Efficient Tandem Copper-Catalyzed Click Synthesis of Multisugar-Modified Oligonucleotides

Nucleic acids in the form of siRNA, antisense oligonucleotides or mRNA are currently explored as new promising modalities in the pharmaceutical industry. Particularly, the success of mRNA-vaccines against SARS-CoV-2, along with the successful development of the first sugar-modified siRNA therapeutics has inspired the field. The development of nucleic acid therapeutics requires efficient chemistry to link oligonucleotides to chemical structures that can improve stability, boost cellular uptake, or enable specific targeting. For the siRNA therapeutics currently in use, modification of the 3'-end of the oligonucleotides with triple-N-acetylgalactosamine (GalNAc)3 was shown to be of significance. This modification is currently achieved through cumbersome multistep synthesis and subsequent loading onto the solid support material. Herein, we report the development of a bifunctional click-reactive linker that allows the modification of oligonucleotides in a tandem click reaction with multiple sugars, regardless of the position within the oligonucleotide, with remarkable efficiency and in a one-pot reaction.

Beyond Lipids: Exploring Advances in Polymeric Gene Delivery in the Lipid Nanoparticles Era

The recent success of gene therapy during the COVID-19 pandemic has underscored the importance of effective and safe delivery systems. Complementing lipid-based delivery systems, polymers present a promising alternative for gene delivery. Significant advances have been made in the recent past, with multiple clinical trials progressing beyond phase I and several companies actively working on polymeric delivery systems which provides assurance that polymeric carriers can soon achieve clinical translation. The massive advantage of structural tunability and vast chemical space of polymers is being actively leveraged to mitigate shortcomings of traditional polycationic polymers and improve the translatability of delivery systems. Tailored polymeric approaches for diverse nucleic acids and for specific subcellular targets are now being designed to improve therapeutic efficacy. This review describes the recent advances in polymer design for improved gene delivery by polyplexes and covalent polymer-nucleic acid conjugates. The review also offers a brief note on novel computational techniques for improved polymer design. The review concludes with an overview of the current state of polymeric gene therapies in the clinic as well as future directions on their translation to the clinic.

Efficient one-pot assembly of higher-order DNA nanostructures by chemically conjugated branched DNA

Chemically conjugated branched DNA was successfully synthesized by a copper-free click reaction to construct sophisticated and higher-order polyhedral DNA nanostructures with pre-defined units in one pot, which can be used as an efficient nanoplatform to precisely organize multiple gold nanoparticles in predesigned patterns.

Chemically Conjugated Branched Staples for Super-DNA Origami

DNA origami, comprising a long folded DNA scaffold and hundreds of linear DNA staple strands, has been developed to construct various sophisticated structures, smart devices, and drug delivery systems. However, the size and diversity of DNA origami are usually constrained by the length of DNA scaffolds themselves. Herein, we report a new paradigm of scaling up DNA origami assembly by introducing a novel branched staple concept. Owing to their covalent characteristics, the chemically conjugated branched DNA staples we describe here can be directly added to a typical DNA origami assembly system to obtain super-DNA origami with a predefined number of origami tiles in one pot. Compared with the traditional two-step coassembly system (yields <10%), a much greater yield (>80%) was achieved using this one-pot strategy. The diverse superhybrid DNA origami with the combination of different origami tiles can be also efficiently obtained by the hybrid branched staples. Furthermore, the branched staples can be successfully employed as the effective molecular glues to stabilize micrometer-scale, super-DNA origami arrays (e.g., 10 × 10 array of square origami) in high yields, paving the way to bridge the nanoscale precision of DNA origami with the micrometer-scale device engineering. This rationally developed assembly strategy for super-DNA origami based on chemically conjugated branched staples presents a new avenue for the development of multifunctional DNA origami-based materials.

Amphiphilic Sialic Acid Derivatives as Potential Dual-Specific Inhibitors of Influenza Hemagglutinin and Neuraminidase

In the shadow of SARS-CoV-2, influenza seems to be an innocent virus, although new zoonotic influenza viruses evolved by mutations may lead to severe pandemics. According to WHO, there is an urgent need for better antiviral drugs. Blocking viral hemagglutinin with multivalent N-acetylneuraminic acid derivatives is a promising approach to prevent influenza infection. Moreover, dual inhibition of both hemagglutinin and neuraminidase may result in a more powerful effect. Since both viral glycoproteins can bind to neuraminic acid, we have prepared three series of amphiphilic self-assembling 2-thio-neuraminic acid derivatives constituting aggregates in aqueous medium to take advantage of their multivalent effect. One of the series was prepared by the azide-alkyne click reaction, and the other two by the thio-click reaction to yield neuraminic acid derivatives containing lipophilic tails of different sizes and an enzymatically stable thioglycosidic bond. Two of the three bis-octyl derivatives produced proved to be active against influenza viruses, while all three octyl derivatives bound to hemagglutinin and neuraminidase from H1N1 and H3N2 influenza types.

Development of nucleic acid medicines based on chemical technology

Oligonucleotide-based therapeutics have attracted attention as an emerging modality that includes the modulation of genes and their binding proteins related to diseases, allowing us to take action on previously undruggable targets. Since the late 2010s, the number of oligonucleotide medicines approved for clinical uses has dramatically increased. Various chemistry-based technologies have been developed to improve the therapeutic properties of oligonucleotides, such as chemical modification, conjugation, and nanoparticle formation, which can increase nuclease resistance, enhance affinity and selectivity to target sites, suppress off-target effects, and improve pharmacokinetic properties. Similar strategies employing modified nucleobases and lipid nanoparticles have been used for developing coronavirus disease 2019 mRNA vaccines. In this review, we provide an overview of the development of chemistry-based technologies aimed at using nucleic acids for developing therapeutics over the past several decades, with a specific emphasis on the structural design and functionality of chemical modification strategies.

Treat the "Untreatable" by a Photothermal Agent: Triggering Heat and Immunological Responses for Rabies Virus Inactivation

Rabies is a fatal neurological zoonotic disease caused by the rabies virus (RABV), and the approved post-exposure prophylaxis (PEP) procedure remains unavailable in areas with inadequate medical systems. Although strategies have been proposed for PEP and postinfection treatment (PIT), because of the complexity of the treatment procedures and the limited curative outcome, developing an effective treatment strategy remains a holy grail in rabies research. Herein, a facile approach is proposed involving photothermal therapy (PTT) and photothermally triggered immunological effects to realize effective PEP and PIT simultaneously. The designed photothermal agent (N⁺ TT-mCB nanoparticles) featured positively charged functional groups and high photo-to-heat efficiency, which are favorable for virus targeting and inactivation. The level of the virus at the site of infection in mice is significantly decreased upon treatment with orthotopic PTT, and the transfer of the virus to the brain is significantly inhibited. Furthermore, the survival ratio of the mice three days postinfection is increased by intracranial injection of N⁺ TT-mCB and laser irradiation. Overall, this work provides a platform for the effective treatment of RABV and opens a new avenue for future antiviral studies.

A covalently conjugated branched DNA aptamer cluster-based nanoplatform for efficiently targeted drug delivery

Targeted delivery of therapeutic drugs is essential for precise treatment of various diseases to reduce possible serious side-effects. A screened DNA aptamer has been widely developed for active targeting delivery. Herein, we report a facile strategy for the construction of a branched DNA aptamer cluster-based nanoplatform for efficiently targeted drug delivery. In our design, the terminal-modified DNA aptamer can be covalently conjugated to form a branched aptamer cluster by click reaction easily. The branched aptamer cluster-modified DNA tetrahedron (TET) demonstrates highly targeted cellular uptake with the modification of only one site. After loading the chemotherapeutic drug (doxorubicin, DOX), the DNA aptamer cluster-based nanoplatform elicits a remarkable and selective inhibition of tumor cell proliferation by much-enhanced targeted delivery. This covalently conjugated branched DNA aptamer cluster-based nanoplatform provides a new strategy for the development of targeted drug delivery.

Chemically Modified DNA Nanostructures for Drug Delivery

Based on predictable, complementary base pairing, DNA can be artificially pre-designed into versatile DNA nanostructures of well-defined shapes and sizes. With excellent addressability and biocompatibility, DNA nanostructures have been widely employed in biomedical research, such as bio-sensing, bio-imaging, and drug delivery. With the development of the chemical biology of nucleic acid, chemically modified nucleic acids are also gradually developed to construct multifunctional DNA nanostructures. In this review, we summarize the recent progress in the construction and functionalization of chemically modified DNA nanostructures. Their applications in the delivery of chemotherapeutic drugs and nucleic acid drugs are highlighted. Furthermore, the remaining challenges and future prospects in drug delivery by chemically modified DNA nanostructures are discussed.

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With excellent addressability and biocompatibility, DNA nanostructures are promising candidates for bio-sensing, bio-imaging, and drug delivery

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The recent progress in chemical modifications of DNA nanostructures is summarized

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Chemically modified DNA nanostructures for efficient delivery of chemotherapeutic drugs and nucleic acid drugs are highlighted

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Challenges and prospects of future development toward chemically modified DNA nanostructures for drug delivery are discussed

Enhanced Immunostimulatory Activity of Covalent DNA Dendrons

The present study focused on the design and synthesis of covalent DNA dendrons bearing multivalent cytosine-phosphate-guanine oligodeoxynucleotides (CpG ODNs) that can stimulate the immune system through the activation of TLR9. These dendrons were synthesized using branching trebler phosphoramidite containing three identical protecting groups that enabled the simultaneous synthesis of multiple strands on a single molecule. Compared with linear ODNs, covalent DNA dendrons were found to be more resistant to nuclease degradation and were more efficiently taken up by macrophage-like RAW264.7 cells. Cellular uptake was suggested to be mediated by macrophage scavenger receptors. The covalent DNA dendrons composed of multivalent immunostimulatory branches enhanced the secretion of proinflammatory cytokines TNF-α and IL-6 from RAW264.7 cells, and 9-branched DNA dendrons showed the highest enhancement. Given their enhanced efficacy, we expect covalent DNA dendrons to be useful structures of oligonucleotide medicines.

Hierarchical Assembly of Super-DNA Origami Based on a Flexible and Covalent-Bound Branched DNA Structure

DNA origami technique provides a programmable way to construct nanostructures with arbitrary shapes. The dimension of assembled DNA origami, however, is usually limited by the length of the scaffold strand. Herein, we report a general strategy to efficiently organize multiple DNA origami tiles to form super-DNA origami using a flexible and covalent-bound branched DNA structure. In our design, the branched DNA structures (B_n: with a certain number of 2-6 branches) are synthesized by a copper-free click reaction. Equilateral triangular DNA origamis with different numbers of capture strands (T_n: T₁, T₂, and T₃) are constructed as the coassembly tiles. After hybridization with the branched DNA structures, the super-DNA origami (up to 13 tiles) can be efficiently ordered in the predesigned patterns. Compared with traditional DNA junctions (J_n: J₂-J₆, as control groups) assembled by base pairing between several DNA strands, a higher yield and more compact structures are obtained using our strategy. The highly ordered and discrete DNA origamis can further precisely organize gold nanoparticles into different patterns. This rationally developed DNA origami ordering strategy based on the flexible and covalent-bound branched DNA structure presents a new avenue for the construction of sophisticated DNA architectures with larger molecular weights.

Click and Bioorthogonal Chemistry: The Future of Active Targeting of Nanoparticles for Nanomedicines?

Over the years, click and bioorthogonal reactions have been the subject of considerable research efforts. These high-performance chemical reactions have been developed to meet requirements not often provided by the chemical reactions commonly used today in the biological environment, such as selectivity, rapid reaction rate, and biocompatibility. Click and bioorthogonal reactions have been attracting increasing attention in the biomedical field for the engineering of nanomedicines. In this review, we study a compilation of articles from 2014 to the present, using the terms "click chemistry and nanoparticles (NPs)" to highlight the application of this type of chemistry for applications involving NPs intended for biomedical applications. This study identifies the main strategies offered by click and bioorthogonal chemistry, with respect to passive and active targeting, for NP functionalization with specific and multiple properties for imaging and cancer therapy. In the final part, a novel and promising approach for "two step" targeting of NPs, called pretargeting (PT), is also discussed; the principle of this strategy as well as all the studies listed from 2014 to the present are presented in more detail.

Cellular Targeting of Oligonucleotides by Conjugation with Small Molecules

Drug candidates derived from oligonucleotides (ON) are receiving increased attention that is supported by the clinical approval of several ON drugs. Such therapeutic ON are designed to alter the expression levels of specific disease-related proteins, e.g., by displaying antigene, antisense, and RNA interference mechanisms. However, the high polarity of the polyanionic ON and their relatively rapid nuclease-mediated cleavage represent two major pharmacokinetic hurdles for their application in vivo. This has led to a range of non-natural modifications of ON structures that are routinely applied in the design of therapeutic ON. The polyanionic architecture of ON often hampers their penetration of target cells or tissues, and ON usually show no inherent specificity for certain cell types. These limitations can be overcome by conjugation of ON with molecular entities mediating cellular 'targeting', i.e., enhanced accumulation at and/or penetration of a specific cell type. In this context, the use of small molecules as targeting units appears particularly attractive and promising. This review provides an overview of advances in the emerging field of cellular targeting of ON via their conjugation with small-molecule targeting structures.

Transition-Metal-Mediated Modification of Biomolecules

The site-selective modification of biomolecules has grown spectacularly in recent years. The presence of a large number of functional groups in a biomolecule makes its chemo- and regioselective modification a challenging goal. In this context, transition-metal-mediated reactions are emerging as a powerful tool owing to their unique reactivity and good functional group compatibility, allowing highly efficient and selective bioconjugation reactions that operate under mild conditions. This Minireview focuses on the current state of organometallic chemistry for bioconjugation, highlighting the potential of transition metals for the development of chemoselective and site-specific methods for functionalization of peptides, proteins and nucleic acids. The importance of the selection of ligands attached to the transition metal for conferring the desired chemoselectivity will be highlighted.

Optimization of Inhibitory Peptides Targeting Phosphoprotein of Rabies Virus

Rabies is a serious zoonosis caused by rabies virus (RABV) of the genus Lyssavirus, and immunotherapy is now the only approved, effective method for post-exposure prophylaxis against rabies in humans, whereas an effective antiviral therapy is still unavailable if the central nervous system is invaded. Phosphoprotein (P) is known to play pivotal roles in the life cycle of RABV, and has been regarded as a prime target for inhibitors of viral replication. This study aimed to carry out intracellular administration of a kind of P-binding peptide for RABV inhibition. A group of reported P-binding peptides were focused on for activity improvement by quantitative structure–activity relationship (QSAR) method, and then were mediated by cell penetrating peptide (CPP) for intracellular activity evaluation. The QSAR models had good performance in reliability and predictability (R² ≥ 0.852, Q² ≥ 0.601, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Q}}_{\text{ext}}^{2}$$\end{document}Qext2 ≥ 0.595), and the peptide screened by partial least squares (PLS) QSAR model (R² = 0.994, Q² = 0.937, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Q}}_{\text{ext}}^{2}$$\end{document}Qext2 = 0.981) exhibited even higher antiviral activity when it was delivered into the cells by CPP. Above all, this study provided an effective way for development of peptide drug against RABV.

Dendrimers in the context of nanomedicine

Dendrimers are globular structures, presenting an initiator core, repetitive layers starting radially from the core and terminal groups on the surface, resembling tree architecture. These structures have been studied in many biological applications, as drug, DNA, RNA and proteins delivery, as well as imaging and radiocontrast agents. With reference to that, this review focused in providing examples of dendrimers used in nanomedicine. Although most studies emphasize cancer, there are others which reveal action in the neurosystem, reducing either neuroinflammation or protein aggregation. Dendrimers can carry bioactive compounds by covalent bond (dendrimer prodrug), or by ionic interaction or adsortion in the internal space of the nanostructure. Additionally, dendrimers can be associated with other polymers, as PEG (polyethylene glycol), and with targeting structures as aptamers, antibodies, folic acid and carbohydrates. Their products in preclinical/clinical trial and those in the market are also discussed, with a total of six derivatives in clinical trials and seven products available in the market.

Zwitterion-Functionalized Detonation Nanodiamond with Superior Protein Repulsion and Colloidal Stability in Physiological Media

Nanodiamond (ND) is a versatile and promising material for bioapplications. Despite many efforts, agglomeration of nanodiamond and the nonspecific adsorption of proteins on the ND surface when exposed to biofluids remains a major obstacle for biomedical applications. Here, the functionalization of detonation nanodiamond with zwitterionic moieties in combination with tetraethylene glycol (TEG) moieties immobilized by click chemistry to improve the colloidal dispersion in physiological media with strong ion background and for the simultaneous prevention of nonspecific interactions with proteins is reported. Based on five building blocks, a series of ND conjugates is synthesized and their performance is compared in biofluids, such as fetal bovine serum (FBS) and Dulbecco's modified Eagle medium (DMEM). The adsorption of proteins is investigated via dynamic light scattering (DLS) and thermogravimetric analysis. The colloidal stability is tested with DLS monitoring over prolonged periods of time in various ratios of water/FBS/DMEM and at different pH values. The results show that zwitterions efficiently promote the anti-fouling properties, whereas the TEG linker is essential for the enhanced colloidal stability of the particles.

Effect of PEGylated Magnetic PLGA-PEI Nanoparticles on Primary Hippocampal Neurons: Reduced Nanoneurotoxicity and Enhanced Transfection Efficiency with Magnetofection

Despite broad application of nanotechnology in neuroscience, the nanoneurotoxicity of magnetic nanoparticles in primary hippocampal neurons remains poorly characterized. In particular, understanding how magnetic nanoparticles perturb neuronal calcium homeostasis is critical when considering magnetic nanoparticles as a nonviral vector for effective gene therapy in neuronal diseases. Here, we address the pressing need to systematically investigate the neurotoxicity of magnetic nanoparticles with different surface charges in primary hippocampal neurons. We found that unlike negative and neutral nanoparticles, positively charged magnetic nanoparticles (magnetic poly(lactic-co-glycolic acid) (PLGA)-polyethylenimine (PEI) nanoparticles, MNP-PLGA-PEI NPs) rapidly elevated cytoplasmic calcium levels in primary hippocampal neurons, mainly via extracellular calcium influx regulated by voltage-gated calcium channels. We went on to show that this perturbation of intracellular calcium homeostasis elicited serious cytotoxicity in primary hippocampal neurons. However, our next experiment demonstrated that PEGylation on the surface of MNP-PLGA-PEI NPs shielded the surface charge, thereby preventing the perturbation of intracellular calcium homeostasis. That is, PEGylated MNP-PLGA-PEI NPs reduced nanoneurotoxicity. Importantly, biocompatible PEGylated MNP-PLGA-PEI NPs under an external magnetic field enhanced transfection efficiency (>7%) of plasmid DNA encoding GFP in primary hippocampal neurons compared to NPs without external magnetic field mediation. Moreover, under an external magnetic field, this system achieved gene transfection in the hippocampus of the C57 mouse. Overall, this study is the first to successfully employ biocompatible PEGylated MNP-PLGA-PEI NPs for transfection using a magnetofection strategy in primary hippocampal neurons, thereby providing a nanoplatform as a new perspective for treating neuronal diseases or modulating neuron activities.

Efficient siRNA-peptide conjugation for specific targeted delivery into tumor cells

Despite the broad applicability of the Huisgen cycloaddition reaction, the click functionalization of RNAs with peptides still remains a challenge. Here we describe a straightforward method for the click functionalization of siRNAs with peptides of different sizes and complexities. Among them, a promising peptide carrier for the selective siRNA delivery into HER2+ breast cancer cell lines has been reported.

Library siRNA-generating RNA nanosponges for gene silencing by complementary rolling circle transcription

Since the discovery of RNA interference (RNAi), small interfering RNA (siRNA) has been powerful tools for gene downregulation in biomedical applications. Despite the outstanding efficacy of siRNA, the development of a therapeutic delivery system remains a challenge owing to the instability of RNA. In this study, we describe a new method for the design of siRNA-generating nanosponges by using complementary rolling circle transcription (cRCT), a technique that requires two complementary circular DNA. The sequences of one of the circular DNA are designed to have complete complementarity to the target mRNA resulting in double stranded RNA (dsRNA) that can be digested to siRNA by cellular Dicer activity. This siRNA design, called 'library siRNA', could be universally applied to fabricate RNA nanosponges targeting any known mRNA sequence.

Preparation and In Vitro Photodynamic Activity of Glucosylated Zinc(II) Phthalocyanines as Underlying Targeting Photosensitizers

Two novel glucosylated zinc(II) phthalocyanines 7a–7b, as well as the acetyl-protected counterparts 6a–6b, have been synthesized by the Cu(I)-catalyzed 1,3-dipolar cycloaddition between the propargylated phthalocyanine and azide-substituted glucoses. All of these phthalocyanines were characterized with various spectroscopic methods and studied for their photo-physical, photo-chemical, and photo-biological properties. With glucose as the targeting unit, phthalocyanines 7a–7b exhibit a specific affinity to MCF-7 breast cancer cells over human embryonic lung fibroblast (HELF) cells, showing higher cellular uptake. Upon illumination, both photosensitizers show high cytotoxicity with IC₅₀ as low as 0.032 µM toward MCF-7 cells, which are attributed to their high cellular uptake and low aggregation tendency in the biological media, promoting the generation of intracellular reactive oxygen species (ROS). Confocal laser fluorescence microscopic studies have also revealed that they have high and selective affinities to the lysosomes, but not the mitochondria, of MCF-7 cells. The results show that these two glucosylated zinc(II) phthalocyanines are potential anticancer agents for targeting photodynamic therapy.

SiRNA Crosslinked Nanoparticles for the Treatment of Inflammation-induced Liver Injury

RNA interference mediated by small interfering RNA (siRNA) provides a powerful tool for gene regulation, and has a broad potential as a promising therapeutic strategy. However, therapeutics based on siRNA have had limited clinical success due to their undesirable pharmacokinetic properties. This study presents pH-sensitive nanoparticles-based siRNA delivery systems (PNSDS), which are positive-charge-free nanocarriers, composed of siRNA chemically crosslinked with multi-armed poly(ethylene glycol) carriers via acid-labile acetal linkers. The unique siRNA crosslinked structure of PNSDS allows it to have minimal cytotoxicity, high siRNA loading efficiency, and a stimulus-responsive property that enables the selective intracellular release of siRNA in response to pH conditions. This study demonstrates that PNSDS can deliver tumor necrosis factor alpha (TNF-α) siRNA into macrophages and induce the efficient down regulation of the targeted gene in complete cell culture media. Moreover, PNSDS with mannose targeting moieties can selectively accumulate in mice liver, induce specific inhibition of macrophage TNF-α expression in vivo, and consequently protect mice from inflammation-induced liver damages. Therefore, this novel siRNA delivering platform would greatly improve the therapeutic potential of RNAi based therapies.

Synthesis of Poly Linear shRNA Expression Cassettes Through Branch-PCR

A facile and universal strategy to construct the poly linear small hairpin RNA (shRNA) expression cassettes with multiple shRNA transcription templates through polymerase chain reaction with flexible branched primers (branch-PCR) is described in this protocol. Double-stranded RNA (dsRNA) is not stable enough for the study of RNA interference (RNAi) delivery in mammalian cells. Therefore, the more stable shRNA transcription template is employed to produce the endogenous transcribed dsRNA. Then, the covalent crosslinked linear shRNA expression cassettes are constructed through the branch-PCR for the long-lasting RNAi effect in this protocol. The branched primer pair is efficiently synthesized through classic click chemistry. In one step of PCR, the much more stable poly linear shRNA expression cassettes can be produced in large scale. This strategy of efficient synthesis of the poly linear gene expression cassettes can also be applied in the field for other target gene delivery. © 2016 by John Wiley & Sons, Inc.

Agglutination of bacteria using polyvalent nanoparticles of aggregation-induced emissive thiophthalonitrile dyes

A novel class of aggregation-induced emissive bis(phenylthio)phthalonitrile dyes were synthesized. These dyes assembled into nanoparticles that were equipped with mannose units. The nanoparticles underwent selective interactions with lectins and bacteria. The bright fluorescent aggregates aid in the visualization of the agglutination of bacteria.

How to Tackle the Challenge of siRNA Delivery with Sequence-Defined Oligoamino Amides

RNA interference (RNAi) as a mechanism of gene regulation provides exciting opportunities for medical applications. Synthetic small interfering RNA (siRNA) triggers the knockdown of complementary mRNA sequences in a catalytic fashion and has to be delivered into the cytosol of the targeted cells. The design of adequate carrier systems to overcome multiple extracellular and intracellular roadblocks within the delivery process has utmost importance. Cationic polymers form polyplexes through electrostatic interaction with negatively charged nucleic acids and present a promising class of carriers. Issues of polycations regarding toxicity, heterogeneity, and polydispersity can be overcome by solid-phase-assisted synthesis of sequence-defined cationic oligomers. These medium-sized highly versatile nucleic acid carriers display low cytotoxicity and can be modified and tailored in multiple ways to meet specific requirements of nucleic acid binding, polyplex size, shielding, targeting, and intracellular release of the cargo. In this way, sequence-defined cationic oligomers can mimic the dynamic and bioresponsive behavior of viruses.

Conjugates of small targeting molecules to non-viral vectors for the mediation of siRNA

To use siRNA (small interfering RNA) for gene therapy, a gene delivery system is often necessary to overcome several challenging requirements including rapid excretion, low stability in blood serum, non-specific accumulation in tissues, poor cellular uptake and inefficient intracellular release. Active and/or passive targeting should help the delivery system to reach the desired tissue or cell, to be internalized, and to deliver siRNA to the cytoplasm so that siRNA can inhibit protein synthesis. This review covers conjugates of small targeting molecules and non-viral delivery systems for the mediation of siRNA, with a focus on their transfection properties in order to help the development of new and efficient siRNA delivery systems, as the therapeutic solutions of tomorrow.

STATEMENT OF SIGNIFICANCE

The delivery of siRNA into cells or tissues remains to be a challenge for its applications, an alternative strategy for siRNA delivery systems is direct conjugation of non-viral vectors with targeting moieties for cellular delivery. In comparison to macromolecules, small targeting molecules have attracted great attention due to their many potential advantages including significant simplicity and ease of production, good repeatability and biodegradability. This review will focus on the most recent advances in the delivery of siRNA using conjugates of small targeting molecules and non-viral delivery systems. Based the editor's suggestions, we hope the revised manuscript could provide more profound understanding to the conjugates of targeting molecules to vectors for mediation of siRNA.

Genetically designed biomolecular capping system for mesoporous silica nanoparticles enables receptor-mediated cell uptake and controlled drug release

Effective and controlled drug delivery systems with on-demand release and targeting abilities have received enormous attention for biomedical applications. Here, we describe a novel enzyme-based cap system for mesoporous silica nanoparticles (MSNs) that is directly combined with a targeting ligand via bio-orthogonal click chemistry. The capping system is based on the pH-responsive binding of an aryl-sulfonamide-functionalized MSN and the enzyme carbonic anhydrase (CA). An unnatural amino acid (UAA) containing a norbornene moiety was genetically incorporated into CA. This UAA allowed for the site-specific bio-orthogonal attachment of even very sensitive targeting ligands such as folic acid and anandamide. This leads to specific receptor-mediated cell and stem cell uptake. We demonstrate the successful delivery and release of the chemotherapeutic agent Actinomycin D to KB cells. This novel nanocarrier concept provides a promising platform for the development of precisely controllable and highly modular theranostic systems.

Branch-PCR Constructed Stable shRNA Transcription Nanoparticles Have Long-Lasting RNAi Effect

RNA interference (RNAi) is a cellular process for gene silencing. Because of poor serum stability, transferring dsRNA directly into the target cells is a challenge. We report a facile and universal strategy to construct short hairpin RNA (shRNA) transcription nanoparticles with multiple shRNA transcription templates by PCR with flexible branched primers (branch-PCR). Compared with conventional linear shRNA transcription templates, these shRNA transcription nanoparticles show excellent stability against digestion by exonuclease III. Importantly, we found that our highly stable shRNA transcription nanoparticles can also be transcribed and thus induce efficient and long-lasting RNAi with picomolar activity in living mammalian cells. These chemically well-defined branch-PCR-generated stable shRNA transcription nanoparticles might facilitate RNAi delivery with a long-lasting RNAi effects.

DNA as Tunable Adaptor for siRNA Polyplex Stabilization and Functionalization

siRNA and microRNA are promising therapeutic agents, which are engaged in a natural mechanism called RNA interference that modulates gene expression posttranscriptionally. For intracellular delivery of such nucleic acid triggers, we use sequence-defined cationic polymers manufactured through solid phase chemistry. They consist of an oligoethanamino amide core for siRNA complexation and optional domains for nanoparticle shielding and cell targeting. Due to the small size of siRNA, electrostatic complexes with polycations are less stable, and consequently intracellular delivery is less efficient. Here we use DNA oligomers as adaptors to increase size and charge of cargo siRNA, resulting in increased polyplex stability, which in turn boosts transfection efficiency. Extending a single siRNA with a 181-nucleotide DNA adaptor is sufficient to provide maximum gene silencing aided by cationic polymers. Interestingly, this simple strategy was far more effective than merging defined numbers (4–10) of siRNA units into one DNA scaffolded construct. For DNA attachment, the 3′ end of the siRNA passenger strand was beneficial over the 5′ end. The impact of the attachment site however was resolved by introducing bioreducible disulfides at the connection point. We also show that DNA adaptors provide the opportunity to readily link additional functional domains to siRNA. Exemplified by the covalent conjugation of the endosomolytic influenza peptide INF-7 to siRNA via a DNA backbone strand and complexing this construct with a targeting polymer, we could form a highly functional polyethylene glycol–shielded polyplex to downregulate a luciferase gene in folate receptor–positive cells.

Recent advances in click chemistry applied to dendrimer synthesis

Dendrimers are monodisperse polymers grown in a fractal manner from a central point. They are poised to become the cornerstone of nanoscale devices in several fields, ranging from biomedicine to light-harvesting. Technical difficulties in obtaining these molecules has slowed their transfer from academia to industry. In 2001, the arrival of the "click chemistry" concept gave the field a major boost. The flagship reaction, a modified Hüisgen cycloaddition, allowed researchers greater freedom in designing and building dendrimers. In the last five years, advances in click chemistry saw a wider use of other click reactions and a notable increase in the complexity of the reported structures. This review covers key developments in the click chemistry field applied to dendrimer synthesis from 2010 to 2015. Even though this is an expert review, basic notions and references have been included to help newcomers to the field.