Background: The Sialyl-Thomsen-nouveau antigen (STn) is abundantly produced on many types of human epithelial cancers including epithelial ovarian cancer (EOC). We previously developed an EOC-specific lectin sandwich immunoassay (CA125MGL) using a human macrophage galactose-binding lectin coated on fluorescent europium nanoparticles (Eu+3-NPs) as a tracer and an anti-CA125–specific mAb for capture. Here we have identified a novel STn-mAb that efficiently recognizes the EOC-associated STn antigen on CA125 when coated on Eu+3-NPs.

Method: CA125 from the ovarian cancer cell line OVCAR-3, placental homogenate, and ascites fluid from patients with liver cirrhosis was captured by anti-CA125 antibody immobilized on microtitration wells and traced with anti-STn-mAb-Eu+3-NPs. Samples from EOC or patients with endometriosis with marginally increased conventional CA125 immunoassay (CA125IA; 35–200 U/mL) and healthy controls were analyzed.

Results: An analytically sensitive CA125STn assay that specifically recognized the CA125 isoform produced by OVCAR-3 was achieved. Serum CA125STn concentration was significantly higher in EOC patients than in those with endometriosis (P < 0.001). Furthermore, the sensitivity for detection of EOC with CA125STn assay was 73.3% when 95% of endometriosis cases were undetectable.

Conclusion: Our findings suggest that Eu+3-NPs–based CA125STn assay could help reduce the false-positive rates of CA125IA to improve differential diagnosis. The results encourage studying further the potential use of CA125STn to detect EOC at earlier clinical stages. This approach warrants further investigation in other cancers as well.