Background: MEDI3726 is an ADC targeting PSMA. Once bound to PSMA and internalized, the released pyrrolobenzodiazepine dimer toxin crosslinks DNA and triggers cell death. This phase 1 study evaluated the safety and efficacy of MEDI3726 in mCRPC after failure of abiraterone and/or enzalutamide and a taxane-based therapy. Methods: The starting dose was 0.015 mg/kg MEDI3726 IV Q3W until disease progression or unacceptable toxicity. Dose escalation used a modified toxicity probability interval algorithm (mTPI). The primary objectives were safety, adverse events (AEs) and dose-limiting toxicities (DLTs) and to determine the maximum tolerated (MTD) or administered (MAD) dose. Secondary objectives included antitumor activity, pharmacokinetics and immunogenicity. The endpoint for activity was composite response: confirmed response by RECIST v1.1, and/or PSA decrease of ≥ 50% after ≥ 12 wks, and/or confirmed conversion in circulating tumor cell count, defined as a decrease from ≥ 5 to < 5 cells/7.5 mL. Efficacy analyses were based on Prostate Working Group Criteria. Mutational profiles were evaluated in ctDNA. Results: As of Sept 27 2019, 33 pts received MEDI3726. Median age was 71.0 yr. Median number of prior regimens was 4. Median follow-up was 5.4 mo. Drug-related AEs occurred in 30 (90.9%), being grade 3/4 in 15 (45.5%), serious in 11 (33.3%) and causing discontinuation in 13 (39.4%). There were no drug-related deaths. One pt at 0.3 mg/kg had a DLT of Grade 3 thrombocytopenia. No MTD was identified per mTPI; the MAD was 0.3 mg/kg. MEDI3726 had nonlinear PK with a short t_1/2 (0.3–2 d). Three pts (15.8%) at baseline and 6 (33.3%) post-baseline had antidrug antibodies, with no correlation to PK exposure. Composite response rate across all doses was 2/33 (6.1%). Time to response was 0.3 mo; duration of response was 1.8–3.8 mo. Median progression-free survival was 3.9 mo and median overall survival was 10.6 mo. Conclusions: An MTD was not identified, but drug-related AEs (skin toxicities and effusions) prevented raising the dose over 0.3 mg/kg and limited the number of cycles. Responses were seen at higher doses, but were not durable as pts discontinued due to drug-related AEs. Clinical trial information: NCT02991911