Fibroblast activation protein imaging in reperfused ST-elevation myocardial infarction: comparison with cardiac magnetic resonance imaging

99mTc-HFAPi SPECT Imaging Predicts Left Ventricular Remodeling After Acute Myocardial Infarction

AIMS

This study aimed to evaluate the predictive utility of 99mTc-radiolabeled fibroblast activation protein inhibitor (99mTc-HFAPi) single-photon emission computed tomography (SPECT) imaging in post- acute myocardial infarction (AMI) patients for assessing 12-month left ventricular (LV) remodeling.

METHODS

A cohort of 58 AMI patients (46 males, median age 61 [53, 67] years) underwent baseline 99mTc-HFAPi imaging (5 ± 2 days post-MI), perfusion imaging (6 ± 2 days post-MI), and echocardiography (2 ± 2 days post-MI). Additionally, 15 patients had follow-up 99mTc-HFAPi and perfusion imaging, while 30 patients had follow-up echocardiography. Myocardial 99mTc-HFAPi activity was assessed at patient level. LV remodeling was defined as a ≥10% increase in LV end-diastolic diameter (LVEDD) or LV end-systolic diameter (LVESD) from baseline to follow-up echocardiography.

RESULTS

AMI patients displayed localized but non-uniform 99mTc-HFAPi uptake, exceeding perfusion defects. Baseline 99mTc-HFAPi activity exhibited significant correlations with BNPmax, LDHmax, cTNImax, and WBCmax, inversely correlating with LVEF. After 12 months, 11 patients (36.66%) experienced LV remodeling. Univariate regression analysis demonstrated an association between baseline 99mTc-HFAPi uptake extent and LV remodeling (OR= 2.14, 95%CI, 1.04, 4.39, p=0.038).

CONCLUSIONS

99mTc-HFAPi SPECT imaging holds promise in predicting LV remodeling post-MI, providing valuable insights for patient management and prognosis.

LAY SUMMARY

Our study introduces 99mTc-radiolabeled fibroblast activation protein inhibitor (99mTc-HFAPi) single-photon emission computed tomography (SPECT) imaging as a potentially cost-effective modality for evaluating fibrotic process in the context of post-AMI LV remodeling. The study establishes a positive correlation between 99mTc-HFAPi activity, particularly 99mTc-HFAPi uptake extent, and LV remodeling, suggesting 99mTc-HFAPi SPECT imaging as a promising tool for risk prediction in post-AMI patients. The findings from this study have the potential to revolutionize post-AMI patient management by enabling early identification of those at risk for adverse LV remodeling. This identification could pave the way for tailored interventions, potentially improving clinical outcomes and reducing the development of heart failure.

Myocardial work by pressure-strain loop is associated with molecular imaging of fibroblast activation in hypertensive hearts using 99mTc-HFAPI SPECT

99mTc-HFAPI can visualize fibroblast activation in hypertensive hearts. Myocardial work (MW) reflects the cardiac mechanical properties after accounting for the afterload in hypertensive patients. We investigated whether MW was associated with increased uptake of 99mTc-HFAPI. A total of 97 hypertensive patients and 41 healthy volunteers were prospectively recruited. Global work index (GWI), global constructive work (GCW), global wasted work (GWW) and global work efficiency (GWE) were analyzed. According to whether myocardial uptake of FAPI was higher than the adjacent blood pool, hypertensive patients were divided into two groups, namely: FAPI + and FAPI- group, respectively. GWI, GCW and GWE of the FAPI + group were lower than the FAPI- group. The value of GWW in the FAPI + group was higher than in the FAPI- group. Multiple regression analyses revealed GWI, GWW and GWE were independently associated with early myocardial fibrosis. According to receiver operating characteristics (ROC) analysis, the best cutoff points for FAPI + of GWI, GWW and GWE were 1968.50 mmHg% (AUC: 0.687, 95% CI: 0.581-0.793, P = 0.002), 133.00 mmHg% (AUC: 0.778, 95% CI: 0.688-0.869, P < 0.001) and 95.07% (AUC: 0.813, 95% CI: 0.730-0.896, P < 0.001), respectively. GWI, GWW and GWE were impaired in hypertensive patients with cardiac 99mTc-HFAPI uptake and were associated with fibroblast activation in hypertensive hearts.

Diagnostic and Therapeutic Application of Fibroblast Activation Protein Inhibitors in Oncologic and Nononcologic Diseases

ABSTRACT

Fibroblast activation protein inhibitor positron emission tomography (PET) has gained interest for its ability to demonstrate uptake in a diverse range of tumors. Its molecular target, fibroblast activation protein, is expressed in cancer-associated fibroblasts, a major cell type in tumor microenvironment that surrounds various types of cancers. Although existing literature on FAPI PET is largely from single-center studies and case reports, initial findings show promise for some cancer types demonstrating improved imaging when compared with the widely used 18F-fludeoxyglucose PET for oncologic imaging. As we expand our knowledge of the utility of FAPI PET, accurate understanding of noncancerous uptake seen on FAPI PET is crucial for accurate evaluation. In this review, we summarize potential diagnostic and therapeutic applications of radiolabeled FAP inhibitors in oncological and nononcological disease processes.

Tissue-level evidence of fibroblast activation protein inhibitor imaging in hypertrophic obstructive cardiomyopathy: a case series

Background

Myocardial fibrosis is a key pathological factor for the occurrence of ventricular arrhythmias in hypertrophic obstructive cardiomyopathy (HOCM).

Case summary

This case series reports on two patients diagnosed with HOCM who underwent 18F-fibroblast activation protein inhibitor (FAPI) positron-emission tomography/computed tomography imaging and Morrow myotomy procedure. The collected myocardial tissue was examined histopathologically. Both patients exhibited intense and heterogeneous 18F-FAPI uptake in the septum, with significant number of activated fibroblasts.

Discussion

Enhanced 18F-FAPI uptake was observed before irreversible fibrosis, and the degree of 18F-FAPI uptake was higher in tissue with greater fibrosis. 18F-FAPI imaging may provide a promising tool for guiding surgical strategy in HOCM, and further research is needed to fully explore its potential in clinical practice.

68Ga-DOTA-D-Alanine-BoroPro Radiotracer for Imaging of the Fibroblast Activation Protein in Malignant and Non-Malignant Diseases

Recently, we reported a new fibroblast activation protein (FAP) inhibitor radiopharmaceutical based on the 99mTc-((R)-1-((6-hydrazinylnicotinoyl)-D-alanyl) pyrrolidin-2-yl) boronic acid (99mTc-HYNIC-D-Alanine-BoroPro)(99mTc-HYNIC-iFAP) structure for tumor microenvironment SPECT imaging. This research aimed to synthesize 68Ga-[2,2',2″,2‴-(2-(4-(2-(5-(((S)-1-((S)-2-boronopyrrolidin-1-yl)-1-oxopropan-2-yl)carbamoyl)pyridin-2-yl)hydrazine-1-carbothioamido)benzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid] (68Ga-DOTA-D-Alanine-BoroPro)(68Ga-iFAP) as a novel radiotracer for PET imaging and evaluate its usefulness for FAP expression in malignant and non-malignant tissues. The coupling of p-SCN-benzene DOTA with HYNIC-iFAP was used for the chemical synthesis and further labeling with 68Ga. Radiochemical purity was verified by radio-HPLC. The specificity of 68Ga-iFAP was evaluated in HCT116 cells, in which FAP expression was verified by immunofluorescence and Western blot. Biodistribution and biokinetic studies were performed in murine models. 68Ga-iFAP uptake at the myocardial level was assessed in mice with induced infarction. First-in-human images of 68Ga-iFAP in healthy subjects and patients with myocardial infarction, glioblastoma, prostate cancer, and breast cancer were also obtained. DOTA-D-Alanine BoroPro was prepared with a chemical purity of 98% and was characterized by UPLC mass spectroscopy, FT-IR, and UV-vis. The 68Ga-iFAP was obtained with a radiochemical purity of >95%. In vitro and in vivo studies demonstrated 68Ga-iFAP-specific recognition for FAP, rapid renal elimination, and adequate visualization of the glioblastoma, breast tumor, prostate cancer, and myocardial infarction sites. The results of this research justify further dosimetry and clinical trials to establish the specificity and sensitivity of 68Ga-iFAP PET for FAP expression imaging.

Molecular Imaging of Heart Failure: An Update and Future Trends

Molecular imaging can detect and quantify pathophysiological processes underlying heart failure, complementing evaluation of cardiac structure and function with other imaging modalities. Targeted tracers have enabled assessment of various cellular and subcellular mechanisms of heart failure aiming for improved phenotyping, risk stratification, and personalized therapy. This review outlines the current status of molecular imaging in heart failure, accompanied with discussion on novel developments. The focus is on radionuclide methods with data from clinical studies. Imaging of myocardial metabolism can identify left ventricle dysfunction caused by myocardial ischemia that may be reversible after revascularization in the presence of viable myocardium. In vivo imaging of active inflammation and amyloid deposition have an established role in the detection of cardiac sarcoidosis and transthyretin amyloidosis. Innervation imaging has well documented prognostic value in predicting heart failure progression and arrhythmias. Tracers specific for inflammation, angiogenesis and myocardial fibrotic activity are in earlier stages of development, but have demonstrated potential value in early characterization of the response to myocardial injury and prediction of cardiac function over time. Early detection of disease activity is a key for transition from medical treatment of clinically overt heart failure towards a personalized approach aimed at supporting repair and preventing progressive cardiac dysfunction.

Imaging of Cardiac Fibrosis: How Far Have We Moved From Extracellular to Cellular?

Cardiovascular disease is the leading cause of morbidity and mortality worldwide. Myocardial fibrosis plays an important role in adverse outcomes such as heart failure and arrhythmias. As the pathological response and degree of scarring, and therefore clinical presentation varies from patient to patient, early detection of fibrosis is crucial for identifying the appropriate treatment approach and forecasting the progression of a disease along with the likelihood of disease-related mortality. Current imaging modalities provides information about either decreased function or extracellular signs of fibrosis. Targeting activated fibroblasts represents a burgeoning approach that could offer insights prior to observable functional alterations, presenting a promising focus for potential anti-fibrotic therapeutic interventions at cellular level. In this article, we provide an overview of imaging cardiac fibrosis and discuss the role of different advanced imaging modalities with the focus on novel non-invasive imaging of activated fibroblasts.

Case Report: A case report of myocardial fibrosis activation assessment after unstable angina using 68Ga-FAPI-04 PET/CT

Myocardial ischemia may induce myocardial fibrosis, a condition that progressively leads to ventricular remodeling, heightening the risk of heart failure. The timely detection of myocardial fibrosis is crucial for intervention and improved outcomes. 68Ga-FAPI-04 PET/CT shows promise in assessing fibroblast activation in patients with early myocardial infarction characterized by prolonged myocardial ischemia. However, there is a notable absence of data regarding patients with short-term myocardial ischemia, such as those experiencing unstable angina (UA). In this report, we evaluated a 49-year-old male with UA and severe stenosis in multiple coronary arteries using 68Ga-FAPI-04 PET/CT. The results demonstrated tracer-specific uptake (SUVmax = 4.6) in the left anterior descending artery (LAD) territory, consistent with myocardial anterior wall ischemia indicated by the electrocardiogram. Following vascular recanalization therapy and regular medication treatment, the patient remained free of angina recurrence. A subsequent review at 2 months revealed a significant reduction in myocardial tracer uptake (SUVmax = 1.8). This case illustrates the validity of 68Ga-FAPI-04 PET/CT in assessing the extent of early myocardial fibroblast activation in patients with UA. This approach offers valuable insights for early detection and visual evidence, providing information on disease progression and treatment response.

Recent topics in fibroblast activation protein inhibitor-PET/CT: clinical and pharmacological aspects

Recently, positron emission tomography (PET) with fibroblast activation protein inhibitor (FAPI) has gained significant attention as an advanced tumor diagnostic imaging tool. FAPI PET has a promising potential owing to its ability to accurately depict most malignant tumors. It has an accuracy that is comparable to or surpassing the diagnostic accuracy of PET using 18F-fluorodeoxyglucose (FDG). Moreover, FAPI PET can identify malignant lesions that may be inconclusive on FDG PET. Beyond its application in neoplastic disorders, there have been encouraging reports suggesting the utility of FAPI PET in non-neoplastic conditions such as respiratory or cardiac diseases. This article aimed to provide a comprehensive overview of the recently published articles investigating FAPI and discuss its clinical utility with an emphasis on its application in tumor diagnostics. Numerous radiopharmaceutical FAPIs, including 18F- and 68Ga-labeled compounds, have been developed, and they offer various advantages and applications. With the progress in the FAPI PET synthesis to enhance accumulation and retention in pathological lesions, future studies are expected to provide valuable data on its therapeutic efficacy.

Fibroblast activation protein: Pivoting cancer/chemotherapeutic insight towards heart failure

An important mechanism for cancer progression is degradation of the extracellular matrix (ECM) which is accompanied by the emergence and proliferation of an activated fibroblast, termed the cancer associated fibroblast (CAF). More specifically, an enzyme pathway identified to be amplified with local cancer progression and proliferation of the CAF, is fibroblast activation protein (FAP). The development and progression of heart failure (HF) irrespective of the etiology is associated with left ventricular (LV) remodeling and changes in ECM structure and function. As with cancer, HF progression is associated with a change in LV myocardial fibroblast growth and function, and expresses a protein signature not dissimilar to the CAF. The overall goal of this review is to put forward the postulate that scientific discoveries regarding FAP in cancer as well as the development of specific chemotherapeutics could be pivoted to target the emergence of FAP in the activated fibroblast subtype and thus hold translationally relevant diagnostic and therapeutic targets in HF.

Fibroblast activation protein imaging in atrial fibrillation: a proof-of-concept study

BACKGROUND

To evaluate the feasibility of using radiolabeled fibroblast activation protein inhibitor (FAPI) PET/CT imaging to assess activated fibroblasts in the atria of individuals with AF and to identify factors contributing to enhanced atrial activity.

METHODS

We constructed left atrial appendage (LAA) pacing beagle dog AF models (n = 5) and conducted 18F-FAPI PET/CT imaging at baseline and eight weeks after pacing. Right atrial (RA) specimens were collected from these models. Additionally, 28 AF patients and ten age- and sex-matched healthy volunteers underwent 18F-FAPI PET/CT imaging.

RESULTS

RA of AF beagles showed increased 18F-FAPI uptake. Among AF patients, 18 out of 28 (64.3%) exhibited enhanced atrial FAPI activity. No atrial 18F-FAPI uptake was observed in the sham beagle and healthy volunteers. In animal RA specimens, 18F-FAPI activity correlated positively with FAP mRNA (r = .98, P = .002) and protein (r = .82, P = .03) levels, as well as collagen I mRNA expression (r = .85, P = .02). B-type natriuretic peptide levels were associated with atrial 18F-FAPI activity (OR = 3.01, P = .046).

CONCLUSION

This proof-of-concept study suggests that 18F-FAPI PET/CT imaging may be a feasible method for evaluating activated fibroblasts in the atria of AF patients.

Functional significance of myocardial activity at 18F-FAPI PET/CT in hypertrophic cardiomyopathy identified by cardiac magnetic resonance feature-tracking strain analysis

PURPOSE

This study aimed to evaluate the functional significance of 18F-labeled fibroblast activation protein inhibitor (18F-FAPI) activity in hypertrophic cardiomyopathy (HCM) by comparison with cardiac magnetic resonance feature-tracking (CMR-FT) strain analysis.

METHODS

A total of 49 HCM patients were included in this study. Two independent control groups of healthy participants with a matched age and sex to the HCM patients were also enrolled. Left ventricular (LV) 18F-FAPI activity was analyzed for extent (FAPI%) and intensity (maximum target-to-background ratio, TBRmax). The CMR tissue characterization parameters of the LV included late gadolinium enhancement, native T1 value, and extracellular volume fraction. LV strain analysis was performed in radial, circumferential, and longitudinal peak strains (PS).

RESULTS

Intense LV myocardial 18F-FAPI uptake was observed in HCM patients, whereas no obvious uptake was detected in healthy participants (median TBRmax, 9.1 vs. 1.2, p < 0.001). The strain parameters of HCM patients, compared with healthy participants, were significantly impaired (mean radial PS, 23.5 vs. 36.0, mean circumferential PS, -14.5 vs. -20.0, and mean longitudinal PS, -9.9 vs. -16.0, all p < 0.001). At segmental levels, there was a moderate correlation between 18F-FAPI activity and strain parameters. The number of positive 18F-FAPI uptake segments (n = 653) was higher than that of hypertrophic segments (n = 190) and positive CMR tissue characterization segments (n = 525) (all p < 0.001). In segments with negative CMR tissue characterization findings, the strain capacity of positive 18F-FAPI uptake segments was lower than that of negative 18F-FAPI uptake segments (median radial PS, 30.5 vs. 36.1, p = 0.026 and median circumferential PS, -18.4 vs. -19.7, p = 0.041).

CONCLUSION

18F-FAPI imaging can partially reflect the potential strain reduction in HCM patients. 18F-FAPI imaging detects more involved myocardium than CMR tissue characterization techniques, and the additionally identified myocardium has impaired strain capacity.

99mTc-HFAPi imaging identifies early myocardial fibrosis in the hypertensive heart

BACKGROUND

This study aimed to explore whether 99mTc-radiolabeled fibroblast activation protein inhibitor (99mTc-HFAPi) imaging can detect early myocardial fibrosis in the hypertensive heart.

METHODS

In the experimental model, spontaneously hypertensive rats (SHRs) and age-matched Wistar Kyoto rats (WKYs) were randomly divided into three groups (8, 16, and 28 weeks). The animals underwent 99mTc-HFAPi imaging and echocardiography. Autoradiography and histological analyses were performed in the left ventricle. The mRNA and protein expression level of the fibroblast activation protein (FAP) and collagen I were measured using quantitative PCR and western blot. In the clinical investigation, a total of 106 patients with essential hypertension and 20 gender-matched healthy controls underwent 99mTc-HFAPi imaging and echocardiography.

RESULTS

In-vivo and in-vitro autographic images demonstrated diffusely enhanced 99mTc-HFAPi uptake in the SHR heart starting at week 8, before irreversible collagen deposition. The mRNA and protein levels of FAP in SHRs began to increase from week 8, whereas changes in collagen I levels were not detected until week 28. In the clinical investigation, even in hypertensive patients with normal diastolic indicators, normal left ventricular geometry, and normal global longitudinal strain (GLS), the prevalence of increased 99mTc-HFAPi uptake reached 34, 41, and 20%, respectively, indicating that early fibrogenesis precedes structural and functional myocardial abnormalities.

CONCLUSION

In hypertension, 99mTc-HFAPi imaging can detect early fibrotic process before myocardial functional and structural changes.

Highlighting Fibroblasts Activation in Fibrosis: The State-of-The-Art Fibroblast Activation Protein Inhibitor PET Imaging in Cardiovascular Diseases

Fibrosis is a common healing process that occurs during stress and injury in cardiovascular diseases. The evolution of fibrosis is associated with cardiovascular disease states and causes adverse effects. Fibroblast activation is responsible for the formation and progression of fibrosis. The incipient detection of activated fibroblasts is important for patient management and prognosis. Fibroblast activation protein (FAP), a membrane-bound serine protease, is almost specifically expressed in activated fibroblasts. The development of targeted FAP-inhibitor (FAPI) positron emission tomography (PET) imaging enabled the visualisation of FAP, that is, incipient fibrosis. Recently, research on FAPI PET imaging in cardiovascular diseases increased and is highly sought. Hence, we comprehensively reviewed the application of FAPI PET imaging in cardiovascular diseases based on the state-of-the-art published research. These studies provided some insights into the value of FAPI PET imaging in the early detection of cardiovascular fibrosis, risk stratification, response evaluation, and prediction of the evolution of left ventricular function. Future studies should be conducted with larger populations and multicentre patterns, especially for response evaluation and outcome prediction.

FAPI PET/CT Imaging-An Updated Review

Despite revolutionizing the field of oncological imaging, Positron Emission Tomography (PET) with [¹⁸F]Fluorodeoxyglucose (FDG) as its workhorse is limited by a lack of specificity and low sensitivity in certain tumor subtypes. Fibroblast activation protein (FAP), a type II transmembrane glycoprotein, is expressed by cancer-associated fibroblasts (CAFs) that form a major component of the tumor stroma. FAP holds the promise to be a pan-cancer target, owing to its selective over-expression in a vast majority of neoplasms, particularly epithelial cancers. Several radiolabeled FAP inhibitors (FAPI) have been developed for molecular imaging and potential theranostic applications. Preliminary data on FAPI PET/CT remains encouraging, with extensive multi-disciplinary clinical research currently underway. This review summarizes the existing literature on FAPI PET/CT imaging with an emphasis on diagnostic applications, comparison with FDG, pitfalls, and future directions.

Pulmonary artery imaging with 68 Ga-FAPI-04 in patients with chronic thromboembolic pulmonary hypertension

BACKGROUND

The feasibility and significance of imaging pulmonary artery (PA) remodeling with 68 Ga-fibroblast activating protein inhibitor (FAPI) in patients with chronic thromboembolic pulmonary hypertension (CTEPH) have not yet been addressed.

METHODS

68 Ga-FAPI-04 uptake in the PA and ascending artery was evaluated in 13 patients with CTEPH and 13 matched non-CTEPH controls. The correlations of PA 68 Ga-FAPI-04 uptake and remodeling parameters derived from right heart catheterization (RHC) were analyzed.

RESULTS

Of the 13 patients with CTEPH, nine (69%) showed visually enhanced 68 Ga-FAPI-04 uptake, whereas none of the control subjects had increased 68 Ga-FAPI-04 uptake in the PA. The prevalence of enhanced uptake in the main, lobar, and segmental PAs was 45% (17/38), 33% (16/48), and 28% (44/159), respectively. 68 Ga-FAPI-04 activity in the PA was positively correlated with pulmonary arterial diastolic pressure (r = 0.571, P = 0.041).

CONCLUSION

68 Ga-FAPI-04 has the potential for imaging fibroblast activation in the PA wall, and 68 Ga-FAPI-04 activity in PA is positively correlated with pulmonary arterial diastolic pressure.

The Potential of Fibroblast Activation Protein-Targeted Imaging as a Biomarker of Cardiac Remodeling and Injury

PURPOSE OF REVIEW

Cardiovascular disease features adverse fibrotic processes within the myocardium, leading to contractile dysfunction. Activated cardiac fibroblasts play a pivotal role in the remodeling and progression of heart failure, but conventional diagnostics struggle to identify early changes in cardiac fibroblast dynamics. Emerging imaging methods visualize fibroblast activation protein (FAP) as a marker of activated fibroblasts, enabling non-invasive quantitative measurement of early cardiac remodeling.

RECENT FINDINGS

Retrospective analysis of oncology patient cohorts has identified cardiac uptake of FAP radioligands in response to various cardiovascular conditions. Small scale studies in dedicated cardiac populations have revealed FAP upregulation in injured myocardium, wherein the area of upregulation predicts subsequent ventricle dysfunction. Recent studies have demonstrated that silencing of FAP-expressing fibroblasts can reverse cardiac fibrosis in disease models. The parallel growth of FAP-targeted imaging and therapy provides the opportunity for imaging-based monitoring and refinement of treatments targeting cardiac fibroblast activation.

Post-myocardial infarction fibrosis: Pathophysiology, examination, and intervention

Cardiac fibrosis plays an indispensable role in cardiac tissue homeostasis and repair after myocardial infarction (MI). The cardiac fibroblast-to-myofibroblast differentiation and extracellular matrix collagen deposition are the hallmarks of cardiac fibrosis, which are modulated by multiple signaling pathways and various types of cells in time-dependent manners. Our understanding of the development of cardiac fibrosis after MI has evolved in basic and clinical researches, and the regulation of fibrotic remodeling may facilitate novel diagnostic and therapeutic strategies, and finally improve outcomes. Here, we aim to elaborate pathophysiology, examination and intervention of cardiac fibrosis after MI.

Current research topics in FAPI theranostics: a bibliometric analysis

PURPOSE

The study aimed to provide a comprehensive bibliometric overview of the current scientific publications on fibroblast activation protein inhibitor (FAPI) positron emission tomography imaging and radionuclide therapy.

METHODS

A PubMed search was performed to identify all MEDLINE-indexed publications on FAPI imaging and radionuclide therapy. The last update was performed on 31 May 2022. An online database of this literature was created, and hierarchical topic-related tags were subsequently assigned to all relevant studies. Frequency analysis was used to evaluate the distribution of the following characteristics: first author's country of origin, journal of publication, study design, imaging techniques and radiopharmaceutical used, histopathological correlation, the type of cancer, and benign disease/uptake types evaluated.

RESULTS

A total of 294 relevant publications on original studies were identified, consisting of 209 (71%) case reports/series and 85 cohort studies (29%). The majority of studies focused on imaging topics, predominantly comparing uptake on FAPI-PET/CT with 2-[¹⁸F]FDG-PET/CT, anatomical imaging, and/or histopathology results. 68% of studies focused on malignancies, with gastro-intestinal cancer, hepato-pancreato-biliary cancer, mixed cancers/metastases, lung cancer, sarcoma, head and neck cancer, and breast cancer being the most frequently reported. 42% of studies focused on benign disease categories, with cardiovascular, musculoskeletal, HPB, head and neck, and IgG4-related disease as most common categories. 16/294 (5%) studies focused on radionuclide therapy, with preliminary reports of acceptable toxicity profiles, tumour activity retention, and suggestion of disease control.

CONCLUSION

FAPI research is rapidly expanding from diagnostic studies in malignancies and benign diseases to the first reports of salvage radionuclide therapy. The research activity needs to shift now from low-level-of-evidence case reports and series to prospectively designed studies in homogenous patient groups to provide evidence on how and in which clinical situations FAPI theranostics can be of added value to clinical care. We have provided an overview of current research topics to build upon.

Non-oncologic incidental uptake on FAPI PET/CT imaging

Fibroblast-activation protein (FAP) is a serine protease classified in the dipeptidyl peptidase 4 (DPP4) family. FAP is predominantly expressed in activated fibroblasts such as the cancer-associated fibroblasts (CAFs). FAP expression in CAFs is associated with tumor progression and poor prognosis in solid cancers. Recently, radiolabeled FAP inhibitors (FAPI) has been developed, which enables positron emission tomography (PET) imaging of FAP. FAPI PET/CT can provide a higher tumor-to-background ratio (TBR) than 18F-fludeoxyglucose PET/CT in various cancers, and thus has attracted substantial attention. As studies on FAPI PET grow in number and size, incidental findings related to non-oncologic conditions have been increasingly reported. FAPI PET uptake has been reported in various conditions such as benign tumors, fibrotic, granulomatosis, scarring/wound, degenerative diseases, and inflammatory diseases.The knowledge of physiological and non-oncologic causes of FAPI uptake is indispensable for accurate FAPI PET/CT interpretation and can help appropriate management of incidental findings on FAPI PET/CT in patients referred for cancer staging indications. In this review article, we describe for each organ system (Brain, Oral mucosa, Salivary Glands, Thyroid, Lung, Myocardium, Breast, Esophagus, Stomach, Intestine, Liver, Gallbladder, Pancreas, Spleen, Kidney, , Uterus, Bone marrow, Joints, Muscle, Vessels, Lymph nodes), the patterns of physiological FAPI uptake and the main causes of non-oncological uptake reported from the literature with FAPI-02, FAPI-04 and FAPI-46. We also illustrate some examples from our institutional database at UCLA.

Myocardial Activity at 18F-FAPI PET/CT and Risk for Sudden Cardiac Death in Hypertrophic Cardiomyopathy

Background Myocardial fibrosis contributes to adverse cardiovascular events in hypertrophic cardiomyopathy (HCM). Purpose To explore the characteristics of cardiac fibroblast activation protein inhibitor (FAPI) PET/CT imaging and its relationship with the risk of sudden cardiac death (SCD) in HCM. Materials and Methods In this prospective study from July 2021 to January 2022, participants with HCM and healthy control participants underwent cardiac fluorine 18 (¹⁸F)-labeled FAPI PET/CT imaging. Myocardial FAPI activity was quantified as intensity (target-to-background uptake ratio), extent (the percent of FAPI-avid myocardium of the left ventricle [LV]), and amount (the percent of FAPI-avid myocardium of LV × target-to-background ratio). Regional wall thickness was analyzed at cardiac MRI. The 5-year SCD risk score was calculated from the 2014 European Society of Cardiology guidelines. Univariable and multivariable linear regression analyses were used to identify factors related to the FAPI amount. The correlation between FAPI amount and 5-year SCD risk was explored. Results Fifty study participants with HCM (mean age, 43 years ± 13 [SD]; 32 men) and 22 healthy control participants (mean age, 45 years ± 17; 14 men) were included. All participants with HCM had intense and inhomogeneous cardiac FAPI activity in the LV myocardium that was higher than that in healthy control participants (median target-to-background ratio, 8.8 vs 2.1, respectively; P < .001). In HCM, more segments with FAPI activity were detected than the number of hypertrophic segments (median, 14 vs five, respectively; P < .001); 84% of nonhypertrophic segments showed FAPI activity. Log-transformed FAPI amount had a positive relationship with log-transformed N-terminal probrain natriuretic peptide, high-sensitive troponin I, and left atrial diameter and a negative relationship with LV ejection fraction z-score. Degree of FAPI activity positively correlated with the 5-year SCD risk score (r = 0.32; P = .03). Conclusion Fibroblast activation protein inhibitor (FAPI) PET/CT imaging indicated intense and heterogeneous activity in hypertrophic cardiomyopathy, and FAPI uptake was associated with 5-year risk of sudden cardiac death. © RSNA, 2022 Online supplemental material is available for this article.

[68Ga]Ga-DOTA-FAPI-04 PET/MR in patients with acute myocardial infarction: potential role of predicting left ventricular remodeling

PURPOSE

To assess predictive value of ⁶⁸Ga-labeled fibroblast activation protein inhibitor-04 ([⁶⁸Ga]Ga-DOTA-FAPI-04) PET/MR for late left ventricular (LV) remodeling in patients with ST-segment elevated myocardial infarction (STEMI).

METHODS

Twenty-six patients with STEMI were included in the study. [⁶⁸Ga]Ga-DOTA-FAPI-04 PET/MR was performed at baseline and at average 12 months after STEMI. LV remodeling was defined as >10% increase in LV end-systolic volume (LVESV) from baseline to 12 months.

RESULTS

The LV remodeling group demonstrated higher [⁶⁸Ga]Ga-DOTA-FAPI-04 uptake volume (UV) at baseline than the non-LV remodeling group (p < 0.001). [⁶⁸Ga]Ga-DOTA-FAPI-04 UV at baseline was a significant predictor (OR = 1.048, p = 0.011) for LV remodeling at 12 months after STEMI. Compared to clinical information, MR imaging and cardiac function parameters at baseline, [⁶⁸Ga]Ga-DOTA-FAPI-04 UV demonstrated better predictive ability (AUC = 0.938, p < 0.001) for late LV remodeling, with sensitivity of 100.0% and specificity of 81.3%.

CONCLUSIONS

[⁶⁸Ga]Ga-DOTA-FAPI-04 PET/MR is an effective tool to non-invasively quantify myocardial fibroblasts activation, and baseline [⁶⁸Ga]Ga-DOTA-FAPI-04 UV may have potential predictive value for late LV remodeling.

Fibroblast Activation Protein Inhibitor Imaging in Nonmalignant Diseases: A New Perspective for Molecular Imaging

Fibroblast activation protein-α (FAP-α) is a type II transmembrane glycoprotein that is overexpressed in activated fibroblasts such as those in the stroma of tumors or in the fibrotic processes accompanying various benign diseases. The recent development and clinical implementation of radiolabeled quinolone-based tracers suitable for PET that act as FAP inhibitors (FAPIs) have opened a new perspective in molecular imaging. Although multiple studies have investigated the use of FAPI imaging in cancer, evidence concerning its use in nonmalignant diseases is still scarce. Herein, we provide a comprehensive review of FAPI imaging in nonmalignant diseases to clarify the current and potential role of this class of molecules in nuclear medicine.

Feasibility of 68Ga-Labeled Fibroblast Activation Protein Inhibitor PET/CT in Light-Chain Cardiac Amyloidosis

BACKGROUND

Systemic amyloid light chain (AL) amyloidosis is the most common type of amyloidosis, leading to cardiomyocyte necrosis and interstitial fibrosis. Gallium-68-labeled fibroblast activation protein inhibitor 04 (⁶⁸Ga-FAPI-04) has recently been introduced for imaging fibroblast activation in cardiac diseases. To date, cardiac fibroblast and cardiac amyloidosis (CA) phenotype activities have not been mapped.

OBJECTIVES

The aim of this study was to evaluate the feasibility of ⁶⁸Ga-FAPI-04 positron emission tomography (PET)/computed tomography (CT) in assessing AL CA.

METHODS

Thirty consecutive patients (mean age: 59.1 ± 7.7 years; 20 men, 10 women) with biopsy-proven AL amyloidosis were enrolled prospectively (including 27 with AL CA and 3 without AL CA). All patients underwent ⁶⁸Ga-FAPI-04 PET/CT (107.4 ± 26.5 MBq). Global standardized uptake values and left ventricular (LV) molecular volume were calculated in correlation to echocardiography (n = 30), cardiac magnetic resonance (n = 18), and clinical biomarkers. Subsequently, the patients were categorized as having patchy (PET-patchy), extensive (PET-extensive), and negative (PET-negative) patterns.

RESULTS

Of all patients, 80% (24 of 30) showed increased LV uptake (PET-patchy [n = 4] vs PET-extensive [n = 20]), whereas 6 patients did not show visible myocardial uptake. Standardized uptake value ratio and LV molecular volume were significantly higher in the PET-extensive than the PET-patchy group (2.79 mL ± 1.22 mL vs 1.53 mL ± 0.66 mL [P = 0.045] and 198.3 mL ± 59.97 mL vs 127.8 mL ± 25.82 [P = 0.005], respectively). Additionally, ⁶⁸Ga-FAPI-04 uptake was significantly correlated with clinical biomarkers (Mayo stage and N-terminal pro-brain natriuretic peptide), interventricular septal thickness, left ventricular ejection fraction (LVEF), LV end-systolic volume, extracellular volume, and LV global strain (P < 0.05).

CONCLUSIONS

⁶⁸Ga-FAPI-04 PET/CT is feasible in detecting myocardial fibroblast activation in patients with AL CA in correlation with myocardial remodeling. It might provide complementary information on cardiac molecular characterization and staging of disease.

Detection of myocardial fibrosis: Where we stand

Myocardial fibrosis, resulting from the disturbance of extracellular matrix homeostasis in response to different insults, is a common and important pathological remodeling process that is associated with adverse clinical outcomes, including arrhythmia, heart failure, or even sudden cardiac death. Over the past decades, multiple non-invasive detection methods have been developed. Laboratory biomarkers can aid in both detection and risk stratification by reflecting cellular and even molecular changes in fibrotic processes, yet more evidence that validates their detection accuracy is still warranted. Different non-invasive imaging techniques have been demonstrated to not only detect myocardial fibrosis but also provide information on prognosis and management. Cardiovascular magnetic resonance (CMR) is considered as the gold standard imaging technique to non-invasively identify and quantify myocardial fibrosis with its natural ability for tissue characterization. This review summarizes the current understanding of the non-invasive detection methods of myocardial fibrosis, with the focus on different techniques and clinical applications of CMR.

A clinical study on relationship between visualization of cardiac fibroblast activation protein activity by Al18F-NOTA-FAPI-04 positron emission tomography and cardiovascular disease

Objective

FAP plays a vital role in myocardial injury and fibrosis. Although initially used to study imaging of primary and metastatic tumors, the use of FAPI tracers has recently been studied in cardiac remodeling after myocardial infarction. The study aimed to investigate the application of FAPI PET/CT imaging in human myocardial fibrosis and its relationship with clinical factors.

Materials and methods

Retrospective analysis of FAPI PET/CT scans of twenty-one oncological patients from 05/2021 to 03/2022 with visual uptake of FAPI in the myocardium were applying the American Heart Association 17-segment model of the left ventricle. The patients’ general data, echocardiography, and laboratory examination results were collected, and the correlation between PET imaging data and the above data was analyzed. Linear regression models, Kendall’s TaU-B test, the Spearman test, and the Mann–Whitney U test were used for the statistical analysis.

Results

21 patients (60.1 ± 9.4 years; 17 men) were evaluated with an overall mean LVEF of 59.3 ± 5.4%. The calcific plaque burden of LAD, LCX, and RCA are 14 (66.7%), 12 (57.1%), and 9 (42.9%). High left ventricular SUVmax correlated with BMI (P < 0.05) and blood glucose level (P < 0.05), and TBR correlated with age (P < 0.05). A strong correlation was demonstrated between SUVmean and CTnImax (r = 0.711, P < 0.01). Negative correlation of SUVmean and LVEF (r = −0.61, P < 0.01), SUVmax and LVEF (r = −0.65, P < 0.01) were found. ROC curve for predicting calcified plaques by myocardial FAPI uptake (SUVmean) in LAD, LCX, and RCA territory showed AUCs were 0.786, 0.759, and 0.769.

Conclusion

FAPI PET/CT scans might be used as a new potential method to evaluate cardiac fibrosis to help patients’ management further. FAPI PET imaging can reflect the process of myocardial fibrosis. High FAPI uptakes correlate with cardiovascular risk factors and the distribution of coronary plaques.