Ligands of the asialoglycoprotein receptor for targeted gene delivery, part 1: Synthesis of and binding studies with biotinylated cluster glycosides containing N-acetylgalactosamine

Glycomimetics for the inhibition and modulation of lectins

Carbohydrates are essential mediators of many processes in health and disease. They regulate self-/non-self- discrimination, are key elements of cellular communication, cancer, infection and inflammation, and determine protein folding, function and life-times. Moreover, they are integral to the cellular envelope for microorganisms and participate in biofilm formation. These diverse functions of carbohydrates are mediated by carbohydrate-binding proteins, lectins, and the more the knowledge about the biology of these proteins is advancing, the more interfering with carbohydrate recognition becomes a viable option for the development of novel therapeutics. In this respect, small molecules mimicking this recognition process become more and more available either as tools for fostering our basic understanding of glycobiology or as therapeutics. In this review, we outline the general design principles of glycomimetic inhibitors (Section 2). This section is then followed by highlighting three approaches to interfere with lectin function, i.e. with carbohydrate-derived glycomimetics (Section 3.1), novel glycomimetic scaffolds (Section 3.2) and allosteric modulators (Section 3.3). We summarize recent advances in design and application of glycomimetics for various classes of lectins of mammalian, viral and bacterial origin. Besides highlighting design principles in general, we showcase defined cases in which glycomimetics have been advanced to clinical trials or marketed. Additionally, emerging applications of glycomimetics for targeted protein degradation and targeted delivery purposes are reviewed in Section 4.

Combination treatment of mannose and GalNAc conjugated small interfering RNA protects against lethal Marburg virus infection

Marburg virus (MARV) infection results in severe viral hemorrhagic fever with mortalities up to 90%, and there is a pressing need for effective therapies. Here, we established a small interfering RNA (siRNA) conjugate platform that enabled successful subcutaneous delivery of siRNAs targeting the MARV nucleoprotein. We identified a hexavalent mannose ligand with high affinity to macrophages and dendritic cells, which are key cellular targets of MARV infection. This ligand enabled successful siRNA conjugate delivery to macrophages both in vitro and in vivo. The delivered hexa-mannose-siRNA conjugates rendered substantial target gene silencing in macrophages when supported by a mannose functionalized endosome release polymer. This hexa-mannose-siRNA conjugate was further evaluated alongside our hepatocyte-targeting GalNAc-siRNA conjugate, to expand targeting of infected liver cells. In MARV-Angola-infected guinea pigs, these platforms offered limited survival benefit when used as individual agents. However, in combination, they achieved up to 100% protection when dosed 24 h post infection. This novel approach, using two different ligands to simultaneously deliver siRNA to multiple cell types relevant to infection, provides a convenient subcutaneous route of administration for treating infection by these dangerous pathogens. The mannose conjugate platform has potential application to other diseases involving macrophages and dendritic cells.

Novel derivatives of arabinogalactan, pullulan & lactobionic acid for targeting asialoglycoprotein receptor: Biomolecular interaction, synthesis & evaluation

Targeted-drug administration to liver reduces side effects by minimising drug distribution to non-target organs and increases therapeutic efficacy by boosting drug concentration in target cells. In this study, arabinogalactan-(AG), pullulan-(PL) and lactobionic acid-(LA) were selected as natural ligands to target asialoglycoprotein receptor-(ASGPR-1) present on hepatocytes. In silico docking studies were performed and binding affinities of novel ligands viz. palmitoylated AG-(PAG), lauroylated AG-(LAG), palmitoylated PL-(PPL), lauroylated PL-(LPL) and lactobionic acid-adipic acid dihydrazide conjugate-(LAD) were compared with AG, PL and LA. These novel ligands were successfully synthesized and characterized. The ligands were incorporated into drug loaded nanostructured lipid carriers-(NLCs) for surface functionalization. HepG2 cellular internalization of hepatocyte-targeted NLCs was studied using fluorescence microscopy and LAD-decorated-drug loaded NLCs giving maximum cellular uptake were studied using confocal microscopy. Toxicity potential of LAD-decorated NLCs was assessed in vivo. Molecular docking results suggested that among the ligands, order of binding affinity was found to be LAD>PAG > PPL > LPL > LAG. Acute toxicity studies revealed hemocompatibility and absence of organ toxicity for ligand LAD. Additionally, the results establish proof-of-concept of enhanced targeting efficacy of novel ASGPR targeting ligands. These ligands can be used for surface modification of nanocarriers for future targeted delivery in treating various liver disorders.

Mega-High-Throughput Screening Platform for the Discovery of Biologically Relevant Sequence-Defined Non-Natural Polymers

Combinatorial methods enable the synthesis of chemical libraries on scales of millions to billions of compounds, but the ability to efficiently screen and sequence such large libraries has remained a major bottleneck for molecular discovery. We developed a novel technology for screening and sequencing libraries of synthetic molecules of up to a billion compounds in size. This platform utilizes the fiber-optic array scanning technology (FAST) to screen bead-based libraries of synthetic compounds at a rate of 5 million compounds per minute (∼83 000 Hz). This ultra-high-throughput screening platform has been used to screen libraries of synthetic "self-readable" non-natural polymers that can be sequenced at the femtomole scale by chemical fragmentation and high-resolution mass spectrometry. The versatility and throughput of the platform were demonstrated by screening two libraries of non-natural polyamide polymers with sizes of 1.77M and 1B compounds against the protein targets K-Ras, asialoglycoprotein receptor 1 (ASGPR), IL-6, IL-6 receptor (IL-6R), and TNFα. Hits with low nanomolar binding affinities were found against all targets, including competitive inhibitors of K-Ras binding to Raf and functionally active uptake ligands for ASGPR facilitating intracellular delivery of a nonglycan ligand.

Ligand conjugate SAR and enhanced delivery in NHP

N-Acetylgalactosamine (GalNAc) conjugated short interfering RNAs (siRNAs) are a leading RNA interference (RNAi) platform allowing targeted inhibition of disease-causing genes in hepatocytes. More than a decade of development has recently resulted in the first approvals for this class of drugs. While substantial effort has been made to improve nucleic acid modification patterns for better payload stability and efficacy, relatively little attention has been given to the GalNAc targeting ligand. In addition, the lack of an intrinsic endosomal release mechanism has limited potency. Here, we report a stepwise analysis of the structure activity relationships (SAR) of the components comprising these targeting ligands. We show that there is relatively little difference in biological performance between bi-, tri-, and tetravalent ligand structures while identifying other features that affect their biological activity more significantly. Further, we demonstrate that subcutaneous co-administration of a GalNAc-functionalized, pH responsive endosomal release agent markedly improved the activity and duration of effect for siRNA conjugates, without compromising tolerability, in non-human primates. These findings could address a significant bottleneck for future siRNA ligand conjugate development.

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.

Lectin recognition and hepatocyte endocytosis of GalNAc-decorated nanostructured lipid carriers

Liver is the main organ for metabolism but is also subject to various pathologies, from viral, genetic, cancer or metabolic origin. There is thus a crucial need to develop efficient liver-targeted drug delivery strategies. Asialoglycoprotein receptor (ASGPR) is a C-type lectin expressed in the hepatocyte plasma membrane that efficiently endocytoses glycoproteins exposing galactose (Gal) or N-acetylgalactosamine (GalNAc). Its targeting has been successfully used to drive the uptake of small molecules decorated with three or four GalNAc, thanks to an optimisation of their spatial arrangement. Herein, we assessed the biological properties of highly stable nanostructured lipid carriers (NLC) made of FDA-approved ingredients and formulated with increasing amounts of GalNAc. Cellular studies showed that a high density of GalNAc was required to favour hepatocyte internalisation via the ASGPR pathway. Interaction studies using surface plasmon resonance and the macrophage galactose-lectin as GalNAc-recognising lectin confirmed the need of high GalNAc density for specific recognition of these NLC. This work is the first step for the development of efficient nanocarriers for prolonged liver delivery of active compounds.

Therapeutic antisense oligonucleotides for movement disorders

Movement disorders are a group of neurological conditions characterized by abnormalities of movement and posture. They are broadly divided into akinetic and hyperkinetic syndromes. Until now, no effective symptomatic or disease-modifying therapies have been available. However, since many of these disorders are monogenic or have some well-defined genetic component, they represent strong candidates for antisense oligonucleotide (ASO) therapies. ASO therapies are based on the use of short synthetic single-stranded ASOs that bind to disease-related target RNAs via Watson-Crick base-pairing and pleiotropically modulate their function. With information arising from the RNA sequence alone, it is possible to design ASOs that not only alter the expression levels but also the splicing defects of any protein, far exceeding the intervention repertoire of traditional small molecule approaches. Following the regulatory approval of ASO therapies for spinal muscular atrophy and Duchenne muscular dystrophy in 2016, there has been tremendous momentum in testing such therapies for other neurological disorders. This review article initially focuses on the chemical modifications aimed at improving ASO effectiveness, the mechanisms by which ASOs can interfere with RNA function, delivery systems and pharmacokinetics, and the common set of toxicities associated with their application. It, then, describes the pathophysiology and the latest information on preclinical and clinical trials utilizing ASOs for the treatment of Parkinson's disease, Huntington's disease, and ataxias 1, 2, 3, and 7. It concludes with issues that require special attention to realize the full potential of ASO-based therapies.

ASGR1 and Its Enigmatic Relative, CLEC10A

The large family of C-type lectin (CLEC) receptors comprises carbohydrate-binding proteins that require Ca²⁺ to bind a ligand. The prototypic receptor is the asialoglycoprotein receptor-1 (ASGR1, CLEC4H1) that is expressed primarily by hepatocytes. The early work on ASGR1, which is highly specific for N-acetylgalactosamine (GalNAc), established the foundation for understanding the overall function of CLEC receptors. Cells of the immune system generally express more than one CLEC receptor that serve diverse functions such as pathogen-recognition, initiation of cellular signaling, cellular adhesion, glycoprotein turnover, inflammation and immune responses. The receptor CLEC10A (C-type lectin domain family 10 member A, CD301; also called the macrophage galactose-type lectin, MGL) contains a carbohydrate-recognition domain (CRD) that is homologous to the CRD of ASGR1, and thus, is also specific for GalNAc. CLEC10A is most highly expressed on immature DCs, monocyte-derived DCs, and alternatively activated macrophages (subtype M2a) as well as oocytes and progenitor cells at several stages of embryonic development. This receptor is involved in initiation of T_H1, T_H2, and T_H17 immune responses and induction of tolerance in naïve T cells. Ligand-mediated endocytosis of CLEC receptors initiates a Ca²⁺ signal that interestingly has different outcomes depending on ligand properties, concentration, and frequency of administration. This review summarizes studies that have been carried out on these receptors.

Asymmetrically Branched Precision Glycooligomers Targeting Langerin

Asymmetrically branched precision glycooligomers are synthesized by solid-phase polymer synthesis for studying multivalent carbohydrate-protein interactions. Through the stepwise assembly of Fmoc-protected oligo(amidoamine) building blocks and Fmoc/Dde-protected lysine, straightforward variation of structural parameters such as the number and length of arms, as well as the number and position of carbohydrate ligands, is achieved. Binding of 1-arm and 3-arm glycooligomers toward lectin receptors langerin and concanavalin A (ConA) was evaluated where the smallest 3-arm glycooligomer shows the highest binding toward langerin, and stepwise elongation of one, two, or all three arms leads to decreased binding. When directly comparing binding toward langerin and ConA, we find that structural variation of the scaffold affects glycomimetic ligand binding differently for the different targets, indicating the potential to tune such ligands not only for their avidity but also for their selectivity toward different lectins.

Therapeutic Oligonucleotides: State of the Art

Oligonucleotides (ONs) can interfere with biomolecules representing the entire extended central dogma. Antisense gapmer, steric block, splice-switching ONs, and short interfering RNA drugs have been successfully developed. Moreover, antagomirs (antimicroRNAs), microRNA mimics, aptamers, DNA decoys, DNAzymes, synthetic guide strands for CRISPR/Cas, and innate immunity-stimulating ONs are all in clinical trials. DNA-targeting, triplex-forming ONs and strand-invading ONs have made their mark on drug development research, but not yet as medicines. Both design and synthetic nucleic acid chemistry are crucial for achieving biologically active ONs. The dominating modifications are phosphorothioate linkages, base methylation, and numerous 2'-substitutions in the furanose ring, such as 2'-fluoro, O-methyl, or methoxyethyl. Locked nucleic acid and constrained ethyl, a related variant, are bridged forms where the 2'-oxygen connects to the 4'-carbon in the sugar. Phosphorodiamidate morpholino oligomers, carrying a modified heterocyclic backbone ring, have also been commercialized. Delivery remains a major obstacle, but systemic administration and intrathecal infusion are used for treatment of the liver and brain, respectively.

Novel Cluster and Monomer-Based GalNAc Structures Induce Effective Uptake of siRNAs in Vitro and in Vivo

GalNAc conjugation is emerging as a dominant strategy for delivery of therapeutic oligonucleotides to hepatocytes. The structure and valency of the GalNAc ligand contributes to the potency of the conjugates. Here we present a panel of multivalent GalNAc variants using two different synthetic strategies. Specifically, we present a novel conjugate based on a support-bound trivalent GalNAc cluster, and four others using a GalNAc phosphoramidite monomer that was readily assembled into tri- or tetravalent designs during solid phase oligonucleotide synthesis. We compared these compounds to a clinically used trivalent GalNAc cluster both in vitro and in vivo. In vitro, cluster-based and phosphoramidite-based scaffolds show a similar rate of internalization in primary hepatocytes, with membrane binding observed as early as 5 min. All tested compounds provided potent, dose-dependent silencing, with 2-4% of injected dose recoverable from liver after 1 week. The two preassembled trivalent GalNAc clusters showed higher tissue accumulation and gene silencing relative to di-, tri-, or tetravalent GalNAc conjugates assembled via phosphoramidite chemistry.

Synthesis and Excellent Duplex Stability of Oligonucleotides Containing 2'-Amino-LNA Functionalized with Galactose Units

A convenient method for the preparation of oligonucleotides containing internally-attached galactose and triantennary galactose units has been developed based on click chemistry between 2′-N-alkyne 2′-amino-LNA nucleosides and azido-functionalized galactosyl building blocks. The synthesized oligonucleotides show excellent binding affinity and selectivity towards complementary DNA/RNA strands with an increase in the melting temperature of up to +23.5 °C for triply-modified variants.

Synthesis and Biophysical Investigations of Oligonucleotides Containing Galactose-Modified DNA, LNA, and 2'-Amino-LNA Monomers

Galactose-modified thymidine, LNA-T, and 2'-amino-LNA-T nucleosides were synthesized, converted into the corresponding phosphoramidite derivatives and introduced into short oligonucleotides. Compared to the unmodified control strands, the galactose-modified oligonucleotides in general, and the N2'-functionalized 2'-amino-LNA derivatives in particular, showed improved duplex thermal stability against DNA and RNA complements and increased ability to discriminate mismatches. In addition, the 2'-amino-LNA-T derivatives induced remarkable 3'-exonuclease resistance. These results were further investigated using molecular modeling studies.

Oligonucleotide Therapies: The Past and the Present

In this review we address the development of oligonucleotide (ON) medicines from a historical perspective by listing the landmark discoveries in this field. The various biological processes that have been targeted and the corresponding ON interventions found in the literature are discussed together with brief updates on some of the more recent developments. Most ON therapies act through antisense mechanisms and are directed against various RNA species, as exemplified by gapmers, steric block ONs, antagomirs, small interfering RNAs (siRNAs), micro-RNA mimics, and splice switching ONs. However, ONs binding to Toll-like receptors and those forming aptamers have completely different modes of action. Similar to other novel medicines, the path to success has been lined with numerous failures, where different therapeutic ONs did not stand the test of time. Since the first ON drug was approved for clinical use in 1998, the therapeutic landscape has changed considerably, but many challenges remain until the expectations for this new form of medicine are met. However, there is room for cautious optimism.

Targeting hepatic cancer cells with pegylated dendrimers displaying N-acetylgalactosamine and SP94 peptide ligands

Poly(amidoamine) (PAMAM) dendrimers are branched water-soluble polymers defined by consecutive generation numbers (Gn) indicating a parallel increase in size, molecular weight, and number of surface groups available for conjugation of bioactive agents. In this article, we compare the biodistribution of N-acetylgalactosamine (NAcGal)-targeted [(14) C]1 -G5-(NH2 )5 -(Ac)108 -(NAcGal)14 particles to non-targeted [(14) C]1 -G5-(NH2 )127 and PEGylated [(14) C]1 -G5-(NH2 )44 -(Ac)73 -(PEG)10 particles in a mouse hepatic cancer model. Results show that both NAcGal-targeted and non-targeted particles are rapidly cleared from the systemic circulation with high distribution to the liver. However, NAcGal-targeted particles exhibited 2.5-fold higher accumulation in tumor tissue compared to non-targeted ones. In comparison, PEGylated particles showed a 16-fold increase in plasma residence time and a 5-fold reduction in liver accumulation. These results motivated us to engineer new PEGylated G5 particles with PEG chains anchored to the G5 surface via acid-labile cis-aconityl linkages where the free PEG tips are functionalized with NAcGal or SP94 peptide to investigate their potential as targeting ligands for hepatic cancer cells as a function of sugar conformation (α versus β), ligand concentration (100-4000 nM), and incubation time (2 and 24 hours) compared to fluorescently (Fl)-labeled and non-targeted G5-(Fl)6 -(NH2 )122 and G5-(Fl)6 -(Ac)107 -(cPEG)15 particles. Results show G5-(Fl)6 -(Ac)107 -(cPEG[NAcGalβ ])14 particles achieve faster uptake and higher intracellular concentrations in HepG2 cancer cells compared to other G5 particles while escaping the non-specific adsorption of serum protein and phagocytosis by Kupffer cells, which make these particles the ideal carrier for selective drug delivery into hepatic cancer cells.

Sugar-appended polyamidoamine dendrimer conjugates with cyclodextrins as cell-specific non-viral vectors

The widespread use of various cyclodextrin (CyD)-appended polymers and polyrotaxanes as gene carriers has been reported. Among the various polyamidoamine dendrimer (dendrimer) conjugates with CyDs (CDE), the dendrimer (G3) conjugate with α-CyD having an average degree of substitution (DS) of 2.4 (α-CDE (G3, DS 2)) displayed remarkable properties as DNA carriers. In an attempt to develop cell-specific gene transfer carriers, we prepared some sugar-appended α-CDEs, e.g. mannosylated, galactosylated, and lactosylated α-CDEs. In addition, PEGylated Lac-α-CDEs (G3) were prepared and evaluated as a hepatocyte-selective and serum-resistant gene transfer carrier. Moreover, PEGylated-α-CDE/CyD polypseudorotaxane systems for novel sustained DNA release system have been developed. Interestingly, glucronylglucosyl-β-cyclodextrin (GUG-β-CyD) conjugates with dendrimer (G2) (GUG-β-CDE (G2)) had superior gene transfer activity to α-CDE (G2), expecting a development of new series of sugar-appended CDEs over α-CDEs (G2). Collectively, sugar-appended α-CDEs have the potential as novel cell-specific and safe carriers for DNA.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2003-2004

This review is the third update of the original review, published in 1999, on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings the topic to the end of 2004. Both fundamental studies and applications are covered. The main topics include methodological developments, matrices, fragmentation of carbohydrates and applications to large polymeric carbohydrates from plants, glycans from glycoproteins and those from various glycolipids. Other topics include the use of MALDI MS to study enzymes related to carbohydrate biosynthesis and degradation, its use in industrial processes, particularly biopharmaceuticals and its use to monitor products of chemical synthesis where glycodendrimers and carbohydrate-protein complexes are highlighted.