Login to Access Video or Poster Abstract: MP83-06
Sources of Funding: 2R01CA105304

Introduction

Following the results of the TOPARP-A Phase II trial, Olaparib, an oral PARP inhibitor was recently recognized as a FDA breakthrough therapy for metastatic castration recurrent prostate cancer (mCRPC) patients who have germline mutations in DNA repair genes. Although these results are noteworthy, the problem remains of how to treat the remaining mCRPC patients who do not have detectable germline mutations of DNA repair genes. Androgen receptor (AR) signaling regulates DNA repair in prostate cancer and AR modulating drugs induce DNA damage. Thus a combination approach using AR modulating drugs with a PARP inhibitor could be a promising option for the treatment of mCRPC. All currently approved AR modulating drugs, such as enzalutamide 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 (ARV-7, ARv567es) that are constitutively active and lack a LBD. EPI-002 is a first-in-class AR antagonist that targets both full-length AR and AR splice variants. Here we present data to support that a combination of Olaparib, a PARP inhibitor, and EPI-002 have beneficial effects in vitro.

Methods

Combination therapy using EPI-002 and Olaparib were evaluated in vitro using human prostate cancer cells, LNCaP (androgen sensitive and expresses full-length AR) and LNCaP95, an androgen-independent cell line that expresses full-length AR and AR-V7 and is resistant to enzalutamide. The effects of monotherapy and combination therapy on cell cycle and DNA damage were analysed using FACS and Western blot.

Results

Unexpectedly, EPI-002 caused an enormous decrease of Checkpoint kinase 1 (Chk1) protein levels in LNCaP and LNCaP95 cells. Chk1 is one of the important mediators in cell cycle checkpoint during the DNA damage response. Whereas, enzalutamide also decreased the expression of Chk1 in LNCaP, it had no effect on Chk1 levels in LNCaP95 cells. Consistent with these data, AR knockdown in LNCaP cells also decreased Chk1 levels. The PARP inhibitor, Olaparib, induced phosphorylation of Chk1. EPI-002 induced G1 cell cycle arrest whereas Olaparib induced G2/M cell cycle arrest. FACS analysis of γH2AX showed increased DNA damage with combination therapy compared to monotherapies.

Conclusions

EPI-002 decreased the expression of Chk1 in prostate cancer cells that expressed both full-length AR and AR-V7. Combination therapy of EPI-002 plus Olaparib may provide a therapeutic approach for prostate cancer that expresses AR-V7.

Funding

2R01CA105304