Androgen Receptor (Ar) And Non-Ar Aberrations Associated With Outcomes In Metastatic Castration-Sensitive Prostate Cancer (Mcspc) Treated With Apalutamide (Apa) Plus Androgen Deprivation Therapy (Adt) In Titan

Background: In TITAN, APA improved radiographic progression-free survival (rPFS), overall survival (OS), and second progression-free survival (PFS2) vs placebo (PBO) in patients (pts) with mCSPC receiving ADT. We determined the presence of genomic and transcriptional aberrations and evaluated associations of these aberrations with clinical outcomes in TITAN. Methods: Whole-blood samples from 130 pts were collected at baseline (BL, n = 59) and end of treatment (EOT: APA, n = 47; PBO, n = 83). Circulating tumor (ct)DNA and non-AR aberrations (Table) were assessed from isolated cell-free (cf)DNA (n = 129), and AR aberrations at EOT were assessed from cfDNA and cellular RNA (n = 127) using next-generation sequencing and real-time PCR. Detected biomarkers at BL and EOT were assessed for associations with rPFS, OS, and PFS2 and with OS and PFS2, respectively, using Cox proportional hazards model and multivariate analyses (MVA) in pooled APA and PBO pts. Results: From BL to EOT, detection of ctDNA increased 1.8-fold and similarly with APA and PBO; detection of AR aberrations increased 2.7-fold and was less frequent with APA vs PBO (Table; p = 0.04, Fisher exact test). Among non-AR aberrations at EOT, those in TP53, RB1, PI3KCA were the most frequent; detection increased ≥ 2.8-fold from BL to EOT and similarly with APA and PBO (Table). Detection of ctDNA, AR aberrations, and aberrations in TP53, RB1, PI3KCA at EOT were associated with shorter PFS2 and OS (Table). Detection of other non-AR aberrations at EOT and of all aberrations at BL was low for robust conclusions. Only detectable ctDNA at EOT maintained association with shorter OS in MVA. Conclusions: In TITAN pts with mCSPC, AR and several non-AR aberrations were most frequent at progression. APA was associated with decreased frequency of AR aberrations at EOT and similar frequency of detectable ctDNA vs PBO. ctDNA was an independent prognostic variable for shorter PFS2 and OS in pts with mCSPC. Citation Format: Kim N. Chi, Shibu Thomas, Michael Gormley, Dong Shen, Neeraj Agarwal, Felix Feng, Gerhardt Attard, Alex Wyatt, Deborah S. Ricci, Angela Lopez-Gitlitz, Julie S. Larsen, Branko Miladinovic, Simon Chowdhury. Androgen receptor (AR) and non-AR aberrations associated with outcomes in metastatic castration-sensitive prostate cancer (mCSPC) treated with apalutamide (APA) plus androgen deprivation therapy (ADT) in TITAN [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 776.