Folate Receptor β-Targeted PET Imaging of Macrophages in Autoimmune Myocarditis

Currently available imaging techniques have limited specificity for the detection of active myocardial inflammation. Aluminum 18F-labeled 1,4,7-triazacyclononane- N,N',N″-triacetic acid conjugated folate ( 18F-FOL) is a PET tracer targeting folate receptor β (FR-β), which is expressed on activated macrophages at sites of inflammation. We evaluated 18F-FOL PET for the detection of myocardial inflammation in rats with autoimmune myocarditis and studied the expression of FR-β in human cardiac sarcoidosis specimens. Methods: Myocarditis was induced by immunizing rats ( n = 18) with porcine cardiac myosin in complete Freund adjuvant. Control rats ( n = 6) were injected with Freund adjuvant alone. 18F-FOL was intravenously injected, followed by imaging with a small-animal PET/CT scanner and autoradiography. Contrast-enhanced high-resolution CT or 18F-FDG PET images were used for coregistration. Rat tissue sections and myocardial autopsy samples from 6 patients with cardiac sarcoidosis were studied for macrophages and FR-β. Results: The myocardium of 10 of 18 immunized rats showed focal macrophage-rich inflammatory lesions, with FR-β expression occurring mainly in M1-polarized macrophages. PET images showed focal myocardial 18F-FOL uptake colocalizing with inflammatory lesions (SUV mean, 2.1 ± 1.1), whereas uptake in the remote myocardium of immunized rats and controls was low (SUV mean, 0.4 ± 0.2 and 0.4 ± 0.1, respectively; P < 0.01). Ex vivo autoradiography of tissue sections confirmed uptake of 18F-FOL in myocardial inflammatory lesions. Uptake of 18F-FOL in inflamed myocardium was efficiently blocked by a nonlabeled FR-β ligand folate glucosamine in vivo. The myocardium of patients with cardiac sarcoidosis showed many FR-β-positive macrophages in inflammatory lesions. Conclusion: In a rat model of autoimmune myocarditis, 18F-FOL shows specific uptake in inflamed myocardium containing macrophages expressing FR-β, which were also present in human cardiac sarcoid lesions. Imaging of FR-β expression is a potential approach for the detection of active myocardial inflammation.