Preparation of Well‐Defined Antibody–Drug Conjugates through Glycan Remodeling and Strain‐Promoted Azide–Alkyne Cycloadditions

Antibody–drug conjugates hold considerable promise as anticancer agents, however, producing them remains a challenge and there is a need for mild, broadly applicable, site‐specific conjugation methods that yield homogenous products. It was envisaged that enzymatic remodeling of the oligosaccharides of an antibody would enable the introduction of reactive groups that can be exploited for the site‐specific attachment of cytotoxic drugs. This is based on the observation that glycosyltransferases often tolerate chemical modifications in their sugar nucleotide substrates, thus allowing the installation of reactive functionalities. An azide was incorporated because this functional group is virtually absent in biological systems and can be reacted by strain‐promoted alkyne–azide cycloaddition. This method, which does not require genetic engineering, was used to produce an anti‐CD22 antibody modified with doxorubicin to selectively target and kill lymphoma cells.     

In‐Cell Dual Drug Synthesis by Cancer‐Targeting Palladium Catalysts

Transition metals have been successfully applied to catalyze non‐natural chemical transformations within living cells, with the highly efficient labeling of subcellular components and the activation of prodrugs. In vivo applications, however, have been scarce, with a need for the specific cellular targeting of the active transition metals. Here, we show the design and application of cancer‐targeting palladium catalysts, with their specific uptake in brain cancer (glioblastoma) cells, while maintaining their catalytic activity. In these cells, for the first time, two different anticancer agents were synthesized simultaneously intracellularly, by two totally different mechanisms (in situ synthesis and decaging), enhancing the therapeutic effect of the drugs. Tumor specificity of the catalysts together with their ability to perform simultaneous multiple bioorthogonal transformations will empower the application of in vivo transition metals for drug activation strategies.     

Affinity and chemical enrichment strategies for mapping low‐abundance protein modifications and protein‐interaction networks

Protein post‐translational modifications and protein interactions are the central research areas in mass‐spectrometry‐based proteomics. Protein post‐translational modifications affect protein structures, stabilities, activities, and all cellular processes are achieved by interactions among proteins and protein complexes. With the continuing advancements of mass spectrometry instrumentations of better sensitivity, speed, and performance, selective enrichment of modifications/interactions of interest from complex cellular matrices during the sample preparation has become the overwhelming bottleneck in the proteomics workflow. Therefore, many strategies have been developed to address this issue by targeting specific modifications/interactions based on their physical properties or chemical reactivities, but only a few have been successfully applied for systematic proteome‐wide study. In this review, we summarized the highlights of recent developments in the affinity enrichment methods focusing mainly on low stoichiometric protein lipidations. Besides, to identify potential glyoxal modified arginines, a small part was added for profiling reactive arginine sites using an enrichment reagent. A detailed section was provided for the enrichment of protein interactions by affinity purification and chemical cross‐linking, to shed light on the potentials of different enrichment strategies, along with the unique challenges in investigating individual protein post‐translational modification or protein interaction network.     

pH‐Responsive Drug‐Delivery Systems

In many biomedical applications, drugs need to be delivered in response to the pH value in the body. In fact, it is desirable if the drugs can be administered in a controlled manner that precisely matches physiological needs at targeted sites and at predetermined release rates for predefined periods of time. Different organs, tissues, and cellular compartments have different pH values, which makes the pH value a suitable stimulus for controlled drug release. pH‐Responsive drug‐delivery systems have attracted more and more interest as “smart” drug‐delivery systems for overcoming the shortcomings of conventional drug formulations because they are able to deliver drugs in a controlled manner at a specific site and time, which results in high therapeutic efficacy. This focus review is not intended to offer a comprehensive review on the research devoted to pH‐responsive drug‐delivery systems; instead, it presents some recent progress obtained for pH‐responsive drug‐delivery systems and future perspectives. There are a large number of publications available on this topic, but only a selection of examples will be discussed.     

Molecular Engineering‐Based Aptamer–Drug Conjugates with Accurate Tunability of Drug Ratios for Drug Combination Targeted Cancer Therapy

Polytherapy (or drug combination cancer therapy (DCCT)), targeting multiple mechanisms associated with tumor proliferation, can efficiently maximize therapeutic efficacy, decrease drug dosage, and reduce drug resistance. However, most DCCT strategies cannot coordinate the specific delivery of a drug combination in an accurately tuned ratio into cancer cells. To address these limitations, the present work reports the engineering of circular bivalent aptamer–drug conjugates (cb‐ApDCs). The cb‐ApDCs exhibit high stability, specific recognition, excellent cellular uptake, and esterase‐triggered release. Furthermore, the drug ratios in cb‐ApDCs can be tuned for an enhanced synergistic effect without the need for complex chemistry. Therefore, cb‐ApDCs provide a promising platform for the development of DCCT strategies for different drug combinations and ratios.     

ROS‐Responsive N‐Alkylaminoferrocenes for Cancer‐Cell‐Specific Targeting of Mitochondria

Mitochondrial membrane potential is more negative in cancer cells than in normal cells, allowing cancer targeting by delocalized lipophilic cations (DLCs). However, as the difference is rather small, these drugs affect also normal cells. Now a concept of pro‐DLCs is proposed based on an N‐alkylaminoferrocene structure. These prodrugs are activated by the reaction with reactive oxygen species (ROS) forming ferrocenium‐based DLCs. Since ROS are overproduced in cancer, the high‐efficiency cancer‐cell‐specific targeting of mitochondria could be achieved as demonstrated by fluorescence microscopy in combination with two fluorogenic pro‐DLCs in vitro and in vivo. We prepared a conjugate of another pro‐DLC with a clinically approved drug carboplatin and confirmed that its accumulation in mitochondria was higher than that of the free drug. This was reflected in the substantially higher anticancer effect of the conjugate.     

Small‐Molecule PROTACS: New Approaches to Protein Degradation

The current inhibitor‐based approach to therapeutics has inherent limitations owing to its occupancy‐based model: 1) there is a need to maintain high systemic exposure to ensure sufficient in vivo inhibition, 2) high in vivo concentrations bring potential for off‐target side effects, and 3) there is a need to bind to an active site, thus limiting the drug target space. As an alternative, induced protein degradation lacks these limitations. Based on an event‐driven model, this approach offers a novel catalytic mechanism to irreversibly inhibit protein function by targeting protein destruction through recruitment to the cellular quality control machinery. Prior protein degrading strategies have lacked therapeutic potential. However, recent reports of small‐molecule‐based proteolysis‐targeting chimeras (PROTACs) have demonstrated that this technology can effectively decrease the cellular levels of several protein classes.     

Nano‐Star‐Shaped Polymers for Drug Delivery Applications

With the advancement of polymer engineering, complex star‐shaped polymer architectures can be synthesized with ease, bringing about a host of unique properties and applications. The polymer arms can be functionalized with different chemical groups to fine‐tune the response behavior or be endowed with targeting ligands or stimuli responsive moieties to control its physicochemical behavior and self‐organization in solution. Rheological properties of these solutions can be modulated, which also facilitates the control of the diffusion of the drug from these star‐based nanocarriers. However, these star‐shaped polymers designed for drug delivery are still in a very early stage of development. Due to the sheer diversity of macromolecules that can take on the star architectures and the various combinations of functional groups that can be cross‐linked together, there remain many structure–property relationships which have yet to be fully established. This review aims to provide an introductory perspective on the basic synthetic methods of star‐shaped polymers, the properties which can be controlled by the unique architecture, and also recent advances in drug delivery applications related to these star candidates.     

Cartilage‐Specific Near‐Infrared Fluorophores for Biomedical Imaging

A novel class of near‐infrared fluorescent contrast agents was developed. These agents target cartilage with high specificity and this property is inherent to the chemical structure of the fluorophore. After a single low‐dose intravenous injection and a clearance time of approximately 4 h, these agents bind to all three major types of cartilage (hyaline, elastic, and fibrocartilage) and perform equally well across species. Analysis of the chemical structure similarities revealed a potential pharmacophore for cartilage targeting. Our results lay the foundation for future improvements in tissue engineering, joint surgery, and cartilage‐specific drug development.     

Controlled Multi‐functionalization Facilitates Targeted Delivery of Nanoparticles to Cancer Cells

A major objective of nanomedicine is to combine in a controlled manner multiple functional entities into a single nanoscale device to target particles with great spatial precision, thereby increasing the selectivity and potency of therapeutic drugs. A multifunctional nanoparticle is described for controlled conjugation of a cytotoxic drug, a cancer cell targeting ligand, and an imaging moiety. The approach is based on the chemical synthesis of polyethylene glycol that at one end is modified by a thioctic acid for controlled attachment to a gold core. The other end of the PEG polymers is modified by a hydrazine, amine, or dibenzocyclooctynol moiety for conjugation with functional entities having a ketone, activated ester, or azide moiety, respectively. The conjugation approach allowed the controlled attachment of doxorubicin through an acid‐labile hydrazone linkage, an Alexa Fluor dye through an amide bond, and a glycan‐based ligand for the cell surface receptor CD22 of B‐cells using strain promoted azide‐alkyne cycloaddition. The incorporation of the ligand for CD22 led to rapid entry of the nanoparticle by receptor‐mediated endocytosis. Covalent attachment of doxorubicin via hydrazone linkage caused pH‐responsive intracellular release of doxorubicin and significantly enhanced the cytotoxicity of nanoparticles. A remarkable 60‐fold enhancement in cytotoxicity of CD22 (+) lymphoma cells was observed compared to non‐ targeted nanoparticles.     

An Aptamer Intrinsically Comprising 5‐Fluoro‐2′‐deoxyuridine for Targeted Chemotherapy

An aptamer specifically binding the interleukin‐6 receptor and intrinsically comprising multiple units of the nucleoside analogue 5‐fluoro‐2′‐deoxyuridine can exert a cytostatic effect direcly on certain cells presenting the receptor. Thus the modified aptamer fulfils the requirements for active drug targeting in an unprecedented manner. It can easily be synthesized in a single enzymatic step and it binds to a cell surface receptor that is conveyed into the lysosome. Upon degradation of the aptamer by intracellular nucleases the active drug is released within the targeted cells exclusively. In this way the aptamer acts as a prodrug meeting two major prerequisites of a drug delivery system: specific cell targeting and the controlled release of the drug triggered by an endogenous stimulus.     

Copper‐Mediated Late‐Stage Functionalization of Heterocycle‐Containing Molecules

One long‐standing issue in directed C−H functionalization is that either nitrogen or sulfur atoms present in heterocyclic substrates may bind preferentially to a transition‐metal catalyst rather than to the desired directing group. This competitive binding has largely hindered the application of C−H functionalization in late‐stage heterocycle drug discovery. Reported here is the use of an oxazoline‐based directing group capable of overriding the poisoning effect of a wide range of heterocycle substrates. The potential use of this directing group in pharmaceutical drug discovery is illustrated by diversification of Telmisartan (an antagonist for the angiotensin II receptor) through copper‐mediated C−H amination, hydroxylation, thiolation, arylation, and trifluoromethylation.     

Near‐Infrared‐Light‐Triggered Antimicrobial Indium Phosphide Quantum Dots

The emergence of multidrug‐resistant (MDR) pathogens represents one of the most urgent global public health crises. Light‐activated quantum dots (QDs) are alternative antimicrobials, with efficient transport, low cost, and therapeutic efficacy, and they can act as antibiotic potentiators, with a mechanism of action orthogonal to small‐molecule drugs. Furthermore, light‐activation enhances control over the spatiotemporal release and dose of the therapeutic superoxide radicals from QDs. However, the limited deep‐tissue penetration of visible light needed for QD activation, and concern over trace heavy metals, have prevented further translation. Herein, we report two indium phosphide (InP) QDs that operate in the near‐infrared and deep‐red light window, enabling deeper tissue penetration. These heavy‐metal‐free QDs eliminate MDR pathogenic bacteria, while remaining non‐toxic to host human cells. This work provides a pathway for advancing QD nanotherapeutics to combat MDR superbugs.     

The siRNAsome: A Cation‐Free and Versatile Nanostructure for siRNA and Drug Co‐delivery

Nanoparticles show great potential for drug delivery. However, suitable nanostructures capable of loading a range of drugs together with the co‐delivery of siRNAs, which avoid the problem of cation‐associated cytotoxicity, are lacking. Herein, we report an small interfering RNA (siRNA)‐based vesicle (siRNAsome), which consists of a hydrophilic siRNA shell, a thermal‐ and intracellular‐reduction‐sensitive hydrophobic median layer, and an empty aqueous interior that meets this need. The siRNAsome can serve as a versatile nanostructure to load drug agents with divergent chemical properties, therapeutic proteins as well as co‐delivering immobilized siRNAs without transfection agents. Importantly, the inherent thermal/reduction‐responsiveness enables controlled drug loading and release. When siRNAsomes are loaded with the hydrophilic drug doxorubicin hydrochloride and anti‐P‐glycoprotein siRNA, synergistic therapeutic activity is achieved in multidrug resistant cancer cells and a tumor model.     

Organelle‐Specific Activity‐Based Protein Profiling in Living Cells

A multimodal activity‐based probe for targeting acidic organelles was developed to measure subcellular native enzymatic activity in cells by fluorescence microscopy and mass spectrometry. A cathepsin‐reactive warhead conjugated to a weakly basic amine and a clickable alkyne, for subsequent appendage of a fluorophore or biotin reporter tag, accumulated in lysosomes as observed by structured illumination microscopy (SIM) in J774 mouse macrophage cells. Analysis of in vivo labeled J774 cells by mass spectrometry showed that the probe was very selective for cathepsins B and Z, two lysosomal cysteine proteases. Analysis of starvation‐induced autophagy, a catabolic pathway involving lysosomes, showed a large increase in the number of tagged proteins and an increase in cathepsin activity. The organelle‐targeting of activity‐based probes holds great promise for the characterization of enzyme activities in the myriad diseases linked to specific subcellular locations, particularly the lysosome.     

Mitochondria‐Targeted Cancer Therapy Using a Light‐Up Probe with Aggregation‐Induced‐Emission Characteristics

Subcellular organelle‐specific reagents for simultaneous tumor targeting, imaging, and treatment are of enormous interest in cancer therapy. Herein, we present a mitochondria‐targeting probe (AIE‐mito‐TPP) by conjugating a triphenylphosphine (TPP) with a fluorogen which can undergo aggregation‐induced emission (AIE). Owing to the more negative mitochondrial membrane potential of cancer cells than normal cells, the AIE‐mito‐TPP probe can selectively accumulate in cancer‐cell mitochondria and light up its fluorescence. More importantly, the probe exhibits selective cytotoxicity for studied cancer cells over normal cells. The high potency of AIE‐mito‐TPP correlates with its strong ability to aggregate in mitochondria, which can efficiently decrease the mitochondria membrane potential and increase the level of intracellular reactive oxygen species (ROS) in cancer cells. The mitochondrial light‐up probe provides a unique strategy for potential image‐guided therapy of cancer cells.     

Lysine‐Targeting Covalent Inhibitors

Targeted covalent inhibitors have gained widespread attention in drug discovery as a validated method to circumvent acquired resistance in oncology. This strategy exploits small‐molecule/protein crystal structures to design tightly binding ligands with appropriately positioned electrophilic warheads. Whilst most focus has been on targeting binding‐site cysteine residues, targeting nucleophilic lysine residues can also represent a viable approach to irreversible inhibition. However, owing to the basicity of the ϵ ‐amino group in lysine, this strategy generates a number of specific challenges. Herein, we review the key principles for inhibitor design, give historical examples, and present recent developments that demonstrate the potential of lysine targeting for future drug discovery.     

Cathepsin B‐Specific Metabolic Precursor for In Vivo Tumor‐Specific Fluorescence Imaging

Recently, metabolic glycoengineering with bioorthogonal click reactions has focused on improving the tumor targeting efficiency of nanoparticles as delivery vehicles for anticancer drugs or imaging agents. It is the key technique for developing tumor‐specific metabolic precursors that can generate unnatural glycans on the tumor‐cell surface. A cathepsin B‐specific cleavable substrate (KGRR) conjugated with triacetylated N‐azidoacetyl‐d ‐mannosamine (RR‐S‐Ac₃ManNAz) was developed to enable tumor cells to generate unnatural glycans that contain azide groups. The generation of azide groups on the tumor cell surface was exogenously and specifically controlled by the amount of RR‐S‐Ac₃ManNAz that was fed to target tumor cells. Moreover, unnatural glycans on the tumor cell surface were conjugated with near infrared fluorescence (NIRF) dye‐labeled molecules by a bioorthogonal click reaction in cell cultures and in tumor‐bearing mice. Therefore, our RR‐S‐Ac₃ManNAz is promising for research in tumor‐specific imaging or drug delivery.     

Ring‐Opening Polymerization of Prodrugs: A Versatile Approach to Prepare Well‐Defined Drug‐Loaded Nanoparticles

The synthesis of polymer–drug conjugates from prodrug monomers consisting of a cyclic polymerizable group that is appended to a drug through a cleavable linker is achieved by organocatalyzed ring‐opening polymerization. The monomers polymerize into well‐defined polymer prodrugs that are designed to self‐assemble into nanoparticles and release the drug in response to a physiologically relevant stimulus. This method is compatible with structurally diverse drugs and allows different drugs to be copolymerized with quantitative conversion of the monomers. The drug loading can be controlled by adjusting the monomer(s)/initiator feed ratio and drug release can be encoded into the polymer by the choice of linker. Initiating these monomers from a poly(ethylene glycol) macroinitiator results in amphiphilic diblock copolymers that spontaneously self‐assemble into micelles with a long plasma circulation, which is useful for systemic therapy.     

Loading‐Dependent Structural Model of Polymeric Micelles Encapsulating Curcumin by Solid‐State NMR Spectroscopy

Detailed insight into the internal structure of drug‐loaded polymeric micelles is scarce, but important for developing optimized delivery systems. We observed that an increase in the curcumin loading of triblock copolymers based on poly(2‐oxazolines) and poly(2‐oxazines) results in poorer dissolution properties. Using solid‐state NMR spectroscopy and complementary tools we propose a loading‐dependent structural model on the molecular level that provides an explanation for these pronounced differences. Changes in the chemical shifts and cross‐peaks in 2D NMR experiments give evidence for the involvement of the hydrophobic polymer block in the curcumin coordination at low loadings, while at higher loadings an increase in the interaction with the hydrophilic polymer blocks is observed. The involvement of the hydrophilic compartment may be critical for ultrahigh‐loaded polymer micelles and can help to rationalize specific polymer modifications to improve the performance of similar drug delivery systems.