Citations

9525 Background: Melanocortin-1 receptor (MC1R) is a novel target for radiopharmaceutical therapy and is highly expressed on melanoma tumor cells. VMT01 is an MC1R-targeted peptide. 203 Pb is used for patient selection via SPECT imaging. Peptide radiolabeling with 212 Pb delivers alpha-particle therapy. Here, we present data on the treatment of adult patients with MC1R-positive metastatic melanoma receiving [ 212 Pb]VMT01 either as a monotherapy or in combination with the programmed death-1 (PD-1) checkpoint inhibitor, nivolumab. Methods: This is an ongoing phase I/IIa, first-in-human, prospective, multicenter, open-label, radioactive dose-finding and dose-expansion trial. The objectives are to investigate safety, pharmacokinetics, dosimetry, and efficacy. Participants receive up to 3 treatment cycles with either [ 212 Pb]VMT01 3 mCi, 5 mCi, or 1.5 mCi monotherapy, or combination therapy with [ 212 Pb]VMT01 (1.5 mCi) + nivolumab (480 mg, Q4W) or [ 212 Pb]VMT01 (3 mCi) + nivolumab. Participants are evaluated for any DLTs for the first 6 weeks after cycle 1. Efficacy is assessed by RECIST v1.1 criteria by the investigator. Results: As of 24 December 2025, 26 participants (53.8% male; median age: 68 years [range: 27-81]) with MC1R-positive metastatic melanoma were enrolled. The median prior systemic therapies was 4. All 26 enrolled participants received ≥1 dose of [²¹²Pb]VMT01. No DLTs were reported. All SAEs (including one grade 5 SAE) were attributed to disease progression. Most TEAEs were grade 1 (19.2%) and grade 2 (34.6%). Grade 3 TEAEs occurred in 8 participants (30.8%), 5 on monotherapy and 3 on combination therapy (3 mCi + nivolumab). Of the 22 evaluable participants, 1 PR, 9 SD, and 12 PD were reported. Four participants had progression-free survival of ≥6 months. Conclusions: [ 212 Pb]VMT01 as a monotherapy and in combination with nivolumab was generally safe and well-tolerated in 26 enrolled participants, with antitumor activity observed at the 3 mCi dose level. These data may be supplemented with a more complete follow-up at the time of the meeting. Clinical trial information: NCT05655312 .

8579 Background: LOH may provide a means to target tumor versus normal cells, to augment the efficacy and safety of MSLN-targeted programs (Hecht et al. JCO . 2022). A2B694 is an autologous, logic-gated, Tmod CAR T therapy designed to improve tumor selectivity and decrease toxicity by integrating an MSLN CAR activator with an HLA-A*02 blocker (Hamburger et al. Mol Imm. 2020). Methods: The first-in-human, open-label, phase 1/2 EVEREST-2 (NCT06051695) study of A2B694 in patients with recurrent/metastatic MSLN-expressing cancers with tumor-associated HLA-A*02 LOH. The prescreening study BASECAMP-1 (NCT04981119) identifies eligible patients and cryopreserves leukapheresis product. Upon progression, A2B694 is manufactured and administered after lymphodepletion. Phase 1 primary objective: evaluate the safety and tolerability of A2B694 and identify a recommended phase 2 dose. Results: As of 05 January 2026, 13 patients were enrolled: 8 women/5 men, median age 59 years, 11 non-Hispanic White/2 Hispanic with unknown race. Tumor types included colorectal (n = 4), ovarian (n = 3), pancreatic (n = 3), non–small cell lung adenocarcinoma (NSCLC), gastro-esophageal, and mesothelioma (n = 1 each). A2B694 dose level (DL) groups were: DL1: 1×10 8 cells (n = 3), DL2: 2×10 8 cells (n = 4), DL3: 4×10 8 cells (n = 5), and DL4: 6×10 8 cells plus low-dose IL-2 (n = 1). Lymphodepletion prior to administration of A2B694 was well-tolerated, with expected, transient cytopenias. The only adverse event reported in more than 1 patient was grade 3 neutropenia. One patient had grade 3 ICANS and 1 patient had grade 1 CRS. There were no dose-limiting toxicities or new safety signals after up to 17 months follow-up. All 13 patients received A2B694, were efficacy-evaluable, and had A2B694 detected post-infusion in peripheral blood. While A2B694 was not detected in tumor biopsies collected in patients treated at DL1 (0/2), all patients treated at DL2-DL4 with available biopsies (3/3) had detectable A2B694 in the tumor microenvironment. A patient with KRAS G12V /STK11 co-mutated NSCLC who had progressed on carboplatin, pemetrexed, and pembrolizumab achieved a complete response (CR) at D90 post-infusion per RECIST 1.1 by central review and had a confirmed CR at D180. In addition, PET-CT scan and ctDNA on D190 demonstrated no evidence of disease. On D243, the patient had a CNS relapse, with an ongoing non-CNS CR per RANO-BM at D284. At M12, the patient's CT showed no new findings and persistence of A2B694 in the blood was confirmed by ddPCR. Conclusions: We report the first patient with NSCLC to have a CR after CAR T. Overall, A2B694 demonstrated manageable safety and tolerability in patients with advanced solid MSLN-expressing tumors with tumor-associated HLA-A*02 LOH. The maximum tolerated dose has not been reached; dose-escalation continues. Clinical trial information: NCT06051695 .

8519 Background: Non-classical mutations (NCM) of epidermal growth factor receptor (EGFR) represent a broad group of oncogenic mutations in non-small cell lung cancer (NSCLC), often associated with resistance to osimertinib (e.g., P-loop and αC-helix compressing [PACC] mutations). Patients (pts) with NCM have inferior clinical outcomes with a high incidence of CNS metastases (CNSm). Silevertinib (BDTX-1535) is a fourth-generation, covalent EGFR TKI with high CNS penetrance targeting both EGFR classical mutations and NCM. The antitumor activity and safety of silevertinib in pts with advanced NSCLC were evaluated in a Phase 2 trial (NCT05256290). Methods: Pts with advanced EGFR NCM NSCLC who received no prior systemic therapy were enrolled (Cohort 3) based on a local molecular test. Silevertinib 200 mg was administered orally once daily. The primary endpoint of objective response rate (ORR) was assessed using RECIST v1.1. CNS ORR was assessed using Response Assessment in Neuro-Oncology for Brain Metastases (RANO-BM) in pts with no prior CNS therapy. Secondary endpoints include progression-free survival (PFS), duration of response (DOR), disease control rate (DCR), and safety. Results: From February 2024 to July 2025, 43 pts with 35 unique NCMs (including 25 pts with PACC mutations) were enrolled in Cohort 3: median age 70.0 years, 72% female, 74% white, and 42% never smoked cigarettes. As of November 3, 2025, median follow-up was 7.2 months. ORR by RECIST v1.1 is shown in the table. Sixteen pts (37%) had CNSm, including 7 pts (16%) with measurable CNS disease. RANO-BM CNS response was 86% (n=6/7). Complete clearance of EGFR VAF ctDNA was observed in 21/26 (81%) evaluable pts. Serious treatment-related adverse events (TRAEs) were reported in 5 pts (12%), and 4 pts (9%) discontinued treatment due to TRAEs. The most common TRAEs included rash (19%, grade 3), diarrhea (19%, grade 3), stomatitis (9%, grade 3), and paronychia (5%, grade 3). While 77% of pts had a dose reduction, 19/22 (86%) patients with a radiographic response maintained or deepened their response after dose reduction, including all patients with CNS response. Conclusions: Silevertinib demonstrated robust antitumor activity in treatment-naïve pts with a broad spectrum of NCM EGFR driver mutations with high intracranial activity in pts with brain metastases. The safety profile was consistent with the known AEs of the EGFR TKI class. Clinical trial information: NCT05256290 . Data as of November 3, 2025 N ORR n (%) 95% CI DCR n (%) 95% CI ITT Population * 43 26 (60) (44.4, 75.0) 39 (91) (77.9, 97.4) PACC mutations 25 14 (56) (34.9, 75.6) 22 (88) (68.8, 97.5) Compound mutations † 16 11 (69) (41.3, 89.0) 15 (94) (69.8, 99.8) *No pts with classical (E746_A750del or L858R) mutations were enrolled. † Presence of ≥ 2 NCMs. As of December 27, 2025, the 6-month PFS rate was 86% overall and 80% in pts with CNSm; available PFS and DOR data will be presented.

8057 Background: The NCI Adjuvant Lung Cancer Enrichment Marker Identification and Sequencing Trials (ALCHEMIST) study tested resected stage II-IIIB(N2) NSCLC for targetable EGFR and ALK alterations and PD-L1 expression to screen patients for biomarker-driven adjuvant therapy trials (NCT02194738). Local lab results were requested when available for comparison to central lab results. Methods: We identified patients with both central and local PD-L1 results in the ALCHEMIST adjuvant immunotherapy study screening population. Discrepancies in tumor proportion score (TPS) between central and local PD-L1 were defined using prespecified cut-points (granular: <1%, 1-<5%, 5-<10%, 10-<20%, 20-<30%, 30-<40%, 40-<50%, 50-60%, 60-<70%, 70-<80%, 80-<90%, 90-<100%; clinical: <1%, 1-<50%, 50-100%). Large discrepancies were defined as ≥ 50 percentage point differences; for these cases, local PD-L1 and hematoxylin & eosin-stained slides were requested. These and all central slides were digitally scanned at 40X. Three expert pathologists reviewed the scanned slides in a blinded fashion. Differences between patient or tumor characteristics were assessed by Chi-square and Wilcoxon rank sum tests. Percent agreement was computed within and between the reviewers. Results: Central and local PD-L1 results were available for 497 patients. 177 cases (36%) had discrepancies that placed patients in different clinical TPS categories (<1%, 1-<50%, 50-100%). Among these, 43 cases had large TPS discrepancies; 14 had available local testing slides. 46 of the 320 cases without discrepancy were randomly selected as controls. No patient or tumor characteristics were associated with discrepancy, including tumor location, histology, grade or cancer stage (p > 0.05). Pairwise comparisons of TPS according to three pathologists re-reading the same scanned PD-L1 slides showed similar levels of disagreement in clinical TPS category whether there was initial discrepancy between central and local results (24%, 34%, and 23%; mean 27%) or not (33%, 30%, and 20%; mean 28%). Among the 14 cases with large initial discrepancies and local slides available, there was agreement in clinical TPS category when the anonymized central and local scanned slides were re-read by the pathologists in 54%, 38%, and 43% of cases, despite 100% non-agreement in initial reads. There were higher rates of agreement when the tested slides both originated from the same (100%) versus different (35%) blocks, when the same (48%) versus different (40%) PD-L1 antibodies were used, and when the cases were of non-squamous (52%) versus squamous (14%) cell carcinoma. Conclusions: There are substantial differences in PD-L1 TPS read by pathologists that can result in placement of patients in different categories commonly used for clinical management. These differences have clear implications in clinical trials and routine patient care.

8001 Background: The ALCHEMIST clinical trial platform, launched by the National Clinical Trials Network (NCTN) in 2014, screened patients with resected non-small cell lung cancer (NSCLC) for molecular markers and tailored post-operative adjuvant therapy trials. A081105 evaluated whether adjuvant erlotinib improved overall survival (OS) following complete resection in patients with early stage EGFR mutated lung adenocarcinoma. Methods: A081105 was a randomized phase III trial enrolling patients with completely resected stage IB (≥4 cm), II, or IIIA non-squamous NSCLC (per the 7 th TNM staging classification) harboring EGFR exon 19 deletion or L858R mutations. Patients were randomized 1:1 to erlotinib 150 mg daily for up to 2 years or placebo/observation, stratified by stage, prior chemotherapy, EGFR mutation subtype, and ECOG performance status. The primary endpoint was OS in the per-protocol (PP) population with centrally confirmed EGFR mutations, designed to detect a hazard ratio (HR) of 0.67 in favor of erlotinib with a power of 86% and one sided type I error rate of 5%. Secondary endpoints included OS and disease free survival (DFS) in a modified intent-to-treat (mITT) population with local or centrally confirmed EGFR mutations, and safety. A prespecified backstop analysis was conducted after a minimum of 5 years of follow-up on all patients. Results: Trial enrollment was discontinued when 390 of the intended 450 patients had been enrolled due to the proven benefit of adjuvant osimertinib. Among 390 enrolled patients, the arms were balanced for age (median: 67.0 years), sex (70% female), race (70% white), stage (II: 51%, IIIA: 37%), prior chemotherapy (82%), ECOG PS 0 (60%), and exon 19 deletion (57%). Adjuvant erlotinib did not significantly improve OS compared with placebo/observation in the overall PP population (364 patients, 108 events, HR 0.89; 95% CI, 0.59–1.34; 1-sided P = 0.29) or within exon 19 deletion (HR: 0.94) or L858R mutation (HR: 0.82) subgroups. Five-year OS rates were 78.6% versus 77.9%, respectively. Results were consistent in the mITT analysis (379 patients, 113 events, HR 0.86; 95% CI, 0.58–1.28). Erlotinib improved DFS (mITT: 181 events, median: 68 vs 50 months, HR 0.74; 95% CI, 0.55–1.01). Across both arms, 11% of patients started non-protocol treatment (Osimertinib: 9%) prior to disease progression, and 9% withdrew consent for all follow-up prior to recurrence. In the erlotinib arm, 3 deaths on study were reported; grade 3 or higher adverse events were reported on 39% of patients. Median duration of treatment on Erlotinib arm was 16 months; only 37% of patients completed the planned treatment. Conclusions: In resected EGFR -mutant NSCLC, adjuvant erlotinib did not improve overall survival but improved DFS. Additional studies are exploring efficacy in selected subsets of patients using genomics and proteomic analyses. Clinical trial information: NCT02193282 .

8000 Background: The NCI’s ALCHEMIST program has screened thousands of patients with resected lung adenocarcinoma for mutations in EGFR and ALK. EA5142 studied the efficacy of adjuvant nivolumab after standard of care adjuvant therapy in patients with resected lung adenocarcinoma without sensitizing EGFR and ALK alterations and squamous cell carcinoma. Methods: Patients enrolled in the ALCHEMIST screening trial (A151216) with resected NSCLC tumors at least 4 cm and/or lymph node positive (N1/2) (for lung adenocarcinoma, without sensitizing EGFR and ALK alterations), were centrally tested for tumor cell PD-L1 expression using the DAKO 28-8 clone. After completion of all planned adjuvant therapy (chemotherapy and/or post-operative radiation) patients were randomized 1:1 to adjuvant nivolumab 480 mg IV every 4 weeks for up to 1 year or best supportive care. Patients were stratified by tumor histology, stage (AJCC 7 th edition IB/IIA vs IIB/IIIA), prior adjuvant treatment, and PD-L1 status (<1% vs ≥1%). The primary study endpoints were disease free survival (DFS) in all patients and in patients with high tumor PD-L1 expression (≥50%). Overall survival was powered to be tested hierarchically, if DFS was positive. Results: Between July 2016 and October 2019, 935 patients were randomized across 378 sites. The treatment arms were balanced for age (median 66 years), sex (52% male), smoking status (10% never smoked), histology (29% squamous), stage and PD-L1 expression (29% PD-L1 high). With a median follow-up of 72.6 months, nivolumab did not improve DFS in the intention to treat population (median DFS 71.3 months vs. 68.8 months in observation arm), hazard ratio (HR) 0.97 (95% CI 0.81-1.17, p=0.78) or the PD-L1 ≥50% subset (median DFS 89.8 months vs. 78.5 months in observation arm), HR 0.86 (95% CI 0.59-1.25, p=0.43). In multivariate analysis, adjusting for age, gender, and smoking history, the DFS HR was 0.99 (95% CI 0.82-1.19, p=0.89) in all randomized patients and 0.82 (95% CI 0.56-1.2, p=0.31) in the PD-L1 ≥50% subset. As the primary recurrence endpoint included any lung cancer, including new primaries, a sensitivity analysis was performed excluding new primary lung cancers, DFS HR 0.97 (95% CI 0.80-1.18, p=0.76) in all patients and HR 0.88 (95% CI 0.60-1.30, p=0.52 in the ≥50% subset. The median duration of treatment with adjuvant nivolumab was 9.4 months (range 0-12.7 months). The most common reasons for nivolumab discontinuation were adverse event (29%), patient withdrawal (11%), and disease progression while on treatment (10%). Treatment related grade 3-5 toxicities were reported in 25% of patients: Grade 3: 23%, Grade 4: 2%, Grade 5: <1%. Conclusions: In patients with resected NSCLC (≥4cm or LN+, EGFR/ALK -) adjuvant nivolumab did not improve DFS, irrespective of tumor PD-L1 expression. Clinical trial information: NCT02595944 .

7028 Background: Axi-cel, autologous anti-CD19 CAR T-cell therapy, showed long-term survival and curative potential in R/R LBCL (ZUMA-1; Neelapu et al. Blood . 2023). Few studies have examined long-term outcomes beyond clinical trials. Here we report long-term real-world outcomes of axi-cel in R/R LBCL using large-sample CIBMTR data. Methods: A total of 1500 adults from 79 US centers receiving commercial axi-cel after ≥2 LOT for LBCL from Oct 2017-Aug 2020 were enrolled in a post-marketing requirement study. A protocol was developed prior to the study’s implementation, reviewed by Kite and CIBMTR. This is a secondary analysis of said study. Key outcomes were progression-free survival (PFS), overall survival (OS), time to next therapy (TTNT), disease-specific mortality (competing risks were non-relapse deaths), subsequent malignancies, and non-relapse mortality (NRM). Outcomes were analyzed descriptively. Cases with missing data were excluded separately for each endpoint. Results: As of Aug 2025, 1446 patients (pts) were included in the analysis. Median age at infusion was 62.2 y (range, 19.6-90.8: 38% of pts ≥65 y); 65% were male. Five percent of pts had ECOG PS ≥2 and 16% had high-grade lymphoma. Pts had a median of 3 (range, 2-4) prior LOT, 28% had prior autologous hematopoietic cell transplantation, and 50% received bridging therapy. At 59.7 mo median follow-up, median PFS (95% CI) was 7.9 mo (6.3-10.9) and 5-y PFS was 30% (27-32). The median OS was 25.7 mo (21.6-30.2); 5-y OS was 38% (35-41). The 5-y cumulative incidence of disease-specific mortality was 51%. Among pts who were progression-free at 2, 3, 4, or 5 y post-infusion, the OS at 6 y (95% CI) was 73% (68-78), 81% (76-86), 90% (84-94), and 94% (89-97), respectively. The median duration of response (DOR) was 25.3 mo (95% CI, 20.7-32.8) and 5-y DOR was 38% (35-41). The median TTNT was 12.2 mo and 30% did not need additional LOT at 5 y. Of pts who received subsequent salvage therapy, 12 (2%) received cell therapy, 39 (7%) received hematopoietic cell transplant, and 475 (90%) received other therapy. The 1, 2, 3, 4, and 5-y cumulative incidence of relapse (95% CI) was 49% (46-51), 52% (50-55), 54% (52-57), 55% (53-58), and 56% (53-59), respectively. No new safety signals were observed. The primary causes of death (n=864; 60%) were primary disease (540; 63%), infection (111; 13%), and organ failure (57; 7%). The 5-y cumulative incidence of NRM was 16% (95% CI, 14-18). At 5 y, 11% (95% CI, 10-13) had subsequent malignancies, mostly therapy-related myeloid neoplasms (6%) and non-melanoma skin cancer (3%); none were directly attributed to axi-cel. Conclusions: This real-world study showed sustained survival at 5 y among pts with R/R LBCL treated with axi-cel after ≥2 LOT, consistent with ZUMA-1. Infection and organ failure were common causes of NRM. These results continue to support the use of axi-cel with curative intent in R/R LBCL.

566 Background: Endocrine therapy (ET) resistance (ETR) remains a primary challenge in ER+ breast cancer. Analyzing pretreatment tumor transcriptomes across trials with early response endpoints can reveal shared and specific ETR signatures. This study utilizes baseline RNA data from the Phase III ALTERNATE trial (Alliance A011106, NCT01953588; Anastrozole [A], Fulvestrant [F], or AF) and the ACOSOG Z1031B trial (NCT00824941) to identify predictors of early Ki67 response in postmenopausal ER+/HER2– patients. Methods: ETR was defined as week-4 on-treatment Ki67 >10%. Baseline gene expression from ALTERNATE was analyzed to identify differentially expressed (DE) genes (Wilcoxon test, P<0.05) and Hallmark pathways associated with ETR, both across and within individual treatment arms. Feature selection was performed using mixOmics. A Pan-Endocrine Therapy Signature (PETS) was developed by uniting DE genes identified across all three ALTERNATE arms and Z1031B. All statistical analyses were conducted in R (P<0.05). Results: Overall ETR rate in the ALTERNATE RNA-seq cohort (n=733) was 26%. ETR was associated with high Risk of Recurrence (ROR), Oncotype RS, and Mammaprint scores (calculated from RNA-seq data in research setting). In luminal tumors (n=649), ETR was linked to chr 3q13.33, 8q24.13, and 20q13.12 cytoband upregulation and 17q21, 18q23, 3p21.1, and 10q24.32 cytoband downregulation. ETR tumors showed T-cell, E2F target, and interferon-γ enrichment; sensitive tumors favored early estrogen response and muscle differentiation. At individual gene level, high MYBL2 , PIF1 , TROAP and with low HJURP predicted ETR across all samples (AUC>0.70). A deep learning model using all protein-coding genes achieved AUC 0.82 (training) and 0.79 (test) in predicting ETR. Cross-trial integration identified ETR-associated PETS, enriched for genomic instability. PETS performed comparably to established signatures and strongly correlated with MYBL2 signature (r=0.93). Top ETR predictors were MYBL2 , AURKB , and EME1 for Arm A and IL4I1 , TNFAIP6 , and ANLN for Arm AF. AF-resistant tumors were enriched for systemic lupus and RIG-I–like receptor signaling; sensitive tumors favored PI3K–AKT, EGFR TKI resistance, AMPK, and insulin signaling. Conclusions: Baseline transcriptomics identify shared and therapy-specific ETR markers. The 15-gene PETS defines a convergent resistance signature, performing similar to established signatures in predicting ETR and correlating with MYBL2. Enrichment of cell-cycle and immune pathways in resistant tumors may suggest patient stratification approach for alternative or combinatorial strategies to overcome early ETR in ER+ breast cancer. Acknowledgement: https://acknowledgments.alliancefound.org. Support: U10CA180821, U10CA180882, U24CA1. Clinical trial information: NCT01953588 .

6028 Background: RET alterations are targetable oncogenic drivers in multiple solid tumors and TCs. Pralsetinib is an oral, potent, selective RET inhibitor granted accelerated approval by the FDA for adults and children aged ≥12 y with advanced/metastatic RET fusion-positive TC who require systemic therapy and are radioactive iodine-refractory, based on ARROW trial (NCT03037385) results. Accelerated approval for medullary TC (MTC) was withdrawn by the sponsor in 2023 due to recruitment challenges in the confirmatory trial. We report final efficacy and safety in ARROW patients with RET -altered TC and MTC. Methods: Phase 2 ARROW patients were enrolled from 84 sites in 13 countries. Primary end points were overall response rate (ORR; per RECIST v1.1) and safety. Progression-free survival (PFS), overall survival (OS), and safety were assessed in patients with advanced/metastatic TC and MTC who received ≥1 dose of pralsetinib (Efficacy Population [EP]). ORR and duration of response (DOR) were assessed in the Measurable Disease Population (MDP). Results: At the May 20, 2024, data lock, 28 patients with RET -altered TC and prior systemic treatment and 145 patients with RET -altered MTC received pralsetinib 400 mg/d (median treatment duration: 24.7 mo [TC]; and 35.4 mo [MTC]). In the TC and MTC groups, median age was 58 and 57 y; 39% and 64% were male; median (range) prior lines of treatment was 2 (1, 9) and 1 (1, 6). 53.8% of MTC patients had prior systemic treatment. In TC and MTC patients, ORRs (95% CI) were 91.7% (73.0, 99.0) and 68.2% (59.5, 76.0) (Table). Treatment-related adverse events (TRAEs) occurred in 27 (96%) TC and 142 (98%) MTC patients; 68% and 67% had grade ≥3. TRAEs in ≥35% of TC and MTC patients were increased AST (50% and 38%) and ALT (43% and 30%), decreased white blood cell count (39% and 30%), and anemia (39% each). Death due to a TRAE occurred in 1 TC patient (liver injury) and 1 MTC patient (pneumonia). Safety was consistent with prior reports. Conclusions: The final analysis of ARROW confirms that pralsetinib yields clinically meaningful and durable responses in patients with RET -altered TC and MTC with a manageable safety profile consistent with prior reports. Clinical trial information: NCT03037385 . Efficacy summary. All MTC MTCPrior Cabozantinib/Vandetanib MTC Treatment-Naïve TCPrior Systemic Treatment MDP, n 132 60 62 24 ORR, % (95% CI) 68.2(59.5, 76.0) 56.7(43.2, 69.4) 79.0(66.8, 88.3) 91.7(73.0, 99.0) Complete response, n (%) 12 (9.1) 2 (3.3) 7 (11.3) 4 (16.7) Partial response, n (%) 78 (59.1) 32 (53.3) 42 (67.7) 18 (75.0) DOR, median, mo (95% CI) 39.6(29.4, NE) 21.7(15.1, 34.8) NR(36.8, NE) NR(16.0, NE) DOR follow-up, median, mo (95% CI) 45.7(41.0, 48.4) 48.9(36.9, 58.7) 45.1(40.4, 47.8) 35.4(26.9, 40.0) EP, n 145 67 67 28 OS, median, mo (95% CI) NR(54.6, NE) 42.2(31.2, NE) NR(NE, NE) NR(25.4, NE) PFS, median, mo (95% CI) 37.2(27.5, 55.5) 24.9(19.9, 35.0) 55.3(36.8, NE) NR(14.7, NE) NE, not evaluable; NR, not reached.

5068 Background: 177 Lu-PSMA-617 (LuPSMA) is a groundbreaking therapy for mPC, but biomarkers are urgently needed to improve patient selection. Plasma-derived EVs are emerging blood-based markers of tumor burden. Here, we analyzed levels of EVs of 123 patients (pts) from 4 cohorts: a pre- and during LuPSMA treatment for a contemporary prospective Mayo Clinic cohort, pre-treatment plasma levels of EVs for a historical Tulane cohort, localized prostate cancer (LPC) after radical prostatectomy (RP), and male healthy donors (HD). Methods: Blood samples were collected from 51 pts pre-cycles (pre-C) 1-3 of LuPSMA at Mayo Clinic from 12/2024-12/2025 and 26 pts pre-C1 at Tulane Cancer Center between 12/2018-1/2025. Small and large EVs were incubated with fluorescently conjugated antibodies (PSMA, STEAP1, CEACAM5, and TROP2) and enumerated using nanoscale flow cytometry. EVs from blood samples of 10 HD and 36 LPC post-RP from 6/2020-8/2024 at Mayo Clinic as controls. AUC-ROC and logistic regression analyses were performed to identify marker signatures associated with PSA50 response. Results: Baseline demographics are shown in the table. Median levels of PSMA, STEAP1, CEACAM5, and TROP2 EV/milliliter (mL) were all significantly higher for pts with mPC (pooled Tulane and Mayo cohorts) compared to LPC post-RP (p<0.001) and HD (p<0.001). In the Mayo cohort, 10 of 51 pts (20%) had undetectable PSA pre-C1. There were no significant differences in median pre-C1 plasma EV levels for these 4 different types of EVs comparing pts with undetectable vs detectable PSA. For 41 Mayo pts with detectable PSA pre-C1, 20 achieved PSA50 response at time of data cutoff (49%), while 7 pts (17%) had persistent PSA rise on LuPSMA. For pts who achieved PSA50, there was a significant decline in both CEACAM5 and TROP2 EVs/mL pre-C1 compared to pre-C3 (19 x 10 6 vs 5.8 x 10 6 , p=0.003 and 16 x 10 6 vs 5.5 x 10 6 , p<0.001, respectively), while no significant changes occurred for the 7 pts with no PSA response. A composite score of pre-C1 CEACAM5 and TROP2 EVs (large- and small-sized EVs) demonstrated the strongest association with PSA50 response (OR=2.96, 95% CI: 1.27-6.44, p=0.038), achieving AUC of 0.73. PSMA and STEAP1 EVs did not show any significant changes from pre-C1 to pre-C3. Conclusions: Plasma-derived EVs are a promising biomarker to identify patients who will benefit from LuPSMA. Longer follow-up and enrolling a larger cohort are underway. Clinical trial information: NCT06200103 . Median values for pre-LuPSMA (Mayo and Tulane), post-RP for LPC, and HD. Mayo mPC (n=51) Tulane mPC (n=26) LPC (n=36) HD (n=10) Age (range) 73 (57-87) 71 (54-83) 66 (56-77) 55 (50-57) Prior lines (range) 1 (1-3) 4 (1-11) - - Gleason score 9 8 7 - PSA (range) 1.0 (0-182) 75 (0.8-503) - - PSMA EV/mL 3.7 x 10 6 14 x 10 6 2.0 x 10 6 1.4 x 10 6 STEAP1 EV/mL 30 x 10 6 45 x 10 6 12 x 10 6 12 x 10 6 CEACAM5 EV/mL 17 x 10 6 5.1 x 10 6 3.7 x 10 6 3.9 x 10 6 TROP2 EV/mL 13 x 10 6 5.2 x 10 6 4.5 x 10 6 3.6 x 10 6