A radiotracer method to study efflux transport of iodide liberated from thyroid hormones via deiodination metabolism in the brain

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Abstract

Aims:

Thyroid hormones (TH) play an important role in the development and functional maintenance of the central nervous system. The purpose of this study was to develop a radiotracer method for studying the in vivo efflux transport of iodide liberated by the TH metabolism in the brain. The rationale of our method is as follows: a radioiodinated compound can enter the brain and rapidly release iodide in situ; the iodide efflux rate can be estimated from the clearance of brain radioactivity after disappearance of the iodinated compound.

Main methods:

6-[125I]Iodo-9-pentylpurine ([125I]9Pe6IP) was designed to enter the brain and release 125I− by the reaction with glutathione and synthesized from the corresponding bromo derivative in a Br/125I exchange reaction. The brain kinetics of radioactivity and radioactive metabolites were investigated after intravenous injection of [125I]9Pe6IP into mice. The iodide efflux rate was estimated in mice pretreated with perchlorate, an inhibitor of iodide transport from the brain.

Key findings:

High brain uptake (5.3% injected dose/g) was observed at 1 min, and almost complete conversion of [125I]9Pe6IP to 125I− occurred 10 min after injection. The 125I− uptake from the blood was negligible. 125I− was eliminated from the brain along a single-exponential curve with a half-life of 6.0 min. Furthermore, dose-dependent inhibition of 125I− efflux was observed in mice pretreated with perchlorate.

Significance:

We conclude that 9Pe6IP labeled with 124I (positron emitter) or 123I (single-photon emitter) may be useful for studying the in vivo efflux transport of iodide in the brain using nuclear medicine imaging devices.

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