Fluorescence theranostic PROTACs for real-time visualization of ERα degradation

Proteolysis targeting chimera (PROTAC) technology, a groundbreaking strategy for degradation of pathogenic proteins by hijacking of the ubiquitin-proteasome-system has become a promising strategy in drug design. However, the real-time monitoring and visualization of protein degradation processes have been long-standing challenges in the realm of drug development. In this research, we sought to amalgamate the highly efficient protein-degrading capabilities of PROTAC technology with the visualization attributes of fluorescent probes, with the potential to pave the path for the design and development of a novel class of visual PROTACs. These novel PROTACs uniquely possess both fluorescence imaging and therapeutic characteristics, all with the goal of enabling real-time observations of protein degradation processes. Our approach involved the utilization of a high ER-targeting fluorescent probe, previously reported in our laboratory, which served as a warhead that specifically binds to the protein of interest (POI). Additionally, a VHL ligand for recruiting E3 ligase and linkers of various lengths were incorporated to synthesize a series of novel ER-inherent fluorescence PROTACs. Among them, compound A3 demonstrated remarkable efficiency in degrading ERα proteins (DC₅₀ = 0.12 μM) and displaying exceptional anti-proliferative activity against MCF-7 cells (IC₅₀ = 0.051 μM). Furthermore, it exhibited impressive fluorescence imaging performance, boasting an emission wavelength of up to 582 nm, a Stokes shift of 116 nm, and consistent optical properties. These attributes make it especially suitable for the real-time, in situ tracking of ERα protein degradation processes, thus may serve as a privileged visual theranostic PROTAC for ERα⁺ breast cancer. This study not only broadens the application spectrum of PROTAC technology but also introduces a novel approach for real-time visualization of protein degradation processes, ultimately enhancing the diagnostic and treatment efficacy of PROTACs.