Background: Inhibition of poly(ADP-ribose) polymerase (PARP) shows promise in prostate cancer, but is limited to the ~20% of men with defects in genes encoding for DNA repair proteins BRCA1, BRCA2 or ATM (homologous recombination defect positive, HRD+). The effect is modest for HRD+ patients with a progression free survival of ~7 months. Pharmacologically simulating genetic DNA repair defects may expand who benefits to homologous recombination defect negative (HRD-) patients and improve HRD+ response. The ataxia telangiectasia and Rad3-related protein (ATR) is ideal with its roles in cell cycle regulation, replication fork resolution and both single and double strand break repair. Preclinical studies on HRD-/HRD+ cell lines support this. We hypothesize co-inhibition of ATR and PARP will respond regardless of HRD status. Methods: TRAP is a prospective, multi-institutional, phase 2 clinical trial testing AZD6738 combined with olaparib in HRD+ and HRD- mCRPC patients. Primary endpoint is the response rate (RR) by RECIST radiographic response or PSA decline ≥50% in 35 HRD- patients, with a secondary objective of RR in 12 HRD+ patients. HRD+ is mono/biallelic loss of ATM or biallelic loss of BRCA1/2. Tissue based sequencing is done unless completed prior in mCRPC, known BRCA germline loss, treating provider deems biopsy unsafe or biopsy fails. Those unable or failing biopsy are designated as HRD-, but BRCA1/2 and ATM are tested via circulating tumor DNA in a commercial test. Eligible patients must progress after ≥1 line of mCRPC therapy. Progression on a second generation anti-androgen (e.g. apalutamide), abiraterone or within 6 months of docetaxel in hormone sensitive disease are eligible. Treatment entails 160 mg PO daily of AZD6738 on days 1-7 and 300 mg PO BID of olaparib on days 1-28 of a 28-day cycle. Statistical analysis will provide RR with 95% binomial confidence intervals. Analysis of tumor specimens, circulating tumor cells and DNA will be performed for predictors of response and acquired resistance. The study is at four sites in the US, participates in the Prostate Cancer Clinical Trials Consortium, LLC, is managed by the University of Michigan and funded by AstraZeneca. Clinical trial information: NCT03787680