Advances in the development of a targeted N-Terminal Domain androgen receptor (AR) degrader (ANITAC) for the treatment of prostate cancer

Background: Androgen receptor (AR) signaling is a main driver of prostate cancer progression and remains a crucial target for therapeutic intervention even in late stages of the disease. While current antiandrogen therapies that directly or indirectly target the AR ligand binding domain (LBD) are initially effective, resistance ultimately develops and new methods of inhibiting the AR pathway are needed. Recent advances in targeted protein degradation using the PROteolysis TArgeting Chimera (PROTAC) technology demonstrate that AR-LBD targeted PROTACs can selectively degrade AR but not splice variant forms of AR that are found in advanced castration-resistant prostate cancer (CRPC) patients. We recently demonstrated that targeting the N-terminal domain (NTD) of the AR by anitens (small molecule inhibitors) represents a novel method of blocking AR signaling that can bypass LBD-related drug resistance mechanisms. By developing an aniten-based bifunctional degrader (ANITAC™ for ANITen bAsed Chimera), our goal is to eliminate any forms of AR proteins found in CRPC patients including LBD mutants and LBD truncated variants that can potentially drive disease progression. Material and Methods: Selective AR degradation was monitored by various in vitro assays. AR transcriptional activity and selectivity was assessed in cellular models expressing different forms of AR using reporter and cell viability assays. NanoString was used to qualitatively demonstrate ANITAC activity on AR signaling. In vivo efficacy was assessed using mouse xenograft models of prostate cancer. Results: Herein we report the first series of AR degraders, ANITACs, which target the NTD of AR. ANITACs degrade AR in all cell lines tested through an E3 ligase dependent mechanism with an observed 50% degradation concentration (DC50) in the low nanomolar range measured using HiBiT assays. ANITACs degrade clinically relevant AR mutants and splice variants and show robust inhibition of AR transcriptional activity in multiple cell lines expressing different forms of AR including AR full length (AR-FL), AR-V7, and AR-V567es. In addition, AR degradation mediated by ANITACs effectively suppresses AR-regulated gene expression in both LNCaP and LNCaP95 cells and decreases prostate cancer cell viability. Moreover, ANITACs induce degradation of AR-FL and AR-V7 in vivo and induce tumor regression in castrated mice bearing VCaP tumor xenografts. In vitro ADME (Absorption, Distribution, Metabolism, and Excretion) studies show the compounds can be metabolically stable and exhibit favorable ADME properties needed for oral dosing. Conclusion: ANITAC molecules are the first bifunctional degraders targeting the AR NTD that can be orally bioavailable and active against all forms of AR expressed in late stage CRPC patients. Conflict of interest: Other Substantive Relationships: All authors listed are shareholders/ employees of ESSA Pharma.