Effects of the fibroblast activation protein inhibitor, PT100, in a murine model of pulmonary fibrosis

Evaluation of drug resistance for EGFR-TKIs in lung cancer via multicellular lung-on-a-chip

Drug resistance to irreversible epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) is a primary factor affecting their therapeutic efficacy in human non-small cell lung cancer (NSCLC). NSCLC cells can undergo epithelial-mesenchymal transition (EMT) induced by many factors in the tumour microenvironment (TME), which plays a crucial role in tumour drug resistance. In this study, a multicellular lung-on-a-chip that can realise the cell co-culture of the human non-small cell lung cancer cell line HCC827, human foetal lung fibroblasts (HFL-1), and human umbilical vein endothelial cells (HUVECs) is prepared. The TME was simulated on the chip combined with perfusion and other factors, and the drug evaluation of osimertinib was performed to explore the drug resistance mechanism of EGFR-TKIs. In the early stages, a two-dimensional static cell co-culture was achieved by microchip, and the results showed that HFL-1 cells could be transformed into cancer-associated fibroblasts (CAFs), and HCC827 cells could undergo EMT, both of which were mediated by Interleukin-6 (IL-6). Vimentin (VIM) and Alpha Skeletal Muscle Actin (a-SMA) expression of HFL-1 was upregulated, whereas E-cadherin (E-cad) expression of HCC827 was down-regulated. Further, N-cadherin (N-cad) expression of HCC827 was upregulated. In both the static cell co-culture and multicellular lung-on-a-chip, HCC827 cells with CAFs co-culture or IL-6 treatment developed resistance to osimertinib. Further use of the IL-6 antibody inhibitor tocilizumab could reverse EGFR-TKI resistance to a certain extent. Combination therapy with tocilizumab and EGFR-TKIs may provide a novel therapeutic strategy for overcoming EGFR-TKI resistance caused by EMT in NSCLC. Furthermore, the lung-on-a-chip can simulate complex TME and can be used for evaluating tumour resistance and exploring mechanisms, with the potential to become an important tool for personalised diagnosis, treatment, and biomedical research.

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.

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.

FAPi PET/CT for assessment and visualisation of active myositis-related interstitial lung disease: a prospective observational pilot study

Background

Interstitial lung disease (ILD) is a common manifestation of idiopathic inflammatory myopathies (IIM) and a substantial contributor to hospitalisation, increased morbidity, and mortality. In-vivo evidence of ongoing tissue remodelling in IIM-ILD is scarce. We aimed to evaluate fibroblast activation in lungs of IIM-patients and control individuals using ⁶⁸Ga-labelled inhibitor of Fibroblast-Activation-Protein (FAPi) based positronic emission tomography and computed tomography imaging (PET/CT).

Methods

In this prospective observational pilot study, consecutive patients with IIM and participants without rheumatic conditions or ILD serving as a control group were recruited at the Medical University of Vienna, Austria, and underwent FAPi PET/CT imaging. Standard-of-care procedures including clinical examination, assessment of severity of dyspnoea, high-resolution computed tomography (HR-CT), and pulmonary function testing (PFT) were performed on all patients with IIM at baseline and for patients with IIM-ILD at follow-up of 12 months. Baseline pulmonary FAPi-uptake was assessed by the maximum (SUVmax) and mean (SUVmean) standardized uptake values (SUV) over the whole lung (wl). SUV was corrected for blood pool background activity and target-to-background ratios (TBR) were calculated. We compared pulmonary FAPi-uptake between patients with IIM-ILD and those without ILD, as well as controls, and correlated baseline FAP-uptake with standard diagnostic tools such as HR-CT and PFT. For predictive implications, we investigated whether patients with IIM and progressive ILD exhibited higher baseline FAPi-uptake compared to those with stable ILD. Metrics are reported as mean with standard deviation (±SD).

Findings

Between November 16, 2021 and October 10, 2022, a total of 32 patients were enrolled in the study. Three participants from the control group were excluded due to cardiopulmonary disease. In individuals with IIM-ILD (n = 14), wlTBRmax and wlTBRmean were significantly increased as compared with both non-ILD-IIM patients (n = 5) and the control group (n = 16): wlTBRmax: 2.06 ± 1.04 vs. 1.04 ± 0.22 (p = 0.019) and 1.08 ± 0.19 (p = 0.0012) and wlTBRmean: 0.45 ± 0.19 vs. 0.26 ± 0.06 (p = 0.025) and 0.27 ± 0.07 (p = 0.0024). Similar values were observed in wlTBRmax or wlTBRmean between non-ILD IIM patients and the control group. Patients with progressive ILD displayed significantly enhanced wlTBRmax and wlTBRmean values at baseline compared to patients with stable ILD: wlTBRmax: 1.30 ± 0.31 vs. 2.63 ± 1.04 (p = 0.0084) and wlTBRmean: 0.32 ± 0.08 vs. 0.55 ± 0.19 (p = 0.021). Strong correlations were found between FAPi-uptake and disease extent on HR-CT (wlTBRmax: R = 0.42, p = 0.07; wlTBRmean: R = 0.56, p = 0.013) and severity of respiratory symptoms determined by the New York Heart Association (NYHA) classification tool (wlTBRmax: R = 0.52, p = 0.022; wlTBRmean: R = 0.59, p = 0.0073). Further, pulmonary FAPi-uptake showed inverse correlation with forced vital capacity (FVC) (wlTBRmax: R = -0.56, p = 0.012; wlTBRmean: R = -0.64, p = 0.0033) and diffusing capacity of the lungs for carbon monoxide (DLCO) (wlTBRmax: R = -0.52, p = 0.028; wlTBRmean: R = -0.68, p = 0.0017).

Interpretation

Our study demonstrates higher fibroblast activation in patients with IIM-ILD compared to non-ILD patients and controls. Intensity of pulmonary FAPi accumulation was associated with progression of ILD. Considering that this study was carried out on a small population, FAPi PET/CT may serve as a useful non-invasive tool for risk stratification of lung disease in IIM.

Funding

The Austrian Research Fund.

Fibroblast activation protein-sensitive polymeric nanobeacon for early diagnosis of renal fibrosis

Early diagnosis and treatment of renal fibrosis (RF) significantly affect the clinical outcomes of chronic kidney diseases (CKDs). As the typical fibrotic ailment, RF is characterized by remodeling of the extracellular matrix, and the activation of fibroblast activation protein (FAP) plays a crucial role in the mediation of extracellular matrix protein degradation. Therefore, FAP can serve as a biomarker for RF. However, up to now, no effective tools have been reported to diagnose early-stage RF via detecting FAP. In this work, a polymeric nanobeacon integrating an FAP-sensitive amphiphilic polymer and fluorophores was proposed, which was used to diagnose early RF by sensing FAP. The FAP can be detected in the range of 0 to 200 ng/mL with a detection limit of 0.132 ng/mL. Furthermore, the fluorescence imaging results demonstrate that the polymeric nanobeacon can sensitively image fibrotic kidneys in mice with unilateral ureteral occlusion (UUO), suggesting its potential for early RF diagnosis and guidance of FAP-targeted treatments. Importantly, when employed alongside with non-invasive diagnostic techniques like magnetic resonance imaging (MRI) and serological tests, this nanobeacon exhibits excellent biocompatibility, low biological toxicity, and sustained imaging capabilities, making it a suitable fluorescent tool for diagnosing various FAP-related fibrotic conditions. To our knowledge, this study represents the first attempt to image RF in early stage by detecting FAP, offering a promising fluorescent molecular tool for diagnosing various FAP-associated diseases in the future.

Transcriptomic Signature and Pro-Osteoclastic Secreted Factors of Abnormal Bone-Marrow Stromal Cells in Fibrous Dysplasia

Fibrous dysplasia (FD) is a mosaic skeletal disorder caused by somatic activating variants of GNAS encoding for Gαs and leading to excessive cyclic adenosine monophosphate signaling in bone-marrow stromal cells (BMSCs). The effect of Gαs activation in the BMSC transcriptome and how it influences FD lesion microenvironment are unclear. We analyzed changes induced by Gαs activation in the BMSC transcriptome and secretome. RNAseq analysis of differential gene expression of cultured BMSCs from patients with FD and healthy volunteers, and from an inducible mouse model of FD, was performed, and the transcriptomic profiles of both models were combined to build a robust FD BMSC genetic signature. Pathways related to Gαs activation, cytokine signaling, and extracellular matrix deposition were identified. To assess the modulation of several key secreted factors in FD pathogenesis, cytokines and other factors were measured in culture media. Cytokines were also screened in a collection of plasma samples from patients with FD, and positive correlations of several cytokines to their disease burden score, as well as to one another and bone turnover markers, were found. These data support the pro-inflammatory, pro-osteoclastic behavior of FD BMSCs and point to several cytokines and other secreted factors as possible therapeutic targets and/or circulating biomarkers for FD.

Value of [18F]AlF-NOTA-FAPI-04 PET/CT for differential diagnosis of malignant and various inflammatory lung lesions: comparison with [18F]FDG PET/CT

OBJECTIVE

Uptake of the imaging tracers [18F]AlF-NOTA-FAPI-04 and [18F]FDG varies in some inflammatory lesions, which may result in false-positive findings for malignancy on PET/CT. Our aim was to compare the [18F]AlF-NOTA-FAPI-04 and [18F]FDG PET/CT imaging features of malignant and various inflammatory lung lesions and to analyze their value for differential diagnosis.

METHODS

We retrospectively analyzed [18F]AlF-NOTA-FAPI-04 PET/CT scans from 67 cancer patients taken between December 2020 and January 2022, as well as the scans of 32 patients who also underwent [18F]FDG PET/CT imaging. The maximum and mean standardized uptake values (SUVmax and SUVmean, respectively) and lesion-to-background ratio (LBR) were calculated. The predictive capabilities of semiquantitative PET/CT parameters were analyzed by receiver operating characteristic curve analysis.

RESULTS

A total of 70 inflammatory and 37 malignant lung lesions were evaluated by [18F]AlF‑NOTA‑FAPI‑04 PET/CT, and 33 inflammatory and 26 malignant lung lesions also were evaluated by [18F]FDG PET/CT. Inflammatory lesions exhibited lower [18F]AlF-NOTA-FAPI-04 and [18F]FDG uptake compared to malignant lesions, with statistically significant differences in SUVmax, SUVmean, and LBR (all p < 0.001). [18F]AlF-NOTA-FAPI-04 uptake also varied among different types of inflammatory lesions (SUVmax, p = 0.005; SUVmean, p = 0.008; LBR, p < 0.001), with the highest uptake observed in bronchiectasis with infection, followed by postobstructive pneumonia, and the lowest in pneumonia. [18F]FDG uptake was higher in postobstructive pneumonia than in pneumonia (SUVmax, p = 0.009; SUVmean, p = 0.016; LBR, p = 0.004).

CONCLUSION

[18F]AlF-NOTA-FAPI-04/[18F]FDG PET/CT showed significantly lower uptake in inflammatory lesions than malignancies as well as variation in different types of inflammatory lesions, and thus, may be valuable for distinguishing malignant and various inflammatory findings.

CLINICAL RELEVANCE STATEMENT

Our study confirmed that the uptake of [18F]AlF-NOTA-FAPI-04/[18F]FDG PET/CT in inflammatory and malignant lung lesions is different, which is beneficial to distinguish inflammatory and malignant lung lesions in clinic.

KEY POINTS

• Malignant and different inflammatory lung lesions showed varying degrees of uptake of [18F]AlF-NOTA-FAPI-04 and [18F]FDG. • Inflammatory lung lesions showed significantly less uptake than malignancies, and uptake varied among different types of inflammatory lesions. • Both types of PET/CT could differentiate malignant and various inflammatory lung findings.

Transcriptomic signature and pro-osteoclastic secreted factors of abnormal bone marrow stromal cells in fibrous dysplasia

Fibrous dysplasia (FD) is a mosaic skeletal disorder caused by somatic activating variants in GNAS, encoding for Gαs, which leads to excessive cAMP signaling in bone marrow stromal cells (BMSCs). Despite advancements in our understanding of FD pathophysiology, the effect of Gαs activation in the BMSC transcriptome remains unclear, as well as how this translates into their local influence in the lesional microenvironment. In this study, we analyzed changes induced by Gαs activation in BMSC transcriptome and performed a comprehensive analysis of their production of cytokines and other secreted factors. We performed RNAseq of cultured BMSCs from patients with FD and healthy volunteers, and from an inducible mouse model of FD, and combined their transcriptomic profiles to build a robust FD BMSC genetic signature. Pathways related to Gαs activation, cytokine signaling, and extracellular matrix deposition were identified. In addition, a comprehensive profile of their secreted cytokines and other factors was performed to identify modulation of several key factors we hypothesized to be involved in FD pathogenesis. We also screened circulating cytokines in a collection of plasma samples from patients with FD, finding positive correlations of several cytokines to their disease burden score, as well as to one another and bone turnover markers. Overall, these data support a pro-inflammatory, pro-osteoclastic behavior of BMSCs bearing hyperactive Gαs variants, and point to several cytokines and other secreted factors as possible therapeutic targets and/or circulating biomarkers for FD.

Bone Metastasis in Prostate Cancer: Bone Scan Versus PET Imaging

Prostate cancer is the second most common cause of malignancy among men, with bone metastasis being a significant source of morbidity and mortality in advanced cases. Detecting and treating bone metastasis at an early stage is crucial to improve the quality of life and survival of prostate cancer patients. This objective strongly relies on imaging studies. While CT and MRI have their specific utilities, they also possess certain drawbacks. Bone scintigraphy, although cost-effective and widely available, presents high false-positive rates. The emergence of PET/CT and PET/MRI, with their ability to overcome the limitations of standard imaging methods, offers promising alternatives for the detection of bone metastasis. Various radiotracers targeting cell division activity or cancer-specific membrane proteins, as well as bone seeking agents, have been developed and tested. The use of positron-emitting isotopes such as fluorine-18 and gallium-68 for labeling allows for a reduced radiation dose and unaffected biological properties. Furthermore, the integration of artificial intelligence (AI) and radiomics techniques in medical imaging has shown significant advancements in reducing interobserver variability, improving accuracy, and saving time. This article provides an overview of the advantages and limitations of bone scan using SPECT and SPECT/CT and PET imaging methods with different radiopharmaceuticals and highlights recent developments in hybrid scanners, AI, and radiomics for the identification of prostate cancer bone metastasis using molecular imaging.

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.

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.

Fibroblast Activation Protein Targeting Probe with Gly-Pro Sequence for PET of Glioblastoma

As an important cancer-associated fibroblast-specific biomarker, fibroblast activation protein (FAP) has become an attractive target for tumor diagnosis and treatment. However, most FAP-based radiotracers showed inadequate uptake and short retention in tumors. In this study, we designed and synthesized a novel FAP ligand (DOTA-GPFAPI-04) through assembling three functional moieties: a quinoline-based FAP inhibitor for specifically targeting FAP, a FAP substrate Gly-Pro as a linker for increasing the FAP protein interaction, and a 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA) chelator for radiolabeling with different radionuclides. The FAP targeting ability of DOTA-GPFAPI-04 was investigated by molecular docking studies. DOTA-GPFAPI-04 was then radiolabeled with 68Ga to give [68Ga]Ga-DOTA-GPFAPI-04 for positron emission tomography (PET) imaging of glioblastoma. [68Ga]Ga-DOTA-GPFAPI-04 exhibited a purity of >98% and high stability analyzed by radio-HPLC in saline and mouse serum. Cell uptake studies demonstrated the targeting specificity of the probe. Further in vivo pharmacokinetic studies in normal mice demonstrated the quick clearance of the probe. Moreover, compared with the widely studied [68Ga]Ga-FAPI-04, [68Ga]Ga-DOTA-GPFAPI-04 showed much higher U87MG tumor uptake values (4.467 ± 0.379 for [68Ga]Ga-DOTA-GPFAPI-04 and 1.267 ± 0.208% ID/g for [68Ga]Ga-FAPI-04 at 0.5 h post-injection, respectively). The area under the curve based on time-activity curve (TAC) analysis for tumor radioactivity in small animal models was 422.5 for [68Ga]Ga-DOTA-GPFAPI-04 and 98.14 for [68Ga]Ga-FAPI-04, respectively, demonstrating that the former had longer tumor retention time. The tumor-to-muscle (T/M) ratio for [68Ga]Ga-DOTA-GPFAPI-04 reached 9.15 in a U87MG xenograft animal model. PET imaging and blocking assays showed that [68Ga]Ga-DOTA-GPFAPI-04 had specific tumor uptake. In summary, this study demonstrates the successful synthesis and evaluation of a novel FAPI targeting probe, [68Ga]Ga-DOTA-GPFAPI-04, with a Gly-Pro sequence. It shows favorable in vivo glioblastoma imaging properties and relatively long tumor retention, highlighting DOTA-GPFAPI-04 as a promising molecular scaffold for developing FAP targeting tumor theranostic agents.

Magnetic resonance imaging and ultrasound elastography in the context of preclinical pharmacological research: significance for the 3R principles

The 3Rs principles-reduction, refinement, replacement-are at the core of preclinical research within drug discovery, which still relies to a great extent on the availability of models of disease in animals. Minimizing their distress, reducing their number as well as searching for means to replace them in experimental studies are constant objectives in this area. Due to its non-invasive character in vivo imaging supports these efforts by enabling repeated longitudinal assessments in each animal which serves as its own control, thereby enabling to reduce considerably the animal utilization in the experiments. The repetitive monitoring of pathology progression and the effects of therapy becomes feasible by assessment of quantitative biomarkers. Moreover, imaging has translational prospects by facilitating the comparison of studies performed in small rodents and humans. Also, learnings from the clinic may be potentially back-translated to preclinical settings and therefore contribute to refining animal investigations. By concentrating on activities around the application of magnetic resonance imaging (MRI) and ultrasound elastography to small rodent models of disease, we aim to illustrate how in vivo imaging contributes primarily to reduction and refinement in the context of pharmacological research.

Fibroblast Activation Protein Inhibitor-Based Radionuclide Therapies: Current Status and Future Directions

Metastatic malignancies have limited management strategies and variable treatment responses. Cancer cells develop beside and depend on the complex tumor microenvironment. Cancer-associated fibroblasts, with their complex interaction with tumor and immune cells, are involved in various steps of tumorigenesis, such as growth, invasion, metastasis, and treatment resistance. Prooncogenic cancer-associated fibroblasts emerged as attractive therapeutic targets. However, clinical trials have achieved suboptimal success. Fibroblast activation protein (FAP) inhibitor-based molecular imaging has shown encouraging results in cancer diagnosis, making them innovative targets for FAP inhibitor-based radionuclide therapies. This review summarizes the results of preclinical and clinical FAP-based radionuclide therapies. We will describe advances and FAP molecule modification in this novel therapy, as well as its dosimetry, safety profile, and efficacy. This summary may guide future research directions and optimize clinical decision-making in this emerging field.

Synthesis and Evaluation of 99mTc-Labeled FAP Inhibitors with Different Linkers for Imaging of Fibroblast Activation Proteins in Tumors

Fibroblast activation protein (FAP) is a potential target for tumor diagnosis and treatment due to its selective expression on cancer-associated fibroblasts (CAFs) in most solid tumor stroma. Two FAP inhibitor (FAPI) derived ligands (L1 and L2) containing different lengths of _DPro-Gly (PG) repeat units as linkers were designed and synthesized with high affinity for FAP. Two stable hydrophilic ^99mTc-labeled complexes ([^99mTc]Tc-L1 and [^99mTc]Tc-L2) were obtained. In vitro cellular studies show that the uptake mechanism is correlated with FAP uptake, and [^99mTc]Tc-L1 shows a higher cell uptake and specific binding to FAP. A nanomolar K_d value for [^99mTc]Tc-L1 indicates its significantly high target affinity for FAP. The biodistribution and microSPECT/CT images obtained for U87MG tumor mice show that [^99mTc]Tc-L1 has high tumor uptake with specificity to FAP and high tumor-to-nontarget ratios. As an inexpensive, easily made, and widely available tracer, [^99mTc]Tc-L1 holds great promise for clinical applications.

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.

Enhancing Clinical Diagnosis for Patients With Persistent Pulmonary Abnormalities After COVID-19 Infection: The Potential Benefit of 68 Ga-FAPI PET/CT

PATIENTS AND METHODS

Six post COVID-19 patients suspected for pulmonary fibrosis were scheduled for dual-tracer PET/CT with 18 F-FDG and 68 Ga-fibroblast activation protein inhibitor (FAPI)-46. The uptake of 68 Ga-FAPI-46 in the involved lung was compared with a control group of 9 non-COVID-19 patients. Clinical data and PET/CT imaging were collected and analyzed.

RESULTS

PET/CT revealed in all 6 pulmonary impaired patients the reduced glucose avidity on 18 F-FDG and clear positivity on 68 Ga-FAPI-46 PET/CT in comparison to the control group.

CONCLUSIONS

Enhancing fibrotic repair mechanisms, 68 Ga-FAPI PET/CT may improve noninvasive clinical diagnostic performance in patients with long-term CT abnormalities after severe COVID-19. Although this study shows promising results, additional studies in larger populations are required to establish a general diagnostic guideline.

Diagnostic and therapeutic potential of protease inhibition

Proteases are enzymes that hydrolyze peptide bonds in proteins and peptides; thus, they control virtually all biological processes. Our understanding of protease function has advanced considerably from nonselective digestive enzymes to highly specialized molecular scissors that orchestrate complex signaling networks through a limited proteolysis. The catalytic activity of proteases is tightly regulated at several levels, ranging from gene expression through trafficking and maturation to posttranslational modifications. However, when this delicate balance is disturbed, many diseases develop, including cancer, inflammatory disorders, diabetes, and neurodegenerative diseases. This new understanding of the role of proteases in pathologic physiology indicates that these enzymes represent excellent molecular targets for the development of therapeutic inhibitors, as well as for the design of chemical probes to visualize their redundant activity. Recently, numerous platform technologies have been developed to identify and optimize protease substrates and inhibitors, which were further used as lead structures for the development of chemical probes and therapeutic drugs. Due to this considerable success, the clinical potential of proteases in therapeutics and diagnostics is rapidly growing and is still not completely explored. Therefore, small molecules that can selectively target aberrant protease activity are emerging in diseases cells. In this review, we describe modern trends in the design of protease drugs as well as small molecule activity-based probes to visualize selected proteases in clinical settings.

Mesoporous Polydopamine Loaded Pirfenidone Target to Fibroblast Activation Protein for Pulmonary Fibrosis Therapy

Recently, fibroblast activation protein (FAP), an overexpressed transmembrane protein of activated fibroblast in pulmonary fibrosis, has been considered as the new target for diagnosing and treating pulmonary fibrosis. In this work, mesoporous polydopamine (MPDA), which is facile prepared and easily modified, is developed as a carrier to load antifibrosis drug pirfenidone (PFD) and linking FAP inhibitor (FAPI) to realize lesion-targeted drug delivery for pulmonary fibrosis therapy. We have found that PFD@MPDA-FAPI is well biocompatible and with good properties of antifibrosis, when ICG labels MPDA-FAPI, the accumulation of the nanodrug at the fibrosis lung in vivo can be observed by NIR imaging, and the antifibrosis properties of PFD@MPDA-FAPI in vivo were also better than those of pure PFD and PFD@MPDA; therefore, the easily produced and biocompatible nanodrug PFD@MPDA-FAPI developed in this study is promising for further clinical translations in pulmonary fibrosis antifibrosis therapy.

FAPI-PET/CT in Cancer Imaging: A Potential Novel Molecule of the Century

Fibroblast activation protein (FAP), a type II transmembrane serine protease, is highly expressed in more than 90% of epithelial tumors and is closely associated with various tumor invasion, metastasis, and prognosis. Using FAP as a target, various FAP inhibitors (FAPIs) have been developed, most of which have nanomolar levels of FAP affinity and high selectivity and are used for positron emission tomography (PET) imaging of different tumors. We have conducted a systematic review of the available data; summarized the biological principles of FAPIs for PET imaging, the synthesis model, and metabolic characteristics of the radiotracer; and compared the respective values of FAPIs and the current mainstream tracer ¹⁸F-Fludeoxyglucose (¹⁸F-FDG) in the clinical management of tumor and non-tumor lesions. Available research evidence indicates that FAPIs are a molecular imaging tool complementary to ¹⁸F-FDG and are expected to be the new molecule of the century with better imaging effects than ¹⁸F-FDG in a variety of cancers, including gastrointestinal tumors, liver tumors, breast tumors, and nasopharyngeal carcinoma.

[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.

Serum Exosomal lncRNA AC007099.1 Regulates the Expression of Neuropeptide-Related FAP, as a Potential Biomarker for Hepatocarcinogenesis

Neuropeptide-associated fibroblast activation protein (FAP) may be an important risk factor for neurovascular metastasis in hepatocellular carcinoma. Analysis of The Cancer Genome Atlas (TCGA) database showed that FAP mRNA was highly expressed in most human tumor tissues. The HPA database then verified that FAP was highly expressed in tumor tissues following protein translation. Survival analysis then showed that the level of FAP expression significantly affected the overall survival (OS), progress free interval (PFI), and disease specific survival (DSS) of patients with hepatocellular carcinoma. A high expression of FAP in tumor tissue is associated with poor patient prognosis. According to the results of spearman correlation, AC009099 and FAP were negatively correlated with miR-7152 expression, while AC009099 and FAP expression were positively correlated. The lncRNA AC007099.1, which may serve as a potential target for the treatment of hepatocellular carcinoma, was associated with liver cancer. AC007099.1/miR-7152/FAP was found to be associated with immune infiltration in patients with hepatocellular carcinoma. Enrichment analysis suggests that the AC009099/miR-7152/FAP ceRNA regulatory network is associated with neuropeptide functional pathways. In conclusion, a neuropeptide-related AC009099/miR-7152/FAP ceRNA regulatory network was constructed in this study.

Therapeutic Targeting of Intestinal Fibrosis in Crohn's Disease

Intestinal fibrosis is one of the most threatening complications of Crohn’s disease. It occurs in more than a third of patients with this condition, is associated with increased morbidity and mortality, and surgery often represents the only available therapeutic option. The mechanisms underlying intestinal fibrosis are partly known. Studies conducted so far have shown a relevant pathogenetic role played by mesenchymal cells (especially myofibroblasts), cytokines (e.g., transforming growth factor-β), growth factors, microRNAs, intestinal microbiome, matrix stiffness, and mesenteric adipocytes. Further studies are still necessary to elucidate all the mechanisms involved in intestinal fibrosis, so that targeted therapies can be developed. Although several pre-clinical studies have been conducted so far, no anti-fibrotic therapy is yet available to prevent or reverse intestinal fibrosis. The aim of this review is to provide an overview of the main therapeutic targets currently identified and the most promising anti-fibrotic therapies, which may be available in the near future.

Fibroblast Activation Protein-Specific PET/CT Imaging in Fibrotic Interstitial Lung Diseases and Lung Cancer: A Translational Exploratory Study

Interstitial lung diseases (ILDs) comprise over 200 parenchymal lung disorders. Among them, fibrosing ILDs, especially idiopathic pulmonary fibrosis, are associated with a poor prognosis, whereas some other ILDs, such as sarcoidosis, have a much better prognosis. A high proportion manifests as fibrotic ILD (fILD). Lung cancer (LC) is a frequent complication of fILD. Activated fibroblasts are crucial for fibrotic processes in fILD. The aim of this exploratory study was to evaluate the imaging properties of static and dynamic fibroblast activation protein (FAP) inhibitor (FAPI) PET/CT in various types of fILD and to confirm FAP expression in fILD lesions by FAP immunohistochemistry of human fILD biopsy samples and of lung sections of genetically engineered (Nedd4-2-/- ) mice with an idiopathic pulmonary fibrosislike lung disease. Methods: PET scans of 15 patients with fILD and suspected LC were acquired 10, 60, and 180 min after the administration of 150-250 MBq of a 68Ga-labeled FAPI tracer (FAPI-46). In 3 patients, dynamic scans over 40 min were performed instead of imaging after 10 min. The SUVmax and SUVmean of fibrotic lesions and LC were measured and CT-density-corrected. Target-to-background ratios (TBRs) were calculated. PET imaging was correlated with CT-based fibrosis scores. Time-activity curves derived from dynamic imaging were analyzed. FAP immunohistochemistry of 4 human fILD biopsy samples and of fibrotic lungs of Nedd4-2-/- mice was performed. Results: fILD lesions as well as LC showed markedly elevated 68Ga-FAPI uptake (density-corrected SUVmax and SUVmean 60 min after injection: 11.12 ± 6.71 and 4.29 ± 1.61, respectively, for fILD lesions and 16.69 ± 9.35 and 6.44 ± 3.29, respectively, for LC) and high TBR (TBR of density-corrected SUVmax and SUVmean 60 min after injection: 2.30 ± 1.47 and 1.67 ± 0.79, respectively, for fILD and 3.90 ± 2.36 and 2.37 ± 1.14, respectively, for LC). SUVmax and SUVmean decreased over time, with a stable TBR for fILD and a trend toward an increasing TBR in LC. Dynamic imaging showed differing time-activity curves for fILD and LC. 68Ga-FAPI uptake showed a positive correlation with the CT-based fibrosis index. Immunohistochemistry of human biopsy samples and the lungs of Nedd4-2-/- mice showed a patchy expression of FAP in fibrotic lesions, preferentially in the transition zone to healthy lung parenchyma. Conclusion:68Ga-FAPI PET/CT imaging is a promising new imaging modality for fILD and LC. Its potential clinical value for monitoring and therapy evaluation of fILD should be investigated in future studies.

Cerebral Venous Sinus Thrombosis Caused by Neuro-Behçet Disease Accidentally Detected by 68Ga-FAPI PET/CT

ABSTRACT

A 26-year-old man presented with recurrent oral ulcer, temporal headache, and blurred vision for 4 months. The giant cell arteritis was suspected, and 18F-FDG and 68Ga-FAPI PET/CT were done. There was no hypermetabolic lesion in 18F-FDG PET/CT. However, 68Ga-FAPI PET/CT showed multiple increased FAPI uptake lesions in the cerebral venous sinus. High-resolution MR venography revealed multiple chronic cerebral venous sinus thrombosis. Neuro-Behçet disease was diagnosed finally. This case showed 68Ga-FAPI PET/CT may play some role in the management of neuro-Behçet disease.

Regulation of Fibroblast Activation Protein-α Expression: Focus on Intracellular Protein Interactions

The prolyl-specific peptidase fibroblast activation protein-α (FAP-α) is expressed at very low or undetectable levels in nondiseased human tissues but is selectively induced in activated (myo)fibroblasts at sites of tissue remodeling in fibrogenic processes. In normal regenerative processes involving transient fibrosis FAP-α⁺(myo)fibroblasts disappear from injured tissues, replaced by cells with a normal FAP-α^- phenotype. In chronic uncontrolled pathological fibrosis FAP-α⁺(myo)fibroblasts permanently replace normal tissues. The mechanisms of regulation and elimination of FAP-α expression in(myo)fibroblasts are unknown. According to a yeast two-hybrid screen and protein databanks search, we propose that the intracellular (co)-chaperone BAG6/BAT3 can interact with FAP-α, mediated by the BAG6/BAT3 Pro-rich domain, inducing proteosomal degradation of FAP-α protein under tissue homeostasis. In this Perspective, we discuss our findings in the context of current knowledge on the regulation of FAP-α expression and comment potential therapeutic strategies for uncontrolled fibrosis, including small molecule degraders (PROTACs)-modified FAP-α targeted inhibitors.

Myofibroblasts: Function, Formation, and Scope of Molecular Therapies for Skin Fibrosis

Myofibroblasts are contractile, α-smooth muscle actin-positive cells with multiple roles in pathophysiological processes. Myofibroblasts mediate wound contractions, but their persistent presence in tissues is central to driving fibrosis, making them attractive cell targets for the development of therapeutic treatments. However, due to shared cellular markers with several other phenotypes, the specific targeting of myofibroblasts has long presented a scientific and clinical challenge. In recent years, myofibroblasts have drawn much attention among scientific research communities from multiple disciplines and specialisations. As further research uncovers the characterisations of myofibroblast formation, function, and regulation, the realisation of novel interventional routes for myofibroblasts within pathologies has emerged. The research community is approaching the means to finally target these cells, to prevent fibrosis, accelerate scarless wound healing, and attenuate associated disease-processes in clinical settings. This comprehensive review article describes the myofibroblast cell phenotype, their origins, and their diverse physiological and pathological functionality. Special attention has been given to mechanisms and molecular pathways governing myofibroblast differentiation, and updates in molecular interventions.

Targeting Dermal Fibroblast Subtypes in Antifibrotic Therapy: Surface Marker as a Cellular Identity or a Functional Entity?

Fibroblasts are the chief effector cells in fibrotic diseases and have been discovered to be highly heterogeneous. Recently, fibroblast heterogeneity in human skin has been studied extensively and several surface markers for dermal fibroblast subtypes have been identified, holding promise for future antifibrotic therapies. However, it has yet to be confirmed whether surface markers should be looked upon as merely lineage landmarks or as functional entities of fibroblast subtypes, which may further complicate the interpretation of cellular function of these fibroblast subtypes. This review aims to provide an update on current evidence on fibroblast surface markers in fibrotic disorders of skin as well as of other organ systems. Specifically, studies where surface markers were treated as lineage markers and manipulated as functional membrane proteins are both evaluated in parallel, hoping to reveal the underlying mechanism behind the pathogenesis of tissue fibrosis contributed by various fibroblast subtypes from multiple angles, shedding lights on future translational researches.

Targeted inhibition of PI3 kinase/mTOR specifically in fibrotic lung fibroblasts suppresses pulmonary fibrosis in experimental models

Idiopathic pulmonary fibrosis (IPF) is a lethal disease with an average life expectancy of 3 to 5 years. IPF is characterized by progressive stiffening of the lung parenchyma due to excessive deposition of collagen, leading to gradual failure of gas exchange. Although two therapeutic agents have been approved from the FDA for IPF, they only slow disease progression with little impact on outcome. To develop a more effective therapy, we have exploited the fact that collagen-producing myofibroblasts express a membrane-spanning protein, fibroblast activation protein (FAP), that exhibits limited if any expression on other cell types. Because collagen-producing myofibroblasts are only found in fibrotic tissues, solid tumors, and healing wounds, FAP constitutes an excellent marker for targeted delivery of drugs to tissues undergoing pathologic fibrosis. We demonstrate here that a low-molecular weight FAP ligand can be used to deliver imaging and therapeutic agents selectively to FAP-expressing cells. Because induction of collagen synthesis is associated with phosphatidylinositol 3-kinase (PI3K) activation, we designed a FAP-targeted PI3K inhibitor that selectively targets FAP-expressing human IPF lung fibroblasts and potently inhibited collagen synthesis. Moreover, we showed that administration of the inhibitor in a mouse model of IPF inhibited PI3K activation in fibrotic lungs, suppressed production of hydroxyproline (major building block of collagen), reduced collagen deposition, and increased mouse survival. Collectively, these studies suggest that a FAP-targeted PI3K inhibitor might be promising for treating IPF.

Identify. Quantify. Predict. Why Immunologists Should Widely Use Molecular Imaging for Coronavirus Disease 2019

Molecular imaging using PET/CT or PET/MRI has evolved from an experimental imaging modality at its inception in 1972 to an integral component of diagnostic procedures in oncology, and, to lesser extent, in cardiology and neurology, by successfully offering in-vivo imaging and quantitation of key pathophysiological targets or molecular signatures, such as glucose metabolism in cancerous disease. Apart from metabolism probes, novel radiolabeled peptide and antibody PET tracers, including radiolabeled monoclonal antibodies (mAbs) have entered the clinical arena, providing the in-vivo capability to collect target-specific quantitative in-vivo data on cellular and molecular pathomechanisms on a whole-body scale, and eventually, extract imaging biomarkers possibly serving as prognostic indicators. The success of molecular imaging in mapping disease severity on a whole-body scale, and directing targeted therapies in oncology possibly could translate to the management of Coronavirus Disease 2019 (COVID-19), by identifying, localizing, and quantifying involvement of different immune mediated responses to the infection with SARS-COV2 during the course of acute infection and possible, chronic courses with long-term effects on specific organs. The authors summarize current knowledge for medical imaging in COVID-19 in general with a focus on molecular imaging technology and provide a perspective for immunologists interested in molecular imaging research using validated and immediately available molecular probes, as well as possible future targets, highlighting key targets for tailored treatment approaches as brought up by key opinion leaders.

A Trifunctional Theranostic Ligand Targeting Fibroblast Activation Protein-α (FAPα)

PURPOSE

Fibroblast activation protein-α (FAPα) is uniquely expressed in activated fibroblasts, including cancer-associated fibroblasts that populate tumor stroma and contribute to proliferation and immunosuppression. Radiolabeled FAPα inhibitors enable imaging of multiple human cancers, but time-dependent clearance from tumors currently limits their utility as FAPα-targeted radiotherapeutics. We sought to increase the area under the curve (AUC) by constructing a trifunctional ligand that binds FAPα with high affinity and also binds albumin and theranostic radiometals.

PROCEDURES

RPS-309 comprised a FAPα-targeting moiety, an albumin-binding group, and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Inhibition of recombinant human FAPα (rhFAPα) was determined by colorimetric assay. Affinity for human serum albumin (HSA) was determined by high-performance affinity chromatography. The tissue distribution of [⁶⁸Ga]Ga-RPS-309 in SW872 tumor xenograft-bearing mice was imaged by microPET/CT and quantified by biodistribution studies performed from 30 min to 3 h post injection (p.i.). The biodistribution of [¹⁷⁷Lu]Lu-RPS-309 was determined at 4, 24, and 96 h p.i.

RESULTS

RPS-309 inhibits rhFAPα with IC₅₀ = 7.3 ± 1.4 nM. [⁶⁸Ga]Ga-RPS-309 is taken up specifically by FAPα-expressing cells and binds HSA with K_d = 4.6 ± 0.1 μM. Uptake of the radiolabeled ligand in tumors was evident from 30 min p.i. (> 5 %ID/g) and was significantly reduced by co-injection of RPS-309. Specific skeletal uptake was also observed. Activity in tumors was constant through 4 h p.i., but cleared significantly by 24 h. The AUC in this period was 127 (%ID/g) × h.

CONCLUSIONS

RPS-309 is a high-affinity FAPα inhibitor with prolonged plasma residence. Introduction of the albumin-binding group did not compromise FAPα binding. Although initial tumor uptake was high and FAPα-specific, RPS-309 also progressively cleared from tumors. Nevertheless, RPS-309 incorporates multiple sites in which structural diversity can be introduced, and therefore serves as a platform for future structure-activity relationship studies.

68Ga-FAPI-04 PET-CT for molecular assessment of fibroblast activation and risk evaluation in systemic sclerosis-associated interstitial lung disease: a single-centre, pilot study

BACKGROUND

Interstitial lung disease (ILD) is the most common cause of death in systemic sclerosis. To date, the progression of systemic sclerosis-associated ILD is judged by the accrual of lung damage on CT and pulmonary function tests. However, diagnostic tools to assess disease activity are not available. Here, we tested the hypothesis that quantification of fibroblast activation by PET-CT using a 68Ga-labelled selective inhibitor of prolyl endopeptidase FAP (68Ga-FAPI-04) would correlate with ILD activity and disease progression in patients with systemic sclerosis-associated ILD.

METHODS

Between Sept 10, 2018, and April 8, 2020, 21 patients with systemic sclerosis-associated ILD confirmed by high-resolution CT (HRCT) within 12 months of inclusion and with onset of systemic sclerosis-associated ILD within 5 years or signs of progressive ILD and 21 controls without ILD were consecutively enrolled. All participants underwent 68Ga-FAPI-04 PET-CT imaging and standard-of-care procedures, including HRCT and pulmonary function tests at baseline. Patients with systemic sclerosis-associated ILD were followed for 6 months with HRCT and pulmonary function tests. We compared baseline 68Ga-FAPI-04 PET-CT uptake with standard diagnostic tools and predictors of ILD progression. The association of 68Ga-FAPI-04 uptake with changes in forced vital capacity was analysed using mixed-effects models. Follow-up 68Ga-FAPI-04 PET-CT scans were obtained in a subset of patients treated with nintedanib (follow-up between 6-10 months) to assess change over time.

FINDINGS

68Ga-FAPI-04 accumulated in fibrotic areas of the lungs in patients with systemic sclerosis-associated ILD compared with controls, with a median standardised uptake value (SUV) mean over the whole lung of 0·80 (IQR 0·60-2·10) in the systemic sclerosis-ILD group and 0·50 (0·40-0·50) in the control group (p<0·0001) and a mean whole lung maximal SUV of 4·40 (range 3·05-5·20) in the systemic sclerosis-ILD group compared with 0·70 (0·65-0·70) in the control group (p<0·0001). Whole-lung FAPI metabolic active volume (wlFAPI-MAV) and whole-lung total lesion FAPI (wlTL-FAPI) were not measurable in control participants, because no 68Ga-FAPI-04 uptake above background level was observed. In the systemic sclerosis-ILD group the median wlFAPI-MAV was 254·00 cm3 (IQR 163·40-442·30), and the median wlTL-FAPI was 183·60 cm3 (98·04-960·70). 68Ga-FAPI-04 uptake was higher in patients with extensive disease, with previous ILD progression, or high EUSTAR activity scores than in those with with limited disease, previously stable ILD, or low EUSTAR activity scores. Increased 68Ga-FAPI-04 uptake at baseline was associated with progression of ILD independently of extent of involvement on HRCT scan and the forced vital capacity at baseline. In consecutive 68Ga-FAPI-04 PET-CTs, changes in 68Ga-FAPI-04 uptake was concordant with the observed response to the fibroblast-targeting antifibrotic drug nintedanib.

INTERPRETATION

Our study presents the first in-human evidence that fibroblast activation correlates with fibrotic activity and disease progression in the lungs of patients with systemic sclerosis-associated ILD and that 68Ga-FAPI-04 PET-CT might improve risk assessment of systemic sclerosis-associated ILD.

FUNDING

German Research Foundation, Erlangen Anschubs-und Nachwuchsfinanzierung, Interdisziplinäres Zentrum für Klinische Forschung Erlangen, Bundesministerium für Bildung und Forschung, Deutsche Stiftung Systemische Sklerose, Wilhelm-Sander-Foundation, Else-Kröner-Fresenius-Foundation, European Research Council, Ernst-Jung-Foundation, and Clinician Scientist Program Erlangen.

The role of fibroblast activation protein in health and malignancy

Fibroblast activation protein-α (FAP) is a type-II transmembrane serine protease expressed almost exclusively to pathological conditions including fibrosis, arthritis, and cancer. Across most cancer types, elevated FAP is associated with worse clinical outcomes. Despite the clear association between FAP and disease severity, the biological reasons underlying these clinical observations remain unclear. Here we review basic FAP biology and FAP's role in non-oncologic and oncologic disease. We further explore how FAP may worsen clinical outcomes via its effects on extracellular matrix remodeling, intracellular signaling regulation, angiogenesis, epithelial-to-mesenchymal transition, and immunosuppression. Lastly, we discuss the potential to exploit FAP biology to improve clinical outcomes.

The emerging role of cell surface receptor and protein binding radiopharmaceuticals in cancer diagnostics and therapy

Targeting specific cell membrane markers for both diagnostic imaging and radionuclide therapy is a rapidly evolving field in cancer research. Some of these applications have now found a role in routine clinical practice and have been shown to have a significant impact on patient management. Several molecular targets are being investigated in ongoing clinical trials and show promise for future implementation. Advancements in molecular biology have facilitated the identification of new cancer-specific targets for radiopharmaceutical development.

Imaging Biomarkers in Animal Models of Drug-Induced Lung Injury: A Systematic Review

For drug-induced interstitial lung disease (DIILD) translational imaging biomarkers are needed to improve detection and management of lung injury and drug-toxicity. Literature was reviewed on animal models in which in vivo imaging was used to detect and assess lung lesions that resembled pathological changes found in DIILD, such as inflammation and fibrosis. A systematic search was carried out using three databases with key words “Animal models”, “Imaging”, “Lung disease”, and “Drugs”. A total of 5749 articles were found, and, based on inclusion criteria, 284 papers were selected for final data extraction, resulting in 182 out of the 284 papers, based on eligibility. Twelve different animal species occurred and nine various imaging modalities were used, with two-thirds of the studies being longitudinal. The inducing agents and exposure (dose and duration) differed from non-physiological to clinically relevant doses. The majority of studies reported other biomarkers and/or histological confirmation of the imaging results. Summary of radiotracers and examples of imaging biomarkers were summarized, and the types of animal models and the most used imaging modalities and applications are discussed in this review. Pathologies resembling DIILD, such as inflammation and fibrosis, were described in many papers, but only a few explicitly addressed drug-induced toxicity experiments.

NLRP1 variant M1184V decreases inflammasome activation in the context of DPP9 inhibition and asthma severity

Background

NLRP1 is an innate immune sensor that can form cytoplasmic inflammasome complexes. Polymorphisms in NLRP1 are linked to asthma; however, there is currently no functional or mechanistic explanation for this.

Objective

We sought to clarify the role of NLRP1 in asthma pathogenesis.

Methods

Results from the GALA II cohort study were used to identify a link between NLRP1 and asthma in Mexican Americans. In vitro and in vivo models for NLRP1 activation were applied to investigate the role of this inflammasome in asthma at the molecular level.

Results

We document the association of an NLRP1 haplotype with asthma for which the single nucleotide polymorphism rs11651270 (M1184V) individually is the most significant. Surprisingly, M1184V increases NLRP1 activation in the context of N-terminal destabilization, but decreases NLRP1 activation on dipeptidyl peptidase 9 inhibition. In vitro studies demonstrate that M1184V increases binding to dipeptidyl peptidase 9, which can account for its inhibitory role in this context. In addition, in vivo data from a mouse model of airway inflammation reveal a protective role for NLRP1 inflammasome activation reducing eosinophilia in this setting.

Conclusions

Linking our in vitro and in vivo results, we found that the NLRP1 variant M1184V reduces inflammasome activation in the context of dipeptidyl peptidase 9 inhibition and could thereby increase asthma severity. Our studies may have implications for the treatment of asthma in patients carrying this variant of NLRP1.

FAPI PET/CT: Will It End the Hegemony of 18F-FDG in Oncology?

For over 40 years, ¹⁸F-FDG has been the dominant PET tracer in neurology, cardiology, inflammatory diseases, and, most particularly, oncology. Combined with the ability to perform whole-body scanning, ¹⁸F-FDG has revolutionized the evaluation of cancer and has stifled the adoption of other tracers, except in situations where low avidity or high background activity limits diagnostic performance. The strength of ¹⁸F-FDG has generally been its ability to detect disease in the absence of structural abnormality, thereby enhancing diagnostic sensitivity, but its simultaneous weakness has been a lack of specificity due to diverse pathologies with enhanced glycolysis. Radiotracers that leverage other hallmarks of cancer or specific cell-surface targets are gradually finding a niche in the diagnostic armamentarium. However, none have had sufficient sensitivity to realistically compete with ¹⁸F-FDG for evaluation of the broad spectrum of malignancies. Perhaps, this situation is about to change with development of a class of tracers targeting fibroblast activation protein that have low uptake in almost all normal tissues but high uptake in most cancer types. In this review, the development and exciting preliminary clinical data relating to various fibroblast activation protein-specific small-molecule inhibitor tracers in oncology will be discussed along with potential nononcologic applications.

The Latest Developments in Imaging of Fibroblast Activation Protein

Fibroblast activation protein (FAP), a membrane-anchored peptidase, is highly expressed in cancer-associated fibroblasts in more than 90% of epithelial tumors and contributes to progression and worse prognosis of different cancers. Therefore, FAP is considered a promising target for radionuclide-based approaches for diagnosis and treatment of tumors and for the diagnosis of nonmalignant diseases associated with a remodeling of the extracellular matrix. Accordingly, a variety of quinolone-based FAP inhibitors (FAPIs) coupled to chelators were developed displaying specific binding to human and murine FAP with a rapid and almost complete internalization. Because of a high tumor uptake and a very low accumulation in normal tissues, as well as a rapid clearance from the circulation, a high contrast is obtained for FAPI PET/CT imaging even at 10 min after tracer administration. Moreover, FAPI PET/CT provides advantages over ¹⁸F-FDG PET/CT in several tumor entities for initial staging and detection of tumor recurrence and metastases, including peritonitis carcinomatosa.

Impact of Primary Staging with Fibroblast Activation Protein Specific Enzyme Inhibitor (FAPI)-PET/CT on Radio-Oncologic Treatment Planning of Patients with Esophageal Cancer

Purpose

Quinoline-based ligands targeting cancer-associated fibroblasts have emerged as promising radiopharmaceuticals in different tumor entities. The aim of this retrospective study was to explore the potential of FAPI-PET/CT in the initial staging of esophageal cancer patients and its usefulness in radiotherapy planning as a first clinical analysis.

Methods

Seven patients with treatment-naive esophageal cancer underwent FAPI-PET/CT. Tracer uptake was quantified by standardized uptake values (SUV)max and (SUV)mean. Six patients received definitive and one neoadjuvant (chemo)radiation therapy. Endo-esophageal clipping, the gold standard to define tumor margins not delineable per CT, was performed in three patients.

Results

Primary tumors demonstrated high FAPI uptake with a median SUVmax of 17.2. Excellent tumor-to-background ratios resulted in accurate target volume delineation and were found in perfect match with clipping. Detection of regional lymph node metastases facilitated the use of simultaneous integrated boost radiotherapy plans for these patients.

Conclusion

FAPI-PET/CT may be beneficial for the management of esophageal cancer particularly in planning radiotherapy, but further research is necessary to increase patient number and statistical reliability.

Imaging Biomarkers and Pathobiological Profiling in a Rat Model of Drug-Induced Interstitial Lung Disease Induced by Bleomycin

A large number of systemically administered drugs have the potential to cause drug-induced interstitial lung disease (DIILD). We aim to characterize a model of DIILD in the rat and develop imaging biomarkers (IBs) for detection and quantification of DIILD. In this study, Sprague-Dawley rats received one single dose of intratracheal (i.t.) bleomycin and were longitudinally imaged at day 0, 3, 7, 14, 21, and 28 post dosing, applying the imaging techniques magnetic resonance imaging (MRI) and positron emission tomography (PET)/computed tomography (CT). Bronchoalveolar lavage fluid (BALF) was analyzed for total protein and inflammatory cells. Lungs were saved for further evaluation by gene analysis using quantitative-PCR and by histology. Lung sections were stained with Masson's-Trichrome staining and evaluated by modified Ashcroft score. Gene expression profiling of inflammatory and fibrotic markers was performed on lung tissue homogenates. Bleomycin induced significant increase in total protein concentration and total cell count in bronchoalveolar lavage (BAL), peaking at day 3 (p > 0.001) and day 7 (p > 0.001) compared to control, respectively. Lesions measured by MRI and PET signal in the lungs of bleomycin challenged rats were significantly increased during days 3-14, peaking at day 7. Two subgroups of animals were identified as low- and high-responders by their different change in total lung volume. Both groups showed signs of inflammation initially, while at later time points, the low-responder group recovered toward control, and the high-responder group showed sustained lung volume increase, and significant increase of lesion volume (p < 0.001) compared to control. Lastly, important inflammatory and pro-fibrotic markers were assessed from lung tissue, linking observed imaging pathological changes to gene expression patterns. In conclusion, bleomycin-induced lung injury is an adequate animal model for DIILD studies and for translational lung injury assessment by MRI and PET imaging. The scenario comprised disease responses, with different fractions of inflammation and fibrosis. Thereby, this study improved the understanding of imaging and biological biomarkers in DIILD and lung injury.

Design and validation of fibroblast activation protein alpha targeted imaging and therapeutic agents

Background: Cancer-associated fibroblasts (CAFs) comprise a major cell type in the tumor microenvironment where they support tumor growth and survival by producing extracellular matrix, secreting immunosuppressive cytokines, releasing growth factors, and facilitating metastases. Because tumors with elevated CAFs are characterized by poorer prognosis, considerable effort is focused on developing methods to quantitate, suppress and/or eliminate CAFs. We exploit the elevated expression of fibroblast activation protein (FAP) on CAFs to target imaging and therapeutic agents selectively to these fibroblasts in solid tumors. Methods: FAP-targeted optical imaging, radioimaging, and chemotherapeutic agents were synthesized by conjugating FAP ligand (FL) to either a fluorescent dye, technetium-99m, or tubulysin B hydrazide. In vitro and in vivo studies were performed to determine the specificity and selectivity of each conjugate for FAP in vitro and in vivo. Results: FAP-targeted imaging and therapeutic conjugates showed high binding specificity and affinity in the low nanomolar range. Injection of FAP-targeted 99mTc into tumor-bearing mice enabled facile detection of tumor xenografts with little off-target uptake. Optical imaging of malignant lesions was also readily achieved following intravenous injection of FAP-targeted near-infrared fluorescent dye. Finally, systemic administration of a tubulysin B conjugate of FL promoted complete eradication of solid tumors with no evidence of gross toxicity to the animals. Conclusion: In view of the near absence of FAP on healthy cells, we conclude that targeting of FAP on cancer-associated fibroblasts can enable highly specific imaging and therapy of solid tumors.

Molecular recognition of fibroblast activation protein for diagnostic and therapeutic applications

Fibroblast activation protein (FAP) is a non-classical serine protease expressed predominantly in conditions accompanied by tissue remodeling, particularly cancer. Due to its plasma membrane localization, FAP represents a promising molecular target for tumor imaging and treatment. The unique enzymatic activity of FAP facilitates development of diagnostic and therapeutic tools based on molecular recognition of FAP by substrates and small-molecule inhibitors, in addition to conventional antibody-based strategies. In this review, we provide background on the pathophysiological role of FAP and discuss its potential for diagnostic and therapeutic applications. Furthermore, we present a detailed analysis of the structural patterns crucial for substrate and inhibitor recognition by the FAP active site and determinants of selectivity over the related proteases dipeptidyl peptidase IV and prolyl endopeptidase. We also review published data on targeting of the tumor microenvironment with FAP antibodies, FAP-targeted prodrugs, activity-based probes and small-molecule inhibitors. We describe use of a recently developed, selective FAP inhibitor with low-nanomolar potency in inhibitor-based targeting strategies including synthetic antibody mimetics based on hydrophilic polymers and inhibitor conjugates for PET imaging. In conclusion, recent advances in understanding of the molecular structure and function of FAP have significantly contributed to the development of several tools with potential for translation into clinical practice.

microRNA-30a attenuates TGF-β1-induced activation of pulmonary fibroblast cell by targeting FAP-α

Idiopathic interstitial pulmonary fibrosis is a common diffuse interstitial lung disease and has poor prognosis. And one of the pathological features of it is persistent fibroblast activation. It was reported that microRNA‐30a was down‐regulated in bronchoalveolar lavage fluid from idiopathic pulmonary fibrosis patients. But whether miR‐30a is involved in fibroblast activation and its specific mechanism is unclear. In this study, we aimed to investigate the role of miR‐30a in fibroblast activation induced by TGF‐β1. We found miR‐30a could targetedly suppress FAP‐α expression. In MRC5 cells, miR‐30a was not only involved in regulating the expression of FAP‐α, col1a and α‐SMA induced by TGF‐β1 but also had a role in cell proliferation with or without TGF‐β1 treatment via regulating FAP‐α expression. Thus, the results indicated that miR‐30a alleviated fibroblast activation by regulating the expression of FAP‐α.

Targeting of activated fibroblasts for imaging and therapy

Tumors form a complex environment consisting of a variety of non-malignant cells. Especially cancer-associated fibroblasts have been shown to have an important role for different aspects of malignant tumors such as migration, metastasis, resistance to chemotherapy and immunosuppression. Therefore, a targeting of these cells may be useful for both imaging and therapy. In this respect, an interesting target is the fibroblast activation protein (FAP) which is expressed in activated fibroblasts, but not in quiescent fibroblasts, giving the opportunity to use this membrane-anchored enzyme as a target for radionuclide-based approaches for diagnosis and treatment of tumors and for the diagnosis of non-malignant disease associated with a remodelling of the extracellular matrix.

Identification of Novel Natural Substrates of Fibroblast Activation Protein-alpha by Differential Degradomics and Proteomics

Fibroblast activation protein-alpha (FAP) is a cell-surface transmembrane-anchored dimeric protease. This unique, constitutively active serine protease has both dipeptidyl aminopeptidase and endopeptidase activities and can hydrolyze the post-proline bond. FAP expression is very low in adult organs but is upregulated by activated fibroblasts in sites of tissue remodeling, including fibrosis, atherosclerosis, arthritis and tumors. To identify the endogenous substrates of FAP, we immortalized primary mouse embryonic fibroblasts (MEFs) from FAP gene knockout embryos and then stably transduced them to express either enzymatically active or inactive FAP. The MEF secretomes were then analyzed using degradomic and proteomic techniques. Terminal amine isotopic labeling of substrates (TAILS)-based degradomics identified cleavage sites in collagens, many other extracellular matrix (ECM) and associated proteins, and lysyl oxidase-like-1, CXCL-5, CSF-1, and C1qT6, that were confirmed in vitro In addition, differential metabolic labeling coupled with quantitative proteomic analysis also implicated FAP in ECM-cell interactions, as well as with coagulation, metabolism and wound healing associated proteins. Plasma from FAP-deficient mice exhibited slower than wild-type clotting times. This study provides a significant expansion of the substrate repertoire of FAP and provides insight into the physiological and potential pathological roles of this enigmatic protease.