ImmunoPET of Trophoblast Cell-surface Antigen 2 (Trop-2) Expression in Pancreatic Cancer

Purpose: Without a standard test for pancreatic carcinomas, this highly lethal disease is normally diagnosed at its advanced stage, leading to a low survival rate of patients. Trophoblast cell-surface antigen 2 (Trop-2), a transmembrane glycoprotein, is associated with cell proliferation and highly expressed in most of solid epithelial tumors, including pancreatic cancer. A non-invasive method of imaging Trop-2 would greatly benefit clinical diagnosis and monitoring of pancreatic cancer. In the current study, Zr-labeled anti-Trop-2 antibody (AF650) was recruited for the systemic evaluation of Trop-2 as an immunoPET target for pancreatic cancer imaging.

Methods: AF650 was conjugated with desferrioxamine (DFO) and then radiolabeled with Zr. Trop-2 expression levels were determined in three pancreatic cancer cell lines (BxPC-3, MIA PaCa-2, and AsPC-1) via western blot, flow cytometry, saturation binding assay, and immunofluorescence staining. The targeting capacity of Zr-DFO-AF650 was evaluated in mouse models with subcutaneous xenograft of pancreatic cancers via PET imaging and bio-distribution studies. In addition, a Trop-2-positive orthotopic cancer model was recruited for further validating the targeting specificity of Zr-DFO-AF650.

Results: BxPC-3 cells expressed high levels of Trop-2, while AsPC-1 and MIA PaCa-2 cells expressed low levels of Trop-2. Additionally, Zr-DFO-AF650 exhibited high specificity to Trop-2 in BxPC-3 cells (K = 22.34 ± 2.509 nM). In subcutaneous xenograft models, about 28.8 ± 7.63%ID/g tracer accumulated in the BxPC-3 tumors at 120 h post injection, which was much higher than those reaching MIA PaCa-2 (6.76 ± 2.08%ID/g) and AsPC-1 (3.51 ± 0.69%ID/g) tumors (n = 4). More importantly, Zr-DFO-AF650 could efficiently distinguish primary tumors in the orthotopic BxPC-3 cancer model, showing high correlation between PET imaging and bio-distribution and sensitivity.

Conclusions: Zr-DFO-AF650 can be effectively used to detect pancreatic cancer via Trop-2-mediated immunoPET in vivo, clearly revealing the great potential of Trop-2-based non-invasive imaging in pancreatic cancer detection and treatment monitoring.