Pirimiphos-methyl

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Data source

Reference

Reference 1

Reference Type:

study report

Title:

Unnamed

Year:

1997

Report date:

1997

Reference 2

Reference Type:

study report

Title:

Unnamed

Year:

1997

Report date:

1997

Materials and methods

Objective of study:

absorption

distribution

excretion

metabolism

Test guideline

Test guideline 1

Qualifier:

according to guideline

Guideline:

OECD Guideline 417 (Toxicokinetics)

Version / remarks:

1984

Test guideline 2

Qualifier:

according to guideline

Guideline:

EU Method B.36 (Toxicokinetics)

Test guideline 3

Qualifier:

according to guideline

Guideline:

EPA OPP 85-1 (Metabolism and Pharmacokinetics)

GLP compliance:

yes

Test material

Test material information

Constituent 1

Reference substance name:

Pirimiphos-methyl

EC Number:

249-528-5

EC Name:

Pirimiphos-methyl

Cas Number:

29232-93-7

Molecular formula:

C11H20N3O3PS

IUPAC Name:

O-2-(diethylamino)-6-methylpyrimidin-4-yl O,O-dimethyl phosphorothioate

Radiolabelling:

yes

Test animals

Species:

rat

Strain:

other: Alpk:APfSD

Sex:

male/female

Administration / exposure

Route of administration:

oral: gavage

Vehicle:

corn oil

Doses / concentrations

Doses / concentrations 1

Dose / conc.:

1 mg/kg bw (total dose)

Doses / concentrations 2

Dose / conc.:

50 mg/kg bw (total dose)

Doses / concentrations 3

Dose / conc.:

250 mg/kg bw (total dose)

Doses / concentrations 4

Dose / conc.:

1 mg/kg bw/day

Remarks:

14 days non-radiolabelled test substance and 1 day radiolabelled test substance

Results and discussion

Any other information on results incl. tables

The test substance is rapidly and extensively absorbed from the gastrointestinal tract following gavage administration of doses of 1, 50 or 250 mg/kg bw and repeated doses of 1 mg/kg bw/day (Table 2. the results at 50 mg/kg bw seem anomalous and are used for comparison with bile-duct cannulated rats only). Excretion is rapid, predominantly in the urine (>70%) at 48 hours, with approximately 50 % of the administered dose present in 12 hour urine samples. Some of the radiolabel present in faeces is due to biliary excretion. Overall the extent of absorption is considered to be >80% based on the
radioactivity found in the urine of animals dosed with a single dose of 1 or 250 mg/kg bw and the repeat dose at 1 mg/kg bw/d (>60%) plus the significant level of radioactivity found in the bile of animals dosed with a single dose of 50 mg/kg bw (17%). Variations in the levels of radiolabel in consecutive faecal samples indicated that test substance metabolites were subject to entero-hepatic circulation, which was more predominant in females and animals receiving repeated doses. This was consistent with the data from bile-duct cannulated animals, but could also involve deposition and release
in adipose tissue. Two days after dosing, less than 2% of the administered dose remained in the carcass and tissues, though abdominal fat had particularly high concentrations in females dosed with 250 mg/kg bw. Other than a marked increase in deposition in fat at 250 mg/kg bw, the results were essentially independent of dose level and duration. Indications of a sex difference in some aspects of the toxicokinetics of the test substance were consistent with metabolite data.

Table 2. Urine, faecal and tissue levels (means) in Alpk rats following administration of the test substance.

Dose	1 × 50 mg/kg bw (bile duct cannulated)		1 × 1 mg/kg bw		1 × 250 mg/kg bw		Repeat (14 + 1) × 1 mg/kg bw/d
	Male	female	male	female	male	female	Male	female
Urine 12h (% of dose)	33	28	59	46	53	50	54	57
Urine 12h - cannulated (% of dose)	22	7	NP	NP	NP	NP	NP	NP
Urine 48h non-cannulated (including terminal cage wash) (% of dose)	50	49	72	64	70	79	69	73
Urine 48h - cannulated (including terminal cage wash) (% of dose)	38	33	NP	NP	NP	NP	NP	NP
Faeces 12h (% of dose)	6	11	11	16	10	3	12	11
Faeces 12h - cannulated (% of dose)	2	<1	NP	NP	NP	NP	NP	NP
Faeces 48h non-cannulated (% of dose)	22	22	-	-	-	-	-	-
Faeces 48 h - cannulated (% of dose)	30	16	-	-	-	-	-	-
Total excretion 48 h (% of dose)	71	68	95	93	95	93	98	94
Total excretion 48 h - cannulated (% of dose)	85	69	NP	NP	NP	NP	NP	NP
Bile 6h (% of dose)	9	4	NP	NP	NP	NP	NP	NP
Bile 12h (% of dose)	14	11	NP	NP	NP	NP	NP	NP
Bile 48h (% of dose)	17	21	NP	NP	NP	NP	NP	NP
Tissue + carcass day 2 (% of dose)	NP	NP	0.3	1.3	0.7	1.9	0.8	1.0
Liver day 2 (mg equiv./g)	NP	NP	0.004	0.003	1.2	1.1	0.005	0.003
Kidney day 2  (mg equiv./g)	NP	NP	0.003	0.004	1.2	1.5	0.004	0.004
Abdominal fat day 4 (mg equiv/g)	NP	NP	0.004	0.009	5.6	56.0	0.007	0.011
Brain day 2 (mg equiv./g)	NP	NP	0.001	0.001	0.2	0.3	0.001	0.001
Blood day 2 (mg equiv./g)	NP	NP	0.002	0.002	1.1	1.1	0.002	0.003
Plasma day 2 (mg equiv./g)	NP	NP	0.001	0.001	1.1	0.7	0.001	0.001
Heart day 2 (mg equiv./g)	NP	NP	0.001	0.001	0.4	0.4	0.001	0.001

Pooled samples of urine, bile and faeces from the studies were examined for the presence of metabolites using a variety of chromatography techniques, mass spectroscopy (MS) and nuclear magnetic resonance spectroscopy (NMR). A total of 20 metabolites were detected, 13 of which were characterised. In most groups, >75% of the administered dose was characterised.Absorbed test substance is extensively metabolised, with no parent compound found in either urine or bile. Administration of a single dose or 15 doses of the test substance at 1 mg/kg bw produced a similar metabolic profile in both sexes with the main reactions being i) cleavage of the ester bond followed by glucuronidation or N-dethylation (M3, M11) or ii) O-demethylation (M6) followed by N-dethylation. Administration of 50 or 250 mg/kg bw indicated saturation in females of the pyrimidinyl esterase and the N-dethylation of M6. This difference in metabolism may be linked with the high levels of radiolabel in abdominal fat of females administered 250 mg/kg bw. Several metabolites retain functional groups consistent with cholinesterase inhibiting potential (M5). There was no direct evidence, in the samples analysed, of transformation to test substance oxon, though other oxons were detected in urine.

Table 3. Main metabolites in urine, faeces and bile (means; % of administered dose) in Alpk rats following administration of the test substance. 

Dose	1 × 50 mg/kg bw (bile duct cannulated)		1 × 1 mg/kg bw		1 × 250 mg/kg bw		Repeat (14 + 1) × 1 mg/kg bw/d
Metabolite#	Male	Female	Male	Female	Male	Female	Male	Female
Urine
parent	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.
M1	1	5	3	4	7	9	3	6
M3	14	5	28	28	15	8	30	28
M11	1	<1	2	1	1	N.D.	1	1
De-ethyl M6	12	7	8	5	24	13	10	10
M6	1	11	N.D.	2	3	35	N.D.	2
M12	2	1	5	4	4	3	8	5
O-Glucuronide of M1	3	2	4	4	5	6	1	5
ethoxy-glucuronide of M12	<1	<1	3	1	1	-	2	2
Faeces
parent	29	15	13	15	4	3	15	5
M1	N.D.	N.D.	N.D.	N.D.	N.D.	N.D.	1	1
M3	N.D.	N.D.	2	3	6	5	7	3
M11	N.D.	N.D.	6	5	5	2	5	3
O-Glucuronide of M1	N.D.	N.D.	N.D.	1	3	1	1	1
Bile
parent	N.D.	N.D.	-	-	-	-	-	-
M5	3	2	-	-	-	-	-	-
M1	1	1	-	-	-	-	-	-
M3	2	1	-	-	-	-	-	-
M11	<1	N.D.	-	-	-	-	-	-
De-ethyl M6	3	2	-	-	-	-	-	-
M6	<1	<1	-	-	-	-	-	-
M12	<1	N.D.	-	-	-	-	-	-
O-Glucuronide of M1	6	12	-	-	-	-	-	-

N.D. - Not Detected

Applicant's summary and conclusion

Conclusions:

Excretion is rapid, predominantly in the urine (>70%) at 48 hours, with approximately 50 % of the administered dose present in 12 hour urine samples. Some of the radiolabel present in faeces is due to biliary excretion. Overall the extent of absorption is considered to be >80% based on the radioactivity found in the urine of animals dosed with a single dose of 1 or 250 mg/kg bw and the repeat dose at 1 mg/kg bw/d (>60%) plus the significant level of radioactivity found in the bile of animals dosed with a single dose of 50 mg/kg bw (17%). Variations in the levels of radiolabel in consecutive faecal samples indicated that the test substance metabolites were subject to entero-hepatic circulation, which was more predominant in females and animals receiving repeated doses. This was consistent with the data from bile-duct cannulated animals, but could also involve deposition and release in adipose tissue. Two days after dosing, less than 2% of the administered dose remained in the carcass and tissues, though abdominal fat had particularly high concentrations in females dosed with 250 mg/kg bw. Other than a marked increase in deposition in fat at 250 mg/kg bw, the results were essentially independent of dose level and duration. Indications of a sex difference in some aspects of the toxicokinetics of the test substance were consistent with metabolite data.
Absorbed test substance is extensively metabolised, with no parent compound found in either urine or bile. A total of 20 metabolites were detected, 13 of which were characterised. In most groups, >75% of the administered dose was characterised.

Executive summary:

The basic ADME of the test substance, following single and repeat dosing, was investigated in Alpk:APfSD rats in a series of studies performed between 1996 and 1998.All studies were performed according to GLP and the following Guidelines without significant deviations:- 87/302/EEC B.36 / OECD 417 (1984) / FIFRA § 85-1. All studies used unlabelled material of 99.6% purity and 2-14C-pyrimidyl labelled material of >98% purity and specific activity of 2.15 GBq/mMol. Doses were administered by gavage in corn oil. Urine samples were taken at 6, 12, 24, 36 and 48 hours after the 14 C dose, with faecal samples taken at 12, 24, 36 and 48 hours. Bile samples were taken on eight occasions between 2 and 48 hours after dosing. Animals were sacrificed at 48 hours (except 50 mg/kg bw non-cannulated animals - 72 hours) and samples of cage washes, blood and a range of tissues taken. Samples were analysed by liquid scintillation counting following appropriate processing. Autoradiography was performed on animals sacrificed 6 or 24 hours after a dose of 1 mg/kg bw.

The test substance is rapidly and extensively absorbed from the gastrointestinal tract following gavage administration of doses of 1, 50 or 250 mg/kg bw and repeated doses of 1 mg/kg bw/d (Table 2. the results at 50 mg/kg bw seem anomalous and are used for comparison with bile-duct cannulated rats only). Excretion is rapid, predominantly in the urine (>70%) at 48 hours, with approximately 50 % of the administered dose present in 12 hour urine samples. Some of the radiolabel present in faeces is due to biliary excretion. Overall the extent of absorption is considered to be >80% based on the radioactivity found in the urine of animals dosed with a single dose of 1 or 250 mg/kg bw and the repeat dose at 1 mg/kg bw/d (>60%) plus the significant level of radioactivity found in the bile of animals dosed with a single dose of 50 mg/kg bw (17%). Variations in the levels of radiolabel in consecutive faecal samples indicated that the test substance metabolites were subject to entero-hepatic circulation, which was more predominant in females and animals receiving repeated doses. This was consistent with the data from bile-duct cannulated animals, but could also involve deposition and release in adipose tissue. Two days after dosing, less than 2% of the administered dose remained in the carcass and tissues, though abdominal fat had particularly high concentrations in females dosed with 250 mg/kg bw. Other than a marked increase in deposition in fat at 250 mg/kg bw, the results were essentially independent of dose level and duration. Indications of a sex difference in some aspects of the toxicokinetics of the test substance were consistent with metabolite data.

Pooled samples of urine, bile and faeces from the studies were examined for the presence of metabolites using a variety of chromatography techniques, mass spectroscopy (MS) and nuclear magnetic resonance spectroscopy (NMR). A total of 20 metabolites were detected, 13 of which were characterised. In most groups, >75% of the administered dose was characterised.Absorbed test substance is extensively metabolised, with no parent compound found in either urine or bile. Administration of a single dose or 15 doses of the test substance at 1 mg/kg bw produced a similar metabolic profile in both sexes with the main reactions being i) cleavage of the ester bond followed by glucuronidation or N-dethylation (M3, M11) or ii) O-demethylation (M6) followed by N-dethylation. Administration of 50 or 250 mg/kg bw indicated saturation in females of the pyrimidinyl esterase and the N-dethylation of M6. This difference in metabolism may be linked with the high levels of radiolabel in abdominal fat of females administered 250 mg/kg bw. Several metabolites retain functional groups consistent with cholinesterase inhibiting potential (M5). There was no direct evidence, in the samples analysed, of transformation to test substance oxon, though other oxons were detected in urine.