Dual Radionuclide Theranostic Pretargeting

High-Spatial-Resolution Benchtop X-ray Fluorescence Imaging through Bragg-Diffraction-Based Focusing with Bent Mosaic Graphite Crystals: A Simulation Study

X-ray fluorescence imaging (XFI) can localize diagnostic or theranostic entities utilizing nanoparticle (NP)-based probes at high resolution in vivo, in vitro, and ex vivo. However, small-animal benchtop XFI systems demonstrating high spatial resolution (variable from sub-millimeter to millimeter range) in vivo are still limited to lighter elements (i.e., atomic number Z≤45). This study investigates the feasibility of focusing hard X-rays from solid-target tubes using ellipsoidal lens systems composed of mosaic graphite crystals with the aim of enabling high-resolution in vivo XFI applications with mid-Z (42≤Z≤64) elements. Monte Carlo simulations are performed to characterize the proposed focusing-optics concept and provide quantitative predictions of the XFI sensitivity, in silico tumor-bearing mice models loaded with palladium (Pd) and barium (Ba) NPs. Based on simulation results, the minimum detectable total mass of PdNPs per scan position is expected to be on the order of a few hundred nanograms under in vivo conform conditions. PdNP masses as low as 150 ng to 50 ng could be detectable with a resolution of 600 μm when imaging abdominal tumor lesions across a range of low-dose (0.8 μGy) to high-dose (8 μGy) exposure scenarios. The proposed focusing-optics concept presents a potential step toward realizing XFI with conventional X-ray tubes for high-resolution applications involving interesting NP formulations.

A proof-of-concept study on bioorthogonal-based pretargeting and signal amplify radiotheranostic strategy

BACKGROUND

Radiotheranostics differs from the vast majority of other cancer therapies in its capacity for simultaneous imaging and therapy, and it is becoming more widely implemented. A balance between diagnostic and treatment requirements is essential for achieving effective radiotheranostics. Herein, we propose a proof-of-concept strategy aiming to address the profound differences in the specific requirements of the diagnosis and treatment of radiotheranostics.

RESULTS

To validate the concept, we designed an s-tetrazine (Tz) conjugated prostate-specific membrane antigen (PSMA) ligand (DOTA-PSMA-Tz) for 68Ga or 177Lu radiolabeling and tumor radiotheranostics, a trans-cyclooctene (TCO) modified Pd@Au nanoplates (Pd@Au-PEG-TCO) for signal amplification, respectively. We then demonstrated this radiotheranostic strategy in the tumor-bearing mice with the following three-step procedures: (1) i.v. injection of the [68Ga]Ga-PSMA-Tz for diagnosis; (2) i.v. injection of the signal amplification module Pd@Au-PEG-TCO; (3) i.v. injection of the [177Lu]Lu-PSMA-Tz for therapy. Firstly, this strategy was demonstrated in 22Rv1 tumor-bearing mice via positron emission tomography (PET) imaging with [68Ga]Ga-PSMA-Tz. We observed significantly higher tumor uptake (11.5 ± 0.8%ID/g) with the injection of Pd@Au-PEG-TCO than with the injection [68Ga]Ga-PSMA-Tz alone (5.5 ± 0.9%ID/g). Furthermore, we validated this strategy through biodistribution studies of [177Lu]Lu-PSMA-Tz, with the injection of the signal amplification module, approximately five-fold higher tumor uptake of [177Lu]Lu-PSMA-Tz (24.33 ± 2.53% ID/g) was obtained when compared to [177Lu]Lu-PSMA-Tz alone (5.19 ± 0.26%ID/g) at 48 h post-injection.

CONCLUSION

In summary, the proposed strategy has the potential to expand the toolbox of pretargeted radiotherapy in the field of theranostics.

Click Chemistry and Radiochemistry: An Update

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.

Theranostic Nuclear Medicine with Gallium-68, Lutetium-177, Copper-64/67, Actinium-225, and Lead-212/203 Radionuclides

Molecular changes in malignant tissue can lead to an increase in the expression levels of various proteins or receptors that can be used to target the disease. In oncology, diagnostic imaging and radiotherapy of tumors is possible by attaching an appropriate radionuclide to molecules that selectively bind to these target proteins. The term "theranostics" describes the use of a diagnostic tool to predict the efficacy of a therapeutic option. Molecules radiolabeled with γ-emitting or β+-emitting radionuclides can be used for diagnostic imaging using single photon emission computed tomography or positron emission tomography. Radionuclide therapy of disease sites is possible with either α-, β-, or Auger-emitting radionuclides that induce irreversible damage to DNA. This Focus Review centers on the chemistry of theranostic approaches using metal radionuclides for imaging and therapy. The use of tracers that contain β+-emitting gallium-68 and β-emitting lutetium-177 will be discussed in the context of agents in clinical use for the diagnostic imaging and therapy of neuroendocrine tumors and prostate cancer. A particular emphasis is then placed on the chemistry involved in the development of theranostic approaches that use copper-64 for imaging and copper-67 for therapy with functionalized sarcophagine cage amine ligands. Targeted therapy with radionuclides that emit α particles has potential to be of particular use in late-stage disease where there are limited options, and the role of actinium-225 and lead-212 in this area is also discussed. Finally, we highlight the challenges that impede further adoption of radiotheranostic concepts while highlighting exciting opportunities and prospects.

Targeted radionuclide therapy directed to the tumor phenotypes: A dosimetric approach using MC simulations

BACKGROUND

In Targeted Radionuclide Therapy (TRT), the continuous technological effort in imaging tumor phenotypes (i.e. sub-volumes with different phenotypic characteristics) and in precise radiopharmaceutical tumor-targeting, is allowing for a better dosimetric optimization at the tumor phenotype level. The aim of this study was to evaluate the dosimetric efficiency (considering strategic absorbed dose delivery to the phenotypes) of personalized TRT directed to the tumor phenotypes.

METHODS

The dosimetric assessment was performed using a four-phenotype realistic tumor model implemented within the ICRP reference voxel phantom and simulations using the state-of-the-art Monte Carlo program PENELOPE. The dose assessment was performed for five radionuclides commonly used in therapy and/or diagnostic procedures: ¹²⁵I, ^99mTc, ¹⁷⁷Lu, ¹⁶¹Tb and ⁶⁷Ga. Two irradiation scenarios were considered: (i) the Whole Tumor Treatment Planning Scenario (WTTPS), i.e. the four phenotypes irradiated with the same radionuclide; (ii) the Phenotype Treatment Planning Scenario (PTPS), i.e. each phenotype irradiated by a single radionuclide. The optimal radionuclide configurations were studied considering the maximization of the absorbed dose delivered to the tumor and the minimization of dose to healthy tissues.

RESULTS

In WTTPS, ¹²⁵I outperforms the other radionuclides in terms of the ratio of the maximum absorbed dose delivered to the tumor and the minimum absorbed dose delivered to healthy tissues. In the PTPS, the use of ¹⁶¹Tb in combination with the other radionuclides maximizes the absorbed dose in the tumor tissues while simultaneously minimizing dose to healthy tissue, compared to the WTTPS. In agreement with recent pre-clinical studies, our computational results confirm and indicate the beneficial additive dosimetric effects of Auger and conversion electrons of ¹⁶¹Tb with respect to ¹⁷⁷Lu, when considering the same cumulated activity for both. Interestingly, in considering a realistic tumor model, the better dosimetric performances of ¹⁶¹Tb were confirmed also for tumor volumes ranging from 1.98 cm³ to 33.32 cm³.

CONCLUSIONS

Dose assessment in realistic non-homogeneous tumor models could provide more insights with respect to consider only homogenous water-spheres tumor models and should be taken into account in dosimetry-based TRT planning studies.

In Vivo Applications of Bioorthogonal Reactions: Chemistry and Targeting Mechanisms

Bioorthogonal chemistry involves selective biocompatible reactions between functional groups that are not normally present in biology. It has been used to probe biomolecules in living systems, and has advanced biomedical strategies such as diagnostics and therapeutics. In this review, the challenges and opportunities encountered when translating in vitro bioorthogonal approaches to in vivo settings are presented, with a focus on methods to deliver the bioorthogonal reaction components. These methods include metabolic bioengineering, active targeting, passive targeting, and simultaneously used strategies. The suitability of bioorthogonal ligation reactions and bond cleavage reactions for in vivo applications is critically appraised, and practical considerations such as the optimum scheduling regimen in pretargeting approaches are discussed. Finally, we present our own perspectives for this area and identify what, in our view, are the key challenges that must be overcome to maximise the impact of these approaches.

Development of a multi faceted platform containing a tetrazine, fluorophore and chelator: synthesis, characterization, radiolabeling, and immuno-SPECT imaging

BACKGROUND

Combining optical (fluorescence) imaging with nuclear imaging has the potential to offer a powerful tool in personal health care, where nuclear imaging offers in vivo functional whole-body visualization, and the fluorescence modality may be used for image-guided tumor resection. Varying chemical strategies have been exploited to fuse both modalities into one molecular entity. When radiometals are employed in nuclear imaging, a chelator is typically inserted into the molecule to facilitate radiolabeling; the availability of the chelator further expands the potential use of these platforms for targeted radionuclide therapy if a therapeutic radiometal is employed. Herein, a novel mixed modality scaffold which contains a tetrazine (Tz)--for biomolecule conjugation, fluorophore-for optical imaging, and chelator-for radiometal incorporation, in one construct is presented. The novel platform was characterized for its fluorescence properties, radiolabeled with single-photon emission computed tomography (SPECT) isotope indium-111 (¹¹¹In³⁺) and therapeutic alpha emitter actinium-225 (²²⁵Ac³⁺). Both radiolabels were conjugated in vitro to trans-cyclooctene (TCO)-modified trastuzumab; biodistribution and immuno-SPECT imaging of the former conjugate was assessed.

RESULTS

Key to the success of the platform synthesis was incorporation of a 4,4'-dicyano-BODIPY fluorophore. The route gives access to an advanced intermediate where final chelator-incorporated compounds can be easily accessed in one step prior to radiolabeling or biomolecule conjugation. The DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) conjugate was prepared, displayed good fluorescence properties, and was successfully radiolabeled with ¹¹¹In & ²²⁵Ac in high radiochemical yield. Both complexes were then separately conjugated in vitro to TCO modified trastuzumab through an inverse electron demand Diels-Alder (IEDDA) reaction with the Tz. Pilot small animal in vivo immuno-SPECT imaging with [¹¹¹In]In-DO3A-BODIPY-Tz-TCO-trastuzumab was also conducted and exhibited high tumor uptake (21.2 ± 5.6%ID/g 6 days post-injection) with low uptake in non-target tissues.

CONCLUSIONS

The novel platform shows promise as a multi-modal probe for theranostic applications. In particular, access to an advanced synthetic intermediate where tailored chelators can be incorporated in the last step of synthesis expands the potential use of the scaffold to other radiometals. Future studies including validation of ex vivo fluorescence imaging and exploiting the pre-targeting approach available through the IEDDA reaction are warranted.

AIE materials for nucleus imaging

Emergence of a captivating phenomenon aggregation induced emission (AIE) in the early years of 21st century attracted worldwide researchers. In the last two decades various novel AIE active biocompatible small molecules, macromolecules and polymers have been developed for diverse biomedical applications. Imaging of specific organelle such as mitochondria, ribosomes, nuclei and many others play important in the controlling and successful treatment of various diseases. Conventional luminescent probe molecules used in the imaging at cellular or subcellular level exhibit very weak emission on dispersion or on aggregation in aqueous media. AIE luminogens development is indispensable to overcome the notorious aggregation-caused quenching (ACQ) issue inherited by conventional fluorophores. In the present chapter we mostly highlighted over one decade development of various AIE active luminogens utilized for imaging of cell nucleus, nucleon and nucleic acids. The development of those AIE luminogens exhibits promising results in the early diagnosis of cancer diseases.

Dual-Modality PET-SPECT Image-Guided Pretargeting Delivery in HER2(+) Breast Cancer Models

Pretargeted drug delivery has been explored for decades as a promising approach in cancer therapy. An image-guided pretargeting strategy significantly enhances the intrinsic advantages of this approach since imaging the pretargeting step can be used for diagnostic purposes, while imaging of the drug delivery step can be utilized to evaluate drug distribution and assess therapeutic response. A trastuzumab (Tz)-based HER2 pretargeting component (Tz-TCO-[89Zr-DFO]) was developed by conjugating with trans-cyclooctene (TCO) bioorthogonal click chemistry functional groups and deferoxamine (DFO) to enable radiolabeling with a 89Zr PET tracer. The drug delivery component (HSA-DM1-Tt-[99mTc-HyNic]) was developed by conjugating human serum albumin (HSA) with mertansine (DM1), tetrazine (Tt) functional groups, and a HyNic chelator and radiolabeling with 99mTc. For ex vivo biodistribution studies, pretargeting and delivery components (without drug) were administered subsequently to mice bearing human HER2(+) breast cancer xenografts, and a high tumor uptake of Tz-TCO-[89Zr-DFO] (26.4% ID/g) and HSA-Tt-[99mTc-HyNic] (4.6% ID/g) was detected at 24 h postinjection. In vivo treatment studies were performed in the same HER2(+) breast cancer model using PET-SPECT image guidance. The increased tumor uptake of the pretargeting and drug delivery components was detected by PET-CT and SPECT-CT, respectively. The study showed a significant 92% reduction of the relative tumor volume in treated mice (RTV = 0.08 in 26 days), compared to the untreated control mice (RTV = 1.78 in 11 days) and to mice treated with only HSA-DM1-Tt-[99mTc-HyNic] (RTV = 1.88 in 16 days). Multimodality PET-SPECT image-guided and pretargeted drug delivery can be utilized to maximize efficacy, predict therapeutic response, and minimize systemic toxicity.

Development of the First Aliphatic 18F-Labeled Tetrazine Suitable for Pretargeted PET Imaging-Expanding the Bioorthogonal Tool Box

Pretargeted imaging of nanomedicines have attracted considerable interest because it has the potential to increase imaging contrast while reducing radiation burden to healthy tissue. Currently, the tetrazine ligation is the fastest bioorthogonal reaction for this strategy and, consequently, the state-of-art choice for in vivo chemistry. We have recently identified key properties for tetrazines in pretargeting. We have also developed a method to ¹⁸F-label reactive tetrazines using an aliphatic nucleophilic substitution strategy. Here, we combined this knowledge and developed an ¹⁸F-labeled tetrazine for pretargeted imaging. In order to develop this ligand, a small SAR study was performed. The most promising compound was selected for labeling and subsequent positron-emission-tomography in vivo imaging. Radiolabeling was achieved in satisfactory yields, molar activities, and high radiochemical purities. [¹⁸F]15 displayed favorable pharmacokinetics and remarkable target-to-background ratios-as early as 1 h post injection. We believe that this agent could be a promising candidate for translation into clinical studies.

Disease diagnosis and treatment; could theranostics change everything?

There has always been an intimate and complex relationship between the diagnosis of a disease and its treatment. The approach dubbed theranostics aims to combine diagnostic techniques with therapeutic ones by deploying the same molecule in two roles, exploiting the specificity of its function to render disease treatment more effective. Does this technical development have the potential to change our conception of disease diagnosis? With the treatment approach so intimately linked to the diagnostic tool, might it be possible to treat a disease without having first made an independent clinical or laboratory diagnosis? Here we discuss medical diagnosis, arguing for three categories of diagnosis, before presenting an example of a theranostic approach using radioactive prostate-specific membrane antigen ligands. This example allows us to envision a form of theranostic agent that would be able to diagnose a cancer, for example, and engage directly in its treatment, opening up the possibility of treating patients at risk of developing this cancer without any other clinical diagnostic steps. Would it be a problem if these approaches eventually became independent of any specialist clinical diagnostic supervision? If a theranostic technique is shown to work, following its own logic, do we still need an independent 'traditional' diagnosis prior to its use? We argue that such a diagnosis would no longer be necessary provided certain conditions are fulfilled.

IEDDA: An Attractive Bioorthogonal Reaction for Biomedical Applications

The pretargeting strategy has recently emerged in order to overcome the limitations of direct targeting, mainly in the field of radioimmunotherapy (RIT). This strategy is directly dependent on chemical reactions, namely bioorthogonal reactions, which have been developed for their ability to occur under physiological conditions. The Staudinger ligation, the copper catalyzed azide-alkyne cycloaddition (CuAAC) and the strain-promoted [3 + 2] azide–alkyne cycloaddition (SPAAC) were the first bioorthogonal reactions introduced in the literature. However, due to their incomplete biocompatibility and slow kinetics, the inverse-electron demand Diels-Alder (IEDDA) reaction was advanced in 2008 by Blackman et al. as an optimal bioorthogonal reaction. The IEDDA is the fastest bioorthogonal reaction known so far. Its biocompatibility and ideal kinetics are very appealing for pretargeting applications. The use of a trans-cyclooctene (TCO) and a tetrazine (Tz) in the reaction encouraged researchers to study them deeply. It was found that both reagents are sensitive to acidic or basic conditions. Furthermore, TCO is photosensitive and can be isomerized to its cis-conformation via a radical catalyzed reaction. Unfortunately, the cis-conformer is significantly less reactive toward tetrazine than the trans-conformation. Therefore, extensive research has been carried out to optimize both click reagents and to employ the IEDDA bioorthogonal reaction in biomedical applications.

Inverse electron demand Diels-Alder click chemistry for pretargeted PET imaging and radioimmunotherapy

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 ([64Cu]Cu-SarAr-Tz) and one labeled with the β-emitting radiolanthanide lutetium-177 ([177Lu]Lu-DOTA-PEG7-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.

Enzymatic Methods for the Site-Specific Radiolabeling of Targeting Proteins

Site-specific conjugation of proteins is currently required to produce homogenous derivatives for medicine applications. Proteins derivatized at specific positions of the polypeptide chain can actually show higher stability, superior pharmacokinetics, and activity in vivo, as compared with conjugates modified at heterogeneous sites. Moreover, they can be better characterized regarding the composition of the derivatization sites as well as the conformational and activity properties. To this aim, several site-specific derivatization approaches have been developed. Among these, enzymes are powerful tools that efficiently allow the generation of homogenous protein-drug conjugates under physiological conditions, thus preserving their native structure and activity. This review will summarize the progress made over the last decade on the use of enzymatic-based methodologies for the production of site-specific labeled immunoconjugates of interest for nuclear medicine. Enzymes used in this field, including microbial transglutaminase, sortase, galactosyltransferase, and lipoic acid ligase, will be overviewed and their recent applications in the radiopharmaceutical field will be described. Since nuclear medicine can benefit greatly from the production of homogenous derivatives, we hope that this review will aid the use of enzymes for the development of better radio-conjugates for diagnostic and therapeutic purposes.

A Theranostic Cellulose Nanocrystal-Based Drug Delivery System with Enhanced Retention in Pulmonary Metastasis of Melanoma

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.

Pre-targeting with ultra-small nanoparticles: boron carbon dots as drug candidates for boron neutron capture therapy

Boron neutron capture therapy (BNCT) is a promising cancer treatment exploiting the neutron capture capacity and subsequent fission reaction of boron-10. The emergence of nanotechnology has encouraged the development of nanocarriers capable of accumulating boron atoms preferentially in tumour cells. However, a long circulation time, required for high tumour accumulation, is usually accompanied by accumulation of the nanosystem in organs such as the liver and the spleen, which may cause off-target side effects. This could be overcome by using small-sized boron carriers via a pre-targeting strategy. Here, we report the preparation, characterisation and in vivo evaluation of tetrazine-functionalised boron-rich carbon dots, which show very fast clearance and low tumour uptake after intravenous administration in a mouse HER2 (human epidermal growth factor receptor 2)-positive tumour model. Enhanced tumour accumulation was achieved when using a pretargeting approach, which was accomplished by a highly selective biorthogonal reaction at the tumour site with trans-cyclooctene-functionalised Trastuzumab.

Radiolabeling of Theranostic Nanosystems

In the recent years, progress in nanotechnology has significantly contributed to the development of novel pharmaceutical formulations to overcome the drawbacks of conventional treatments and improve the therapeutic outcome in many diseases, especially cancer. Nanoparticle vectors have demonstrated the potential to concomitantly deliver diagnostic and therapeutic payloads to diseased tissue. Due to their special physical and chemical properties, the characteristics and function of nanoparticles are tunable based on biological molecular targets and specific desired features (e.g., surface chemistry and diagnostic radioisotope labeling). Within the past decade, several theranostic nanoparticles have been developed as a multifunctional nanosystems which combine the diagnostic and therapeutic functionalities into a single drug delivery platform. Theranostic nanosystems can provide useful information on a real-time systemic distribution of the developed nanosystem and simultaneously transport the therapeutic payload. In general, the diagnostic functionality of theranostic nanoparticles can be achieved through labeling gamma-emitted radioactive isotopes on the surface of nanoparticles which facilitates noninvasive detection using nuclear molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT), meanwhile, the therapeutic effect arises from the potent drug released from the nanoparticle. Moreover, some radioisotopes can concurrently emit both gamma radiation and high-energy particles (e.g., alpha, beta, and Auger electrons), prompting the use either alone for radiotheranostics or synergistically with chemotherapy. This chapter provides an overview of the fundamentals of radiochemistry and relevant radiolabeling strategies for theranostic nanosystem development as well as the methods for the preclinical evaluation of radiolabeled nanoparticles. Furthermore, preclinical case studies of recently developed theranostic nanosystems will be highlighted.

Harnessing 64Cu/67Cu for a theranostic approach to pretargeted radioimmunotherapy

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 (⁶⁴Cu, t _1/2 = 12.7 h) and beta particle-emitting copper-67 (⁶⁷Cu, 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 ⁶⁷Cu-labeled Tz radioligand ([⁶⁷Cu]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 [⁶⁷Cu]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 [⁶⁷Cu]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 [⁶⁴Cu]Cu-MeCOSar-Tz and [⁶⁷Cu]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 [⁶⁴Cu]Cu-MeCOSar-Tz and the subsequent therapeutic response to [⁶⁷Cu]Cu-MeCOSar-Tz.

Theranostic Pretargeting Drug Delivery and Imaging Platforms in Cancer Precision Medicine

Theranostics are nano-size or molecular-level agents serving for both diagnosis and therapy. Structurally, they are drug delivery systems integrated with molecular or targeted imaging agents. Theranostics are becoming popular because they are targeted therapeutics and can be used with no or minimal changes for diagnostic imaging to aid in precision medicine. Thus, there is a close relation between theranostics and image-guided therapy (IGT), and theranostics are actually a subclass of IGT in which both therapeutic and imaging functionalities are attributed to a single platform. An important theranostics strategy is biological pretargeting. In pretargeted IGT, first, the target is identified by a target-specific natural or synthetic bioligand followed by a nano-scale or molecular drug delivery component, which form therapeutic clusters by in situ conjugation reactions. If pretargeted drug delivery platforms are labeled with multimodal imaging probes, they can be used as theranostics for both diagnostic imaging and therapy. Optical and nuclear imaging techniques have mostly been used in proof-of-concept studies with pretargeted theranostics. The concept of pretargeting in theranostics is comparatively novel and generally requires a confirmed overexpression of surface receptors on targeted cells/tissue. In addition, the receptors should have natural or synthetic bioligands to be used as pretargeting components. Therefore, applications of pretargeting theranostics are still limited to several cancer types, which overexpress cell-surface markers on the target cancer cells. In this review, recent discoveries of pretargeting theranostics in breast, ovarian, prostate, and colorectal cancers are discussed to highlight main strengths and potential limitations the strategy.

ImmunoPET: Concept, Design, and Applications

Immuno-positron emission tomography (immunoPET) is a paradigm-shifting molecular imaging modality combining the superior targeting specificity of monoclonal antibody (mAb) and the inherent sensitivity of PET technique. A variety of radionuclides and mAbs have been exploited to develop immunoPET probes, which has been driven by the development and optimization of radiochemistry and conjugation strategies. In addition, tumor-targeting vectors with a short circulation time (e.g., Nanobody) or with an enhanced binding affinity (e.g., bispecific antibody) are being used to design novel immunoPET probes. Accordingly, several immunoPET probes, such as 89Zr-Df-pertuzumab and 89Zr-atezolizumab, have been successfully translated for clinical use. By noninvasively and dynamically revealing the expression of heterogeneous tumor antigens, immunoPET imaging is gradually changing the theranostic landscape of several types of malignancies. ImmunoPET is the method of choice for imaging specific tumor markers, immune cells, immune checkpoints, and inflammatory processes. Furthermore, the integration of immunoPET imaging in antibody drug development is of substantial significance because it provides pivotal information regarding antibody targeting abilities and distribution profiles. Herein, we present the latest immunoPET imaging strategies and their preclinical and clinical applications. We also emphasize current conjugation strategies that can be leveraged to develop next-generation immunoPET probes. Lastly, we discuss practical considerations to tune the development and translation of immunoPET imaging strategies.

Antibody Pretargeting Based on Bioorthogonal Click Chemistry for Cancer Imaging and Targeted Radionuclide Therapy

Bioorthogonal click chemistry-employing antibody-conjugated trans-cyclooctenes (TCO) and tetrazine (Tz)-based radioligands able to covalently bind in vivo-appeared recently as a potential alternative to circumvent the hematotoxicity induced by radioimmunotherapy of solid tumors. This Review focuses on the recent advances concerning TCO/Tz pretargeting in both cancer imaging and targeted-radionuclide therapy for prospective clinical transfer. We exhaustively identified 25 PubMed publications reporting preclinical imaging and 5 therapy studies with full mAbs as targeting vectors, since its first application in 2010. The fast, safe, modulable, and specific TCO/Tz pretargeting showed high potential as a theranostic tool to get more personalized and precise cancer care. The recent optimizations reported here highlighted a possible first clinical evaluation of IEDDA pretargeting in the coming years.