Formulation and Preclinical Testing of Tc-99m-Labeled HYNIC-Glc-FAPT as a FAP-Targeting Tumor Radiotracer.

ABSTRACT

Molecular imaging and targeted radiotherapy with radiolabeled fibroblast activation protein inhibitor (FAPI) targeting peptide probes hold great potential for enhancing the clinical management of patients with FAP-expressing cancers. However, the high cost of PET probes has prompted us to search for new FAP-targeting single-photon imaging agents. In this study, HYNIC-Glc-FAPT is synthesized and radiolabeled with technetium-99m using tricine/EDDA or dimer tricine as coligands to produce [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT and [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT. Both [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT and [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT were effectively synthesized with an excellent radiochemistry yield (both >97%, n = 6) in a single-step technique, and their stability in PBS and human serum was satisfactory. Compared to [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT, [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT exhibited a more hydrophilic nature with a log P of -3.53 ± 0.12. In vitro cellular uptake and blocking assays, internalization, efflux experiments, and affinity experiments all suggested a mechanism with high FAP-specificity and affinity. SPECT imaging and biodistribution of [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT demonstrated sustained high tumor uptake in BALB/c nude mice bearing U87MG tumors for 6 h. It demonstrated a long-range retention characteristic and more rapid clearance ability from nontarget organs. Collectively, we successfully synthesized [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT and [99mTc]Tc-tricine(2)-HYNIC-Glc-FAPT, and the excellent targeting properties of [99mTc]Tc-tricine/EDDA-HYNIC-Glc-FAPT suggest a potential diagnostic value in future clinical studies for advanced-stage FAP-expressing malignancies, especially in prognostic evaluation of tumors for it low price and convenient source.