MP34-05 COMBINATION THERAPY FOR CASTRATION-RESISTANT PROSTATE CANCER USING ANTAGONISTS OF THE N-TERMINAL DOMAIN OF ANDROGEN RECEPTOR WITH IONIZING RADIATION

Abstract

INTRODUCTION AND OBJECTIVES: Emerging evidence supports that androgen receptor (AR) signaling regulates DNA repair in prostate cancer. Therefore, a combination approach using AR modulating drugs with radiation could be a promising option for the treatment of metastatic castration resistant prostate cancer (mCRPC). All currently approved AR modulating drugs, such as enzalutamide (ENZA) and abiraterone, either directly or indirectly target the AR C-terminus ligand-binding domain (LBD). Such drugs are often unsuccessful due to the emergence of AR splice variants (e.g., AR-V7) that are constitutively active and lack a LBD. EPI- 002 is a first-in-class AR antagonist that binds to its N-terminus domain to inhibit the transcriptional activities of both full-length AR and AR splice variants. A next generation compound, EPI-7170 has been developed. Here we present data to support that a combination of EPI compounds and ionizing radiation maybe beneficial for the treatment of mCRPC. METHODS: Androgen-independent LNCaP95 cells that are resistant to ENZA and endogenously express both full-length AR and ARV7 were used. The effect of EPI compounds on the expression of DNA repair genes was measured using TaqMan array and Western blot analysis. Monotherapies versus combination therapies using EPI-002, EPI-7170, or ENZA, with ionizing radiation were evaluated for effects on proliferation, colony formation, cell cycle and DNA damage using BrdU incorporation, FACS and Western blot and immune fluorescent staining. RESULTS: EPI-7170 was more potent than EPI-002. Both EPI- 002 and EPI-7170 decreased expression of DNA repair genes contrary to ENZA. EPI-002 induced G1 cell cycle arrest whereas radiation induced G2/M cell cycle arrest. FACS analysis revealed a dosedependent decrease of BrdU incorporation with increased accumulation of gammaH2AX with combination therapy. A synergistic inhibitory effect on proliferation of ENZA-resistant LNCaP95 cells was achieved with a combination of EPI compounds with ionizing radiation. CONCLUSIONS: Combination therapy with radiation and an antagonist to the AR NTD such as EPI-002 or EPI-7170 that inhibits the transcriptional activities of both AR-Vs and full-length AR, may provide a new therapeutic approach for mCRPC.