The production and characterization of medium and high specific activity 125I-T3 and 125I-T4 utilizing substitution and exchange mechanisms

Article abstract of DOI:10.1002/jlcr.2580150166

The reactions of ¹²⁵I with 3,5-diiodo-L-thyronine (T₂), 3,3',5-triiodo-L-thyronine (T₃), and L-thyroxine (T₄) have been studied under various pH and concentration conditions. It was found that when the pH is 7.5 and approximately equimolar amounts of thyronine substrate and ¹²⁵I are reacted, T₂ produces as the major product monolabeled T₃ of high specific activity (~3300 mCi/mg) and long term stability while T₃ produces as the major product approximately 30% dilabeled ¹²⁵I-T₄ with short term stability and a specific activity of ~3000 mCi/mg. Simple methods for purification of this product to make it useful are discussed. When the pH is raised to 8.7 and the molar ratio of ¹²⁵I to thyronine substrate is increased to 1:6 then T₃ produces as the major product T₃ of medium specific activity (~500 mCi/mg) and long term stability while T₄ produces very stable monolabeled T₄ of medium specific activity (~250 mCi/mg). The mechanisms of substitution and exchange, as they relate to the formation of these and other side products in these reactions, are discussed.