Synthesis and estrogen receptor affinity of a 4-hydroxytamoxifen-labeled ligand for diagnostic imaging

A 10-step synthesis of a novel 4-hydroxytamoxifen-DTPA ligand (HOTam-DTPA) is reported. Tamoxifen and its primary metabolite 4-hydroxytamoxifen are common estrogen receptor ligands. Consequently, tamoxifen has found utility as the targeting component of various diagnostic agents for selective imaging of estrogen receptor-rich tissue, specifically breast cancer. An L-aspartic acid-derived DTPA analogue was attached to the ethyl side chain of 4-hydroxy-tamoxifen using N,N′-dimethylethylenediamine as a hydrophilic linker. A competitve estrogen receptor binding assay using [³H]-17β-estradiol was performed to determine the effect of the ethyl side chain modification on estrogen receptor affinity. The results show that while the relative affinity of HOTam-DTPA for the estrogen receptor is ∼10-fold lower than that of tamoxifen, it still remains a potent ligand at relatively low concentrations.