pymetrozine (ISO); (E)-4,5-dihydro-6-methyl-4-(3-pyridylmethyleneamino)-1,2,4-triazin-3(2H)-one

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Study period:

16 Oct 1991 to 15 Mar 1993

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Data source

Reference

Reference Type:

study report

Title:

Unnamed

Year:

1993

Report date:

1993

Materials and methods

Objective of study:

absorption

distribution

excretion

metabolism

toxicokinetics

Test guideline

Test guideline 1

Qualifier:

according to guideline

Guideline:

EPA OPP 85-1 (Metabolism and Pharmacokinetics)

Version / remarks:

Adopted 1984

Deviations:

no

Test guideline 2

Qualifier:

according to guideline

Guideline:

other: Agricultural Chemicals Laws and Regulations, Japan (II) Testing Guidelines of Toxicology Studies Society of Agricultural Chemical Industry

Version / remarks:

Adopted 1985

Deviations:

no

Test guideline 3

Qualifier:

according to guideline

Guideline:

OECD Guideline 417 (Toxicokinetics)

Version / remarks:

Adopted April 1984

Deviations:

no

GLP compliance:

yes

Test material

Test material information

Constituent 1

Reference substance name:

6-methyl-4-[(E)-[(pyridin-3-yl)methylidene]amino]-2,3,4,5-tetrahydro-1,2,4-triazin-3-one

EC Number:

602-927-1

Cas Number:

123312-89-0

Molecular formula:

C10H11N5O

IUPAC Name:

6-methyl-4-[(E)-[(pyridin-3-yl)methylidene]amino]-2,3,4,5-tetrahydro-1,2,4-triazin-3-one

Radiolabelling:

yes

Test animals

Species:

rat

Strain:

other: Tif: RAI f (SPF)

Details on species / strain selection:

Laboratory rats were selected as standard for rodent species to comply with the corresponding toxicological studies.

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Age at study initiation: 7 to 9 weeks
- Weight at study initiation: 175 to 214 g (for males at dosing), 189 to 204 g (for females at dosing).
- Housing: Different housing per group:
A1, B1, D1, D2, E1-E4, F2 & F4: Closed all-glass metabolism cages, suitable for the collection of expired air (cage system 1)
C1: Wire floor polycarbonate cages during 13 days of pre-treatment and in cage system 1 immediately after receiving the last dose of non-radiolabelled test substance.
F1 & F3: Open plexiglass metabolism cages (cage-system 2)
Control: Cage-System 2, except one animal each of Group K1 (control animals for Group A1), K2 (control animals for Group B1, C1, D1), K3 (control animals for Group D2), and all animals of Group K4 (control animals for Group E1 through E4), which were housed in Cage-System 1.
- Diet: libitum, certified standard diet (except the night before administration of the radiolabelled test substance).
- Water: ad libitum, throughout the study.
- Acclimation period: for all groups at least two days to the laboratory environment including one day to the metabolism cages under test conditions.
- Identification: The animals were identified by individual numbers marked on ear tags and on the cage. The animal numbers were allocated randomly.

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 19 to 23
- Humidity (%): 40 to 80
- Photoperiod (hrs dark / hrs light): 12/12

IN-LIFE DATES:
16 Oct 1991 to 15 Mar 1993

Administration / exposure

Route of administration:

other: Oral gavage or intravenous administration (only for group A1)

Vehicle:

other: Oral gavage: 0.5% sodium carboxymethyl cellulose containing 0.4% Tween 80. Intravenous administration: physiological saline (0.9% NaCl in water)

Duration and frequency of treatment / exposure:

See for details 'any other information incl tables'.

Doses / concentrations

Doses / concentrations 1

Dose / conc.:

0.5 mg/kg bw/day

Remarks:

low dose, actual dose received: 0.49 to 0.61 mg/kg

Doses / concentrations 2

Dose / conc.:

100 mg/kg bw/day

Remarks:

high dose, actual dose received: 91.8 to 109.6 mg/kg

No. of animals per sex per dose / concentration:

Groups A1, B1, C1, D1 & D2: 5 (both sexes)
Groups E1 - E4: 4 (only males)
Groups F1 - F4: 4 (only males)

Control animals:

yes, concurrent vehicle

Details on dosing and sampling:

TOXICOKINETIC STUDY
- Tissues and body fluids sampled: urine, faeces, expired air, cage washes, blood, plasma, tissues (bone, brain, fat (abdominal, heart, kidneys, liver , lungs, muscle (skeletal), spleen, gonads, uterus, carcass and total residues as % of total dose.
- Time and frequency of sampling:
[Groups A1, B1, C1, D1 & D2]
Urine and faeces: 0-24, 24-48 and 48-168 hours
Expired air 0-24, 24-48 and 48-72 hours
[Groups E1-E4]
Blood: 0.25, 0.5, 1, 2 and 4 hours (first two animals), 4, 8, 12, 24 and 48 hours (second two animals.
[Groups F1-F4]
Tissues:
F1: 15 min, 1.75, 3.25, 5.25 hours
F2: 4, 11, 17 and 21 hours
F3: 15 min, 50 min, 1.75 and 3.25 hours
F4: 4, 11, 17 and 21 hours

METABOLITE CHARACTERISATION STUDIES
- Tissues and body fluids sampled: urine, faeces
- Time and frequency of sampling: urine 0-24 hours, faeces 0-48 hours
- From how many animals: urine samples and faeces samples were pooled according to animal group and sex
- Method type for identification: HPLC-UV

DOSE ADMINISTRATION
For the oral administrations the test substance was dissolved (low dose level) or suspended (high dose level) in 0.5% sodium carboxymethyl cellulose containing 0.4% Tween 80. Two dose levels were used. Each animal received about 0.8 mL of the administration solution by stomach tube, except the animals of Group E3 (0.9 mL).
For the pre-treatment period the animals of Group C1 received the respective amount of non-radiolabelled test substance dissolved in 0.6 - 0.8 mL of the administration solution.
For the intravenous administration (Group A1) the test substance was dissolved in physiological saline (0.9% NaCl in water). Each animal received about 0.5 mL of the administration solution by syringe into the tail vein.

Results and discussion

Main ADME results

Main ADME results 1

Type:

absorption

Results:

Radioactivity was rapidly and almost completely absorbed from the GI tract into the general circulation (84.7% of dose). Maximum concentrations in the blood were reached 15 min and 4 h after administration at the low and high dose level, respectively.

Main ADME results 2

Type:

distribution

Results:

Seven days after administration, mean residue levels were below 0.038 ppm in all tissues. At high dose levels, residue concentrations in fat were 200 times higher than other tissues.

Main ADME results 3

Type:

metabolism

Results:

HPLC analysis revealed complex urinary and faecal metabolite pattern, independent of the sex, pre-treatment, dose level. The cleavage of the bridge between the triazine and pyridine ring is of minor importance in the metabolism in the rat.

Main ADME results 4

Type:

excretion

Results:

Within 24 hours totally 80 to 90% of the administered dose were excreted, mainly with urine.

Toxicokinetic / pharmacokinetic studies

Details on absorption:

An overview of absorption data of group A1, B1, C1, D1 & D2 can be found in Table 1 in ‘any other information on results incl. tables’.
Following oral administration in the first study, radioactivity was rapidly and almost completely absorbed from the gastrointestinal tract into the general circulation (range 57.4 to 84.7% of dose). Independent of the label, maximum concentrations in the blood were reached 15 minutes and 4 hours after administration at the low and high dose level, respectively. These maximum values accounted for about 0.3 ppm and 60 ppm, for the 0.5 and 100 mg/kg dose levels.
The renal elimination was used for the calculation of the bioavailability of the oral dose. This factor F can approximately be calculated by the ratio of the total urinary excretion after p.o. and i.v. administration. The bioavailability was determined to be 0.9 for both sexes, demonstrating also a high extent of absorption.

Details on distribution in tissues:

An overview of the tissues residues for groups B1, C1, D1 & D2 can be found in Table 2 in ‘any other information on results incl. tables’.
An overview of depletion kinetics in several tissues for groups F1, F2, F3 & F4 can be found in Table 3 in 'any other information on results incl. tables’.
Seven days after a single oral administration of [14C-triazine] labelled test substance at a low dose level, tissue residues were low. However, detectable residues were measured in all tissues. The mean values were below 0.025 ppm for all tissues except for heart with 0.038 ppm. At the 100 mg/kg dose level the tissue residues were only in fat accordingly higher, i.e. about 200 times. In brain, heart and muscle the residues were about 5 times, and in all other tissues and organs they were 20 to 50 times higher than at the low dose. This indicates a saturation of tissue binding sites and an unfettered distribution of radioactivity (most probably unchanged test substance) in the fat. This assumption is supported by the distribution of residual radioactivity in the pre-treated animals.
The experiments with [5-14C] pyridine labelled test substance revealed only slight differences compared to the triazine label. While the excretion pattern was independent of the label, and the urinary and faecal metabolite patterns were qualitatively similar, tissue residues were higher with the [5-14C] pyridine labelled compound. The total residual radioactivity accounted for about 3 - 4% of the dose compared to about 1% of the dose after administration of [6- 14C] triazine labelled test substance.
Seven days after administration only about 10 to 15% and 5% of the radioactivity covered in the whole blood were associated with the plasma in the animals of Group D1 and D2, respectively. This ratio was higher at earlier time points after dosage, i.e. about 60% at 4 hours after administration, decreasing to values of about 50% and 15% at 21 hours after dosing for Group D1 and D2, respectively.
The calculated half-life times (t½) for the depuration of the residual radioactivity from the tissues, assuming to follow (monophasic) first order kinetics, were in the range of 1 to 2 hours at the 0.5 mg/kg dose level (both labels). At the high dose level the half-life times were between 3 and 6 hours (except fat: 11 hours) and 2 to 11 hours for the triazine and pyridine label, respectively.

Details on excretion:

An overview of the excretion data for groups A1, B1, C1, D1 & D2 can be found in Table 4 in ‘any other information on results incl. tables’.
The principal route of excretion was via urine with a total of 56 to 80% of the dose. Excretion was rapid. Within 24 hours 52 - 74%, 10 - 37%, and 0.2 - 1.2% of the administered dose were detected in urine, faeces, and expired air, respectively. The excretion pattern was essentially independent of sex, route of administration and pre-treatment with non-radiolabelled test substance. At the high dose level, both sexes eliminated significantly more via kidneys than at the low dose level. The excretion data after intravenous and oral administration (Group A and B1) were on the other side only slightly different, confirming the complete absorption from the gastrointestinal tract.

Metabolite characterisation studies

Metabolites identified:

no

Remarks:

A quantitative distribution of the metabolite fractions is provided

Details on metabolites:

An overview of the quantitative distribution of the metabolite fraction in urine and faeces given as percent of dose for groups A1, B1, C1, D1 & D2 can be found in tables 5 and 6 in 'any other information on results incl, tables'.
URINE: The chromatography revealed a complex metabolite pattern, consisting of up to 14 metabolite fractions. The pattern were qualitatively similar for both sexes, both dose levels, and both labels. The most unpolar metabolite fraction, i.e. U14, accounted for 15 - 22% of the dose (high dose, both labels), 2% (low dose, i.v.), and less than 1 % (low dose, p.o.), cochromatographed with unchanged test substance.
FAECES: HPLC analysis of the extracts revealed up to 13 metabolite fractions. The pattern were qualitatively similar for all groups. Some fractions showed quantitative differences especially between both labels. One of the major metabolite fractions, i.e. F5, showed the same retention time as the urinary fraction U8. The metabolite fraction F11 cochromatographed with unchanged test substance and corresponded thus to the urinary fraction U14. The metabolite pattern revealed only slight differences between the labels indicating that the cleavage between the triazine and pyridine ring represents a minor metabolic pathway. But the resulting pyridine related metabolite(s) is (are) responsible for the observed significant higher tissue residues after administration the [14C-pyridine] labelled test substance.

Bioaccessibility (or Bioavailability)

Bioaccessibility (or Bioavailability) testing results:

not measured

Any other information on results incl. tables

Clinical signs: the appearance and the behaviour of the animals were observed at each sampling time. No test substance related or otherwise unusual behaviour of the animals were observed.

Dosing formulation stability: The stability of the test substance in the administration vehicle, at the time of dosing, was found to be stable. The test substance represented more than 95% of the radioactivity.

For the repeated dosing experiment (Group C1, pre-treatment with non-radiolabelled test substance) the stability of the test substance in the administration vehicle was investigated over a period of 19 days prior to the first dosing using radiolabelled material. After 19 days in the refrigerator the radiopurity determined by TLC was still above 96%.

Table 1. Absorption of radiolabelled test substance after intravenous and oral administration

Route of Adm.	intravenous		per oral
Label	[6-14C] triazine								[5-14C] pyridine
Group	A1		B1		C1		D1		D2
Sex	male	female	male	female	male	female	male	female	male	female
Dose[mg/kg]	0.54	0.52	0.52	0.53	0.51	0.55	97.8	93.8	99.4	99.6
Urine	65.6	69.9	56.3	62.1	56.8	64.2	72.5	78.3	76.9	80.3
Expired Air	0.3	0.6	0.2	0.4	0.2	0.5	1.2	1.4	0.4	0.6
Tissues	0.4	0.3	3.7	2.4	0.4	0.4	0.6	1.0	3.7	3.8
Sum	66.3	70.8	60.2	64.9	57.4	65.1	74.3	80.7	81.0	84.7

 

Table 2. Tissue residues in ppm test substance equivalents, 7 days after administration

Label	[6-14C]triazine						[5-14C]pyridine
Group	B1		C1		D1		D2
Sex	male	female	male	female	male	female	male	female
Dose (mg/kg) [mg/kg]	0.52	0.53	0.51	0.55	97.8	93.8	99.4	99.6
Blood	0.0043	0.0035	= LD	< LQ	0.153	0.183	0.962	1.125
Bone	0.0034	0.0026	0.0010	= LQ	0.131	0.127	0.679	0.656
Brain	0.0155	0.0110	< LQ	< LQ	0.078	0.096	3.190	3.600
Fat (abdominal)	0.0041	0.0039	0.0036	0.0035	1.088	0.833	0.813	0.671
Heart	0.0377	0.0275	< LQ	= LQ	0.112	0.141	7.461	8.648
Kidneys	0.0218	0.0171	0.0046	0.0041	0.912	0.864	4.094	4.666
Liver	0.0249	0.0188	0.0080	0.0073	0.985	0.969	3.750	3.940
Lungs	0.0077	0.0063	0.0011	0.0014	0.166	0.236	1.590	1.753
Muscle (skeletal)	0.0244	0.0171	0.0008	= LQ	0.151	0.119	5.057	5.240
Plasma	< LQ	< LQ	< LQ	= LQ	0.034	0.054	0.072	0.097
Spleen	0.0110	0.0087	< LQ	< LQ	0.138	0.183	1.692	1.970
Gonads	0.0069	0.0105	< LQ	< LQ	0.094	0.265	1.248	2.837
Uterus	---	0.0047	---	< LQ	---	0.179	---	1.551
Carcass	0.0141	0.0110	0.0012	0.0016	0.472	0.900	2.780	3.340
Total residues [% of dose]	3.7	2.4	0.4	0.4	0.6	1.0	3.7	3.8

Table 3. Depletion kinetics in half-life time (h)

Label	[6-14C]triazine		[5-14C]pyridine
Group	F1	F2	F3	F4
Dose (mg/kg)	0.53	99.7	0.57	105.3
Time interval (h)	¼-5¼	4- 21	¼-3¼	11- 21
Blood	1.2	3.1	0.9	3.4
Bone	1.9	3.6	1.2	6.0
Brain	1.4	3.3	1.2	8.8
Fat (abdominal)	1.5	10.9	1.7	10.6
Heart	1.1	2.9	1.0	6.6
Kidneys	1.4	4.4	1.4	7.8
Liver	1.5	5.5	2.3	10.8
Lungs	1.2	3.2	1.4	6.9
Muscle (skeletal)	1.5	2.9	1.1	4.8
Plasma	1.2	3.0	0.8	2.2
Spleen	1.3	3.3	1.4	7.7
Testes	1.8	2.8	1.9	4.3
Carcass	1.7	3.8	1.8	5.0

Table 4. Excretion after intravenous and oral administration [% of dose]

Label	[6-14C] triazine								[5-14C] pyridine
Administration	intravenously		orally						orally
Group	A1		B1		C1		D1		D2
Sex	male	female	male	female	male	female	male	female	male	female
Dose[mg/kg]	0.54	0.52	0.52	0.53	0.51	0.55	97.8	93.8	99.4	99.6
Urine 0 - 24h 24 -48h 48 -168h Subtotal	63.6 0.9 1.1 65.6	68.3 0.9 0.7 69.9	52.0 1.8 2.5 56.3	58.4 1.8 1.9 62.1	55.4 0.9 0.5 56.8	63.0 0.8 0.4 64.2	69.6 2.0 0.9 72.5	73.5 3.9 1.0 78.3	70.2 3.0 3.7 76.9	73.5 3.1 3.7 80.3
Faeces 0 - 24h 24 - 48h 48 -168h Subtotal	24.9 1.1 0.5 26.5	20.3 1.2 1.1 22.6	26.6 2.7 1.1 30.3	23.0 3.7 1.1 27.8	36.6 1.8 0.5 38.9	27.1 2.7 0.6 30.3	20.8 3.9 0.7 25.4	11.6 5.0 1.4 18.0	15.6 2.8 1.0 19.5	10.2 4.3 1.0 15.4
Expired Air	0.3	0.6	0.2	0.4	0.2	0.5	1.2	1.4	0.4	0.6
Cage Wash	0.2	0.7	0.5	0.4	0.3	0.4	0.3	0.4	0.3	0.3
Total Excretion	92.7	93.6	87.3	90.7	96.1	95.4	99.4	98.1	97.0	96.6

Table 5. Quantitative distribution of the metabolite fraction in urine given as percent of dose

Label	[6-14C]triazine								[5-14C]pyridine
Group	A1		B1		C1		D1		D2
Sex	male	female	male	female	male	female	male	female	male	female
Metabolite Fraction
U1	5.2	3.7	1.2	0.9	11.2	6.1	2.4	1.7	0.9	1.3
U2	9.0	7.2	1.7	1.3	5.2	4.9	5.1	3.6	1.8	1.7
U3	2.3	1.8	3.0	2.1	2.7	2.3	1.6	1.9	1.6	2.6
U4	1.5	2.4	1.1	0.7	2.5	2.8	1.7	3.0	1.3	1.1
U5	1.3	1.3	8.5	5.9	1.7	1.0	1.0	1.1	4.1	5.4
U6	3.1	2.5	6.1	5.0	5.7	4.4	2.0	1.2	1.5	1.7
U7	3.7	4.2	4.1	7.2	5.0	6.5	2.8	2.7	4.9	4.4
U8	5.4	8.8	5.3	11.2	6.3	12.6	4.0	5.0	4.4	3.6
U9	3.5	4.1	3.8	4.4	4.4	5.5	2.9	2.9	2.1	2.1
U10	3.1	2.7	2.8	3.1	2.4	3.0	3.1	4.0	3.8	2.6
U11	16.1	15.9	9.5	9.9	4.1	5.8	17.7	16.2	18.5	17.8
U12	1.1	1.3	0.7	0.8	1.2	1.8	3.4	2.4	4.1	3.2
U13	6.3	9.9	3.6	5.3	2.2	5.5	7.2	9.5	5.3	4.3
U14	2.1	2.6	0.6	0.7	0.6	0.8	14.5	18.3	16.9	21.7
Sum	63.6	68.3	52.0	58.4	55.4	63.0	69.6	73.5	70.2	73.5

 

Table 6. Quantitative distribution of the metabolite fraction in faeces given as percent of dose

Label	[6-14C]triazine								[5-14C]pyridine
Group	A1		B1		C1		D1		D2
Sex	male	female	male	female	male	female	male	female	male	female
Metabolite Fraction
0	5.3	4.7	2.3	2.2	6.2	6.6	3.9	2.5	1.1	0.9
F2	1.0	1.3	1.7	1.4	1.7	1.4	1.5	0.9	0.9	0.6
F3	1.6	1.1	2.2	1.3	2.1	2.0	1.9	1.3	0.3	0.3
F4	1.7	1.5	3.0	3.0	2.6	2.3	1.7	1.1	1.4	1.3
F5	4.9	3.1	6.0	5.7	8.3	4.7	2.5	1.3	2.4	1.1
F6	0.7	0.7	1.0	0.9	0.7	0.7	0.6	0.2	0.6	0.4
F7	0.3	0.3	1.5	1.9	0.7	0.3	0.5	0.3	1.1	1.6
F8	0.1	0.2	0.2	0.2	0.2	0.1	0.5	0.7	0.6	0.5
F9	0.8	1.4	0.8	0.9	0.7	0.7	1.5	1.4	1.9	1.7
F10	1.0	0.6	1.4	1.5	1.2	0.9	0.8	0.6	0.6	0.6
F11	0.3	0.2	0.3	0.5	0,3	0.1	0 7	0.4	1.1	1.1
F12	0.7	0.3	0.2	0.6	0.6	0.4	0.2	0.2	0.3	0.2
F13	0.7	0.5	1.4	1.3	1.2	1.2	0.9	0.6	0.7	0.6
Sum Eluat 2	19.0	16.0	21.9	21.3	26.7	21.5	17.1	11.5	13.1	10.8
Eluat 1 + 3	2.2	1.5	2.6	1.7	2.6	1.5	1.5	0.4	3.0	1.9
Extract 2	1.7	1.0	2.1	1.1	4.9	2.1	3.1	2.3	0.9	0.6
Nonextract.	3.2	3.1	2.7	2.5	4.2	4.6	3.1	2.3	1.5	1.1
Total	26.1	21.5	29.2	26.7	38.4	29.7	24.8	16.6	18.5	14.5

Applicant's summary and conclusion

Conclusions:

- Absorption: Radioactivity was rapidly and almost completely absorbed from the GI tract into the general circulation (maximum, D2 mean 84.7). Maximum concentrations in the blood were reached 15 minutes and 4 hours after administration at the low and high dose level, respectively.
- Distribution: Seven days after administration, mean residue levels were below 0.038 ppm in all tissues. At high dose levels, residue concentrations in fat were 200 times higher than other tissues. This indicates a saturation of tissue binding sites and an unfettered distribution of radioactivity (most probably unchanged test substance) in the fat at high doses. This assumption is supported by the distribution of residual radioactivity in the pre-treated animals. This finding is not considered to be an indication for bioaccumulation based on the efficient metabolism and excretion
- Metabolism: HPLC analysis revealed complex urinary and faecal metabolite pattern, independent of the sex, pre-treatment, dose level. The cleavage of the bridge between the triazine and pyridine ring is of minor importance in the metabolism in the rat.
- Excretion: The principal route of excretion was via urine with a total of 56 to 80% of the dose. Excretion was rapid. Within 24 hours 52 - 74%, 10 - 37%, and 0.2 - 1.2% of the administered dose were detected in urine, faeces, and expired air, respectively

Executive summary:

In this GLP compliant ADME study performed according to OECD 417, single oral doses of the labelled test substance were administrated at two dose levels (low dose: 0.5 mg/kg; high dose: 100 mg/kg body weight) to several groups of male and female rats (strain: Tif: RAI f (SPF)). Group A1 received a single intravenous administration at the low dose level. Group B1, C1, D1 and D2 were exposed orally and urine, faeces and expired air were collected at different times. After 7 days the rats were sacrificed and tissues were analysed for radiolabelled content. Group E1 to E4 (all males) were designated for a timewise analysis of radiolabelled content in the blood. Groups F1 to F4 were designated for analysis for radiolabelled content in tissues at several time points.

No test substance related or otherwise unusual behaviour of the animals were observed. The orally administered test substance was fast and almost completely absorbed from the gastrointestinal tract into the general circulation (maximum, D2 mean 84.7% of dose, minimum, C1 mean 57.4% of dose). Independent of the label maximum concentrations in the blood were reached 15 minutes and 4 hours after administration at the low and high dose level, respectively. These maximum values accounted for about 0.3 ppm and 60 ppm, for the 0.5 and 100 mg/kg dose level. The residual radioactivity was determined in several tissues and organs of male rats at different time points after oral administration using both labels at both dose levels. The calculated half-life times (t½) for the depuration of the residual radioactivity from the tissues, assuming to follow first order kinetics, were in the range of 1 to 2 hours at the 0.5 mg/kg dose level (both labels) and between 3 and 6 hours (except fat: 11 hours) for the triazine label at the high dose level. For the high dose pyridine label the half-life times were in the range of 2 to 11 hours. Seven days after a single oral administration of 0.5 mg/kg, low but detectable residues were measured in all tissues and organs. The mean values were below 0.025 ppm test substance equivalents, except in the heart (0.038 ppm). Highest amounts besides the heart were found in liver (0.025 ppm), muscle (0.024 ppm, and kidneys (0.022 ppm).At the high dose level (Group D1, 100 mg/kg), the tissue residues were only in fat accordingly higher, i.e. about 200 times. This indicates a saturation of tissue binding sites and an unfettered distribution of radioactivity (most probably unchanged test substance) in the fat at high doses. This assumption is supported by the distribution of residual radioactivity in the pre-treated animals. This finding is not considered to be an indication for bioaccumulation based on the efficient metabolism and excretion. In brain, heart, and muscle the residues were about 5 times in all other tissues and organs 20 to 50 times higher. The highest residues were detected in fat (about 1 ppm test substance equivalents). These data indicate a saturation of tissue binding sites, except in fat, where an unhindered distribution of radioactivity (most probably unchanged test substance) was observed. The residual radioactivity determined in the pre-treated animals support this assumption. The principal route of excretion was urine (totally 56 to 80% of the dose). The absorbed amount was rapidly excreted. Within 24 hours 52 - 74%, 10 - 37%, and 0.2 - 1.2% of the administered dose were detected in urine, faeces, and expired air, respectively. The excretion pattern was essentially independent of the sex, the route of administration, and pre-treatment with non-radiolabelled test substance. Both sexes eliminated significantly more via the kidneys at the high dose level, i.e. 73 - 80% instead of 55 - 65% at the low dose level.

HPLC analysis revealed complex urinary and faecal metabolite pattern. The chromatograms were qualitatively similar for both sexes, both dose levels, the i.v. and p.o. administration, and the pre-treated animals. Some metabolite fractions showed quantitative differences between the sexes and the dose levels. The most unpolar urinary fraction, accounting for 15 - 18% and 1 - 2% of the dose at the high and low dose level, respectively, cochromatographed with unchanged test substance. In the faeces <1% of the administered dose were characterized as unchanged test substance.

The experiments with the [5-14C] pyridine labelled test substance revealed only slight differences compared to the triazine label. The excretion pattern was independent of the label. The urinary and faecal metabolite pattern were qualitatively similar for both labels but showed some quantitative differences.

In summary, the test substance was after oral administration to rats was fast and almost completely absorbed and rapidly eliminated, mainly with urine. The test substance was extensively metabolized and the metabolic pathways are independent of the sex, pre- treatment, and the dose level. The cleavage of the bridge between the triazine and pyridine ring is of minor importance in the metabolism of the test substance in the rat.