Background: DDR alterations are associated with poorer prognosis, including shorter overall survival (OS), in patients with mHSPC. In ARCHES (NCT02677896), patients with mHSPC treated with ENZA + ADT had a reduced risk of radiographic progression or death and improved OS versus PBO + ADT. This post hoc analysis assessed the prevalence of DDR alterations and associated baseline characteristics in patients with mHSPC in ARCHES. Methods: Patients with mHSPC (n=1150) were randomized 1:1 to ENZA (160 mg/day) + ADT or PBO + ADT. Germline DDR alteration testing was performed in blood using Ambry Genetics CustomNext-Cancer panel; 16 DDR-related genes were reported in the subset of patients who consented to participate in a pharmacogenomic study (n=664; Table). Descriptive analyses of baseline demographics and disease characteristics by DDR status were performed. Results: Of 664 patient samples tested, 652 were evaluable for analysis (ENZA + ADT, n=326; PBO + ADT, n=326). Baseline characteristics of these patients were similar to the ARCHES intent-to-treat population. The prevalence of DDR+ patients was lower than expected (n=34/652, 5.2%; Table). Of DDR+ patients, 13 (38.2%) had low-volume disease, compared with 228 (36.9%) of DDR− patients. High-volume disease was present in 21 (61.8%) DDR+ and 390 (63.1%) DDR− patients. Of DDR+ patients, 6 (17.6%) had localized disease at initial diagnosis (M0), compared with 145 (23.5%) DDR− patients. De novo metastatic disease at initial diagnosis (M1) was present in 28 (82.4%) DDR+ patients and 465 (75.2%) DDR− patients. Of DDR+ patients, 29 (85.3%) were from Europe, four (11.8%) were from North America, and one (2.9%) was from the Asia-Pacific region. Conclusions: This post hoc analysis found a lower prevalence of DDR alterations in patients with mHSPC in ARCHES, compared with the 7–12% previously reported in patients with metastatic castration-resistant prostate cancer (Lozano et al. Br J Cancer 2021; Pritchard et al. N Engl J Med 2016). We did not identify differences in baseline disease characteristics based on DDR status. Clinical trial information: NCT02677896.

^aDDR: ATM, BRCA1, BRCA2, CHEK2, MLH1, MRE11A, NBN, PALB2, RAD50, RAD51C, FANCC, MSH2, MSH3, MSH6, POLD1, POLE. No patient had >1 alteration.