Background: Second-generation AR signaling inhibitors have improved outcomes from mCRPC; drug resistance, however, invariably evolves with AR overexpression, AR mutation, or AR-SVs and continued AR signaling. TAS3681 is an oral and selective AR antagonist with AR and AR-SV down-regulation and has antitumor efficacy in AR-SV+, ENZ-resistant CRPC models. We report the dose escalation part of the first-in-human trial of TAS3681 in pts with mCRPC (NCT02566772). Methods: mCRPC pts with progressing disease after ABI and/or ENZ, and ≥1 additional CT, received TAS3681 in a 3+3 dose escalation design; QD or BID tablets were given in 28-day cycles; BID dosing at ≤600 mg was introduced to increase daily exposure while limiting C_max. Primary endpoints: incidence of dose limiting toxicities (DLTs) and adverse events (AEs); other endpoints: pharmacokinetics (PK), and antitumor activity per Prostate Cancer Clinical Trials Working Group 3 (PCWG3). Results: As of January 22, 2021, 56 pts in 10 cohorts were dosed (QD: 25, 50, 100, 200, 400, 600, 800, and 1000 mg; and BID: 300 and 400 mg). Median age was 66 (56–79) yrs; pts had a median of 6 prior lines of systemic therapy. Of 41 pts evaluable for DLTs, 3 had confirmed DLTs: 1/10 DLT evaluable pts at 600 mg QD (QT prolongation >480 ms) and 2/3 DLT evaluable pts at 400 mg BID (1 pt with QT prolongation >480 ms, 1 pt with G3 hypertension). The most common treatment-related AEs (TRAEs) were: nausea (32 pts, 57.1%), hyperbilirubinemia (21 pts, 37.5%), fatigue (18 pts, 32.1%), vomiting (17 pts, 30.4%) and diarrhea (16 pts, 28.6%); AEs of QT prolongation were seen in 11 pts (19.6%). TRAEs ≥G3 were reported in 12 pts (21.4%). TAS3681 exposure increased dose-dependently up to 600 mg QD and then plateaued. Steady state was reached by Day 8 and accumulation ratios of AUC were approximately 2 to 6 times. Confirmed PSA declines of >50% from baseline were seen in the 600 mg QD (2 pts) and 300 mg BID cohorts (1 pt). Antitumor activity was observed; tumor response rate was 23.1% in the 600 mg QD cohort (3 confirmed partial responses (cPRs) in 13 dosed pts), 22.2% in the 300 mg BID cohort (1 cPR and 1 unconfirmed complete response in 9 dosed pts), and 14.3% in the 400 mg BID cohort (cPR in 1/7 dosed pts). Responses occurred after 2–4 cycles of treatment. The longest duration of response (DoR) to date is 16.2 mo. The 5 pts with cPR had DoR >6 mo with the exception of 1 pt who had a short follow-up period. 300 mg BID was found to be the best tolerated dose with antitumor activity. Conclusions: The recommended phase 2 dose is 300 mg BID. TAS3681 has a manageable safety profile and has antitumor activity against heavily pretreated, multi-drug resistant mCRPC. The study expansion phase is enrolling pts who have progressed on ABI or ENZ +/- taxane CT. Clinical trial information: NCT02566772