Estrogen receptor based imaging agents. 1. Synthesis and receptor binding affinity of some aromatic and D-ring...

Steroidal and nonsteroidal estrogens substituted with halogens ortho to the phenolic hydroxyl group in the D ring at C-16 have been prepared as potential estrogen receptor-based imaging agents for human breast tumors. Estrogens bearing an aromatic fluorine ortho to a phenolic hydroxyl group were prepared by the Schiemann reaction on the corresponding methyl esters; other ortho-halogenated estrogens were prepared by direct halogenation. Steroidal estrogens substituted at the 16 alpha position were prepared by halogenation of estrone 3-acetate (17-enol acetate) followed by hydride reduction, and those substituted at the 16 beta position were prepared by epimerization prior to reduction. The binding affinity of these halogenated estrogens to the uterine estrogen receptor was measured relative to that of [3H]estradiol by a competitive binding assay. All of the monosubstituted ortho-fluorinated estrogens show very high binding affinity for the receptor (64--250% that of estradiol). The monosubstituted and symmetrically disubstituted bromo- and iodohexestrols and 2- and 4-substituted estradiols have binding affinities considerably lower than those of the fluoro compounds, the 4-substituted estradiols have affinities greater than the corresponding 2-substituted isomers. Introduction of a halogen (Cl, Br, I) at the 16 alpha position of 17 beta-estradiol results in compounds with receptor affinities comparable to that of 17 beta-estradiol itself; the 16 beta-epimers and the estrone derivatives are bound less well. Thus, provided that they can be labeled with suitable gamma-emitting radioisotopes at sufficiently high specific activity, it appears that the A-ring fluoroestrogens and 16 alpha-bromo- and 16 alpha-iodoestradiol-17 beta are excellent candidates for receptor-based imaging of human breast tumors.