Kill Two Birds with One Stone: An Efficient and Potent AR CBS-Targeted Degrader Reverses Breast Cancer Resistance via Concurrent AR Degradation and ERα Transcriptional Suppression

The mechanism of endocrine resistance in breast cancer (BC) extends beyond ESR1 mutations to include compensatory androgen receptor (AR) signaling. However, current therapeutic agents directed at the AR ligand-binding pocket (LBP) face the risk of failure due to mutations within this pocket. Herein, we reported a series of PROTACs targeting the AR coactivator binding site (AR-CBS) to overcome endocrine resistance in AR⁺ BC. Among which, degrader 18o could not only effectively degrade AR, but also potently inhibit the proliferation of MCF-7 and multiple mutant or resistant BC cells. Simultaneously, 18o effectively blocked estrogen receptor α (ERα) signaling through a dual mechanism involving ERα protein downregulation and suppression of its transcriptional activity. Importantly, 18o exhibited excellent antitumor activity in both MCF-7 and LCC2 xenografted models. This proof-of-concept study confirms that AR-CBS is a promising target for BC treatment and highlights degrader 18o as a potential candidate for overcoming the endocrine resistance.