Small Molecules Labeled with Fluorine-18 as Inhibitors of Fibroblast Activation Protein (FAP) for PET Imaging of Tumors

This review provides a comprehensive overview of the latest advancements made in PET radiotracers based on low-molecular-mass inhibitors of fibroblast (major component of tumor microenvironment) activation protein (FAPIs). Two approaches to the synthesis of the radiotracers based on an effective FAPI, N-(4-quinolinoyl)-glycyl-2-cyanopyrrolidine (UAMC1110), are discussed. The first approach is “pseudo-chelation” of fluorine-18 in the form of the [18F]AlF2+ cation by NOTA chelator. The second approach is based on classical methods for preparing FAPI radiotracers with 18F–C covalent bond by aliphatic nucleophilic substitution, copper-catalyzed radiofluorination of nonactivated arenes, indirect label introduction into a linker-modified UAMC1110 molecule using various radio synthons, etc. Special attention is paid to the characteristics of the new FAPI agents with the 18F–C covalent bond, such as affinity for FAP, selectivity, lipophilicity, and tumor uptake, which determine their potential application in cancer diagnostics.