The Synthesis of 125I-2-Iodohistamine for Radioimmunoassay1

Abstract

Recently radioactively labeled iodohistamines have been claimed to have superior shelf-life to the iodophenols more commonly used in radioimmunoassay of small molecules. This claim is based largely on theoretical considerations; no systematic study has appeared. We found that iodination of histamine on macroscale proceeds rapidly at pH 7–8.4 to yield principally 2-iodohistamine. With large excess of iodine, substantial diiodi-nation can be achieved. In 0.5 N sodium hydroxide solution, tri-iodination produces 1,2,5-triiodohistamine; the N-I bond, how-ever, is somewhat labile. ¹²⁵I-2,5-Diiodohistamine is also some-what unstable, having first order decomposition rate of 1.6 × 10^?3 day^?1 (t^1/2, 182 days), while ¹²⁵I-2-iodohistamine shows barely perceptible change in 60 days (7.5 × 10^?5 day^?1) The assignment of the first iodine introduced to C-2 is based on a comparison of the NMR spectra of monoiodohistamine and histamine. Iodination with carrier-free iodine-125 using the Hunter-Greenwood procedure (chloramine-T) produces a 76% yield of mono- and a 19% yield of diiodo product which are easily isolable by a single TLC using silica gel in the solvent system, ethano1:ethyl ether:water, 5:5:2.

Nothing has appeared on the chemistry of iodination of histamine. Since the proposal of Jeffcoate et al.⁴ that iodohistamines might be more stable than iodophenols, notably iodotyrosine derivates, a number of investigators have used ¹²⁵I-iodohistamines for the radioimmunoassay of estradiol-17β⁵, progesterone^6–9, norethisterone^6,10,11, testosterone⁶ and bile acids¹². Usually an undefined mixture of iodohistamines has been coupled to the assay ligand.

In this paper we report the preparation of standards (2-, 2,5- and l,2,5-iodohistamines) and the effect of experimental parameters on the synthesis of ¹²⁵I-2-iodohistamine and ²⁵¹I-2,5-diiodohistamine.