Importance of the 6'-hydroxy group and its configuration for apramycin activity

Biosynthetic Origin of the Octose Core and Its Mechanism of Assembly during Apramycin Biosynthesis

Apramycin is an aminoglycoside antibiotic isolated from Streptoalloteichus tenebrarius and S. hindustanus that has found clinical use in veterinary medicine. The apramycin structure is notable for its atypical eight-carbon bicyclic dialdose (octose) moiety. While the apramycin biosynthetic gene cluster (apr) has been identified and several of the encoded genes functionally characterized, how the octose core itself is assembled has remained elusive. Nevertheless, recent gene deletion studies have hinted at an N-acetyl aminosugar being a key precursor to the octose, and this hypothesis is consistent with the additional feeding experiments described in the present report. Moreover, bioinformatic analysis indicates that AprG may be structurally similar to GlcNAc-2-epimerase and hence recognize GlcNAc or a structurally similar substrate suggesting a potential role in octose formation. AprG with an extended N-terminal sequence was therefore expressed, purified, and assayed in vitro demonstrating that it does indeed catalyze a transaldolation reaction between GlcNAc or GalNAc and 6'-oxo-lividamine to afford 7'-N-acetyldemethylaprosamine with the same 6'-R and 7'-S stereochemistry as those observed in the apramycin product. Biosynthesis of the octose core in apramycin thus proceeds in the [6 + 2] manner with GlcNAc or GalNAc as the two-carbon donor, which has not been previously reported for biological octose formation, as well as novel inverting stereochemistry of the transferred fragment. Consequently, AprG appears to be a new transaldolase that lacks any apparent sequence similarity to the currently known aldolases and catalyzes a transaldolation for which there is no established biological precedent.

Aprosamine Derivatives Active against Multidrug-Resistant Gram-Negative Bacteria

Novel aprosamine derivatives were synthesized for the development of aminoglycoside antibiotics active against multidrug-resistant Gram-negative bacteria. The synthesis of aprosamine derivatives involved glycosylation at the C-8' position and subsequent modification (epimerization and deoxygenation at the C-5 position and 1-N-acylation) of the 2-deoxystreptamine moiety. All 8'-β-glycosylated aprosamine derivatives (3a-h) showed excellent antibacterial activity against carbapenem-resistant Enterobacteriaceae and 16S ribosomal RNA methyltransferase-producing multidrug-resistant Gram-negative bacteria compared to the clinical drug, arbekacin. The antibacterial activity of 5-epi (6a-d) and 5-deoxy derivatives (8a,b and 8h) of β-glycosylated aprosamine was further enhanced. On the other hand, the derivatives (10a,b and 10h) in which the amino group at the C-1 position was acylated with (S)-4-amino-2-hydroxybutyric acid showed excellent activity (MICs 0.25-0.5 μg/mL) against resistant bacteria that produce the aminoglycoside-modifying enzyme, aminoglycoside 3-N-acetyltransferase IV, which induces high resistance against parent apramycin (MIC > 64 μg/mL). In particular, 8b and 8h showed approximately 2- to 8-fold antibacterial activity against carbapenem-resistant Enterobacteriaceae and 8- to 16-fold antibacterial activity against resistant Gram-positive bacteria, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci, compared to apramycin. Our results showed that aprosamine derivatives have immense potential in the development of therapeutic agents for multidrug-resistant bacteria.

Importance of Co-operative Hydrogen Bonding in the Apramycin-Ribosomal Decoding A-Site Interaction

An intramolecular hydrogen bond between the protonated equatorial 7'-methylamino group of apramycin and the vicinal axial 6'-hydroxy group acidifies the 6'-hydroxy group leading to a strong hydrogen bond to A1408 in the ribosomal drug binding pocket in the decoding A site of the small ribosomal subunit. In 6'-epiapramycin, the trans-nature of the 6'-hydroxy group and the 7'-methylamino group results in a much weaker intramolecular hydrogen bond, and a consequently weaker cooperative hydrogen bonding network with A1408, resulting overall in reduced inhibition of protein synthesis and antibacterial activity.

Synthesis, Antibacterial and Antiribosomal Activity of the 3C-Aminoalkyl Modification in the Ribofuranosyl Ring of Apralogs (5-O-Ribofuranosyl Apramycins)

The synthesis and antiribosomal and antibacterial activity of both anomers of a novel apralog, 5-O-(5-amino-3-C-dimethylaminopropyl-D-ribofuranosyl)apramycin, are reported. Both anomers show excellent activity for the inhibition of bacterial ribosomes and that of MRSA and various wild-type Gram negative pathogens. The new compounds retain activity in the presence of the aminoglycoside phosphoryltransferase aminoglycoside modifying enzymes that act on the primary hydroxy group of typical 4,5-(2-deoxystreptamine)-type aminoglycoside and related apramycin derivatives. Unexpectedly, the two anomers have comparable activity both for the inhibition of bacterial ribosomes and of the various bacterial strains tested.

Structure-Activity Relationships for 5''-Modifications of 4,5-Aminoglycoside Antibiotics

Modification at the 5''-position of 4,5-disubstituted aminoglycoside antibiotics (AGAs) to circumvent inactivation by the APH(3',5'') class of aminoglycoside modifying enzymes (AMEs) has been widely reported.  Such modifications, however, impact activity against wild type bacteria and affect target selectivity in unpredictable ways thereby hindering drug development.  We present a systematic survey of modifications to the 5''-position of the 4,5-AGAs and of the related 5- O -furanosyl apramycin derivatives. In the neomycin and the apralog series, all modifications were well-tolerated, but other 4,5-AGAs require the presence of a hydrogen bonding group at the 5''-position for maintenance of high antibacterial activity. Though the 5''-amino modification resulted in comparable activity to the parent compounds, reduced selectivity against the human cytosolic decoding A site renders this modification generally unfavorable in paromomycin, propylamycin, and ribostamycin. Installation of a 5''-formamido group and, to a lesser degree, a 5''-ureido group resulted in comparable activity to the parents without the selectivity cost of the 5''-amino modification. The lessons learned from this work will aid in the design of next-generation AGAs capable of circumventing susceptibility to AMEs while maintaining high antibacterial activity and target selectivity.

An Advanced Apralog with Increased in vitro and in vivo Activity toward Gram-negative Pathogens and Reduced ex vivo Cochleotoxicity

We describe the convergent synthesis of a 5-O-β-D-ribofuranosyl-based apramycin derivative (apralog) that displays significantly improved antibacterial activity over the parent apramycin against wild-type ESKAPE pathogens. In addition, the new apralog retains excellent antibacterial activity in the presence of the only aminoglycoside modifying enzyme (AAC(3)-IV) acting on the parent, without incurring susceptibility to the APH(3') mechanism that disables other 5-O-β-D-ribofuranosyl 2-deoxystreptamine type aminoglycosides by phosphorylation at the ribose 5-position. Consistent with this antibacterial activity, the new apralog has excellent 30 nM activity (IC50 ) for the inhibition of protein synthesis by the bacterial ribosome in a cell-free translation assay, while retaining the excellent across-the-board selectivity of the parent for inhibition of bacterial over eukaryotic ribosomes. Overall, these characteristics translate into excellent in vivo efficacy against E. coli in a mouse thigh infection model and reduced ototoxicity vis à vis the parent in mouse cochlear explants.

Chemical Modifications Reduce Auditory Cell Damage Induced by Aminoglycoside Antibiotics

Although aminoglycoside antibiotics are effective against Gram-negative infections, these drugs often cause irreversible hearing damage. Binding to the decoding site of the eukaryotic ribosomes appears to result in ototoxicity, but there is evidence that other effects are involved. Here, we show how chemical modifications of apramycin and geneticin, considered among the least and most toxic aminoglycosides, respectively, reduce auditory cell damage. Using molecular dynamics simulations, we studied how modified aminoglycosides influence the essential freedom of movement of the decoding site of the ribosome, the region targeted by aminoglycosides. By determining the ratio of a protein translated in mitochondria to that of a protein translated in the cytoplasm, we showed that aminoglycosides can paradoxically elevate rather than reduce protein levels. We showed that certain aminoglycosides induce rapid plasma membrane permeabilization and that this nonribosomal effect can also be reduced through chemical modifications. The results presented suggest a new paradigm for the development of safer aminoglycoside antibiotics.

Apralogs: Apramycin 5-O-Glycosides and Ethers with Improved Antibacterial Activity and Ribosomal Selectivity and Reduced Susceptibility to the Aminoacyltranserferase (3)-IV Resistance Determinant

Apramycin is a structurally unique member of the 2-deoxystreptamine class of aminoglycoside antibiotics characterized by a monosubstituted 2-deoxystreptamine ring that carries an unusual bicyclic eight-carbon dialdose moiety. Because of its unusual structure, apramycin is not susceptible to the most prevalent mechanisms of aminoglycoside resistance including the aminoglycoside-modifying enzymes and the ribosomal methyltransferases whose widespread presence severely compromises all aminoglycosides in current clinical practice. These attributes coupled with minimal ototoxocity in animal models combine to make apramycin an excellent starting point for the development of next-generation aminoglycoside antibiotics for the treatment of multidrug-resistant bacterial infections, particularly the ESKAPE pathogens. With this in mind, we describe the design, synthesis, and evaluation of three series of apramycin derivatives, all functionalized at the 5-position, with the goals of increasing the antibacterial potency without sacrificing selectivity between bacterial and eukaryotic ribosomes and of overcoming the rare aminoglycoside acetyltransferase (3)-IV class of aminoglycoside-modifying enzymes that constitutes the only documented mechanism of antimicrobial resistance to apramycin. We show that several apramycin-5-O-β-d-ribofuranosides, 5-O-β-d-eryrthofuranosides, and even simple 5-O-aminoalkyl ethers are effective in this respect through the use of cell-free translation assays with wild-type bacterial and humanized bacterial ribosomes and of extensive antibacterial assays with wild-type and resistant Gram negative bacteria carrying either single or multiple resistance determinants. Ex vivo studies with mouse cochlear explants confirm the low levels of ototoxicity predicted on the basis of selectivity at the target level, while the mouse thigh infection model was used to demonstrate the superiority of an apramycin-5-O-glycoside in reducing the bacterial burden in vivo.

Synthesis of saccharocin from apramycin and evaluation of its ribosomal selectivity

We describe a straightforward synthesis of the apramycin biosynthetic precursor saccharocin from apramycin by regioselective partial azidation followed by stereoretentive oxidative deamination. Saccharocin was found to exhibit excellent selectivity for inhibition of the bacterial ribosome over the eukaryotic ribosomes indicating that its presence as a minor impurity in apramycin itself should not be problematic in the development of the latter as a clinical candidate.

Aminoglycoside Revival: Review of a Historically Important Class of Antimicrobials Undergoing Rejuvenation

Aminoglycosides are cidal inhibitors of bacterial protein synthesis that have been utilized for the treatment of serious bacterial infections for almost 80 years. There have been approximately 15 members of this class approved worldwide for the treatment of a variety of infections, many serious and life threatening. While aminoglycoside use declined due to the introduction of other antibiotic classes such as cephalosporins, fluoroquinolones, and carbapenems, there has been a resurgence of interest in the class as multidrug-resistant pathogens have spread globally. Furthermore, aminoglycosides are recommended as part of combination therapy for empiric treatment of certain difficult-to-treat infections. The development of semisynthetic aminoglycosides designed to overcome common aminoglycoside resistance mechanisms, and the shift to once-daily dosing, has spurred renewed interest in the class. Plazomicin is the first new aminoglycoside to be approved by the FDA in nearly 40 years, marking the successful start of a new campaign to rejuvenate the class.

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.

Exploration of Strategies for Mechanism-Based Inhibitor Design for Family GH99 endo-α-1,2-Mannanases

endo-α-1,2-Mannosidases and -mannanases, members of glycoside hydrolase family 99 (GH99), cleave α-Glc/Man-1,3-α-Man-OR structures within mammalian N-linked glycans and fungal α-mannan, respectively. They are proposed to act through a two-step mechanism involving a 1,2-anhydrosugar "epoxide" intermediate incorporating two conserved catalytic carboxylates. In the first step, one carboxylate acts as a general base to deprotonate the 2-hydroxy group adjacent to the fissile glycosidic bond, and the other provides general acid assistance to the departure of the aglycon. We report herein the synthesis of two inhibitors designed to interact with either the general base (α-mannosyl-1,3-(2-aminodeoxymannojirimycin), Man2NH2 DMJ) or the general acid (α-mannosyl-1,3-mannoimidazole, ManManIm). Modest affinities were observed for an endo-α-1,2-mannanase from Bacteroides thetaiotaomicron. Structural studies revealed that Man2NH2 DMJ binds like other iminosugar inhibitors, which suggests that the poor inhibition shown by this compound is not a result of a failure to achieve the expected interaction with the general base, but rather the reduction in basicity of the endocyclic nitrogen caused by introduction of a vicinal, protonated amine at C2. ManManIm binds with the imidazole headgroup distorted downwards, a result of an unfavourable interaction with a conserved active site tyrosine. This study has identified important limitations associated with mechanism-inspired inhibitor design for GH99 enzymes.

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.

Characterization of a C3 Deoxygenation Pathway Reveals a Key Branch Point in Aminoglycoside Biosynthesis

Apramycin is a clinically interesting aminoglycoside antibiotic (AGA) containing a highly unique bicyclic octose moiety, and this octose is deoxygenated at the C3 position. Although the biosynthetic pathways for most 2-deoxystreptamine-containing AGAs have been well characterized, the pathway for apramycin biosynthesis, including the C3 deoxygenation process, has long remained unknown. Here we report detailed investigation of apramycin biosynthesis by a series of genetic, biochemical and bioinformatical studies. We show that AprD4 is a novel radical S-adenosyl-l-methionine (SAM) enzyme, which uses a noncanonical CX3CX3C motif for binding of a [4Fe-4S] cluster and catalyzes the dehydration of paromamine, a pseudodisaccharide intermediate in apramycin biosynthesis. We also show that AprD3 is an NADPH-dependent reductase that catalyzes the reduction of the dehydrated product from AprD4-catalyzed reaction to generate lividamine, a C3' deoxygenated product of paromamine. AprD4 and AprD3 do not form a tight catalytic complex, as shown by protein complex immunoprecipitation and other assays. The AprD4/AprD3 enzyme system acts on different pseudodisaccharide substrates but does not catalyze the deoxygenation of oxyapramycin, an apramycin analogue containing a C3 hydroxyl group on the octose moiety, suggesting that oxyapramycin and apramycin are partitioned into two parallel pathways at an early biosynthetic stage. Functional dissection of the C6 dehydrogenase AprQ shows the crosstalk between different AGA biosynthetic gene clusters from the apramycin producer Streptomyces tenebrarius, and reveals the remarkable catalytic versatility of AprQ. Our study highlights the intriguing chemistry in apramycin biosynthesis and nature's ingenuity in combinatorial biosynthesis of natural products.

A review of patents (2011-2015) towards combating resistance to and toxicity of aminoglycosides

Since the discovery of the first aminoglycoside (AG), streptomycin, in 1943, these broad-spectrum antibiotics have been extensively used for the treatment of Gram-negative and Gram-positive bacterial infections. The inherent toxicity (ototoxicity and nephrotoxicity) associated with their long-term use as well as the emergence of resistant bacterial strains have limited their usage. Structural modifications of AGs by AG-modifying enzymes, reduced target affinity caused by ribosomal modification, and decrease in their cellular concentration by efflux pumps have resulted in resistance towards AGs. However, the last decade has seen a renewed interest among the scientific community for AGs as exemplified by the recent influx of scientific articles and patents on their therapeutic use. In this review, we use a non-conventional approach to put forth this renaissance on AG development/application by summarizing all patents filed on AGs from 2011-2015 and highlighting some related publications on the most recent work done on AGs to overcome resistance and improving their therapeutic use while reducing ototoxicity and nephrotoxicity. We also present work towards developing amphiphilic AGs for use as fungicides as well as that towards repurposing existing AGs for potential newer applications.

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.

Selective Protection of Secondary Amines as the N-Phenyltriazenes. Application to Aminoglycoside Antibiotics

Selective protection of secondary amines as triazenes in the presence of multiple primary amines is demonstrated, with subsequent protection of the primary amines as either azides or carbamates in the same pot. Aminoglycoside antibiotic examples reveal broad functional group compatibility. The triazene group is removed with trifluoroacetic acid and, because of the low barrier to rotation, affords sharp (1)H NMR spectra at room temperature.

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

A series of 20 4'-O-glycosides of the aminoglycoside antibiotic paromomycin were synthesized and evaluated for their ability to inhibit protein synthesis by bacterial, mitochondrial and cytosolic ribosomes. Target selectivity, i.e., inhibition of the bacterial ribosome over eukaryotic mitochondrial and cytosolic ribosomes, which is predictive of antibacterial activity with reduced ototoxicity and systemic toxicity, was greater for the equatorial than for the axial pyranosides, and greater for the d-pentopyranosides than for the l-pentopyranosides and d-hexopyranosides. In particular, 4'-O-β-d-xylopyranosyl paromomycin shows antibacterioribosomal activity comparable to that of paromomycin, but is significantly more selective showing considerably reduced affinity for the cytosolic ribosome and for the A1555G mutant mitochondrial ribosome associated with hypersusceptibility to drug-induced ototoxicity. Compound antibacterioribosomal activity correlates with antibacterial activity, and the ribosomally more active compounds show activity against Escherichia coli, Klebsiella pneumonia, Enterobacter cloacae, Acinetobacter baumannii, and methicillin-resistant Staphylococcus aureus (MRSA). The paromomycin glycosides retain activity against clinical strains of MRSA that are resistant to paromomycin, which is demonstrated to be a consequence of 4'-O-glycosylation blocking the action of 4'-aminoglycoside nucleotidyl transferases by the use of recombinant E. coli carrying the specific resistance determinant.

Fluorine-decoupled carbon spectroscopy for the determination of configuration at fully substituted, trifluoromethyl- and perfluoroalkyl-bearing carbons: comparison with 19F-1H heteronuclear Overhauser effect spectroscopy

The synthesis of a series of α-trifluoromethylcyclohexanols and analogous trimethylsilyl ethers by addition of the Ruppert-Prakash reagent to substituted cyclohexanones is presented. A method for the assignment of configuration of such compounds, of related α-trifluoromethylcyclohexylamines and of quaternary trifluoromethyl-substituted carbons is described based on the determination of the (3)J(CH) coupling constant between the fluorine-decoupled (13)CF3 resonance and the vicinal hydrogens. This method is dubbed fluorine-decoupled carbon spectroscopy and abbreviated FDCS. The method is also applied to the configurational assignment of substances bearing mono-, di-, and perfluoroalkyl rather than trifluoromethyl groups. The configuration of all substances was verified by either (19)F-(1)H heteronuclear Overhauser spectroscopy (HOESY) or X-ray crystallography. The relative merits of FDCS and HOESY are compared and contrasted. (2)J(CH), (3)J(CH), and (4)J(CH) coupling constants to (19)F decoupled CF3 groups in alkenes and arenes have also been determined and should prove to be useful in the structural assignment of trifluoromethylated alkenes and arenes.