PROTAC-induced BET protein degradation as a therapy for castration-resistant prostate cancer

Significance We describe the development of a small molecule that mediates the degradation of bromodomain and extra-terminal (BET) proteins and its application in the treatment of castration-resistant prostate cancer (CRPC). Few therapeutic options exist to treat CRPC, especially CRPC tumors expressing constitutively active androgen receptor (AR) splice variants that lack the ligand-binding domain and can effect androgen-independent transactivation of target genes. Importantly, we demonstrate that targeted degradation of BET proteins using proteolysis-targeting chimera (PROTAC) technology causes cell death in cultured prostate cancer cells and results in tumor growth inhibition or regression in mouse models of CRPC, including models that express high levels of AR splice variant 7. Our work thus contains a significant potential therapeutic advance in the treatment of this cancer. Prostate cancer has the second highest incidence among cancers in men worldwide and is the second leading cause of cancer deaths of men in the United States. Although androgen deprivation can initially lead to remission, the disease often progresses to castration-resistant prostate cancer (CRPC), which is still reliant on androgen receptor (AR) signaling and is associated with a poor prognosis. Some success against CRPC has been achieved by drugs that target AR signaling, but secondary resistance invariably emerges, and new therapies are urgently needed. Recently, inhibitors of bromodomain and extra-terminal (BET) family proteins have shown growth-inhibitory activity in preclinical models of CRPC. Here, we demonstrate that ARV-771, a small-molecule pan-BET degrader based on proteolysis-targeting chimera (PROTAC) technology, demonstrates dramatically improved efficacy in cellular models of CRPC as compared with BET inhibition. Unlike BET inhibitors, ARV-771 results in suppression of both AR signaling and AR levels and leads to tumor regression in a CRPC mouse xenograft model. This study is, to our knowledge, the first to demonstrate efficacy with a small-molecule BET degrader in a solid-tumor malignancy and potentially represents an important therapeutic advance in the treatment of CRPC.