Fibroblast activation protein targeted therapy using [177Lu]FAPI-46 compared with [225Ac]FAPI-46 in a pancreatic cancer model

Synthesis and Preclinical Evaluation of Three Novel 68Ga-Labeled Bispecific PSMA/FAP-Targeting Tracers for Prostate Cancer Imaging

Tumor heterogeneity limits the efficacy and reliability of monospecific radiopharmaceuticals in prostate cancer diagnosis and therapy. To overcome this limitation and improve lesion detection sensitivity, we developed and evaluated three bispecific radiotracers that can target both prostate-specific membrane antigen (PSMA) and fibroblast activation protein (FAP), which are the two key proteins overexpressed in prostate cancer. Three FAP-targeting ligands with various linker lengths were synthesized through multistep organic synthesis, and then connected to the PSMA-targeting motif. IC₅₀(PSMA) and IC₅₀(FAP) values of Ga-complexed bispecific ligands, Ga-AV01017, Ga-AV01030, and Ga-AV01038 were 25.2-71.6 and 1.25-2.74 nM, respectively. The uptake values in PSMA-expressing LNCaP tumor xenografts were 4.38 ± 0.55, 5.17 ± 0.51, and 4.25 ± 0.86 %ID/g for [⁶⁸Ga]Ga-AV01017, [⁶⁸Ga]Ga-AV01030, and [⁶⁸Ga]Ga-AV01038, respectively, which were lower than the monospecific PSMA-targeting tracer [⁶⁸Ga]Ga-HTK03041 (23.1 ± 6.11 %ID/g). The uptake values in FAP-expressing HEK293T:hFAP tumor xenografts were 2.99 ± 0.37, 3.69 ± 0.81, 3.64 ± 0.83 %ID/g for [⁶⁸Ga]Ga-AV01017, [⁶⁸Ga]Ga-AV01030, and [⁶⁸Ga]Ga-AV01038, respectively, which were also lower than the monospecific FAP-targeting tracer, [⁶⁸Ga]Ga-FAPI-04 (12.5 ± 2.00 %ID/g). We observed that the bispecific tracers had prolonged blood retention, in which tracers with a longer linker tend to have a higher blood uptake and lower tumor uptake. Further investigations are needed to optimize the linker selection to generate promising bispecific PSMA/FAP-targeting tracers for prostate cancer imaging.

Recent Trends in Diagnostic Biomarkers of Tumor Microenvironment

The tumor microenvironment (TME) play critical roles in tumor survival, progression, and metastasis and can be considered potential targets for molecular imaging of cancer. The targeting agents for imaging of TME components (e.g., fibroblasts, mesenchymal stromal cells, immune cells, extracellular matrix, blood vessels) provide a promising strategy to target these biomarkers for the early diagnosis of cancers. Moreover, various cancer types have similar tumor immune microenvironment (TIME) features that targeting those biomarkers and offer clinically translatable molecular imaging of cancers. In this review, we categorize and summarize the components in TME which have been targeted for molecular imaging. Moreover, this review updated the recent progress in targeted imaging of TIME biological molecules by various modalities for the early detection of cancer.

A Dimeric FAP-Targeting Small-Molecule Radioconjugate with High and Prolonged Tumor Uptake

Imaging procedures based on small-molecule radioconjugates targeting fibroblast activation protein (FAP) have recently emerged as a powerful tool for the diagnosis of a wide variety of tumors. However, the therapeutic potential of radiolabeled FAP-targeting agents is limited by their short residence time in neoplastic lesions. In this work, we present the development and in vivo characterization of BiOncoFAP, a new dimeric FAP-binding motif with an extended tumor residence time and favorable tumor-to-organ ratio. Methods: The binding properties of BiOncoFAP and its monovalent OncoFAP analog were assayed against recombinant human FAP. Preclinical experiments with ¹⁷⁷Lu-OncoFAP-DOTAGA (¹⁷⁷Lu-OncoFAP) and ¹⁷⁷Lu-BiOncoFAP-DOTAGA (¹⁷⁷Lu-BiOncoFAP) were performed on mice bearing FAP-positive HT-1080 tumors. Results: OncoFAP and BiOncoFAP displayed comparable subnanomolar dissociation constants toward recombinant human FAP in solution, but the bivalent BiOncoFAP bound more avidly to the target immobilized on solid supports. In a comparative biodistribution study, ¹⁷⁷Lu-BiOncoFAP exhibited a more stable and prolonged tumor uptake than ¹⁷⁷Lu-OncoFAP (∼20 vs. ∼4 percentage injected dose/g, respectively, at 24 h after injection). Notably, ¹⁷⁷Lu-BiOncoFAP showed favorable tumor-to-organ ratios with low kidney uptake. Both ¹⁷⁷Lu-OncoFAP and ¹⁷⁷Lu-BiOncoFAP displayed potent antitumor efficacy when administered at therapeutic doses to tumor-bearing mice. Conclusion: ¹⁷⁷Lu-BiOncoFAP is a promising candidate for radioligand therapy of cancer, with favorable in vivo tumor-to-organ ratios, a long tumor residence time, and potent anticancer efficacy.

Performance and Prospects of [68Ga]Ga-FAPI PET/CT Scans in Lung Cancer

Fibroblast activation protein (FAP) could be a promising target for tumor imaging and therapy, as it is expressed in >90% of epithelial cancers. A high level of FAP-expression might be associated with worse prognosis in several cancer types, including lung cancer. FAPI binds this protein and allows for labelling to Gallium-68, as well as several therapeutic radiopharmaceuticals. As FAP is only expressed at insignificant levels in adult normal tissue, FAPI provides a highly specific tumor-marker for many epithelial cancers. In this review, current information on the use of [68Ga]Ga-FAPI PET/CT in lung cancer is presented. [68Ga]Ga-FAPI shows a high uptake (standardized uptake value = SUVmax) and tumor-to-background ratio (TBR) in primary lung cancer lesions, as well as in metastatic lesions of other tumor types located in the lung and in lung cancer metastases located throughout the body. Where a comparison was made to [18F]FDG PET/CT, [68Ga]Ga-FAPI showed a similar or higher SUVmax and TBR. In brain and bone metastases, [68Ga]Ga-FAPI PET/CT outperformed [18F]FDG PET/CT. In addition to this strong diagnostic performance, a possible prognostic value of [68Ga]Ga-FAPI PET/CT in lung cancer is proposed.

Imaging-guided targeted radionuclide tumor therapy: From concept to clinical translation

Since the first introduction of sodium iodide I-131 for use with thyroid patients almost 80 years ago, more than 50 radiopharmaceuticals have reached the markets for a wide range of diseases, especially cancers. The nuclear medicine paradigm also shifts from solely molecular imaging or radionuclide therapy to imaging-guided radionuclide therapy, which is deemed a vital component of precision cancer therapy and an emerging medical modality for personalized medicine. The imaging-guided radionuclide therapy highlights the systematic integration of targeted nuclear diagnostics and radionuclide therapeutics. Regarding this, nuclear imaging serves to "visualize" the lesions and guide the therapeutic strategy, followed by administration of a precise patient specific dose of radiotherapeutics for treatment according to the absorbed dose to different organs and tumors calculated by dosimetry tools, and finally repeated imaging to predict the prognosis. This strategy leads to significantly enhanced therapeutic efficacy, improved patient outcomes, and manageable adverse events. In this review, we provide an overview of imaging-guided targeted radionuclide therapy for different tumors such as advanced prostate cancer and neuroendocrine tumors, with a focus on development of new radioligands and their preclinical and clinical results, and further discuss about challenges and future perspectives.

Fibroblast activation protein-based theranostics in pancreatic cancer

Fibroblast activation protein-α (FAP) is a type II transmembrane serine protease that has specific endopeptidase activity. Given its well-established selective expression in the activated stromal fibroblasts of epithelial cancers, although not in quiescent fibroblasts, FAP has received substantial research attention as a diagnostic marker and therapeutic target. Pancreatic cancer is characterized by an abundant fibrotic or desmoplastic stroma, leading to rapid progression, therapeutic resistance, and poor clinical outcomes. Numerous studies have revealed that the abundant expression of FAP in cancer cells, circulating tumor cells, stromal cells, and cancer-associated fibroblasts (CAFs) of pancreatic adenocarcinoma is implicated in diverse cancer-related signaling pathways, contributing to cancer progression, invasion, migration, metastasis, immunosuppression, and resistance to treatment. In this article, we aim to systematically review the recent advances in research on FAP in pancreatic adenocarcinoma, including its utility as a diagnostic marker, therapeutic potential, and correlation with prognosis. We also describe the functional role of FAP-overexpressing stromal cells, particulary CAFs, in tumor immuno- and metabolic microenvironments, and summarize the mechanisms underlying the contribution of FAP-overexpressing CAFs in pancreatic cancer progression and treatment resistance. Furthermore, we discuss whether targeting FAP-overexpressing CAFs could represent a potential therapeutic strategy and describe the development of FAP-targeted probes for diagnostic imaging. Finally, we assess the emerging basic and clinical studies regarding the bench-to-bedside translation of FAP in pancreatic cancer.

Impact of radiopharmaceutical therapy (177Lu, 225Ac) microdistribution in a cancer-associated fibroblasts model

Background

The aim of this study is to elucidate the difference in absorbed dose (D_abs) patterns in radiopharmaceutical therapies between alpha emitters (²²⁵Ac) and beta emitters (¹⁷⁷Lu) when targeting cancer-associated fibroblasts (CAF) or tumor cells. Five spherical models with 3 mm diameter were created, representing spherical tumor masses that contain tumor clusters, interspersed with CAFs. The mean distance from a tumor cell to the nearest CAF (L_mean) varied throughout these models from 92 to 1030 µm. D_abs calculations were performed while selecting either CAFs or tumor cells as sources, with Convolution/Superposition with ¹⁷⁷Lu and Monte Carlo simulations (GATE) with ²²⁵Ac. Analyses were conducted with Dose Volume Histograms and efficacy ratios (ER), which represents the ratio of mean D_abs that is deposited in the target volume.

Results

²²⁵Ac is the most optimal radionuclide when CAFs are both targeted and irradiating themselves, as ERs increase from 1.5 to 3.7 when L_mean increases from 92 to 1030 µm. With ¹⁷⁷Lu, these numbers vary from 1.2 to 2.7. Conversely, when CAFs are sources and tumors are targets with ²²⁵Ac, ERs decreased from 0.8 to 0.1 when L_mean increases from 92 to 1030 µm. With ¹⁷⁷Lu, these numbers vary from 0.9 to 0.3

Conclusion

When targeting CAFs to irradiate tumors, the efficacy of using ²²⁵Ac decreases as the average size of the tumor clusters (or L_mean) increases. In such situations, ¹⁷⁷Lu will be more effective than ²²⁵Ac when targeting CAFs due to the longer beta particle range.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-022-00497-5.

Advancement and Future Perspective of FAPI PET/CT In Gynecological Malignancies

Fibroblast activation protein (FAP) is ubiquitously present in healthy tissue, and additionally upregulated by cancer associated fibroblasts (CAFs) leading to high levels of FAP. Thus, neoplastic tissue, which is containing CAFs, characterized by a high presence of FAP. Moreover, in more than 90% of all epithelial tumors this phenomenon seems to occur, including many gynecological tumors, providing the foundation for a successful application of FAP-ligands. However, FAP upregulation, can also be initiated by benign conditions such as inflammation, hormonal-influence, and wound healing. Gynecological cancers seem to represent a field of interest for the utilization of FAPI-PET/CT to potentially improve staging, restaging and therapeutic management. First highly promising investigations demand further research in order to validate these preliminary findings.

86Y-Labeled Albumin-Binding Fibroblast Activation Protein Inhibitor for Late-Time-Point Cancer Diagnosis

Fibroblast activation protein inhibitor (FAPI) is a novel quinoline-based radiopharmaceutical that has theranostic potential, yet the limited tumor retention hinders late-time diagnosis and radionuclide treatment. This study synthesized four albumin-binding FAPIs (TE-FAPI-01 to 04) and evaluated their in vitro stability, binding affinity, in vivo biodistribution, and tumor uptake with ⁶⁸Ga, ⁸⁶Y, and ¹⁷⁷Lu labeling, aiming to select the best molecule that has favorable pharmacokinetics to extend the blood circulation and tumor uptake in FAP-expressing tumors. All TE-FAPIs were stable in saline and plasma and displayed high FAP-binding affinity, with IC₅₀ values ranging from 3.96 to 34.9 nmol/L. The capabilities of TE-FAPIs to be retained in circulation were higher than that of FAPI-04, and TE-FAPI-04 displayed minimum physiological uptake in major organs compared with other molecules. TE-FAPI-03 and TE-FAPI-04 exhibited persistent tumor accumulation, with tumor radioactivity 24 h after administration of 2.84 ± 1.19%ID/g and 3.86 ± 1.15%ID/g for ¹⁷⁷Lu-TE-FAPI-03 and ¹⁷⁷Lu-TE-FAPI-04, respectively, both of which outperformed ¹⁷⁷Lu-FAPI-04 (0.34 ± 0.07%ID/g). TE-FAPI-04 was recognized as the albumin-binding FAPI with the most favorable pharmacokinetics and imaging performance. The enhanced circulation half-life and tumor uptake of TE-FAPI-04 aided the theranostics of malignant tumors and warrant further clinical investigations.

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-DATA5m.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-DATA5m.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-DATA5m.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.

Radiosynthesis and First Preclinical Evaluation of the Novel 11C-Labeled FAP Inhibitor 11C-FAPI: A Comparative Study of 11C-FAPIs and (68Ga) Ga-DOTA-FAPI-04 in a High–FAP-Expression Mouse Model

Purpose: ⁶⁸Ga-labeled fibroblast activation protein inhibitors, such as [⁶⁸Ga]Ga-DOTA-FAPI-04 and [⁶⁸Ga]Ga-DOTA-FAPI-46, have been successfully applied in positron emission tomography imaging of various tumor types. To broaden the PET tracers of different positron nuclides for imaging studies of FAP-dependent diseases, we herein report the radiosynthesis and preclinical evaluation of two ¹¹C-labeled FAP inhibitors, ¹¹C-RJ1101 and ¹¹C-RJ1102. Methods: Two phenolic hydroxyl precursors based on a quinoline amide core coupled with a 2-cyanopyrrolidine moiety were coupled with [¹¹C]CH₃I to synthesize ¹¹C-RJ1101 and ¹¹C-RJ1102. In vivo small-animal PET and biological distribution studies of ¹¹C-RJ1101 and ¹¹C-RJ1102 compared to [⁶⁸Ga]Ga-DOTA-FAPI-04 were conducted in nude mice bearing U87MG tumor xenografts at 30, 60, and 90min, respectively. Results: ¹¹C-RJ1101 and ¹¹C-RJ1102 were synthesized in over 15% radiochemical yields, with specific activities of 67 GBq/μmol and 34 GBq/μmol, respectively, at the end of synthesis and radiochemical purities greater than 99%. In U87MG tumor xenograft PET studies, the three tracers experienced higher specific uptake at the tumor site. However, because of significant differences in metabolism and clearance, [⁶⁸Ga]Ga-DOTA-FAPI-04 experienced high uptake in the kidney, whereas ¹¹C-RJ1101 and ¹¹C-RJ1102 showed high uptake in the liver and intestine. Biodistribution studies revealed significant hepatobiliary excretion of ¹¹C-RJ1101 and ¹¹C-RJ1102. 11C-RJ1102 showed higher specific tumor uptake in U87MG xenografts (1.71 ± 0.08% injected dose per Gram of tissue [ID/g]) than ¹¹C-RJ1101 (1.34 ± 0.10%ID/g) and [⁶⁸Ga]Ga-DOTA-FAPI-04 (1.29 ± 0.04%ID/g) after 30 min p. i. In orthotopic glioma models, the uptake values were 0.07 ± 0.03% ([⁶⁸Ga]Ga-DOTA-FAPI-04) and 0.16 ± 0.03% (¹¹C-RJ1102), respectively. Conclusion: ¹¹C-RJ1101 and ¹¹C-RJ1102 are interesting candidates for translation to the clinic, taking advantage of the shorter half-life and physical imaging properties of C-11.

Development of Fibroblast Activation Protein Inhibitor-Based Dimeric Radiotracers with Improved Tumor Retention and Antitumor Efficacy

Fibroblast activation protein (FAP), a fundamental component of the tumor stroma, is overexpressed in cancer-associated fibroblasts (CAFs). As a promising theranostic probe, we evaluated whether the FAP inhibitor (FAPI) dimer (DOTA-2P[FAPI]₂) is more effective than its monomeric analogs for FAP-targeted radionuclide therapy. [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ were assayed in a stability study, small-animal positron emission tomography (PET) and single-photon emission computed tomography (SPECT), biodistribution, and radionuclide therapy to comprehensively evaluate their preclinical pharmacokinetics. The pharmacokinetics of [⁶⁸Ga]Ga-DOTA-2P(FAPI)₂ and [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ were determined in FAP-positive hepatocellular carcinoma patient-derived xenografts (PDXs) and HT-1080-FAP cell-derived xenografts (CDXs). [⁶⁸Ga]Ga-DOTA-2P(FAPI)₂ and [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ were stable in phosphate-buffered saline for 4 h. The tumor retention of [⁶⁸Ga]Ga-DOTA-2P(FAPI)₂ was better than that of [⁶⁸Ga]Ga-FAPI-46 in HT-1080-FAP CDXs, while healthy organs showed low tracer uptake and fast body clearance. In single-photon emission computed tomography, [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ showed a higher uptake and longer retention for tumors in both PDXs and CDXs from 1-48 h. [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ showed the best inhibition of tumor growth in PDXs and CDXs. DOTA-2P(FAPI)₂ has increased tumor uptake and retention properties compared to FAPI-46, which significantly improves the use of FAPI-based vectors for PET imaging and radionuclide therapy. [¹⁷⁷Lu]Lu-DOTA-2P(FAPI)₂ may be safe and effective for the treatment of FAP-positive malignant tumors.

Construction of a 124I-Labeled Specific Antibody for the Noninvasive Detection of Mesothelin-Overexpressing Tumors

Mesothelin (MSLN) is a molecular biomarker of many types of solid tumors, such as mesothelioma, pancreatic cancer, and colon cancer. Owing to the significant difference in expression between cancer cells and normal cells, mesothelin has been widely used as a key target in cancer immunotherapy. In this study, we used iodine isotope (^nat/124/125I)-labeled mesothelin antibodies to noninvasively detect MSLN expression in mice with LS174T colon cancer. The ¹²⁴I-labeled MSLN antibody showed a high radiochemical purity (RCP, >99%) and specific activity (20.8-67.8 GBq/μmol) after purification and was stable in 5% HSA and PBS (>95% RCP at 8 days). Western blot analysis indicated that the LS174T cells showed a higher MSLN protein level than the HepG2 cells. The half maximal effective concentration (EC₅₀) values of the MSLN antibody and ^natI-anti-MSLN were 34.77 ± 3.72 ng/mL and 32.60 ± 2.52 ng/mL (P = 0.63), respectively. The dissociation constant of ¹²⁴I-anti-MSLN binding to MSLN protein was 16.0 nM. The radiotracer showed a significantly higher uptake in LS174T cells than in HepG2 tumor cells (1.56 ± 0.09 vs 0.81 ± 0.03, P = 0.0016) 2 days postinjection. The LS174T mouse models showed extremely low organ uptake and high tumor uptake 96 h after the injection of ¹²⁴I-anti-MSLN, and the T/M values were much higher than those of the other imaging groups (10.56 ± 1.20 for ¹²⁴I-anti-MSLN in LS174T mice vs 3.27 ± 0.20 for ¹²⁴I-anti-MSLN in HepG2 mice vs 3.53 ± 0.2 for ¹²⁴I-IgG in LS174T mice). The immunochemical histology results showed that LS174T tumors were strongly positive (+++) for MSLN, while those in the HepG2 group showed slight expression (+). The dosimetry estimation study showed that the effective dose of ¹²⁴I-anti-MSLN was 0.185 mSv/MBq, which is within the range of acceptable doses for further nuclear medicine translational research. Taken together, these results suggest that this radiotracer has the potential for detecting mesothelin-overexpressing tumors.

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.

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

Fibroblast activation protein (FAP) is a type II transmembrane protein, which is over-expressed in cancer-associated fibroblasts (CAFs). CAFs are tumor stromal cells that constitute a major component of cancer volume and are reportedly related to tumorigenesis, angiogenesis, metastasis, promotion of drug resistance and induction of tumor immunity. FAP is widely acknowledged as the signature protein of CAFs. At present, FAP inhibitors (FAPI) have achieved ideal results in tumor PET/computed tomography (CT) imaging. Theoretically, FAP-targeted drugs can inhibit tumor progression. Nonetheless, no satisfactory therapeutic effect has been observed so far, which has impeded their implementation in clinical practice. In this review, we describe the characteristics of FAP and its role in the occurrence and development of cancer. We also highlight the potential value of targeting FAP to improve current diagnostic and therapeutic approaches.

Repair of α-particle-induced DNA damage in peripheral blood mononuclear cells after internal ex vivo irradiation with 223Ra

Purpose

As α-emitters for radiopharmaceutical therapies are administered systemically by intravenous injection, blood will be irradiated by α-particles that induce clustered DNA double-strand breaks (DSBs). Here, we investigated the induction and repair of DSB damage in peripheral blood mononuclear cells (PBMCs) as a function of the absorbed dose to the blood following internal ex vivo irradiation with [²²³Ra]RaCl₂.

Methods

Blood samples of ten volunteers were irradiated by adding [²²³Ra]RaCl₂ solution with different activity concentrations resulting in absorbed doses to the blood of 3 mGy, 25 mGy, 50 mGy and 100 mGy. PBMCs were isolated, divided in three parts and either fixed directly (d-samples) or after 4 h or 24 h culture. After immunostaining, the induced γ-H2AX α-tracks were counted. The time-dependent decrease in α-track frequency was described with a model assuming a repair rate R and a fraction of non-repairable damage Q.

Results

For 25 mGy, 50 mGy and 100 mGy, the numbers of α-tracks were significantly increased compared to baseline at all time points. Compared to the corresponding d-samples, the α-track frequency decreased significantly after 4 h and after 24 h. The repair rates R were (0.24 ± 0.05) h⁻¹ for 25 mGy, (0.16 ± 0.04) h⁻¹ for 50 mGy and (0.13 ± 0.02) h⁻¹ for 100 mGy, suggesting faster repair at lower absorbed doses, while Q-values were similar.

Conclusion

The results obtained suggest that induction and repair of the DSB damage depend on the absorbed dose to the blood. Repair rates were similar to what has been observed for irradiation with low linear energy transfer.

Associations between Normal Organs and Tumor Burden in Patients Imaged with Fibroblast Activation Protein Inhibitor-Directed Positron Emission Tomography

Simple Summary

Several radiolabeled fibroblast activation protein targeted inhibitors (FAPI) have been developed for molecular imaging and therapy. A potential correlation of radiotracer uptake in normal organs and extent of tumor burden may have consequences for a theranostic approach using ligands structurally associated with [⁶⁸Ga]Ga-FAPI, as one may anticipate decreased doses to normal organs in patients with extensive tumor load. In the present proof-of-concept study investigating patients with solid tumors, we aimed to quantitatively determine the normal organ biodistribution of [⁶⁸Ga]Ga-FAPI-04, depending on the extent of tumor. Except for a trend towards significance in the myocardium, we did not observe any relevant associations between PET-based tumor burden and normal organs. Those preliminary findings may trigger future studies to determine possible implications for theranostic approaches and FAP-directed drugs, as one may expect an unchanged dose for normal organs even in patients with higher tumor load.

Abstract

(1) Background: We aimed to quantitatively investigate [⁶⁸Ga]Ga-FAPI-04 uptake in normal organs and to assess a relationship with the extent of FAPI-avid tumor burden. (2) Methods: In this single-center retrospective analysis, thirty-four patients with solid cancers underwent a total of 40 [⁶⁸Ga]Ga-FAPI-04 PET/CT scans. Mean standardized uptake values (SUV_mean) for normal organs were established by placing volumes of interest (VOIs) in the heart, liver, spleen, pancreas, kidneys, and bone marrow. Total tumor burden was determined by manual segmentation of tumor lesions with increased uptake. For tumor burden, quantitative assessment included maximum SUV (SUV_max), tumor volume (TV), and fractional tumor activity (FTA = TV × SUV_mean). Associations between uptake in normal organs and tumor burden were investigated by applying Spearman’s rank correlation coefficient. (3) Results: Median SUV_mean values were 2.15 in the pancreas (range, 1.05–9.91), 1.42 in the right (range, 0.57–3.06) and 1.41 in the left kidney (range, 0.73–2.97), 1.2 in the heart (range, 0.46–2.59), 0.86 in the spleen (range, 0.55–1.58), 0.65 in the liver (range, 0.31–2.11), and 0.57 in the bone marrow (range, 0.26–0.94). We observed a trend towards significance for uptake in the myocardium and tumor-derived SUV_max (ρ = 0.29, p = 0.07) and TV (ρ = −0.30, p = 0.06). No significant correlation was achieved for any of the other organs: SUV_max (ρ ≤ 0.1, p ≥ 0.42), TV (ρ ≤ 0.11, p ≥ 0.43), and FTA (ρ ≤ 0.14, p ≥ 0.38). In a sub-analysis exclusively investigating patients with high tumor burden, significant correlations of myocardial uptake with tumor SUV_max (ρ = 0.44; p = 0.03) and tumor-derived FTA with liver uptake (ρ = 0.47; p = 0.02) were recorded. (4) Conclusions: In this proof-of-concept study, quantification of [⁶⁸Ga]Ga-FAPI-04 PET showed no significant correlation between normal organs and tumor burden, except for a trend in the myocardium. Those preliminary findings may trigger future studies to determine possible implications for treatment with radioactive FAP-targeted drugs, as higher tumor load or uptake may not lead to decreased doses in the majority of normal organs.

[99mTc]Tc-iFAP Radioligand for SPECT/CT Imaging of the Tumor Microenvironment: Kinetics, Radiation Dosimetry, and Imaging in Patients

Tumor microenvironment fibroblasts overexpress the fibroblast activation protein (FAP). We recently reported the preclinical evaluation of [99mTc]Tc-iFAP as a new SPECT radioligand capable of detecting FAP. This research aimed to evaluate the kinetic and dosimetric profile of [99mTc]Tc-iFAP in healthy volunteers, and to assess the radioligand uptake by different solid tumors in three cancer patients. [99mTc]Tc-iFAP was obtained from lyophilized formulations prepared under GMP conditions with >98% radiochemical purity. Whole-body scans of six healthy subjects were obtained at 0.5, 2, 4, and 24 h after [99mTc]Tc-iFAP (740 MBq) administration. A 2D-planar/3D-SPECT hybrid activity quantitation method was used to fit the biokinetic models of the source organs (volume of interest: VOI) as exponential functions (A(t)VOI). The total nuclear transformations (N) that occurred in the source organs were calculated from the mathematical integration (0,∞) of A(t)VOI. The OLINDA code was used to estimate the radiation doses. Three treatment-naive patients (breast, lung, and cervical cancer) with a prior [18F]FDG PET/CT scan underwent whole-body, chest, and abdominal SPECT/CT scanning after [99mTc]Tc-iFAP (740 MBq) administration. Both imaging methods were compared visually and quantitatively. Oncological diagnoses were performed histopathologically. The results showed favorable [99mTc]Tc-iFAP biodistribution and kinetics due to rapid blood activity removal (t1/2α = 2.22 min and t1/2β = 90 min) and mainly renal clearance. The mean radiation equivalent doses were 5.2 ± 0.8 mSv for the kidney and 1.7 ± 0.3 mSv for the liver after administration of 740 MBq. The effective dose was 2.3 ± 0.4 mSv/740 MBq. [99mTc]Tc-iFAP demonstrated high and reliable uptake in the primary tumor lesions and lymph node metastases in patients with breast, cervical, and lung cancer, which correlated with that detected by [18F]FDG PET/CT. The tumor microenvironment molecular imaging from cancer patients obtained in this research validates the performance of additional clinical studies to determine the utility of [99mTc]Tc-iFAP in the diagnosis and prognosis of different types of solid tumors.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

Background

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biyearly highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

Results

This commentary of highlights has resulted in 23 different topics selected by each member of the Editorial Board addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals and also a contribution in relation to MRI-agents is included.

Conclusion

Trends in (radio)chemistry and radiopharmacy are highlighted demonstrating the progress in the research field being the scope of EJNMMI Radiopharmacy and Chemistry.

The Clinical Value of Breast Specific Gamma Imaging and Positron Imaging: An Update

In the management of patients with breast cancer (BC), a mammography contributed to screen an early-stage patient, to plan a therapy strategy, to evaluate a therapy outcome, to detect a recurrence, and to reduce a mortality. Currently, various imaging modalities, such as CT, MR, Ultrasound (US), SPECT/CT, PET/CT, PET/MR have been utilized for the management of BC patients. In order to overcome a limited spatial resolution and sensitivity of whole-body systems in nuclear medicine imaging, dedicated breast imaging modalities were developed. One is a gamma imaging system with single/dual head scintillation detectors or semiconductor detectors associated with light compression device for breast parenchyma. Radiopharmaceutical for the gamma imaging is ^99mTc-sestamibi. Another is a positron imaging system with opposite-type panel detectors and ring-shaped type detectors. Radiopharmaceutical for positron imaging is ¹⁸F-fluorodeoxyglucose. The breast-specific gamma and positron imaging systems were utilized mainly to detect small lesions less than 1 cm in diameter especially in patients with dense breast, to evaluate an effect of preoperative neo-adjuvant therapy, to plan surgical procedures (conservative-surgery vs mastectomy), and to detect a recurrence. By combining higher sensitivity and spatial resolution scanners with new radiopharmaceuticals, an information on molecular-level pathology of BC is increasingly available in an individual patient. This article reviewed clinical impact and future perspective of this field.

18F- or 177Lu-labeled bivalent ligand of fibroblast activation protein with high tumor uptake and retention

PURPOSE

Fibroblast activation protein (FAP) has become a promising cancer-related target for diagnosis and therapy. The aim of this study was to develop a bivalent FAP ligand for both diagnostic PET imaging and endoradiotherapy.

METHODS

We synthesized a bivalent FAP ligand (ND-bisFAP) and labeled it with ¹⁸F or ¹⁷⁷Lu. FAP-positive A549-FAP cells were used to study competitive binding to FAP, cellular internalization, and efflux properties in vitro. Micro-PET imaging with [¹⁸F]AlF-ND-bisFAPI was conducted in mice bearing A549-FAP or U87MG tumors. Biodistribution and therapeutic efficacy of [¹⁷⁷Lu]Lu-ND-bisFAPI were conducted in mice bearing A549-FAP tumors.

RESULTS

The FAP binding affinity of ND-bisFAPI is 0.25 ± 0.05 nM, eightfold higher in potency than the monomeric DOTA-FAPI-04 (IC₅₀ = 2.0 ± 0.18 nM). In A549-FAP cells, ND-bisFAPI showed specific uptake, a high internalized fraction, and slow cellular efflux. Compared to the monomeric [¹⁸F]AlF-FAPI-42, micro-PET imaging with [¹⁸F]AlF-ND-bisFAPI showed higher specific tumor uptake and retention for at least 6 h. Biodistribution studies showed that [¹⁷⁷Lu]Lu-ND-bisFAPI had higher tumor uptake than [¹⁷⁷Lu]Lu-FAPI-04 at the 24, 72, 120, and 168 h time points (all P < 0.01). [¹⁷⁷Lu]Lu-ND-bisFAPI delivered fourfold higher radiation than [¹⁷⁷Lu]Lu-FAPI-04 to A549-FAP tumors. For the endoradiotherapy study, 37 MBq of [¹⁷⁷Lu]Lu-ND-bisFAPI significantly reduced tumor growth compared to the same dose of [¹⁷⁷Lu]Lu-FAPI-04. Half of the dose of [¹⁷⁷Lu]Lu-ND-bisFAPI (18.5 MBq) has comparable median survival as 37 MBq of [¹⁷⁷Lu]Lu-FAPI-04 (37 vs 36 days).

CONCLUSION

The novel bivalent FAP ligand was developed as a theranostic radiopharmaceutical and showed promising properties including higher tumor uptake and retention compared to the established radioligands [¹⁸F]AlF-FAPI-42 and [¹⁷⁷Lu]Lu-FAPI-04. Preliminary experiments with ¹⁸F- or ¹⁷⁷Lu-labeled ND-bisFAPI showed promising imaging properties and favorable anti-tumor responses.

Fibroblast activation protein-based theranostics in cancer research: A state-of-the-art review

In recent years, quinoline-based fibroblast activation protein (FAP) inhibitors (FAPI) have shown promising results in the diagnosis of cancer and several other diseases, making them the hotspot of much productive research. This review summarizes the literature for the state-of-the-art FAPI-PET imaging for cancer diagnosis compared with fluorodeoxyglucose (FDG)-PET. We also summarize the use of FAPI-PET for therapeutic regimen improvement and fibroblast activation protein (FAP)-targeted molecule modification strategies, as well as preliminary clinical studies regarding FAP-targeted radionuclide therapy. Our qualitative summary of the literature to date can inform future research directions, medical guidelines, and optimal clinical decision-making.

Evans blue-modified radiolabeled fibroblast activation protein inhibitor as long-acting cancer therapeutics

Rationale: Fibroblast activation protein (FAP) targeted molecular imaging radiotracers have shown promising preclinical and clinical results in tumor diagnosis. However, rapid clearance and inadequate tumor retention of these molecules have hindered them for further clinical translation in cancer therapy. In this study, we aimed to develop a series of albumin binder-truncated Evans blue (EB) modified FAP targeted radiotracers, and optimize the pharmacokinetic (PK) characteristics to overcome the existing limitations in order to apply in the radionuclide therapy of cancer.

Methods: A series of compounds with the general structure of EB-FAPI-Bn were synthesized based on a FAP inhibitor (FAPI) variant (FAPI-02) and radiolabeled with ¹⁷⁷LuCl₃. To verify the binding affinity and FAP targeting specificity of these tracers in vitro, U87MG cell uptake and competition assays were performed. Preclinical PK was evaluated in U87MG tumor-bearing mice using SPECT imaging and biodistribution studies. The lead compound EB-FAPI-B1 was selected and cancer therapeutic efficacy of ¹⁷⁷Lu-EB-FAPI-B1 was assessed in U87MG tumor-bearing mice.

Results:¹⁷⁷Lu-EB-FAPI-B1, B2, B3, B4 were stable in PBS (pH 7.4) and saline for at least 24 h. EB-FAPI-B1 showed high binding affinity (IC₅₀ = 16.5 nM) to FAP in vitro, which was comparable with that of FAPI-02 (IC₅₀ = 10.9 nM). SPECT imaging and biodistribution studies of ¹⁷⁷Lu-EB-FAPI-B1, B2, B3, B4 have proved their prominently improved tumor accumulation and retention at 96 h post-injection, especially for ¹⁷⁷Lu-EB-FAPI-B1, high tumor uptake and low background signal make it the optimal compound. Compared to the saline group, noteworthy tumor growth inhibitions of ¹⁷⁷Lu-EB-FAPI-B1 have been observed after administration of different dosages.

Conclusion: In this study, several EB modified FAPI-02 related radiopharmaceuticals have been synthesized successfully and evaluated. High binding affinity and FAP targeting specificity were identified in vitro and in vivo. Remarkably enhanced tumor uptake and retention of EB-FAPI-B1 were found over the unmodified FAPI-02. ¹⁷⁷Lu-EB-FAPI-B1 showed remarkable tumor growth suppression in U87MG tumor model with negligible side effects, indicating that ¹⁷⁷Lu-EB-FAPI-B1 is promising for clinical application and transformation.

Design, Synthesis and Preclinical Assessment of 99mTc-iFAP for In Vivo Fibroblast Activation Protein (FAP) Imaging

Fibroblast activation protein (FAP) is expressed in the microenvironment of most human epithelial tumors. ⁶⁸Ga-labeled FAP inhibitors based on the cyanopyrrolidine structure (FAPI) are currently used for the detection of the tumor microenvironment by PET imaging. This research aimed to design, synthesize and preclinically evaluate a new FAP inhibitor radiopharmaceutical based on the ^99mTc-((R)-1-((6-hydrazinylnicotinoyl)-D-alanyl) pyrrolidin-2-yl) boronic acid (^99mTc-iFAP) structure for SPECT imaging. Molecular docking for affinity calculations was performed using the AutoDock software. The chemical synthesis was based on a series of coupling reactions of 6-hidrazinylnicotinic acid (HYNIC) and D-alanine to a boronic acid derivative. The iFAP was prepared as a lyophilized formulation based on EDDA/SnCl₂ for labeling with ^99mTc. The radiochemical purity (R.P.) was verified via ITLC-SG and reversed-phase radio-HPLC. The stability in human serum was evaluated by size-exclusion HPLC. In vitro cell uptake was assessed using N30 stromal endometrial cells (FAP positive) and human fibroblasts (FAP negative). Biodistribution and tumor uptake were determined in Hep-G2 tumor-bearing nude mice, from which images were acquired using a micro-SPECT/CT. The iFAP ligand (Ki = 0.536 nm, AutoDock affinity), characterized by UV-Vis, FT-IR, ¹H–NMR and UPLC-mass spectroscopies, was synthesized with a chemical purity of 92%. The ^99mTc-iFAP was obtained with a R.P. >98%. In vitro and in vivo studies indicated high radiotracer stability in human serum (>95% at 24 h), specific recognition for FAP, high tumor uptake (7.05 ± 1.13% ID/g at 30 min) and fast kidney elimination. The results found in this research justify additional dosimetric and clinical studies to establish the sensitivity and specificity of the ^99mTc-iFAP.

Dimeric FAPI with potential for tumor theranostics

Radionuclide-labeled fibroblast activation protein inhibitors (FAPIs) are popular nuclear imaging probes in recent years. It's of great significance for tumor diagnosis and has great potential in tumor treatment. However, optimization of the probes is needed to further increase tumor uptake and prolong tumor retention for improved treatment efficacy and fewer side effects. In this issue of AJNMMI, Moon et al. reported two squaramide coupled FAPI conjugates (DOTA.(SA.FAPi)2 and DOTAGA.(SA.FAPi)2) and labeled them with 68Ga. The resulted tracers showed increased tumor accumulation and persistent retention, which led to an advance in PET imaging. The use of dimeric structures provides a feasible strategy to develop radiotherapeutic analogs of FAP inhibitors.

Overview and Future Perspectives on Tumor-Targeted Positron Emission Tomography and Fluorescence Imaging of Pancreatic Cancer in the Era of Neoadjuvant Therapy

BACKGROUND

Despite recent advances in the multimodal treatment of pancreatic ductal adenocarcinoma (PDAC), overall survival remains poor with a 5-year cumulative survival of approximately 10%. Neoadjuvant (chemo- and/or radio-) therapy is increasingly incorporated in treatment strategies for patients with (borderline) resectable and locally advanced disease. Neoadjuvant therapy aims to improve radical resection rates by reducing tumor mass and (partial) encasement of important vascular structures, as well as eradicating occult micrometastases. Results from recent multicenter clinical trials evaluating this approach demonstrate prolonged survival and increased complete surgical resection rates (R0). Currently, tumor response to neoadjuvant therapy is monitored using computed tomography (CT) following the RECIST 1.1 criteria. Accurate assessment of neoadjuvant treatment response and tumor resectability is considered a major challenge, as current conventional imaging modalities provide limited accuracy and specificity for discrimination between necrosis, fibrosis, and remaining vital tumor tissue. As a consequence, resections with tumor-positive margins and subsequent early locoregional tumor recurrences are observed in a substantial number of patients following surgical resection with curative intent. Of these patients, up to 80% are diagnosed with recurrent disease after a median disease-free interval of merely 8 months. These numbers underline the urgent need to improve imaging modalities for more accurate assessment of therapy response and subsequent re-staging of disease, thereby aiming to optimize individual patient's treatment strategy. In cases of curative intent resection, additional intra-operative real-time guidance could aid surgeons during complex procedures and potentially reduce the rate of incomplete resections and early (locoregional) tumor recurrences. In recent years intraoperative imaging in cancer has made a shift towards tumor-specific molecular targeting. Several important molecular targets have been identified that show overexpression in PDAC, for example: CA19.9, CEA, EGFR, VEGFR/VEGF-A, uPA/uPAR, and various integrins. Tumor-targeted PET/CT combined with intraoperative fluorescence imaging, could provide valuable information for tumor detection and staging, therapy response evaluation with re-staging of disease and intraoperative guidance during surgical resection of PDAC.

METHODS

A literature search in the PubMed database and (inter)national trial registers was conducted, focusing on studies published over the last 15 years. Data and information of eligible articles regarding PET/CT as well as fluorescence imaging in PDAC were reviewed. Areas covered: This review covers the current strategies, obstacles, challenges, and developments in targeted tumor imaging, focusing on the feasibility and value of PET/CT and fluorescence imaging for integration in the work-up and treatment of PDAC. An overview is given of identified targets and their characteristics, as well as the available literature of conducted and ongoing clinical and preclinical trials evaluating PDAC-targeted nuclear and fluorescent tracers.

Evaluation of Aminopolycarboxylate Chelators for Whole-Body Clearance of Free 225Ac: A Feasibility Study to Reduce Unexpected Radiation Exposure during Targeted Alpha Therapy

Actinium-225 (²²⁵Ac) is a promising radionuclide used in targeted alpha therapy (TAT). Although ²²⁵Ac labeling of bifunctional chelating ligands is effective, previous in vivo studies reported that free ²²⁵Ac can be released from the drugs and that such free ²²⁵Ac is predominantly accumulated in the liver and could cause unexpected toxicity. To accelerate the clinical development of ²²⁵Ac TAT with a variety of drugs, preparing methods to deal with any unexpected toxicity would be valuable. The aim of this study was to evaluate the feasibility of various chelators for reducing and excreting free ²²⁵Ac and compare their chemical structures. Nine candidate chelators (D-penicillamine, dimercaprol, Ca-DTPA, Ca-EDTA, CyDTA, GEDTA TTHA, Ca-TTHA, and DO3A) were evaluated in vitro and in vivo. The biodistribution and dosimetry of free ²²⁵Ac were examined in mice before an in vivo chelating study. The liver exhibited pronounced ²²⁵Ac uptake, with an estimated human absorbed dose of 4.76 Sv_RBE5/MBq. Aminopolycarboxylate chelators with five and six carboxylic groups, Ca-DTPA and Ca-TTHA, significantly reduced ²²⁵Ac retention in the liver (22% and 30%, respectively). Significant ²²⁵Ac reductions were observed in the heart and remainder of the body with both Ca-DTPA and Ca-TTHA, and in the lung, kidney, and spleen with Ca-TTHA. In vitro interaction analysis supported the in vivo reduction ability of Ca-DTPA and Ca-TTHA. In conclusion, aminopolycarboxylate chelators with five and six carboxylic groups, Ca-DTPA and Ca-TTHA, were effective for whole-body clearance of free ²²⁵Ac. This feasibility study provides useful information for reducing undesirable radiation exposure from free ²²⁵Ac.