Abstract 1974: Prostate cancer lineage states detected by circulating tumor cell RNA sequencing are prognostic biomarkers for overall survival and resistance to androgen receptor pathway inhibitors and177Lutetium-PSMA-617

Abstract Background: The development of treatment resistance remains universal for metastatic prostate cancer patients, driven by AR alterations and lineage state transitions. Identifying the evolution of molecular alterations during treatment has been limited by the challenges of serial tumor biopsies, which can be overcome by blood-based liquid biopsies. However, standard cell-free DNA approaches cannot directly assess changes in gene expression, crucial to understanding resistance. Patients and Methods: We developed a novel circulating tumor cell (CTC) isolation approach with purity comparable to tissue biopsies and performed CTC RNA sequencing of 273 blood samples from a prospective multi-institutional cohort of 117 metastatic prostate cancer patients. We examined prostate cancer lineage states associated with prognosis and response to 177Lu-PSMA-617 in men with metastatic prostate cancer. Results: Single sample pathway analysis of high CTC purity samples identified four transcriptional phenotypes: luminal A-like (LumA), luminal B-like (LumB), low proliferation (LP), and neuroendocrine (NE). Compared to patients with low CTC burden/purity (median OS not reached), patients with LumA and LP phenotypes showed similar survival, while patients with LumB and NE CTC phenotypes had shorter survival (LumB: median OS 6m, HR 9.1[3.8-21.8], p<0.0001, NE: median OS 3.7m, HR 11.8[2.4-57.2], p=0.0019). We isolated CTCs longitudinally from 37 patients receiving 177Lu-PSMA-617. LumA/LP/Low burden samples had significantly greater benefit from ARPIs compared to LumB and NEPC, as hypothesized. Pretreatment LumA/LP/Low burden CTC phenotypes had a significantly higher clinical benefit rate with 177Lu-PSMA-617 than the LumB phenotype (64% vs 14%, Fisher’s Exact p=0.033), which was associated with early progression on 177Lu-PSMA-617 (Fisher’s Exact p=0.038). Conclusion: We report the largest CTC RNA-seq cohort of patients with metastatic prostate cancer and demonstrate that high-purity CTC RNA-seq identifies prostate cancer lineage states mirroring those in tissue, including an NE phenotype concordant with small cell/neuroendocrine histology. We also identified a more common LumB phenotype defined by persistent AR signaling and high proliferation and associated with poor prognosis and poor response to 177Lu-PSMA-617. Citation Format: Joshua M. Lang, Marina Sharifi, Jamie Sperger, Amy K. Taylor, David Kosoff, Hamid Emamekhoo, Christos Kyriakopolous, Andrew Armstrong, Xiao Wei, Rana McKay. Prostate cancer lineage states detected by circulating tumor cell RNA sequencing are prognostic biomarkers for overall survival and resistance to androgen receptor pathway inhibitors and177Lutetium-PSMA-617 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1974.