Background: PARP inhibitors (PARPi) show antitumor activity in mCRPC/DDRm pts treated with novel hormonal therapy (NHT). TALAPRO-1 is an open-label study evaluating TALA (potent PARP inhibitor/trapper) in men with mCRPC/DDRm. We report a planned IA (Dec 2019). Methods: TALAPRO-1 (NCT03148795) is enrolling pts (N ≈ 100) with measurable soft tissue disease, progressive mCRPC, and DDRm likely to sensitize to PARPi (ATM, ATR, BRCA1/2, CHEK2, FANCA, MLH1, MRE11A, NBN, PALB2, RAD51C), who received 1–2 chemotherapy regimens (≥1 taxane-based) for metastatic disease and progressed on ≥1 NHT (enzalutamide/abiraterone acetate) given for mCRPC. DDRm are defined as known/likely pathogenic variants or homozygous deletions. Pts receive oral TALA 1 mg/day (moderate renal impairment 0.75 mg/day) until radiographic progression, unacceptable toxicity, consent withdrawal or death. Primary endpoint is objective response rate (ORR). Secondary endpoints: time to OR; response duration; PSA decrease ≥50%; circulating tumor cell (CTC) count conversion (to CTC = 0 and <5 per 7.5 mL blood); time to PSA progression; radiographic PFS (rPFS); overall survival; safety. A planned efficacy/safety IA was done when 60 pts with DDRm and measurable disease completed ≥6 months of TALA/no longer followed (DDR population [DDRp]). Radiographic responses are based on investigator assessments. Results: 113 pts received TALA (cutoff Dec 12, 2019); 75 pts were DDRp, had measurable disease, received ≥16 wk treatment, and were evaluable for ORR (54.7% BRCA1/2, 4.0% PALB2, 22.7% ATM; 18.7% other DDRm).All DDRp pts had prior docetaxel; 45.3% cabazitaxel. Confirmed ORR, rPFS, and composite response (investigator-assessed) in pts who received TALA for ≥16 weeks are in the table. Most common treatment-emergent adverse events: anemia (42.5%); nausea (32.7%). Conclusions: TALA monotherapy has encouraging antitumor activity in docetaxel-pretreated mCRPC pts with BRCA1/2 alterations and was generally well tolerated. Clinical trial information: NCT03148795.

^aMeasurable soft tissue disease per investigator at screening; ^bDDR deficient population; ^cOR and/or PSA response ≥50% and/or CTC conversion (from CTC ≥5 to <5)