Radiosynthesis and Preclinical Evaluation of [68Ga]Ga-NOTA-Folate for PET Imaging of Folate Receptor β Positive Macrophages

Abstract

Folate receptor β (FR-β) is one of the markers expressed on macrophages and a promising target for imaging of inflammation. Here, we report the radiosynthesis and preclinical evaluation of [68Ga]Ga-NOTA-folate (68Ga-FOL). First, we determined the affinity of 68Ga-FOL using human FR-β expressing cells. Then, we studied atherosclerotic mice with 68Ga-FOL and 18F-FDG PET/CT. After sacrifice, the tissues excised were measured with a γ-counter for ex vivo biodistribution. Further, the tracer distribution and co-localization with macrophages in aorta cryosections were studied using autoradiography, hematoxylin-eosin staining and immunostaining with anti-Mac-3 antibody. Specificity of 68Ga-FOL was assessed in a blocking study with excess of folate glucosamine. As a last step, human radiation doses were extrapolated from rat PET data. We were able to produce 68Ga-FOL at high radioactivity concentration, with high molar activity and radiochemical purity. The cell binding studies showed high (5.1 ± 1.1 nM) affinity of 68Ga-FOL to FR-β. The myocardial uptake of 68Ga-FOL (SUV 0.43 ± 0.06) was 20-folds lower compared to 18F-FDG (SUV 10.6 ± 1.8, P = 0.001). The autoradiography and immunohistochemistry of aorta revealed that 68Ga-FOL radioactivity co-localized with Mac-3-positive macrophage-rich atherosclerotic plaques. The plaque-to-healthy vessel wall ratio of 68Ga-FOL (2.44 ± 0.15) was significantly higher than that of 18F-FDG (1.93 ± 0.22, P = 0.005). Blocking studies verified 68Ga-FOL specificity to FR. As estimated from rat data the human effective dose was 0.0105 mSv/MBq. The organ with highest absorbed dose was kidney (0.1420 mSv/MBq). In conclusion, 68Ga-FOL is a promising new FR-β-targeted tracer for imaging macrophage-associated inflammation.