Preparation of no-carrier-added [124I]A14-iodoinsulin as a radiotracer for positron emission tomography

A₁₄-Iodoinsulin is a close biochemical and pharmacological mimic of insulin. Human insulin was labelled in its A chain tyrosine-14 residue by direct iodination with the positron-emitter iodine-124 (β⁺ = 25.6%; t_1/2 = 4.15 days) to provide a radiotracer for imaging with positron emission tomography (PET). Several reagents were compared for conversion of cyclotron-produced [¹²⁴I]iodide into a reactive species for the labelling reaction. Radiochemical yields from the use of Iodo-Gen®, Chloramine-T, N-bromosuccinimide or lactoperoxidase-hydrogen peroxide were similar [35% (n = 1), 33 ± 9% (n = 10), 32 ± 11% (n = 25) or 33% (n = 2), respectively]. [¹²⁴I]A₁₄-Iodoinsulin was separated from unreacted insulin and radioactive by-products by tandem reverse phase HPLC and rapidly formulated for intravenous injection by adsorption on a Sep-Pak ^tC18-Plus® cartridge, followed by elution with 10 mM hydrochloric acid-ethanol (1:1 v/v, 1 ml). This radiotracer can now be obtained in useful radioactivities at high effective specific radioactivity and is now being applied to PET studies of its biodistribution in living subjects.