[68Ga]Ga-FAPI-04 PET/CT on assessing Crohn's disease intestinal lesions

Differences in Fibroinflammatory Activity Shown on 68Ga-FAPI-04 and 18F-FDG PET/CT in the Two Subtypes of IgG4-Related Disease

IgG4-related disease (IgG4-RD) is a highly heterogeneous autoimmune disease. Recently, 2 subtypes of IgG4-RD, proliferative and fibrotic, were defined according to patients' clinicopathologic characteristics. The purpose of this study was to determine the difference in fibroinflammatory activity shown on 68Ga-FAPI-04 and 18F-FDG PET/CT in the proliferative and fibrotic IgG4-RD subtypes. Methods: Thirty-seven newly diagnosed IgG4-RD patients (29 of the proliferative subtype and 8 of the fibrotic subtype) who had undergone 68Ga-FAPI-04 and 18F-FDG PET/CT were enrolled. SUVmax and target-to-background ratio (TBR) of IgG4-RD lesions were measured. To evaluate the weight of fibroinflammatory activity, the PET index of a lesion was calculated as the quotient of SUVmax or TBR of 68Ga-FAPI-04 and that of 18F-FDG. For the assessment of the global disease in an individual patient, the PET index was defined as the ratio of SUVmean of all involved lesions in 68Ga-FAPI-04 PET/CT to that in 18F-FDG. Results: The 18F-FDG uptake values of the most prominent lesions in the proliferative and fibrotic subtypes were similar; however, the proliferative subtype showed significantly higher uptake of 68Ga-FAPI-04 than did the fibrotic subtype (SUVmax, 17.67 ± 7.46 vs. 10.93 ± 2.22, P = 0.005; TBR, 15.49 ± 8.23 vs. 9.25 ± 3.00, P = 0.015). The PET index of proliferative-subtype patients was higher than that of fibrotic-subtype patients (1.46 ± 0.41 vs. 1.14 ± 0.39, P = 0.039). The PET index of pancreatobiliary disease was significantly higher than that of head-and-neck disease, fibrosis or aortitis, lymph nodes, and another disease subtype (P < 0.05). After first-line treatment, patients with a PET index of at least 1.5 had significantly shorter relapse-free survival than those with a PET index of less than 1.5 (22.0 mo vs. not reached, P < 0.0001; hazard ratio, 13.46; 95% CI, 2.236-81.03). Conclusion: The proliferative subtype of IgG4-RD had a greater weight of fibroinflammatory activity than that of the fibrotic subtype. The PET index, a marker of the weight of fibroinflammatory activity, is predictive of relapse-free survival of IgG4-RD.

Head-to-Head Comparison of 68Ga-FAPI-04 and 18F-FDG PET/CT for the Assessment of Crohn's Disease: A Prospective Pilot Study

BACKGROUND

Crohn disease is a chronic granulomatous inflammatory disease of gastrointestinal tract. Previous studies showed Crohn disease strictures overexpress fibroblast activation protein and had active 68Ga-FAPI-04 uptake. Our study was to compare the diagnostic performance of 68Ga-FAPI-04 and 18F-FDG PET/CT in Crohn disease.

PATIENTS AND METHODS

This is a prospective cohort study recruiting patients with newly diagnosed or relapsed Crohn disease. All the patients underwent 68Ga-FAPI-04 and 18F-FDG PET/CT. The diagnostic performance of the 2 PET/CT modalities and their uptake values were compared. The correlation of PET semiquantitative parameters [metabolic intestinal volume (MIVFDG and MIVFAPI), total intestinal uptake (TIUFDG and TIUFAPI)] with clinical biomarkers were also analyzed.

RESULTS

Seventeen participants (13 men and 4 women, age 32.3 ± 15.9 y) were recruited. The sensitivity of 68Ga-FAPI-04 and 18F-FDG PET/CT in detecting Crohn lesions were 90.0% and 85.0%, and the specificities were 93.0% and 88.4%, respectively. In receiver operating characteristic curve analysis, 68Ga-FAPI-04 PET/CT [area under the curve = 0.92 (95% CI: 0.83-0.97), P < 0.001] showed better diagnostic performance compared with 18F-FDG PET/CT [area under the curve = 0.87 (95% CI: 0.78-0.93), P < 0.001; P = 0.043]. The SUVmax of 68Ga-FAPI and 18F-FDG in stricture/fistula lesions were significantly higher than those in non-stricture/fistula lesions (68Ga-FAPI, 10.9 ± 6.7 vs 5.0 ± 3.5, P = 0.0002; 18F-FDG, 9.5 ± 4.9 vs 5.3 ± 1.8, P = 0.0016). TIUFAPI and MIVFAPI of 68Ga-FAPI-04 PET/CT were significantly correlated with high sensitivity C-reactive protein and simple endoscopic score for Crohn disease (P < 0.05). TIUFDG and MIVFDG of 18F-FDG PET/CT were also correlated with simple endoscopic scores for Crohn disease (P < 0.05).

CONCLUSIONS

68Ga-FAPI-04 PET/CT showed better diagnostic performance than 18F-FDG PET/CT in Crohn disease.

Distinguishing benign from malignant lesions with high [68 Ga]Ga-FAPI-04 uptake in oncology patients: Insights from dynamic total-body [68 Ga]Ga-FAPI-04 PET/CT

BACKGROUND AND PURPOSE

While [68 Ga]Ga-FAPI-04 PET/CT is widely used in various malignant tumors diagnosis, its specificity is challenged by high uptake in benign lesions such as inflammatory lymphadenopathy, bone fractures, and degenerative changes. This study aimed to quantitatively assess and characterize the metabolic heterogeneity of [68 Ga]Ga-FAPI-04 uptake in benign and malignant lesions using dynamic total-body PET/CT.

METHODS

Dynamic total-body [68 Ga]Ga-FAPI-04 PET/CT scans (0-60 min post-injection) were performed on 17 oncology patients. Time-activity curves (TACs) were generated for benign and malignant lesions with high [68 Ga]Ga-FAPI-04 uptake. The reversible two-tissue compartment model (2T4k) was used to derive kinetic metrics, including K1, k2, k3, k4, vB and VT. Receiver operating characteristic (ROC) curve analysis was used to determine the cut-off values for differentiating benign and malignant lesions.

RESULTS

The study included 58 malignant and 55 inflammatory lesions with high [68 Ga]Ga-FAPI-04 uptake. Malignant lesions exhibited higher K1 (0.277 ± 0.217 ml/ccm/min vs. 0.221 ± 0.216 ml/ccm/min, P = 0.011), vB (0.042 ± 0.007 vs. 0.013 ± 0.004, P < 0.001), and lower k3 (0.267 ± 0.041 1/min vs. 0.481 ± 0.085 1/min, P = 0.008) compared to benign lesions. Lesions were classified into low, medium, and high-probability groups for being malignant based on K1, k3 and vB values, with probabilities of 0%, 50.7%, and 92.0%, respectively (P < 0.001).

CONCLUSIONS

Kinetic metrics, particularly K1, k3 and vB values, show promise as imaging biomarkers for distinguishing between benign and malignant lesions with high [68 Ga]Ga-FAPI-04 uptake in oncology patients. These metrics reflect the metabolic heterogeneity of the lesions and may improve diagnostic accuracy in oncological imaging.

Discrimination between Inflammatory and Fibrotic Activity in Crohn's Disease-Associated Ileal-Colonic Anastomotic Strictures by Combined Ga-68-FAPI-46 and F-18-FDG-PET/CT Imaging

Introduction

The development of an intestinal stricture in patients with Crohn's disease represents an important and frequent complication, reflecting the progressive nature of the disease. Depending on the inflammatory and fibrotic composition of the stricture, intensified medical therapy, interventional endoscopy, or surgical intervention is required. However, currently available diagnostic approaches can only assess the level of inflammation, but not the degree of fibrosis, limiting rational therapeutic management of Crohn's disease patients. Recently, prolyl endopeptidase fibroblast activating protein (FAP) has been functionally implicated in fibrotic tissue remodelling, indicating it as a promising target for detection of sites of fibrotic tissue remodelling. Thus, intestinal fibrosis might be visualized using Gallium-68 labelled inhibitors of FAP (FAPI). While F-18-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/CT is a standard diagnostic tool for visualizing inflammatory processes, we combined Ga-68-FAPI-46-PET/CT and F-18-FDG-PET/CT to differentiate predominantly fibrotic or inflammatory areas in Crohn's disease patients with ileo-colonic strictures.

Methods

In our study, we analysed three Crohn's disease patients with anastomotic ileo-colonic strictures who underwent both dynamic Ga-68-FAPI-46-PET/CT and static F-18-FDG-PET/CT imaging to assess the level of visualized fibrotic areas within the stricture and differentiate it from inflammatory ones. PET images were analysed both visually and quantitatively. Furthermore, conventional MR enterography and endoscopy were performed in parallel to correlate observed findings. Two of the included patients underwent surgery and the histological specimen were analysed for the level of inflammation and fibrosis, which results were similarly compared to the findings of the PET imaging procedures.

Results

Different uptake patterns of Ga-68-FAPI-46 could be observed in the anastomotic ileo-colonic strictures of the examined Crohn's disease patients, respectively. Immunohistochemical analyses demonstrated that there was a correlation between the level of Ga-68-FAPI-46 uptake and severity of fibrosis, while FDG uptake correlated with the inflammatory activity in the analysed strictures.

Discussion

The combination with F-18-FDG-PET/CT represents a promising imaging modality to distinguish inflammation from fibrosis and guide subsequent therapy in stricturing Crohn's disease patients, warranting further studies.

Intense fibroblast activation protein inhibitor localization around the site of embolized gastroduodenal artery in a patient with metachronous pancreatic adenocarcinoma: A potential pitfall in positron imaging

68Gallium-Fibroblast activation protein inhibitor (68Ga-FAPI) positron emission tomography/computed tomography (PET/CT) is increasingly used for evaluating various epithelial neoplasms. Despite addressing some pitfalls, many remain unacknowledged. This report details a 77-year-old man with suspected pancreatic malignancy who underwent a 68Ga-FAPI PET/CT scan post-gastroduodenal coil embolization for upper gastrointestinal bleeding. The scan revealed intense 68Ga-FAPI uptake in the pancreatic body and tail malignancy and around the embolized gastroduodenal artery, indicating a healing process. This highlights the importance of understanding FAPI expression in recent surgical sites for accurate radiologic interpretation.

68 Ga-FAPI PET/CT Depicted the Fibrosis of Ischemic Colitis

ABSTRACT

Ischemic colitis is the most common form of ischemic bowel disease; identification of ischemic colitis with PET/CT is rare. We present a case of unexplained ischemic colitis in a 66-year-old man who underwent 18 F-FDG PET/CT to exclude intestinal malignancy and was subsequently recruited in a clinical trial regarding 68 Ga-FAPI. 18 F-FDG PET/CT showed intense activity in the intestinal lumen where mucosa had chronic inflammation. Conversely, 68 Ga-FAPI PET/CT revealed high tracer uptake in the intestinal wall and adjacent mesentery. Our case showed the different distribution pattern of 18 F-FDG and 68 Ga-FAPI in ischemic colitis.

The fibroblast activation protein alpha as a biomarker of pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a rare, chronic, and progressive interstitial lung disease with an average survival of approximately 3 years. The evolution of IPF is unpredictable, with some patients presenting a relatively stable condition with limited progression over time, whereas others deteriorate rapidly. In addition to IPF, other interstitial lung diseases can lead to pulmonary fibrosis, and up to a third have a progressive phenotype with the same prognosis as IPF. Clinical, biological, and radiological risk factors of progression were identified, but no specific biomarkers of fibrogenesis are currently available. A recent interest in the fibroblast activation protein alpha (FAPα) has emerged. FAPα is a transmembrane serine protease with extracellular activity. It can also be found in a soluble form, also named anti-plasmin cleaving enzyme (APCE). FAPα is specifically expressed by activated fibroblasts, and quinoline-based specific inhibitors (FAPI) were developed, allowing us to visualize its distribution in vivo by imaging techniques. In this review, we discuss the use of FAPα as a useful biomarker for the progression of lung fibrosis, by both its assessment in human fluids and/or its detection by imaging techniques and immunohistochemistry.

Role of artificial intelligence in Crohn's disease intestinal strictures and fibrosis

Crohn's disease (CD) is a chronic inflammatory disorder of the gastrointestinal tract. Intestinal fibrosis or stricture is one of the most prevalent complications in CD with a high recurrence rate. Manual examination of intestinal fibrosis or stricture by physicians may be biased or inefficient. A rapid development of artificial intelligence (AI) technique in recent years facilitates the detection of existing or possible intestinal fibrosis and stricture in CD through various modalities, including endoscopy, imaging examination, and serological biomarkers. We reviewed the articles on AI application in diagnosing intestinal fibrosis and stricture in CD during the past decade and categorized them into three aspects based on the detection methods, and found that AI helps accurate and expedient identification and prediction of intestinal fibrosis and stenosis in CD.

The Role of Fibroblast Activation Protein Inhibitor Positron Emission Tomography in Inflammatory and Infectious Diseases: An Updated Systematic Review

The role of fibroblast activation protein inhibitor (FAPI) positron emission tomography/computed tomography (PET/CT) is emerging for the assessment of non-oncological diseases, such as inflammatory and infectious diseases, even if the evidence in the literature is still in its initial phases. We conducted a systematic search of Scopus, PubMed/MEDLINE, Embase, and Cochrane library databases for studies published before 31 December 2023 reporting infectious and inflammatory disease imaging with FAPI PET/CT. We included twenty-one studies for a total of 1046 patients. The most frequent disease studied was lung interstitial disease, investigated in six studies for a total of 200 patients, followed by bone and joint diseases in two studies and 185 patients, IgG4-related disease in 53 patients, and Crohn's disease in 30 patients. Despite the heterogeneity of studies in terms of study design and technical features, FAPI PET/CT showed a high detection rate and diagnostic role. Moreover, when compared with 2-[18F]FDG PET/CT (n = 7 studies), FAPI PET/CT seems to have better diagnostic performances. The presence of chronic inflammation and tissue remodeling, typical of immune-mediated inflammatory conditions, may be the underlying mechanism of FAPI uptake.

Fibroblast Activation Protein Inhibitor Tracers and Their Preclinical, Translational, and Clinical Status in China

Quinoline-based fibroblast activation protein (FAP) inhibitors (FAPIs) have recently emerged as a focal point in global nuclear medicine, underscored by their promising applications in cancer theranostics and the diagnosis of various nononcological conditions. This review offers an in-depth summary of the existing literature on the evolution and use of FAPI tracers in China, tracing their journey from preclinical to clinical research. Moreover, this review also assesses the diagnostic accuracy of FAPI PET for the most common cancers in China, analyzes its impact on oncologic management paradigms, and investigates the potential of FAP-targeted radionuclide therapy in patients with advanced or metastatic cancer. This review also summarizes studies using FAPI PET for nononcologic disorders in China. Thus, this qualitative overview presents a snapshot of China's engagement with FAPI tracers, aiming to guide future research endeavors.

Application Value of 68 Ga-FAPI PET/CT in the Evaluation of Myelofibrotic Diseases

PURPOSE

Fibroblast activation protein is highly expressed in neoplastic lesions and various fibrotic tissues, making it an attractive target for disease evaluation. 68 Ga-labeled fibroblast activation protein inhibitor (FAPI), a new tumor interstitial imaging agent, holds promise for evaluating myelofibrosis. Therefore, this study aimed to use 68 Ga-FAPI PET/CT for the noninvasive visualization and quantification of the extent of myelofibrosis.

PATIENTS AND METHODS

This was a prospective clinical study involving 22 patients with myelofibrosis who underwent 68 Ga-FAPI PET/CT. The uptake of 68 Ga-FAPI was measured in their respective bone marrow and spleen, and the obtained imaging findings were compared with laboratory, cytogenetic, and histopathological data.

RESULTS

68 Ga-FAPI uptake in the bone marrow was significantly and positively correlated with the myelofibrosis grade ( r > 0.8, P < 0.001). 68 Ga-FAPI PET/CT showed visually negative results in patients with grades 0-1 myelofibrosis and positive in those with grades 2-3, but the level of involvement varied. 68 Ga-FAPI PET/CT provides a noninvasive means of visualizing the extent of systemic bone marrow involvement and differentiation between the early and advanced stages of fibrosis.

CONCLUSIONS

68 Ga-FAPI PET/CT shows promise as a method for visualizing and quantifying myelofibrosis, providing suitable sites for bone marrow biopsy. The extent of 68 Ga-FAPI uptake by bone marrow increases with the progression of myelofibrosis, thus it is a simple and noninvasive measurement that can be used to evaluate the progression of myelofibrosis. Nevertheless, although 68 Ga-FAPI PET/CT has demonstrated a potential value in prognostic assessment, further confirmation is needed.

Contemporary Imaging Assessment of Strictures and Fibrosis in Crohn Disease, With Focus on Quantitative Biomarkers: From the AJR Special Series on Imaging of Fibrosis

Patients with Crohn disease commonly have bowel strictures develop, which exhibit varying degrees of inflammation and fibrosis. Differentiation of the distinct inflammatory and fibrotic components of strictures is key for the optimization of therapeutic management and for the development of antifibrotic drugs. Cross-sectional imaging techniques, including ultrasound, CT, and MRI, allow evaluation of the full thickness of the bowel wall as well as extramural complications and associated mesenteric abnormalities. Although promising data have been reported for a range of novel imaging biomarkers for detection of fibrosis and quantification of the degree of fibrosis, these biomarkers lack sufficient validation and standardization for clinical use. Additional methods, including PET with emerging radiotracers, artificial intelligence, and radiomics, are also under investigation for stricture characterization. In this review, we highlight the clinical relevance of identifying fibrosis in Crohn disease, review the histopathologic aspects of strictures in Crohn disease, summarize the morphologic imaging findings of strictures, and explore contemporary developments in the use of cross-sectional imaging techniques for detecting and characterizing intestinal strictures, with attention given to emerging quantitative biomarkers.

Tissue fibrosis induced by radiotherapy: current understanding of the molecular mechanisms, diagnosis and therapeutic advances

Cancer remains the leading cause of death around the world. In cancer treatment, over 50% of cancer patients receive radiotherapy alone or in multimodal combinations with other therapies. One of the adverse consequences after radiation exposure is the occurrence of radiation-induced tissue fibrosis (RIF), which is characterized by the abnormal activation of myofibroblasts and the excessive accumulation of extracellular matrix. This phenotype can manifest in multiple organs, such as lung, skin, liver and kidney. In-depth studies on the mechanisms of radiation-induced fibrosis have shown that a variety of extracellular signals such as immune cells and abnormal release of cytokines, and intracellular signals such as cGAS/STING, oxidative stress response, metabolic reprogramming and proteasome pathway activation are involved in the activation of myofibroblasts. Tissue fibrosis is extremely harmful to patients' health and requires early diagnosis. In addition to traditional serum markers, histologic and imaging tests, the diagnostic potential of nuclear medicine techniques is emerging. Anti-inflammatory and antioxidant therapies are the traditional treatments for radiation-induced fibrosis. Recently, some promising therapeutic strategies have emerged, such as stem cell therapy and targeted therapies. However, incomplete knowledge of the mechanisms hinders the treatment of this disease. Here, we also highlight the potential mechanistic, diagnostic and therapeutic directions of radiation-induced fibrosis.

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.