Presentation Authors: Yukiyoshi Hirayama*, Kunzhong Jian, Raymond J. Anderson, Marianne D. Sadar, Vancouver, Canada
Introduction: Expression of androgen receptor splice variants (AR-Vs) is associated with resistance to current androgen deprivation therapies and antiandrogens. Constitutively active AR-Vs lack the ligand binding domain of androgen receptor (AR-LBD) which is the direct or indirect target of anti-androgens such as enzalutamide (ENZ) and abiraterone which blocks steroidogenesis. Recently the PLATO trial showed that the addition of abiraterone to ongoing ENZ did not improve progression-free survival in patients with castration resistant prostate cancer (CRPC). These data imply a limitation of combining AR-LBD targeting therapies. Therefore, novel therapeutic approaches are needed to improve the outcomes of CRPC patients. Both AR and truncated AR-Vs require a functional N-terminal domain (NTD) for activity and hence first-in-class antagonists of the AR NTD have been developed such as EPI-002 (ralaniten). Herein a next-generation EPI compound (EPI-7170) was evaluated as a monotherapy or in combination of ENZ in ENZ resistant prostate cancer cells that express AR-Vs.
Methods: We developed ENZ resistant VCaP cells (VCaP-MDVR) by chronic exposure to ENZ. These cells and C4-2B-MDVR cells were used as ENZ resistant prostate cancer cells to test monotherapies versus combination therapy with ENZ and EPI-7170. BrdU incorporation, clonogenic assays and flow cytometry were used to analyze effects on proliferation and cell cycle. Inhibition of expression of AR and AR-V7 target genes by ENZ, EPI-7170 or a combination was analyzed by qPCR.
Results: EPI-7170 had 10 times better potency than EPI-002 as measured using proliferation and PSA-reporter gene assays. VCaP-MDVR cells expressed significantly higher levels of AR-V7 protein and mRNA compared to the parental cell line. Knockdown of AR-V7 restored sensitivity of VCaP-MDVR cells to ENZ. A combination of EPI-7170 and ENZ caused synergistic inhibition of proliferation of ENZ resistant cells. Consistent results were also obtained with the clonogenic assay. While ENZ did not inhibited AR-V7 target genes, EPI-7170 inhibited both AR and AR-V7 target genes. A combination of EPI-7170 with ENZ led to a complete inhibition of DNA synthesis in S phase.
Conclusions: The role of AR-Vs in the mechanism of ENZ resistance was provided by knockdown experiments and response to EPI-7170. Synergic inhibition was achieved with a combination of EPI-7170 with ENZ. These results suggest that targeting AR-NTD in addition to AR-LBD to block both FL-AR and AR-Vs could be a potential treatment option for CRPC.
Source of Funding: US National Cancer Institute (R01 CA105304) awarded to Marianne D. Sadar