First-generation high-affinity ST14 radiopharmaceutical: Design, synthesis, and preclinical PET imaging evaluation for pancreatic cancer detection

The non-specificity of ¹⁸F-FDG, coupled with high false-positive rates in pancreatitis, underscores an unmet clinical need for using specific positron emission tomography (PET) radiopharmaceuticals in noninvasive pancreatic cancer detection. ST14, a trypsin-like protease and a member of the type II transmembrane serine protease family, is overexpressed in various solid malignancies, including pancreatic cancer. This study aimed to develop a ⁶⁸Ga-labeled PET radiopharmaceutical targeting ST14 for pancreatic cancer detection. A precursor ST14-06 was designed, and molecular docking was employed to preliminarily predict the binding mode. ST14-06 emerged as the preferred precursor with experimental inhibition assays confirming its high affinity for ST14 (IC₅₀ = 1.06 ± 0.08 nM). ⁶⁸Ga-ST14-06 was successfully synthesized with high radiochemical purity (RCP, >95 %) and molar activity (25-30 GBq/μmol) and was stable in saline and serum. In vitro studies demonstrated specific binding of the tracer to ST14-positive AsPC-1 cells compared to the blocking group (11.45 ± 0.12 % vs. 2.48 ± 0.34 %, P < 0.01). PET/CT imaging in AsPC-1 tumor-bearing mice confirmed ST14-specific uptake, which was reduced by co-administration of an excess blocking agent. Biodistribution studies revealed higher distribution in AsPC-1 tumors (0.99 ± 0.08 %ID/g) than in PANC-1 tumors (0.32 ± 0.02 %ID/g) and the blocking group (0.32 ± 0.04 %ID/g) at 1 h post-injection. Immunohistochemistry results showed that ST14 was highly positive in AsPC-1 tumors, but was negative in PANC-1 tumors. These preliminary findings suggest that ⁶⁸Ga-ST14-06 has potential as a first-generation PET radiopharmaceutical for ST14-specific imaging, offering a promising tool for pancreatic cancer detection.