sodium 2-{N-[(3,5-difluorophenyl)carbamoyl]ethanehydrazonoyl}nicotinate

Endpoint summary

Description of key information

The test item was partially absorbed, partially metabolized and rapidly eliminated from the body of rats, hens and goats after oral administration. Hydrolytic and hydroxylation products were identified in a low percentage of the dose. The elimination half-life was found to be about 6 hours in rats. No significant differences between male and female rats were observed. Metabolites identified are comparable among the species.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

50

Additional information

ADME studies

 

Metabolism in Rats

Read-across to free acid of the registered substance

The metabolism of 14C-labeled test item at the phenyl and pyridinyl rings was investigated separately using male and female Wistar rats. In part 1 study [phenyl-UL-14C]-test item was used while in part 2 study [pyridinyl-4,6-14C]-test item was used. In each part of the study, there were 4 dosing groups: intravenous at 1 mg/kg bw, oral at 10 mg/kg bw and 1000 mg/kg bw, and repeated unlabeled test item oral at 10 mg/kg bw/day for 14 days followed by a 14C dose at 10 mg/kg bw. With the exception of repeated unlabeled group, each dosing group consisted of 3 subgroups: rats killed 72 hr after 14C dose, rats for periodic blood sampling and killed 24 hr after dosing, and bile-duct cannulated rats killed 48 hr after dosing. Each subgroup consisted of 5 male and 5 female rats. Urine and feces samples were collected from each subgroup at 7 hr, 24 hr and then each 24 hr until sacrifice. Bile samples from bile-duct cannulated rats were collected periodically for 48 hr. For the repeated unlabeled group, only 2 subgroups were used without the bile-duct cannulated subgroup and rats were killed at either 24 or 72 hr after 14C dose.

The test item was partially absorbed, partially metabolized and rapidly eliminated from the body. By intravenous administration, 73-89% of the dose was eliminated in urine with 7-20% in feces. Biliary elimination accounted for 3-19% of the dose by intravenous or oral administration. By oral administration 20-44% of the dose was eliminated in urine and 57-79% in feces. It was estimated that 30-50% of the oral dose was absorbed by rats. The half-life of radiocarbone elimination in urine and feces was about 6 hr. Radiocarbon retained in tissues and organs were low. Residue in selected tissues and blood decreased rapidly from about 0.2% at 24 hr to about 0.02% at 72 hr. Generally, kidney, liver, spleen and lung had higher residue level than heart, fat, muscle, gonad and bone. Brain had the lowest residue level in all groups administered either the phenyl or the pyridinyl label. Total material balance in excreta and selected tissues for all groups sacrificed at 72 hr ranged between 84-102% of administered dose with an average of 96%.

The test item was eliminated in urine, feces and bile primarily as unchanged parent compound. Hydrolytic products M1, M5 and M6 were also found in excreta but in low percentage of the dose. A trace amount of M2 was detected in urine. Hydroxylation products (M9, M10 and M19) of pyridinyl ring and/or methyl group were also identified. These hydroxylation products existed in low percentage of the dose. For animals sacrificed at 72 hr, total identified radiocarbon in excreta for phenyl label ranged between 69-81% of the dose with an average of 76%. For pyridinyl label, the identified radiocarbon ranged between 75-93% of the dose with an average of 85%. The remaining radiocarbon in excreta was characterized.

It appeared that there was no significant difference in absorption, distribution, metabolism and elimination for the test item in rats between male and female. There was a slight increase in elimination rate for rats orally dosed at 1000 mg/kg bw. The elimination half-life was also slightly increased for multiple doses.

M9, M10 and M19 isolated from rat urine matched the respective metabolite isolated from corn silage by TLC and HPLC cochromatography. Thus these metabolites isolated from rat and corn were identical. M9, M10 and M19 were also found in goat and hen excreta. Thus, parts of the test items metabolic pathways in rat are common in other animal species and in corn.

 

Metabolism in hens

Read-across to free acid of the registered substance

The tissue retention, distribution and metabolism of the test item was investigated in laying hens. 14C-labeled test item at two different rings and with radiochemical purity of >98% were used in two separate studies. In Part 1 study [phenyl-UL-14C]-test item was used, and in Part 2 study [pyridine-4,6-14C]-test item was used. For each label, 3 hens per group were orally dosed with a low dose level of 0.6 mg/kg/day and a high dose level of 60 mg/kg/day for four consecutive days. The low dose rate (0.6 mg/kg/day) is equivalent to approximately 10 ppm in the diet while the high dose rate (60 mg/kg/day) is equivalent to 1,000 ppm when feed was consumed at 60 g/kg/day. Each daily dose contained about 0.05 mCi of 14C-test item. Hens were killed at 24 hr after the last dose for the low dose groups and 7 hr after the last dose for the high dose groups. Hens dosed at 0.6 mg/kg/day were used for detailed metabolite characterization while all groups were analyzed for material balance calculations. The use of the high dose groups was to generate larger amounts of metabolites for instrumental analysis if needed. A third group was included in each study that served as a control in which a single hen was dosed with the dosing vehicle only otherwise treated the same as the other groups. Eggs were collected daily and edible tissues obtained upon necropsy.

The test item was minimally retained in tissues, partially metabolized and rapidly excreted by the laying hens. The material balance was over 90% for each label 24 hr after the last dose. At the dose rate of 0.6 mg/kg/day, the average total radiocarbon residue (TRR) levels in liver was 0.005 to 0.022 ppm which was the highest among the edible tissues. Egg white TRR increased over time to a maximum level in the pyridine label of only 0.015 ppm. Egg yolk TRR remained low ranging between 0.001 to 0.008 ppm. Residue levels in muscle ranged from 0.000 ppm to 0.005 ppm, and in fat from 0.001 ppm to 0.006 ppm. Total radiocarbon in edible tissues accounted for between 0.012% (pyridine label) and 0.067% (phenyl label) of the dose. During the study a total of 0.025% (phenyl label) to 0.051% (pyridine label) of the dose was excreted in the eggs.

The test item was partially metabolized by hens as 31 to 48% of unchanged test item were recovered from the excreta. A major metabolite identified in the excreta was M5 (carbamoyl phthalazinone) at 20 to 37% TRR in excreta. M5 was further metabolized to Ml (phthalazinone, 7% TRR). The test item was also metabolized to M6 (2-acetyl nicotinic acid, 10% TRR). Trace amounts of corn metabolites M9 (2-keto-M1, 0.003% TRR), M10 (8-hydroxymethyl-Ml, 0.01% TRR) and M19 (2-keto-8-hydroxymethyl-M1, 0.69% TRR) were also detected in excreta.

Liver radiocarbon from phenyl label (0.022 ppm) was characterized. About 0.013 ppm was freely organoextractable and 0.007 ppm was acid-released. Egg white radiocarbon from pyridine label (0.016 ppm) was also characterized. Organoextractables accounted for 0.014 ppm of TRR. M1 was identified in egg white which represented 58% (0.010 ppm) of TRR.

All of these metabolites have been identified in the rat metabolism study. No single metabolites found in any hen matrix were greater than 0.01 ppm level. Therefore, no cold residue analytical method was conducted on samples as all threshold levels are at or below the 0.01 ppm detection limit.

 

Metabolism in lacting goats

Read-across to free acid of the registered substance

Metabolism of the test item labeled at [phenyl] and [pyridine] rings was conducted in Alpine lactating goats. In part I study, [phenyl-UL-14C]-test item was used; while in part II, [pyridine-4,6-14C]-test item was used. Three lactating goats were used in each part of the study. One goat from each part of the study was orally dosed with the test item once a day for four consecutive days at the rate of 0.4 mg/kg bw/day (equivalent to 10 ppm in the diet) and the goats were killed 24 hr after the last dosing. Goats A and D (0.4 mg/kg/day) were used for material balance and detailed metabolite characterization and identification. Another 1 goat from each part of the study was orally dosed once daily for four consecutive days at 40 mg/kg/day (equivalent to 1,000 ppm in diet) and the goats were killed 7 hr after the last dosing. The purpose of goats dosed at 40 mg/kg/day was to generate metabolites if needed for instrumental analysis. The third goat from each part (Goats C and F) was dosed with carrier only without the test item to serve as a control.

The test item was partially absorbed, partially metabolized, and rapidly excreted by the lactating goats. At sacrifice, 90% to 94% of the total dose had been eliminated in excreta (50% to 63% in urine and 40% to 31% in feces). Milk accounted for 0.1% to 0.7% of the dose; while edible tissues accounted for 0.3% to 0.5% of the dose. A material balance of 90% to 95% was achieved for the study.

Residue levels in edible tissues 24 hr after the last dose were low. In part I study (Goat A, phenyl label), residue levels in descending order were: liver, 0.068 ppm equivalent; kidney, 0.054 ppm; fat, 0.014 ppm; muscle, 0.004 ppm. In part II study (Goat D, pyridine label), residue levels in descending order were: kidney, 0.113 ppm equivalent; liver, 0.042 ppm; fat, 0.024 ppm; muscle, 0.010 ppm. The unchanged test item (0.01 to 0.05 ppm) and carbamoyl phthalazinone (M5, 0.01 ppm) were detected in both liver and kidney for phenyl and pyridine labels. In addition, phthalazinone (Ml, 0.004 ppm) and 2-acetyl nicotininc acid (M6, 0.005 ppm) were identified in both liver and kidney for pyridine label. Characterization of radiocarbon in fat samples was conducted. Due to the low residue concentration in muscle (0.004-0.01 ppm), radiocarbon characterization in muscle was not performed.

Total residue levels in milk were low, with an average concentration of 0.035 to 0.09 ppm equivalent. The unchanged test item (0.01 ppm) and the carbamoyl phthalazinone (M5, 0.01 ppm) were detected in both labels. Phthalazinone (M1, 0.03 ppm) was the major metabolite in milk for pyridine label. 2-Acetyl nicotinic acid (M6, 0.005 ppm) was also detected.

Radiocarbon in urine was primarily the unchanged test item (68%) for goat administered Pyridine label, followed by carbamoyl phthalazinone (M5, 16%). 2-Acetyl nicotinic acid (M6, 10%), and phthalazinone (M1, 4%), were also detected in urine. A trace amount (0.59%) of 2-keto M1 (M9, a corn metabolite) was detected in urine; small amount of M10 (1.4%, a corn metabolite) and 13.3% of 2-keto-8 hydroxymethyl M1 (M19, a corn metabolite) were also detected in urine. For phenyl label, carbamoyl phthalazinone (M5) was the major metabolite (68%) followed by the unchanged test item (16%). Identification was based on TLC and HPLC cochromatography with authentic reference standards.

It appears that the test item was initially cyclized to form the intermediate carbamoyl phthalazinone (M5), then the intermediate degraded to form phthalazinone (M1) which was oxidized to 2-keto M1 (M9) and 8-hydroxymethyl M1 (M10). M9 and/or M10 was further metabolized to M19 (2-keto-8-hydroxymethyl M1). The test item also metabolized to 2-acetyl nicotinic acid (M6).