〔Abstract〕 Objective To synthesis a Gd-labeled molecular probe capable of responding to fibroblast activation protein(FAP) highly expressed in tumor-associated fibroblasts for magnetic resonance imaging studies of pancreatic cancer.Methods The chemical structure of Z-Gly-Pro-Cys-Lys(Gd-DOTA)-CBT(contrast agent 1) was characterized through mass spectrometry analysis.High-performance liquid chromatography(HPLC) was used to verify the contrast agent 1 formed a dimer under the cleavage of fibroblast activation protease(FAP) in vitro,and further self-assembled to form Gd-nanoparticles.The cell counting Kit-8(CCK-8) assay was used for biological safety analysis.BxPc-3 tumor-bearing mice were established and randomly divided into three groups,3 mice intravenously(i.v.) injected with 0.08 mmol/kg contrast agent 1 through tail vein were designated as the experimental group,3mice i.v.injected with 0.08 mmol/kg Gd-DTPA were designated as the control group,and 3 mice i.v.injected with normal saline were designated as the blank control group(n=3).Dynamic MR scanning from 0 to 3 hours was performed for them,respectively.The enhancement effects of T1-and T2-weighted imaging were observed and the rate of change in the tumor-to-muscle ratio(T/M) over time was analyzed.Results Contrast agent 1 efficiently responded to FAP in vitro,forming dimers under the action of FAP,and further self-assembled to form nanoparticle structures.Contrast agent 1 has good biocompatibility.The BxPc-3 tumor-bearing mice showed that the T1-weighted contrast of the tumor reached its highest level at 2 h post injection of contrast agent 1 or Gd-DTPA.In T1-weighted imaging,the T/M at 2 h in the experimental group was 135.20%±0.06% of 0 h,the T/M at 2 h in the Gd-DTPA group was 115.70%±0.05% of 0 h and the T/M at 2 h in the saline group was 113.5%±0.02% of 0h.Compared with Gd-DTPA control group,the experimental group had significantly enhanced T1-weighted MR contrast,and the difference was statistically significant(P<0.01).In tumor T2-weighted imaging,the T/M quantitative analysis showed that the tumor T2-weighted contrast enhancement of the three groups was relatively small,and the T/M of the experimental group and the control group at 0.5 h was 94.60±0.03% and 106.30±0.04% of the 0 h,respectively.In the normal saline group,T/M at 0.5 h was 102.20±0.002% of that at 0 h,and no significant enhancement of the T2-weighted MR contrast of tumor was observed.Conclusion Successfully construct a FAP-responsive molecular probe which can self-assemble to form a Gd-nanoparticle structure in tumor cells under the cleavage of FAP,effectively improving the enrichment of Gd in the target region,and enhancing T1-weighted magnetic resonance imaging of pancreatic cancer,which has potential clinical application value.