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263 Background: Prostate cancer (PCa) is characterized by considerable genetic heterogeneity, and complex genomic features may influence prognosis and treatment response. We created a database of aggressive PCa that integrates comprehensive genomic sequencing with detailed clinical outcomes to better understand the optimal use of genomic sequencing. Methods: From 4/2005-7/2021, PCa cancer patients older than 18 years of age underwent tissue collection for tumoral RNA-sequencing and tumor/normal whole exome sequencing at our institution (HUM00046018, HUM00048105, HUM00067928, SU2C). Genomic and transcriptomic sequencing data was processed using Turnkey Precision Oncology. Genetic alterations, including ETS fusions, SPOP, FOXA1 class 1, and CDK12 mutations, as well as TP53 and RB1 mutations were analyzed. Clinical data was collected from 05/2021-01/2022, and clinical associations (metastasis free survival (MFS), time to castrate resistant prostate cancer (CRPC), and overall survival (OS)) were determined. Results: Data was available for 325 men. Median follow up from diagnosis was 106 months (IQR, 90-121), median age at diagnosis was 61 (IQR, 54-67), and most (91%) presented with PCa adenocarcinoma (n=292/325). At diagnosis, 51% (n=165) had localized, 5% (n=18) had clinical node positive, and 40% (n=128) had de-novo M1 disease. At time of tissue sampling, 87% (n=283) had metastatic disease, and 59% (n=192) were castrate resistant. Established PCa driver mutations included 140 ETS fusions (49%), 26 SPOP mutations (9%), 22 FOXA1 class 1 mutations (8%), and 15 (5%) CDK12 mutations. For men with localized disease at diagnosis (n=197/325), ETS fusion was associated with improved MFS (HR: 0.55; 95% CI: 0.37-0.81), time to CRPC (HR: 0.53; 0.35-0.80), and OS (HR: 0.56; 0.35-0.89). SPOP mutations were also associated with improved prognosis in this population (n=197/325): MFS (HR: 0.45; 0.24-0.84), time to CRPC (HR: 0.36; 0.18-0.73), and OS (HR: 0.46; 0.21-0.99). TP53 mutations were identified in 38% (n=122) of all patients and were associated with worse OS from the time of biopsy after adjusting for PCa castration state and disease spread at biopsy (HR: 2.2; 1.7-2.9, p

TPS5113 Background: Molecular imaging (i.e. PSMA directed agents) identifies metastatic prostate cancer at an earlier disease state than conventional imaging resulting in a new clinical entity within metastatic hormone-sensitive prostate cancer (mHSPC): molecular positive HSPC (mpHSPC). Historically, patients in this category with prior local therapy would have been classified as having a biochemical recurrence only, and observation was routine. Approaches to mpHSPC include observation or the use of focal and/or systemic therapies. Focal radiation for mpHSPC may delay the need for systemic therapy, yet many patients require further focal or systemic treatment. Androgen deprivation therapy (ADT) with abiraterone (abi) benefits men with high-risk localized disease and standard mHSPC. mpHSPC hypothetically resides somewhere between these two disease states. Finally, the inhibition of poly(ADP-ribose) polymerase (PARP) with olaparib plus abi shows promise in metastatic castration resistant prostate, suggesting this approach may be worthy of testing in earlier disease states. Methods: FAALCON is a single-site, phase 2 clinical trial testing olaparib with abi, ADT and radiation therapy in oligometastatic mpHSPC. Oligometastatic mpHSPC is defined as up to 5 radiation treatment sites (5 cm maximum size each) and must encompass all visible disease on the molecular scan. Patients must have had their prostate previously treated. The primary endpoint is the percentage of patients without treatment failure 24 months from study start. Treatment failure is defined as one of the following: new or progressive metastases on CT/MRI, new lesions on bone scan without alternate explanation, clinical progression, or a PSA doubling time under 6 months with an absolute final PSA over 1.5 ng/mL. Additional radiation therapy is deemed progression. Select secondary endpoints include time to any subsequent therapy, and percentage of patients with undetectable PSA with a recovered testosterone at multiple timepoints. Correlative work will analyze quality of life and prior prostatic tissue. ADT and abi (1000 mg daily) are given for 6 months, and radiation is completed by day 40. Olaparib (300 mg PO twice daily) is started 2 weeks after radiation completes and continues for the remaining ̃5 months. After therapy completion, patients are monitored by PSA q3 months with imaging based on predetermined PSA cutoffs. Molecular imaging (PSMA-PET) may be offered on study. Historical disease control at 24 months is estimated at 40% from prior molecular guided radiation studies and intermittent ADT. With 80% power and a one-sided 5% type-I error, we can detect a hazard ratio of 0.5 (80% control rate) at 24 months with 26 patients. To account for dropout, 29 patients will be accrued. Clinical trial information: NCT04748042.

TPS201 Background: Androgen receptor signaling inhibitors (ARSI), such as enzalutamide (Enza), and abiraterone (Abi), are standard therapies for metastatic hormone-sensitive and metastatic castration-resistant prostate cancer (mHSPC, mCRPC). Patients who respond to the initial ARSI are frequently prescribed a second ARSI upon progression. However, a suboptimal response to first line ARSI, including the ̃ 20% treated with an ARSI for mHSPC who progress within 12 months of treatment initiation, may enrich for cancers harboring AR-independent mechanisms of resistance including treatment-emergent neuroendocrine prostate cancer (t-NEPC). Recently, BETi have been shown pre-clinically to block the neuroendocrine prostate cancer lineage plasticity program through modulating E2F1, a transcription factor involved in stemness and cell differentiation. Prior results from a mCRPC Ph. 1b/2a trial of ZEN-3694+ Enza support this notion, as lower AR transcriptional activity in baseline tumor biopsies was associated with longer radiographic progression-free survival (rPFS). Additionally, mCRPC patients who were primary refractory to first-line abiraterone had prolonged rPFS with ZEN-3694 + Enza, suggesting that the patients with primary resistance may benefit from the combination. To test this hypothesis, a Ph. 2b randomized trial has been initiated, enriching for mCRPC with suboptimal response to first-line ARSI. Methods: This is a multi-national, open-label, randomized, two cohort, Ph. 2b study of ZEN-3694 + Enza vs. Enza in mCRPC patients who have progressed on Abi (NCT04986423). Cohort A (N = 150): Patients with poor response to Abi defined either as progression in < 12 months or failure to achieve PSA nadir of 0.2 ng/mL while taking Abi in HSPC setting, or progression in < 6 months and/or failure to achieve a PSA50 response while taking Abi in the CRPC setting. Cohort B (N = 50): Patients who responded to Abi, defined as > 12 months duration without progression while on Abi in the HSPC setting and achieving a nadir PSA < 0.2 ng/mL, or > 6 months duration without progression while on Abi in the CRPC setting and confirmed PSA50 response. The primary endpoint is radiographic progression-free survival (rPFS) by blinded independent central review (BICR) in Cohort A evaluated by PCWG3. Key secondary endpoints include rPFS by BICR for Cohorts A + B, PFS by investigator assessment, overall survival, PSA50 response rate, objective response rate by RECIST 1.1, and patient-reported health status and quality of life, evaluated in Cohorts A, and Cohorts A + B together. The trial, conducted in collaboration with Newsoara is expected to enroll 200 participants, and the first patient was dosed in October 2021. Astellas is providing the enzalutamide for this study. Clinical trial information: NCT04986423.

Purpose Metastatic castration-resistant prostate cancer (CRPC) is the lethal form of the disease. Many groups have performed mutational or immunohistochemistry (IHC) testing in metastatic CRPC to identify treatment targets. However, the frequency with which mutational or IHC data have an impact on clinical decision making and the outcomes of molecularly guided therapy in CRPC are largely unknown. We report our institution’s experience with mutational and IHC testing in patients with metastatic CRPC and its impact on clinical decision making and patient outcomes. Methods Between 2012 and 2015, 59 patients with CRPC underwent metastatic tissue biopsies and were genotyped with a 37–cancer gene panel in a Clinical Laboratory Improvement Amendments–certified laboratory. PTEN expression by IHC testing was also measured in 35 of these samples. A retrospective chart review was performed to determine whether the genomic information was acted upon and the outcome of patients whose treatment was guided by molecular testing. Results Forty-six of 59 patients with CRPC (78.0%) had biopsies with adequate tumor for mutational testing. Thirty-one of 46 subjects (67.4%) had mutations identified by sequencing. Of the 35 patients with CRPC whose biopsies were evaluated for PTEN expression by IHC testing, 13 had PTEN loss. Two patients had treatment on the basis of molecular testing, and one of these subjects had greater tumor control with molecularly guided therapy than his immediate prior therapy. Conclusion Targeted sequencing and IHC can identify clinically informative molecular abnormalities in CRPC. Despite this, a small minority of patients in our series underwent therapies guided by mutational or IHC testing. Actionability of abnormalities identified in metastatic CRPC may be improved with access to clinical trials, insurance approval for unapproved uses of existing anticancer drugs, and larger gene sequencing panels that include more frequently mutated genes.

148 Background: 5-hydroxymethylcytosine (5hmC) is an epigenetic modification which regulates gene expression and is associated with active transcription. The optimization of 5hmC sequencing in cell-free DNA (cfDNA) could therefore enable assessment of gene activity through a liquid biopsy. We aimed to investigate the 5hmC landscape of metastatic castration-resistant prostate cancer (mCRPC) and to evaluate the potential of 5hmC modifications in cfDNA as biomarkers of outcome in mCRPC patients. Methods: Genome-wide 5hmC modifications were analyzed with a low-input whole-genome 5hmC sequencing method based on selective chemical labeling in DNA from 93 mCRPC tissue biopsies previously profiled with whole-genome sequencing (WGS), RNA-sequencing and whole-genome bisulfite sequencing (WGBS). In addition, we analyzed 64 cell-free DNA (cfDNA) samples, from men with mCRPC before first-line abiraterone or enzalutamide, with both 5hmC sequencing and a conventional targeted ctDNA panel assessing common genomic alterations. Results: In mCRPC tissue samples, 5hmC enrichment was more strongly associated with gene expression than promoter methylation or copy number. Among cancer hallmark pathways, the androgen response genes had the strongest association between 5hmC and gene expression, suggesting a disease specific marking of gene activation. 5hmC patterns in cfDNA could be used to estimate the circulating tumor DNA fraction (ct-fraction), which was prognostic for overall survival (tertiles of ct-fraction, HR = 1.6 95%CI 1.1-2.3, p = 0.007). Further, 5hmC levels were indicative of gain of oncogene activity (such as AR, MYC, and PIK3CA) and loss of tumor suppressor gene activity (such as RB1, TP53 and BRCA2). The number of alterations, by 5hmC levels, of common drivers of mCRPC was prognostic for overall survival, also after adjusting for ct-fraction (adjusted p = 0.00001), and the prognostic value of common alterations detected by 5hmC sequencing versus conventional targeted ctDNA sequencing was similar. Finally, 5hmC levels in cfDNA of genes not significantly altered by copy number gain or loss (and thus not routinely included in targeted ctDNA sequencing assays), such as TOP2A and EZH2, identified a high-risk subgroup of mCRPC, which was highly prognostic for overall survival independent of ct-fraction (adjusted HR = 1.8 95%CI 1.2-2.8, p = 0.007). Conclusions: 5hmC in mCRPC tissue demonstrated an association with gene expression that was highest for prostate cancer driver genes, highlighting the ability to track disease-specific biology. 5hmC in cfDNA from men with mCRPC can be used to estimate the ct-fraction of the sample, infer activity gain and loss of common drivers of mCRPC, and identify high-risk groups of mCRPC based on alterations not commonly detected with conventional ctDNA sequencing, showing its potential as a liquid biomarker. Further studies are aimed at optimizing and validating 5hmC-based biomarkers in larger cohorts.

5507 Background: While recent studies have delineated the genomic landscape of mCRPC, its epigenomic landscape has not been as well characterized. The goal of this study was to define the comprehensive methylation landscape of mCRPC. Methods: mCRPC patients (pts) underwent a metastasis biopsy as part of a multi-institutional study (NCT02432001). Deep whole-genome bisulfite sequencing (mean depth 46x) was performed on fresh frozen tissue from 100 mCRPC patients; data was paired with deep whole-genome and transcriptome sequencing from the same samples. Unbiased hierarchical clustering of the mCRPC methylome was undertaken, and the survival of patients in each cluster was calculated using the Kaplan Meier method. Results: Unbiased hierarchical clustering revealed several distinct subtypes. 22% of mCRPC samples exhibited a novel epigenomic subtype associated with hyper-methylation. This hypermethylated (HM) cluster was significantly associated with somatic mutations in genes known to be involved in methylation, eg TET2 and DNMT3B, as well as in genes in which mutations have been associated with hyper-methylation in other cancer types ( IDH1 in glioblastoma and BRAF in colon cancer). mCRPC survival was 56.1 mos in pts with HM cancers compared to 35.6 mos in non-HM (p = .055). Methylome clustering also identified a unique cluster comprised of all patients with treatment-induced small cell/neuroendocrine cancer, a subtype previously associated with poor survival. Conclusions: This integrated study of whole-genome, whole methylome and whole-transcriptome sequencing provides the first comprehensive overview of the important regulatory role of methylation in metastatic castration-resistant prostate cancer, and has identified at least two distinct subtypes. The clinical and therapeutic implications of methylation subtypes should be explored in future studies. Clinical trial information: NCT02432001 .

158 Background: A subset of mCRPC patients (pts) that develop resistance to next-gen androgen signaling inhibitors are subsequently found to harbor t-SCNC. We have undertaken a longitudinal prospective biopsy (bx) study to clinically and molecularly characterize these pts. Methods: Eligible pts underwent an image-guided mCRPC bx at one of 5 WCDT centers. Tissue was both frozen and formalin fixed/paraffin embedded (FFPE). FFPE tissue was used for targeted next generation sequencing (NGS) and independent pathologic review by 3 pathologists. Frozen specimens underwent laser capture micro-dissection prior to RNA sequencing (seq). Machine learning was used to derive histology-specific expression signatures. Linear Discriminant Analysis (LDA) was applied to the RNA seq data from the first 131 bx using 3 observed histologic subtypes as labels to test for an intermediate subtype. Results: Of 391 mCRPC metastasis (53% bone, 26% node, 12% liver, 9% other soft tissue), 295(75%) yielded cancer. Overall, 89% of the bx contained a single histologic subtype: pure adeno in 156(53%); pure t-SCNC in 35(12%). A novel morphologically distinct subtype, IAC, with features of both tSCNC and adeno was seen in 72(24%). Samples comprised of two subtypes made up 32(11%) of all bx. Overall, 50(17%) of bx had a t-SCNC component. TP53 and/or RB1 loss of function variants were significantly more frequent in t-SCNC. NGS revealed no difference between adeno and IAC. LDA classified IAC as intermediate between adeno and t-SCNC 93% of the time, across 100 bootstrap replicates, significantly better than the 33% expected by chance (p −17). Conclusions: Nearly half of mCRPC bx exhibit non-adeno features with t-SCNC found in 17%. IAC is a reproducible mCRPC histologic subtype found in 24% of bx, with a gene expression signature that is intermediate to adeno and t-SCNC, and targeted NGS results similar to adeno. Understanding the role of IAC in disease evolution and resistance is critical for improving outcomes in mCRPC. Clinical trial information: NCT02432001.

320 Background: While there is no standard therapy for BCR PC following local therapy, intermittent ADT is widely used. We evaluated utility of APA alone, ADT (luteinizing hormone–releasing hormone agonist [LHRHa]) alone, or APA + LHRHa in ADT-naïve BCR PC pts. Methods: Pts with BCR PC after primary definitive local therapy and prostate-specific antigen (PSA) doubling time (PSADT) ≤ 12 mo were randomized 1:1:1 to open-label 240 mg APA daily, LHRHa alone, or APA + LHRHa for 12 mo, followed by a 12-mo observation period off therapy. Pts were stratified by PSADT (< 6 vs 6-12 mo) and age (≤ 70 vs > 70 y). Primary end point: mean change from baseline (BL) in health-related quality of life (HRQoL) per Functional Assessment of Cancer Therapy-Prostate total score at 12 mo. Secondary end points included PSA nadir < 0.2 ng/mL by 7 mo, time to PSA progression (TTPpsa), and time to testosterone (T) recovery. Results: 90 pts (APA, n = 29; LHRHa, n = 30; APA + LHRHa, n = 31) were treated for median of 12 mo with similar distribution of BL characteristics across groups: 67% age ≤ 70 y; 67% PSADT < 6 mo. There was no significant difference in HRQoL in APA vs LHRHa at 12 mo, or between LHRHa vs APA + LHRHa groups. At median follow-up of 30-33 mo, TTPpsa in APA, LHRHa, and APA + LHRHa groups was 26 mo, 31 mo, and 36 mo, respectively. Compared to LHRHa alone, APA + LHRHa resulted in a trend toward improved TTPpsa (HR [95% CI] 0.56 [0.23-1.36], p = 0.196), and APA alone resulted in a trend for shorter TTPpsa (HR 1.09 [0.49-2.43], p = 0.824). PSA nadir < 0.2 ng/mL was reached in 89%, 89%, and 97% in APA, LHRHa, and APA + LHRHa pts. Median time to T recovery was similar in LHRHa and APA + LHRHa groups (23 mo vs 24 mo). Grade 3-4 adverse events (AEs) occurred in 17% of APA, 14% of LHRHa, and 29% of APA + LHRHa pts. The only grade 3-4 AE reported in > 1 pt per group was hypertension (APA, 3%; LHRHa, 0; APA + LHRHa, 13%). Conclusions: Addition of APA to LHRHa resulted in a trend for longer TTPpsa and a higher proportion of pts achieving optimal PSA nadir without significant difference in HRQoL or time to T recovery. Observed AEs were consistent with known safety profiles. Results support further evaluation of APA + LHRHa for a specified duration in BCR PC. Clinical trial information: NCT01790126.

86 Background: There are six agents that improve survival in mCRPC, each administered as a single agent. Combinations of agents with distinct mechanisms of action have the potential to improve outcomes. Methods: We performed a multi-institution phase I/II study to examine safety and efficacy of CAB plus ENZ with mandatory granulocyte-colony stimulating factor support in mCRPC. Results: A sample size of 3 to 12 subjects for the phase I portion and 33 for the phase II portion provided 82% power to detect PSA response rate (decrease ≥90%) of 50% compared to the null hypothesis of 24%. The main eligibility criteria allowed prior abiraterone/prednisone (AAP) and docetaxel (in the metastatic hormone sensitive setting). Baseline characteristics: median age 69 years (47 - 82), median PSA 20.2 ng/dl (0.2 - 966.3); 7 subjects had visceral disease, 10 received prior AAP, and 8 received prior docetaxel. In the phase I portion, there were no dose limiting toxicities using CAB 25 mg/m2 IV Q3wks up to 10 cycles and ENZ 160 mg PO QD, hence this dosing was used for the phase II portion. 33 men with mCRPC were treated with CAB plus ENZ in the phase II arm. PSA response rates are listed in Table. Prior exposure to AAP decreased PSA response, but subjects who had prior AAP also had higher pre-treatment PSA. There were no treatment related deaths. Dose reduction of CAB to 20 mg/m2 was needed in 7 subjects. Over the course of the study, 14 Grade 3 adverse events occurred that were deemed possibly related to treatment: fatigue (n=2, 6%), febrile neutropenia (n=2, 6%), leukopenia (n=2, 6%), thrombocytopenia (n=2, 6%), anemia (n=1, 3%), hypertension (n=1, 3%), leukocytosis (n=2, 6%), fracture (n=1, 3%), failure to thrive (n=1, 3%). Conclusions: CAB plus ENZ was tolerable and associated with promising anti-tumor activity, particularly in abiraterone-naïve subjects. Further evaluation of this regimen is warranted. This project was managed by the Prostate Cancer Clinical Trials Consortium and funded by Astellas Inc. and Sanofi. Clinical trial information: NCT02522715. [Table: see text]

145 Background: The net oncogenic effect of the G protein-coupled receptor β2 adrenergic receptor ADRB2, which may induce neuroendocrine differentiation via cyclic AMP and protein kinase A and whose expression is epigenetically regulated by EZH2, is controversial. ADRB2 expression and associated clinical outcomes in metastatic castration-resistant prostate cancer (mCRPC) are unknown. Methods: This was a retrospective analysis of a cohort of men with mCRPC who were prospectively enrolled in the multi-center SU2C/PCF/AACR West Coast Prostate Cancer Dream Team study, in which biopsies of a metastatic site were obtained at disease progression. Specimens underwent laser capture microdissection and RNA-seq. ADRB2 expression was stratified by histology and transcriptional cluster based on prior unsupervised hierarchical transcriptome clustering, and correlated with EZH2 expression. ADRB2 expression (lowest quartile) was correlated with OS from time of biopsy by log rank test and a multivariable Cox proportional hazard model. Results: One-hundred and twenty-seven men with progressive mCRPC underwent metastatic biopsies and had sufficient tumor for RNA-seq. ADRB2 expression was lowest in the small cell-enriched transcriptional cluster (P

It was not until 2004 that the first clinical trials in metastatic castration-resistant prostate cancer (mCRPC) demonstrated a survival benefit for men with this disease. The results of the TAX327 and SWOG 9916 trials demonstrated extended survival and improved quality-of-life benefit of docetaxel, offering a new option for patients with mCRPC. Since then, phase III clinical trials of sipuleucel-T, cabazitaxel, abiraterone, enzalutamide, and radium-223 have established the efficacy of these agents for the treatment of mCRPC. Furthermore, recent work also demonstrates a significant improvement in overall survival with earlier use of docetaxel in the metastatic castration-naive setting. Despite these notable successes, failure has been much more common in mCRPC phase III clinical trials (Table 1). As demonstrated in the article accompanying this editorial, there are several reasons why the phase III trial of cabozantinib versus prednisone might have been negative, despite the apparently promising results of earlier-stage trials. Improvements in progressionfree survival, bone scan response, circulating tumor cell conversion—end points commonly associated with overall survival benefits—were seen in the phase II clinical trials of cabozantinib. However, despite these promising signals, declines in serum prostate-specific antigen concentrations—an imperfect and yet important marker of antitumor activity—were rare in prior phase II studies of cabozantinib, and PSA responses remained uncommon in the phase III trial. Moreover, it is possible that changes in bone scintigraphy seen with cabozantinib reflected, at least in part, interference with the imaging modality rather than true antitumor activity. This hypothesis is supported by the observation that enzalutamide and abiraterone (agents that have well-established antitumor activity inmCRPC and produce substantial improvements in progression-free and overall survival) do not cause the dramatic improvements in bone scintigraphy that are seen with cabozantinib. Thus, the true level of antitumor activity of cabozantinib was likely lower than hoped for. Other factors that are not unique to cabozantinib may also have contributed to the negative outcome of this phase III trial. These factors include the molecular and clinical heterogeneity of the unselected patient population studied, access to postprotocol life-prolonging therapies, and early discontinuation of cabozantinib because of toxicity. However, even if the trial proved to be positive in a statistical sense, would it have produced a clinically meaningful result? Would it have yielded a new, well-tolerated treatment option that patients with mCRPC and their providers would embrace? Although we will not know the answer to those questions, concern is raised by themagnitude of overall survival improvement this trial was designed to detect. The trial sample size was selected based on the assumption that the median survival in the control armwould be 7months, and success was defined as a 25% reduction in the risk of death (a hazard ratio of 0.75). This might be a meaningful amount of benefit to a patient whose life expectancy is 3 years, but it translates into only a 2.3-month improvement in median survival in the patient population tested. Even acknowledging that treatment benefit varies considerably between patients, one is left to ask: Is this a bar worth jumping over? Furthermore, the trial was designed with 90% power. If we were to repeat the power calculation with the commonly used 80%

5011 Background: Although enza prolongs life in mCRPC pts, the development of drug resistance and subsequent disease progression is nearly universal. Seeking to clarify molecular mechanisms that underlie enza resistance, we analyzed whole genome sequencing (WGS) and RNA sequencing (seq) of tumors obtained from patients with enza-naive or -resistant mCRPC. Methods: One hundred and one men with mCRPC who underwent image-guided biopsy and subsequent WGS were included (n = 64 with enza-naive and n = 37 with enza-resistant mCRPC). The differential copy number alteration (CNA) events enriched in enza-resistant vs. naïve samples were determined, and the prognostic significance of differential CNAs was assessed. RNA-seq data were evaluated to confirm that CNAs correlated with changes in gene expression of relevant loci and to identify potentially druggable targets selectively activated in tumors with specific CNAs. Results: Copy number loss was more common than gain in enza-resistant tumors. Specifically, we identified 123 protein-coding genes that were more commonly lost in enza-resistant samples—eight of which were previously described tumor suppressor genes. There was a strong concordance of copy number loss and reduced mRNA expression of these genes. We identified one gene from this list of eight genes whose copy number loss was associated with poor overall survival (median overall survival from date of CRPC was 19.1 months in tumors with gene loss vs. 42.0 months in intact tumors, hazard ratio 3.8 [1.46–9.8], log-rank p = 0.003). Finally, Master Regulator analysis determined that tumors with copy number loss of this poor prognosis gene had activation of several potentially targetable factors, including the kinases Akt and PLK1. Conclusions: Copy number loss of specific tumor suppressor genes is associated with enza resistance in mCRPC patients. Previously unappreciated molecular subsets of enza-resistant CRPC were identified, including one subset associated with poor clinical outcome.

146 Background: Metastatic castration-resistant prostate cancer (mCRPC) is the lethal form of the disease. Several recent efforts have identified genomic alterations in mCRPC, but the clinical implications of these alterations have not been fully elucidated. We conducted a prospective cohort study (n = 101) using whole genome sequencing (WGS) to analyze the association between key driver gene alterations and overall survival. We also performed whole-transcriptome RNA sequencing (RNA-seq) analyses to identify potential mechanisms of enzalutamide resistance in mCRPC. Methods: Metastasis biopsies were obtained in 101 mCRPC patients as part of the multi-institutional West Coast Prostate Cancer Dream Team project. Samples underwent WGS and RNA-seq. The resulting mutation, copy number, and structural variant calls were integrated to determine functional copy number status of candidate genes for downstream clinical analyses. We performed univariate and multivariable analyses to assess the prognostic significance of candidate genomic events with respect to overall survival. To nominate and investigate genomic pathways associated with enzalutamide resistance, we performed expression-based gene set enrichment analysis followed by cross-sectional enrichment and survival analyses related to the top nominated pathway. Results: RB1 loss was associated with poor overall survival (median 14.1 vs. 42.0 months, p < 0.001). When we compared enzalutamide resistant versus naïve samples using gene set enrichment analysis, we identified the Wnt/beta-catenin pathway as the top differentially expressed pathway in enzalutamide-resistant patients. Furthermore, CTNNB1 (beta-catenin) activating mutations were exclusive to enzalutamide-resistant patients (p = 0.013) and predictive of poor overall survival (median 13.6 vs. 41.7 months, p < 0.001). Conclusions: Impaired survival in mCRPC patients is associated with RB1 loss, identified by integrated genomic analysis of CRPC metastasis biopsies. Among men with mCRPC that was enzalutamide-resistant, the Wnt/beta-catenin pathway is nominated as an important predictive (and potentially therapeutic) pathway.

5047Background: Anti-PD1 treatment with Pembro is a promising treatment strategy in many solid tumors. Here, we report clinical outcomes of adding Pembro to men with mCRPC progressing on Enz. Metho...

e14521Background: To implement real-time multi-omic analysis of proximal tumor biopsies to support precision-guided therapeutic intervention, a tissue acquisition and analysis trial was implemented...

5039Background: Genomic alterations in DNA repair genes are present in approximately 20-30% of patients with mCRPC. t-SCNC may be increasing in prevalence and is associated with adverse clinical ou...

TPS5097Background: The management of mCRPC has been both enhanced and complicated by the rapid emergence of at least five new agents that can lengthen survival. While great strides have been made i...

278 Background: Detection of t-SCNC in mCRPC patients relies primarily on histopathologic evaluation (Histo) of a metastatic tumor biopsy (bx), likely leading to underdiagnosis. The clinical features of t-SCNC and the diagnostic utility of serum NE markers (neuron-specific enolase (NSE) and chromogranin (CGA)) were evaluated. Methods: Eligible patients (pts) underwent a metastatic bx at one of 5 centers. Histo was performed by 3 independent pathologists (JH, GT, LT). NE markers were evaluated in a central lab (lower limit = 1 ng/mL). Kruskal-Wallis and chi-square test were used to compare continuous and categorical variables, respectively. Receiver-operative-curve (ROC) analysis of serum NE markers was undertaken. Results: 160 consecutive pts with available Histo and NE markers were included. t-SCNC was found in 27 pts (17%). Detection of t-SCNC was observed in all bx sites, including liver (14%), lymph node (19%) and bone (14%). Clinical features are shown in the Table. By ROC analysis, if both serum NSE was > 6.05 ng/mL and chromogranin was > 3.1 ng/mL, the sensitivity, specificity, negative predictive value (NPV), and positive predictive value (PPV) for the detection of t-SCNC were 95%, 50%, 98%, and 22%, respectively. Conclusions: Many of the classic features of de novo SCNC, including low PSA levels, do not reliably distinguish t-SCNC. In contrast, serum NE markers have diagnostic utility with high sensitivity and NPV, but low specificity and PPV. Heterogeneous NE differentiation may partially account for these findings. Clinical trial information: NCT02432001. [Table: see text]

197 Background: The PAM50 gene expression classifier (PAM50) identifies luminal and basal subtypes and predicts response to androgen deprivation therapy in localized prostate cancer. The clinical utility of using PAM50 to molecularly subtype mCRPC was evaluated. Methods: PAM50 was applied to RNA expression data from 86 metastatic tumor biopsies from the SU2C-AACR-PCF West Coast Prostate Cancer Dream Team (WCDT; NCT02432001). Overall survival (OS) differences between luminal A (LuA), luminal B (LuB), or basal patients (pts) were determined using Kaplan-Meier analyses. PAM50 was also applied to 15,136 prospectively collected radical prostatectomy (RP) samples from the Decipher GRiD database (NCT02609269). Drug response signatures (DRS) for 89 drugs were derived using publicly available data from the NCI60 cell line panel, and applied to gene expression data from the RP samples to predict patient-specific drug sensitivities. Differences in DRS as a function of PAM50 subtype were assessed using the Pearson’s chi-squared test. Results: The application of PAM50 to mCRPC transcriptional data segregated pts into LuA, LuB, and basal populations (43%, 14%, and 43%, respectively). The median OS for LuA, LuB, and basal pts was 20.6 months, 9.5 months, and 10.4 months, respectively (p=0.04), which was consistent with localized prostate cancer where LuB pts have the worst prognosis. DRS analyses revealed statistically significant differences in drug sensitivities, with LuA and LuB pts predicted to be markedly more sensitive to docetaxel than basal pts (p