[¹²⁵I]Iododesethyl tamoxifen aziridine: Synthesis and covalent labeling of the estrogen receptor with an iodine-labeled affinity label

Iododesethyl tamoxifen aziridine (I-Tam-Az), an analog of the estrogen receptor-affinity label tamoxifen aziridine (Tam-Az) in which the ethyl group has been replaced by an iodine, has been prepared by two routes: (a) metallation of a bromotriarylethylene system, followed by reaction with iodine, and aziridinylation, and (b) direct iodination of a trimethylstannyl triarylethylene system that is the immediate precursor of I-Tam-Az. The latter method can be used to prepare [¹²⁵I]I-Tam-Az rapidly and in good yield, both at carrier-added and no-carrier-added levels; specific activities greater than 200 Ci mmol have been obtained. In competitive radiometric binding assays with the estrogen receptor, I-Tam-Az has an apparent affinity of ca. 20%, equivalent to that of Tam-Az. It also undergoes rapid and selective time-dependent, irreversible binding to the estrogen receptor. [¹²⁵I]I-Tam-Az reacts covalently with estrogen receptor in uterine cytosol preparations; its attachment is rapid and efficient, but somewhat less selective than that of Tam-Az. Estrogen receptor in intact MCF-7 human breast cancer cells can also be labeled with [¹²⁵I]I-Tam-Az, and autoradiographic analysis of salt extracts of labeled nuclear estrogen receptor on SDS-polyacrylamide slab gels shows highly selective labeling of a 65K protein. [¹²⁵I]I-Tam-Az is an efficient, selective affinity label for the estrogen receptor, available at high specific activity, and should be useful in studies on estrogen receptor structure, dynamics, and chromatin interactions.