Monovalent and Divalent Designs of Copper Radiotheranostics Targeting Fibroblast Activation Protein in Cancer.

: Fibroblast activation protein (FAP)-targeted theranostic radiopharmaceuticals have shown desired tumor-to-background organ selectivity due to the ubiquitous presence of FAP within the tumor microenvironment. However, suboptimal tumor retention and fast clearance have hindered their use to deliver effective cancer therapies. With well-documented FAP-targeting moieties and linkers appending them to optimal chelators, the development of copper radiopharmaceuticals has attracted considerable interest, given the fact that an ideal theranostic pair of copper radionuclides (Cu: t = 12.7 h; 17.4% β; E = 653 keV and Cu: t = 2.58 d; 100% β; E = 562 keV) are available. Herein, we report our design, synthesis, and comparative evaluation of monovalent and divalent FAP-targeted theranostic conjugates constructed from our previously reported bifunctional chelator scaffold (BFS) based on 1,4,8,11-tetraaza-bicyclo [6.6.2]hexadecane-4,11-diacetic acid (CB-TE2A), which forms the most stable complex with Cu(II). : After synthesis and characterization, the monovalent and divalent conjugates were radiolabeled with Cu for in vitro cell assays, followed by in vivo positron emission tomography (PET) imaging evaluation in relevant mouse models. : Both Cu-labeled conjugates showed high in vitro stability and anticipated FAP-mediated cell binding and internalization. The divalent one showed significantly higher FAP-specific tumor uptake than its monovalent counterpart. : Our results demonstrate that the BFS-based multivalent approach can be practically used to generate FAP-targeted radiotheranostic agents for effective cancer diagnosis and treatment.