Preparation of Bioconjugates through an Ugi Reaction This work was financially supported by the Fonds der Chemischen Industrie and Aventis Research & Technologies, Frankfurt am Main

Chemical Conjugation in Drug Delivery Systems

Cancer is one of the diseases with the highest mortality rate. Treatments to mitigate cancer are usually so intense and invasive that they weaken the patient to cure as dangerous as the own disease. From some time ago until today, to reduce resistance generated by the constant administration of the drug and improve its pharmacokinetics, scientists have been developing drug delivery system (DDS) technology. DDS platforms aim to maximize the drugs’ effectiveness by directing them to reach the affected area by the disease and, therefore, reduce the potential side effects. Erythrocytes, antibodies, and nanoparticles have been used as carriers. Eleven antibody–drug conjugates (ADCs) involving covalent linkage has been commercialized as a promising cancer treatment in the last years. This review describes the general features and applications of DDS focused on the covalent conjugation system that binds the antibody carrier to the cytotoxic drug.

Linker Molecules Convert Commercial Fluorophores into Tailored Functional Probes during Biolabelling

Many life‐science techniques and assays rely on selective labeling of biological target structures with commercial fluorophores that have specific yet invariant properties. Consequently, a fluorophore (or dye) is only useful for a limited range of applications, e.g., as a label for cellular compartments, super‐resolution imaging, DNA sequencing or for a specific biomedical assay. Modifications of fluorophores with the goal to alter their bioconjugation chemistry, photophysical or functional properties typically require complex synthesis schemes. We here introduce a general strategy that allows to customize these properties during biolabelling with the goal to introduce the fluorophore in the last step of biolabelling. For this, we present the design and synthesis of ‘linker’ compounds, that bridge biotarget, fluorophore and a functional moiety via well‐established labeling protocols. Linker molecules were synthesized via the Ugi four‐component reaction (Ugi‐4CR) which facilitates a modular design of linkers with diverse functional properties and bioconjugation‐ and fluorophore attachment moieties. To demonstrate the possibilities of different linkers experimentally, we characterized the ability of commercial fluorophores from the classes of cyanines, rhodamines, carbopyronines and silicon‐rhodamines to become functional labels on different biological targets in vitro and in vivo via thiol‐maleimide chemistry. With our strategy, we showed that the same commercial dye can become a photostable self‐healing dye or a sensor for bivalent ions subject to the linker used. Finally, we quantified the photophysical performance of different self‐healing linker–fluorophore conjugates and demonstrated their applications in super‐resolution imaging and single‐molecule spectroscopy.

GlyConnect-Ugi: site-selective, multi-component glycoprotein conjugations through GlycoDelete expressed glycans

Recently, the GlyConnect-oxime (GC) protein conjugation strategy was developed to provide a site-selective glycan-based conjugation strategy as an extension to the in-house developed GlycoDelete (GD) technology. GD gives access to glycoproteins with single GlcNAc, LacNAc, or LacNAc-Sia type glycans on their N-glycosylation sites. We have previously shown that these glycans provide a unique handle for site-selective conjugation as they provide a short, homogeneous and hydrophilic link to the protein backbone. GC focused on the use of chemical and chemo-enzymatic pathways for conjugation of a single molecule of interest via oxime formation or reductive amination. In the current work, we explore multicomponent reactions (MCR), namely Ugi and Passerini reactions, for GlycoDelete glycan directed, site-specific protein conjugation (MC-GC). The use of the Ugi and Passerini multicomponent reactions holds the potential of introducing multiple groups of interest in a single reaction step while creating a hydrophilic peptide-like linker.

One-pot synthesis of Acanthus ilicifolius Linn alkaloid 2-benzoxazolinone derivatives via a tandem Ugi 4-component coupling/haloform cyclization

A one-pot, base-mediated approach to Acanthus ilicifolius Linn alkaloid 2-benzoxazolinone derivatives is developed. Starting from trichloroacetic acid, o-aminophenol, substituted benzaldehydes and alkyl isocyanides, the desired 2-benzoxazolinone derivatives are obtained in good yields via a tandem Ugi condensation and intramolecular haloform cyclization at room temperature in the presence of Et3N.

Polyoxazolines with a Vicinally Double-Bioactivated Terminus for Biomacromolecular Affinity Assessment

Interactions between proteins and carbohydrates with larger biomacromolecules, e.g., lectins, are usually examined using self-assembled monolayers on target gold surfaces as a simplified model measuring setup. However, most of those measuring setups are either limited to a single substrate or do not allow for control over ligand distance and spacing. Here, we develop a synthetic strategy, consisting of a cascade of a thioesterification, native chemical ligation (NCL) and thiol-ene reaction, in order to create three-component polymer conjugates with a defined double bioactivation at the chain end. The target architecture is the vicinal attachment of two biomolecule residues to the α telechelic end point of a polymer and a thioether group at the ω chain end for fixating the conjugate to a gold sensor chip surface. As proof-of-principle studies for affinity measurements, we demonstrate the interaction between covalently bound mannose and ConA in surface acoustic wave (SAW) and surface plasmon resonance (SPR) experiments.

Isocyanides: Promising Functionalities in Bioorthogonal Labeling of Biomolecules

Isocyanides have drawn increasing attention in biological applications due to their attractive properties and unique reactivities, which can undergo various reactions, such as multicomponent reactions, α-addition reactions, [4 + 1] cycloaddition reactions, and the reaction scope keeps expanding. In addition to acting as reactants for the preparation of structurally interesting and diverse N-heterocycles or peptidomimetics, this type of functionality may be a good choice in the labeling and modulation of biomolecules due to the high biocompatibility and small size to minimize modifications on the parent molecule. It has been demonstrated that isocyanides can participate in biomolecule labeling through three strategies, including the two-component bioorthogonal reaction, multicomponent reaction, and metal chelation. Among them, the isocyanide-tetrazine reaction has been better studied recently, augmenting the potency of isocyanide as a bioorthogonal handle. This review will focus on the recent progress in isocyanide chemistry for labeling of biomolecules. Meanwhile, methods to introduce isocyano groups into biomacromolecules are also described to facilitate wider applications of this unique functionality.

From Ugi Multicomponent Reaction to Linkers for Bioconjugation

Bioconjugation is a key approach for the development of novel molecular entities with clinical applications. The biocompatibility and specificity of biomolecules such as peptides, proteins, and antibodies make these macromolecules ideal carriers for selective targeted therapies. In this context, there is a need to develop new molecular units that cover the requirements of the next generation of targeted pharmaceuticals. Here, we present the design and development of a versatile and stable linker based on a N-alkylated α,α-dialkyl dipeptide for bioconjugation, with a particular focus on antibody-drug conjugates (ADCs). Starting with the well-known Ugi multicomponent reaction, the convenient chemical modification of the prepared adducts allowed us the obtention of versatile bifunctional linkers for bioconjugation. A conjugation strategy was tested to demonstrate the efficiency of the linker. In addition, a novel cytotoxic anti-HER2 ADC was prepared using the Ugi-linker approach.

Addition of Isocyanide-Containing Amino Acids to the Genetic Code for Protein Labeling and Activation

Site-specific introduction of bioorthogonal handles into biomolecules provides powerful tools for studying and manipulating the structures and functions of proteins. Recent advances in bioorthogonal chemistry demonstrate that tetrazine-based bioorthogonal cycloaddition is a particularly useful methodology due to its high reactivity, biological selectivity, and turn-on property for fluorescence imaging. Despite its broad applications in protein labeling and imaging, utilization of tetrazine-based bioorthogonal cycloaddition has been limited to date by the requirement of a hydrophobic strained alkene reactive moiety. Circumventing this structural requirement, we report the site-specific incorporation of noncanonical amino acids (ncAAs) with a small isocyanide (or isonitrile) group into proteins in both bacterial and mammalian cells. We showed that under physiological conditions and in the absence of a catalyst these isocyanide-containing ncAAs could react selectively with tetrazine molecules via [4 + 1]-cycloaddition, thus providing a versatile bioorthogonal handle for site-specific protein labeling and protein decaging. Significantly, these bioorthogonal reactions between isocyanides and tetrazines also provide a unique mechanism for the activation of tetrazine-quenched fluorophores. The addition of these isocyanide-containing ncAAs to the list of 20 commonly used, naturally occurring amino acids expands our repertoire of reagents for bioorthogonal chemistry, therefore enabling new biological applications ranging from protein labeling and imaging studies to the chemical activation of proteins.

Multicomponent Reaction Toolbox for Peptide Macrocyclization and Stapling

In the past decade, multicomponent reactions have experienced a renaissance as powerful peptide macrocyclization tools enabling the rapid creation of skeletal complexity and diversity with low synthetic cost. This review provides both a historical and modern overview of the development of the peptide multicomponent macrocyclization as a strategy capable to compete with the classic peptide cyclization methods in terms of chemical efficiency and synthetic scope. We prove that the utilization of multicomponent reactions for cyclizing peptides by either their termini or side chains provides a key advantage over those more established methods; that is, the possibility to explore the cyclic peptide chemotype space not only at the amino acid sequence but also at the ring-forming moiety. Owing to its multicomponent nature, this type of peptide cyclization process is well-suited to generate diversity at both the endo- and exo-cyclic fragments formed during the ring-closing step, which stands as a distinctive and useful characteristic for the creation and screening of cyclic peptide libraries. Examples of the novel multicomponent peptide stapling approach and heterocycle ring-forming macrocyclizations are included, along with multicomponent methods incorporating macrocyclization handles and the one-pot syntheses of macromulticyclic peptide cages. Interesting applications of this strategy in the field of drug discovery and chemical biology are provided.

Multicomponent Reactions in Ligation and Bioconjugation Chemistry

Multicomponent reactions (MCRs) encompass an exciting class of chemical transformations that have proven success in almost all fields of synthetic organic chemistry. These convergent procedures incorporate three or more reactants into a final product in one pot, thus combining high levels of complexity and diversity generation with low synthetic cost. Striking applications of these processes are found in heterocycle, peptidomimetic, and natural product syntheses. However, their potential in the preparation of large macro- and biomolecular constructs has been realized just recently. This Account describes the most relevant results of our group in the utilization of MCRs for ligation/conjugation of biomolecules along with significant contributions from other laboratories that validate the utility of this special class of bioconjugation process. Thus, MCRs have proven to be efficient in the ligation of lipids to peptides and oligosaccharides as well as the ligation of steroids, carbohydrates, and fluorescent and affinity tags to peptides and proteins. In the field of glycolipids, we highlight the power of isocyanide-based MCRs with the one-pot double lipidation of glycan fragments functionalized as either the carboxylic acid or amine. In peptide chemistry, the versatility of the multicomponent ligation strategy is demonstrated in both solution-phase lipidation protocols and solid-phase procedures enabling the simultaneous lipidation and biotinylation of peptides. In addition, we show that MCRs are powerful methods for synchronized lipidation/labeling and macrocyclization of peptides, thus accomplishing in one step what usually requires long sequences. In the realm of protein bioconjugation, MCRs have also proven to be effective in labeling, site-selective modification, immobilization, and glycoconjugation processes. For example, we illustrate a successful application of multicomponent polysaccharide-protein conjugation with the preparation of multivalent glycoconjugate vaccine candidates by the ligation of two antigenic capsular polysaccharides of a pathogenic bacterium to carrier proteins. By highlighting the ability to join several biomolecules in only one synthetic operation, we hope to encourage the biomolecular chemistry community to apply this powerful chemistry to novel biomedicinal challenges.

Multicomponent polysaccharide-protein bioconjugation in the development of antibacterial glycoconjugate vaccine candidates

A new synthetic strategy for the development of multivalent antibacterial glycoconjugate vaccines is described. The approach comprises the utilization of an isocyanide-based multicomponent process for the conjugation of functionalized capsular polysaccharides of S. pneumoniae and S. Typhi to carrier proteins such as diphtheria and tetanus toxoids. For the first time, oxo- and carboxylic acid-functionalized polysaccharides could be either independently or simultaneously conjugated to immunogenic proteins by means of the Ugi-multicomponent reaction, thus leading to mono- or multivalent unimolecular glycoconjugates as vaccine candidates. Despite the high molecular weight of the two or three reacting biomolecules, the multicomponent bioconjugation proved highly efficient and reproducible. The Ugi-derived glycoconjugates showed notable antigenicity and elicited good titers of functional specific antibodies. To our knowledge, this is the only bioconjugation method that enables the incorporation of two different polysaccharidic antigens to a carrier protein in a single step. Applications in the field of self-adjuvanting, eventually anticancer, multicomponent vaccines are foreseeable.

Giant hollow M(n)L(2n) spherical complexes: structure, functionalisation and applications

Drawing inspiration from the self-assembly of hollow spherical virus capsids and protein cages found in nature, a family of roughly spherical coordination polyhedra with general formula MnL2n was designed and several members of the series have been synthesised. These spherical complexes are self-assembled upon reaction of bent bis(pyridine) ligands with Pd(2+) ions. The introduction of functional side chains into the ligands is straightforward, making the synthesis of both exo- and endohedrally functionalised spherical complexes possible. Accumulation of a high density of functional groups at the periphery of the spherical framework results in an enhancement of the weak interactions used in biomolecular recognition processes and the strong and selective interaction of the complex with a variety of substrates. Discrete and well-defined environments are generated within the spherical framework by functionalisation of the interior of the complex. These environments can be used for the selective encapsulation of guest molecules, including species as diverse as simple metal ions, fluoroalkanes and fullerenes. The well-defined cavity of the spherical complexes can also be exploited for the synthesis of precisely size-controlled nanoparticles and polymers. Most recently, a protein was successfully enclosed within a hollow self-assembled spherical complex, with a long-term view towards the control of protein functions for the development of new applications.

Carbohydrate-steroid conjugation by Ugi reaction: one-pot synthesis of triple sugar/pseudo-peptide/spirostane hybrids

The one-pot synthesis of novel molecular chimeras incorporating sugar, pseudo-peptide, and steroidal moieties is described. For this, a new carbohydrate-steroid conjugation approach based on the Ugi four-component reaction was implemented for the ligation of glucose and chacotriose to spirostanic steroids. The approach proved wide substrate scope, as both mono and oligosaccharides functionalized with amino, carboxy, and isocyano groups were conjugated to steroidal substrates in an efficient, multicomponent manner. Two alternative strategies based on the hydrazoic acid variant of the Ugi reaction were employed for the synthesis of tetrazole-based chacotriose-diosgenin conjugates resembling naturally occurring spirostan saponins. This is the first time that triple sugar/pseudo-peptide/steroid hybrids are produced, thus opening up an avenue of opportunities for applications in drug discovery and biological chemistry.

Use of a multicomponent reaction for chemoselective derivatization of multiple classes of metabolites

We demonstrate that the Ugi reaction enables chemoselective derivatization of biological amines, carboxylic acids, aldehydes, or ketones with a chromophore under one set of reaction conditions, even in the presence of water. Derivatization of neurotransmitters, hormones, disease biomarkers and other metabolites bodes well for systems biology and diagnostic medicine.

Molecular diversity through sugar scaffolds

Monosaccharides provide an excellent platform to tailor molecular diversity by appending desired substituents at selected positions around the sugar scaffold. The presence of five functionalized and stereo-controlled centres on the sugar scaffolds gives the chemist plenty of scope to custom design molecules to a pharmacophore model. This review focuses on the peptidomimetic developments in this area, as well as the concept of tailoring structural and functional diversity in a library using carbohydrate scaffolds and how this can lead to increased hit rates and rapid identification of leads, which has promising prospects for drug development.

Chiral recognition: design and preparation of chiral stationary phases using selectors derived from ugi multicomponent condensation reactions and a combinatorial approach

Combinatorial approaches together with high-throughput screening have been used to develop highly selective stationary phases for chiral recognition. Libraries of potential chiral selectors have been prepared by the Ugi multicomponent condensation reactions and screened for their enantioselectivity using the reciprocal approach involving a chiral stationary phase with immobilized model target compound N-(3,5-dinitrobenzoyl)-alpha-l-leucine. The best candidates were identified from the library of phenyl amides of 2-oxo-azetidineacetic acid derivatives. This screening also enabled specification of the functionalities of the selector desired to achieve the highest level of chiral recognition. The substituents of the phenyl ring adjacent to the chiral center of the selector candidates exhibited the most profound effect on the chiral recognition. The best candidate was then synthesized on a larger scale, resolved into single enantiomers using preparative enantioselective HPLC, and attached to porous poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) beads via an ester linkage to afford the desired stationary phase. Selectivities alpha as high as 3.2 were found for the separation of a variety of amino acid derivatives.

Application of Ugi reactions in synthesis of divalent neoglycoconjugates: evidence that the sugars are presented in restricted conformation

[reaction: see text]. The Ugi reaction has been used to prepare divalent galactose derivatives. NMR analysis shows that a divalent neoglycoconjugate, where the glycopeptides are bridged by a terephthaloyl group, is an 83:17 mixture of two conformers; the amide groups of the major isomer have E-anti conformations. The spatial relationship and the relative orientation of the sugars are restricted, which may have consequences for the recognition of this and related structures in biological systems.