Translational imaging of the fibroblast activation protein (FAP) using the new ligand [${}^{68}$Ga]Ga-OncoFAP-DOTAGA

The fibroblast activation protein (FAP) is an emerging target for molecular imaging and therapy in cancer. OncoFAP is a novel small organic ligand for FAP with very high affinity. In this translational study, we establish [${}^{68}$Ga]Ga-OncoFAP-DOTAGA (${}^{68}$Ga-OncoFAP) radiolabeling, benchmark its properties in preclinical imaging, and evaluate its application in clinical PET scanning. ${}^{68}$Ga-OncoFAP was synthesized in a cassette-based fully automated labeling module. Lipophilicity, affinity, and serum stability of ${}^{68}$Ga-OncoFAP were assessed by determining logD${}_{7.4}$, IC${}_{50}$ values, and radiochemical purity. ${}^{68}$Ga-OncoFAP tumor uptake and imaging properties were assessed in preclinical dynamic PET/MRI in murine subcutaneous tumor models. Finally, biodistribution and uptake in a variety of tumor types were analyzed in 12 patients based on individual clinical indications that received 163 ± 50 MBq ${}^{68}$Ga-OncoFAP combined with PET/CT and PET/MRI. ${}^{68}$Ga-OncoFAP radiosynthesis was accomplished with high radiochemical yields. Affinity for FAP, lipophilicity, and stability of ${}^{68}$Ga-OncoFAP measured are ideally suited for PET imaging. PET and gamma counting-based biodistribution demonstrated beneficial tracer kinetics and high uptake in murine FAP-expressing tumor models with high tumor-to-blood ratios of 8.6 ± 5.1 at 1 h and 38.1 ± 33.1 at 3 h p.i. Clinical ${}^{68}$Ga-OncoFAP-PET/CT and PET/MRI demonstrated favorable biodistribution and kinetics with high and reliable uptake in primary cancers (SUV${}_{max}$ 12.3 ± 2.3), lymph nodes (SUV${}_{max}$ 9.7 ± 8.3), and distant metastases (SUV${}_{max}$ up to 20.0). Favorable radiochemical properties, rapid clearance from organs and soft tissues, and intense tumor uptake validate ${}^{68}$Ga-OncoFAP as a powerful alternative to currently available FAP tracers.