Background: Immune checkpoint blockade (ICB) therapies have had a major impact across a wide range of cancers. However, only subsets of patients across all malignancies benefit from ICB. In particular, metastatic castrate-resistant prostate cancers (mCRPC) have shown very limited responses to ICB. While there is ongoing work to identify predictive biomarkers to ICB responsiveness, early preclinical data from our group suggests that targeting fundamental DNA repair pathways could markedly increase the fraction of patients responsive to immunotherapeutic interventions. Based on these preclinical studies, we are conducting an investigator-initiated Phase Ib/IIa co-clinical trial of rucaparib and nivolumab singly and in combination, in mCRPC patients. Methods: Patients are randomized to one of three arms – rucaparib, nivolumab, or both drugs in combination for 4 weeks. Metastatic biopsy samples are collected at baseline and after 4 weeks on treatment, after which all arms switch to combination therapy. The primary objective is to assess feasibility of the combination, and to elucidate changes in T cell infiltration by RNA-seq analysis using established T-cell non-inflamed and inflamed gene signatures. Secondary objectives are to assess changes in immune cell infiltration via flow cytometry, multiplex IHC, transparent tissue tomography (3D mapping) and single-cell RNA-seq. We will correlate changes in the metastatic tumor microenvironment (TME) at baseline and following 4 weeks of treatment, with genomic alterations and clinical responses. We have currently enrolled 12 patients to the study, and collected pre- and 4 week on-treatment biopsies. This study utilizes novel emerging technologies for in-depth TME analysis that will unravel the impact of PARP inhibition, singly and in combination with PD-1 blockade, on specific immune subsets within the TME. The correlative analyses will also lead to the discovery of novel biomarkers of response/resistance, and suggest additional immuno-oncology combinations for specific genomic subsets of mCRPC. Clinical trial information: NCT03572478