Background: Rad is an alpha particle emitter that causes DNA double strand breaks and has been FDA-approved for use in mCRPC pts with bone metastases. PARP-1 activity critically supports androgen receptor (AR) activity in mCRPC and potentiates AR-dependent DNA damage response pathways that promote prostate cancer cell survival. Nira is a potent and selective PARP-1/2 inhibitor that has shown single agent clinical activity in mCRPC. We previously reported the safety of targeting the PARP-1/AR axis with Nira in combination with Rad. Herein we describe the results of an exploratory biomarker analysis. Methods: The primary objective of NiraRad is to determine the optimum Ph II dose of Nira plus Rad (55 kBq/kg of body weight IV every 4 weeks (wks) x 6) in pts with and without prior chemotherapy (docetaxel). Pts were enrolled to one of three dose levels of Nira (100, 200, or 300 mg PO daily). All cohorts were combined for exploratory biomarker analysis using Nanostring PanCancer Driver and Immune Pathways panels and the nSolver Advanced analysis module was performed on blood obtained from 23 pts at baseline, cycle (C) 1 day (D) 15, and C3D15. A favorable response was defined as any PSA reduction at week 12 or treatment (tx) duration > 18 wks, the median time on tx in the cohort of pts analyzed. A threshold of > 2 fold (X) differentially expressed genes was used. Results: Of the 23 pts with biomarker data, 7 (30%) experienced PSA reductions and 11 (48%) received tx for > 18 wks, 6 of which also had PSA reductions. Exploratory analysis revealed that the PI3K/Ras, MAPK, and transcriptional misregulation pathways were differentially regulated in pts who had favorable responses. The top downregulated gene, PAX5, which has been shown to promote prostate cancer growth, was decreased at C1D15 (2.7X, p < 0.01) and C3D15 (4.8X, p < 0.001) in pts with tx duration > 18 wks and at baseline in pts who had PSA reductions (3.1X, p < 0.05). Immune pathways analysis suggested downregulation of immunosuppressive B-cell (plasma cell) and upregulation of NK and T cell pathways in pts with tx duration > 18 wks. Conclusions: Previously Nira and Rad have been shown to have acceptable tolerability in mCRPC pts. This exploratory analysis suggests potential response biomarkers that warrant further investigation. Managed by: the Prostate Cancer Clinical Trials Consortium; Funded by: Janssen Pharmaceuticals and Bayer Healthcare Pharmaceuticals, Inc. Clinical trial information: NCT03076203