Fibroblast activation protein α in tumor microenvironment: recent progression and implications (review)

Carborane-FAPI conjugate: A potential FAP-targeted boron agent with improved boron content

Boron neutron capture therapy (BNCT) has received extensive attention as an advanced binary radiotherapy method. However, BNCT still faces poor selectivity of boron agent and is insufficient boron content in tumor tissues. To improve the tumor-targeted ability and boron content, this research aims to design, synthesize and preliminary evaluate a new borane agent Carborane-FAPI, which coupling the o-carborane to the compound skeleton of a mature fibroblast activating protein (FAP) inhibitor (FAPI). FAP is a tumor-associated antigen. FAP expressed lowly in normal organs and highly expressed in tumors, so it is a potential target for diagnosis and treatment. Boronophenylalanine (BPA) is the most widely investigated BNCT drug in present. Compared with BPA, the boron content of a single molecule is increased and drug targeting is enhanced. The results show that Carboaren-FAPI has low toxicity to normal cells, and selective enrichment in tumor tissues. It is a promising boron drug that has the potential to be used in BNCT.

Multidimensional biological characteristics of ground glass nodules

The detection rate of ground glass nodules (GGNs) has increased in recent years because of their malignant potential but relatively indolent biological behavior; thus, correct GGN recognition and management has become a research focus. Many scholars have explored the underlying mechanism of the indolent progression of GGNs from several perspectives, such as pathological type, genomic mutational characteristics, and immune microenvironment. GGNs have different major mutated genes at different stages of development; EGFR mutation is the most common mutation in GGNs, and p53 mutation is the most abundant mutation in the invasive stage of GGNs. Pure GGNs have fewer genomic alterations and a simpler genomic profile and exhibit a gradually evolving genomic mutation profile as the pathology progresses. Compared to advanced lung adenocarcinoma, GGN lung adenocarcinoma has a higher immune cell percentage, is under immune surveillance, and has less immune escape. However, as the pathological progression and solid component increase, negative immune regulation and immune escape increase gradually, and a suppressive immune environment is established gradually. Currently, regular computer tomography monitoring and surgery are the main treatment strategies for persistent GGNs. Stereotactic body radiotherapy and radiofrequency ablation are two local therapeutic alternatives, and systemic therapy has been progressively studied for lung cancer with GGNs. In the present review, we discuss the characterization of the multidimensional molecular evolution of GGNs that could facilitate more precise differentiation of such highly heterogeneous lesions, laying a foundation for the development of more effective individualized treatment plans.

Diagnostic and Therapeutic Application of Fibroblast Activation Protein Inhibitors in Oncologic and Nononcologic Diseases

ABSTRACT

Fibroblast activation protein inhibitor positron emission tomography (PET) has gained interest for its ability to demonstrate uptake in a diverse range of tumors. Its molecular target, fibroblast activation protein, is expressed in cancer-associated fibroblasts, a major cell type in tumor microenvironment that surrounds various types of cancers. Although existing literature on FAPI PET is largely from single-center studies and case reports, initial findings show promise for some cancer types demonstrating improved imaging when compared with the widely used 18F-fludeoxyglucose PET for oncologic imaging. As we expand our knowledge of the utility of FAPI PET, accurate understanding of noncancerous uptake seen on FAPI PET is crucial for accurate evaluation. In this review, we summarize potential diagnostic and therapeutic applications of radiolabeled FAP inhibitors in oncological and nononcological disease processes.

Heterogeneity of primary and metastatic CAFs: From differential treatment outcomes to treatment opportunities (Review)

Compared with primary tumor sites, metastatic sites appear more resistant to treatments and respond differently to the treatment regimen. It may be due to the heterogeneity in the microenvironment between metastatic sites and primary tumors. Cancer‑associated fibroblasts (CAFs) are widely present in the tumor stroma as key components of the tumor microenvironment. Primary tumor CAFs (pCAFs) and metastatic CAFs (mCAFs) are heterogeneous in terms of source, activation mode, markers and functional phenotypes. They can shape the tumor microenvironment according to organ, showing heterogeneity between primary tumors and metastases, which may affect the sensitivity of these sites to treatment. It was hypothesized that understanding the heterogeneity between pCAFs and mCAFs can provide a glimpse into the difference in treatment outcomes, providing new ideas for improving the rate of metastasis control in various cancers.

Molecular imaging for evaluation of synovitis associated with osteoarthritis: a narrative review

Recent evidence highlights the role of low-grade synovial inflammation in the progression of osteoarthritis (OA). Inflamed synovium of OA joints detected by imaging modalities are associated with subsequent progression of OA. In this sense, detecting and quantifying synovitis of OA by imaging modalities may be valuable in predicting OA progressors as well as in improving our understanding of OA progression. Of the several imaging modalities, molecular imaging such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) has an advantage of visualizing the cellular or subcellular events of the tissues. Depending on the radiotracers used, molecular imaging method can potentially detect and visualize various aspects of synovial inflammation. This narrative review summarizes the recent progresses of imaging modalities in assessing inflammation and OA synovitis and focuses on novel radiotracers. Recent studies about imaging modalities including ultrasonography (US), magnetic resonance imaging (MRI), and molecular imaging that were used to detect and quantify inflammation and OA synovitis are summarized. Novel radiotracers specifically targeting the components of inflammation have been developed. These tracers may show promise in detecting inflamed synovium of OA and help in expanding our understanding of OA progression.

Nano-drug delivery system targeting FAP for the combined treatment of oral leukoplakia

Oral leukoplakia (OLK) has received much attention due to its potential risk of malignant transformation. Studies have shown that when drug therapy is combined with photothermal therapy (PTT), not only can the cytotoxicity of the drug be enhanced, but also the heat energy can be used to kill the lesion cells, so we can combine drug therapy with PTT to enhance the therapeutic effect on OLK. However, with certain drawbacks due to its lack of targeting, fibroblast activating protein (FAP) has become an attractive target for OLK combination therapy. In this study, we used NGO-PEG loaded with FAP-targeting peptide (F-TP) and celecoxib (CXB) to construct a nano-drug delivery system CGPF for targeting OLK with high FAP expression and confirmed the biocompatibility and therapeutic efficacy of CGPF by in vitro and in vivo experiments. Overall, the novel nano-drug delivery system CGPF proposed in this study showed a very significant potential for the combination therapy of OLK.

Fibroblast activation protein: Pivoting cancer/chemotherapeutic insight towards heart failure

An important mechanism for cancer progression is degradation of the extracellular matrix (ECM) which is accompanied by the emergence and proliferation of an activated fibroblast, termed the cancer associated fibroblast (CAF). More specifically, an enzyme pathway identified to be amplified with local cancer progression and proliferation of the CAF, is fibroblast activation protein (FAP). The development and progression of heart failure (HF) irrespective of the etiology is associated with left ventricular (LV) remodeling and changes in ECM structure and function. As with cancer, HF progression is associated with a change in LV myocardial fibroblast growth and function, and expresses a protein signature not dissimilar to the CAF. The overall goal of this review is to put forward the postulate that scientific discoveries regarding FAP in cancer as well as the development of specific chemotherapeutics could be pivoted to target the emergence of FAP in the activated fibroblast subtype and thus hold translationally relevant diagnostic and therapeutic targets in HF.

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.

18F-FAPI-04 PET/CT parameters predict PD-L1 expression in esophageal squamous cell carcinoma

Purpose

This prospective study examined whether metabolism parameters obtained using the tracer 18F-AlFNOTA-fibroblast activation protein inhibitor (FAPI)-04 (denoted as 18F-FAPI-04) in positron emission tomography/computed tomography (PET/CT) can predict programmed death ligand-1 (PD-L1) expression in patients with locally advanced esophageal squamous cell carcinoma (LA-ESCC).

Patients and methods

The 24 enrolled LA-ESCC patients underwent an 18F-FAPI-04 PET/CT scan. The maximum, mean, peak and standard deviation standard uptake values (SUVmax, SUVmean, SUVpeak and SUVsd), metabolic tumor volume (MTV), and total lesion FAP (TLF) expression of the primary tumor were collected. Additionally, we evaluated PD-L1 expression on cancer cells by immunohistochemistry and immunofluorescence methods. Patients were divided into negative and positive expressions according to the expression of PD-L1 (CPS < 10 and CPS ≥ 10), and the variables were compared between the two groups.

Results

The SUVmax, SUVmean, SUVpeak and SUVsd were significantly higher in patients with positive expression than in negative expression (all p < 0.05). Receiver operating characteristic curve analysis identified SUVmean (area under the curve [AUC] = 0.882, p = 0.004), SUVsd (AUC = 0.874, p = 0.005), SUVpeak (AUC = 0.840, p = 0.010) and SUVmax (AUC = 0.765, p = 0.045) as significant predictors of the PD-L1 positive expression, with cutoff values of 9.67, 1.90, 9.67 and 13.71, respectively. On univariate logistic regression analysis, SUVmean (p = 0.045), SUVsd (p = 0.024), and SUVpeak (p = 0.031) were significantly correlated with the PD-L1 positive expression. On multivariable logistic regression analysis, SUVsd (p = 0.035) was an optimum predictor factor for PD-L1 positive expression.

Conclusion

18F-FAPI-04 PET/CT parameters, including SUVmean, SUVpeak, and SUVsd, correlated with PD-L1 expression in patients with LA-ESCC, and thus SUVsd was an optimum predictor for PD-L1 positive expression, which could help to explore the existence of immune checkpoints and select ESCC candidates for immunotherapy.

Tumor Progression from a Fibroblast Activation Protein Perspective: Novel Diagnostic and Therapeutic Scenarios for Colorectal Cancer

In 2020, the Global Cancer Observatory estimated the incidence of colorectal cancer (CRC) at around 10.7% coupled with a mortality rate of 9.5%. The explanation for these values lies in the tumor microenvironment consisting of the extracellular matrix and cancer-associated fibroblasts (CAFs). Fibroblast activation protein (FAP) offers a promising target for cancer therapy since its functions contribute to tumor progression. Immunohistochemistry examination of FAP, fibronectin ED-B, and CXCR4 in primary tumors and their respective synchronous and/or metachronous metastases along with semiquantitative analysis have been carried out on histological samples of 50 patients diagnosed with metastatic CRC. The intensity of FAP, articulated by both "Intensity %" and "Intensity score", is lower in the first metastasis compared to the primary tumor with a statistically significant correlation. No significant correlations have been observed regarding fibronectin ED-B and CXCR4. Tumors that produce FAP have an ambivalent relationship with this protein. At first, they exploit FAP, but later they reduce its expressiveness. Although our study has not directly included FAP-Inhibitor (FAPI) PET/CT, the considerable expression of FAP reveals its potential as a diagnostic and therapeutic tool worthy of further investigation. This dynamic relationship between cancer and FAP has substantial diagnostic and therapeutic implications.

FAP promotes clear cell renal cell carcinoma progression via activating the PI3K/AKT/mTOR signaling pathway

OBJECTIVE

Herein, we aimed at exploring the FAP expression in clear cell renal cell carcinoma (ccRCC) along with its clinical implication.

METHODS

Using computational tools analysis of different freely accessible gene databases, the expression pattern, clinical importance, co-expressed genes, and signaling pathways of FAP in ccRCC were thoroughly investigated. FAP expression was examined in clinical ccRCC specimens through qRT-PCR, western blotting and immunohistochemistry. Furthermore, in vitro and in vivo experiments were carried out using flow cytometry, CCK-8, wound-healing and Transwell assays, as well as xenograft tumor model, respectively.

RESULTS

FAP levels were found to be significantly elevated in ccRCC based on bioinformatic data from public databases. Patients who exhibited higher expression levels of FAP had poorer prognoses, according to Kaplan-Meier analysis of survival data. In addition, diagnostic and prognostic value of FAP in ccRCC was figured out by ROC curve and prognostic nomogram model. In vitro study revealed that the over-expression FAP accelerated cell proliferation, migration as well as invasion, and suppressed cell apoptosis, but silencing of FAP had the opposite effect. FAP suppression reduced the PI3K/AKT/mTOR pathway's stimulation, whereas FAP up-regulation increased the stimulation of the pathway. Blocking the PI3K/AKT/mTOR signaling pathway with the dual PI3K/mTOR inhibitor BEZ235 repressesed cancer-promoting effect of FAP. Additionally, we found that the downregulation of FAP was effective at slowing tumor progression in vivo.

CONCLUSION

It is possible that FAP could be a reliable biomarker for the diagnosis and prognosis of ccRCC because of its role in the ccRCC progression via triggering the PI3K/AKT/mTOR signaling pathway.

Automated Synthesis of [68Ga]Ga-FAPI-46 on a Scintomics GRP Synthesizer

[68Ga]Ga-FAPI-46 is a radiolabelled fibroblast activation protein inhibitor that selectively binds to fibroblast activation protein (FAP), which is overexpressed by cancer-associated fibroblasts (CAFs) in the tumour microenvironment. In recent years, radiolabelled FAP inhibitors (FAPIs) are becoming increasingly important in cancer diagnostics and also for targeted radionuclide therapy. Because of the increasing demand for radiolabelled FAPIs, automating the synthesis of these compounds is of great interest. In this work, we present a newly programmed automatic synthesis process of [68Ga]Ga-FAPI-46 on a Scintomics GRP module using two Galli Ad generators as a radionuclide source. Dedicated cassettes for the labelling of 68Ga-peptides were used without any modifications. The generators were connected via a three-way valve to the module and eluted automatically over a strong cation exchange (SCX) cartridge by using the vacuum pump of the synthesis module, eliminating the need to transfer the eluates into a separate vial. After a reaction step in HEPES buffer, the compound was purified by solid-phase extraction (SPE) over a Sep-Pak Light C18 cartridge. The evaluation of 10 routine syntheses of [68Ga]Ga-FAPI-46 resulted in a radiochemical yield of 72.6 ± 4.9%. The radiochemical purity was 97.6 ± 0.3%, and the amount of free gallium-68 and colloid was <2%. The final product fulfilled the quality criteria, which were adapted from relevant monographs of the European Pharmacopoeia (Ph. Eur.). This work presents the successful preparation of multiple doses of [68Ga]Ga-FAPI-46 in a GMP-compliant automated process for clinical use.

Investigation of target genes and potential mechanisms related to compound Xiao-ai-fei honey ointment based on network pharmacology and bioinformatics analysis

BACKGROUND

Compound Xiao-ai-fei honey ointment (CXHO) is an anticancer preparation with a long history in Uyghur folk medicine in China and has been used for the treatment of gastric cancer (GC) in Xinjiang, China. Nevertheless, the mechanism of its anticancer effect remains to be investigated.

METHODS

Bioactive ingredients of CXHO were retrieved from the Traditional Chinese Medicine Systems Pharmacology Database and Analysis Platform database. Target genes of ingredients were acquired via the PubChem and Swiss target prediction database. Gene expression profiling of GC was obtained from GSE54129 in the GEO database and analyzed using the limma package in R. The hub genes associated with CXHO in GC were validated using the TIMER2.0 database, GEPIA2 database and Auto Dock tools. The effect of CXHO on migration of GC cells was detected by Transwell chamber assay and Wound healing assay. The effect of CXHO on expression levels of MMP2/MMP9 and NF-κb, PI3K/AKT signaling pathway was detected by Western blot assay.

RESULTS

Forty-five bioactive ingredients and their 819 related genes were found. A total of 462 differentially expressed genes were identified between GC patients and healthy controls. Seventeen common target genes were identified as hub genes CXHO against GC. Among them, MMP2 and MMP9 were significantly associated with tumor immune infiltrates and had good binding affinity with effective ingredients. Moreover, we validated the mRNA and protein expression levels and prognostic value of MMP2 and MMP9 by different databases. In addition, Kyoto encyclopedia of genes and genomes and gene ontology analyses showed that the 17 common target genes were mainly involved in steroid hormone biosynthesis and cancer-related pathways. Experimental results showed that CXHO inhibited migration of GC cells and down regulated the expression levels of MMP2/MMP9, NF-κb. In addition, CXHO can inhibited PI3K/AKT signaling pathway.

CONCLUSION

We identified and experimental validated 2 pivotal target genes of CXHO against GC and preliminarily analyzed the potential mechanisms by which CXHO inhibits the development of GC. All these findings support CXHO as a promising drug for the treatment of GC.

The Role of 68Ga FAPI-04 and 18F-FDG PET/CT in Detecting Liver Metastases in Different Types of Cancer

OBJECTIVE

We compared the diagnostic accuracies of 68Ga FAPI-04 PET/CT and 18F-FDG PET/CT for detecting liver metastases (LMs) in patients with different cancer types.

MATERIALS AND METHODS

This retrospective study included 63 patients with liver lesions who underwent 18F-FDG PET/CT and 68Ga-FAPI-04 PET/CT between May 2020 and May 2022. Patients with histopathologically confirmed primary diagnoses, data that could be accessed retrospectively, liver lesions confirmed by biopsy over at least 3-6 months of follow-up (via ultrasonography, CT, magnetic resonance imaging, PET/CT, or laboratory tests) were included. Patients with secondary malignancies or primary liver malignancies, and/or who could not be followed-up, were excluded.

RESULTS

Of the 63 total patients, 34 (54%) were female, and the mean age was 61 (30-92) years. There were 582 LMs in 51 patients and 35 benign liver lesions in 20 (12 patients had only benign and 8 both benign and malignant lesions). Of the 582 LMs, 472 (81.1%) evidenced 18F-FDG uptake and 572 (98.2%) 68Ga-FAPI uptake. The diagnostic accuracies of 68Ga-FAPI PET/CT and 18F-FDG PET/CT were 98% and 82%, respectively (p < 0.001; McNemar test). When the LMs were compared, the maximum standardized uptake (SUVmax) value was significantly higher on 18F-FDG PET/CT than 68Ga-FAPI PET/CT (median: 6.0 vs. 5.4; p = 0.016). However, the LM-to-background ratio (TBR) was significantly higher on 68Ga-FAPI PET/CT than 18F-FDG PET/CT (median: 4.1 vs. 2.1; p < 0.001).

CONCLUSION

68Ga-FAPI PET/CT detected more LMs than did 18F-FDG PET/CT, and TBR was significantly higher on 68Ga-FAPI PET/CT than 18F-FDG PET/CT.

Targeting fibroblast activation protein (FAP): advances in CAR-T cell, antibody, and vaccine in cancer immunotherapy

Fibroblast activation protein (FAP) is a serine protease with dual enzymatic activities overexpressed in cancer-associated fibroblasts (CAFs) in several tumor types, while its expression in healthy adult tissues is scarce. FAP overexpression on CAFs is associated with poor prognosis and plays an important role in tumor development, progression, and invasion. Therefore, FAP is considered a robust therapeutic target for cancer therapy. Here, we try to review and highlight the recent advances in immunotherapies for FAP targeting including the anti-FAP antibodies and immunoconjugates, FAP chimeric antigen receptor (CAR)-T cell, and various FAP vaccines in a preclinical and clinical setting. Subsequently, a discussion on the challenges and prospects associated with the development and translation of effective and safe therapies for targeting and depletion of FAP is provided. We proposed that new CAR-T cell engineering strategies and nanotechnology-based systems as well as advanced functional biomaterials can be used to improve the efficiency and safety of CAR-T cells and vaccines against FAP for more personalized immunotherapy. This review emphasizes the immune targeting of FAP as an emerging stromal candidate and one of the crucial elements in immunotherapy and shows the potential for improvement of current cancer therapy. A summary of different immunotherapy approaches to target fibroblast activation protein (FAP) for cancer therapy.

The Prognostic and therapeutic value and clinical implications of fibroblast activation protein-α as a novel biomarker in colorectal cancer

The identification of contributing factors leading to the development of Colorectal Cancer (CRC), as the third fatal malignancy, is crucial. Today, the tumor microenvironment has been shown to play a key role in CRC progression. Fibroblast-Activation Protein-α (FAP) is a type II transmembrane cell surface proteinase expressed on the surface of cancer-associated fibroblasts in tumor stroma. As an enzyme, FAP has di- and endoprolylpeptidase, endoprotease, and gelatinase/collagenase activities in the Tumor Microenvironment (TME). According to recent reports, FAP overexpression in CRC contributes to adverse clinical outcomes such as increased lymph node metastasis, tumor recurrence, and angiogenesis, as well as decreased overall survival. In this review, studies about the expression level of FAP and its associations with CRC patients' prognosis are reviewed. High expression levels of FAP and its association with clinicopathological factors have made as a potential target. In many studies, FAP has been evaluated as a therapeutic target and diagnostic factor into which the current review tries to provide a comprehensive insight. Video Abstract.

Nanoparticles Targeted to Fibroblast Activation Protein Outperform PSMA for MRI Delineation of Primary Prostate Tumors

Accurate delineation of gross tumor volumes remains a barrier to radiotherapy dose escalation and boost dosing in the treatment of solid tumors, such as prostate cancer. Magnetic resonance imaging (MRI) of tumor targets has the power to enable focal dose boosting, particularly when combined with technological advances such as MRI-linear accelerator. Fibroblast activation protein (FAP) is overexpressed in stromal components of >90% of epithelial carcinomas. Herein, the authors compare targeted MRI of prostate specific membrane antigen (PSMA) with FAP in the delineation of orthotopic prostate tumors. Control, FAP, and PSMA-targeting iron oxide nanoparticles were prepared with modification of a lymphotropic MRI agent (FerroTrace, Ferronova). Mice with orthotopic LNCaP tumors underwent MRI 24 h after intravenous injection of nanoparticles. FAP and PSMA nanoparticles produced contrast enhancement on MRI when compared to control nanoparticles. FAP-targeted MRI increased the proportion of tumor contrast-enhancing black pixels by 13%, compared to PSMA. Analysis of changes in R2 values between healthy prostates and LNCaP tumors indicated an increase in contrast-enhancing pixels in the tumor border of 15% when targeting FAP, compared to PSMA. This study demonstrates the preclinical feasibility of PSMA and FAP-targeted MRI which can enable targeted image-guided focal therapy of localized prostate cancer.

Distinguishing Benign and Malignant Findings on [68 Ga]-FAPI PET/CT Based on Quantitative SUV Measurements

AIM/PURPOSE

Fibroblast activation protein (FAP) is overexpressed by cancer-associated fibroblasts. However, activated fibroblasts have been shown to play a significant role also in certain benign conditions such as wound healing or chronic inflammation. Therefore, the current study aimed to identify whether FAPI uptake might differ between malignant lesions and benign conditions.

MATERIAL AND METHODS

We retrospectively analyzed 155 patients with various cancer types who received [⁶⁸ Ga]-FAPI-04/02-PET/CT between July 2017 and March 2020. SUV_max, SUV_mean, and lesion-to-background ratios (LBR) of FAPI uptake were measured in benign processes compared to malignant lesions (primary and/or 2 exemplary metastases). In addition, receiver operating characteristic (ROC) curve analysis was conducted to compare the predictive capabilities of semiquantitative PET/CT parameters. Furthermore, the sensitivity, specificity, optimal cutoff value, and 95% confidence interval (CI) were determined for each parameter.

RESULTS

Benign lesions exhibited significantly lower FAPI uptake compared to malignant lesions (mean SUV_max benign vs. malignant: 4.2 vs. 10.6; p < 0.001). In ROC analysis, cutoff values of these lesions (benign vs. malignant) were established based on SUV_max, SUV_mean, and LBR. The SUV_max cutoff value for all lesions was 5.5 and the corresponding sensitivity, specificity, accuracy, and AUC were 78.8%, 85.1%, 82.0%, and 0.89%, respectively.

CONCLUSION

Our aim was to systematically analyze the pattern of FAPI uptake in benign and malignant processes. This investigation demonstrates that FAPI uptake might be useful to differentiate malignant and benign findings due to different patho-physiological origins.

Usefulness of [68Ga]FAPI-04 and [18F]FDG PET/CT for the detection of primary tumour and metastatic lesions in gastrointestinal carcinoma: a comparative study

OBJECTIVE

To assess and compare the diagnostic performance of gallium-68-labelled fibroblast activation protein inhibitor ([⁶⁸Ga]FAPI-04) and fluorine-18 fluorodeoxyglucose ([¹⁸F]FDG) positron emission tomography/computed tomography (PET/CT) in gastrointestinal cancer.

METHODS

Fifty-one patients who underwent both [¹⁸F]FDG and [⁶⁸Ga]FAPI-04 PET/CT for initial staging or restaging were enrolled. Histopathological findings, typical radiological appearances, and clinical imaging follow-up were used as the reference standard. The diagnostic performance of the two tracers was calculated and compared. The maximum standardised uptake value (SUVmax), mean SUV (SUVmean), tumour-to-mediastinal blood pool ratio (TBR), and tumour-to-liver ratio (TLR) of primary and metastatic lesions were measured and compared between two imaging modalities.

RESULTS

In patient-based analysis, [⁶⁸Ga]FAPI-04 showed much better diagnostic sensitivity than [¹⁸F]FDG in detecting primary tumour (94.44% [17/18] vs. 61.11% [11/18]), postoperative recurrence and metastases (95.65% [22/23] vs. 69.57% [16/23]), and peritoneal carcinomatosis (100% [28/28] vs. 60.71% [17/28]) (all p < 0.05). In lesion-based analysis, [⁶⁸Ga]FAPI-04 showed higher sensitivity than [¹⁸F]FDG for detecting lymph node metastases. In peritoneal carcinomatosis, the median SUVmax (12.12 vs. 7.18) and SUVmean (6.84 vs. 4.11) with [⁶⁸Ga]FAPI-04 were significantly higher than those with [¹⁸F]FDG (all p < 0.005). The TBR and TLR of [⁶⁸Ga]FAPI-04 were significantly higher than those of [¹⁸F]FDG for detecting primary tumour, lymph node, liver, and peritoneal metastases (all p < 0.005). Therapeutic management changed in 13 patients according to [⁶⁸Ga]FAPI-04 PET/CT compared with conventional imaging.

CONCLUSIONS

[⁶⁸Ga]FAPI-04 is superior to [¹⁸F]FDG PET/CT for detecting primary tumour, postoperative recurrence and metastasis, and peritoneal carcinomatosis in gastrointestinal cancer.

KEY POINTS

• [⁶⁸Ga]FAPI-04 PET/CT showed significantly higher sensitivity than [¹⁸F]FDG PET/CT in the detection of primary tumour and postoperative recurrence and metastasis in patients with gastrointestinal carcinoma. • [⁶⁸Ga]FAPI-04 PET/CT had obvious advantages over [¹⁸F]FDG PET/CT in the detection of peritoneal carcinomatosis from gastrointestinal carcinoma with a much higher FAPI uptake value, TBR, and TLR. • Although the median SUVmax and SUVmean of [⁶⁸Ga]FAPI-04 were similar to those of [¹⁸F]FDG for the primary tumour, lymph node metastases, and liver metastases in gastrointestinal carcinoma, the TBR and TLR of the SUVmax and SUVmean were significantly higher on [⁶⁸Ga]FAPI-04 PET/CT, causing the lesions to be displayed more clearly.

Diagnostic performance of Ga-68 FAPI 04 PET/CT in colorectal malignancies

AIM

To evaluate the role of Ga-68 fibroblast activation protein inhibitor 04 PET/computed tomography (FAPI) in colorectal cancers (CRCs) in terms of diagnostic accuracy and impact on clinical management. FAPI is compared with FDG PET/CT and conventional imaging in staging, restaging, recurrence detection, and response evaluation of CRC.

METHODS

Twenty-nine consecutive patients of histopathologically confirmed primary or relapsed CRC were included in the study. Patients who underwent FAPI PET/CT along with either FDG PET/CT or conventional imaging were included. Primary lesions, recurrence sites, lymph nodes, and metastatic lesions were recorded on all the scans. Maximum standardized uptake value (SUVmax) was measured from both primary and metastatic lesions.

RESULTS

The sensitivity of FAPI in primary and recurrence detection is 100% compared to 88% for FDG/conventional imaging. The overall sensitivity of FAPI stands at 98% with accuracy at 95% whereas for FDG/conventional imaging the sensitivity and accuracy are 78% and 77%, respectively, with P  < 0.002. Significant difference was noted in the detection of peritoneal metastasis (96% vs. 66%).

CONCLUSION

FAPI PET/CT shows better sensitivity and accuracy in the evaluation of CRCs, especially in peritoneal disease compared to FDG PET/CT and conventional imaging. FAPI has the potential to replace FDG in CRCs.

Fibroblast Activation Protein Inhibitor Theranostics

The theranostic use of fibroblast activation protein inhibitors (FAPIs) is a novel approach in oncology. Sarcomas are a heterogenous group of rare malignant tumors. Prognosis remains poor in advanced/metastatic disease due to limited therapeutic options. Sarcoma frequently demonstrate high expression of fibroblast activation protein alpha on the tumor cells themselves, in contrast to other solid tumors, where it is mainly expressed on cancer-associated fibroblasts. Consequently, high in vivo uptake of FAPI in PET is observed in sarcoma. Moreover, retrospective case reports and series demonstrated feasibility of FAPI radioligand therapy with signs of tumor response.

FAPI PET: Fibroblast Activation Protein Inhibitor Use in Oncologic and Nononcologic Disease

Gallium 68 (68Ga)-labeled fibroblast activation protein (FAP) inhibitor (FAPI) PET is based on the molecular targeting of the FAP, which is known to be highly expressed in the major cell population in tumor stroma, termed cancer-associated fibroblasts. Among many FAP-targeted radiopharmaceuticals developed so far, 68Ga-FAPI exhibits rapid tracer accumulation in target lesions and low background signal, which results in excellent imaging features. FAPI PET can be integrated in the clinical workflow and enables the detection of small primary or metastatic lesions, especially in the brain, liver, pancreas, and gastrointestinal tract due to the low tracer accumulation in these organs. Moreover, the DOTA (1,4,7,10-tetraazacylclododecane-1,4,7,10-tetrayl tetraacetic acid) chelator in the molecular structure allows coupling of the FAPI molecules with therapeutic emitters such as yttrium 90 for theranostic applications. This review provides an overview of the state of the art in FAP imaging, summarizes the current knowledge of relevant cancer biology, and highlights the latest findings in the clinical use of 68Ga-FAPI PET and other current FAPI tracers. Published under a CC BY 4.0 license.

From Automated Synthesis to In Vivo Application in Multiple Types of Cancer-Clinical Results with [68Ga]Ga-DATA5m.SA.FAPi

Radiolabeled FAPI (fibroblast activation protein inhibitors) recently gained attention as widely applicable imaging and potential therapeutic compounds targeting CAF (cancer-associated fibroblasts) or DAF (disease-associated fibroblasts in benign disorders). Moreover, the use of FAPI has distinct advantages compared to FDG (e.g., increased sensitivity in regions with high glucose metabolism, no need for fasting, and rapid imaging). In this study, we wanted to evaluate the radiochemical synthesis and the clinical properties of the new CAF-targeting tracer [⁶⁸Ga]Ga-DATA^5m.SA.FAPi. The compound consists of a (radio)chemically easy to use hybrid chelate DATA.SA, which can be labeled at low temperatures, making it an interesting molecule for 'instant kit-type' labeling, and a squaric acid moiety that provides distinct advantages for synthesis and radiolabeling. Our work demonstrates that automatic synthesis of the FAP inhibitor [⁶⁸Ga]Ga-DATA^5m.SA.FAPi is feasible and reproducible, providing convenient access to this new hybrid chelator-based tracer. Our studies demonstrated the diagnostic usability of [⁶⁸Ga]Ga-DATA^5m.SA.FAPi for the unambiguous detection of cancer-associated fibroblasts of various carcinomas and their metastases (NSCLC, liposarcoma, parotid tumors, prostate cancer, and pancreas adenocarcinoma), while physiological uptake in brain, liver, intestine, bone, and lungs was very low.

Advances in nuclear medicine-based molecular imaging in head and neck squamous cell carcinoma

Head and neck squamous cell carcinomas (HNSCCs) are often aggressive, making advanced disease very difficult to treat using contemporary modalities, such as surgery, radiation therapy, and chemotherapy. However, targeted therapy, e.g., cetuximab, an epidermal growth factor receptor inhibitor, has demonstrated survival benefit in HNSCC patients with locoregional failure or distant metastasis. Molecular imaging aims at various biomarkers used in targeted therapy, and nuclear medicine-based molecular imaging is a real-time and non-invasive modality with the potential to identify tumor in an earlier and more treatable stage, before anatomic-based imaging reveals diseases. The objective of this comprehensive review is to summarize recent advances in nuclear medicine-based molecular imaging for HNSCC focusing on several commonly radiolabeled biomarkers. The preclinical and clinical applications of these candidate imaging strategies are divided into three categories: those targeting tumor cells, tumor microenvironment, and tumor angiogenesis. This review endeavors to expand the knowledge of molecular biology of HNSCC and help realizing diagnostic potential of molecular imaging in clinical nuclear medicine.

Elevated CXCL12 in the plasma membrane of locally advanced rectal cancer after neoadjuvant chemoradiotherapy: a potential prognostic marker

Background: Neoadjuvant chemoradiotherapy (nCRT) in locally advanced rectal cancer (LARC) has been shown to improve sphincter preservation and local pelvic control, but the efficacy of nCRT plateaus due to metastasis. CXC chemokine ligand 12 (CXCL12) has a critical impact on cancer development and metastasis.

Methods: By investigating public databases containing LARC patient data, CXCL12, CXCR4 and FAPα expression was analyzed via the Tumor Immune Estimation Resource (TIMER) and GSEA. Immunohistochemistry was applied to a total of 121 surgically resected specimens consisting of 61 LARCs after nCRT and 60 LARCs with no nCRT and 16 cases with endoscopic resection of high-grade colorectal adenoma.

Results: By investigating public databases containing LARC patient data, CXCL12 expression is correlated with poor prognosis, immune cell infiltration, epithelial- mesenchymal transition, and angiogenesis in LARC. Furthermore, radiation selectively induced CXCL12, CXCR4 and FAPα expression in tumor tissues. Immunohistochemistry results showed that the levels of CXCL12, CXCR4, and FAPα in LARC cells after nCRT were higher than in LARC cells untreated with nCRT (p < 0.001 for each). Elevated levels of CXCL12 in the plasma membrane of LARC cells after nCRT demonstrated an association with the period of freedom from recurrence (FFR) in univariate and multivariate survival analyses (p = 0.005 and p = 0.031, respectively).

Conclusions: The expression of CXCL12 may influence the survival and invasive properties of LARC cells during nCRT and promote cancer recurrence. We suggest that CXCL12 expression in the plasma membrane of radioresistant LARC cells may be a predictive factor of recurrence and a viable therapeutic strategy to control radioresistant LARC recurrence.

Radiotheranostics - Precision Medicine in Nuclear Medicine and Molecular Imaging

'See what you treat and treat what you see, at a molecular level', could be the motto of theranostics. The concept implies diagnosis (imaging) and treatment of cells (usually cancer) using the same molecule, thus guaranteeing a targeted cytotoxic approach of the imaged tumor cells while sparing healthy tissues. As the brilliant late Sam Gambhir would say, the imaging agent acts like a 'molecular spy' and reveals where the tumoral cells are located and the extent of disease burden (diagnosis). For treatment, the same 'molecular spy' docks to the same tumor cells, this time delivering cytotoxic doses of radiation (treatment). This duality represents the concept of a 'theranostic pair', which follows the scope and fundamental principles of targeted precision and personalized medicine. Although the term theranostic was noted in medical literature in the early 2000s, the principle is not at all new to nuclear medicine. The first example of theranostic dates back to 1941 when Dr. Saul Hertz first applied radioiodine for radionuclide treatment of thyroid cells in patients with hyperthyroidism. Ever since, theranostics has been an integral element of nuclear medicine and molecular imaging. The more we understand tumor biology and molecular pathology of carcinogenesis, including specific mutations and receptor expression profiles, the more specific these 'molecular spies' can be developed for diagnostic molecular imaging and subsequent radionuclide targeted therapy (radiotheranostics). The appropriate selection of the diagnostic and therapeutic radionuclide for the 'theranostic pair' is critical and takes into account not only the type of cytotoxic radiation emission, but also the linear energy transfer (LET), and the physical half-lives. Advances in radiochemistry and radiopharmacy with new radiolabeling techniques and chelators are revolutionizing the field. The landscape of cytotoxic systemic radionuclide treatments has dramatically expanded through the past decades thanks to all these advancements. This article discusses present and promising future theranostic applications for various types of diseases such as thyroid disorders, neuroendocrine tumors (NET), pediatric malignancies, and prostate cancer (PC), and provides an outlook for future perspectives.

Preparation and Bioevaluation of 99mTc-Labeled FAP Inhibitors as Tumor Radiotracers to Target the Fibroblast Activation Protein

Fibroblast activation protein (FAP) is overexpressed in cancer-associated fibroblasts (CAFs) in a majority of human epithelial cancers. With low expression in normal organs, FAP has become a promising molecular target for tumor theranostics. To develop a lower cost and more widely available alternative to positron emission tomography (PET), two isocyanide-containing FAP inhibitors (CN-C₅-FAPI and CN-PEG₄-FAPI) were synthesized and radiolabeled with ^99mTc to obtain [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺ and [^99mTc][Tc-(CN-PEG₄-FAPI)₆]⁺ in high yields (>95%). They showed good stability in saline and mouse serum. The partition coefficient (log P) values of [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺ and [^99mTc^][Tc-(CN-PEG₄-FAPI)₆]⁺ were -0.86 ± 0.03 and -2.38 ± 0.07, respectively, indicating that they were good hydrophilic complexes. The low nanomolar IC₅₀ values of CN-C₅-FAPI and CN-PEG₄-FAPI indicated that they had specificity to FAP. In vitro cellular uptake and blocking experiments implied a FAP-targeted uptake mechanism. The nanomolar K_d values from the saturation binding assay indicated that they had significantly high target affinity to FAP. The biodistribution and blocking study in BALB/c nude mice bearing U87MG tumors showed that both exhibited specific tumor uptake. [^99mTc][Tc-(CN-PEG₄-FAPI)₆]⁺ showed a higher tumor uptake and a higher tumor/nontarget ratio than [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺. The results of micro-single-photon emission computed tomography (SPECT) imaging studies of [^99mTc][Tc-(CN-C₅-FAPI)₆]⁺ and [^99mTc^][Tc-(CN-PEG₄-FAPI)₆]⁺ were in accordance with the biodistribution results, suggesting that [^99mTc][Tc-(CN-PEG₄-FAPI)₆]⁺ is a promising tumor imaging agent for targeting FAP.

Noncontrast Magnetic Resonance Radiomics and Multilayer Perceptron Network Classifier: An approach for Predicting Fibroblast Activation Protein Expression in Patients With Pancreatic Ductal Adenocarcinoma

BACKGROUND

Fibroblast activation protein (FAP) in pancreatic ductal adenocarcinoma (PDAC) is closely related to the prognosis and treatment of patients. Accurate preoperative FAP expression can better identify the population benefitting from FAP-targeting drugs.

PURPOSE

To develop and validate a machine learning classifier based on noncontrast MRI for the preoperative prediction of FAP expression in patients with PDAC.

STUDY TYPE

Retrospective cohort study.

POPULATION

Altogether, 129 patients with pathology-confirmed PDAC undergoing MR scan and surgical resection; 90 patients in a training cohort, and 39 patients in a validation cohort. FIELD STRENGTH/SEQUENCE/3T: Breath-hold single-shot fast-spin echo T2-weighted sequence and unenhanced and noncontrast T1-weighted fat-suppressed sequences.

ASSESSMENT

FAP expression was quantified using immunohistochemistry. For each patient, 1409 radiomics features were extracted from T1- and T2-weighted images and reduced using the least absolute shrinkage and selection operator logistic regression algorithm. A multilayer perceptron (MLP) network classifier was developed using the training and validation set. The MLP network classifier performance was determined by its discriminative ability, calibration, and clinical utility.

STATISTICAL TESTS

Kaplan-Meier estimates, student's t-test, the Kruskal-Wallis H test, and the chi-square test, univariable regression analysis, receiver operating characteristic curve, and decision curve analysis were used.

RESULTS

A log-rank test showed that the survival of patients with low FAP expression (24.43 months) was significantly longer (P < 0.05) than that in the FAP-high group (13.50 months). The prediction model showed good discrimination in the training set (area under the curve [AUC], 0.84) and the validation set (AUC, 0.77). The sensitivity, specificity, accuracy, positive predictive value, and negative predictive value for the training set were 75.00%, 79.41%, 0.77, 0.86, and 0.66, respectively, whereas those for the validation set were 85.00%, 63.16%, 0.74, 0.71, and 0.80, respectively.

DATA CONCLUSIONS

The MLP network classifier based on noncontrast MRI can accurately predict FAP expression in patients with PDAC.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 2.

Potential Targets Other Than PSMA for Prostate Cancer Theranostics: A Systematic Review

Background: Prostate-specific membrane antigen (PSMA) is not sufficiently overexpressed in a small proportion of prostate cancer (PCa) patients, who require other strategies for imaging and/or treatment. We reviewed potential targets other than PSMA for PCa theranostics in nuclear medicine that have already been tested in humans. Methods: We performed a systematic web search in the PubMed and Cochrane databases, with no time restrictions by pooling terms (“prostate cancer”, “prostatic neoplasms”) and (“radioligand”, “radiotracer”). Included articles were clinical studies. The results were synthetized by the target type. Results: We included 38 studies on six different targets: gastrin-releasing peptide receptors (GRPRs) (n = 23), androgen receptor (n = 11), somatostatin receptors (n = 6), urokinase plasminogen activator surface receptor (n = 4), fibroblast activation protein (n = 2 studies) and integrin receptors (n = 1). GRPRs, the most studied target, has a lower expression in high-grade PCa, CRPC and bone metastases. Its use might be of higher interest in treating earlier stages of PCa or low-grade PCa. Radiolabeled fibroblast activation protein inhibitors were the most recent and promising molecules, but specific studies reporting their interest in PCa are needed. Conclusion: Theranostics in nuclear medicine will continue to develop in the future, especially for PCa patients. Targets other than PSMA exist and deserve to be promoted.

New Frontiers in Cancer Imaging and Therapy Based on Radiolabeled Fibroblast Activation Protein Inhibitors: A Rational Review and Current Progress

Over the past decade, the tumor microenvironment (TME) has become a new paradigm of cancer diagnosis and therapy due to its unique biological features, mainly the interconnection between cancer and stromal cells. Within the TME, cancer-associated fibroblasts (CAFs) demonstrate as one of the most critical stromal cells that regulate tumor cell growth, progression, immunosuppression, and metastasis. CAFs are identified by various biomarkers that are expressed on their surfaces, such as fibroblast activation protein (FAP), which could be utilized as a useful target for diagnostic imaging and treatment. One of the advantages of targeting FAP-expressing CAFs is the absence of FAP expression in quiescent fibroblasts, leading to a controlled targetability of diagnostic and therapeutic compounds to the malignant tumor stromal area using radiolabeled FAP-based ligands. FAP-based radiopharmaceuticals have been investigated strenuously for the visualization of malignancies and delivery of theranostic radiopharmaceuticals to the TME. This review provides an overview of the state of the art in TME compositions, particularly CAFs and FAP, and their roles in cancer biology. Moreover, relevant reports on radiolabeled FAP inhibitors until the year 2021 are highlighted-as well as the current limitations, challenges, and requirements for those radiolabeled FAP inhibitors in clinical translation.

FAP and FAPI-PET/CT in Malignant and Non-Malignant Diseases: A Perfect Symbiosis?

A fibroblast activation protein (FAP) is an atypical type II transmembrane serine protease with both endopeptidase and post-proline dipeptidyl peptidase activity. FAP is overexpressed in cancer-associated fibroblasts (CAFs), which are found in most epithelial tumors. CAFs have been implicated in promoting tumor cell invasion, angiogenesis and growth and their presence correlates with a poor prognosis. However, FAP can generally be found during the remodeling of the extracellular matrix and therefore can be detected in wound healing and benign diseases. For instance, chronic inflammation, arthritis, fibrosis and ischemic heart tissue after a myocardial infarction are FAP-positive diseases. Therefore, quinoline-based FAP inhibitors (FAPIs) bind with a high affinity not only to tumors but also to a variety of benign pathologic processes. When these inhibitors are radiolabeled with positron emitting radioisotopes, they provide new diagnostic and prognostic tools as well as insights into the role of the microenvironment in a disease. In this respect, they deliver additional information beyond what is afforded by conventional FDG PET scans that typically report on glucose uptake. Thus, FAP ligands are considered to be highly promising novel tracers that offer a new diagnostic and theranostic potential in a variety of diseases.

Fibroblast activation protein targeted therapy using [177Lu]FAPI-46 compared with [225Ac]FAPI-46 in a pancreatic cancer model

Purpose

Fibroblast activation protein (FAP), which has high expression in cancer-associated fibroblasts of epithelial cancers, can be used as a theranostic target. Our previous study used ⁶⁴Cu and ²²⁵Ac-labelled FAP inhibitors (FAPI-04) for a FAP-expressing pancreatic cancer xenograft imaging and therapy. However, the optimal therapeutic radionuclide for FAPI needs to be investigated further. In this study, we evaluated the therapeutic effects of beta-emitter (¹⁷⁷Lu)-labelled FAPI-46 and alpha-emitter (²²⁵Ac)-labelled FAPI-46 in pancreatic cancer models.

Methods

PET scans (1 h post injection) were acquired in PANC-1 xenograft mice (n = 9) after the administration of [¹⁸F]FAPI-74 (12.4 ± 1.7 MBq) for the companion imaging. The biodistribution of [¹⁷⁷Lu]FAPI-46 and [²²⁵Ac]FAPI-46 were evaluated in the xenograft model (total n = 12). For the determination of treatment effects, [¹⁷⁷Lu]FAPI-46 and [²²⁵Ac]FAPI-46 were injected into PANC-1 xenograft mice at different doses: 3 MBq (n = 6), 10 MBq (n = 6), 30 MBq (n = 6), control (n = 4) for [¹⁷⁷Lu]FAPI-46, and 3 kBq (n = 3), 10 kBq (n = 2), 30 kBq (n = 6), control (n = 7) for [²²⁵Ac]FAPI-46. Tumour sizes and body weights were followed.

Results

[¹⁸F]FAPI-74 showed rapid clearance by the kidneys and high accumulation in the tumour and intestine 1 h after administration. [¹⁷⁷Lu]FAPI-46 and [²²⁵Ac]FAPI-46 also showed rapid clearance by the kidneys and relatively high accumulation in the tumour at 3 h. Both [¹⁷⁷Lu]FAPI-46 and [²²⁵Ac]FAPI-46 showed tumour-suppressive effects, with a mild decrease in body weight. The treatment effects of [¹⁷⁷Lu]FAPI-46 were relatively slow but lasted longer than those of [²²⁵Ac]FAPI-46.

Conclusion

This study suggested the possible application of FAPI radioligand therapy in FAP-expressing pancreatic cancer. Further evaluation is necessary to find the best radionuclide with shorter half-life, as well as the combination with therapies targeting tumour cells directly.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05554-2.

The Role of Fibroblast Activation Protein Ligands in Oncologic PET Imaging

Fibroblast activation protein inhibitor emerges as a novel and highly promising agent for diagnostic and possibly theranostic application in various malignant and non-malignant diseases. FAPI impresses with its selective expression in several pathologies, ligand induced internalization, and presence in a large variety of malignancies. Current studies indicate that FAPI is equal or even superior to the current standard oncological tracer fluorodeoxyglucose in several oncological diseases. It seems to present lower background activity, stronger uptake in tumorous lesions and thus sharper contrasts. For improved comprehension of fibroblast activation, protein expression and clinicopathologic conditions, further studies are of essence.

State-of-the-art of FAPI-PET imaging: a systematic review and meta-analysis

INTRODUCTION

Fibroblast activation protein-α (FAPα) is overexpressed on cancer-associated fibroblasts in approximately 90% of epithelial neoplasms, representing an appealing target for therapeutic and molecular imaging applications. [⁶⁸ Ga]Ga-labelled radiopharmaceuticals-FAP-inhibitors (FAPI)-have been developed for PET. We systematically reviewed and meta-analysed published literature to provide an overview of its clinical role.

MATERIALS AND METHODS

The search, limited to January 1st, 2018-March 31st, 2021, was performed on MedLine and Embase databases using all the possible combinations of terms "FAP", "FAPI", "PET/CT", "positron emission tomography", "fibroblast", "cancer-associated fibroblasts", "CAF", "molecular imaging", and "fibroblast imaging". Study quality was assessed using the QUADAS-2 criteria. Patient-based and lesion-based pooled sensitivities/specificities of FAPI PET were computed using a random-effects model directly from the STATA "metaprop" command. Between-study statistical heterogeneity was tested (I²-statistics).

RESULTS

Twenty-three studies were selected for systematic review. Investigations on staging or restaging head and neck cancer (n = 2, 29 patients), abdominal malignancies (n = 6, 171 patients), various cancers (n = 2, 143 patients), and radiation treatment planning (n = 4, 56 patients) were included in the meta-analysis. On patient-based analysis, pooled sensitivity was 0.99 (95% CI 0.97-1.00) with negligible heterogeneity; pooled specificity was 0.87 (95% CI 0.62-1.00), with negligible heterogeneity. On lesion-based analysis, sensitivity and specificity had high heterogeneity (I² = 88.56% and I² = 97.20%, respectively). Pooled sensitivity for the primary tumour was 1.00 (95% CI 0.98-1.00) with negligible heterogeneity. Pooled sensitivity/specificity of nodal metastases had high heterogeneity (I² = 89.18% and I² = 95.74%, respectively). Pooled sensitivity in distant metastases was good (0.93 with 95% CI 0.88-0.97) with negligible heterogeneity.

CONCLUSIONS

FAPI-PET appears promising, especially in imaging cancers unsuitable for [¹⁸F]FDG imaging, particularly primary lesions and distant metastases. However, high-level evidence is needed to define its role, specifically to identify cancer types, non-oncological diseases, and clinical settings for its applications.

CD40 Agonist Targeted to Fibroblast Activation Protein α Synergizes with Radiotherapy in Murine HPV-Positive Head and Neck Tumors

PURPOSE

The incidence of human papillomavirus-associated head and neck squamous cell carcinoma (HPV⁺-HNSCC) is rising worldwide and although current therapeutic modalities are efficient in the majority of patients, there is a high rate of treatment failures. Thus, novel combination approaches are urgently needed to achieve better disease control in patients with HPV⁺-HNSCC. We investigated the safety and therapeutic efficacy of a novel fibroblast activation protein (FAP)-targeted CD40 agonist (FAP-CD40) in combination with local hypofractionated radiation in a syngeneic HPV⁺-HNSCC model.

EXPERIMENTAL DESIGN

Using an established orthotopic model, we treated tumor-bearing mice with local hypofractionated radiotherapy (2 × 6 Gy) alone or in combination with a systemic administration of the FAP-CD40 antibody. Following up the mice, we evaluated the changes in the tumor microenvironment (TME) by immunofluorescence, FACS, and NanoString RNA analysis.

RESULTS

The suboptimal radiotherapy regimen chosen failed to control tumors in the treated mice. The FAP-CD40 administered in monotherapy transiently controlled tumor growth, whereas the combined therapy induced durable complete responses in more than 80% of the tumor-bearing mice. This notable efficacy relied on the radiotherapy-induced remodeling of the TME and activation of the CD8⁺ T-cell-cDC1 axis and was devoid of the systemic toxicity frequently associated with CD40-targeted therapy. Moreover, the robust immunologic memory developed effectively prevented tumor relapses, a common feature in patients with HNSCC.

CONCLUSIONS

Our study provides proof of concept, as well as mechanistic insights of the therapeutic efficacy of a bispecific FAP-CD40 combined with local radiotherapy in a FAP⁺-HNSCC model increasing overall survival and inducing long-term antitumor immunity.

Microenvironmental markers are correlated with lymph node metastasis in invasive submucosal colorectal cancer

Aims

Recent studies have shown that the microenvironment can include cancer cells and cancer‐associated fibroblasts (CAFs), and that both play important roles in the progression and metastasis of CRC. Here, we aimed to analyse the expression patterns of cancer cell‐ and CAF‐related proteins in submucosal invasive colorectal cancer (SiCRC) and whether such markers are correlated with lymph node metastasis (LNM).

Methods and results

Quantitative analysis was conducted for Ki‐67, p53, β‐catenin and matrix metalloproteinase‐7 (MMP7) to assess cancer cell markers. In addition, we examined CAF markers, including smooth muscle alpha‐actin (α‐SMA), CD10, podoplanin, fibroblast‐specific protein 1 (FSP‐1), platelet‐derived growth factor receptor (PDGFR)‐α, PDGFR‐β, adipocyte enhancer‐binding protein 1 (AEBP1), fibroblast‐associated protein 1 (FAP‐1), zinc finger E‐box binding homeobox 1 (ZEB1) and TWIST‐related protein 1 (TWIST1). In both cases, we conducted digital pathology with Aperio software. We also examined the expression patterns of biomarkers using hierarchical cluster analysis. Two subgroups were established based on the expression patterns of cancer cell‐ and CAF‐ related markers, and the associations of these subgroups with clinicopathological variables. In multivariate analysis, subgroup 2, which was characterised by high expression of Ki‐67, p53, FAP‐1, platelet‐derived growth factor receptor (PDGFR)‐α, PDGFR‐β and TWIST1, was correlated with LNM (P < 0.01). Next, we examined the associations of individual biomarkers with LNM. Multivariate analysis showed that high expression levels of Ki‐67 and FAP‐1 were significantly associated with LNM (P < 0.05).

Conclusions

Our findings showed that expression patterns of cancer cell‐ and CAF‐related proteins may allow for stratification of patients into risk categories for LNM in SiCRC. In addition, Ki‐67‐ and FAP‐1‐expressing microenvironmental cells might be helpful for identification of correlations with LNM in SiCRC.

In Vitro and In Situ Activity-Based Labeling of Fibroblast Activation Protein with UAMC1110-Derived Probes

Fibroblast activation protein (FAP) is a proline-selective protease that belongs to the S9 family of serine proteases. It is typically highly expressed in the tumor microenvironment (TME) and especially in cancer-associated fibroblasts, the main cell components of the tumor stroma. The exact role of its enzymatic activity in the TME remains largely unknown. Hence, tools that enable selective, activity-based visualization of FAP within the TME can help to unravel FAP’s function. We describe the synthesis, biochemical characterization, and application of three different activity-based probes (biotin-, Cy3-, and Cy5-labeled) based on the FAP-inhibitor UAMC1110, an in-house developed molecule considered to be the most potent and selective FAP inhibitor available. We demonstrate that the three probes have subnanomolar FAP affinity and pronounced selectivity with respect to the related S9 family members. Furthermore, we report that the fluorescent Cy3- and Cy5-labeled probes are capable of selectively detecting FAP in a cellular context, making these chemical probes highly suitable for further biological studies. Moreover, proof of concept is provided for in situ FAP activity staining in patient-derived cryosections of urothelial tumors.

Imaging of Fibroblast Activation Protein in Cancer Xenografts Using Novel (4-Quinolinoyl)-glycyl-2-cyanopyrrolidine-Based Small Molecules

Fibroblast activation protein (FAP) has become a favored target for imaging and therapy of malignancy. We have synthesized and characterized two new (4-quinolinoyl)-glycyl-2-cyanopyrrolidine-based small molecules for imaging of FAP, QCP01 and [111In]QCP02, using optical and single-photon computed tomography/CT, respectively. Binding of imaging agents to FAP was assessed in six human cancer cell lines of different cancer types: glioblastoma (U87), melanoma (SKMEL24), prostate (PC3), NSCLC (NCIH2228), colorectal carcinoma (HCT116), and lung squamous cell carcinoma (NCIH226). Mouse xenograft models were developed with FAP-positive U87 and FAP-negative PC3 cells to test pharmacokinetics and binding specificity in vivo. QCP01 and [111In]QCP02 demonstrated nanomolar inhibition of FAP at Ki values of 1.26 and 16.20 nM, respectively. Both were selective for FAP over DPP-IV, a related serine protease. Both enabled imaging of FAP-expressing tumors specifically in vivo. [111In]QCP02 showed high uptake at 18.2 percent injected dose per gram in the U87 tumor at 30 min post-administration.

Altered Tissue and Plasma Levels of Fibroblast Activation Protein-α (FAP) in Renal Tumours

Simple Summary

Malignant epithelial tumour’s behaviour in the kidney has traditionally been analysed attending to different prognostic parameters focussed on the proliferating neoplastic cell. This is the case of renal cell carcinoma (RCC), in which a large tumour diameter, high histological grade, and the presence of necrosis, among other factors, have been related with a high risk of distant metastasis and, therefore, worse survival. Recently, several elements of the tumour microenviroment, such as cancer-associated fibroblasts (CAFs), are being studied in order to develop more accurate diagnostic and therapeutic approaches. We present data that support that the fibroblast activation protein-α (FAP), a CAF biomarker, provides interesting information both in tumour tissues and in plasma from patients with RCC.

Abstract

(1) Background: Renal cell carcinoma (RCC) is a heterogeneous and complex disease with only partial response to therapy, high incidence of metastasis and recurrences, and scarce reliable biomarkers indicative of progression and survival. Cancer-associated fibroblasts (CAFs) play an important role supporting and promoting renal cancer progression. (2) Methods: In this study, we analysed fibroblast activation protein-α (FAP) immunohistochemical expression and its soluble isoform (sFAP) in tumour tissues and plasma from 128 patients with renal tumours. (3) Results: FAP is expressed in the cell surface of CAFs of the tumour centre and infiltrating front from clear cell renal cell carcinomas (CCRCC, n = 89), papillary renal cell carcinomas (PRCC, n = 21), and chromophobe renal cell carcinomas (ChRCC, n = 8), but not in the benign tumour renal oncocytoma (RO, n = 10). A high expression of FAP and low levels sFAP are significantly associated with high tumour diameter, high grade, and high pT stage, lymph node invasion, development of early metastases, and worse 5-year cancer specific survival of CCRCC patients. (4) Conclusions: These findings corroborate the potential usefulness of FAP immunohistochemistry and plasma sFAP as a biomarker of CCRCC progression and point to CAF-related proteins as promising immunohistochemical biomarkers for the differential diagnosis of ChRCC and RO.

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.

AND-gate contrast agents for enhanced fluorescence-guided surgery

The surgical resection of tumours requires the precise location and definition of the margins between lesions and normal tissue. However, this is made difficult by irregular margin borders. Although molecularly targeted optical contrast agents can be used to define tumour margins during surgery in real time, the selectivity of the contrast agents is often limited by the target being expressed in both healthy and tumour tissues. Here, we show that AND-gate optical imaging probes requiring the processing of two substrates by multiple tumour-specific enzymes produce a fluorescent signal with significantly improved specificity and sensitivity to tumour tissue. We evaluated the performance of the probes in mouse models of mammary tumours and of metastatic lung cancer, and during fluorescence-guided robotic surgery. Imaging probes relying on multivariate activation to selectively target complex patterns of enzymatic activity should be useful in disease detection, treatment and monitoring.

Targeting fibroblast activation protein (FAP): next generation PET radiotracers using squaramide coupled bifunctional DOTA and DATA5m chelators

Background

Fibroblast activation protein (FAP) is a proline selective serine protease that is overexpressed in tumor stroma and in lesions of many other diseases that are characterized by tissue remodeling. In 2014, a most potent FAP-inhibitor (referred to as UAMC1110) with low nanomolar FAP-affinity and high selectivity toward related enzymes such as prolyl oligopeptidase (PREP) and the dipeptidyl-peptidases (DPPs): DPP4, DPP8/9 and DPP2 were developed. This inhibitor has been adopted recently by other groups to create radiopharmaceuticals by coupling bifunctional chelator-linker systems. Here, we report squaric acid (SA) containing bifunctional DATA^5m and DOTA chelators based on UAMC1110 as pharmacophor. The novel radiopharmaceuticals DOTA.SA.FAPi and DATA^5m.SA.FAPi with their non-radioactive derivatives were characterized for in vitro inhibitory efficiency to FAP and PREP, respectively and radiochemical investigated with gallium-68. Further, first proof-of-concept in vivo animal study followed by ex vivo biodistribution were determined with [⁶⁸Ga]Ga-DOTA.SA.FAPi.

Results

[⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DATA^5m.SA.FAPi showed high complexation > 97% radiochemical yields after already 10 min and high stability over a period of 2 h. Affinity to FAP of DOTA.SA.FAPi and DATA^5m.SA.FAPi and its ^natGa and ^natLu-labeled derivatives were excellent resulting in low nanomolar IC₅₀ values of 0.7–1.4 nM. Additionally, all five compounds showed low affinity for the related protease PREP (high IC₅₀ with 1.7–8.7 μM). First proof-of-principle in vivo PET-imaging animal studies of the [⁶⁸Ga]Ga-DOTA.SA.FAPi precursor in a HT-29 human colorectal cancer xenograft mouse model indicated promising results with high accumulation in tumor (SUV_mean of 0.75) and low background signal. Ex vivo biodistribution showed highest uptake in tumor (5.2%ID/g) at 60 min post injection with overall low uptake in healthy tissues.

Conclusion

In this work, novel PET radiotracers targeting fibroblast activation protein were synthesized and biochemically investigated. Critical substructures of the novel compounds are a squaramide linker unit derived from the basic motif of squaric acid, DOTA and DATA^5m bifunctional chelators and a FAP-targeting moiety. In conclusion, these new FAP-ligands appear promising, both for further research and development as well as for first human application.

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.

Are Synapse-Like Structures a Possible Way for Crosstalk of Cancer with Its Microenvironment?

The failure of therapies directed at targets within cancer cells highlight the necessity for a paradigm change in cancer therapy. The attention of researchers has shifted towards the disruption of cancer cell interactions with the tumor microenvironment. A typical example of such a disruption is the immune checkpoint cancer therapy that disrupts interactions between the immune and the cancer cells. The interaction of cancer antigens with T cells occurs in the immunological synapses. This is characterized by several special features, i.e., the proximity of the immune cells and their target cells, strong intercellular adhesion, and secretion of signaling cytokines into the intercellular cleft. Earlier, we hypothesized that the cancer-associated fibroblasts interacting with cancer cells through a synapse-like adhesion might play an important role in cancer tumors. Studies of the interactions between cancer cells and cancer-associated fibroblasts showed that their clusterization on the membrane surface determined their strength and specificity. The hundreds of interacting pairs are involved in the binding that may indicate the formation of synapse-like structures. These interactions may be responsible for successful metastasis of cancer cells, and their identification and disruption may open new therapeutic possibilities.

The prognostic significance of fibroblast activation protein-α in human lung adenocarcinoma

Background

Fibroblast activation protein (FAP) is a type II cell surface-bound integral serine protease, which is an important biomarker of cancer-associated fibroblasts. FAP-α performs several biological activities, including remolding extracellular matrix and acting as an immunosuppressor in the tumor microenvironment. However, the proliferation role of FAP-α in human lung adenocarcinoma has not been fully elucidated.

Methods

The expression of FAP-α in 94-paired human lung adenocarcinoma tissues was identified by immunohistochemistry test. The effect of FAP on cell proliferation was examined by CCK-8 assay. RNA-sequencing and bioinformatics analysis were utilized to investigate the underlying mechanism. Western blot analysis, quantitative polymerase chain reaction (qPCR), and nude mice experiments, were also conducted for further validation.

Results

The proliferation rates of human fibroblast strains FAP-HFF and FAP-BJ, and human lung adenocarcinoma cell line FAP-SPC-A-1 were higher than those of controls. The nude mice experiment also showed that FAP could promote the proliferation of SPC-A-1 cell line in vivo. qPCR and Western blot analysis indicated that CCNB1 was upregulated by the overexpression of FAP in the lung adenocarcinoma cell line. The expression of FAP-α was higher in both the cytoplasm and stroma of lung adenocarcinoma than in adjacent normal tissues. Survival analysis indicated that patients with higher expression of FAP-α in tumor stroma had a poor prognosis (P=0.019). The Cancer Genome Atlas Program (TCGA) data also showed that the expression of FAP within tumor tissues was higher (in both cytoplasm and stroma) compared with that in normal tissues (P<0.05).

Conclusions

Our study indicates that FAP-α could facilitate the proliferation of lung adenocarcinoma cells and can be a prognostic marker in human lung adenocarcinoma.

Cell surface-anchored serine proteases in cancer progression and metastasis

Over the last two decades, a novel subgroup of serine proteases, the cell surface-anchored serine proteases, has emerged as an important component of the human degradome, and several members have garnered significant attention for their roles in cancer progression and metastasis. A large body of literature describes that cell surface-anchored serine proteases are deregulated in cancer and that they contribute to both tumor formation and metastasis through diverse molecular mechanisms. The loss of precise regulation of cell surface-anchored serine protease expression and/or catalytic activity may be contributing to the etiology of several cancer types. There is therefore a strong impetus to understand the events that lead to deregulation at the gene and protein levels, how these precipitate in various stages of tumorigenesis, and whether targeting of selected proteases can lead to novel cancer intervention strategies. This review summarizes current knowledge about cell surface-anchored serine proteases and their role in cancer based on biochemical characterization, cell culture-based studies, expression studies, and in vivo experiments. Efforts to develop inhibitors to target cell surface-anchored serine proteases in cancer therapy will also be summarized.

Expression of fibroblast activation protein and the clinicopathological relevance in distal cholangiocarcinoma

Objectives: The current survival of patients with distal cholangiocarcinoma (dCCA) is poor. There is a need to develop new prognostic and predictive biomarkers to improve the survival of patients. Fibroblast activation protein (FAP) expression has been associated with survival in several solid malignancies. The goal of this study was to evaluate the expression pattern and prognostic significance of FAP in dCCA.Materials and methods: FAP expression was examined in 57 resected dCCA specimens and 28 paired lymph node metastasis specimens, as well as 10 benign bile ducts using immunohistochemistry. FAP expression was scored in the epithelial and stromal component of the dCCA specimens. The association between FAP expression and prognosis was evaluated using univariable and multivariable statistical modeling.Results: FAP expression was absent in the benign controls. FAP expression was evident in the epithelial 43 (75%) and stromal compartment 34 (60%) of dCCA. There was no association between epithelial or stromal FAP expression and clinicopathological factors. Epithelial FAP expression (HR 0.4 95% CI 0.20-0.78; p=.007) but not stromal FAP expression was significantly associated with better survival in univariable and multivariable analysis.Conclusions: FAP overexpression is evident in dCCA. There was a positive association between epithelial FAP expression and better survival which merits further evaluation.