Background: Bavdegalutamide (ARV-110) is a novel, oral PROteolysis TArgeting Chimera (PROTAC) protein degrader that targets wild-type androgen receptor (AR) and clinically relevant mutants. Bavdegalutamide demonstrated tumor growth inhibition in multiple xenograft models (eg, AR gene amplification, AR mutation, enzalutamide resistance, and enzalutamide insensitivity). In a phase 1/2 study (NCT03888612), bavdegalutamide showed clinical activity in patients with metastatic castration-resistant prostate cancer (mCRPC) who had previously received 1–2 prior novel hormonal agents (eg, abiraterone and/or enzalutamide), including heavily pretreated patients. Abiraterone is approved, in combination with a corticosteroid, to treat patients with mCRPC or with high-risk castration-sensitive prostate cancer (CSPC). Up to a third of patients treated with abiraterone develop primary resistance to this drug and nearly all patients experience disease progression. Here we describe a phase 1b study that will evaluate the combination of bavdegalutamide with abiraterone at the initiation of progression on abiraterone (prostate-specific antigen [PSA] progression without radiographic progression) to test if the addition of bavdegalutamide will overcome resistance to abiraterone and re-establish the AR pathway blockade in patients with prostate cancer. Methods: Eligible patients are men ≥18 years of age with histologically, pathologically, or cytologically confirmed adenocarcinoma of the prostate and Eastern Cooperative Oncology Group performance status of 0 or 1. Patients must be receiving ongoing treatment with stable doses of abiraterone and a concomitant corticosteroid for mCRPC or CSPC and have PSA progression ≥16 weeks after initiation of abiraterone, ≥2 rising PSA values measured ≥1 week apart, and no radiographic evidence of disease progression while receiving abiraterone. Ongoing androgen deprivation therapy with a gonadotropin-releasing hormone analogue or inhibitor or orchiectomy is required. Prior treatment with enzalutamide, apalutamide, darolutamide, or experimental AR-directed therapies is not permitted. Bavdegalutamide, abiraterone, and a corticosteroid will be administered daily in 28-day cycles. Primary objectives are to evaluate the safety and tolerability of bavdegalutamide plus abiraterone and determine the recommended phase 2 dose and schedule of this combination (based on the incidence of first-cycle dose-limiting toxicities and the frequency and severity of adverse events and laboratory abnormalities). Clinical trial information: NCT05177042.