Word count abstract: 293Word count text: 2592 This is the accepted manuscript of the article, which has been published in European Urology. 2019, 75(4), 667-675. http://dx. AbstractBackground: Several systemic therapeutic options exist for metastatic castratesensitive prostate cancer (mCSPC). Circulating tumour DNA (ctDNA) can molecularly profile metastatic castration-resistant prostate cancer (mCRPC) and can influence decision-making, but remains untested in mCSPC. Objective:To determine ctDNA abundance at de novo mCSPC diagnosis and whether ctDNA provides complementary clinically-relevant information to a prostate biopsy. Design, Setting, and Participants:We collected plasma cell-free DNA (cfDNA) from 53 newly diagnosed patients with mCSPC and, where possible, during treatment.Targeted sequencing was performed on cfDNA and DNA from diagnostic prostate tissue. Results and Limitations:Median ctDNA fraction was 11% (range 0-84) among untreated patients but lower (1.0%, range 0-51) in patients after short term (median 22 days) androgen deprivation therapy (ADT). TP53 mutations and DNA repair defects were identified in 47% and 21% of the cohort, respectively. Concordance for mutation detection in matched samples was 80%. Combined ctDNA and tissue analysis identified potential driver alterations in 94% of patients, whereas ctDNA or prostate biopsy alone was insufficient in 19 cases (36%). Limitations include the use of a narrow gene panel and undersampling of primary disease by prostate biopsy.Conclusions: ctDNA provides additional information to a prostate biopsy in men with de novo mCSPC, but ADT rapidly reduces ctDNA availability. Primary tissue and ctDNA share relevant somatic alterations, suggesting that either are suitable for molecular subtyping in de novo mCSPC. The optimal approach for biomarker development should ! 3 utilize both a tissue and liquid biopsy at diagnosis, as neither captures clinically-relevant somatic alterations in all patients. Patient summary:In men with advanced prostate cancer, tumour DNA shed into the bloodstream can be measured by a blood test. The information from this test provides complementary information to a prostate needle biopsy and could be used to guide management strategies.! 4

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Purpose: DNA mismatch repair defects (MMRd) and tumor hypermutation are rare and under-characterized in metastatic prostate cancer (mPC). Furthermore, because hypermutated MMRd prostate cancers can respond to immune checkpoint inhibitors, there is an urgent need for practical detection tools.Experimental Design: We analyzed plasma cell-free DNAtargeted sequencing data from 433 patients with mPC with circulating tumor DNA (ctDNA) purity !2%. Samples with somatic hypermutation were subjected to 185 Â whole-exome sequencing and capture of mismatch repair gene introns. Archival tissue was analyzed with targeted sequencing and IHC.Results: Sixteen patients (3.7%) had somatic hypermutation with MMRd etiology, evidenced by deleterious alterations in MSH2, MSH6, or MLH1, microsatellite instability, and characteristic trinucleotide signatures. ctDNA was concordant with mismatch repair protein IHC and DNA sequencing of tumor tissue. Tumor suppressors such as PTEN, RB1, and TP53 were inactivated by mutation rather than copy-number loss. Hotspot mutations in oncogenes such as AKT1, PIK3CA, and CTNNB1 were common, and the androgen receptor (AR)-ligand binding domain was mutated in 9 of 16 patients. We observed high intrapatient clonal diversity, evidenced by subclonal driver mutations and shifts in mutation allele frequency over time. Patients with hypermutation and MMRd etiology in ctDNA had a poor response to AR inhibition and inferior survival compared with a control cohort.Conclusions: Hypermutated MMRd mPC is associated with oncogene activation and subclonal diversity, which may contribute to a clinically aggressive disposition in selected patients. In patients with detectable ctDNA, cell-free DNA sequencing is a practical tool to prioritize this subtype for immunotherapy.See related commentary by Schweizer and Yu, p. 981

Purpose: Cross-resistance renders multiple lines of androgen receptor (AR) signaling inhibitors increasingly futile in metastatic castration-resistant prostate cancer (mCRPC). We sought to determine acquired genomic contributors to cross-resistance. Experimental Design: We collected 458 serial plasma cell-free DNA samples at baseline and progression timepoints from 202 patients with mCRPC receiving sequential AR signaling inhibitors (abiraterone and enzalutamide) in a randomized phase II clinical trial (NCT02125357). We utilized deep targeted and whole-exome sequencing to compare baseline and posttreatment somatic genomic profiles in circulating tumor DNA (ctDNA). Results: Patient ctDNA abundance was correlated across plasma collections and independently prognostic for sequential therapy response and overall survival. Most driver alterations in established prostate cancer genes were consistently detected in ctDNA over time. However, shifts in somatic populations after treatment were identified in 53% of patients, particularly after strong treatment responses. Treatment-associated changes converged upon the AR gene, with an average 50% increase in AR copy number, changes in AR mutation frequencies, and a 2.5-fold increase in the proportion of patients carrying AR ligand binding domain truncating rearrangements. Conclusions: Our data show that the dominant AR genotype continues to evolve during sequential lines of AR inhibition and drives acquired resistance in patients with mCRPC.

Purpose: DNA damage repair (DDR) defects are common across cancer types and can indicate therapeutic vulnerability. Optimal exploitation of DDR defects in prostate cancer requires new diagnostic strategies and a better understanding of associated clinical genomic features. Experimental Design: We performed targeted sequencing of 1,615 plasma cell-free DNA samples from 879 patients with metastatic prostate cancer. Depth-based copy-number calls and heterozygous SNP imbalance were leveraged to expose DDR-mutant allelic configuration and categorize mechanisms of biallelic loss. We used split-read structural variation analysis to characterize tumor suppressor rearrangements. Patient-matched archival primary tissue was analyzed identically. Results: BRCA2, ATM, and CDK12 were the most frequently disrupted DDR genes in circulating tumor DNA (ctDNA), collectively mutated in 15% of evaluable cases. Biallelic gene disruption via second somatic alteration or mutant allele–specific imbalance was identified in 79% of patients. A further 2% exhibited homozygous BRCA2 deletions. Tumor suppressors TP53, RB1, and PTEN were controlled via disruptive chromosomal rearrangements in BRCA2-defective samples, but via oncogene amplification in context of CDK12 defects. TP53 mutations were rare in cases with ATM defects. DDR mutations were re-detected across 94% of serial ctDNA samples and in all available archival primary tissues, indicating they arose prior to metastatic progression. Loss of BRCA2 and CDK12, but not ATM, was associated with poor clinical outcomes. Conclusions: BRCA2, ATM, and CDK12 defects are each linked to distinct prostate cancer driver genomics and aggression. The consistency of DDR status in longitudinal samples and resolution of allelic status underscores the potential for ctDNA as a diagnostic tool.

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Small-cell prostate carcinoma (SCPC) is an aggressive malignancy that is managed similarly to small-cell lung cancer. SCPC can evolve from prostate adenocarcinoma in response to androgen deprivation therapy, but, in rare cases, is present at initial cancer diagnosis. The molecular aetiology of de novo SCPC is incompletely understood, owing to the scarcity of tumour tissue and the short life-expectancy of patients. Through a retrospective search of our regional oncology pharmacy database, we identified 18 patients diagnosed with de novo SCPC between 2004 and 2017. Ten patients had pure SCPC pathology, and the remainder had some admixed adenocarcinoma foci, but all were treated with first-line platinum-based chemotherapy. The median overall survival was 28 months. We performed targeted DNA sequencing, whole exome sequencing and mRNA profiling on formalin-fixed paraffin-embedded archival tumour tissue. We observed frequent biallelic deletion and/or mutation of the tumour suppressor genes TP53, RB1, and PTEN, similarly to what was found in treatment-related SCPC. Indeed, at the RNA level, pure de novo SCPC closely resembled treatment-related SCPC. However, five patients had biallelic loss of DNA repair genes, including BRCA1, BRCA2, ATM, and MSH2/6, potentially underlying the high genomic instability of this rare disease variant. Two patients with pure de novo SCPC harboured ETS gene rearrangements involving androgen-driven promoters, consistent with the evolution of de novo SCPC from an androgen-driven ancestor. Overall, our results reveal a highly aggressive molecular landscape that underlies this unusual pathological variant, and suggest opportunities for targeted therapy strategies in a disease with few treatment options. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.