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Basic toxicokinetics

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Endpoint:
basic toxicokinetics
Type of information:
experimental study
Adequacy of study:
key study
Study period:
26/04/2010 to 03/05/2010
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
GLP; comparable to guideline study. Read-across is justified on the following basis: The family of zinc borates that include Zinc Borate 500, Zinc Borate 2335 and Zinc Borate 415 (also known as Zinc Borate 411). Zinc borate 500 is anhydrous Zinc Borate 2335 and Zinc Borate 415 has different zinc to boron ratio. Zinc borate 2335 (in common with other zinc borates such as Zinc borate 415 and 500) breaks down to Zinc Hydroxide (via Zinc oxide) and Boric Acid, therefore the family of zinc borates shares the same toxicological properties. Zinc borates are sparingly soluble salts. Hydrolysis under high dilution conditions leads to zinc hydroxide via zinc oxide and boric acid formation. Zinc hydroxide and zinc oxide solubility is low under neutral and basic conditions. This leads to a situation where zinc borate hydrolyses to zinc hydroxide, zinc oxide and boric acid at neutral pH quicker than it solubilises. Therefore, it can be assumed that at physiological conditions and neutral and lower pH zinc borate will be hydrolysed to boric acid, zinc oxide and zinc hydroxide. Hydrolysis and the rate of hydrolysis depend on the initial loading and time. At a loading of 5% (5g/100ml) zinc borate hydrolysis equilibrium may take 1-2 months, while at 1 g/l hydrolysis is complete after 5 days. At 50 mg/l hydrolysis and solubility is complete (Schubert et al., 2003). At pH 4 hydrolysis is complete. Zinc Borate 2335 breaks down as follows: 2ZnO • 3B2O3 •3.5H2O + 3.5H2O + 4H+ ↔ 6H3BO3 + 2Zn2+ 2Zn2+ + 4OH- ↔ 2Zn(OH)2 ____________________________________________________________ Overall equation 2ZnO • 3B2O3 •3.5H2O + 7.5H2O ↔ 2Zn(OH)2 + 6H3BO3 The relative zinc oxide and boric oxide % are as follows: Zinc borate 2335:zinc oxide = 37.45% (30.09% Zn) B2O3 = 48.05% (14.94% B) Water 14.5% Zinc borate 415: zinc oxide = 78.79%; (63.31% Zn) B2O3 = 16.85% (5.23% B) Water 4.36% Zinc borate, anhydrous: Zinc oxide = 45 % B2O3= 55% (17.1 % B)

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2010
Report date:
2010

Materials and methods

Objective of study:
distribution
excretion
Test guideline
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 417 (Toxicokinetics)
Deviations:
not specified
Principles of method if other than guideline:
The test substance was administered to rats as a single oral (gavage) dose at a level of 1000 mg/kg to evaluate the toxicokinetic profile of the test article with blood, urine, faeces and tissue samples at time points up to 168 h (7 days) after exposure. Samples were analysed for zinc and boron using an inductively coupled plasma-mass spectrometer (ICP-MS) and toxicokinetic parameters were evaluated using the composite mean plasma zinc and boron concentration versus time profiles.
GLP compliance:
yes

Test material

Constituent 1
Reference substance name:
138265-88-0
EC Number:
604-070-9
Cas Number:
138265-88-0
IUPAC Name:
138265-88-0
Details on test material:
- Name of test material: Zinc borate 2335 (Firebrake ZB)
- Lot/batch No.: 29K16
- Storage condition of test material: At room temperature in original container.
Radiolabelling:
no

Test animals

Species:
rat
Strain:
Sprague-Dawley
Sex:
male/female
Details on test animals or test system and environmental conditions:
TEST ANIMALS
- Source: Charles River Laboratories.
- Age at study initiation: Males date of birth 28/02/2010; females 02/03/2010
- Weight at study initiation: The weight of a representative sample of four rats per sex were used. Males - 165 to 184 g; females - 132 to 141 g.
- Fasting period before study: No
- Housing: Individually in stainless steel cages equipped with automatic watering systems. Absorbent cage boards were placed underneath animal cages to contain liquid and solid wastes.
- Individual metabolism cages: Yes, during the final 24 h immediately preceding termination
- Diet: Ad libitum via automatic watering systems.
- Water: Ad libitum. A single lot of diet was used for the entire study to avoid variations in dietary zinc or boron content.
- Acclimation period: Rats were held in quarantine for 10 days prior to dosing.

ENVIRONMENTAL CONDITIONS
- Temperature (°C): 20 - 23 °C (actual measured)
- Humidity (%): 42 - 54 % (actual measured)
- Photoperiod (hrs dark / hrs light): 12 h dark/12 h light.

IN-LIFE DATES: From 26/04/2010 to 03/05/2010

Administration / exposure

Route of administration:
oral: gavage
Vehicle:
CMC (carboxymethyl cellulose)
Details on exposure:
PREPARATION OF DOSING SOLUTIONS: On the day of dosing, the zinc borate 2335 dosing formulation was prepared as a suspension in 1 % CMC at a concentration of 200 mg/mL. The prepared dosing formulation was stored refrigerated at approximately 2 to 8 C with continuous stirring prior to dosing, and was used for dosing within 3 h of preparation. The pH of the dosing formulation was measured at the time of preparation and the dosing formulation was analysed for homogeneity and to verify the concentration of the test article.

VEHICLE
- Amount of vehicle: 5 mL/kg
- Lot/batch no. (if required): 118K0021

HOMOGENEITY AND STABILITY OF TEST MATERIAL: A frozen sample of the diet was analysed along with a sample of the water source.
Duration and frequency of treatment / exposure:
Single administration
Doses / concentrations
Remarks:
Doses / Concentrations:
1000 mg/kg (200 mg/mL at a dosing volume of 5 mL/kg). The dose was calculated based on the most recent individual animal body weight.
No. of animals per sex per dose / concentration:
Six/sex/group
Control animals:
yes, concurrent vehicle
Details on study design:
- Dose selection rationale: The dose of 1000 mg/kg/day was the maximum tolerated dose in a 28-day subacute oral toxicity study in the rat for a similar zinc borate compound.
- Rationale for animal assignment: Animals were randomised by body weight using a "measure random" method that produced similar group mean values using ToxData software.
Details on dosing and sampling:
PHARMACOKINETIC STUDY (Absorption, distribution, excretion)
- Tissues and body fluids sampled: Urine, faeces, blood, plasma and tissues
- Time and frequency of sampling: The amount of zinc and boron in blood was determined once in the control group and at 13 time points up to 168 (7 days) after exposure in the treated groups. Blood samples were collected from 12 animals (6 males and 6 females) per time point:
Group 1: 0 h (control); pre-dose
Group 2: 24 h after dose administration
Group 3: 1, 5 and 48 h after dose administration
Group 4: 2, 6 and 72 h after dose administration
Group 5: 3, 9 and 96 h after dose administration
Group 6: 4, 12 and 168 h after dose administration

The amount of zinc and boron in urine and faeces was determined by collecting samples over the course of 24 h (immediately preceding termination) form rats in all groups at time points as follows:
Group 1: 0 - 24 h after administration of vehicle (control)
Group 2: 0 - 24 h after dose administration
Group 3: 24 - 48 h after dose administration
Group 4: 48 - 72 h after dose administration
Group 5: 72 - 96 h after dose administration
Group 6: 144 - 168 h after dose administration

Immediately following urine and faeces collection, animals were euthanized and subjected to a limited necropsy. Necropsy included examination of the animal's external surface, all orifices and the cranial, thoracic, abdominal and pelvic cavities (including viscera). The following tissues were collected, washed, blotted and placed in polyporpylene tubes on ice and stored at -20 °C:
Adipose tissue, bone with marrow, brain (cerebrum level 1and 2; cerebellum with medulla/pons) epididymides (2), femur with joint, heart, intestine (duodenum, ileum, jejunum) kidneys (2), liver, lings, ovaries (2 ) with oviducts (2) pancreas, prostate, seminal vesicles (2), skeletal muscle (rectus femoris) and testes (2). Frozen tissue from Groups 3, 4, 5 and 6 were stored frozen for possible future analysis. The following tissues from groups 1 and 2 were kept frozen for analysis for boron and zinc using ICPMS:
Epididymides, femur with joint, kidneys, liver, ovaries, pancreas, seminal vesicles and testes.
- Other:
Toxicokinetic analysis:
the following TK parameters were calculated and/or considered for zinc and boron:
- contribution of diet and drinking water to the overall dose of zinc and boron
- maximum zinc and boron concentration of plasma after administration (Cmax)
- time needed to reach Cmax (Tmax) for zinc and boron
- absorption half life(t1/2a) for zinc and boron
- elimination half-life (t1/2) for zinc and boron
- terminal elimination half-life (T1/2) for zinc and boron
- area under concentration-time curve [0 to the last measureable concentration and 0 to infinity; AUC0-t and AUC0-∞, respectively] for zinc and boron.
- apparent volume of distribution (Vz/F) for zinc and boron
- total body clearance (CL/F) for zinc and boron
- urinary zinc and boron excretion (μg/24 h)
- % zinc and boron recovered in urine in 24, 48 and 72 h
- % zinc and boron recovered in faeces in 24, 48 and 72 h
-mean urinary zinc and boron concentration per mg creatinine at 24, 48 and 72 h post-dose.
- tissue zinc and boron levels.
Analysis of plasma TK parameters were performed on the composite mean plasma zinc and boron concentration
Statistics:
Analysis of plasma toxicokinetic parameters was performed on the composite mean plasma zinc and boron concentration versus time profiles obtained from Groups 2 - 6 animals using WinNolin Professional Edition software version 4.1 (Pharsight Inc, Mountian view, CA). Mean plasma zinc and boron levels for Group 1 (control) animals were considered as background concentrations, and were subtracted prior to the model analysis. Nominal doses of 303 mg zinc/kg and 150 mg boron/kg (equivalent to zinc borate dose level of 1000 mg/kg) were used in the TK analysis of zinc and boron respectively.
The noncompartmental model for extravascular administration was used for TK analysis, with one-compartmental model analyses. Elimination rate constants (λ2) were calculated by log-linear regression on mean data points of the terminal phase; the plasma terminal elimination half-life (T1/2) was calculated as In(2)/λ2. The area under the plasma concentration-time curve from time zero to the last measured concentration (AUC0-t) was calculated by the linear-up/log-down trapezoidal rule; the area under the plasma concentration-time curve extrapolated to infinity (AUC0-∞) was calculated by AUC0-t+Ct/λ2, where Ct is the last measurable plasma concentration. Total body clearance (CL/F) was calculated by dose/AUC0-∞, where the nominal doses of 303 and 150 mg/kg were used for zinc and boron, respectively.
The composite mean plasma concentration-time profile [C(t)] was also analysed with a first-order absorption and elimination one-compartmental model: C(t) = Z x [exp(-K10 x t) - exp(-K01 x t)], where Z is the scaling factor, K10 is the rate constant in descending phase (i.e., absorption rate constant). Analysis was performed using compartmental model 3 in WinNonlin Professional Edition 4.1 with uniform weight. The derived parameters t1/2a (absorption half-life) and t1/2 (elimination half-life) were calculated as 1n(2)/K01 and 1n(2)/K10, respectively.

Results and discussion

Main ADME resultsopen allclose all
Type:
distribution
Results:
Pancreas tissue had the highest level of zinc, ranging from 72.7 to 76.8 μg/mg tissue, followed by liver and femur tissue. Kidney tissue had the highest level of boron ranging from 14 to 32.7 μg/mg tissue followed by femur tissue.
Type:
excretion
Results:
Urinary excretion for zinc reached a maximum rate range of 20.4 to 44.9 μg/24 h. Urinary excretion of boron reached a maximum rate range of 16245 to 20981 μg/24 h.
Type:
excretion
Results:
A negligible amount of intake zinc was recovered from urine. Total recovery of boron in urine was 57.8 % (males) to 60.8 % (females) during the 0 - 72 h post-dose collection period (groups combined per sex).
Type:
excretion
Results:
5 % or less of intake boron was recovered from faeces between 0 and 24 h post-dose. Total recovery of zinc in faeces was 59.3 % (males) to 79.4 % (females) during the 0 - 72 h post-dose collection period (groups combined per sex)

Toxicokinetic / pharmacokinetic studies

Details on absorption:
Zinc and boron appeared in organ tissue, indicating hydrolysis of zinc borate 2335 in the gastrointestinal tract and subsequent systemic absorption of zinc and boron.
Details on distribution in tissues:
Zinc and boron levels in tissue:
Pancreas tissue had the highest level of zinc, ranging from 72.7 to 76.8 μg/mg tissue, followed by liver and femur tissue. Kidney tissue had the highest level of boron ranging from 14 to 32.7 μg/mg tissue followed by femur tissue.
Details on excretion:
Net zinc and boron urinary excretion rate:
Following oral administration of zinc borate 2335, urinary excretion for zinc reached a maximum rate range of 20.4 to 44.9 μg/24 h, using the 24 - 48 h post-dose collection range (Group 3) and urinary excretion of boron reached a maximum rate range of 16245 to 20981 μg/24 h during the 0 - 24 h post-dose collection range (Group 2).

Zinc/boron to creatinine rations: For each mg of creatinine, zinc was present in urine at a range of 3.68 to 4.95 μg for the first 48 h collection period, decreasing to 1.71 to 2.42 μg during the 48 - 72 h post-dose collection. For each mg of creatinine, boron was present in urine at a range of 2684 to 3330 μg during the 0 - 24 h collection period, as compared to a range of 19.3 to 296 μg between 24 and 72 h post-dose.

Recovery of zinc and boron from urine and faeces:
A negligible amount of intake zinc was recovered from urine, with a total recovery of less than 0.1 % during the 0 - 72 h post-dose collection period (groups combined). For boron, at least 50 % of intake boron was excreted during the first 24 h post-dose falling to a range of 5 to 7 % between 24 and 48 h post-dose. Total recovery of boron in urine was 57.8 % (males) to 60.8 % (females) during the 0 - 72 h post-dose collection period (groups combined per sex).
Zinc was recovered in faeces at a range of 25.3 to 43.6 % during the 0 - 48 h post-dose collection period, decreasing to 4.68 to 6.27 % during the 48 - 72 h post-dose collection period. Total recovery of zinc in faeces was 59.3 % (males) to 79.4 % (females) during the 0 - 72 h post-dose collection period (groups combined per sex). Only 5 % or less of intake boron was recovered from faeces between 0 and 24 h post-dose followed by undetectable amounts of boron in faeces from 24 - 72 h.

Zinc and boron levels in tissue:
Zinc and boron appeared in organ tissue, indicating hydrolysis of zinc borate 2335 in the gastrointestinal tract and subsequent systemic absortpion of zinc and boron. Pancreas tissue had the highest level of zinc, ranging from 72.7 to 76.8 μg/mg tissue, followed by liver and femur tissue. Kidney tissue had the highest level of boron ranging from 14 to 32.7 μg/mg tissue followed by femur tissue.
Toxicokinetic parametersopen allclose all
Toxicokinetic parameters:
Tmax: 5 - 6 h after administration.
Toxicokinetic parameters:
Cmax: 9.63 - 11.7 μg/mL for zinc and 26.7 to 27 μg/mL for boron were reached before concentrations decreased to background levels by 72 h post-dose.
Toxicokinetic parameters:
AUC: AUC0-∞ ranged from 9201 to 10396 min*μg/mL for zinc and 26023 to 32687 min*μg/mL for boron.
Toxicokinetic parameters:
other: CL/F (total body clearance) was 1.75 to 1.98 L/h/kg for zinc and 0.275 to 0.346 L/h/kg for boron.
Toxicokinetic parameters:
other: Vz/F was 18.7 to 21.8 L/kg for zinc and 1.97 to 3.48 L/kg for boron.
Toxicokinetic parameters:
half-life 1st: T1/2 ranged from 5.0 to 7.7 h for both zinc and boron.
Toxicokinetic parameters:
half-life 1st: T1/2a ranged from 0.79 to 1.7 h for zinc and from 5.1 to 5.2 h for boron.
Toxicokinetic parameters:
half-life 1st: T1/2 ranged from 10.1 to 15.1 h for zinc and from 5.1 to 7.4 h for boron.

Metabolite characterisation studies

Metabolites identified:
not measured

Any other information on results incl. tables

Zinc and boron in feed and drinking water: The average levels of zinc and boron in the feed samples analysed were 72 and 11 μg/g (ppm) respectively. The median zinc level in the water samples analysed for this study was 44 ng/mL (ppb) while boron was not detected in the water samples. Although food and water consumption were not measured for each animal, in general the daily feed and water consumption for a rat would be 100 g/kg/day and 120 mL/kg/day respectively. At the measured element levels, this would result in less than 2.5 % of the intake (nominal) dose of zinc and less than 1 % of the intake dose of boron as coming from feed and water. Therefore the zinc and boron levels contributed by feed and drinking water in this study were considered negligible in terms of TK analysis.

Applicant's summary and conclusion

Conclusions:
Interpretation of results (migrated information): no bioaccumulation potential based on study results
Following a single oral dose (1000 mg/kg) of zinc borate 2335, zinc and boron appeared in rat plasma and tissue samples, indicating the hydrolysis of zinc borate 2335 in the gastrointestinal tract and subsequent systemic absorption of zinc and boron. In plasma, Tmax occurred between 5 and 6 h after administration, and Cmax ranges of 9.63 to 11.7 μg/mL for zinc and 26.7 to 27.9 μg/mL for boron were reached before concentrations decreased to background levels by 72 h post-dose; T1/2 ranged form 5.0 to 7.7 h (zinc and boron, respectively).
The gastrointestinal route was the primary elimination route for zinc, while urinary excretion via the kidneys was the primary elimination route for boron. Overall, approximately 70 % of intake zinc and 63 % of intake boron was recovered from excreta. Pancreas tissue had the highest level of zinc, followed by liver and femur tissue. Kidney had the highest level of boron, followed by femur tissue.
Read-across is justified on the basis detailed in the rationale for reliability above. This study is therefore considered to be of sufficient adequacy and reliability to be used as a key study.