Caged xanthone derivatives to promote mitochondria-mediated apoptosis in breast cancer cells

Caged xanthones represent a class of natural secondary metabolites exhibiting significant potential as antitumor agents. These compounds are characterized by their distinct cage-like structures, which offer novel and compelling frameworks for drug design. Nonetheless, there exists a dearth of research focused on the structural modification of these compounds, particularly in relation to their cage-like architectures. This study aims to address this gap by introducing an innovative synthetic method for constructing a novel caged structure that incorporates a widely employed maleimide group. Drawing upon the well-established synthetic approach for dihydroxanthones previously developed within our research group, we successfully synthesized 13 new caged xanthones using the Diels-Alder reaction. Subsequently, we evaluated their anti-proliferative activity against HepG2, A549, and MDA-MB-231 cell lines. The results revealed that compound 10i exhibited IC50 values of 15.86 µM ± 1.29, 19.27 µM ± 1.58, and 12.96 µM ± 0.09 against these cell lines, respectively. Further investigations into the mechanism of action of 10i demonstrated its ability to induce G2/M cell cycle arrest and initiate mitochondria-mediated apoptosis in breast cancer cells.

From molecules to medicine: thiol selective bioconjugation in synthesis of diagnostic and therapeutic radiopharmaceuticals

Radiolabeling of biomolecules and cells with radiolabeled prosthetic groups has significant implications for nuclear medicine, imaging, and radiotherapy. Achieving site-specific and controlled incorporation of radiolabeled prostheses under mild reaction conditions is crucial for minimizing the impact on the bioactivity of the radiolabeled compounds. The targeting of natural and abundant amino acids during radiolabeling of biomolecules often results in nonspecific and uncontrolled modifications. Cysteine is distinguished by its low natural abundance and unique nucleophilicity. It is therefore an optimal target for site-selective and site-specific radiolabeling of biomolecules under controlled parameters. This review extensively discusses thiol-specific radiolabeled prosthetic groups and provides a critical analysis and comprehensive study of the synthesis of these groups, their in vitro and in vivo stability profiles, reaction kinetics, stability of resulting adducts, and overall impact on the targeting ability of radiolabeled biomolecules. The insights presented here aim to facilitate the development of highly efficient radiopharmaceuticals, initially in preclinical settings and ultimately in clinical applications.

The development of potent, competitive CXCR4 antagonists for the prevention of cancer metastasis

Cancer metastasis is the cause of up to 90 % of cancer related mortality. The CXCR4 receptor and its cognate ligand, CXCL12, have major roles in enabling cancer metastasis and consequently, the CXCR4 receptor has become an attractive therapeutic target for the prevention of metastasis. Despite this, CXCR4 antagonists have had limited success in clinical trials due to cellular toxicity and poor stability and efficacy. In this study, we developed a novel, competitive CXCR4 antagonist (IS4) that through copper-catalysed-azide-alkyne-cycloaddition can be clicked to other chemical moieties such as fluorescent dyes (IS4-FAM) for CXCR4-based imaging. We determined that these CXCR4 antagonists were non-toxic and could be used to specifically label the CXCR4 receptor. Furthermore, IS4 and IS4-FAM inhibited CXCL12-stimulated cancer cell migration and Ca2+ release in both adherent and suspension cell lines with similar or improved potency as compared to two literature CXCR4 antagonists. Our results highlight the potential of IS4 and IS4-FAM as research tools and as potent CXCR4 antagonists for the prevention of metastasis.

Photochemical [2+2] Cycloaddition of Alkynyl Boronates

A photochemical [2+2] cycloaddition of alkynyl boronates and maleimides is reported. The developed protocol provided 35-70 % yield of maleimide-derived cyclobutenyl boronates and demonstrated wide compatibility with various functional groups. The synthetic utility of the prepared building blocks was demonstrated for a range of transformations, including Suzuki cross-coupling, catalytic or metal-hydride reduction, oxidation, and cycloaddition reactions. With aryl-substituted alkynyl boronates, the products of double [2+2] cycloaddition were obtained predominantly. Using the developed protocol, a cyclobutene-derived analogue of Thalidomide was prepared in one step. Mechanistic studies supported the participation of the triplet-excited state maleimides and ground state alkynyl boronates in the key step of the process.

Maleimide as the PEG end-group promotes macrophage-targeted drug delivery of PEGylated nanoparticles in vivo by enhancing interaction with circulating erythrocytes

Radiotherapy (IR) is capable of enhancing antitumor immune responses. However, IR treatment also aggravates the infiltration of peripheral macrophages into the tumor, resulting in reversing the therapeutic effects of antitumor immunity. Thus, a strategy to effectively prevent tumor infiltration by macrophages may further improved the therapeutic efficacy of radiotherapy. Herein, we found that PEGylated solid lipid nanoparticles with maleimide as PEG end-group (SLN-PEG-Mal) show significantly enhanced adsorption onto RBCs through reacting with reactive sulfhydryl groups on RBCs' surface both in vitro and in vivo, and caused significant changes in the surface properties and morphology of RBCs. These RBCs adsorbed by SLN-PEG-Mal were rapidly removed from circulation due to efficient engulfment by reticuloendothelial macrophages, supporting the usefulness of SLN-PEG-Mal for macrophage-targeted drug delivery. While lacking the use of radioisotope tracing (considered the gold standard for PK/BD studies), our data align with the expected pathway of host defense activation through surface-loaded RBCs. Importantly, injection of paclitaxel-loaded SLN-PEG-Mal effectively inhibited the tumor-infiltration by macrophages, and significantly improved the antitumor immune responses in tumor-bearing mice treated with low-dose irradiation. This study provides insights into the effects of maleimide as PEG end-group on enhancing the interaction between PEGylated nanoparticles and RBCs and offers an effective strategy to inhibit tumor infiltration by circulating macrophages.

Cysteine-Assisted Click-Chemistry for Proximity-Driven, Site-Specific Acetylation of Histones

Post-translational modifications of histones are essential in the regulation of chromatin structure and function. Among these modifications, lysine acetylation is one of the most established. Earlier studies relied on the use of chromatin containing heterogeneous mixtures of histones acetylated at multiple sites. Differentiating the individual contribution of single acetylation events towards chromatin regulation is thus of great relevance. However, it is difficult to access homogeneous samples of histones, with a single acetylation, in sufficient quantities for such studies. By engineering histone H3 with a cysteine in proximity of the lysine of interest, we demonstrate that conjugation with maleimide-DBCO followed by a strain-promoted alkyne-azide cycloaddition reaction results in the acetylation of a single lysine in a controlled, site-specific manner. The chemical precision offered by our click-to-acetylate approach will facilitate access to and the study of acetylated histones.

Novel [18F]-labeled thiol for the labeling of Dha- or maleimide-containing biomolecules

Background

Prosthetic approach for the radiolabeling of biologics with fluorine-18 is a robust strategy and has been employed for many years. It requires fast, biocompatible and selective reactions suited to these fragile molecules. Michael addition of a nucleophilic thiol moiety on α,β-unsaturated carbonyl entities is an interesting compromise between simplicity of preparation of the prosthetic reagent and control of the selectivity of the addition. The α,β-unsaturated carbonyl entity of the biologic can easily be generated by addition of a maleimide function using adequate heterobifunctional linkers or generated by selective modification of a cysteine residue leading to a dehydroalanine moiety. We report here the design, synthesis and radiosynthesis of a new fluoropyridine-based thiol [¹⁸F]FPySH and its conjugation via Michael addition on model dehydroalanine- or maleimide-containing biologics.

Results

The preparation of cold reference and labeling precursor of [¹⁸F]FPySH was achieved and its radiosynthesis was fully automated, enabling production of the thiol prosthetic group with a 7 ± 2.1% radiochemical yield after two steps. The conjugation of [¹⁸F]FPySH to two model Dha-containing molecules was then carried out in reducing conditions, yielding the corresponding adducts in 30–45 min reaction time. Furthermore, [¹⁸F]FPySH was employed to radiolabel the maleimide-modified c(RGDfK) peptide, affording the radiofluorinated analogue in 15 min.

Conclusion

We have developed an original [¹⁸F]-labeled thiol for site-selective conjugation and radiolabeling of Dha or maleimide-containing biomolecules of interest. Labeling of three model compounds was successfully carried out and gave the expected radiofluorinated adducts in less than 45 min, thus compatible with fluorine-18 half-life.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41181-022-00160-5.

Labeling of Proteins for Single-Molecule Fluorescence Spectroscopy

Single-molecule fluorescence spectroscopy has become an important technique for studying the conformational dynamics and folding of proteins. A key step for performing such experiments is the availability of high-quality samples. This chapter describes a simple and widely applicable strategy for preparing proteins that are site-specifically labeled with a donor and an acceptor dye for single-molecule Förster resonance energy transfer (FRET) experiments. The method is based on introducing two cysteine residues that are labeled with maleimide-functionalized fluorophores, combined with high-resolution chromatography. We discuss how to optimize site-specific labeling even in the absence of orthogonal coupling chemistry and present purification strategies that are suitable for samples ranging from intrinsically disordered proteins to large folded proteins. We also discuss common problems in protein labeling, how to avoid them, and how to stringently control sample quality.

Introducing Cysteines into Nanobodies for Site-Specific Labeling

We have developed a generally applicable methodology for cysteine mutagenesis of nanobody (Nb) framework region serine residues. This strategy allows for subsequent labeling with thiol-reactive compounds without disrupting Nb antigen binding. We provide a protocol for production, labeling, and affinity determination of cysteine-engineered Nbs (cys-Nbs) with Alexa Fluor 488-maleimide and the mercury compound para-chloromercuribenzoic acid (PCMB). Alexa Fluor 488- and PCMB-labeled cys-Nbs can be used for immunofluorescence microscopy and experimental phasing in crystallography, respectively.

Maleimide structure: a promising scaffold for the development of antimicrobial agents

Natural compounds bearing maleimide rings are a series of secondary metabolites derived from fungi/marine microorganisms, which are characterized by a general structure -CO-N(R)-CO-, and the R group is normally substituted with alkyl or aryl groups. Maleimide compounds show various biological activities such as antibacterial, antifungal, and anticancer activity. In this review, the broad-spectrum antimicrobial activities of 15 maleimide compounds from natural sources and 32 artificially synthesized maleimides were summarized, especially against Candida albicans, Sclerotinia sclerotiorum, and Staphylococcus aureus. It highlights that maleimide scaffold has tremendous potential to be utilized in the development of novel antimicrobial agents.

Boron neutron capture therapy using dodecaborated albumin conjugates with maleimide is effective in a rat glioma model

Introduction Boron neutron capture therapy (BNCT) is a biologically targeted, cell-selective particle irradiation therapy that utilizes the nuclear capture reaction of boron and neutron. Recently, accelerator neutron generators have been used in clinical settings, and expectations for developing new boron compounds are growing. Methods and Results In this study, we focused on serum albumin, a well-known drug delivery system, and developed maleimide-functionalized closo-dodecaborate albumin conjugate (MID-AC) as a boron carrying system for BNCT. Our biodistribution experiment involved F98 glioma-bearing rat brain tumor models systemically administered with MID-AC and demonstrated accumulation and long retention of boron. Our BNCT study with MID-AC observed statistically significant prolongation of the survival rate compared to the control groups, with results comparable to BNCT study with boronophenylalanine (BPA) which is the standard use of in clinical settings. Each median survival time was as follows: untreated control group; 24.5 days, neutron-irradiated control group; 24.5 days, neutron irradiation following 2.5 h after termination of intravenous administration (i.v.) of BPA; 31.5 days, and neutron irradiation following 2.5 or 24 h after termination of i.v. of MID-AC; 33.5 or 33.0 days, respectively. The biological effectiveness factor of MID-AC for F98 rat glioma was estimated based on these survival times and found to be higher to 12. This tendency was confirmed in BNCT 24 h after MID-AC administration. Conclusion MID-AC induces an efficient boron neutron capture reaction because the albumin contained in MID-AC is retained in the tumor and has a considerable potential to become an effective delivery system for BNCT in treating high-grade gliomas.

Maleimide-functionalized phospholipid/Pluronic F127 mixed micelles for efficient ophthalmic delivery of voriconazole against Candida albicans

Drugs that are topically applied on the eyes have low bioavailability, which has always been an important problem. In this study, maleimide functionalized, voriconazole (VCZ) loaded mixed micelles (Mal-VCZ-MM) were designed. Pluronic F127 and phospholipid were used as materials, and maleimide was used as an adhesive. The prepared Mal-VCZ-MM was nearly spherical with a particle size of 84.45 ± 1.39 nm and a zeta potential of - 20.3 ± 0.29 mV. The encapsulation efficiency of Mal-VCZ-MM was 95.33 ± 0.06%, and it had high stability with a critical micelle concentration value of 1.28 × 10^-4 mg/mL. CCK-8 assay showed that its cytotoxicity was lower than that of free VCZ solution (VCZ-Sol). Both quantitative and qualitative analyses of the HCE-T cellular uptake showed that the cellular internalization of Mal-C6-MM was significantly stronger than that of C6-MM. The endocytosis pathway was macropinocytosis-mediated, cavernous-mediated, and energy-dependent. In vitro results against Candida albicans showed that the diameters of the antifungal inhibition zones of VCZ-Sol, VCZ-MM, and Mal-VCZ-MM were 15.5 ± 0.50 mm, 24.0 ± 0.71 mm, and 31.5 ± 1.12 mm, respectively. The antifungal effect of Mal-VCZ-MM was significantly higher than that of VCZ-Sol and VCZ-MM (P < 0.001). This study shows that Mal-VCZ-MM is a highly effective hydrophobic ophthalmic drug-delivery carrier that can improve the therapeutic effect of the drug.

Theoretical study on the grafting reaction of maleimide containing 2-hydroxy-benzophenone onto polyethylene

A theoretical study on the multi-channel hydrogen addition of maleimide containing 2-hydroxy-benzophenone onto polyethylene in ultra-violet (UV) radiation cross-linking process was carried out using density functional theory (DFT) method at the B3LYP/6-311 + G(d,p) level. The energetic information and the minimum energy path (MEP) are calculated for nine reaction channels. The electrophilic addition reactions at two positions in the target molecule (maleimide containing 2-hydroxy-benzophenone) were investigated, which are on the C atom of C = C groups and on the O atom of C = O groups. Frontier molecular orbitals (MOs) and natural bond orbital (NBO) charge population of the target molecule have been analyzed in detail. As a result, the reaction site of C in C = C group is more active than the site of O in C = O groups. The target molecule can be used as a multi-functional additive candidate. The predicted mechanism may provide a theoretical basis for the real application of cross-linked polyethylene (XLPE) high-voltage insulation cables.

A Fast and Ratiometric Method for Quantification of Cysteine-Bound Persulfides Based on Alkylation and Gel-Shift Assays

Cysteine-bound persulfides (Cys-SSH) in proteins are sulfur carrier intermediates in the synthesis of essential cofactors such as iron-sulfur clusters, molybdenum cofactor, vitamin (thiamine), and thionucleosides (thiolated tRNA). Protein-bound persulfides are also used for signaling purposes as a carrier of the "H₂S" signal. Several methods have been developed to detect and quantify cysteine-bound persulfides in protein and monitor their exchange. The main challenge in developing these techniques is to discriminate persulfidated cysteine from cysteine and other cysteine modifications. It is also critical to develop ratiometric methods to quantify the level of persulfidation in the protein of interest. We describe here a ratiometric method to label and quantify protein-bound persulfides relying on alkylation and gel-shift assays. This method is based on the derivation of cysteine and persulfides with "heavy" alkylating agents, followed by specific cleavage of the sulfur-sulfur bond of the alkylated persulfide by a reducing agent and separation of the alkylated species by electrophoresis. A persulfide is thus revealed by the appearance of a species lacking one alkylation unit under reducing conditions. We call this alkylation-reduction band-shift (ARBS) assay. Moreover, the quantification of the bands corresponding to the persulfidated and non-persulfidated species in the same lane provides a ratiometric quantification allowing determination of the level of persulfidation of individual cysteine. Other cysteine modifications such as disulfides, sulfenic, sulfinic, sulfonic acids, nitrosothiols, and sulfenamides preclude alkylation. Thus, they may appear as false positives, but they are ruled out by the analysis under nonreducing conditions since these species do not behave as persulfides under these conditions.

Pure redox-sensitive paclitaxel-maleimide prodrug nanoparticles: Endogenous albumin-induced size switching and improved antitumor efficiency

A commercial albumin-bound paclitaxel nano-formulation has been considered a gold standard against breast cancer. However, its application still restricted unfavorable pharmacokinetics and the immunogenicity of exogenous albumin carrier. Herein, we report an albumin-bound tumor redox-responsive paclitaxel prodrugs nano-delivery strategy. Using diverse linkages (thioether bond and disulfide bond), paclitaxel (PTX) was conjugated with an albumin-binding maleimide (MAL) functional group. These pure PTX prodrugs could self-assemble to form uniform and spherical nanoparticles (NPs) in aqueous solution without any excipients. By immediately binding to blood circulating albumin after intravenous administration, NPs are rapidly disintegrated into small prodrug/albumin nanoaggregates in vivo, facilitating PTX prodrugs accumulation in the tumor region via albumin receptor-mediated active targeting. The tumor redox dual-responsive drug release property of prodrugs improves the selectivity of cytotoxicity between normal and cancer cells. Moreover, disulfide bond-containing prodrug/albumin nanoaggregates exhibit long circulation time and superior antitumor efficacy in vivo. This simple and facile strategy integrates the biomimetic characteristic of albumin, tumor redox-responsive on-demand drug release, and provides new opportunities for the development of the high-efficiency antitumor nanomedicines.

Development of a novel human adrenomedullin derivative: human serum albumin-conjugated adrenomedullin

Adrenomedullin is a biologically active peptide with multiple functions. Here, we have developed a novel human serum albumin-adrenomedullin (HSA-AM) conjugate, which was synthesized by the covalent attachment of a maleimide derivative of adrenomedullin to the 34th cysteine residue of human serum albumin via a linker. Denaturing gel electrophoresis and Western blotting for HSA-AM yielded a single band with adrenomedullin immunoreactivity at the position corresponding to a molecular weight (MW) of 73 kDa. Following gel-filtration chromatography, the purified HSA-AM showed a single main peak corresponding with a MW of 73 kDa, indicating that HSA-AM is a monomer. Both adrenomedullin and HSA-AM stimulated the intracellular accumulation of cyclic AMP in HEK-293 cells stably expressing the adrenomedullin 1 receptor. The pEC50 values for adrenomedullin and HSA-AM were 8.660 and 7.208 (equivalent to 2.19 nM and 61.9 nM as EC50), respectively. The bioavailability of HSA-AM compared with that of adrenomedullin was much improved after subcutaneous administration in the rat, which was probably due to the superior resistance of HSA-AM toward endogenous proteases and its reduced clearance from the blood. HSA-AM may be a promising drug candidate for clinical application.

Ultrasensitive and selective fluorescent sensor for cysteine and application to drug analysis and bioimaging

A fluorescent sensor based on coumarin-maleimide conjugate was developed for efficient discrimination of Cys from Hcy and GSH in both organic and aqueous solution. Addition of Cys to the non-fluorescent sensor solution in DMF induced bright blue fluorescence and enhanced the fluorescence intensity by 320-fold while other amino acids and biothiols (Gly, Hcy, GSH, Glu, Val, Tyr, Arg, Trp, Lys, His, Leu, Phe, Asp and Met) did not bring about remarked change. The sensor responds to Cys extremely rapidly. If Cys was added to the sensor solution, the fluorescence intensity increased by 170-fold immediately and attained the maximum value in 5 min. A linear relationship was observed between Cys concentration within 2-20 μM and the fluorescence intensity of the sensor solution. The detection limit of the sensor toward Cys is as low as 4.7 nM. The sensor is also effective for specific detection of Cys in aqueous (DMF/H₂O = 9:1, v/v) solution. Practical application of the sensor to drug analysis and bioimaging of living Hela cells has been verified. Possible sensing mechanism of the sensor toward Cys has been proposed.

Synthesis and Photophysical Study of [60]Fullerene-Maleimide Dyads

Novel [60]fullerene-maleimide dyads were synthesized by covalent linking of maleimide fluorophore to the [60]fullerene (C₆₀) via Bingel reaction. The dyads were well characterized and studied for their absorption and emission properties. The fluorescence quenching of maleimide moiety by C₆₀ was observed, indicating the intramolecular energy transfer from maleimide fluorophore to C₆₀ moiety.

Site-Specific Conjugation to Cys-Engineered THIOMAB™ Antibodies

Antibodies bearing engineered cysteine residues (termed THIOMAB™ antibodies) enable the site-selective attachment of a drug, label or other payload for specific delivery to certain tissues (e.g., tumors). This Chapter describes detailed methods we have developed and optimized for the conjugation, purification and analysis of THIOMAB™ antibody drug conjugates (TDCs).

Maleimide-functionalised PLGA-PEG nanoparticles as mucoadhesive carriers for intravesical drug delivery

Low permeability of the urinary bladder epithelium, poor retention of the chemotherapeutic agents due to dilution and periodic urine voiding as well as intermittent catheterisations are the major limitations of intravesical drug delivery used in the treatment of bladder cancer. In this work, maleimide-functionalised poly(lactide-co-glycolide)-block-poly(ethylene glycol) (PLGA-PEG-Mal) nanoparticles were developed. Their physicochemical characteristics, including morphology, architecture and molecular parameters have been investigated by means of dynamic light scattering, transmission electron microscopy and small-angle neutron scattering techniques. It was established that the size of nanoparticles was dependent on the solvent used in their preparation and molecular weight of PEG, for example, 105 ± 1 nm and 68 ± 1 nm particles were formed from PLGA_20K-PEG_5K in dimethyl sulfoxide and acetone, respectively. PLGA-PEG-Mal nanoparticles were explored as mucoadhesive formulations for drug delivery to the urinary bladder. The retention of fluorescein-loaded nanoparticles on freshly excised lamb bladder mucosa in vitro was evaluated and assessed using a flow-through fluorescence technique and Wash Out₅₀ (WO₅₀) quantitative method. PLGA-PEG-Mal nanoparticles (NPs) exhibited greater retention on urinary bladder mucosa (WO₅₀ = 15 mL) compared to maleimide-free NPs (WO₅₀ = 5 mL). The assessment of the biocompatibility of PEG-Mal using the slug mucosal irritation test revealed that these materials are non-irritant to mucosal surfaces.

Determination of Protein S-Acylation State by Enhanced Acyl-Switch Methods

S-Acylation is increasingly being recognized as an important dynamic posttranslational modification of cysteine residues in proteins. Various approaches have been described for assaying protein S-acylation with acyl-switch approaches being the most common and accessible. However, these approaches can be time-consuming with low reproducibility as a result of multiple protein precipitation/resuspension cleanup steps. Here we present a faster, cleaner, and more sensitive acyl-switch approach for detecting the S-acylation state of any protein, from any cell or tissue type, that can be detected by western blotting. In the case of acyl-RAC, the procedure is now performed without protein precipitation, greatly increasing speed and improving sample handling in the assay. This also allows for more samples to be processed simultaneously and opens the way for medium-throughput assays. Overall, maleimide scavenging improves the reliability of determination and quantification of protein S-acylation state by acyl-switch methods.

Thioredoxin-1 PEGylation as an In Vitro Method for Drug Target Identification

Redox signaling in the cell, which is essential for cell physiology, involves proteins with free sulfhydryl groups (-SH). Among them, the thioredoxin system plays the most significant role. Many conditions associated with cell malignancies feature the higher expression of thioredoxin, making it an attractive target for new therapeutic drug development. Here we present a simple in vitro model of testing the interaction between thioredoxin and the putative drug. This method is relatively inexpensive and gives the Investigator a first screen of the drug properties, which can be essential for further experimental approaches.

Bioconjugation with Maleimides: A Useful Tool for Chemical Biology

Maleimide chemistry stands out in the bioconjugation toolbox by virtue of its synthetic accessibility, excellent reactivity, and practicability. The second-generation of clinically approved antibody-drug conjugates (ADC) and much of the current ADC pipeline in clinical trials contain the maleimide linkage. However, thiosuccinimide linkages are now known to be less robust than once thought, and ergo, are correlated with suboptimal pharmacodynamics, pharmacokinetics, and safety profiles in some ADC constructs. Rational design of novel generations of maleimides and maleimide-type reagents have been reported to address the shortcomings of classical maleimides, allowing for the formation of robust bioconjugate linkages. This review highlights the main strategies for rational reagent design that have allowed irreversible bioconjugations in cysteines, reversible labelling strategies and disulfide re-bridging.

Preparation of stable tau oligomers for cellular and biochemical studies

Increasing evidence suggests that small oligomers are the principal neurotoxic species of tau in Alzheimer's disease and other tauopathies. However, mechanisms of tau oligomer-mediated neurodegeneration are poorly understood. The transience of oligomers due to aggregation can compromise the stability of oligomers prepared in vitro. Consequently, we sought to develop an efficient method which maintains the stability and globular conformation of preformed oligomers. This study demonstrates that labeling a single-cysteine form of the pro-aggregant tau four-repeat region (K18) with either Alexa Fluor 488-C5-maleimide or N-ethylmaleimide in reducing conditions stabilizes oligomers by impeding their further aggregation. Furthermore, the use of this approach to study the propagation of labeled extracellular tau K18 oligomers into human neuroblastoma cells and human stem cell-derived neurons is described. This method is potentially applicable for preparing stabilized oligomers of tau for diagnostic and biomarker tests, as well as for in vitro structure-activity relationship assays.

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The transient nature of tau aggregation makes it difficult to maintain the stability of preformed oligomers.

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Efficient labeling of tau K18 with Alexa Fluor-488-C5-maleimide or N-ethyl maleimide stabilizes the resulting oligomers.

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Oligomers applied exogenously are propagated intracellularly by cultured human iPSC neurons and neuroblastoma cells.

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Oligomer preparation by maleimide labeling allows mechanistic studies of tau aggregation and its link to neurodegeneration.

Asperimides A-D, anti-inflammatory aromatic butenolides from a tropical endophytic fungus Aspergillus terreus

Four new aromatic butenolides, asperimides A-D (1-4), together with a known analogue, butyrolactone I (5), were isolated from solid cultures of a tropical endophytic fungus Aspergillus terreus. The structures of these compounds were elucidated on the basis of spectroscopic methods and electronic circular dichroism calculations. Compounds 1-4 represent the first examples of butenolides with a maleimide core isolated from Aspergillus sp. Inhibitory effects of the isolated compounds on nitric oxide production were investigated in lipopolysaccaride (LPS)-mediated RAW 264.7 cells. Compounds 3 and 4 exhibited potent anti-inflammatory with IC₅₀ values of 0.78 ± 0.06 and 1.26 ± 0.11 μM, respectively.

Structure-activity relationship studies of lipophilic teicoplanin pseudoaglycon derivatives as new anti-influenza virus agents

Six series of semisynthetic lipophilic glycopeptide antibiotic derivatives were evaluated for in vitro activity against influenza A and B viruses. The new teicoplanin pseudoaglycon-derived lipoglycopeptides were prepared by coupling one or two side chains to the N-terminus of the glycopeptide core, using various conjugation methods. Three series of derivatives bearing two lipophilic groups were synthesized by attaching bis-alkylthio maleimides directly or through linkers of different lengths to the glycopeptide. Access to the fourth and fifth series of compounds was achieved by click chemistry, introducing single alkyl/aryl chains directly or through a tetraethylene glycol linker to the same position. A sixth group of semisynthetic derivatives was obtained by sulfonylation of the N-terminus. Of the 42 lipophilic teicoplanin pseudoaglycon derivatives tested, about half showed broad activity against influenza A and B viruses, with some of them having reasonable or no cytotoxicity. Minor differences in the side chain length as well as lipophilicity appeared to have significant impact on antiviral activity and cytotoxicity. Several lipoglycopeptides were also found to be active against human coronavirus.

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Multiple series of lipophilic teicoplanin pseudoaglycon derivatives were prepared.

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Alkyl or aryl chains were coupled to the N-terminus by various conjugation methods.

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The activity of new antibiotic derivatives was evaluated against influenza viruses.

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Half of the 42 derivatives showed high activity against influenza A and B viruses.

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The length and lipophilicity of the side chains influence the antiviral activity.

Comparison of thermally actuated retro-diels-alder release groups for nanoparticle based nucleic acid delivery

The present study explores alternate pericyclic chemistries for tethering amine-terminal biomolecules onto silver nanoparticles. Employing the versatile tool of the retro-Diels-Alder (rDA) reaction, three thermally-labile cycloadducts are constructed that cleave at variable temperature ranges. While the reaction between furan and maleimide has widely been reported, the current study also evaluates the reverse reaction kinetics between thiophene-maleimide, and pyrrole-maleimide cycloadducts. Density Functional Theorem (DFT) calculations used to model and plan the experiments, predict energy barriers for the thiophene-maleimide reverse reaction to be greatest, and the pyrrole-maleimide barriers the lowest. Based on the computational analyses, it is projected that the cycloreversion rate would occur slowest with the thiophene, followed by furan, and finally pyrrole would yield the promptest release. These thermally-responsive linkers, characterized by Electrospray Ionization Mass Spectrometry, ¹H and ¹³C NMR, are thiol-linked to silver nanoparticles and conjugate single stranded siRNA mimics with 5' fluorescein tag. Second harmonic generation spectroscopy (SHG) and fluorescence spectroscopy are used to measure release and rate of release. The SHG decay constants and fluorescence release profiles obtained for the three rDA reactions confirm the trends obtained from the DFT computations.

Injectable synthetic hydrogel for bone regeneration: Physicochemical characterisation of a high and a low pH gelling system

Hybrid poly(ethylene glycol)-co-peptide hydrogels are a versatile platform for bone regeneration. For the use as injectable scaffolds, a good understanding of reaction kinetics and physical properties is vital. However, these factors have not yet been comprehensively illuminated. We show that gelation time can be effectively controlled by pH without affecting the elasticity of the formed hydrogels. Maleimide functionalised PEG gels at lower pH and produces more densely cross-linked networks than vinylsulfone functionalised PEG. Both form non-ideal networks. The elastic moduli on the order of a few kPa are in good agreement with the structural characterisation. Primary human osteoblasts cultured in proximity to bulk gels were not adversely affected in vitro. The results demonstrate that hybrid PEG-peptide hydrogels can be tailored to the requirements of in situ gelation. Attributed to their increased structural properties and a higher tolerance towards low pH, maleimide functionalised hydrogels might provide a better alternative for injectable applications.

Insights into maleimide-thiol conjugation chemistry: Conditions for efficient surface functionalization of nanoparticles for receptor targeting

Maleimide-thiol chemistry is widely used for the design and preparation of ligand-decorated drug delivery systems such as poly(lactide-co-glycolide) (PLGA) based nanoparticles (NPs). While many publications on nanocarriers functionalized exploiting this strategy are available in the literature, the conditions at which this reaction takes place vary among publications. This paper presents a comprehensive study on the conjugation of the peptide cRGDfK and the nanobody 11A4 (both containing a free thiol group) to maleimide functionalized PLGA NPs by means of the maleimide-thiol click reaction. The influence of different parameters, such as the nanoparticles preparation method and storage conditions as well as the molar ratio of maleimide to ligand used for conjugation, on the reaction efficiency has been evaluated. The NPs were prepared by a single or double emulsion method using different types and concentrations of surfactants and stored at 4 or 20 °C before reaction with the targeting moieties. Several maleimide to ligand molar ratios and different reaction times were studied and the conjugation efficiency was determined by quantification of the not-bound ligand by liquid chromatography. The kind of emulsion used to prepare the NPs as well as the type and concentration of surfactant used had no effect on the conjugation efficiency. Reaction between the maleimide groups present in the NPs and cRGDfK was optimal at a maleimide to thiol molar ratio of 2:1, reaching a conjugation efficiency of 84 ± 4% after 30 min at room temperature in 10 mM HEPES pH 7.0. For 11A4 nanobody the optimal reaction efficiency, 58 ± 12%, was achieved after 2 h of incubation at room temperature in PBS pH 7.4 using a 5:1 maleimide to protein molar ratio. Storage of the NPs at 4 °C for 7 days prior to their exposure to the ligands resulted in approximately 10% decrease in the reactivity of maleimide in contrast to storage at 20 °C which led to almost 40% of the maleimide being unreactive after the same storage time. Our findings demonstrate that optimization of this reaction, particularly in terms of reactant ratios, can represent a significant increase in the conjugation efficiency and prevent considerable waste of resources.

Maleimide-acetylcholine headed bolaamphiphilic vesicles made from ricinoleic acid: Prospective active targeted drug delivery systems

Based on ricinoleic acid, two asymmetric bolaamphiphiles with unsymmetrical hydrophobic skeletons and two different hydrophilic head groups were designed and synthesized. The first bola compound had acetylcholine (ACh) and maleimide (MAL) head groups while the second was derived from the first bolaamphiphile by thiol-ene conjugation of its maleimide moiety with l-glutathione and possessed ACh and l-glutathione-MAL head groups. Both synthetic bolaamphiphiles were characterized by common spectroscopic methods. The asymmetric bola compound with ACh and MAL head groups was investigated for its ability to self-aggregate into nanoparticles and showed to form in aqueous media nano-sized vesicles that were stable, positively charged and had symmetrical monolayer membrane with antiparallel packing. These vesicles prepared with or without membrane stabilizers such as cholesterol (CHOL) and cholesteryl hemisuccinate (CHEMS) were able to encapsulate carboxyfluorescein (CF), a water soluble and self-quenching marker and particularly those without additives were more CF encapsulating. The synthesis of bolaamphiphile with ACh-l-glutathione-MAL head groups gives evidence that the bola with ACh and MAL head groups can be utilized as a precursor of a plethora of asymmetric bolas.

The Taming of the Maleimide: Fabrication of Maleimide-Containing 'Clickable' Polymeric Materials

Functional polymers are widely employed in various areas of biomedical sciences. In order to tailor them for desired applications, facile and efficient functionalization of these polymeric materials under mild and benign conditions is important. Polymers containing reactive maleimide groups can be employed for such applications since they provide an excellent handle for conjugation of thiol- and diene-containing molecules and biomolecules. Until recently, fabrication of maleimide containing polymeric materials has been challenging due to the interference from the highly reactive double bond. A Diels-Alder/retro Diels-Alder reaction sequence based strategy to transiently mask the maleimide group provides access to such polymeric materials. In this personal account, we summarize contributions from our group towards the fabrication and functionalization of maleimide-containing polymeric materials over the past decade.

Monocyte activation drives preservation of membrane thiols by promoting release of oxidised membrane moieties via extracellular vesicles

The redox state of cellular exofacial molecules is reflected by the amount of available thiols. Furthermore, surface thiols can be considered as indicators of immune cell activation. One group of thiol containing proteins, peroxiredoxins, in particular, have been associated with inflammation. In this study, we assessed surface thiols of the U937 and Thp1 monocyte cell lines and primary monocytes in vitro upon inflammatory stimulation by irreversibly labelling the cells with a fluorescent derivative of maleimide. We also investigated exofacial thiols on circulating blood mononuclear cells in patients with rheumatoid arthritis and healthy controls. When analysing extracellular vesicles, we combined thiol labelling with the use of antibodies to specific CD markers to exclude extracellular vesicle mimicking signals from thiol containing protein aggregates. Furthermore, differential detergent lysis was applied to confirm the vesicular nature of the detected extracellular events in blood plasma. We found an increase in exofacial thiols on monocytes upon in vitro stimulation by LPS or TNF, both in primary monocytes and monocytic cell lines (p<0.0005). At the same time, newly released extracellular vesicles showed a decrease in their exofacial thiols compared with those from unstimulated cells (p<0.05). We also found a significant elevation of surface thiols on circulating monocytes in rheumatoid arthritis patients (p<0.05) and newly released extracellular vesicles of isolated CD14⁺ cells from rheumatoid arthritis patients had decreased thiol levels compared with healthy subjects (p<0.01). Exofacial peroxiredoxin 1 was demonstrated on the surface of primary and cultured monocytes, and the number of peroxiredoxin 1 positive extracellular vesicles was increased in rheumatoid arthritis blood plasma (p<0.05). Furthermore, an overoxidised form of peroxiredoxin was detected in extracellular vesicle-enriched preparations from blood plasma. Our data show that cell surface thiols play a protective role and reflect oxidative stress resistance state in activated immune cells. Furthermore, they support a role of extracellular vesicles in the redox regulation of human monocytes, possibly representing an antioxidant mechanism.

A bivalent dendrimeric peptide bearing a T-cell epitope from foot-and-mouth disease virus protein 3A improves humoral response against classical swine fever virus

Three dendrimeric peptides were synthesized in order to evaluate their immunogenicity and their potential protection against classical swine fever virus (CSFV) in domestic pigs. Construct 1, an optimized version of a previously used dendrimer, had four copies of a B-cell epitope derived from CSFV E2 glycoprotein connected to an also CSFV-derived T-cell epitope through maleimide instead of thioether linkages. Construct 2 was similarly built but included only two copies of the B-cell epitope, and in also bivalent construct 3 the CSFV T-cell epitope was replaced by a previously described one from the 3A protein of foot-and-mouth disease virus (FMDV). Animals were inoculated twice with a 21-day interval and challenged 15days after the second immunization. Clinical signs were recorded daily and ELISA tests were performed to detect antibodies against specific peptide and E2. The neutralising antibody response was assessed 13days after challenge. Despite the change to maleimide connectivity, only partial protection against CSFV was again observed. The best clinical protection was observed in group 3. Animals inoculated with constructs 2 and 3 showed higher anti-peptide humoral response, suggesting that two copies of the B-cell epitope are sufficient or even better than four copies for swine immune recognition. In addition, for construct 3 higher neutralizing antibody titres against CSFV were detected. Our results support the immunogenicity of the CSFV B-cell epitope and the cooperative role of the FMDV 3A T-cell epitope in inducing a neutralising response against CSFV in domestic pigs. This is also the first time that the FMDV T-cell epitope shows effectivity in improving swine immune response against a different virus. Our findings highlight the relevance of dendrimeric peptides as a powerful tool for epitope characterization and antiviral strategies development.

Site-Specifically Labeled Immunoconjugates for Molecular Imaging--Part 1: Cysteine Residues and Glycans

Due to their remarkable selectivity and specificity for cancer biomarkers, immunoconjugates have emerged as extremely promising vectors for the delivery of diagnostic radioisotopes and fluorophores to malignant tissues. Paradoxically, however, these tools for precision medicine are synthesized in a remarkably imprecise way. Indeed, the vast majority of immunoconjugates are created via the random conjugation of bifunctional probes (e.g., DOTA-NCS) to amino acids within the antibody (e.g., lysines). Yet antibodies have multiple copies of these residues throughout their macromolecular structure, making control over the location of the conjugation reaction impossible. This lack of site specificity can lead to the formation of poorly defined, heterogeneous immunoconjugates with suboptimal in vivo behavior. Over the past decade, interest in the synthesis and development of site-specifically labeled immunoconjugates—both antibody-drug conjugates as well as constructs for in vivo imaging—has increased dramatically, and a number of reports have suggested that these better defined, more homogeneous constructs exhibit improved performance in vivo compared to their randomly modified cousins. In this two-part review, we seek to provide an overview of the various methods that have been developed to create site-specifically modified immunoconjugates for positron emission tomography, single photon emission computed tomography, and fluorescence imaging. We will begin with an introduction to the structure of antibodies and antibody fragments. This is followed by the core of the work: sections detailing the four different approaches to site-specific modification strategies based on cysteine residues, glycans, peptide tags, and unnatural amino acids. These discussions will be divided into two installments: cysteine residues and glycans will be detailed in Part 1 of the review, while peptide tags and unnatural amino acids will be addressed in Part 2. Ultimately, we sincerely hope that this review fosters interest and enthusiasm for site-specific immunoconjugates within the nuclear medicine and molecular imaging communities.

Maleimide-functionalized closo-dodecaborate albumin conjugates (MID-AC): Unique ligation at cysteine and lysine residues enables efficient boron delivery to tumor for neutron capture therapy

Maleimide-conjugating closo-dodecaborate sodium form 5c (MID) synthesized by the nucleophilic ring-opening reaction of closo-dodecaborate-1,4-dioxane complex 2 with tetrabutylammonium (TBA) azide was found to conjugate to free SH of cysteine and lysine residues in BSA under physiological conditions, forming highly boronated BSA that showed high and selective accumulation in tumor and significant tumor growth inhibition in colon 26 tumor-bearing mice subjected to thermal neutron irradiation.

Anticancer activity of Ru- and Os(arene) compounds of a maleimide-functionalized bioactive pyridinecarbothioamide ligand

With the aim of increasing the accumulation of Ru anticancer agents in the tumor, a targeted delivery strategy based on a maleimide anchor for the biological vector human serum albumin (HSA) was developed. A group of piano stool Ru- and Os(η⁶-arene) complexes carrying a maleimide-functionalized N-phenyl-2-pyridinecarbothioamide (PCA) ligand was designed allowing for covalent conjugation to biological thiols. The complexes were characterized by NMR spectroscopy, ESI-MS, elemental analysis and single-crystal X-ray diffraction analysis. The compounds were shown to undergo halido/aqua ligand exchange reactions in aqueous solution, depending mainly on the metal center and the nature of the halide. In vitro cytotoxicity studies revealed low potency which is explained by the observed high reactivity of the maleimide to the thiol of l-cysteine (Cys), while the metal center itself shows little affinity to amino acids of the model protein lysozyme.

A new method for the rapid analysis of 1H-Pyrrole-2,5-diones (maleimides) in environmental samples by two-dimensional gas chromatography time-of-flight mass spectrometry

Maleimides (1H-Pyrrole-2,5-diones) are monopyrrolic pigment derivatives with specific alkyl side chains that can be directly linked to their tetrapyrrole precursors, most notably chlorophylls and bacteriochlorophylls. These compounds can be used as palaeoenvironmental indicators such as algal productivity and redox conditions in ancient and modern aquatic systems. Here, we present a new method using two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-ToF-MS), which enables the rapid analysis of maleimides in complex mixtures and different matrices (e.g. sediments and soils), therefore largely simplifying the previous intricate maleimide purification protocol. This method also reduces the potential for bias associated with partial losses due to low recovery and the high volatility of maleimides. The maleimide distributions and concentrations obtained by GC × GC-ToF-MS were reproducible and in agreement with the previously used purification procedure followed by analysis with traditional gas chromatography-mass spectrometry (GC-MS). The new method also resolved previously unrecognised, partial coelution of some maleimides with unknown compounds by quantification with the m/z 75 fragment ion. Furthermore, the higher sensitivity enabled the detection of previously unrecognised and preliminarily identified maleimides based on their relative retention times. The new, easier, rapid and more sensitive GC×GC-ToF-MS method greatly facilitates the analysis of maleimides in environmental samples to study tetrapyrrole degradation processes and will further the development of maleimides as biomarkers for palaeoenvironmental reconstructions.

Stabilization of cysteine-linked antibody drug conjugates with N-aryl maleimides

Maleimides are often used to covalently attach drugs to cysteine thiols for production of antibody-drug conjugates (ADCs). However, ADCs formed with traditional N-alkyl maleimides have variable stability in the bloodstream leading to loss of drug. Here, we report that N-aryl maleimides form stable antibody conjugates under very mild conditions while also maintaining high conjugation efficiency. Thiol-maleimide coupling and ADC stabilization via thiosuccinimide hydrolysis were accelerated by addition of N-phenyl or N-fluorophenyl groups to the ring-head nitrogen. Cysteine-linked ADCs prepared with N-aryl maleimides exhibited less than 20% deconjugation in both thiol-containing buffer and serum when incubated at 37 °C over a period of 7 days, whereas the analogous ADCs prepared with N-alkyl maleimides showed 35-67% deconjugation under the same conditions. ADCs prepared with the anticancer drug N-phenyl maleimide monomethyl-auristatin-E (MMAE) maintained high cytotoxicity following long-term exposure to serum whereas the N-alkyl maleimide MMAE ADC lost potency over time. These data demonstrate that N-aryl maleimides are a convenient and flexible platform to improve the stability of ADCs through manipulation of functional groups attached to the maleimide ring-head nitrogen.

Synthesis, biological evaluation and docking study of maleimide derivatives bearing benzenesulfonamide as selective COX-2 inhibitors and anti-inflammatory agents

A series of maleimide analogs bearing benzenesulfonamide were synthesized (4a-r). The anti-inflammatory activity of synthesized derivatives was evaluated using carrageenan induced rat paw edema model. COX-1 and COX-2 potency was evaluated through in vitro cyclooxygenase assays. The results revealed that, compounds 4a, 4h, 4 j, 4 k and 4r had potent COX-2 percentage inhibition as well as in vivo anti-inflammatory activity. The potent compound 4 j was docked into the COX-2 active site to determine the probable binding model. The results of in vivo and in vitro studies demonstrate that phenyl ring with electron withdrawing groups on maleimide ring would generate more potent anti-inflammatory agents. Thus, these compounds can serve as potential leads for further anti-inflammatory studies.

Micropattern-guided assembly of overlapping pairs of dynamic microtubules

Interactions between antiparallel microtubules are essential for the organization of spindles in dividing cells. The ability to form immobilized antiparallel microtubule pairs in vitro, combined with the ability to image them via TIRF microscopy, permits detailed biochemical characterization of microtubule cross-linking proteins and their effects on microtubule dynamics. Here, we describe methods for chemical micropatterning of microtubule seeds on glass surfaces in configurations that specifically promote the formation of antiparallel microtubule overlaps in vitro. We demonstrate that this assay is especially well suited for reconstitution of minimal midzone overlaps stabilized by the antiparallel microtubule cross-linking protein PRC1 and its binding partners. The micropatterning method is suitable for use with a broad range of proteins, and the assay is generally applicable to any microtubule cross-linking protein.