Development and Application of Radioactive Ligands Targeting Fibroblasts with Albumin-Binding Sites

Fibroblast activation protein (FAP) is overexpressed on cancer-associated fibroblasts, making it an important target for cancer diagnosis and treatment, but limited tumor retention hinders late-stage diagnosis and radionuclide therapy. In this study, three albumin-bound FAP inhibitor (FAPI) radioligands, 68Ga/177Lu-DOTA-ALB-01, 68Ga/177Lu-DOTA-ALB-02, and 68Ga/177Lu-DOTA-ALB-03, were synthesized and evaluated for their in vitro stability, binding affinity, in vivo biodistribution, and tumor uptake using 68Ga and 177Lu labeling. All radioligands are stable in saline and plasma and exhibit high FAP-binding affinity. 177Lu-DOTA-ALB-02 has longer retention in circulation than 177Lu-FAPI-46 and other radioligands. Continuous tumor accumulation was observed during imaging with both 177Lu-DOTA-ALB-01 and 177Lu-DOTA-ALB-02. Notably, 177Lu-DOTA-ALB-02 had a significant tumor/nontarget ratio as indicated by biodistribution data. The outstanding tumor retention properties of 177Lu-DOTA-ALB-02 have been demonstrated in small-animal single-photon emission computed tomography imaging and biodistribution studies; therefore, it is considered the albumin-binding FAPI with the most favorable pharmacokinetic and imaging properties, worthy of further clinical investigation.