The cloudy fluid that accumulates in the abdomen of patients with peritoneal metastasis has long been viewed as little more than a symptom of advanced disease, a sign that cancer has seeded the lining of the abdominal cavity and that the prognosis is grim. Now, a large-scale molecular study of that fluid, known as malignant ascites, suggests it may instead function as a molecular directory, cataloguing the very signals tumors use to grow, spread and resist treatment. Publishing in the journal Molecular Cancer, a multi-institutional team led by researchers at the National Cancer Centre Singapore, Singapore General Hospital and Westlake University has carried out one of the most comprehensive proteotranscriptomic surveys of malignant ascites to date, and in the process has identified a series of druggable paracrine factors, including the fibrinolysis regulator PAI-1, for which they developed and tested a therapeutic nanobody.
Peritoneal metastasis occurs when cancer cells detach from a primary tumor, most commonly of gastrointestinal, gynecological or appendiceal origin, and implant on the peritoneum, the membrane lining the abdominal wall and covering the organs within. As the disease progresses, many patients develop ascites, an abnormal buildup of protein-rich fluid that causes distension, discomfort and breathing difficulties. Clinically, the appearance of malignant ascites portends poor survival, yet the molecular contents of that fluid and their biological significance have remained surprisingly underexplored. Because ascites sits in direct contact with peritoneal tumor deposits, any signaling molecules it contains are, by definition, delivered to the tumor microenvironment, making it an ideal starting point for hunting paracrine factors, the secreted ligands that cells use to influence their neighbors.
To interrogate that signaling milieu systematically, the team assembled an extraordinary cohort: 397 samples of malignant cell-free ascites representing 21 distinct histological origins of peritoneal metastasis. Cell-free ascites, the liquid fraction left after cells and debris are removed, concentrates the soluble protein complement of the peritoneal cavity. The researchers applied quantitative mass spectrometry, using tandem mass tag labeling to measure protein abundance across large batches of samples with high reproducibility, complemented by rigorous quality control based on metrics such as coefficients of variation. This proteomic backbone was then integrated with transcriptomic data, allowing the team to confirm that the proteins they detected were not passive leakage from dying cells but were actively produced and secreted by defined cell populations within the tumor ecosystem.
The mass spectrometry analysis revealed multiple coherent biological themes woven through the ascites proteome. Chief among them was the coagulation cascade and its counterbalancing system, fibrinolysis, the enzymatic machinery that dissolves blood clots. The team identified both previously recognized paracrine factors and a set of novel candidates that appear amenable to therapeutic perturbation, including proteins such as S100A10, macrophage migration inhibitory factor, peroxiredoxin 1, phosphoglycerate kinase 1 and lactate dehydrogenase A. Each of these has known roles in tumor biology, from plasminogen activation and extracellular matrix remodeling to inflammatory signaling and metabolic adaptation, and their enrichment in ascites points to a peritoneal microenvironment actively shaped by tumor-driven secretion.
To determine which cells were responsible for producing the key ligands, the researchers turned to single-cell transcriptomic sequencing of cells recovered from peritoneal metastasis ascites. This analysis delivered one of the study’s most conceptually important findings: two functionally antagonistic regulators of fibrinolysis, S100A10 and plasminogen activator inhibitor-1, or PAI-1, are secreted by distinct cell populations within the tumor microenvironment. S100A10, which promotes plasmin generation and matrix degradation, and PAI-1, which inhibits it, sit on opposite sides of the fibrinolytic balance. Their co-elevation, attributed to separate cellular sources, indicates that upregulation of these ligands reflects active, coordinated secretion rather than passive homeostatic drift. In other words, the tumors and their stromal partners are not merely accumulating proteins; they are engineering the extracellular fluid to favor their own agenda.
Why would a cancer care about fibrinolysis? The answer lies in the mechanics of peritoneal spread. Plasmin activity remodels the extracellular matrix, clears fibrin barriers and facilitates cell motility, all processes that help disseminated tumor cells implant, invade and expand on peritoneal surfaces. By mapping the components of this system, including tissue-type and urokinase plasminogen activators, annexin 2, and their inhibitors, the study frames the ascites compartment as a regulated enzymatic workspace where coagulation and dissolution are choreographed to tumor advantage. That framing immediately suggests intervention points: if specific ligands sustain the peritoneal ecosystem, neutralizing them could degrade the environment on which the metastases depend.
The team went beyond cataloguing. They selected PAI-1, a well-studied but therapeutically elusive target, and engineered a nanobody, a single-domain antibody fragment derived from camelid heavy-chain antibodies, designed to bind and neutralize it. Nanobodies are roughly a tenth the size of conventional antibodies, offering deep tissue penetration, high stability and straightforward manufacturing, properties well suited to an intraperitoneal disease. Using surface plasmon resonance the researchers confirmed the nanobody bound PAI-1 with high affinity, and they then challenged it across a battery of preclinical models: in vitro assays with cancer and stromal cells, in vivo animal models of peritoneal metastasis, and ex vivo human explant systems that preserve fragments of actual patient tumor in their native architecture.
The results provided proof of concept that intercepting PAI-1 signaling can constrain peritoneal tumor growth with minimal toxicity. In the experimental systems, the anti-PAI-1 nanobody suppressed the fibrinolytic and pro-metastatic programs the multiomic analysis had flagged, while sparing normal physiology, an encouraging safety signal for a target embedded in the hemostatic system. Because PAI-1 is secreted into ascites and acts on neighboring cells, antibody-based neutralization is a rational modality: the drug circulates in the same fluid compartment where the ligand operates. The strategy also generalizes, since the same proteotranscriptomic pipeline that surfaced PAI-1 can be reapplied to the other paracrine factors the study identified, each now carrying a mechanistic rationale for drug development.
The broader significance of the work lies in its scale and integration. By pairing deep proteomics across nearly four hundred ascites samples and twenty-one tumor types with single-cell resolution attribution of ligand sources and functional validation in vitro, in vivo and ex vivo, the study demonstrates a template for target discovery in fluid-biopsied tumor microenvironments. It reframes malignant ascites from a prognostic liability into an analytical asset, a lens through which the signaling economy of peritoneal metastasis becomes visible. For a disease whose treatment options remain limited largely to cytoreductive surgery with hyperthermic intraperitoneal chemotherapy and systemic regimens of modest efficacy, the identification of actionable paracrine targets opens a genuinely new therapeutic front.
Considerable work remains before patients benefit. The nanobody must advance through formal preclinical development and clinical trials, biomarkers must be established to select patients whose tumors depend on PAI-1, and the logistics of intraperitoneal delivery will need clinical refinement. Yet the conceptual shift is already tangible. The study shows that the fluid bathing peritoneal tumors can be read, almost like a molecular transcript of the tumor’s intentions, and that reading it carefully reveals not just how the disease progresses but precisely where it is vulnerable. In turning the fluid of a poor prognosis into a map of therapeutic opportunity, the Singapore-led consortium has offered one of the more hopeful reframings of advanced abdominal cancer in recent years.
The scale of the ascites cohort deserves particular emphasis. Studies of malignant fluid have often been limited to a few dozen samples drawn from a single tumor type, making it difficult to distinguish cancer-specific signals from generic inflammatory noise. By sampling 397 specimens across 21 histological origins, the investigators could identify protein themes that recur regardless of whether the primary tumor arose in the stomach, ovary, colon or appendix, strengthening the case that the observed ligands reflect shared biology of the peritoneal metastatic niche rather than the idiosyncrasies of any one malignancy.
The choice of PAI-1 as the validation target also carries scientific weight. PAI-1, formally known as serpin E1, is a member of the serine protease inhibitor family and functions as the principal physiological brake on both tissue-type and urokinase plasminogen activators. Although elevated PAI-1 levels have been associated with poor outcomes in numerous cancers, the protein has historically been considered difficult to drug, partly because it interacts with multiple partners, including vitronectin, and because its role in hemostasis raises concerns about systemic interference. A nanobody format, with its compact size and high affinity confirmed here by surface plasmon resonance, offers one route around these obstacles, particularly when delivered locally into the peritoneal cavity where the ligand is concentrated.
The single-cell finding that S100A10 and PAI-1 originate from separate cell populations adds a layer of ecological interpretation to the fibrinolytic data. It implies a division of labor within the tumor microenvironment, in which different cellular compartments contribute opposing components of the same pathway, potentially allowing the tumor to tune plasmin activity spatially and temporally. Spatially resolved methods such as Stereo-seq, referenced in the study’s methodology, are increasingly enabling this kind of anatomical attribution of secreted signals.
Finally, the open-access release of the proteomic dataset itself represents a resource for the field. Researchers studying individual tumor types can query the ascites proteome for their own candidate ligands, extending the paracrine target list well beyond the factors validated in this initial report.
Subject of Research: Proteotranscriptomic identification of paracrine therapeutic targets in malignant ascites from peritoneal metastases
Article Title: Identification of novel paracrine therapeutic targets in peritoneal metastases through integrative proteotranscriptomic analysis of ascites
Article References: Liu, Y., Chong, C. Y. L., Wang, Y., Xu, L., Lim, H. J., Tan, Q. X., Hendrikson, J., Tan, J. W.-S., Ng, G., Guo, W., Wang, Y., Huang, L., Ge, W., Rethineswaran, V. K., Ong, J. Z. L., Quek, Y. E., Ang, A. J. Y., Teo, F., Chen, O. W., … Singapore Peritoneal Oncology Study (SPOS) Group and Singapore Gastric Cancer Consortium (SGCC) (2026). Identification of novel paracrine therapeutic targets in peritoneal metastases through integrative proteotranscriptomic analysis of ascites. Molecular Cancer. https://doi.org/10.1186/s12943-026-02787-3
Image Credits: AI Generated
DOI: 10.1186/s12943-026-02787-3
Keywords: peritoneal metastases, ascites, proteomics, PAI-1, S100A10, fibrinolysis, nanobody, paracrine signaling, single-cell sequencing, coagulation cascade, drug targets, Molecular Cancer
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Nathaniel Bowman. (September 12, 2026). Ascites Proteins Reveal New Drug Targets for Peritoneal Cancer Spread. Scienmag. https://scienmag.com/ascites-proteins-reveal-new-drug-targets-for-peritoneal-cancer-spread/
Nathaniel Bowman. “Ascites Proteins Reveal New Drug Targets for Peritoneal Cancer Spread.” Scienmag, 12 September 2026, https://scienmag.com/ascites-proteins-reveal-new-drug-targets-for-peritoneal-cancer-spread/. Accessed 12 September 2026.
Nathaniel Bowman. “Ascites Proteins Reveal New Drug Targets for Peritoneal Cancer Spread.” Scienmag. September 12, 2026. https://scienmag.com/ascites-proteins-reveal-new-drug-targets-for-peritoneal-cancer-spread/
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