PT - JOURNAL ARTICLE AU - Annika Hess AU - Jochen Tillmanns AU - Tobias Ross AU - Frank Bengel AU - James Thackeray TI - Preliminary characterization of <sup>68</sup>Ga-FAPI-46 for molecular imaging of cardiac fibroblast activation <strong/> DP - 2021 May 01 TA - Journal of Nuclear Medicine PG - 134--134 VI - 62 IP - supplement 1 4099 - http://jnm.snmjournals.org/content/62/supplement_1/134.short 4100 - http://jnm.snmjournals.org/content/62/supplement_1/134.full SO - J Nucl Med2021 May 01; 62 AB - 134Objectives: Cardiac injury including myocardial infarction (MI) precipitates the activation and transdifferentiation of quiescent cardiac fibroblasts, leading to replacement and reactive fibrosis. Excessive fibrosis and collagen deposition contribute to adverse ventricular remodeling and may be associated with worse functional outcome. Early identification of fibroblast activation may provide prognostic information for heart failure progression. Here, we characterized the tracer 68Ga-FAPI-46 for imaging cardiac fibroblast activation protein (FAP) expressed by activated myofibroblasts. Methods: In vitro uptake assays assessed accumulation of 68Ga-FAPI-46 (kindly provided by Univ. of Heidelberg) in HT1080 fibrosarcoma cells including wildtype (WT), and cells overexpressing either human FAP (hFAP) or mouse FAP (mFAP). Blocking specificity was confirmed by co-incubation with 1nM unlabeled compound. Uptake was further characterized in adult human left ventricle cardiac fibroblasts (HCF) with and without blocking agent. Preliminary positron emission tomography imaging determined cardiac FAP expression in healthy control C57BL/6N mice (n=5), and serially after permanent left coronary artery ligation (MI, n=16) at 7d and 21d after MI. 68Ga-FAPI-46 (13±2MBq) was administered and static PET scans were acquired over 50-60min after injection. Co-injection of 30nmol cold compound measured in vivo tracer specificity in a subgroup of MI mice (n=6). Ex vivo autoradiography and Masson trichrome histology validated in vivo results. Results: In vitro assays in HT1080 cells demonstrated high uptake in both hFAP and mFAP overexpressing cells compared to WT (%Uptake; hFAP: 7.77±5.82; mFAP: 36.30±8.94; vs WT: 0.22±0.18, p&lt;0.01). Co-incubation with cold compound markedly lowered tracer accumulation to the level of WT cells in hFAP (0.26±0.18, p=0.01) and mFAP (0.49±0.41, p&lt;0.001), indicating tracer binding specificity. In basal unstimulated adult HCF, 68Ga-FAPI-46 uptake was relatively low, but was significantly reduced in the presence of blocking agent (0.36±0.10 vs 0.07±0.05, p=0.001), consistent with low FAP expression in quiescent fibroblasts. Evaluation in stimulated adult HCF is in progress. Imaging in healthy control mice showed low background tracer accumulation in myocardial regions and liver, and displayed rapid renal clearance. At 7d post-MI, infarct FAP expression tended to be higher compared to control mice (%ID/g; 0.95±0.36 vs 0.71±0.12, p=0.18), which declined by 21d (%ID/g; 0.75±0.24 vs 0.71±0.12, p=0.71), consistent with the timeframe of FAP upregulation. Co-injection of unlabeled precursor lowered the infarct FAP signal on d7 (%ID/g; 0.95±0.36 vs 0.58±0.33, p=0.14). Localization of the signal was confirmed by ex vivo autoradiography, wherein the FAP signal exceeded the Masson trichrome derived infarct area, suggesting sensitivity to replacement and reactive fibrosis. Conclusions: The molecular imaging radiotracer 68Ga-FAPI-46 binds selectively to human and mouse FAP. Rapid renal clearance, low background, and specific binding after MI support feasibility of FAP imaging for cardiac fibroblast activation in mice, but the low absolute uptake in mice may complicate quantitative measurements in vivo.