Research progress on the role of fibroblast activation protein in diagnosis and treatment of cancer

Evaluation of a Novel Gd-FAPI Dimer Molecular Probe Targeting Fibroblast Activation Protein for Imaging of Solid Tumors

Cancer-associated fibroblasts (CAFs) are essential components of the tumor microenvironment. Fibroblast activation protein (FAP) is overexpressed in CAFs. FAP-targeted molecular imaging agents, including the FAP inhibitors (FAPIs), have shown promising results in tumor diagnosis. We aimed to design a Gd-labeled FAPI Dimer, Gd-DOTA-Suc-Lys-(FAPI04)2, to optimize the pharmacokinetics and evaluate its potential capacity for targeting FAP-positive solid tumors in vivo. The Gd-labeled FAPI Dimer was successfully synthesized with exceeding 98% purity. Preclinical pharmacokinetics were determined in assessed FAP-positive U87 cell-derived xenografts and FAP-negative C6-derived xenografts using small-animal T1-weighted 7.0T MR imaging. The longitudinal correlation coefficient (r1) of the agent was 3.813 mM-1·S-1. The administration of the Gd-FAPI04 Dimer probe showed a notable enhancement of tumor contrast on T1-weighted whole-body MRI. At 10 and 30 minutes post-injection, the U87 subcutaneous tumor demonstrated significantly greater contrast enhancement than the C6 subcutaneous tumor (P <0.05). In vivo, the safety of the Gd-FAPI-04 Dimer probe was evaluated, which showed no tissue damage in vital organs like the heart, liver, spleen, lung, and kidneys, as indicated by unchanged morphology compared to a normal saline control group. The novel Gd-FAPI04 Dimer molecular probe, Gd-DOTA-Suc-Lys-(FAPI-04)2 specifically targeting FAP may serve as a safe and promising tool for the diagnostic imaging of solid tumors.

Preclinical evaluation and first-in-human study of [18F]AlF-FAP-NUR for PET imaging cancer-associated fibroblasts

BACKGROUND

Fibroblast activation protein (FAP) has gained attention as a promising molecular target with potential utility for cancer diagnosis and therapy. [68Ga]Ga-labeled FAP-targeting peptides have been successfully applied to positron emission tomography (PET) imaging of various tumor types. To meet the applicable demand for peptide-based FAP tracers with high patient throughput, we herein report the radiosynthesis, preclinical evaluation, and the first-in-human imaging of a novel [18F]F-labeled FAP-targeting peptide.

RESULTS

[18F]AlF-FAP-NUR was automatedly prepared within 45 min with a non-decay corrected radiochemical yield of 18.73 ± 4.25% (n = 3). Compared to [68Ga]Ga-FAP-2286, the [18F]F-labeled peptide demonstrated more rapid, higher levels of cellular uptake and internalization, and lower levels of cellular efflux in HT1080-FAP cells. Micro-PET imaging and biodistribution studies conducted on xenograft mice models revealed a similar distribution pattern between the two tracers. However, [18F]AlF-FAP-NUR demonstrated significantly higher tumor-specific uptake resulting in improved Tumor-Background Ratios (TBRs). In the patients, a significant accumulation of [18F]AlF-FAP-NUR was found in the primary tumor. High uptake of the tracer within the bladder indicated that its major route of excretion was through urine.

CONCLUSIONS

Based on the physical imaging properties and longer half-life of [18F]F, [18F]AlF-FAP-NUR exhibited promising characteristics such as enhanced tumor-specific accumulation and elevated TBRs, which made it a viable candidate for further clinical investigation.

TRIAL REGISTRATION

www.Chictr.org.cn , ChiCTR2300076976 Retrospectively registered 25 October 2023. at, URL: https://www.chictr.org.cn/showproj.html?proj=206753 .

Synthesis and Preclinical Evaluation of Novel 99mTc-Labeled FAPI-46 Derivatives with Significant Tumor Uptake and Improved Tumor-to-Nontarget Ratios

Fibroblast activation protein (FAP), which is expressed on the cell membranes of fibroblasts in most solid tumors, has become an important target for tumor diagnosis and treatment. However, previously reported 99mTc-labeled FAPI-04 complexes have high blood uptake, limiting their use in the clinic. In this work, six 99mTc-labeled FAPI-46 derivatives with different linkers (different amino acids, peptides, or polyethylene glycol) were prepared and evaluated. They had good in vitro stability, hydrophilicity, and good specificity for FAP. The biodistribution and MicroSPECT images revealed that they all had high specific tumor uptake for FAP, and their blood uptake was significantly decreased. Among them, [99mTc]Tc-6-1 exhibited the highest target-to-nontarget ratios (tumor/blood: 6.06 ± 1.19; tumor/muscle: 10.26 ± 0.44) and good tumor uptake (16.15 ± 0.83%ID/g), which also had significantly high affinity for FAP, good in vivo stability, and safety. Therefore, [99mTc]Tc-6-1 holds great potential as a promising molecular tracer for FAP tumor imaging.

Theranostics in targeting fibroblast activation protein bearing cells: Progress and challenges

Cancer cells are surrounded by a complex and highly dynamic tumor microenvironment (TME). Cancer-associated fibroblasts (CAFs), a critical component of TME, contribute to cancer cell proliferation as well as metastatic spread. CAFs express a variety of biomarkers, which can be targeted for detection and therapy. Most importantly, CAFs express high levels of fibroblast activation protein (FAP) which contributes to progression of cancer, invasion, metastasis, migration, immunosuppression, and drug resistance. As a consequence, FAP is an attractive theranostic target. In this review, we discuss the latest advancement in targeting FAP in oncology using theranostic biomarkers and imaging modalities such as single-photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT), fluorescence imaging, and magnetic resonance imaging (MRI).

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

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