Background: Docetaxel is the standard first-line chemotherapy for patients with metastatic castration-resistant prostate cancer (mCRPC) whose disease progressed on an androgen receptor-targeted agent (ARTA; typically, abiraterone, enzalutamide, apalutamide, or darolutamide). However, most patients develop chemotherapy resistance, and median overall survival (OS) remains under 3 years. Preclinical studies have associated phosphoinositide 3-kinase (PI3K)/protein kinase (AKT)/phosphatase and tensin homolog (PTEN) pathway signaling with the development of mCRPC and with resistance to taxane chemotherapy. The ProCAID Phase II trial (NCT02121639) examined whether adding capivasertib, a potent, selective inhibitor of all three AKT isoforms (AKT1/2/3), to docetaxel chemotherapy improved clinical outcomes. Although no difference in the primary endpoint (composite progression-free survival [PFS]) was detected, OS, a secondary endpoint, was extended in the capivasertib plus docetaxel treatment group. A subsequent follow-up analysis at advanced maturity suggested that the OS benefit in ProCAID was restricted to patients who had received prior treatment with an ARTA. Methods: CAPItello-280 is a Phase III study to confirm the efficacy and safety of capivasertib in combination with docetaxel compared with placebo and docetaxel in patients with mCRPC who have not previously received chemotherapy for mCRPC, but whose disease has progressed despite treatment with an ARTA in any setting. Approximately 790 patients will be randomized 1:1 to receive either capivasertib or matching placebo (320 mg orally, twice daily [BD], 4 days on, 3 days off) plus docetaxel (75 mg/m² on day 1 of each 21-day cycle for 6–10 cycles, with prednisone or prednisolone 5 mg BD or 10 mg once daily) with a background of continued androgen deprivation therapy until disease progression, unacceptable toxicity, death, or withdrawal of consent. The primary endpoint is OS; secondary endpoints include radiographic PFS, time to pain progression, symptomatic skeletal events, safety, and tolerability. Recruitment began in March 2022 and is planned at 207 study sites in 20 countries, including the USA, Canada, Europe, South America, and Asia-Pacific. Clinical trial information: NCT05348577.