Androgen deprivation therapy is an efficient first line therapy for advanced prostate cancer. However, in most cases resistance inevitably develops resulting in metastatic castration-resistant prostate cancer (mCRPC). Abiraterone and enzalutamide are approved to treat mCRPC and act by inhibiting androgen biosynthesis and androgen receptor (AR) signaling, respectively. Poly (ADP-ribose) polymerase 1 (PARP1) is a key enzyme in the DNA damage repair pathway and has also been implicated in AR signaling cascade. Recent reports suggest significant efficacy combining anti-androgens and inhibitors of Poly (ADP-ribose) polymerase (PARP) particularly against cells harboring mutations in the DNA repair pathway. Here, we provide evidence of synergistic cytotoxicity combining PARP inhibition and androgen signaling ablation in the AR positive human CRPC cell line 22RV1 harboring mutation in the DNA repair gene BRCA2. The PARP inhibitor talazoparib (IC50 - 0.05 μM) was upto 20 times more potent than olaparib (IC50 – 1.0 μM) in inducing cell death in 22RV1 as seen by cell viability and clonogenic assays. When combined with AR antagonist enzalutamide (IC50 – 0.25 μM) or androgen synthesis inhibitor abiraterone (IC50 – 1.25 μM) both PARP inhibitors showed significant synergy, although talazoparib was more prominent owing to its superior potency. Higher concentrations of enzalutamide and abiraterone (>10μM) induced PARP cleavage and increased expression of γ-H2AX, a marker for DNA damage. Our results suggest that AR signaling inhibition involves PARP mediated apoptosis and highlight the crosstalk between PARP and AR pathways. In summary, combining anti-androgen therapy and PARP inhibitors carry significant therapeutic potential for CRPC patients harboring mutations in DNA repair pathway.

Citation Format: Sahithi Pamarthy, Vinay Sagar, Rajita J. Vatapalli, Zachary Reinstein, Muhammad Zayd Ansari, Benedito A. Carneiro Filho, Francis J. Giles, Sarki A. Abdulkadir. Combining anti-androgen therapy and PARP inhibition results in synergistic cytotoxicity of metastatic castration-resistant prostate cancer (mCRPC) cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1114. doi:10.1158/1538-7445.AM2017-1114