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Toxicological information

Basic toxicokinetics

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Administrative data

Endpoint:
basic toxicokinetics in vitro / ex vivo
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study well documented, meets generally accepted principles, acceptable for assessment.

Data source

Reference
Reference Type:
publication
Title:
Studies of structure activity relationship of nitroxide free radicals and their precursors as modifiers against oxidative damage
Author:
Krishna, M.C., DeGraff, W., Hankovszky, O.H., Sár, C.P., Kálai, T., Jeko, J., Russo, A., Mitchell, J.B., Hideg, K.
Year:
1998
Bibliographic source:
J. Med.Chem. 41, 3477-3492

Materials and methods

Objective of study:
toxicokinetics
Test guideline
Qualifier:
no guideline followed
Principles of method if other than guideline:
The protective effects of stable nitroxides and their hydroxylamines were tested in Chinese Hamster V79 cells treated with either hydrogen peroxide or ionising radiation. Cytotoxicity was evaluated by monitoring the viability of the cells in a colonogenic assay.
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Type:
Constituent
Test material form:
solid: flakes

Administration / exposure

Details on exposure:
Chinese Hamster V79 cells were treated with either hydrogen peroxide 500 µM for 1 h or ionising radiation (dose: 12 Gy).
Details on dosing and sampling:
Cytotoxicity was evaluated by monitoring the viability of the cells by the colonogeninc assay.

Results and discussion

Metabolite characterisation studies

Metabolites identified:
not measured

Any other information on results incl. tables

The mean surviving fraction following hydrogen peroxide exposure alone was 0.14+-0.016. Possible modulation of H2O2modulated toxicity was evaluated using a fixed concentration of 100 µM present during the hydrogen peroxide exposure. In a pre-test with TEMPOL (4-Hydroxy TEMPO) this dose provided partial protection against toxicity, while concentrations of 1000 µM completely protected. The survival fraction after 12-GY radiation was 0.02 +- 0.002. Modulation was evaluated using a fixed final concentration of 10 mM present 10 min prior to radiation. In a pre-test with TEMPOL this concentration again provided partial protection.

By undergoing one electron transfer reactions nitroxides are readily reduced to the hydroxylamine or oxidised to the oxoammonium cation. The Nitroxide/oxoammonium cation behaves as an efficient electrochemically reversible redox pair and is involved in the catalytic decomposition of superoxide and induces catalase like activity in heme proteins. The hydroxylamine can function as a typical reducing agent to scavenge oxidants.

 

Most agents tested provided protection against hydrogen peroxide exposure. TEMPO had a protection factor of 3.5 whereas TEMPOL had a protection factor of 2.5. The unsubstituted TEMP-hydroxylamine had only a protection factor of 0.4. Against radiation, the protection factors were 4.2 for TEMPO, 5.1 for TEMPOL and 0.9 for the hydroxylamine.

 

The catalytic efficiency of superoxide dismutation 

(R-NO. + H++ HO2. →R-NO++ H2O2

RNO++ O2-. → RNO. + O2) depends on the ease of oxidation of the nitroxide to the corresponding oxoammonium cation (rate limiting step). Nitroxides can also prevent generation of oxidants via the Fenton reaction by maintaining the metals in an oxidised state. 

RNO. + H+ + Fe2+chelate → RNOH + Fe3+-chelate.

The reaction of hydroxylamines is a classic reduction:

R-NOH + .OH → RNO. + H2O

The nitroxide reduction product is then also able to interrupt chain propagation reactions of radical reactions.

The corresponding amines were generally not protective and in some cases enhanced hydrogen peroxide toxicity.

 

Nitroxides, and to a lesser extend hydroxylamines were protective against radiation. With the amines, in some cases, sensitisation to radiation was also observed. Nitroxides can provide radiation protection by radical scavenging and chemical repair. Hydroxylamines can also mediate both reactions and provide protective effects.

Applicant's summary and conclusion