PET imaging of fibroblast activation protein alpha (FAP) detects incipient cardiotoxicity due to anthracycline chemotherapy

ABSTRACT Background Anthracycline chemotherapy is associated with a risk of cardiotoxicity leading to heart disease, particularly in pediatric cancer patients. Gold standard methods of detecting cardiotoxicity are insufficiently sensitive to early damage and specific pathophysiologies driving disease. Positron emission tomography (PET) couples anatomical resolution with biochemical mechanistic selectivity and potentially addresses the current diagnostic limitations in cardio-oncology. We aimed to validate PET imaging biomarkers targeting fibroblast activation protein alpha (FAP), Translocator protein (TSPO), and norepinephrine receptor (NET) for detection of incipient anthracycline-induced cardiotoxicity. Methods Cardiotoxicity was established in male C57BL/6J mice by a cumulative dose of 24 mg/kg doxorubicin (DOX) over 2 weeks. DOX mice and their age-matched controls were imaged with echocardiography and PET, using [ 68 Ga]Ga-FAPI-04, [ 18 F]DPA-714, and [ 18 F]MFBG, over 12 weeks. Fractional shortening (FS) was determined from the echocardiograms, and cardiac uptake of the radioligands was quantified from the PET images. Heart sections were collected and used for the analysis of bulk RNA-seq, RT-qPCR, Western blot, in situ hybridization (ISH), and histopathological analysis. Results DOX mice exhibited cardiotoxicity and cardiac atrophy. Cardiac [ 68 Ga]Ga-FAPI-04 PET signal was significantly higher in DOX mice from 2 weeks through the study endpoint. By contrast, no cardiac dysfunction was evident by echocardiography until 10 weeks, at which point FS was significantly reduced in DOX mice. There were no differences in [ 18 F]DPA-714 and [ 18 F]MFBG signals. Transcription and translation of FAP, but not TSPO or NET, was detected in cardiomyocytes and were elevated in the DOX hearts, in agreement with the PET data. Genes related to cell adhesion and extracellular remodeling were significantly upregulated in the DOX mice relative to controls. Conclusions FAP is a sensitive and selective imaging biomarker for incipient cardiotoxicity and FAPI PET is a promising non-invasive imaging tool for identifying patients at risk of cardiotoxicity during or after anthracycline chemotherapy. GRAPHICAL ABSTRACT NOVELTY AND SIGNIFICANCE What is Known? Anthracycline chemotherapy results in cardiotoxicity for a sizeable population of treated patients. Cardiotoxocity manifests as cardiac dysfunction, and may result in long-term cardiac disease and heart failure, particularly in survivors of pediatric cancer. Cardiotoxicity is typically defined in terms of left ventricular ejection fraction (LVEF) deficits, as measured by echocardiography. However, this metric is often poorly sensitive to early disease and agnostic to underlying pathophysiology. Early treatment of cardiotoxicity improves recovery and long-term survival, emphasizing the need for accurate diagnostics in incipient disease. What New Information Does This Article Contain? [ 68 Ga]Ga-FAPI-04 accumulates in the hearts of mice experiencing doxorubicin-induced cardiotoxicity as a function of fibroblast activation protein alpha (FAP) expression and activity. By contrast, cardiac uptake of radioligands targeting the translocator protein 18-kDa (TSPO) and the norepinephrine transporter (NET) do not differ between DOX animals and controls. Positron emission tomography (PET) imaging following administration of [ 68 Ga]Ga-FAPI-04 detects abnormal cardiac remodeling significantly earlier than LVEF decrease is observed, indicating that it may be more sensitive to incipient disease. Our study identifies fibroblast activation protein alpha (FAP) as a promising diagnostic imaging biomarker in anthracycline-induced cardiotoxicity. We show that cardiac PET signal increases immediately after doxorubicin treatment, and the signal increase is sustained for at least 10 weeks. In addition, we demonstrate that FAP inhibitor (FAPI) PET correlates with expression of FAP protein and gene. Thus, we provide mechanistic insight into potentially-treatable pathophysiologies driving cardiac atrophy and toxicity, and have identified a translational PET tracer that can image the activation of these processes at an early stage.