Background: Androgen deprivation therapy (ADT) with gonadotropin-releasing hormone (GnRH) analogs or receptor antagonists is a cornerstone of prostate cancer treatment. As disease progresses, agents with complementary mechansims are co-prescribed with ADT to suppress extra-testicular testosterone production or block androgen receptors on cancer cells. Relugolix (120 mg) is an oral non-peptide GnRH receptor antagonist approved in the US for the treatment of aPC. In the phase 3 study, relugolix maintained suppression of testosterone to castration levels in 96.7% of men for up to 48 weeks (wks), with superiority to leuprolide acetate. Relugolix was well tolerated and associated with a 54% lower risk of major adverse cardiovascular events relative to leuprolide acetate (Shore N, NEJM 2020;382;23). To formally assess the safety and tolerability of combination treatment with relugolix, a phase 1 study in men with aPC has been undertaken. Methods: This is a three-part, open-label, parallel-cohort safety and tolerability study of relugolix in combination with abiraterone acetate in men with metastatic castration-sensitive prostate cancer (mCSPC) or metastatic castration-resistant prostate cancer (mCRPC) (Part 1), apalutamide in men with mCSPC or non-metastatic catration-resistant prostate cancer (nmCRPC) (Part 2), or docetaxel in men with mCSPC or mCRPC (Part 3). Each part of the study consists of a 45-day screening period, a 12-wk primary study treatment period and a 40-wk safety extension treatment period. All of the men are required to have been treated with leuprolide acetate or a GnRH receptor antagonist (eg, degarelix) in combination with abiraterone acetate for a minimum of 12 wks, apalutamide for a minimum of 6 wks, or docetaxel for a minimum of one treatment cycle prior to the baseline (Day 1) visit. Men will be transitioned from leuprolide acetate or degarelix to relugolix (120 mg [Part 1 and 3] or 240 mg [Part 2] once daily after a single loading dose of 360 mg); on the approximate date the next analog or antagonist injection is scheduled; treatment with each combination treatment will continue as previously prescribed. Hence, the study will provide safety and tolerability of relugolix and the three different combination agents for up to 1 year and in addition, will provide safety and tolerability data as men transition from injectable leuprolide acetate or degarelix to oral treatment with relugolix. Enrollment into the study began in March 2021. A protocol amendment was approved in July 2021 to include Part 2 and Part 3 of the study and to add the 40-wk safety extension treatment period. Screening for Part 2 was initiated in August 2021 and for Part 3 it is expected to initiate in January 2022. Clinical trial information: NCT04666129.