Preclinical Evaluation and a Pilot Clinical Positron Emission Tomography Imaging Study of [68Ga]Ga-FAPI-FUSCC-II

Fibroblast activation protein (FAP), a type II integral membrane serine protease, is a promising target for tumor diagnosis and therapy. OncoFAP has been recently discovered for PET imaging procedures for various solid malignancies. In this study, we presented the development of manual radiolabeling procedures for the preparation of OncoFAP-based radiopharmaceuticals for cancer imaging. A novel series of [⁶⁸Ga/¹⁷⁷Lu]Ga/Lu-FAPI-FUSCC-I/II were produced with high radiochemical yields. [⁶⁸Ga]Ga-FAPI-FUSCC-I/II and [¹⁷⁷Lu]Lu-FAPI-FUSCC-I/II were stable in phosphate-buffered saline, fetal bovine serum, and human serum for at least 3 h. In vitro cellular uptake and blocking experiments implied that they had specificity to FAP. Additionally, the low nanomolar IC₅₀ values of FAPI-FUSCC-II indicated that it had a high target affinity to FAP. The in vivo biodistribution and blocking study in mice bearing HT-1080-FAP tumors showed that both exhibited specific tumor uptake. [⁶⁸Ga]Ga-FAPI-FUSCC-II showed a higher tumor uptake and a higher tumor/nontarget ratio than [⁶⁸Ga]Ga-FAPI-FUSCC-I and [⁶⁸Ga]Ga-FAPI-04. The results of ex vivo biodistribution were in accordance with the biodistribution results. Clinical [⁶⁸Ga]Ga-FAPI-FUSCC-II-PET/CT imaging further demonstrated its favorable biodistribution and kinetics with elevated and reliable uptake by primary tumors (maximum standardized uptake value (SUV_max), 12.17 ± 6.67) and distant metastases (SUV_max, 9.24 ± 4.28). In summary, [⁶⁸Ga]Ga-FAPI-FUSCC-II displayed increased tumor uptake and retention compared to [⁶⁸Ga]Ga-FAPI-04, giving it potential as a promising tracer for the diagnostic imaging of malignant tumors with positive FAP expression.