Radiosynthesis and identification of a molecular probe I-FAPI targeting cancer-associated fibroblasts.

Purpose: Fibroblast activation protein (FAP) is highly expressed in the mesenchyme of most malignant epithelial tumors, while its expression is low in normal tissues. FAP inhibitors (FAPIs) bind specifically to FAP and are used for tumor-targeted diagnosis and therapy. The aim of this study was to radiosynthesize a novel molecular probe I-FAPI and evaluate its targeting and biological characteristics.

Methods: The structurally modified FAPI was labelled with I through the chloramine-T method. The radiolabeling rate was then detected by thin-layer chromatography (TLC). The stability of I-FAPI was determined at PBS (room temperature) and serum (37°C). Its hydrophilicity was calculated by measuring its lipid-water partition coefficient. Pancreatic cancer PANC-1 cell line and glioma U87 cell line were cultured . Cell uptake assay was used to show the binding ability of I-FAPI. The CCK-8 assay was used to calculate the inhibitory effects of I-FAPI at different time points (4h, 8h, 12h, 24h, 48h) after comparing with the I and FAPI. The before-and-after-24h scratch areas of the two cells were determined in order to verify the effect of I-FAPI on the migration ability of the cells.

Results: The radiolabeling rate was (84.9 ± 1.02) %. The radiochemical purity of I-FAPI remained over 80% in both 25°C PBS and 37°C serum. The value of the lipid-water partition coefficient was -0.869 ± 0.025, indicating the hydrophilic of the probe. The cellular uptake assay showed that U87 cells had a specific binding capacity for I-FAPI. In cell inhibition assays, the inhibitory effect of I-FAPI on U87 cells increased with time. The results of cell scratch assay showed that I-FAPI had the strongest inhibitory effect on the migratory ability of U87 cells compared with I and FAPI (<0.001).

Conclusion: I-FAPI was synthesized with good stability and hydrophilic properties. It can be specifically bound by U87 cells. The proliferation and migration of U87 cells can be effectively inhibited. I-FAPI is promising to become a therapeutic probe.