Synthesis and Preliminary Evaluation of FAPα Targeted Tracer ¹³¹I-FAPI-03

Using N-(4-quinolinoyl)-Gly-(2-cyanopyrrolidine) as scaffold, we prolonged the linker with serine to obtain a FAPI derivative ATE-FAPI-03, which was subsequently labeled with ¹³¹I by electrophilic substitution. Then the in vitro stability, Log P value, binding affinity, targeting properties and biodistribution behavior of ¹³¹I-FAPI-03 was evaluated. Results show that ¹³¹I-FAPI-03 was lipophilic and stable in vitro, capable of specifically binding to FAPα-positive U87MG cells fast with a major proportion trapped intracellularly. After 10 min of incubation, ¹³¹I-FAPI-03 showed a specific binding rate of (22.00 ± 0.35)%, and the binding rate increased with the incubation time, to a peak of (37.5 ± 0.83)% at 180 min. However, the FAPα-negative MCF-7 cells exhibited very low uptake of ¹³¹I-FAPI-03 at any time point. The IC₅₀ measured by the competition assay indicated significant binding property of ¹³¹I-FAPI-03. Biodistribution studies revealed that ¹³¹I-FAPI-03 could rapidly accumulate in tumor sites with an uptake of (14.90 ± 3.21)% ID/g at 5 min post injection. At 2 h post injection, ¹³¹I-FAPI-03 displayed the highest tumor-to-muscle ratio of 43.7 ± 16.7. All above results provided important reference for the development of novel FAPα-targeting tracers.