No clinical, gross or microscopic changes reported at 3 months. The following observations were reported for rats from 3 months up to and including the 26 month sacrifice: 25 mg/kg b.w./day - One female died at 51 weeks with no drug-related changes noted. All 13 remaining rats were sacrificed by 73 weeks due to palpable abdominal masses. At necropsy all rats had multiple hepatic nodules. Ten of 13 rats were diagnosed microscopically to have benign nodular hyperplasia, while the remaining 3 rats were determined to have malignant transformation based on metastatic foci observed in other organs (lung, kidney and lymph nodes). 10 mg/kg b.w./day - One rat died after 67 weeks with reticuloendothelial neoplasia, a common tumour of aged rats. The remaining 13 rats were sacrificed between the 64th and 112th weeks. Eleven of these were observed to have hepatic benign nodular hyperplasia. 5 mg/kg b.w./day - One male died at 20 weeks due to pulmonary abscesses. The remaining 13 died or were sacrificed between the 61st and 112th weeks. Five or these were observed to have hepatic benign nodular hyperplasia. Controls One male was sacrificed at 33 weeks with a forestomach papilloma and pulmonary atelectasis and a second male died at 93 weeks with myocarditis, nephrosclerosis and peribronchitis. The remaining animals were sacrificed between weeks 80 and 112 of the study. No animals in the control group were observed to have hepatic nodular hyperplasia. The authors suggested that the observed nodular hyperplasia in treated groups was associated with hepatic injury and concluded that the rats failed to tolerate carbadox at any of the tested dose levels. A treatment-related increase in total tumours was also reported for carbadox-treated rats. The tumour incidence was lower in the high-dose group, however mortality occurred earlier in this group than in other treatment groups (Stebbins & Coleman, 1967). A follow-up study was performed to determine a chronic carbadox level tolerated by rats. Sixty male Charles River rats (124-173 gms) and 60 female rats (107-152 gms) of the same strain were divided into 3 groups (20/sex/dose) and received 2.5, 1.0, or 0 mg/kg b.w./day of carbadox in the diet. Signs, weights and food consumption were recorded weekly and drug levels were adjusted accordingly. Haematology, complete urinalysis, and ophthalmoscopic examination were done at 3, 6, 12, 18, and 24 months on 5 rats/sex/dose. An interim sacrifice of 5 rats/sex/dose was conducted at 54 weeks with the remaining rats sacrificed at 106 weeks. Each rat received a gross necropsy and microscopic examination of standard tissues including: brain, spinal cord, eye, sub-maxillary gland, thyroid gland, thymus, heart, aorta, lung, sternum, liver, spleen, pancreas, kidney, adrenal, stomach, mesenteric lymph node, reproductive tract, urinary bladder, femoral nerve, femoral bone marrow, and mammary gland with skin. All parameters evaluated, including histopathologic examination, were within normal limits at 54 weeks. Survival at 2 years was as follows: control - 38%, 1.0 mg/kg b.w./day - 45%, 2.5 mg/kg b.w./day - 43%. At the 2.5 mg/kg b.w./day level 7/27 rats displayed hepatic benign nodular hyperplasia and 7/27 showed peliosis hepatis. At the 1.0 mg/kg b.w./day level 1/29 rats was found to have hepatic benign nodular hyperplasia and 3/29 displayed peliosis hepatis. In the control group 3/29 rats had benign nodular hyperplasia and 2/29 had peliosis hepatis. Additionally, an increase in total mammary tumours was reported in the 2.5 mg/kg b.w./day dose group. The authors concluded that the 1.0 mg/kg b.w./day dose level was tolerated by the rats for 2 years with no adverse effects, based on the similar incidence of both hepatic lesions and tumours in other organs in treatment and control groups (Sigler, 1969a). Three groups of 28 rats (14/sex) were treated with a target dose of 25 mg/kg b.w./day of carbadox, or one of two lots of desoxycarbadox for 11 months to compare the oncogenic activity of carbadox and desoxycarbadox. An equal sized group served as untreated controls. The test material was given in the feed for 10 months. Two to four rats were sacrificed at 30, 60, 90, 191, and 309 days. Animals were observed daily for toxicity and mortality. Weights and food consumption were recorded weekly until the 38th week and then every other week. Rats received a gross necropsy at sacrifice and liver and adrenals were evaluated histopathologically. A moderate decrease in weight gain occurred in both sexes receiving carbadox. No other significant clinical observations were made. At gross necropsy all groups treated for 10 months showed evidence of hepatic changes including necrosis and nodule formation. Changes in the carbadox- treated group were less severe than in the desoxycarbadox-treated group. Histopathologically all desoxycarbadox-treated rats showed evidence of hepatocellular carcinoma, while 2/18 rats receiving carbadox showed evidence of hepatocellular carcinoma. There was a significant increase in adrenal cortical hemorrhage in both carbadox and desoxycarbadox-treated groups (King, 1976). The tumorigenicity of carbadox administered in various dosing regimens was studied more recently in rats. A total of 119 Wistar rats of each sex were divided into groups of various size (2 - 18 rats) following parturition and treated with various regimens of carbadox i.p. (prior to weaning for 8 to 20 days) and/or in the feed at 300 ppm until study termination at 1 year. Rats were examined and weighed routinely during the study. Tumours were evaluated grossly and histopathologically at the end of the study. Weight gains were reduced. Animals receiving 300 ppm carbadox in feed all developed hepatic tumours. Rats which received carbadox i.p. developed a higher incidence of hepatic tumour than untreated controls. The largest number of spontaneous and rare tumours occurred in groups receiving carbadox via both routes (Sykora & Vortel, 1986). 2.2.3.2 Monkeys A study was performed to assess the long-term toxicity of carbadox in primates. Twenty-eight monkeys were divided into 4 groups of 7 animals (3 or 4/sex) and were dosed with carbadox in gelatin capsules 5 days/week. The doses were 20 (5 mg/kg QID), 10 (5 mg/kg BID) or 5 mg/kg b.w./day and controls. A variety of parameters were evaluated at 1, 3, 6, 12, and 24 months including haemoglobin, haematocrit, RBC's, WBC's, clotting time, prothrombin time, complete urinalysis, blood glucose, BUN, alkaline phosphatase, SGOT and SGPT. Ophthalmoscopic examinations were performed at the same intervals. At 3 months and 2 years animals were sacrificed and necropsied. The diet contained 0.2% isoniazid as prophylaxis for tuberculosis. Elevated transaminase levels were detected at the 3 and 6 month evaluations in both treated and control animals. The authors concluded that monkeys tolerated 20 mg/kg/day for two years with no adverse effect (Coleman, 1967). 2.2.4 Reproduction studies 2.2.4.1 Rats A 3-generation reproduction study with 2 litters per generation was conducted using feed levels adjusted to provide target doses of 2.5, 1.0 and 10 mg/kg b.w./day. Feed levels were identical to those used for a concurrent 2 year chronic study. Sixty female and 30 male Charles River C-D rats were divided into groups of 20 females and 10 males per level for the 1st generation. Two females were placed with each male for mating. First litter offspring were sacrificed following a gross examination on lactation day 21. Following a 10 day rest period the females were rebred to different males. Twenty females and 10 males from the 2nd litter of each dose level were selected to become the 2nd generation. These rats were mated at 100 days of age. Dose levels were maintained and the 2nd and 3rd generation were treated from weaning throughout the breeding period. Reproductive parameters evaluated included: fertility, length of gestation, number of livebirths, stillbirths, number alive at 4 days, number weaned, litter weight at 24 hours, litter weight at weaning, sex at weaning, viability index, lactation index, growth index, live birth index, and gross abnormalities. In addition 10 weanlings/dose level from the 2nd litter of the 3rd generation received an ophthalmoscopic examination, gross necropsy, and a standard array of tissues were evaluated microscopically. Although occasional differences from control group parameters were noted, these were sporadic and not thought to be treatment related. A reduced pregnancy rate was reported for all groups of the third generation, with only the first litter of the 1.0 mg/kg b.w./day treatment group above 50%. Additionally a high incidence of cannibalism (30 - 44%) occurred in treated groups of the 3rd generation as well as in the second litter of the untreated control (19%). This cannibalism produced a non dose-related reduction in lactation index in the treated groups of the 3rd generation (59 - 79% in treated group litters vs 93 - 98% in controls). No effects were reported in the first two generations. The authors concluded that carbadox had no effect on fertility, lactation or the neonate at any dose level (Sigler, 1969b). 2.2.5 Special studies on genotoxicity The results of genotoxicity studies with carbadox are shown in Table 2. 2.2.6 Special studies on relay toxicity 2.2.6.1 Rats A study was done to assess the possible risk of toxicity of pork from pigs fed carbadox. To accomplish the study pigs weighing 60-80 kg were fed carbadox at the rate of 0, 20, and 200 ppm in their feed for 30 days at which time they were slaughtered. Meat from these pigs was divided into pieces of approximately 200 grams and frozen at -18 C. Weanling Wistar rats were divided into groups of 12/sex/dose (except the high dose which had 24 females). Rats received a dietary supplement of 10% pork liver from swine fed the carbadox regimens outlined above. This dietary regimen was continued for 23 - 25 months. All rats received gross necropsies and the following organs were evaluated microscopically: liver, kidney, adrenal, testes, ovary, and thyroid. Weights and mortality were recorded. There was no effect on mortality. No treatment-related histologic changes were reported (Ferrando et al., 1975). 2.2.6.2 Dogs In another study 12 beagle dogs weighing about 4.5 kg were fed for one month with 100 g of control pork as part of a diversified diet. The dogs were then distributed into 3 treatment groups and received pork from hogs treated with the above regimens of carbadox at the rate of 100 gm/day for the first 5 months and then 200 gms for the remainder of the treatment period (85 months). The dogs were weighed periodically. Every fourth month haematology, clinical chemistry, and urinalysis parameters were evaluated. All dogs were in good health at sacrifice. The dogs received a gross necropsy at study termination and a standard set of tissues were evaluated histopathologically. No treatment-related effects were reported in this study. The authors concluded that there is no danger to the public health when carbadox is used in pig feed as a growth promoter at 50 ppm (Ferrando, et al.,1978). Table 2: Results of genotoxicity assays on carbadox Test System Test Object Concentration Results Reference Ames test S. typhimurium 0.0004 - 0.125 Positive Pfizer, undated b TA1535 (mg/plate) TA1536, TA1537, Equivocal TA1538, C340 Ames test TA100 3.1 - 61.7 Positive Beutin et al, TA98 (+/- S9) (nM) 1981 Ames test TA100 2.5 - 15 µg/plate Positive Yoshimura et al, TA98 (+/- S9) 1981 Ames test TA100, TA98 .1 umole/plate Positive Negishi, et al, 1980 Ames test TA100 0.001-.02 Positive Voogd et al., 1980 Mutagenicity Ames E. coli WP2hcr Positive Ohta et al, 1980 test TA100+/-S9 5 - 50 µg/plate Positive TA98 Positive TA1538 Negative TA1537 Negative TA1535 Negative Host mediated Assay S. typhimurium 0,1,2, 2.5, 5, 10, Negative Pfizer, undated b TA1534, TA1952 0,1,2.5 5,7.5, Positive 10,12.5, 20 mg/kg Repair test B. subtilis (rec) 1 - 100 µg/disc Positive Yoshimura et al., S. typhimurium (urv) 1981 Fluctuation test K. pneumoniae .00002 - Positive Voogd et al, 1980 E. coli K12 .005 mmoles/l Table 2 (contd) Test System Test Object Concentration Results Reference Mutagenicity test S. cerevisiae D4 .02% Positive Voogd et al, 1980 Micronucleus test rat bone marrow 5 - 240 mg/kg i.p. Positive Cihak and Srb, 1983 Repair test B. subtilis M45 .1 - 100 Positive Ohta et al, umole/disk 1980 Chromosomal damage mouse bone marrow 200 mg/kg acute, Positive Pfizer, undated b 10 mg/kg day seven days Chromosomal damage in vitro human 0 - 250 µg/ml Positive Pfizer, undated b lymphocytes Dominant lethal CD-1 mice 10,50,150,30 mg/kg Negative Pfizer, undated b assay QUINOXALINE-2-CARBOXYLIC ACID 2.2 Toxicological studies 2.2.1 Acute toxicity studies The results of acute toxicity studies with quinoxaline-2-carboxylic acid are given in Table 3. Table 3: Acute toxicity of quinoxaline-2-carboxylic acid LD50 Species Sex Route (mg/kg) Reference Rats M Oral 4450 Briggs, 1971 Mice M Oral 1360 2.2.3 Long-term/carcinogenicity Studies 2.2.3.1 Mouse Groups of 50 Charles River CD mice/sex/dose (100 in controls) received quinoxaline-2-carboxylic acid in feed for 19 months in a study of its chronic oral toxicity and tumorigenic potential. Fresh diets were prepared weekly. Feed and water intake were monitored weekly for the initial three months and monthly thereafter. Feed levels of quinoxaline-2-carboxylic acid were adjusted at these intervals to provide target doses of 0, 25, 50, and 100 mg/kg b.w./day. Haematology and clinical chemistry were evaluated once, prior to sacrifice. Necropsy was performed on all animals. Organ weights were obtained for brain, heart, kidneys, liver, testes, and ovaries. Histopathologic examination was performed on a standard array of tissues. No treatment-related effects were noted in any parameter examined during the study. Cumulative mortalities were 20% and 15% for control females and males, respectively, while treatment group cumulative mortalities ranged from 10% to 20%. Incidence of all tumours was 33% in female and 46% in male control groups, while the incidences in treatment groups ranged from 32% to 42%. The authors concluded that oral administration of quinoxaline-2-carboxylic acid to mice for 19 months produced no evidence of toxicity (Faccini et al., 1979). 2.2.3.2 Rats Nine male and 9 female Charles River C-D rats were divided into groups of 3/sex/dose level. Rats received quinoxaline-2-carboxylic acid in the feed for 2 years. Weight and food consumption were recorded weekly through week 30 and monthly thereafter. Drug levels were adjusted to provide target doses of 100, 50 or 0 mg/kg b.w./day. Rats received clinical examinations weekly and ophthalmoscopic examinations on days 0, 30, 92, 186, 365, 554, and 733. Haematology and urinalysis parameters were evaluated on days 31, 93, 182, 274, 370, 461, 547, 644, and 730. Terminal sacrifice was performed on day 735 of the study. All rats received gross necropsies and standard tissues were evaluated microscopically from each rat. No treatment- related changes were reported. The authors concluded that quinoxaline-2-carboxylic acid was tolerated at up to 100 mg/kg b.w./day when given to rats via feed (Coleman, 1971). An additional study was conducted to determine whether quinoxaline-2-carboxylic acid has tumorigenic potential. To accomplish the study quinoxaline-2-carboxylic acid was administered in the diet of Charles River Sprague-Dawley rats (20/sex/dose level) for two years to provide target doses of 0, 10, 25, and 50 mg/kg b.w./day. Weight and food consumption were monitored weekly and drug concentration adjusted accordingly. Rats were examined for clinical signs weekly. Ophthalmologic examinations were performed on all rats prior to treatment and at 6, 12, 20 and 24 months. Clinical chemistry, haematology and urine parameters were evaluated in 5 rats at 6 and 12 months, 2-4 rats at 18 months, and all rats at termination. At 12 months 5 rats/sex/dose and at 24 months all remaining rats were sacrificed and received a gross necropsy and tissues were obtained for microscopic evaluation. Organ weights were determined for heart, liver, lungs, kidneys, gonads, adrenal, spleen, and brain. A standard array of tissues was examined histopathologically. No treatment-related effects were noted on any of the parameters evaluated. Cumulative incidences of tumours (based on summing males and females with 40 rats/dose) occurring in treatment groups for selected organs of interest are summarized in Table 4. It was concluded that quinoxaline-2-carboxylic acid in doses of 10, 25, and 50 mg/kg b.w./day over a 2 year period to rats does not produce any toxicity or elevated tumor incidence (Pfizer, 1971). Table 4: Cumulative incidence of tumours (males and females combined) after treatment of rats with quinoxaline-2-carboxylic acid Treatment Group Mammary gland Pituitary Pancreas Liver Control 28% 30% 3% 5% 10 mg/kg b.w./day 18 23 10 5 25 mg/kg b.w./day 13 28 3 5 50 mg/kg b.w./day 33 28 3 3 A follow-up chronic study was performed using rats obtained from the 3-generation study F1 litter. An additional high dose level, 100 mg/kg b.w./day, was added for this study. To accomplish the study Charles River C-D rats were divided into groups of 50/sex/dose and received 100, 50, 25, or 0 mg/kg b.w./day of quinoxaline-2-carboxylic acid in the diet for 29 months. In addition 10 rats/sex in the control and 100 mg/kg b.w./day groups were added for clinical chemistry studies at 12 months. Weights were monitored weekly. Food consumption was recorded weekly during the first 5 months, twice during the 6th month, and monthly thereafter, and treatment levels were adjusted accordingly. Clinical examinations were performed daily. Clinical chemistry parameters were evaluated at 6, 12, and 29 months. Haematology was evaluated at the conclusion of the study. All rats received a gross necropsy. Liver and kidneys were weighed from rats sacrificed at 12 months and heart, liver, kidneys, brain, and testes were weighed during the gross necropsies at 29 months. Tissues from many organs showing macroscopic abnormality plus a standard array of grossly normal tissues were evaluated microscopically. The general health of the rats was unaffected by treatment. An outbreak of sialoacryoadenitis occurred between ages 89 and 97 days. Slightly reduced weight gain (3-5%) was noted in the high-dose groups in both sexes. The cumulative mortality for controls was 56% in males and 70% in females, while treatment group cumulative mortality ranged from 46% to 74%. There was no difference in the incidence of nonneoplastic lesions among the control and treatment groups, nor did treatment shorten the lifespan of treatment groups. Treatment of rats for 2 years did not produce an increase in the incidence of animals with malignant or benign tumours compared to the control group. The authors concluded that the NOAEL was 100 mg/kg b.w./day (King, 1979a). 2.2.4 Reproduction studies 2.2.4.1 Rats The potential reproductive toxicity of quinoxaline-2-carboxylic acid was studied in rats. The study was performed in conjunction with a 2-year chronic study. Treatment route was via feed. The study utilized 2 litters per generation for 3 generations with twenty rats/sex/dose level using target doses of 25, 50, and 100 mg/kg b.w./day. The F0 rats were bred following approximately 60 days of treatment with quinoxaline-2-carboxylic acid. Fifty rats/sex/dose were selected for the concurrent chronic study from the initial F1 litter. Following a 7 day rest period the females were rebred to different males to produce the second litter. The subsequent first litters of each generation were sacrificed at weaning while the second litters were used to constitute the next generation. These first litters were reduced to 10 pups/litter (5/sex if possible) for rearing, then sacrificed, and necropsied at weaning. The 2nd litters were used to produce the next generation. Subsequent generations of rats were mated at 100 days of age. Dose levels were maintained and test animals were treated from weaning throughout the breeding period. Rats were inspected daily. Food consumption and weights were determined weekly except for mating periods. Adults of all generations were sacrificed after weaning of the 2nd litter and received a gross necropsy with liver weights determined. Blood levels were determined in F0 rats (5/sex/level) just prior to sacrifice. Pups were examined daily until weaning and weighed on days 1, 4, and 21 after birth. Gross necropsies were carried out on all pups that died or were stillborn and on 5 weanlings/sex/dose. Reproductive parameters evaluated included: fertility, length of gestation, number of livebirths, stillbirths, number alive at 1, 4, and 21 days, number weaned, sex ratios, mean pup weights, lactation index, and gross abnormalities. No treatment-related alterations were observed in any parameter monitored during the study (King, 1979b). 2.2.5 Special studies on embryotoxicity/teratogenicity 2.2.5.1 Rabbits The potential embryotoxicity or teratogenicity of quinoxaline-2- carboxylic acid was also studied in the New Zealand White Rabbit. Rabbits were randomly assigned to 4 treatment groups of 19 - 20 animals receiving 0, 25, 50, or 100 mg/kg b.w. quinoxaline-2- carboxylic acid via gavage on gestation days 7-18. Animals were observed daily for toxicity and weighed on gestation days 0, 3, 7, 9, 12, 15, 18, 21, 24, and 28. Rabbits were sacrificed on day 28 and the number of corpora lutea and number and position of live and dead implants in the uteri were recorded. All fetuses were examined for external malformations and weighed. Half the fetuses were cleared, stained with alizarin red and examined for skeletal abnormalities. The remaining half were fixed in Bouin Allen fluid and examined by serial sectioning. No treatment related maternal toxicity, embryotoxicity or malformations were observed (Holmes, 1976a). 2.2.6 Special studies on genotoxicity The results of genotoxicity studies with quinoxaline-2-carboxylic acid are given in table 5. Table 5: Quinoxaline-2-carboxylic acid genetic toxicology Test System Test Object Concentration Results Reference Ames test S. typhimurium .008, .032, Negative Pfizer, TA1535 .125, .5, 1975 TA1537 2 mg/plate TA1538 TA 1535+S9 .16, .63, Negative Pfizer, 2.5 mg/plate 1975 Chromosomal Human 50, 100 Negative Pfizer, aberrations lymphocytes µg/ml 1975 in vitro DESOXYCARBADOX 2.2 Toxicological studies 2.2.3 Long-term/carcinogenicity studies 2.2.3.1 Rats A long term study was performed to determine the tumorigenic potential of desoxycarbadox, a carbadox metabolite. Four hundred Charles River C-D rats (5 weeks of age) were divided into groups of 50/sex/dose level. Rats were distributed randomly according to weight levels. Desoxycarbadox was administered at target doses of 0, 5, 10, and 25 mg/kg b.w./day continuously in the diet. Weights were recorded weekly. Food consumption was monitored weekly the first 3 months and monthly thereafter. Treatment levels were adjusted accordingly. Although treatment was planned for 2 years, the test material was withdrawn from all rats in the 25 mg/kg b.w./day group and 50% of the rats in each sex in the other 2 treatment groups on day 350, due to high morbidity and mortality. Administration of desoxycarbadox was stopped completely on day 416. The study was terminated after 447 days. Clinical examinations were performed twice weekly. A number of treatment-related signs were observed. These included: tumours in the mammary region of both sexes, small cutaneous nodules, and enlargement of the liver with nodules preceded by weight loss and polyphagia. There was also a dose-related decrease in weight gain and a dose-related decrease in survival rate. Clinical chemistry parameters were evaluated. Desoxycarbadox administration resulted in dose-related increases in plasma enzyme activities, urea, and bilirubin. The abnormalities were consistent with hepatic disorders. The high degree of variability from animal to animal and lack of reversibility following drug withdrawal indicates a correlation with tumour progression. A dose-related hypoglycaemia was noted in both sexes. Haematologic parameters were evaluated in ten rats/sex/dose at day 413 and the remainder at 447 days. The two primary changes in haematology in the treated rats were a moderate hypochromic microcytic anaemia in the 25 mg/kg b.w./day group, a slight anaemia in the 10 mg/kg b.w./day group, and a neutrophilia in the 25 mg/kg b.w./day group. All rats received gross necropsies. Selected organs were weighed. Histopathologic examination was performed on all grossly abnormal tissue and routinely on a standard array of tissues. There was a dose-related increase in pigmentation of renal tubules and nephrosis. The major pathologic findings related to increased tumour incidences are summarized in Table 6. The authors concluded that desoxycarbadox was a potent hepatocarcinogen in the rat and that there were dose-related increases in other tumours (Reinert, 1976). Table 6: Incidence of tumours after administration of desoxycarbadox to rats Treatment Hepatic Subcutaneous Haemanigiomas Mammary Group tumours fibromas tumours Males and females combined Females only Control 0% 0% 0% 12% 5 mg/kg 75 2 0 24% b.w./day 10 mg/kg 95 10 5 28 b.w./day 25 mg/kg 100 18 47 27 b.w./day 2.2.5.2 Special Studies on Genotoxicity The results of genotoxicity studies with desoxycarbadox are summarized in Table 7. Table 7: Desoxycarbadox genetic toxicology Test System Test Object Concentration Result Reference Ames test TA1535 .0008, .0032, Negative Pfizer, 1975 TA1537 .0125, 0.05, .125, TA1538 .5, 2, 20 mg/plate TA1535+S9 .0125, .05, .2, 1, Negative Pfizer, 1975 5 ml/plate Ames test TA1535 .032, .25, .4, .5 Negative Pfizer, 1975 TA1537 ml/plate plasma; TA1538 .032, .25. .5 ml/plate urine1 Host mediated TA1950 500 mg/kg acute Negative Pfizer, 1975 assay Chromosomal damage Human lymphocytes 200 µg/ml Negative Pfizer, 1975 Rat bone marrow 200 mg/kg acute, Negative Pfizer, 1975 or 100 mg/kg 5 days 10 mg or 25 mg per kg Positive Pfizer, 1975 9 months in feed Host mediated TA1950 400 or 500 mg/kg Negative Pfizer, 1975 assay in mice Host mediated TA1950 300 mg/kg with 100 mg/ Negative Pfizer, 1975 assay in rats kg/day previous 31 days TA1535 200 mg/kg Negative Table 7 (contd) Test System Test Object Concentration Result Reference Yeast assay S. cerevisiae D3 10 mg/ml Negative Holmes, 1976b & D4 Host mediated TA1950 400 mg/kg +DMNA Negative Holmes, 1976b assay (mouse) Ames test TA1537 .01 - 10 mg/plate Negative Holmes, 1976b TA100 TA98 Ames test TA1535+/- TA100 .005 - 2 mg/plate Positive Holmes, 1976b S9(rat) Ames test TA1537+S9 .005 - 2 mg/plate Negative Holmes, 1976b TA98 Ames test TA1535+S9 from .005 - 2 mg/plate Holmes, 1976b rat mouse Positive hamster Positive dog Negative monkey Negative Negative Ames test TA1535 Quantitative plate Negative Holmes, 1976b assay on plasma, liver & urine at selected intervals from chronic study rats fed carbadox or desoxy-carbadox Cell transformation BALB/C Swiss 3T3 .1-10 µg/ml Positive Holmes, 1976b test 1 Urine and plasma collected from mice or rats administered 400 mg/kg desoxycarbadox administered in 3 hourly doses or 100 mg/kg/day for 5 days. METHYL CARBAZATE 2.2 Toxicological studies 2.2.3 Long-term/carcinogenicity studies 2.2.3.1 Rats Methyl carbazate is a metabolite of carbadox. The chronic oral toxicity of methyl carbazate was studied in the rat. Wistar rats were divided into groups of 12/sex/dose and received methyl carbazate at target doses of 1 and 10 mg/kg b.w./day in the feed for 21 months. The colony served as controls. Food consumption was determined every 2 days and weights every 2-3 weeks. Necropsy was performed on all animals. Organ weights were determined for liver, kidney, adrenals, testes, and thyroid. Histology was performed on liver, kidneys, adrenals, thyroid, testes, ovaries, and tissues containing tumours observed grossly. Three males and 3 females in the high dose group died before termination. No low dose rats died prior to termination. No histopathologic evidence of toxicity or evidence of an elevation in tumours was reported. The authors concluded that administration of methyl carbazate to rats at dose levels of 1 and 10 mg/kg b.w./day in the diet produced no evidence of toxicity or carcinogenic potential (Rutty, 1972). In a second study weanling Wistar rats were divided into groups of 24/sex and treated with target doses of 0, 2.5, 5, and 10 mg/kg b.w./day methyl carbazate in the feed for 710 days. Weights were determined weekly and food consumption weekly the first 3 months and monthly thereafter. Drug concentrations were adjusted accordingly. Clinical examinations were conducted weekly. Blood samples for clinical chemistry were obtained at terminal sacrifice and from moribund animals. Haematology was also conducted on these samples and at 12 months on 6 rats. Rats were sacrificed and necropsied at 710 days. An interim sacrifice was also performed at 12 months. All moribund and dead rats also received a gross necropsy. Histopathology was performed on all gross lesions and the following organs: liver, pituitary, thyroid, parathyroid, adrenal, lung, kidney, urinary bladder, ovary, and testis. There were no treatment-related effects noted in any parameters evaluated in the study. There was histologic evidence of widespread chronic respiratory disease in all groups. It was concluded that methyl carbazate had no adverse effect when given to rats in the diet for 2 years (Ferrando, 1980). 2.2.5 Special studies on genotoxicity The results of genotoxicity studies with methyl carbazate are summarized in Table 8. Table 8: Genetic toxicity studies with methyl carbazate Test System Test Object Concentration Results Reference Ames test TA1535 .0005, .002, Negative Pfizer, 1975 TA1537 .008, .032, TA1538 .125, 0.5, 2, 5 mg/plate TA1535+S9 .0125, .05, .2 Negative Pfizer, 1975 1, 5, 10 mg/plate Host- TA1950 50, 100, 300 Equivocal Pfizer, 1975 mediated mg/kg assay Chromosomal Human 50, 100 Negative Pfizer, 1975 damage lymphocytes µg/ml in vitro Ames test TA1535+S9 0.5 - 10 Negative Holmes, 1976b mg/plate HYDRAZINE 2.1 Biochemical aspects 2.1.1 Absorption, distribution, metabolism, and excretion Hydrazine toxicokinetic parameters have been determined in the rabbit. Rabbits received a bolus of 12 mg/kg b.w. Parameters were derived using a one-compartment model, and spectrophotometric serum analysis for hydrazine, with 5 rabbits/group. The serum half-life was 2.3 hours and the volume of distribution was 0.63 liters/kg (Keller et al., 1981). The metabolism, distribution, and elimination of hydrazine in rats was studied using 15N-labelled hydrazine or a spectrophotometric method. About 15% of the 15N label was found as N2. About 20% and 30% of the infused hydrazine was recovered in the urine as acetylated metabolite and hydrazine, respectively. The serum half-lives were determined, using a two-compartment model, to be 0.74 hours for the majority of the hydrazine elimination, and 26.9 hours for a much slower, smaller elimination component (Dost et al., 1981). 2.1.3 Effects on enzymes and other biochemical parameters Hydrazine-induced short-term biochemical parameter changes were investigated following oral administration of hydrazine in Sprague- Dawley rats. Groups of 5 rats were given doses of hydrazine (0-200 mg/kg b.w.). Some rats received pyruvate azine, phenobarbital, piperonyl butoxide, or diethylmaleate to more clearly elucidate the hepatic effects of hydrazine. Hydrazine caused a dose-related increase in liver triglycerides and liver weight and a decrease in hepatic glutathione. The threshold dose for toxicity was about 10 mg/kg b.w. While liver weight and glutathione alterations were evident within 30 minutes, triglyceride alteration was not evident for 4 hours. Pretreatment with phenobarbital decreased toxicity, while pretreatment with piperonyl butoxide increased toxicity suggesting hepatotoxicity is due to the parent - not a metabolite. Depletion of hepatic glutathione was without effect on hepatic toxicity, and studies with pyruvate azine - a possible oxidative metabolite - also supported this conclusion (Timbrell, et al., 1982). Hydrazine-induced short-term histologic and ultrastructural changes were investigated following oral administration of hydrazine to Sprague-Dawley rats. Groups of 5 rats were given doses of hydrazine (0-200 mg/kg b.w.). Some rats received pyruvate azine, phenobarbital, piperonyl butoxide, or diethylmaleate to more clearly elucidate the hepatic effects of hydrazine. A dose of 20 mg/kg caused accumulation of lipid, swelling of mitochondria, and the appearance of microbodies in periportal and midzonal hepatocytes. Accumulation of lipid droplets and mitochondrial swelling were detected by electron microscopy 30 minutes after dosing. Piperonyl butoxide or phenobarbital treatment increased or decreased fatty liver respectively, while pyruvate azine was much less effective than hydrazine in producing this effect (Scales & Timbrell, 1982). 2.2 Toxicological studies 2.2.3 Long-term carcinogenicity studies 2.2.3.1 Mice Oral administration of hydrazine sulfate to Balb/c female mice at 1.13 mg/day for 46 weeks produced a 100% incidence of lung tumours (Biancifiori & Ribacchi, 1962). In another study hydrazine was administrated via gavage to 25 female Swiss mice 5 days/week for 40 weeks at the rate of 0.25 mg/day. Eighty-five mice served as controls. This dose regimen produced a 46% incidence of lung tumours in treated mice vs 10% in controls (Roe et al., 1967). A study with CBA/Cb/Se mice was performed using hydrazine sulfate given for 36 weeks by gavage. Twenty-one 8- week-old rats/sex received 1.13 mg/day. Treated rats had an incidence of 76% and 90% lung tumours in males and females, respectively, while controls had 3%. Hepatomas were found in 62% of the males and 71% of the females treated with hydrazine, while 11% of control males and 4% control females had hepatomas (Severi & Biancifiori 1968). In another study in mice, 8-week-old CBA mice were divided into groups of 40-59 mice/sex. Hydrazine sulfate was given by gavage daily for 150 days at rates of 45, 22, 11, 5.6 and 0 mg/kg b.w./day. Mice were examined at natural death or following sacrifice when moribund. Control male mice had a hepatoma incidence of 10% while females had a 3.4% rate. Treated males had hepatoma rates of 60, 48, 28, and 3.8% at 45, 22, 11, and 5.6 mg/kg b.w./day, respectively. Treated females had 62.5, 66.6, 8 and 0% hepatomas, respectively. The author reported many lung tumours in treated groups but did not provide the incidence rates (Biancifiori, 1970). 2.2.3.2 Hamsters Eight-week old golden hamsters were divided into 3 groups of 23, 35 and 56 animals which were administered 60 doses of 3.0 mg hydrazine sulfate via intubation over a 15 week period, 2.8 mg 100 times during a 20 week period or no hydrazine, respectively. Hepatic lesions reported include: Fibrosis, reticuloendothelial cell proliferation, and bile duct proliferation. Hepatic lesions were found in 60-80% of treated hamsters but in none of the controls. No hepatic tumours were reported (Biancifiori, 1970). Negative tumorigenicity results were also obtained in a follow up chronic study with golden hamsters given 2.3 mg/day over a lifetime. In this study 50 hamsters/sex received hydrazine in drinking water from the age of 9 weeks for a lifetime (Toth, 1972). 2.2.3.3 Rats A chronic oral study was performed in conjunction with the above mouse study using 8 week old Cb/Se rats. Aqueous hydrazine sulfate was administered daily by stomach tube at doses of 18 mg/rat to 14 males and 12 mg/rat to 18 females for 68 weeks. Lung tumours were found in 21 and 27% of dosed male and female rats, respectively. Control groups of 28 males and 22 females had no lung tumours. Hepatic tumours were found in 30% of males, while no hepatic tumours were found in females or controls (Severi & Biancifiori, 1968). 2.2.7.5 Special studies on genotoxicity The results of genotoxicity studies with hydrazine are summarized in Table 9. Table 9: Genetic toxicity studies with hydrazine Test System Test Object Concentration Results Reference Bacterial E. coli WP2 0.5 - 2.0 µmole Positive von Wright uvr A trp & Tikkanen, 1980 Mouse L5178Y 0.1 - 5 mmole Positive Rogers & lymphoma Back, 1981 cells Ames test S. typhimurium Positive Ames, 1971 2.3. Observations in humans No information available 3. COMMENTS The Committee considered data from short-term, long-term and "relay" toxicity studies, and from studies on carcinogenicity, mutagenicity, and reproduction with carbadox, together with data from mutagenicity and long-term toxicity studies of its metabolites. The metabolism of carbadox has been studied in rats, monkeys and pigs using [14C]carbadox, labelled in either the phenyl ring or the carbonyl group of the side-chain. The metabolism of carbadox was characterized by the rapid reduction of the N-oxide groups, the cleavage of the methylcarbazate side-chain, and the liberation of respired CO2. The primary metabolite in the urine was quinoxaline-2- carboxylic acid, which was also excreted in conjugated form. The detectable residues in tissues, up to 24 hours after drug withdrawal, were carbadox, desoxycarbadox, quinoxaline-1,4-di-N-oxide-2- carboxaldehyde and quinoxaline-2-carboxylic acid. Hydrazine was a minor metabolite but would be expected to be present only for a short time before undergoing further metabolism. The concentration of carbadox and desoxycarbadox in the tissues declined rapidly (half-life about 6 hours), and, after 3 days, was less than 5 µg/kg. In a study in pigs, in which carbonyl-labelled [14C]carbadox was administered by gavage, the concentration of total residues in liver at 5 days withdrawal time corresponded to 0.12 mg/kg methyl carbazate equivalent. However, some of this radioactivity was due to the presence of amino acids, which had been labelled by incorporation of 14CO2 when ring-labelled [14C]carbadox was administered to pigs, extremely low levels of unidentified metabolites remained in the liver at withdrawal periods longer than 7 days. These residues were partially released and converted to quinoxaline-2- carboxylic acid by alkaline digestion of the liver. The Committee also reviewed recent reports of the effect of carbadox on adrenal function in pigs, although the information did not effect the outcome of the evaluation. Several long-term feeding studies in rats were evaluated which demonstrated dose-related increases in the incidence of benign and malignant liver tumours at doses of carbadox above 1.0 mg per kg of body weight per day. Doses above 25 mg per kg of body weight per day produced pronounced toxic effects which precluded chronic treatment. Data from a variety of mammalian and non-mammalian genotoxicity studies were also reviewed. Positive findings were reported in 14 of the 15 tests carried out. The Committee concluded that carbadox appears to be both genotoxic and carcinogenic. The Committee evaluated a "relay" toxicity study in which pigs were fed carbadox at up to 200 mg/kg in the diet for 30 days and killed at zero withdrawal time, their livers were fed to rats as a 10% dietary component for 2 years. Although this study was conducted satisfactorily and no treatment-related effects were reported, the Committee did not use it for assessing the carcinogenicity of carbadox residues because of the number of animals used and because only small quantities of residues can be consumed by rats in this type of study. In a long-term study in rats, desoxycarbadox was reported to produce an increase in the incidence of tumours. All rats that received 25 mg per kg of body weight desoxycarbadox daily for 10 months in the diet developed hepatic tumours. The incidence of tumours was increased in all treated groups, at doses of 5-25 mg per kg of body weight per day. While the most pronounced change occurred in the liver, tumour incidence was also elevated at other sites, including the skin and mammary glands. The mutagenic potential of desoxycarbadox was investigated in a range of studies. The Committee noted that, while desoxycarbadox produced negative results in most mutagenicity test systems, positive findings were recorded in the cell transformation test and in the Ames test using liver microsomes from rats treated with polychlorinated biphenyls. The Committee noted that the tumorigenic potential of desoxycarbadox was apparently greater than that of the parent compound, and that desoxycarbadox would therefore probably make a significant contribution to the tumorigenic activity of carbadox in rats. However, the Committee also noted that the metabolism data for carbadox supported the conclusion that desoxycarbadox was a relatively short-lived intermediate between carbadox and quinoxaline-2-carboxylic acid. The Committee also considered information on the carbadox side- chain metabolite, methyl carbazate. This information consisted of results from two long-term studies in rats and a number of mutagenicity assays. In the more informative long-term study (in which more animals per dose and concurrent controls were used), rats received methyl carbazate in the diet for 710 days. There was no increase in the incidence of tumours and no other treatment-related effects were observed at 10 mg per kg of body weight per day, which was the highest dose tested. The lack of effect limited the usefulness of this study. The no-observed-effect level for the study was 10 mg per kg of body weight per day. The useful mutagenicity studies were reported to be negative. The results of studies on the pharmacokinetics, mutagenicity, and carcinogenicity of hydrazine were also considered by the Committee. Pharmacokinetic data from rats and rabbits as well as hydrazine's known chemical reactivity suggest that it should be rapidly eliminated. The structure of carbadox and its known metabolic pathways indicate that hydrazine is probably a further metabolite of methyl carbazate. Hydrazine showed mutagenic and carcinogenic potential. In evaluating the potential toxicity of residues of carbadox, the Committee also considered toxicity data on the carbadox metabolite quinoxaline-2-carboxylic acid to be useful. In a carcinogenicity study in mice in which quinoxaline-2-carboxylic acid was administered in the diet for 19 months, no treatment related increases in tumour incidence or any other effects on the animals were reported. The no- observed-effect level was 100 mg per kg of body weight per day. Several chronic toxicity and carcinogenicity studies in which quinoxaline-2-carboxylic acid was administered for 24 months were conducted in rats. There were no effects on the incidence of tumours, the only findings being a slight reduction in weight gain at 100 mg per kg of body weight per day. The no-observed-effect level was 50 mg per kg of body weight per day. In a three-generation reproduction study in rats, quinoxaline-2- carboxylic acid was given in the diet at levels of up to 100 mg per kg of body weight per day. No treatment-related effects were observed. Developmental studies were conducted in rats and rabbits in which the compound was administered by gavage at levels of up to 100 mg per kg of body weight per day. There was no evidence of maternal toxicity, embryotoxicity, or teratogenicity in either species. The no-observed- effect levels in these studies were 100 mg per kg of body weight per day. 4. EVALUATION With current analytical procedures, quinoxaline-2-carboxylic acid is the only carbadox metabolite that can be identified in liver from pigs treated according to good practice in the use of veterinary drugs. However, uncertainty remains because of the presence of unidentified residues in liver. Because of the genotoxic and carcinogenic nature of carbadox and some of its metabolites, the Committee was not able to establish an ADI. On the basis of data from studies on the toxicity of quinoxaline- 2-carboxylic acid, and on the metabolism and depletion of carbadox and the nature of the compounds released from the bound residues, the Committee concluded that residues resulting from the use of carbadox in pigs were acceptable, provided that the recommended MRLs are not exceeded. 5. REFERENCES AMES, B. (1971). Chemical Mutagens, Vol. 1, Hollaender, A. (ed), Plenum Press, New York. p. 267. BEUTIN, L., PRELLER, E., & KOWALSKI, B. (1981). 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See Also: Toxicological Abbreviations Carbadox (ICSC) Carbadox (JECFA Food Additives Series 51) CARBADOX (JECFA Evaluation)