A Study of ARV-766 Given by Mouth in Men With Metastatic Prostate Cancer

The study consists of multiple parts. 1. Part A evaluates JSB462 monotherapy in a dose-escalation setting to determine safety, tolerability, dose-limiting toxicities, and the maximum tolerated dose or recommended Phase 2 dose (RP2D). Dose escalation follows an accelerated titration design transitioning to a 3+3 design. 2. Part B is a randomized cohort expansion evaluating JSB462 monotherapy at selected dose levels (e.g., 100 mg or 300 mg once daily) to further assess safety and preliminary antitumor activity at the RP2D. Participants are randomized in a 1:1 ratio. 3. Part C evaluates JSB462 in combination with abiraterone, administered with corticosteroid and ongoing androgen deprivation therapy. This part assesses safety, tolerability, pharmacokinetics, and defines a recommended dose for the combination regimen. An abiraterone lead-in period is implemented prior to combination treatment. The study population includes adult males with metastatic prostate cancer. In Parts A and B, participants have metastatic castration-resistant prostate cancer (mCRPC) following prior systemic therapy. Part C includes participants with metastatic prostate cancer, including those who may be naïve to novel hormonal agents. Primary objectives focus on safety and tolerability, including incidence of adverse events, laboratory abnormalities, and dose-limiting toxicities. In the expansion phase, antitumor activity is evaluated using prostate-specific antigen (PSA) response rates (e.g., PSA30 and PSA50). Secondary objectives include characterization of pharmacokinetics and further assessment of antitumor activity using endpoints such as objective response rate, radiographic progression-free survival, duration of response, and time to PSA progression. Tumor assessments are performed using modified Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1 for soft tissue disease and Prostate Cancer Working Group 3 (PCWG3) criteria for bone disease, with imaging conducted at regular intervals. This study aims to characterize the clinical profile of JSB462 and explore its potential to overcome resistance mechanisms associated with androgen receptor signaling in metastatic prostate cancer.