Radioligands Targeting Fibroblast Activation Protein (FAP)

Single-cell transcriptome analysis reveals tumoral microenvironment heterogenicity and hypervascularization in human carotid body tumor

Carotid body tumor (CBT) is a rare neck tumor located at the adventitia of the common carotid artery bifurcation. The prominent pathological features of CBT are high vascularization and abnormal proliferation. However, single-cell transcriptome analysis of the microenvironment composition and molecular complexity in CBT has yet to be performed. In this study, we performed single-cell RNA sequencing (scRNA-seq) analysis on human CBT to define the cells that contribute to hypervascularization and chronic hyperplasia. Unbiased clustering analysis of transcriptional profiles identified 16 distinct cell populations including endothelial cells (ECs), smooth muscle cells (SMCs), neuron cells, macrophage cells, neutrophil cells, and T cells. Within the ECs population, we defined subsets with angiogenic capacity plus clear signs of later endothelial progenitor cells (EPCs) to normal ECs. Two populations of macrophages were detectable in CBT, macrophage1 showed enrichment in hypoxia-inducible factor-1 (HIF-1) and as well as an early EPCs cell-like population expressing CD14 and vascular endothelial growth factor. In addition to HIF-1-related transcriptional protein expression, macrophages1 also display a neovasculogenesis-promoting phenotype. SMCs included three populations showing platelet-derived growth factor receptor beta and vimentin expression, indicative of a cancer-associated fibroblast phenotype. Finally, we identified three types of neuronal cells, including chief cells and sustentacular cells, and elucidated their distinct roles in the pathogenesis of CBT and abnormal proliferation of tumors. Overall, our study provided the first comprehensive characterization of the transcriptional landscape of CBT at scRNA-seq profiles, providing novel insights into the mechanisms underlying its formation.

Chelator impact: investigating the pharmacokinetic behavior of copper-64 labeled PD-L1 radioligands

BACKGROUND

Programmed cell death ligand 1 (PD-L1) plays a critical role in the tumor microenvironment and overexpression in several solid cancers has been reported. This was associated with a downregulation of the local immune response, specifically of T-cells. Immune checkpoint inhibitors showed a potential to break this localized immune paralysis, but only 30% of patients are considered responders. New diagnostic approaches are therefore needed to determine patient eligibility. Small molecule radiotracers targeting PD-L1, may serve as such diagnostic tools, addressing the heterogeneous PD-L1 expression between and within tumor lesions, thus aiding in therapy decisions.

RESULTS

Four biphenyl-based small-molecule PD-L1 ligands were synthesized using a convergent synthetic route with a linear sequence of up to eleven steps. As a chelator NODA-GA, CB-TE2A or DiAmSar was used to allow radiolabeling with copper-64 ([64Cu]Cu-14-[64Cu]Cu-16). In addition, a dimeric structure based on DiAmSar was synthesized ([64Cu]Cu-17). All four radioligands exhibited high proteolytic stability (> 95%) up to 48 h post-radiolabeling. Saturation binding yielded moderate affinities toward PD-L1, ranging from 100 to 265 nM. Real-time radioligand binding provided more promising KD values around 20 nM for [64Cu]Cu-14 and [64Cu]Cu-15. In vivo PET imaging in mice bearing both PC3 PD-L1 overexpressing and PD-L1-mock tumors was performed at 0-2, 4-5 and 24-25 h post injection (p.i.). This revealed considerably different pharmacokinetic profiles, depending on the substituted chelator. [64Cu]Cu-14, substituted with NODA-GA, showed renal clearance with low liver uptake, whereas substitution with the cross-bridged cyclam chelator CB-TE2A resulted in a primarily hepatobiliary clearance. Notably, the monomeric DiAmSar radioligand [64Cu]Cu-16 demonstrated a higher liver uptake than [64Cu]Cu-15, but was still renally cleared as evidenced by the lack of uptake in gall bladder and intestines. The dimeric structure [64Cu]Cu-17 showed extensive accumulation and trapping in the liver but was also cleared via the renal pathway. Of all tracer candidates and across all timepoints, [64Cu]Cu-17 showed the highest accumulation at 24 h p.i. in the PD-L1-overexpressing tumor of all timepoints and all radiotracers, indicating drastically increased circulation time upon dimerization of two PD-L1 binding motifs.

CONCLUSIONS

This study shows that chelator choice significantly influences the pharmacokinetic profile of biphenyl-based small molecule PD-L1 radioligands. The NODA-GA-conjugated radioligand [64Cu]Cu-14 exhibited favorable renal clearance; however, the limited uptake in tumors suggests the need for structural modifications to the binding motif for future PD-L1 radiotracers.

Initial Experience with 68Ga-FAP-2286 PET Imaging in Patients with Urothelial Cancer

Improved imaging modalities are needed to accurately stage patients with muscle-invasive bladder cancer (MIBC) and metastatic urothelial carcinoma. Imaging with small-molecule ligands or inhibitors of fibroblast activation protein (FAP) is a promising modality that has demonstrated initial efficacy across a broad range of tumors. We present our experience with the novel FAP-peptide binder 68Ga-FAP-2286 in patients with MIBC. Methods: Patients with histopathologically confirmed bladder cancer who had either localized disease at diagnosis (localized cohort, n = 13) or known metastatic disease (metastatic cohort, n = 8) were imaged with 68Ga-FAP-2286 PET as part of a clinical trial (NCT04621435). The SUVmax of 68Ga-FAP-2286 PET-positive lesions and lesion size were documented. In patients who had available 18F-FDG PET performed within 45 d of 68Ga-FAP-2286 PET (n = 5), uptake on the 2 scans was compared. When there was a discrepancy between imaging modalities on retrospective review, biopsy of suggestive lesions was performed as the standard of care. Results: In the metastatic and localized cohorts, 36 and 18 68Ga-FAP-2286-avid lesions, respectively, were identified across multiple anatomic locations, including lymph nodes, visceral metastases, and bones. Fourteen of 36 lesions in the metastatic cohort and 14 of 18 lesions in the localized cohort were lymph nodes measuring less than 1 cm. Among lesions measuring less than 0.5 cm, 0.5-1 cm, and more than 1 cm, average SUVmax was 5.2 ± 2.6, 9.6 ± 3.7, and 13.0 ± 4.3, respectively, in the metastatic cohort and 10.5 ± 5.1, 10.8 ± 5.7, and 9.9 ± 5.4, respectively, in the localized cohort. Five patients had 18F-FDG PET available for comparison. The average SUVmax for lesions avid on 68Ga-FAP-2286 PET and 18F-FDG PET was 9.9 ± 3.4 versus 4.2 ± 1.9, respectively (n = 16 lesions). For 3 patients in the localized cohort, 68Ga-FAP-2286 PET informed clinical management, including identification of both false-positive findings on 18F-FDG PET and false-negative findings on conventional CT. Conclusion: 68Ga-FAP-2286 imaging is highly sensitive in patients with urothelial cancer and is effective in identifying metastatic lesions across a variety of anatomic sites, including subcentimeter lymph nodes that would not have raised suspicion on conventional scans. This novel imaging modality may inform clinical decision-making in patients with MIBC both by refining local nodal staging and by defining metastatic disease that would otherwise be undetectable on conventional imaging.

Emerging role of cancer-associated fibroblasts in esophageal squamous cell carcinoma

Esophageal carcinoma is the sixth leading cause of cancer death globally and the majority of global cases are esophageal squamous cell carcinoma (ESCC). Difficulty in diagnosis exists as more than 70% of ESCC patients are diagnosed at the intermediate or advanced stage. Cancer-associated fibroblasts (CAFs) have been considered one of the crucial components in the process of tumor growth, promoting communications between cancer cells and the tumor microenvironment (TME). CAFs grow alongside malignancies dynamically and interact with ESCC cells to promote their progression, proliferation, invasion, tumor escape, chemo- and radio-resistance, etc. It is believed that CAFs qualify as a promising direction for treatment. Analyzing CAFs' subtypes and functions will elucidate the involvement of CAFs in ESCC and aid in therapeutics. This review summarizes current information on CAFs in ESCC and focuses on the latest interaction between CAFs and ESCC cancer cell discoveries. The origin of CAFs and their communication with ESCC cells and TME are also demonstrated. On the foundation of a thorough analysis, we highlight the clinical prospects and CAFs-related therapies in ESCC in the future.

Recent progress on the effect of extracellular matrix on occurrence and progression of breast cancer

Breast cancer (BC) metastasis is an enormous challenge targeting BC therapy. The extracellular matrix (ECM), the principal component of the BC metastasis niche, is the pivotal driver of breast tumor development, whose biochemical and biophysical characteristics have attracted widespread attention. Here, we review the biological effects of ECM constituents and the influence of ECM stiffness on BC metastasis and drug resistance. We provide an overview of the relative signal transduction mechanisms, existing metastasis models, and targeted drug strategies centered around ECM stiffness. It will shed light on exploring more underlying targets and developing specific drugs aimed at ECM utilizing biomimetic platforms, which are promising for breast cancer treatment.

Cross-species evaluation of fibroblast activation protein alpha as potential imaging target for soft tissue sarcoma: a comparative immunohistochemical study in humans, dogs, and cats

Introduction

Complete surgical tumor resection is paramount in the management of soft tissue sarcoma (STS) in humans, dogs, and cats alike. Near-infrared targeted tracers for fluorescence-guided surgery (FGS) could facilitate intraoperative visualization of the tumor and improve resection accuracy. Target identification is complicated in STS due to the rarity and heterogeneity of the disease. This study aims to validate the expression of fibroblast activation protein alpha (FAP) in selected human, canine, and feline STS subtypes to assess the value of FAP as a target for FGS and to validate companion animals as a translational model.

Methods

Formalin-fixed and paraffin-embedded tissue samples from 53 canine STSs (perivascular wall tumor (PWT), canine fibrosarcoma (cFS), and STS not further specified (NOS)), 24 feline fibrosarcomas, and 39 human STSs (myxofibrosarcoma, undifferentiated pleomorphic sarcoma, dermatofibrosarcoma protuberans, and malignant peripheral nerve sheath tumor) as well as six canine and seven feline healthy controls and 10 inflamed tissue samples were immunohistochemically stained for their FAP expression. FAP labeling in tumor, peritumoral, healthy skin, and inflamed tissue samples was quantified using a visually assessed semiquantitative expression score and digital image analysis. Target selection criteria (TASC) scoring was subsequently performed as previously described.

Results

Eighty-five percent (85%) of human (33/39), 76% of canine (40/53), and 92% of feline (22/24) STSs showed FAP positivity in over 10% of the tumor cells. A high expression was determined in 53% canine (28/53), 67% feline (16/24), and 44% human STSs (17/39). The average FAP-labeled area of canine, feline, and human STSs was 31%, 33%, and 42%, respectively (p > 0.8990). The FAP-positive tumor area was larger in STS compared to healthy and peritumoral tissue samples (p < 0.0001). TASC scores were above 18 for all feline and human STS subtypes and canine PWTs but not for canine STS NOS and cFS.

Conclusion

This study represents the first cross-species target evaluation of FAP for STS. Our results demonstrate that FAP expression is increased in various STS subtypes compared to non-cancerous tissues across species, thereby validating dogs and cats as suitable animal models. Based on a TASC score, FAP could be considered a target for FGS.

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.

FAP expression in alpha cells of Langherhans insulae-implications for FAPI radiopharmaceuticals' use

PURPOSE

Radiopharmaceuticals targeting fibroblast activation protein (FAP) alpha are increasingly studied for diagnostic and therapeutic applications. We discovered FAP expression at immunohistochemistry (IHC) in the alpha cells of the Langerhans insulae of few patients. Therefore, we planned an investigation aimed at describing FAP expression in the pancreas and discussing the implications for radioligand applications.

METHODS

We retrospectively included 40 patients from 2 institutions (20 pts each) according to the following inclusion/exclusion criteria: (i) pathology proven pancreatic ductal adenocarcinoma and neuroendocrine tumors (NET), 10 pts per each group at each center; (ii) and availability of paraffin-embedded tissue; and (iii) clinical-pathological records. We performed IHC analysis and applied a semiquantitative visual scoring system (0, negative staining; 1, present in less than 30%; 2, present in more than 30% of the area). FAP expression was assessed according to histology-NET (n = 20) vs ductal adenocarcinoma (n = 20)-and to previous treatments within the adenocarcinoma group. The local ethics committee approved the study (No. INT 21/16, 28 January 2016).

RESULTS

The population consisted of 24 males and 16 females, with a median age of 68 and a range of 14-84 years; 8/20 adenocarcinoma patients received chemotherapy. In all the Langerhans insulae (40/40), pancreatic alpha cells were found to express FAP, with a score of 2. No difference was found among NET (20/20) and adenocarcinoma (20/20), nor according to neoadjuvant chemotherapy in the adenocarcinoma cohort (received or not received).

CONCLUSION

Pancreatic Langerhans islet alpha cells normally express FAP. This is not expected to influence the diagnostic accuracy of FAP-targeting tracers. In the therapeutic setting, our results suggest the need to better elucidate FAPI radioligands' effects on the Langerhans insulae function.

The Potential Role of the T2 Ribonucleases in TME-Based Cancer Therapy

In recent years, there has been a growing interest in developing innovative anticancer therapies targeting the tumor microenvironment (TME). The TME is a complex and dynamic milieu surrounding the tumor mass, consisting of various cellular and molecular components, including those from the host organism, endowed with the ability to significantly influence cancer development and progression. Processes such as angiogenesis, immune evasion, and metastasis are crucial targets in the search for novel anticancer drugs. Thus, identifying molecules with "multi-tasking" properties that can counteract cancer cell growth at multiple levels represents a relevant but still unmet clinical need. Extensive research over the past two decades has revealed a consistent anticancer activity for several members of the T2 ribonuclease family, found in evolutionarily distant species. Initially, it was believed that T2 ribonucleases mainly acted as anticancer agents in a cell-autonomous manner. However, further investigation uncovered a complex and independent mechanism of action that operates at a non-cell-autonomous level, affecting crucial processes in TME-induced tumor growth, such as angiogenesis, evasion of immune surveillance, and immune cell polarization. Here, we review and discuss the remarkable properties of ribonucleases from the T2 family in the context of "multilevel" oncosuppression acting on the TME.

Clinical Advances and Perspectives in Targeted Radionuclide Therapy

Targeted radionuclide therapy has become increasingly prominent as a nuclear medicine subspecialty. For many decades, treatment with radionuclides has been mainly restricted to the use of iodine-131 in thyroid disorders. Currently, radiopharmaceuticals, consisting of a radionuclide coupled to a vector that binds to a desired biological target with high specificity, are being developed. The objective is to be as selective as possible at the tumor level, while limiting the dose received at the healthy tissue level. In recent years, a better understanding of molecular mechanisms of cancer, as well as the appearance of innovative targeting agents (antibodies, peptides, and small molecules) and the availability of new radioisotopes, have enabled considerable advances in the field of vectorized internal radiotherapy with a better therapeutic efficacy, radiation safety and personalized treatments. For instance, targeting the tumor microenvironment, instead of the cancer cells, now appears particularly attractive. Several radiopharmaceuticals for therapeutic targeting have shown clinical value in several types of tumors and have been or will soon be approved and authorized for clinical use. Following their clinical and commercial success, research in that domain is particularly growing, with the clinical pipeline appearing as a promising target. This review aims to provide an overview of current research on targeting radionuclide therapy.

Stromal depletion by TALEN-edited universal hypoimmunogenic FAP-CAR T cells enables infiltration and anti-tumor cytotoxicity of tumor antigen-targeted CAR-T immunotherapy

Adoptive cell therapy based on chimeric antigen receptor (CAR)-engineered T-cells has proven to be lifesaving for many cancer patients. However, its therapeutic efficacy has so far been restricted to only a few malignancies, with solid tumors proving to be especially recalcitrant to efficient therapy. Poor intra-tumor infiltration by T cells and T cell dysfunction due to a desmoplastic, immunosuppressive microenvironment are key barriers for CAR T-cell success against solid tumors. Cancer-associated fibroblasts (CAFs) are critical components of the tumor stroma, evolving specifically within the tumor microenvironment (TME) in response to tumor cell cues. The CAF secretome is a significant contributor to the extracellular matrix and a plethora of cytokines and growth factors that induce immune suppression. Together they form a physical and chemical barrier which induces a T cell-excluding 'cold' TME. CAF depletion in stroma rich solid tumors can thus provide an opportunity to convert immune evasive tumors susceptible to tumor-antigen CAR T-cell cytotoxicity. Using our TALEN-based gene editing platform we engineered non-alloreactive, immune evasive CAR T-cells (termed UCAR T-cells) targeting the unique CAF marker Fibroblast Activation Protein, alpha (FAP). In an orthotopic mouse model of triple-negative breast cancer (TNBC) composed of patient derived-CAFs and tumor cells, we demonstrate the efficacy of our engineered FAP UCAR T-cells in CAF depletion, reduction of desmoplasia and successful tumor infiltration. Furthermore, while previously resistant, pre-treatment with FAP UCAR T-cells now sensitized these tumors to Mesothelin (Meso) UCAR T-cell infiltration and anti-tumor cytotoxicity. Combination therapy of FAP UCAR, Meso UCAR T cells and the checkpoint inhibitor anti-PD-1 significantly reduced tumor burden and prolonged mice survival. Our study thus proposes a novel treatment paradigm for successful CAR T-cell immunotherapy against stroma-rich solid tumors.

Advanced Fibroblast Activation Protein-Ligand Developments: FAP Imaging Agents: A Review of the Structural Requirements

Fibroblast activation protein-α (FAP) has attracted increasing attention as a selective marker of cancer-associated fibroblasts (CAFs) and more broadly, of activated fibroblasts in tissues undergoing remodeling of their ECM due to chronic inflammation, fibrosis, or wound healing. Since FAP is critical to the initiation of metastatic growth, its expression will serve as a molecular marker to detect tumors at an earlier stage of development compared to currently available methods. The design of high affinity small molecule FAP inhibitor will allow for noninvasive imaging of activated fibroblast in cancer patients. Small molecule inhibitors of FAP are being developed for targeted radiotherapy of tumors.

A Simple Kit Formulation for Preparation and Exploratory Human Studies of a Novel 99mTc-Labeled Fibroblast Activation Protein Inhibitor Tracer for Imaging of the Fibroblast Activation Protein in Cancers

Fibroblast activation protein (FAP) is a potential target for tumor diagnosis and treatment because it is selectively expressed on the cell membrane of cancer-associated fibroblasts in most solid tumor stroma. The aim of this study was to develop a ^99mTc-labeled fibroblast activation protein inhibitor (FAPI) tracer, evaluate its imaging efficacy in nude mice, and further explore its biodistribution in healthy volunteers and uptake in tumor patients. An FAPI-derived ligand (DP-FAPI) containing d-proline was designed and synthesized as a linker, and a stable hydrophilic ^99mTc-labeled complex ([^99mTc]Tc-DP-FAPI) was obtained by kit formulation. In vitro cellular uptake and saturation binding assays were performed in FAP-transfected HT-1080 cells (FAP-HT-1080). The biodistribution was characterized, and micro-single-photon emission computed tomography (SPECT) imaging was performed in BALB/c nude mice bearing U87 MG tumors. Furthermore, a first-in-man application was performed in four healthy volunteers and three patients with gastrointestinal tumors. In vitro, the nanomolar K_d values of [^99mTc]Tc-DP-FAPI indicated that it had significantly high target affinity for FAP. Biodistribution and micro-SPECT imaging studies showed that [^99mTc]Tc-DP-FAPI exhibited high uptake and high tumor-to-nontargeted ratios. The calculated effective dose for [^99mTc]Tc-DP-FAPI was approximately <5 mSv in four healthy volunteers. In three patients with gastrointestinal tumors, [^99mTc]Tc-DP-FAPI quantitative SPECT/CT revealed high and reliable uptake. [^99mTc]Tc-DP-FAPI exhibited high selectivity and affinity for FAP in vitro. The safety and effectiveness of [^99mTc]Tc-DP-FAPI in primary tumor imaging have been confirmed by animal and clinical studies, revealing the potential clinical application value of this tracer.

Fibroblast Activation Protein Inhibitor Theranostics: Early Clinical Translation

Fibroblast activation protein (FAP)-targeted radioligand therapy offers a possibility of a novel cancer therapeutic strategy, aiming at tumor stroma1. Early clinical translations of FAP-tracers occurred as early as in the 1990s using antibodies, without substantial achievement further than the clinical phase II trial. The essential step toward the theranostic approach, with a conceptual combination of diagnostic and therapeutic emitters in a specific tracer, began with the implementation of small-molecule FAP-enzyme inhibitors (FAPI) in 2018. Currently, FAPI-04 and FAPI-46, containing DOTA-chelators with the possibility of radionuclide combination (Ga-68, Y-90, and Lu-177), are the compounds most widely used in the theranostic regimen.

Synthesis and Preclinical Evaluation of a Novel FAPI-04 Dimer for Cancer Theranostics

Overexpression of fibroblast activation protein (FAP) in cancer-associated fibroblasts in a wide variety of tumors enables a highly selective targeting strategy using FAP inhibitors (FAPIs). Quinoline-based FAPIs labeled with radionuclides have been widely developed for tumor-targeted nuclear medicine imaging. However, the short retention time of FAPIs at the tumor site limits their application in radionuclide therapy. In this study, a novel FAPI-04 dimer was synthesized and labeled with radionuclides to prolong the retention time in tumors for imaging and therapy. To prepare the FAPI-04 dimer complex, DOTA-Suc-Lys-(FAPI-04)₂, we used Fmoc-Lys(Boc)-OH as the linker to conjugate two FAPI-04 structures by an amide reaction. The resulting product was further modified by DOTA groups to allow for conjugation with radioactive metals. Both [⁶⁸Ga]Ga-(FAPI-04)₂ and [¹⁷⁷Lu]Lu-(FAPI-04)₂ showed a radiochemical purity of >99% and remained stable in vitro. In vivo, micro-PET images of SKOV3, A431, and H1299 xenografts revealed that the tumor uptake of [⁶⁸Ga]Ga-(FAPI-04)₂ was about twice that of [⁶⁸Ga]Ga-FAPI-04 and that the accumulation of [⁶⁸Ga]Ga-(FAPI-04)₂ at the tumor site did not significantly decrease even 3h after injection. The tumor-abdomen ratio of [⁶⁸Ga]Ga-(FAPI-04)₂ images was significantly higher than that of [¹⁸F]F-FDG images. For radionuclide therapy, [¹⁷⁷Lu]Lu-(FAPI-04)₂ effectively retarded tumor growth and displayed good tolerance. In conclusion, the DOTA-Suc-Lys-(FAPI-04)₂ design enhanced its uptake in FAP-expressing tumors, improved its retention time at the tumor site, and produced high-contrast imaging in xenografts after radionuclide labeling. Furthermore, it showed a noticeable antitumor effect. DOTA-Suc-Lys-(FAPI-04)₂ provides a new approach for applying FAPI derivatives in tumor theranostics.

[68Ga]Ga-HBED-CC-FAPI Derivatives with Improved Radiolabeling and Specific Tumor Uptake

Fibroblast activation protein (FAP) is selectively expressed in tumors and highly important for maintaining the microenvironment in malignant tumors. Radioisotope-labeled FAP inhibitors (FAPIs) were proven to be useful for diagnosis and radionuclide therapy of cancer and are under active clinical investigations. Ga-HBED complex displays a higher in vivo stability constant (log K_GaL: 38.5), compared to that of Ga-DOTA (log K_GaL: 21.3). Such advantage in stability constant suggests that it may be useful for development of alternative FAPI imaging agents. In this study, previously reported [⁶⁸Ga]Ga-DOTA-FAPI-02 and -04 were converted to the corresponding [⁶⁸Ga]Ga-HBED-CC-FAPI-02 and -04 derivatives ([⁶⁸Ga]Ga-4, [⁶⁸Ga]Ga-5, [⁶⁸Ga]Ga-6, and [⁶⁸Ga]Ga-7). It was found that substituting the DOTA chelating group with HBED-CC led to several unique and desirable tumor-targeting properties: (1) robust, fast, and high yield labeling─readily adaptable to a kit formulation; (2) high stabilities in vitro; (3) excellent FAP binding affinities (IC₅₀ ranging between 4 and 7 nM) and improved cell uptake and retention (in HT1080 (FAP+) cells); and (4) excellent selective in vivo tumor uptake in nude mice bearing U87MG tumor. It appeared that Ga(III) chelation with HBED-CC improved the in vivo kinetics favoring higher tumor uptake and retention compared to the corresponding Ga-DOTA complex. Out of the four tested ligands the new [⁶⁸Ga]Ga-HBED-CC-FAPI dimer, [⁶⁸Ga]Ga-6, displayed the best tumor localization properties, and further studies are warranted to demonstrate that it is an alternative FAP imaging agent for cancer patients.

FAPI PET/CT in Diagnostic and Treatment Management of Colorectal Cancer: Review of Current Research Status

FAPI PET/CT is a novel imaging tool targeting fibroblast activation protein (FAP), with high tumor uptake rate and low background noise. Therefore, the appearance of FAPI PET/CT provides a good tumor-to-background ratio between tumor and non-tumor tissues, which is beneficial to staging, tumor description and detection. Colorectal cancer has the biological characteristics of high expression of FAP, which provides the foundation for targeted FAP imaging. FAPI PET/CT may have a potential role in changing the staging and re-staging of colorectal cancer, monitoring recurrence and treatment management, and improving the prognosis of patients. This review will summarize the application status of FAPI PET/CT in colorectal cancer and provide directions for further application research.

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.

Positron Emission Tomography Radiopharmaceuticals in Differentiated Thyroid Cancer

Differentiated thyroid cancer (DTC), arising from thyroid follicular epithelial cells, is the most common type of thyroid cancer. Despite the well-known utilization of radioiodine treatment in DTC, i.e., iodine-131, radioiodine imaging in DTC is typically performed with iodine-123 and iodine-131, with the current hybrid scanner performing single photon emission tomography/computed tomography (SPECT/CT). Positron emission tomography/computed tomography (PET/CT) provides superior visualization and quantification of functions at the molecular level; thus, lesion assessment can be improved compared to that of SPECT/CT. Various types of cancer, including radioiodine-refractory DTC, can be detected by 2-[¹⁸F]fluoro-2-deoxy-D-glucose ([¹⁸F]FDG), the most well-known and widely used PET radiopharmaceutical. Several other PET radiopharmaceuticals have been developed, although some are limited in availability despite their potential clinical utilizations. This article aims to summarize PET radiopharmaceuticals in DTC, focusing on molecular pathways and applications.

Radionuclide imaging and therapy directed towards the tumor microenvironment: a multi-cancer approach for personalized medicine

Targeted radionuclide theranostics is becoming more and more prominent in clinical oncology. Currently, most nuclear medicine compounds researched for cancer theranostics are directed towards targets expressed in only a small subset of cancer types, limiting clinical applicability. The identification of cancer-specific targets that are (more) universally expressed will allow more cancer patients to benefit from these personalized nuclear medicine–based interventions. A tumor is not merely a collection of cancer cells, it also comprises supporting stromal cells embedded in an altered extracellular matrix (ECM), together forming the tumor microenvironment (TME). Since the TME is less genetically unstable than cancer cells, and TME phenotypes can be shared between cancer types, it offers targets that are more universally expressed. The TME is characterized by the presence of altered processes such as hypoxia, acidity, and increased metabolism. Next to the ECM, the TME consists of cancer-associated fibroblasts (CAFs), macrophages, endothelial cells forming the neo-vasculature, immune cells, and cancer-associated adipocytes (CAAs). Radioligands directed at the altered processes, the ECM, and the cellular components of the TME have been developed and evaluated in preclinical and clinical studies for targeted radionuclide imaging and/or therapy. In this review, we provide an overview of the TME targets and their corresponding radioligands. In addition, we discuss what developments are needed to further explore the TME as a target for radionuclide theranostics, with the hopes of stimulating the development of novel TME radioligands with multi-cancer, or in some cases even pan-cancer, application.

AAZTA-Derived Chelators for the Design of Innovative Radiopharmaceuticals with Theranostic Applications

With the development of 68Ga and 177Lu radiochemistry, theranostic approaches in modern nuclear medicine enabling patient-centered personalized medicine applications have been growing in the last decade. In conjunction with the search for new relevant molecular targets, the design of innovative chelating agents to easily form stable complexes with various radiometals for theranostic applications has gained evident momentum. Initially conceived for magnetic resonance imaging applications, the chelating agent AAZTA features a mesocyclic seven-membered diazepane ring, conferring some of the properties of both acyclic and macrocyclic chelating agents. Described in the early 2000s, AAZTA and its derivatives exhibited interesting properties once complexed with metals and radiometals, combining a fast kinetic of formation with a slow kinetic of dissociation. Importantly, the extremely short coordination reaction times allowed by AAZTA derivatives were particularly suitable for short half-life radioelements (i.e., 68Ga). In view of these particular characteristics, the scope of this review is to provide a survey on the design, synthesis, and applications in the nuclear medicine/radiopharmacy field of AAZTA-derived chelators.

Nuclear Molecular Imaging of Cardiac Remodeling after Myocardial Infarction

The role of molecular imaging technologies in detecting, evaluating, and monitoring cardiovascular disease and their treatment is expanding rapidly. Gradually replacing the conventional anatomical or physiological approaches, molecular imaging strategies using biologically targeted markers provide unique insight into pathobiological processes at molecular and cellular levels and allow for cardiovascular disease evaluation and individualized therapy. This review paper will discuss currently available and developing molecular-based single-photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging strategies to evaluate post-infarction cardiac remodeling. These approaches include potential targeted methods of evaluating critical biological processes, such as inflammation, angiogenesis, and scar formation.