Tributyl citrate

Basic toxicokinetics

Administrative data

Endpoint:

basic toxicokinetics in vitro / ex vivo

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Acceptable well documented study publication

Data source

Reference

Reference Type:

publication

Title:

Technical Report on the Metabolism of Acetyltributylcitrate (ATBC) and Tributylcitrate (TBC) in Human Serum and Rat Liver Homogenates

Author:

Davis P.

Year:

1991

Bibliographic source:

Univ. Texas, Austin, TX, USA (cited in US EPA (2003) High Production Volume (HPV) Chemicals Challenge Program: Assessment of Data Availability and Test Plan for Acetyl Tributyl Citrate (ATBC) (CAS NR 77-90-7))

Materials and methods

Objective of study:

metabolism

Test guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

The studies were performed to demonstrate that human serum and rat liver homogenates are capable of the metabolism of ATBC and its metabolite TBC and that butanol is a stoichiometric metabolite of both.

GLP compliance:

not specified

Test material

Test material information

Constituent 1

Reference substance name:

Tributyl citrate

EC Number:

201-071-2

EC Name:

Tributyl citrate

Cas Number:

77-94-1

Molecular formula:

C18H32O7

IUPAC Name:

tributyl citrate

Test material form:

not specified

Details on test material:

- Name of test material (as cited in study report): Tributyl citrate.

- Other substances tested: Acetyl tributyl citrate and n- butanol

Radiolabelling:

no

Remarks:

detection by GC

Test animals

Species:

other: not applicable (in vitro study)

Details on test animals or test system and environmental conditions:

not applicable

Administration / exposure

Route of administration:

other: in vitro incubation of test material with human serum and rat liver homogenates

Vehicle:

DMSO

Details on exposure:

- Human serum (50 ml) was secured from a single volunteer by venous puncture. The serum was stored at 4°C (never frozen) for three hours prior to use.
- Rat liver homogenate was obtained as follows: one 250 g adult rat was anaesthetised with ether and the liver removed. The liver was thoroughly washed (with phosphate buffer) and then minced and homogenized in the same buffer. The homogenate was centrifuged and the supernatant obtained. Protein determination indicated a protein concentration of 65 mg/ml. This homogenate was diluted with an additional 20 ml buffer to yield a final protein concentration of approximately 32 mg/ml. The homogenate was held at 4°C (never frozen) for 24 hours prior to use.

Duration and frequency of treatment / exposure:

single exposure

Doses / concentrations

Remarks:

Doses / Concentrations:
100 μg TBC/mL (252 nmoles/mL);
100 μg ATBC/mL (248 nmoles/mL);
14.8 μg n-butanol/mL (200 nmoles/mL).

No. of animals per sex per dose / concentration:

one human volunteer and one rat

Control animals:

other: not applicable

Positive control reference chemical:

no

Details on study design:

- Dose selection rationale: defined by author

Details on dosing and sampling:

METABOLITE CHARACTERISATION STUDIES
- Tissues and body fluids sampled: human serum and rat liver homogenate
- Time and frequency of sampling: single sample
- From how many animals: 1 human volunteer and 1 rat
- Method type(s) for identification: GC (detector is not reported)

Statistics:

not reported

Results and discussion

Main ADME results

Main ADME results 1

Type:

metabolism

Results:

An estimated half-life of 4 hours was obtained in human serum

Main ADME results 2

Type:

metabolism

Results:

A half-life of seconds could only be estimated in rat liver homogenates

Metabolite characterisation studies

Metabolites identified:

yes

Details on metabolites:

The metabolism of TBC in human serum showed an exponential decline in the levels of TBC with complete conversion observed in the 24 hour sample.
The metabolism of TBC in rat liver homogenate showed a nearly instantaneous and complete metabolism of TBC in 15 minutes. The metabolism was so rapid that the T0 data indicated only 35 μg/ml even though 100 μg/ml was added. A repeat incubation was conducted to try to capture an earlier time point in the conversion, but a comparable value (42 μg/ml) was again obtained. Thus, a half-life of seconds could only be estimated.

Any other information on results incl. tables

Metabolism of acetyl tributyl citrate in human serum

The metabolism of ATBC in human serum was a linear decline in the concentration of ATBC of the 48 hour period, after which only 25% of the starting material remained. An estimated half-life of 32 hours was obtained. In addition, only traces of TBC were detected from the deacetylation of ATBC to TBC.

Metabolism of acetyl tributyl citrate in rat liver homogenates

The metabolism of ATBC in liver homogenate was linear and rapid decline in the concentration of ATBC the first hour of the 9 hour period examined. From the slope of the linear decline, an estimated half-life of only 10 minutes can be obtained. Not even traces of TBC were detected from the deacetylation of ATBC to TBC was seen.

Results with butanol capillary GC analysis in human serum and rat liver homogenate

Butanol levels generated from ATBC were maximal at 1 to 2 hours, representing a level of 279 nmoles/ml at 2 hours. This represents a 37% (279 nmoles/750 nmoles) of the theoretical amount produced. Butanol levels generated from TBC, also maximal at 1 to 2 hours yielded 436 nmoles/ml, or 58% (436 nmoles/750 nmoles) of the theoretical amount. Therefore, with 3 moles of butanol theoretically produced from one mole of ATBC or TBC, the amounts observed were 1.11 mole equivalents from ATBC and 1.74 mole equivalents from TBC.

Applicant's summary and conclusion

Conclusions:

Interpretation of results (migrated information): no bioaccumulation potential based on study results
Tributyl citrate undergo rapid metabolism in both human serum and in rat liver homogenate.

Executive summary:

The studies were performed to demonstrate that human serum and rat liver homogenates are capable of the metabolism of ATBC and its metabolite TBC and that butanol is a stoichiometric metabolite of both. Dose levels of test material were as follows: 100 μg ATBC/mL (248 nmoles/mL), 100 μg TBC/mL (252 nmoles/mL) 14.8 μg n-butanol/ml (200 nmoles/mL). Both ATBC and the intermediate metabolite TBC undergo rapid metabolism in both human serum and rat liver homogenates. The half-lives are: 32 hours for ATBC, 4 hours for TBC and only seconds for n-butanol. It would be expected yielding the principal metabolites acetic acid, citric acid and butanol. The butanol would then be expected to further oxidize to butanoic acid and assimilated by ß-oxidation. Although a direct stoichiometry of butanol formed from ATBC and TBC was not observed, these results are partially explained based on the fact that butanol also is metabolized in the rat liver homogenate at a rate of 37 nmoles/ml/hr. It also may be suggested that an initial single or double debutylation may yield products which are less readily hydrolysed in the system; products which would be, as fully ionisable carboxylic acids, readily excreted in vivo (Author of report). The study provides additional information on the metabolism of ATBC (Hiser et al., 1992; Morflex, Inc.)