Preparation of the N-(4'-hydroxy-[3'-125I]iodophenethyl)-6-(4-O-diethylstilbestryl)hexanam ide for diethylstilbestrol radioimmunoassays.

The synthesis of a radioiodinated diethylstilbestrol (DES) derivative is described. This derivative was prepared by coupling the previously synthesized active ester of 6-(4-O-diethylstilbestryl)hexanoic acid with mono-[125I]iodotyramine in dry tetrahydrofuran (20 to 22 C, 16 hours). The mono-[125I]iodotyramine was prepared using a chloramine-T method and purified by paper electrophoresis. The final product, N-(4'-hydroxy-[3'-125I]iodophenethyl)-6-(4-O- diethylstilbestryl)hexanamide, was separated by thin-layer chromatography (cyclohexane/ethanol/NH4OH 2.5 N/acetone; 40:50:5:20, v/v/v/v); it was stable for 2 months in ethanol at 4 C and had a specific activity higher than 540 Ci/mmol. The [125I]DES amide synthesized was found to retain the immunoreactivity of DES, since it competed with [3H]DES or DES in an in vitro radioimmunoassay system for the binding sites of a rabbit anti-DES antibody; thus, it seems to be capable of replacing the tritiated tracer used so far in DES radioimmunoassays.