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PURPOSE Docetaxel and abiraterone acetate plus prednisone or prednisolone (AAP) both improve survival when commenced alongside standard of care (SOC) androgen deprivation therapy in locally advanced or metastatic hormone-sensitive prostate cancer. Thus, patient-reported quality of life (QOL) data may guide treatment choices. METHODS A group of patients within the STAMPEDE trial were contemporaneously enrolled with the possibility of being randomly allocated to receive either docetaxel + SOC or AAP + SOC. A mixed-model assessed QOL in those who had completed at least one QLQ-C30 + PR25 questionnaire. The primary outcome measure was difference in global-QOL (QLQ-C30 Q29&30) between patients allocated to docetaxel + SOC or AAP + SOC over the 2 years after random assignment, with a predefined criterion for clinically meaningful difference of > 4.0 points. Secondary outcome measures included longitudinal comparison of functional domains, pain, and fatigue, plus global-QOL at defined timepoints. RESULTS Five hundred fifteen patients (173 docetaxel + SOC and 342 AAP + SOC) were included. Baseline characteristics, proportion of missing data, and mean baseline global-QOL scores (docetaxel + SOC 77.8 and AAP + SOC 78.0) were similar. Over the 2 years following random assignment, the mean modeled global-QOL score was +3.9 points (95% CI, +0.5 to +7.2; P = .022) higher in patients allocated to AAP + SOC. Global-QOL was higher for patients allocated to AAP + SOC over the first year (+5.7 points, 95% CI, +3.0 to +8.5; P < .001), particularly at 12 (+7.0 points, 95% CI, +3.0 to +11.0; P = .001) and 24 weeks (+8.3 points, 95% CI, +4.0 to +12.6; P < .001). CONCLUSION Patient-reported QOL was superior for patients allocated to receive AAP + SOC, compared with docetaxel + SOC over a 2-year period, narrowly missing the predefined value for clinical significance. Patients receiving AAP + SOC reported clinically meaningful higher global-QOL scores throughout the first year following random assignment.

241 Background: We previously reported on the use of gene expression profiling to characterize four primary subtypes in an analysis of over 100,000 prostate cancer primary tumors. Here we examine these subtypes and response to radiation (RT) after prostatectomy or chemotherapy in addition to androgen deprivation therapy (ADT) in hormone-sensitive metastatic prostate cancer (mHSPC). Methods: We examined a 215-gene prostate subtyping classifier (PSC) which groups luminal and basal gene expression profiles into one of four subtypes; luminal differentiated (LD), luminal proliferating (LP), basal immune (BI) and basal neuroendocrine-like (BN). The log-rank method was used to compare (i) metastasis-free survival between patients who did and did not receive salvage RT in the META855 cohort of localized disease patients treated with radical prostatectomy (n=855) and the (ii) overall survival of patients on the Phase III mHSPC EA3805 CHAARTED trial of ADT or ADT + docetaxel chemotherapy (n=160). Results: After radical prostatectomy, patients with BI tumors derived benefit from RT in terms of metastasis-free survival ( P=9.23e-4) while those with other subtype tumors did not (Each Log-rank P≥0.5). Patients with metastatic disease and LP tumors derived benefit from docetaxel (Log-rank P=0.002) while those with other subtypes tumors did not (Log-rank P=0.2). Conclusions: Basal-luminal subtyping segments tumors by relevant biological processes with potential implications for identifying patients who benefit from salvage radiation post prostatectomy and addition of docetaxel to ADT in mHSPC.

170 Background: In the primary analysis of the phase 3 MAGNITUDE study, NIRA/AAP significantly improved outcomes in pts with mCRPC and HRR gene alterations. Here, we report results from IA2 of secondary endpoints in MAGNITUDE. Methods: 423 eligible pts with mCRPC and HRR alterations (HRR+ cohort) were randomized 1:1 to receive NIRA/AAP (n = 212) or placebo (PBO)/AAP (n = 211). At the prespecified IA2, secondary endpoints (time to cytotoxic chemotherapy [TCC], time to symptomatic progression [TSP], overall survival [OS]) were formally assessed and the primary rPFS endpoint was updated in the HRR+ cohort, with sensitivity analysis performed for the subgroup of pts with BRCA alterations. Results: Updated descriptive rPFS results at IA2 (cutoff: June 17, 2022) were consistent with the primary analysis in the HRR+ cohort. In the BRCA subgroup, NIRA/AAP extended median rPFS to 19.5 mos vs 10.9 mos with PBO/AAP. NIRA/AAP led to statistically significant benefit in TSP in the HRR+ cohort with consistent benefit in the BRCA subgroup. Continued consistent improvement of TCC was seen with NIRA/AAP in the HRR+ cohort and in the BRCA subgroup. There was a trend towards improved OS with NIRA/AAP in the BRCA subgroup in the primary stratified analysis and the multivariate analysis (MVA), accounting for imbalances in key baseline characteristics. BRCA pts treated with NIRA/AAP experienced delayed time to worst pain intensity (HR, 0.70; 95% CI, 0.44, 1.12; nominal P = 0.1338) and pain interference (HR, 0.67; 95% CI, 0.40, 1.12; nominal P = 0.1275) compared to PBO/AAP. The safety profile at IA2 was consistent with that of the primary analysis, with no new safety signals observed. Conclusions: With 26.8 months of median follow-up, there was a statistically significant and meaningful clinical benefit in TSP and meaningful clinical benefit in TCC. Additionally, updated rPFS results from MAGNITUDE IA2 were consistent with the primary analysis; OS benefit was not conclusive due to immaturity and will be followed through to final analysis. Taken together, these data continue to support the use of NIRA/AAP in pts with mCRPC and BRCA alterations or select other HRR gene alterations. Clinical trial information: NCT03748641 . [Table: see text]

172 Background: NIRA/AAP significantly improved outcomes in pts with mCRPC and HRR gene alterations, particularly in BRCA, in the phase 3 MAGNITUDE study. As a practical measure, pts were permitted to receive up to 4 mos of AAP (in 1L mCRPC) prior to randomization to allow time for genomic testing. We evaluated the impact of AAP run-in treatment on the efficacy of NIRA/AAP. Methods: 423 pts with mCRPC and HRR gene alterations were randomized 1:1 to receive NIRA/AAP or placebo (PBO)/AAP. At the prespecified second interim analysis, a sensitivity analysis based on the duration of AAP run-in was conducted. Pts with BRCA alterations were also analyzed separately. Results: Median duration of prior AAP treatment received was 1.9 (range, 0.3–4.1) mos. Pts receiving AAP ≤2 mos had similar benefit (radiographic progression-free survival [rPFS] hazard ratio [HR], 0.69 [95% confidence interval [CI], 0.36-1.30]; time to cytotoxic chemotherapy [TCC] HR, 0.52 [95% CI, 0.24-1.11]; time to symptomatic progression [TSP] HR, 0.32 [95% CI, 0.13-0.79]; Table) to pts not receiving any prior AAP. rPFS benefit was not demonstrated in pts who had previously received AAP >2 – 4 mos: HR, 1.47 (95% CI, 0.66-3.30). Findings were consistent in the BRCA population. Conclusions: Pts receiving a short run-in (≤2 mos) of AAP alone obtained similar benefit from NIRA/AAP as those who received both NIRA/AAP together for initial treatment of mCRPC. While interpretation of data is limited by the small sample size and event numbers, for pts where NIRA/AAP is being considered as therapy, AAP may be initiated during HRR testing and combination treatment should be initiated expeditiously once HRR positivity is established to attain maximal treatment benefit. Clinical trial information: NCT03748641 . [Table: see text]

5060 Background: Results from the international, randomized, double-blind, phase 3 MAGNITUDE study demonstrated that NIRA + AAP improved radiographic progression-free survival, time to cytotoxic chemotherapy, and time to symptomatic progression, with manageable toxicity in pts with mCRPC and HRR alterations (9-gene panel). Here, we report HRQoL and pain in MAGNITUDE. Methods: Eligible pts with mCRPC and HRR alterations were randomized 1:1 to NIRA + AAP or placebo (PBO) + AAP orally daily in 28-day cycles. Pts had ECOG status ≤1 and a Brief Pain Inventory–Short Form (BPI-SF) worst pain score ≤3 in prescreening. HRQoL assessments on day 1 of specified cycles included Functional Assessment of Cancer Therapy–Prostate (FACT-P) and BPI-SF. Changes from baseline were compared between treatment arms using repeated measures analysis. Proportional hazards regression models were used to compare time to deterioration (TTD) in worst pain intensity between arms. Results: Compliance for FACT-P and BPI-SF was > 80%. Most pts maintained low pain levels over time. Repeated measures analyses showed no clinically meaningful differences in pain over time or between arms. Median TTD in pain intensity was not reached in either arm. At the 25th percentile, there was a trend toward longer TTD in pain intensity with NIRA + AAP vs PBO + AAP (11.1 vs 10.1 mo; HR, 0.87; 95% CI, 0.61-1.24). HRQoL was maintained with NIRA + AAP, with no clinically meaningful differences in FACT-P total score over time or between arms. There was a trend toward greater worsening in early cycles on FACT-P physical wellbeing with NIRA + AAP vs PBO + AAP, driven by events within the known safety profile of NIRA + AAP (worsening of side effect bother, lack of energy, and nausea); however, overall, most pts reported minimal side effect burden (Table). Conclusions: In MAGNITUDE, most pts maintained low pain levels and positive HRQoL over time, with no clinically meaningful differences between treatment arms, further supporting the use of NIRA + AAP in pts with mCRPC and HRR alterations. Side effect burden was perceived as low in both arms. Although more pts on NIRA+AAP reported worsening side effects, the symptoms were generally perceived as mild. Clinical trial information: NCT03748641. [Table: see text]

5020 Background: NIRA + AAP significantly improved outcomes in pts with mCRPC and HRR gene alterations in the Phase 3 MAGNTUDE study. There is a paucity of data supporting use of PARP inhibitors in pts with HRR gene alterations other than BRCA1/2. We report on the efficacy of NIRA + AAP in pts with mCRPC and a qualifying single gene HRR alteration other than BRCA1/2. Methods: A pre-specified analysis was undertaken of the primary endpoint (radiographic progression-free survival [rPFS] by BICR), secondary endpoints (time to cytotoxic chemotherapy [TCC], time to symptomatic progression [TSP], overall survival [OS]), as well as time to PSA progression (TPSA) and overall response rate (ORR) across 186 pts (91 randomized to NIRA + AAP, 95 to PBO + AAP) with an alteration in the ATM, BRIP1, CDK12, CHEK2, FANCA, HDAC2, or PALB2 gene (excluding cooccurring alterations) . This analysis of individual alterations was not powered for formal statistical inference. Given the rarity of some alterations, groups based on functional similarity are also presented. Results: (Table). Pts with PALB2 or CHEK2 alterations had consistent improvement across all endpoints. In pts with ATM alterations benefit was observed in TCC, TSP, TPSA and ORR. There was benefit only in TPSA and ORR for pts with CDK12 alterations. When combined into functional groups, pts with an alteration in the HRR-Fanconi pathway ( BRIP1, FANCA, and PALB2) as well as pts with a HRR associated alteration ( CHEK2 or HDAC2) showed improvement in all endpoints. Conclusions: These data support the overall conclusions of the MAGNITUDE primary analysis and support benefit of NIRA + AAP in pts with HRR mutations beyond BRCA1/2. Clinical trial information: NCT03748641. [Table: see text]

12 Background: Approximately 20% of mCRPC has alterations in genes associated with HRR and is responsive to PARP inhibitors (PARPi) such as NIRA. Combined PARPi with androgen receptor pathway targeting may also benefit unselected mCRPC. MAGNITUDE assessed whether adding NIRA to AAP improves outcomes in pts with mCRPC with or without alterations in HRR associated genes. Methods: MAGNITUDE (NCT03748641) is a randomized, double-blind phase 3 study. In eligible mCRPC pts, ≤4 mos of prior AAP for mCRPC was allowed. Pts with (HRR biomarker [BM]+; ATM, BRCA1, BRCA2, BRIP1, CDK12, CHEK2, FANCA, HDAC2, PALB2) and without specified gene alterations (HRR BM-) were randomized 1:1 to receive NIRA 200 mg once daily + AAP or placebo (PBO) + AAP. Primary endpoint was radiographic progression-free survival (rPFS) assessed by blinded independent central review (BICR) in the BRCA1/2 group followed by all HRR BM+ pts. Secondary endpoints were time to initiation of cytotoxic chemotherapy (TTCC), time to symptomatic progression (TTSP) and overall survival (OS). Other endpoints included time to PSA progression (TTPP) and objective response rate (ORR). Results: 423 HRR BM+ pts were randomized to NIRA + AAP (n = 212) or PBO + AAP (n = 211). Median age was 69, 23% had prior AAP, 21% had visceral metastases, and 53% had BRCA1/2 mutations. Median follow-up was 18.6 mos. NIRA + AAP significantly improved rPFS by BICR in the BRCA1/2 subgroup and in all HRR BM+ pts, reducing the risk of progression or death by 47% (16.6 vs 10.9 mo) and 27% (16.5 vs 13.7 mo) respectively (Table), vs PBO + AAP. Investigator assessed rPFS was consistent with BICR. NIRA + AAP delayed TTCC, TTSP, and TTPP and improved ORR in HRR BM+ pts (Table). First interim analysis of OS is immature. The preplanned futility analysis in 233 HRR BM- pts showed no benefit of adding NIRA to AAP in the prespecified composite endpoint (first of PSA progression or rPFS; HR, 1.09; 95% CI, 0.75-1.57). No new safety signals were seen. In HRR BM+ pts, 67% and 46.4% had grade 3/4 AEs and 9% and 3.8% discontinued treatment in the NIRA + AAP and PBO + AAP arms, respectively. There were no clinically significant differences in overall quality of life (FACT-P). Conclusions: NIRA + AAP improves rPFS and other clinically relevant outcomes in pts with mCRPC and alterations in HRR associated genes. There was no evidence of benefit with the addition of NIRA to AAP in HRR BM- pts with mCRPC. Clinical trial information: NCT03748641. [Table: see text]

142 Background: In ACIS, APA + AAP improved radiographic progression-free survival in mCRPC vs AAP; difference in overall survival (OS) was not statistically significant. We report a prespecified analysis of androgen receptor (AR) or non-AR genomic/transcriptional aberrations, known to be associated with poor prognosis, and their associations with OS in patients (pts) with mCRPC. Methods: Circulating tumor (ct)DNA and aberrations in 17 PC-relevant genes were assessed using next-generation sequencing at baseline (BL; n = 197) and at end of study treatment (EOST; n = 140) (biomarker population). ctDNA was summarized qualitatively; genomic aberrations were normalized by ctDNA detection. AR splice variant version 7 (ARv7) was detected by quantitative RT-PCR. Cox proportional hazards model assessed association of OS with biomarkers in univariate/multivariate analyses overall and in pts receiving subsequent treatment (tx). Results: BL characteristics of biomarker and total study populations were similar. From BL to EOST: ctDNA detection did not differ (123/196 [63%] to 92/140 [66%], p=0.6); significant increases occurred in ARv7, AR mutations, and any AR aberration (Table). Prevalence of aberrations in PI3K and homologous recombination repair (HRR) pathways did not change from BL to EOST (40/123 [33%] to 34/92 [37%], p=0.5 and 18/123 [15%] to 14/92 [15%], p>0.9, respectively). Worse OS was associated with ctDNA detection or HRR pathway inactivation at BL (HR, 2.5 or 2.2; all p < 0.001) or presence of ctDNA, ARv7, AR amplification or inactivation of TP53, RB1, or RB1/TP35 pathway at EOST (2.2, p < 0.001; 2.1, p < 0.001; 2.6, p < 0.001; 1.5, p < 0.05; 1.7, p < 0.05; 2.1, p < 0.001, respectively) in univariate analyses. EOST ARv7, AR amplification, and inactivation of RB1, TP53, or RB1/TP53 pathway was associated with worse OS in pts receiving subsequent tx. Detection of ctDNA and RB1 inactivation at BL and RB1/TP53 pathway inactivation at EOST was strongly associated with worse OS (2.1, 3.3, 2.9, respectively, all p < 0.01) in multivariate analyses. Conclusions: In ACIS, AAP or APA + AAP tx was associated with increased AR mutations and ARv7 expression. Detection of ctDNA and select AR/non-AR aberrations may predict poor survival in mCRPC. These markers may be used to improve tx selection following confirmation of their predictive value. Clinical trial information: NCT02257736. [Table: see text]

15 Background: PRESIDE (NCT02288247) evaluated the benefit of continued ENZA + androgen deprivation therapy (ADT) with DOC + PDN in men with mCRPC who progressed on ENZA + ADT. Methods: PRESIDE (Dec 2014–Apr 2020) enrolled chemotherapy-naïve men with mCRPC and disease progression while on a luteinizing hormone-releasing hormone (LHRH) agonist/antagonist (ADT) or after bilateral orchiectomy. Pts received open-label ENZA (160 mg) + ADT in Period 1 (P1). Those with a prostate-specific antigen (PSA) response of ≥50% change from baseline to week (wk) 13 and later progression were eligible for Period 2 (P2). P2 pts received DOC (75 mg/m2), PDN (10 mg), and ADT, and were randomized to ENZA (160 mg) or PBO. The primary endpoint was progression-free survival (PFS) in P2 (from randomization to radiographic/clinical progression or death). Secondary endpoints included time to PSA progression (TTPP) [≥25% increase; absolute increase ≥2 ng/mL] and PSA response in P2. Hazard ratios (HRs) were from a Cox proportional hazards model with covariates for treatment and P1 progression. Adverse events (AEs) were recorded to assess safety. Results: 687 pts received ENZA in P1; 273 pts were randomized and 271 were treated in P2. Baseline demographics and characteristics were balanced between P2 arms. Median ENZA exposure was 62.6 wks in P1 and 36.1 and 30.1 wks in P2 with ENZA and PBO, respectively. At P2 data cut-off (Apr 30, 2020), 269 (99.3%) pts had discontinued therapy; 93 pts in each arm (ENZA, 74.4%; PBO, 75.6%) had progression. PFS was significantly improved with ENZA (HR 0.72; 95% confidence interval [CI] 0.53, 0.96; p = 0.027), with a higher median PFS with ENZA (9.53 months; 95% CI 8.25, 10.87) than with PBO (8.28 months; 95% CI 6.28, 8.71). ENZA also significantly delayed TTPP (8.44 vs. 6.24 months with PBO; HR 0.58; 95% CI 0.41, 0.82; p = 0.002) and improved PSA response at any time (ENZA, n = 76 [55.9%]; PBO, n = 50 [37.0%]). There were 46 (6.7%) deaths in P1 and 20 (ENZA, n = 13 [9.6%]; PBO, n = 7 [5.2%]) in P2. In P2, 264 (97.4%) pts had a treatment-emergent AE (TEAE) [ENZA, n = 133 (97.8%); PBO, n = 131 (97.0%)]. Grade 3/4 TEAEs were reported by 84 (61.8%) pts on ENZA and 84 (62.2%) on PBO, and 12 (8.8%) and 9 (6.7%) pts, respectively, had TEAEs leading to discontinuation. Neutropenia (ENZA, 16.9%; PBO, 20.7%) was the most common grade 4 TEAE. Drug-related TEAEs (ENZA, 46.3%; PBO, 41.5%) were similar, and 90.4% of pts in each arm reported DOC-related TEAEs. Conclusions: Continued ENZA therapy in men with mCRPC who progressed on ENZA + ADT and received post-progression DOC + PDN significantly improved PFS compared to PBO. Treatment was well tolerated and ENZA AEs were consistent with its known safety profile. Clinical trial information: NCT02288247.

5048 Background: Cancer is a risk factor for VTE. In mCRPC, ptDNA is an independent predictor of outcome ( Romanel, Sci Transl Med 2015; Conteduca, Br J Cancer 2020). We firstly aimed to investigate the association between ptDNA and VTE in mCRPC men treated with androgen receptor signaling inhibitors (ARSI) Methods: This prospective biomarker study included mCRPC patients treated with abiraterone and enzalutamide from April 2013 to December 2018. We excluded patients with a previous VTE history and/or ongoing anticoagulation. Targeted next-generation sequencing was performed to determine ptDNA fraction from pre-treatment plasma samples. Assessment of VTE risk based on survival analysis was performed using cumulative incidence function and estimating sub-distributional hazard ratio (SHR) Results: Median age of 180 enrolled patients was 74 years (range 42-90). Of these, 60 (33.3%) were chemotherapy-naive. At a median follow-up of 58 months (range 0.5-111.0), 21 patients experienced VTE (venous thrombosis and/or pulmonary embolism) with a cumulative incidence of VTE at 12 months of 17.1% (95% CI 10.3-23.9). Before starting ARSI, ptDNA fraction above median value of 0.175, presence of visceral metastasis (mets), prior chemotherapy and serum lactate dehydrogenase (LDH) were significantly associated with higher incidence of VTE compared with patients with no thrombosis (12-month estimate, 18.6 vs 3.5%, P=0.0003; 44.4 vs 14.8%, P=0.015; 24.7 vs 4.5%, P=0.006; and 30.0 vs 13.5%, P=0.050, respectively). In the multivariate analysis (Table) including baseline ptDNA level, visceral, liver and lung mets, number of lesions, LDH, high ptDNA fraction was the only independent factor associated with the risk of thrombosis (HR 5.78, 95% confidence interval [CI] 1.63-20.44, P=0.006). All these patients received anticoagulant therapy for VTE. No ARSI discontinuation and VTE-related death were reported, and no significant difference in progression-free/overall survival was observed in mCRPC patients with and without VTE. Conclusions: These results suggest that baseline ptDNA fraction in mCRPC patients treated with ARSI is associated with increased risk of VTE. These patients may be followed up more closely for the risk of VTE and the need for a primary thromboprophylaxis should be taken into account in mCRPC with elevated ptDNA concentration. Validation of these findings in larger multicenter trials is warranted.[Table: see text]

5076 Background: Treatment intensification with docetaxel or abiraterone improved survival for advanced prostate cancer starting androgen deprivation therapy (ADT) in the Systemic Therapy in Advancing or Metastatic Prostate Cancer: Evaluation of Drug Efficacy (STAMPEDE, NCT00268476) trial. However, survival and time-to-progression is highly variable on ADT, introducing the risk of unnecessary toxicity from additional treatments for some patients. Here we test the prognostic association of proliferation index using Ki67 scores in the control arm of the STAMPEDE population of high-risk localised (M0) and metastatic (M1) prostate cancer. Methods: Pre-ADT diagnostic needle biopsies were obtained from 517 men randomized in STAMPEDE arm A between 2006 and 2015. These were assessed for proliferation using an analytically optimised Ki67 immunohistochemistry assay. Ki67 was tested for associations with baseline clinico-pathological variables (Grade group, pre-ADT serum PSA and imaging metastatic burden) in univariable linear-regression models, and for associations with survival outcomes in multivariable Cox-regression models adjusted for these and additional confounding variables. Primary outcome measure was overall survival, secondary outcomes were prostate cancer-specific, failure-free, progression-free and metastatic progression-free survival. Results: Ki67 was available for 475 patients who received ADT only for at least 2 years ± radiotherapy. Of 202 M0, 74 were node positive. Of 273 M1, 116, 127 and 30 were respectively low, high and unknown radiological M1 volume. Ki67 score associated with higher Gleason (p=7.15x10-11) and presence of extra-pelvic metastases (p=1.41x10-8). Increasing Ki67 scores showed a strong linear association with poorer overall survival, with an estimated 2% increase in the hazard of death per percentage increase in the score (adjusted HR=1.02, 95% CI 1.01-1.02; p=1.04x10-5). There was also strong evidence that Ki67 associated positively with all secondary outcomes, including prostate cancer-specific survival (adjusted p=5.50x10-6) and metastatic progression-free survival (adjusted p=3.50x10-9). Conclusions: Ki67 immuno-score is strongly prognostic in clinically advanced prostate cancer independent of Gleason score and the other clinicopathological variables tested in this study. Ki67 is a clinically scalable assay that could improve selection for treatment intensification and provide a tool for screening patients most likely to benefit from further molecular investigation.

5021 Background: Men with advanced hormone-sensitive prostate cancer (HSPC) starting long term androgen deprivation therapy (ADT) follow a highly variable clinical course. Treatment intensification with docetaxel or AR targeted therapies improves outcomes but there is a risk of overtreatment, especially in non-metastatic (M0) or metastatic (M1) low volume disease. We established a framework for biomarker evaluation in the STAMPEDE trial. We aimed to evaluate the feasibility and clinical utility of assessing the burden of copy number (CN) aberrations in newly diagnosed advanced HSPC. We hypothesised that increased percentage genome altered (PGA) would associate with higher disease burden and worse prognosis. Methods: We implemented a scalable strategy using low coverage whole genome sequencing (lpWGS) of formalin fixed paraffin embedded (FFPE) diagnostic core biopsies from STAMPEDE participants randomised to the standard of care ADT arm, between 2005 and 2016. Tissue was retrieved from 136 trial sites. 315 cases were randomly selected, aiming for a biomarker population of 300, anticipating an assay failure rate ̃5%. We defined 40% as the minimum histopathologically determined tumor cellularity (TC) for inclusion. We performed a survival analysis investigating PGA at diagnosis as a continuous measure with fractional polynomial specification in Cox models adjusting for disease burden, Gleason grade, pre-ADT PSA (log-transformed), age at randomisation and TC. We pre-specified that all hypothesis tests required evidence at the 5% significance level to consider rejecting the null hypothesis. Results: We successfully CN profiled 300/315 cases. There were no significantly different baseline clinico-pathological features between the full trial comparison n = 3106 and final biomarker population n = 300, 290/300 cases were de novo presentations. PGA in the core with highest Gleason grade and TC was median 18% (range 0%-75%; n = 300). PGA was significantly higher in M1 (n = 169) compared to M0 (n = 131) cases (median: 21% vs 14%; p = 0.00006). 284/300 were subclassified by disease burden into M0 node negative and node positive, and M1 low and high volume. PGA was significantly associated with increased disease burden (p = 0.00002). Increased PGA was significantly and non-linearly associated with an increased hazard of failure-free survival (p = 0.004), progression-free survival (p = 0.002), metastatic progression-free survival (p = 0.003), overall survival (p = 0.045) and prostate cancer-specific survival (p = 0.011). Conclusions: Evaluation of the burden of CN aberrations in archival, poor quality FFPE diagnostic tissue from men randomised in the STAMPEDE trial is feasible using lpWGS and has potential clinical utility to identify better prognosis advanced HSPC patients, who may not require treatment intensification.

5037 Background: In the double-blind PBO-controlled ACIS study, investigator-assessed radiographic progression-free survival (rPFS) was significantly improved with APA + AAP vs PBO + AAP in chemo-naive mCRPC, with no significant new safety signals (Rathkopf ASCO GU 2021). Among prespecified subgroups, efficacy and safety were explored in two difficult to treat subgroups: pts with visceral disease (VD; liver, lung, and/or adrenal gland metastasis) or age ≥ 75 y. Methods: Pts with mCRPC with ongoing ADT and no prior life-prolonging treatment were randomized 1:1 to APA (240 mg QD) + AA (1000 mg QD) + P (5 mg BID) or PBO + AAP. Stratified: presence or absence of VD, ECOG PS 0 or 1, geographic region. Primary end point: rPFS (randomization to radiographic progression or death); secondary end points: overall survival (OS), time to initiation of cytotoxic chemotherapy, time to chronic opioid use, time to pain progression, safety. Results: 982 pts enrolled. 14.6% had VD and 35.9% were ≥ 75 y. Median rPFS, OS, and time to pain progression favored APA + AAP vs AAP (HR < 1) in both subgroups (Table). For pts ≥ 75 y, rPFS and OS were ≥ 7 mo longer with APA + AAP. Overall, treatment-emergent adverse events (TEAEs) were similar (all > 94%) in pts with VD, ≥ 75 y, and overall safety population; hypertension was more frequent with APA + AAP vs AAP mainly in pts ≥ 75 y (31.7% vs 17.6%). Grade 3/4 TEAEs (APA + AAP vs AAP): VD, 60.8%, n = 74 vs 48.5%, n = 68; ≥ 75 y, 71.5%, n = 186 vs 68.5%, n = 165; overall, 63.3%, n = 490 vs 56.2%, n = 489. TEAEs leading to discontinuation: VD, 17.6% vs 5.9%; ≥ 75 y, 26.3% vs 20.6%; overall, 16.9% vs 12.5%. TEAEs leading to death: VD, 6.8% vs 5.9%; ≥ 75 y, 5.4% vs 13.9%; overall, 3.5% vs 7.6%. Conclusions: In this analysis of two difficult to treat subgroups, addition of APA to AAP favored rPFS and OS. Safety, while generally consistent with the overall population, showed higher hypertension rate in ≥ 75 y and TEAEs leading to discontinuation in VD. Clinical trial information: NCT02257736. [Table: see text]

9 Background: As mCRPC is driven by activated androgen receptors and elevated intratumoral androgens, androgen annihilation may require dual inhibition. APA and AA are approved prostate cancer therapies that have distinct receptor inhibition and ligand suppression actions, respectively. ACIS compared radiographic progression-free survival (rPFS) of APA + AAP vs PBO + AAP in chemo-naive mCRPC. Methods: mCRPC pts with ongoing androgen deprivation therapy (ADT) but no other life-prolonging treatment since diagnosis were randomized 1:1 to APA (240 mg po QD) + AA (1000 mg po QD) + P (5 mg po BID) or PBO + AAP (stratified by presence or absence of visceral metastases, Eastern Cooperative Oncology Group performance status 0 or 1, and geographic region). The primary end point was rPFS (investigator assessed; defined from randomization date to radiographic progression date or death). Secondary/other end points included prostate-specific antigen (PSA) response, overall survival (OS), safety, time to PSA progression, chronic opioid use, initiation of cytotoxic chemotherapy, and pain progression. Results: 982 pts were enrolled (Dec 2014 to Aug 2016). Median rPFS was extended 6 mos with APA + AAP (22.6 mos) vs AAP (16.6 mos) (HR 0.69 [95% CI 0.58-0.83]; p < 0.0001) at the final analysis for rPFS, coinciding with the first interim analysis of OS (median follow-up time 25.7 mo). At final analysis, treatment with APA + AAP vs AAP showed a significantly higher rate of confirmed ≥ 50% PSA decline. Although not statistically significant, treatment with APA + AAP vs AAP showed a longer median OS. Time to PSA progression, chronic opioid use, initiation of cytotoxic chemotherapy, and pain progression did not differ statistically significantly between arms. The safety profile was consistent with prior drug experience, with no new safety concerns. Grade 3/4 treatment-emergent adverse events (TEAEs) were reported in 63.3% (310/490) of APA + AAP–treated pts vs 56.2% (275/489) of AAP-treated pts, with more grade 3 (56.1%, 275/490 vs 45.6%, 223/489) and less grade 4 (7.1%, 35/490 vs 10.6%, 52/489) TEAEs, respectively. Results based on biomarker subpopulations and quality of life will be presented. Conclusions: The ACIS final analysis met the primary end point and demonstrated a 31% reduction in risk of radiographic progression or death in chemo-naive mCRPC pts treated with the combination of APA + AAP with ADT vs AAP with ADT. Funding: Janssen Research & Development. Trial registration: NCT02257736. Clinical trial information: NCT02257736. [Table: see text]

TPS176 Background: Clinical benefit has been observed with poly(ADP-ribose) polymerase (PARP) inhibition in patients with metastatic castration-resistant prostate cancer tumors that carry HRR gene defects. Niraparib is a highly selective and potent PARP inhibitor, with activity against PARP-1 and PARP-2 deoxyribonucleic acid (DNA)-repair polymerases. The objective of AMPLITUDE is to assess the efficacy and safety of niraparib/placebo added to AAP with androgen deprivation therapy (ADT) in a biomarker-selected population with mCSPC. Methods: AMPLITUDE (NCT04497844) is a phase 3, randomized, double-blind, placebo-controlled study with an expected enrollment of 788 patients at ~350 sites in ~30 countries. Clinical trial information: NCT04497844. [Table: see text]

141 Background: Recently, plasma tumour DNA (ptDNA) has been identified as a potential early noninvasive biomarker of treatment response in mCRPC patients ( Conteduca, Br J Cancer 2020). In this study, we sought to determine whether pre-treatment ptDNA could accurately reflect metabolic tumor burden in mCRPC and if it could be in combination with functional imaging could provide better prognostication. Methods: Between October 2011 and June 2016, 102 plasma samples from mCRPC patients treated with abiraterone or enzalutamide were collected. Targeted next-generation sequencing was performed to determine baseline ptDNA fraction. Maximum standardized uptake value (SUVmax), total lesion activity (TLA), and metabolic tumour volume (MTV) were calculated on 18F-fluorocholine positron emission tomography/computed tomography. A Weibull multiple regression model was adopted to evaluate the combined impact of clinical, molecular and imaging features on overall survival (OS) and to obtain a prognostic score. Each variable was allotted a “partial score” that depended on the size of the regression coefficient. Total scores ranged between 0 and 5.85 and assigned patients to 3 different risk groups according to 18-months survival probability: group I, >70%; group II 30%-70%; and group III, 0.188) and median SUVmax (p

5562 Background: PC is characterized by a relatively low prevalence of recurrent somatic point mutations. ctDNA is shed from PC and can be analyzed to profile somatic point mutations and copy number changes. We evaluated a computational approach to detect ctDNA (ie. ctDNA+) in PC based on allele frequencies of polymorphisms and mutations. We then sought to confirm the association of this biomarker with disease burden and clinical outcome. Methods: Customized, hybrid capture, high-depth next-generation sequencing was performed on pre-treatment (PT) plasma samples from a phase 2 line 3+ metastatic castration-resistant PC (mCRPC) study (NCT02854436, GALAHAD) and PT and end of treatment (EOT) samples from randomized Phase 3 study in non-metastatic (nm) CRPC (NCT01946204, SPARTAN) and from metastatic castration-sensitive PC (mCSPC) (NCT02489318, TITAN). Associations of ctDNA+ with bone lesions (number), visceral metastases (+/-), circulating tumor cells count (CTCc), and serum prostate specific antigen (PSA), alkaline phosphatase (AP) and lactate dehydrogenase (LD) were tested. Also, associations of ctDNA+ with overall survival (OS) and second progression free survival (PFS2) were evaluated in randomized studies using Cox regression. Results: ctDNA+ at PT was 7.5% in nmCRPC, 23.7% in mCSPC and 66% in heavily pre-treated mCRPC. ctDNA+ increased from PT to EOT in nmCRPC (7.5% to 27%) and mCSPC (23.7% to 63.6%). Disease burden metrics were evaluated in ctDNA+ vs ctDNA- patients. ctDNA+ was associated with higher disease burden in mCRPC (Table), nmCRPC and mCSPC. At EOT, ctDNA+ patients had shorter OS and PFS2 in nmCRPC (HR [95% CI] OS: 2.73 [1.83, 4.08], p < 0.0001; PFS2: 2.00 [1.38, 2.90], p = 0.0002) and mCSPC (HR [95% CI] OS: 7.59 [3.22, 17.91], p < 0.0001; PFS2: 4.84 [2.47, 9.47], p < 0.0001). Conclusions: ctDNA+ assessed using our novel, composite biomarker increases with advanced disease state and disease progression, is significantly associated with disease burden and poor clinical outcome in PC and could be a clinically relevant metric for monitoring response to therapy. Clinical trial information: NCT02854436 . [Table: see text]

5546 Background: Plasma AR status has been identified as a potential biomarker of response in CRPC patients receiving docetaxel or the AR-targeted therapies abiraterone or enzalutamide. However, the relevance of plasma AR in the overall management of CRPC patients (pts) receiving docetaxel at different dose due to the toxicity profiles and physician-patient preferences is unknown. Methods: This was a multi-institution study of associations between baseline plasma AR-copy-number status assessed by droplet digital PCR and outcome in 325 CRPC pts. Between September 2011 and July 2019 pts started treatment with docetaxel administered at standard regimen 75mg/m2 every three weeks or adapted regimen (75-80% of standard recommended dose or 30mg/m2 weekly administration) at the discretion of the treating physician. Patients were assigned randomly into 2 sets with a ratio 2:1 to either training (n=217) and internal validation (n=108) cohorts. Results: In our study, adapted regimen of docetaxel was administered in 68 (31.3%) and 35 (32.4%) of training and validation cohorts, respectively. Based on plasma AR status, 67 (30.9%) and 39 (36.1%) validation and training set pts were classified as AR gain, respectively. In men treated with standard docetaxel regimen, no difference in progression-free/overall survival (PFS/OS) was seen between plasma AR normal and gain in both cohorts. In patients treated with adapted docetaxel regimen, we observed a significantly shorter median PFS (3.9 vs. 6.4 months, HR 4.77, 95%CI 1.48-3.80, p=0.0003) and median OS (11.2 vs . 20.4 months, HR 2.87, 95%CI 1.73-2.13, p=0.0008) in the training cohort. This finding was confirmed in the validation cohort (median PFS: 4.8 vs. 7.4 months, HR 2.54, 95%CI 1.40-4.58, p=0.005, and median OS: 11.8 vs. 26.4 months, HR 5.00, 95%CI 2.59-9.65, p

e17500 Background: IT heterogeneity (ITH) is well recognised in prostate cancer (PC), but its role in high-risk disease and patient outcomes are uncertain. Methods: This was a prospective, single-arm, translational study using targeted multi-region prostate biopsies to study genomic and T-cell ITH in clinically high-risk PC aiming to identify drivers and potential therapeutic strategies. A pre-defined exploratory objective was to evaluate the association of ITH, median inflammatory infiltrate (MII), total, clonal and neoantigens, with overall survival (OS) and time-to-next-line of therapy (TTNL). Results: 49 men with elevated prostate-specific antigen (PSA) and multiparametric-magnetic resonance imaging (MRI) detected PC underwent image-guided multiregion transperineal biopsy. 79 tumour regions from 25 patients (pts) with PC underwent sequencing, analysis of mutations, copy number and neoepitopes combined with tumour infiltrating T-cell subset quantification. The median follow-up was 60 months (m) with 10 death events. For %ITH, MII, total, clonal and subclonal neoantigens, patients were split by the median. Conclusions: The PROGENY study provides a diagnostic platform suitable for studying tumour ITH and has the potential to identify poor outcome subgroups in PC. Larger sample numbers in prospective trials are warranted to further investigate this hypothesis. [Table: see text]

5535 Background: In TITAN, addition of APA to ADT improved radiographic progression-free survival (rPFS) and overall survival (OS) versus PBO plus ADT in patients (pts) with mCSPC. In this post hoc analysis, we performed transcriptome-wide profiling of tumor samples and assessed association of molecular subtypes with rPFS. Methods: The DECIPHER platform (Decipher Biosciences, Inc.) was used to assess gene expression in archival primary prostate tumors from TITAN. Samples were classified into high versus low to average risk of metastases (DECIPHER genomic classifier [GC] > 0.6 and ≤ 0.6, respectively), basal and luminal A/B (PAM50 classifier), and androgen receptor activity (AR-A) signature high and low. Associations between subtypes with rPFS were assessed with Cox proportional hazards model. Results: The biomarker population included 222 pts (APA, 110; PBO, 112). Benefit in rPFS from APA in the biomarker population (HR [95% CI]; p value; 0.49 [0.31-0.78]; 0.002) resembled that in the overall study population (0.49 [0.40-0.61]; < 0.0001). The majority of TITAN pts had GC high scores (n = 166, 75%). GC high risk subtype in the PBO group had poorer prognosis for rPFS than GC low to average risk subtype (median rPFS 18.2 mos for GC high vs not reached [NR] for GC low to average, 0.28 [0.11-0.69]; 0.006), but there was no difference in prognosis between high and low to average GC risk subtypes in the APA group (GC high NR vs GC low to average NR; 0.81 [0.35-1.89]; 0.625). Pts were further stratified based on basal/luminal and AR-A signatures. Basal (n = 112, 50%) and AR-A low (n = 96, 43%) subtypes, known to be nonresponsive to ADT, both showed significant benefit from APA vs PBO (0.30 [0.16-0.57]; < 0.001 and 0.25 [0.12-0.52]; < 0.001, respectively). The majority of AR-A low subtype (74%, 71/96) overlapped with basal subtype. Further conclusions for risk of rPFS in GC low, luminal, and AR-A high subtypes and OS across all subtypes will be assessed as more events occur. Conclusions: In TITAN, addition of APA to ADT improved rPFS for all subtypes of pts with mCSPC. APA overcame the poor prognosis of GC high risk subtype and prolonged rPFS in ADT-resistant AR-A low and basal molecular subtypes, suggesting APA is beneficial especially for the highest risk molecular subtypes. Clinical trial information: NCT02489318 .