Pretargeted PET Imaging of trans-Cyclooctene-Modified Porous Silicon Nanoparticles

Development of a 99mTc-labeled tetrazine for pretargeted SPECT imaging using an alendronic acid-based bone targeting model

Pretargeting, which is the separation of target accumulation and the administration of a secondary imaging agent into two sequential steps, offers the potential to improve image contrast and reduce radiation burden for nuclear imaging. In recent years, the tetrazine ligation has emerged as a promising approach to facilitate covalent pretargeted imaging due to its unprecedented kinetics and bioorthogonality. Pretargeted bone imaging with TCO-modified alendronic acid (Aln-TCO) is an attractive model that allows the evaluation of tetrazines in healthy animals without the need for complex disease models or targeting regimens. Recent structure-activity relationship studies of tetrazines evaluated important parameters for the design of potent tetrazine-radiotracers for pretargeted imaging. However, limited information is available for 99mTc-labeled tetrazines. In this study, four tetrazines intended for labeling with fac-[99mTc(OH2)3 (CO)3]+ were synthesized and evaluated using an Aln-TCO mouse model. 3,6-bis(2-pyridyl)-1,2,4,5-Tz without additional linker showed higher pretargeted bone uptake and less background activity compared to the same scaffold with a PEG8 linker or 3-phenyl-1,2,4,5-Tz-based compounds. Additionally, improved bone/blood ratios were observed in pretargeted animals compared to animals receiving directly labeled Aln-TCO. The results of this study implicate 3,6-bis(2-pyridyl)-1,2,4,5-Tz as a promising scaffold for potential 99mTc-labeled tetrazines.

[18F]-Radiolabelled Nanoplatforms: A Critical Review of Their Intrinsic Characteristics, Radiolabelling Methods, and Purification Techniques

A wide range of nano-objects is found in many applications of our everyday life. Recognition of their peculiar properties and ease of functionalization has prompted their engineering into multifunctional platforms that are supposed to afford efficient tools for the development of biomedical applications. However, bridging the gap between bench to bedside cannot be expected without a good knowledge of their behaviour in vivo, which can be obtained through non-invasive imaging techniques, such as positron emission tomography (PET). Their radiolabelling with [18F]-fluorine, a technique already well established and widely used routinely for PET imaging, with [18F]-FDG for example, and in preclinical investigation using [18F]-radiolabelled biological macromolecules, has, therefore, been developed. In this context, this review highlights the various nano-objects studied so far, the reasons behind their radiolabelling, and main in vitro and/or in vivo results obtained thereof. Then, the methods developed to introduce the radioelement are presented. Detailed indications on the chemical steps involved are provided, and the stability of the radiolabelling is discussed. Emphasis is then made on the techniques used to purify and analyse the radiolabelled nano-objects, a point that is rarely discussed despite its technical relevance and importance for accurate imaging. The pros and cons of the different methods developed are finally discussed from which future work can develop.

A PEG-assisted membrane coating to prepare biomimetic mesoporous silicon for PET/CT imaging of triple-negative breast cancer

Triple-negative breast cancer (TNBC) diagnosis remains challenging without expressing critical receptors. Cancer cell membrane (CCm) coating has been extensively studied for targeted cancer diagnostics due to attractive features such as good biocompatibility and homotypic tumor-targeting. However, the present study found that widely used CCm coating approaches, such as extrusion, were not applicable for functionalizing irregularly shaped nanoparticles (NPs), such as porous silicon (PSi). To tackle this challenge, we proposed a novel approach that employs polyethylene glycol (PEG)-assisted membrane coating, wherein PEG and CCm are respectively functionalized on PSi NPs through chemical conjugation and physical absorption. Meanwhile, the PSi NPs were grafted with the bisphosphonate (BP) molecules for radiolabeling. Thanks to the good chelating ability of BP and homotypic tumor targeting of cancer CCm coating, a novel PSi-based contrast agent (CCm-PEG-89Zr-BP-PSi) was developed for targeted positron emission tomography (PET)/computed tomography (CT) imaging of TNBC. The novel imaging agent showed good radiochemical purity (∼99 %) and stability (∼95 % in PBS and ∼99 % in cell medium after 48 h). Furthermore, the CCm-PEG-89Zr-BP-PSi NPs had efficient homotypic targeting ability in vitro and in vivo for TNBC. These findings demonstrate a versatile biomimetic coating method to prepare novel NPs for tumor-targeted diagnosis.

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.

In Vivo Imaging of [60]Fullerene-Based Molecular Spherical Nucleic Acids by Positron Emission Tomography

18F-Labeled [60]fullerene-based molecular spherical nucleic acids (MSNAs), consisting of a human epidermal growth factor receptor 2 (HER2) mRNA antisense oligonucleotide sequence with a native phosphodiester and phosphorothioate backbone, were synthesized, site-specifically labeled with a positron emitting fluorine-18 and intravenously administrated via tail vein to HER2 expressing HCC1954 tumor-bearing mice. The biodistribution of the MSNAs was monitored in vivo by positron emission tomography/computed tomography (PET/CT) imaging. MSNA with a native phosphodiester backbone (MSNA-PO) was prone to rapid nuclease-mediated degradation, whereas the corresponding phosphorothioate analogue (MSNA-PS) with improved enzymatic stability showed an interesting biodistribution profile in vivo. One hour after the injection, majority of the radioactivity was observed in spleen and liver but also in blood with an average tumor-to-muscle ratio of 2. The prolonged radioactivity in blood circulation may open possibilities to the targeted delivery of the MSNAs.

Recent Advances in Bioorthogonal Click Chemistry for Enhanced PET and SPECT Radiochemistry

Due to their high reaction rate and reliable selectivity, bioorthogonal click reactions have been extensively investigated in numerous research fields, such as nanotechnology, drug delivery, molecular imaging, and targeted therapy. Previous reviews on bioorthogonal click chemistry for radiochemistry mainly focus on ¹⁸F-labeling protocols employed to produce radiotracers and radiopharmaceuticals. In fact, besides fluorine-18, other radionuclides such as gallium-68, iodine-125, and technetium-99m are also used in the field of bioorthogonal click chemistry. Herein, to provide a more comprehensive perspective, we provide a summary of recent advances in radiotracers prepared using bioorthogonal click reactions, including small molecules, peptides, proteins, antibodies, and nucleic acids as well as nanoparticles based on these radionuclides. The combination of pretargeting with imaging modalities or nanoparticles, as well as the clinical translations study, are also discussed to illustrate the effects and potential of bioorthogonal click chemistry for radiopharmaceuticals.

Synthesis and ex vivo biodistribution of two 68Ga-labeled tetrazine tracers: Comparison of pharmacokinetics

Pretargeted PET imaging allows the use of radiotracers labeled with short-living PET radionuclides for tracing drugs with slow pharmacokinetics. Recently, especially methods based on bioorthogonal chemistry have been under intensive investigation for pretargeted PET imaging. The pharmacokinetics of the radiotracer is one of the factors that determine the success of the pretargeted strategy. Here, we report synthesis and biological evaluation of two ⁶⁸Ga-labeled tetrazine (Tz)-based radiotracers, [⁶⁸Ga]Ga-HBED-CC-PEG₄-Tz ([⁶⁸Ga]4) and [⁶⁸Ga]Ga-DOTA-PEG₄-Tz ([⁶⁸Ga]6), aiming for development of new tracer candidates for pretargeted PET imaging based on the inverse electron demand Diels-Alder (IEDDA) ligation between a tetrazine and a strained alkene, such as trans-cyclooctene (TCO). Excellent radiochemical yield (RCY) was obtained for [⁶⁸Ga]4 (RCY > 96%) and slightly lower for [⁶⁸Ga]6 (RCY > 88%). Radiolabeling of HBED-CC-Tz proved to be faster and more efficient under milder conditions compared to the DOTA analogue. The two tracers exhibited excellent radiolabel stability both in vitro and in vivo. Moreover, [⁶⁸Ga]4 was successfully used for radiolabeling two different TCO-functionalized nanoparticles in vitro: Hepatitis E virus nanoparticles (HEVNPs) and porous silicon nanoparticles (PSiNPs).

Recent Advances in the Development of Tetrazine Ligation Tools for Pretargeted Nuclear Imaging

Tetrazine ligation has gained interest as a bio-orthogonal chemistry tool within the last decade. In nuclear medicine, tetrazine ligation is currently being explored for pretargeted approaches, which have the potential to revolutionize state-of-the-art theranostic strategies. Pretargeting has been shown to increase target-to-background ratios for radiopharmaceuticals based on nanomedicines, especially within early timeframes. This allows the use of radionuclides with short half-lives which are more suited for clinical applications. Pretargeting bears the potential to increase the therapeutic dose delivered to the target as well as reduce the respective dose to healthy tissue. Combined with the possibility to be applied for diagnostic imaging, pretargeting could be optimal for theranostic approaches. In this review, we highlight efforts that have been made to radiolabel tetrazines with an emphasis on imaging.

Recent developments of porous silicon nanovectors with various imaging modalities in the framework of theranostics

The number of in vitro, ex vivo, and in vivo studies on porous silicon (PSi) nanoparticles for biomedical applications has increased extensively over the last decade. The focus of the reports has been on the carrier properties of PSi concerning the therapeutic aspect due to several beneficial nanovector characteristics including high payload capacity, biocompatibility, and versatile surface chemistry. Recently, increasing attention has been paid to the diagnostic aspects of PSi, which is typically attributed to the biotraceability of the nanovector. Also, PSi has been studied as a contrast agent. When both these aspects, therapy and diagnosis, are integrated into one nanovector, we can discuss a real nanotheranostics approach. Herein, we review the recent progress developing PSi for various imaging modalities, specifically focusing on optical imaging, magnetic resonance imaging, and nuclear medicine imaging. Furthermore, we summarized the knowledge gaps that must be covered before applying PSi in clinical imaging, highlighting future research trends.

Click and Bioorthogonal Chemistry: The Future of Active Targeting of Nanoparticles for Nanomedicines?

Over the years, click and bioorthogonal reactions have been the subject of considerable research efforts. These high-performance chemical reactions have been developed to meet requirements not often provided by the chemical reactions commonly used today in the biological environment, such as selectivity, rapid reaction rate, and biocompatibility. Click and bioorthogonal reactions have been attracting increasing attention in the biomedical field for the engineering of nanomedicines. In this review, we study a compilation of articles from 2014 to the present, using the terms "click chemistry and nanoparticles (NPs)" to highlight the application of this type of chemistry for applications involving NPs intended for biomedical applications. This study identifies the main strategies offered by click and bioorthogonal chemistry, with respect to passive and active targeting, for NP functionalization with specific and multiple properties for imaging and cancer therapy. In the final part, a novel and promising approach for "two step" targeting of NPs, called pretargeting (PT), is also discussed; the principle of this strategy as well as all the studies listed from 2014 to the present are presented in more detail.

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.

Extracellular vesicles-based pre-targeting strategy enables multi-modal imaging of orthotopic colon cancer and image-guided surgery

BACKGROUD

Colon cancer contributes to high mortality rates as the result of incomplete resection in tumor surgery. Multimodal imaging can provide preoperative evaluation and intraoperative image-guiding. As biocompatible nanocarriers, extracellular vesicles hold great promise for multimodal imaging. In this study, we aim to synthesized an extracellular vesicles-based nanoprobe to visualize colon cancer with positron-emission tomography/computed tomography (PET/CT) and near-infrared fluorescence (NIRF) imaging, and investigated its utility in image-guided surgery of colon cancer in animal models.

RESULTS

Extracellular vesicles were successfully isolated from adipose-derived stem cells (ADSCs), and their membrane vesicles were observed under TEM. DLS detected that the hydrodynamic diameters of the extracellular vesicles were approximately 140 nm and the zeta potential was - 7.93 ± 0.24 mV. Confocal microscopy showed that extracellular vesicles had a strong binding ability to tumor cells. A click chemistry-based pre-targeting strategy was used to achieve PET imaging in vivo. PET images and the biodistribution results showed that the best pre-targeting time was 20 h, and the best imaging time was 2 h after the injection of ⁶⁸ Ga-L-NETA-DBCO. The NIRF images showed that the tumor had clear images at all time points after administration of nanoparticles and the Tumor/Muscle ratio peaked at 20 h after injection. Our data also showed that both PET/CT and NIRF imaging clearly visualized the orthotopic colon cancer models, providing preoperative evaluation. Under real-time NIRF imaging, the tumor location and tumor boundary could be clearly observed.

CONCLUSIONS

In brief, this novel nanoprobe may be useful for multi-modal imaging of colon cancer and NIRF image-guided surgery. More importantly, this study provides a new possibility for clinical application of extracellular vesicles as nanocarriers.

Nanoparticles and bioorthogonal chemistry joining forces for improved biomedical applications

Bioorthogonal chemistry comprises chemical reactions that can take place inside complex biological environments, providing outstanding tools for the investigation and elucidation of biological processes. Its use in combination with nanotechnology can lead to further developments in diverse areas of biomedicine, such as molecular bioimaging, targeted delivery, in situ drug activation, study of cell-nanomaterial interactions, biosensing, etc. Here, we summarise the recent efforts to bring together the unique properties of nanoparticles and the remarkable features of bioorthogonal reactions to create a toolbox of new or improved biomedical applications. We show how, by joining forces, bioorthogonal chemistry and nanotechnology can overcome some of the key current limitations in the field of nanomedicine, providing better, faster and more sensitive nanoparticle-based bioimaging and biosensing techniques, as well as therapeutic nanoplatforms with superior efficacy.

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.

Labeling a TCO-functionalized single domain antibody fragment with 18F via inverse electron demand Diels Alder cycloaddition using a fluoronicotinyl moiety-bearing tetrazine derivative

Single domain antibody fragments (sdAbs) exhibit a rapid tumor uptake and fast blood clearance amenable for labeling with ¹⁸F (t_½ = 110 min) but suffer from high kidney accumulation. Previously, we developed a method for ¹⁸F-labeling of sdAbs via trans-cyclooctene (TCO)-tetrazine (Tz) inverse electron demand Diel's Alder cycloaddition reaction (IEDDAR) that incorporated a renal brush border enzyme (RBBE)-cleavable linker. Although >15 fold reduction in kidney activity levels was achieved, tumor uptake was compromised. Here we investigate whether replacing the [¹⁸F]AlF-NOTA moiety with [¹⁸F]fluoronicotinyl would rectify this problem. Anti-HER2 sdAb 5F7 was first derivatized with a TCO-containing agent that included the RBBE-cleavable linker GlyLys (GK) and a PEG chain, and then subjected to IEDDAR with 6-[¹⁸F]fluoronicotinyl-PEG₄-methyltetrazine to provide [¹⁸F]FN-PEG₄-Tz-TCO-GK-PEG₄-5F7 ([¹⁸F]FN-GK-5F7). For comparisons, a control lacking GK linker and 5F7 labeled using residualizing N-succinimidyl 3-guanidinomethyl-5-[¹²⁵I]iodobenzoate (iso-[¹²⁵I]SGMIB) also were synthesized. Radiochemical purity, affinity (K_D) and immunoreactive fraction of [¹⁸F]FN-GK-5F7 were 99%, 5.4 ± 0.7 nM and 72.5 ± 4.3%, respectively. Tumor uptake of [¹⁸F]FN-GK-5F7 in athymic mice bearing subcutaneous SKOV3 xenografts (3.7 ± 1.2% ID/g and 3.4 ± 1.0% ID/g at 1 h and 3 h, respectively) was 2- to 3-fold lower than for co-injected iso-[¹²⁵I]SGMIB-5F7 (6.9 ± 1.9 %ID/g and 8.7 ± 3.0 %ID/g). However, due to its 6-fold lower kidney activity levels, tumor-to-kidney ratios for [¹⁸F]FN-GK-5F7 were 3-4 times higher than those for co-injected iso-[¹²⁵I]SGMIB-5F7 as well as those observed for the ¹⁸F conjugate lacking the RBBE-cleavable linker. Micro-PET/CT imaging of [¹⁸F]FN-GK-5F7 in mice with SKOV-3 subcutaneous xenografts clearly delineated tumor as early as 1 h with minimal activity in the kidneys; however, there was considerable activity in gallbladder and intestines. Although the tumor uptake of [¹⁸F]FN-GK-5F7 was unexpectedly disappointing, incorporating an alternative RBBE-cleavable linker into this labeling strategy may ameliorate this problem.

Enhancement of spontaneous emission of semiconductor quantum dots inside one-dimensional porous silicon photonic crystals

Controlling spontaneous emission by modifying the local electromagnetic environment is of great interest for applications in optoelectronics, biosensing and energy harvesting. Although the development of devices based on one-dimensional porous silicon photonic crystals with embedded luminophores is a promising approach for applications, the efficiency of the embedded luminophores remains a key challenge because of the strong quenching of the emission due to the contact of the luminophores with the surface of porous silicon preventing the observation of interesting light-matter coupling effects. Here, we experimentally demonstrate an increase in the quantum dot (QD) spontaneous emission rate inside a porous silicon microcavity and almost an order of magnitude enhancement of QD photoluminescence intensity in the weak light-matter coupling regime. Furthermore, we have demonstrated drastic alteration of the QD spontaneous emission at the edge of the photonic band gap in porous silicon distributed Bragg reflectors and proved its dependence on the change in the density of photonic states.

Design and preclinical evaluation of nanostars for the passive pretargeting of tumor tissue

INTRODUCTION

Pretargeting strategies that do not rely on the expression of molecular targets have expanded imaging and therapy options for cancer patients. Nanostars with designed multivalency and which highly accumulate in tumor tissue via the enhanced permeability and retention (EPR) effect may therefore be the ideal vectors for the development of a passive pretargeting approach.

METHODS

Nanostars were synthesized, consisting of 7-8 center-cross-linked arms that were modified with trans-cyclooctene (TCO) using poly(ethylene glycol) (PEG) linkers of 12 or 106 monomer units or without linker. The bioorthogonal click reaction with radiofluorinated 2,2'-(7-(2-(tetrazine-poly(ethyleneglycol)11-amino)-2-oxoethyl)-1,4,7-triazonane-1,4-diyl)diacetic acid ([18F]F-Tz-PEG11-NODA) or 2,2'-(7-(2-(tetrazine-amino)-2-oxoethyl)-1,4,7-triazonane-1,4-diyl)diacetic acid ([18F]F-Tz-NODA) was measured by ex vivo biodistribution studies and positron emission tomography (PET) in mice bearing tumors with high EPR characteristics. Bioorthogonal masking was performed using a tetrazine-functionalized dextran polymer (Tz-DP).

RESULTS

Highest tumor accumulation of [18F]F-Tz-PEG11-NODA was observed for nanostars functionalized with TCO without linker, with a tumor uptake of 3.2 ± 0.4%ID/g and a tumor-to-muscle ratio of 12.8 ± 4.2, tumor-to-large intestine ratio of 0.5 ± 0.3 and tumor-to-kidney ratio of 2.0 ± 0.3, being significantly higher than for nanostars functionalized with TCO-PEG12 (P < 0.05) or TCO-PEG106 (P < 0.05). Tumor uptake and tumor-to-tissue ratios did not improve upon bioorthogonal masking with Tz-DP or when using a smaller, more lipophilic tetrazine([18F]F-Tz-NODA).

CONCLUSIONS

A pretargeting strategy was developed based on the passive delivery of TCO-functionalized nanostars. Such a strategy would allow for the imaging and treatment of tumors with apparent EPR characteristics, with high radioactive tumor doses and minimal doses to off-target tissues.

Image-guided mathematical modeling for pharmacological evaluation of nanomaterials and monoclonal antibodies

While plasma concentration kinetics has traditionally been the predictor of drug pharmacological effects, it can occasionally fail to represent kinetics at the site of action, particularly for solid tumors. This is especially true in the case of delivery of therapeutic macromolecules (drug-loaded nanomaterials or monoclonal antibodies), which can experience challenges to effective delivery due to particle size-dependent diffusion barriers at the target site. As a result, disparity between therapeutic plasma kinetics and kinetics at the site of action may exist, highlighting the importance of target site concentration kinetics in determining the pharmacodynamic effects of macromolecular therapeutic agents. Assessment of concentration kinetics at the target site has been facilitated by non-invasive in vivo imaging modalities. This allows for visualization and quantification of the whole-body disposition behavior of therapeutics that is essential for a comprehensive understanding of their pharmacokinetics and pharmacodynamics. Quantitative non-invasive imaging can also help guide the development and parameterization of mathematical models for descriptive and predictive purposes. Here, we present a review of the application of state-of-the-art imaging modalities for quantitative pharmacological evaluation of therapeutic nanoparticles and monoclonal antibodies, with a focus on their integration with mathematical models, and identify challenges and opportunities. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Diagnostic Tools > in vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Preparation and in vivo evaluation of red blood cell membrane coated porous silicon nanoparticles implanted with 155Tb

INTRODUCTION

Porous silicon (PSi) nanoparticles are capable of delivering therapeutic payloads providing targeted delivery and sustained release of the payloads. In this work we describe the development and proof-of-concept in vivo evaluation of thermally hydrocarbonized porous silicon (PSi) nanoparticles that are implanted with radioactive ¹⁵⁵Tb atoms and coated with red blood cell (RBC) membrane (¹⁵⁵Tb-THCPSi). The developed nanocomposites can be utilized as an intravenous delivery platform for theranostic radionuclides.

METHODS

THCPSi thin films were implanted with ¹⁵⁵Dy ions that decay to ¹⁵⁵Tb at the ISOLDE radioactive ion-beam (RIB) facility at CERN. The films were processed to nanoparticles by ball-milling and sonication, and subsequently coated with either a solid lipid and RBC membrane or solely with RBC membrane. The nanocomposites were evaluated in vitro for stability and in vivo for circulation half-life and ex vivo for biodistribution in Balb/c mice.

RESULTS

Nanoporous THCPSi films were successfully implanted with ¹⁵⁵Tb and processed to coated nanoparticles. The in vitro stability of the particles in plasma and buffer solutions was not significantly different between the particle types, and therefore the RBC membrane coated particles with less laborious processing method were chosen for the biological evaluation. The RBC membrane coating enhanced significantly the blood half-life compared to bare THCPSi particles. In the ex vivo biodistribution study a pronounced accumulation to the spleen was found, with lower uptake in the liver and a minor uptake in the lung, gall bladder and bone marrow.

CONCLUSIONS

We have demonstrated, using ¹⁵⁵Tb RIB-implanted PSi nanoparticles coated with mouse RBC membranes, the feasibility of using such a theranostic nanosystem for the delivery of RIB based radionuclides with prolonged circulation time.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

For the first time, the RIB implantation technique has been utilized to produce PSi nanoparticle with a surface modified for better persistence in circulation. When optimized, these particles could be used in targeted radionuclide therapy with a combination of chemotherapeutic payload within the PSi structure.

Pre-targeted Imaging of Protease Activity through In Situ Assembly of Nanoparticles

The pre-targeted imaging of enzyme activity has not been reported, likely owing to the lack of a mechanism to retain the injected substrate in the first step for subsequent labeling. Herein, we report the use of two bioorthogonal reactions-the condensation reaction of aromatic nitriles and aminothiols and the inverse-electron demand Diels-Alder reaction between tetrazine and trans-cyclooctene (TCO)-to develop a novel strategy for pre-targeted imaging of the activity of proteases. The substrate probe (TCO-C-SNAT4) can be selectively activated by an enzyme target (e.g. caspase-3/7), which triggers macrocyclization and subsequent in situ self-assembly into nanoaggregates retained at the target site. The tetrazine-imaging tag conjugate labels TCO in the nanoaggregates to generate selective signal retention for imaging in vitro, in cells, and in mice. Owing to the decoupling of enzyme activation and imaging tag immobilization, TCO-C-SNAT4 can be repeatedly injected to generate and accumulate more TCO-nanoaggregates for click labeling.

Evaluation of Organo [18F]Fluorosilicon Tetrazine as a Prosthetic Group for the Synthesis of PET Radiotracers

Fluorine-18 is the most widely used positron emission tomography (PET) radionuclide currently in clinical application, due to its optimal nuclear properties. The synthesis of ¹⁸F-labeled radiotracers often requires harsh reaction conditions, limiting the use of sensitive bio- and macromolecules as precursors for direct radiolabeling with fluorine-18. We aimed to develop a milder and efficient in vitro and in vivo labeling method for trans-cyclooctene (TCO) functionalized proteins, through the bioorthogonal inverse-electron demand Diels-Alder (IEDDA) reaction with fluorine-18 radiolabeled tetrazine ([¹⁸F]SiFA-Tz). Here, we used TCO-modified bovine serum albumin (BSA) as the model protein, and isotopic exchange (IE) (¹⁹F/¹⁸F) chemistry as the labeling strategy. The radiolabeling of albumin-TCO with [¹⁸F]SiFA-Tz ([¹⁸F]6), providing [¹⁸F]fluoroalbumin ([¹⁸F]10) in high radiochemical yield (99.1 ± 0.2%, n = 3) and a molar activity (MA) of 1.1 GBq/µmol, confirmed the applicability of [¹⁸F]6 as a quick in vitro fluorination reagent for the TCO functionalized proteins. While the biological evaluation of [¹⁸F]6 demonstrated defluorination in vivo, limiting the utility for pretargeted applications, the in vivo stability of the radiotracer was dramatically improved when [¹⁸F]6 was used for the radiolabeling of albumin-TCO ([¹⁸F]10) in vitro, prior to administration. Due to the detected defluorination in vivo, structural optimization of the prosthetic group for improved stability is needed before further biological studies and application of pretargeted PET imaging.

Evaluation of a 68Ga-Labeled DOTA-Tetrazine as a PET Alternative to 111In-SPECT Pretargeted Imaging

The bioorthogonal reaction between a tetrazine and strained trans-cyclooctene (TCO) has garnered success in pretargeted imaging. This reaction was first validated in nuclear imaging using an ¹¹¹In-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-linked bispyridyl tetrazine (Tz) ([¹¹¹In]In-DOTA-PEG₁₁-Tz) and a TCO functionalized CC49 antibody. Given the initial success of this Tz, it has been paired with TCO functionalized small molecules, diabodies, and affibodies for in vivo pretargeted studies. Furthermore, the single photon emission tomography (SPECT) radionuclide, ¹¹¹In, has been replaced with the β-emitter, ¹⁷⁷Lu and α-emitter, ²¹²Pb, both yielding the opportunity for targeted radiotherapy. Despite use of the ‘universal chelator’, DOTA, there is yet to be an analogue suitable for positron emission tomography (PET) using a widely available radionuclide. Here, a ⁶⁸Ga-labeled variant ([⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz) was developed and evaluated using two different in vivo pretargeting systems (Aln-TCO and TCO-CC49). Small animal imaging and ex vivo biodistribution studies were performed and revealed target specific uptake of [⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz in the bone (3.7 %ID/g, knee) in mice pretreated with Aln-TCO and tumor specific uptake (5.8 %ID/g) with TCO-CC49 in mice bearing LS174 xenografts. Given the results of this study, [⁶⁸Ga]Ga-DOTA-PEG₁₁-Tz can serve as an alternative to [¹¹¹In]In-DOTA-PEG₁₁-Tz.

Functionalizing nanoparticles with cancer-targeting antibodies: A comparison of strategies

Standard cancer therapies sometimes fail to deliver chemotherapeutic drugs to tumor cells in a safe and effective manner. Nanotechnology takes the lead in providing new therapeutic options for cancer due to major potential for selective targeting and controlled drug release. Antibodies and antibody fragments are attracting much attention as a source of targeting ligands to bind specific receptors that are overexpressed on cancer cells. Therefore, researchers are devoting time and effort to develop targeting strategies based on nanoparticles functionalized with antibodies, which hold great promise to enhance therapeutic efficacy and circumvent severe side effects. Several methods have been described to immobilize antibodies on the surface of nanoparticles. However, selecting the most appropriate for each application is challenging but also imperative to preserve antigen binding ability and yield stable antibody-conjugated nanoparticles. From this perspective, we aim to provide considerable knowledge on the most widely used methods of functionalization that can be helpful for decision-making and design of conjugation protocols as well. This review summarizes adsorption, covalent conjugation (carbodiimide, maleimide and "click" chemistries) and biotin-avidin interaction, while discussing the advantages, limitations and relevant therapeutic approaches currently under investigation.

Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals

In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions.

Pretargeted Nuclear Imaging and Radioimmunotherapy Based on the Inverse Electron-Demand Diels-Alder Reaction and Key Factors in the Pretargeted Synthetic Design

The exceptional speed and biorthogonality of the inverse electron-demand Diels-Alder (IEDDA) click chemistry between 1,2,4,5-tetrazines and strained alkene dienophiles have made it promising in the realm of pretargeted imaging and therapy. During the past 10 years, the IEDDA-pretargeted strategies have been tested and have already proven capable of producing images with high tumor-to-background ratios and improving therapeutic effect. This review will focus on recent applications of click chemistry ligations in the pretargeted imaging studies of single photon emission computed tomography (SPECT), positron emission tomography (PET), and pretargeted radioimmunotherapy investigations. Additionally, the influence factors of stability, reactivity, and pharmacokinetic properties of TCO tag modified immunoconjugates and radiolabeled Tz derivatives were also summarized in this article, which should be carefully considered in the system design in order to develop a successful pretargeted methodology. We hope that this review will not only equip readers with a knowledge of pretargeted methodology based on IEDDA click chemistry but also inspire synthetic chemists and radiochemists to develop pretargeted radiopharmaceutical components in a more innovative way with various influence factors considered.

Recent Advances in Targeting Nuclear Molecular Imaging Driven by Tetrazine Bioorthogonal Chemistry

Molecular imaging techniques apply sophisticated technologies to monitor, directly or indirectly, the spatiotemporal distribution of molecular or cellular processes for biomedical, diagnostic, or therapeutic purposes. For example, Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) imaging, the most representative modalities of molecular imaging, enable earlier and more accurate diagnosis of cancer and cardiovascular diseases. New possibilities for noninvasive molecular imaging in vivo have emerged with advances in bioorthogonal chemistry. For example, tetrazine-related Inverse Electron Demand Diels-Alder (IEDDA) reactions can rapidly generate short-lived radioisotope probes in vivo that provide strong contrast for SPECT and PET. Here, we review pretargeting strategies for molecular imaging and novel radiotracers synthesized via tetrazine bioorthogonal chemistry. We systematically describe advances in direct radiolabeling and pretargeting approaches in SPECT and PET using metal and nonmetal radioisotopes based on tetrazine bioorthogonal reactions, and we discuss prospects for the future of such contrast agents.

Synthesis and reactivity/stability study of double-functionalizable strained trans-cyclooctenes for tetrazine bioorthogonal reactions

The unique ability of the bioorthogonal pairs to withstand and unaffect biological processes while maintaining high selectivity towards each other sparked the interest in better probing and controlling biological functions. In early years, trans-cyclooctene (TCO)/tetrazine ligation readily standed out by encompassing most of the bioorthogonal criteria such as its excellent biocompatibility, selectivity and efficiency, and as a result of high HOMO-LUMO gap. Modifications on the TCO scaffold such as cyclopropanation render bicyclononene-based TCOs with high enhancement of its reactivity, whereas other modifications focused on improving the solubility, stability, or enabling the scaffold to act as click-to-release drug delivery system. The implementation of facile methods to enhance its versatility is essential for potentiating drug-delivery strategies and expanding the dynamic range of bioorthogonal on/off control. Considering the remarkable properties of bicyclononene-based TCOs we envisioned that the incorporation of an additional vector for functionalization at the cyclopropane moiety could allow access to more complex and double-functionalized TCO probes. Herein we report the synthesis and study of a double-functionalizable strained trans-cyclooctenes for tetrazine bioorthogonal reactions.

Organocatalytic and Scalable Syntheses of Unsymmetrical 1,2,4,5-Tetrazines by Thiol-Containing Promotors

Despite the growing application of tetrazine bioorthogonal chemistry, it is still challenging to access tetrazines conveniently from easily available materials. Described here is the de novo formation of tetrazine from nitriles and hydrazine hydrate using a broad array of thiol-containing catalysts, including peptides. Using this facile methodology, the syntheses of 14 unsymmetric tetrazines, containing a range of reactive functional groups, on the gram scale were achieved with satisfactory yields. Using tetrazine methylphosphonate as a building block, a highly efficient Horner-Wadsworth-Emmons reaction was developed for further derivatization under mild reaction conditions. Tetrazine probes with diverse functions can be scalably produced in yields of 87-93 %. This methodology may facilitate the widespread application of tetrazine bioorthogonal chemistry.

Efficient cartridge purification for producing high molar activity [18F]fluoro-glycoconjugates via oxime formation

INTRODUCTION

¹⁸F-fluoroglycosylation via oxime formation is a chemoselective and mild radiolabeling method for sensitive molecules. Glycosylation can also improve the bioavailability, in vivo kinetics, and stability of the compound in blood, as well as accelerate clearance of biomolecules. A typical synthesis procedure for ¹⁸F-fluoroglycosylation with [¹⁸F]FDG (2-deoxy-2-[¹⁸F]fluoro-d-glucose) and [¹⁸F]FDR (5-deoxy-5-[¹⁸F]fluoro-d-ribose) involves two HPLC (high performance liquid chromatography) purifications: one after ¹⁸F-fluorination of the carbohydrate to remove its labeling precursor, and a second one after the oxime formation step to remove the aminooxy precursor. The two HPLC purifications can be time consuming and complicate the adaptation of the synthetic strategy in nuclear medicine applications and automated synthesis. We have developed a procedure in which SPE (solid phase extraction) and resin purification methods replace both of the needed HPLC purification steps.

METHODS

We used [¹⁸F]FDR and [¹⁸F]FDG as prosthetic groups to radiolabel two aminooxy-modified model molecules, a tetrazine and a PSMA (prostate specific membrane antigen) inhibitor. After fluorination, the excess carbohydrate precursor was removed by derivatizing it with 4,4'-dimethoxytrityl chloride (DMT-Cl). The DMT moiety increases the hydrophobicity of the unreacted precursor making the separation from the fluorinated precursor possible with simple C18 Sep-Pak cartridge. For removal of the aminooxy precursor, we used a commercially available aldehyde resin (AminoLink, Thermo Fisher Scientific). C18 Sep-Pak SPE cartridge was used to separate [¹⁸F]FDR and [¹⁸F]FDG from the ¹⁸F-fluoroglycoconjugate end product.

RESULTS

[¹⁸F]FDR and [¹⁸F]FDG were efficiently purified from their precursors, free fluorine-18, and other impurities. The aldehyde resin quantitatively removed the unreacted aminooxy precursors after the oxime formation. The fluorine-18 labeled oxime end products were obtained with high radiochemical purity (>99%) and molar activity (>600 GBq μmol^-1).

CONCLUSIONS

We have developed an efficient cartridge purification method for producing high molar activity ¹⁸F-glycoconjugates synthesized via oxime formation.

Pretargeting in nuclear imaging and radionuclide therapy: Improving efficacy of theranostics and nanomedicines

Pretargeted nuclear imaging and radiotherapy have recently attracted increasing attention for diagnosis and treatment of cancer with nanomedicines. This is because it conceptually offers better imaging contrast and therapeutic efficiency while reducing the dose to radiosensitive tissues compared to conventional strategies. In conventional imaging and radiotherapy, a directly radiolabeled nano-sized vector is administered and allowed to accumulate in the tumor, typically on a timescale of several days. In contrast, pretargeting is based on a two-step approach. First, a tumor-accumulating vector carrying a tag is administered followed by injection of a fast clearing radiolabeled agent that rapidly recognizes the tag of the tumor-bound vector in vivo. Therefore, pretargeting circumvents the use of long-lived radionuclides that is a necessity for sufficient tumor accumulation and target-to-background ratios using conventional approaches. In this review, we give an overview of recent advances in pretargeted imaging strategies. We will critically reflect on the advantages and disadvantages of current state-of-the-art conventional imaging approaches and compare them to pretargeted strategies. We will discuss the pretargeted imaging concept and the involved chemistry. Finally, we will discuss the steps forward in respect to clinical translation, and how pretargeted strategies could be applied to improve state-of-the-art radiotherapeutic approaches.

Multimodality Imaging of Silica and Silicon Materials In Vivo

Recent progress in the development of silica- and silicon-based multimodality imaging nanoprobes has advanced their use in image-guided drug delivery, and the development of novel systems for nanotheranostic and diagnostic applications. As biocompatible and flexibly tunable materials, silica and silicon provide excellent platforms with high clinical potential in nanotheranostic and diagnostic probes with well-defined morphology and surface chemistry, yielding multifunctional properties. In vivo imaging is of great value in the exploration of methods for improving site-specific nanotherapeutic delivery by silica- and silicon-based drug-delivery systems. Multimodality approaches are essential for understanding the biological interactions of nanotherapeutics in the physiological environment in vivo. The aim here is to describe recent advances in the development of in vivo imaging tools based on nanostructured silica and silicon, and their applications in single and multimodality imaging.

Thermally Carbonized Porous Silicon and Its Recent Applications

Recent progress in research on thermally carbonized porous silicon (TCPSi) and its applications is reported. Despite a slow start, thermal carbonization has now started to gain interest mainly due to new emerging areas for applications. These new areas, such as optical sensing, drug delivery, and energy storage, require stable surface chemistry and physical properties. TCPSi is known to have all of these desired properties. Herein, the above-listed properties of TCPSi are summarized, and the carbonization processes, functionalization, and characterization of TCPSi are reviewed. Moreover, some of the emerging fields of TCPSi applications are discussed and recent advances in the fields are introduced.

Inverse electron demand Diels-Alder reactions in chemical biology

The emerging inverse electron demand Diels-Alder (IEDDA) reaction stands out from other bioorthogonal reactions by virtue of its unmatchable kinetics, excellent orthogonality and biocompatibility. With the recent discovery of novel dienophiles and optimal tetrazine coupling partners, attention has now been turned to the use of IEDDA approaches in basic biology, imaging and therapeutics. Here we review this bioorthogonal reaction and its promising applications for live cell and animal studies. We first discuss the key factors that contribute to the fast IEDDA kinetics and describe the most recent advances in the synthesis of tetrazine and dienophile coupling partners. Both coupling partners have been incorporated into proteins for tracking and imaging by use of fluorogenic tetrazines that become strongly fluorescent upon reaction. Selected notable examples of such applications are presented. The exceptional fast kinetics of this catalyst-free reaction, even using low concentrations of coupling partners, make it amenable for in vivo radiolabelling using pretargeting methodologies, which are also discussed. Finally, IEDDA reactions have recently found use in bioorthogonal decaging to activate proteins or drugs in gain-of-function strategies. We conclude by showing applications of the IEDDA reaction in the construction of biomaterials that are used for drug delivery and multimodal imaging, among others. The use and utility of the IEDDA reaction is interdisciplinary and promises to revolutionize chemical biology, radiochemistry and materials science.

Pretargeting of internalizing trastuzumab and cetuximab with a 18F-tetrazine tracer in xenograft models

BACKGROUND

Pretargeting-based approaches are being investigated for radioimmunoimaging and therapy applications to reduce the effective radiation burden to the patient. To date, only a few studies have used short-lived radioisotopes for pretargeting of antibodies, and such examples with internalizing antibodies are even rarer. Herein, we have investigated pretargeting methodology using inverse electron-demand Diels-Alder (IEDDA) for tracing two clinically relevant, internalizing monoclonal antibodies, cetuximab and trastuzumab.

RESULTS

Bioorthogonal reaction between tetrazine and trans-cyclooctene (TCO) was used for tracing cetuximab and trastuzumab in vivo with a fluorine-18 (t _½ = 109.8 min) labelled tracer. TCO-cetuximab or TCO-trastuzumab was administered 24, 48, or 72 h prior to the injection of tracer to A431 or BT-474 tumour-bearing mice, respectively. With cetuximab, the highest tumour-to-blood ratios were achieved when the lag time between antibody and tracer injections was 72 h. With trastuzumab, no difference was observed between different lag times. For both antibodies, the tumour could be clearly visualized in the PET images with the highest tumour uptake of 3.7 ± 0.1%ID/g for cetuximab and 1.5 ± 0.1%ID/g for trastuzumab as quantified by ex vivo biodistribution. In vivo IEDDA reaction was observed in the blood for both antibodies, but with trastuzumab, this was to a much lower degree than with cetuximab.

CONCLUSIONS

We could successfully visualize the tumours by using cetuximab and trastuzumab in pretargeted PET imaging despite the challenging circumstances where the antibody is internalized and there is still some unbound antibody circulating in the blood flow. This clearly demonstrates the potential of a pretargeted approach for targeting internalizing antigens and warrants development of pharmacokinetic optimization of the biorthogonal reactants to this end.

Antibody PEGylation in bioorthogonal pretargeting with trans-cyclooctene/tetrazine cycloaddition: in vitro and in vivo evaluation in colorectal cancer models

Bioorthogonal chemistry represents a challenging approach in pretargeted radioimmunotherapy (PRIT). We focus here on mAb modifications by grafting an increase amount of trans-cyclooctene (TCO) derivatives (0 to 30 equivalents with respect to mAb) bearing different polyethylene glycol (PEG) linkers between mAb and TCO (i.e. PEG₀ (1), PEG₄ (2) and PEG₁₂ (3)) and assessing their functionality. We used colorectal xenograft (HT29/Ts29.2) and peritoneal carcinomatosis (A431-CEA-Luc/35A7) as tumor cells/mAbs models and fluorescent tetrazines (TZ). MALDI-TOF MS shows that grafting with 2,3 increases significantly the number of TCO per mAb compared with no PEG. In vitro immunofluorescence showed that Ts29.2 and 35A7 labeling intensity is correlated with the number of TCO when using 1,3 while signals reach a maximum at 10 equivalents when using 2. Under 10 equivalents conditions, the capacity of resulting mAbs-1–3 for antigen recognition is similar when reported per grafted TCO and comparable to mAbs without TCO. In vivo, on both models, pretargeting with mAbs-2,3 followed by TZ injection induced a fluorescent signal two times lower than with mAbs-1. These findings suggest that while PEG linkers allow a better accessibility for TCO grafting, it might decrease the number of reactive TCO. In conclusion, mAb-1 represents the best candidate for PRIT.