FAP-IN-9 (compound P12) is a fibroblast activation protein (FAP) inhibitor with an IC50 of 0.38 nM for human FAP. This compound effectively inhibits FAP activity by binding specifically, as confirmed through in vitro and in vivo blocking experiments. FAP-IN-9 forms stable complexes and serves as a tumor-targeting agent. When labeled with 99mTc, FAP-IN-9 can be used as a SPECT probe for research related to glioblastoma.