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Fluid-filled cysts are the key feature of polycystic kidney disease, which eventually leads to renal failure. We analyzed the composition of cyst fluid from a rat model of autosomal recessive polycystic kidney disease, the PCK rat, and identified sexual differences. Our results demonstrate that the ion composition of cyst fluid differs from that of urine or plasma. Untargeted metabolomics combined with transcriptomic data identified tryptophan metabolism, enzyme metabolism, steroid hormone biosynthesis, and fatty acid metabolism as pathways differing between male and female PCK rats. We quantified 42 amino acids in the cyst fluid (PCK only), plasma, and urine of male and female PCK rats and Sprague Dawley rats. Taurine was the most concentrated amino acid present in the cyst fluid, and PCK rat urinary taurine excretion was over 3-fold greater than Sprague Dawley rats. Understanding the composition of cyst fluid provides valuable insights into disease pathophysiology and may help identify potential dietary or pharmacological interventions to mitigate disease progression and improve patient outcomes., Competing Interests: Competing interests: The authors declare no competing interests., (© 2025. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Glycosylation changes of circulating proteins carrying the CA19-9 antigen may offer new targets for detection methods to be explored for the diagnosis of epithelial ovarian cancer (EOC). Search for assay designs for targets initially captured by a CA19-9 antigen reactive antibody from human body fluids by probing with fluorescent nanoparticles coated with lectins or antibodies to known EOC associated proteins. CA19-9 antigens were immobilized from ascites fluids, ovarian cyst fluids or serum samples using monoclonal antibody C192 followed by probing of carrier proteins using anti-MUC16, anti-MUC1 and, anti STn antibodies and seven lectins, all separately coated on nanoparticles. Compared to reference CA19-9 and CA125 immunoassays, nanoparticle aided detection using MUC16, Ma695 and STn antibodies and lectin WGA provided, both separately and combined, improved discrimination of EOC and borderline cancers from benign samples when applied to 60 cyst fluid specimens. When applied to a panel of 44 serum samples (EOC N = 24, healthy and benign samples N = 20) two assays, CA19-9 Ma695 and CA19-9 MUC1 , stood out with equally superior separations (p-values < 10 -8 ) of the two groups compared to conventional CA19-9 immunoassay (p-value 0.03).Eu +3 -NP based CA19-9 MUC16 , CA19-9 Ma695 , CA19-9 STn and CA19-9 WGA show promise for improved EOC detection when applied to ascites & cyst fluids. When applied to circulation-derived samples, the two MUC1 based assays, CA19-9 Ma695 and CA19-9 Ma552 outperformed other assay constructs. Our results call for further validation in larger EOC cohorts preferentially with early stage ovarian cancers and all major histotypes against commonly occurring benign conditions., Competing Interests: Declarations. Competing interests: The authors declare no competing interests. Ethics approval statement: The study was approved by the local ethics committee Regionala etikprövningsnämnden i Göteborg (Dnr 201–15) in Gothenburg and was performed in accordance with the Declaration of Helsinki and all its amendments. Patient consent statement: Informed consent was obtained from all individual participants included in the study., (© 2025. The Author(s).)