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Monoclonal antibodies and antibody fragments have proven to be highly effective vectors for the delivery of radionuclides to target tissues for positron emission tomography (PET) and single-photon emission computed tomography (SPECT). However, the stochastic methods that have traditionally been used to attach radioisotopes to these biomolecules inevitably produce poorly defined and heterogeneous probes and can impair the ability of the immunoglobulins to bind their molecular targets. In response to this challenge, an array of innovative site-specific and site-selective bioconjugation strategies have been developed, and these approaches have repeatedly been shown to yield better-defined and more homogeneous radioimmunoconjugates with superior in vivo performance than their randomly modified progenitors. In this Current Opinion in Chemical Biology review, we will examine recent advances in this field, including the development - and, in some cases, clinical translation - of nuclear imaging agents radiolabeled using strategies that target the heavy chain glycans, peptide tags, and unnatural amino acids., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Brian Zeglis has patent #US11000604B2 issued to None. Brian Zeglis has patent #CA3230848A1 pending to Zentera Therapeutics. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Gynecological pathologies account for approximately 4.5% of the overall global disease burden. Although cancers of the female reproductive system have understandably been the focus of a great deal of research, benign gynecological conditions-such as endometriosis, polycystic ovary syndrome, and uterine fibroids-have remained stubbornly understudied despite their astonishing ubiquity and grave morbidity. This historical inattention has frequently become manifested in flawed diagnostic and treatment paradigms. Molecular imaging could be instrumental in improving patient care on both fronts. In this Focus on Molecular Imaging review, we will examine recent advances in the use of PET, SPECT, MRI, and fluorescence imaging for the diagnosis and management of benign gynecological conditions, with particular emphasis on recent clinical reports, areas of need, and opportunities for growth., (© 2024 by the Society of Nuclear Medicine and Molecular Imaging.)

Purpose: Cadherin-17 (CDH17) is a calcium-dependent cell adhesion protein that is overexpressed in several adenocarcinomas, including gastric, colorectal, and pancreatic adenocarcinoma. High levels of CDH17 have been linked to metastatic disease and poor prognoses in patients with these malignancies, fueling interest in the protein as a target for diagnostics and therapeutics. Herein, we report the synthesis, in vitro validation, and in vivo evaluation of a CDH17-targeted 89 Zr-labeled immunoPET probe., Methods: The CDH17-targeting mAb D2101 was modified with an isothiocyanate-bearing derivative of desferrioxamine (DFO) to produce a chelator-bearing immunoconjugate - DFO-D2101 - and flow cytometry and surface plasmon resonance (SPR) were used to interrogate its antigen-binding properties. The immunoconjugate was then radiolabeled with zirconium-89 (t 1/2 ~ 3.3 days), and the serum stability and immunoreactive fraction of [ 89 Zr]Zr-DFO-D2101 were determined. Finally, [ 89 Zr]Zr-DFO-D2101's performance was evaluated in a trio of murine models of pancreatic ductal adenocarcinoma (PDAC): subcutaneous, orthotopic, and patient-derived xenografts (PDX). PET images were acquired over the course of 5 days, and terminal biodistribution data were collected after the final imaging time point., Results: DFO-D2101 was produced with a degree of labeling of ~ 1.1 DFO/mAb. Flow cytometry with CDH17-expressing AsPC-1 cells demonstrated that the immunoconjugate binds to its target in a manner similar to its parent mAb, while SPR with recombinant CDH17 revealed that D2101 and DFO-D2101 exhibit nearly identical K D values: 8.2 × 10 -9 and 6.7 × 10 -9 M, respectively. [ 89 Zr]Zr-DFO-D2101 was produced with a specific activity of 185 MBq/mg (5.0 mCi/mg), remained >80% stable in human serum over the course of 5 days, and boasted an immunoreactive fraction of >0.85. In all three murine models of PDAC, the radioimmunoconjugate yielded high contrast images, with high activity concentrations in tumor tissue and low uptake in non-target organs. Tumoral activity concentrations reached as high as >60 %ID/g in two of the cohorts bearing PDXs., Conclusion: Taken together, these data underscore that [ 89 Zr]Zr-DFO-D2101 is a highly promising probe for the non-invasive visualization of CDH17 expression in PDAC. We contend that this radioimmunoconjugate could have a significant impact on the clinical management of patients with both PDAC and gastrointestinal adenocarcinoma, most likely as a theranostic imaging tool in support of CDH17-targeted therapies., (© 2024. The Author(s).)

This paper is the first of a Series on theranostics that summarises the current landscape of the radiopharmaceutical sciences as they pertain to oncology. In this Series paper, we describe exciting developments in radiochemistry and the production of radionuclides, the development and translation of theranostics, and the application of artificial intelligence to our field. These developments are catalysing growth in the use of radiopharmaceuticals to the benefit of patients worldwide. We also highlight some of the key issues to be addressed in the coming years to realise the full potential of radiopharmaceuticals to treat cancer., Competing Interests: Declaration of interests SEL reports research support from Navidea Biopharmaceuticals, Fusion Pharmaceuticals, Cytosite Biopharma, Viewpoint Molecular Targeting, and Genzyme Corporation; has acted as an advisor for NorthStar Medical Radioisotopes and Trevarx Biomedical; and is supported by the Department of Energy as part of the Department of Energy University Isotope Network under grant DESC0021269. PJHS reports research support from Bristol Myers Squibb, Telix Pharmaceuticals, and Radionetics Oncology; has acted as an adviser to Synfast Consulting and Telix Pharmaceuticals; holds equity in Bristol Myers Squibb, Telix Pharmaceuticals, and Novartis; and is supported by National Institutes of Health R01 EB021155. AMS reports trial funding from EMD Serono, ITM, Telix Pharmaceuticals, AVID Radiopharmaceuticals, Fusion Pharmaceuticals, and Cyclotek; research funding from Medimmune, AVID Radiopharmaceuticals, Adalta, Antengene, Humanigen, Telix Pharmaceuticals, and Theramyc; is on the advisory boards of Imagion and ImmunOs; and is supported by Australian National Health and Medical Research Council grant number 1177837. ADW reports his role as the Editor in Chief of Nuclear Medicine and Biology. BMZ holds equity in Summit Biomedical Imaging. RPB is an advisor to 3B Pharmaceuticals (Berlin, Germany), ITM, Full Life Technologies, Sinotau, Jiangsu Huayi Technology, and Telix Pharmaceuticals. APK reports clinical trial funding from Novartis, Bayer, POINT, and Merck; and unpaid consulting for Novartis. JK reports an unrestricted grant from Janssen, and consulting fees from Telix and Novartis. JSL reports research support from Clarity Pharmaceuticals and Avid Radiopharmaceuticals; has acted as an adviser of Alpha-9 Theranostics, Clarity Pharmaceuticals, Earli, Evergreen Theragnostics, Inhibrix, Precirix, and Telix Pharmaceuticals; is a co-inventor on technologies licensed to Diaprost, Elucida Oncology, Theragnostics, CheMatech, Clarity Pharmaceuticals, and Samus Therapeutics; is the co-founder of pHLIP; holds equity in Summit Biomedical Imaging, Telix Pharmaceuticals, Clarity Pharmaceuticals, and Evergreen Theragnostics; and is supported by National Institutes of Health R35 CA232130. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)

Although the promise of radionuclides for the diagnosis and treatment of disease was recognised soon after the discovery of radioactivity in the late 19th century, the systematic use of radionuclides in medicine only gradually increased over the subsequent hundred years. The past two decades, however, has seen a remarkable surge in the clinical application of diagnostic and therapeutic radiopharmaceuticals, particularly in oncology. This development is an exciting time for the use of theranostics in oncology, but the rapid growth of this area of nuclear medicine has created challenges as well. In particular, the infrastructure for the manufacturing and distribution of radiopharmaceuticals remains in development, and regulatory bodies are still optimising guidelines for this new class of drug. One issue of paramount importance for achieving equitable access to theranostics is building a sufficiently trained workforce in high-income, middle-income, and low-income countries. Here, we discuss the key challenges and opportunities that face the field as it seeks to build its workforce for the 21st century., Competing Interests: Declaration of interests Outside the submitted work: AMS reports trial funding from EMD Serono, ITM, Telix Pharmaceuticals, AVID Radiopharmaceuticals, Fusion Pharmaceuticals, and Cyclotek; research funding from Medimmune, AVID Radiopharmaceuticals, Adalta, Antengene, Humanigen, Telix Pharmaceuticals, and Theramyc; and payment for participation in advisory boards of Imagion and Immunos. BMZ reports research funding from Evergreen Theragnostics, equity in Summit Biomedical Imaging, and has licensed technologies to Clarity Pharmaceuticals. SEL reports research support from Navidea Biopharmaceuticals, Fusion Pharmaceuticals, Cytosite Biopharma, Viewpoint Molecular Targeting, and Genzyme Corporation, and has acted as an advisor for NorthStar Medical Radioisotopes and Trevarx biomedical. PJHS reports research support from Bristol Myers Squibb, Telix Pharmaceuticals, and Radionetics Oncology; has acted as an adviser to Synfast Consulting and Telix Pharmaceuticals; and holds equity in Bristol Myers Squibb, Telix Pharmaceuticals, and Novartis. ADW reports their role as Editor-in-Chief of Nuclear Medicine and Biology. JSL reports research support from Clarity Pharmaceuticals and Avid Radiopharmaceuticals; has acted as an adviser of Alpha-9 Theranostics, Clarity Pharmaceuticals, Earli, Evergreen Theragnostics, Inhibrix, Precirix, and Telix Pharmaceuticals; is a co-inventor on technologies licensed to Diaprost, Elucida Oncology, Theragnostics, CheMatech, Clarity Pharmaceuticals, and Samus Therapeutics; is the co-founder of pHLIP; and holds equity in Summit Biomedical Imaging, Telix Pharmaceuticals, Clarity Pharmaceuticals, and Evergreen Theragnostics. AMS is supported by National Health and Medical Research Council grant number 1177837. SEL is supported by the Department of Energy as part of the DOE University Isotope Network, under grant DESC0021269. PJHS is supported by grant R01 EB021155. JSL is supported by National Institutes of Health grant R35 CA232130. MA-W, AJ, AK, PK, SV, STL, SKL, DP, JZ, JLU, and FG report no competing interests. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved, including those for text and data mining, AI training, and similar technologies.)

Protease-cleavable supramolecular oligopeptide nanofilaments are promising materials for targeted therapeutics and diagnostics. In these systems, single amino acid substitutions can have profound effects on the supramolecular structure and consequent proteolytic degradation, which are critical parameters for their intended applications. Herein, we describe changes to the self-assembly and proteolytic cleavage of iodine containing sequences for future translation into matrix metalloprotease (MMP-9)-activated supramolecular radio-imaging probes. We use a systematic single amino acid exchange in the tripeptide linker region of these peptide amphiphiles to provide insights into the role of each residue in the supramolecular assemblies. These modifications resulted in dramatic changes in the nature of the assembled structures formed, including an unexpected chiral inversion. By using circular dichroism, atomic force microscopy, Fourier transform infrared spectroscopy, and molecular dynamics simulations, we found that the GD loop, a common motif in β-turn elements, induced a reversal of the chiral orientation of the assembled nanofibers. In addition to the impact on peptide packing and chirality, MMP-9-catalyzed hydrolysis was evaluated for the four peptides, with the β-sheet content found to be a stronger determinant of enzymatic hydrolysis than supramolecular chirality. These observations provide fundamental insights into the sequence design in protease cleavable amphiphilic peptides with the potential for radio-labeling and selective biomedical applications.

Immunoglobulins, both full-length antibodies and smaller antibody fragments, have long been regarded as effective platforms for diagnostic and therapeutic radiopharmaceuticals. The construction of radiolabeled immunoglobulins (i.e., radioimmunoconjugates) requires the manipulation of the biomolecule through the attachment of a radiohalogen or the bioconjugation of a chelator that is subsequently used to coordinate a radiometal. Both synthetic approaches have historically relied upon the stochastic modification of amino acids within the immunoglobulin, a process which poses a risk to the structural and functional integrity of the biomolecule itself. Not surprisingly, radioimmunoconjugates with impaired antigen binding capacity will inevitably exhibit suboptimal in vivo performance. As a result, the biological characterization of any newly synthesized radioimmunoconjugate must include an assessment of whether it has retained its ability to bind its antigen. Herein, we provide straightforward and concise protocols for three assays that can be used to determine the immunoreactivity of a radioimmunoconjugate: (1) a cell-based linear extrapolation assay; (2) a cell-based antigen saturation assay; and (3) a resin- or bead-based assay. In addition, we will provide a critical analysis of the relative merits of each assay, an examination of the inherent limitations of immunoreactivity assays in general, and a discussion of other approaches that may be used to interrogate the biological behavior of radioimmunoconjugates., (© 2024. The Author(s).)

Despite decades of work, small-cell lung cancer (SCLC) remains a frustratingly recalcitrant disease. Both diagnosis and treatment are challenges: low-dose computed tomography (the approved method used for lung cancer screening) is unable to reliably detect early SCLC, and the malignancy's 5 year survival rate stands at a paltry 7%. Clearly, the development of novel diagnostic and therapeutic tools for SCLC is an urgent, unmet need. CD133 is a transmembrane protein that is expressed at low levels in normal tissue but is overexpressed by a variety of tumors, including SCLC. We previously explored CD133 as a biomarker for a novel autoantibody-to-immunopositron emission tomography (PET) strategy for the diagnosis of SCLC, work that first suggested the promise of the antigen as a radiotheranostic target in the disease. Herein, we report the in vivo validation of a pair of CD133-targeted radioimmunoconjugates for the PET imaging and radioimmunotherapy of SCLC. To this end, [ 89 Zr]Zr-DFO-αCD133 was first interrogated in a trio of advanced murine models of SCLC─i.e., orthotopic, metastatic, and patient-derived xenografts─with the PET probe consistently producing high activity concentrations (>%ID/g) in tumor lesions combined with low uptake in healthy tissues. Subsequently, a variant of αCD133 labeled with the β-emitting radiometal 177 Lu─[ 177 Lu]Lu-DTPA-A″-CHX-αCD133─was synthesized and evaluated in a longitudinal therapy study in a subcutaneous xenograft model of SCLC, ultimately revealing that treatment with a dose of 9.6 MBq of the radioimmunoconjugate produced a significant increase in median survival compared to a control cohort. Taken together, these data establish CD133 as a viable target for the nuclear imaging and radiopharmaceutical therapy of SCLC.

Radioimmunoconjugates targeting human epidermal growth factor receptor 2 (HER2) have shown potential to noninvasively visualize HER2-positive tumors. However, the stochastic approach that has been traditionally used to radiolabel these antibodies yields poorly defined and heterogeneous products with suboptimal in vivo performance. Here, we describe a first-in-human PET study on patients with HER2-positive breast cancer evaluating the safety, biodistribution, and dosimetry of 89 Zr-site-specific (ss)-pertuzumab PET, a site-specifically labeled radioimmunoconjugate designed to circumvent the limitations of random stochastic lysine labeling. Methods: Six patients with HER2-positive metastatic breast cancer were enrolled in a prospective clinical trial. Pertuzumab was site-specifically modified with desferrioxamine (DFO) via a novel chemoenzymatic strategy and subsequently labeled with 89 Zr. Patients were administered 74 MBq of 89 Zr-ss-pertuzumab in 20 mg of total antibody intravenously and underwent PET/CT at 1 d, 3-4 d, and 5-8 d after injection. PET imaging, whole-body probe counts, and blood draws were performed to assess the pharmacokinetics, biodistribution, and dosimetry. Results: 89 Zr-ss-pertuzumab PET/CT was used to assess HER2 status and heterogeneity to guide biopsy and decide the next line of treatment at progression. The radioimmunoconjugate was able to detect known sites of malignancy, suggesting that these tumor lesions were HER2-positive. The optimal imaging time point was 5-8 d after administration, and no toxicities were observed. Dosimetry estimates from OLINDA showed that the organs receiving the highest doses (mean ± SD) were kidney (1.8 ± 0.5 mGy/MBq), liver (1.7 ± 0.3 mGy/MBq), and heart wall (1.2 ± 0.1 mGy/MBq). The average effective dose for 89 Zr-ss-pertuzumab was 0.54 ± 0.03 mSv/MBq, which was comparable to both stochastically lysine-labeled 89 Zr-DFO-pertuzumab and 89 Zr-DFO-trastuzumab. One patient underwent PET/CT with both 89 Zr-ss-pertuzumab and 89 Zr-DFO-pertuzumab 1 mo apart, with 89 Zr-ss-pertuzumab demonstrating improved lesion detection and higher tracer avidity. Conclusion: This study demonstrated the safety, dosimetry, and potential clinical applications of 89 Zr-ss-pertuzumab PET/CT. 89 Zr-ss-pertuzumab may detect more lesions than 89 Zr-DFO-pertuzumab. Potential clinical applications include real-time evaluation of HER2 status to guide biopsy and assist in treatment decisions., (© 2024 by the Society of Nuclear Medicine and Molecular Imaging.)

Microplastics and nanoplastics have become ubiquitous environmental pollutants. The threat these plastics pose to human health has fueled research focused on their pathophysiology and toxicology, yet many of their fundamental properties - for example, their in vivo pharmacokinetics - remain poorly understood. In this investigation, we have harnessed positron emission tomography (PET) to track the in vivo fate of micro- and nanoplastics administered to mice intratracheally and intravenously. To this end, 1 μm and 20 nm diameter amine-functionalized polystyrene particles were modified with an isothiocyanate-bearing variant of desferrioxamine (DFO) and radiolabeled with the positron-emitting radiometal [ 89 Zr]Zr 4+ . Both radioplastics - [ 89 Zr]Zr-DFO-PS1000 and [ 89 Zr]Zr-DFO-PS20 - were produced in ∼95% radiochemical yield and found to be >85% stable to demetallation over one week at 37 °C in human serum and simulated lung fluid. The incubation of [ 89 Zr]Zr-DFO-PS1000 and [ 89 Zr]Zr-DFO-PS20 with MH-S cells revealed that the majority of the former were phagocytosed by alveolar macrophages within 4 h, while the latter largely evaded consumption. Finally, the in vivo behavior of the radioplastics was interrogated in mice upon intravenous and intratracheal administration. PET imaging and biodistribution experiments revealed that the intravenously injected plastics accumulated primarily in the liver and spleen, yielding hepatic radioactivity concentrations of 101 ± 48 %ID/g and 92 ± 22 %ID/g at 168 h post-injection for [ 89 Zr]Zr-DFO-PS1000 and [ 89 Zr]Zr-DFO-PS20 , respectively. In contrast, the mice that received the radioplastics via intratracheal installation displayed the highest uptake in the lungs at the end of one week: 4 ± 2 %ID/g for [ 89 Zr]Zr-DFO-PS1000 and 32 ± 6 %ID/g for [ 89 Zr]Zr-DFO-PS20. Ultimately, this work illustrates the critical role that the route of exposure plays in the bioaccumulation of plastic particles, reveals that size dramatically influences the pulmonary retention of inhaled particles, and underscores the value of PET imaging as a tool for studying the pharmacokinetics of environmental pollutants., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

Purpose: Site-specific approaches to bioconjugation produce well-defined and homogeneous immunoconjugates with potential for superior in vivo behavior compared to analogs synthesized using traditional, stochastic methods. The possibility of incorporating photoaffinity chemistry into a site-specific bioconjugation strategy is particularly enticing, as it could simplify and accelerate the preparation of homogeneous immunoconjugates for the clinic. In this investigation, we report the synthesis, in vitro characterization, and in vivo evaluation of a site-specifically modified, 89 Zr-labeled radioimmunoconjugate created via the reaction between an mAb and an Fc-binding protein bearing a photoactivatable 4-benzoylphenylalanine residue., Procedures: A variant of the Fc-binding Z domain of protein A containing a photoactivatable, 4-benzoylphenylalanine residue - Z(35BPA) - was modified with desferrioxamine (DFO), combined with the A33 antigen-targeting mAb huA33, and irradiated with UV light. The resulting immunoconjugate - DFO Z(35BPA) -huA33 - was purified and characterized via SDS-PAGE, MALDI-ToF mass spectrometry, surface plasmon resonance, and flow cytometry. The radiolabeling of DFO Z(35BPA) -huA33 was optimized to produce [ 89 Zr]Zr-DFO Z(35BPA) -huA33, and the immunoreactivity of the radioimmunoconjugate was determined with SW1222 human colorectal cancer cells. Finally, the in vivo performance of [ 89 Zr]Zr-DFO Z(35BPA) -huA33 in mice bearing subcutaneous SW1222 xenografts was interrogated via PET imaging and biodistribution experiments and compared to that of a stochastically labeled control radioimmunoconjugate, [ 89 Zr]Zr-DFO-huA33., Results: HuA33 was site-specifically modified with Z(35BPA)-DFO, producing an immunoconjugate with on average 1 DFO/mAb, high in vitro stability, and high affinity for its target. [ 89 Zr]Zr-DFO Z(35BPA) -huA33 was synthesized in 95% radiochemical yield and exhibited a specific activity of 2 mCi/mg and an immunoreactive fraction of ~ 0.85. PET imaging and biodistribution experiments revealed that high concentrations of the radioimmunoconjugate accumulated in tumor tissue (i.e., ~ 40%ID/g at 120 h p.i.) but also that the Z(35BPA)-bearing immunoPET probe produced higher uptake in the liver, spleen, and kidneys than its stochastically modified cousin, [ 89 Zr]Zr-DFO-huA33., Conclusions: Photoaffinity chemistry and an Fc-binding variant of the Z domain were successfully leveraged to create a novel site-specific strategy for the synthesis of radioimmunoconjugates. The probe synthesized using this method - DFO Z(35BPA) -huA33 - was well-defined and homogeneous, and the resulting radioimmunoconjugate ([ 89 Zr]Zr-DFO Z(35BPA) -huA33) boasted high specific activity, stability, and immunoreactivity. While the site-specifically modified radioimmunoconjugate produced high activity concentrations in tumor tissue, it also yielded higher uptake in healthy organs than a stochastically modified analog, suggesting that optimization of this system is necessary prior to clinical translation., (© 2023. The Author(s), under exclusive licence to World Molecular Imaging Society.)

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

The worldwide proliferation of persistent environmental pollutants is accelerating at an alarming rate. Not surprisingly, many of these pollutants pose a risk to human health. In this review, we examine recent literature in which molecular imaging and radiochemistry have been harnessed to study environmental pollutants. Specifically, these techniques offer unique ways to interrogate the pharmacokinetic profiles and bioaccumulation patterns of pollutants at environmentally relevant concentrations, thereby helping to determine their potential health risks., (© 2023 by the Society of Nuclear Medicine and Molecular Imaging.)

We report the in vitro characterization and in vivo evaluation of a novel 89 Zr-labeled radioimmunoconjugate synthesized using a site-selective bioconjugation strategy based on the oxidation of tyrosinase residues exposed by the deglycosylation of the IgG and the subsequent strain-promoted oxidation-controlled 1,2-quinone cycloaddition between these amino acids and trans -cyclooctene-bearing cargoes. More specifically, we site-selectively modified a variant of the A33 antigen-targeting antibody huA33 with the chelator desferrioxamine (DFO), thereby producing an immunoconjugate (DFO- SPOCQ huA33) with equivalent antigen binding affinity to its parent immunoglobulin but attenuated affinity for the FcγRI receptor. This construct was subsequently radiolabeled with [ 89 Zr]Zr 4+ to create a radioimmunoconjugate - [ 89 Zr]Zr-DFO- SPOCQ huA33 - in high yield and specific activity that exhibited excellent in vivo behavior in two murine models of human colorectal carcinoma., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)

Galectin-3 binding protein (Gal-3BP) is a glycoprotein that is overexpressed and secreted by several cancers and has been implicated as a marker of both tumor progression and poor prognosis in melanoma, non-small cell lung cancer, head and neck squamous cell carcinoma, and breast cancer. The expression of Gal-3BP by a variety of neoplasms makes it an enticing target for both diagnostics and therapeutics, including immuno-positron emission tomography (immunoPET) probes and antibody-drug conjugates (ADCs). Herein, we report the development, in vitro characterization, and in vivo evaluation of a pair of Gal-3BP-targeting radioimmunoconjugates for 89 Zr-immunoPET. A humanized anti-Gal-3BP antibody, 1959, and its corresponding ADC, 1959-sss/DM4 (DM4 = ravtansine), were modified with desferrioxamine (DFO) to yield DFO-1959 and DFO-1959-sss/DM4 immunoconjugates bearing 1-2 DFO/monoclonal antibody. Both DFO-modified immunoconjugates retained their affinity for Gal-3BP in enzyme-linked immunosorbent assay experiments. The chelator-bearing antibodies were radiolabeled with zirconium-89 ( t 1/2 ≈ 3.3 d) to produce radioimmunoconjugates ─ [ 89 Zr]Zr-DFO-1959 and [ 89 Zr]Zr-DFO-1959-sss/DM4 ─ with high specific activity (>444 MBq/mg, >12 mCi/mg) and stability (>80% intact after 168 h in human serum at 37 °C). In mice bearing subcutaneous Gal-3BP-secreting A375-MA1 xenografts, [ 89 Zr]Zr-DFO-1959 clearly delineated tumor tissue, reaching a maximum tumoral activity concentration (54.8 ± 15.8%ID/g) and tumor-to-background contrast (tumor-to-blood = 8.0 ± 4.6) at 120 h post-injection. The administration of [ 89 Zr]Zr-DFO-1959 to mice bearing subcutaneous Gal-3BP-expressing melanoma patient-derived xenografts produced similarly promising results. [ 89 Zr]Zr-DFO-1959 and [ 89 Zr]Zr-DFO-1959-sss/DM4 exhibited nearly identical pharmacokinetic profiles in the mice bearing A375-MA1 tumors, though the latter produced higher uptake in the spleen and kidneys. Both [ 89 Zr]Zr-DFO-1959 and [ 89 Zr]Zr-DFO-1959-sss/DM4 effectively visualized Gal-3BP-secreting tumors in murine models of melanoma. These results suggest that both probes could play a role in the clinical imaging of Gal-3BP-expressing malignancies, particularly as companion theranostics for the identification of patients likely to respond to Gal-3BP-targeted therapeutics such as 1959-sss/DM4.

The 2022 Nobel Prize in Chemistry was awarded to Professors K. Barry Sharpless, Morten Meldal and Carolyn Bertozzi for their pioneering roles in the advent of click chemistry. Sharpless and Meldal worked to develop the canonical click reaction-the copper-catalyzed azide-alkyne cycloaddition-while Bertozzi opened new frontiers with the creation of the bioorthogonal strain-promoted azide-alkyne cycloaddition. These two reactions have revolutionized chemical and biological science by facilitating selective, high yielding, rapid and clean ligations and by providing unprecedented ways to manipulate living systems. Click chemistry has affected every aspect of chemistry and chemical biology, but few disciplines have been impacted as much as radiopharmaceutical chemistry. The importance of speed and selectivity in radiochemistry make it an almost tailor-made application of click chemistry. In this Perspective, we discuss the ways in which the copper-catalyzed azide-alkyne cycloaddition, the strain-promoted azide-alkyne cycloaddition and a handful of 'next-generation' click reactions have transformed radiopharmaceutical chemistry, both as tools for more efficient radiosyntheses and as linchpins of technologies that have the potential to improve nuclear medicine., (© 2023. Springer Nature Limited.)

Site-specifically modified radioimmunoconjugates exhibit superior in vitro and in vivo behavior compared to analogues synthesized via traditional stochastic methods. However, the development of approaches to site-specific bioconjugation that combine high levels of selectivity, simple reaction conditions, and clinical translatability remains a challenge. Herein, we describe a novel solution to this problem: the use of dual-variable domain immunoglobulins (DVD-IgG). More specifically, we report the synthesis, in vitro evaluation, and in vivo validation of a 177 Lu-labeled radioimmunoconjugate based on HER2 DVD, a DVD-IgG containing the HER2-targeting variable domains of trastuzumab and the catalytic variable domains of IgG h38C2. To this end, we first modified HER2 DVD with a phenyloxadiazolyl methlysulfone-modified variant of the chelator CHX-A″-DTPA (PODS-CHX-A''-DTPA) and verified the site-specificity of the conjugation for the reactive lysines within the catalytic domains via chemical assay, MALDI-ToF mass spectrometry, and SDS-PAGE. The chelator-bearing immunoconjugate was subsequently labeled with [ 177 Lu]Lu 3+ to produce the completed radioimmunoconjugate, [ 177 Lu]Lu-CHX-A″-DTPA PODS - HER2 DVD, in >80% radiochemical conversion and a specific activity of 29.5 ± 7.1 GBq/μmol. [ 177 Lu]Lu-CHX-A″-DTPA PODS - HER2 DVD did not form aggregates upon prolonged incubation in human serum, displayed 87% stability to demetalation over a 7 days of incubation in serum, and exhibited an immunoreactive fraction of 0.95 with HER2-coated beads. Finally, we compared the pharmacokinetic profile of [ 177 Lu]Lu-CHX-A″-DTPA PODS - HER2 DVD to that of a 177 Lu-labeled variant of trastuzumab in mice bearing subcutaneous HER2-expressing BT-474 human breast cancer xenografts. The in vivo performance of [ 177 Lu]Lu-CHX-A″-DTPA PODS - HER2 DVD matched that of 177 Lu-labeled trastuzumab, with the former producing a tumoral activity concentration of 34.1 ± 12.1 %ID/g at 168 h and tumor-to-blood, tumor-to-liver, and tumor-to-kidney activity concentration ratios of 10.5, 9.6, and 21.8, respectively, at the same time point. Importantly, the DVD-IgG did not exhibit a substantially longer serum half-life than the traditional IgG despite its significantly larger size (202 kDa for the former vs 148 kDa for the latter). Taken together, these data suggest that DVD-IgGs represent a viable platform for the future development of highly effective site-specifically labeled radioimmunoconjugates for diagnostic imaging, theranostic imaging, and radioimmunotherapy.

Because of their rapid tumor accumulation and normal tissue clearance, single-domain antibody fragments (sdAbs) are an attractive vehicle for developing radiotherapeutics labeled with the α-emitter 211 At. Herein, we have evaluated iso -[ 211 At]AGMB-PODS, a prosthetic agent that combines a functionality for residualizing radiohalogens with a phenyloxadiazolyl methylsulfone (PODS) moiety for site-specific sdAb conjugation. Iso -[ 211 At]AGMB-PODS and its radioiodinated analogue were evaluated for thiol-selective conjugation to anti-HER2 5F7 sdAb bearing a C-terminus GGC tail. Both radiohalogenated PODS-5F7GGC conjugates were synthesized in good radiochemical yields and retained high binding affinity on HER2-positive BT474 breast carcinoma cells. Iso -[ 211 At]AGMB-PODS-5F7GGC was considerably more stable in vitro than its maleimide analogue in the presence of cysteine and human serum albumin (HSA) and exhibited excellent tumor uptake and high in vivo stability. Superior tumor-to-kidney activity ratios were seen for both radiohalogenated PODS-5F7GGC conjugates compared with [ 177 Lu]Lu-DOTA-PODS-5F7GGC. These results suggest that iso -[ 211 At]AGMB-PODS-5F7GGC warrants further evaluation for the treatment of HER2-expressing malignancies.

Small cell lung cancer (SCLC) is a deadly neuroendocrine tumor for which there are no screening methods sensitive enough to facilitate early, effective intervention. We propose targeting the neuroendocrine tumor neoantigen CD133 via antibody-based early detection and PET (immunoPET) to facilitate earlier and more accurate detection of SCLC. Methods: RNA sequencing datasets, immunohistochemistry, flow cytometry, and Western blots were used to quantify CD133 expression in healthy and SCLC patients. CD133 was imaged in vivo using near-infrared fluorescence (NIRF) immunoimaging, and 89 Zr immunoPET. Anti(α)-CD133 autoantibody levels were measured in SCLC patient plasma using antibody microarrays. Results: Across 6 publicly available datasets, CD133 messenger RNA was found to be higher in SCLC tumors than in other tissues, including healthy or normal adjacent lung and non-SCLC samples. Critically, the upregulation of CD133 messenger RNA in SCLC was associated with a significant increase (hazard ratio, 2.62) in death. CD133 protein was expressed in primary human SCLC, in SCLC patient-derived xenografts, and in both SCLC cell lines tested (H82 and H69). Using an H82 xenograft mouse model, we first imaged CD133 expression with NIRF. Both in vivo and ex vivo NIRF clearly showed that a fluorophore-tagged αCD133 homed to lung tumors. Next, we validated the noninvasive visualization of subcutaneous and orthotopic H82 xenografts via immunoPET. An αCD133 antibody labeled with the positron-emitting radiometal 89 Zr demonstrated significant accumulation in tumor tissue while producing minimal uptake in healthy organs. Finally, plasma αCD133 autoantibodies were found in subjects from cohort studies up to 1 year before SCLC diagnosis. Conclusion: In light of these findings, we conclude that the presence of αCD133 autoantibodies in a blood sample followed by CD133-targeted 89 Zr-immunoPET could be an effective early detection screening strategy for SCLC., (© 2022 by the Society of Nuclear Medicine and Molecular Imaging.)

VlsE (variable major protein-like sequence, expressed) is an outer surface protein of the Lyme disease pathogen ( Borreliella species) responsible for its within-host antigenic variation and a key diagnostic biomarker of Lyme disease. However, the high sequence variability of VlsE poses a challenge to the development of consistent VlsE-based diagnostics and therapeutics. In addition, the standard diagnostic protocols detect immunoglobins elicited by the Lyme pathogen, not the presence of the pathogen or its derived antigens. Here, we described the development of recombinant monoclonal antibodies (rMAbs) that bound specifically to conserved epitopes on VlsE. We first quantified amino-acid sequence variability encoded by the vls genes from 13 B. burgdorferi genomes by evolutionary analyses. We showed broad inconsistencies of the sequence phylogeny with the genome phylogeny, indicating rapid gene duplications, losses, and recombination at the vls locus. To identify conserved epitopes, we synthesized peptides representing five long conserved invariant regions (IRs) on VlsE. We tested the antigenicity of these five IR peptides using sera from three mammalian host species including human patients, the natural reservoir white-footed mouse (Peromyscus leucopus), and VlsE-immunized New Zealand rabbits (Oryctolagus cuniculus). The IR4 and IR6 peptides emerged as the most antigenic and reacted strongly with both the human and rabbit sera, while all IR peptides reacted poorly with sera from natural hosts. Four rMAbs binding specifically to the IR4 and IR6 peptides were identified, cloned, and purified. Given their specific recognition of the conserved epitopes on VlsE, these IR-specific rMAbs are potential novel diagnostic and research agents for direct detection of Lyme disease pathogens regardless of strain heterogeneity. IMPORTANCE Current diagnostic protocols of Lyme disease indirectly detect the presence of antibodies produced by the patient upon infection by the bacterial pathogen, not the pathogen itself. These diagnostic tests tend to underestimate early-stage bacterial infections before the patients develop robust immune responses. Further, the indirect tests do not distinguish between active or past infections by the Lyme disease bacteria in a patient sample. Here, we described novel monoclonal antibodies that have the potential to become the basis of direct and definitive diagnostic detection of the Lyme disease pathogen, regardless of its genetic heterogeneity.

The synthesis of radioimmunoconjugates via the stochastic attachment of bifunctional chelators to lysines can yield heterogeneous products with suboptimal in vitro and in vivo behavior. In response to this, several site-selective approaches to bioconjugation have been developed, yet each has intrinsic drawbacks, such as the need for expensive reagents or the complexity of incorporating unnatural amino acids into IgGs. Herein, we describe the use of a simple and facile approach to lysine-directed site-selective bioconjugation for the generation of radioimmunoconjugates. This strategy relies upon on the selective modification of single lysine residues within each light chain of the monoclonal antibody (mAb) with a branched azide-bearing perfluorophenyl ester (PFP-bisN 3 ) followed by the ligation of dibenzocyclooctyne (DBCO)-bearing payloads to these bioorthogonal handles via the strain-promoted azide-alkyne cycloaddition. This methodology was used to create [ 89 Zr]Zr- SSK DFO-pertuzumab, a radioimmunoconjugate of the HER2-targeting mAb pertuzumab labeled with desferrioxamine (DFO) and the positron-emitting radiometal zirconium-89 ( 89 Zr). [ 89 Zr]Zr- SSK DFO-pertuzumab was compared to a pair of analogous probes: one synthesized via random lysine modification ([ 89 Zr]Zr-DFO-pertuzumab) and another via thiol-maleimide chemistry ([ 89 Zr]Zr- mal DFO-pertuzumab). The bioconjugation strategy was assessed using ESI mass spectrometry, SDS-PAGE, and autoradiography. All three immunoconjugates demonstrated comparable binding to HER2 via flow cytometry and surface plasmon resonance (SPR), and 89 Zr-labeled variants of each were synthesized in >99% radiochemical yield and molar activities of up to ∼55.5 GBq/μmol (10 mCi/mg). Subsequently, the in vivo behavior of this trio of 89 Zr-immunoPET probes was interrogated in athymic nude mice bearing subcutaneous HER2-expressing BT-474 human breast cancer xenografts. [ 89 Zr]Zr- SSK DFO-pertuzumab, [ 89 Zr]Zr- mal DFO-pertuzumab, and [ 89 Zr]Zr-DFO-pertuzumab produced positron emission tomography (PET) images with high tumoral uptake and high tumor-to-healthy organ activity concentration ratios. A terminal biodistribution study complemented the PET results, revealing tumoral activity concentrations of 126.9 ± 50.3%ID/g, 86.9 ± 53.2%ID/g, and 92.5 ± 27.2%ID/g at 144 h post-injection for [ 89 Zr]Zr- SSK DFO-pertuzumab, [ 89 Zr]Zr- mal DFO-pertuzumab, and [ 89 Zr]Zr-DFO-pertuzumab, respectively. Taken together, the data clearly illustrate that this highly modular and facile approach to site-selective bioconjugation produces radioimmunoconjugates that are better-defined and more homogeneous than stochastically modified constructs and also exhibit excellent in vitro and in vivo performance. Furthermore, we contend that this lysine-directed strategy holds several key advantages over extant approaches to site-selective bioconjugation, especially in the context of production for the clinic.

The last decade has witnessed the creation of a highly effective approach to in vivo pretargeting based on the inverse electron demand Diels-Alder (IEDDA) click ligation between tetrazine (Tz) and trans -cyclooctene (TCO). Despite the steady progression of this technology toward the clinic, concerns have persisted regarding whether this in vivo chemistry will work in humans given their larger size and blood volume. In this work, we describe the use of a 64 Cu-labeled Tz radioligand ([ 64 Cu]Cu-SarAr-Tz) and a TCO-bearing bisphosphonate (TCO-BP) for the pretargeted positron emission tomography (PET) imaging of osteodestructive lesions in a large animal model: companion dogs. First, in a small animal pilot study, healthy mice were injected with TCO-BP followed after 1 or 6 h by [ 64 Cu]Cu-SarAr-Tz. PET images were collected 1, 6, and 24 h after the administration of [ 64 Cu]Cu-SarAr-Tz, revealing that this approach produced high activity concentrations in the bone (>20 and >15%ID/g in the femur and humerus, respectively, at 24 h post injection) as well as high target-to-background contrast. Subsequently, companion dogs ( n = 5) presenting with osteodestructive lesions were administered TCO-BP (5 or 10 mg/kg) followed 1 h later by [ 64 Cu]Cu-SarAr-Tz (2.2-7.3 mCi; 81.4-270.1 MBq). PET scans were collected for each dog 4 h after the administration of the radioligand, and SUV values for the osteodestructive lesions, healthy bones, and kidneys were determined. In these animals, pretargeted PET clearly delineated healthy bone and produced very high activity concentrations in osteodestructive lesions. Low levels of uptake were observed in all healthy organs except for the kidneys and bladder due to the renal excretion of excess radioligand. Ultimately, this work not only illustrates that pretargeted PET with TCO-BP and [ 64 Cu]Cu-SarAr-Tz is an effective tool for the visualization of osteodestructive lesions but also demonstrates for the first time that in vivo pretargeting based on IEDDA click chemistry is feasible in large animals.

Radiolabeled antibodies have become indispensable tools in nuclear medicine. However, the natural roles of antibodies within the immune system mean that they have several intrinsic limitations as a platform for radiopharmaceuticals. In recent years, the field has increasingly turned to antibody engineering to circumvent these issues while retaining the manifold benefits of the immunoglobulin framework. In this "Focus on Molecular Imaging" review, we cover recent advances in the application of antibody engineering to immunoPET, immunoSPECT, and radioimmunotherapy. Specifically, we address how antibody engineering has been used to improve radioimmunoconjugates on four fronts: optimizing pharmacokinetics, facilitating site-specific bioconjugation, modulating Fc interactions, and creating bispecific constructs., (© 2022 by the Society of Nuclear Medicine and Molecular Imaging.)

Purpose: Advances in our understanding of the contribution of aberrant glycosylation to the pro-oncogenic signaling and metastasis of tumor cells have reinvigorated the development of mucin-targeted therapies. Here, we validate the tumor-targeting ability of a novel monoclonal antibody (mAb), AR9.6, that binds MUC16 and abrogates downstream oncogenic signaling to confer a therapeutic response., Experimental Design: The in vitro and ex vivo validation of the binding of AR9.6 to MUC16 was achieved via flow cytometry, radioligand binding assay (RBA), and immunohistochemistry (IHC). The in vivo MUC16 targeting of AR9.6 was validated by creating a 89Zr-labeled radioimmunoconjugate of the mAb and utilizing immunoPET and ex vivo biodistribution studies in xenograft models of human ovarian and pancreatic cancer., Results: Flow cytometry, RBA, and IHC revealed that AR9.6 binds to ovarian and pancreatic cancer cells in an MUC16-dependent manner. The in vivo radiopharmacologic profile of 89Zr-labeled AR9.6 in mice bearing ovarian and pancreatic cancer xenografts confirmed the MUC16-dependent tumor targeting by the radioimmunoconjugate. Radioactivity uptake was also observed in the distant lymph nodes (LNs) of mice bearing xenografts with high levels of MUC16 expression (i.e., OVCAR3 and Capan-2). IHC analyses of these PET-positive LNs highlighted the presence of shed antigen as well as necrotic, phagocytized, and actively infiltrating neoplastic cells. The humanization of AR9.6 did not compromise its ability to target MUC16-expressing tumors., Conclusions: The unique therapeutic mechanism of AR9.6 combined with its excellent in vivo tumor targeting makes it a highly promising theranostic agent. huAR9.6 is poised for clinical translation to impact the management of metastatic ovarian and pancreatic cancers., (©2021 The Authors; Published by the American Association for Cancer Research.)

Matrix metalloproteinase (MMP) enzymes are over-expressed by some metastatic cancers, in which they are responsible for the degradation and remodeling of the extracellular matrix. In recent years, MMPs have emerged as promising targets for enzyme-responsive diagnostic probes because oligopeptides can be designed to be selectively hydrolyzed by exposure to these enzymes. With the ultimate goal of developing radio-iodinated peptides as supramolecular building blocks for MMP-sensitive tools for nuclear imaging and therapy, we designed three MMP-9-responsive peptides containing either tyrosine or iodotyrosine to assess the impact of iodotyrosine introduction to the peptide structure and cleavage kinetics. We found that the peptides containing iodotyrosine underwent more rapid and more complete hydrolysis by MMP-9. While the peptides under investigation were predominantly disordered, it was found that iodination increased the degree of aromatic residue-driven aggregation of the peptides. We determined that these iodination-related trends stem from the improved overall intramolecular order through H- and halogen bonding, in addition to intermolecular organization of the self-assembled peptides due to steric and electrostatic effects introduced by the halogenated tyrosine. These fundamental observations provide insights for the development of enzyme-triggered peptide aggregation tools for localized radioactive iodine-based tumor imaging.

Delta-like ligand 3 (DLL3) is a therapeutic target for the treatment of small cell lung cancer, neuroendocrine prostate cancer, and isocitrate dehydrogenase mutant glioma. In the clinic, DLL3-targeted 89 Zr-immunoPET has the potential to aid in the assessment of disease burden and facilitate the selection of patients suitable for therapies that target the antigen. The overwhelming majority of 89 Zr-labeled radioimmunoconjugates are synthesized via the random conjugation of desferrioxamine (DFO) to lysine residues within the immunoglobulin. While this approach is admittedly facile, it can produce heterogeneous constructs with suboptimal in vitro and in vivo behavior. In an effort to circumvent these issues, we report the development and preclinical evaluation of site-specifically labeled radioimmunoconjugates for DLL3-targeted immunoPET. To this end, we modified a cysteine-engineered variant of the DLL3-targeting antibody SC16-MB1 with two thiol-reactive variants of DFO: one bearing a mal eimide moiety (Mal-DFO) and the other containing a p henyl o xa d iazolyl methyl s ulfone group (PODS-DFO). In an effort to obtain immunoconjugates with a D FO-to- a ntibody r atio (DAR) of 2, we explored both the reduction of the antibody with tris(2-carboxyethyl) phosphine (TCEP) as well as the use of a combination of glutathione and arginine as reducing and stabilizing agents, respectively. While exerting control over the DAR of the immunoconjugate proved cumbersome using TCEP, the use of glutathione and arginine enabled the selective reduction of the engineered cysteines and thus the formation of homogeneous immunoconjugates. A head-to-head comparison of the resulting 89 Zr-radioimmunoconjugates in mice bearing DLL3-expressing H82 xenografts revealed no significant differences in tumoral uptake and showed comparable radioactivity concentrations in most healthy nontarget organs. However, 89 Zr-DFO PODS - DAR2 SC16-MB1 produced 30% lower uptake (3.3 ± 0.5 %ID/g) in the kidneys compared to 89 Zr-DFO Mal - DAR2 SC16-MB1 (4.7 ± 0.5 %ID/g). In addition, H82-bearing mice injected with a 89 Zr-labeled isotype-control radioimmunoconjugate synthesized using PODS exhibited ∼40% lower radioactivity in the kidneys compared to mice administered its maleimide-based counterpart. Taken together, these results demonstrate the improved in vivo performance of the PODS-based radioimmunoconjugate and suggest that a stable, well-defined DAR2 radiopharmaceutical may be suitable for the clinical immunoPET of DLL3-expressing cancers.

Radiolabeled antibodies have shown promise as tools for both the nuclear imaging and endoradiotherapy of cancer, but the protracted circulation time of radioimmunoconjugates can lead to high radiation doses to healthy tissues. To circumvent this issue, we have developed an approach to positron emission tomography (PET) imaging and radioimmunotherapy (RIT) predicated on radiolabeling the antibody after it has reached its target within the body. This in vivo pretargeting strategy is based on the rapid and bio-orthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans-cyclooctene (TCO). Pretargeted PET imaging and RIT using TCO-modified antibodies in conjunction with Tz-bearing radioligands produce high activity concentrations in target tissues as well as reduced radiation doses to healthy organs compared to directly labeled radioimmunoconjugates. Herein, we describe how to prepare a TCO-modified antibody (humanized A33-TCO) as well as how to synthesize two Tz-bearing radioligands: one labeled with the positron-emitting radiometal copper-64 ([ 64 Cu]Cu-SarAr-Tz) and one labeled with the β-emitting radiolanthanide lutetium-177 ([ 177 Lu]Lu-DOTA-PEG 7 -Tz). We also provide a detailed description of pretargeted PET and pretargeted RIT experiments in a murine model of human colorectal carcinoma. Proper training in both radiation safety and the handling of laboratory mice is required for the successful execution of this protocol.

The proliferation of plastics in the environment continues at an alarming rate. Plastic particles have been found to be persistent and ubiquitous pollutants in a variety of environments, including sea water, fresh water, soil, and air. In light of this phenomenon, the scientific and medical communities have become increasingly wary of the dangers posed to human health by chronic exposure to microplastics (< 5 mm diameter) and nanoplastics (< 100 nm diameter). A critical component of the study of the health effects of these pollutants is the accurate determination of their pharmacokinetic behavior in vivo. Herein, we report the first use of molecular imaging to track polystyrene (PS) micro- and nanoplastic particles in mammals. To this end, we have modified PS particles of several sizes-diameters of 20 nm, 220 nm, 1 µm, and 6 µm-with the chelator desferrioxamine (DFO) and radiolabeled these DFO-bearing particles with the positron-emitting radiometal zirconium-89 ( 89 Zr; t 1/2  ~ 3.3 d). Subsequently, positron emission tomography (PET) was used to visualize the biodistribution of these radioplastics in C57BL/6J mice at 6, 12, 24, and 48 h after ingestion. The imaging data reveal that the majority of the radioplastics remain in the gastrointestinal tract and are eliminated through the feces by 48 h post-ingestion, a result reinforced by acute biodistribution studies. Ultimately, this work suggests that nuclear imaging-and PET in particular-can be a sensitive and effective tool in the urgent and rapidly growing effort to study the in vivo behavior and potential toxicity of micro- and nanoplastics.

Metastatic melanoma can be difficult to detect until at the advanced state that decreases the survival rate of patients. Several FDA-approved BRAF inhibitors have been used for treatment of metastatic melanoma, but overall therapeutic efficacy has been limited. Lutetium-177 ( 177 Lu) enables simultaneous tracking of tracer accumulation with single-photon emission computed tomography and radiotherapy. Therefore, the codelivery of 177 Lu alongside chemotherapeutic agents using nanoparticles (NPs) might improve the therapeutic outcome in metastatic melanoma. Cellulose nanocrystals (CNC NPs) can particularly deliver payloads to lung capillaries in vivo. Herein, 177 Lu-labeled CNC NPs loaded with vemurafenib ([ 177 Lu]Lu-CNC-V NPs) is developed and the therapeutic effect in BRAF V600E mutation-harboring YUMM1.G1 murine model of lung metastatic melanoma is investigated. The [ 177 Lu]Lu-CNC-V NPs demonstrate favorable radiolabel stability, drug release profile, cellular uptake, and cell growth inhibition in vitro. In vivo biodistribution reveals significant retention of the [ 177 Lu]Lu-CNC-V NPs in the lung, liver, and spleen. Ultimately, the median survival time of animals is doubly increased after treatment with [ 177 Lu]Lu-CNC-V NPs compared to control groups. The enhanced therapeutic efficacy of [ 177 Lu]Lu-CNC-V NPs in the lung metastatic melanoma animal model provides convincing evidence for the potential of clinical translation for theranostic CNC NP-based drug delivery systems after intravenous administration., (© 2021 The Authors. Small published by Wiley-VCH GmbH.)

Triple negative breast cancer (TNBC) has no targeted detection or treatment method. Mutant p53 (mtp53) is overexpressed in >80% of TNBCs, and the stability of mtp53 compared to the instability of wild-type p53 (wtp53) in normal cells makes mtp53 a promising TNBC target for diagnostic and theranostic imaging. We generated Cy5p53Tet, a novel nucleus-penetrating mtp53-oligomerization-domain peptide (mtp53ODP) to the tetramerization domain (TD) of mtp53. This mtp53ODP contains the p53 TD sequence conjugated to a Cy5 fluorophore for near-infrared fluorescence imaging (NIRF). In vitro co-immunoprecipitation and glutaraldehyde cross-linking showed a direct interaction between mtp53 and Cy5p53Tet. Confocal microscopy and flow cytometry demonstrated higher uptake of Cy5p53Tet in the nuclei of TNBC MDA-MB-468 cells with mtp53 R273H than in ER-positive MCF7 cells with wtp53. Furthermore, depletion of mtp53 R273H caused a decrease in the uptake of Cy5p53Tet in nuclei. In vivo analysis of the peptide in mice bearing MDA-MB-468 xenografts showed that Cy5p53Tet could be detected in tumor tissue 12 min after injection. In these in vivo experiments, significantly higher uptake of Cy5p53Tet was observed in mtp53-expressing MDA-MB-468 xenografts compared with the wtp53-expressing MCF7 tumors. Cy5p53Tet has clinical potential as an intraoperative imaging agent for fluorescence-guided surgery, and the mtp53ODP scaffold shows promise for modification in the future to enable the delivery of a wide variety of payloads including radionuclides and toxins to mtp53-expressing TNBC tumors.

Chemoselective reactions with thiols have long held promise for the site-specific bioconjugation of antibodies and antibody fragments. Yet bifunctional probes bearing monovalent maleimides-long the "gold standard" for thiol-based ligations-are hampered by two intrinsic issues: the in vivo instability of the maleimide-thiol bond and the need to permanently disrupt disulfide linkages in order to facilitate bioconjugation. Herein, we present the synthesis, characterization, and validation of DiPODS, a novel bioconjugation reagent containing a pair of oxadiazolyl methyl sulfone moieties capable of irreversibly forming covalent bonds with two thiolate groups while simultaneously rebridging disulfide linkages. The reagent was synthesized from commercially available starting materials in 8 steps, during which rotamers were encountered and investigated both experimentally and computationally. DiPODS is designed to be modular and can thus be conjugated to any payload through a pendant terminal primary amine (DiPODS-PEG 4 -NH 2 ). Subsequently, the modification of a HER2-targeting Fab with a fluorescein-conjugated variant of DiPODS (DiPODS-PEG 4 -FITC) reinforced the site-specificity of the reagent, illustrated its ability to rebridge disulfide linkages, and produced an immunoconjugate with in vitro properties superior to those of an analogous construct created using traditional stochastic bioconjugation techniques. Ultimately, we believe that this work has particularly important implications for the synthesis of immunoconjugates, specifically for ensuring that the attachment of cargoes to immunoglobulins is robust, irreversible, and biologically and structurally benign.

Over the past decade, theranostic imaging has emerged as a powerful clinical tool in oncology for identifying patients likely to respond to targeted therapies and for monitoring the response of patients to treatment. Herein, we report a theranostic approach to pretargeted radioimmunotherapy (PRIT) based on a pair of radioisotopes of copper: positron-emitting copper-64 ( 64 Cu, t 1/2 = 12.7 h) and beta particle-emitting copper-67 ( 67 Cu, t 1/2 = 61.8 h). This strategy is predicated on the in vivo ligation between a trans-cyclooctene (TCO)-bearing antibody and a tetrazine (Tz)-based radioligand via the rapid and bioorthogonal inverse electron-demand Diels-Alder reaction. Longitudinal therapy studies were conducted in a murine model of human colorectal carcinoma using an immunoconjugate of the huA33 antibody modified with TCO (huA33-TCO) and a 67 Cu-labeled Tz radioligand ([ 67 Cu]Cu-MeCOSar-Tz). The injection of huA33-TCO followed 72 h later by the administration of 18.5, 37.0, or 55.5 MBq of [ 67 Cu]Cu-MeCOSar-Tz produced a dose-dependent therapeutic response, with the median survival time increasing from 68 d for the lowest dose to >200 d for the highest. Furthermore, we observed that mice that received the highest dose of [ 67 Cu]Cu-MeCOSar-Tz in a fractionated manner exhibited improved hematological values without sacrificing therapeutic efficacy. Dual radionuclide experiments in which a single administration of huA33-TCO was followed by separate injections of [ 64 Cu]Cu-MeCOSar-Tz and [ 67 Cu]Cu-MeCOSar-Tz revealed that the positron emission tomography images produced by the former accurately predicted the efficacy of the latter. In these experiments, a correlation was observed between the tumoral uptake of [ 64 Cu]Cu-MeCOSar-Tz and the subsequent therapeutic response to [ 67 Cu]Cu-MeCOSar-Tz., Competing Interests: Competing interest statement: E.v.D., M.H., A.H., and C.B. are/were employed by Clarity Pharmaceuticals, the licensee of the intellectual property for the sarcophagine chelators and the MeCoSar-Tz construct. P.S.D. and N.Z. are inventors of intellectual property in this area of research which has been licensed from the University of Melbourne to Clarity Pharmaceuticals. P.S.D. and J.S.L. serve on the Scientific Advisory Board of Clarity Pharmaceuticals., (Copyright © 2020 the Author(s). Published by PNAS.)

Ovarian cancer is the fifth leading cause of cancer deaths among women, accounting for more deaths than any other cancer of the female reproductive system. The foundation of its management consists of cytoreductive surgery (CRS) followed by systemic chemotherapy, with the completeness of surgical resection consistently identified as one of the most important prognostic factors for the disease. The goal of our investigation is the development of a near-infrared fluorescence (NIRF) imaging agent for the intraoperative imaging of high-grade serous ovarian cancer (HGSOC). As surgeons are currently limited to the visual and manual assessment of tumor tissue during CRS, this technology could facilitate more complete resections as well as serve important functions at other points in the surgical management of the disease. Elevated levels of cancer antigen 125 (CA125) have proven a useful biomarker of HGSOC, and the CA125-targeting antibody B43.13 has shown potential as a platform for immunoPET imaging in murine models of ovarian cancer. Herein, we report the development of a NIRF imaging agent based on B43.13: ss B43.13-IR800. We site-specifically modified the heavy chain glycans of B43.13 with the near-infrared dye IRDye 800CW using a chemoenzymatic approach developed in our laboratories. SDS-PAGE analysis confirmed the specificity of the conjugation reaction, and flow cytometry, immunostaining, and fluorescence microscopy verified the specific binding of ss B43.13-IR800 to CA125-expressing OVCAR3 human ovarian cancer cells. NIRF imaging studies demonstrated that ss B43.13-IR800 can be used to image CA125-expressing HGSOC tumors in subcutaneous, orthotopic, and patient-derived xenograft mouse models. Finally, ex vivo analyses confirmed that ss B43.13-IR800 can bind and identify CA125-expressing cells in primary tumor and metastatic lymph node samples from human patients with HGSOC.

Background Human epidermal growth factor receptor 2 (HER2)-targeted therapies are successful in patients with HER2-positive malignancies; however, spatial and temporal heterogeneity of HER2 expression may prevent identification of optimal patients for these therapies. Purpose To determine whether imaging with the HER2-targeted PET tracer zirconium 89 ( 89 Zr)-pertuzumab can depict HER2-positive metastases in women with HER2-negative primary breast cancer. Materials and Methods From January to June 2019, women with biopsy-proven HER2-negative primary breast cancer and biopsy-proven metastatic disease were enrolled in a prospective clinical trial ( ClinicalTrials.gov NCT02286843) and underwent 89 Zr-pertuzumab PET/CT for noninvasive whole-biopsy evaluation of potential HER2-positive metastases. 89 Zr-pertuzumab-avid foci that were suspicious for HER2-positive metastases were tissue sampled and examined by pathologic analysis to document HER2 status. Results Twenty-four women (mean age, 55 years ± 11 [standard deviation]) with HER2-negative primary breast cancer were enrolled. Six women demonstrated foci at 89 Zr-pertuzumab PET/CT that were suspicious for HER2-positive disease. Of these six women, three had biopsy-proven HER2-positive metastases, two had pathologic findings that demonstrated HER2-negative disease, and one had a fine-needle aspirate with inconclusive results. Conclusion Human epidermal growth factor receptor 2 (HER2)-targeted imaging with zirconium 89-pertuzumab PET/CT was successful in detecting HER2-positive metastases in women with HER2-negative primary breast cancer. This demonstrates the ability of targeted imaging to identify patients for targeted therapies that might not otherwise be considered. © RSNA, 2020 Online supplemental material is available for this article. See the editorial by Mankoff and Pantel in this issue.

Purpose: Glioblastoma multiforme is a highly aggressive form of brain cancer whose location, tendency to infiltrate healthy surrounding tissue, and heterogeneity significantly limit survival, with scant progress having been made in recent decades., Experimental Design: 123 I-MAPi (Iodine-123 Meitner-Auger PARP1 inhibitor) is a precise therapeutic tool composed of a PARP1 inhibitor radiolabeled with an Auger- and gamma-emitting iodine isotope. Here, the PARP inhibitor, which binds to the DNA repair enzyme PARP1, specifically targets cancer cells, sparing healthy tissue, and carries a radioactive payload within reach of the cancer cells' DNA., Results: The high relative biological efficacy of Auger electrons within their short range of action is leveraged to inflict DNA damage and cell death with high precision. The gamma ray emission of 123 I-MAPi allows for the imaging of tumor progression and therapy response, and for patient dosimetry calculation. Here we demonstrated the efficacy and specificity of this small-molecule radiotheranostic in a complex preclinical model. In vitro and in vivo studies demonstrate high tumor uptake and a prolonged survival in mice treated with 123 I-MAPi when compared with vehicle controls. Different methods of drug delivery were investigated to develop this technology for clinical applications, including convection enhanced delivery and intrathecal injection., Conclusions: Taken together, these results represent the first full characterization of an Auger-emitting PARP inhibitor which demonstrate a survival benefit in mouse models of GBM and confirm the high potential of 123 I-MAPi for clinical translation., (©2020 American Association for Cancer Research.)

The N -linked biantennary glycans on the heavy chain of immunoglobulin G (IgG) antibodies (mAbs) are instrumental in the recognition of the Fc region by Fc-gamma receptors (FcγR). In the case of full-length mAb-based imaging tracers targeting immune cell populations, these Fc:FcγR interactions can potentially deplete effector cells responsible for tumor clearance. To bypass this problem, we hypothesize that the enzymatic removal of the Fc glycans will disrupt Fc:FcγR interactions and spare tracer-targeted immune cells from depletion during immunopositron emission tomography (immunoPET) imaging. Herein, we compared the in vitro and in vivo properties of 89 Zr-radiolabeled CD8-specific murine mAb (anti-CD8 wt , clone 2.43), a well-known depleting mAb, and its deglycosylated counterpart (anti-CD8 degly ). Deglycosylation was achieved via enzymatic treatment with the peptide: N- glycosidase F (PNGaseF). Both anti-CD8 wt and anti-CD8 degly mAbs were conjugated to p -SCN-Bn-desferrioxamine (DFO) and labeled with 89 Zr. Bindings of both DFO-conjugated mAbs to FcγR and CD8 + splenocytes were compared. In vivo imaging and distribution studies were conducted to examine the specificity and pharmacokinetics of the radioimmunoconjugates in tumor-naive and CT26 colorectal tumor-bearing mice. Ex vivo analysis of CD8 + T cell population in spleens and tumors obtained postimaging were measured via flow cytometry and qRT-PCR. The removal of the Fc glycans from anti-CD8 wt was confirmed via SDS-PAGE. A reduction in FcγR interaction was exhibited by DFO-anti-CD8 degly , while its binding to CD8 remained unchanged. Tissue distribution showed similar pharmacokinetics of [ 89 Zr]Zr-DFO-anti-CD8 degly and the wt radioimmunoconjugate. In vivo blocking studies further demonstrated retained specificity of the deglycosylated radiotracer for CD8. From the imaging studies, no difference in accumulation in both spleens and tumors was observed between both radiotracers. Results from the flow cytometry analysis confirmed depletion of CD8 + T cells in spleens of mice administered with DFO-anti-CD8 wt , whereas an increase in CD8 + T cells was shown with DFO-anti-CD8 degly . No statistically significant difference in tumor infiltrating CD8 + T cells was observed in cohorts administered with the probes when compared to control unmodulated mice. CD8 mRNA levels from excised tumors showed increased transcripts of the antigen in mice administered with [ 89 Zr]Zr-DFO-anti-CD8 degly compared to mice imaged with [ 89 Zr]Zr-DFO-anti-CD8 wt . In conclusion, the removal of Fc glycans offers a straightforward approach to develop full length antibody-based imaging probes specifically for detecting CD8 + immune molecules with no consequential depletion of their target cell population in peripheral tissues.

High expression levels of the tumor-associated antigen MUC1 have been correlated with tumor aggressiveness, poor response to therapy, and poor survival in several tumor types, including breast, pancreatic, and epithelial ovarian cancer. Herein, we report the synthesis, characterization, and in vivo evaluation of a novel radioimmunoconjugate for the immuno-positron emission tomography (immunoPET) imaging of MUC1 expression based on the AR20.5 antibody. To this end, we modified AR20.5 with the chelator desferrioxamine (DFO) and labeled it with the positron-emitting radiometal zirconium-89 (t 1/2 ~3.3 d) to produce [ 89 Zr]Zr-DFO-AR20.5. In subsequent in vivo experiments in athymic nude mice bearing subcutaneous MUC1-expressing ovarian cancer xenografts, [ 89 Zr]Zr-DFO-AR20.5 clearly delineated tumor tissue, producing a tumoral activity concentration of 19.1 ± 6.4 percent injected dose per gram (%ID/g) at 120 h post-injection and a tumor-to-muscle activity concentration ratio of 42.4 ± 10.6 at the same time point. Additional PET imaging experiments in mice bearing orthotopic MUC1-expressing ovarian cancer xenografts likewise demonstrated that [ 89 Zr]Zr-DFO-AR20.5 enables the visualization of tumor tissue-including metastatic lesions-with promising tumor-to-background contrast.

Poly(ADP-ribose)polymerase-1 (PARP1) is a DNA repair enzyme highly expressed in the nuclei of mammalian cells, with a structure and function that have attracted interest since its discovery. PARP inhibitors, moreover, can be used to induce synthetic lethality in cells where the homologous recombination (HR) pathway is deficient. Several small molecule PARP inhibitors have been approved by the FDA for multiple cancers bearing this deficiency These PARP inhibitors also act as radiosensitizing agents by delaying single strand break (SSB) repair and causing subsequent double strand break (DSB) generation, a concept that has been leveraged in various preclinical models of combination therapy with PARP inhibitors and ionizing radiation. Researchers have determined the efficacy of various PARP inhibitors at sub-cytotoxic concentrations in radiosensitizing multiple human cancer cell lines to ionizing radiation. Furthermore, several groups have begun evaluating combination therapy strategies in mouse models of cancer, and a fluorescent imaging agent that allows for subcellular imaging in real time has been developed from a PARP inhibitor scaffold. Other PARP inhibitor scaffolds have been radiolabeled to create PET imaging agents, some of which have also entered clinical trials. Most recently, these highly targeted small molecules have been radiolabeled with therapeutic isotopes to create radiotherapeutics and radiotheranostics in cancers whose primary interventions are surgical resection and whole-body radiotherapy. In this review we discuss the utilization of these small molecules in combination therapies and in scaffolds for imaging agents, radiotherapeutics, and radiotheranostics. Development of these radiolabeled PARP inhibitors has presented promising results for new interventions in the fight against some of the most intractable cancers., (Copyright © 2020 Jannetti, Zeglis, Zalutsky and Reiner.)

Pretargeting is widely explored in immunoPET as a strategy to reduce radiation exposure of non-target organs and allow the use of short-lived radionuclides that would not otherwise be compatible with the slow pharmacokinetic profiles of antibodies. Here we investigate a pretargeting strategy based on gallium-68 and the chelator THP Me as a high-affinity pair capable of combining in vivo. After confirming the ability of THP Me to bind 68 Ga in vivo at low concentrations, the bifunctional THP Me -NCS was conjugated to a humanised huA33 antibody targeting the A33 glycoprotein. Imaging experiments performed in nude mice bearing A33-positive SW1222 colorectal cancer xenografts compared pretargeting (100 μg of THP Me -NCS-huA33, followed after 24 h by 8-10 MBq of 68 Ga 3+ ) with both a directly labelled radioimmunoconjugate ( 89 Zr-DFO-NCS-huA33, 88 μg, 7 MBq) and a 68 Ga-only negative control (8-10 MBq of 68 Ga 3+ ). Imaging was performed 25 h after antibody administration (1 h after 68 Ga 3+ administration for negative control). No difference between pretargeting and the negative control was observed, suggesting that pretargeting via metal chelation is not feasible using this model. However, significant accumulation of "unchelated" 68 Ga 3+ in the tumour was found (12.9 %ID/g) even without prior administration of THP Me -NCS-huA33, though tumour-to-background contrast was impaired by residual activity in the blood. Therefore, the 68 Ga-only experiment was repeated using THP Me (20 μg, 1 h after 68 Ga 3+ administration) to clear circulating 68 Ga 3+ , producing a three-fold improvement of the tumour-to-blood activity concentration ratio. Although preliminary, these results highlight the potential of THP Me as a 68 Ga clearing agent in imaging applications with gallium citrate.

Rationale : The overwhelming majority of radioimmunoconjugates are produced via random conjugation methods predicated on attaching bifunctional chelators to the lysines of antibodies. However, this approach inevitably produces poorly defined and heterogeneous immunoconjugates because antibodies have several lysines distributed throughout their structure. To circumvent this issue, we have previously developed a chemoenzymatic bioconjugation strategy that site-specifically appends cargoes to the biantennary heavy chain glycans attached to C H 2 domains of the immunoglobulin's Fc region. In the study at hand, we explore the effects of this approach to site-specific bioconjugation on the Fc receptor binding and in vivo behavior of radioimmunoconjugates. Methods : We synthesized three desferrioxamine (DFO)-labeled immunoconjugates based on the HER2-targeting antibody pertuzumab: one using random bioconjugation methods (DFO- nss pertuzumab) and two using variants of our chemoenzymatic protocol (DFO- ss pertuzumab-EndoS and DFO- ss pertuzumab-βGal). Subsequently, we characterized these constructs and evaluated their ability to bind HER2, human FcγRI (huFcγRI), and mouse FcγRI (muFcγRI). After radiolabeling the immunoconjugates with zirconium-89, we conducted PET imaging and biodistribution studies in two different mouse models of HER2-expressing breast cancer. Results : MALDI-ToF and SDS-PAGE analysis confirmed the site-specific nature of the bioconjugation, and flow cytometry and surface plasmon resonance (SPR) revealed that all three immunoconjugates bind HER2 as effectively as native pertuzumab. Critically, however, SPR experiments also illuminated that DFO- ss pertuzumab-EndoS possesses an attenuated binding affinity for huFcγRI (17.4 ± 0.3 nM) compared to native pertuzumab (4.7 ± 0.2 nM), DFO- nss pertuzumab (4.1 ± 0.1 nM), and DFO- ss pertuzumab-βGal (4.7 ± 0.2 nM). ImmunoPET and biodistribution experiments in athymic nude mice bearing HER2-expressing BT474 human breast cancer xenografts yielded no significant differences in the in vivo behavior of the radioimmunoconjugates. Yet experiments in tumor-bearing humanized NSG mice revealed that 89 Zr-DFO- ss pertuzumab-EndoS produces higher activity concentrations in the tumor (111.8 ± 39.9 %ID/g) and lower activity concentrations in the liver and spleen (4.7 ± 0.8 %ID/g and 13.1 ± 4.0 %ID/g, respectively) than its non-site-specifically labeled cousin, a phenomenon we believe stems from the altered binding of the former to huFcγRI. Conclusion : These data underscore that this approach to site-specific bioconjugation not only produces more homogeneous and well-defined radioimmunoconjugates than traditional methods but may also improve their in vivo performance in mouse models by reducing binding to FcγRI., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

Metallic radionuclides have been instrumental in the field of nuclear imaging for over half a century. While recent years have played witness to a dramatic rise in the use of radiometals as labels for chelator-bearing biomolecules, imaging agents based solely on coordination compounds of radiometals have long played a critical role in the discipline as well. In this work, we seek to provide a brief overview of metal complex-based radiopharmaceuticals for positron emission tomography (PET) and single photon emission computed tomography (SPECT). More specifically, we have focused on imaging agents in which the metal complex itself rather than a pendant biomolecule or targeting moiety is responsible for the in vivo behavior of the tracer. This family of compounds contains metal complexes based on an array of different nuclides as well as probes that have been used for the imaging of a variety of pathologies, including infection, inflammation, cancer, and heart disease. Indeed, two of the defining traits of transition metal complexes-modularity and redox chemistry-have both been creatively leveraged in the development of imaging agents. In light of our audience, particular attention is paid to structure and mechanism, though clinical data is addressed as well. Ultimately, it is our hope that this review will not only educate readers about some of the seminal work performed in this space over the last 30 years but also spur renewed interest in the creation of radiopharmaceuticals based on small metal complexes.

Recent years have played witness to the advent of nuclear theranostics: the synergistic use of "matched pair" radiopharmaceuticals for diagnostic imaging and targeted radiotherapy. In this investigation, we report the extension of this concept to in vivo pretargeting based on the rapid and bioorthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans -cyclooctene (TCO). We demonstrate that a single injection of a TCO-modified immunoconjugate can be used as a platform for pretargeted PET imaging and radiotherapy via the sequential administration of a pair of Tz-bearing radioligands labeled with the positron-emitting radiometal copper-64 ( t 1/2 ≈ 12.7 h) and the beta-emitting radiometal lutetium-177 ( t 1/2 ≈ 6.7 days). More specifically, a mouse model of human colorectal carcinoma received a dose of the A33 antigen-targeting immunoconjugate huA33-TCO, followed 24 and 48 h later by injections of [ 64 Cu]Cu-SarAr-Tz and [ 177 Lu]Lu-DOTA-PEG 7 -Tz, respectively. This approach produces high activity concentrations of both radioligands in tumor tissue (16.4 ± 2.7 %ID/g for [ 64 Cu]Cu-SarAr-Tz at 48 h post-injection and 18.1 ± 2.1 %ID/g for [ 177 Lu]Lu-DOTA-PEG 7 -Tz at 120 h post-injection) as well as promising tumor-to-healthy organ activity concentration ratios. Ultimately, we believe that this work could not only have important implications in nuclear theranostics-most excitingly with isotopologue-based radioligand pairs such as [ 64 Cu]Cu-SarAr-Tz and [ 67 Cu]Cu-SarAr-Tz-but also in the delivery of fractionated doses during pretargeted radioimmunotherapy.

Rationale : Pretargeted radioimmunotherapy (PRIT) based upon bioorthogonal click chemistry has been investigated for the first time in the context of peritoneal carcinomatosis using a CEA-targeting 35A7 mAb bearing trans -cyclooctene (TCO) moieties and several 177 Lu-labeled tetrazine (Tz) radioligands. Starting from three Tz probes containing PEG linkers of varying lengths between the DOTA and Tz groups ( i.e . PEG n = 3, 7, or 11, respectively, for Tz- 1 , Tz- 2 , and Tz- 3 ), we selected [ 177 Lu]Lu-Tz -2 as the most appropriate for pretargeted SPECT imaging and demonstrated its efficacy in tumor growth control. Methods: An orthotopic model of peritoneal carcinomatosis (PC) was obtained following the intraperitoneal (i.p.) injection of A431-CEA-Luc cells in nude mice. Tumor growth was assessed using bioluminescence imaging. Anti-CEA 35A7 mAb was grafted with 2-3 TCO per immunoglobulin. Pretargeted SPECT imaging and biodistribution experiments were performed to quantify the activity concentrations of [ 177 Lu]Lu-Tz- 1-3 in tumors and non-target organs to determine the optimal Tz probe for the PRIT of PC. Results: The pharmacokinetic profiles of [ 177 Lu]Lu-Tz- 1-3 alone were determined using both SPECT imaging and biodistribution experiments. These data revealed that [ 177 Lu]Lu-Tz- 1 was cleared via both the renal and hepatic systems , while [ 177 Lu]Lu-Tz- 2 and [ 177 Lu]Lu-Tz- 3 were predominantly excreted via the renal system. In addition, these results illuminated that the longer the PEG linker, the more rapidly the Tz radioligand was cleared from the peritoneal cavity. The absorbed radiation dose corresponding to pretargeting with 35A7-TCO followed 24 h later by [ 177 Lu]Lu-Tz- 1-4 was higher for tumors following the administration of [ 177 Lu]Lu-Tz- 2 ( i.e . 0.59 Gy/MBq) compared to either [ 177 Lu]Lu-Tz- 1 ( i.e . 0.25 Gy/MBq) and [ 177 Lu]Lu-Tz- 3 ( i.e . 0.18 Gy/MBq). In a longitudinal PRIT study, we showed that the i.p. injection of 40 MBq of [ 177 Lu]Lu-Tz- 2 24 hours after the systemic administration of 35A7-TCO significantly slowed tumor growth compared to control mice receiving only saline or 40 MBq of [ 177 Lu]Lu-Tz- 2 alone. Ex vivo measurement of the peritoneal carcinomatosis index (PCI) confirmed that PRIT significantly reduced tumor growth (PCI = 15.5 ± 2.3 after PRIT vs 30.0 ± 2.3 and 30.8 ± 1.4 for the NaCl and [ 177 Lu]Lu-Tz- 2 alone groups, respectively). Conclusion : Our results clearly demonstrate the impact of the length of PEG linkers upon the biodistribution profiles of 177 Lu-labeled Tz radioligands. Furthermore, we demonstrated for the first time the possibility of using bioorthogonal chemistry for both the pretargeted SPECT and PRIT of peritoneal carcinomatosis., Competing Interests: Competing Interests: The authors have declared that no competing interest exists., (© The author(s).)

Antibodies are promising vectors for PET imaging. However, the high uptake of radioimmunoconjugates in nontarget tissues such as the liver and spleen hampers their performance as radiotracers. This off-target uptake can lead to suboptimal tumor-to-background activity concentration ratios, decreasing the contrast of images and reducing their diagnostic utility. A possible cause of this uptake is the sequestration of radioimmunoconjugates by immune cells bearing Fc-γ-receptors (FcγR) that bind to the Fc regions of antibodies. Methods: Since the heavy chain glycans influence the affinity of FcγR for the Fc domain, we set out to investigate whether radioimmunoconjugates with truncated glycans would exhibit altered binding to FcγRI and, in turn, improved in vivo performance. Using the HER2-targeting antibody trastuzumab, we synthesized a series of desferrioxamine-bearing immunoconjugates with differing glycosylation states and interrogated their FcγRI binding via surface plasmon resonance, enzyme-linked immunosorbent assay, and flow cytometry. Furthermore, we labeled these immunoconjugates with 89 Zr and explored their biodistribution in athymic nude, NSG, and humanized NSG mice bearing human epidermal growth factor receptor 2-expressing human breast cancer xenografts. Results: We observed a strong correlation between the impaired in vitro FcγRI binding of deglycosylated immunoconjugates and significant decreases in the in vivo off-target uptake of the corresponding 89 Zr-labeled radioimmunoconjugates (i.e., liver activity concentrations are reduced by ∼3.5-fold in humanized NSG mice). These reductions in off-target uptake were accompanied by concomitant increases in the tumoral activity concentrations of the glycoengineered radioimmunoconjugates, ultimately yielding improved tumor-to-healthy organ contrast and higher quality PET images. Conclusion: Our findings suggest that the deglycosylation of antibodies represents a facile strategy for improving the quality of immuno-PET in animal models as well as in certain patient populations., (© 2019 by the Society of Nuclear Medicine and Molecular Imaging.)

Pretargeted radioimmunotherapy (PRIT) based on the inverse electron demand Diels-Alder reaction has shown promise in murine models of disease, yet the radiation dosimetry of this approach must be optimized to make it a viable clinical option. To this end, we have leveraged two recent developments in pretargeted imaging-dendritic scaffolds and masking agents-to improve the dosimetric profile of a proof-of-concept PRIT system that is based on the huA33 antibody, a 177 Lu-labeled tetrazine radioligand ([ 177 Lu]Lu-DOTA-PEG 7 -Tz), and a mouse model of A33 antigen-expressing colorectal carcinoma. Pretargeting using an huA33 immunoconjugate bearing a trans-cyclooctene-decorated dendritic scaffold ( ss huA33-DEN-TCO) produced significantly higher tumoral activity concentrations at 120 h post-injection (23.0 ± 2.2 %ID/g) than those achieved with an analogous, dendrimer-lacking immunoconjugate (12.7 ± 2.6 %ID/g). However, pretargeting using ss huA33-DEN-TCO also resulted in increased activity concentrations in the blood at the same time point (1.9 ± 0.4 %ID/g) compared to the dendrimer-lacking construct (0.7 ± 0.2 %ID/g), thereby curtailing improvements to the tumor-to-blood therapeutic ratio of the system. In order to circumvent this issue, a tetrazine-labeled, dextran-based masking agent (Tz-DP) was injected prior to the radioligand to prevent the ligation between [ 177 Lu]Lu-DOTA-PEG 7 -Tz and circulating immunoconjugate. This approach dramatically decreased the absorbed dose to the blood but also attenuated the absorbed dose to the tumor and increased the absorbed dose to the lungs. Ultimately, these data suggest that dendritic scaffolds and masking agents could be used to improve the dosimetry of PRIT, but the combination of these technologies will require extensive optimization.

Maleimide-bearing bifunctional probes have been employed for decades for the site-selective modification of thiols in biomolecules, especially antibodies. Yet maleimide-based conjugates display limited stability in vivo because the succinimidyl thioether linkage can undergo a retro-Michael reaction. This, of course, can lead to the release of the radioactive payload or its exchange with thiol-bearing biomolecules in circulation. Both of these processes can produce elevated activity concentrations in healthy organs as well as decreased activity concentrations in target tissues, resulting in reduced imaging contrast and lower therapeutic ratios. In 2018, we reported the creation of a modular, stable, and easily accessible phenyloxadiazolyl methyl sulfone reagent - dubbed 'PODS' - as a platform for thiol-based bioconjugations. We have clearly demonstrated that PODS-based site-selective bioconjugations reproducibly and robustly create homogenous, well-defined, highly immunoreactive, and highly stable radioimmunoconjugates. Furthermore, preclinical experiments in murine models of colorectal cancer have shown that these site-selectively labeled radioimmunoconjugates exhibit far superior in vivo performance compared to radiolabeled antibodies synthesized via maleimide-based conjugations. In this protocol, we will describe the four-step synthesis of PODS, the creation of a bifunctional PODS-bearing variant of the ubiquitous chelator DOTA (PODS-DOTA), and the conjugation of PODS-DOTA to the HER2-targeting antibody trastuzumab.

While radioimmunotherapy (RIT) is a promising approach for the treatment of cancer, the long pharmacokinetic half-life of radiolabeled antibodies can result in high radiation doses to healthy tissues. Perhaps not surprisingly, several different strategies have been developed to circumvent this troubling limitation. One of the most promising of these approaches is pretargeted radioimmunotherapy (PRIT). PRIT is predicated on decoupling the radionuclide from the immunoglobulin, injecting them separately, and then allowing them to combine in vivo at the target tissue. This approach harnesses the exceptional tumor-targeting properties of antibodies while skirting their pharmacokinetic drawbacks, thereby lowering radiation doses to non-target tissues and facilitating the use of radionuclides with half-lives that are considered too short for use in traditional radioimmunoconjugates. Over the last five years, our laboratory and others have developed an approach to in vivo pretargeting based on the inverse electron-demand Diels-Alder (IEDDA) reaction between trans-cyclooctene (TCO) and tetrazine (Tz). This strategy has been successfully applied to pretargeted positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging with a variety of antibody-antigen systems. In a pair of recent publications, we have demonstrated the efficacy of IEDDA-based PRIT in murine models of pancreatic ductal adenocarcinoma and colorectal carcinoma. In this protocol, we describe protocols for PRIT using a 177 Lu-DOTA-labeled tetrazine radioligand ([ 177 Lu]Lu-DOTA-PEG7-Tz) and a TCO-modified variant of the colorectal cancer targeting huA33 antibody (huA33-TCO). More specifically, we will describe the construction of huA33-TCO, the synthesis and radiolabeling of [ 177 Lu]Lu-DOTA-PEG7-Tz, and the performance of in vivo biodistribution and longitudinal therapy studies in murine models of colorectal carcinoma.