Influence of 4'-O-Glycoside Constitution and Configuration on Ribosomal Selectivity of Paromomycin

Enantioselective Total Synthesis of Fortimicin B

Fortimicins, featuring a pseudodisaccharide scaffold, are an unusual class of aminoglycosides (AGs) with potent efficacy against several aminoglycoside-resistant bacterial strains. Notably, these molecules also exhibit lower inherent ototoxicity and nephrotoxicity than common aminoglycosides. Consequently, fortimicins are a promising type of protoypical molecules for the development of the next generation of aminoglycoside antibiotics. Here, we report the asymmetric total synthesis of fortimicin B in 12 steps (longest linear sequence, LLS) from readily available starting materials. An enantioselective Cu(II)-catalyzed inverse-electron-demand Diels-Alder (IEDDA) reaction of 2-pyrones and N-substituted 2-oxazolones was developed for the efficient synthesis of the fortamine fragment, which previously required a lengthy multistep synthesis owing to its complex stereochemistry. The 6-epi-purpurosamine B fragment was efficiently synthesized through a Cr(II)/Co(I)-mediated C─C bond coupling between aldehydes and alkyl halides. Within these two fragments, the stereoselective construction of the α-glycosidic bond of fortimicin B was realized via the gold(I)-catalyzed glycosylation. Overall, this study provides an efficient synthetic platform for future investigations into the structure-activity relationships of fortimicins.

Site-Selective Palladium-catalyzed Oxidation of Unprotected Aminoglycosides and Sugar Phosphates

The site-selective modification of complex biomolecules by transition metal-catalysis is highly warranted, but often thwarted by the presence of Lewis basic functional groups. This study demonstrates that protonation of amines and phosphates in carbohydrates circumvents catalyst inhibition in palladium-catalyzed site-selective oxidation. Both aminoglycosides and sugar phosphates, compound classes that up till now largely escaped direct modification, are oxidized with good efficiency. Site-selective oxidation of kanamycin and amikacin was used to prepare a set of 3'-modified aminoglycoside derivatives of which two showed promising activity against antibiotic-resistant E. coli strains.

Synthesis of 4-O-(4-Amino-4-deoxy-β-D-xylopyranosyl)paromomycin and 4-S-(β-D-Xylopyranosyl)-4-deoxy-4'-thio-paromomycin and Evaluation of their Antiribosomal and Antibacterial Activity

The design, synthesis and antiribosomal and antibacterial activity of two novel glycosides of the aminoglycoside antibiotic paromomycin are described. The first carries of 4-amino-4-deoxy-β-D-xylopyranosyl moiety at the paromomycin 4'-position and is approximately two-fold more active than the corresponding β-D-xylopyranosyl derivative. The second is a 4'-(β-D-xylopyranosylthio) derivative of 4'-deoxyparomomycin that is unexpectedly less active than the simple β-D-xylopyranosyl derivative, perhaps because of the insertion of the conformationally more mobile thioglycosidic linkage.

En Route to the Transformation of Glycoscience: A Chemist's Perspective on Internal and External Crossroads in Glycochemistry

Carbohydrate chemistry is an essential component of the glycosciences and is fundamental to their progress. This Perspective takes the position that carbohydrate chemistry, or glycochemistry, has reached three crossroads on the path to the transformation of the glycosciences, and illustrates them with examples from the author's and other laboratories. The first of these potential inflexion points concerns the mechanism of the glycosylation reaction and the role of protecting groups. It is argued that the experimental evidence supports bimolecular SN2-like mechanisms for typical glycosylation reactions over unimolecular ones involving stereoselective attack on naked glycosyl oxocarbenium ions. Similarly, it is argued that the experimental evidence does not support long-range stereodirecting participation of remote esters through bridged bicyclic dioxacarbenium ions in organic solution in the presence of typical counterions. Rational design and improvement of glycosylation reactions must take into account the roles of the counterion and of concentration. A second crossroads is that between mainstream organic chemistry and glycan synthesis. The case is made that the only real difference between glycan and organic synthesis is the formation of C-O rather than C-C bonds, with diastereocontrol, strategy, tactics, and elegance being of critical importance in both areas: mainstream organic chemists should feel comfortable taking this fork in the road, just as carbohydrate chemists should traveling in the opposite direction. A third crossroads is that between carbohydrate chemistry and medicinal chemistry, where there are equally many opportunities for traffic in either direction. The glycosciences have advanced enormously in the past decade or so, but creativity, input, and ingenuity of scientists from all fields is needed to address the many sophisticated challenges that remain, not the least of which is the development of a broader and more general array of stereospecific glycosylation reactions.

Synthesis of a Pseudodisaccharide Suitable for Synthesis of Ring I Modified 4,5-2-Deoxystreptamine Type Aminoglycoside Antibiotics

To facilitate the synthesis of paromomycin and/or neomycin analogues, we describe a cleavage of ring I from paromomycin that proceeds in the presence of azides and affords a glycosyl acceptor for the installation of a modified ring I. A paromomycin 4',6'-diol is oxidized by the Dess-Martin periodinane followed by m-chloroperoxybenzoic acid. Base treatment then affords a protected pseudodisaccharide, which functions as a glycosyl acceptor. The method should also apply to the cleavage of pyranosyl 4,6-diols from oligosaccharides and glycoconjugates.

Selective Modification of Streptozotocin at the C3 Position to Improve Its Bioactivity as Antibiotic and Reduce Its Cytotoxicity towards Insulin-Producing β Cells

With the increasing resistance of bacteria to current antibiotics, novel compounds are urgently needed to treat bacterial infections. Streptozotocin (STZ) is a natural product that has broad-spectrum antibiotic activity, albeit with limited use because of its toxicity to pancreatic β cells. In an attempt to derivatize STZ through structural modification at the C3 position, we performed the synthesis of three novel STZ analogues by making use of our recently developed regioselective oxidation protocol. Keto-STZ (2) shows the highest inhibition of bacterial growth (minimum inhibitory concentration (MIC) and viability assays), but is also the most cytotoxic compound. Pre-sensitizing the bacteria with GlcNAc increased the antimicrobial effect, but did not result in complete killing. Interestingly, allo-STZ (3) revealed moderate concentration-dependent antimicrobial activity and no cytotoxicity towards β cells, and deoxy-STZ (4) showed no activity at all.

Aminoglycosides: Time for the Resurrection of a Neglected Class of Antibacterials?

This Viewpoint addresses the question of whether, after years of declining use, the time is ripe for renewed investigations into the aminoglycoside class of antibiotics for the development of novel therapeutic agents for the treatment of drug-resistant bacterial infections, particularly of the Gram-negative type. The reasons underlying the decline in use of the aminoglycosides are briefly considered and found to be outweighed by the ever-increasing clinical need for improved antibacterials with which to combat modern day multidrug resistant pathogens. The potential of the aminoglycosides builds on their well-established pharmacokinetics/pharmacodynamics (PK/PD), mechanisms of action, toxicity, and resistance and the extensive existing structure-activity relationship (SAR) databases, which permit rational, informed drug design. When coupled with the power of modern synthetic organic chemistry and improved funding scenarios, these multiple attributes open the door for the development of structurally novel, potent, and less toxic aminoglycosides to address the pressing societal problem of multidrug-resistant (MDR) infectious disease.

Modification at the 2'-Position of the 4,5-Series of 2-Deoxystreptamine Aminoglycoside Antibiotics To Resist Aminoglycoside Modifying Enzymes and Increase Ribosomal Target Selectivity

A series of derivatives of the 4,5-disubstituted class of 2-deoxystreptamine aminoglycoside antibiotics neomycin, paromomycin, and ribostamycin was prepared and assayed for (i) their ability to inhibit protein synthesis by bacterial ribosomes and by engineered bacterial ribosomes carrying eukaryotic decoding A sites, (ii) antibacterial activity against wild type Gram negative and positive pathogens, and (iii) overcoming resistance due to the presence of aminoacyl transferases acting at the 2′-position. The presence of five suitably positioned residual basic amino groups was found to be necessary for activity to be retained upon removal or alkylation of the 2′-position amine. As alkylation of the 2′-amino group overcomes the action of resistance determinants acting at that position and in addition results in increased selectivity for the prokaryotic over eukaryotic ribosomes, it constitutes an attractive modification for introduction into next generation aminoglycosides. In the neomycin series, the installation of small (formamide) or basic (glycinamide) amido groups on the 2′-amino group is tolerated.

Design, Multigram Synthesis, and in Vitro and in Vivo Evaluation of Propylamycin: A Semisynthetic 4,5-Deoxystreptamine Class Aminoglycoside for the Treatment of Drug-Resistant Enterobacteriaceae and Other Gram-Negative Pathogens

Infectious diseases due to multidrug-resistant pathogens, particularly carbapenem-resistant Enterobacteriaceae (CREs), present a major and growing threat to human health and society, providing an urgent need for the development of improved potent antibiotics for their treatment. We describe the design and development of a new class of aminoglycoside antibiotics culminating in the discovery of propylamycin. Propylamycin is a 4'-deoxy-4'-alkyl paromomycin whose alkyl substituent conveys excellent activity against a broad spectrum of ESKAPE pathogens and other Gram-negative infections, including CREs, in the presence of numerous common resistance determinants, be they aminoglycoside modifying enzymes or rRNA methyl transferases. Importantly, propylamycin is demonstrated not to be susceptible to the action of the ArmA resistance determinant whose presence severely compromises the action of plazomicin and all other 4,6-disubstituted 2-deoxystreptamine aminoglycosides. The lack of susceptibility to ArmA, which is frequently encoded on the same plasmid as carbapenemase genes, ensures that propylamycin will not suffer from problems of cross-resistance when used in combination with carbapenems. Cell-free translation assays, quantitative ribosome footprinting, and X-ray crystallography support a model in which propylamycin functions by interference with bacterial protein synthesis. Cell-free translation assays with humanized bacterial ribosomes were used to optimize the selectivity of propylamycin, resulting in reduced ototoxicity in guinea pigs. In mouse thigh and septicemia models of Escherichia coli, propylamycin shows excellent efficacy, which is better than paromomycin. Overall, a simple novel deoxy alkyl modification of a readily available aminoglycoside antibiotic increases the inherent antibacterial activity, effectively combats multiple mechanisms of aminoglycoside resistance, and minimizes one of the major side effects of aminoglycoside therapy.

Synthesis of Glycosidic (β-1''→6, 3' and 4') Site Isomers of Neomycin B and their Effect on RNA and DNA Triplex Stability

Glycosidic (β-1''→6, 3' and 4') site isomers of neomycin B (i.e., neobiosamine (β-1''→6, 3' and 4') neamines) have been synthesized in a straightforward manner. Peracetylated neomycin azide was used as a common starting material to obtain neobiosamine glycosyl donor and 6, 3',4'-tri-O-acetyl neamine azide that after simple protecting group manipulation was converted to three different glycosyl acceptors (i.e., 5,6,4'-, 5,3',4'- and 5,6,3'-tri-O-acetyl neamine azide). Glycosylation between the neobiosamine glycosyl donor and the neamine-derived acceptors gave the protected pseudo-tetrasaccharides, which were converted, via global deprotection (deacetylation and reduction of the azide groups), to the desired site isomers of neomycin. The effect of these aminoglycosides on the RNA and DNA triplex stability was studied by UV-melting profile analysis.

Effects of the 1- N-(4-Amino-2 S-hydroxybutyryl) and 6'- N-(2-Hydroxyethyl) Substituents on Ribosomal Selectivity, Cochleotoxicity, and Antibacterial Activity in the Sisomicin Class of Aminoglycoside Antibiotics

Syntheses of the 6'- N-(2-hydroxyethyl) and 1- N-(4-amino-2 S-hydroxybutyryl) derivatives of the 4,6-aminoglycoside sisomicin and that of the doubly modified 1- N-(4-amino-2 S-hydroxybutyryl)-6'- N-(2-hydroxyethyl) derivative known as plazomicin are reported together with their antibacterial and antiribosomal activities and selectivities. The 6'- N-(2-hydroxyethyl) modification results in a moderate increase in prokaryotic/eukaryotic ribosomal selectivity, whereas the 1- N-(4-amino-2 S-hydroxybutyryl) modification has the opposite effect. When combined in plazomicin, the effects of the two groups on ribosomal selectivity cancel each other out, leading to the prediction that plazomicin will exhibit ototoxicity comparable to those of the parent and the current clinical aminoglycoside antibiotics gentamicin and tobramycin, as borne out by ex vivo studies with mouse cochlear explants. The 6'- N-(2-hydroxyethyl) modification restores antibacterial activity in the presence of the AAC(6') aminoglycoside-modifying enzymes, while the 1- N-(4-amino-2 S-hydroxybutyryl) modification overcomes resistance to the AAC(2') class but is still affected to some extent by the AAC(3) class. Neither modification is able to circumvent the ArmA ribosomal methyltransferase-induced aminoglycoside resistance. The use of phenyltriazenyl protection for the secondary amino group of sisomicin facilitates the synthesis of each derivative and their characterization through the provision of sharp NMR spectra for all intermediates.

Comprehensive review of chemical strategies for the preparation of new aminoglycosides and their biological activities

A systematic analysis of all synthetic and chemoenzymatic methodologies for the preparation of aminoglycosides for a variety of applications (therapeutic and agricultural) reported in the scientific literature up to 2017 is presented. This comprehensive analysis of derivatization/generation of novel aminoglycosides and their conjugates is divided based on the types of modifications used to make the new derivatives. Both the chemical strategies utilized and the biological results observed are covered. Structure-activity relationships based on different synthetic modifications along with their implications for activity and ability to avoid resistance against different microorganisms are also presented.

Pseudouridine modifications influence binding of aminoglycosides to helix 69 of bacterial ribosomes

Development of antibiotics that target new regions of functionality is a possible way to overcome antibiotic resistance. In this study, the interactions of aminoglycoside antibiotics with helix 69 of the E. coli 23S rRNA in the context of complete 70S ribosomes or the isolated 50S subunit were investigated by using chemical probing and footprinting analysis. Helix 69 is a dynamic RNA motif that plays major roles in bacterial ribosome activity. Neomycin, paromomycin, and gentamicin interact with the stem region of helix 69 in complete 70S ribosomes, but have diminished binding to the isolated 50S subunit. Pseudouridine modifications in helix 69 were shown to impact the aminoglycoside interactions. These results suggest a requirement for a specific conformational state of helix 69 for efficient aminoglycoside binding, and imply that this motif may be a suitable target for mechanism-based therapeutics.

Structure-Based Design and Synthesis of Apramycin-Paromomycin Analogues: Importance of the Configuration at the 6'-Position and Differences between the 6'-Amino and Hydroxy Series

The preparation of a series of four analogues of the aminoglycoside antibiotics neomycin and paromomycin is described in which ring I, involved in critical binding interactions with the ribosomal target, is replaced by an apramycin-like dioxabicyclo[4.4.0]octane system. The effect of this modification is to lock the hydroxymethyl side chain of the neomycin or paromomycin ring I, as part of the dioxabicyclooctane ring, into either the gauche-gauche or the gauche-trans conformation (respectively, axial or equatorial to the bicyclic system). The antiribosomal activity of these compounds is investigated with cell-free translation assays using both bacterial ribosomes and recombinant hybrid ribosomes carrying eukaryotic decoding A site cassettes. Compounds substituted with an equatorial hydroxyl or amino group in the newly formed ring are considerably more active than their axial diastereomers, lending strong support to crystallographically derived models of aminoglycoside-ribosome interactions. One such bicyclic compound carrying an equatorial hydroxyl group has activity equal to that of the parent yet displays better ribosomal selectivity, predictive of an enhanced therapeutic index. A paromomycin analog lacking the hydroxymethyl ring I side chain is considerably less active than the parent. Antibacterial activity against model Gram negative and Gram positive bacteria is reported for selected compounds, as is activity against ESKAPE pathogens and recombinant bacteria carrying specific resistance determinants. Analogues with a bicyclic ring I carrying equatorial amino or hydroxyl groups mimicking the bound side chains of neomycin and paromomycin, respectively, show excellent activity and, by virtue of their novel structure, retain this activity in strains that are insensitive to the parent compounds.

N6', N6''', and O4' Modifications to Neomycin Affect Ribosomal Selectivity without Compromising Antibacterial Activity

The synthesis of a series of neomycin derivatives carrying the 2-hydroxyethyl substituent on N6' and/or N6‴ both alone and in combination with a 4'-O-ethyl group is described. By means of cell-free translation assays with wild-type bacterial ribosomes and their hybrids with eukaryotic decoding A sites, we investigate how individual substituents and their combinations affect activity and selectivity at the target level. In principle, and as shown by cell-free translation assays, modifications of the N6' and N6‴ positions allow enhancement of target selectivity without compromising antibacterial activity. As with the 6'OH aminoglycoside paromomycin, the 4'-O-ethyl modification affects the ribosomal activity, selectivity, and antibacterial profile of neomycin and its 6'-N-(2-hydroxyethyl) derivatives. The modified aminoglycosides show good antibacterial activity against model Gram-positive and Gram-negative microbes including the ESKAPE pathogens Staphylococcus aureus, Klebsiella pneumoniae, Enterobacter cloacae, and Acinetobacter baumannii.

Effects of 5-O-Ribosylation of Aminoglycosides on Antimicrobial Activity and Selective Perturbation of Bacterial Translation

We studied six pairs of aminoglycosides and their corresponding ribosylated derivatives synthesized by attaching a β-O-linked ribofuranose to the 5-OH of the deoxystreptamine ring of the parent pseudo-oligosaccharide antibiotic. Ribosylation of the 4,6-disubstituted 2-deoxystreptamine aminoglycoside kanamycin B led to improved selectivity for inhibition of prokaryotic relative to cytosolic eukaryotic in vitro translation. For the pseudodisaccharide aminoglycoside scaffolds neamine and nebramine, ribosylated derivatives were both more potent antimicrobials and more selective to inhibition of prokaryotic translation. On the basis of the results of this study, we suggest that modification of the 5-OH position of the streptamine ring of other natural or semisynthetic pseudodisaccharide aminoglycoside scaffolds containing an equatorial amine at the 2' sugar position with a β-O-linked ribofuranose is a promising avenue for the development of novel aminoglycoside antibiotics with improved efficacy and reduced toxicity.

Synthesis and Antiribosomal Activities of 4'-O-, 6'-O-, 4″-O-, 4',6'-O- and 4″,6″-O-Derivatives in the Kanamycin Series Indicate Differing Target Selectivity Patterns between the 4,5- and 4,6-Series of Disubstituted 2-Deoxystreptamine Aminoglycoside Antibiotics

Chemistry for the efficient modification of the kanamycin class of 4,6-aminoglycosides at the 4'-position is presented. In all kanamycins but kanamycin B, 4'-O-alkylation is strongly detrimental to antiribosomal and antibacterial activity. Ethylation of kanamycin B at the 4″-position entails little loss of antiribosomal and antibacterial activity, but no increase of ribosomal selectivity. These results are contrasted with those for the 4,5-aminoglycosides, where 4'-O-alkylation of paromomycin causes only a minimal loss of activity but results in a significant increase in selectivity with a concomitant loss of ototoxicity.