Rational modifications on N-(4-quinolinoyl)-Gly-2-cyanopyrrolidine to develop fibroblast activation protein-targeted radioligands with improved affinity and tumor uptake

Fibroblast activation protein (FAP) has been an attractive target for cancer imaging and therapy. Radiolabeled FAP-targeting ligands have shown promising results for clinical applications. However, further improvements are ongoing in pursuit of increasing tumor uptake, prolonging tumor residence, and maintenance good tumor–to–background contrast for extensive theranostic application. Achieving a higher affinity of the precursor is one of the ways in research. In this study, we designed a series of FAP inhibitors based on N-(4-quinolinoyl)-Gly-2-cyanopyrrolidine and found compound QI-18 with an IC₅₀ value of 0.50 nM, which is a 6.5-fold increase in potency over that of UAMC-1110 (IC₅₀ of 3.25 nM). QI-18 was then functionalized with a DOTA chelator to obtain the ligand CY03 for further radiolabeling with ⁶⁸Ga to obtain the radiotracer [⁶⁸Ga]Ga-CY03. In BALB/c nude mice bearing U87MG tumor models, [⁶⁸Ga]Ga-CY03 exhibited a high and specific uptake (10.30 ± 0.63 % ID/g at 1 h post-injection and 9.28 ± 1.60 % ID/g at 2 h post-injection), which represented 3.2- and 4.1-fold increases over those for [⁶⁸Ga]Ga-FAPI-04 (3.24 ± 0.53 % ID/g and 2.25 ± 0.33 % ID/g, respectively). [⁶⁸Ga]Ga-CY03 also showed higher tumor–to–blood and tumor–to–kidney ratios (7.62 ± 0.44 and 2.59 ± 0.27, respectively) than [⁶⁸Ga]Ga-FAPI-04 (2.38 ± 0.47 and 0.98 ± 0.19, respectively). The results indicate that [⁶⁸Ga]Ga-CY03 is a promising imaging agent to target FAP.