5003
Background: The treatment for poor prognosis mCRPC includes taxanes and androgen receptor (AR) targeted therapy, however the optimal treatment is undefined. Methods: Patients (pts) with poor prognosis (liver metastases, early CRPC ( < 12 months from ADT start), and/or > 3 of 6 poor prognostic criteria (Chi et al, Annals of Oncol, 2016)) were randomized to receive CAB (Arm A) or AR targeted therapy (Arm B, ABI or ENZ by investigator choice) with cross over at progression. Primary objective was to determine the clinical benefit rate (CBR) (PSA decline ≥50% (PSA50), objective response (OR), or stable disease (SD) ≥ 12 weeks). Plasma was sampled serially for circulating tumour DNA (ctDNA). Results: 95 pts were randomized (Arm A: 45, Arm B: 50): 18% had liver mets, 88% early CRPC and 30% had > 3 of 6 poor prognostic criteria. 52% of pts had prior docetaxel, half for castration sensitive disease. Table summarizes 1st-line therapy outcomes. Baseline ctDNA fraction > 15% (median) was associated with shorter 1st-line progression-free survival (PFS) (median 2.8 vs 8.4 m, HR = 2.54, P < 0.001) and overall survival (OS) (median 14.0 vs 38.7 m, HR = 2.64, P = 0.001). ctDNA alterations in AR, TP53, PI3K pathway, RB1 and DNA repair were detected in 53%, 45%, 31%, 23%, and 21% of pts. Shorter PFS and OS were associated with AR gain (HR 2.57 (95% CI 1.63-4.06); HR 3.59, (1.9-6.69), respectively) and TP53 defects (HR 2.62 (CI 1.65-4.15); HR 3.33 (CI 1.8-6.14), respectively). Pts with concurrent defects in TP53 and RB1 had a trend for worse PFS/OS than pts with TP53 defect alone. AR rearrangements predicted to disrupt the ligand binding domain were detected in 6% of pts and had a shorter PFS (HR = 2.60 (1.11 - 6.09)) with a trend for shorter OS (HR = 2.27 (0.89 - 5.81)). Conclusions: In this poor prognosis cohort, 1st-line treatment with CAB had a higher clinical benefit rate than treatment with ABI/ENZA. Elevated ctDNA and genomic alterations in AR and TP53 were prognostic. Supported in part by Sanofi. Clinical trial information: NCT02254785. [Table: see text]