Matsubara, Nobuaki

TPS5127 Background: Androgen deprivation therapy (ADT) ± androgen receptor pathway inhibitor therapy is a primary treatment for metastatic hormone-sensitive prostate cancer, but is noncurative and has significant toxicities when used long-term. In patients with OMPC for whom delaying ADT is appropriate, metastasis-directed therapy such as stereotactic body radiation therapy (SBRT) has been shown to provide local disease control. However, many patients do not experience a complete prostate-specific antigen (PSA) response and develop poly-metastatic disease. [ 177 Lu]Lu-PSMA-617 ( 177 Lu-PSMA-617) is a prostate-specific membrane antigen (PSMA)-targeted radioligand therapy with demonstrated efficacy and a manageable safety profile in patients with PSMA-positive metastatic castration-resistant prostate cancer in the VISION and PSMAfore trials. PSMA-DC (NCT05939414) is an ongoing, international, randomized phase 3 trial to evaluate the efficacy of 177 Lu-PSMA-617 versus observation after SBRT in delaying castration and disease progression in patients with PSMA-positive OMPC. Methods: Eligible patients have histologically confirmed prostate cancer, biochemical recurrence post-definitive treatment, OMPC with ≤ 5 PSMA-positive metastatic lesions including ≥ 1 distant metastasis on PSMA PET/CT scans (all must be amenable to SBRT), PSA doubling time < 10 months and non-castration testosterone levels (> 100 ng/dL). Exclusion criteria include distant metastasis by conventional imaging (CI; CT/MRI and bone scans) at screening, prior ADT (except adjuvant ADT completed > 12 months before randomization), or other systemic therapy for metastatic prostate cancer. Patients (N = ~450) will be randomized 2:1 to 177 Lu-PSMA-617 or observation and will receive SBRT to all metastatic lesions within 14 days, completed within 3 weeks. Patients will then receive either intravenous 177 Lu-PSMA-617 (7.4 GBq/6 weeks; 4 cycles), starting 7–21 days after SBRT, or undergo observation only. Additional SBRT for new lesions is allowed. ADT is allowed after a metastasis-free survival (MFS) event by CI confirmed by blinded independent review committee (BIRC). Safety follow-up will occur 42 days after the last 177 Lu-PSMA-617 dose and at the week 24 visit for the observational arm. Long-term follow-up for the 177 Lu-PSMA-617 arm will include safety assessments every ~32 weeks. The primary endpoint is MFS by CI as assessed by BIRC using RECIST v1.1, or death. To provide 90% power to detect a hazard ratio of 0.6, 187 MFS events are required. The key secondary endpoint is time to next hormonal therapy. Additional secondary endpoints include time to PSA progression, radiographic progression-free survival, symptomatic progression, patient-reported health-related quality of life, overall survival and safety. Shore et al. PSMA-delay castration (DC): an open-label, multicenter, randomized phase 3 study of [ 177 Lu]Lu-PSMA-617 versus observation in patients with metachronous PSMA-positive oligometastatic prostate cancer (OMPC). J Urol 2025;213 (5S2_suppl):e28. https://www.auajournals.org/doi/10.1097/01.JU.0001110444.53548.eb . Reused with permission; ©American Urological Association, 2025. This abstract previously presented at 2025 AUA Annual Meeting. Clinical trial information: NCT05939414 .

TPS5118 Background: The median overall survival of patients with mCRPC remains under 2 years even with newer therapies. Xaluritamig, an XmAb 2+1 T-cell engager that targets the six-transmembrane epithelial antigen of prostate 1 (STEAP1), facilitates lysis of STEAP1-expressing cancer cells, such as those in advanced prostate cancer. In a first-in-human study, xaluritamig demonstrated encouraging efficacy and a manageable safety profile for patients with mCRPC refractory to standard of care therapies (Kelly WK, et al. Cancer Discov. 2024;14(1):76-89). Methods: XALute is a randomized, multicenter, open-label, phase 3 study to evaluate the efficacy and safety of xaluritamig vs cabazitaxel or second ARDT in men with mCRPC previously treated with taxane chemotherapy. Enrollment in the control arm treatments will be split evenly between cabazitaxel and second ARDT. Stratification factors include LDH ≤ or > 260 IU/L, liver metastases (Y/N), prior prostate-specific membrane antigen radioligand therapy (PSMA-RLT) (Y/N) and the intention to treat with cabazitaxel or ARDT switch. Approximately 675 patients will be enrolled. Participants will be randomly assigned in a 2:1 ratio to xaluritamig monotherapy or standard care. Participants will receive treatment until radiographic disease progression per Prostate Cancer Clinical Trials Working Group 3 (PCWG3), unacceptable toxicity, initiation of other anticancer therapy, withdrawal of consent, death, or end of study as determined by the sponsor. The primary efficacy endpoint is overall survival. The key secondary efficacy endpoint is radiographic progression-free survival per PCWG3 by blinded independent central review. Key inclusion criteria are pathological/cytological confirmation of prostate adenocarcinoma; mCRPC with at least one metastatic lesion; evidence of progressive disease; prior treatment with at least one ARDT; one taxane therapy in the mCRPC setting, and ongoing androgen deprivation with serum testosterone levels (<50 ng/dL or <1.7 nmol/L). Prior treatment with PSMA-RLT, poly ADP-ribosylation inhibitors, and immune checkpoint inhibitors are permitted. Exclusion criteria include prior STEAP1-targeted therapy, any anticancer therapy within 4 weeks prior to first dose of study treatment (not including androgen deprivation therapy), prior PSMA-RLT within 2 months of first dose of study treatment unless less than 2 cycles received, and prior radionuclide therapy (radium-223) within 2 months of first dose of study treatment To mitigate risk of cytokine release syndrome, xaluritamig will be administered with step dosing. Cabazitaxel or second ARDT will be administered according to regional prescribing information. Funded by Amgen Inc. Clinical trial information: NCT06691984 .

Abstract Purpose: The three-arm, phase III KEYNOTE-361 study did not meet its dual primary endpoints of progression-free survival (PFS) or overall survival (OS) with first-line pembrolizumab plus chemotherapy versus chemotherapy in advanced urothelial carcinoma. This prespecified exploratory analysis assessed the association of tumor mutational burden (TMB) and PD-L1 combined positive score (CPS) with clinical outcomes. Patients and Methods: TMB and PD-L1 CPS were determined via whole-exome sequencing and PD-L1 IHC 22C3 pharmDx, respectively. The association was evaluated in each treatment arm using logistic regression [objective response rate (ORR)] and Cox proportional hazards regression models (PFS and OS); one-sided (pembrolizumab monotherapy; pembrolizumab plus chemotherapy) and two-sided (chemotherapy) nominal P values were calculated. Significance was prespecified at α = 0.05 without multiplicity adjustment. Efficacy was evaluated by prespecified cutoffs of 175 mutations/exome (TMB) and CPS 10 (PD-L1). Results: Of the 993 treated patients, 820 (82.6%) and 993 (100%) had evaluable TMB and CPS data, respectively. Continuous TMB was positively associated with ORR, PFS, and OS for pembrolizumab monotherapy (one-sided P &lt; 0.001, P &lt; 0.001, and P = 0.007, respectively); PFS and OS for pembrolizumab plus chemotherapy (one-sided P = 0.007 and P = 0.010, respectively); and OS for chemotherapy alone (two-sided P = 0.040). Continuous PD-L1 CPS showed evidence of anticipated association with ORR and PFS for pembrolizumab monotherapy. The subgroup with TMB ≥175 mutations/exome and PD-L1 CPS ≥10 had the highest PFS and OS improvements with pembrolizumab alone or with chemotherapy versus chemotherapy alone. Conclusions: These data suggest that TMB may be predictive of the response to pembrolizumab alone or with chemotherapy in advanced urothelial carcinoma.

540 Background: The 3-arm, open-label, phase 3 KEYNOTE-361 study (NCT02853305) evaluated first-line pembro ± chemo vs chemo in advanced UC regardless of PD-L1 status. The trial did not meet its primary end points of superior PFS and OS with pembro + chemo vs chemo and thus analysis of pembro monotherapy (mono) vs chemo was exploratory. We explored the association of TMB status and PD-L1 combined positive score (CPS) with clinical outcomes in KEYNOTE-361. Methods: In pts with TMB and/or PD-L1 data, the association between TMB (via whole exome sequencing) and PD-L1 (via PD-L1 IHC 22C3 pharmDx) and clinical outcomes (ORR, PFS, and OS) was evaluated. In each treatment arm, the hypotheses regarding the associations were evaluated using logistic regression (ORR) and Cox proportional hazards regression (PFS; OS), and 1-sided (pembro; pembro + chemo) and 2-sided (chemo) P values were calculated; significance was prespecified at α = 0.05 without multiplicity adjustment. Clinical utility was assessed using prespecified cutoffs of 175 mut/exome (TMB) and CPS 10 (PD-L1). Clinical data cutoff was April 29, 2020. Results: 820/993 pts (82.6%) had evaluable TMB data (pembro, 252; pembro + chemo, 282; chemo, 286). TMB (log10) was significantly positively associated with ORR, PFS, and OS for pembro ( P < 0.001, < 0.001, and 0.007, respectively) and PFS and OS for pembro + chemo ( P= 0.007 and 0.010, respectively). The area under the receiver operating characteristics (AUROC) curve (95% CI) for discriminating response was 0.64 (0.56-0.71) for pembro, 0.53 (0.46-0.60) for pembro + chemo, and 0.52 (0.45-0.59) for chemo. Efficacy by TMB cutoff is reported in the Table. All 993 pts had PD-L1 data (pembro, 302; pembro + chemo, 349; chemo, 342). PD-L1 was significantly positively associated with PFS for pembro ( P= 0.006) and ORR for pembro + chemo ( P= 0.042) but not chemo. Efficacy by PD-L1 CPS is reported in the Table. Conclusions: Strong associations were observed between TMB and all 3 clinical outcomes (ORR, PFS, and OS) with pembro mono in the first-line setting and a reduced association was observed between TMB and clinical outcomes with pembro + chemo. No consistent associations were observed between PD-L1 and clinical outcomes with pembro mono or pembro + chemo. Clinical trial information: NCT02853305. [Table: see text]