High Affinity Fluorescent Ligands for the Estrogen Receptor

Fluorescent binders of the estrogen receptor (ER) are used in binding assays and in detection or imaging studies. However, fluorescence labelling of ER ligands usually leads to substantial decreases in binding affinity. In this study, we describe the development of high affinity fluorescent ER ligands. Cyanine dyes of the MiDye series were directly attached to the SERMs 4-hydroxytamoxifen (OHT) and raloxifene (Ral); linkers were deliberately omitted. This approach yielded conjugates with ER binding affinities superior to the natural ligand estradiol. The OHT- and Ral-MiDye conjugates emitted in the 600-800 nm range. First round staining experiments showed that the conjugates, but not the dyes alone, accumulate in cells expressing estrogen-binding receptors.