Radiosynthesis And Preclinical Evaluation Of [Ga-68]Ga-Nota-Folate For Pet Imaging Of Folate Receptor Beta-Positive Macrophages

Folate receptor beta (FR-beta), a marker expressed on macrophages, is a promising target for imaging of inflammation. Here, we report the radiosynthesis and preclinical evaluation of [Ga-68]Ga-NOTA-folate (Ga-68-FOL). After determining the affinity of Ga-68-FOL using cells expressing FR-beta, we studied atherosclerotic mice with Ga-68-FOL and F-18-FDG PET/CT. In addition, we studied tracer distribution and co-localization with macrophages in aorta cryosections using autoradiography, histology, and immunostaining. The specificity of Ga-68-FOL was assessed in a blocking study with folate glucosamine. As a final step, human radiation doses were extrapolated from rat PET data. We were able to produce Ga-68-FOL with high radiochemical purity and moderate molar activity. Cell binding studies revealed that Ga-68-FOL had 5.1 nM affinity for FR-beta. Myocardial uptake of Ga-68-FOL was 20-fold lower than that of F-18-FDG. Autoradiography and immunohistochemistry of the aorta revealed that Ga-68-FOL radioactivity co-localized with Mac-3-positive macrophage-rich atherosclerotic plaques. The plaque-to-healthy vessel wall ratio of Ga-68-FOL was significantly higher than that of F-18-FDG. Blocking studies verified that Ga-68-FOL was specific for FR. Based on estimations from rat data, the human effective dose was 0.0105 mSv/MBq. Together, these findings show that Ga-68-FOL represents a promising new FR-beta -targeted tracer for imaging macrophage-associated inflammation.