Molecular imaging of fibroblast activity after myocardial infarction using a ⁶⁸Ga-labelled fibroblast activation protein inhibitor FAPI-04

Abstract

Heart failure (HF) remains a major source of late morbidity and mortality after myocardial infarction (MI). Temporospatial presence of the activated fibroblasts in the injured myocardium predicts the quality of cardiac remodelling post-MI. Therefore, monitoring of activated fibroblasts is of great interest for studying cardiac remodelling after MI. Fibroblast activation protein (FAP) expression is upregulated in activated fibroblasts. This study investigates the feasibility of imaging activated fibroblasts with a new ⁶⁸Ga-labelled FAP inhibitor (⁶⁸Ga-FAPI-04) for PET imaging of fibroblast activation in a preclinical model of MI. Methods: MI and sham-operated rats were scanned with ⁶⁸Ga-FAPI-04−PET/CT (1, 3, 6, 14, 23, and 30 days post-MI) and with ¹⁸F-FDG (3 days post-MI). Dynamic ⁶⁸Ga-FAPI-04−PET and blocking studies were performed on MI rats, 7 days after coronary ligation. After in vivo scans, animals were euthanized and hearts were harvested for ex vivo analyses. Cryosections were prepared for autoradiography, haematoxylin and eosin (H&E), and immunofluorescence staining. Results: ⁶⁸Ga-FAPI-04 uptake in the injured myocardium peaked on day 6 after coronary ligation. The tracer accumulated intensely in the MI territory, as identified by decreased ¹⁸F-FDG uptake and confirmed by PET/MR and H&E staining. Autoradiography and H&E staining of cross-sections revealed that ⁶⁸Ga-FAPI-04 accumulates mainly at the border zone of the infarcted myocardium. In contrast, there was only minimal uptake in the infarct of the blocked rats, comparable to the uptake in the remote non-infarcted myocardium (PET image-derived infarct-to-remote uptake ratio: 6±2). Immunofluorescence staining confirmed the presence of FAP-positive (FAP+) myofibroblasts in the injured myocardium. Morphometric analysis of the whole heart sections demonstrated 3- and 8-fold higher FAP+ fibroblast density in the border zone compared to infarct centre and remote area, respectively. Conclusion: ⁶⁸Ga-FAPI-04 represents a promising radiotracer for in vivo imaging of post-MI fibroblast activation. Non-invasive imaging of activated fibroblasts may have significant diagnostic and prognostic values, which could aid clinical management of patients after MI.