Exploring the tumor microenvironment (TME) in patients with mismatch repair-deficient (MMRd) endometrial cancer (EC) undergoing immune checkpoint inhibitor (ICI) therapy: Proteomic insights into immune resistance mechanisms.

5609 Background: MMRd status is a robust predictive biomarker for ICI in EC, however half of these pts do not respond. Most studies exploring biomarkers of response to ICI have focused on tumor or immune cell factors. We aimed to explore the tumor microenvironment (TME) in ICI-Responder (R) versus Non-Responder (NR) MMRd EC pts to identify new predictive biomarkers of response. Methods: Clinical data and outcomes of metastatic MMRd EC pts, treated with ICI at Gustave Roussy Institute (2016-2023), were retrospectively collected. Pts were classified as Rs (CR, PR, or SD ≥12 months) or NRs (PD or SD 0. 58. CD8+ T cells were quantified in the intratumoral area and expressed as number of cells per mm 2. Spearman correlation test (r) was used for lineal correlation calculations. Functional enrichment analysis was conducted to elucidate differentially expressed molecular signatures (Odds ratio; adjusted p-value) in Rs vs NRs, utilizing GOBiological Process database. Results: Twenty-three tumor samples were included in the analysis: 15 from Rs and 8 from NRs. Overall, 11, 187 proteins were quantified. Only 72 were found to be differentially abundant between Rs and NRs. Protein candidates present within the tumor microenvironment that were significantly UP-regulated in NRs related to: non-specific inflammation (CRP), arachidonic acid metabolism (LOX15, PTGR3), extracellular matrix synthesis (COL5A2, COL22A1, PXDN), and TGF-b (TFBR1). CRP, LOX15, PXDN and COL5A2, were negatively correlated with CD8 + T cell infiltration (r -0. 23, r -0. 33, r -0. 51, r -0. 21). Based on functional enrichment analysis, collagen fibril organization (OR 21. 4; p 1. 1 e -6), extracellular structure organization (OR 10. 2; p 1. 2 e -6), extracellular matrix (ECM) organization (OR 7. 5; p 1. 1e -6), supramolecular fiber organization (OR 3. 4; p 0. 03), and TGFBR signaling pathway (OR 6. 4; p 0. 08) were among the top 30 biological process upregulated in NRs. Conclusions: Our preliminary data on high throughput proteomic analysis identified the arachidonic acid pathway, ECM structural organization and TGF-b signaling as potential mediators of ICI resistance in MMRd EC, all of which may be actionable in an effort to enhance sensitivity to ICI.