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80 Background: The poly(ADP-ribose) polymerase (PARP) inhibitor rucaparib was granted accelerated approval by the US Food and Drug Administration for patients with BRCA+ mCRPC based on results from the phase 2 TRITON2 study (NCT02952534). The TP53 tumor suppressor gene is among the most frequently mutated genes in human cancers, including mCRPC, and alterations in TP53, PTEN, and RB1 are associated with poor prognosis in patients with prostate cancer and other tumor types. We present data on co-occurring alterations in patients with BRCA+ mCRPC treated with rucaparib in TRITON2. Methods: Patients had progressed on 1–2 lines of androgen receptor-directed therapy and 1 taxane-based chemotherapy and were treated with rucaparib 600 mg BID. Tissue and/or cell-free DNA extracted from plasma samples were profiled comprehensively for genomic alterations using Foundation Medicine, Inc., next-generation sequencing assays. Objective response rate (ORR) was assessed per modified Response Evaluation Criteria in Solid Tumors and Prostate Cancer Working Group 3 criteria by independent radiologic review of patients with measurable disease. Prostate-specific antigen (PSA) response rate (≥50% decrease from baseline) was assessed in all patients. Results: Tissue and/or plasma samples were available for 114/115 patients with BRCA+ mCRPC (visit cutoff date: Dec. 23, 2019). Among patients with BRCA+ mCRPC who had samples available for comprehensive genomic profiling, 36.8% (42/114) had a co-occurring alteration in TP53. Deleterious alterations in PTEN were observed in 34.2% (39/114) of patients, 44% (17/39) of which were homozygous deletions of PTEN. RB1 loss was observed in 12.3% (14/114) of patients and was seen more frequently in patients with measurable disease (18.0%, 11/61) than in patients with non-measurable disease (5.7%, 3/53). Although patients with and without TP53 mutations had generally similar baseline demographics and disease characteristics, visceral disease was more prevalent in patients with TP53 mutations (54.8%; 23/42) than in those without them (29.2%; 21/72). Similar ORR and PSA response rates were seen in patients with BRCA+ mCRPC with or without TP53 mutation, with a non-significant trend towards lower response rates in patients with co-occurring TP53 alterations. Conclusions: Results from TRITON2 showed antitumor activity for rucaparib in patients with BRCA+ mCRPC associated with or without co-occurring alterations in TP53. Demographics and additional efficacy analyses in genomic subgroups with co-occurring alterations in TP53, PTEN, and RB1 will be reported. Clinical trial information: NCT02952534. [Table: see text]

335 Background: Systematic reviews are outdated quickly when the evidence is rapidly evolving as the process is laborious and there is little incentive for primary author team of an index SRMA to update the evidence. Consequently, there is an epidemic of redundant SRMAs performed by different teams—sometimes with conflicted results—for treatment of first line mRCC. Methods: We have created a living, interactive systematic review (LISR)and network meta-analysis(LINMA) for the treatment of first line mRCC using an Artificial intelligence (AI) assisted framework for evidence synthesis (Living, Interactive evidence synthesis framework) (LIvE) . The framework is implemented in five-layered architecture (application layer, shared module layer, core service layer, middleware layer, and storage layer) which work together to automate the identification of new studies and analysis and semi-automate the screening and data extraction. Dynamic features such as interactive tables, figures and evidence maps are enabled using Python and JavaScript programming languages. Results: We have maintained a living, interactive evidence profile for the first line treatment mRCC since September 2019 ( LIVING WEBSITE) . Living search strategy identifies new studies as they become available. As of October 13, 2020 LISR, includes data 14 clinical trials ( PRISMA ). Baseline characteristics are summarized in an interactive table ( TABLE) . Cabozantinib& Nivolumab (Cabo-Nivo) is the highest ranked drug for improving Overall Response (OR), Progression Free Survival (PFS) and Overall Survival (OS) whereas Ipilimumab in combination with Nivolumab (Ipi-Nivo) is highest ranked drug for achieving complete response (CR). Ipi-Nivo and Atezolizumab & Bevacizumab (Ate-Bev) ranked highest and Cabo-Nivo ranked lowest for treatment related Adverse events (TRAEs). Results of network meta-analysis are summarized as interactive tables and plots ( NMA ), summary of findings tables ( MULTIPLE COMAPRISONS ) and evidence maps ( MAP ). Conclusions: LISRs can potentially reduce redundancy, increase transparency, reproducibility, enable shared-decision making (at a guideline level, or in a patient-clinician dyad) and support living guidelines.

256 Background: PARP inhibitors, olaparib and rucaparib recently improved survival outcomes (in the Profound and Triton-2 trials) in pts with aPC harboring HRRm. Notably, ~35% of pts screened for the PROfound trial did not meet eligibility solely due to the lack of adequate quality/quantity of tumor tissue required for CGP (De Bono, NEJM, 2020). cfDNA analysis non-invasively assesses tumor-derived genomic alterations. We evaluated the prevalence of HRRm in a real-world aPC population, who had commercially available cfDNA assay results available. Methods: cfDNA based CGP (using a clinically-validated 73- to 74-gene panel i.e. Guardant360 or G360) from consecutive aPC pts between 11/2016–8/2020 were used for detection of HRRm. Frameshift and nonsense mutations were included as pathogenic. Variants of unknown significance were excluded. In this unmatched study, the prevalence of each HRRm was compared with those reported in literature using the chi square test. Results: cfDNA CGP from 7701 aPC pts were available for interrogation of BRCA1/BRCA2 mutations, & from 4671 aPC pts for ATM mutations. Prevalence of BRCA1 and 2 as detected by cfDNA was similar to historical CGP of primary tissue. Prevalence of BRCA1 was higher than historical CGP of metastatic tissue. Prevalence of ATM in cfDNA was higher than historic tumor tissue CGP but similar to prior reports from cfDNA testing (Table). Conclusions: In this large real-world population of pts with aPC undergoing routine cfDNA CGP by a CLIA certified lab, prevalence of HRRm was similar (or higher for BRCA1 and ATM) to what has been previously reported from the tumor tissue. These data provide the rationale for utilizing cfDNA CGP as a routine test for detection of HRRm in men with aPC to identify men who are candidates for PARPi. [Table: see text]

Abstract Background Genomic biomarkers that predict response to anti-PD1 therapy in prostate cancer are needed. Frameshift mutations are predicted to generate more neoantigens than missense mutations; therefore, we hypothesized that the number or proportion of tumor frameshift mutations would correlate with response to anti-PD1 therapy in prostate cancer. Methods To enrich for response to anti-PD1 therapy, we assembled a multicenter cohort of 65 men with mismatch repair-deficient (dMMR) prostate cancer. Patient characteristics and outcomes were determined by retrospective chart review. Clinical somatic DNA sequencing was used to determine tumor mutational burden (TMB), frameshift mutation burden, and frameshift mutation proportion (FSP), which were correlated to outcomes on anti-PD1 treatment. We subsequently used data from a clinical trial of pembrolizumab in patients with nonprostatic dMMR cancers of various histologies as a biomarker validation cohort. Results Nineteen of 65 patients with dMMR metastatic castration-resistant prostate cancer were treated with anti-PD1 therapy. The PSA50 response rate was 65%, and the median progression-free survival (PFS) was 24 (95% confidence interval 16–54) weeks. Tumor FSP, more than overall TMB, correlated most strongly with prolonged PFS and overall survival (OS) on anti-PD1 treatment and with density of CD8+ tumor-infiltrating lymphocytes. High FSP similarly identified patients with longer PFS as well as OS on anti-PD1 therapy in a validation cohort. Conclusion Tumor FSP correlated with prolonged efficacy of anti-PD1 treatment among patients with dMMR cancers and may represent a new biomarker of immune checkpoint inhibitor sensitivity. Implications for Practice Given the modest efficacy of immune checkpoint inhibition (ICI) in unselected patients with advanced prostate cancer, biomarkers of ICI sensitivity are needed. To facilitate biomarker discovery, a cohort of patients with DNA mismatch repair-deficient (dMMR) prostate cancer was assembled, as these patients are enriched for responses to ICI. A high response rate to anti-PD1 therapy in these patients was observed; however, these responses were not durable in most patients. Notably, tumor frameshift mutation proportion (FSP) was identified as a novel biomarker that was associated with prolonged response to anti-PD1 therapy in this cohort. This finding was validated in a separate cohort of patients with nonprostatic dMMR cancers of various primary histologies. This works suggests that FSP predicts response to anti-PD1 therapy in dMMR cancers, which should be validated prospectively in larger independent cohorts.

PURPOSE BRCA1 or BRCA2 ( BRCA) alterations are common in men with metastatic castration-resistant prostate cancer (mCRPC) and may confer sensitivity to poly(ADP-ribose) polymerase inhibitors. We present results from patients with mCRPC associated with a BRCA alteration treated with rucaparib 600 mg twice daily in the phase II TRITON2 study. METHODS We enrolled patients who progressed after one to two lines of next-generation androgen receptor–directed therapy and one taxane-based chemotherapy for mCRPC. Efficacy and safety populations included patients with a deleterious BRCA alteration who received ≥ 1 dose of rucaparib. Key efficacy end points were objective response rate (ORR; per RECIST/Prostate Cancer Clinical Trials Working Group 3 in patients with measurable disease as assessed by blinded, independent radiology review and by investigators) and locally assessed prostate-specific antigen (PSA) response (≥ 50% decrease from baseline) rate. RESULTS Efficacy and safety populations included 115 patients with a BRCA alteration with or without measurable disease. Confirmed ORRs per independent radiology review and investigator assessment were 43.5% (95% CI, 31.0% to 56.7%; 27 of 62 patients) and 50.8% (95% CI, 38.1% to 63.4%; 33 of 65 patients), respectively. The confirmed PSA response rate was 54.8% (95% CI, 45.2% to 64.1%; 63 of 115 patients). ORRs were similar for patients with a germline or somatic BRCA alteration and for patients with a BRCA1 or BRCA2 alteration, while a higher PSA response rate was observed in patients with a BRCA2 alteration. The most frequent grade ≥ 3 treatment-emergent adverse event was anemia (25.2%; 29 of 115 patients). CONCLUSION Rucaparib has antitumor activity in patients with mCRPC and a deleterious BRCA alteration, but with a manageable safety profile consistent with that reported in other solid tumor types.

To report a multi-institutional case series of patients with advanced microsatellite instability high (MSI-H) prostate adenocarcinoma identified with clinical cell-free DNA (cfDNA) next-generation sequencing (NGS) testing and treated with immune checkpoint inhibitors. Retrospective analysis of patients with metastatic castration-resistant prostate cancer (mCRPC) and MSI-H tumor detected by a commercially available cfDNA NGS assay Guardant360 (G360, Guardant Health) at eight different Academic Institutions in the USA, from September 2018 to April 2020. From a total of 14 MSI-H metastatic prostate cancer patients at participating centers, nine patients with mCRPC with 56% bone, 33% nodal, 11% liver and 11% soft-tissue metastases and a median PSA of 29.3 ng/dL, were treated with pembrolizumab after 2 lines of therapy for CRPC. The estimated median time on pembrolizumab was 9.9 (95% CI 1.0 to 18.8) months. Four patients (44%) achieved PSA50 after a median of 4 (3–12) weeks after treatment initiation including three patients with >99% PSA decline. Among the patients evaluable for radiographic response (n=5), the response rate was 60% with one complete response and two partial responses. Best response was observed after a median of 3.3 (1.4–7.6) months. At time of cut-off, four patients were still on pembrolizumab while four patients discontinued therapy due to progressive disease and one due to COVID-19 infection. Half of the patients with PSA50 had both MSI-H and pathogenic alterations in BRCA1 and BRCA2 in their G360 assays. The use of liquid biopsy to identify metastatic prostate cancer patients with MSI-H is feasible in clinical practice and may overcome some of the obstacles associated with prostate cancer tumor tissue testing. The robust activity of pembrolizumab in selected patients supports the generalized testing for MSI-H.

e17587 Background: Radium (Ra)-223 has become the cornerstone treatment in patients with metastatic castrate-resistant prostate cancer (mCRPC) with bone metastases. Ra-223 has been shown to suppress abnormal bone formation by inducing DNA double-strand breaks in tumor cells. This can reduce skeletal-related events, as well as improve survival and quality of life. Studies have indicated mutations that regulate DNA damage response in prostate cancer as susceptible to PARP inhibitors and platinum-based therapies. This study aims to evaluate mCRPC response to Ra-223 stratified by tumor genomics. Methods: This is a retrospective study of mCRPC patients who received Ra-223 and genetic testing within the Mayo Clinic database (Arizona, Florida, and Minnesota). Patient demographics, genetic mutations, treatment responses in terms of alkaline phosphatase (ALP) and prostate-specific antigen (PSA), and survival were assessed. Primary end points were progression-free survival (PFS) and overall survival (OS) from time of first radium treatment. Results: 239 mCRPC patients treated with Ra-223 were identified. Germline and/or somatic genetic sequencing was available in 50 patients, who had a median age of 61 years at time of diagnosis and 68 years at time of Ra-223 treatment. Median Gleason score at time of diagnosis was 8.0. 100% of patients received prior androgen deprivation therapy. 72% received prior docetaxel, and 18% received prior cabazitaxel. Notable mutations included TP53 (45.2%), TMPRSS2-ERG (28.6%), PTEN (26.2%), BRCA1/2 (14.3%), ATM (9.3%), CDK12 (7.1%), and Rb (4.9%). TMPRSS2-ERG was the second most commonly found mutation, with median OS 12.7 mo versus 15.4 mo in patients without TMPRSS2-ERG mutations (p = 0.099). In patients with CDK12 mutations (2 of 28 tested positive), PFS was 10.4 mo versus 5.6 mo in patients without CDK12 mutations (p = 0.157). The assessment of ALP and PSA response to radium treatment stratified by genetic mutation did not reach statistical significance. Conclusions: Among mCRPC patients treated with Ra-223 at Mayo Clinic, TMPRSS2-ERG was the second most commonly found genetic mutation and had observed worse survival outcomes that approached statistical significance. Prospective studies and increased sample sizes are needed to determine the impact of genetic mutations in response to Ra-223.

e17540 Background: The clinical course in metastatic castrate-resistant prostate cancer (mCRPC) can be complicated when patients have disease progression after treatment with 2nd generation hormone therapy (2nd-HT), such as enzalutamide or abiraterone. Currently, limited data exist regarding the optimal choice of chemotherapy for mCRPC after failing 2nd-HT. We sought to evaluate three common chemotherapy regimens in this setting. Methods: We retrospectively identified 150 patients with mCRPC with disease progression on enzalutamide or abiraterone. 92 patients were chemo-naïve, while 58 patients had previously received docetaxel chemotherapy prior to 2nd-HT. After failing 2nd-HT, 90 patients received docetaxel-alone (group A), 33 patients received carboplatin plus docetaxel (group B), while 27 patients received cabazitaxel-alone (Group C). Favorable response was defined by ≥50% reduction in PSA level from baseline after a complete course of chemotherapy. Survival outcome was assessed for 30-month overall survival. Results: Mean (SD) age was 71.2 (8.28), 69.5(8.38) and 67.2 (8.36) for group (A), (B) and (C), respectively. Mean (SD) pre-chemotherapy PSA was 63.8 (138.18), 58.5 (118.15) and 53.7 (88.15) for group (A), (B) and (C), respectively. Mean (SD) Gleason score was 7.9 (1.1), 8.4 (0.88) and 8.1 (1.06) for group (A), (B) and (C), respectively. Patients in group (B) were 2.6 times more likely to have a favorable response compared to group (A) (OR = 2.625, 95%CI: 1.15 - 5.99) and almost 3 times compared to patients in group (C) (OR = 2.975, 95%CI: 1.04 – 8.54) (p-value = 0.0442). We report a Hazard Ratio (HR) of 3.1 (95% CI 1.31-7.35; p = 0.0037) between patients in group (A) versus group (B), and a HR of 4.18 (95% CI 1.58-11.06; p = 0.0037) between patients in group (C) versus group (B). Thirty-month overall survival was 70.7%, 38.9%, and 30.3% for group (B), (A), and (C) respectively (p-value = 0.008). Conclusions: Our data demonstrate improved response and cancer-specific survival in patients with treatment-refractory mCRPC on docetaxel plus carboplatin compared to docetaxel or cabazitaxel alone. Selection bias is inherent in any retrospective study; however, our finding suggests that clinicians may consider docetaxel plus carboplatin in mCRPC patients who fail 2nd-HT. Further prospective studies are warranted.

e17539 Background: Chemotherapy with Docetaxel (D) or androgen pathway inhibition (API) with Abiraterone Acetate plus prednisone (AAP), Aplautamide(APA) and Enzalutamide(E) are acceptable, FDA approved treatment options for mCSPC. It is not clear whether the magnitude of benefit varies by the choice of initial agent [chemotherapy vs API] or by volume of disease [High vs Low]. Data is now available from all registration trials by volume status and motivated this analysis to inform initial treatment choice in mCSPC. Methods: We systematically searched MEDLINE(Ovid), Embase, and Scopus for randomized controlled trials of chemotherapy(D) or APIs (AAP, APA, ENZ) that had available hazard ratios (HRs) for overall survival (OS) and Progression Free Survival (PFS) according to patient’s volume of disease. We also reviewed abstracts and presentations from all major conference proceedings. We calculated the pooled overall survival HR and 95% CI by chemotherapy and APIs and by high volume(HVD) and low volume(LVD) using a random effect model, and tested for heterogeneity to assess the null hypothesis that no difference in the survival advantage exists by choice of initial agent and volume of disease. Results: Of 4456 studies identified in our search, there were 8 eligible randomized controlled trials that were included in the analysis. Both D and APIs significantly improved PFS [HR 0.48; 0.45-0.51] and OS [0.72; 0.64-0.81] when added to ADT, however the latter was associated with significantly higher improvement in PFS( P < 0.01) and OS (P = 0.03). In patients treated with D, patients with HVD derive significantly more benefit as compared to LVD( P = 0.046) and patients treated with APIs both HVD and LVD patients derive similar benefit( P = 0.80) (Table). Conclusions: mCSPC patients derive higher magnitude of survival benefit when treated with APIs as compared to D; however, D may be preferred in HVD patients. [Table: see text]