Synthesis of ¹²⁵ I-reverse T₃ using a low and a high 3,3′-di-iodothyronine/¹²⁵I ratio

Abstract

The possibility was tested of obtaining ¹²⁵I-3,3′,5′-tri-iodothyronine (rT₃) with high specific activity and high stability for use in radioimmunoassays by employing a high 3,3′-di-iodothyronine (3,3′-T₂)/¹²⁵ I ratio during iodination. It was found that ¹²⁵I-rT₃ prepared by a high 3,3′-T₂/¹²⁵I ratio (molar ratio approximately 6.5:1) was stable, since only 6% of the ¹²⁵I was liberated as ¹²⁵I- during 18 weeks storage. In comparison the release of ¹²⁵I was more than 20% when a low 3,3′-T₂/¹²⁵I (1:2) was employed. However, while the specific activity of the low 3,3′-T₂/¹²⁵I-rT₃ tracer was 3.49 Ci/mg (2.27 Ci/μmol) when new and decreased to 2.40 Ci/mg (1.56 Ci/μmol) after 3 months, the specific activity of the high 3,3′-T,/¹²⁵I-rT₃ tracer was 710 mCi/mg (462 Ci/mmol) when new and decreased to 290 mCi/mg (189 Ci/mmol) after 3 months. In accordance with this the sensitivity of the radioimmunoassay was considerably less and decreasing much faster using the high 3,3′-T₂/¹²⁵I-rT₃ tracer. The cause for the low specific activity is probably dilution of the ¹²⁵I-tracer with ¹²⁷I-released from the 3,3′-T₂ during the oxidative iodination procedure.

Thus for obtaining high sensitivity in the rT₃ radioimmunoassay the ¹²⁵I-rT₃ prepared by a low 3,3′-T₂/¹²⁵I ratio should be employed—necessitating relatively frequent iodinations or repurification of the tracer after a certain period of storage.

Key Words:

• iodothyronine labeling
• reverse T₃ radioimmunoassay