Design and optimization of Oestrogen Receptor PROTACs based on 4-hydroxytamoxifen

In the last four decades, treatment of oestrogen receptor positive (ER+) breast cancer (BCa), has focused on targeting the estrogenic receptor signaling pathway. This signaling function is pivotal to sustain cell proliferation. Tamoxifen, a competitor inhibitor of oestrogen has played a major role in therapeutics. However, primary and acquired resistance to hormone blockade occurred in a large subset of these cancers, and new approaches were urgently needed. Aromatase inhibitors and receptor degraders were approved and alternatively used. Yet, resistance appears in the metastatic setting. Here we report the design and synthesis of a series of proteolysis targeting chimeras (PROTACs) that induce the degradation of estrogen receptor alpha in breast cancer MCF-7 (ER+) cells at nanomolar concentration. Using a warhead based on 4-hydroxitamoxifen, bifunctional degraders recruiting either cereblon or the Von Hippel Lindau E3 ligases were synthesized. Our efforts resulted in the discovery of TAM-VHL-1, a potent ERα degrader (DC50: 4.5 nM) that we envisage as a useful tool for biology study and a platform for potential therapeutics.