eFAP-51, a novel FAPI dimer with enhanced tumor uptake and retention for theranostic applications

Overexpression of fibroblast activation protein (FAP) in cancer-associated fibroblasts enables a highly selective targeting strategy using FAP inhibitors. However, radiolabeled small-molecule FAP inhibitors (FAPIs) show limited tumor retention, restricting therapeutic applications. Multimerization represents a promising strategy to enhance tumor uptake and retention through polyvalent binding. Here, we report the rational design and evaluation of dimeric FAP-targeted radiopharmaceuticals for theranostic applications. Based on the (4-quinolinoyl)-glycyl-2-cyanopyrrolidine (QCP) scaffold, a series of monomeric FAPIs were assembled into dimeric radioligands using three distinct tripod-based platforms and equipped with a DOTA-GA chelator for efficient radiolabeling with indium-111 and lutetium-177; systematic in vitro and in vivo evaluation of binding affinity, stability, cellular uptake, and tumor targeting identified eFAP-51 and eFAP-52 as lead candidates, both showing high radiochemical yield and purity, favorable stability, and sub-nanomolar affinity for FAP with strong selectivity over DPP4. Further studies demonstrated superior tumor targeting and pharmacokinetics for eFAP-51 compared with the clinical reference FAPI-46, with [ 111 In]In-eFAP-51 exhibiting more than 4-fold higher tumor uptake at 48 h post-injection (3.2 ± 0.5 vs. 0.71 ± 0.1%ID/g) and prolonged retention. eFAP-51 was subsequently radiolabeled with lutetium-177 to yield a stable therapeutic candidate, and dose-dependent studies revealed that intermediate molar doses (0.3–0.6 nmol) provided an optimal balance between tumor uptake and tumor-to-background contrast. The FAPI dimer eFAP-51 showed favorable tumor targeting and pharmacokinetics, supporting its potential for clinical translation in theranostic oncology.