Background: Nucleosomes (DNA wound around histone proteins) were reported as potential cancer biomarkers (1). The links between circulating nucleosomes and clinical endpoints in advanced ovarian cancer (OC) patients treated with neo-adjuvant chemotherapy (NACT) +/- interval debulking surgery (IDS) were retrospectively assessed in the randomized phase II CHIVA trial (2). Methods: The diagnostic concentration and the longitudinal kinetics of 2 nucleosomes (H3K27 and H3K36, log-scale) were investigated at baseline, during NACT, after IDS, and at progression. The baseline titer measured with Nu.Q prototype immunoassays (H3K27Me3, H3K36Me3 (Belgian Volition SRL, Belgium)) was compared to those of a cohort of 201 cancer-free subjects. Their prognostic values regarding the probability of obtaining a complete IDS and progression-free survival (PFS) were assessed with respect to the modeled CA-125 KELIM (3), classical clinical covariates and treatment arms. The maximum selected rank statistics was used to determine the optimal cut-off at different disease management times. Results: H3K27 and H3K36 nucleosome concentrations were available for 148/188 patients. H3K36 and H3K27 baseline concentrations were correlated and significantly higher in OC patients compared to cancer-free subjects (H3K36, 2.8 vs 2.4 ng/mL, P<0.0001; H3K27, 3.2 vs 2.1, P<0.0001), without difference according to BRCA status. The baseline titer of H3K36 was an independent predictor of the completeness of IDS (OR=0.33; 95% CI=0.13-0.71, p=0.009). The baseline concentration of H3K27 was independently associated with PFS (< or ≥3.8 ng/mL, HR=1.84, 95%CI=1.11-3.02, p=0.016), complementary to the CA-125 KELIM. Both nucleosome longitudinal kinetics showed a decrease during NACT, but there was no significant statistical association between their kinetics and radiological response rate, completeness of IDS or PFS. Conclusions: Baseline values of H3K36 and H3K27 could represent a non-invasive method for detecting OC, with potential relevant prognostic value of initial titers in newly diagnosed OC patients. 1. McAnena, Cancers 2017. 2. Ferron, Gynecol Oncol 2023. 3. You, CCR 2020.