Fibroblast activation protein: a serine protease expressed at the remodeling interface in idiopathic pulmonary fibrosis

Synthesis of piperazine-based fluorinated fibroblast activation protein inhibitors for the development of a novel 18F-labeled PET imaging probe

Radiolabeled probes addressing the fibroblast activation protein (FAP) expressed among others by cancer associated fibroblasts in the microenvironment of tumors emerged as promising drugs for diagnostic imaging and therapy of tumors. 68Ga-chelator-based FAP inhibitors are clinically used for the diagnosis of various tumor types. To enhance the imaging quality and improve the applicability, we started to develop covalently 18F-labeled PET tracers for imaging of FAP in various diseases. For this purpose, four fluorinated quinolinecarboxamides 4a-d were synthesized and biologically evaluated. The seven-step synthesis of 4a comprised a Pd-catalyzed Buchwald-Hartwig reaction of bromoquinoline 6 and a Cu-catalyzed 1,3-dipolar cycloaddition of alkyne 12 with 1-azido-2-fluoroethane (Click reaction) as key steps. The fluorinated quinolinecarboxamides 4a-d showed low nanomolar inhibitory activity on FAP and high selectivity against related enzymes. Due to its low lipophilicity (logD7.4 = 0.08) and high metabolic stability (78 % intact after incubation with murine liver microsomes for 90 min), fluoroethyltriazole 4a (IC50 = 1.7 nM) was selected for radiosynthesis. The two-step radiosynthesis with [18F]-1-azido-2-fluoroethane provided the PET tracer [18F]4a in acceptable radiochemical yields (10.8 %) and high radiochemical purities (>97.0 %) within a total synthesis time of 156 min. The molar activities were 0.1-5.8 GBq/μmol. In vitro [18F]4a was stable in human and mouse serum over 90 min. In the biodistribution studies in mice [18F]4a showed fast renal and hepatobiliary elimination. In a mouse xenograft model with a tumor expressing FAP only very low accumulation in the tumor tissue was observed. This unexpected result was confirmed by the relative low uptake of [18F]4a by FAP expressing HT1080 cells.

Detecting radiation esophagitis using 18F-FAPI-04 PET/CT in patients with LA-ESCC treated with concurrent chemoradiotherapy

PURPOSE

This prospective study examined whether 18F-AlF-NOTA-fibroblast activation protein inhibitor (FAPI)-04 (denoted as 18F-FAPI-04) positron emission tomography/computed tomography (PET/CT) can detect the development and severity of radiation esophagitis (RE) in patients with locally advanced esophageal squamous cell carcinoma (LA-ESCC) treated with concurrent chemoradiotherapy.

MATERIALS AND METHODS

From June 2021 to March 2022, images were collected from LA-ESCC patients who underwent 18F-FAPI-04 PET/CT examinations before and during radiotherapy. The development of RE was evaluated weekly according to Radiation Therapy Oncology Group criterion. The target-to-background ratio in blood (TBRblood) was analyzed at each time point and correlated with the onset and severity of RE. Factors that predicted RE were identified by multivariate logistic analyses.

RESULTS

Thirty patients were evaluated. Significantly higher TBRblood (during radiotherapy, P = 0.003) and change in TBRblood compared with pre-RT (ΔTBRblood, P = 0.002) were observed in patients with RE than patients without RE. Those with grade 3 RE had a significantly higher TBRblood (during radiotherapy, P = 0.003) and ΔTBRblood (P = 0.003) compared with those with RE < grade 3. On multivariate analysis, ΔTBRblood was identified as a significant detection of any grade RE (P = 0.021) and grade 3 RE (P = 0.038).

CONCLUSION

The ΔTBRblood on 18F-FAPI-04 PET/CT may be effective at identifying patients with RE, especially grade 3 RE.

Head-to-Head Comparison Between [ 68 Ga]Ga-DOTA.SA.FAPi And [ 18 F]F-FDG PET/CT Imaging in Patients With Sarcoma

PURPOSE

This study aimed to compare the diagnostic efficacy of [ 68 Ga]Ga-DOTA.SA.FAPi and [ 18 F]F-FDG PET/CT for detecting primary and metastatic lesions in sarcoma patients.

MATERIALS AND METHODS

The analysis included both patient-based and lesion-based comparisons of PET/CT scans in individuals with histologically confirmed sarcoma.

RESULTS

A total of 23 sarcoma patients (mean age 43.0 ± 16.5 years; range: 21-76 years) underwent both [ 18 F]F-FDG and [ 68 Ga]Ga-DOTA.SA.FAPi PET/CT scans. Histological distribution included 30% synovial sarcoma, 13% liposarcoma, and 21.7% leiomyosarcoma, with 70% of patients presenting with distant metastases. Detection rates for primary tumors were similar between [ 68 Ga]Ga-DOTA.SA.FAPi and [ 18 F]F-FDG PET/CT (85.7% vs 100%, P  = 0.149). Lymph node detection rates were also comparable (80% vs 100%, P  = 0.146). Lesion-based analysis revealed that [ 68 Ga]Ga-DOTA.SA.FAPi detected 220 lesions (83% efficiency) compared with 249 lesions (94% efficiency) for [ 18 F]F-FDG ( P  < 0.0001). Notably, [ 68 Ga]Ga-DOTA.SA.FAPi demonstrated superior detection of liver (54 vs 38 lesions, P  < 0.0001) and bone metastases (125 vs 102 lesions, P  < 0.0001).

CONCLUSIONS

Our study shows that although [ 18 F]F-FDG PET/CT offers superior overall lesion detection efficiency, [ 68 Ga]Ga-DOTA.SA.FAPi PET/CT excels in identifying specific metastatic sites, particularly in bone and liver. These findings highlight the complementary roles of both imaging modalities in sarcoma evaluation.

Physiological Uptake of 68Ga-FAPI-04 in Female Reproductive System

PURPOSE

Since the avid uptake of 68Ga-labeled fibroblast activation protein inhibitor (68Ga-FAPI-04) observed in female reproductive organs, our objectives were to investigate the physiological uptake characteristics and provide preliminary reference ranges for clinical use.

PROCEDURES

We reviewed the findings of female patients who underwent 68Ga-FAPI-04 PET/CT at our institution between April 2022 and June 2023. The standard uptake value (SUV) of reproductive organs and menstrual information were collected. All patients were categorized into reproductive period group, perimenopause group, and postmenopause group. We analysed the uptake levels among the three groups, and their association with age and menstrual cycle.

RESULTS

A total of 109 patients were included in this study. Higher ovarian SUVs were detected in reproductive patients (SUVright: 2.75 ± 0.84, IQR: 1.39-5.26; SUVleft: 2.72, IQR: 2.34-3.20; p = 0.315) than in postmenopausal patients (SUVright: 2.27, IQR: 2.01-2.75; SUVleft: 2.38 ± 0.55, IQR: 1.35-3.60; p = 0.767), as well as uterine 68Ga-FAPI-04 accumulations. The SUVs of uterine fundus and corpus were approximately three times higher than that of the cervix. In reproductive period group, higher SUVs were observed in bilateral ovaries around the ovulatory phase to the early luteal phase, and higher uterine SUVs were noted in the menstrual and proliferative phases. The SUVs in all reproductive organs (except the ovaries) showed significant negative correlations with age in all patients (all p < 0.01).

CONCLUSIONS

68Ga-FAPI-04 SUVs in reproductive organs are higher in premenopausal patients than in postmenopausal patients. The 68Ga-FAPI-04 accumulation in reproductive organs might be associated with menstrual cycle. Including more patients from different menstrual phases could contribute to investigating the uptake characteristics and their underlying mechanisms.

Context-specific eQTLs provide deeper insight into causal genes underlying shared genetic architecture of COVID-19 and idiopathic pulmonary fibrosis

Most genetic variants identified through genome-wide association studies (GWASs) are suspected to be regulatory in nature, but only a small fraction colocalize with expression quantitative trait loci (eQTLs, variants associated with expression of a gene). Therefore, it is hypothesized but largely untested that integration of disease GWAS with context-specific eQTLs will reveal the underlying genes driving disease associations. We used colocalization and transcriptomic analyses to identify shared genetic variants and likely causal genes associated with critically ill COVID-19 and idiopathic pulmonary fibrosis. We first identified five genome-wide significant variants associated with both diseases. Four of the variants did not demonstrate clear colocalization between GWAS and healthy lung eQTL signals. Instead, two of the four variants colocalized only in cell type- and disease-specific eQTL datasets. These analyses pointed to higher ATP11A expression from the C allele of rs12585036, in monocytes and in lung tissue from primarily smokers, which increased risk of idiopathic pulmonary fibrosis (IPF) and decreased risk of critically ill COVID-19. We also found lower DPP9 expression (and higher methylation at a specific CpG) from the G allele of rs12610495, acting in fibroblasts and in IPF lungs, and increased risk of IPF and critically ill COVID-19. We further found differential expression of the identified causal genes in diseased lungs when compared to non-diseased lungs, specifically in epithelial and immune cell types. These findings highlight the power of integrating GWASs, context-specific eQTLs, and transcriptomics of diseased tissue to harness human genetic variation to identify causal genes and where they function during multiple diseases.

Proapoptotic Bcl-2 inhibitor as potential host directed therapy for pulmonary tuberculosis

Mycobacterium tuberculosis establishes within host cells by inducing anti-apoptotic Bcl-2 family proteins, triggering necrosis, inflammation, and fibrosis. Here, we demonstrate that navitoclax, an orally bioavailable, small-molecule Bcl-2 inhibitor, significantly improves pulmonary tuberculosis (TB) treatments as a host-directed therapy. Addition of navitoclax to standard TB treatments at human equipotent dosing in mouse models of TB, inhibits Bcl-2 expression, leading to improved bacterial clearance, reduced tissue necrosis, fibrosis and decreased extrapulmonary bacterial dissemination. Using immunohistochemistry and flow cytometry, we show that navitoclax induces apoptosis in several immune cells, including CD68+ and CD11b+ cells. Finally, positron emission tomography (PET) in live animals using clinically translatable biomarkers for apoptosis (18F-ICMT-11) and fibrosis (18F-FAPI-74), demonstrates that navitoclax significantly increases apoptosis and reduces fibrosis in pulmonary tissues, which are confirmed in postmortem analysis. Our studies suggest that proapoptotic drugs such as navitoclax can potentially improve pulmonary TB treatments, reduce lung damage / fibrosis and may be protective against post-TB lung disease.

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.

The Role of Activated Stromal Cells in Fibrotic Foci Formation and Reversion

Fibrotic focus is a pivotal morphofunctional unit in developing fibrosis in various tissues. For most fibrotic diseases, including progressive forms, the foci are considered unable to remodel and contribute to the worsening of prognosis. Unfortunately, the dynamics of the fibrotic focus formation and resolution remains understudied. A number of data suggest that the key cell type for focus formation are activated stromal cells marked by fibroblast activated protein alpha (FAPα) due to their high capacity for extracellular matrix (ECM) remodeling. We evaluated the dynamics of fibrotic focus formation and the contribution of the main cell types, including FAPα+ cells, in this process using a murine model of bleomycin-induced lung fibrosis. We revealed the very early appearance of FAPα+ cells in lungs after injury and assumed their important involvement to the myofibroblast pool formation. During the first month after bleomycin administration, FAPα+ cells colocalize with CD206+ M2 macrophages. Interestingly, during the reversion stage, we unexpectedly observed the specific structured foci formed by CD90+FAPα+ cells, which we suggested calling "remodeling foci". Our findings highlight the crucial role of activated stromal cells in fibrosis initiation, progression, and reversion and provide emerging issues regarding the novel targets for antifibrotic therapy.

Correlation of FAPI PET Uptake with Immunohistochemistry in Explanted Lungs from Patients with Advanced Interstitial Lung Disease

Recent studies have demonstrated promising results of fibroblast activation protein (FAP) inhibitor (FAPI) PET in prognosticating and monitoring interstitial lung diseases (ILDs). As a first step toward successful translation, our primary aim was to validate the FAPI PET uptake through immunohistochemistry in patients with advanced ILD who underwent lung transplantation after a FAPI PET scan. Methods: This is a preliminary analysis of a single-center, open-label, single-arm, prospective exploratory biodistribution study of 68Ga-FAPI-46 PET imaging in patients with ILD (NCT05365802). Patients with ILD confirmed by high-resolution CT and scheduled for lung transplant were included. Tissue samples of explanted lungs were obtained from both the central and peripheral lung parenchyma of each lobe. Additional samples were obtained from areas of the lung corresponding to regions of FAPI PET activity. Immunohistochemical staining was performed with an anti-FAP antibody. Percentages of FAP immunohistochemistry-positive area were measured semiautomatically using QuPath software. SUVs in the areas of pathologic samples were measured on FAPI PET/CT by referencing the gross photomap of the explanted lung. A Spearman correlation coefficient test was used to assess the relationship between FAPI PET uptake and FAP immunohistochemical expression in each specimen. Results: Four patients with advanced ILD who underwent FAPI PET/CT before lung transplantation were included. The types of ILD were idiopathic pulmonary fibrosis (n = 2), rheumatoid arthritis-associated ILD (n = 1), and nonspecific interstitial pneumonia (n = 1). FAPI uptake was visualized mainly in the fibrotic area on CT. Twenty-nine surgical pathology samples from 3 patients were analyzed. FAP staining was predominantly positive in fibroblastic foci. FAPI PET SUVmax and SUVmean showed a positive correlation with the immunohistochemical FAP expression score (SUVmax: r = 0.57, P = 0.001; SUVmean: r = 0.54, P = 0.002). Conclusion: In this analysis conducted in patients who underwent lung transplantation after a FAPI PET scan, FAPI PET uptake was positively correlated with FAP immunohistochemistry. These findings provide a rationale for further investigation of FAPI PET as a potential imaging biomarker for ILD.

Enzymatic activity of fibroblast activation protein-α is essential for TGF-β1-induced fibroblastic differentiation of human periodontal ligament cells

Human periodontal ligament cells (hPDLCs) contain multipotent postnatal stem cells that can differentiate into PDL fibroblasts, osteoblasts, and cementoblasts. Interaction between the extracellular environment and stem cells is an important factor for differentiation into other progenitor cells. To identify cell surface molecules that induce PDL fibroblastic differentiation, we developed a series of monoclonal antibodies against membrane/ECM molecules. One of these antibodies, an anti-PDL25 antibody, recognizes approximately a 100 kDa protein, and this antigenic molecule accumulates in the periodontal ligament region of tooth roots. By mass spectrometric analysis, we found that the antigenic molecule recognized by the anti-PDL25 antibody is fibroblast activation protein α (FAPα). The expression level of FAPα/PDL25 increased in TGF-β1-induced PDL fibroblasts, and this protein was localized in the cell boundaries and elongated processes of the fibroblastic cells. Ectopic expression of FAPα induced fibroblastic differentiation. In contrast, expression of representative markers for PDL differentiation was decreased by knock down and antibody blocking of FAPα/PDL25. Inhibition of dipeptidyl peptidase activity by a potent FAPα inhibitor dramatically inhibited PDL fibroblastic marker expression but did not affect in cell proliferation and migration.

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.

Proapoptotic Bcl-2 inhibitor as host directed therapy for pulmonary tuberculosis

Mycobacterium tuberculosis establishes within host cells by inducing anti-apoptotic Bcl-2 family proteins, triggering necrosis, inflammation, and fibrosis. Here, we demonstrate that navitoclax, an orally bioavailable, small-molecule Bcl-2 inhibitor, significantly improves pulmonary tuberculosis (TB) treatments as a host-directed therapy. Addition of navitoclax to standard TB treatments at human equipotent dosing in mouse models of TB, inhibits Bcl-2 expression, leading to improved bacterial clearance, reduced tissue damage / fibrosis and decreased extrapulmonary bacterial dissemination. Using immunohistochemistry and flow cytometry, we show that navitoclax induces apoptosis in several immune cells, including CD68 + and CD11b + cells. Finally, positron emission tomography (PET) in live animals using novel, clinically translatable biomarkers for apoptosis (18F-ICMT-11) and fibrosis (18F-FAPI-74) demonstrates that navitoclax significantly increases apoptosis and reduces fibrosis in pulmonary tissues, which are confirmed using post-mortem studies. Our studies suggest that proapoptotic drugs such as navitoclax can improve pulmonary TB treatments, and should be evaluated in clinical trials.

Fibroblast activation protein and disease severity, progression, and survival in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive fibrosis in the lungs. Activated fibroblasts play a central role in fibrogenesis and express fibroblast activation protein α. A truncated, soluble form (sFAP) can be measured in blood and is a potential novel biomarker of disease activity. The aim was to study the association between sFAP and clinical, radiological, and histopathological measures of disease severity, progression, and survival in a prospective, multicentre, real-world cohort of patients with IPF. Patients with IPF were recruited from the tertiary interstitial lung disease centres in Denmark and followed for up to 3 years. Baseline serum levels of sFAP were measured by ELISA in patients with IPF and compared to healthy controls. Pulmonary function tests, 6-minute walk test and quality of life measures were performed at baseline and during follow-up. The study included 149 patients with IPF. Median sFAP in IPF was 49.6 ng/mL (IQR: 43.1-61.6 ng/mL) and in healthy controls 73.8 ng/mL (IQR: 62.1-92.0 ng/mL). Continuous sFAP was not associated with disease severity, progression or survival (p > 0.05). After dichotomization of sFAP below or above mean sFAP + 2 SD for healthy controls, higher levels of sFAP were associated with lower FVC % predicted during follow-up (p < 0.01). Higher than normal serum levels of sFAP were associated with longitudinal changes in FVC % predicted, but sFAP did not show clear associations with other baseline or longitudinal parameters. As such, sFAP has limited use as a biomarker of disease progression or survival in patients with IPF.

Fibroblast activation protein is a cellular marker of fibrotic activity in canine idiopathic pulmonary fibrosis

Canine idiopathic pulmonary fibrosis (CIPF) is a progressive fibrotic interstitial lung disease of unknown etiology, afflicting aging West Highland white terriers (WHWTs) and leading to progressive respiratory failure. Fibroblast activation protein (FAP), a protease overexpressed in many cancers, is upregulated in idiopathic pulmonary fibrosis in humans. The aim of this study was to investigate FAP as a marker of active fibrosis in lung biopsies from WHWTs affected with CIPF, as well as the potential of plasmatic FAP as a biomarker. After establishing a scoring system to evaluate the severity and activity of fibrosis on histopathological lung sections, anti-FAP immunohistochemistry was performed on healthy and CIPF samples. FAP expression was characterized using both visual and digital quantitative pathology software analyses and then correlated to fibrosis severity and activity. Levels of plasmatic FAP in WHWTs affected with CIPF were measured by enzyme-linked immunosorbent assay and compared with healthy dogs. Lung samples from 22 WHWTs affected with CIPF were collected. According to the fibrosis scoring system, they were classified as cases of mild (5), moderate (9) and severe (8) fibrosis and were attributed scores of fibrosis activity. Fifteen healthy lung samples were classified as non-fibrotic. Healthy lung samples were FAP-negative, whereas fibroblasts were FAP-positive in 20 CIPF samples. FAP immunohistochemical expression correlated mildly with fibrosis severity (p < 0.05; R 2 = 0.22) but highly with fibrosis activity scores (p < 0.001; R 2 = 0.68). Digital image analysis detected a higher percentage of FAP-positive cells in areas of active fibrosis (p < 0.001) and FAP-positive cells were distributed outside mature fibrosis lesions, clustered in active fibrosis areas or scattered within alveolar septa. On the other hand, plasmatic FAP was significantly lower in dogs affected with CIPF compared with healthy dogs (p < 0.01). In conclusion, this study provides a valuable histological scoring system to assess the severity and activity of fibrosis in CIPF. It demonstrates that FAP is a good cellular marker of fibrotic activity in CIPF, and thus constitutes a promising target to be exploited for diagnostic and therapeutic applications. Additionally, it suggests that plasmatic FAP, although non-specific, could be altered in CIPF.

Context-specific eQTLs reveal causal genes underlying shared genetic architecture of critically ill COVID-19 and idiopathic pulmonary fibrosis

Most genetic variants identified through genome-wide association studies (GWAS) are suspected to be regulatory in nature, but only a small fraction colocalize with expression quantitative trait loci (eQTLs, variants associated with expression of a gene). Therefore, it is hypothesized but largely untested that integration of disease GWAS with context-specific eQTLs will reveal the underlying genes driving disease associations. We used colocalization and transcriptomic analyses to identify shared genetic variants and likely causal genes associated with critically ill COVID-19 and idiopathic pulmonary fibrosis. We first identified five genome-wide significant variants associated with both diseases. Four of the variants did not demonstrate clear colocalization between GWAS and healthy lung eQTL signals. Instead, two of the four variants colocalized only in cell-type and disease-specific eQTL datasets. These analyses pointed to higher ATP11A expression from the C allele of rs12585036, in monocytes and in lung tissue from primarily smokers, which increased risk of IPF and decreased risk of critically ill COVID-19. We also found lower DPP9 expression (and higher methylation at a specific CpG) from the G allele of rs12610495, acting in fibroblasts and in IPF lungs, and increased risk of IPF and critically ill COVID-19. We further found differential expression of the identified causal genes in diseased lungs when compared to non-diseased lungs, specifically in epithelial and immune cell types. These findings highlight the power of integrating GWAS, context-specific eQTLs, and transcriptomics of diseased tissue to harness human genetic variation to identify causal genes and where they function during multiple diseases.

Enhanced Detection of Early Pulmonary Fibrosis Disease Using 68Ga-FAPI-LM3 PET

Early detection of pulmonary fibrosis is a critical yet insufficiently met clinical necessity. This study evaluated the effectiveness of FAPI-LM3, a 68Ga-radiolabeled heterobivalent molecular probe that targets fibroblast activating protein (FAP) and somatostatin receptor 2 (SSTR2), in the early detection of pulmonary fibrosis, leveraging its potential for early disease identification. A bleomycin-induced early pulmonary fibrosis model was established in C57BL/6 mice for 7 days. FAP and SSTR2 expression levels were quantitatively assessed in human idiopathic pulmonary fibrosis lung tissue samples and bleomycin-treated mouse lung tissues by using western blotting, real-time quantitative PCR (RT-qPCR), and immunofluorescence techniques. The diagnostic performance of FAPI-LM3 was investigated by synthesizing monomeric radiotracers 68Ga-FAPI-46 and 68Ga-DOTA-LM3 alongside the heterobivalent probe 68Ga-FAPI-LM3. These imaging radiopharmaceuticals were used in small-animal PET to compare their uptake in fibrotic and normal lung tissues. Results indicated significant upregulation of FAP and SSTR2 at both RNA and protein levels in fibrotic lung tissues compared with that in normal controls. PET imaging demonstrated significantly enhanced uptake of the 68Ga-FAPI-LM3 probe in fibrotic lung tissues, with superior visual effects compared to monomeric tracers. At 60 min postinjection, early stage fibrotic tissues (day 7) demonstrated low-to-medium uptake of monomeric probes, including 68Ga-DOTA-LM3 (0.45 ± 0.04% ID/g) and 68Ga-FAPI-46 (0.78 ± 0.09% ID/g), whereas the uptake of the heterobivalent probe 68Ga-FAPI-LM3 (1.90 ± 0.10% ID/g) was significantly higher in fibrotic lesions than in normal lung tissue. Blockade experiments confirmed the specificity of 68Ga-FAPI-LM3 uptake, which was attributed to synergistic targeting of FAP and SSTR2. This study demonstrates the potential of 68Ga-FAPI-LM3 for early pulmonary fibrosis detection via molecular imaging, offering significant benefits over monomeric tracers 68Ga-FAPI-46 and 68Ga-DOTA-LM3. This strategy offers new possibilities for noninvasive and precise early detection of pulmonary fibrosis.

[68Ga]FAPI PET for Imaging and Treatment Monitoring in a Preclinical Model of Pulmonary Fibrosis: Comparison to [18F]FDG PET and CT

PURPOSE

This study aimed to evaluate the feasibility of using [68Ga]-fibroblast-activating protein inhibitor (FAPI) positron emission tomography (PET) imaging for diagnosing pulmonary fibrosis in a mouse model. We also examined its value in monitoring treatment response and compared it with traditional [18F]-fluorodeoxyglucose (FDG) PET and computed tomography (CT) imaging.

METHODS

A model of idiopathic pulmonary fibrosis was established using intratracheal injection of bleomycin (BLM, 2 mg/kg) into C57BL/6 male mice. For the treatment of IPF, a daily oral dose of 400 mg/kg/day of pirfenidone was administered from 9 to 28 days after the establishment of the model. Disease progression and treatment efficacy were assessed at different stages of the disease every week for four weeks using CT, [18F]FDG PET, and [68Ga]FAPI PET (baseline imaging performed at week 0). Mice were sacrificed and lung tissues were harvested for hematoxylin-eosin staining, picrosirius red staining, and immunohistochemical staining for glucose transporter 1 (GLUT1) and FAP. Expression levels of GLUT1 and FAP in pathological sections were quantified. Correlations between imaging parameters and pathological quantitative values were analyzed.

RESULTS

CT, [18F]FDG PET and [68Ga]FAPI PET revealed anatomical and functional changes in the lung that reflected progression of pulmonary fibrosis. In untreated mice with pulmonary fibrosis, lung uptake of [18F]FDG peaked on day 14, while [68Ga]FAPI uptake and mean lung density peaked on day 21. In mice treated with pirfenidone, mean lung density and lung uptake of both PET tracers decreased. Mean lung density, [18F]FDG uptake, and [68Ga]FAPI uptake correlated well with quantitative values of picrosirius red staining, GLUT1 expression, and FAP expression, respectively. Conclusions: Although traditional CT and [18F]FDG PET reflect anatomical and metabolic status in fibrotic lung, [68Ga]FAPI PET provides a means of evaluating fibrosis progression and monitoring treatment response.

FAP expression dynamics and role in silicosis: Insights from epidemiological and experimental models

Prolonged exposure to free silica leads to the development of silicosis, wherein activated fibroblasts play a pivotal role in its pathogenesis and progression. Fibroblast Activation Protein (FAP), as a biomarker for activated fibroblasts, its expression pattern and role in key aspects of silicosis pathogenesis remain unclear. This study elucidated the expression pattern and function of FAP through population-based epidemiological investigations, establishment of mouse models of silicosis, and in vitro cellular models. Results indicated a significant elevation of FAP in plasma from silicosis patients and lung tissues from mouse models of silicosis. In the cellular model, we observed a sharp increase in FAP expression early in the differentiation process, which remained high expression. Inhibition of FAP suppressed fibroblast differentiation, while overexpression of FAP produced the opposite effect. Moreover, fibroblast-derived FAP can alter the phenotype and function of neighboring macrophages. In summary, we revealed a high expression pattern of FAP in silicosis and its potential mechanistic role in fibrosis, suggesting FAP as a potential therapeutic target for silicosis.

Progressive changes in the protein expression profile of alveolar septa in early-stage lung adenocarcinoma

BACKGROUND

Adenocarcinomas show a stepwise progression from atypical adenomatous hyperplasia (AAH) through adenocarcinoma in situ (AIS) to invasive adenocarcinoma (IA). Immunoglobulin superfamily containing leucine-rich repeat (ISLR) is a marker of tumor-restraining cancer-associated fibroblasts (CAFs), which are distinct from conventional, strongly α-smooth muscle actin (αSMA)-positive CAFs. Fibroblast activation protein (FAP) has been focused on as a potential therapeutic and diagnostic target of CAFs.

METHODS

We investigated the changes in protein expression during adenocarcinoma progression in the pre-existing alveolar septa by assessing ISLR, αSMA, and FAP expression in normal lung, AAH, AIS, and IA. Fourteen AAH, seventeen AIS, and twenty IA lesions were identified and randomly sampled. Immunohistochemical analysis was performed to evaluate cancer-associated changes and FAP expression in the pre-existing alveolar structures.

RESULTS

Normal alveolar septa expressed ISLR. The ISLR level in the alveolar septa decreased in AAH and AIS tissues when compared with that in normal lung tissue. The αSMA-positive area gradually increased from the adjacent lung tissue (13.3% ± 15%) to AIS (87.7% ± 14%), through AAH (70.2% ± 21%). Moreover, the FAP-positive area gradually increased from AAH (1.69% ± 1.4%) to IA (11.8% ± 7.1%), through AIS (6.11% ± 5.3%). Protein expression changes are a feature of CAFs in the pre-existing alveolar septa that begin in AAH. These changes gradually progressed from AAH to IA through AIS.

CONCLUSIONS

FAP-positive fibroblasts may contribute to tumor stroma formation in early-stage lung adenocarcinoma, and this could influence the development of therapeutic strategies targeting FAP-positive CAFs for disrupting extracellular matrix formation.

Fibroblast activating protein promotes the proliferation, migration, and activation of fibroblasts in oral submucous fibrosis

OBJECTIVES

Fibroblast activating protein (FAP) is associated with various organ fibrosis. However, the expression and molecular function of FAP in oral submucous fibrosis (OSF) is still unclear.

MATERIALS AND METHODS

The high-performance liquid chromatography was used to detect the presence of alkaloids in areca nut extract (ANE). Real-time qPCR, Western blot, and Immunohistochemistry assay were used to analyze the expression of FAP mRNA or protein in OSF and normal oral tissue. A chi-squared test analyzed the relationship between FAP protein expression and clinicopathological data of OSF patients. CCK-8, Wound-healing, and Transwell migration assay were employed to assess the effect of the proliferation and migration ability of hOMF cells with FAP overexpression or knockdown. The expression level of a-SMA, FSP1, and P13K-Akt signaling pathways-related protein in hOMF cells transfected with FAP overexpression or knockdown plasmid was verified by western blot assay.

RESULTS

The four specific areca alkaloids (Arecoline, Guvacine, Arecaidine, and Guvacoline) were successfully detected in the ANE. The viability of hOMF cells was significantly improved in the 50 μg/mL ANE group and was inhibited in the 5 and 50 mg/mL ANE groups. The expression of FAP was upregulated in OSF tissues, and hOMF cells treated with 50 μg/mL ANE and was related to pathology grade, clinical stage, and history of chewing betel nut. Additionally, FAP may promote the proliferation, migration, and activation of hOMF cells through the P13K-Akt signaling pathway.

CONCLUSIONS

This study found that ANE had a bidirectional effect on the viability of hOMF cells, and the FAP gene was a potential therapeutic target in OSF.

Preclinical Development of PNT6555, a Boronic Acid-Based, Fibroblast Activation Protein-α (FAP)-Targeted Radiotheranostic for Imaging and Treatment of FAP-Positive Tumors

The overexpression of fibroblast activation protein-α (FAP) in solid cancers relative to levels in normal tissues has led to its recognition as a target for delivering agents directly to tumors. Radiolabeled quinoline-based FAP ligands have established clinical feasibility for tumor imaging, but their therapeutic potential is limited due to suboptimal tumor retention, which has prompted the search for alternative pharmacophores. One such pharmacophore is the boronic acid derivative N-(pyridine-4-carbonyl)-d-Ala-boroPro, a potent and selective FAP inhibitor (FAPI). In this study, the diagnostic and therapeutic (theranostic) potential of N-(pyridine-4-carbonyl)-d-Ala-boroPro-based metal-chelating DOTA-FAPIs was evaluated. Methods: Three DOTA-FAPIs, PNT6555, PNT6952, and PNT6522, were synthesized and characterized with respect to potency and selectivity toward soluble and cell membrane FAP; cellular uptake of the Lu-chelated analogs; biodistribution and pharmacokinetics in mice xenografted with human embryonic kidney cell-derived tumors expressing mouse FAP; the diagnostic potential of 68Ga-chelated DOTA-FAPIs by direct organ assay and small-animal PET; the antitumor activity of 177Lu-, 225Ac-, or 161Tb-chelated analogs using human embryonic kidney cell-derived tumors expressing mouse FAP; and the tumor-selective delivery of 177Lu-chelated DOTA-FAPIs via direct organ assay and SPECT. Results: DOTA-FAPIs and their natGa and natLu chelates exhibited potent inhibition of human and mouse sources of FAP and greatly reduced activity toward closely related prolyl endopeptidase and dipeptidyl peptidase 4. 68Ga-PNT6555 and 68Ga-PNT6952 showed rapid renal clearance and continuous accumulation in tumors, resulting in tumor-selective exposure at 60 min after administration. 177Lu-PNT6555 was distinguished from 177Lu-PNT6952 and 177Lu-PNT6522 by significantly higher tumor accumulation over 168 h. In therapeutic studies, all 3 177Lu-DOTA-FAPIs exhibited significant antitumor activity at well-tolerated doses, with 177Lu-PNT6555 producing the greatest tumor growth delay and animal survival. 225Ac-PNT6555 and 161Tb-PNT6555 were similarly efficacious, producing 80% and 100% survival at optimal doses, respectively. Conclusion: PNT6555 has potential for clinical translation as a theranostic agent in FAP-positive cancer.

The Role and Application of Fibroblast Activating Protein

Fibroblast activation protein-α (FAP), a type-II transmembrane serine protease, is rarely expressed in normal tissues but highly abundant in pathological diseases, including fibrosis, arthritis, and cancer. Ever since its discovery, we have deciphered its structure and biological properties and continue to investigate its roles in various diseases while attempting to utilize it for targeted therapy. To date, no significant breakthroughs have been made in terms of efficacy. However, in recent years, several practical applications in the realm of imaging diagnosis have been discovered. Given its unique expression in a diverse array of pathological tissues, the fundamental biological characteristics of FAP render it a crucial target for disease diagnosis and immunotherapy. To obtain a more comprehensive understanding of the research progress of FAP, its biological characteristics, involvement in diseases, and recent targeted application research have been reviewed. Moreover, we explored its development trend in the direction of clinical diagnoses and treatment.

[Localization features of FAP-positive activated stromal cells in fibrosis in patients with new coronavirus infection COVID-19]

OBJECTIVE

To evaluate the representation and localization of FAP-positive activated stromal cells depending on the severity of fibrotic changes in tissues of patients with a confirmed diagnosis of COVID-19.

MATERIAL AND METHODS

20 autopsy observations of patients who died from COVID-19 were studied. Immunohistochemical studies were performed using antibodies to CD90, FAP and aSMA and a dual imaging system. The severity of fibrosis in lung tissue was assessed according to the R.Hubner scale (2008).

RESULTS

FAP-positive cells are detected in the lungs, kidneys, liver and myocardium, while only in the lungs FAP expression is associated with fibrotic tissue areas. FAP expression of varying intensity was observed in capillaries, alveolar and bronchial epithelium, as well as in fibrotic nodules in the lungs, where it partially colocalizes with the key myofibroblast marker aSMA. CD90-positive stromal cells were also predominantly found in fibrotic foci.

CONCLUSION

The results obtained allow us to clarify the localization of activated stromal precursors in human tissues during injury and suggest an important role of FAP in the formation of fibrotic foci and the progression of fibrotic changes in tissues, primarily the lungs, which makes it a promising pharmaceutical target for the treatment of diseases associated with the development of fibrosis.

Fibroblast Activation Protein-Targeted PET/CT With Al18F-NODA-FAPI-04 for In Vivo Imaging of Tendon Healing in Rat Achilles Tendon Injury Models

BACKGROUND

Fibroblast activation protein (FAP) has shown high expression in inflammatory responses and fibrosis.

HYPOTHESIS

We speculated that FAP could serve as a diagnostic and monitoring target in the tendon healing process.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 72 Sprague-Dawley rats were randomly divided into a tendon crush group and a half-partial tendon laceration group. Four rats in each group were injected with radiotracers weekly for 4 weeks after surgery, with aluminum fluoride-labeled 1,4,7-triazacyclononane-N,N',N″-triacetic acid-conjugated FAP inhibitor (Al18F-NODA-FAPI-04) administered on the first day of each week and 18F-fludeoxyglucose (18F-FDG) on the next day. Small animal positron emission tomography (PET) imaging was performed, and tendon tissue was collected for pathology and quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis each week after surgery.

RESULTS

One week after surgery, both radiotracers showed signal concentration at the lesion site, which was the highest radioactive uptake observed during 4 weeks postoperatively, consistent with the severity of the lesion. Consistent trends were observed for inflammatory cytokines during qRT-PCR analysis. Additionally, Al18F-NODA-FAPI-04 PET exhibited a more precise lesion pattern, attributed to its high specificity for naive fibroblasts when referring to histological findings. Over time, the uptake of both radiotracers at the injury site gradually decreased, with 18F-FDG experiencing a more rapid decrease than Al18F-NODA-FAPI-04. In the fourth week after surgery, the maximum standardized uptake values of Al18F-NODA-FAPI-04 in the injured lesion almost reverted to the baseline levels, indicating a substantial decrease in naive fibroblasts and inflammatory cells and a reduction in inflammation and fibrosis, especially compared with the first week. Corresponding trends were also revealed in pathological and qRT-PCR results.

CONCLUSION

Our findings suggest that inflammation is a prominent feature during the early stage of tendon injury. Al18F-NODA-FAPI-04 PET allows accurate localization and provides detailed morphological imaging, enabling continuous monitoring of the healing progress and assessment of injury severity.

Fibroblast Activation Protein Alpha (FAPα) in Fibrosis: Beyond a Perspective Marker for Activated Stromal Cells?

The development of tissue fibrosis is a complex process involving the interaction of multiple cell types, which makes the search for antifibrotic agents rather challenging. So far, myofibroblasts have been considered the key cell type that mediated the development of fibrosis and thus was the main target for therapy. However, current strategies aimed at inhibiting myofibroblast function or eliminating them fail to demonstrate sufficient effectiveness in clinical practice. Therefore, today, there is an unmet need to search for more reliable cellular targets to contribute to fibrosis resolution or the inhibition of its progression. Activated stromal cells, capable of active proliferation and invasive growth into healthy tissue, appear to be such a target population due to their more accessible localization in the tissue and their high susceptibility to various regulatory signals. This subpopulation is marked by fibroblast activation protein alpha (FAPα). For a long time, FAPα was considered exclusively a marker of cancer-associated fibroblasts. However, accumulating data are emerging on the diverse functions of FAPα, which suggests that this protein is not only a marker but also plays an important role in fibrosis development and progression. This review aims to summarize the current data on the expression, regulation, and function of FAPα regarding fibrosis development and identify promising advances in the area.

Practice of 18F-FDG-PET/CT in ICU Patients: A Systematic Review

18F-FDG-PET/CT imaging has become a key tool to evaluate infectious and inflammatory diseases. However, application of 18F-FDG-PET/CT in patients in the intensive care unit (ICU) is limited, which is remarkable since the development of critical illness is closely linked to infection and inflammation. This limited use is caused by perceived complexity and risk of planning and executing 18F-FDG-PET/CT in such patients. The aim of this systematic review was to investigate the feasibility of 18F-FDG-PET/CT in ICU patients with special emphasis on patient preparation, transport logistics and safety. Therefore, a systematic search was performed in PubMed, Embase, and Web of Science using the search terms: intensive care, critically ill, positron emission tomography and 18F-FDG or derivates. A total of 1183 articles were found of which 10 were included. Three studies evaluated the pathophysiology of acute respiratory distress syndrome, acute lung injury and acute chest syndrome. Three other studies applied 18F-FDG-PET/CT to increase understanding of pathophysiology after traumatic brain injury. The remaining four studies evaluated infection of unknown origin. These four studies showed a sensitivity and specificity between 85%-100% and 57%-88%, respectively. A remarkable low adverse event rate of 2% was found during the entire 18F-FDG-PET/CT procedure, including desaturation and hypotension. In all studies, a team consisting of an intensive care physician and nurse was present during transport to ensure continuation of necessary critical care. Full monitoring during transport was used in patients requiring mechanical ventilation or vasopressor support. None of the studies used specific patient preparation for ICU patients. However, one article described specific recommendations in their discussion. In conclusion, 18F-FDG-PET/CT has been shown to be feasible and safe in ICU patients, even when ventilated or requiring vasopressors. Specific recommendations regarding patient preparation, logistics and scanning are needed. Including 18F-FDG-PET/CT in routine workup of infection of unknown origin in ICU patients showed potential to identify source of infection and might improve outcome.

Radiotracers for Imaging of Fibrosis: Advances during the Last Two Decades and Future Directions

Fibrosis accompanies various pathologies, and there is thus an unmet medical need for non-invasive, sensitive, and quantitative methods for the assessment of fibrotic processes. Currently, needle biopsy with subsequent histological analysis is routinely used for the diagnosis along with morphological imaging techniques, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US). However, none of these imaging techniques are sufficiently sensitive and accurate to detect minor changes in fibrosis. More importantly, they do not provide information on fibrotic activity on the molecular level, which is critical for fundamental understanding of the underlying biology and disease course. Molecular imaging technology using positron emission tomography (PET) offers the possibility of imaging not only physiological real-time activity, but also high-sensitivity and accurate quantification. This diagnostic tool is well established in oncology and has exhibited exponential development during the last two decades. However, PET diagnostics has only recently been widely applied in the area of fibrosis. This review presents the progress of development of radiopharmaceuticals for non-invasive detection of fibrotic processes, including the fibrotic scar itself, the deposition of new fibrotic components (fibrogenesis), or the degradation of existing fibrosis (fibrolysis).

Usefulness of FAPα assessment in bronchoalveolar lavage as a marker of fibrogenesis: results of a preclinical study and first report in patients with idiopathic pulmonary fibrosis

BACKGROUND

Fibroblast activation protein-α (FAPα) is a marker of activated fibroblasts that can be selectively targeted by an inhibitor (FAPI) and visualised by PET/CT imaging. We evaluated whether the measurement of FAPα in bronchoalveolar lavage fluids (BALF) and the uptake of FAPI by PET/CT could be used as biomarkers of fibrogenesis.

METHODS

The dynamics of lung uptake of 18F-labeled FAPI ([18F]FAPI-74) was assessed in the bleomycin mouse model at various time points and using different concentrations of bleomycin by PET/CT. FAPα was measured in BALFs from these bleomycin-treated and control mice. FAPα levels were also assessed in BALFs from controls and patients with idiopathic pulmonary fibrosis (IPF).

RESULTS

Bleomycin-treated mice presented a significantly higher uptake of [18F]FAPI-74 during lung fibrinogenesis (days 10 and 16 after instillation) compared to control mice. No significant difference was observed at initial inflammatory phase (3 days) and when fibrosis was already established (28 days). [18F]FAPI-74 tracer was unable to show a dose-response to bleomycin treatment. On the other hand, BALF FAPα levels were steeply higher in bleomycin-treated mice at day 10 and a significant dose-response effect was observed. Moreover, FAPα levels were strongly correlated with lung fibrosis as measured by the modified Aschroft histological analysis, hydroxyproline and the percentage of weight loss. Importantly, higher levels of FAPα were observed in IPF patients where the disease was progressing as compared to stable patients and controls. Moreover, patients with FAPα BALF levels higher than 192.5 pg/mL presented a higher risk of progression, transplantation or death compared to patients with lower levels.

CONCLUSIONS

Our preclinical data highlight a specific increase of [18F]FAPI-74 lung uptake during the fibrotic phase of the bleomycin murine model. The measurement of FAPα in BALF appears to be a promising marker of the fibrotic activity in preclinical models of lung fibrosis and in IPF patients. Further studies are required to confirm the role of FAPα in BALF as biomarker of IPF activity and assess the relationship between FAPα levels in BALF and [18F]FAPI-74 uptake on PET/CT in patients with fibrotic lung disease.

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.

99mTc-Labeled FAPI SPECT Imaging in Idiopathic Pulmonary Fibrosis: Preliminary Results

AIM

Idiopathic pulmonary fibrosis (IPF) is associated with a poor prognosis, presenting the most aggressive form of interstitial lung diseases (ILDs). Activated fibroblasts are crucial for pathological processes. Fibroblast activation protein (FAP) inhibitor (FAPI) tracers would be promising imaging agents for these diseases. The purpose of this study was to evaluate a 99mTc-labeled FAPI tracer, 99mTc-HFAPI imaging in IPF patients.

METHODS

Eleven IPF patients (nine males and two females; age range 55-75 year) were included in this pilot study. 99mTc-HFAPI serial whole-body scintigraphy at 5 min, 20 min, 40 min, 1 h, 2 h, 3 h, 4 h, and 6 h was acquired for dynamic biodistribution and dosimetry estimation in seven representative patients. SPECT/CT tomography fusion imaging of the chest region was performed in all patients at 4 h post-injection, which was considered as the optimal acquisition time. Dosimetry was calculated using OLINDA/EXM software (version 2.0; HERMES Medical Solutions). The quantified or semi-quantified standardized uptake values (SUVs) and lesion-to-background ratios (LBRs) of affected lung parenchyma were also calculated. The high-resolution CT (HRCT) stage was determined with visual evaluation, and the total HRCT score of each patient was measured using a weighting factor formula. Pulmonary function tests (PFTs) were recorded as well. Then, the relationships between the 99mTc-HFAPI results, disease extent on HRCT, and PFT results were investigated.

RESULTS

Normal physiological uptake of 99mTc-HFAPI was observed mainly in the liver, intestinal tract, pancreas, gallbladder, and to a lesser extent in the spleen, kidneys, and thyroid, with no apparent retention in the blood circulation at the late time point. The mean injected activity of 99mTc-HFAPI was 813.4 MBq (range 695.6-888.0 MBq). No subjective side effects were noticed. The average whole-body effective dose was 0.0041 mSv/MBq per patient. IPF patients exhibited elevated pulmonary 99mTc-HFAPI uptake in abnormal lung regions, which was correlated with fibrotic regions on HRCT. Among different HRCT stage groups, both SUVmax and LBR showed significant differences (p < 0.001). The higher HRCT stage demonstrated significantly higher SUVmax and LBR. A linear correlation between 99mTc-HFAPI uptake and total HRCT score was observed for SUVmax (r = 0.7839, F = 54.41, p = 0.0094) and LBR (r = 0.7402, F = 56.33, p = 0.0092). 99mTc-HFAPI uptake also had moderate correlations with PFT results.

CONCLUSIONS

Our preliminary data show that the 99mTc-HFAPI SPECT imaging is a promising new imaging modality in IPF patients. Investigations of its clinical value in monitoring disease progression and treatment response are needed in the future.

FAP-retargeted Ad5 enables in vivo gene delivery to stromal cells in the tumor microenvironment

Fibroblast activation protein (FAP) is a cell surface serine protease that is highly expressed on reactive stromal fibroblasts, such as cancer-associated fibroblasts (CAFs), and generally absent in healthy adult tissues. FAP expression in the tumor stroma has been detected in more than 90% of all carcinomas, rendering CAFs excellent target cells for a tumor site-specific adenoviral delivery of cancer therapeutics. Here, we present a tropism-modified human adenovirus 5 (Ad5) vector that targets FAP through trivalent, designed ankyrin repeat protein-based retargeting adapters. We describe the development and validation of these adapters via cell-based screening assays and demonstrate adapter-mediated Ad5 retargeting to FAP+ fibroblasts in vitro and in vivo. We further show efficient in vivo delivery and in situ production of a therapeutic payload by CAFs in the tumor microenvironment (TME), resulting in attenuated tumor growth. We thus propose using our FAP-Ad5 vector to convert CAFs into a "biofactory," secreting encoded cancer therapeutics into the TME to enable a safe and effective cancer treatment.

Assessment of pulmonary fibrosis induced by paraquat using Al18F-NODA-FAPI-04 PET/CT

The lack of a highly sensitive method to evaluate paraquat (PQ)-induced pulmonary fibrosis and predict disease progression remains an unresolved clinic issue. Fibroblast activation protein (FAP) may play an important role in the pathogenesis of PQ-induced pulmonary fibrosis. We aimed to evaluate the role of FAP in the PQ-induced pulmonary fibrosis and the utility of fibroblast activation protein inhibitor (FAPI) for positron emission tomography (PET) imaging in PQ-induced pulmonary fibrosis. In our study, two cases of PQ poisoning were presented and FAPI PET/CT was performed as a novel imaging technique. The uptake of FAPI increased in both cases of PQ poisoning. Animal experiments were then performed to validate the findings in the patients. Physiological FAPI lung uptake was higher in mice of the PQ group than in the control group. The results of histological analysis and Western blot were consistent with the findings of PET/CT imaging. The pulmonary fibrosis animal model was developed by intragastric gavage of PQ. PET/CT imaging was performed after injection of FAPI. Lung tissues of mice were collected for fibrosis assessment after imaging. Immunohistochemistry for FAP, histology and Western blot for collagen were performed to further validate the imaging findings. In conclusion, FAPI was involved in the pathogenesis of fibrosis induced by PQ, and PET/CT with FAPI could detect lung fibrogenesis, making it a promising tool to assess early disease activity and predict disease progression.

Exosomal miR-103a-3p from Crohn's Creeping Fat-Derived Adipose-Derived Stem Cells Contributes to Intestinal Fibrosis by Targeting TGFBR3 and Activating Fibroblasts

BACKGROUND AND AIMS

Mesenteric adipose tissue hypertrophy is a hallmark of Crohn's disease [CD], and creeping fat [CF] is unique to CD. Adipose-derived stem cells [ASCs] from inflammatory tissue exhibited altered biological functions. The role of ASCs isolated from CF in intestinal fibrosis and the potential mechanism remain unclear.

METHODS

ASCs were isolated from CF [CF-ASCs] and disease-unaffected mesenteric adipose tissue [Ctrl-ASCs] of patients with CD. A series of in vitro and in vivo experiments were conducted to study the effects of exosomes from CF-ASCs [CF-Exos] on intestinal fibrosis and fibroblast activation. A micro-RNA microarray analysis was performed. Western blot, luciferase assay and immunofluorescence were performed to further detect the underlying mechanisms.

RESULTS

The results indicated that CF-Exos promoted intestinal fibrosis by activating fibroblasts in a dose-dependent manner. They continuously promoted progression of intestinal fibrosis even after dextran sulphate sodium withdrawal. Further analysis showed that exosomal miR-103a-3p was enriched in CF-Exos and participated in exosome-mediated fibroblast activation. TGFBR3 was identified as a target gene of miR-103a-3p. Mechanistically, CF-ASCs released exosomal miR-103a-3p and promoted fibroblast activation by targeting TGFBR3 and promoting Smad2/3 phosphorylation. We also found that the expression of miR-103a-3p in diseased intestine was positively associated with the degree of CF and fibrosis score.

CONCLUSION

Our findings show that exosomal miR-103a-3p from CF-ASCs promotes intestinal fibrosis by activating fibroblasts via TGFBR3 targeting, suggesting that CF-ASCs are potential therapeutic targets for intestinal fibrosis in CD.

Mesenchymal stromal cells facilitate resolution of pulmonary fibrosis by miR-29c and miR-129 intercellular transfer

To date, pulmonary fibrosis remains an unmet medical need. In this study, we evaluated the potency of mesenchymal stromal cell (MSC) secretome components to prevent pulmonary fibrosis development and facilitate fibrosis resolution. Surprisingly, the intratracheal application of extracellular vesicles (MSC-EVs) or the vesicle-depleted secretome fraction (MSC-SF) was not able to prevent lung fibrosis when applied immediately after the injury caused by bleomycin instillation in mice. However, MSC-EV administration induced the resolution of established pulmonary fibrosis, whereas the vesicle-depleted fraction did not. The application of MSC-EVs caused a decrease in the numbers of myofibroblasts and FAPa+ progenitors without affecting their apoptosis. Such a decrease likely occurred due to their dedifferentiation caused by microRNA (miR) transfer by MSC-EVs. Using a murine model of bleomycin-induced pulmonary fibrosis, we confirmed the contribution of specific miRs (miR-29c and miR-129) to the antifibrotic effect of MSC-EVs. Our study provides novel insights into possible antifibrotic therapy based on the use of the vesicle-enriched fraction of the MSC secretome.

Fibroblast Activation Protein Expression in Sarcomas

Objectives

Fibroblast activation protein alpha (FAP) is highly expressed by cancer-associated fibroblasts in multiple epithelial cancers. The aim of this study was to characterize FAP expression in sarcomas to explore its potential utility as a diagnostic and therapeutic target and prognostic biomarker in sarcomas.

Methods

Available tissue samples from patients with bone or soft tissue tumors were identified at the University of California, Los Angeles. FAP expression was evaluated via immunohistochemistry (IHC) in tumor samples (n = 63), adjacent normal tissues (n = 30), and positive controls (n = 2) using semiquantitative systems for intensity (0 = negative; 1 = weak; 2 = moderate; and 3 = strong) and density (none, <25%, 25-75%; >75%) in stromal and tumor/nonstromal cells and using a qualitative overall score (not detected, low, medium, and high). Additionally, RNA sequencing data in publicly available databases were utilized to compare FAP expression in samples (n = 10,626) from various cancer types and evaluate the association between FAP expression and overall survival (OS) in sarcoma (n = 168).

Results

The majority of tumor samples had FAP IHC intensity scores ≥2 and density scores ≥25% for stromal cells (77.7%) and tumor cells (50.7%). All desmoid fibromatosis, myxofibrosarcoma, solitary fibrous tumor, and undifferentiated pleomorphic sarcoma samples had medium or high FAP overall scores. Sarcomas were among cancer types with the highest mean FAP expression by RNA sequencing. There was no significant difference in OS in patients with sarcoma with low versus high FAP expression.

Conclusion

The majority of the sarcoma samples showed FAP expression by both stromal and tumor/nonstromal cells. Further investigation of FAP as a potential diagnostic and therapeutic target in sarcomas is warranted.

Synthesis and Evaluation of 68Ga-Labeled (2S,4S)-4-Fluoropyrrolidine-2-Carbonitrile and (4R)-Thiazolidine-4-Carbonitrile Derivatives as Novel Fibroblast Activation Protein-Targeted PET Tracers for Cancer Imaging

Fibroblast activation protein α (FAP-α) is a cell-surface protein overexpressed on cancer-associated fibroblasts that constitute a substantial component of tumor stroma and drive tumorigenesis. FAP is minimally expressed by most healthy tissues, including normal fibroblasts. This makes it a promising pan-cancer diagnostic and therapeutic target. In the present study, we synthesized two novel tracers, [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058, bearing a (2S,4S)-4-fluoropyrrolidine-2-carbonitrile or a (4R)-thiazolidine-4-carbonitrile pharmacophore, respectively. [⁶⁸Ga]Ga-SB03045 and [⁶⁸Ga]Ga-SB03058 were evaluated for their FAP-targeting capabilities using substrate-based in vitro binding assays, and in PET/CT imaging and ex vivo biodistribution studies in an HEK293T:hFAP tumor xenograft mouse model. The IC₅₀ values of ^natGa-SB03045 (1.59 ± 0.45 nM) and ^natGa-SB03058 (0.68 ± 0.09 nM) were found to be lower than those of the clinically validated ^natGa-FAPI-04 (4.11 ± 1.42 nM). Contrary to the results obtained in the FAP-binding assay, [⁶⁸Ga]Ga-SB03058 demonstrated a ~1.5 fold lower tumor uptake than that of [⁶⁸Ga]Ga-FAPI-04 (7.93 ± 1.33 vs. 11.90 ± 2.17 %ID/g), whereas [⁶⁸Ga]Ga-SB03045 (11.8 ± 2.35 %ID/g) exhibited a tumor uptake comparable to that of [⁶⁸Ga]Ga-FAPI-04. Thus, our data suggest that the (2S,4S)-4-fluoropyrrolidine-2-carbonitrile scaffold holds potential as a promising pharmacophore for the design of FAP-targeted radioligands for cancer diagnosis and therapy.

Joint analysis identified FAP as a prognostic and diagnostic biomarker correlated immune infiltration in gastric cancer

Gastric cancer is one of the most malignant types of cancer in the digestive system because of its high incidence and mortality. There is a notable association between gastric cancer progression and the level and sort of immune cells infiltrating the tumor microenvironment. First, 41 up-regulated differentially expressed genes (DEGs) and 91 down-regulated DEGs were identified from the Gene Expression Omnibus (GEO) database. Among the 21 core genes, prognosis biomarkers FAP, ASPN and CTHRC1 were identified for further study via Kaplan-Meier Plotter, with FAP having the highest prognostic value among them. In addition, the ROC curves of FAP (AUC=0.992), ASPN (AUC=0.955) and CTHRC1 (AUC=0.983) also showed high diagnostic value. Then the expression and mutation levels of the biomarkers were verified by GEPIA and cBioPortal. Their high expression levels were closely correlated to the clinical stages and metastasis status of gastric cancer. Furthermore, their expression was strongly relevant to immune infiltration and macrophage marker levels. In drug response analysis, gastric cancer cell lines with overexpression of FAP and ASPN were more sensitive to PI3K and MET inhibitors, respectively. Importantly, the meta-analysis showed that FAP had an overall positive rate of 68 % (63-73 %, 95 % CI; n = 382) and the patients with high expression of FAP showed a poor prognosis in terms of OS (HR=1.82, 1.33-2.48, 95 % CI) in gastric cancer. In short, FAP, ASPN and CTHRC1 were identified as potential prognostic and diagnostic biomarkers related with immunity and might be effective therapeutic targets of gastric cancer, and the significance of FAP for the prognosis was further assessed by meta-analysis.

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

BACKGROUND

Fibrosis and inflammation are major pathological changes of Crohn's disease (CD). Early detection and accurate severity evaluation of CD are critical for patient's prognosis. Endoscopy is widely used to evaluate CD progression. Herein, we evaluated the efficacy of [⁶⁸Ga]Ga-FAPI-04 PET/CT to identify lesions and assess the progression of CD.

METHODS

All CD patients received computed tomography enterography (CTE), [⁶⁸Ga]Ga-FAPI-04 PET/CT examination, and ileocolonoscopy within 1 week. Two independent gastroenterologists computed the Crohn's disease activity index (CDAI) of all patients. Two radiology physicians assessed the CTE images separately, and the CTE scores were calculated. Lastly, two nuclear medicine physicians independently examined the [⁶⁸Ga]Ga-FAPI-04 PET/CT images. Once the FAPI uptake of the intestinal segment was equal or higher relative to the liver (considered FAPI-positive), the target-to-background ratio (TBR) and global FAPI PET/CT score were computed, representing the independent intestinal activity and activity of all intestinal segments, respectively. Levels of fecal calprotectin (FCP) and C-reactive protein (CRP) were determined before the endoscopy. The Crohn's disease endoscopy index of severity (CDEIS) and the simple endoscopic score for Crohn's disease (SES-CD) were calculated during the endoscopy. Finally, all data were obtained and analyzed.

RESULTS

There were 74 intestinal segments in 16 patients were assessed. [⁶⁸Ga]Ga-FAPI-04 PET/CT identified 42 of 45 endoscopically lesioned segments (endoscopic lesions detection sensitivity: 93.3%), while CTE identified 39 of them (endoscopic lesions detection sensitivity: 86.7%). According to the receiver operating characteristic (ROC) analysis, [⁶⁸Ga]Ga-FAPI-04 PET/CT showed better performance in the detection of endoscopic lesions compared with CTE (P < 0.05). The TBR was significantly associated with the CTE score (r = 0.81; (95% CI): 0.736-0.869; P < 0.0001) and SES-CD values (r = 0.86; (95% CI): 0.776-0.908; P < 0.0001). In addition, the global FAPI PET/CT score was significantly correlated with FCP (r = 0.52; 95% CI, 0.02-0.81; P = 0.039), CRP (r = 0.60; 95% CI, 0.13-0.85; P = 0.014), CDEIS (r = 0.55; 95% CI, 0.06-0.83; P = 0.028), and CDAI (r = 0.81; 95% CI, 0.50-0.93; P < 0.0001).

CONCLUSION

In summary, [⁶⁸Ga]Ga-FAPI-04 PET/CT correlated well with endoscopic, CTE, clinical, and biomarkers of CD. It was also highly sensitive in the detection of different classes of lesions in all intestinal segments, and unlike other examinations, this technique required no patient fasting or bowel preparation. Therefore, [⁶⁸Ga]Ga-FAPI-04 PET/CT may be a promising method for assessing the activity of CD.

Imaging Fibrosis

Tissue injury in nonmalignant human disease can develop from either disproportionate inflammation or exaggerated fibrotic responses. The molecular and cellular fundamental of these 2 processes, their impact on disease prognosis and the treatment concept deviates fundamentally. Consequently, the synchronous assessment and quantification of these 2 processes in vivo is extremely desirable. Although noninvasive molecular techniques such as ¹⁸F-fluorodeoxyglucose PET offer insights into the degree of inflammatory activity, the assessment of the molecular dynamics of fibrosis remains challenging. The ⁶⁸Ga-fibroblast activation protein inhibitor-46 may improve noninvasive clinical diagnostic performance in patients with both fibroinflammatory pathology and long-term CT-abnormalities after severe COVID-19.

A Polymeric Nanoparticle Formulation for Targeted mRNA Delivery to Fibroblasts

Messenger RNA (mRNA)-based therapies offer enhanced control over the production of therapeutic proteins for many diseases. Their clinical implementation warrants formulations capable of delivering them safely and effectively to target sites. Owing to their chemical versatility, polymeric nanoparticles can be designed by combinatorial synthesis of different ionizable, cationic, and aromatic moieties to modulate cell targeting, using inexpensive formulation steps. Herein, 152 formulations are evaluated by high-throughput screening using a reporter fibroblast model sensitive to functional delivery of mRNA encoding Cre recombinase. Using in vitro and in vivo models, a polymeric nanoformulation based on the combination of 3 specific monomers is identified to transfect fibroblasts much more effectively than other cell types populating the skin, with superior performance than lipid-based transfection agents in the delivery of Cas9 mRNA and guide RNA. This tropism can be explained by receptor-mediated endocytosis, involving CD26 and FAP, which are overexpressed in profibrotic fibroblasts. Structure-activity analysis reveals that efficient mRNA delivery required the combination of high buffering capacity and low mRNA binding affinity for rapid release upon endosomal escape. These results highlight the use of high-throughput screening to rapidly identify chemical features towards the design of highly efficient mRNA delivery systems targeting fibrotic diseases.

Indocyanine-enhanced mouse model of bleomycin-induced lung fibrosis with hallmarks of progressive emphysema

The development of new drugs for idiopathic pulmonary fibrosis strongly relies on preclinical experimentation, which requires the continuous improvement of animal models and integration with in vivo imaging data. Here, we investigated the lung distribution of bleomycin (BLM) associated with the indocyanine green (ICG) dye by fluorescence imaging. A long-lasting lung retention (up to 21 days) was observed upon oropharyngeal aspiration (OA) of either ICG or BLM + ICG, with significantly more severe pulmonary fibrosis, accompanied by the progressive appearance of emphysema-like features, uniquely associated with the latter combination. More severe and persistent lung fibrosis, together with a progressive air space enlargement uniquely associated with the BLM + ICG group, was confirmed by longitudinal micro-computed tomography (CT) and histological analyses. Multiple inflammation and fibrosis biomarkers were found to be increased in the bronchoalveolar lavage fluid of BLM- and BLM + ICG-treated animals, but with a clear trend toward a much stronger increase in the latter group. Similarly, in vitro assays performed on macrophage and epithelial cell lines revealed a significantly more marked cytotoxicity in the case of BLM + ICG-treated mice. Also unique to this group was the synergistic upregulation of apoptotic markers both in lung sections and cell lines. Although the exact mechanism underlying the more intense lung fibrosis phenotype with emphysema-like features induced by BLM + ICG remains to be elucidated, we believe that this combination treatment, whose overall effects more closely resemble the human disease, represents a valuable alternative model for studying fibrosis development and for the identification of new antifibrotic compounds.

Comprehensive Analysis of Fibroblast Activation Protein Expression in Interstitial Lung Diseases

Rationale: Sustained activation of lung fibroblasts and the resulting oversynthesis of the extracellular matrix are detrimental events for patients with interstitial lung diseases (ILDs). Lung biopsy is a primary evaluation technique for the fibrotic status of ILDs, and is also a major risk factor for triggering acute deterioration. Fibroblast activation protein (FAP) is a long-known surface biomarker of activated fibroblasts, but its expression pattern and diagnostic implications in ILDs are poorly defined. Objectives: The present study aims to comprehensively investigate whether the expression intensity of FAP could be used as a potential readout to estimate or measure the amounts of activated fibroblasts in ILD lungs quantitatively. Methods: FAP expression in human primary lung fibroblasts as well as in clinical lung specimens was first tested using multiple experimental methods, including real-time quantitative PCR (qPCR), Western blot, immunofluorescence staining, deep learning measurement of whole slide immunohistochemistry, as well as single-cell sequencing. In addition, FAP-targeted positron emission tomography/computed tomography imaging PET/CT was applied to various types of patients with ILD, and the correlation between the uptake of FAP tracer and pulmonary function parameters was analyzed. Measurements and Main Results: Here, it was revealed, for the first time, FAP expression was upregulated significantly in the early phase of lung fibroblast activation event in response to a low dose of profibrotic cytokine. Single-cell sequencing data further indicate that nearly all FAP-positive cells in ILD lungs were collagen-producing fibroblasts. Immunohistochemical analysis validated that FAP expression level was closely correlated with the abundance of fibroblastic foci on human lung biopsy sections from patients with ILDs. We found that the total standard uptake value (SUV) of FAP inhibitor (FAPI) PET (SUVtotal) was significantly related to lung function decline in patients with ILD. Conclusions: Our results strongly support that in vitro and in vivo detection of FAP can assess the profibrotic activity of ILDs, which may aid in early diagnosis and the selection of an appropriate therapeutic window.

Early detection of radiation-induced myocardial damage by [18F]AlF-NOTA-FAPI-04 PET/CT imaging

PURPOSE

Retrospective analysis revealed increased [18F]AlF-NOTA-FAPI-04 uptake in the myocardium of patients with esophageal squamous cell cancer (ESCC) treated with concurrent chemoradiotherapy (CCRT). This study investigated and verified the feasibility of [18F]AlF-NOTA-FAPI-04 PET/CT for detecting radiation-induced myocardial damage (RIMD).

METHODS

Myocardial FAPI uptake was analyzed before and during radiotherapy in thirteen ESCC patients treated with CCRT. In the animal study, a single dose of 50 Gy was delivered to the cardiac apex of Wistar rats (24 rats, including 16 RIMD model rats and 8 control model rats). RIMD model rats were scanned with [18F]AlF-NOTA-FAPI-04 PET/CT weekly for 12 weeks, and left ventricular ejection fraction (LVEF) was measured by magnetic resonance imaging. Dynamic, blocking, and [18F]FDG PET/CT studies (4 rats/group) were performed on RIMD rats at 5 weeks post-radiation, and histopathological analyses were conducted.

RESULTS

Increased FAPI uptake in the myocardium was found after CCRT (1.53 ± 0.53 vs 1.88 ± 0.70, P = 0.015). In RIMD rats, significantly increased FAPI uptake in the damaged myocardium was observed from the 2nd week post-radiation exposure and peaked in the 5th week. Significantly more intense tracer accumulation was observed in the damaged myocardium than in the remote myocardium, as identified by decreased [18F]FDG uptake and confirmed by autoradiography, hematoxylin-eosin, Masson's trichrome, and immunohistochemical staining. The LVEF remained unchanged at the 3rd week post-radiation exposure but was remarkably decreased compared with that in the control group at the 8th week.

CONCLUSION

Through clinical phenomena and animal experimental studies, this study indicated that [18F]AlF-NOTA-FAPI-04 PET/CT imaging can detect RIMD noninvasively and before a decrease in LVEF, indicating the clinical potential of [18F]AlF-NOTA-FAPI-04 as a PET/CT tracer for early monitoring of RIMD.

Non-tumoral uptake of 68Ga-FAPI-04 PET: A retrospective study

OBJECTIVE

Fibroblast activation protein (FAP)-targeting radiopharmaceutical based on the FAP-specific inhibitor (FAPI) is considered as a potential alternative agent to FDG for tumor-specific imaging. However, FAP is also expressed in normal adult tissues. The aim of this study was to explore the image features of non-tumoral regions with high uptake of ⁶⁸Ga-FAPI-04 in positron emission tomography (PET) imaging and to reveal the physiological mechanisms of these regions.

MATERIAL

A total of 137 patients who underwent whole-body ⁶⁸Ga-FAPI-04 PET/MR (n=46) or PET/CT (n=91) were included in this retrospective study. Three experienced nuclear medicine physicians determined the non-tumoral regions according to other imaging modalities (CT, MRI, ¹⁸F-FDG PET, or ultrasound), clinical information, or pathological results. The regions of interest (ROIs) were drawn manually, and the maximum standardized uptake value (SUV_max) was measured.

RESULTS

A total of 392 non-tumoral uptake regions were included in this study. The included physiological regions were uterus (n=38), submandibular gland (n=118), nipple (n=37), gingiva (n=65), and esophagus (n=31). The incidence of ⁶⁸Ga-FAPI-04 uptake in physiological regions was independent of age, the tracer uptakes in the gingiva and esophagus were more common in male patients (p=0.006, 0.009), while that in the nipple was more common in female patients (p < 0.001). The included benign regions were inflammatory lymph node (n =10), pneumonia (n=13), atherosclerosis (n=10), pancreatitis (n=18), osteosclerosis (n=45), and surgical scar (n=7). No significant difference was observed in SUV_max between physiological and benign regions.

CONCLUSIONS

A number of organs exhibit physiological uptakes of ⁶⁸Ga-FAPI-04. Our study showed that regions with high ⁶⁸Ga-FAPI-04 uptake did not necessarily represent malignancy. Being familiar with physiological and typical benign ⁶⁸Ga-FAPI-04 uptake regions can be helpful for physicians to interpret images and to make an accurate diagnosis.

Novel tracers for molecular imaging of interstitial lung disease: A state of the art review

Interstitial lung disease is an overarching term for a wide range of disorders characterized by inflammation and/or fibrosis in the lungs. Most prevalent forms, among others, include idiopathic pulmonary fibrosis (IPF) and connective tissue disease associated interstitial lung disease (CTD-ILD). Currently, only disease modifying treatment options are available for IPF and progressive fibrotic CTD-ILD, leading to reduction or stabilization in the rate of lung function decline at best. Management of these patients would greatly advance if we identify new strategies to improve (1) early detection of ILD, (2) predicting ILD progression, (3) predicting response to therapy and (4) understanding pathophysiology. Over the last years, positron emission tomography (PET) and single photon emission computed tomography (SPECT) have emerged as promising molecular imaging techniques to improve ILD management. Both are non-invasive diagnostic tools to assess molecular characteristics of an individual patient with the potential to apply personalized treatment. In this review, we encompass the currently available pre-clinical and clinical studies on molecular imaging with PET and SPECT in IPF and CTD-ILD. We provide recommendations for potential future clinical applications of these tracers and directions for future research.

Epitope-based minigene vaccine targeting fibroblast activation protein α induces specific immune responses and anti-tumor effects in 4 T1 murine breast cancer model

Fibroblast activation protein (FAPα) is a tumor stromal antigen expressed by cancer-associated fibroblasts (CAFs) in more than 90 % of malignant epithelial carcinomas. FAPα-based immunotherapy has been reported and showed that FAPα-specific immune response can remold immune microenvironment and contribute to tumor regression. Many FAPα-based vaccines have been investigated in preclinical trials, which can elicit strong and durable cytolytic T lymphocytes (CTL) with good safety. However, epitope-based FAPα vaccines are rarely reported. To break tolerance against self-antigens, analogue epitopes with modified peptides at the anchor residues are typically used to improve epitope immunogenicity. To investigate the feasibility of a FAPα epitope-based vaccine for cancer immunotherapy in vivo, we conducted a preclinical study to identify a homologous CTL epitope of human and mouse FAPα and obtained its analogue epitope in BALB/c mice, and explored the anti-tumor activity of their minigene vaccines in 4 T1 tumor-bearing mice. By using in silico epitope prediction tools and immunogenicity assays, immunodominant epitope FAP.291 (YYFSWLTWV) and its analogue epitope FAP.291I9 (YYFSWLTWI) were identified. The FAP.291-based epitope minigene vaccine successfully stimulated CTLs targeting CAFs and exhibited anti-tumor activity in a 4 T1 murine breast cancer model. Furthermore, although the analogue epitope FAP.291I9 enhanced FAP.291-specific immune responses, improvement of anti-tumor immunity effects was not observed. Check of immunosuppressive factors revealed that the high levels of IL-10, IL-13, myeloid-derived suppressor cells and iNOS induced by FAP.291I9 increased, which considered the main cause of the failure of the analogue epitope-based vaccine. Thus, we demonstrated for the first time that the FAP.291 minigene vaccine could induce mouse CTLs and also function as a tumor regression antigen, providing the basis for future studies of FAPα epitope-based vaccines. This study may also be valuable for further improvement of the immunogenicity of analogue epitope vaccines.

Preclinical evaluation of FAP-2286 for fibroblast activation protein targeted radionuclide imaging and therapy

PURPOSE

Fibroblast activation protein (FAP) is a membrane-bound protease that has limited expression in normal adult tissues but is highly expressed in the tumor microenvironment of many solid cancers. FAP-2286 is a FAP-binding peptide coupled to a radionuclide chelator that is currently being investigated in patients as an imaging and therapeutic agent. The potency, selectivity, and efficacy of FAP-2286 were evaluated in preclinical studies.

METHODS

FAP expression analysis was performed by immunohistochemistry and autoradiography on primary human cancer specimens. FAP-2286 was assessed in biochemical and cellular assays and in in vivo imaging and efficacy studies, and was further evaluated against FAPI-46, a small molecule-based FAP-targeting agent.

RESULTS

Immunohistochemistry confirmed elevated levels of FAP expression in multiple tumor types including pancreatic, breast, and sarcoma, which correlated with FAP binding by FAP-2286 autoradiography. FAP-2286 and its metal complexes demonstrated high affinity to FAP recombinant protein and cell surface FAP expressed on fibroblasts. Biodistribution studies in mice showed rapid and persistent uptake of 68Ga-FAP-2286, 111In-FAP-2286, and 177Lu-FAP-2286 in FAP-positive tumors, with renal clearance and minimal uptake in normal tissues. 177Lu-FAP-2286 exhibited antitumor activity in FAP-expressing HEK293 tumors and sarcoma patient-derived xenografts, with no significant weight loss. In addition, FAP-2286 maintained longer tumor retention and suppression in comparison to FAPI-46.

CONCLUSION

In preclinical models, radiolabeled FAP-2286 demonstrated high tumor uptake and retention, as well as potent efficacy in FAP-positive tumors. These results support clinical development of 68Ga-FAP-2286 for imaging and 177Lu-FAP-2286 for therapeutic use in a broad spectrum of FAP-positive tumors.

[68 Ga]Ga-FAPI-46 PET for non-invasive detection of pulmonary fibrosis disease activity

PURPOSE

The lack of effective molecular biomarkers to monitor idiopathic pulmonary fibrosis (IPF) activity or treatment response remains an unmet clinical need. Herein, we determined the utility of fibroblast activation protein inhibitor for positron emission tomography (FAPI PET) imaging in a mouse model of pulmonary fibrosis.

METHODS

Pulmonary fibrosis was induced by intratracheal administration of bleomycin (1 U/kg) while intratracheal saline was administered to control mice. Subgroups from each cohort (n = 3-5) underwent dynamic 1 h PET/CT after intravenously injecting FAPI-46 radiolabeled with gallium-68 ([68 Ga]Ga-FAPI-46) at 7 days and 14 days following disease induction. Animals were sacrificed following imaging for ex vivo gamma counting and histologic correlation. [68 Ga]Ga-FAPI-46 uptake was quantified and reported as percent injected activity per cc (%IA/cc) or percent injected activity (%IA). Lung CT density in Hounsfield units (HU) was also correlated with histologic examinations of lung fibrosis.

RESULTS

CT only detected differences in the fibrotic response at 14 days post-bleomycin administration. [68 Ga]Ga-FAPI-46 lung uptake was significantly higher in the bleomycin group than in control subjects at 7 days and 14 days. Significantly (P = 0.0012) increased [68 Ga]Ga-FAPI-46 lung uptake in the bleomycin groups at 14 days (1.01 ± 0.12%IA/cc) vs. 7 days (0.33 ± 0.09%IA/cc) at 60 min post-injection of the tracer was observed. These findings were consistent with an increase in both fibrinogenesis and FAP expression as seen in histology.

CONCLUSION

CT was unable to assess disease activity in a murine model of IPF. Conversely, FAPI PET detected both the presence and activity of lung fibrogenesis, making it a promising tool for assessing early disease activity and evaluating the efficacy of therapeutic interventions in lung fibrosis patients.