Registration Dossier

Administrative data

Endpoint:
basic toxicokinetics in vivo
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles.

Data source

Reference
Reference Type:
publication
Title:
Pharmacokinetics of TRIS (hydroxymethyl-)aminomethane in the Rabbit.
Author:
Brasch, H. and Iven, H.
Year:
1981
Bibliographic source:
Arch. int. Pharmacodyn. 254:4-12

Materials and methods

Objective of study:
distribution
Principles of method if other than guideline:
Pharmocokinetics of TRIS were investigated in rabbits after a short-time infusion of a single low dose.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Details on test material:
- Analytical purity: analytical grade
Radiolabelling:
no

Test animals

Species:
rabbit
Strain:
not specified
Sex:
male/female
Details on test animals and environmental conditions:
TEST ANIMALS
- Weight at study initiation: 3600-4400 g

Administration / exposure

Route of administration:
intravenous
Vehicle:
not specified
Duration and frequency of treatment / exposure:
24 h
Doses / concentrations
Remarks:
Doses / Concentrations:
121 mg/kg (1 mmol/kg)
No. of animals per sex per dose:
6
Control animals:
no
Details on dosing and sampling:
PHARMACOKINETIC STUDY
- Tissues and body fluids sampled: blood
- Time and frequency of sampling: 2, 4, 8, 15 and 30 min, 1, 2, 3, 4, 6, 8, 12 and 24 h
- Method type(s) for identification: Hewlett-Packard 5710A GC equipped with a FID together with a Hewlett Packard 3380A integrator

Statistics:
The experimental data were fitted by mulitexponential functions, using the method of least squares. The kinetic parameters of two- and three compartment models were calculated according to Gibaldi and Perrier (1975) and Wagner (1975) with a Hewlett-Packard 9820 A desk top calculator.

Results and discussion

Main ADME results
Type:
distribution
Results:
A half-life of 12.9 h was calculated. The final volume of distribution (3641 mL/kg) indicated drug accumulation inside cells, but equilibration between compartments was slow compared to drug elimination.

Toxicokinetic / pharmacokinetic studies

Details on distribution in tissues:
TRIS
Two min after the end of infusion the TRIS (pH 7.4) concentration in plasma was 550 µg/mL. The drug concentration then declined steadily and was only 2.9 µg/mL after 24 h. The drug exhibited three-compartment characteristics with long terminal half-life (12.9 h).
TRIS concentration in erythrocytes increased during the first hour after infusion. After 2 h drug levels in erythrocytes were about 2.5 times those in plasma and remained well above plasma levels during the rest of the observation period. This was found in all animals.
To calculate pharmacokinetic parameters a linear three-compartment open model with elimination from the central compartment was used. With these parameters the drug concentrations in the peripheral compartments were simulated.
The final volume of distribution (3641 mL/kg) indicated drug accumulation inside cells, but equilibration between the body compartments was slow compared to drug elimination. The rate constants of distriubtion to the peripheral compartments (0.5 h and 0.3 h) were smaller than the elimination constant (0.9 h).
A reasonable correspondence between the expected concentrations in the small fast-exchanging compartment and the drug levels observed in erythrocytes was obtained. The large discrepancies during the first minues after infusion, when irregular changes of TRIS concentrations were found, may be due to a partial equilibration between plasma and erythrocytes in the time between drawing a blood sample and centrifuging it.
To evaluate a possible influence of the pH of the TRIS solution on drug distribution, the experiments with TRIS solution of pH 10.9 were repeated in the same animals. The results were the same as with TRIS solution of pH 7.4 (data of pH 10.9 not shown in the publication).

Inulin
A biexponential decline of inulin plasma levels was found in experiments with netural alkaline TRIS. A half-life of 42 min and a final volume of distribution of 275 mL/kg was calculated.

Metabolite characterisation studies

Metabolites identified:
not measured

Any other information on results incl. tables

The volume of the central compartment (223 mL/kg) was similar to the volume of the inulin space (data not given). Therefore the authors concluded that TRIS initially distributes to the extracellular space. The final volume of distribution of TRIS (3641 mL/kg) however, is much larger than the inulin space (275 mL/kg), which can only be explained by an accumulation of the drug inside cells.

Intracellular accumulation would be expected from the physicochemical properties of TRIS. Since the drug is a weak base with a pK-value of 7.92, 70% of the administered amount exists in the cationic form at normal plasma pH. Assuming that only the undissociated from can pass the cell membrance, the normal transmembrane pH-gradient may create a 2 -3 -fold higher intracellular drug concentration due to pH-partitioning. The observed drug levels in erythrocytes and the drug concentrations calculated for the peripheral compartments fit into this concept. In situations of elevated extracellular pH the accumulation of TRIS inside cells should be even greater. But the buffer capacity of plasma probably prevented a long-lasting rise of extracellular pH after infusion of TRIS solution of pH 10.9 and thus the results obtained with both solutions were identical.

Applicant's summary and conclusion

Conclusions:
Interpretation of results (migrated information): bioaccumulation potential cannot be judged based on study results