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Toxicological information

Basic toxicokinetics

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Administrative data

Endpoint:
basic toxicokinetics
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Data source

Reference
Reference Type:
publication
Title:
Toxicokinetic modeling of dose-dependent formate elimination in rats: in vivo-in vitro correlations using the perfused rat liver.
Author:
Damian P and Raabe OG
Year:
2001
Bibliographic source:
Toxicol Appl Pharmacol 139, 22-32.

Materials and methods

Objective of study:
toxicokinetics
Principles of method if other than guideline:
Toxicokinetic modeling: in vivo - in vitro correlations using the perfused rat liver. Development of a toxicokinetic model using data from perfused rat liver experiments, and model evaluation in vivo.
GLP compliance:
not specified

Test material

Constituent 1
Chemical structure
Reference substance name:
Sodium formate
EC Number:
205-488-0
EC Name:
Sodium formate
Cas Number:
141-53-7
Molecular formula:
CHO2Na
IUPAC Name:
sodium formate
Details on test material:
- Name of test material (as cited in study report): sodium formate
Specific details on test material used for the study:
- Name of test material (as cited in study report): sodium formate
Radiolabelling:
no

Test animals

Species:
rat

Administration / exposure

Route of administration:
other: C34-001:perfusate (perfused liver experiments) and i.v. (in-vivo experiments)
Vehicle:
water
Doses / concentrationsopen allclose all
Dose / conc.:
2 other: mM
Remarks:
in vitro test; in terms of sodium formate
Dose / conc.:
4 other: mM
Remarks:
in vitro test; in terms of sodium formate
Dose / conc.:
8 other: mM
Remarks:
in vitro test; in terms of sodium formate
Dose / conc.:
12 other: mM
Remarks:
in vitro test; in terms of sodium formate
Dose / conc.:
41 mg/kg bw (total dose)
Remarks:
in vivo test; in terms of sodium formate
Dose / conc.:
164 mg/kg bw (total dose)
Remarks:
in vivo test; in terms of sodium formate
Dose / conc.:
328 mg/kg bw (total dose)
Remarks:
in vivo test; in terms of sodium formate
Dose / conc.:
492 mg/kg bw (total dose)
Remarks:
in vivo test; in terms of sodium formate
No. of animals per sex per dose / concentration:
Males: 92
Control animals:
yes
Details on study design:
- Dose selection rationale: dose levels were estimated from target plasma concentration assuming a rapid mixing after intravenous injection and a total body water volume of 60% of the body weight.
Details on dosing and sampling:
PHARMACOKINETIC STUDY (in vivo)
- Tissues and body fluids sampled: urine, blood
- Time and frequency of sampling:
Blood: at 15 min before dosing and 5, 10, 15, 20, 30, 40, 50 min and 1, 1.25, 1.5, 1.75., 2, 2.25, 2.5, 2.75, and 3 hr after dosing.
Urine: 20 min and 3, 8, 12, 20, and 28 hr after dosing.

Results and discussion

Toxicokinetic / pharmacokinetic studies

Details on absorption:
not applicable (i.v. injection)
Details on distribution in tissues:
not examined
Details on excretion:
Urinary excretion at 3 hours after dosing accounted for 8/34/45/55% at plasma target levels of 1/4/8/12 mM.

Any other information on results incl. tables

1) Perfused liver
i) experiment without liver: formate was not lost from the perfusion system without liver.
ii) experiments with liver: elimination of formate was dose-dependent. Saturation of metabolic elimination was

evident beginning at 4 mM. Formate elimination was virtually linear when plotted on a linear scale. Elimination rates calculated from the slope of the elimination curves for the 2, 4, 8, and 12 mmM dose were 7.1, 16.0, 22.8, and 26.4 µmol/hour/g liver, respectively. Good agreement between the model prediction and the measured perfusate concentrations were obtained (correlation coefficient r²= 0.9996). 

Table 1: statistical results for the perfused liver model:  

Maximal rate

Vmax  = 0.0100 mmol/min

Michaelis constant

KM    = 1.324 mM

endogenous liver formate production

KO    = 0.002 mmol/min



2) In-vivo studies

Elimination of formate from plasma was very rapid at all dose levels. Endogenous levels (approx. 0.06 mM) were

reached at approx. 3 hours.  The plasma curves showed nonlinearities in the elimination of formate across doses in

the log concentration-time plot.

Urinary excretion was initially very rapid. The cumulative excretion had reached the steady state condition of the endogenous formate excretion 

3 h after dosing.

Table 2: Urinary formate excretion in the rat

 

Target concentration
 (mM)

Dose

(mg/kg bw)

Total dose

(mmol)

Urinary excretion

(% of dose by 3 hours)

Experiment

(mean, n=4)

Model prediction

1

41

0.21

19

8

4

164

0.84

35.1

34

8

328

1.68

42.7

45

12

492

2.52

48.3

55


3) The toxicokinetic model 
The toxicokinetic model predicted both the rapid initial excretion and the steady state concentrations in good

agreement with the in-vivo  experiments, indicating a good fit of the model. Predictions for the in-vivo experiments (tabulated below) were in good agreement with the perfusion data.

Table 3: Model predictions for the in vivo situation


Maximal rate

Vmax  = 0.016 mmol/min

Michaelis constant

KM    = 1.84 mM

endogenous liver formate production

KO    = 0.00052 mmol/min

 

The simulated proportions of formate excretion via hepatic metabolism and urinary excretion indicate that the liver can account for virtually all formate metabolism in vivo:
 

Dose level (mM)

% of dose elimiminated by 3 hours)

Hepatic metabolism

Urinary excretion

1

92

8

4

66

34

8

55

45

12

45

55

 

Regarding the toxicological behavior: Elimination follows Michaelis-Menten kinetics. Hepatic metabolism  prevails at plasma levels up to 1 mM. Urinary elimination

 accounts for up  to 50% at high plasma levels. No accumulation to be considereed in the rat in-vivo.

Applicant's summary and conclusion

Conclusions:
Data obtained in rat perfused liver experiments indicated that formate is rapidly eliminated from the perfusate at all tested perfusate concentrations of 2, 4, 8, and 12 mM. The elimination was dose dependent, and metabolic saturation was seen at 4 mM and above.
A 2-compartment toxicokinetic model was developed and used to predict the elimination of formate. The model prediction fitted well with the
measured values and allowed to calculate the Michaelis-Menten constants of the metabolic reactions.
In-vivo experiments confirmed both the model prediction and the results of the liver perfusions.
Key findings of the presented studies include:
i) The rat perfused liver model and the toxicokinetic model were both suitable to model the in-vivo situation following low and high formate doses.
ii) Elimination of formate in vivo is saturable and follows Michaelis-Menten kinetics. Predicted constants are Vmax=0.016 mmol/min and
KM=1.84 mM.
iii) The endogenous formate generation in liver is calculated to be KO=0.00052 mmol/min.
iv) Hepatic metabolism of formate prevails (92% of elimination) over urinary excretion (8%) at the low dose (1 mM). At physiological plasma levels
(approx. 0.06 mM) hepatic metabolism is considered to play the major role for elimination. At higher plasma levels the urinary excretion
accounted for op to approx. 50% of the elimination.
v) In the in-vivo experiments, plasma levels of up to 12 mM returned to normal (0.06 mM) within 3 hours after dosing.
Thus, there was no indication of accumulation of formate in the rat.

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