Abstract 5679: Androgen receptor degraders overcome common resistance mechanisms developed during prostate cancer treatment

Background: Androgen receptor (AR) signaling is critical for prostate development and homeostasis as well as for the initiation and progression of prostate cancer (PCa). Indeed, the clinical development of second-generation AR antagonists, including enzalutamide, has confirmed that AR remains a key oncogene in castration-resistant prostate cancer (CRPC). Furthermore, response to enzalutamide is temporary and incremental, and PCa cells that develop resistance to AR-targeted therapy usually maintain AR expression. Therefore, with the hypothesis that AR protein is still active even during castration- and enzalutamide-resistant states, we employed the proteolysis targeting chimera (PROTAC) strategy to build compounds that targeted the AR through proteasomal degradation. We developed an optimized functional molecule, ARD-61, that targets enzalutamide-resistant and AR splice variant (including AR-V7, which lacks the ligand binding domain) overexpressing cells in vitro and in vivo. Methods/Results: To characterize ARD-61, we performed western blotting, qRT-PCR, growth assays, xenograft studies, as well as proteomic and RNA-sequencing analyses. Additionally, to investigate the sensitivity of AR splice variants to ARD-61, we created CRISPR-Cas9 mediated stable AR-V7 overexpression in LNCaP cells. ARD-61 functions to rapidly and specifically degrade AR protein, and thus decreases downstream AR signaling in a wide variety (e.g. enzalutamide sensitive, AR-mutated, enzalutamide resistant) of AR-positive cell lines and tumor xenografts. Enzalutamide-sensitive and -resistant cell lines were similarly sensitive to ARD-61 in vivo and in vitro. CRISPR-induced endogenous overexpression of AR-V7 had no effect on the sensitivity of cells to ARD-61 or enzalutamide. Conclusions: Our data reveal the necessity of full-length AR in all Pca stages. Despite continued AR-antagonization, enzalutamide-resistant cell lines require full-length AR for sustained survival. Further, AR-variant overexpression, previously shown to promote resistance to AR-targeted therapies, still required AR protein with an intact ligand-binding domain to maintain growth in our models. Our data illustrate the importance of continued targeting of AR in advanced Pca, as well as demonstrate the efficacy of ARD-61. Ultimately, through its clinical translation, we anticipate the development of ARD-61 to be a therapeutic advance for patients with the metastatic CRPC. Citation Format: Steven Kregel, Chao Wang, Xin Ha