Preclinical evaluation and pilot clinical study of [18F]AlF-NOTA-FAPI-04 for PET imaging of rheumatoid arthritis

The molar dose of FAPI administered impacts on the FAP-targeted PET imaging and therapy in mouse syngeneic tumor models

PURPOSE

Since fibroblast activation protein (FAP), one predominant biomarker of cancer associated fibroblasts (CAFs), is highly expressed in the tumor stroma of various epidermal-derived cancers, targeting FAP for tumor diagnosis and treatment has shown substantial potentials in both preclinical and clinical studies. However, in preclinical settings, tumor-bearing mice exhibit relatively low absolute FAP expression levels, leading to challenges in acquiring high-quality PET images using radiolabeled FAP ligands (FAPIs) with low molar activity, because of which a saturation effect in imaging is prone to happen. Moreover, how exactly the molar dose of FAPI administered to a mouse influences the targeted PET imaging and radiotherapy remains unclear now. Therefore, this study aims to investigate the impacts of the molar dose of the administered FAPI on FAP-targeted PET imaging and radiotherapy in mouse syngeneic tumor models.

METHODS

[68Ga]Ga-FAPI-04 with various molar doses of FAPI-04 was administered to wild-type 4T1 tumor-bearing mice, followed by static PET imaging. Sigmoidal curves were generated to analyze the correlation between the standard uptake value (SUV) and the administered molar doses of FAPI-04. Similarly, [177Lu]Lu-DOTAGA.(SA.FAPi)2 with a consistent dose of radioactivity but containing different moles of DOTAGA.(SA.FAPi)2 were injected into 4T1 tumor-bearing mice to assess the therapeutic effect. [68Ga]Ga-FAPI-04 was also applied to different tumor models for PET/CT imaging.

RESULTS

A gradient blocking effect was observed with increasing FAPI molar dose in [68Ga]Ga-FAPI-04 PET imaging and [177Lu]Lu-DOTAGA.(SA.FAPi)2 treatment, with various imaging and therapeutic outcomes. [68Ga]Ga-FAPI-04 PET exhibit potentials to characterize murine derived FAP expression with low molar dose of administered FAPI-04 using various tumor models.

CONCLUSION

The molar dose of FAPI in [68Ga]Ga/[177Lu]Lu-FAPI had a substantial impact on FAP-targeted imaging and therapy in mouse syngeneic tumor models. To acquire enhanced reliability and reproducibility in preclinical situation, it is critical to carefully consider the molar dose of the radiotracer when applying radiolabeled FAP ligands to FAP-targeted imaging and radiotherapy.

Current status of FAP-directed cancer theranostics: a bibliometric analysis

Fibroblast activation protein (FAP) is a key molecule in the field of oncology, with significant impacts on tumor diagnosis and treatment. Importantly, it has paved the way for the development of radiotracers for quinoline-based FAP inhibitors (FAPIs), which are currently among the most promising radiotracers for PET imaging in cancer. We performed a bibliometric analysis of scientific publications related to FAP and FAPI-based radiotracers, which included the quantification and visualization of current research trends and prospects based on various bibliometric indicators. In our survey of FAP-related studies in the Web of Science Core Collection databases, R and VOSviewer were used for visualization and bibliometric analyses based on country, institute, author, journal, and keywords. We also examined the methodology, radionuclide type, imaging instruments, and major diseases associated with studies on FAPI-based radiotracers. The results revealed 2,664 FAP-related publications from 1992 to the present. Germany, the USA, and China dominated paper publications, multinational collaborations, and societal impacts on FAP research. Southwest Medical University was the most productive institute, while Haberkorn Uwe authored the most cited papers and the highest H-index. The European Journal of Nuclear Medicine and Molecular Imaging and the Journal of Nuclear Medicine were the most influential periodicals. Keywords "FAP", "68Ga-FAPI", and "PET/CT" emerged as the most significant in this field. This study may help elucidate current research trends, hotspots, and directions for future research.

Preclinical Evaluation and Pilot Clinical Study of CD137 PET Radiotracer for Noninvasive Monitoring Early Responses of Immunotherapy

Given the variability in the effectiveness of immune checkpoint blocking therapy among patients and tumor types, development of noninvasive methods for longitudinal assessment of immune cell function and early tumor response is crucial for precision immunotherapy. CD137 (4-1BB), a marker of activated T cells, plays a significant role in immunotherapy. However, its potential as an imaging biomarker for activated T cells in the tumor microenvironment has not been explored. This study introduces a bicyclic peptide-based probe that targets CD137 for noninvasive PET imaging of tumor-infiltrating activated T cells. Methods: A bicyclic peptide-based probe, [18F]AlF-NOTA-BCP137, was first designed and synthesized for quantitative and longitudinal whole-body visualization of CD137 dynamics. Initially, [18F]AlF-NOTA-BCP137 was assessed in mouse models with varying CD137 expression levels. Next, [18F]AlF-NOTA-BCP137 was used for longitudinal monitoring of systemic CD137 changes in a humanized tumor-bearing mouse model. Lastly, the probe was further evaluated in a small group of patients with hepatocellular carcinoma undergoing immunotherapy or combination immunotherapy. Results: [18F]AlF-NOTA-BCP137 PET accurately characterized CD137 expression in homologous transplanted mouse models and tumor patients. The findings from animal studies indicated that uptake of [18F]AlF-NOTA-BCP137 was predictive of the early therapeutic response to combination immunotherapies and was positively associated with the increased survival rates of mice with tumors. A preliminary clinical study involving small patient cohorts demonstrated that [18F]AlF-NOTA-BCP137 imaging effectively predicted early patient responses to immunotherapeutic interventions. Conclusion: [18F]AlF-NOTA-BCP137 PET imaging of CD137 is a promising and reliable method for evaluating the efficacy of multiple combination immunotherapies and merits further validation in larger-scale clinical trials. This approach has the potential for early noninvasive visualization of individual patient responses in combination cancer immunotherapy and will aid in tailoring personalized strategies for patients.

Unveiling the Tumor Microenvironment Through Fibroblast Activation Protein Targeting in Diagnostic Nuclear Medicine: A Didactic Review on Biological Rationales and Key Imaging Agents

The tumor microenvironment (TME) is a dynamic and complex medium that plays a central role in cancer progression, metastasis, and treatment resistance. Among the key elements of the TME, cancer-associated fibroblasts (CAFs) are particularly important for their ability to remodel the extracellular matrix, promote angiogenesis, and suppress anti-tumor immune responses. Fibroblast activation protein (FAP), predominantly expressed by CAFs, has emerged as a promising target in both cancer diagnostics and therapeutics. In nuclear medicine, targeting FAP offers new opportunities for non-invasive imaging using radiolabeled fibroblast activation protein inhibitors (FAPIs). These FAP-specific radiotracers have demonstrated excellent tumor detection properties compared to traditional radiopharmaceuticals such as [18F]FDG, especially in cancers with low metabolic activity, like liver and biliary tract tumors. The most recent FAPI derivatives not only enhance the accuracy of positron emission tomography (PET) imaging but also hold potential for theranostic applications by delivering targeted radionuclide therapies. This review examines the biological underpinnings of FAP in the TME, the design of FAPI-based imaging agents, and their evolving role in cancer diagnostics, highlighting the potential of FAP as a target for precision oncology.

MSC-microvesicles protect cartilage from degradation in early rheumatoid arthritis via immunoregulation

OBJECTIVE

As research into preclinical rheumatoid arthritis (pre-RA) has advanced, a growing body of evidence suggests that abnormalities in RA-affected joint cartilage precede the onset of arthritis. Thus, early prevention and treatment strategies are imperative. In this study, we aimed to explore the protective effects of mesenchymal stem cell (MSC)-derived microvesicles (MVs) on cartilage degradation in a collagen-induced arthritis (CIA) mouse model.

METHODS

A CIA mouse model was established to observe early pathological changes in cartilage (days 21-25) through histological and radiological examinations. On day 22, MSCs-MVs were intravenously injected into the mice with CIA. Radiological, histological, and flow cytometric examinations were conducted to observe inflammation and cartilage changes in these mice compared to the mice with CIA and the control mice. In vitro, chondrocytes were cultured with inflammatory factors such as IL-1β and TNFα to simulate inflammatory damage to cartilage. After the addition of MVs, changes in inflammatory levels and collagen expression were measured via Western blotting, immunofluorescence, enzyme-linked immunosorbent assays (ELISAs), and quantitative PCR to determine the role of MVs in maintaining chondrocytes.

RESULTS

MSC-MVs expressed vesicular membrane proteins (CD63 and Annexin V) and surface markers characteristic of MSCs (CD44, CD73, CD90, and CD105). In the early stages of CIA in mice, a notable decrease in collagen content was observed in the joint cartilage. In mice with CIA, injection of MSCs-MVs resulted in a significant reduction in the peripheral blood levels of IL-1β, TNFα, and IL-6, along with a decrease in the ratio of proinflammatory T and B cells. Additionally, MSC-MVs downregulated the expression of IL-1β, TNFα, MMP-13, and ADAMTS-5 in cartilage while maintaining the stability of type I and type II collagen. These MVs also attenuated the destruction of cartilage, which was evident on imaging. In vitro experiments demonstrated that MSC-MVs effectively suppressed the secretion of the inflammatory factors IL-1β, TNFα, and IL-6 in stimulated peripheral blood mononuclear cells (PBMCs).

CONCLUSIONS

MSCs-MVs can inhibit the decomposition of the inflammation-induced cartilage matrix by regulating immune cell inflammatory factors to attenuate cartilage destruction. MSC-MVs are promising effective treatments for the early stages of RA.

Magnetic Resonance Imaging of Fibroblast Activation Protein Using a Targeted Gadolinium-Based Contrast Agent

The aim of this study was to synthesize a quinoline-based MRI contrast agent, Gd-DOTA-FAPI04, and assess its capacity for targeting fibroblast activation protein (FAP)-positive tumors in vivo. Gd-DOTA-FAPI04 was synthesized by attaching a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) complex of gadolinium(III) to FAP inhibitor FAPI04. The longitudinal relaxation time (T1) of the contrast agent was measured using a Siemens Prisma 3.0T MR system, and the CCK-8 assay was performed to evaluate its potential cytotoxicity. Male nude mice bearing tumors grown from FAP-expressing fibrosarcoma cells were divided into experimental (n = 4) and control (n = 4) groups, and T1-weighted image enhancement was measured at different times (0, 10, 30, 60, 90, and 120 min) postinjection of Gd-DOTA-FAPI04. The control group received an additional preinjection of excess FAPI04. FAP expression in tumor tissue was investigated by using immunohistochemistry with an anti-FAP antibody. The longitudinal relaxivities of gadodiamide and Gd-DOTA-FAPI04 were measured to be 3.734 mM-1 s-1 and 5.323 mM-1 s-1, respectively. The CCK-8 assay demonstrated that Gd-DOTA-FAPI04 has minimal toxicity to cultured human fibrosarcoma cells. In vivo MRI showed that peak accumulation of Gd-DOTA-FAPI04 in FAP-expressing tumors occurred 1 h postinjection and could be blocked by preinjection of excess FAPI04. Immunohistochemical analysis of harvested tumor tissue supported the above findings. Gd-DOTA-FAPI04 is a promising contrast agent for in vivo imaging of FAP.

Emerging Landscape of In Vitro Models for Assessing Rheumatoid Arthritis Management

Rheumatoid arthritis (RA) is a complex condition that is influenced by various causes, including immunological, genetic, and environmental factors. Several studies using animal models have documented immune system dysfunction and described the clinical characteristics of the disease. These studies have provided valuable insights into the pathogenesis of inflammatory arthritis and the identification of new targets for treatment. Nevertheless, none of these animal models successfully replicated all the characteristics of RA. Additionally, numerous experimental medications, which were developed based on our enhanced comprehension of the immune system's function in RA, have shown potential in animal research but ultimately proved ineffective during different stages of clinical trials. There have been several novel therapy alternatives, which do not achieve a consistently outstanding therapeutic outcome in all patients. This underscores the importance of employing the progress in in vitro models, particularly 3D models like tissue explants, and diverse multicomponent approaches such as coculture strategies, synovial membrane, articular cartilage, and subchondral bone models that accurately replicate the structural characteristics of RA pathophysiology. These methods are crucial for the advancement of potential therapeutic strategies. This review discusses the latest advancements in in vitro models and their potential to greatly impact research on managing RA.

Antitumor efficacy and potential mechanism of FAP-targeted radioligand therapy combined with immune checkpoint blockade

Radiotherapy combined with immune checkpoint blockade holds great promise for synergistic antitumor efficacy. Targeted radionuclide therapy delivers radiation directly to tumor sites. LNC1004 is a fibroblast activation protein (FAP)-targeting radiopharmaceutical, conjugated with the albumin binder Evans Blue, which has demonstrated enhanced tumor uptake and retention in previous preclinical and clinical studies. Herein, we demonstrate that 68Ga/177Lu-labeled LNC1004 exhibits increased uptake and prolonged retention in MC38/NIH3T3-FAP and CT26/NIH3T3-FAP tumor xenografts. Radionuclide therapy with 177Lu-LNC1004 induced a transient upregulation of PD-L1 expression in tumor cells. The combination of 177Lu-LNC1004 and anti-PD-L1 immunotherapy led to complete eradication of all tumors in MC38/NIH3T3-FAP tumor-bearing mice, with mice showing 100% tumor rejection upon rechallenge. Immunohistochemistry, single-cell RNA sequencing (scRNA-seq), and TCR sequencing revealed that combination therapy reprogrammed the tumor microenvironment in mice to foster antitumor immunity by suppressing malignant progression and increasing cell-to-cell communication, CD8+ T-cell activation and expansion, M1 macrophage counts, antitumor activity of neutrophils, and T-cell receptor diversity. A preliminary clinical study demonstrated that 177Lu-LNC1004 was well-tolerated and effective in patients with refractory cancers. Further, scRNA-seq of peripheral blood mononuclear cells underscored the importance of addressing immune evasion through immune checkpoint blockade treatment. This was emphasized by the observed increase in antigen processing and presentation juxtaposed with T cell inactivation. In conclusion, our data supported the efficacy of immunotherapy combined with 177Lu-LNC1004 for cancer patients with FAP-positive tumors.

Targeting pathogenic fibroblast-like synoviocyte subsets in rheumatoid arthritis

Fibroblast-like synoviocytes (FLSs) play a central role in RA pathogenesis and are the main cellular component in the inflamed synovium of patients with rheumatoid arthritis (RA). FLSs are emerging as promising new therapeutic targets in RA. However, fibroblasts perform many essential functions that are required for sustaining tissue homeostasis. Direct targeting of general fibroblast markers on FLSs is challenging because fibroblasts in other tissues might be altered and side effects such as reduced wound healing or fibrosis can occur. To date, no FLS-specific targeted therapies have been applied in the clinical management of RA. With the help of high-throughput technologies such as scRNA-seq in recent years, several specific pathogenic FLS subsets in RA have been identified. Understanding the characteristics of these pathogenic FLS clusters and the mechanisms that drive their differentiation can provide new insights into the development of novel FLS-targeting strategies for RA. Here, we discuss the pathogenic FLS subsets in RA that have been elucidated in recent years and potential strategies for targeting pathogenic FLSs.

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.

Evaluating the Patterns of FAPI Uptake in the Shoulder Joint: a Preliminary Study Comparing with FDG Uptake in Oncological Studies

BACKGROUND

Fibroblast activation protein inhibitor (FAPI) targeting PET has been introduced as a novel molecular imaging modality for visualizing cancer-associated fibroblasts. There have also been reports suggesting incidental findings of localized accumulation in the shoulder joints. However, further characterization in a larger patient cohort is still lacking.

METHODS

77 consecutive patients (28 females; mean age, 63.1 ± 11.6) who underwent Ga-68 FAPI-04 PET/CT for diagnosis of solid tumors were included. The incidence and localization of tracer uptake in shoulder joints were investigated and compared with available F-18 FDG scans serving as reference.

RESULTS

Ga-68 FAPI-04 uptake was evaluated in 77 patients (154 shoulder joints), of whom 54 subjects (108 shoulder joints) also had available F-18 FDG scans for head-to-head comparison. On FAPI-targeted imaging, 67/154 shoulders (43.5%) demonstrated increased radiotracer accumulation in target lesions, which were distributed as follows: acromioclavicular (AC) joints in 25/67 (37.3%), followed by glenohumeral and subacromial (GH + SA) joints in 23/67 (34.3%), or both (AC and GH + SA joints) in the remaining 19/67 (28.4%). Ga-68 FAPI-04 correlated with quantified F-18 FDG uptake (r = 0.69, p < 0.0001). Relative to the latter radiotracer, however, in-vivo FAP expression in the shoulders was significantly increased (Ga-68 FAPI-04, 4.7 ± 3.2 vs F-18 FDG, 3.6 ± 1.3, p < 0.001).

CONCLUSION

Our study revealed focal accumulation of Ga-68 FAPI-04 in the shoulders, particularly in the AC joints, with higher uptake compared to the inflammatory-directed PET radiotracer F-18 FDG in oncological studies. As a result, further trials are warranted to investigate the potential of FAPI-directed molecular imaging in identifying chronic remodeling in shoulder joints. This could have implications for initiating anti-FAP targeted photodynamic therapy based on PET signal strength.

CD200+ fibroblasts form a pro-resolving mesenchymal network in arthritis

Fibroblasts are important regulators of inflammation, but whether fibroblasts change phenotype during resolution of inflammation is not clear. Here we use positron emission tomography to detect fibroblast activation protein (FAP) as a means to visualize fibroblast activation in vivo during inflammation in humans. While tracer accumulation is high in active arthritis, it decreases after tumor necrosis factor and interleukin-17A inhibition. Biopsy-based single-cell RNA-sequencing analyses in experimental arthritis show that FAP signal reduction reflects a phenotypic switch from pro-inflammatory MMP3+/IL6+ fibroblasts (high FAP internalization) to pro-resolving CD200+DKK3+ fibroblasts (low FAP internalization). Spatial transcriptomics of human joints indicates that pro-resolving niches of CD200+DKK3+ fibroblasts cluster with type 2 innate lymphoid cells, whereas MMP3+/IL6+ fibroblasts colocalize with inflammatory immune cells. CD200+DKK3+ fibroblasts stabilized the type 2 innate lymphoid cell phenotype and induced resolution of arthritis via CD200-CD200R1 signaling. Taken together, these data suggest a dynamic molecular regulation of the mesenchymal compartment during resolution of inflammation.

Role of PET/CT in diagnosing and monitoring disease activity in rheumatoid arthritis: a review

Rheumatoid Arthritis (RA) is a systemic inflammatory disorder that commonly presents with polyarthritis but can have multisystemic involvement and complications, leading to increased morbidity and mortality. The diagnosis of RA continues to be challenging due to its varied clinical presentations. In this review article, we aim to determine the potential of PET/CT to assist in the diagnosis of RA and its complications, evaluate the therapeutic response to treatment, and predict RA remission. PET/CT has increasingly been used in the last decade to diagnose, monitor treatment response, predict remissions, and diagnose subclinical complications in RA. PET imaging with [18F]-fluorodeoxyglucose ([18F]-FDG) is the most commonly applied radiotracer in RA, but other tracers are also being studied. PET/CT with [18F]-FDG, [18F]-NaF, and other tracers might lead to early identification of RA and timely evidence-based clinical management, decreasing morbidity and mortality. Although PET/CT has been evolving as a promising tool for evaluating and managing RA, more evidence is required before incorporating PET/CT in the standard clinical management of RA.

Metabolic and molecular imaging in inflammatory arthritis

It is known that metabolic shifts and tissue remodelling precede the development of visible inflammation and structural organ damage in inflammatory rheumatic diseases such as the inflammatory arthritides. As such, visualising and measuring metabolic tissue activity could be useful to identify biomarkers of disease activity already in a very early phase. Recent advances in imaging have led to the development of so-called 'metabolic imaging' tools that can detect these changes in metabolism in an increasingly accurate manner and non-invasively.Nuclear imaging techniques such as 18F-D-glucose and fibroblast activation protein inhibitor-labelled positron emission tomography are increasingly used and have yielded impressing results in the visualisation (including whole-body staging) of inflammatory changes in both early and established arthritis. Furthermore, optical imaging-based bedside techniques such as multispectral optoacoustic tomography and fluorescence optical imaging are advancing our understanding of arthritis by identifying intra-articular metabolic changes that correlate with the onset of inflammation with high precision and without the need of ionising radiation.Metabolic imaging holds great potential for improving the management of patients with inflammatory arthritis by contributing to early disease interception and improving diagnostic accuracy, thereby paving the way for a more personalised approach to therapy strategies including preventive strategies. In this narrative review, we discuss state-of-the-art metabolic imaging methods used in the assessment of arthritis and inflammation, and we advocate for more extensive research endeavours to elucidate their full field of application in rheumatology.

Design, Synthesis, and Evaluation of 18F-Labeled Tracers Targeting Fibroblast Activation Protein for Brain Imaging

Fibroblast activation protein (FAP) is closely related to central nervous system diseases such as stroke and brain tumors, but PET tracers that can be used for brain imaging have not been reported. Here, we designed, synthesized, and evaluated 18F-labeled UAMC1110 derivatives suitable for brain imaging targeting FAP. By substituting the F atom for the H atom on the aromatic ring of compound UAMC1110, 1a-c were designed and prepared. 1a-c were confirmed to have a high affinity for FAP through molecular docking and enzyme assay. [18F]1a-c were successfully prepared and confirmed to have high affinity. The stability in vivo indicates that no obvious metabolites of [18F]1a,b were found in the plasma 1 h after injection, which is beneficial for brain imaging. In vitro cell uptake experiments showed that [18F]1a,b and [68Ga]FAPI04 exhibited similar uptake and internalization rates. PET imaging of U87MG subcutaneous tumor showed that [18F]1a,b could penetrate the blood-brain barrier with higher uptake and longer retention time than [68Ga]FAPI04 (uptake at 62.5 min, 1.06 ± 0.23, 1.09 ± 0.25% ID/g vs 0.21 ± 0.10% ID/g, respectively). The brain-to-blood ratios of [18F]1a,b were better than [68Ga]FAPI04. Biodistribution and PET imaging showed that [18F]1a had better uptake on tumors and a higher tumor-to-muscle ratio than [18F]1b and [68Ga]FAPI04. Further imaging of U87MG intracranial glioma showed that [18F]1a outlined high-contrast gliomas in a short period of time compared to [18F]1b. Therefore, [18F]1a is expected to be useful in the diagnosis of FAP-related brain diseases.

Experimental training in molecular pharmacology education based on drug-target interactions

INTRODUCTION

Drug development has been challenged by the dual drawbacks involving unpredictable disease outcomes and drug resistance, which has placed greater demands on pharmacology education. Molecular pharmacology, as a frontier crossover field of pharmacology, focuses on the research of new drugs and targets. However, due to the lack of a systematic experimental training system, molecular pharmacology has not made a corresponding contribution in promoting the training of innovative talent in pharmacology. We aim to establish an experimental training program suitable for molecular pharmacology to improve students' ability to engage in drug development in future.

METHODS

Based on the feasibility of drug-target projects, a comprehensive training program containing molecular docking, target stability experiment, and fluorescent probe detection of protein expression in living cells and mice was conducted among 20 pharmacy graduate students. The experimental training was assessed by the experimental training report and the student recognition questionnaires.

RESULTS

All 20 students mastered the experimental principles and operations required for the training program. The experimental reports proved that the students were in good command of the experimental principles, operations and applications. The results of the Likert questionnaire indicated that the training program promoted the understanding of the drug research process and increased motivation to learn.

CONCLUSION

The designed experimental training program has a positive effect on the training of pharmacology talents, and can be implemented as a part of molecular pharmacology education.

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.

Diagnostic efficiency of [68 Ga]Ga-DOTA-FAPI-04 in differentiating periprosthetic hip joint infection and aseptic failure

PURPOSE

To assess the efficiency of [68 Ga]Ga-DOTA-FAPI-04 in diagnosing periprosthetic hip joint infection and establish a diagnostic standard of clinical significance based on uptake pattern.

METHODS

[68 Ga]Ga-DOTA-FAPI-04 PET/CT was performed in patients with symptomatic hip arthroplasty from December 2019 to July 2022. The reference standard was based on the 2018 Evidence-Based and Validation Criteria. Two diagnostic criteria, SUVmax and uptake pattern, were used to diagnose PJI. Meanwhile, original data were imported into IKT-snap to draw the view of interest, A.K. was used to extract features of clinical cases, and unsupervised clustering analysis was applied according to the groups.

RESULTS

A total of 103 patients were included, 28 of whom had PJI. The area under the curve of SUVmax was 0.898, which was better than that of all of the serological tests. The cutoff value of SUVmax was 7.53, and the sensitivity and specificity were 100 and 72%, respectively. The sensitivity, specificity and accuracy of the uptake pattern were 100, 93.1 and 95%, respectively. In radiomics analysis, the features of PJI were significantly different from those of aseptic failure.

CONCLUSION

The efficiency of [68 Ga]Ga-DOTA-FAPI-04 PET/CT in diagnosing PJI showed promising results, and the diagnostic criteria of the uptake pattern were more clinically instructive. Radiomics also showed certain application prospects in the field of PJI.

TRIAL REGISTRATION NUMBER

Trial registration: ChiCTR2000041204. Registered 24 September 2019.

Evaluation of 18F-FAPI-04 Imaging in Assessing the Therapeutic Response of Rheumatoid Arthritis

PURPOSE

Fibroblast activating protein (FAP) is highly expressed in the synovial tissues of rheumatoid arthritis (RA) patients. The aim of this study was to determine the feasibility of PET imaging with an Al[¹⁸F] F-NOTA-labeled FAP inhibitor 04(¹⁸F-FAPI-04) for the evaluation of arthritic progression and therapeutic response in experimental arthritis.

METHODS

Fibroblast-like synoviocytes (FLSs) were obtained from patients with RA or osteoarthritis (OA), and the relationship between ¹⁸F-FAPI-04 uptake and the inflammatory activity of RA FLSs was investigated. Collagen-induce arthritis (CIA) mice models were established and treated with methotrexate (MTX) or etanercept (ETC). Then, PET imaging was performed 24 h following ¹⁸F-FAPI-04 injection. The imaging results were compared by assessing macroscopic arthritis scores and histological staining.

RESULTS

¹⁸F-FAPI-04 uptake was obvious in RA FLSs that characterizing FAP activation. The higher the uptake of ¹⁸F-FAPI-04, the more severity of the inflammatory phenotype in RA FLS. Furthermore, the uptake of ¹⁸F-FAPI-04 in inflamed joints could be found even before the deformity of the parental joints could be observed by histological examination. Both MTX and ETC were effective in inhibiting the progression of arthritis in CIA mice was confirmed by macroscopic, histological, and radiographic pathology scores. Importantly, ¹⁸F-FAPI-04 uptake declined accordingly in CIA models following MTX and ETC treatment.

CONCLUSIONS

These findings suggest that PET imaging of ¹⁸F-FAPI-04 can be used to monitor treatment response in RA, and is more sensitive in disease speculation than macroscopic arthritis scoring.

Role and mechanism of fibroblast-activated protein-α expression on the surface of fibroblast-like synoviocytes in rheumatoid arthritis

Fibroblast-activated protein-α (FAP) is a type II integrated serine protease expressed by activated fibroblasts during fibrosis or inflammation. Fibroblast-like synoviocytes (FLSs) in rheumatoid arthritis (RA) synovial sites abundantly and stably overexpress FAP and play important roles in regulating the cellular immune, inflammatory, invasion, migration, proliferation, and angiogenesis responses in the synovial region. Overexpression of FAP is regulated by the initial inflammatory microenvironment of the disease and epigenetic signaling, which promotes RA development by regulating FLSs or affecting the signaling cross-linking FLSs with other cells at the local synovium and inflammatory stimulation. At present, several treatment options targeting FAP are in the process of development. This review discusses the basic features of FAP expressed on the surface of FLSs and its role in RA pathophysiology and advances in targeted therapies.

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.

PET imaging to assess fibroblast activation protein inhibitor biodistribution: A training program adapted to pharmacology education

In the process of pharmacology education, practical teaching is an important complement to theoretical teaching. These activities include the use of experimental animals to obtain certain pharmacological parameters or to help students understand certain classical concepts. However, the growing interest in laboratory animal welfare, the rapid development of pharmacology research and the challenges of cultivating innovative pharmacy talent create a need for innovative and flexible in vitro experiments for teaching purposes. Here, we report the application of positron emission tomography (PET) imaging of 18 F-labeled fibroblast activation protein inhibitor (18 F-FAPi) to practical pharmacology teaching, enabling dynamic visualization of the distribution and excretion process of FAPi in mice. Students can quantitatively analyze the distribution of FAPi in various tissues and organs without sacrificing the mice. Furthermore, the newly implemented method resulted in highly reproducible results and was generally appreciated by the students. Additionally, the application of PET imaging in pharmacokinetic teaching can not only greatly reduce the use of experimental animals but also need not sacrificing animals. Of note is that dynamic scanning data from this project can be used for online practical teaching during COVID-19 pandemic.