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PURPOSE Nearly all men with prostate cancer treated with androgen receptor (AR) signaling inhibitors (ARSIs) develop resistance via diverse mechanisms including constitutive activation of the AR pathway, driven by AR genomic structural alterations, expression of AR splice variants (AR-Vs), or loss of AR dependence and lineage plasticity termed neuroendocrine prostate cancer. Understanding these de novo acquired ARSI resistance mechanisms is critical for optimizing therapy. MATERIALS AND METHODS A novel liquid biopsy technology was used to collect mRNA from circulating tumor cells (CTCs) to measure expression of AR-Vs, AR targets, and neuroendocrine prostate cancer markers. An institutional review board–approved prospective cohort (N = 99) was used to identify patterns of gene expression. Two prospective multicenter phase II clinical trials of ARSIs for men with castration-resistant prostate cancer (ClinicalTrials.gov: NCT01942837 [enzalutamide, N = 21] and NCT02025010 [abiraterone, N = 27]) were used to further validate these findings. RESULTS Hierarchical clustering of CTC transcripts identified two distinct clusters. Cluster 2 (C2) exhibited increased expression of AR-regulated genes and was associated with worse overall survival (median 8.6 v 22.4 months; P < .01; hazard ratio [HR] = 3.45 [1.9 to 6.14]). In multivariable analysis, C2 was prognostic independent of other clinicopathologic variables. AR-V status was not significant when accounting for C2. Upon further validation in pooled multicenter phase II trials, C2 was associated with worse overall survival (15.2 months v not reached; P < .01; HR = 8.43 [2.74 to 25.92]), prostate-specific antigen progression-free survival (3.6 v 12 months; P < .01; HR = 4.64 [1.53 to 14.11]), and radiographic progression-free survival (2.7 v 40.6 months; P < .01; HR = 4.64 [1.82 to 17.41]). CONCLUSION We demonstrate that a transcriptional profile detectable in CTCs obtained from liquid biopsies can serve as an independent prognostic marker beyond AR-V7 in patients with metastatic prostate cancer and can be used to identify the emergence of multiple ARSI resistance mechanisms. This is currently being investigated in additional prospective trials.

PURPOSE Alterations in DNA damage repair (DDR) genes occur in up to 25% of patients with metastatic castration-resistant prostate cancer (mCRPC) and may sensitize to platinum chemotherapy. We aimed to evaluate the efficacy of platinum-based chemotherapy in DDR-mutant (DDRmut) mCRPC. METHODS We assessed response to platinum chemotherapy based on DDR gene alteration status in men with mCRPC who underwent tumor and germline genomic profiling. Patients with deleterious alterations in a gene panel that included BRCA2, BRCA1, ATM, PALB2, FANCA, and CDK12 were considered DDRmut. RESULTS A total of 109 patients with mCRPC received platinum-based chemotherapy between October 2013 and July 2018. Sixty-four of 109 patients were taxane refractory and poly (ADP-ribose) polymerase inhibitor (PARPi) naïve. Within this subset, DDRmut was found in 16/64 patients (25%) and was associated with an increased likelihood of achieving a prostate-specific antigen (PSA) decline of 50% or more from baseline (PSA50; odds ratio, 7.0; 95% CI, 1.9 to 29.2). Time on platinum chemotherapy tended to be longer in the DDRmut group (median, 3.0 v 1.6 months; hazard ratio, 0.55, 95% CI, 0.29 to 1.24). No difference in survival was detected. Of 8 patients with DDRmut disease who received platinum-based therapy after a PARPi, 3/7 evaluable patients had radiographic partial response or stable disease, and 2/7 had a PSA50 response. None of 4 patients with ATM mutations had platinum responses regardless of prior PARPi exposure. CONCLUSION Patients with DDRmut disease had better response to platinum-based chemotherapy, suggesting that DDR status warrants prospective validation as a potential biomarker for patient selection. Responses to platinum chemotherapy were observed in BRCA-altered prostate cancer after PARPi progression. Additional studies are needed to determine the predictive role of individual genes on platinum sensitivity in the context of other clinical and genomic factors.

TPS292 Background: PT-112 is a novel small I-O molecule currently in development for several cancers. Preclinical studies have shown the robust induction of immunogenic cell death by PT-112, leading to an anti-cancer adaptive immune response. Biodistribution experiments in mice showed PT-112’s partial bone affinity. In two prior phase I studies, late-line metastatic castration-resistant prostate cancer patients (mCRPC pts) were treated with evidence of anti-cancer activity, including tumor volume reductions, improvements in PET and bone scans, PSA and alkaline phosphatase (ALP) reductions, and anecdotal cases of improvement in disease-related pain. This, taken together with the immunogenic and osteotropic properties observed in preclinical work, provided a strong rationale to further explore PT-112 in mCRPC, an immunologically “cold” disease with a high prevalence of bone metastases. Moreover, in an analysis of >6,000 mCRPC pts, circulating tumor cell (CTC) declines (CTC0 and CTC conversions) were correlated with improvements in survival to a greater extent than PSA declines (PSA50) (Heller et al., JCO, 36:6 2017), prompting incorporation of CTC changes as a secondary endpoint. Methods: The primary objective of the study is to define the dose regimen of PT-112 for pivotal study. Pts are randomized to one of three arms, receiving IV PT-112 on 28-day cycles via one hour infusions on (1) Days 1 and 15 at 250 mg/m2, (2) Days 1 and 15 at 360 mg/m2, or (3) Days 1 and 15 of cycle 1 at 360 mg/m2 followed by 250 mg/m2 on Day 15 of subsequent cycles. Key eligibility criteria include radiographically progressive disease at study entry; ≥3 life-prolonging therapies for metastatic disease including 1-2 taxanes, ≥1 new generation anti-androgen therapies, and other drugs FDA approved on the basis of survival; and allowing bone-only metastatic disease. Efficacy assessments comprise disease control rate at 4 months (DCR4) using RECIST and PCWG3 criteria, objective response rate, CTC0 and CTC conversion, PSA50 and ALP reductions. Additionally, T cell receptor sequencing is conducted to characterize treatment-induced changes in T cell fraction and clonal expansions, as a means of generating meaningful supportive data on the immune mechanism of PT-112 monotherapy. A Fleming two-stage design will be used to assess each arm, with a 20% DCR4 as the null hypotheses, requiring ≥6 of 25 pts responding in stage one and ≥14 of 45 in total to reject the null. In addition, other efficacy measures, such as CTC responses, as well as exposure/response and exposure/safety analyses will be applied to characterize the risk/benefit ratio and select the optimal dose regimen for PT-112. As of October 11th, 2022, 38 pts have been enrolled out of a planned maximum of 135. Clinical trial information: NCT02266745 .

251 Background: The heterogeneity of individual sites of disease in prostate cancer is well recognized and increases over time as the therapies administered promote divergent evolution. In clinical practice, this is often depicted in radiology reports as a mixed response, wherein some lesions improve and others progress or emerge, for which individualizing management is challenging. The problem is exacerbated in patients with high volume disease by the inability to concisely and effectively assess overall disease status which may mask the presence of emerging resistance that could benefit from early intervention and/or a change in therapy. Methods: Thirty-one sets of serial baseline and on-treatment FDG-PET/CT images done for the clinical management of patients with progressing mCRPC were analyzed using TRAQinform IQ software (AIQ Solutions). Individual regions of interest (ROI) identified and tracked across imaging time-points were analyzed for a range of features. The univariate prognostic weight of each feature was assessed with Cox regression models. Imaging features from single timepoints, heterogeneity of response features, and PSA values/dynamics were input into the separate TRAQinform Profile software, which was calculated to predict either time on treatment or overall survival using 3-fold cross-validation of a random survival forest. Individual case reviews were performed on select patients including TRAQinform IQ analytics, PSA trends, radiology reports, and physician notes to evaluate the potential additive benefit of the TRAQinform IQ output. Results: In general, imaging features were more strongly correlated with overall survival than PSA dynamics. After iterative feature selection, only imaging features were selected for TRAQinform Profile scores. In the case of predicting OS, the most important features were baseline total lesion glycolysis (TLG), and the number of new/progressing lesions. Patients with high TRAQinform Profile scores had shorter median survival times than those with low scores (630 vs 1326 days, p

PURPOSE Androgen receptor splice variant 7 (AR-V7) results in a truncated receptor, which leads to ligand-independent constitutive activation that is not inhibited by anti-androgen therapies, including abiraterone or enzalutamide. Given that previous reports suggested that circulating tumor cell (CTC) AR-V7 detection is a poor prognostic indicator for the clinical efficacy of secondary hormone therapies, we conducted a prospective multicenter validation study. PATIENTS AND METHODS PROPHECY ( ClinicalTrials.gov identifier: NCT02269982) is a multicenter, prospective-blinded study of men with high-risk mCRPC starting abiraterone acetate or enzalutamide treatment. The primary objective was to validate the prognostic significance of baseline CTC AR-V7 on the basis of radiographic or clinical progression free-survival (PFS) by using the Johns Hopkins University modified-AdnaTest CTC AR-V7 mRNA assay and the Epic Sciences CTC nuclear-specific AR-V7 protein assay. Overall survival (OS) and prostate-specific antigen responses were secondary end points. RESULTS We enrolled 118 men with mCRPC who were starting abiraterone or enzalutamide treatment. AR-V7 detection by both the Johns Hopkins and Epic AR-V7 assays was independently associated with shorter PFS (hazard ratio, 1.9 [95% CI, 1.1 to 3.3; P = .032] and 2.4 [95% CI, 1.1 to 5.1; P = .020], respectively) and OS (hazard ratio, 4.2 [95% CI, 2.1 to 8.5] and 3.5 [95% CI, 1.6 to 8.1], respectively) after adjusting for CTC number and clinical prognostic factors. Men with AR-V7–positive mCRPC had fewer confirmed prostate-specific antigen responses (0% to 11%) or soft tissue responses (0% to 6%). The observed percentage agreement between the two AR-V7 assays was 82%. CONCLUSION Detection of AR-V7 in CTCs by two blood-based assays is independently associated with shorter PFS and OS with abiraterone or enzalutamide, and such men with mCRPC should be offered alternative treatments.

5029 Background: The incidence of transformation to neuroendocrine prostate cancer (NEPC) has increased in castration resistant prostate cancer (CRPC) in parallel with treatment advances inhibiting androgen receptor signaling. The current understanding is that this occurs in ̃10-20% of CRPC cases. Missing is a determination of the timing of molecular events that drive the process. Methods: Under an IRB-approved protocol, retrospective annotation of all MSK-reviewed pathology reports was conducted for 1447 prostate cancer patients with MSK-IMPACT sequencing data. For patients with pathologically confirmed NEPC, the date of the first sample with unequivocally reported NEPC (described as “neuroendocrine carcinoma” or as having “neuroendocrine differentiation” or “neuroendocrine features”) was recorded. Patients with early signs of histologic transformation not specifically reported as NEPC (double negative prostate cancer or rare/focal staining for NE markers) were analyzed separately. Sequencing results were described by castration-status at collection (CRPC vs castration-sensitive) and, if applicable, the relationship to NEPC diagnosis (i.e. pre- vs post-NEPC). Genomic enrichment analysis was used to identify differentially altered genes between groups. Results: In total, 95 (6.6%) patients had pathologically confirmed NEPC during their disease course, from whom 150 samples with sequencing results were available: including 18 patients with matched pre- and post-NEPC samples. CRPC samples from patients with NEPC (n = 70) were significantly enriched for RB1 alterations (50% vs 12%, p < 10-10, q < 10-10). AR alterations were significantly enriched in CRPC samples from patients without NEPC (n = 380) (63% vs 21%, p < 10-10, q < 1.27*10-8). Further, alterations in numerous genes including TP53, AMER1, ARID5B, YAP1, SOX2, and NKX2.1 were enriched in NEPC patients at the 95% confidence interval (CI) without correction for repeat testing. Matched pre- and post-NEPC samples demonstrated that TP53 alterations in post-NEPC samples are detected in the majority of pre-NEPC samples (8 of 10 patients), but RB1 alterations in post-NEPC samples are detected in a minority of pre-NEPC samples (1 of 8 patients). 54 (3.7%) patients had evidence of early histologic transformation. CRPC samples from these patients (n = 29) were enriched for mutations in RB1, MAP2K2, MUTYH, and CTNNB1 at the 95% CI without correction. FOXA1 mutations were enriched in patients without transformation. Conclusions: RB1, consistent with previous findings, is enriched in NEPC. The inability to detect RB1 alterations in pre-NEPC samples supports divergent evolution, although technical limits of tissue panel sequencing make it difficult to rule out the presence of sub-clonal alterations. Further study of additional genes which contribute to histologic transformation and the development of NEPC is warranted.

64 Background: Routine clinical data from the electronic medical record are indispensable for retrospective and prospective observational studies and clinical trials. Their reproducibility is often not assessed. We sought to develop a prostate cancer-specific database with a defined source hierarchy for clinical annotations and to evaluate data reproducibility. Methods: At a comprehensive cancer center, we designed and implemented a clinical database for men with prostate cancer and clinical-grade paired tumor–normal sequencing for whom we performed team-based retrospective clinical data annotation from the electronic medical record, using a prostate cancer-specific data dictionary. We developed an open-source R package for data processing. We then evaluated completeness of data elements, reproducibility of team-based annotation using blinded repeat annotation by a medical oncologist as the reference, and the impact of measurement error on bias in survival analyses. Results: Data elements on demographics, diagnosis and staging, disease state at the time of procuring a genomically characterized sample, and clinical outcomes were piloted and then abstracted for 2,261 patients and their 2,631 genomically profiled samples. Completeness of data elements was generally high, between 55% to 99% for elements of clinical TNM staging, self-reported race, biopsy Gleason score, and presence of variant histologies, both for the team-based annotation and the repeat annotation. Comparing team-based annotation to the repeat annotation (100 patients/samples), reproducibility of annotations was high to very high. For 7 binary data elements, both sensitivity and specificity of the team-based annotation reached or exceeded 90%. The T stage, metastasis date, and presence and date of castration resistance had lower reproducibility. Impact of measurement error on estimates for strong prognostic factors was modest. Conclusions: With a prostate cancer-specific data dictionary and quality control measures, manual team-based annotations can be scalable and reproducible. The data dictionary and the R package for reproducible data processing tools provided (https://stopsack.github.io/prostateredcap) are freely available to help increase data quality in clinical prostate cancer research.

22 Background: The initiative aimed to increase the rate of advance care planning (ACP) activities for outpatients with metastatic cancer. Methods: The project was sponsored by the institution’s Quality Committee in collaboration with the Cancer Quality Committee, Oncology Division, Tumor Boards, and Medical Group. Metastatic cancer patients were identified by ICD-10 coding and later by oncologist electronic health record (EHR) documentation of metastatic status. ACP activities were defined as either an ACP note, Advance Directive, Physician’s Order for Life Sustaining Therapy, or a palliative medicine (PM) consultation. The EHR was revised to include a section for ACP documentation. Quarterly reports were sent to oncologists with data comparing their rate of ACP activities for patients with metastatic cancer with peers. Oncologists’ identities were initially blinded and later unblinded. Oncologists received a monthly list of their metastatic patients without any ACP activities. Results: The study covered 5201 unique cancer patients cared for by 60 oncologists each year. The rate of ACP activities for metastatic cancer patients increased from 37% in 2017 to 57% at the end of 2020. Data on individual ACP activities are pending analysis. The ACP activities were driven most by PM consultations, which rose from 12% to 39%. Conclusions: This initiative successfully increased ACP activities for patients with metastatic cancer. ACP activities are an essential step to achieve goal concordant care. Given that the main driver of increased ACP activities was PM referrals, further work will be required to strengthen oncologist’s ACP skills and improve access to PM.

Purpose A long natural history and a predominant osseous pattern of metastatic spread are impediments to the adoption of precision medicine in patients with prostate cancer. To establish the feasibility of clinical genomic profiling in this disease, we performed targeted deep sequencing of tumor and normal DNA from patients with locoregional, metastatic noncastrate, and metastatic castration-resistant prostate cancer. Patients and Methods Patients consented to genomic analysis of their tumor and germline DNA. A hybridization capture-based clinical assay was used to identify single-nucleotide variations, small insertions and deletions, copy number alterations, and structural rearrangements in more than 300 cancer-related genes in tumors and matched normal blood. Results We successfully sequenced 504 tumors from 451 patients with prostate cancer. Potentially actionable alterations were identified in DNA damage repair, phosphatidylinositol 3-kinase, and mitogen-activated protein kinase pathways. Twenty-seven percent of patients harbored a germline or a somatic alteration in a DNA damage repair gene that may predict for response to poly (ADP-ribose) polymerase inhibition. Profiling of matched tumors from individual patients revealed that somatic TP53 and BRCA2 alterations arose early in tumors from patients who eventually developed metastatic disease. In contrast, comparative analysis across disease states revealed that APC alterations were enriched in metastatic tumors, whereas ATM alterations were specifically enriched in castration-resistant prostate cancer. Conclusion Through genomic profiling of prostate tumors that represent the disease clinical spectrum, we identified a high frequency of potentially actionable alterations and possible drivers of disease initiation, metastasis, and castration resistance. Our findings support the routine use of tumor and germline DNA profiling for patients with advanced prostate cancer for the purpose of guiding enrollment in targeted clinical trials and counseling families at increased risk of malignancy.

Purpose Adjuvant therapy for intermediate-risk and high-risk localized prostate cancer decreases the number of deaths from this disease. Surrogates for overall survival (OS) could expedite the evaluation of new adjuvant therapies. Methods By June 2013, 102 completed or ongoing randomized trials were identified and individual patient data were collected from 28 trials with 28,905 patients. Disease-free survival (DFS) and metastasis-free survival (MFS) were determined for 21,140 patients from 24 trials and 12,712 patients from 19 trials, respectively. We evaluated the surrogacy of DFS and MFS for OS by using a two-stage meta-analytic validation model by determining the correlation of an intermediate clinical end point with OS and the correlation of treatment effects on both the intermediate clinical end point and OS. Results Trials enrolled patients from 1987 to 2011. After a median follow-up of 10 years, 45% of 21,140 men and 45% of 12,712 men experienced a DFS and MFS event, respectively. For DFS and MFS, 61% and 90% of the patients, respectively, were from radiation trials, and 63% and 66%, respectively, had high-risk disease. At the patient level, Kendall’s τ correlation with OS was 0.85 and 0.91 for DFS and MFS, respectively. At the trial level, R2 was 0.86 (95% CI, 0.78 to 0.90) and 0.83 (95% CI, 0.71 to 0.88) from weighted linear regression of 8-year OS rates versus 5-year DFS and MFS rates, respectively. Treatment effects—measured by log hazard ratios—for the surrogates and OS were well correlated ( R2, 0.73 [95% CI, 0.53 to 0.82] for DFS and 0.92 [95% CI, 0.81 to 0.95] for MFS). Conclusion MFS is a strong surrogate for OS for localized prostate cancer that is associated with a significant risk of death from prostate cancer.

5012 Background: Standard of care in VHR PCa is radiation therapy (RT) with 18-36 months (mos) of androgen-deprivation therapy (ADT). With this regimen, chronic ADT toxicity is significant and biochemical recurrence (BCR) frequent. We sought to improve tumor control and minimize toxicity with intensified short course ADT with dual androgen receptor signaling inhibitors (ARSI) and UHRT. Methods: 64 patients (pts) with VHR, N0 PCa were enrolled from 4 centers. VHR PCa was defined as Gleason score (GS) 9-10, >4 cores of GS 8 disease, or 2 high-risk features (including rT3/T4 disease). Treatment (tx) involved 6 mos of A, AA+P, and leuprolide with prostate/seminal vesicle-directed RT (7.5-8 Gy x 5 fractions). The primary endpoint was BCR defined as nadir PSA + 2ng/mL. Biochemical recurrence-free survival (bRFS) is reported herein. Our hypothesized reduction in BCR from 25% to 10% at 3 years (yrs) required 53 pts to provide a power of 0.84 and an alpha of 0.03. Undetectable PSA was defined as 150 ng/mL. All analyses were intention-to-treat. Toxicity and health-related quality of life measures were evaluated using CTCAEv4.0 and the EPIC-26 questionnaire. Results: Baseline characteristics are summarized in the Table; 63 of 64 pts completed protocol tx. Median time to nadir PSA from tx start was 2 mos (range, 1-9); 63 of 64 pts (98.4%) achieved an undetectable nadir PSA. Median time to post-tx, non-castrate T was 6.5 mos (range, 2.5-25.5). Median follow-up (f/u) for pts without BCR was 30 mos (range, 15-44). Seven pts had BCR; 2-yr bRFS was 95.0% (95% CI, 89.7-100); 3-yr bRFS was 89.7% (95 CI, 81.0-99.3). For the 57 pts without BCR, 56 (98.2%) had T > 150ng/mL at last f/u; median PSA at last f/u was 0.10 ng/mL (IQR

e17039 Background: Schlafen 11 (SLFN11) is a DNA repair protein (DNA/RNA helicase homology) that is recruited to stressed replication forks and leads to cell death. Recent ph II trial data in Extensive Stage Small-Cell Lung Cancer (ES-SCLC) [Byers et al. JCO 2018 PMID: 29906251] and a retrospective analysis of Circulating Tumor Cells (CTCs) and tumor tissue in patients (Pts) with advanced prostate cancer [Conteduca et al. Mol Cancer Therapeutics 2020 PMID: 32127465] suggested that SLFN11 expression predicts sensitivity to DNA damage targeting agents. In both contexts, metastatic tumor biopsies may not provide adequate material for profiling. We assessed the frequency of SLFN11 expression in CTCs isolated from blood in men with progressing metastatic Castration Resistant Prostate Cancer (mCRPC) and related expression to Homologous Recombination Repair (HRR) alterations identified in metastatic tumor biopsies profiled by MSK-IMPACT. Methods: 95 patients with progressing mCRPC about to start a new systemic therapy who had undergone pre-treatment metastatic tumor profiling by MSK-IMPACT and a matched blood draw for CTC profiling were selected. Blood was sent overnight to Epic Sciences and processed onto glass pathology slides and bio-banked until analysis. Detected CTCs (cytokeratin (CK) positive and leukocyte (CD45) negative) were analyzed for SLFN11 protein expression by immunofluorescence and correlated to Homologous Recombination Repair (HRR) alterations in metastatic biopsy from the bone, lymph node, or visceral metastases (15 PROFOUND genes). A mean of 1.2 mL of blood was analyzed per patient. Results: CTCs with SLFN11 expression (DAPI+, CK+, CD45-, SLFN11+) were detected in 28.4% (27/95) of the patient sample analyzed. SLFN11 signal was found to overlap with the nuclear DAPI signal. Individual CTC expression of SLFN11 in a sample was heterogeneous and ranged from a minimum of 2.9% to 100%. Seven of 10 (70%) of patients with a BRCA1/2 or ATM alteration had a least one SLFN11 expressing CTC. In contrast, in patients with other HRR alterations, only 20% (1/5) had CTCs with SLFN11. Sequencing of single CTCs is ongoing. Conclusions: SLFN11 expression is detected with high frequency in CTCs in men with progressing mCRPC. In whom CTCs are detected, the majority of patients with BRCA1/2 or ATM altered tumors also had SLFN11 expressing CTCs. The results support the prospective evaluation of CTC SLFN11 expression as a predictive biomarker for PARPi or platinum agents.

Purpose Evolving treatments, disease phenotypes, and biology, together with a changing drug development environment, have created the need to revise castration-resistant prostate cancer (CRPC) clinical trial recommendations to succeed those from prior Prostate Cancer Clinical Trials Working Groups. Methods An international expert committee of prostate cancer clinical investigators (the Prostate Cancer Clinical Trials Working Group 3 [PCWG3]) was reconvened and expanded and met in 2012-2015 to formulate updated criteria on the basis of emerging trial data and validation studies of the Prostate Cancer Clinical Trials Working Group 2 recommendations. Results PCWG3 recommends that baseline patient assessment include tumor histology, detailed records of prior systemic treatments and responses, and a detailed reporting of disease subtypes based on an anatomic pattern of metastatic spread. New recommendations for trial outcome measures include the time to event end point of symptomatic skeletal events, as well as time to first metastasis and time to progression for trials in the nonmetastatic CRPC state. PCWG3 introduces the concept of no longer clinically benefiting to underscore the distinction between first evidence of progression and the clinical need to terminate or change treatment, and the importance of documenting progression in existing lesions as distinct from the development of new lesions. Serial biologic profiling using tumor samples from biopsies, blood-based diagnostics, and/or imaging is also recommended to gain insight into mechanisms of resistance and to identify predictive biomarkers of sensitivity for use in prospective trials. Conclusion PCWG3 moves drug development closer to unmet needs in clinical practice by focusing on disease manifestations most likely to affect prognosis adversely for therapeutics tested in both nonmetastatic and metastatic CRPC populations. Consultation with regulatory authorities is recommended if a trial is intended to seek support for drug approval.

157 Background: We evaluated the prognostic significance of circulating tumor cell (CTC) number as determined on the Epic Sciences platform in men with metastatic castration resistant prostate cancer (mCRPC) treated with an androgen receptor signaling inhibitor (ARSI). Methods: A pre-treatment blood sample was collected from men with progressing mCRPC starting either abiraterone or enzalutamide as a 1st, 2nd or 3rd line systemic therapy at Memorial Sloan Kettering Cancer Center (Discovery cohort, N=175) or as a 1st and 2nd line therapy as part of the multi-center PROPHECY trial (NCT02269982) (Validation cohort, N=107). Enumeration was performed on the Epic Sciences platform and associated with overall survival (OS) in the Discovery cohort, and progression-free survival (PFS) and OS in the Validation cohort. Matched blood samples from the Validation cohort were to CTC counts measured on the CellSearch Circulating Tumor Cell kit. Results: In the MSKCC Discovery cohort, CTC count was a statistically significant prognostic factor of OS as a dichotomous (< 3 CTCs/mL versus ≥ 3 CTCs/mL; HR = 1.8, (1.3-3.0, 95% CI)) and as a continuous variable when adjusting for line of therapy, presence of visceral metastases, PSA, lactate-dehydrogenase, and alkaline-phosphatase. The findings were validated in an independent dataset (PROPHECY trial) - (HR (95% CI) = 1.8, (1.1-3.0) for OS and 1.7 (1.1-2.9), for PFS). A strong correlation was observed between CTC counts determined in matched samples on the CellSearch and Epic platforms (r = 0.84). Conclusions: The findings validate the prognostic significance of pretreatment CTC number determined on the Epic Sciences platform for predicting OS in men with progressing mCRPC starting an ARSI.

5531 Background: Genomic studies have shown that up to 25% of prostate cancer tissue specimens harbor alterations in DNA Damage Repair (DDR) genes, which may sensitize the tumor to poly ADP-ribose polymerase inhibitors (PARPi). Trials evaluating PARPi in patients with DDR deficiencies have shown varied response rates and differences regarding which genomic alterations predict for sensitivity to these agents, with the majority of objective responses seen in BRCA2-altered tumors. These results highlight the need to develop biomarker assays which can predict benefit from PARPi therapy. Tissue and cell-free DNA (cfDNA) have been the most utilized sources of tumor material for analysis in this setting, but success rates of obtaining sufficient tumor for analysis from bone are low and detecting tumor-derived copy number variants (CNVs) in cfDNA is challenging. Circulating tumor cells (CTCs) represent an alternate source of genetic information, for which assays are available to isolate and sequence individual cells in a manner that eliminates background noise from stroma and healthy cells, while capturing inter-cellular heterogeneity. Methods: Blood samples, collected from 138 progressing metastatic CRPC patients within 30 days of a pre-treatment biopsy intended for sequencing using MSK-IMPACT, were sent to EPIC Sciences for CTC analysis. Detected CTCs underwent single cell, low pass whole genome sequencing. Prevalence and concordance of BRCA2 copy-loss, regardless of whether single copy or homozygous, was compared in matched tissue and CTC samples. Results: BRCA2 copy-loss was identified in 21% (23/108) and 50% (58/115) of successfully sequenced tissue and CTC samples, respectively. In the 58 patients with CTC-detected BRCA2 loss, BRCA2 loss was detected in 36% (220/565) of the sequenced CTCs, representing a median of 46% (range 4-100%) of CTCs found in each individual sample. When both sequencing assays were successful, BRCA2 loss was detected in CTCs in 84% (16/19) of the tissue-positive cases, whereas tissue sequencing detected BRCA2 loss in 35% (16/46) of CTC-positive cases. Conclusions: Data from this study supports the notion that single-cell CTC sequencing can detect BRCA2 copy-loss at a high frequency, including cases that were negative in tissue, while also characterizing inter-cellular heterogeneity. Further studies will investigate whether CTC BRCA2 copy-loss can predict the likelihood of response to PARPi.

5572 Background: The increasing availability and earlier use of life prolonging drugs targeting the androgen receptor signaling axis (ARSI) has resulted in an increase in the frequency of late state tumors with “small cell/neuroendocrine (NESC) phenotypes” similar to small-cell lung cancer (SCLC). Definitive pathologic criteria to diagnose the “entity” are lacking, and the eligibility criteria across trials are inconsistent, limiting the ability to relate outcomes between studies. We hypothesized that an analytically valid assay for a rigorously defined “small-cell CTC” phenotype might serve as a unifying biomarker for the presence of NESC-like tumors in an individual for use in clinical trials. Methods: Using the WHO guidelines for small-cell diagnosis in tissue as reference, we defined an equivalent set of single-cell CTC criteria for defining a CTC with small-cell histology: a small and circular CD45-, CK+ cell with high N/C ratio lacking detectable nucleoli. Small-cell subtype pharmacodynamic changes were studied in 233 patients with progressing mCRPC about to start an AR signaling inhibitor ARSi (N=111), taxane (N=89), or platinum (N=33). Results: CTCs with small-cell morphology had lower AR protein expression compared with non-small-cell CTCs (P

e24174 Background: Incorporation of the PRO-CTCAE during chemotherapy is associated with fewer ER visits and improved survival. Limited data exist using this tool to capture irAE in immunotherapy trials. We collected select PRO-CTCAEs in a clinical trial evaluating an immune checkpoint inhibitor (ICI) in patients (pts) with CSPC. Methods: Cohort A (Coh A) enrolled de novo metastatic CSPC; Cohort B (Coh B) enrolled recurrent CSPC after radical prostatectomy (RP). Treatment consisted of 4 doses of ipi and 8 months of androgen deprivation therapy (ADT) in Coh A & B, and RP in Coh A. PRO-CTCAEs were collected at each ipi or ADT administration, and during follow-up. PRO-CTCAE items included abdominal pain, diarrhea, fatigue, anorexia, nausea, vomiting, rash and pruritus, with the correspondent attributes of frequency (freq), severity (sev), interference (int), and presence. Results: 16 pts were treated (Coh A: 7; Coh B: 9) with 181 matched pairs of PRO-CTCAE and CTCAE. The study was terminated early for an unfavorable risk:benefit ratio. PRO-CTCAE completion at required visits was 85.4% in Coh A and 98.1% in Coh B. Cohen’s kappa coefficients was lowest for pruritus (k = 0.10, slight agreement) and highest for rash (k = 0.64, moderate agreement). CTCAE captured diarrhea in 10 pts, 4 of whom received steroids. 75% (3/4) of pts receiving steroids graded diarrhea as ‘frequently’ or ‘almost constantly’ on the PRO-CTCAE while 66.7% (4/6) of those who did not receive steroids reported the same frequency. Conclusions: IrAE were more commonly reported and of higher grade by patients using the PRO-CTCAE as compared to clinician reporting, consistent with existing literature in non-ICI studies. Diarrhea as reported by the PRO-CTCAE did not associate with steroid use in this small trial. Further studies are needed to evaluate irAE using PRO-CTCAEs and the potential role in management of these toxicities. Clinical trial information: NCT02020070 . [Table: see text]

e17502 Background: Ki-67 and CC3 are markers of cellular proliferation and apoptosis and have been associated with prognosis in localized PC. Testosterone suppression promotes a time-dependent modulation of these indices. We explored changes in Ki-67 and CC3 at different timepoints in a pilot trial using short-term ADT prior to radical prostatectomy (RP). Methods: Pts with intermediate and high-risk localized PC were randomized to receive a single dose of neoadjuvant degarelix (240 mg SC) either 4 ± 1 days (Cohort A) or 7 ± 1 days (Cohort B) prior to RP. Anatomically matched tumor foci from the pre-treatment diagnostic biopsy and the prostatectomy specimens were assessed by immunohistochemistry (IHC) for Ki-67, CC3, and PTEN expression. Results: 32 pts were randomized: 15 to Cohort A and 17 to Cohort B. Ki-67 and CC3 expression in the pre- and post-treatment samples for both cohorts are summarized in the table. A significant reduction in Ki-67 and increase in CC3 expression occurred in both cohorts (all p-values < 0.05). No difference was found when the absolute difference of Ki-67 expression was compared between Cohort A and Cohort B (p = 0.7). A significant reduction of Ki-67 and increase in CC3 expression occurred in pts independent of PTEN status. Conclusions: A reduction of Ki-67 and an increase in CC3 expression was achieved in both cohorts following a single dose of degarelix within 1 week of RP. Although loss of PTEN has been associated with androgen-independent progression, reduction of proliferation and increase in apoptosis levels were achieved independent of PTEN status in this small cohort. Further analyses are ongoing to correlate the pathologic findings with clinical outcomes. Clinical trial information: NCT01542021 . [Table: see text]

e12598 Background: Palbociclib, a small-molecule inhibitor of cyclin-dependent kinases (CDK4 and CDK6), combined with letrozole increases progression-free survival among patients with previously untreated ER-positive, HER2 negative advanced breast cancer. The purpose of our study was to retrospectively evaluate the tolerance of the concomitant association of Palbociclib and radiation therapy (RT) at Curie Institute. Methods: Between April 2017 and August 2019, 30 women with ER-positive, HER2 negative metastatic breast cancer received locoregional (LR) and/or symptomatic irradiation at a metastatic site concurrently with Palbociclib at a daily dose of 125 mg, from d1 to d21 every 28 days. Palbociclib was always associated with endocrine therapy: letrozole (with or without an LHRH analogue) or fulvestrant. Thirty-five sites were irradiated: nine patients received post-operative locoregional RT, including the chest wall or breast and lymph node areas, and 26 sites of metastases were irradiated: 17 at the spine, 7 peripheral skeletal lesions, 1 brain lesion and 1 choroidal lesion. The dose prescribed for locoregional mammary radiotherapy was 50 Gy in 25 fractions and varied for the treatment of metastatic sites: 20 Gy in 5 fractions (n = 13), 30 Gy in 10 fractions (n = 10) and 8 Gy in 1 fraction (n = 2). The brain metastasis was stereotactically treated (1 fraction of 18 Gy). The primary endpoint was toxicity scored according to the common terminology criteria for NCI adverse events, version v5.0. Results: Mean number of days of Palbociclib during RT was 8.8 days (range, 1 to 24 days). The most common acute toxicities were dermatitis (12/35, including 2 grade 2) and neutropenia (12/35, including 9 grade 2). Palbociclib had to be stopped during the RT of two patients (2/30): one patient treated locoregionally (bilateral breast and lymph nodes irradiation) developed a grade 3 dermatitis and febrile neutropenia, another treated locoregionally developed grade 2 dysphagia. After a median follow-up since the end of RT of 17 months (6 – 31 months, SD 8), none of the patients have so far exhibited late toxicity. Conclusions: Concomitant administration of palbociclib with RT was reasonably well tolerated in our series of 30 patients. Given this experience, palbociclib should not be discontinued during radiation therapy. Nevertheless, our findings should be confirmed in prospective registration studies collecting larger number of patients.