Magnetic\nResonance Imaging of Fibroblast Activation\nProtein Using a Targeted Gadolinium-Based Contrast Agent

The aim of this study was to synthesize a quinoline-based\nMRI contrast\nagent, Gd-DOTA-FAPI04, and assess its capacity for targeting fibroblast\nactivation protein (FAP)-positive tumors in vivo. Gd-DOTA-FAPI04 was\nsynthesized by attaching a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic\nacid (DOTA) complex of gadolinium(III) to FAP inhibitor FAPI04. The\nlongitudinal relaxation time (T1) of the contrast agent was measured\nusing a Siemens Prisma 3.0T MR system, and the CCK-8 assay was performed\nto evaluate its potential cytotoxicity. Male nude mice bearing tumors\ngrown from FAP-expressing fibrosarcoma cells were divided into experimental\n(<i>n</i> = 4) and control (<i>n</i> = 4) groups,\nand T1-weighted image enhancement was measured at different times\n(0, 10, 30, 60, 90, and 120 min) postinjection of Gd-DOTA-FAPI04.\nThe control group received an additional preinjection of excess FAPI04.\nFAP expression in tumor tissue was investigated by using immunohistochemistry\nwith an anti-FAP antibody. The longitudinal relaxivities of gadodiamide\nand Gd-DOTA-FAPI04 were measured to be 3.734 mM<sup>–1</sup> s<sup>–1</sup> and 5.323 mM<sup>–1</sup> s<sup>–1</sup>, respectively. The CCK-8 assay demonstrated that Gd-DOTA-FAPI04\nhas minimal toxicity to cultured human fibrosarcoma cells. In vivo\nMRI showed that peak accumulation of Gd-DOTA-FAPI04 in FAP-expressing\ntumors occurred 1 h postinjection and could be blocked by preinjection\nof excess FAPI04. Immunohistochemical analysis of harvested tumor\ntissue supported the above findings. Gd-DOTA-FAPI04 is a promising\ncontrast agent for in vivo imaging of FAP.