Radiosynthesis and preclinical evaluation of [68Ga]Ga-NOTA-folate for PET imaging of folate receptor \u03b2-positive macrophages

Olli Moisio,Päivi Marjamäki ,Merja Heinäniemi ,Fenghua Zhang,Tuula Tolvanen,Heidi Liljenbäck ,Erika Atencio Herre,Senthil Palani,Juhani Knuuti,Meeri Käkelä ,Xiangguo Li ,Tiit Örd ,Petri Elo,Maxwell Miner ,Madduri Srinivasarao,Minna U Kaikkonen,Jenni Virta,Philip S Low,Antti Saraste,Anne Roivainen

doi:10.1038/s41598-020-70394-3

Abstract

Folate receptor β (FR-β), a marker expressed on macrophages, is a promising target for imaging of inflammation. Here, we report the radiosynthesis and preclinical evaluation of [68Ga]Ga-NOTA-folate (68Ga-FOL). After determining the affinity of 68Ga-FOL using cells expressing FR-β, we studied atherosclerotic mice with 68Ga-FOL and 18F-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 68Ga-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 68Ga-FOL with high radiochemical purity and moderate molar activity. Cell binding studies revealed that 68Ga-FOL had 5.1 nM affinity for FR-β. Myocardial uptake of 68Ga-FOL was 20-fold lower than that of 18F-FDG. Autoradiography and immunohistochemistry of the 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 was significantly higher than that of 18F-FDG. Blocking studies verified that 68Ga-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 68Ga-FOL represents a promising new FR-β–targeted tracer for imaging macrophage-associated inflammation.

Highlights

Folate receptor β (FR-β), a marker expressed on macrophages, is a promising target for imaging of inflammation
fluorescence-activated cell sorting (FACS) analyses confirmed that folate receptor (FR)-β was clearly expressed on Chinese hamster ovary (CHO)-FR-β+ cells but not on CHO-FR-β− cells (Fig. 2A–C)
We observed no clear accumulation of 68Ga-FOL on CHO-FR-β− cells, even at concentrations up to 40 nM (Fig. 2D)

Summary

Introduction

Folate receptor β (FR-β), a marker expressed on macrophages, is a promising target for imaging of inflammation. We prepared [ 68Ga]Ga-NOTA-folate (68Ga-FOL, Fig. 1) with generator-produced 68Ga and evaluated its potential for imaging of inflammation. We investigated the uptake and specificity of intravenously (i.v.) administered 68Ga-FOL for the detection of inflamed atherosclerotic lesions in mice and compared the tracer with 18F-FDG.

Results

Conclusion