Covalent Destabilizing Degrader of AR and AR-V7 in Androgen-Independent Prostate Cancer Cells

Androgen-independent prostate cancers, correlated with heightened aggressiveness and poor prognosis, are caused by mutations or deletions in the androgen receptor (AR) or the expression of truncated variants of AR that are constitutively activated. Currently, drugs and drug candidates against AR target the steroid-binding domain to antagonize or degrade AR. However, these compounds cannot therapeutically access largely intrinsically disordered truncated splice variants of AR, such as AR-V7, which only possess the N-terminal transactivation domain and DNA-binding domain and are missing the ligand-binding domain. Targeting intrinsically disordered regions within transcription factors has remained challenging and is considered "undruggable". Herein, we leverage a cysteine-reactive covalent ligand library in a cellular screen to identify the degraders of AR and AR-V7 in androgen-independent prostate cancer cells. We identified a covalent compound, EN1441, that selectively degrades AR and AR-V7 in a proteasome-dependent manner through direct covalent targeting of intrinsically disordered cysteine C125 in the N-terminal transactivation domain of AR and AR-V7. EN1441 causes significant and selective destabilization of AR and AR-V7, leading to the aggregation of AR/AR-V7 and subsequent proteasome-mediated degradation. Consistent with targeting both AR and AR-V7, we find that EN1441 completely inhibits total AR transcriptional activity in androgen-independent prostate cancer cells expressing both AR and AR-V7 compared with AR antagonists or degraders that only target the ligand-binding domain of full-length AR, such as enzalutamide and ARV-110. Our results put forth a pathfinder molecule EN1441 that targets an intrinsically disordered cysteine within AR to destabilize, degrade, and inhibit both AR and AR-V7 in androgen-independent prostate cancer cells and highlights the utility of covalent ligand discovery approaches in directly targeting, destabilizing, inhibiting, and degrading classically undruggable transcription factor targets.