Deconvolving Passive and Active Targeting of Liposomes Bearing LDL Receptor Binding Peptides Using the Zebrafish Embryo Model.

Improving target versus off-target ratio in nanomedicine remains a major challenge for increasing drug bioavailability and reducing toxicity. Active targeting using ligands on nanoparticle surfaces is a key approach but has limited clinical success. A potential issue is the integration of targeting ligands also changes the physicochemical properties of nanoparticles (passive targeting). Direct studies to understand the mechanisms of active targeting and off-targeting in vivo are limited by the lack of suitable tools. Here, the biodistribution of a representative active targeting liposome is analyzed, modified with an apolipoprotein E (ApoE) peptide that binds to the low-density lipoprotein receptor (LDLR), using zebrafish embryos. The ApoE liposomes demonstrated the expected liver targeting effect but also accumulated in the kidney glomerulus. The ldlra-/- zebrafish is developed to explore the LDLR-specificity of ApoE liposomes. Interestingly, liver targeting depends on the LDLR-specific interaction, while glomerular accumulation is independent of LDLR and peptide sequence. It is found that cationic charges of peptides and the size of liposomes govern glomerular targeting. Increasing the size of ApoE liposomes can avoid this off-targeting. Taken together, the study shows the potential of the zebrafish embryo model for understanding active and passive targeting mechanisms, that can be used to optimize the design of nanoparticles.

Utility of Triethyloxonium Tetrafluoroborate for Chloride Removal during Sarcosine N-Carboxyanhydride Synthesis: Improving NCA Purity.

The clinical translation of polysarcosine (pSar) as polyethylene glycol (PEG) replacement in the development of novel nanomedicines creates a broad demand of polymeric material in high-quality making high-purity sarcosine N-carboxyanhydride (Sar-NCA) as monomer for its production inevitable. Within this report, we present the use of triethyloxonium tetrafluoroborate in Sar-NCA synthesis with focus on amino acid and chloride impurities to avoid the sublimation of Sar-NCAs. With a view towards upscaling into kilogram or ton scale, a new methodology of monomer purification is introduced by utilizing the Meerwein's Salt triethyloxonium tetrafluoroborate to remove chloride impurities by covalent binding and converting chloride ions into volatile products within a single step. The novel straightforward technique enables access to monomers with significantly reduced chloride content (<100 ppm) compared to Sar-NCA derived by synthesis or sublimation. The derived monomers enable the controlled-living polymerization in DMF and provide access to pSar polymers with Poisson-like molecular weight distribution within a high range of chain lengths (Xn 25-200). In conclusion, the reported method can be easily applied to Sar-NCA synthesis or purification of commercially available pSar-NCAs and eases access to well-defined hetero-telechelic pSar polymers.

Miktoarm Star-polypept(o)ide-Based Polyion Complex Micelles for the Delivery of Large Nucleic Acids.

Miktoarm star polymers exhibit a captivating range of physicochemical properties, setting them apart from their linear counterparts. This study devised a synthetic pathway to synthesize cationic miktoarm stars utilizing polypept(o)ides (PeptoMiktoStars), comprising 3 or 6 polysarcosine (pSar) arms (AB3×100, AB6×50, overall 300) for shielding and a cross-linkable poly(S-ethylsulfonyl-l-homocysteine) (pHcy(SO2Et)20) block and a poly(l-lysine) ((pLys)20) block for nucleic acid complexation. Precise control over the DPn and narrow molecular weight distributions (D̵ ≈ 1.2) were achieved for both structures. Both PeptoMiktoStars efficiently complexed mRNA and pDNA into polyion complex micelles (PICMs). AB6-PICMs provided modest (mRNA) to high (pDNA) stability against glutathione and heparin sulfate (HS), while even cross-linked AB3-PICMs were susceptible to HS. All PICMs delivered pDNA and mRNA into D1 cells (over 80%) and Jurkat T cells (over 50%) in vitro. Despite payload- and cell-dependency, AB3 showed overall higher transfection efficiency, while AB6 demonstrated better shielding and enhanced stability.

Tuning the Cross-Linking Density and Cross-Linker in Core Cross-Linked Polymeric Micelles and Its Effects on the Particle Stability in Human Blood Plasma and Mice.

Core cross-linked polymeric micelles (CCPMs) are designed to improve the therapeutic profile of hydrophobic drugs, reduce or completely avoid protein corona formation, and offer prolonged circulation times, a prerequisite for passive or active targeting. In this study, we tuned the CCPM stability by using bifunctional or trifunctional cross-linkers and varying the cross-linkable polymer block length. For CCPMs, amphiphilic thiol-reactive polypept(o)ides of polysarcosine-block-poly(S-ethylsulfonyl-l-cysteine) [pSar-b-pCys(SO2Et)] were employed. While the pCys(SO2Et) chain lengths varied from Xn = 17 to 30, bivalent (derivatives of dihydrolipoic acid) and trivalent (sarcosine/cysteine pentapeptide) cross-linkers have been applied. Asymmetrical flow field-flow fraction (AF4) displayed the absence of aggregates in human plasma, yet for non-cross-linked PM and CCPMs cross-linked with dihydrolipoic acid at [pCys(SO2Et)]17, increasing the cross-linking density or the pCys(SO2Et) chain lengths led to stable CCPMs. Interestingly, circulation time and biodistribution in mice of non-cross-linked and bivalently cross-linked CCPMs are comparable, while the trivalent peptide cross-linkers enhance the circulation half-life from 11 to 19 h.

Shining Light on Polymeric Drug Nanocarriers with Fluorescence Correlation Spectroscopy.

The use of nanoparticles as carriers is an extremely promising way for administration of therapeutic agents, such as drug molecules, proteins, and nucleic acids. Such nanocarriers (NCs) can increase the solubility of hydrophobic compounds, protect their cargo from the environment, and if properly functionalized, deliver it to specific target cells and tissues. Polymer-based NCs are especially promising, because they offer high degree of versatility and tunability. However, in order to get a full advantage of this therapeutic approach and develop efficient delivery systems, a careful characterization of the NCs is needed. This review highlights the fluorescence correlation spectroscopy (FCS) technique as a powerful and versatile tool for NCs characterization at all stages of the drug delivery process. In particular, FCS can monitor and quantify the size of the NCs and the drug loading efficiency after preparation, the NCs stability and possible interactions with, e.g., plasma proteins in the blood stream and the kinetic of drug release in the cytoplasm of the target cells.

[11 C]Carboxylated Tetrazines for Facile Labeling of Trans-Cyclooctene-Functionalized PeptoBrushes.

Functionalization of macromolecules (antibodies, polymers, nanoparticles) with click-reactive groups greatly enhances the versatility of their potential applications. Click chemistry based on tetrazine - trans-cyclooctene (TCO) ligation is especially promising and is already widely applied for pretargeted imaging and therapy. Indirect radiolabeling of TCO-functionalized macromolecules with substoichiometric amounts of radioactive tetrazines is a convenient way to monitor the fate of those macromolecules by means of positron emission tomography (PET) imaging after their administration into the test subject. In this work, the preparation is reported of TCO-containing graft copolymers, namely PeptoBrushes (polyglutamic acid-graft-polysarcosine), novel [11 C]carboxylated tetrazines, and their combined use in radiolabeling the polymer by inverse electron demand Diels Alder reaction, to investigate it is potential for an application in pretarget imaging or injectable brachytherapy. The procedure for [11 C]tetrazine production is easy and scalable, while indirect TCO-PeptoBrushes labeling with these [11 C]tetrazines is mild, fast, and quantitative. This strategy allows facile 11 C-labeling of diverse TCO-functionalized macromolecules, so that their localization and distribution shortly after injection can be assessed by PET.

Cross-Linkable Polyion Complex Micelles from Polypept(o)ide-Based ABC-Triblock Copolymers for siRNA Delivery.

ABC-type triblock copolymers are a rising platform especially for oligonucleotide delivery as they offer an additional functionality besides the anyhow needed functions of shielding and complexation. The authors present a polypept(o)ide-based triblock copolymer synthesized by amine-initiated ring-opening polymerization (ROP) of N-carboxyanhydrides (NCAs), comprising a shielding block A of polysarcosine (pSar), a poly(S-ethylsulfonyl-l-cystein) (pCys(SO2 Et)) block B for bioreversible and chemo-selective cross-linking and a poly(l-lysine) (pLys) block C for complexation to construct polyion complex (PIC) micelles as vehicle for small interfering RNA (siRNA) delivery. The self-assembly behavior of ABC-type triblocks is investigated to derive correlations between block lengths of the polymer and PIC micelle structure, showing an enormous effect of the ß-sheet forming pCys(SO2 Et) block. Moreover, the block enables the introduction of disulfide cross-links by reaction with multifunctional thiols to increase stability against dilution. The right content of the additional block leads to well-defined cross-linked 50-60 nm PIC micelles purified from production impurities and determinable siRNA loading. These PIC micelles can deliver functional siRNA into Neuro2A and KB cells evaluated by cellular uptake and specific gene knockdown assays.

Precision Wormlike Nanoadjuvant Governs Potency of Vaccination.

It remains unclear how the precise length of one-dimensional nanovehicles influences the characters of vaccination. Here, a unimolecular nanovehicle with tailored size and aspect ratio (AR) is applied to deliver CpG oligodeoxynucleotide, a Toll-like receptor (TLR) 9 agonist, as an adjuvant of recombinant hepatitis B virus surface antigen (rHBsAg), for treating chronic hepatitis B (CHB). Cationic nanovehicles with fixed width (ca. 45 nm) but varied length (46 nm-180 nm), AR from 1 to 4, are prepared through controlled polymerization and are loaded with CpG by electrostatic interaction. We reveal that the nanoadjuvant with AR = 2 shows the highest retention in proximal lymph nodes. Importantly, it is more easily internalized into antigen-presenting cells and accumulates in the late endosome, where TLR9 is located. Such a nanoadjuvant exhibits the strongest immune response with rHBsAg to clear the hepatitis B virus in the CHB mouse model, showing that the AR of nanovehicles governs the efficiency of vaccination.

Delivery of siHIF-1α to Reconstruct Tumor Normoxic Microenvironment for Effective Chemotherapeutic and Photodynamic Anticancer Treatments.

The tumor hypoxic microenvironment not only induces genetic and epigenetic changes in tumor cells, immature vessels formation for oxygen demand, but also compromises the efficiency of therapeutic interventions. On the other hand, conventional therapeutic approaches which kill tumor cells or destroy tumor blood vessels to block nutrition and oxygen supply usually facilitate even harsher microenvironment. Thus, simultaneously relieving the strained response of tumor cells and blood vessels represents a promising strategy to reverse the adverse tumor hypoxic microenvironment. In the present study, an integrated amphiphilic system (RSCD) is designed based on Angiotensin II receptor blocker candesartan for siRNA delivery against the hypoxia-inducible factor-1 alpha (HIF-1α), aiming at both vascular and cellular "relaxation" to reconstruct a tumor normoxic microenvironment. Both in vitro and in vivo studies have confirmed that the hypoxia-inducible HIF-1α expression is down-regulated by 70% and vascular growth is inhibited by 60%. The "relaxation" therapy enables neovascularization with more complete and organized structures to obviously increase the oxygen level inside tumor, which results in a 50% growth inhibition. Moreover, reconstruction of tumor microenvironment enhances tumor-targeted drug delivery, and significantly improves the chemotherapeutic and photodynamic anticancer treatments.

Secondary Structure-Driven Self-Assembly of Thiol-Reactive Polypept(o)ides.

Secondary structure formation differentiates polypeptides from most of the other synthetic polymers, and the transitions from random coils to rod-like α-helices or ß-sheets represent an additional parameter to direct self-assembly and the morphology of nanostructures. We investigated the influence of distinct secondary structures on the self-assembly of reactive amphiphilic polypept(o)ides. The individual morphologies can be preserved by core cross-linking via chemoselective disulfide bond formation. A series of thiol-responsive copolymers of racemic polysarcosine-block-poly(S-ethylsulfonyl-dl-cysteine) (pSar-b-p(dl)Cys), enantiopure polysarcosine-block-poly(S-ethylsulfonyl-l-cysteine) (pSar-b-p(l)Cys), and polysarcosine-block-poly(S-ethylsulfonyl-l-homocysteine) (pSar-b-p(l)Hcy) was prepared by N-carboxyanhydride polymerization. The secondary structure of the peptide segment varies from α-helices (pSar-b-p(l)Hcy) to antiparallel ß-sheets (pSar-b-p(l)Cys) and disrupted ß-sheets (pSar-b-p(dl)Cys). When subjected to nanoprecipitation, copolymers with antiparallel ß-sheets display the strongest tendency to self-assemble, whereas disrupted ß-sheets hardly induce aggregation. This translates to worm-like micelles, solely spherical micelles, or ellipsoidal structures, as analyzed by atomic force microscopy and cryogenic transmission electron microscopy, which underlines the potential of secondary structure-driven self-assembly of synthetic polypeptides.

Racemic S-(ethylsulfonyl)-dl-cysteine N-Carboxyanhydrides Improve Chain Lengths and Monomer Conversion for ß-Sheet-Controlled Ring-Opening Polymerization.

The secondary structure formation of polypeptides not only governs folding and solution self-assembly but also affects the nucleophilic ring-opening polymerization of α-amino acid-N-carboxyanhydrides (NCAs). Whereby helical structures are known to enhance polymerization rates, ß-sheet-like assemblies reduce the propagation rate or may even terminate chain growth by precipitation or gelation. To overcome these unfavorable properties, racemic mixtures of NCAs can be applied. In this work, racemic S-(ethylsulfonyl)-dl-cysteine NCA is investigated for the synthesis of polypeptides, diblock and triblock copolypept(o)ides. In contrast to the polymerization of stereoregular S-(ethylsulfonyl)-l-cysteine NCA, the reaction of S-(ethylsulfonyl)-dl-cysteine NCA proceeds with a rate constant of up to kp  = 1.70 × 10-3 L mol-1 s -1 and is slightly faster than the enatiopure polymerization. While the polymerization of S-(ethylsulfonyl)-l-cysteine NCA suffers from incomplete monomer conversion and degrees of polymerization (DPs) limited to 30-40, racemic mixtures yield polypeptides with DPs of up to 102 with high conversion rates and well-defined dispersities (1.2-1.3). The controlled living nature of the ring-opening polymerization of S-(ethylsulfonyl)-dl-cysteine NCA thus enables the synthesis of triblock copolymers by sequential monomer addition. This methodology allows for precise control over DPs of individual blocks and yields uniform triblock copolymers with symmetric molecular weight distributions at a reduced synthetic effort.

Precision Anisotropic Brush Polymers by Sequence Controlled Chemistry.

The programming of nanomaterials at molecular length-scales to control architecture and function represents a pinnacle in soft materials synthesis. Although elusive in synthetic materials, Nature has evolutionarily refined macromolecular synthesis with perfect atomic resolution across three-dimensional space that serves specific functions. We show that biomolecules, specifically proteins, provide an intrinsic macromolecular backbone for the construction of anisotropic brush polymers with monodisperse lengths via grafting-from strategy. Using human serum albumin as a model, its sequence was exploited to chemically transform a single cysteine, such that the expression of said functionality is asymmetrically placed along the backbone of the eventual brush polymer. This positional monofunctionalization strategy was connected with biotin-streptavidin interactions to demonstrate the capabilities for site-specific self-assembly to create higher ordered architectures. Supported by systematic experimental and computational studies, we envisioned that this macromolecular platform provides unique avenues and perspectives in macromolecular design for both nanoscience and biomedicine.

Integration of Indocyanine Green Analogs as Near-Infrared Fluorescent Carrier for Precise Imaging-Guided Gene Delivery.

Codelivery of diagnostic probes and therapeutic molecules often suffers from intrinsic complexity and premature leakage from or degradation of the nanocarrier. Inspired by the "Y" shape of indocyanine green (ICG), the dye is integrated in an amphiphilic lipopeptide (RNF). The hydrophilic segment is composed of arginine-rich dendritic peptides, while cyanine dyes are modified with two long carbon chains and employed as the hydrophobic moiety. They are linked through a disulfide linkage to improve the responsivity in the tumor microenvironment. After formulation with other lipopeptides at an optimized ratio, the theranostic system (RNS-2) forms lipid-based nanoparticles with slight positive zeta potential enabling efficient condensation of DNA. The RNS-2 displays glutathione responded gene release, activatable fluorescence recovery, and up to sevenfold higher in vitro transfection than Lipofectamine 2000. Compared with a Cy3 and Cy5 labeled fluorescence resonance energy transfer indicator for gene release, the "turn-on" indocyanine green analogs exhibit longer emission wavelength and better positive correlation with the dynamic processes of gene delivery. More importantly, the RNS-2 system enables efficient near infrared imaging guided gene transfer in tumor-bearing mice and thus provides more precise and accurate information on location of the cargo gene and synthesized carriers.

Zebrafish Embryos Allow Prediction of Nanoparticle Circulation Times in Mice and Facilitate Quantification of Nanoparticle-Cell Interactions.

The zebrafish embryo is a vertebrate well suited for visualizing nanoparticles at high resolution in live animals. Its optical transparency and genetic versatility allow noninvasive, real-time observations of vascular flow of nanoparticles and their interactions with cells throughout the body. As a consequence, this system enables the acquisition of quantitative data that are difficult to obtain in rodents. Until now, a few studies using the zebrafish model have only described semiquantitative results on key nanoparticle parameters. Here, a MACRO dedicated to automated quantitative methods is described for analyzing important parameters of nanoparticle behavior, such as circulation time and interactions with key target cells, macrophages, and endothelial cells. Direct comparison of four nanoparticle (NP) formulations in zebrafish embryos and mice reveals that data obtained in zebrafish can be used to predict NPs' behavior in the mouse model. NPs having long or short blood circulation in rodents behave similarly in the zebrafish embryo, with low circulation times being a consequence of NP uptake into macrophages or endothelial cells. It is proposed that the zebrafish embryo has the potential to become an important intermediate screening system for nanoparticle research to bridge the gap between cell culture studies and preclinical rodent models such as the mouse.

Polymeric Nanoparticles with Neglectable Protein Corona.

The current understanding of nanoparticle-protein interactions indicates that they rapidly adsorb proteins upon introduction into a living organism. The formed protein corona determines thereafter identity and fate of nanoparticles in the body. The present study evaluates the protein affinity of three core-crosslinked polymeric nanoparticles with long circulation times, differing in the hydrophilic polymer material forming the particle surface, namely poly(N-2-hydroxypropylmethacrylamide) (pHPMA), polysarcosine (pSar), and poly(ethylene glycol) (PEG). This includes the nanotherapeutic CPC634, which is currently in clinical phase II evaluation. To investigate possible protein corona formation, the nanoparticles are incubated in human blood plasma and separated by asymmetrical flow field-flow fractionation (AF4). Notably, light scattering shows no detectable differences in particle size or polydispersity upon incubation with plasma for all nanoparticles, while in gel electrophoresis, minor amounts of proteins can be detected in the particle fraction. Label-free quantitative proteomics is additionally applied to analyze and quantify the composition of the proteins. It proves that some proteins are enriched, but their concentration is significantly less than one protein per particle. Thus, most of the nanoparticles are not associated with any proteins. Therefore, this work underlines that polymeric nanoparticles can be synthesized, for which a protein corona formation does not take place.

Influence of Riboflavin Targeting on Tumor Accumulation and Internalization of Peptostar Based Drug Delivery Systems.

Riboflavin carrier protein (RCP) and riboflavin transporters (RFVTs) have been reported to be highly overexpressed in various cancer cells. Hence, targeting RCP and RFVTs using riboflavin may enhance tumor accumulation and internalization of drug delivery systems. To test this hypothesis, butyl-based 3-arm peptostar polymers were synthesized consisting of a lysine core (10 units per arm) and a sarcosine shell (100 units per arm). The end groups of the arms and the core were successfully modified with riboflavin and the Cy5.5 fluorescent dye, respectively. While in phosphate buffered saline the functionalized peptostars showed a bimodal behavior and formed supramolecular structures over time, they were stable in the serum maintaining their hydrodynamic diameter of 12 nm. Moreover, the polymers were biocompatible and the uptake of riboflavin targeted peptostars in A431 and PC3 cells was higher than in nontargeted controls and could be blocked competitively. In vivo, the polymers showed a moderate passive tumor accumulation, which was not significantly different between targeted and nontargeted peptostars. Nonetheless, at the histological level, internalization into tumor cells was strongly enhanced for the riboflavin-targeted peptostars. Based on these results, we conclude that passive accumulation is dominating the accumulation of peptostars, while tumor cell internalization is strongly promoted by riboflavin targeting.

Nanomedicine and macroscale materials in immuno-oncology.

Immunotherapy is revolutionizing the treatment of cancer. It can achieve unprecedented responses in advanced-stage patients, including complete cures and long-term survival. However, immunotherapy also has limitations, such as its relatively low response rates and the development of severe side effects. These drawbacks are gradually being overcome by improving our understanding of the immune system, as well as by establishing combination regimens in which immunotherapy is combined with other treatment modalities. In addition to this, in recent years, progress made in chemistry, nanotechnology and materials science has started to impact immuno-oncology, resulting in more effective and less toxic immunotherapy interventions. In this context, multiple different nanomedicine formulations and macroscale materials have been shown to be able to boost anti-cancer immunity and the efficacy of immunomodulatory drugs. We here review nanotechnological and materials chemistry efforts related to endogenous and exogenous vaccination, to the engineering of antigen-presenting cells and T cells, and to the modulation of the tumor microenvironment. We also discuss limitations, current trends and future directions. Together, the insights provided and the evidence obtained indicate that there is a bright future ahead for engineering nanomedicines and macroscale materials for immuno-oncology applications.

Poly(Sarcosine) Surface Modification Imparts Stealth-Like Properties to Liposomes.

Circulation lifetime is a crucial parameter for a successful therapy with nanoparticles. Reduction and alteration of opsonization profiles by surface modification of nanoparticles is the main strategy to achieve this objective. In clinical settings, PEGylation is the most relevant strategy to enhance blood circulation, yet it has drawbacks, including hypersensitivity reactions in some patients treated with PEGylated nanoparticles, which fuel the search for alternative strategies. In this work, lipopolysarcosine derivatives (BA-pSar, bisalkyl polysarcosine) with precise chain lengths and low polydispersity indices are synthesized, characterized, and incorporated into the bilayer of preformed liposomes via a post insertion technique. Successful incorporation of BA-pSar can be realized in a clinically relevant liposomal formulation. Furthermore, BA-pSar provides excellent surface charge shielding potential for charged liposomes and renders their surface neutral. Pharmacokinetic investigations in a zebrafish model show enhanced circulation properties and reduction in macrophage recognition, matching the behavior of PEGylated liposomes. Moreover, complement activation, which is a key factor in hypersensitivity reactions caused by PEGylated liposomes, can be reduced by modifying the surface of liposomes with an acetylated BA-pSar derivative. Hence, this study presents an alternative surface modification strategy with similar benefits as the established PEGylation of nanoparticles, but with the potential of reducing its drawbacks.

Impact of Branching on the Solution Behavior and Serum Stability of Starlike Block Copolymers.

The size control of nanomedicines for tumor diagnosis and therapy is of high importance, since it enables or disables deep and sufficient tumor penetration. Amphiphilic star-shaped block copolypept(o)ides offer substantial promise to precisely adjust the hydrophobic core and the hydrophilic corona, independent of each other, and therefore simultaneously control the size dimension in the interesting size range from 10 to 30 nm. To gain access to core-shell structures of such sizes, 3-arm and 6-arm PeptoStars, based on poly(γ- tert-butyloxycarbonyl-l-glutamate)- b-polysarcosine (pGlu(O tBu)- b-pSar), were prepared via controlled living ring-opening polymerization (ROP) of the corresponding N-carboxyanhydrides. Moreover, size exclusion chromatography (SEC) proves the presence of well-defined star shaped polymers with molecular weights from 38 to 88 kg/mol with low polymer dispersities of 1.16 to 1.23. By varying the α-helical peptide core and maintain a constant polysarcosine corona, hydrodynamic size analyses revealed the importance of using a sufficiently large and dense hydrophilic shielding corona to prevent aggregation of the hydrophobic core and obtain uniform-sized spherical-shaped particles with hydrodynamic diameters below 24 nm. Fluorescence correlation spectroscopy (FCS) additionally demonstrates the absence of protein adsorption in human plasma for 6-arm polypept(o)ide stars and thus confirms polysarcosine as stealthlike material.

Improved radiosynthesis and preliminary in vivo evaluation of the 11C-labeled tetrazine [11C]AE-1 for pretargeted PET imaging.

Pretargeted nuclear imaging based on the ligation between tetrazines and nano-sized targeting agents functionalized with trans-cyclooctene (TCO) has recently been shown to improve both imaging contrast and dosimetry in nuclear imaging of nanomedicines. Herein, we describe the improved radiosynthesis of a 11C-labeled tetrazine ([11C]AE-1) and its preliminary evaluation in both mice and pigs. Pretargeted imaging in mice was carried out using both a new TCO-functionalized polyglutamic acid and a previously reported TCO-functionalized bisphosphonate system as targeting agents. Unfortunately, pretargeted imaging was not successful using these targeting agents in pair with [11C]AE-1. However, brain imaging in pig indicated that the tracer crossed the blood-brain-barrier. Hence, we suggest that this tetrazine scaffold could be used as a starting point for the development of pretargeted brain imaging, which has so far been a challenging task.