Ethanone, 2,2-dichloro-1-(2,3-dihydro-3-methyl-4H-1,4-benzoxazin-4-yl)-

Endpoint summary

Description of key information

Toxicokinetics, metabolism and distribution of the substance have been extensively studied in different animal species.  

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - dermal (%):

56

Additional information

The metabolism of CGA 154281, Benoxacor, was investigated in Sprague Dawley rats. The pharmacokinetic behaviour was examined in each of 5 rats per sex in three different experiments after

•      a single oral low dose (0.5 mg/kg);

•      a single oral low dose (0.5 mg/kg) after 14 daily low dose applications of non-labelled CGA 154281, and;

•      a single oral high dose (500 mg/kg).

The latter experiment was performed twice. No radioactive CO2 or other volatiles were detected. CGA 154281 is rapidly absorbed and excreted in urine and faeces. Within 72 hours more than 90% of the radioactivity is excreted mainly in urine. The recovery of radioactivity ranged from 77.7 to 100.6%. At lower dose levels excretion was somewhat faster, whereas at higher dose levels the proportion of radioactivity excreted in faeces was slightly higher.

Tissue residues were generally low. Seven days after single administration of 0.5 mg/kg CGA 154281 quantifiable residues were found in liver, blood (red blood cells (RBC)), kidney and in lung and spleen for single animals. The latter organs are highly perfused organs, therefore the residues are likely to result from residual blood. Tissue residues were generally below 0.034 ppm, representing 0.38% of the administered dose.

The residues at the 1000 fold higher dose (500 mg/kg) were accordingly higher. Just as for the low dose, the highest concentrations were found in liver, red blood cells, and kidney. Small amounts were detected in lung and spleen. Tissue residues were generally below 14.5 ppm, representing 0.15% of the administered dose.

Extraction of livers from high and low dose rats revealed that most of the radioactivity is nonextractable, i.e. 78% at the low dose and 75% at the high dose. In the light of the known metabolism this is not surprising, since the reductive dehalogenation of CGA 154281 is likely to result in binding of the molecule to microsomal protein.

Rats metabolise CGA 154281 to a great number of polar metabolites. When a 24-48 hour male urine (500 mg/kg) was partitioned with Chloroform, 35% of the radioactivity was detected in the organic phase. When the same was done with the respective urine from females only 3% of the radioactivity partitioned into the organic phase. However, the metabolite pattern was essentially comparable.

The nature of the metabolites was determined by gas chromatography/mass spectroscopy of the organic phase from the pooled male urine. The metabolism of CGA 154281 proceeds by hydroxylation of the aromatic ring, deacetylation at the nitrogen, and glutathione dependent reductive dehalogenation.

The organosoluble metabolites of CGA 154281 are rapidly conjugated to form the aqueous soluble 14C metabolites found in the urine.

The relative distribution of the different metabolites in the organosoluble phase of the urine of male and female rats dosed with 500 mg/kg CGA 154281 was as follows: A1: 6.01 to 12.13%; A2: 49.42 to 27.19%; B1: 7.69 to 11.86%; B2: 16.54 to 10.58%; C: 3.29 to 3.16%; D: 5.31 to 3.56%. The first value always refers to the males, the second to the females. There seem to be differences in the proportion of the different metabolites between males and females.

No validated experimental data are available to determine dermal absorption for formulated benoxacor in human skin.

Discussion on bioaccumulation potential result:

Rat

In a preliminary metabolism study (McFarland et al., 1986), male and female rats were given single oral doses of 500 mg/kg [14C]-benoxacor. Urine, faeces and CO2/volatiles were collected for 7 days; tissues were sampled at necropsy. Greater than 99% of the radioactivity was excreted in urine and faeces with >97% in the initial 72 hours after dosing. The majority of the radioactivity was present in urine (75.7% in males; 82.3% in females). No radioactivity was quantified in CO2/volatiles. There was no sex difference in either the rate or route of excretion. Total tissue residues were <5% of dose; the greatest amounts were present in liver (~0.15% in males; 0.18% in females) and red blood cells (0.07% in both sexes).

Male and female rats were given oral doses of [14C]-benoxacor; separate groups received either a single low dose (0.5 mg/kg), a single high dose (500 mg/kg) or 14 daily doses of unlabelled benoxacor (0.5 mg/kg) followed by a single dose of [14C]-benoxacor (0.5 mg/kg). Urine and faeces were collected up to 7 days after dosing; tissue and blood samples were retained at necropsy on day 7. Greater than 90% of the radioactivity was excreted within 72 hours; the majority in urine. Tissue residues were <0.034 ppm (0.38% dose) and were predominantly in blood and highly perfused tissues. Metabolite profiles determined using 24-48 hour urine from rats given 500 mg/kg showed the presence of metabolites A1, A2, B1, B2, C and D (see attached metabolic pathway); there were sex differences in the proportions of individual metabolites (Frantz & Tallant, 1986; Capps et al., 1986).

Goats

Lactating goats were given daily oral doses of [14C]-benoxacor (0.09 mg; 2.33 µg/kg bodyweight; equivalent to 0.045 ppm in feed) for up to 28 days. Milk samples were collected twice daily and tissues retained at necropsy; samples were analysed for radioactivity. Radioactive residues in tissues were all <0.002 ppm; with the exception of one goat (0.0002 ppm on days 5 and 7), all milk samples contained <0.0001 ppm (Merricks, 1988b).

In a separate high dose study (Wu, 1988), lactating goats were administered daily oral doses of [14C]-benoxacor (76.7 mg; 1.92 mg/kg bodyweight; 51.13 ppm equivalents in feed) for 3 days. Milk, urine and faeces were collected daily and tissues were retained at necropsy 4 hours after the final dose. The pattern of metabolites identified was similar to that found previously in rat urine; metabolites A1, A2, B1, C and D (see attached metabolite profile) were identified by co-chromatography. Aromatic hydroxylation may occur at different sites from those seen in rat samples.

HensEgg and tissue samples from laying hens administered daily oral doses of [14C]-benoxacor (0.007 mg; equivalent to 0.05 ppm in diet) for up to 28 days were analysed for radioactivity. Residues in eggs, muscle, skin and fat were <0.002 ppm. Low residue levels were detected in some samples of liver (0.003-0.004 ppm on days 7, 14, 21 and 28) and kidney (0.002 ppm on day 28) but these had all declined to <0.002 ppm by 3 days after the final dose (Tweedy, 1988; Merricks, 1988c).

In a high dose study, laying hens were given daily oral doses of [14C]-benoxacor (equivalent to 50 ppm in diet) for 8 days. Daily egg and excreta samples were collected and tissues taken at necropsy 18 hours after the final dose; samples were analysed for radioactivity (Ballantine, 1988). Between 81 and 87% of the administered radioactivity was recovered in the excreta with only 0-0.13% remaining in tissue and blood; small amounts were found in the yolk (0.05%) and whites (<0.01%) of eggs. With the exception of liver (1.6-1.76 ppm) and kidney (0.81-1.34 ppm), tissue concentrations of radioactivity were between 0.021 and 0.078 ppm. Concentrations in whole eggs collected on day 8 were 0.131 ppm.

Radioactivity in excreta was present as [14C]-benoxacor (4.4% of dose) and at least 12 metabolites. Metabolite profiles in liver, kidney and egg yolk were similar to that in excreta and closely matched that found in rat urine. Three metabolites were identified as B1, C and E (Capps, 1988).

Discussion on absorption rate:

Two groups of male rats were exposed dermally for up to 24 hours to either 1 or 10 mg [14C]-benoxacor (Craine, 1987). The dose suspensions were prepared to simulate metalochlor 7.8 EC formulations. The high dose suspension contained [14C]-benoxacor (31.5 mg), metalochlor (1 g), cyclohexanone (92.4 mg) and 10 mL distilled water; the low dose suspension contained [14C]-benoxacor (15.7 mg), metalochlor (0.5 g), cyclohexanone (46.4 mg) and 25 mL distilled water. Doses were applied to an area of 10 cm2 on the upper back and were occluded. Absorption was measured by analysis of radioactivity in excreta, blood, carcass and washed skin. After 10 hours exposure, 49.4 and 25.4% of the low and high doses, respectively had been absorbed; 5-17% of the dosed radioactivity was recovered in excreta, a further 47-73% was removed from the application site by washing. At the end of the 24 hour exposure period absorption was 55.7 and 27.5% at the low and high dose levels, respectively. Excreta contained 11-33% of the dose; 35-66% was removed from the application site by washing. Absorption is inversely proportional to the applied dose (Murphy & Simoneaux, 1987).