Background <p>The gonadotropin-releasing hormone (GnRH) receptor antagonist relugolix is the only oral androgen deprivation therapy (ADT) indicated for advanced prostate cancer (aPC). Combining ADT with androgen receptor signaling inhibitors (ARSIs) has shown improved clinical outcomes.</p> Objective <p>To assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of relugolix in combination with ARSIs in patients with aPC.</p> Methods <p>In this 52-week, open-label study, patients received relugolix (120 mg once daily) with abiraterone (1,000 mg once daily) and corticosteroid (Part 1) or relugolix (240 mg once daily) with apalutamide (240 mg once daily) (Part 2). Metastatic castration-sensitive patients were eligible for both parts, whereas castration-resistant patients were eligible for Part 1 if metastatic and Part 2 if non-metastatic. Adverse events and other safety data were evaluated over 52 weeks, while pharmacodynamic and pharmacokinetic (Part 2 only) data were assessed over 12 weeks. Medication adherence to relugolix was measured by pill count.</p> Results <p>Of 48 patients, 21 completed Part 1 and 20 completed Part 2. Most adverse events were grade 1 or 2, with hypertension (Part 1) and rash (Part 2) being most common. Mean testosterone concentrations remained below castrate level. Median prostate-specific antigen concentration was 0.04 ng/mL at week 12 in both parts. Concentrations of relugolix, apalutamide, and N-desmethyl-apalutamide were stable over 12 weeks similar to previous data. Relugolix adherence rates were &gt;&#xa0;97% in both parts.</p> Conclusions <p>The safety/tolerability profile of both combination therapies was consistent with those of the individual drugs. These findings support using relugolix in combination with abiraterone or apalutamide as treatment of aPC.</p> Clinical Trial Registration <p>ClinicalTrials.gov identifier NCT04666129.</p>