Noninvasive Imaging of Myocardial Inflammation in Myocarditis using ⁶⁸Ga-tagged Mannosylated Human Serum Albumin Positron Emission Tomography

Abstract

The diagnosis of myocarditis traditionally relies on invasive endomyocardial biopsy but none of the imaging studies so far are specific for infiltration of the inflammatory cells itself. We synthesized 68Ga-2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) mannosylated human serum albumin (MSA) by conjugating human serum albumin with mannose, followed by conjugation with NOTA and labeling it with 68Ga. The efficacy of 68Ga-NOTA-MSA positron emission tomography (PET) for imaging myocardial inflammation was tested in a rat myocarditis model. A significant number of mannose receptor-positive inflammatory cells infiltrated the myocardium in both human and rat myocarditis tissue. 68Ga-NOTA-MSA uptake was upregulated in organs of macrophage accumulation, such as liver, spleen, bone marrow and myocardium (0.32 (0.31~0.33) for normal versus 1.02 (0.86~1.06) for myocarditis (median (range), SUV); n=4~6 per group, p-value=0.01). 68Ga-NOTA-MSA uptake in the left ventricle was upregulated in myocarditis compared with normal rats (2.29 (1.42~3.40) for normal versus 4.18 (3.43~6.15) for myocarditis (median (range), average standard uptake value ratio against paraspinal muscle); n=6 per group, p-value<0.01), which was downregulated in rats with cyclosporine-A treated myocarditis (3.69 (2.59~3.86) for myocarditis versus 2.28 (1.76~2.60) for cyclosporine-A treated myocarditis; n=6 per group, p-value<0.01). The specificity of the tracer was verified by administration of excess non-labeled MSA. 68Ga-NOTA-MSA uptake was significantly enhanced earlier in the evolution of myocarditis before any signs of inflammation could be seen on echocardiography. These results demonstrate the potential utility of visualizing infiltration of mannose receptor-positive macrophages with 68Ga-NOTA-MSA PET in the early diagnosis of as well as in the monitoring of treatment response of myocarditis.

Clinical Translation of [68Ga]Ga-NOTA-anti-MMR-sdAb for PET/CT Imaging of Protumorigenic Macrophages

“…), which is about 20-fold lower than that seen with [ 99m Tc]Tc(CO) 3 -anti-MMR-sdAb and 10-fold lower than with the here presented [ 68 Ga]Ga-NOTA-anti-MMR-sdAb [11]. Similar lower kidney retention for 18 Flabeled sdAb has also been reported for anti-HER2, although [13][14][15]. Depending on the chemical structure created, different catabolites will be produced in the kidney cells, with some 18 F-catabolites clearing faster from kidneys.…”

“…Uptake in extra-tumoral MMR-expressing organs, such as the liver, spleen, lymph nodes, and bone, is slightly higher for the 68 Ga-compound when compared to the 18 F version, but substantially lower than what was observed for the 99m Tc compound. These differences may be due to different radiochemistry methods or differences in apparent molar activity (70.1, 31.5, 10.1 GBq/μmol for 99m Tc, 68 Ga, 18 F, respectively (injection time)) [11]. Kidney uptake was also different for the three variants, with the lowest value obtained for the [ 18 F]FB-anti-MMR-sdAb (7.98 ± 0.86 %IA/g at 3 h p.i.…”

“…Other research groups have also investigated macrophage imaging using mannose receptor targeting. Many have focused on mannosylation of the targeting moiety, thereby utilizing the natural ligand of MMR [17][18][19]. However, mannose does not only bind MMR, but also multiple other mannose-binding proteins, such as mannose-binding lectins.…”

“…Previously, we have reported the generation of cross-reactive anti-mouse/human MMR-sdAbs, selection, and validation of the lead compound for imaging of MMR-expressing macrophages using radiolabeling with 99m Tc and 18 F [11]. Here, we report the synthesis and validation of the clinical-grade [ 68 Ga]Ga-NOTAanti-MMR-sdAb compound for PET/CT imaging, its biodistribution, dosimetry, and toxicity studies in animal models, making [ 68 Ga]Ga-NOTA-anti-MMR-sdAb ready for application in a phase I clinical trial.…”

Modern Opportunities of Radionuclide Methods for Diagnosis of Inflammatory Processes in the Myocardium

“…et al suggested that a new 68 Ga-NOTA-MSA PET tracer which has the affinity with macrophage and originally designed for visualization of the lymph nodes, could be used to detect inflammatory infiltrates in the myocardium [85]. In this study the authors used a model of myocarditis in mice, while the presence of inflammation was verified at the end of the PET scan by histological, immunohistochemical and immunofluorocene methods.…”