A DOTA-peptide conjugate by copper-free click chemistry

Label-free detection of biomolecules using inductively coupled plasma mass spectrometry (ICP-MS)

Bioassays using inductively coupled plasma mass spectrometry (ICP-MS) have gained increasing attention because of the high sensitivity of ICP-MS and the various strategies of labeling biomolecules with detectable metal tags. The classic strategy to tag the target biomolecules is through direct antibody-antigen interaction and DNA hybridization, and requires the separation of the bound from the unbound tags. Label-free ICP-MS techniques for biomolecular assays do not require direct labeling: they generate detectable metal ions indirectly from specific biomolecular reactions, such as enzymatic cleavage. Here, we highlight the development of three main strategies of label-free ICP-MS assays for biomolecules: (1) enzymatic cleavage of metal-labeled substrates, (2) release of immobilized metal ions from the DNA backbone, and (3) nucleic acid amplification-assisted aggregation and release of metal tags to achieve amplified detection. We briefly describe the fundamental basis of these label-free ICP-MS assays and discuss the benefits and drawbacks of various designs. Future research is needed to reduce non-specific adsorption and minimize background and interference. Analytical innovations are also required to confront challenges faced by in vivo applications.

Opportunities for Expanding Encoded Chemical Diversification and Improving Hit Enrichment in mRNA-Displayed Peptide Libraries

DNA-encoded small-molecule libraries and mRNA displayed peptide libraries both use numerically large pools of oligonucleotide-tagged molecules to identify potential hits for protein targets. They differ dramatically, however, in the 'drug-likeness' of the molecules that each can be used to discover. We give here an overview of the two techniques, comparing some advantages and disadvantages of each, and suggest areas where particularly mRNA display can benefit from adopting advances developed with DNA-encoded small molecule libraries. We outline cases where chemical modification of the peptide library has already been used in mRNA display, and survey opportunities to expand this using examples from DNA-encoded small molecule libraries. We also propose potential opportunities for encoding such reactions within the mRNA/cDNA tag of an mRNA-displayed peptide library to allow a more diversity-oriented approach to library modification. Finally, we outline alternate approaches for enriching target-binding hits from a pooled and tagged library, and close by detailing several examples of how an adjusted mRNA-display based approach could be used to discover new 'drug-like' modified small peptides.

Aqueous synthesis of a small-molecule lanthanide chelator amenable to copper-free click chemistry

The lanthanides (Ln3+), or rare earth elements, have proven to be useful tools for biomolecular NMR, X-ray crystallographic, and fluorescence analyses due to their unique 4f orbitals. However, their utility in biological applications has been limited because site-specific incorporation of a chelating element is required to ensure efficient binding of the free Ln3+ ion. Additionally, current Ln3+ chelator syntheses complicate efforts to directly incorporate Ln3+ chelators into proteins as the multi-step processes and a reliance on organic solvents promote protein denaturation and aggregation which are generally incompatible with direct incorporation into the protein of interest. To overcome these limitations, herein we describe a two-step aqueous synthesis of a small molecule lanthanide chelating agent amenable to site-specific incorporation into a protein using copper-free click chemistry with unnatural amino acids. The bioconjugate combines a diethylenetriaminepentaacetic acid (DTPA) chelating moiety with a clickable dibenzylcyclooctyne-amine (DBCO-amine) to facilitate the reaction with an azide containing unnatural amino acid. Incorporating the DBCO-amine avoids the use of the cytotoxic Cu2+ ion as a catalyst. The clickable lanthanide chelator (CLC) reagent reacted readily with p-azidophenylalanine (paF) without the need of a copper catalyst, thereby demonstrating proof-of-concept. Implementation of the orthogonal click chemistry reaction has the added advantage that the chelator can be used directly in a protein labeling reaction, without the need of extensive purification. Given the inherent advantages of Cu2+-free click chemistry, aqueous synthesis, and facile labeling, we believe that the CLC will find abundant use in both structural and biophysical studies of proteins and their complexes.

Clickable and High-Sensitivity Metal-Containing Tags for Mass Cytometry

Mass cytometry is a highly multiplexed single-cell analysis platform that uses metal-tagged reagents to identify multiple cellular biomarkers. The current metal-tagged reagent preparation employs thiol-maleimide chemistry to covalently couple maleimide-functionalized metal-chelating polymers (MCPs) with antibodies (Abs), a process that requires partial reduction of the Ab to form reactive thiol groups. However, some classes of Abs (for example, IgM) as well as biomolecules lacking cysteine residues have been challenging to label using this method. This inherent limitation led us to develop a new conjugation strategy for labeling a wide range of biomolecules and affinity reagents. In this report, we present a metal tagging approach using a new class of azide- or transcyclooctene-terminated MCPs with copper(I)-free strain-promoted alkyne-azide cycloaddition or tetrazine-alkene click chemistry reactions, in which biomolecules with -NH₂ functional groups are selectively activated with a dibenzocyclooctyne or tetrazine moiety, respectively. This approach enabled us to generate highly sensitive and specific metal-tagged IgGs, IgMs, small peptides, and lectins for applications in immunophenotyping and glycobiology. We also created dual-tagged reagents for simultaneous detection of markers by immunofluorescence, mass cytometry, and imaging mass cytometry using a two-step conjugation process. The Helios mass cytometer was used to test the functionality of reagents on suspension human leukemia cell lines and primary cells. The dual-tagged Abs, metal-tagged lectins, and phalloidin staining reagent were used to visualize target proteins and glycans on adherent cell lines and frozen/FFPE tissue sections using the Hyperion Imaging System. In some instances, reagents produced by click conjugation showed superior sensitivity and specificity compared to those of reagents produced by thiol-maleimide chemistry. In general, the click chemistry-based conjugation with new MCPs could be instrumental in developing a wide range of highly sensitive metal-containing reagents for proteomics and glycomics applications.

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.

Peptibodies: An elegant solution for a long-standing problem

Chimeric proteins composed of a biologically active peptide and a fragment crystallizable (Fc) domain of immunoglobulin G (IgG) are known as peptibodies. They present an extended half-life due to neonatal Fc receptor (FcRn) salvage pathway, a decreased renal clearance rate owing to its increased size (≈70 kDa) and, depending on the peptide used in the design of the peptibody, an active-targeting moiety. Also, the peptides therapeutic activity is boosted by the number of peptides in the fusion protein (at least two peptides) and to some peptides' alterations. Peptibodies are mainly obtained through recombinant DNA technology. However, to improve peptide properties, "unnatural" changes have been introduced to the original peptides' sequence, for instance, the incorporation of D- or non-natural amino acid residues or even cyclization thus, limiting the application of genetic engineering in the production of peptibodies, since these peptides must be obtained via chemical synthesis. This constrains prompted the development of new methods for conjugation of peptides to Fc domains. Another challenge, subject of intense research, relates to the large-scale production of such peptibodies using these new techniques, which can be minimized by their proved value. To date, two peptibodies, romiplostim and dulaglutide, have been approved and stay as the standard of care in their areas of action. Furthermore, a considerable number of peptibodies are currently in preclinical and clinical development.

Injectable hyaluronic acid/poly(ethylene glycol) hydrogels crosslinked via strain-promoted azide-alkyne cycloaddition click reaction

This paper reports injectable hyaluronic acid (HA)-based hydrogels crosslinked with azide-modified poly(ethylene glycol) (PEG) via the strain-promoted azide-alkyne cycloaddition (SPAAC) between cyclooctyne and azide groups. Cyclooctyne-modified HA (Cyclooctyne-HA) is prepared by the reaction of HA with 2-(aminoethoxy)cyclooctyne. To crosslink the modified HA, quadruply azide-terminated poly(ethylene glycol) (Azide-PEG) is designed and prepared. The mixture of Cyclooctyne-HA and Azide-PEG gelates in a few minutes to form a strong HA-PEG hydrogel. The hydrogel has fast gelation time, good strength, and slow degradation rate, because of the high reactivity of SPAAC, high crosslinking density originated from the quadruply-substituted Azide-PEG, and the good stability of the crosslinking amide bonds. In vitro cell culturing within the hydrogel demonstrated an excellent cell-compatibility. The bioorthogonality of SPAAC makes the hydrogel injectable. With good mechanical properties and biocompatibility, the hydrogel would be useful in a wide range of applications such as injection filling materials for plastic surgery.

An injectable and fast-degradable poly(ethylene glycol) hydrogel fabricated via bioorthogonal strain-promoted azide-alkyne cycloaddition click chemistry

Biocompatible and degradable injectable materials prepared via bioorthogonal reactions are highly promising for biomedical applications because they can be formed in situ and administered in a minimally invasive way. In this work, a PEG-based injectable hydrogel was fabricated via a copper-free, strain-promoted azide-alkyne cycloaddition (SPAAC) click chemistry. Azide and cyclooctyne moieties on the PEG backbones underwent a rapid click reaction to trigger the formation of the hydrogel within several minutes. Resulting from the introduction of ester groups into the cross-linked network, the hydrogel presented pH-dependent hydrolysis and biological fast degradability. Good biocompatibility of the hydrogel was verified by in vitro cytotoxicity assay and in vivo studies. The hydrogel formed in situ after subcutaneously injecting the gel precursors into Kungming (KM) mice. The implanted hydrogel caused a mild inflammatory response in vivo, and the surrounding tissues fully recovered a week after the injection. The injectable and fast-degradable hydrogel fabricated by the bioorthogonal click reaction may be useful as biomaterials such as embolic agents for interventional therapy.

Exploring strain-promoted 1,3-dipolar cycloadditions of end functionalized polymers

Strain-promoted 1,3-dipolar cycloaddition of cyclooctynes with 1,3-dipoles such as azides, nitrones, and nitrile oxides, are of interest for the functionalization of polymers. In this study, we have explored the use of a 4-dibenzocyclooctynol (DIBO)-containing chain transfer agent in reversible addition-fragmentation chain transfer polymerizations. The controlled radical polymerization resulted in well-defined DIBO-terminating polymers that could be modified by 1,3-dipolar cycloadditions using nitrones, nitrile oxides, and azides having a hydrophilic moiety. The self-assembly properties of the resulting block copolymers have been examined. The versatility of the methodology was further demonstrated by the controlled preparation of gold nanoparticles coated with the DIBO-containing polymers to produce materials that can be further modified by strain-promoted cycloadditions.

Multi-Functionalization of Polymers by Strain-Promoted Cycloadditions

We report here a synthetic route to oxime, azide and nitrone-bearing copolymers via reversible addition-fragmentation chain transfer copolymerization of 4-vinylbenzaldehyde and 1-(chloromethyl)-4-vinylbenzene with styrene. The azide and nitrone moieties could be employed in strain-promoted 1,3-dipolar cycloadditions with various functionalized dibenzocyclooctynols (DIBO) for metal-free post-functionalization of the polymers. In situ oxidation of the oximes with hypervalent iodine gave nitrile oxides, which could also be employed as 1,3-dipoles for facile cycloadditions with DIBO derivatives. Kinetic measurements demonstrated that the pendant nitrile oxides reacted approximately twenty times faster compared to similar cycloadditions with azides. A block copolymer, containing azide and oxime groups in segregated blocks, served as a scaffold for attachment of hydrophobic and hydrophilic moieties by sequential strain-promoted alkyne-azide and strain-promoted alkyne-nitrile oxide cycloadditions. This sequential bi-functionalization approach made it possible to prepare in a controlled manner multi-functional polymers that could self-assemble into well-defined nanostructures.

Radiosynthesis of a ⁶⁸Ga labeled matrix metalloproteinase inhibitor as a potential probe for PET imaging

A matrix metalloproteinase inhibitor based on a barbiturate scaffold was conjugated with a cyclooctyne derivative of the (radio)metal chelator DOTA via strain induced azide alkyne cycloaddition. Subsequent radiolabeling with (68)Ga yielded the corresponding radiometal labeled target compound (68)Ga-4 with a yield of 87% (decay corrected). The target molecule was also synthesized by a second synthesis route, the reaction of a pre-labeled (68)Ga-cyclooctyne-DOTA derivative (68)Ga-1 with an azide bearing barbiturate 3. This approach offers a valuable alternative for providing the desired (68)Ga-radiolabeled target compound. But, in this case, the strain induced cycloaddition of the reported pre-labeled cyclooctyne-DOTA derivative (68)Ga-1 with azides was proven to be slow at room temperature and heating was necessary for acceptable reaction times.

Copper-Free Click for PET: Rapid 1,3-Dipolar Cycloadditions with a Fluorine-18 Cyclooctyne

The strain-promoted click 1,3-dipolar cycloaddition reactions involving azides and cyclooctynes for the synthesis of triazoles offer the advantage of being able to be performed in biological settings via copper-free chemistries. While strained reagents conjugated to optical dyes and radiometal conjugates have been reported, cyclooctyne reagents labeled with fluorine-18 ((18)F) and radiochemically evaluated in a copper-free click reaction have yet to be explored. This report describes the conversion of a bifunctional azadibenzocyclooctyne (ADIBO) amine to the (18)F-labeled cyclooctyne 4, the subsequent fast copper-free 1,3-dipolar cycloaddition reaction with alkyl azides at 37 °C (>70% radiochemical conversion in 30 min), and biological evaluations (serum stability of >95% at 2 h). These findings demonstrate the excellent reactivity of the (18)F-labeled cyclooctyne 4 with readily available azides that will allow future work focusing on rapid copper-free in vitro and in vivo click chemistries for PET imaging using (18)F-labeled cyclooctyne derivatives of ADIBO.

Copper, nickel, and zinc cyclam-amino acid and cyclam-peptide complexes may be synthesized with "click" chemistry and are noncytotoxic

We describe the synthesis of cyclam metal complexes derivatized with amino acids or a tripeptide using a copper(I)-catalyzed Huisgen "click" reaction. The linker triazole formed during the synthesis plays an active coordinating role in the complexes. The reaction conditions do not racemize the amino acid stereocenters. However, a methylene group adjacent to the triazole is susceptible to H/D exchange under ambient conditions, an observation which has potentially important implications for structures involving stereocenters adjacent to triazoles in click-derived structures. The successful incorporation of several amino acids is described, including reactive tryptophan and cysteine side chains. All complexes are formed rapidly upon introduction of the relevant metal salt, including synthetically convenient cases where trifluoroacetate salts of cyclam derivatives are used directly in the metalation. None of the metal complexes displayed any cytotoxicity to mammalian cells, suggesting that the attachment of such complexes to amino acids and peptides does not induce toxicity, further supporting their potential suitability for labeling/imaging studies. One Cu(II)-cyclam-triazole-cysteine disulfide complex displayed moderate activity against MCF-10A breast nontumorigenic epithelial cells.

The hydrazide/hydrazone click reaction as a biomolecule labeling strategy for M(CO)3 (M = Re, (99m)Tc) radiopharmaceuticals

Facile reactivity of hydrazides and aldehydes was explored as potential coupling partners for incorporation into M(CO)(3) (M = Re, (99m)Tc) based radiopharmaceuticals. Both 'click, then chelate' and 'prelabel, then click' synthetic routes produced identical products in high yields and lacked metal-hydrazide/-hydrazone interactions, highlighting the potential of this click strategy.

[18F]azadibenzocyclooctyne ([18F]ADIBO): a biocompatible radioactive labeling synthon for peptides using catalyst free [3+2] cycloaddition

N-Terminally azido-modified peptides were labeled with the novel prosthetic labeling synthon [(18)F]azadibenzocyclooctyne ([(18)F]ADIBO) using copper-free azide-alkyne [3+2]-dipolar cycloaddition in high radiochemical yields (RCYs). (18)F-Labeled [(18)F]ADIBO was prepared by nucleophilic substitution of the corresponding tosylate in 21% overall RCY (EOB) in a fully automated synthesis unit within 55 min. [(18)F]ADIBO was incubated with azide-containing peptides at room temperature in the absence of toxic metal catalysts and the formation of the triazole conjugate was confirmed. Finally, the azide-alkyne [3+2]-dipolar cycloaddition was shown to proceed with 95% radiochemical yield in ethanol within 30 min, allowing for a development of a kit-like peptide labeling approach with [(18)F]ADIBO.

Modular strategy for the construction of radiometalated antibodies for positron emission tomography based on inverse electron demand Diels-Alder click chemistry

A modular system for the construction of radiometalated antibodies was developed based on the bioorthogonal cycloaddition reaction between 3-(4-benzylamino)-1,2,4,5-tetrazine and the strained dienophile norbornene. The well-characterized, HER2-specific antibody trastuzumab and the positron emitting radioisotopes ⁶⁴Cu and ⁸⁹Zr were employed as a model system. The antibody was first covalently coupled to norbornene, and this stock of norbornene-modified antibody was then reacted with tetrazines bearing the chelators 1,4,7,10-tetraazacyclo-dodecane-1,4,7,10-tetraacetic acid (DOTA) or desferrioxamine (DFO) and subsequently radiometalated with ⁶⁴Cu and ⁸⁹Zr, respectively. The modification strategy is simple and robust, and the resultant radiometalated constructs were obtained in high specific activity (2.7–5.3 mCi/mg). For a given initial stoichiometric ratio of norbornene to antibody, the ⁶⁴Cu-DOTA- and ⁸⁹Zr-DFO-based probes were shown to be nearly identical in terms of stability, the number of chelates per antibody, and immunoreactivity (>93% in all cases). In vivo PET imaging and acute biodistribution experiments revealed significant, specific uptake of the ⁶⁴Cu- and ⁸⁹Zr-trastuzumab bioconjugates in HER2-positive BT-474 xenografts, with little background uptake in HER2-negative MDA-MB-468 xenografts or other tissues. This modular system—one in which the divergent point is a single covalently modified antibody stock that can be reacted selectively with various chelators—will allow for both greater versatility and more facile cross-comparisons in the development of antibody-based radiopharmaceuticals.

Improved synthesis and biological evaluation of chelator-modified α-MSH analogs prepared by copper-free click chemistry

Radionuclide chelators (DOTA, NOTA) functionalized with a monofluorocyclooctyne group were prepared. These materials reacted rapidly and in high yield with a fully deprotected azide-modified peptide via Cu-free click chemistry under mild reaction conditions (aqueous solution, room temperature). The resulting bioconjugates bind with high affinity and specificity to their cell-surface receptor targets in vitro and appear stable to degradation in mouse serum over 3h of incubation at 37°C.

A practical guide to the construction of radiometallated bioconjugates for positron emission tomography

Positron emission tomography (PET) has become a vital imaging modality in the diagnosis and treatment of disease, most notably cancer. A wide array of small molecule PET radiotracers have been developed that employ the short half-life radionuclides (11)C, (13)N, (15)O, and (18)F. However, PET radiopharmaceuticals based on biomolecular targeting vectors have been the subject of dramatically increased research in both the laboratory and the clinic. Typically based on antibodies, oligopeptides, or oligonucleotides, these tracers have longer biological half-lives than their small molecule counterparts and thus require labeling with radionuclides with longer, complementary radioactive half-lives, such as the metallic isotopes (64)Cu, (68)Ga, (86)Y, and (89)Zr. Each bioconjugate radiopharmaceutical has four component parts: biomolecular vector, radiometal, chelator, and covalent link between chelator and biomolecule. With the exception of the radiometal, a tremendous variety of choices exists for each of these pieces, and a plethora of different chelation, conjugation, and radiometallation strategies have been utilized to create agents ranging from (68)Ga-labeled pentapeptides to (89)Zr-labeled monoclonal antibodies. Herein, the authors present a practical guide to the construction of radiometal-based PET bioconjugates, in which the design choices and synthetic details of a wide range of biomolecular tracers from the literature are collected in a single reference. In assembling this information, the authors hope both to illuminate the diverse methods employed in the synthesis of these agents and also to create a useful reference for molecular imaging researchers both experienced and new to the field.

Preparation of a versatile bifunctional zeolite for targeted imaging applications

Bifunctional zeolite Y was prepared for use in targeted in vivo molecular imaging applications. The strategy involved functionalization of the external surface of zeolite Y with chloropropyltriethoxysilane followed by reaction with sodium azide to form azide-functionalized NaY, which is amenable to copper(1)-catalyzed click chemistry. In this study, a model alkyne (4-pentyn-1-ol) was attached to the azide-terminated surface via click chemistry to demonstrate feasibility for attachment of molecular targeting vectors (e.g., peptides, aptamers) to the zeolite surface. The modified particle efficiently incorporates the imaging radioisotope gallium-68 ((68)Ga) into the pores of the azide-functionalized NaY zeolite to form a stable bifunctional molecular targeting vector. The result is a versatile "clickable" zeolite platform that can be tailored for future in vivo molecular targeting and imaging modalities.