Abstract 2558: Multi-omic single-cell assessment of castration-resistant prostate cancer patients receiving177Lu-PSMA-617 and pembrolizumab shows timing of radioligand therapy alters immune response

Abstract Background: Prostate cancer, a leading cause of cancer-related death in men, is refractory to checkpoint inhibition.1-3 Lutetium-177-labeled PSMA-617 (177Lu-PSMA, Pluvicto) is an FDA-approved radioligand therapy in CRPC. We have shown that a single dose of 177Lu-PSMA is immunomodulatory, and can be safely combined with the PD-1 blockade, pembrolizumab, in a phase 1 clinical trial (NCT03805594).4 However, the schedule by which these treatments should be combined is unknown. Methods: In our phase 1 clinical trial, patients with mCRPC received a single dose of 177Lu-PSMA either before (Schedule 1, n=30), concurrently with (Schedule 2, n=6), or after pembrolizumab (Schedule 3, n=6). Serial peripheral blood mononuclear cells (PBMCs) were collected and analyzed using multi-omic single-cell RNA sequencing (scRNA-seq) to dissect the immune responses with these 3 schedules. Results: We found that a single dose of 177Lu-PSMA in monotherapy led to an expansion of CD4+ effector T cells as well as in Tregs indicative of direct immunomodulatory activity. This was accompanied by a reduction in NK and B cells. When 177Lu-PSMA was combined with pembrolizumab, we saw dynamic changes in non-Treg T cell subsets (CD4+ and CD8+ T cells), dendritic cells (DCs), NK cells, and B cells across all three dosing schedules. Schedules 1 and 2 demonstrated stable T cell levels accompanied by a gradual increase in DCs, whereas Schedule 3 was characterized by low T cell frequencies and a progressive decline in DCs relative to baseline. Conclusions: Our study demonstrates the critical importance of treatment sequencing to fully harness the immunomodulatory potential of RLT using a single dose of 177Lu-PSMA and optimize synergy with PD-1 blockade in mCRPC. While initiating with either 177Lu-PSMA or concurrently starting 177Lu-PSMA with pembrolizumab resulted in an increase in DCs, starting with pembrolizumab first resulted in lower frequencies T cells and DCs suggesting that this schedule could be detrimental to immune outcomes. Citation Format: Anusha Muralidhar, Karen Law, Zenghua Fan, Shloka Shukla, Aram Lyu, Serena Kwek, David Y. Oh, Michael Evans, Rahul R. Aggarwal, Lawrence Fong. Multi-omic single-cell assessment of castration-resistant prostate cancer patients receiving177Lu-PSMA-617 and pembrolizumab shows timing of radioligand therapy alters immune response abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2558.