The comparative utility of FAPI-based PET radiotracers over [18F]FDG in the assessment of malignancies.

Fibroblast activation protein (FAP) is a type II transmembrane serine protease overexpressed in cancer-associated fibroblasts (CAFs) and has been associated with poor prognosis. PET/CT imaging with radiolabeled FAP inhibitors (FAPI) is currently being studied for various malignancies. This review identifies the uses and limitations of FAPI PET/CT in malignancies and compares the advantages and disadvantages of FAPI and 18F-fluorodeoxyglucose ([18F]FDG). Due to high uptake, rapid clearance from the circulation, and limited uptake in normal tissue, FAPI tumor-to-background contrast ratios are equivalent to or better than [18F]FDG in most applications. In several settings, FAPI has shown greater uptake specificity than [18F]FDG and improved sensitivity in detecting lymph node, bone, and visceral tissue metastases. Therefore, FAPI PET/CT may be complementary in distinguishing pathological lesions with conventional imaging, determining the primary site of malignancy, improving tumor staging, and detecting disease recurrence, especially in patients with inconclusive [18F]FDG PET/CT findings. Nevertheless, FAPI has limitations, including certain settings with non-specific uptake, modified uptake with age and menopause status, challenges with clinical access, and limited clinical evidence.

Exploration the feasibility and additional value of [18F]FDG/[68Ga]Ga-FAPI-04 dual-low-activity-tracer one-stop total-body PET imaging at 34 min post-injection of [68Ga]Ga-FAPI-04.

OBJECTIVES: To validate the feasibility of one-stop 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) and [68Ga]Ga-fibroblast activation protein inhibitor-04 ([68Ga]Ga-FAPI-04) dual-low-activity-tracer positron emission tomography/computed tomography (PET/CT) at 34 min post-injection of [68Ga]Ga-FAPI-04 and explore its additional value. METHODS: Thirty pairs of patients with suspected malignancies who underwent dual-tracer imaging were enrolled in this retrospective study. The images were reconstructed at 34-39 and 50-60 min after additional injection of [68Ga]Ga-FAPI-04 (in one-stop FDG-FAPI PET/CT, named PETFDG, PETD34-39, and PETD50-60; in the 2-day protocol, named PETFDG', PETF34-39, and PETF50-60, respectively). Tumour-to-normal ratios (TNR) of lesions in PETFDG, PETD34-39, and PETD50-60 and TNR of lesions in PETF34-39 and PETF50-60 were evaluated separately. To evaluate the potential added value of one-stop FDG-FAPI PET/CT over the 2-day protocol, TNRs of PETFDG, PETD34-39, and PETD50-60 were compared with PETF34-39. The lesion detectability of the two imaging protocols was evaluated by chi-square test. RESULTS: Comparing FAPI-weighted PET (PETD34-39 and PETD50-60) and single-tracer imaging (PETFDG) in one-stop FDG-FAPI PET/CT, TNRs of FAPI-weighted PET were higher than those of PETFDG. PETD34-39 and PETD50-60 showed similar performance in lesion detectability and TNRs (all P > 0.05). In the 2-day protocol, there are no statistically significant differences in TNRs of all lesions at PETF34-39 and PETF50-60. Comparing one-stop FDG-FAPI PET/CT with the 2-day protocol, TNRs of PETF34-39 were significantly higher than those of PETFDG but lower than those of PETD34-39 and PETD50-60. Lesion detectability in the one-stop FDG-FAPI PET/CT was higher than that in the 2-day protocol. The average radiation dose in one-stop FDG-FAPI PET/CT was significantly lower than that in the 2-day protocol (P<0.001). CONCLUSION: One-stop FDG-FAPI PET/CT at 34 min could provide sufficient information to meet clinical diagnosis and showed better lesion detectability than that in the 2-day protocol.

[68Ga]Ga-DOTAGA-Glu(FAPi)2 Shows Enhanced Tumor Uptake and Theranostic Potential in Preclinical PET Imaging.

The use of fibroblast activation protein inhibitors (FAPis) for positron emission tomography (PET) imaging in cancer has garnered significant interest in recent years, yielding promising results in preclinical and clinical settings. FAP is predominantly expressed in pathological conditions such as fibrosis and cancer, making it a compelling target. An optimized approach involves using FAPi homodimers as PET tracers, which enhance tumor uptake and retention, making them more effective candidates for therapy. Here, a UAMC-1110 inhibitor-based homodimer, DOTAGA-Glu(FAPi)2, was synthesized and radiolabeled with gallium-68, and its efficacy was evaluated in vivo for PET imaging in an endogenously FAP-expressing xenografted mouse model, U87MG. Notably, 45 min post-injection, the mean uptake of [68Ga]Ga-DOTAGA-Glu(FAPi)2 was 4.7 ± 0.5% ID/g in the tumor with low off-target accumulation. The ex vivo analysis of the FAP expression in the tumors confirmed the in vivo results. These findings highlight and confirm the tracer's potential for diagnostic imaging of cancer and as a theranostic companion.

Prospective Study Comparing [99mTc]Tc-DP-FAPI Quantitative SPECT/CT with [68Ga]Ga-FAPI-04 PET/CT in Patients with Gastrointestinal Tumors.

Over the past decade, [68Ga]Ga-FAPI-04 positron emission tomography (PET)/CT imaging has been widely used for the treatment of various tumors. However, the application of 99mTC-labeled fibroblast activation protein inhibitors in tumors has been less studied. Our team previously demonstrated the safe biological distribution of [99mTc]Tc-DP-FAPI in the human body. Based on this, this study reports the accuracy of [99mTc]Tc-DP-FAPI in the imaging diagnosis of gastrointestinal tumors and compares it with that of [68Ga]Ga-FAPI-04 to evaluate the differences. A total of 24 patients with clinically diagnosed gastrointestinal tumors were prospectively included. All patients received [99mTc]Tc-DP-FAPI quantitative SPECT/CT imaging on the first day and [68Ga]Ga-FAPI-04 PET/CT imaging on the second day. And the effectiveness of the two imaging probes in detecting suspicious lesions was analyzed and compared. The primary tumors of all 24 patients were well detected by two imaging probes, and the sensitivity of [99mTc]Tc-DP-FAPI and [68Ga]Ga-FAPI-04 to the primary lesions was 100%. [99mTc]Tc-DP-FAPI examined 21 lymph nodes with a sensitivity and specificity of 32.8% and 10.9%, and [68Ga]Ga-FAPI-04 detected 57 lymph nodes with a sensitivity and specificity of 89.1% and 67.2%, respectively. Three distant metastases were detected by [99mTc]Tc-DP-FAPI and nine metastases by [68Ga]Ga-FAPI-04. The study showed that [99mTc]Tc-DP-FAPI is highly sensitive to detecting primary lesions of gastrointestinal tumors. Compared with [68Ga]Ga-FAPI-04, [99mTc]Tc-DP-FAPI has the same sensitivity in detecting primary tumors but has certain limitations in detecting metastases. [99mTc]Tc-DP-FAPI is of great value for preliminary screening of tumor lesions and early diagnosis of disease in patients who are suspected of having gastrointestinal tumors.

Enzymatic activity of fibroblast activation protein-α is essential for TGF-ß1-induced fibroblastic differentiation of human periodontal ligament cells.

Human periodontal ligament cells (hPDLCs) contain multipotent postnatal stem cells that can differentiate into PDL fibroblasts, osteoblasts, and cementoblasts. Interaction between the extracellular environment and stem cells is an important factor for differentiation into other progenitor cells. To identify cell surface molecules that induce PDL fibroblastic differentiation, we developed a series of monoclonal antibodies against membrane/ECM molecules. One of these antibodies, an anti-PDL25 antibody, recognizes approximately a 100 kDa protein, and this antigenic molecule accumulates in the periodontal ligament region of tooth roots. By mass spectrometric analysis, we found that the antigenic molecule recognized by the anti-PDL25 antibody is fibroblast activation protein α (FAPα). The expression level of FAPα/PDL25 increased in TGF-ß1-induced PDL fibroblasts, and this protein was localized in the cell boundaries and elongated processes of the fibroblastic cells. Ectopic expression of FAPα induced fibroblastic differentiation. In contrast, expression of representative markers for PDL differentiation was decreased by knock down and antibody blocking of FAPα/PDL25. Inhibition of dipeptidyl peptidase activity by a potent FAPα inhibitor dramatically inhibited PDL fibroblastic marker expression but did not affect in cell proliferation and migration.

Fibroblast Activation Protein Is Expressed by Altered Osteoprogenitors and Associated to Disease Burden in Fibrous Dysplasia.

Fibrous dysplasia (FD) is a mosaic skeletal disorder involving the development of benign, expansile fibro-osseous lesions during childhood that cause deformity, fractures, pain, and disability. There are no well-established treatments for FD. Fibroblast activation protein (FAPα) is a serine protease expressed in pathological fibrotic tissues that has promising clinical applications as a biomarker and local pro-drug activator in several pathological conditions. In this study, we explored the expression of FAP in FD tissue and cells through published genetic expression datasets and measured circulating FAPα in plasma samples from patients with FD and healthy donors. We found that FAP genetic expression was increased in FD tissue and cells, and present at higher concentrations in plasma from patients with FD compared to healthy donors. Moreover, FAPα levels were correlated with skeletal disease burden in patients with FD. These findings support further investigation of FAPα as a potential imaging and/or biomarker of FD, as well as a pro-drug activator specific to FD tissue.

Fibroblast activation protein inhibitor positron emission tomography imaging in muscles of patients with idiopathic inflammatory myopathy.

OBJECTIVES: Mesenchymal stromal cells in muscles participate in regeneration following muscle injury. This study explored the potential of [18F]fibroblast activation protein inhibitor (FAPI)-42 positron emission tomography (PET) targeting mesenchymal stromal cells to evaluate disease activity of idiopathic inflammatory myopathy (IIM). METHODS: Patients with IIM (n = 26) were prospectively included and underwent [18F]FAPI-42 PET/CT and whole-body MRI between January 2023 and July 2023. Patients with malignancies were retrospectively included in the control group and only underwent [18F]FAPI-42 PET/CT (n = 28). [18F]FAPI-42 PET/CT images were evaluated using for avid-FAPI uptake and the target-to-background ratio (TBR). Whole-body MRI was evaluated for oedema, fatty infiltration, and atrophy in 42 muscles in the IIM group. The global FAPI- and MRI-derived parameters were calculated for each. PATIENT: . Clinical assessment of disease activity and muscle strength were collected. RESULTS: Patients with IIM had significantly higher global FAPI-avid muscle ratios (0.68 [interquartile range (IQR): 0.45, 0.79] vs 0.06 [IQR: 0, 0.11], p< 0.001) and global muscle TBR (2.26 [IQR: 1.71, 2.75] vs 1.23 [IQR: 1.02, 1.52], p< 0.001) compared with controls. In the IIM group, the median TBR was higher in muscles with oedema than in those without (2.44 [IQR: 1.46, 3.27] vs 1.31 [IQR: 0.95, 1.99], p< 0.001). Global FAPI-avid muscle ratios significantly correlated with global oedema score (r = 0.833), muscle strength (r=-0.649), serum creatine kinase (r = 0.456), and disease activity index (r = 0.495-0.621). CONCLUSION: Increased [18F]FAPI-42 uptake was associated with muscle oedema in IIM. FAPI-derived parameters correlated with IIM disease activity. [18F]FAPI-42 is a promising PET tracer for evaluating IIM disease activity.

Efficient α and ß- radionuclide therapy targeting fibroblast activation protein-α in an aggressive preclinical mouse tumour model.

PURPOSE: Targeted radionuclide therapy (TRT) is a cancer treatment with relative therapeutic efficacy across various cancer types. We studied the therapeutic potential of TRT using fibroblast activation protein-α (FAP) targeting sdAbs (4AH29) labelled with 225Ac or 131I in immunocompetent mice in a human FAP (hFAP) expressing lung cancer mouse model. We further explored the combination of TRT with programmed cell death ligand 1 (PD-L1) immune checkpoint blockade (ICB). METHODS: We studied the biodistribution and tumour uptake of [131I]I-GMIB-4AH29 and [225Ac]Ac-DOTA-4AH29 by ex vivo γ-counting. Therapeutic efficacy of [131I]I-GMIB-4AH29 and [225Ac]Ac-DOTA-4AH29 was evaluated in an immunocompetent mouse model. Flow cytometry analysis of tumours from [225Ac]Ac-DOTA-4AH29 treated mice was performed. Treatment with [225Ac]Ac-DOTA-4AH29 was repeated in combination with PD-L1 ICB. RESULTS: The biodistribution showed high tumour uptake of [131I]I-GMIB-4AH29 with 3.5 ± 0.5% IA/g 1 h post-injection (p.i.) decreasing to 0.9 ± 0.1% IA/g after 24 h. Tumour uptake of [225Ac]Ac-DOTA-4AH29 was also relevant with 2.1 ± 0.5% IA/g 1 h p.i. with a less steep decrease to 1.7 ± 0.2% IA/g after 24 h. Survival was significantly improved after treatment with low and high doses [131I]I-GMIB-4AH29 or [225Ac]Ac-DOTA-4AH29 compared to vehicle solution. Moreover, we observed significantly higher PD-L1 expression in tumours of mice treated with [225Ac]Ac-DOTA-4AH29 compared to vehicle solution. Therefore, we combined high dose [225Ac]Ac-DOTA-4AH29 with PD-L1 ICB showing therapeutic synergy. CONCLUSION: [225Ac]Ac-DOTA-4AH29 and [131I]I-GMIB-4AH29 exhibit high and persistent tumour targeting, translating into prolonged survival in mice bearing aggressive tumours. Moreover, we demonstrate that the combination of PD-L1 ICB with [225Ac]Ac-DOTA-4AH29 TRT enhances its therapeutic efficacy.

Preclinical Evaluation of 177Lu-OncoFAP-23, a Multivalent FAP-Targeted Radiopharmaceutical Therapeutic for Solid Tumors.

Fibroblast activation protein (FAP) is abundantly expressed in the stroma of most human solid tumors. Clinical-stage radiolabeled FAP ligands are increasingly used as tools for the detection of various cancer lesions. To unleash the full therapeutic potential of FAP-targeting agents, ligands need to remain at the tumor site for several days after administration. We recently described the discovery of OncoFAP, a high-affinity small organic ligand of FAP with a rapid accumulation in tumors and low uptake in healthy tissues in cancer patients. Trimerization of OncoFAP provided a derivative (named TriOncoFAP, or OncoFAP-23) with improved FAP affinity. In this work, we evaluated the tissue biodistribution profile and the therapeutic performance of OncoFAP-23 in tumor-bearing mice. Methods: OncoFAP-23 was radiolabeled with the theranostic radionuclide 177Lu. Preclinical experiments were conducted on mice bearing SK-RC-52.hFAP (BALB/c nude mice) or CT-26.hFAP (BALB/c mice) tumors. 177Lu-OncoFAP and 177Lu-FAP-2286 were included in the biodistribution study as controls. Toxicologic evaluation was performed on Wistar rats and CD1 mice by injecting high doses of OncoFAP-23 or its cold-labeled counterpart, respectively. Results: 177Lu-OncoFAP-23 emerged for its best-in-class biodistribution profile, high and prolonged tumor uptake (i.e., ∼16 percentage injected dose/g at 96 h), and low accumulation in healthy organs, which correlates well with its potent single-agent anticancer activity at low levels of administered radioactivity. Combination treatment with the tumor-targeted interleukin 2 (L19-IL2, a clinical-stage immunocytokine) further expands the therapeutic window of 177Lu-OncoFAP-23 by potentiating its in vivo antitumor activity. Proteomics studies revealed a potent tumor-directed immune response on treatment with the combination. OncoFAP-23 and natLu-OncoFAP-23 exhibited a favorable toxicologic profile, without showing any side effects or signs of toxicity. Conclusion: OncoFAP-23 presents enhanced tumor uptake and tumor retention and low accumulation in healthy organs, findings that correspond to a strongly improved in vivo antitumor efficacy. The data presented in this work support the clinical development of 177Lu-OncoFAP-23 for the treatment of FAP-positive solid tumors.

Cancer-Associated Fibroblast Proteins as Potential Targets against Colorectal Cancers.

In colorectal cancer (CRC), attempts to identify cancer cell-specific markers to guide antibody-mediated therapeutics have failed to uncover markers that are both exclusive to cancer tissues and abundant across CRCs. Alternatively, cancer-associated fibroblasts (CAFs), which are abundant in the tumor microenvironment and upregulate unique surface markers, are not found in healthy tissues. Here, we evaluated the expression patterns of CAF-associated proteins α-smooth muscle actin (αSMA), fibroblast activation protein (FAP), podoplanin (PDPN), matrix metalloproteinase-2 (MMP2), transgelin (TAGLN), and THY1. While αSMA and THY1 were abundant in cancer tissues, high abundance in normal tissues limited their targeting potential. FAP was present in 94.5% of primary and metastatic CRC tissues and absent in 93.7% of adjacent normal colon and liver tissues assessed. These results indicate that FAP is a promising target for antibody conjugates with potential for broad application in CRC. Co-expression analyses showed that CRCs simultaneously expressing high levels of PDPN, MMP2, and THY1 were enriched for immune-related signatures, indicating potential for antibody-mediated immune engagers. Overall, this work highlights the potential of CAF proteins to act as therapeutic targets for novel anticancer agents and become important therapeutic biomarkers.

Dynamically visualizing profibrotic maladaptive repair after acute kidney injury by fibroblast activation protein imaging.

A major challenge in prevention and early treatment of organ fibrosis is the lack of valuable tools to assess the evolving profibrotic maladaptive repair after injury in vivo in a non-invasive way. Here, using acute kidney injury (AKI) as an example, we tested the utility of fibroblast activation protein (FAP) imaging for dynamic assessment of maladaptive repair after injury. The temporospatial pattern of kidney FAP expression after injury was first characterized. Single-cell RNA sequencing and immunostaining analysis of patient biopsies were combined to show that FAP was specifically upregulated in kidney fibroblasts after AKI and was associated with fibroblast activation and chronic kidney disease (CKD) progression. This was corroborated in AKI mouse models, where a sustained and exaggerated kidney FAP upregulation was coupled to persistent fibroblast activation and a fibrotic outcome, linking kidney FAP level to post-insult maladaptive repair. Furthermore, using positron emission tomography (PET)/CT scanning with FAP-inhibitor tracers ([18F]FAPI-42, [18F]FAPT) targeting FAP, we demonstrated the feasibility of non-invasively tracking of maladaptive repair evolution toward kidney fibrosis. Importantly, a sustained increase in kidney [18F]FAPT (less hepatobiliary metabolized than [18F]FAPI-42) uptake reflected persistent kidney upregulation of FAP and characterized maladaptive repair after AKI. Kidney [18F]FAPT uptake at hour 2-day 7 correlated with kidney fibrosis 14 days after AKI. Similar changes in [18F]FAPI-42 PET/CT imaging were observed in patients with AKI and CKD progression. Thus, persistent kidney FAP upregulation after AKI was associated with maladaptive repair and a fibrotic outcome. Hence, FAP-specific PET/CT imaging enables dynamic visualization of maladaptive repair after AKI and prediction of kidney fibrosis within a clinically actionable window.

Potential of 68Ga-FAPI-04 PET/MR to predict worsening renal function after acute ST-elevation myocardial infarction.

PURPOSE: The fibroblast activation protein inhibitor-04 (FAPI-04) specifically binds to the FAP of activated myocardial fibroblasts, which makes 68Ga-labelled FAPI-04 (68Ga-FAPI-04) positron emission tomography (PET)/magnetic resonance (MR) a new potential imaging technique for the evaluation of myocardial fibrosis. This study aimed to evaluate the potential value of 68Ga-FAPI-04 PET/MR in assessing and predicting changes in renal function in patients with acute ST-elevation myocardial infarction (STEMI). METHODS: Thirty-three patients with STEMI were included in this study. 68Ga-FAPI-04 PET/MR and cardiac magnetic resonance were performed before discharge in all patients. Worsening renal function(WRF) was defined as ≥20% decrease in estimated glomerular filtration rate(eGFR) from baseline to 12 months. RESULTS: The WRF group demonstrated higher 68Ga-FAPI-04 uptake volume (UV) at baseline than the non-WRF group(P = 0.009). 68Ga-FAPI-04 UV at baseline was correlated with follow-up eGFR (r = -0.493, P = 0.004). 68Ga-FAPI-04 UV at baseline was a significant predictor of WRF (OR = 1.014, P = 0.029) at 12 months after STEMI. CONCLUSIONS: As an effective tool to non-invasively quantify myocardial fibroblast activation, 68Ga-FAPI-04 PET/MR has potential value for assessing and predicting worsening renal function in patients with STEMI.

Enhanced Pancreatitis Detection: FAPI PET/CT Emerging Utility in Patient with Pancreatic Mass.

Fibroblast Activation Protein Inhibitor (FAPI) positron emission tomography (PET) imaging has emerged as a useful method for identifying pancreatic disorders, notably pancreatitis. Unlike Fluorine-18 fluorodeoxyglucose (FDG), FAPI uptake is directly proportional to the degree of fibrosis, making it very useful in separating pancreatic tumors from inflammation. Recent investigations have shown that FAPI positron emission tomography/computer tomography (PET/CT) can identify pancreatic inflammation with great sensitivity, providing vital diagnostic information. In this case study, a 52-year-old male with a history of Ewing sarcoma presented with epigastric pain. Pancreatitis was confirmed on a computer tomography (CT) scan showing mild fat stranding in the pancreatic body and tail, in addition to a significant increase in pancreatic head mass, necessitating further evaluation with FDG PET/CT and FAPI PET/CT, as the patient was known to have metastatic sarcoma. While FDG PET/CT revealed an avid infiltrative lesion in the duodenal/pancreatic head area, FAPI PET/CT showed diffuse uptake in the pancreatic body and tail, indicating fibroblast-mediated inflammation consistent with pancreatitis. This case demonstrates the usefulness of FAPI imaging in discriminating between pancreatic metastasis and pancreatitis, with FAPI PET/CT providing crucial diagnostic information when FDG uptake is ambiguous.

Efficacy and safety evaluation of cross-reactive Fibroblast activation protein scFv-based CAR-T cells.

Introduction: Fibroblast activation protein (FAP) overexpression on cancer-associated fibroblasts (CAFs) is associated with poor prognosis and worse clinical outcomes. Selective ablation of pro-tumorgenic FAP+ stromal cells with CAR-T cells may be a new therapeutic strategy. However, the clinical use of FAP-CAR T cells is suggested to proceed with caution for occasional poor efficacy and induction of on-target off-tumor toxicity (OTOT), including lethal osteotoxicity and cachexia. Hence, more investigations and preclinical trials are required to optimize the FAP-CAR T cells and to approve their safety and efficacy. Methods: In this study, we designed second-generation CAR T cells targeting FAP with 4-1BB as a co-stimulatory molecule, and tested their cytotoxicity against FAP-positive cells (hFAP-HT1080 cells and a variety of primary CAFs) in vitro and in Cell line-derived xenograft (CDX) and a patient-derived xenograft (PDX) model. Results: Results showed that our FAP-CAR T cells were powerfully potent in killing human and murine FAP-positive tumor cells and CAFs in multiple types of tumors in BALB/c and C57BL/6 mice and in patient-derived xenografts (PDX) model. And they were proved to be biologically safe and exhibit low-level OTOT. Discussion: Taken together, the human/murine cross-reactive FAP-CAR T cells were powerfully potent in killing human and murine FAP positive tumor cells and CAFs. They were biologically safe and exhibit low-level OTOT, warranting further clinical investigation into our FAP-CAR T cells.

68Ga-labeled fibroblast activation protein inhibitor (FAPI) PET/CT for locally advanced or recurrent pancreatic cancer staging and restaging after chemoradiotherapy.

Purpose: 68Ga-labeled fibroblast activation protein inhibitor (FAPI) is a novel PET tracer with great potential for staging pancreatic cancer. Data on locally advanced or recurrent disease is sparse, especially on tracer uptake before and after high dose chemoradiotherapy (CRT). The aim of this study was to evaluate [68Ga]Ga-FAPI-46 PET/CT staging in this setting. Methods: Twenty-seven patients with locally recurrent or locally advanced pancreatic adenocarcinoma (LRPAC n = 15, LAPAC n = 12) in stable disease or partial remission after chemotherapy underwent FAPI PET/CT and received consolidation CRT in stage M0 with follow-up FAPI PET/CT every three months until systemic progression. Quantitative PET parameters SUVmax, SUVmean, FAPI-derived tumor volume and total lesion FAPI-uptake were measured in baseline and follow-up PET/CT scans. Contrast-enhanced CT (ceCT) and PET/CT data were evaluated blinded and staged according to TNM classification. Results: FAPI PET/CT modified staging compared to ceCT alone in 23 of 27 patients in baseline, resulting in major treatment alterations in 52% of all patients (30%: target volume adjustment due to N downstaging, 15%: switch to palliative systemic chemotherapy only due to diffuse metastases, 7%: abortion of radiotherapy due to other reasons). Regarding follow-up scans, major treatment alterations after performing FAPI PET/CT were noted in eleven of 24 follow-up scans (46%) with switch to systemic chemotherapy or best supportive care due to M upstaging and ablative radiotherapy of distant lymph node and oligometastasis. Unexpectedly, in more than 90 % of the follow-up scans, radiotherapy did not induce local fibrosis related FAPI uptake. During the first follow-up, all quantitative PET metrics decreased, and irradiated lesions showed significantly lower FAPI uptake in locally controlled disease (SUVmax p = 0.047, SUVmean p = 0.0092) compared to local failure. Conclusion: Compared to ceCT, FAPI PET/CT led to major therapeutic alterations in patients with LRPAC and LAPAC prior to and after radiotherapy, which might help identify patients benefiting from adjustments in every treatment stage. FAPI PET/CT should be considered a useful diagnostic tool in LRPAC or LAPAC before and after CRT.

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

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

Predictive performance of [18F]F-fibroblast activation protein inhibitor (FAPI)-42 positron emission tomography/computed tomography (PET/CT) in evaluating response of recurrent or metastatic gastrointestinal stromal tumors: complementary or alternative to [18F]fluorodeoxyglucose (FDG) PET/CT?

Background: Accurately and promptly predicting the response of gastrointestinal stromal tumors (GISTs) to targeted therapy is essential for optimizing treatment strategies. However, some fractions of recurrent or metastatic GISTs present as non-FDG-avid lesions, limiting the value of [18F]fluorodeoxyglucose positron emission tomography/computed tomography ([18F]FDG PET/CT) in treatment evaluation. This study evaluated the efficacy of [18F]F-fibroblast activation protein inhibitor (FAPI)-42 [18F]FAPI-42) PET/CT for assessing the treatment response in recurrent or metastatic GISTs, in comparison to [18F]FDG PET/CT and explores a model integrating PET/CT imaging and clinical parameters to optimize the clinical use of these diagnostic tools. Methods: Our retrospective analysis included 27 patients with recurrent or metastatic GISTs who underwent [18F]FAPI-42 PET/CT and [18F]FDG PET/CT at baseline before switching targeted therapy. Treatment response status was divided into a progression group (PG) and a non-progression group (NPG) based on the Response Criteria in Solid Tumors (RECIST) 1.1, according to the contrast-enhanced computed tomography (CT) scan at six months. [18F]FAPI-42 and [18F]FDG PET/CT parameters including the mean standardized uptake value (SUVmean), the standard uptake value corrected for lean body mass (SULpeak), the maximum standardized uptake value (SUVmax), tumor-to-blood pool SUV ratio (TBR), tumor-to-liver SUV ratio (TLR), metabolic tumor volume (MTV)/FAPI-positive tumor volume (GTV-FAPI), total lesion glycolysis (TLG)/FAPI-positive total lesion accumulation (TLF) were correlated with the response status to identify indicative of treatment response. The predictive performance of them was quantified by generating receiver operating characteristic curves (ROC), calibration curves, and cross-validation. Results: A total of 110 lesions were identified in 27 patients. Compared with PG, NPG was associated with lower levels of TBR and SUVmean in FDG PET/CT (TBR-FDG, SUVmean-FDG; P=0.033 and P=0.038, respectively), with higher SULpeak and TLF in FAPI PET/CT (SULpeak-FAPI, TLF-FAPI; P=0.10 and P=0.049, respectively). The predictive power of a composite-parameter model, including TBR-FDG, SULpeak-FAPI, gene mutation, and type of targeted therapy [area under the curve (AUC) =0.865], was superior to the few-parameter models incorporating TBR-FDG (AUC =0.637, P<0.001), SULpeak-FAPI (AUC =0.665, P<0.001) or both (AUC =0.721, P<0.001). Conclusions: Both [18F]FAPI-42 PET/CT and [18F]FDG PET/CT have value in predicting the treatment response of recurrent or metastatic GISTs. And [18F]FAPI-42 PET/CT offers synergistic value when used in combination with [18F]FDG PET/CT. Notably, the nomogram generated from the model incorporating [18F]FAPI-42 PET/CT, [18F]FDG PET/CT parameters, gene mutation, and type of targeted therapy could yield more precise predictions of the response of recurrent metastatic GISTs.

Exploring the value of FAP-targeted PET/CT in differentiating breast cancer molecular subtypes: a preliminary study.

PURPOSE: This prospective study aims to evaluate the value of [18F]AlF-NOTA-fibroblast activation protein inhibitor (FAPI)-04 positron emission tomography-computed tomography (PET/CT) in predicting molecular subtypes of breast cancer. METHODS: The study consecutively recruited patients suspected of having breast cancer from a single center who were prospectively enrolled from July 2023 to May 2024 and underwent [18F]AlF-NOTA-FAPI-04 PET/CT. This study compared the differences in tracer uptake among breast cancers with different adverse prognostic factors and molecular subtypes. The classification performance for each molecular subtype of breast cancer was assessed using a receiver operating characteristic (ROC) curve. RESULTS: Fifty-three participants (mean age, 51 ± 11 years; 52 females) were evaluated. Breast cancer lesions with adverse prognostic factors showed higher tracer uptake. The five different molecular subtypes exhibited varying levels of uptake. The luminal A and luminal B (HER2-negative) subtypes had relatively low uptake, while the luminal B (HER2-positive), HER2-positive, and triple-negative subtypes had relatively high uptake. ROC analysis identified the max standardized uptake value (SUVmax) as a significant classifier (AUC = 0.912, P = 0.0005) for the luminal A subtype, with 100% sensitivity and 83% specificity. For predicting the luminal B (HER2-negative) subtype, SUVmax had an AUC of 0.770 (P = 0.0015). SUVmax, with an AUC of 0.781 (P = 0.003), was used to identify the triple-negative subtype tumors, resulting in a sensitivity of 100% and specificity of 51%. Lastly, the ROC curve showed the cut-off 15.40 (AUC = 0.921, P < 0.0001) could classify luminal A & luminal B (HER2-negative), and luminal B (HER2-positive) & HER2-positive & triple-negative, yielding a sensitivity of 94% and specificity of 79%. CONCLUSION: The uptake of [18F]AlF-NOTA-FAPI-04 is significantly correlated with the molecular subtypes of breast cancer, and [18F]AlF-NOTA-FAPI-04 PET/CT is a potential tool for noninvasive identification of luminal A subtypes and guidance of FAP-targeted therapies.

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

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

Development of fibroblast activation protein-α radiopharmaceuticals: Recent advances and perspectives.

Fibroblast activation protein-α (FAP) has emerged as a promising target in the field of radiopharmaceuticals due to its selective expression in cancer-associated fibroblasts (CAFs) and other pathological conditions involving fibrosis and inflammation. Recent advancements have focused on developing FAP-specific radioligands for diagnostic imaging and targeted radionuclide therapy. This perspective summarized the latest progress in FAP radiopharmaceutical development, highlighting novel radioligands, preclinical evaluations, and potential clinical applications. Additionally, we analyzed the advantages and existing problems of targeted FAP radiopharmaceuticals, and discussed the key breakthrough directions of this target, so as to improve the development and conversion of FAP-targeted radiopharmaceuticals.