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

Rational Design and Comparison of Novel 99mTc-Labeled FAPI Dimers for Visualization of Multiple Tumor Types

Recognizing the significance of SPECT in nuclear medicine and the pivotal role of fibroblast activation protein (FAP) in cancer diagnosis and therapy, this study focuses on the development of 99mTc-labeled dimeric HF2 with high tumor uptake and image contrast. The dimeric HF2 was synthesized and radiolabeled with 99mTc in one pot using various coligands (tricine, TPPTS, EDDA, and TPPMS) to yield [99mTc]Tc-TPPTS-HF2, [99mTc]Tc-EDDA-HF2, and [99mTc]Tc-TPPMS-HF2 dimers. SPECT imaging results indicated that [99mTc]Tc-TPPTS-HF2 exhibited higher tumor uptake and tumor-to-normal tissue (T/NT) ratio than [99mTc]Tc-EDDA-HF2 and [99mTc]Tc-TPPMS-HF2. Notably, [99mTc]Tc-TPPTS-HF2 exhibited remarkable tumor accumulation and retention in HT-1080-FAP and U87-MG tumor-bearing mice, thereby surpassing the monomeric [99mTc]Tc-TPPTS-HF. Moreover, [99mTc]Tc-TPPTS-HF2 achieved acceptable T/NT ratios in the hepatocellular carcinoma patient-derived xenograft (HCC-PDX) model, which provided identifiable contrast and imaging quality. In conclusion, this study presents proof-of-concept research on 99mTc-labeled FAP inhibitor dimers for the visualization of multiple tumor types. Among these candidate compounds, [99mTc]Tc-TPPTS-HF2 showed excellent clinical potential, thereby enriching the SPECT tracer toolbox.

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.

Preclinical Evaluation and a Pilot Clinical Positron Emission Tomography Imaging Study of [68Ga]Ga-FAPI-FUSCC-II

Fibroblast activation protein (FAP), a type II integral membrane serine protease, is a promising target for tumor diagnosis and therapy. OncoFAP has been recently discovered for PET imaging procedures for various solid malignancies. In this study, we presented the development of manual radiolabeling procedures for the preparation of OncoFAP-based radiopharmaceuticals for cancer imaging. A novel series of [68Ga/177Lu]Ga/Lu-FAPI-FUSCC-I/II were produced with high radiochemical yields. [68Ga]Ga-FAPI-FUSCC-I/II and [177Lu]Lu-FAPI-FUSCC-I/II were stable in phosphate-buffered saline, fetal bovine serum, and human serum for at least 3 h. In vitro cellular uptake and blocking experiments implied that they had specificity to FAP. Additionally, the low nanomolar IC50 values of FAPI-FUSCC-II indicated that it had a high target affinity to FAP. The in vivo biodistribution and blocking study in mice bearing HT-1080-FAP tumors showed that both exhibited specific tumor uptake. [68Ga]Ga-FAPI-FUSCC-II showed a higher tumor uptake and a higher tumor/nontarget ratio than [68Ga]Ga-FAPI-FUSCC-I and [68Ga]Ga-FAPI-04. The results of ex vivo biodistribution were in accordance with the biodistribution results. Clinical [68Ga]Ga-FAPI-FUSCC-II-PET/CT imaging further demonstrated its favorable biodistribution and kinetics with elevated and reliable uptake by primary tumors (maximum standardized uptake value (SUVmax), 12.17 ± 6.67) and distant metastases (SUVmax, 9.24 ± 4.28). In summary, [68Ga]Ga-FAPI-FUSCC-II displayed increased tumor uptake and retention compared to [68Ga]Ga-FAPI-04, giving it potential as a promising tracer for the diagnostic imaging of malignant tumors with positive FAP expression.

Synthesis and Evaluation of [18F]SiFA-Conjugated Ligands for Fibroblast Activation Protein Imaging

In recent years, fibroblast activation protein (FAP) has emerged as an important target for the diagnosis and therapy of various tumors due to its high expression on the cell surface of cancer-associated fibroblasts, which are the major components of the tumor stroma. In this study, we synthesized and evaluated 18F-labeled FAP inhibitors (FAPIs) for FAP imaging. Two silicon fluoride acceptor (SiFA)-conjugated FAPIs were synthesized: one containing a γ-carboxy-l-glutamic acid (Gla) residue (1) and another containing two Gla residues (2). Both ligands exhibited high binding affinities for FAP. 18F/19F exchange reactions on both ligands were performed in the presence of 2% water. This resulted in the formation of radioligands [18F]1 and [18F]2 in high radiochemical yields. Radioligand [18F]2, with a more favorable partition coefficient, was selected for the U87MG cell binding study, and the results showed FAP-specific binding of the radioligand to the cells. An ex vivo biodistribution study in U87MG tumor-bearing mice 60 min after injection demonstrated a 5.8-fold higher tumor accumulation of [18F]2 than that of [18F]1. Furthermore, PET and ex vivo biodistribution studies of [18F]2 in U87MG tumor-bearing mice showed high and persistent tumor uptake over time, which was significantly blocked by the preinjection of FAPI-04. Our results indicate that [18F]SiFA-(Gla)2-conjugated FAPI ([18F]2) has the potential for FAP 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).

Synthesis and Evaluation of 99mTc-Labelled 2-Nitroimidazole Derivatives with Different Linkers for Tumour Hypoxia Imaging

When developing novel radiopharmaceuticals, a linker moiety between the chelator and targeting vector can have a crucial influence on adjusting the affinity of the tracer and its biodistribution in organisms. To develop novel 99mTc-labelled hypoxia imaging radiotracers, in this study, five isocyanide-containing 2-nitroimidazole derivatives with different linkers (L1, L2, L3, L4 and L5) were synthesised and radiolabelled with technetium-99m to obtain five stable 99mTc-complexes ([99mTc]Tc-L1, [99mTc]Tc-L2, [99mTc]Tc-L3, [99mTc]Tc-L4 and [99mTc]Tc-L5). Corresponding rhenium analogues of [99mTc]Tc-L1 were synthesised and suggested the structures of these 99mTc-complexes would be a monovalent cation with a technetium (I) core surrounded by six ligands. [99mTc]Tc-L1 is hydrophilic, while the lipophilicities of [99mTc]Tc-L2, [99mTc]Tc-L3, [99mTc]Tc-L4 and [99mTc]Tc-L5 are close. In vitro cell experiments showed that all five novel 99mTc-complexes had higher uptake in hypoxic cells compared with aerobic cells, which indicates the complexes have good hypoxia selectivity. The biodistribution of the five 99mTc-complexes in S180 tumour-bearing mice showed that they all had certain uptake in the tumours. Among them, [99mTc]Tc-L1 had the highest tumour-to-muscle (4.68 ± 0.44) and tumour-to-blood (3.81 ± 0.46) ratios. The introduction of polyethylene glycol (PEG) chains effectively reduced the lipophilicity and decreased uptake by the liver, intestine and blood but also increased clearance from the tumours. In vivo metabolic studies showed [99mTc]Tc-L1 kept intact and remained stable in tumour, blood and urine at 2 h post-injection. The results of SPECT imaging showed that [99mTc]Tc-L1 had significant tumour uptake at 2 h post-injection, but there was still high uptake in abdominal organs such as the liver and kidney, suggesting that this complex needs to be further optimised before being used for tumour hypoxia imaging.