[ 68 Ga]Ga-HBED-CC-FAPI Derivatives with Improved Radiolabeling and Specific Tumor Uptake.

Fibroblast activation protein (FAP) is selectively expressed in tumors and highly important for maintaining the microenvironment in malignant tumors. Radioisotope-labeled FAP inhibitors (FAPIs) were proven to be useful for diagnosis and radionuclide therapy of cancer and are under active clinical investigations. Ga-HBED complex displays a higher in vivo stability constant (log  K GaL : 38.5), compared to that of Ga-DOTA (log  K GaL : 21.3). Such advantage in stability constant suggests that it may be useful for development of alternative FAPI imaging agents. In this study, previously reported [ 68 Ga]Ga-DOTA-FAPI-02 and -04 were converted to the corresponding [ 68 Ga]Ga-HBED-CC-FAPI-02 and -04 derivatives ([ 68 Ga]Ga- 4 , [ 68 Ga]Ga- 5 , [ 68 Ga]Ga- 6 , and [ 68 Ga]Ga- 7 ). It was found that substituting the DOTA chelating group with HBED-CC led to several unique and desirable tumor-targeting properties: (1) robust, fast, and high yield labeling─readily adaptable to a kit formulation; (2) high stabilities in vitro ; (3) excellent FAP binding affinities (IC 50 ranging between 4 and 7 nM) and improved cell uptake and retention (in HT1080 (FAP+) cells); and (4) excellent selective in vivo tumor uptake in nude mice bearing U87MG tumor. It appeared that Ga(III) chelation with HBED-CC improved the in vivo kinetics favoring higher tumor uptake and retention compared to the corresponding Ga-DOTA complex. Out of the four tested ligands the new [ 68 Ga]Ga-HBED-CC-FAPI dimer, [ 68 Ga]Ga- 6 , displayed the best tumor localization properties, and further studies are warranted to demonstrate that it is an alternative FAP imaging agent for cancer patients.