Wang, Amy

153 Background: The prognostic significance of DDR alterations in mCRPC remains unclear, with conflicting data from prior reports. Whether DDR alterations are predictive of outcomes with therapeutic agents other than PARP inhibitors in mCRPC is also poorly understood. With increasing use of molecular profiling in mCRPC, understanding the full prognostic and predictive utility of plasma DDR alterations is paramount. Methods: A next-generation sequencing Predicine liquid biopsy assay was used to profile cfDNA and germline DNA in 407 mCRPC patients (pts) from two independent international cohorts (n=162 Australia, n=245 US). DDR genes profiled were BRCA2, ATM, BRCA1, MLH1 and MSH2. Kaplan-Meier survival estimates and multivariable Cox regression analyses were used to assess associations between DDR alterations and clinical outcomes including PSA response rate (PSA RR), progression-free survival and overall survival (OS). Results: Median age was 74 (IQR 67-79), with median follow up 74 months and median OS 23 months. 65/407 (16%) harboured pathogenic DDR alterations, including 21 patients with ³1 alteration. Frequency of genomic aberrations are shown in the table. BRCA2 alterations were further classified as heterozygous loss (66%), homozygous loss (14%), monoallelic mutation (11%) and biallelic mutation/loss of heterozygosity (9%). Aberrations in any DDR gene, ATM, MLH1 + MSH2 or BRCA2 were associated with shorter OS on univariable analysis, but only any DDR or BRCA2 aberration remained significant upon adjusting for clinical prognosticators and ctDNA fraction (Table). Pre-treatment BRCA2 aberration was associated with significantly shorter OS and lower PSA RR compared to BRCA2 wt for pts receiving an AR pathway inhibitor (ARPI) (18 vs 32 months, p=0.006; 36 vs 60%, p=0.04 respectively) but not for taxane chemotherapy (17 vs 20 months, p=0.3; 45 vs 66, p=0.1 respectively). Conclusions: Detection of an aberration in any DDR gene or BRCA2 was an independent poor prognostic factor across 2 large independent cohorts of mCRPC patients. Intriguingly, patients with a BRCA2 alteration appeared to have worse outcomes with ARPI but not chemotherapy, suggesting predictive utility of DDR profiling. Our data collectively speak to the potential role of DDR alterations, in particular BRCA2, as prognostic and/ or predictive biomarkers in mCRPC.[Table: see text]

150 Background: With the evolving use of targeted therapies exploiting genomic vulnerabilities in mCRPC, screening patients for sensitizing alterations is of increasing relevance. However, the influence of age on the detection of relevant genomic alterations is incompletely understood. In this study, we compared the cell-free DNA (cfDNA) profiles of younger (age <70) versus older (age ≥70) men with mCRPC and assessed the relative prognostic impact of common genomic alterations. Methods: A next-generation sequencing-based Predicine cfDNA assay was used to profile 276 mCRPC patients from two independent international cohorts. The frequency of genomic alterations across age categories was compared using the chi-squared test, while circulating tumour DNA fraction (ctDNA%) was compared using the Mann–Whitney U test. Cox proportional hazards analysis with interaction testing was used to assess the association between age, genomic alteration and overall survival. Results: The median age of the combined cohort was 72 years (IQR 66-78), with 170 (62%) patients ≥70 years old. Despite differences in baseline characteristics (poorer performance status in older patients, higher Gleason score and de novo metastatic disease in younger patients), there was no significant difference in ctDNA% (median 22% vs 30%, p=0.6). We observed similar frequencies of genomic alterations across the 10 most commonly aberrant genes, including AR (<70 vs ≥70: 46% vs 49%, p=0.6), TP53 (40% vs 36%, p=0.6), BRCA2 (29% vs 21%, p=0.13) and PTEN (39% vs 35%, p=0.5). This similarity persisted when analysing mutations and copy number alterations in isolation. While driver alterations in both age categories were strongly associated with unfavourable outcomes (Table), an exploratory analysis revealed that copy number gain in PIK3CA may have a worse prognostic impact in younger men ( p for interaction = 0.03), with a lesser effect observed in AR gain ( p = 0.1) and MYC gain ( p = 0.1). Conclusions: The genomic profiles of pertinent alterations between younger and older men are comparable. Certain genomic alterations may carry a worse prognosis in younger men. Patients should be considered for precision medicine assessment and directed therapies, regardless of age. [Table: see text]

159 Background: Tumour tissue from metastatic castration-resistant prostate cancer (mCRPC) harbors frequent copy number variations (CNVs) in the phosphatidylinositol-3-kinase (PI3K)/Akt-signaling pathway. However, identifying CNVs in plasma cell-free DNA (cfDNA) has proven challenging. With emerging data supporting Akt inhibition in PTEN-deficient mCRPC, cfDNA assays for robustly identifying PI3K/Akt pathway aberrations including CNVs are urgently required. Methods: In this multi-institutional prospective biomarker study, we used the Predicine cfDNA assay, optimised for CNV detection, to perform targeted sequencing in 231 mCRPC patients in two independent cohorts (Australian, n = 78; US, n = 153). Kaplan-Meier survival estimates and multivariable Cox regression analysis were used to assess associations between genomic aberrations and progression-free survival (PFS) and overall survival (OS). Results: PTEN loss and PIK3CA gain were detected in 37% (85/231) and 17% (39/231) of patients, respectively. Poorer outcomes were observed in patients with PI3K/Akt pathway aberrations, including those with dual PTEN loss and PIK3CA gain (HR 2.3, 95% CI 1.2-4.4). Similarly, cumulative CNV burden in the PI3K/Akt and AR pathways (0 vs 1 vs ≥2 CNVs in Australian cohort: median OS 33.5 vs 17.2 vs 9.7 months, p< 0.001; 0 vs 1 vs ≥2 CNVs in US cohort: median OS 35.5 vs 14.3 vs 9.2 months, p< 0.001) was associated with significantly worse clinical outcomes. Notably, 21% (31/146) of PTEN-neutral patients harbored other alterations in the PI3K/Akt pathway. Conclusions: Our cfDNA assay readily detected PI3K/Akt pathway CNVs, with the prevalence of PTEN loss comparable to prior tissue sequencing studies. CNVs in the PI3K/Akt pathway were associated with deleterious clinical outcomes, especially when concurrent with AR gain. Additional PI3K/Akt pathway aberrations were found in approximately one-fifth of PTEN-neutral mCRPC. Collectively, our data demonstrate the potential utility of profiling cfDNA to facilitate and optimize patient selection for treatment with Akt inhibitors in mCRPC. [Table: see text]

160 Background: mCRPC is a heterogeneous disease state with variable survival and molecular markers that define survival continue to be determined. We applied a 120 gene panel based targeted next-gen sequencing (NGS) approach to probe plasma ctDNA based outcomes for survival. Methods: mCRPC chemo-naive patients (pts) were enrolled in a prospective study and plasma cell free DNA (cfDNA) extracted. ctDNA fractions estimated based on somatic variants. NGS of plasma cfDNA was performed using HiSeq X Ten on a panel of 120 genes (PredicineLDT). Clinical and molecular prognostic factors were determined for overall survival (OS). Cox proportional hazard regression was used to estimate hazard ratios (HR) at the univariate level and only significant associations included in multivariate analysis (significance at P<0.05). Results: 96 mCRPC pts were enrolled. 17/96 pts failed NGS QC. Median cfDNA, ctDNA yields for 79/96 pts are provided in Table. Median follow up for study cohort was 92.5 months (mon) (Range:62.3-109.6); median survival time was 25.9 mon (95% CI:19.9, 31.4) and 72/79 pts had died. TMPRSS2-ERG fusion was detected in 18/79 pts. Significant alterations associated with OS are listed in Table. 49/59 pts underwent subsequent docetaxel chemo. In chemo treated pts significant copy number alterations predictive of survival included AR amplification (p=0.01, HR=2.0, 95% CI[1.2, 3.5], RB1 loss (p=3.20E-06, HR=5.0, 95% CI[2.4, 10.6], PTEN(p=0.009, HR=2.6, 95% CI[1.2, 5.4]), CDH1(p=0.001, HR=4.3, 95% CI[1.6, 11.4]). Conclusions: ctDNA yield in mCRPC state was the most significant prognostic factor for survival, Several specific plasma genomic perturbations were observed topredict docetaxel efficacy and need further evaluation.[Table: see text]

5047 Background: We identified plasma cell free DNA (cfDNA) based copy number variations (CNV); single nucleotide variants (SNVs) & TMPRSS-ERG fusion in four sub states of metastatic prostate cancer (mPCa) and determined the impact on survival. Two mHSPC cohorts included treatment naïve, gp-1 and mHSPC patients (pts) responding to chronic androgen ablation (AA) (gp-2). Two mCRPC cohorts included mHSPC pts with PSA relapse on chronic AA (gp-3) and a clinically progressive mCRPC cohort post AA (gp-4). Methods: Enrollment of mPCa pts was performed from 2009-14 who were followed until 2018. Plasma from collected blood was used for extracting cfDNA. NGS was performed using Illumina HiSeq X for a preselected target panel of 129 genes including DNA damage repair genes. Statistical analyses of genomic aberrations were performed in R 3.5.1 and Cox proportional-hazard models were used for survival analyses. Results: A total of 255 pts were enrolled with 215 having adequate cfDNA that passed NGS QC. Median study follow up was 90.2 (Range:73;99) months. The table highlights pts in each gp with CNV, SNV, fusion events. ARamplification was higher in mCRPC gps3&4 (20/103 pts) compared to 3/106 pts in mHSPC gps1&2 (p < 0.001) and was prognostic for poor survival in mCRPC (p < 0.001;HR:3.34; 95%CI: 1.9-5.76). 54/103 pts in gp3&4 had SNVs in TP53 compared to 34/106 pts in mHSPC gps1&2 (P < 0.01). SNVs in APC, AR, CDK12 and BRIP1 were also increased in gps3&4 (p < 0.01). Gp1 mHSPC pts with SNVs in ATM/ CHEK2 had shorter response to AA (p < 0.001; HR:3.66; 95%: CI:1.81-7.39). Conclusions: Plasma cfDNA based somatic aberrations are detected with increased prevalence in mHSPC to mCRPC states. The ease of specimen collection and the need for molecular profiling in mPCa increases its potential for clinical applications in pt care. [Table: see text]