Background: EPI-7386 (masofaniten) is a next-generation aniten that inhibits androgen receptor (AR) activity by binding the N-terminal domain and blocking transcription, irrespective of ligand-binding domain resistance mechanisms. Preclinically, the combination of masofaniten with Enz results in a deeper blockade of the AR pathway and greater antitumor activity than either agent alone. Methods: This is a Phase 1/2 multicenter clinical trial (NCT05075577) enrolling patients (pts) with mCRPC on androgen deprivation therapy and naïve to second-generation antiandrogens (one line of prior chemotherapy in the metastatic hormone-sensitive setting allowed). P1 examines escalating doses of masofaniten + Enz at 120 or 160 mg QD. Primary and secondary endpoints of P1 evaluate the safety and pharmacokinetics (PK) of masofaniten + Enz when co-administered to establish the recommended P2 combination doses (RP2CDs). P2 is designed as a two arm, 2:1 randomized, open-label trial evaluating the antitumor activity of masofaniten in combination with Enz versus Enz alone; approximately 120 pts will be randomized with masofaniten 600 mg BID + Enz 160 mg QD (n=80) vs single agent Enz at 160 mg QD (n = 40). Results: P1 completed enrollment with 18 pts in 4 cohorts with 16 pts evaluable for efficacy as per protocol. The combination regimen was well tolerated with a safety profile consistent with Enz monotherapy. One grade 3 rash was observed in cohort 4 evaluating masofaniten 600 mg BID + Enz 160 mg QD. PK results demonstrated Enz exposure was not impacted by concomitant administration of masofaniten, allowing testing of the full dose of Enz (160 mg). By contrast, masofaniten exposure was consistently reduced by concomitant administration of Enz (CYP3A4 inducer that metabolizes masofaniten) but remained within the clinically relevant range with the highest exposures and C_min observed using masofaniten BID dosing. Although efficacy data are still maturing, to date, response rates were PSA50 88% (14/16 pts), PSA90 69% (11/16 pts) and PSA <0.2 ng/mL 56% (9/16 pts) in evaluable pts, regardless of their previous chemotherapy status. Conclusions: The RP2CD for P2 was established at masofaniten 600 mg BID + Enz 160 mg QD. Updated results, including long term follow-up of the dose escalation patients, and P2 study design, will be presented. Clinical trial information: NCT05075577.