Synthesis and Preclinical Evaluation of a Novel FAPI-04 Dimer for Cancer Theranostics

Overexpression of fibroblast activation protein (FAP) in cancer-associated fibroblasts in a wide variety of tumors enables a highly selective targeting strategy using FAP inhibitors (FAPIs). Quinoline-based FAPIs labeled with radionuclides have been widely developed for tumor-targeted nuclear medicine imaging. However, the short retention time of FAPIs at the tumor site limits their application in radionuclide therapy. In this study, a novel FAPI-04 dimer was synthesized and labeled with radionuclides to prolong the retention time in tumors for imaging and therapy. To prepare the FAPI-04 dimer complex, DOTA-Suc-Lys-(FAPI-04) 2, we used Fmoc-Lys(Boc)–OH as the linker to conjugate two FAPI-04 structures by an amide reaction. The resulting product was further modified by DOTA groups to allow for conjugation with radioactive metals. Both [ 68 Ga]Ga-(FAPI-04) 2 and [ 177 Lu]Lu-(FAPI-04) 2 showed a radiochemical purity of >99% and remained stable in vitro . In vivo, micro-PET images of SKOV3, A431, and H1299 xenografts revealed that the tumor uptake of [ 68 Ga]Ga-(FAPI-04) 2 was about twice that of [ 68 Ga]Ga-FAPI-04 and that the accumulation of [ 68 Ga]Ga-(FAPI-04) 2 at the tumor site did not significantly decrease even 3h after injection. The tumor–abdomen ratio of [ 68 Ga]Ga-(FAPI-04) 2 images was significantly higher than that of [ 18 F]F-FDG images. For radionuclide therapy, [ 177 Lu]Lu-(FAPI-04) 2 effectively retarded tumor growth and displayed good tolerance. In conclusion, the DOTA-Suc-Lys-(FAPI-04) 2 design enhanced its uptake in FAP-expressing tumors, improved its retention time at the tumor site, and produced high-contrast imaging in xenografts after radionuclide labeling. Furthermore, it showed a noticeable antitumor effect. DOTA-Suc-Lys-(FAPI-04) 2 provides a new approach for applying FAPI derivatives in tumor theranostics.