Translational assessment of a DATA-functionalized FAP inhibitor with facile 68Ga-labeling at room temperature

PURPOSE

The present study aims at evaluating the preclinical and the clinical performance of [68Ga]Ga-DATA5m.SA.FAPi, which has the advantage to be labeled with gallium-68 at room temperature.

METHODS

[68Ga]Ga-DATA5m.SA.FAPi was assessed in vitro on FAP-expressing stromal cells, followed by biodistribution and in vivo imaging on prostate and glioblastoma xenografts. Moreover, the clinical assessment of [68Ga]Ga-DATA5m.SA.FAPi was conducted on six patients with prostate cancer, aiming on investigating, biodistribution, biokinetics, and determining tumor uptake.

RESULTS

[68Ga]Ga-DATA5m.SA.FAPi is quantitatively prepared in an instant kit-type version at room temperature. It demonstrated high stability in human serum, affinity for FAP in the low nanomolar range, and high internalization rate when associated with CAFs. Biodistribution and PET studies in prostate and glioblastoma xenografts revealed high and specific tumor uptake. Elimination of the radiotracer mainly occurred through the urinary tract. The clinical data are in accordance with the preclinical data concerning the organ receiving the highest absorbed dose (urinary bladder wall, heart wall, spleen, and kidneys). Different to the small-animal data, uptake of [68Ga]Ga-DATA5m.SA.FAPi in tumor lesions is rapid and stable and tumor-to-organ and tumor-to-blood uptake ratios are high.

CONCLUSION

The radiochemical, preclinical, and clinical data obtained in this study strongly support further development of [68Ga]Ga-DATA5m.SA.FAPi as a diagnostic tool for FAP imaging.

Molecular recognition of fibroblast activation protein for diagnostic and therapeutic applications

Fibroblast activation protein (FAP) is a non-classical serine protease expressed predominantly in conditions accompanied by tissue remodeling, particularly cancer. Due to its plasma membrane localization, FAP represents a promising molecular target for tumor imaging and treatment. The unique enzymatic activity of FAP facilitates development of diagnostic and therapeutic tools based on molecular recognition of FAP by substrates and small-molecule inhibitors, in addition to conventional antibody-based strategies. In this review, we provide background on the pathophysiological role of FAP and discuss its potential for diagnostic and therapeutic applications. Furthermore, we present a detailed analysis of the structural patterns crucial for substrate and inhibitor recognition by the FAP active site and determinants of selectivity over the related proteases dipeptidyl peptidase IV and prolyl endopeptidase. We also review published data on targeting of the tumor microenvironment with FAP antibodies, FAP-targeted prodrugs, activity-based probes and small-molecule inhibitors. We describe use of a recently developed, selective FAP inhibitor with low-nanomolar potency in inhibitor-based targeting strategies including synthetic antibody mimetics based on hydrophilic polymers and inhibitor conjugates for PET imaging. In conclusion, recent advances in understanding of the molecular structure and function of FAP have significantly contributed to the development of several tools with potential for translation into clinical practice.