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Basic toxicokinetics

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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 Type:
study report
Title:
Unnamed
Year:
2015
Report Date:
2015

Materials and methods

Objective of study:
absorption
distribution
excretion
metabolism
Test guidelineopen allclose all
Qualifier:
according to
Guideline:
EPA OPPTS 870.7485 (Metabolism and Pharmacokinetics)
Deviations:
no
Qualifier:
according to
Guideline:
OECD Guideline 417 (Toxicokinetics)
Deviations:
no
Qualifier:
according to
Guideline:
other: MAFF Japan Test Guidelines for Agricultural Chemicals 2-3-1 Notification 12-Nousan-8147
Deviations:
no
GLP compliance:
yes

Test material

Reference
Name:
Unnamed
Type:
Constituent
Test material form:
solid
Details on test material:
Purity: 99.4-99.9%
Impurities: Not reported
Radiolabelling:
yes

Test animals

Species:
rat
Strain:
Sprague-Dawley
Remarks:
Crl:CD®(SD)IGS BR
Details on species / strain selection:
The Sprague-Dawley rat was chosen for this study because of the extensive experience with this strain and its suitability with respect to longevity, sensitivity, and low incidence of spontaneous diseases. Furthermore, the Sprague-Dawley rat has been used previously for toxicity testing of this chemical.
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Source: Charles River Laboratories, Inc., Raleigh, North Carolina, U.S.A.
- Age at study initiation: 8-11 weeks old (non-cannulated animals were approximately 8 weeks old, except for the high (200 mg/kg bw) dose group administered which had animals that were approximately 9 weeks old. Cannulated animals were approximately 11 weeks old for the pharmacokinetic and biliary elimination experiments).
- Weight at study initiation: 205 (±10) g to 345 (±20) g for male rats and 159 (±6) g to 237 (±20) g for female rats
- Housing: During the pretest period, animals were group housed (non-cannulated) or housed individually (cannulated) in solid bottom caging with bedding and nestlets. Each cage rack contained animals of either sex. Animals were moved to glass metabolism units for the in-life phase of the study. Animals for the Tmax collection times of the tissue distribution experiment were placed in solid bottom cages with a wire metal grate added to accommodate the short collection interval during the in-life phase.
- Diet (e.g. ad libitum): PMI® Nutrition International, LLC Certified Rodent LabDiet® 5002
- Water (e.g. ad libitum): tap water
- Acclimation period: The animals were quarantined for at least 6 days with the exception of cannulated rats, which were quarantined for at least 3 days. Other experiment specific exceptions included rats for the low dose distribution experiment which were quarantined for 5 days. Male rats for the low dose material balance experiment were quarantined for 4 days and female rats for this dose group were quarantined for 6 days.

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 20-26ºC (68-79ºF)
- Humidity (%): 30-70%
- Air changes (per hr):
- Photoperiod (hrs dark / hrs light): Animal rooms were artificially illuminated (fluorescent light) on an approximate 12 hour light/dark cycle.

IN-LIFE DATES: From: To:

Administration / exposure

Route of administration:
oral: gavage
Vehicle:
other: 0.1% Tween 80 in 0.5% methylcellulose
Duration and frequency of treatment / exposure:
Single dose
Doses / concentrationsopen allclose all
Dose / conc.:
10 mg/kg bw (total dose)
Remarks:
[pyridine-2,6-14C]
Dose / conc.:
200 mg/kg bw (total dose)
Remarks:
[pyridine-2,6-14C]
Dose / conc.:
10 mg/kg bw (total dose)
Remarks:
[fused pyrimidine-3-14C]
Dose / conc.:
200 mg/kg bw (total dose)
Remarks:
[fused pyrimidine-3-14C]
Dose / conc.:
10 mg/kg bw (total dose)
Remarks:
[methylene-14C]
Dose / conc.:
200 mg/kg bw (total dose)
Remarks:
[methylene-14C]
No. of animals per sex per dose:
1 male and 1 female for the vehicle in the Pilot Excretion, Material Balance and Tissue Distribution, Pharmacokinetic, Biliary Elimination, and Tissue Distribution studies and 4 males and 4 females in the Main Pilot Excretion, Material Balance and Tissue Distribution, Pharmacokinetic, Biliary Elimination, and Tissue Distribution studies
Control animals:
yes, concurrent vehicle
Details on study design:
- Dose selection rationale: The dose levels were selected to bracket the 2 lowest mean daily intake doses in a 28-day feeding toxicity study and the 2 middle doses in a 13-week feeding study in rats
- Rationale for animal assignment (if not random): Animals were selected for use on study based on adequate body weight gain and freedom from any clinical signs of disease or injury.
Details on dosing and sampling:
TOXICOKINETIC / PHARMACOKINETIC STUDY (Absorption, distribution, excretion)
- Tissues and body fluids sampled: urine, faeces, tissues, carcass, cage washes, exhaled breath, plasma, red blood cells, bile, and residual feed
- Time and frequency of sampling: Urine and feces samples were collected pre-dose and at 0-6, 6-12, 12-24, 24-48, 48-72, 72-96, 96-120, 120-144, and 144-168 hour intervals
- Other:
METABOLITE CHARACTERISATION STUDIES
- Tissues and body fluids sampled: Whole blood, urine, faeces, tissues, carcass, cage washes, and residual feed
- Time and frequency of sampling: 2 hours post-dose, and plasma was submitted for metabolite profiling
- From how many animals: 4 males and 4 females; aliquots of homogenized feces samples were pooled across the four animals per sex per dose group for the 0-24 and 24-48 hour collection intervals, respectively.
- Method type(s) for identification: Metabolite identification was conducted by HPLC/MS chromatography and a combination of mass spectral fragmentation pattern, accurate mass of the molecular ion, and retention time comparison with available reference standards
- Limits of detection and quantification: the limit of detection (LOD) and limit of quantitation (LOQ) values were set to 2 and 3 times the background mean, respectively.
Statistics:
Group data are represented as a mean ± SD. Only the values obtained were used in the mean calculation. Individual animal values reported as
All calculations were carried out with attention to maintaining an adequate number of significant figures so as not to inadvertently misrepresent results. Tables and appendices presented in the report were computer generated and values were rounded appropriately for inclusion in this report. Consequently, the application of manual calculation to report values may in some instances yield a minor variation. These occurrences should not be construed as adversely affecting the integrity or interpretation of the data.

Results and discussion

Preliminary studies:
For the pilot excretion experiments, the percentage of administered dose recovered from all groups by 7 day after dose administration ranged from 89.7 to 114% for all three labels. The majority of the administered dose was recovered in the urine (38.1 to 49.4%) and feces (40.4 to 57.4%) with no apparent difference between sexes. A key finding was that no appreciable radioactivity from any of the three radiolabels was eliminated in exhaled breath.
Main ADME resultsopen allclose all
Type:
absorption
Results:
Very rapid absorption. Absorption was slightly reduced at the high (200 mg/kg bw) dose with a mean of 63 (±12)% that was consistent with slight saturation of absorption.
Type:
distribution
Results:
Mean distribution was 43.3-52.6% in feces, 39.9-47.8% in urine, and 0.364-0.586% in tissues (low dose) and 56.8-68.8% in feces, 25.0-35.3% in urine and 0.116- 0.415% in tissues (high dose).
Type:
metabolism
Results:
Metabolism was similar across dose, sex, and the three radiolabels. The proposed metabolic pathway included a series of hydroxylation, oxidation, hydrolysis, decarboxylation, and rearrangement reactions at various locations throughout the molecule.
Type:
excretion
Results:
A significant portion was excreted in urine and feces as the intact test substance. The high percentage for urinary excretion of the intact test substance can be attributed to its high water solubility (0.23 g/L at 20°C).

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Absorption was rapid. Mean absorption half-lives estimated by compartmental analysis ranged from 0.051-0.14 hour (3-8 minutes) and 0.28-1.1 hours (17-66 min) after low and high dose administration, respectively. Mean Tmax values at the low dose for all three labels ranged from 0.3 to 0.8 hours. Tmax at the high dose ranged from 1.8 to 2.5 hours for all three radiolabels.
Details on distribution in tissues:
The mean distribution of radioactivity for rats administered the low (10 mg/kg bw) dose for all three radiolabels was 43.3-52.6% in feces, 39.9-47.8% in urine, and 0.364-0.586% in tissues. For rats administered the high (200 mg/kg bw) dose of each radiolabel, the mean distribution was 56.8-68.8% in feces, 25.0-35.3% in urine and 0.116- 0.415% in tissues.

The percentages and concentration values of quantifiable [14C]residues in all collected tissues were very low. Mean tissue concentrations, with the exception of the GI tract tissue, were less than 22 μg equivalents/g at Tmax after low dose administration for all three radiolabels. The majority of the dose at Tmax was associated with the gastrointestinal tract contents. By 168 hours, the tissue concentrations had declined to 0.09% and 0.03% after low and high dose administration, respectively. No major radiolabel or sex differences were observed in the overall distribution. Tissue:plasma concentration ratios indicated no potential for accumulation.
Details on excretion:
The majority of the dose was excreted in the feces and urine by 24 hours after dose administration. The mean distribution of radioactivity for rats administered the low (10 mg/kg bw) dose for all three radiolabels was 43.3-52.6% in feces, 39.9-47.8% in urine, and 0.364-0.586% in tissues. For rats administered the high (200 mg/kg bw) dose of each radiolabel, the mean distribution was 56.8-68.8% in feces, 25.0-35.3% in urine and 0.116- 0.415% in tissues.
Toxicokinetic parametersopen allclose all
Key result
Toxicokinetic parameters:
half-life 1st: 1.4-1.8 hours
Remarks:
10 mg/kg dose
Key result
Toxicokinetic parameters:
half-life 1st: 3.8-5.4 hourshours
Remarks:
200 mg/kg dose
Key result
Toxicokinetic parameters:
Tmax: 0.3-0.8 hours
Remarks:
10 mg/kg dose
Key result
Toxicokinetic parameters:
Tmax: 1.8-2.5 hours
Remarks:
200 mg/kg dose
Key result
Toxicokinetic parameters:
AUC: 83.2% (males) and 93.6% (females)
Remarks:
10 mg/kg dose and at 0-48 hours
Key result
Toxicokinetic parameters:
AUC: 88.9% (males) and 93.9% (females)
Remarks:
200 mg/kg dose and at 0-48 hours
Key result
Toxicokinetic parameters:
AUC: 10.8% (males) and 3.86% (females)
Remarks:
10 mg/kg dose and at 48-120 hours
Key result
Toxicokinetic parameters:
AUC: 6.82% (males) and 4.10% (females)
Remarks:
200 mg/kg dose and at 48-120 hours

Metabolite characterisation studies

Metabolites identified:
yes
Details on metabolites:
The metabolism of the test substance was similar across dose, sex, and the three radiolabels. The proposed metabolic pathway included a series of hydroxylation, oxidation, hydrolysis, decarboxylation, and rearrangement reactions at various locations throughout the molecule. The most significant reaction was hydroxylation of the trifluoromethyl phenyl ring to form IN-R6U70, which was found in feces (14-27%) and to a lesser extent in urine (0.7-2.2%). Sulfation and glucuronidation of IN-R6U70 also was observed. Other minor proposed metabolic reactions included hydroxylation (IN-R6U71 and four isomers of OH-RAB55), N-oxidation (IN-R3Z91), oxidative hydrolysis (IN-RPD47 and IN-Y2186), sequential oxidative hydrolysis, decarboxylation, and rearrangement (IN-SBV06 and IN-RUB93), and hydrolytic cleavage (IN-RPA19). Additional proposed reactions on the metabolites of DPX-RAB55 included hydrolytic cleavage of IN-R6U70, IN-R6U71 and IN-R6U72 (IN-RPA19), hydrolytic defluorination of IN-R6U70 (IN-R6U72), amide hydrolysis of IN-SBV06 or IN-RUB93 (IN-RUA92), hydroxylation of IN-SBV06 and IN-RUB93, and glucuronide metabolites of OH-RAB55, IN-R6U71, and OH-RUB93.

Applicant's summary and conclusion

Conclusions:
The test substance was readily excreted as the parent molecule representing 43-63% of the administered dose recovered in urine and feces. Approximately 24-43% of the administered dose was recovered in urine and feces as metabolized residues. The proposed metabolic pathway included a series of hydroxylation, oxidation, hydrolysis, decarboxylation, and rearrangement reactions at various locations throughout the molecule. The most significant reaction was hydroxylation of the trifluoromethyl phenyl ring to form IN-R6U70, which was found in feces (14-27%) and to a lesser extent in urine (0.7-2.2%). Sulfation and glucuronidation of IN-R6U70 also was observed. Other minor metabolites included IN-R6U71, IN-R3Z91, IN-RPD47, IN-SBV06, IN-RUB93, IN-RPA19, IN-R6U72, IN-Y2186, and IN-RUA92. OH-SBV06, OH-RUB93, four isomers of OH-RAB55, and glucuronide metabolites of OH-RAB55, IN-R6U71, and OH-RUB93 were proposed.
Executive summary:

The absorption, distribution, metabolism, and excretion of the test substance was studied in male and female rats. Experiments were performed to understand the disposition and material balance of total 14C residues among tissues, and excreta (including exhaled breath), pharmacokinetic behavior of radioactive residues in plasma and red blood cells, measurement of absorption thru elimination of 14C residues in bile, and the percentage and concentration of 14C residues in tissues at selected times after dosing. The experiments were performed using three radiolabeled forms: [pyridine-2,6-14C], [fused pyrimidine-3-14C], or [methylene-14C], each mixed with the non-radiolabeled test material (OECD Test Guideline 417).

For the pilot excretion experiments, the percentage of administered dose recovered from all groups by day 7 after dose administration ranged from 89.7 to 114% for all three labels. The majority of the administered dose was recovered in the urine (38.1 to 49.4%) and feces (40.4 to 57.4%) with no apparent difference between sexes. A key finding was that no appreciable radioactivity from any of the three radiolabels was eliminated in exhaled breath.

 

In the definitive material balance experiments, the mean percentage of administered dose recovered from all groups by 168 hours after dose administration ranged from 89.0% to 102%. The majority of the dose was excreted in the feces and urine by 24 hours after dose administration. The mean distribution of radioactivity for rats administered the low (10 mg/kg bw) dose for all three radiolabels was 43.3-52.6% in feces, 39.9-47.8% in urine, and 0.364-0.586% in tissues. For rats administered the high (200 mg/kg bw) dose of each radiolabel, the mean distribution was 56.8-68.8% in feces, 25.0-35.3% in urine and 0.116- 0.415% in tissues. The overall material balance was similar across sexes and radiolabels. The tissue burden and urinary excretion at the high dose were respectively 1.4- to 3.1-fold and 1.3- to 1.7-fold lower than the low dose. Fecal excretion increased by 1.1- to 1.4- fold as the administered dose was increased from 10 to 200 mg/kg bw. These results indicated a slight saturation of absorption after high dose administration.

 

The pharmacokinetics of 14C residues in plasma following oral gavage administration showed similar biphasic elimination kinetics among the three radiolabels with very rapid absorption, rapid distribution, and no significant sex differences. Cmax and AUC values were also similar across the respective dose groups. The majority of the dose was eliminated by 24-48 hours based on the rapid tissue distribution half-lives (1.4-5.4 hours) and AUC partial area percentages (82-96%). The remaining residues (4-18%) were eliminated with an average elimination half-life of 46 (±7) hours (n=40 rats). Group mean elimination half-lives at both dose levels ranged from 35 to 54 hours, except for the high dose [methylene-14C] mean elimination half-lives of 12-14 hours. The individual animal values for these means were not included in the overall mean value of 46 hours. The pharmacokinetic results were consistent with the rate and extent of excretion in urine and feces from the material balance experiments. Plasma and red blood cell 14C residue concentrations were similar throughout all sampling times indicating the absence of binding in red blood cells.

 

The biliary elimination results showed that the mean percent absorption at the low (10 mg/kg bw) dose was 84 (±5)% across both sexes and three radiolabels. Absorption was slightly reduced at the high (200 mg/kg bw) dose with a mean of 63 (±12)% that was consistent with slight saturation of absorption.

 

From the tissue distribution experiments, systemic uptake and distribution was evident based on the presence of quantifiable 14C residues in all the collected tissues. However, the percentages and concentration values were very low. Mean tissue concentrations, with the exception of the GI tract tissue, were less than 22 μg equivalents/g at Tmax after low dose administration for all three radiolabels. The majority of the dose at Tmax was associated with the gastrointestinal tract contents. By 168 hours, the tissue concentrations had declined substantially by 2 to 4 orders of magnitude with concentrations ranging between 0.001 to 0.035 μg equivalents/g, and a number of tissues had concentrations that were <LOQ. A similar concentration distribution pattern was observed in tissues of rats administered the high (200 mg/kg bw) dose of each radiolabel. By 168 hours after dose administration, the mean percentages of the dose in each tissue declined substantially, ranging from <LOQ to 0.09% and <LOQ to 0.03% after low and high dose administration, respectively. Slightly lower percentages of 14C recovery in tissues after high (200 mg/kg bw) dose administration was consistent with a reduction in absorption observed in the biliary elimination experiments and increased recovery in feces observed in the material balance experiments. No major radiolabel or sex differences were observed in the overall distribution. Tissue:plasma concentration ratios indicated no potential for accumulation.

 

A significant portion of the low and high dose was excreted in urine (20-41%) and feces (12-37%) as the intact test substance. The high percentage for urinary excretion of the unchanged test substance can be attributed to its high water solubility (0.23 g/L at 20°C). The parent test substance was also the major component in plasma. The metabolism of the test substance was similar across dose, sex, and the three radiolabels. The proposed metabolic pathway included a series of hydroxylation, oxidation, hydrolysis, decarboxylation, and rearrangement reactions at various locations throughout the molecule. The most significant reaction was hydroxylation of the trifluoromethyl phenyl ring to form IN-R6U70, which was found in feces (14-27%) and to a lesser extent in urine (0.7-2.2%). Sulfation and glucuronidation of IN-R6U70 also was observed. Other minor proposed metabolic reactions included hydroxylation (IN-R6U71 and four isomers of OH-RAB55), N-oxidation (IN-R3Z91), oxidative hydrolysis (IN-RPD47 and IN-Y2186), sequential oxidative hydrolysis, decarboxylation, and rearrangement (IN-SBV06 and IN-RUB93), and hydrolytic cleavage (IN-RPA19). Additional proposed reactions on the metabolites of DPX-RAB55 included hydrolytic cleavage of IN-R6U70, IN-R6U71 and IN-R6U72 (IN-RPA19), hydrolytic defluorination of IN-R6U70 (IN-R6U72), amide hydrolysis of IN-SBV06 or IN-RUB93 (IN-RUA92), hydroxylation of IN-SBV06 and IN-RUB93, and glucuronide metabolites of OH-RAB55, IN-R6U71, and OH-RUB93.