Abstract 1180: Integrating methylation-based circulating tumor DNA analysis with PSMA-PET/CT primary staging for enhanced risk stratification in <i>de novo</i> metastatic prostate cancer.

Abstract Background: Metastatic prostate cancer (mPC) is associated with high mortality. Newly diagnosed PC patients suspected of metastasis can undergo imaging-based primary tumor staging by PSMA PET/CT. Accurate staging is crucial for treatment selection, as localized PC may be cured by surgery or radiation therapy, while de novo mPC is treated systemically and with intensification for high-volume disease. However, the prognostic value of PSMA PET/CT remains unclear and novel biomarkers are needed to improve risk stratification and inform treatment selection. To this end, analysis of plasma cell-free DNA (cfDNA) offers promising biomarker potential, as detection of tumor-derived cfDNA (ctDNA) has demonstrated prognostic value in multiple cancer types. In this study, we aimed to establish a methylation-based approach for detecting ctDNA in plasma from mPC patients and evaluate its utility as a prognostic biomarker. Methods: We generated cfDNA methylome profiles for 68 castration-resistant mPC (mCRPC) patients and 62 controls using methylated cfDNA immunoprecipitation and sequencing. From a subset of the cohort (27 mCRPCs, 10 controls), we established an mPC-associated multi-region methylation signature (cfMeCaP). A ctDNA detection cutoff was defined using mean cfMeCaP methylation of controls, yielding 93% sensitivity and 96% specificity in the remaining cohort (41 mCRPCs, 52 controls). Next, to test cfMeCaP ctDNA detection in mPC at diagnosis, we generated cfDNA methylome profiles for another cohort of 117 PC patients who had undergone PSMA PET/CT for primary tumor staging upon suspicion of de novo mPC. Based on PSMA PET/CT findings, patients were grouped into localized PC (n=24), low-volume mPC (n=61), and high-volume mPC (n=28). Results: Using cfMeCaP, ctDNA was detectable in 32/117 (27%) PC patients at primary tumor staging. Highest sensitivity was observed in patients with high-volume mPC (15/28, 54%) relative to low-volume mPC (14/61, 23%) and localized PC (2/24, 8.0%). Mean cfMeCaP methylation was positively correlated with the total tumor volume, as estimated from the PSMA PET/CT scans (ρ=0.47, p&amp;lt;0.01). This correlation was largely driven by the total metastatic tumor volume (ρ=0.52, p&amp;lt;0.01) rather than the total primary tumor volume (ρ=0.16, p=0.08). In mPC patients, ctDNA detection at primary tumor staging was associated with significantly shorter time to mCRPC progression as compared to ctDNA negative patients (median PFS of 25.6 months vs median PFS not reached, p=0.01), suggesting ctDNA positive low-volume mPC patients represent a high-risk subgroup that may benefit from treatment intensification. Conclusion: These findings highlight the promising potential of plasma ctDNA as a minimally invasive biomarker to improve risk stratification in patients with de novo mPC, warranting large-scale clinical validation. Citation Format: Karoline Kondrup, Mads Ryø Jochumsen, Laura Iisager, Paul V. Salachan, Maibritt Nørgaard, Philippe Lamy, Bodil G. Pedersen, Michael Borre, Karina D. Sørensen. Integrating methylation-based circulating tumor DNA analysis with PSMA-PET/CT primary staging for enhanced risk stratification in de novo metastatic prostate cancer [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 1180.