Molecular imaging of fibroblast activation protein after myocardial infarction using the novel radiotracer [⁶⁸Ga]MHLL1

Abstract

Background: Myocardial infarction (MI) evokes an organized remodeling process characterized by the activation and transdifferentiation of quiescent cardiac fibroblasts to generate a stable collagen rich scar. Early fibroblast activation may be amenable to targeted therapy, but is challenging to identify in vivo. We aimed to non-invasively image active fibrosis by targeting the fibroblast activation protein (FAP) expressed by activated (myo)fibroblasts, using a novel positron emission tomography (PET) radioligand [ 68 Ga]MHLL1 after acute MI. Methods: One-step chemical synthesis and manual as well as module-based radiolabeling yielded [ 68 Ga]MHLL1. Binding characteristics were evaluated in murine and human FAP-transfected cells, and stability tested in human serum. Biodistribution in healthy animals was interrogated by dynamic PET imaging, and metabolites were measured in blood and urine. The temporal pattern of FAP expression was determined by serial PET imaging at 7 d and 21 d after coronary artery ligation in mice as percent injected dose per gram (%ID/g). PET measurements were validated by ex vivo autoradiography and immunostaining for FAP and inflammatory macrophages. Results: [ 68 Ga]MHLL1 displayed specific uptake in murine and human FAP-positive cells (p = 0.0208). In healthy mice the tracer exhibited favorable imaging characteristics, with low blood pool retention and dominantly renal clearance. At 7 d after coronary artery ligation, [ 68 Ga]MHLL1 uptake was elevated in the infarct relative to the non-infarcted remote myocardium (1.3 ± 0.3 vs. 1.0 ± 0.2 %ID/g, p < 0.001) which persisted to 21 d after MI (1.3 ± 0.4 vs. 1.1 ± 0.4 %ID/g, p = 0.013). Excess unlabeled compound blocked tracer accumulation in both infarct and non-infarct remote myocardium regions (p < 0.001). Autoradiography and histology confirmed the regional uptake of [ 68 Ga]MHLL1 in the infarct and especially border zone regions, as identified by Masson trichrome collagen staining. Immunostaining further delineated persistent FAP expression at 7 d and 21 d post-MI in the border zone, consistent with tracer distribution in vivo. Conclusion:The simplified synthesis of [ 68 Ga]MHLL1 bears promise for non-invasive characterization of fibroblast activation protein early in remodeling after MI.

Nuclear Molecular Imaging of Cardiac Remodeling after Myocardial Infarction

“…No segment was rated with intense retention of the tracer. Similar findings have been reported for 68 Ga-DOTA-TATE in another clinical study [9]. In a preclinical study, however, 68 Ga-DOTA-TATE signal did not correspond to the regions of inflammation, suggesting that 68 Ga-DOTA-TATE is not an appropriate tracer for evaluation of post-MI inflammation in mice [10].…”

“…The low expression on unstimulated M0 macrophages and alternatively activated M2 macrophages and high expression on pro-inflammatory M1 macrophages make SSTR2 an ideal biomarker to image inflammation [6,7]. The concept of macrophage detection with SSTR-2 PET/CT compared to cardiac MRI in 6 male patients with sub-acute MI is investigated on a compassionate use base using 68 Ga-DOTA-TOC [8]. Mild or moderate radiotracer retention was reported within areas of acute myocardial damage in 36 of 47 MRI positive segments (76.6%).…”

Developing a Novel Positronium Biomarker for Cardiac Myxoma Imaging

“…Concurrent application of the β+γ emitter with a novel radiotracer, e.g. the ligand of serine protease fibroblast activation protein (FAP), which is a biomarker of activated (myo)fibroblasts, will give a new advantage in assessment of post-myocardial infarction remodeling ( 47 ). Moreover, examples of tracers proposed for PET imaging with the aforementioned isotopes are summarized in the reference by Moskal and Stępień ( 1 ).…”

Radioligands Targeting Fibroblast Activation Protein (FAP)

“…Although the synthesis reduces the total number of steps, problems regarding the purification of the final precursor by means of preparative HPLC were reported. While the compound showed a partial biliary elimination and was not tested in an oncological setup, the PET/CT-imaging of the infarcted region at 7 and 21 days post infarction were successful [ 27 ].…”

“… Small-molecule FAP-targeted radiotracers published by Roy, Langer and Millul [ 26 , 27 , 28 ]. R represents the chelating moiety, which is eventually attached by an appropriate linker.…”