Hillman, David W.

Abstract Background: Multiple studies have confirmed the central role of preexisting immune response measured by stromal tumor-infiltrating lymphocytes (sTILs) in triple-negative breast cancer (TNBC). Emerging studies showed that not only the number of TILs but also the location of TILs is important. There are 3 distinct immune architectures described based on the amount and locations of TILs, namely immune enriched (IN), immune excluded (IE), and immune desert (ID). Here we evaluated outcomes and characteristics associated with each immune landscape. Methods: NanoString Digital Spatial Profiling (DSP) and CosMx, a spatial multi-omics single-cell imaging platform, were performed in 75 samples from the Mayo Clinic (MC) TNBC cohort (Leon-Ferre BCRT 2018). NanoString IO360 was performed in 114 samples from the FinXX trial ( NCT00114816) . Firstly, tumors with sTIL quantified by H&E ≤ 30% were classified as ID. The rest of the tumors with high sTIL > 30% were categorized according to the intratumoral CD8 protein expression by DSP, with IE having intratumoral CD8 in the lower median and IN having intratumoral CD8 in the upper median. Chi-square test, gene set enrichment, Cox regression, and Kaplan-Meier analysis were used. Differential expression listed as log 2-fold change (FC) was estimated from the linear mixed model with significance defined as two-sided p< 0.05. Results: ID is associated with low Ki67 < 5% (23.3% vs. 9.6% ID, p 0.02) as well as apocrine (11/13, 84.6%) and metaplastic histology (10/12, 83%). In both univariate and multivariate analysis, patients with IN had significantly improved recurrence-free survival (RFS) compared to those with ID (HR 0.37, 95%CI 0.18-0.74, p 0.005). Despite having high sTILs, IE had poor outcomes similar to ID (HR 0.84, 95%CI 0.36-1.98). Strikingly, we identified that IE patients had significantly lower plasmacytoid dendritic cells (pDCs) compared to IN (mean 0 vs. 0.26/100 tumor cells, 95%CI 0.08-0.43 , p 0.01). Using Gene Set Enrichment Analysis to evaluate differential hallmarks between IN and IE, we identified IF Nɑ and IFNγ (FDR < 0.001) responses as significantly enriched in IN group, consistent wi th the function of pDCs, which are a subset of dendritic cells specialized in secreting high levels of type I interferon. To validate this finding, we further evaluated the 11 leading edge gene IFNɑ signature in the FinXX trial. A high IFNɑ signature score was associated with significantly improved outcomes in the FinXX trial (HR 0.21, 95%CI 0.09-0.51 , p < 0.001). Similar findings were observed using Kaplan-Meier analysis in the FinXX trial with significantly improved RFS (p 0.0006) and overall survival (p 0.0001) in patients with high IFNɑ signature scores. Furthermore, we evaluated the differential gene expression unique to IN tumors in the Mayo cohort. Expressions of MHC class I and class II in tumor cells, including HLA-A, HLA-B, HLA-C, HLA-DRA, HLA-DRB1, HLA-DPA1, and HLA-E, were associated with IN and significantly improved outcomes (p < 0.05) compared to ID and IE. Conclusions: Highlighting the importance of spatial context, we identified that patients with IE tumors had poor outcomes despite having high TILs. Moreover, u sing an in-depth analysis with spatially defined context, we identified the central role of pDC and the significance of IF Nɑ in TNBC. Support: Breast Cancer Research Foundation, Mayo Clinic Breast Cancer SPORE (P50CA116201-17) W81XWH-15-1-0292, P50CA015083, R35CA253187 Citation Format: Saranya Chumsri, Yi Liu, Yaohua Ma, Jodi Carter, Mark Gregory, Sarah Church, Jason Reeves, Heather Ann Brauer, Sarah Warren, Heikki Joensuu, Edith Perez, Roberto Leon-Ferre, David Hillman, Judy Boughey, James Ingle, Krishna Kalari, Fergus Couch, Matthew Goetz, Keith Knutson, E. Thompson. The spatially resolved single-cell atlas of the tumor immune architecture revealed the central role of IFN-alpha and plasmacytoid dendritic cells in triple-negative breast cancer in the Mayo Clinic cohort and FinXX trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS03-03.

Abstract Background: Residual disease after neoadjuvant chemotherapy (NAC) in breast cancer (BC) confers poor outcomes. Trastuzumab emtansine (T-DM1) is approved for HER2+ patients with residual disease after NAC. However, many patients still develop recurrence. H2NVAC is a multi-epitope CD4 helper T cell activating vaccine that includes a pool of 4 degenerate HER2-derived HLA-DR epitopes admixed with GM-CSF. Our previous phase I trial showed that this vaccine was safe and generated robust, long-lasting T and B cell immune responses. Method: We conducted a safety run-in phase of H2NVAC in combination with T-DM1 in stage II-III HER2+ BC with residual disease after NAC. Patients were treated with H2NVAC and T-DM1 for 6 cycles, followed by 2 boosters at 3 and 12 months. Dose-limiting toxicity (DLT) was defined as any grade (G) ≥ 3 toxicity occurring within 21 days after the first vaccination. Results: 20 patients were enrolled (median age 51; 27-69 years), including 80% White and 15% Black. 90% had hormone receptor-positive disease. There was no DLT observed in the safety run-in cohort. All adverse events (AEs) during the DLT period were grades 1 and 2, with fatigue being the most common (60% G1, 5% G2), followed by peripheral sensory neuropathy (55% G1, 5% G2), and injection site reaction (50% G1). Across all treatment cycles, the 10 most common AEs that were at least possibly related to treatment in all cycles are summarized in the table below. The injection site reaction was the most common AE, with 85% G1 and 15% G2. Conclusion: H2NVAC, in combination with T-DM1, was safe, with a favorable side effect profile. There was no DLT observed. The most common AE was injection site reaction, with the majority being G1. A multicenter, randomized, placebo-controlled, phase II trial of H2NVAC vs. placebo in combination with trastuzumab emtansine is currently ongoing through the ACCRU consortium (NCT04197687). Adverse Events Grades 1 2 N % N % Injection site reaction 17 85.0 3 15.0 Aspartate aminotransferase increase 13 65.0 Alanine aminotransferase increase 9 45.0 Fatigue 7 35.0 2 10.0 Peripheral sensory neuropathy 7 35.0 2 10.0 Alkaline phosphatase increase 8 40.0 Nausea 7 35.0 1 5.0 Arthralgia 7 35.0 Myalgia 5 25.0 1 5.0 Platelet count decrease 6 30.0 Citation Format: Saranya Chumsri, Sharmila Giri, Andy J. Ness, David W. Hillman, Nadine Norton, Davitte Cogen, Brian M. Necela, Aziza Nassar, Donald W. Northfelt, Pooja Advani, Rohit Rao, Kostandinos Sideras, Alvaro Moreno-Aspitia, Brenda J. Ernst, Kathryn J. Ruddy, Matthew P. Goetz, Keith L. Knutson. Safety and tolerability of multi-epitope HER2 peptide vaccines in combination with trastuzumab emtansine in HER2-positive breast cancer patients with residual disease after neoadjuvant chemotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5005.

Abstract Background: Previous studies demonstrated that obesity is associated with poor outcomes and significantly worse pathological complete response (pCR) in triple-negative breast cancer (TNBC) treated with conventional chemotherapy. Emerging studies described a phenomenon termed the obesity paradox, which referred to improved outcomes observed in obese patients treated with immune checkpoint inhibitors (ICIs). Our study aimed to determine distinct immune features in TNBC across body mass index (BMI). Methods: Characteristics and outcomes of TNBC patients treated with neoadjuvant pembrolizumab-based chemotherapy at Mayo Clinic were collected. Normal BMI was defined as 18-25 kg/m2, overweight 25-30, and obese BMI ≥ 30. NanoString CosMx, a spatial multi-omics single-cell imaging (SMI), was performed in 75 samples from the Mayo Clinic TNBC cohort (Leon-Ferre BCRT 2018). Differential expression as log 2-fold change (FC) was estimated from the linear mixed model with significance defined as two-sided p< 0.05. Results: Between 2021-2023, there were 108 TNBC patients treated with neoadjuvant pembrolizumab. 25.9% were obese, 32.4% were overweight, and 41.7% had normal BMI. In contrast to previous studies with conventional chemotherapy, obese patients numerically had a higher pCR rate of 71.4% with pembrolizumab-based chemotherapy compared to 55.6% in normal and 62.9% in overweight patients. However, this difference did not reach statistical significance (obese vs. normal weight, OR 2.26, 95%CI 0.89-6.06, p 0.094). We further evaluated spatially-resolved immune features associated with obesity using CosMx SMI in 75 samples from the MC TNBC cohort. Obese patients had significantly higher intratumoral B cells (log2FC 1.88, p 0.017), stromal B cells (log2FC 2.79, p 0.001), stromal plasmablasts (log2FC 1.94, p 0.047), and stromal macrophages (log2FC 1.80, p 0.017) compared to patients with normal BMI. Obese patients also had significantly lower intratumoral myeloid dendritic cells (mDC) (log2FC -1.27, p 0.037). Similar trends were observed after standardizing cell counts per 100 tumor or stromal cells, including higher intratumoral B cells (log2FC 3.00, p 0.001), intratumoral plasmablasts (log2FC 2.00, p 0.006), intratumoral natural killer (NK) cells (log2FC 1.66, p 0.044), and stromal B cells (log2FC 1.50, p 0.008). Differential gene analyses for intratumoral B cells revealed that genes (CD74, HLA-C, HLA-DRB1, HLA-DRA, HLA-DRB) associated with the positive regulation of type 2 immune or chemokine response were upregulated. Conclusions: Using in-depth analysis with spatially defined context, our study showed that obese TNBC patients had significantly higher B cell infiltration, which is associated with improved outcomes with ICIs. Future research on how obesity alters the immune microenvironment is needed to identify potential pathways to improve outcomes with ICIs in TNBC. Citation Format: Songhan Pang, Yi Liu, Yaohua Ma, Zhuo Li, Roberto A. Leon-Ferre, David Zahrieh, David W. Hillman, Judy C. Boughey, James N. Ingle, Krishna R. Kalari, Fergus J. Couch, Matthew P. Goetz, Keith L. Knutson, Saranya Chumsri, E. Aubrey Thompson. Tumor-immune microenvironment associated with obesity in triple-negative breast cancer in the Mayo Clinic cohort [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1162.

PURPOSE Patients with biochemically recurrent prostate cancer (BRPC) after radical prostatectomy and a short PSA doubling time are at risk for distant metastases. Apalutamide, an androgen receptor antagonist, and abiraterone acetate plus prednisone (AAP) prolong survival in the metastatic setting. We evaluated whether intensification of androgen-deprivation therapy (ADT) improves outcomes in BRPC. PATIENTS AND METHODS PRESTO is a randomized phase III, open-label trial in patients with BRPC and PSA doubling time ≤9 months (ClinicalTrials.gov identifier: NCT03009981 ). Patients were randomly assigned 1:1:1 to receive a finite 52-week treatment course with ADT control, ADT + apalutamide, or ADT + apalutamide + AAP. The primary end point was PSA progression-free survival (PSA-PFS), defined as serum PSA >0.2 ng/mL after treatment completion. RESULTS Five hundred three patients were enrolled. The median PSA was 1.8 ng/mL (IQR, 1.0-3.6). At the first planned interim analysis, both experimental arms significantly prolonged PSA-PFS compared with the control arm (median, 24.9 months for ADT + apalutamide v 20.3 months for ADT; hazard ratio [HR], 0.52 [95% CI, 0.35 to 0.77]; P = .00047; median, 26.0 months for ADT + apalutamide + AAP v 20.0 months for ADT; HR, 0.48 [95% CI, 0.32 to 0.71]; P = .00008). Median time to testosterone recovery did not differ across treatment arms. The most common grade ≥3 adverse event was hypertension (7.5%, 7.4%, and 18% in ADT, ADT + apalutamide, and ADT + apalutamide + AAP arms, respectively). CONCLUSION Intensified AR blockade for a finite duration prolongs PSA-PFS with a manageable safety profile, without adversely affecting time to testosterone recovery. The addition of apalutamide to ADT should be considered in patients with high-risk BRPC.

5077 Background: In the phase 3 PRESTO study, intensified androgen deprivation therapy (ADT) with apalutamide (Apa) with or without abiraterone acetate plus prednisone (AAP), administered for a finite treatment period of 52 weeks, prolonged prostate-specific antigen progression-free survival (PSA PFS) in pts with high-risk biochemically relapsed prostate cancer (BRPC). We evaluated the association between PSA nadir within 3 months of treatment initiation with subsequent PSA PFS in this study. Methods: PRESTO is a randomized phase 3, open-label trial in pts with BRPC following radical prostatectomy (RP) and PSA doubling time (PSADT) ≤ 9 months, without distant metastases on conventional imaging (NCT03009981). Pts were randomized 1:1:1 to receive a finite 52-week treatment course with ADT, ADT + Apa, or ADT + Apa + AAP, stratified by PSADT (< 3 vs 3–9 months), with post-treatment follow-up. The association between PSA nadir within the first three months on treatment with subsequent PSA PFS was analyzed in a post hoc fashion. Results: 504 pts were randomized to ADT alone (N = 167), ADT + Apa (N = 168) or ADT + Apa + AAP (N = 169). 463 pts were evaluable for PSA nadir at 3 months. Across the three treatment arms, a PSA nadir of less than 0.1, between 0.1 – 0.2, and ≥ 0.2 ng/mL within the first three months of treatment was observed in 86.3%, 4.4%, and 9.3% of patients, respectively. A PSA nadir of < 0.2 ng/mL was reached within 3 months of treatment initiation in 79.8%, 96.9%, and 95.0% of pts randomized to the ADT, ADT + Apa, and ADT + Apa + AAP arms, respectively. The association between time to PSA nadir and subsequent PSA PFS was -0.30 (standard error = 0.03, Kendall’s tau modified for bivariate censoring), and median time to PSA nadir was 2.0 months (interquartile range 1.12 – 4.76 months). Failure to reach a PSA nadir < 0.2 ng/mL within the first three months of treatment was associated with a significantly shorter PSA PFS compared to pts with a PSA nadir ≤ 0.1 ng/mL (median PSA PFS of 13.9 vs. 22.8 months from 3 months post-treatment initiation; hazard ratio = 5.60, 95% CI: 3.58 – 8.75, p < 0.0001). There was also a significant difference in PSA PFS in those with a three-month PSA nadir between 0.1 – 0.2 vs ≤ 0.1 ng/mL (median PSA PFS 17.4 vs 22.8 months, HR = 2.63, 95:CI: 1.49 – 4.63, p = 0.0008). Conclusions: Using a landmark analysis at 3 months, a failure to reach PSA nadir less than 0.1 ng/mL within three months following initiation of ADT in BRPC is associated with shorter time to subsequent progression following treatment discontinuation. Follow-up is ongoing to integrate genomic profiling of primary prostate cancer tissues and whole blood RNA in pts with suboptimal PSA nadir to identify potential markers of relative androgen pathway inhibitor resistance. Clinical trial information: NCT03009981 .

5021 Background: Somatic mutations in hematopoietic progenitor cells accumulate with age leading to clonal expansion in a process called clonal hematopoiesis (CH). CH increases the risk of developing myeloid neoplasms and shortens overall survival (OS), primarily linked to increased cardiovascular (CV) mortality. The impact of CH in metastatic solid cancers has been conflicting, with shorter OS in a broad cancer cohort but prolonged OS in metastatic colorectal cancer, which may be influenced by prior cytotoxic therapy that can promote emergence of CH. As AR pathway inhibitors (ARPIs) are associated with CV adverse events (AE), exploring the impact of CH in patients (pts) with mCRPC is of interest. Methods: A 9 gene targeted DNA sequencing panel designed to capture common CH mutations ( DNMT3A, TET2, ASXL1, PPM1D, TP53, CHEK2, SRSF2, SF3B1, JAK2) was applied to pre-treatment peripheral blood samples of 957 pts enrolled in the Alliance A031201 phase 3 trial of enzalutamide versus enzalutamide plus abiraterone/prednisone in the first line mCRPC setting. No difference in OS was found between treatment arms. The primary endpoint was the impact of CH on OS; secondary and exploratory endpoints included progression-free survival (PFS) and associations with baseline comorbidities and AEs (hematologic and CV). Cox proportional-hazards and logit models were adjusted for treatment arm, age, and radiation therapy (RT). Results: CH was common with 57% of pts having low CH (VAF>0.5% in 1+ mutation, L-CH), 24% having standard CH (VAF>2%, CH), and 7% having high CH (VAF>10%, H-CH). 4% of pts had 2+ CH mutations and 1% had 3+ mutations. Patients with CH were older (73.1 vs 70.2) and had higher rates of prior RT (58.4% vs 55.7%) and prior chemotherapy (3.9% vs 2.2%). No differences in OS were detected in pts with L-CH, CH, or H-CH. Similarly, no differences in PFS or baseline CV comorbidities were seen in any CH group. Pts with H-CH and TET2-mutated CH had statistically higher odds of any CV event (adjusted p=0.0004 and p=0.010, respectively) There was also a difference in the occurrence of major adverse CV events (MACE) by H-CH status (5.8% vs. 1.9%, adjusted p=0.042), while there was no difference seen for CH (3.4% vs 1.8%, adjusted p=0.147). Regarding hematologic AEs, one pt with a TP53 mutation (VAF 0.94%) developed myelodysplastic syndrome. Conclusions: CH did not affect OS or PFS in pts with mCRPC treated with ARPIs in the A031201 trial. However, CH with VAF>10% and TET2-mutated CH were associated with an increased rate of all CV AEs, suggesting an additive effect of ARPI therapy and CH on adverse CV outcomes. Confirmatory studies are warranted to assess whether CH should be considered in the risk-benefit discussion for pts receiving ARPIs.

581 Background: Several studies have established the crucial role of preexisting immune response measured by tumor-infiltrating lymphocytes (TILs) in triple-negative breast cancer (TNBC). Emerging studies showed that not only the number of TILs but also the location of TILs is as critical. There are 3 distinct immune landscapes described based on the locations of TILs, namely immune enriched (IN), immune excluded (IE), and immune desert (ID), which are associated with outcomes in TNBC treated with immune-checkpoint inhibitors. Here we evaluated characteristics associated with each immune landscape. Methods: NanoString IO360, Digital Spatial Profiling (DSP), and CosMx, a spatial multi-omics single-cell imaging platform, were used. DSP was used to quantify 39 immune-related proteins in stromal and tumor-enriched segments from 44 TNBC samples from the FinXX trial (NCT00114816) and 276 samples from the Mayo Clinic (MC) TNBC cohort (Leon-Ferre BCRT 2018). CosMx was performed in 75 samples from the MC TNBC cohort. First, tumors with TIL quantified by H&E ≤ 30% were classified as ID. The rest of the tumors were categorized according to the intratumoral CD8 protein expression by DSP, with IE having intratumoral CD8 in the lower median and IN having intratumoral CD8 in the upper median. Differential expression listed as log fold change (FC) was estimated from the linear mixed model with significance defined as two-sided p < 0.05. Results: Using DSP in the FinXX trial, intratumoral and stromal higher HLA-DR (FC 1.68, p = 0.001), B2M (FC 0.8, p = 0.005), CD4 (FC 0.74, p = 0.01), and CD40 (FC 1.56, p = 0.001) were associated with IN compared to ID. When comparing IE and IN, higher intratumoral CD11c (FC 0.97, p = 0.01) and stromal CD4 (FC 0.89, p = 0.047), CD20 (FC 0.85, p = 0.016), CD40 (FC 0.95, p = 0.045), and CD27 (FC 0.84, p = 0.024) were associated with IN. Similar findings were observed in the MC cohort. Moreover, intratumoral NY-ESO-1 expression (FC 0.55, p = 0.03) was associated with IN. Using GSEA with IO360 in the FinXX trial, PI3K-Akt signaling was associated with ID compared to IN (p = 0.01). We further evaluated the differential gene expression in a spatially resolved manner using CosMx with single-cell sequencing in the MC cohort. Expressions of MHC class I and class II in tumor cells, including HLA-A, HLA-B, HLA-C, HLA-DRA, HLA-DRB1, HLA-DPA1, and HLA-E, were associated with IN compared to ID and IE. Conclusions: Using an in-depth analysis with spatially defined context, we identified characteristics associated with distinct immune landscapes in TNBC. Our study highlights the potential future implications of intratumoral MHC expression, CD40, and PI3K-AKT as biomarkers and therapeutic targets. Clinical trial information: NCT00114816 .

PURPOSE Enzalutamide and abiraterone both target androgen receptor signaling but via different mechanisms. The mechanism of action of one drug may counteract the resistance pathways of the other. We sought to determine whether the addition of abiraterone acetate and prednisone (AAP) to enzalutamide prolongs overall survival (OS) in patients with metastatic castration-resistant prostate cancer (mCRPC) in the first-line setting. PATIENTS AND METHODS Men with untreated mCRPC were randomly assigned (1:1) to receive first-line enzalutamide with or without AAP. The primary end point was OS. Toxicity, prostate-specific antigen declines, pharmacokinetics, and radiographic progression-free survival (rPFS) were also examined. Data were analyzed using an intent-to-treat approach. The Kaplan-Meier estimate and the stratified log-rank statistic were used to compare OS between treatments. RESULTS In total, 1,311 patients were randomly assigned: 657 to enzalutamide and 654 to enzalutamide plus AAP. OS was not statistically different between the two arms (median, 32.7 [95% CI, 30.5 to 35.4] months for enzalutamide v 34.2 [95% CI, 31.4 to 37.3] months for enzalutamide and AAP; hazard ratio [HR], 0.89; one-sided P = .03; boundary nominal significance level = .02). rPFS was longer in the combination arm (median rPFS, 21.3 [95% CI, 19.4 to 22.9] months for enzalutamide v 24.3 [95% CI, 22.3 to 26.7] months for enzalutamide and AAP; HR, 0.86; two-sided P = .02). However, pharmacokinetic clearance of abiraterone was 2.2- to 2.9-fold higher when administered with enzalutamide, compared with clearance values for abiraterone alone. CONCLUSION The addition of AAP to enzalutamide for first-line treatment of mCRPC was not associated with a statistically significant benefit in OS. Drug-drug interactions between the two agents resulting in increased abiraterone clearance may partly account for this result, although these interactions did not prevent the combination regimen from having more nonhematologic toxicity.

201 Background: Previous studies have evaluated peripheral whole blood mRNA to identify prognostic biomarkers in metastatic prostate cancer, with no consistent genes of prognostic significance between studies and sample collection done at various points of therapy. We aim to identify the prognostic value of 20 previously identified genes in patients mCRPC from a prospective clinical trial. Methods: Between June 2013 and August 2015, whole blood was prospectively collected in PAXgene RNA tubes from 92 men with mCRPC (NCT01953640). Gene expression from samples with RIN > 4.0 were quantified using a customized NanoString CodeSet of 20 candidate genes (C1QA, MCM2, STOM, GABARAPL2, R10K3, HMBS, TFDP1, CDKN1A, ITGAL, TMCC2, TERF2IP, MMP9, SNCA, TIMP1, SLC4A1, SELENBP1, TMEM66, SEMA4D, PROS1, CD22). The raw data from the nCounter Analysis system were normalized to the positive controls and log2 transformed. Univariable Cox proportional hazards models were performed on each target gene sufficiently above negative control (5 times the maximum value of the negative control) to evaluate associations with overall survival (OS). For each gene we considered a linear, quadratic, or categorical model with quantile cutpoints. Categorical models with cutpoints at the 25th percentile for the genes CD22, GABARAPL2, R10K3, and STOM were used as there was a nonlinear relationship between gene expression and death hazard. The remaining genes were modeled with a linear model. Findings were considered statistically significant based on p-values adjusted using the Holm-Bonferroni correction. Results: 18/20 genes were identified and passed quality control (QC) measures, which excluded C1QA and MCM2. 72/92 men had an evaluable pretreatment specimen that passed QC. The median age was 72 years. The study cohort was evenly balanced in terms of Gleason score ( < = 7 v. > 7) and 60% had high volume disease. The median follow-up time was 5.73 years (IQR: 5.39, 6.49) and 58 patients had died. Of the 18 genes evaluated, STOM (Stomatin) [hazard ratio (HR) 3.00, adjusted p = 0.007] and GABARAPL2 (GABA Type A Receptor Associated Protein Like 2) [HR 2.62, adjusted p = 0.033] were significantly associated with overall survival in univariate analysis. In multivariate analysis, after adjusting for volume of metastatic disease and Gleason score, STOM [HR 2.89, adjusted p = 0.014] was significantly associated with overall survival. Conclusions: Elevated whole blood mRNA expression of STOM is adversely associated with OS in mCRPC. Further studies investigating elucidating the molecular significance of STOM in mCRPC are planned. Clinical trial information: NCT01953640 .

208 Background: In the Phase 3 PRESTO study, intensified androgen deprivation therapy (ADT) with apalutamide (APA) with or without abiraterone acetate plus prednisone (AAP), administered for a finite treatment period of 52 weeks, prolonged prostate-specific antigen progression-free survival (PSA PFS) in pts with high-risk biochemically relapsed prostate cancer (BRPC). We evaluated baseline factors associated with PSA PFS in this study. Methods: PRESTO is a randomized phase 3, open-label trial in pts with BRPC following radical prostatectomy (RP) and PSA doubling time (PSADT) ≤ 9 months (mo), without distant metastases on conventional imaging (NCT03009981). Pts were randomized 1:1:1 to receive a finite 52-week treatment course with ADT, ADT + APA, or ADT + APA + AAP, stratified by PSADT (< 3 vs 3–9 mo), with post-treatment follow-up. Baseline factors associated with PSA PFS including Gleason sum at RP (6-7, 8, ≥ 9) were analyzed in a post hoc fashion. Results: 504 pts were randomized to ADT alone (N = 167), ADT + APA (N = 168) or ADT + APA + AAP (N = 169). Baseline patient characteristics including Gleason sum at diagnosis, serum PSA and PSADT at study entry, time interval from radical prostatectomy, and receipt of prior radiation (none, adjuvant, salvage) were well balanced across the three treatment arms. At the first planned interim analysis, both experimental arms significantly prolonged PSA PFS compared to the control arm (median 24.9 mo for ADT + APA vs 20.3 mo for ADT, HR = 0.52 (95% CI: 0.35–0.77); median 26.0 mo for ADT + APA + AAP vs 20.0 mo for ADT, HR = 0.48 (95% CI: 0.32–0.71)). Across the study cohort, Gleason sum ≥ 9 at diagnosis was associated with shorter PSA PFS (median 21.9 mo for Gleason ≥ 9 vs. 31.1 mo for Gleason 8 vs. 25.2 mo for Gleason 6-7, log-rank p-value = 0.0409). In addition, within each treatment arm, a shorter observed median PSA PFS was detected for patients with Gleason ≥ 9 prostate cancer. Serum PSA and PSADT at study entry, time from prior radical prostatectomy, and prior radiation were not associated with PSA PFS in the overall study cohort or in individual study arms. Conclusions: Gleason sum ≥ 9 prostate cancer at diagnosis was associated with shorter time to PSA progression following subsequent intensified ADT administered for a finite treatment interval in BRPC. Follow-up is ongoing to integrate genomic profiling of primary prostate cancer tissue with these results and validate with longer term endpoints including metastasis-free survival. Clinical trial information: NCT03009981 .