Fibroblast Activation Protein Research Articles

Clinical applications of fibroblast activation protein inhibitor positron emission tomography (FAPI-PET)

AbstractThe discovery of fibroblast activation protein inhibitor positron emission tomography (FAPI-PET) has paved the way for a new class of PET tracers that target the tumor microenvironment (TME) rather than the tumor itself. Although 18F-fluorodeoxyglucose (FDG) is the most common PET tracer used in clinical imaging of cancer, multiple studies have now shown that the family of FAP ligands commonly outperform FDG in detecting cancers, especially those known to have lower uptake on FDG-PET. Moreover, FAPI-PET will have applications in benign fibrotic or inflammatory conditions. Thus, even while new FAPI-PET tracers are in development and applications are yet to enter clinical guidelines, a significant body of literature has emerged on FAPI-PET, suggesting it will have important clinical roles. This article summarizes the current state of clinical FAPI-PET imaging as well as potential uses as a theranostic agent.

NIMG-10. THE ROLE OF FIBROBLAST ACTIVATION PROTEIN IN GLIOBLASTOMA AND GLIOSARCOMA – A COMPARISON OF TISSUE, 68GA-FAPI-46 POSITRON EMISSION TOMOGRAPHY AND SURVIVAL DATA

Abstract BACKGROUND Despite their unique histological features,gliosarcomas belong to the group of glioblastomas and are treated according to the same standards. Fibroblast-activation-protein(FAP) is a component of a tumor-specific subpopulation of fibroblasts that play a critical role in tumor growth and invasion.However,the prognostic impact of FAP in glioblastoma and gliosarcoma is unclear. METHODS In a retrospective analysis,patients diagnosed with glioblastoma without sarcomatous differentiation and gliosarcoma were enrolled.Histological examination was performed using immunohistochemical FAP-staining and quantitative analysis with a standardized FAP-score.In a subset of patients,FAPI46-PET has been performed.The SUV-values are planned to be correlated with FAP-expression in the tumor tissue.Data obtained from immunohistochemical analysis and SUV-values are planned to be brought into perspective with clinically relevant prognostic factors,including survival estimates. RESULTS We enrolled 61 patients,including 13 diagnosed with gliosarcoma.Our analysis revealed that gliosarcomas exhibit significantly higher FAP-expression (median FAP-score: 3) compared to glioblastomas without sarcomatous differentiation (median FAP-score: 0.5, p<0.0001),where FAP was predominantly observed in the perivascular space.In gliosarcomas,FAP was also expressed by neoplastic cells.Moreover,a significant relationship was established between the immunohistochemical FAP-scores and the SUVmean and SUVpeak values on PET,suggesting that the PET tracer uptake accurately reflects the tumor’s FAP-expression.However,the differential expression of FAP suggests its potential as both a diagnostic marker and a therapeutic target.This hypothesis is further supported by the application of 68Ga-FAPI-46-PET in a 66-year-old female patient with recurrent gliosarcoma,alongside immunohistochemical analysis of tumor tissue.Both methods identified elevated FAP-expression and uptake,underscoring the feasibility of FAP as a target for nuclear medicine therapies in brain tumors. CONCLUSIONS Our study establishes a significant correlation between SUVmean/SUVpeak in 68Ga-FAPI-46 PET scans and FAP-expression in glioblastoma.Gliosarcomas express significantly more FAP than other glioblastomas. These insights suggest FAP’s potential as a theranostic target.We plan to investigate the effectiveness of FAP-specific tracers in treating recurrent gliosarcoma,aiming to open new therapeutic avenues.

Design, preclinical evaluation, and first-in-human PET study of [68Ga]Ga-PSFA-01: a PSMA/FAP heterobivalent tracer.

Prostate cancer (PCa), characterized by tumor heterogeneity, may exhibit low or absent prostate-specific membrane antigen (PSMA) expression in cancerous lesions, limiting the detection sensitivity of monospecific probes. Given that fibroblast activation protein (FAP) is frequently overexpressed in the tumor microenvironment (TME), we developed a PSMA/FAP dual-targeting tracer to address this limitation. The precursor (PSFA-01) was synthesized by coupling a quinolone-based FAP-targeting scaffold and EuK with HBED-CC via amide bonds. The dual-receptor-binding affinity and cell uptake of PSFA-01 and [natGa]Ga-PSFA-01 was evaluated in vitro. Micro-PET/CT imaging was performed on 22Rv1 and U87MG tumor-bearing mice. The feasibility of [68Ga]Ga-PSFA-01 PET/CT in a clinical setting was evaluated in a metastatic prostate cancer patient, and the results were compared with those of [68Ga]Ga-FAPI-04 and [68Ga]Ga-PSMA-11 PET/CT. PSFA-01 and [natGa]Ga-PSFA-01 showed high affinity for both FAP and PSMA proteins (Ki = 0.14-1.02 nM). On micro-PET/CT imaging, the 22Rv1 tumor uptake of [68Ga]Ga-PSFA-01 (SUVmax = 3.89 ± 0.47) was higher than that of [68Ga]Ga-PSMA-11 (SUVmax = 2.96 ± 0.48). The U87MG tumor uptake of [68Ga]Ga-PSFA-01 was significantly higher (SUVmax = 7.29 ± 1.13) than [68Ga]Ga-FAPI-04 (SUVmax = 0.28 ± 0.12), showing tumor to muscle ratio as 12.68 ± 1.93 at 1h p.i. On clinical trial, the primary tumor and metastatic lesions were distinctly identified by [68Ga]Ga-PSFA-01 (21 lesions), demonstrating superior performance compared to [68Ga]Ga-FAPI-04 (3 lesions) and [68Ga]Ga-PSMA-11 (13 lesions) in terms of lesion count and specificity. [68Ga]Ga-PSFA-01 exhibited satisfactory PSMA and FAP dual-receptor-targeting properties both in vitro and in vivo. This study highlights the clinical feasibility of [68Ga]Ga-PSFA-01 PET/CT for detecting metastatic tumors of prostate cancer more sensitively compared to monomeric [68Ga]Ga-PSMA-11 and [68Ga]Ga-FAPI-04, which also suggests that a PSMA/FAP dual-targeted radionuclide therapy could potentially overcome challenges related to tumor heterogeneity and insufficient PSMA expression in PCa. Clinical trial registry NCT06387381, Registered 1 May 2024.

Cervicovaginal lavages uncover growth factors as key biomarkers for early diagnosis and prognosis of endometrial cancer

Endometrial cancer (EC) rates are continuing to rise and it remains the most common gynecologic cancer in the US. Existing diagnostic methods are invasive and can cause pain and anxiety. Hence, there is a need for less invasive diagnostics for early EC detection. The study objective was to evaluate the utility of growth factors collected through minimally invasive cervicovaginal lavage (CVL) sampling as diagnostic and prognostic biomarkers for EC. CVL samples from 192 individuals undergoing hysterectomy for benign or malignant conditions were collected and used to quantify the concentrations of 19 growth and angiogenic factors using multiplex immunoassays. Patients were categorized based on disease groups: benign conditions (n = 108), endometrial hyperplasia (n = 18), and EC (n = 66). EC group was stratified into grade 1/2 endometrial endometrioid cancer (n = 53) and other EC subtypes (n = 13). Statistical associations were assessed using receiver operating characteristics, Spearman correlations and hierarchical clustering. Growth and angiogenic factors: angiopoietin-2, endoglin, fibroblast activation protein (FAP), melanoma inhibitory activity, and vascular endothelial growth factor-A (VEGF-A) were significantly (p < 0.0001) elevated in EC patients. A multivariate model combining 11 proteins with patient age and body mass index exhibited excellent discriminatory potential (area under curve = 0.918) for EC, with a specificity of 90.7% and a sensitivity of 87.8%. Moreover, angiopoietin-2, FAP and VEGF-A significantly (p < 0.05–0.001) associated with tumor grade, size, myometrial invasion, and mismatch repair status. Our results highlight the innovative use of growth and angiogenic factors collected through CVL sampling for the detecting endometrial cancer, showcasing not only their diagnostic potential but also their prognostic value.

Emerging Radiopharmaceuticals in Pet Imaging for Mesothelioma: A Review of [18F]FDG Alternatives.

Mesothelioma is a malignant tumor associated primarily with asbestos exposure, characterized by an aggressive nature and poor prognosis. Accurate diagnosis, staging, and monitoring of therapeutic response are crucial for effective patient management. Along with a computed tomography (CT) scan, fluorodeoxyglucose labeled with fluorine-18 ([18F]FDG) positron emission tomography (PET) is commonly used in mesothelioma evaluation. However, it has some limitations, including lower sensitivity after pleurodesis and poor accuracy for involved lymph node evaluation. Thus, there is the need to explore other agents. The aim of the present review is to analyze the current literature on the use of alternative radiopharmaceuticals for PET imaging in patients with mesothelioma. A comprehensive search of scientific databases (PubMed, Scopus, and Web of Science) for studies published in the last decade was performed by using the following keywords: "mesothelioma" AND "PET" AND "PET/CT" "radiopharmaceuticals", "[18F]FDG alternatives". Articles focused solely on [18F]FDG, non-English publications or preclinical studies, reviews, meeting abstracts, letters to the editors, and editorials were excluded. A qualitative assessment was made by using the Critical Appraisal Skills Programme (CASP) checklist for diagnostic test studies, when applicable. In total, 14 papers were selected; in seven articles more than five patients were enrolled, while the other seven were only clinical cases (enrolling up to two subjects). [18F]/gallium-68 ([68Ga])-labeled fibroblast activation protein inhibitor (FAPI) compounds, [18F]Fluorothymidine ([18F]FLT), methionine labeled with carbon-11 ([11C]MET), and fluoromisonidazole labeled with fluorine-18 ([18F]FMISO) PET/CT were the alternative agents used most often. In 12 articles, [18F]FDG PET/CT was used as a comparator imaging modality. Detection rate of [18F]FDG was similar to the other radiopharmaceuticals ([68Ga]/[18F]-labeled FAPI compounds, [18F]FLT, [18F]FMISO, [11C]MET, and [68Ga]-Pentaxifor), although radiolabeled FAPI seems to exhibit a higher diagnostic performance. [18F]FDG is still a valuable agent in patients with mesothelioma. However, radiolabeled FAPI appears to be promising and its theranostic properties should therefore be further assessed.

Exploring the FAP-Targeted Therapeutics for Adrenocortical Carcinoma: Choosing the Right Track.

Metastatic or recurrent adrenocortical carcinoma is a potentially lethal malignancy, presenting significant challenges in disease management owing to absence of effective systemic treatments. Significantly diminished survival rates necessitate rapid identification of specific molecules for the development of targeted therapeutics. Fibroblast activation protein (FAP)-expressing cancer-associated fibroblasts have been a major breakthrough causing a paradigm shift in targeted theranostics focusing on the tumor microenvironment. The effectiveness of various FAP inhibitors (FAPis) and FAP targeting peptide has been extensively documented in diverse clinical investigations. We have evaluated 3 molecules, that is, DOTA.SA.FAPi (SA.FAPi), FAPi46, and FAP2286, as potential theranostic probes for adrenocortical carcinoma.

Feasibility of Early 68Ga-FAP-2286 PET Imaging: A Case Study.

Fibroblast activation protein (FAP) has emerged as a promising molecular target for diagnostic and therapeutic strategies. Previous research, including our own study and other published reports, has highlighted the potential of FAP inhibitors labeled with 68Ga as effective diagnostic radiopharmaceuticals. In this study, we present a comparative analysis of early and late PET/CT scans, using 68Ga-FAP-2286, for the detection of tumor lesions in a patient with metastatic breast adenocarcinoma.

DNA-Encoded Noncanonical Substrate Library for Protease Profiling.

Profiling the substrate sequence preferences of proteases is important for understanding both biological functions as well as for designing protease inhibitors. Several methods are available for profiling the sequence specificity of proteases. However, there is currently no rapid and high-throughput method to profile specificity of proteases for noncanonical substrates. In this study, we described a strategy to use a DNA-encoded noncanonical substrate library to identify the protease substrates composed of both canonical and noncanonical amino acids. This approach uses a DNA-encoded peptide library and introduces a biotin molecule at the N-terminus to immobilize the library on a solid support. Upon protease hydrolysis, the released DNA tag of the substrate peptides can be sequenced to identify the substrate structures. Using this approach, we profiled trypsin and fibroblast activation protein α and discovered noncanonical substrates that were more efficiently cleaved than the commonly used substrates. The identified substrates of FAP were further used to design corresponding covalent inhibitors containing non-canonical sequences with high potency for the target protease. Overall, our approach can aid in the development of new protease substrates and inhibitors.

Integrated omics profiling reveals systemic dysregulation and potential biomarkers in the blood of patients with neuromyelitis optica spectrum disorders

BackgroundNeuromyelitis optica spectrum disorders (NMOSD) are autoimmune conditions that affect the central nervous system. The contribution of peripheral abnormalities to the disease's pathogenesis is not well understood.MethodsTo investigate this, we employed a multi-omics approach analyzing blood samples from 52 NMOSD patients and 46 healthy controls (HC). This included mass cytometry, cytokine arrays, and targeted metabolomics. We then analyzed the peripheral changes of NMOSD, and features related to NMOSD's disease severity. Furthermore, an integrative analysis was conducted to identify the distinguishing characteristics of NMOSD from HC. Additionally, we unveiled the variations in peripheral features among different clinical subgroups within NMOSD. An independent cohort of 40 individuals with NMOSD was utilized to assess the serum levels of fibroblast activation protein alpha (FAP).ResultsOur analysis revealed a distinct peripheral immune and metabolic signature in NMOSD patients. This signature is characterized by an increase in monocytes and a decrease in regulatory T cells, dendritic cells, natural killer cells, and various T cell subsets. Additionally, we found elevated levels of inflammatory cytokines and reduced levels of tissue-repair cytokines. Metabolic changes were also evident, with higher levels of bile acids, lactates, triglycerides, and lower levels of dehydroepiandrosterone sulfate, homoarginine, octadecadienoic acid (FA[18:2]), and sphingolipids. We identified distinctive biomarkers differentiating NMOSD from HC and found blood factors correlating with disease severity. Among these, fibroblast activation protein alpha (FAP) was a notable marker of disease progression.ConclusionsOur comprehensive blood profile analysis offers new insights into NMOSD pathophysiology, revealing significant peripheral immune and metabolic alterations. This work lays the groundwork for future biomarker identification and mechanistic studies in NMOSD, highlighting the potential of FAP as a marker of disease progression.

Targeting fibroblast activation protein with chimeric antigen receptor macrophages

Under the rapid advancement of chimeric antigen receptor T cell (CAR-T) technology, CAR-macrophages (CAR-Ms) are also being developed currently in the pre-clinical stage and have been shown to inhibit tumor growth in several mouse tumor models. Fibroblast activation protein (FAP) is a type II transmembrane serine protease, which is expressed in stromal fibroblasts of over 90 % of common human epithelial cancers and is upregulated in fibrotic diseases of the liver, lung and colon, etc. In this study, we firstly constructed FAP-CAR macrophages to target FAP+ cells through in vitro phagocytosis assays. In subsequent in vivo assays, we discovered that FAP-CAR-ΔZETA bone marrow-derived macrophages (BMDMs) rather than FAP-CAR BMDMs, exhibited a pronounced anti-tumor effect in mouse subcutaneous MC38 colon cancer model. In addition, FAP-CAR and FAP-CAR-ΔZETA BMDMs therapy could effectively improve CCl4-induced liver fibrosis in mice. Collectively CAR-Ms targeting FAP demonstrated great therapeutic potential in cancer and liver fibrosis therapy.

Assessing the Value of 68Ga-FAPI PET/CT in Gastric Mucinous Adenocarcinoma or Signet Ring Cell Carcinoma.

Purpose To investigate the clinical impact and prognostic value of gallium 68 (68Ga)-labeled fibroblast activation protein inhibitor (FAPI) PET/CT in gastric mucinous adenocarcinoma (MAC) and signet ring cell carcinoma (SRCC). Materials and Methods Eighty-six participants with newly diagnosed or recurrent gastric MAC or SRCC were prospectively enrolled from April 2021 to October 2021 and underwent both fluorine 18 (18F) fluorodeoxyglucose (FDG) PET/CT and 68Ga-FAPI PET/CT. The sensitivity, specificity, and accuracy of the two scans in primary and metastatic tumors were evaluated using the McNemar test. Changes of treatment strategies were recorded to compare the treatment management value of the two PET/CT scans. The maximum standardized uptake value (SUVmax) and peritoneal cancer index (PCI) were recorded for survival analysis. Progression-free survival (PFS) was defined as the time interval from the date of PET/CT scans to the date of disease progression. Results Eighty-six participants (median age, 62 years [IQR, 45-78 years]; 49 female) were evaluated. 68Ga-FAPI PET/CT showed higher diagnostic accuracy in detecting involved lymph nodes (87% [212 of 244] vs 71% [173 of 244], P < .001) and peritoneal metastases (96% [70 of 73] vs 55% [40 of 73], P < .001) than 18F-FDG PET/CT. Twenty-six participants (30% [26 of 86]) had treatment changes due to more accurate diagnosis with 68Ga-FAPI PET/CT. Additionally, the 68Ga-FAPI PCI was an independent predictor for PFS (hazard ratio, 6.9; 95% CI: 2.1, 23.1; P = .002). Conclusion 68Ga-FAPI PET/CT had higher accuracy in diagnosis of gastric MAC/SRCC compared with 18F-FDG PET/CT and demonstrated the potential to improve treatment strategies and predict prognosis. Keywords: PET/CT, Mucinous Adenocarcinoma, Signet Ring Cell Carcinoma, Oncology, Abdomen/GI, Molecular Imaging Supplemental material is available for this article. © RSNA, 2024.

Elucidating the linagliptin and human fibroblast activation protein binding mechanism through molecular dynamics and binding free energy analysis

Elucidating the linagliptin and human fibroblast activation protein binding mechanism through molecular dynamics and binding free energy analysis

O09 Correlation between Fibroblast Activation Protein inhibitor (99mTc-FAPi/SPECT) and histology in patients submitted to Retroperitoneal Lymph Node Dissection (RPLND) with residual post-chemotherapy disease in Non-Seminomatous Germ Cell Tumors (NSGCT)

O09 Correlation between Fibroblast Activation Protein inhibitor (99mTc-FAPi/SPECT) and histology in patients submitted to Retroperitoneal Lymph Node Dissection (RPLND) with residual post-chemotherapy disease in Non-Seminomatous Germ Cell Tumors (NSGCT)

FAP and PSMA Expression by Immunohistochemistry and PET Imaging in Castration-Resistant Prostate Cancer: A Translational Pilot Study.

Prostate-specific membrane antigen (PSMA) is a theranostic target for metastatic prostate cancer (PCa). However, castration-resistant PCa (CRPC) may lose PSMA expression after systemic therapy. Fibroblast activation protein (FAP), expressed by carcinoma-associated fibroblasts in various cancer types, including PCa, has the potential to be an alternative target. In this study, we evaluated FAP expression in CRPC to assess its potential, using PSMA as a comparison. Methods: FAP expression was assessed using immunohistochemistry in 116 CRPC tumors: 78 adenocarcinomas, 11 small cell carcinomas, and 27 anaplastic carcinomas. Correlation analysis between manual scoring and automated scoring was performed on 54 whole-slide sections of metastatic CRPC. Paired FAP and PSMA stains were assessed in tissue microarray cores of CRPC (n = 62), consisting of locally advanced CRPC (n = 9) and metastatic CRPC (n = 53). FAP and PSMA positivity was defined by an immunohistochemistry score of at least 10. To explore the correlation of PSMA and FAP inhibitor (FAPi) PET imaging and immunohistochemistry, a preliminary analysis of 4 patients included in a [68Ga]-FAPi-46 imaging trial (NCT04457232) was conducted. Results: Manual and automated scoring of FAP yielded results with strong correlations. Overall, FAP expression in CRPC was notably lower than PSMA expression (median immunoscores, 14 vs. 72; P < 0.001). Different histologic subtypes of CRPC demonstrated distinct levels of PSMA expression, whereas their FAP expression levels were comparable. Among the 19 PSMA-negative tumors, 11 (58%) exhibited FAP positivity. FAP expression levels in lymph node metastases were significantly lower than those in nonnodal metastases (P = 0.021). Liver metastases showed significant enrichment of tumors with strong FAP expression compared with nonliver lesions (P = 0.016). In the 4 clinical trial patients, the biopsied metastatic lesions showed lower uptake on FAPi PET than on PSMA PET (median SUVmax, 9.6 vs. 14.5), consistent with FAP expression that was lower than PSMA expression in the corresponding tumor biopsy samples (median immunoscores, 30 vs. 160). Conclusion: Because of the low FAP expression levels in CRPC, the utility of FAPi PET imaging may be limited. Although FAPi PET imaging may be further tested in PSMA-negative CRPC, such as small cell carcinoma, other molecular imaging modalities should be evaluated as alternative choices.

Feasibility, Tolerability, and Preliminary Clinical Response of Fractionated Radiopharmaceutical Therapy with 213Bi-FAPI-46: Pilot Experience in Patients with End-Stage, Progressive Metastatic Tumors.

Radiopharmaceutical therapies (RPTs) based on fibroblast activation protein (FAP) and FAP inhibitors (FAPIs) are a new option for progressive metastatic cancer in patients pretreated multiple times. To date, published in-human data refer to initial experiences with β-emitting 90Y- and 177Lu-based RPT. However, the short tumor retention time of FAPI ligands is considered a major limitation of FAPI RPT. Therefore, fractionated FAPI RPT with 213Bi, an α-emitter with a half-life of 46 min, appears to be a promising FAPI RPT regimen. Here, we report on our initial experiences with regard to the feasibility, tolerability, and response of fractionated 213Bi-FAPI-46 RPT. Methods: Six patients (4 women and 2 men) with progressive metastatic solid tumors (3 colon cancer, 1 anal cancer, 1 breast cancer, and 1 prostate cancer) aged 16-77 y were treated with a mean of 1,609 MBq of 213Bi-FAPI-46, fractionated into 53 single applications (range, 5-12 RPT applications per patient; mean, 8.8 applications) over a period of up to 107 h per patient. Of the 6 patients, 4 patients received adjuvant treatment with pembrolizumab. 18F-FDG (4 patients) and 68Ga-FAPI-46 (5 patients) PET/CT scans were performed before and after RPT. PET images were assessed visually and by calculating total lesion glycolysis and total lesion FAPI. Results: RPT with 213Bi-FAPI-46 was well tolerated without adverse side effects. In terms of visual response assessment, there was 1 partial response (16.7%), 1 patient with stable disease (16.7%), and 4 patients with progressive disease (66.7%). Concordantly, total lesion glycolysis and total lesion FAPI were decreased in the responding patient (not applicable and -24.3%, respectively), slightly decreased in the patient with stable disease (-10.6% and -5.9%, respectively), and increased in the 4 patients with progression (mean, +104.4% and +321.3%, respectively). Conclusion: Fractionated FAPI RPT with the short-half-life α-emitter 213Bi-FAPI-46 is a promising approach that matches the pharmacokinetics of FAPI-46 better than the 177Lu- or 90Y-labeled compounds. In this pilot project, fractionated RPT with 213Bi-FAPI-46 showed good clinical tolerability and even led to regressive or stable disease in the short term in 2 of 6 patients. Further studies with larger patient cohorts are required to evaluate the actual efficacy and long-term effects of this variant of FAPI RPT.

Predicting Malignancy in Adnexal Tumors With FAPI PET/CT and FDG PET/CT: A Case Report on a Borderline Ovarian Tumor.

A 54-year-old woman presented with a 55 × 64-mm tumor in the ovary with high [18F]FDG uptake on [18F]FDG PET/CT highly suggestive of ovarian cancer. Prior to surgery, the patient underwent [68Ga]-Ga- fibroblast activation protein inhibitor (FAPI)-46 PET/CT, which revealed low [68Ga]-Ga-FAPI-46 tumor uptake. Histopathology revealed a borderline ovarian tumor, which has low malignant potential and a 10-year survival rate greater than 93%. High [18F]FDG uptake is considered a fair predictor of malignancy in adnexal tumors. The present case demonstrates the potential superiority of [68Ga]-Ga-FAPI-46 PET/CT over [18F]-FDG PET/CT in differentiating malignant adnexal tumors from borderline ovarian tumors.

Rational modifications on N-(4-quinolinoyl)-Gly-2-cyanopyrrolidine to develop fibroblast activation protein-targeted radioligands with improved affinity and tumor uptake

Fibroblast activation protein (FAP) has been an attractive target for cancer imaging and therapy. Radiolabeled FAP-targeting ligands have shown promising results for clinical applications. However, further improvements are ongoing in pursuit of increasing tumor uptake, prolonging tumor residence, and maintenance good tumor–to–background contrast for extensive theranostic application. Achieving a higher affinity of the precursor is one of the ways in research. In this study, we designed a series of FAP inhibitors based on N-(4-quinolinoyl)-Gly-2-cyanopyrrolidine and found compound QI-18 with an IC50 value of 0.50 nM, which is a 6.5-fold increase in potency over that of UAMC-1110 (IC50 of 3.25 nM). QI-18 was then functionalized with a DOTA chelator to obtain the ligand CY03 for further radiolabeling with 68Ga to obtain the radiotracer [68Ga]Ga-CY03. In BALB/c nude mice bearing U87MG tumor models, [68Ga]Ga-CY03 exhibited a high and specific uptake (10.30 ± 0.63 % ID/g at 1 h post-injection and 9.28 ± 1.60 % ID/g at 2 h post-injection), which represented 3.2- and 4.1-fold increases over those for [68Ga]Ga-FAPI-04 (3.24 ± 0.53 % ID/g and 2.25 ± 0.33 % ID/g, respectively). [68Ga]Ga-CY03 also showed higher tumor–to–blood and tumor–to–kidney ratios (7.62 ± 0.44 and 2.59 ± 0.27, respectively) than [68Ga]Ga-FAPI-04 (2.38 ± 0.47 and 0.98 ± 0.19, respectively). The results indicate that [68Ga]Ga-CY03 is a promising imaging agent to target FAP.

Recent Updates of PET in Lymphoma: FDG and Beyond

Lymphoma is one of the most common cancers worldwide, categorized into Hodgkin lymphoma and non-Hodgkin lymphoma. 18F-fluorodeoxyglucose positron emission tomography (FDG PET) has become an essential imaging tool for evaluating patients with lymphoma in terms of initial diagnosis, staging, prognosis, and treatment response assessment. Recent advancements in imaging technology and methodologies, along with the development of artificial intelligence, have revolutionized the evaluation of complex imaging data, enhancing the diagnostic and predictive power of PET in lymphoma. However, FDG is not cancer-specific, but it primarily reflects glucose metabolism, which has prompted the investigation of alternative PET tracers to address this limitation. Novel PET radiotracers, such as fibroblast activation protein inhibitors targeting the tumor microenvironment, have recently shown promising results in evaluating various malignancies compared to FDG PET. Furthermore, with the rapid advancements in immunotherapy and the favorable imaging properties of 89Zr, immunoPET has emerged as a promising modality, offering insights into the functional and molecular status of the immune system. ImmunoPET can also facilitate the development of new antibody therapeutics and radioimmunotherapy by providing pharmacokinetic and pharmacodynamic data. This review provides comprehensive insights into the current clinical applications of FDG PET in lymphoma, while also exploring novel PET imaging radiotracers beyond FDG, discussing their mechanisms of action and potential impact on patient management.

Comment on: Fibroblast activation protein inhibitor-positron emission tomography in aortitis: fibroblast pathology in active inflammation and remission: Reply.

Comment on: Fibroblast activation protein inhibitor-positron emission tomography in aortitis: fibroblast pathology in active inflammation and remission: Reply.

CTR-FAPI PET enables precision management of medullary thyroid carcinoma.

Medullary thyroid carcinoma (MTC) can only be cured through the excision of all metastatic lesions, but 29-60% patients failed to localize the disease in the current clinical practice. Previously, we developed a fibroblast activation protein inhibitor (FAPI)-based covalent targeted radioligand (CTR) for improved detection sensitivity and accuracy. In this first-in-class clinical trial, we head-to-head compared [68Ga]Ga-CTR-FAPI PET-CT and [18F]FDG PET-CT in 50 MTC patients. The primary endpoint was the patient-based detection rate, with [68Ga]Ga-CTR-FAPI exhibiting higher detection than [18F]FDG (98% vs. 66%, p=0.0002). This improved detection was attributed to increased tumor uptake (SUVmax 11.71±9.16 vs. 2.55±1.73, p<0.0001). Diagnostic accuracy, validated on lesions with gold-standard pathology, was greater for [68Ga]Ga-CTR-FAPI compared to [18F]FDG (96.7% vs. 43.3%, p<0.0001). Notably, 32% patients altered management following [68Ga]Ga-CTR-FAPI PET-CT, and 66.7% patients changed their surgical plan. Overall, [68Ga]Ga-CTR-FAPI PET-CT provided superior detection and diagnostic accuracy compared to [18F]FDG PET-CT, enabling precision management of MTC patients.