Myocardial Fibroblast Activation After Acute Myocardial Infarction: A Positron Emission Tomography and Magnetic Resonance Study

Molecular Imaging of Fibroblast Activation Protein in Response to Cardiac Injury Using [68Ga]Ga-DATA5m.SA.FAPi

Background/Objectives: Fibroblast activation protein (FAP) has gained tremendous traction as a target for tumor imaging and cancer treatment, while also playing a key role in fibrosis. Our study aimed to evaluate [68Ga]Ga-DATA5m.SA.FAPi for PET imaging of replacement fibrosis following myocardial infarction (MI) or interstitial fibrosis associated with hypertrophy. Methods: MI or transverse aortic constriction (TAC)-induced hypertrophy was induced in C57BL/6 mice, with sham-operated animals serving as controls. At multiple time points during disease progression (1, 2, and 6 weeks post-surgery), [68Ga]Ga-DATA5m.SA.FAPi PET/CT scans were performed, followed by ex vivo investigations. Additionally, in vitro cell uptake experiments simulating hypertrophy were conducted. Results: Cardiac uptake of [68Ga]Ga-DATA5m.SA.FAPi significantly increased two weeks after MI induction (MI: 2.1 ± 0.2%ID/g, n = 7 vs. SHAM: 1.1 ± 0.2%ID/g, n = 5; p = 0.002), confirmed by ex vivo autoradiography. No significant difference was observed at six weeks post-MI (MI: 1.1 ± 0.1%ID/g, n = 4 vs. SHAM: 0.8 ± 0.0%ID/g, n = 3), indicating infarct healing completion. In contrast, TAC mice showed increased uptake after six weeks (TAC: 1.8 ± 0.2%ID/g, n = 6; p = 0.007), related to interstitial fibrosis progression. Consistently, high-stretched cardiac fibroblasts demonstrated a higher uptake compared to low-stretched conditioned ones, suggesting the stretch mediates regulation of FAP. Conclusions: This study demonstrated the efficacy of [68Ga]Ga-DATA5m.SA.FAPi for longitudinal imaging of cardiac fibrosis in response to different cardiac injuries. In vivo FAP imaging during cardiac remodeling may serve as a valuable tool for diagnosing and predicting disease progression, ultimately aiding in the clinical management of patients.

FAPI-Targeted Molecular Imaging: Transforming Insights into Post-Ischemic Myocardial Remodeling?

Post-ischemic myocardial remodeling significantly impacts clinical outcomes after acute myocardial infarction (MI), involving structural and functional changes such as ventricular dilation, infarct wall thinning, and fibrosis development. These processes, driven by inflammatory cascades, neurohormonal activation, and extracellular matrix remodeling, result in impaired cardiac output and an increased risk of heart failure. Imaging with fibroblast activation protein inhibitors (FAPI) has emerged as a promising non-invasive tool for assessing myocardial fibrosis via positron emission tomography (PET) or single-photon emission computed tomography (SPECT), targeting activated fibroblasts; the mediators of reparative and fibrotic processes. This innovative approach enables precise visualization and quantification of fibrosis dynamics, surpassing traditional imaging modalities. Preclinical studies using [68Ga]Ga-FAPI PET/computed tomography (CT) demonstrated the tracer's specificity for fibroblast activation and its peak uptake in the infarct border zone at day 6 post-MI. These findings, corroborated by histology and autoradiography, highlight its potential for tracking reparative fibrosis. Clinical translation of FAPI imaging was recently achieved with [68Ga]Ga-FAPI-46 PET/magnetic resonance imaging (MRI), showing persistent fibroblast activity beyond infarct zones and strong correlations with myocardial injury markers. Complementary research on [99mTc]Tc-HFAPi SPECT imaging in patients post-MI established its predictive value for left ventricular remodeling, emphasizing its cost-effectiveness and accessibility compared with PET. These advancements underscore FAPI-based imaging's potential to transform risk stratification and therapeutic guidance in post-MI care.