ImmunoPET: Concept, Design, and Applications

A cutting-edge 68Ga-labeled bicyclic peptide PET molecular probe for noninvasive assessment of Nectin4 expression

The diagnosis and treatment of triple negative breast cancer (TNBC) are huge challenges due to the lack of identifiable molecular targets. The high expression of Nectin4 in a variety of tumors, including TNBC, is associated with the occurrence, invasion, progression and poor prognosis of tumors. Therefore, Nectin4 is an emerging biomarker for the diagnosis and treatment of TNBC. A PET imaging method to non-invasively quantify Nectin4 expression levels may aid in TNBC diagnosis and classification. In this study, a novel bicyclic peptide molecular probe [68Ga]Ga-DN68 was used to evaluate the expression of Nectin4 in tumors. The radiolabeling rate of [68Ga]Ga-DN68 was over 97 %, while maintaining more than 99 % radiochemical purity. In vitro experiments showed that [68Ga]Ga-DN68 could effectively target Nectin4 in tumor cells, and the cellular uptake of MC38-Nectin4 cells (Nectin4+) was significantly higher than that of MC38 cells (Nectin4-). Biodistribution and PET imaging studies consistently showed that [68Ga]Ga-DN68 was specifically accumulated in MC38-Nectin4 and MDA-MB-468 tumors, which was significantly higher than that of MC38. When co-injected with cold DN68, the specific accumulation could block the tumor uptake of MDA-MB-468. Notably, the signal-to-noise ratio at the tumor site gradually increased over time, reaching a peak at 1 h. These results strongly suggest that [68Ga]Ga-DN68 has broad application prospects as a PET tracer in TNBC imaging.

Targeted protein degradation combined with PET imaging reveals the role of host PD-L1 in determining anti-PD-1 therapy efficacy

PURPOSE

Immunohistochemical staining of programmed death-ligand 1 (PD-L1) in tumor biopsies acquired through invasive procedures is routinely employed in clinical practice to identify patients who are most likely to benefit from anti-programmed cell death protein 1 (PD-1) therapy. Nevertheless, PD-L1 expression is observed in various cellular subsets within tumors and their microenvironments, including tumor cells, dendritic cells, and macrophages. The impact of PD-L1 expression across these different cell types on the responsiveness to anti-PD-1 treatment is yet to be fully understood.

METHODS

We synthesized polymer-based lysosome-targeting chimeras (LYTACs) that incorporate both PD-L1-targeting motifs and liver cell-specific asialoglycoprotein receptor (ASGPR) recognition elements. Small-animal positron emission tomography (PET) imaging of PD-L1 expression was also conducted using a PD-L1-specific radiotracer 89Zr-αPD-L1/Fab.

RESULTS

The PD-L1 LYTAC platform was capable of specifically degrading PD-L1 expressed on liver cancer cells through the lysosomal degradation pathway via ASGPR without impacting the PD-L1 expression on host cells. When coupled with whole-body PD-L1 PET imaging, our studies revealed that host cell PD-L1, rather than tumor cell PD-L1, is pivotal in the antitumor response to anti-PD-1 therapy in a mouse model of liver cancer.

CONCLUSION

The LYTAC strategy, enhanced by PET imaging, has the potential to surmount the limitations of knockout mouse models and to provide a versatile approach for the selective degradation of target proteins in vivo. This could significantly aid in the investigation of the roles and mechanisms of protein functions associated with specific cell subsets in living subjects.

Molecular imaging supports the development of multispecific cancer antibodies

Multispecific antibodies are engineered antibody derivatives that can bind to two or more distinct epitopes or antigens. Unlike mixtures of monospecific antibodies, the binding properties of multispecific antibodies enable two specific molecules to be physically linked, a characteristic with important applications in cancer therapy. The field of multispecific antibodies is highly dynamic and expanding rapidly; to date, 15 multispecific antibodies have been approved for clinical use, of which 11 were approved for oncological indications, and more than 100 new antibodies are currently in clinical development. Nevertheless, substantial challenges limit the applications of multispecific antibodies in cancer therapy, particularly inefficient targeting of solid tumours and substantial adverse effects. Both PET and single photon emission CT imaging can reveal the biodistribution and complex pharmacology of radiolabelled multispecific antibodies. This Review summarizes the insights obtained from preclinical and clinical molecular imaging studies of multispecific antibodies, focusing on their structural properties, such as molecular weight, shape, target specificity, affinity and avidity. The opportunities associated with use of molecular imaging studies to support the clinical development of multispecific antibody therapies are also highlighted.

Spatiotemporal metabolomic approaches to the cancer-immunity panorama: a methodological perspective

Metabolic reprogramming drives the development of an immunosuppressive tumor microenvironment (TME) through various pathways, contributing to cancer progression and reducing the effectiveness of anticancer immunotherapy. However, our understanding of the metabolic landscape within the tumor-immune context has been limited by conventional metabolic measurements, which have not provided comprehensive insights into the spatiotemporal heterogeneity of metabolism within TME. The emergence of single-cell, spatial, and in vivo metabolomic technologies has now enabled detailed and unbiased analysis, revealing unprecedented spatiotemporal heterogeneity that is particularly valuable in the field of cancer immunology. This review summarizes the methodologies of metabolomics and metabolic regulomics that can be applied to the study of cancer-immunity across single-cell, spatial, and in vivo dimensions, and systematically assesses their benefits and limitations.

Scission-Enhanced Molecular Imaging (SEMI)

Positron emission tomography (PET) imaging methods have advanced our understanding of human biology, while targeted radiotherapeutic drug treatments are now routinely used clinically. The field is expected to grow considerably based on an expanding repertoire of available affinity ligands, radionuclides, conjugation chemistries, and their FDA approvals. With this increasing use, strategies for dose reduction have become of high interest to protect patients from unnecessary and off-target toxicity. Here, we describe a simple and powerful method, scission-enhanced molecular imaging (SEMI). The technique allows for rapid corporeal elimination of radionuclides once imaging or theranostic treatment is completed and relies on "click-to-release" bioorthogonal linkers.

ImmunoPET imaging of EpCAM in solid tumours with nanobody tracers: a preclinical study

PURPOSE

Epithelial cell adhesion molecule (EpCAM) is a potential therapeutic target and anchoring molecule for circulating and disseminated tumour cells (CTC/DTC) in liquid biopsy. In this study, we aimed to construct EpCAM-specific immuno-positron emission tomography (immunoPET) imaging probes and assess the diagnostic abilities in preclinical cancer models.

METHODS

By engineering six single-domain antibodies (e.g., EPCD1 - 6) targeting EpCAM of different binding properties and labelling with 68Ga (T1/2 = 1.1 h) and 18F (T1/2 = 110 min), we developed a series of EpCAM-targeted immunoPET imaging probes. The probes' pharmacokinetics and diagnostic accuracies were investigated in cell-derived human colorectal (LS174T) and esophageal cancer (OE19) tumour models.

RESULTS

Based on in vitro binding affinities and in vivo pharmacokinetics of the first three tracers ([68Ga]Ga-NOTA-EPCD1, [68Ga]Ga-NOTA-EPCD2, and [68Ga]Ga-NOTA-EPCD3), we selected [68Ga]Ga-NOTA-EPCD3 for tumour imaging which showed an average tumour uptake of 2.06 ± 0.124%ID/g (n = 3) in LS174T cell-derived tumour model. Development and characterisation of [18F]AIF-RESCA-EPCD3 showed comparable tumour uptake of 1.73 ± 0.0471%ID/g (n = 3) in the same tumour model. Further validation of [68Ga]Ga-NOTA-EPCD3 in OE19 cell-derived tumour model showed an average tumour uptake of 4.27 ± 1.16%ID/g and liver uptake of 13.5 ± 1.30%ID/g (n = 3). Near-infrared fluorescence imaging with Cy7-EPCD3 confirmed the in vivo pharmacokinetics and relatively high liver accumulation. We further synthesized another three 18F-labeled nanobody tracers ([18F]AIF-RESCA-EPCD4, [18F]AIF-RESCA-EPCD5, and [18F]AIF-RESCA-EPCD6) and found that [18F]AIF-RESCA-EPCD6 had the best pharmacokinetics with low background. [18F]AIF-RESCA-EPCD6 showed explicit uptake in the subcutaneously inoculated OE19 tumour model with an average uptake of 4.70 ± 0.26%ID/g (n = 3). In comparison, the corresponding tumour uptake (0.17 ± 0.25%ID/g, n = 3) in the EPCD6 blocking group was substantially lower (P < 0.001), indicating the targeting specificity of the tracer.

CONCLUSIONS

We developed a series of 68Ga/18F-labeled nanobody tracers targeting human EpCAM. ImmunoPET imaging with [18F]AIF-RESCA-EPCD6 may facilitate better use of EpCAM-targeted therapeutics by noninvasively displaying the target's expression dynamics.

Trends in cancer imaging

Molecular imaging of cancer is a collaborative endeavor, uniting scientists and physicians from diverse fields. Such collaboration is actively developing and translating cutting-edge molecular imaging approaches to enhance the diagnostic landscape of human malignancies. The advent of positron emission tomography (PET) and PET imaging tracers has realized non-invasive target annotation and tumor characterization at the molecular level. In surgical procedures, novel imaging techniques, such as fluorescence or Cherenkov luminescence, help identify tumors and enhance surgical precision. Simultaneously, progress in imaging equipment, innovative algorithms, and artificial intelligence has opened avenues for next-generation cancer screening and imaging, augmenting the efficiency and accuracy of cancer diagnosis. In this review, we provide a panorama of molecular cancer imaging and ongoing developments in the field.

Target engagement of an anti-MT1-MMP antibody for triple-negative breast cancer PET imaging and beta therapy

PURPOSE

Triple-negative breast cancer (TNBC) is a highly aggressive subtype of breast cancer that lacks effective diagnostic and therapeutic options. Membrane type 1 matrix metalloproteinase (MT1-MMP) is an attractive biomarker for improving patient selection. This study aimed to develop a theranostic tool using a highly tumour-selective anti-MT1-MMP antibody (LEM2/15) radiolabelled with 89Zr for PET and 177Lu for therapy in a TNBC murine model.

METHODS

The LEM2/15 antibody and IgG isotype control were radiolabelled with 89Zr. PET imaging was performed in a TNBC orthotopic mouse model at 1, 2, 4, and 7 days after administration. Tissue biodistribution and pharmacokinetic parameters were analysed and Patlak linearisation was used to calculate the influx rate of irreversible uptake. The TNBC mice were treated with [177Lu]Lu-DOTA-LEM2/15 (single- or 3-dose regimen) or saline. Efficacy of [177Lu]Lu-DOTA-LEM2/15 was evaluated as tumour growth and DNA damage (γH2AX) in MDA 231-BrM2-831 tumours.

RESULTS

At 7 days post-injection, PET uptake in tumour xenografts revealed a 1.6-fold and 2.4-fold higher tumour-to-blood ratio for [89Zr]Zr-Df-LEM2/15 in the non-blocked group compared to the blocked and IgG isotype control groups, respectively. Specific uptake of LEM2/15 in TBNC tumours mediated by MT1-MMP-binding was demonstrated by the Patlak linearisation method, providing insights into the potential efficacy of LEM2/15-based treatments. A similar uptake was found for [89Zr]Zr-Df-LEM2/15 and [177Lu]Lu-DOTA-LEM2/15 in tumours 7 days post-injection (6.80 ± 1.31 vs. 5.61 ± 0.66 %ID/g). Tumour doubling time was longer in the [177Lu]Lu-DOTA-LEM2/15 3-dose regimen treated group compared to the control (50 vs. 17 days, respectively). The percentage of cells with γH2AX-foci was higher in tumours treated with [177Lu]Lu-DOTA-LEM2/15 3-dose regimen compared to tumours non-treated or treated with [177Lu]Lu-DOTA-LEM2/15 single-dose (12 % vs. 4-5 %).

CONCLUSIONS

The results showed that the 89Zr/177Lu-labelled anti-MT1-MMP mAb (LEM2/15) pair facilitated immune-PET imaging and reduced tumour growth in a preclinical TNBC xenograft model.

18F-FDG-PET/MR imaging to monitor disease activity in large vessel vasculitis

Disease-monitoring in large vessel vasculitis (LVV) is challenging. Simultaneous 18F-fluorodeoxyglucose positron emission tomography with magnetic resonance imaging (PET/MRI) provides functional assessment of vascular inflammation alongside high-definition structural imaging with a relatively low burden of radiation exposure. Here, we investigate the ability of PET/MRI to monitor LVV disease activity longitudinally in a prospective cohort of patients with active LVV. We demonstrate that both the PET and MRI components of the scan can distinguish active from inactive disease using established quantification methods. Using logistic-regression modelling of PET/MRI metrics, we devise a novel PET/MRI-specific Vasculitis Activity using MR PET (VAMP) score which is able to distinguish active from inactive disease with more accuracy than established methods and detects changes in disease activity longitudinally. These findings are evaluated in an independent validation cohort. Finally, PET/MRI improves clinicians' assessment of LVV disease activity and confidence in disease management, as assessed via clinician survey. In summary, PET/MRI may be useful in tracking disease activity and assessing treatment-response in LVV. Based on our findings, larger, prospective studies assessing PET/MRI in LVV are now warranted.

Radioimmunotheragnosis in Cancer Research

The combination of immunoPET-where an antibody (Ab) is labeled with an isotope for PET imaging-and radioimmunotherapy (RIT), using the same antibody with a therapeutic isotope, offers significant advantages in cancer management. ImmunoPET allows non-invasive imaging of antigen expression, which aids in patient selection for subsequent radioimmunotherapy. It also facilitates the assessment of tumor response to therapy, allowing for treatment adjustments if necessary. In addition, immunoPET provides critical pharmacokinetic data, including antibody biodistribution and clearance rates, which are essential for dosimetry calculations and treatment protocol optimization. There are still challenges to overcome. Identifying appropriate target antigens that are selectively expressed on cancer cells while minimally expressed on normal tissues remains a major hurdle to reduce off-target toxicity. In addition, it is critical to optimize the pharmacokinetics of radiolabeled antibodies to maximize tumor uptake and minimize normal tissue uptake, particularly in vital organs such as the liver and kidney. This approach offers the potential for targeted and personalized cancer therapy with reduced systemic toxicity by exploiting the specificity of monoclonal antibodies and the cytotoxic effects of radiation. However, further research is needed to address remaining challenges and to optimize these technologies for clinical use.

Construction of [89Zr]Zr-Labeled HuL13 for ImmunoPET Imaging of LAG-3 Checkpoint Expression on Tumor-Infiltrating T Cells

Lymphocyte activation gene 3 (LAG-3) has attracted much attention as a potentially valuable immune checkpoint. Individual identification of LAG-3 expression at screening and during treatment could improve the successful implementation of anti-LAG-3 therapies. HuL13 is a human IgG1 monoclonal antibody that binds to the LAG-3 receptor in T cells. Here, we used [89Zr]Zr-labeled HuL13 to delineate LAG-3+ T-cell infiltration into tumors via positron emission tomography (PET) imaging. A549/LAG-3 cells, which stably express LAG-3, were generated by infection with lentivirus. The uptake of [89Zr]Zr-DFO-HuL13 in A549/LAG-3 cells was greater than that in the negative control (A549/NC) cells at each time point. The equilibrium dissociation constant (Kd) of [89Zr]Zr-DFO-HuL13 for the LAG-3 receptor was 8.22 nM. PET imaging revealed significant uptake in the tumor areas of A549/LAG-3 tumor-bearing mice from 24 h after injection (SUVmax = 2.43 ± 0.06 at 24 h). As a proof of concept, PET imaging of the [89Zr]Zr-DFO-HuL13 tracer was further investigated in an MC38 tumor-bearing humanized LAG-3 mouse model. PET imaging revealed that the [89Zr]Zr-DFO-HuL13 tracer specifically targets human LAG-3 expressed on tumor-infiltrating lymphocytes (TILs). In addition to the tumors, the spleen was also noticeably visible. Tumor uptake of the [89Zr]Zr-DFO-HuL13 tracer was lower than its uptake in the spleen, but high uptake in the spleen could be reduced by coinjection of unlabeled antibodies. Coinjection of unlabeled antibodies increases tracer activity in the blood pool, thereby improving tumor uptake. Dosimetry evaluation of the healthy mouse models revealed that the highest absorbed radiation dose was in the spleen, followed by the liver and heart wall. In summary, these studies demonstrate the feasibility of using the [89Zr]Zr-DFO-HuL13 tracer for the detection of LAG-3 expression on TILs. Further clinical evaluation of the [89Zr]Zr-DFO-HuL13 tracer may be of significant help in the stratification and management of patients suitable for anti-LAG-3 therapy.

Engineered Antibodies as Cancer Radiotheranostics

Radiotheranostics is a rapidly growing approach in personalized medicine, merging diagnostic imaging and targeted radiotherapy to allow for the precise detection and treatment of diseases, notably cancer. Radiolabeled antibodies have become indispensable tools in the field of cancer theranostics due to their high specificity and affinity for cancer-associated antigens, which allows for accurate targeting with minimal impact on surrounding healthy tissues, enhancing therapeutic efficacy while reducing side effects, immune-modulating ability, and versatility and flexibility in engineering and conjugation. However, there are inherent limitations in using antibodies as a platform for radiopharmaceuticals due to their natural activities within the immune system, large size preventing effective tumor penetration, and relatively long half-life with concerns for prolonged radioactivity exposure. Antibody engineering can solve these challenges while preserving the many advantages of the immunoglobulin framework. In this review, the goal is to give a general overview of antibody engineering and design for tumor radiotheranostics. Particularly, the four ways that antibody engineering is applied to enhance radioimmunoconjugates: pharmacokinetics optimization, site-specific bioconjugation, modulation of Fc interactions, and bispecific construct creation are discussed. The radionuclide choices for designed antibody radionuclide conjugates and conjugation techniques and future directions for antibody radionuclide conjugate innovation and advancement are also discussed.

ImmunoPET imaging of LAG-3 expression in tumor microenvironment with 68Ga-labelled cyclic peptides tracers: from bench to bedside

BACKGROUND

Lymphocyte activation gene 3 (LAG-3) has been considered as the next generation of immune checkpoint and a promising prognostic biomarker of immunotherapy. As with programmed cell death protein-1/programmed death-ligand 1 and cytotoxic T-lymphocyte antigen-4 inhibitors, positron emission tomography (PET) imaging strategies could benefit the development of clinical decision-making of LAG-3-related therapy. In this study, we developed and validated 68Ga-labeled cyclic peptides tracers for PET imaging of LAG-3 expression in bench-to-bedside studies.

METHODS

A series of LAG-3-targeted cyclic peptides were modified and radiolabeled with 68GaCl3 and evaluated their affinity and specificity, biodistribution, pharmacokinetics, and radiation dosimetry in vitro and in vivo. Furthermore, hu-PBL-SCID (PBL) mice models were constructed to validate the capacity of [68Ga]Ga-CC09-1 for mapping of LAG-3+ lymphocytes infiltrates using longitudinal PET imaging. Lastly, [68Ga]Ga-CC09-1 was translated into the first-in-human studies to assess its safety, biodistribution and potential for imaging of LAG-3 expression.

RESULTS

A series of cyclic peptides targeting LAG-3 were employed as lead compounds to design and develop 68Ga-labeled PET tracers. In vitro binding assays showed higher affinity and specificity of [68Ga]Ga-CC09-1 in Chinese hamster ovary-human LAG-3 cells and peripheral blood mononuclear cells. In vivo PET imaging demonstrated better imaging capacity of [68Ga]Ga-CC09-1 with a higher tumor uptake of 1.35±0.33 per cent injected dose per gram and tumor-to-muscle ratio of 17.18±3.20 at 60 min post-injection. Furthermore, [68Ga]Ga-CC09-1 could detect the LAG-3+ lymphocyte infiltrates in spleen, lung and salivary gland of PBL mice. In patients with melanoma and non-small cell lung cancer, primary lesions with modest tumor uptake were observed in [68Ga]Ga-CC09-1 PET, as compared with that of [18F]FDG PET. More importantly, [68Ga]Ga-CC09-1 delineated the heterogeneity of LAG-3 expression within large tumors.

CONCLUSION

These findings consolidated that [68Ga]Ga-CC09-1 is a promising PET tracer for quantifying the LAG-3 expression in tumor microenvironment, indicating its potential as a companion diagnostic for patients stratification and therapeutic response monitoring in anti-LAG-3 therapy.

A Concept for Preoperative and Intraoperative Molecular Imaging and Detection for Assessing Extent of Disease of Solid Tumors

The authors propose a concept of "systems engineering," the approach to assessing the extent of diseased tissue (EODT) in solid tumors. We modeled the proof of this concept based on our clinical experience with colorectal carcinoma (CRC) and gastrinoma that included short and long-term survival data of CRC patients. This concept, applicable to various solid tumors, combines resources from surgery, nuclear medicine, radiology, pathology, and oncology needed for preoperative and intraoperative assessments of a patient's EODT. The concept begins with a patient presenting with biopsy-proven cancer. An appropriate preferential locator (PL) is a molecule that preferentially binds to a cancer-related molecular target (i.e., tumor marker) lacking in non-malignant tissue and is the essential element. Detecting the PL after an intravenous injection requires the PL labeling with an appropriate tracer radionuclide, a fluoroprobe, or both. Preoperative imaging of the tracer's signal requires molecular imaging modalities alone or in combination with computerized tomography (CT). These include positron emission tomography (PET), PET/CT, single-photon emission computed tomography (SPECT), SPECT/CT for preoperative imaging, gamma cameras for intraoperative imaging, and gamma-detecting probes for precise localization. Similarly, fluorescent-labeled PLs require appropriate cameras and probes. This approach provides the surgeon with real-time information needed for R0 resection.

Generic semi-automated radiofluorination strategy for single domain antibodies: [18F]FB-labelled single domain antibodies for PET imaging of fibroblast activation protein-α or folate receptor-α overexpression in cancer

BACKGROUND

Radiofluorination of single domain antibodies (sdAbs) via N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) has shown to be a promising strategy in the development of sdAb-based PET tracers. While automation of the prosthetic group (PG) [18F]SFB production, has been successfully reported, no practical method for large scale sdAb labelling has been reported. Therefore, we optimized and automated the PG production, enabling a subsequently efficient manual conjugation reaction to an anti-fibroblast activation protein (FAP)-α sdAb (4AH29) and an anti-folate receptor (FR)-α sdAb (2BD42). Both the alpha isoform of FAP and the FR are established tumour markers. FAP-α is known to be overexpressed mainly by cancer-associated fibroblasts in breast, ovarian, and other cancers, while its expression in normal tissues is low or undetectable. FR-α has an elevated expression in epithelial cancers, such as ovarian, brain and lung cancers. Non-invasive imaging techniques, such as PET-imaging, using tracers targeting specific tumour markers can provide molecular information over both the tumour and its environment, which aides in the diagnosis, therapy selection and assessment of the cancer treatment.

RESULTS

[18F]SFB was synthesized using a fully automated three-step, one-pot reaction. The total procedure time was 54 min and results in [18F]SFB with a RCP > 90% and a RCY d.c. of 44 ± 4% (n = 13). The manual conjugation reaction after purification produced [18F]FB-sdAbs with a RCP > 95%, an end of synthesis activity > 600 MBq and an apparent molar activity > 10 GBq/µmol. Overall RCY d.c., corrected to the trapping of [18F]F- on the QMA, were 9% (n = 1) and 5 ± 2% (n = 3) for [18F]FB-2BD42 and [18F]FB-4AH29, respectively.

CONCLUSION

[18F]SFB synthesis was successfully automated and upscaled on a Trasis AllInOne module. The anti-hFAP-α and anti-hFR-α sdAbs were radiofluorinated, yielding similar RCYs d.c. and RCPs, showing the potential of this method as a generic radiofluorination strategy for sdAbs. The radiofluorinated sdAbs showed a favourable biodistribution pattern and are attractive for further characterization as new PET tracers for FAP-α and FR-α imaging.

Synthesis and evaluation of bifunctional DFO2K: a modular chelator with ideal properties for zirconium-89 chelation

The synthesis and evaluation of the newest generation of our DFO2 chelator family-DFO2K-is described. DFO2K was designed with a simple synthetic route to access different bifunctional derivatives, with each derivative having similar metal ion coordination spheres and high denticity (up to 12 coordinate) to ensure stable coordination of zirconium-89. The high denticity could potentially enhance stability with other large oxophilic radiometals. Zirconium-89 is the most popular radionuclide to pair with large macromolecules such as antibodies (immunoPET) for positron emission tomography applications. Although clinically successful, the stability of the "gold standard" chelator desferrioxamine B (DFO) can be improved as significant bone uptake is observed in animal models, despite no obvious stability issues in humans. Following the synthesis of DFO2K we assessed its radiolabeling efficiency with zirconium-89 and compared with DFO, which revealed rapid and nearly identical radiolabeling kinetics to DFO. The resultant [89Zr]Zr-DFO2K complex showed improved stability over [89Zr]Zr-DFO in different in vitro stability assays such as hydroxyapatite and 1000-fold molar excess EDTA challenges. Furthermore, biodistribution studies of the non-bifunctional chelators in healthy mice showed that [89Zr]Zr-DFO2K had a similar distribution profile and clearance to [89Zr]Zr-DFO. The bifunctional derivative p-SCN-Ph-DFO2K was conjugated to a non-specific human IgG antibody and evaluated after 2 weeks circulating in healthy female CD1 mice. Mice administered [89Zr]Zr-DFO2K-IgG showed substantially lower bone uptake in PET-CT images than [89Zr]Zr-DFO-IgG, with PET ROI data and ex vivo biodistribution revealing a statistically significantly lower bone uptake for DFO2K. Overall, owing to its high denticity, ease of synthesis, improved solubility over DFO2 and DFO2p, and stable chelation of zirconium-89, DFO2K appears to be an improved alternative chelator to DFO for zirconium-89 chelation.

Illuminating immunotherapy response via precision T cell-targeted PET imaging

Traditionally, immunotherapy agent selection and treatment strategies are guided by biopsy-based histological information. However, biopsies are limited in that they are invasive, provide static information regarding the tumor immune microenvironment, and only sample a small part of one tumor site. The tumor microenvironment is dynamic and heterogenous. As a result, the immune milieu at one site may be distinct from other metastatic sites. These factors make identifying which patients are likely to respond to different immunotherapies and which harbor intrinsic resistance mechanisms difficult to identify based on a biopsy alone. As such, there is significant interest in alternative methodologies that better characterize the tumor immune microenvironment and monitor immunotherapy response. PET imaging potentially offers a non-invasive way to characterize the tumor immune microenvironment at the primary tumor and metastases and allow for longitudinal characterization. Herein, we review pre-clinically and clinically tested T cell-targeted PET radiopharmaceuticals, as T cells have been the dominant immunotherapy target, and their utility in both evaluating response to immunotherapy and in understanding the systemic immune response to treatment with immunotherapeutics.

Multimodal Imaging Approach for Tumor Treatment Response Evaluation in the Era of Immunotherapy

ABSTRACT

Immunotherapy is likely the most remarkable advancement in lung cancer treatment during the past decade. Although immunotherapy provides substantial benefits, their therapeutic responses differ from those of conventional chemotherapy and targeted therapy, and some patients present unique immunotherapy response patterns that cannot be judged under the current measurement standards. Therefore, the response monitoring of immunotherapy can be challenging, such as the differentiation between real response and pseudo-response. This review outlines the various tumor response patterns to immunotherapy and discusses methods for quantifying computed tomography (CT) and 18F-fluorodeoxyglucose positron emission tomography (PET) in the field of lung cancer. Emerging technologies in magnetic resonance imaging (MRI) and non-FDG PET tracers are also explored. With immunotherapy responses, the role for imaging is essential in both anatomical radiological responses (CT/MRI) and molecular changes (PET imaging). Multiple aspects must be considered when assessing treatment responses using CT and PET. Finally, we introduce multimodal approaches that integrate imaging and nonimaging data, and we discuss future directions for the assessment and prediction of lung cancer responses to immunotherapy.

The role of CD8 PET imaging in guiding cancer immunotherapy

Currently, immunotherapy is being widely used for treating cancers. However, the significant heterogeneity in patient responses is a major challenge for its successful application. CD8-positive T cells (CD8+ T cells) play a critical role in immunotherapy. Both their infiltration and functional status in tumors contribute to treatment outcomes. Therefore, accurate monitoring of CD8+ T cells, a potential biomarker, may improve therapeutic strategy. Positron emission tomography (PET) is an optimal option which can provide molecular imaging with enhanced specificity. This review summarizes the mechanism of action of CD8+ T cells in immunotherapy, and highlights the recent advancements in PET-based tracers that can visualize CD8+ T cells and discusses their clinical applications to elucidate their potential role in cancer immunotherapy.

ImmunoPET/CT imaging of clear cell renal cell carcinoma with [18F]RCCB6: a first-in-human study

PURPOSE

The cluster of differentiation (CD70) is a potential biomarker of clear cell renal cell carcinoma (ccRCC). This study aims to develop CD70-targeted immuno-positron emission tomography/computed tomography (immunoPET/CT) imaging tracers and explore the diagnostic value in preclinical studies and the potential value in detecting metastases in ccRCC patients.

METHODS

Four novel CD70-specific single-domain antibodies (sdAbs) were produced and labelled with 18F by the aluminium fluoride restrained complexing agent (AlF-RESCA) method to develop radiotracers. The visualisation properties of the tracers were evaluated in a subcutaneous ccRCC patient-derived xenograft (PDX) model. In a registered prospective clinical trial (NCT06148220), six patients with pathologically confirmed RCC were included and underwent immunoPET/CT examination exploiting one of the developed tracers (i.e., [18F]RCCB6).

RESULTS

We engineered four sdAbs (His-tagged RCCB3 and RCCB6, His-tag-free RB3 and RB6) specifically targeting recombinant human CD70 without cross-reactivity to murine CD70. ImmunoPET/CT imaging with [18F]RCCB3 and [18F]RCCB6 demonstrated a high tumour-to-background ratio in a subcutaneous ccRCC PDX model, with the latter showing better diagnostic potential supported by higher tumour uptake and lower bone accumulation. In comparison, [18F]RB6, developed by sequence optimisation, has significantly lower kidney accumulation than that of [18F]RCCB6. In a pilot translational study, [18F]RCCB6 immunoPET/CT displayed ccRCC metastases in multiple patients and demonstrated improved imaging contrast and diagnostic value than 18F-FDG PET/CT in a patient with ccRCC.

CONCLUSION

The work successfully developed a series of CD70-targeted immunoPET/CT imaging tracers. Of them, [18F]RCCB6 clearly and specifically identified inoculated ccRCCs in preclinical studies. Clinical translation of [18F]RCCB6 suggests potential for identifying recurrence and/or metastasis in ccRCC patients.

Zirconium 89 and Copper 64 for ImmunoPET: From Antibody Bioconjugation and Radiolabeling to Molecular Imaging

Immunotherapy has transformed cancer treatment. Nevertheless, given the heterogeneity of clinical efficacy, the multiplicity of treatment options available and the possibility of serious adverse effects, selecting the most effective treatment has become the greatest challenge. Molecular imaging offers an attractive way for this purpose. ImmunoPET provides specific imaging with positron emission tomography (PET) using monoclonal antibodies (mAb) or its fragments as vector. By combining the high targeting specificity of mAb and the sensitivity of PET technique, immunoPET could noninvasively and dynamically reveal tumor antigens expression and provide theranostic tools of several types of malignancies. Because of their slow kinetics, mAbs require radioelements defined by a consistent half-life. Zirconium 89 (89Zr) and Copper 64 (64Cu) are radiometals with half-lives suitable for mAb labeling. Radiolabeling with a radiometal requires the prior use of a bifunctional chelate agent (BFCA) to functionalize mAb for radiometal chelation, in a second step. There are a number of BFCA available and much research is focused on antibody functionalization techniques or on developing the optimum chelating agent depending the selected radiometal. In this manuscript, we present a critical account of radiochemical techniques with radionuclides 89Zr and 64Cu and their applications in preclinical and clinical immuno-PET imaging.

Navigating the landscape of PD-1/PD-L1 imaging tracers: from challenges to opportunities

Immunotherapy targeted to immune checkpoint inhibitors, such as the program cell death receptor (PD-1) and its ligand (PD-L1), has revolutionized cancer treatment. However, it is now well-known that PD-1/PD-L1 immunotherapy response is inconsistent among patients. The current challenge is to customize treatment regimens per patient, which could be possible if the PD-1/PD-L1 expression and dynamic landscape are known. With positron emission tomography (PET) imaging, it is possible to image these immune targets non-invasively and system-wide during therapy. A successful PET imaging tracer should meet specific criteria concerning target affinity, specificity, clearance rate and target-specific uptake, to name a few. The structural profile of such a tracer will define its properties and can be used to optimize tracers in development and design new ones. Currently, a range of PD-1/PD-L1-targeting PET tracers are available from different molecular categories that have shown impressive preclinical and clinical results, each with its own advantages and disadvantages. This review will provide an overview of current PET tracers targeting the PD-1/PD-L1 axis. Antibody, peptide, and antibody fragment tracers will be discussed with respect to their molecular characteristics and binding properties and ways to optimize them.

One-day examination of triple nuclear medicine imaging and application in evaluating transarterial embolization

A diagnosis based on multiple nuclear medicine imaging (NMI) was more comprehensive in approaching the nature of pathological changes. In this research, a method to realize triple NMIs within one day was developed based on the reasonable arrangements of 68Ga-RGD PET/CT specialized on neovascularization, 99mTc-HL-91 SPECT/CT specialized on hypoxia and 18F-FDG PET/CT specialized on tumor metabolism. Feasibility was verified in evaluating the therapeutic effects of transarterial embolization (TAE) performed on rabbit models with VX2 tumor. Radiation dosimetry was carried out to record the radiation exposure from multiple injections of radiopharmaceuticals. In results, the one-day examination of triple NMIs manifested the diversity of the postoperative histological changes, including the local neovascularization induced by embolization, hypoxic state of embolized tissues, and suppression of tumor metabolism. More importantly, radiation dosage from radiopharmaceuticals was limited below 5.70 ± 0.90 mSv. In conclusion, the strong timeliness and complementarity of one-day examination of triple nuclear medicine imaging made it clinically operative and worthy of popularizing. There was flexibility in combining distinct NMIs according to the clinical demands, so as to provide comprehensive information for diagnosis.

68Ga labeled EphA2-targeted cyclic peptide: a novel positron imaging tracer for triple-negative breast cancer?

The absence of better biomarkers currently limits early diagnosis and treatment of triple-negative breast cancer (TNBC). Our previously published study reported that the cyclic-peptide SD01 exhibited specific binding to EphA2 (Ephrin type-A receptor 2) on TNBC. To develop a novel PET imaging agent, we prepared gallium-68 (68Ga) labeled-DOTA-SD01 and evaluated its specificity and effectiveness through micro PET/CT imaging in a TNBC-bearing mouse model. SD01 and a control linear peptide YSA were conjugated to DOTA and subsequently labeled with 68Ga, obtaining 68Ga-DOTA-SD01 and 68Ga-DOTA-YSA. Both showed high radiochemical purity, stability, good hydrophilicity, and high binding affinity to 4T1 cells. Micro PET/CT imaging showed high radioactivity accumulation in tumors; SUVmean (mean standardized uptake value) of tumors in the group of 68Ga-DOTA-SD01 was 3.34 ± 0.25 and 2.65 ± 0.32 in the group of 68Ga-DOTA-YSA; T/NT ratios (target to non-target, SUVmean ratios of tumor to muscle) were 3.12 ± 0.06 and 2.77 ± 0.11 at 30 min, respectively (p < 0.05). The biodistribution study showed that tumor uptake % ID per g (percentage of injected dose per gram of tissue) in the group of 68Ga-DOTA-SD01 was 2.73 ± 0.34, and 1.77 ± 0.38 in the group of 68Ga-DOTA-YSA; T/NT ratios (radioactivity of tumor to muscle) were 3.55 ± 0.12 and 3.05 ± 0.10 for both groups at 30 min, respectively (p < 0.05). All these suggest that 68Ga-DOTA-SD01 may act as a better novel PET imaging agent for EphA2 positive tumors, such as TNBC.

Diagnostic Positron Emission Tomography Imaging with Zirconium-89 Desferrioxamine B Squaramide: From Bench to Bedside

Molecular imaging with antibodies radiolabeled with positron-emitting radionuclides combines the affinity and selectivity of antibodies with the sensitivity of Positron Emission Tomography (PET). PET imaging allows the visualization and quantification of the biodistribution of the injected radiolabeled antibody, which can be used to characterize specific biological interactions in individual patients. This characterization can provide information about the engagement of the antibody with a molecular target such as receptors present in elevated levels in tumors as well as providing insight into the distribution and clearance of the antibody. Potential applications of clinical PET with radiolabeled antibodies include identifying patients for targeted therapies, characterization of heterogeneous disease, and monitoring treatment response.Antibodies often take several days to clear from the blood pool and localize in tumors, so PET imaging with radiolabeled antibodies requires the use of a radionuclide with a similar radioactive half-life. Zirconium-89 is a positron-emitting radionuclide that has a radioactive half-life of 78 h and relatively low positron emission energy that is well suited to radiolabeling antibodies. It is essential that the zirconium-89 radionuclide be attached to the antibody through chemistry that provides an agent that is stable in vivo with respect to the dissociation of the radionuclide without compromising the biological activity of the antibody.This Account focuses on our research using a simple derivative of the bacterial siderophore desferrioxamine (DFO) with a squaramide ester functional group, DFO-squaramide (DFOSq), to link the chelator to antibodies. In our work, we produce conjugates with an average ∼4 chelators per antibody, and this does not compromise the binding of the antibody to the target. The resulting antibody conjugates of DFOSq are stable and can be easily radiolabeled with zirconium-89 in high radiochemical yields and purity. Automated methods for the radiolabeling of DFOSq-antibody conjugates have been developed to support multicenter clinical trials. Evaluation of several DFOSq conjugates with antibodies and low molecular weight targeting agents in tumor mouse models gave PET images with high tumor uptake and low background. The promising preclinical results supported the translation of this chemistry to human clinical trials using two different radiolabeled antibodies. The potential clinical impact of these ongoing clinical trials is discussed.The use of DFOSq to radiolabel relatively low molecular weight targeting molecules, peptides, and peptide mimetics is also presented. Low molecular weight molecules typically clear the blood pool and accumulate in target tissue more rapidly than antibodies, so they are usually radiolabeled with positron-emitting radionuclides with shorter radioactive half-lives such as fluorine-18 (t1/2 ∼ 110 min) or gallium-68 (t1/2 ∼ 68 min). Radiolabeling peptides and peptide mimetics with zirconium-89, with its longer radioactive half-life (t1/2 = 78 h), could facilitate the centralized manufacture and distribution of radiolabeled tracers. In addition, the ability to image patients at later time points with zirconium-89 based agents (e.g. 4-24 h after injection) may also allow the delineation of small or low-uptake disease sites as the delayed imaging results in increased clearance of the tracer from nontarget tissue and lower background signal.

Bifunctional octadentate pseudopeptides as Zirconium-89 chelators for immuno-PET applications

BACKGROUND

Positron emission tomography (PET) is a highly sensitive method that provides fine resolution images, useful in the field of clinical diagnostics. In this context, Zirconium-89 (89Zr)-based imaging agents have represented a great challenge in molecular imaging with immuno-PET, which employs antibodies (mAbs) as biological vectors. Indeed, immuno-PET requires radionuclides that can be attached to the mAb to provide stable in vivo conjugates, and for this purpose, the radioactive element should have a decay half-life compatible with the time needed for the biodistribution of the immunoglobulin. In this regard, 89Zr is an ideal radioisotope for immuno-PET because its half-life perfectly matches the in vivo pharmacokinetics of mAbs.

RESULTS

The main objective of this work was the design and synthesis of a series of bifunctional octadentate pseudopeptides able to generate stable 89Zr complexes. To achieve this, here we investigated hydroxamate, N-methylhydroxamate and catecholate chelating moieties in complexing radioactive zirconium. N-methylhydroxamate proved to be the most effective 89Zr-chelating group. Furthermore, the increased flexibility and hydrophilicity obtained by using polyoxyethylene groups spacing the hydroxamate units led to chelators capable of rapidly forming (15 min) stable and water-soluble complexes with 89Zr under mild reaction conditions (aqueous environment, room temperature, and physiological pH) that are mandatory for complexation reactions involving biomolecules. Additionally, we report challenge experiments with the competitor ligand EDTA and metal ions such as Fe3+, Zn2+ and Cu2+. In all examined conditions, the chelators demonstrated stability against transmetallation. Finally, a maleimide moiety was introduced to apply one of the most promising ligands in bioconjugation reactions through Thiol-Michael chemistry.

CONCLUSION

Combining solid phase and solution synthesis techniques, we identified novel 89Zr-chelating molecules with a peptide scaffold. The adopted chemical design allowed modulation of molecular flexibility, hydrophilicity, as well as the decoration with different zirconium chelating groups. Best results in terms of 89Zr-chelating properties were achieved with the N-methyl hydroxamate moiety. The Zirconium complexes obtained with the most effective compounds were water-soluble, stable to transmetallation, and resistant to peptidases for at least 6 days. Further studies are needed to assess the potential of this novel class of molecules as Zirconium-chelating agents for in vivo applications.

The Role of Total-Body PET in Drug Development and Evaluation: Status and Outlook

Total-body PET, an emerging technique, enables high-quality simultaneous total-body dynamic PET acquisition and accurate kinetic analysis. It has the potential to facilitate the study of multiple tracers while minimizing radiation dose and improving tracer-specific imaging. This advancement holds promise for enhancing the development and clinical evaluation of drugs, particularly radiopharmaceuticals. Multiple clinical trials are using a total-body PET scanner to explore existing and innovative radiopharmaceuticals. However, challenges persist, along with the opportunities, with regard to the use of total-body PET in drug development and evaluation. Specifically, considerations relate to the role of total-body PET in clinical pharmacologic evaluations and its integration into the theranostic paradigm. In this review, state-of-the-art total-body PET and its potential roles in pharmaceutical research are explored.

Enhanced Theranostic Efficacy of 89Zr and 177Lu-Labeled Aflibercept in Renal Cancer: A Viable Option for Clinical Practice

Vascular endothelial growth factor (VEGF) targeted therapy serves as an important therapeutic approach for renal cancer, but its clinical effectiveness is unsatisfactory. Moreover, there is a lack of reliable biomarkers for preoperative assessment of tumor VEGF expression. This study aimed to explore the potential for further applications of 177Lu/89Zr-labeled aflibercept (Abe), a VEGF-binding agent, in imaging visualization of VEGF expression and therapy for renal cancer. To determine specificity uptake in renal cancer, BALB/c mice with VEGF-expressing Renca tumor were intravenously injected with [89Zr]Zr-Abe, [177Lu]Lu-Abe, or Cy5.5-Abe and the blocking group was designed as a control group. PET, SPECT, and fluorescence images were acquired, and the biodistribution of [89Zr]Zr-Abe and [177Lu]Lu-Abe was performed. Additionally, the [177Lu]Lu-Abe, [177Lu]Lu-Abe-block, 177Lu only, Abe only, and PBS groups were compared for evaluation of the therapeutic effect. To assess the safety, we monitored and evaluated the body weight, blood biochemistry analysis, and whole blood analysis and major organs were stained with hematoxylin and eosin after [177Lu]Lu-Abe treatment. DOTA-Abe was successfully labeled with 177Lu and Df-Abe with 89Zr in our study. The uptake in tumor of [89Zr]Zr-Abe was significantly higher than that of [89Zr]Zr-Abe-block (P < 0.05) and provided excellent tumor contrast in PET images. [177Lu]Lu-Abe demonstrated promising tumor-specific targeting capability with a high and persistent tumor uptake. The standardized tumor volume of [177Lu]Lu-Abe was significantly smaller than those of other treatment groups (P < 0.05). [177Lu]Lu-Abe also had smaller tumor volumes and reduced expression of VEGF and CD31 compared to those of the control groups. Fluorescence images demonstrate higher tumor uptake in the Cy5.5-Abe group compared to the Cy5.5-Abe-block group (P < 0.05). In conclusion, [89Zr]Zr-Abe enables noninvasive analysis of VEGF expression, serving as a valuable tool for assessing the VEGF-targeted therapy effect. Additionally, all of the findings support the enhanced therapeutic efficacy and safety of [177Lu]Lu-Abe, making it a viable option for clinical practice in renal cancer.

PET Imaging for Monitoring Cellular and Immunotherapy of Cancer

ABSTRACT

Cancer immunotherapy, including checkpoint blockade and cellular therapy, has become a cornerstone in cancer treatment. However, understanding the factors driving patient response or resistance to these therapies remains challenging. The dynamic interplay between the immune system and tumors requires new approaches for characterization. Biopsies and blood tests provide valuable information, but their limitations have led to increased interest in positron emission tomography (PET)/computed tomography imaging to complement these strategies. The noninvasive nature of PET imaging makes it ideal for monitoring the dynamic tumor immune microenvironment. This review discusses various PET imaging approaches, including immune cell lineage markers, immune functional markers, immune cell metabolism, direct cell labeling, and reporter genes, highlighting their potential in targeted immunotherapies and cell-based approaches. Although PET imaging has limitations, its integration into diagnostic strategies holds promise for improving patient outcomes and accelerating drug development in cancer immunotherapy.

Recent advances in the development of 225Ac- and 211At-labeled radioligands for radiotheranostics

Radiotheranostics utilizes a set of radioligands incorporating diagnostic or therapeutic radionuclides to achieve both diagnosis and therapy. Imaging probes using diagnostic radionuclides have been used for systemic cancer imaging. Integration of therapeutic radionuclides into the imaging probes serves as potent agents for radionuclide therapy. Among them, targeted alpha therapy (TAT) is a promising next-generation cancer therapy. The α-particles emitted by the radioligands used in TAT result in a high linear energy transfer over a short range, inducing substantial damage to nearby cells surrounding the binding site. Therefore, the key to successful cancer treatment with minimal side effects by TAT depends on the selective delivery of radioligands to their targets. Recently, TAT agents targeting biomolecules highly expressed in various cancer cells, such as sodium/iodide symporter, norepinephrine transporter, somatostatin receptor, αvβ3 integrin, prostate-specific membrane antigen, fibroblast-activation protein, and human epidermal growth factor receptor 2 have been developed and have made remarkable progress toward clinical application. In this review, we focus on two radionuclides, 225Ac and 211At, which are expected to have a wide range of applications in TAT. We also introduce recent fundamental and clinical studies of radiopharmaceuticals labeled with these radionuclides.

ImmunoPET imaging of Trop2 in patients with solid tumours

Accurately predicting and selecting patients who can benefit from targeted or immunotherapy is crucial for precision therapy. Trophoblast cell surface antigen 2 (Trop2) has been extensively investigated as a pan-cancer biomarker expressed in various tumours and plays a crucial role in tumorigenesis through multiple signalling pathways. Our laboratory successfully developed two 68Ga-labelled nanobody tracers that can rapidly and specifically target Trop2. Of the two tracers, [68Ga]Ga-NOTA-T4, demonstrated excellent pharmacokinetics in preclinical mouse models and a beagle dog. Moreover, [68Ga]Ga-NOTA-T4 immuno-positron emission tomography (immunoPET) allowed noninvasive visualisation of Trop2 heterogeneous and differential expression in preclinical solid tumour models and ten patients with solid tumours. [68Ga]Ga-NOTA-T4 immunoPET could facilitate clinical decision-making through patient stratification and response monitoring during Trop2-targeted therapies.

Reduction of the hepatic radioactivity levels of [111In]In-DOTA-labeled antibodies via cleavage of a linkage metabolized in lysosomes

INTRODUCTION

Radiolabeled antibodies are promising tools for cancer diagnosis using nuclear medicine. A DOTA-chelating system is useful for preparing immuno-positron emission tomography and immuno-single-photon emission computed tomography probes with various radiometals. Radiolabeled antibodies are generally metabolized in the reticuloendothelial system, producing radiometabolites after proteolysis in hepatic lysosomes. Because of the bulkiness and extremely high hydrophilicity of DOTA, radiometabolites containing a radiometal-DOTA complex typically exhibit high and persistent localization in hepatic lysosomes. Radioactivity in the liver impairs the accurate diagnosis of cancer surrounding the liver and liver metastasis, and a high tumor/liver ratio is desirable. In this study, we reduced the hepatic radioactivity of radiometal-labeled antibodies containing a DOTA-chelating system. A cleavable linkage was inserted to liberate the radiometabolite, which exhibited a short residence time in hepatocytes.

METHODS

Using indium-111 (111In)-labeled antibodies, we prepared 111In-labeled galactosyl-neoglycoalbumins (NGAs) because they are useful for evaluating the residence time of radiometabolites in the liver. An 111In-labeled NGA with a cleavable linkage ([111In]In-DO3AiBu-Bn-FGK-NGA) was administered to normal mice, and biodistribution studies and metabolic analyses of urinary and fecal samples were performed with comparison to an 111In-labeled NGA prepared by a conventional method ([111In]In-DOTA-Bn-SCN-NGA). Then, 111In-labeled antibodies ([111In]In-DO3AiBu-Bn-FGK-IgG and [111In]In-DOTA-Bn-SCN-IgG) were prepared using a procedure similar to that for 111In-labeled NGAs. In vitro plasma stability and biodistribution were investigated for both 111In-labeled antibodies in U87MG tumor-bearing mice.

RESULTS

Through the liberation of radiometabolites including [111In]In-DO3AiBu-Bn-F, [111In]In-DO3AiBu-Bn-FGK-NGA was cleared more rapidly from the liver than [111In]In-DOTA-Bn-SCN-NGA (4.07 ± 1.54%ID VS 71.68 ± 3.03%ID at 6 h postinjection). [111In]In-DO3AiBu-Bn-FGK-IgG exhibited lower tumor accumulation (8.83 ± 1.48%ID/g) but a significantly higher tumor/liver ratio (2.21 ± 0.53) than [111In]In-DOTA-Bn-SCN-IgG (11.65 ± 2.17%ID/g in the tumor and a tumor/liver ratio of 0.85 ± 0.18) at 72 h after injection.

CONCLUSION

A molecular design that reduces the high and persistent hepatic radioactivity of radiolabeled antibodies by liberating radiometabolites with a short hepatic residence time in lysosomes would be applicable for radiometal-labeled antibodies using a DOTA-chelating system.

In Vivo Detection of Staphylococcus aureus Infections Using Radiolabeled Antibodies Specific for Bacterial Toxins

Purpose

The Gram-positive Staphylococcus aureus bacterium is one of the leading causes of infection in humans. The lack of specific noninvasive techniques for diagnosis of staphylococcal infection together with the severity of its associated complications support the need for new specific and selective diagnostic tools. This work presents the successful synthesis of an immunotracer that targets the α-toxin released by S. aureus.

Methods

[89Zr]Zr-DFO-ToxAb was synthesized based on radiolabeling an anti-α-toxin antibody with zirconium-89. The physicochemical characterization of the immunotracer was performed by high-performance liquid chromatography (HPLC), radio-thin layer chromatography (radio-TLC), and electrophoretic analysis. Its diagnostic ability was evaluated in vivo by positron emission tomography/computed tomography (PET/CT) imaging in an animal model of local infection-inflammation (active S. aureus vs. heat-killed S. aureus) and infective osteoarthritis.

Results

Chemical characterization of the tracer established the high radiochemical yield and purity of the tracer while maintaining antibody integrity. In vivo PET/CT image confirmed the ability of the tracer to detect active foci of S. aureus. Those results were supported by ex vivo biodistribution studies, autoradiography, and histology, which confirmed the ability of [89Zr]Zr-DFO-ToxAb to detect staphylococcal infectious foci, avoiding false-positives derived from inflammatory processes.

Conclusions

We have developed an immuno-PET tracer capable of detecting S. aureus infections based on a radiolabeled antibody specific for the staphylococcal alpha toxins. The in vivo assessment of [89Zr]Zr-DFO-ToxAb confirmed its ability to selectively detect staphylococcal infectious foci, allowing us to discern between infectious and inflammatory processes.

Preclinical development of novel PD-L1 tracers and first-in-human study of [68Ga]Ga-NOTA-RW102 in patients with lung cancers

BACKGROUND

The programmed cell death protein-1 (PD-1)/programmed death receptor ligand 1 (PD-L1) axis critically facilitates cancer cells' immune evasion. Antibody therapeutics targeting the PD-1/PD-L1 axis have shown remarkable efficacy in various tumors. Immuno-positron emission tomography (ImmunoPET) imaging of PD-L1 expression may help reshape solid tumors' immunotherapy landscape.

METHODS

By immunizing an alpaca with recombinant human PD-L1, three clones of the variable domain of the heavy chain of heavy-chain only antibody (VHH) were screened, and RW102 with high binding affinity was selected for further studies. ABDRW102, a VHH derivative, was further engineered by fusing RW102 with the albumin binder ABD035. Based on the two targeting vectors, four PD-L1-specific tracers ([68Ga]Ga-NOTA-RW102, [68Ga]Ga-NOTA-ABDRW102, [64Cu]Cu-NOTA-ABDRW102, and [89Zr]Zr-DFO-ABDRW102) with different circulation times were developed. The diagnostic efficacies were thoroughly evaluated in preclinical solid tumor models, followed by a first-in-human translational investigation of [68Ga]Ga-NOTA-RW102 in patients with non-small cell lung cancer (NSCLC).

RESULTS

While RW102 has a high binding affinity to PD-L1 with an excellent KD value of 15.29 pM, ABDRW102 simultaneously binds to human PD-L1 and human serum albumin with an excellent KD value of 3.71 pM and 3.38 pM, respectively. Radiotracers derived from RW102 and ABDRW102 have different in vivo circulation times. In preclinical studies, [68Ga]Ga-NOTA-RW102 immunoPET imaging allowed same-day annotation of differential PD-L1 expression with specificity, while [64Cu]Cu-NOTA-ABDRW102 and [89Zr]Zr-DFO-ABDRW102 enabled longitudinal visualization of PD-L1. More importantly, a pilot clinical trial shows the safety and diagnostic value of [68Ga]Ga-NOTA-RW102 immunoPET imaging in patients with NSCLCs and its potential to predict immune-related adverse effects following PD-L1-targeted immunotherapies.

CONCLUSIONS

We developed and validated a series of PD-L1-targeted tracers. Initial preclinical and clinical evidence indicates that immunoPET imaging with [68Ga]Ga-NOTA-RW102 holds promise in visualizing differential PD-L1 expression, selecting patients for PD-L1-targeted immunotherapies, and monitoring immune-related adverse effects in patients receiving PD-L1-targeted treatments.

TRIAL REGISTRATION NUMBER

NCT06165874.

Rethinking the potential role of dose painting in personalized ultra-fractionated stereotactic adaptive radiotherapy

Fractionated radiotherapy was established in the 1920s based upon two principles: (1) delivering daily treatments of equal quantity, unless the clinical situation requires adjustment, and (2) defining a specific treatment period to deliver a total dosage. Modern fractionated radiotherapy continues to adhere to these century-old principles, despite significant advancements in our understanding of radiobiology. At UT Southwestern, we are exploring a novel treatment approach called PULSAR (Personalized Ultra-Fractionated Stereotactic Adaptive Radiotherapy). This method involves administering tumoricidal doses in a pulse mode with extended intervals, typically spanning weeks or even a month. Extended intervals permit substantial recovery of normal tissues and afford the tumor and tumor microenvironment ample time to undergo significant changes, enabling more meaningful adaptation in response to the evolving characteristics of the tumor. The notion of dose painting in the realm of radiation therapy has long been a subject of contention. The debate primarily revolves around its clinical effectiveness and optimal methods of implementation. In this perspective, we discuss two facets concerning the potential integration of dose painting with PULSAR, along with several practical considerations. If successful, the combination of the two may not only provide another level of personal adaptation ("adaptive dose painting"), but also contribute to the establishment of a timely feedback loop throughout the treatment process. To substantiate our perspective, we conducted a fundamental modeling study focusing on PET-guided dose painting, incorporating tumor heterogeneity and tumor control probability (TCP).

Synthesis and evaluation of fluorine-18 labelled tetrazines as pre-targeting imaging agents for PET

BACKGROUND

The brain is a challenging target for antibody-based positron emission tomography (immunoPET) imaging due to the restricted access of antibody-based ligands through the blood-brain barrier (BBB). To overcome this physiological obstacle, we have previously developed bispecific antibody ligands that pass through the BBB via receptor-mediated transcytosis. While these radiolabelled ligands have high affinity and specificity, their long residence time in the blood and brain, typical for large molecules, poses another challenge for PET imaging. A viable solution could be a two-step pre-targeting approach which involves the administration of a tagged antibody that accumulates at the target site in the brain and then clears from the blood, followed by administration of a small radiolabelled molecule with fast kinetics. This radiolabelled molecule can couple to the tagged antibody and thereby make the antibody localisation visible by PET imaging. The in vivo linkage can be achieved by using the inverse electron demand Diels-Alder reaction (IEDDA), with trans-cyclooctene (TCO) and tetrazine groups participating as reactants. In this study, two novel 18F-labelled tetrazines were synthesized and evaluated for their potential use as pre-targeting imaging agents, i.e., for their ability to rapidly enter the brain and, if unbound, to be efficiently cleared with minimal background retention.

RESULTS

The two compounds, a methyl tetrazine [18F]MeTz and an H-tetrazine [18F]HTz were radiolabelled using a two-step procedure via [18F]F-Py-TFP synthesized on solid support followed by amidation with amine-bearing tetrazines, resulting in radiochemical yields of 24% and 22%, respectively, and a radiochemical purity of > 96%. In vivo PET imaging was performed to assess their suitability for in vivo pre-targeting. Time-activity curves from PET-scans showed [18F]MeTz to be the more pharmacokinetically suitable agent, given its fast and homogenous distribution in the brain and rapid clearance. However, in terms of rection kinetics, H-tetrazines are advantageous, exhibiting faster reaction rates in IEDDA reactions with dienophiles like trans-cyclooctenes, making [18F]HTz potentially more beneficial for pre-targeting applications.

CONCLUSION

This study demonstrates a significant potential of [18F]MeTz and [18F]HTz as agents for pre-targeted PET brain imaging due to their efficient brain uptake, swift clearance and appropriate chemical stability.

Biodistribution and pharmacokinetics of [89Zr]-anti-VEGF mAbs using PET in glioblastoma rat models

INTRODUCTION

Glioblastomas present intensive angiogenesis, thus anti-Vascular Endothelial Growth Factor (VEGF) antibodies (mAbs) have been proposed as promising therapies. However, the results of clinical trials reported moderate toxicity and limited effectiveness. This study evaluates the in vivo pharmacokinetics and biodistribution of these mAbs in a growing model of glioblastoma in rats using Positron Emission Tomography (PET).

MATERIAL

&Methods: mAbs were radiolabeled with zirconium-89. Four days after the model induction, animals were injected with 2.33 ± 1.3 MBq of [89Zr]-DFO-bevacizumab (n = 8) or 2.35 ± 0.26 MBq of [89Zr]-DFO-aflibercept (n = 6). PETs were performed at 0H, 48H, 168H, 240H, and 336H post-injection. Tumor induction was confirmed using [18F]-Fluorodeoxyglucose-PET and immunohistochemistry. Radiotracer uptake was estimated in all pre-defined Volumes-of-Interest.

RESULTS

Anti-VEGF mAbs showed 100 % Radiochemical-Purity. [89Zr]-DFO-bevacizumab showed a significantly higher bioavailability in whole-blood. A significant increase in the tumor uptake was detectable at 168H PET with [89Zr]-DFO-bevacizumab meanwhile with [89Zr]-DFO-aflibercept it was only detectable at 336H. [89Zr]-DFO-bevacizumab tumor uptake was significantly higher than that of [89Zr]-DFO-aflibercept in all the scans. Tumor induction was confirmed in all animal models.

CONCLUSION

MAbs detect VEGF-expression in glioblastoma models. Tumors were earlier targeted by Bevacizumab. The lower blood availability of aflibercept resulted in a lower tumor uptake than bevacizumab in all the scans.

PET Imaging of Peptide Probe Al[18F]F-NOTA-PCP1 for Monitoring the Engagement of PD-L1 Antibodies in Tumors

Immune checkpoint inhibitors (ICIs) are a powerful treatment modality for various types of cancer. The effectiveness of ICIs is intimately connected to the binding status of antibodies to receptors. However, validated means to accurately evaluate target specificity and predict antibody efficacy in vivo are lacking. A novel peptide-based probe called Al[18F]F-NOTA-PCP1 was developed and validated for its specificity to PD-L1 in A549, U87MG, GL261, and GL261-iPDL1 cell lines, as well as in xenograft models. Then the probe was used in PET/CT scans to determine the binding status of PD-L1 antibodies (atezolizumab, avelumab, and durvalumab) in U87MG xenograft model mice. Moreover, Al[18F]F-NOTA-PCP1 was used to evaluate the impact of different treatment times and doses. Al[18F]F-NOTA-PCP1 PET/CT can be used to evaluate the interaction between PD-L1 and antibodies to determine the effectiveness of immunotherapy. By quantifying target engagement, the probe has the potential to predict the efficacy of immunotherapy and optimize the dose and treatment schedules for PD-L1 immunotherapy. This imaging agent could be a valuable tool in guiding personalized treatment strategies and improving cancer patient outcomes.

An iodine‑labelled Antibody-drug conjugate PET probe for noninvasive monitoring of Nectin-4 expression in urothelial carcinoma

BACKGROUND AND PURPOSE

Some kinds of antibody-drug conjugate (ADC) with high affinity to Nectin-4 have demonstrated breakthrough progress in the third-line setting for bladder cancer. However, many patients are still difficult to benefit from treatment based on the heterogeneity of tumour. As the most advanced auxiliary treatment technology, treatment visualization can most intuitively predict the effectiveness of drug treatment, and timely detect the occurrence of drug resistance. Among them, nuclear medicine molecular probes play an important role in this field.

METHODS

124/125I-EV was prepared by labelling Enfortumad Vedetin (EV), an ADC drugs widely used in clinic targeted Nectin-4, with Na124/125I using N-bromine succinimide as oxidant. The radiochemical purity was analyzed via radio-TLC and bioactivity was measured by enzyme-linked immunosorbent assay. Cell uptake assay and small-animal PET imaging were performed to verified the specificity and targeting.

KEY RESULTS

124/125I-EV was prepared with high labeling yield and radiochemical purity. ELISA assays demonstrated that 124I-EV maintained the same high bioactivity as EV with significantly higher uptake in SW780 cells (Nectin-4 positive, 4.05 ± 0.32 %IA/5 × 105 cells at 8 h) than that in T24 cells (Nectin-4 negative, 1.34 ± 0.18 %IA/5 × 105 cells, p < 0.001). In PET imaging, 124I-EV had a significantly higher accumulation in SW780 tumour than that in T24 tumour and the uptake in SW780 tumour could be specifically blocked when co-injected with cold EV. The signal-to-noise ratio at the tumour site gradually increased with time, and peaked at 72 h.

CONCLUSION AND IMPLICATIONS

124I-EV was successfully prepared with high specificity and binding affinity of Nectin-4. This radioactive probe completely simulates the internal circulation of ADC drugs and tumour uptake and retention, which will greatly improve the clinical application of ADC therapy.

Ultralight crystalline hybrid composite material for highly efficient sequestration of radioiodine

Considering the importance of sustainable nuclear energy, effective management of radioactive nuclear waste, such as sequestration of radioiodine has inflicted a significant research attention in recent years. Despite the fact that materials have been reported for the adsorption of iodine, development of effective adsorbent with significantly improved segregation properties for widespread practical applications still remain exceedingly difficult due to lack of proper design strategies. Herein, utilizing unique hybridization synthetic strategy, a composite crystalline aerogel material has been fabricated by covalent stepping of an amino-functionalized stable cationic discrete metal-organic polyhedra with dual-pore containing imine-functionalized covalent organic framework. The ultralight hybrid composite exhibits large surface area with hierarchical macro-micro porosity and multifunctional binding sites, which collectively interact with iodine. The developed nano-adsorbent demonstrate ultrahigh vapor and aqueous-phase iodine adsorption capacities of 9.98 g.g-1 and 4.74 g.g-1, respectively, in static conditions with fast adsorption kinetics, high retention efficiency, reusability and recovery.

Straightforward Synthesis of DFO* - An Octadentate Chelator for Zirconium-89

DFO* is an octadentate chelator able to form highly stable chelates with Zirconium-89 (89 Zr) for nuclear medicinal applications in Positron Emission Tomography (PET).[1,2] The synthesis of DFO* and its scale-up remains challenging by reported synthetic protocols. For this reason, we set out to develop a de novo synthesis of a hydroxamate-containing building block suitable for the coupling to the commercially available DFO (desferrioxamine B, mesylate salt) yielding, after deprotection, the desired chelator DFO* in a more efficient procedure. Highlights of the new synthesis of DFO* reported herein are less synthetic steps and the isolation of the desired product DFO* by using solid phase extraction (SPE), thus avoiding tedious HPLC purification. DFO* is obtained in excellent purity (92-98 %) and an overall yield of approximately 29 %. In addition, the isolated trifluoroacetic acid (TFA)-salt of DFO* displays an improved solubility in organic solvents (DMSO, DMF, methanol), which will facilitate its use for the preparation of structurally diverse derivatives suitable for bioconjugation chemistry and the development of 89 Zr-labeled radiotracers.

β-Galactosidase-Activated and Red Light-Induced RNA Modification Strategy for Prolonged NIR Fluorescence/PET Bimodality Imaging

Improving the retention of small-molecule-based therapeutic agents in tumors is crucial to achieve precise diagnosis and effective therapy of cancer. Herein, we propose a β-galactosidase (β-Gal)-activated and red light-induced RNA modification (GALIRM) strategy for prolonged tumor imaging. A β-Gal-activatable near-infrared (NIR) fluorescence (FL) and positron emission tomography (PET) bimodal probe 68Ga-NOTA-FCG consists of a triaaza triacetic acid chelator NOTA for 68Ga-labeling, a β-Gal-activated photosensitizer CyGal, and a singlet oxygen (1O2)-susceptible furan group for RNA modification. Studies have demonstrated that the probe emits an activated NIR FL signal upon cleavage by endogenous β-Gal overexpressed in the lysosomes, which is combined with the PET imaging signal of 68Ga allowing for highly sensitive imaging of ovarian cancer. Moreover, the capability of 68Ga-NOTA-FCG generating 1O2 under 690 nm illumination could be simultaneously unlocked, which can trigger the covalent cross-linking between furan and nucleotides of cytoplasmic RNAs. The formation of the probe-RNA conjugate can effectively prevent exocytosis and prolong retention of the probe in tumors. We thus believe that this GALIRM strategy may provide entirely new insights into long-term tumor imaging and efficient tumor treatment.

ImmunoPET provides a novel way to visualize the CD103+ tissue-resident memory T cell to predict the response of immune checkpoint inhibitors

BACKGROUND

Immune checkpoint inhibitors (ICIs) have made significant progress in oncotherapy improving survival of patients. However, the benefits are limited to only a small subgroup of patients who could achieve durable responses. Early prediction of response may enable treatment optimization and patient stratification. Therefore, developing appropriate biomarkers is critical to monitoring efficacy and assessing patient response to ICIs.

MAIN BODY

Herein, we first introduce a new potential biomarker, CD103, expressed on tissue-resident memory T cells, and discuss the potential application of CD103 PET imaging in predicting immune checkpoint inhibitor treatment. In addition, we describe the current targets of ImmunoPET and compare these targets with CD103. To assess the benefit of PET imaging, a comparative analysis between ImmunoPET and other imaging techniques commonly employed for tumor diagnosis was performed. Additionally, we compare ImmunoPET and immunohistochemistry (IHC), a widely utilized clinical method for biomarker identification with respect to visualizing the immune targets.

CONCLUSION

CD103 ImmunoPET is a promising method for determining tumor-infiltrating lymphocytes (TILs) load and response to ICIs, thereby addressing the lack of reliable biomarkers in cancer immunotherapy. Compared to general T cell markers, CD103 is a specific marker for tissue-resident memory T cells, which number increases during successful ICI therapy. ImmunoPET offers noninvasive, dynamic imaging of specific markers, complemented by detailed molecular information from immunohistochemistry (IHC). Radiomics can extract quantitative features from traditional imaging methods, while near-infrared fluorescence (NIRF) imaging aids tumor detection during surgery. In the era of precision medicine, combining such methods will offer a more comprehensive approach to cancer diagnosis and treatment.

ImmunoPET imaging of Trop2 expression in solid tumors with nanobody tracers

PURPOSE

The high expression of the transmembrane glycoprotein trophoblast cell-surface antigen 2 (Trop2) was strongly associated with the progression of solid tumors, including pancreatic and gastric cancers. Our study aimed to construct Trop2-specific immuno-positron emission tomography (immunoPET) probes and assess the diagnostic abilities in preclinical pancreatic and gastric cancer models.

METHODS

The expression of Trop2 in pancreatic cancer was determined by single-cell sequencing and immunohistochemistry on tissue microarray (TMA). Flow cytometry was used to screen the expression of Trop2 in pancreatic cancer cell lines. Two nanobodies (i.e., RTD98 and RTD01) targeting Trop2 were developed and labeled with gallium-68 (68Ga, T1/2 = 1.1 h) to construct immunoPET imaging probes. The agents were researched in cell-derived pancreatic and patient-derived gastric cancer models expressing varying Trop2.

RESULTS

Single-cell sequencing results showed high expression of Trop2 in pancreatic ductal cells as well as acinar cells and immunohistochemical staining of TMA from pancreatic cancers showed significantly higher expression of Trop2 in cancerous than in paracancerous tissues. ImmunoPET utilizing [68Ga]Ga-NOTA-RTD98 could clearly delineate subcutaneous tumors, both in cell-derived pancreatic cancer models and patient-derived gastric cancer models, superior to imaging using [18F]-FDG or a non-specific probe [68Ga]Ga-NOTA-RTD161. Another probe with improved pharmacokinetics targeting Trop2, [68Ga]Ga-NOTA-RTD01, was further prepared and showed advantageous diagnostic capabilities in preclinical pancreatic cancer models.

CONCLUSION

In the work, we reported two nanobody tracers targeting human Trop2 which may facilitate better use of Trop2-targeted therapeutics by noninvasively displaying expression dynamics of the target.

Imaging diagnosis and efficacy monitoring by [89Zr]Zr-DFO-KN035 immunoPET in patients with PD-L1-positive solid malignancies

Rationale: Although programmed death-ligand 1 (PD-L1) inhibitors have achieved efficacy in cancer therapy, their response rate is low. Differences in the prognosis of patients with cancer under anti-PD-L1 treatment are related to the PD-L1 level in tumors. Accurate PD-L1 detection can optimize the accuracy of tumor immunotherapy and avoid ineffective clinical diagnosis and treatments. Methods: We investigated the imaging efficiency and therapy monitoring capacity of [89Zr]Zr-DFO-KN035 immunoPET for tumors. We labeled the monodomain anti-PD-L1 antibody KN035 with the radionuclide zirconium-89 and used this tracer for PET imaging. [89Zr]Zr-DFO-KN035 uptakes in patients with PD-L1-positive tumors, including primary and metastatic tumors, as well as in normal tissues, were comparatively assessed by using positron emission tomography/computed tomography imaging. Results: In PD-L1-positive patients, [89Zr]Zr-DFO-KN035 was sensitive in tumor-targeting imaging and could detect multiple metastatic foci, including multiple bone metastases (tumor-to-muscle ratios of 7.102 and 6.118 at 55 and 120 h, respectively) and lymph-node metastases (tumor-to-muscle ratios of 11.346 and 6.542 at 55 and 120 h, respectively). The needed radioactive dose of [89Zr]Zr-DFO-KN035 (55.5-92.5 MBq) used in this study was considerably lower than that of [18F]FDG (370-555 MBq). [89Zr]Zr-DFO-KN035 monitored and predicted the site of adverse reactions in antitumor immunotherapy. Moreover, after antitumor treatment, [89Zr]Zr-DFO-KN035 enabled observational imaging for therapeutic efficacy evaluation, which can help predict patient prognosis. Conclusion: [89Zr]Zr-DFO-KN035 can be used for the diagnosis and therapy monitoring of PD-L1-positive tumors and provide noninvasive and comprehensive observations for tumor diagnostic imaging, prognosis prediction, and efficacy evaluation.

Methods for the Production of Radiolabeled Bioagents for ImmunoPET

Immunoglobulin-based positron emission tomography (ImmunoPET) is making increasingly significant contributions to the nuclear imaging toolbox. The exquisite specificity of antibodies combined with the high-resolution imaging of PET enables clinicians and researchers to localize diseases, especially cancer, with a high degree of spatial certainty. This review focuses on the radiopharmaceutical preparation necessary to obtain those images-the work behind the scenes, which occurs even before the patient or animal is injected with the radioimmunoconjugate. The focus of this methods review will be the chelation of four radioisotopes to their most common and clinically relevant chelators.

Annotation of CD8+ T-cell function via ICAM-1 imaging identifies FAK inhibition as an adjuvant to augment the antitumor immunity of radiotherapy

Background: Radiotherapy (RT) may trigger systemic antitumor immunity, manifesting as regression of non-irradiated lesions (abscopal effect). Intracellular adhesion molecule-1 (ICAM-1) is a key molecule involved in the abscopal effect of RT. However, the specific function of ICAM-1 in CD8+ T cells during antitumor immune responses remains unclear. Herein, we investigated whether noninvasive imaging of ICAM-1 can be used to annotate CD8+ T-cell function, thereby better selecting combinational therapy to enhance the antitumor immunity induced by RT. Methods: Using knockout mouse models, we investigated the role of ICAM-1 expressed on CD8+ T cells in the antitumor immunity of RT and conducted drug screening guided by ICAM-1-targeted noninvasive imaging. Results: The systemic antitumor effect of RT relies on the expression of ICAM-1 on CD8+ T cells. ICAM-1 expression is essential for CD8+ T-cell activation, proliferation, and effector function. Noninvasive annotation of the proliferation and effector function of CD8+ T cells by ICAM-1-targeted imaging identified VS-6063, a focal adhesion kinase inhibitor, as a new adjuvant to augment systemic antitumor immunity of RT in an immunologically "cold" tumor model. Mechanistically, VS-6063 overcomes the physical barriers in tumors and promotes the migration and infiltration of CD8+ T cells primed by RT into distant tumors. Conclusion: Our findings highlight that molecular imaging of ICAM-1 levels provides a dynamic readout of the proliferation and effector function of tumor-infiltrating CD8+ T cells, which facilitates the high-throughput exploitation of new combinational drugs to maximize the systemic antitumor effect of RT.

Preclinical Evaluation of 99mTc-MAG3-5-Fab Targeting TREM2 in Lung Cancer Mouse Models: A Comparison with 99mTc-MAG3-5-F(ab')2

Triggering receptor expressed on myeloid cells-2 (TREM2), which is expressed on the surface of tumor-associated macrophages (TAMs), has been found to play a major role in the diagnosis and treatment of tumors. TREM2 expression is significantly upregulated in tumor tissues, and therefore, targeting TREM2 for tumor imaging may be of value. Previously, we performed TREM2 targeting imaging by using 68Ga-NOTA-COG1410 or a 124I-labeled monoclonal antibody (mAb) and F(ab')2 in mouse models of colon and gastric tumors. However, some of the shortcomings of these probes (i.e., the high uptake of 68Ga-NOTA-COG1410 in the liver, the difficulty of obtaining iodine-124, and the long half-life of iodine-124) have hindered their clinical use. Herein, we sought to synthesize novel molecular probes targeting TREM2 that are more conducive to clinical translation, eliminating the interference of isotope availability and in vivo probe biodistribution issues. Therefore, we established A549 cell lines with negative human TREM2 (hTREM2) expression (GFP tag; hTREM2- A549) or upregulated hTREM2 expression (GFP tag; hTREM2+ A549) using lentiviral transfection and confirmed these with Western blotting and immunocytochemistry. We then prepared a mouse anti-human TREM2 (5-mAb) by immunizing with the hTREM2 antigen. The antibody fragments 5-F(ab')2 and 5-Fab were prepared from 5-mAb, and 99mTc-MAG3-5-F(ab')2 and 99mTc-MAG3-5-Fab were then synthesized with excellent stability and specificity. 99mTc-MAG3-5-F(ab')2 had a slightly higher in vitro affinity than 99mTc-MAG3-5-Fab (Kd = 3.32 ± 0.05 nmol versus 4.62 ± 0.85 nmol). 99mTc-MAG3-5-F(ab')2 and 99mTc-MAG3-5-Fab both showed excellent specificity: after adding a 100-fold precursor, the two probes binding to the cells were almost blocked. In vivo pharmacokinetics showed that the distribution and elimination half-lives of 99mTc-MAG3-5-Fab (T1/2α = 1.25 ± 0.30 min and T1/2β = 21.98 ± 2.80 min, respectively) were significantly reduced compared to those of 99mTc-MAG3-5-F(ab')2 (T1/2α = 2.64 ± 0.37 min and T1/2β = 86.55 ± 26.86 min, respectively). In micro single-photon emission computed tomography/computed tomography (micro-SPECT/CT) imaging, the tumor was clearly displayed at 1 h after 99mTc-MAG3-5-Fab injection, while the blood background was extremely low at 3 h, and the probe was mainly excreted through the kidneys and biliary tract. 99mTc-MAG3-5-F(ab')2 uptake was also detected at the tumor site, although the blood background was consistently high. The biodistribution results were consistent with the micro-SPECT/CT imaging results. 99mTc-MAG3-5-Fab could clearly display hTREM2+ A549 tumors in a short time (1 h) with low uptake in nontumor organs and tissues and thus has clinical application prospects.

Dive into the details of radionuclide antibody conjugates: what role do EPR effects and LETs of different radionuclides play?

Radionuclide antibody conjugate (RAC) is a promising diagnostic and therapeutic tool. It combines radionuclides and antibodies by connecting arms and chelating agents, offering precise targeting and potent killing of tumor cells. However, further development and optimization of this radiopharmaceutical is needed to enhance the ultimate substantive efficacy for clinical translation. In this issue of AJNMMI, Strand et al. evaluated the enhanced permeability effect and different linear energy transfer (LET) of radionuclides in a prostate cancer xenograft model. The results showed that specific targeting might negatively influence normal organ uptake when targeting secreted antigens and different LETs of radionuclides might have diverse effects on receptor expression and cell proliferation in tumors. The findings provide new thinking for the development of antibody-based radiopharmaceuticals.

ImmunoPET Targeting Receptor Tyrosine Kinase: Clinical Applications

Receptor tyrosine kinases, or RTKs, are one large family of cell surface receptors involved in signal transduction, which represent an integral part of the signaling pathways. They play a crucial role in most important cellular processes, starting with the cell cycle, proliferation and differentiation, as well as cell migration, metabolism and survival. The introduction of ImmunoPET evaluating the expression of RTKs by specific monoclonal antibodies (mAbs) or antibody fragments is regarded as a promising tool for imaging treatment efficacy and developing anticancer therapeutics. Our review focuses mainly on the current clinical research regarding ImmunoPET targeting RTKs, with particular interest in the epidermal growth factor family, or HER family, and vascular endothelial-derived growth factor/receptor.