[18F]FAPI-42 PET/CT versus [18F]FDG PET/CT for imaging of recurrent or metastatic gastrointestinal stromal tumors

Diagnostic Performances of PET/CT Using Fibroblast Activation Protein Inhibitors in Patients with Primary and Metastatic Liver Tumors: A Comprehensive Literature Review

PET/CT using radiolabeled fibroblast activation protein inhibitors (FAPIs) is a promising diagnostic tool in oncology, especially when non-increased and/or physiologically high [18F]FDG uptake (as in liver parenchyma) is observed. We aimed to review the role of PET/CT using radiolabeled FAPIs in primary and/or metastatic liver lesions, and to compare their performances with more "conventional" radiopharmaceuticals. A search algorithm based on the terms "FAPI" AND ("hepatic" OR "liver") was applied, with the last update on 1st January 2024. Out of 177 articles retrieved, 76 studies reporting on the diagnostic application of radiolabeled FAPI PET/CT in at least one patient harboring primary or metastatic liver lesion(s) were fully analyzed. Although there was some heterogeneity in clinical conditions and/or study methodology, PET/CT with radiolabeled FAPIs showed an excellent performance in common primary liver malignancies (hepatocarcinoma, intrahepatic cholangiocarcinoma) and liver metastases (mostly from the gastrointestinal tract and lungs). A higher tumor-to-background ratio for FAPIs than for [18F]FDG was found in primary and metastatic liver lesions, due to lower background activity. Despite limited clinical evidence, radiolabeled FAPIs may be used to assess the suitability and effectiveness of FAPI-derived therapeutic agents such as [177Lu]Lu-FAPI. However, future prospective research on a wider population is needed to confirm the excellent performance.

A Systematic Review on the Diagnostic Value of Fibroblast Activation Protein Inhibitor PET/CT in Genitourinary Cancers

In contemporary oncologic diagnostics, molecular imaging modalities are pivotal for precise local and metastatic staging. Recent studies identified fibroblast activation protein as a promising target for molecular imaging across various malignancies. Therefore, we aimed to systematically evaluate the current literature on the utility of fibroblast activation protein inhibitor (FAPI) PET/CT for staging patients with genitourinary malignancies. Methods: A systematic Embase and Medline search was conducted, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) process, on August 1, 2023. Relevant publications reporting on the diagnostic value of FAPI PET/CT in genitourinary malignancies were identified and included. Studies were critically reviewed using a modified version of a tool for quality appraisal of case reports. Study results were summarized using a narrative approach. Results: We included 22 retrospective studies with a cumulative total of 69 patients, focusing on prostate cancer, urothelial carcinoma of the bladder and of the upper urinary tract, renal cell carcinoma, and testicular cancer. FAPI PET/CT was able to visualize both local and metastatic disease, including challenging cases such as prostate-specific membrane antigen (PSMA)-negative prostate cancer. Compared with radiolabeled 18F-FDG and PSMA PET/CT, FAPI PET/CT showed heterogeneous performance. In selected cases, FAPI PET/CT demonstrated superior tumor visualization (i.e., better tumor-to-background ratios and visualization of small tumors or metastatic deposits visible in no other way) over 18F-FDG PET/CT in detecting local or metastatic disease, whereas comparisons with PSMA PET/CT showed both superior and inferior performances. Challenges in FAPI PET/CT arise from physiologic urinary excretion of most FAPI radiotracers, hindering primary-lesion visualization in the bladder and upper urinary tract, despite generally providing high tumor-to-background ratios. Conclusion: The current findings suggest that FAPI PET/CT may hold promise as a future tool to aid clinicians in detecting genitourinary malignancies. Given the substantial heterogeneity among the included studies and the limited number of patients, caution in interpreting these findings is warranted. Subsequent prospective and comparative investigations are anticipated to delve more deeply into this innovative imaging modality and elucidate its role in clinical practice.

Comparative analysis of [18F]F-FAPI PET/CT, [18F]F-FDG PET/CT and magnetization transfer MR imaging to detect intestinal fibrosis in Crohn's disease: A prospective animal model and human cohort study

PURPOSE

Accurately and early detection of intestinal fibrosis in Crohn's disease (CD) is crucial for clinical management yet remains an unmet need. Fibroblast activation protein inhibitor (FAPI) PET/CT has emerged as a promising tool to assess fibrosis. We aimed to investigate the diagnostic capability of [18F]F-FAPI PET/CT in detecting intestinal fibrosis and compared it with[18F]F-FDG PET/CT and magnetization transfer MR imaging (MTI).

METHODS

Twenty-two rats underwent TNBS treatment to simulate fibrosis development, followed by three quantitative imaging sessions within one week. Mean and maximum standardized uptake values (SUVmean and SUVmax) were calculated on[18F]F-FAPI and [18F]F-FDG PET/CT, along with normalized magnetization transfer ratio on MTI. Intestinal fibrosis was assessed pathologically, with MTI serving as imaging standard for fibrosis. The diagnostic efficacy of imaging parameters in fibrosis was compared using pathological and imaging standards. Ten patients with 34 bowel strictures were prospectively recruited to validate their diagnostic performance, using the identical imaging protocol.

RESULTS

In CD patients, the accuracy of FAPI uptake (both AUCs = 0.87, both P ≤ 0.01) in distinguishing non-to-mild from moderate-to-severe fibrosis was higher than FDG uptake (both AUCs = 0.82, P ≤ 0.01) and comparable to MTI (AUCs = 0.90, P ≤ 0.001). In rats, FAPI uptake responded earlier to fibrosis development than FDG and MTI; consistently, during early phase, FAPI uptake showed a stronger correlation (SUVmean: R = 0.69) with pathological fibrosis than FDG (SUVmean: R = 0.17) and MTI (R = 0.52).

CONCLUSION

The diagnostic efficacy of [18F]F-FAPI PET/CT in detecting CD fibrosis is superior to [18F]F-FDG PET/CT and comparable to MTI, exhibiting great potential for early detection of intestinal fibrosis.

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.

How to Perform FAPI PET? An Expedited Systematic Review Providing a Recommendation for FAPI PET Imaging With Different FAPI Tracers

This expedited systematic review aims to provide the first overview of the different Fibroblast activation protein inhibitor (FAPI) PET scan procedures in the literature and discuss how to efficiently obtain optimal FAPI PET images based on the best available evidence. The PubMed, Embase, Cochrane Library, and Web of Science databases were systematically searched in April 2023. Peer-reviewed cohort studies published in English and used FAPI tracers were included. Articles were excluded if critical scan procedure information was missing, or the article was not retrievable from a university library within 30 days. Data were grouped according to the FAPI tracer applied. Meta-analysis with proper statistics was deemed not feasible based on a pilot study. A total of 946 records were identified. After screening, 159 studies were included. [68Ga]Ga-FAPI-04 was applied in 98 studies (61%), followed by [68Ga]Ga-FAPI-46 in 19 studies (12%). Most studies did not report specific patient preparation. A mean/median administered activity of 80-200 MBq was most common; however, wide ranges were seen in [68Ga]Ga-FAPI-04 PET studies (56-370 MBq). An injection-to-scan-time of 60 minutes was dominant for all FAPI PET studies. A possible trend toward shorter injection-to-scan times was observed for [68Ga]Ga-FAPI-46. Three studies evaluated [68Ga]Ga-FAPI-46 PET acquisition at multiple time points in more than 593 cancer lesions, all yielding equivalent tumor detection at 10 minutes vs later time points despite slightly lower tumor-to-background Ratios. Despite the wide ranges, most institutions administer an average of 80-200 MBq [68Ga]Ga-FAPI-04/46 and scan patients at 60 minutes postinjection. For [68Ga]Ga-FAPI-46, the present evidence consistently supports the feasibility of image acquisition earlier than 30 minutes. Currently, data on the optimal FAPI PET scan procedure are limited, and more studies are encouraged. The current review can serve as a temporary guideline for institutions planning FAPI PET studies.

From basic research to clinical application: targeting fibroblast activation protein for cancer diagnosis and treatment

PURPOSE

This study aims to review the multifaceted roles of a membrane protein named Fibroblast Activation Protein (FAP) expressed in tumor tissue, including its molecular functionalities, regulatory mechanisms governing its expression, prognostic significance, and its crucial role in cancer diagnosis and treatment.

METHODS

Articles that have uncovered the regulatory role of FAP in tumor, as well as its potential utility within clinical realms, spanning diagnosis to therapeutic intervention has been screened for a comprehensive review.

RESULTS

Our review reveals that FAP plays a pivotal role in solid tumor progression by undertaking a multitude of enzymatic and nonenzymatic roles within the tumor stroma. The exclusive presence of FAP within tumor tissues highlights its potential as a diagnostic marker and therapeutic target. The review also emphasizes the prognostic significance of FAP in predicting tumor progression and patient outcomes. Furthermore, the emerging strategies involving FAPI inhibitor (FAPI) in cancer research and clinical trials for PET/CT diagnosis are discussed. And targeted therapy utilizing FAP including FAPI, chimeric antigen receptor (CAR) T cell therapy, tumor vaccine, antibody-drug conjugates, bispecific T-cell engagers, FAP cleavable prodrugs, and drug delivery system are also introduced.

CONCLUSION

FAP's intricate interactions with tumor cells and the tumor microenvironment make it a promising target for diagnosis and treatment. Promising strategies such as FAPI offer potential avenues for accurate tumor diagnosis, while multiple therapeutic strategies highlight the prospects of FAP targeting treatments which needs further clinical evaluation.

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.

Head-to-Head Comparison of FDG and Radiolabeled FAPI PET: A Systematic Review of the Literature

FAPI-based radiopharmaceuticals are a novel class of tracers, mainly used for PET imaging, which have demonstrated several advantages over [18F]FDG, especially in the case of low-grade or well-differentiated tumors. We conducted this systematic review to evaluate all the studies where a head-to-head comparison had been performed to explore the potential utility of FAPI tracers in clinical practice. FAPI-based radiopharmaceuticals have shown promising results globally, in particular in detecting peritoneal carcinomatosis, but studies with wider populations are needed to better understand all the advantages of these new radiopharmaceuticals.