[ ⁶⁸ Ga]Ga-PG2-2FAPI: A DPro-Gly-Modified Dimeric FAPI Probe with Enhanced Tumor Uptake and Retention

In this study, a novel fibroblast activation protein (FAP) targeting ligand based on the bivalent DPro-Gly structure, DOTA-PG2-2FAPI, was designed and synthesized. Molecular docking analysis revealed that compared with monomeric FAPI-46, DOTA-PG2-2FAPI had a higher binding score (-15.00 vs -11.36), with an <i>in vitro</i> IC₅₀ value of 6.65 nM, indicating a significantly increased binding affinity toward FAP. The ⁶⁸Ga-labeled complex ([⁶⁸Ga]Ga-PG2-2FAPI) demonstrated radiochemical purity exceeding 95% with good stability and high hydrophilicity (Log<i>D</i>_7.4 = -3.29 ± 0.06). In HT-1080-FAP cells, the cellular uptake of [⁶⁸Ga]Ga-PG2-2FAPI reached 10.98 ± 0.10% ID, which decreased by 92% upon FAP inhibition. <i>In vivo</i> studies using tumor-bearing mice revealed that the tumor uptake of [⁶⁸Ga]Ga-PG2-2FAPI was 17.60 ± 1.33% ID/g in HT-1080-FAP tumors and 32.71 ± 0.98% ID/g in U87MG tumors, which was significantly greater than that in nontargeted tissues. Positron emission tomography (PET) imaging revealed rapid tumor accumulation and sustained retention, with high-specificity imaging across all four tumor models (HT-1080-FAP, U87MG, HT-29, and PANC-1). On the basis of these characteristics, this probe holds promise as a broad-spectrum tumor imaging agent with significant clinical application value.