A Subtype-specific Probe for Estrogen Receptor Alpha (ERα) Reveals Imaging of PROTAC-induced Degradation

Hormone receptor-positive breast cancers depend on estrogen receptor (ER) signaling for growth, yet the chemistry to directly visualize ERα dynamics under pharmacological perturbation remains limited. Proteolysis targeting chimera (PROTAC) offers an alternative approach by promoting degradation of ERα rather than inhibiting alone. Here, we present the design and chemical development of a nonsteroidal ERα-targeted probe, ERi-FL, enabling real-time visualization of ERα-specific dynamics in live cells, tissues, and animal models. ERi-FL derived from a diaryltetrahydronaphthalene scaffold, exhibits high-affinity for ERα (IC 50 = 3-5 nM) and bright imaging contrast with pronounced selectivity over ERbeta (ERβ). Based on relative binding affinity (RBA) to estradiol, ERi-FL exhibits greater than 14-fold selectivity for ERα over ERβ, and greater than 11-fold relative to lasofoxifene. At nanomolar concentrations, ERi-FL permits imaging of ERα under hormone- and estrogen-regulated conditions, allowing insights into ER signaling in biological systems. Applied in the context of PROTAC-induced therapy, ERi-FL revealed ERα degradation in MCF-7 via imaging, with signal intensities tightly correlated to ERα levels. We demonstrate that ERi-FL provides a chemically defined platform for interrogating ERα pharmacology directly. We show it to measure target occupancy, assess subtype selectivity, and evaluate other therapeutic strategies, including imaging the efficacy of targeted protein degraders, such as PROTACs in ER+ cancers.