A click chemistry approach to pleuromutilin conjugates with nucleosides or acyclic nucleoside derivatives and their binding to the bacterial ribosome

Hit-to-Lead Identification and Validation of a Triaromatic Pleuromutilin Antibiotic Candidate

Herein, we report the hit-to-lead identification of a drug-like pleuromutilin conjugate 16, based on a triaromatic hit reported in 2020. The lead arose as the clear candidate from a hit-optimization campaign in which Gram-positive antibacterial activity, solubility, and P-gp affinity were optimized. Conjugate 16 was extensively evaluated for its in vitro ADMET performance which, apart from solubility, was overall on par with lefamulin. This evaluation included Caco-2 cell permeability, plasma protein binding, hERG inhibition, cytotoxicity, metabolism in microsomes and CYP3A4, resistance induction, and time-kill kinetics. Intravenous pharmacokinetics of 16 proved satisfactory in both mice and pigs; however, oral bioavailability was limited likely due to insufficient solubility. The in vivo efficacy was evaluated in mice, systemically infected with Staphylococcus aureus, where 16 showed rapid reduction in blood bacteriaemia. Through our comprehensive studies, lead 16 has emerged as a highly promising and safe antibiotic candidate for the treatment of Gram-positive bacterial infections.

Preliminary SAR of Novel Pleuromutilin-Polyamine Conjugates

While pleuromutilin (1) and its clinically available derivatives (2-6) are highly effective against Gram-positive bacteria, they remain inactive against many pathogenic Gram-negative bacteria due to the efflux pump AcrAB-TolC. In an effort to broaden the spectrum of activity of pleuromutilin (1), we developed a series of novel pleuromutilin-polyamine conjugates (9a-f) which exhibited promising intrinsic antimicrobial properties, targeting both Gram-positive and Gram-negative bacteria, including Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and Escherichia coli, along with the fungal strain Cryptococcus neoformans, and were devoid of cytotoxic and hemolytic properties with the exception of one conjugate. Furthermore, this series displayed moderate to low antibiotic potentiation of legacy antibiotics doxycycline and erythromycin, with three conjugates enhancing the activity four-fold in combination with doxycycline. In comparison to pleuromutilin (1) and tiamulin (2), one of the conjugates exhibited an expanded spectrum of activity, including Gram-negative bacteria and fungi, making it a promising option for combating microbial infections.

New Nucleic Base-Tethered Trithiolato-Bridged Dinuclear Ruthenium(II)-Arene Compounds: Synthesis and Antiparasitic Activity

Aiming toward compounds with improved anti-Toxoplasma activity by exploiting the parasite auxotrophies, a library of nucleobase-tethered trithiolato-bridged dinuclear ruthenium(II)-arene conjugates was synthesized and evaluated. Structural features such as the type of nucleobase and linking unit were progressively modified. For comparison, diruthenium hybrids with other type of molecules were also synthesized and assessed. A total of 37 compounds (diruthenium conjugates and intermediates) were evaluated in a primary screening for in vitro activity against transgenic Toxoplasma gondii tachyzoites constitutively expressing β-galactosidase (T. gondii β-gal) at 0.1 and 1 µM. In parallel, the cytotoxicity in non-infected host cells (human foreskin fibroblasts, HFF) was determined by alamarBlue assay. Twenty compounds strongly impairing parasite proliferation with little effect on HFF viability were subjected to T. gondii β-gal half maximal inhibitory concentration determination (IC₅₀) and their toxicity for HFF was assessed at 2.5 µM. Two promising compounds were identified: 14, ester conjugate with 9-(2-oxyethyl)adenine, and 36, a click conjugate bearing a 2-(4-(hydroxymethyl)-1H-1,2,3-triazol-1-yl)methyl substituent, with IC₅₀ values of 0.059 and 0.111 µM respectively, significantly lower compared to pyrimethamine standard (IC₅₀ = 0.326 µM). Both 14 and 36 exhibited low toxicity against HFF when applied at 2.5 µM and are candidates for potential treatment options in a suitable in vivo model.

Semisynthetic pleuromutilin antimicrobials with therapeutic potential against methicillin-resistant Staphylococcus aureus by targeting 50S ribosomal subunit

A series of pleuromutilin analogs with a substituted 1,2,4-triazole were designed, synthesized and assessed for their in vitro and in vivo antibacterial activity. Initially, the MIC of the synthesized derivatives against five strains of Staphylococcus aureus (MRSA ATCC 43300, S. aureus ATCC 29213, clinical isolation of S. aureus AD3, S. aureus 144 and S. aureus SA17) were tested by the broth dilution method. Compounds 30a, 31b and 32a were the most active antibacterial agents in vitro against MRSA (MIC = 0.0625 μg/mL). The results of the time-kill curves showed that compounds 30a and 32a could reduce the amount of MRSA in vitro quickly (-7.70 log10 CFU/mL and -7.10 log10 CFU/mL reduction). In the experiment to further evaluate the in vivo antibacterial activity of compound 30a against MRSA, compound 30a (-1.71 log10 CFU/g) was effective in reducing MRSA load in thigh infected mice. Compound 30a (survival rate was 50%) displayed superior in vivo efficacy to that of tiamulin (survival rate was 30%) in the mouse systemic model. The results of further pharmacokinetic studies on compound 30a showed that the half-life (t1/2), clearance rate (Cl) and the area under the plasma concentration time curve (AUC0→∞) of compound 30a were 0.37 h, 5.43 L/h/kg and 1.84 μg h/mL, respectively. After affinity measurement by surface plasmon resonance (SPR), compound 30a exhibited high affinity with the 50S ribosome, with KD value of 1.95 × 10-6 M. Furthermore, the results of molecular docking studies revealed that compound 30a was successfully localized inside the binding pocket of 50S ribosomal subunit (ΔGb = -9.40 kcal/mol). Meanwhile, most of these compounds had no significant inhibitory effect on RAW 264.7 cells and 16HBE cells at the concentration of 8 μg/mL. The obtained outcomes showed that compound 30a could be utilized as an encouraging perspective in the development of a new therapeutic candidate for bacterial infection.

Design, synthesis and antibacterial evaluation of pleuromutilin derivatives

We report herein the design, synthesis, and structure-activity relationship studies of pleuromutilin derivatives containing urea/thiourea functionalities. The antibacterial activities of these new pleuromutilin derivatives were evaluated in vitro against Gram-positive pathogens (GPPs) (Staphylococcus aureus, Staphylococcus epidermidis and Enterococcus faecium) and Mycoplasma pneumoniae by the broth dilution method. Most of the targeted compounds exhibit good potency in inhibiting the growth of pathogens including Methicillin-susceptible S. aureus (MSSA, ATCC29213, MIC: 0.0625-16 μg/mL), Methicillin-resistant S. aureus (MRSA, ATCC43300, MIC: 0.125-16 μg/mL) and M. pneumoniae (ATCC15531 MIC: 0.125-1 μg/mL, ATCC29342 MIC: 0.0625-0.25 μg/mL and drug resistant strain MIC: 0.5-2 μg/mL). In particular, the compounds 6m and 6n containing phenyl-urea group showed excellent activity with the MIC value less than 0.0625 μg/mL against S. aureus ATCC29213. The compound 6h exhibited better activity than tiamulin against Methicillin-resistant S. aureus ATCC43300.

Green synthesis of triazolo-nucleoside conjugates via azide–alkyne C–N bond formation

Modified nucleosides are the core precursors for the synthesis of artificial nucleic acids, and are important in the field of synthetic and medicinal chemistry. In order to synthesize various triazolo-compounds, copper and ruthenium catalysed azide–alkyne 1,3-dipolar cycloaddition reactions also known as click reaction have emerged as a facile and efficient tool due to its simplicity and convenient conditions. Introduction of a triazole ring in nucleosides enhances their therapeutic value and various photophysical properties. This review primarily focuses on the plethora of synthetic methodologies being employed to synthesize sugar modified triazolyl nucleosides, their therapeutic importance and various other applications.

A click chemistry approach to pleuromutilin derivatives, evaluation of anti-MRSA activity and elucidation of binding mode by surface plasmon resonance and molecular docking

Novel series of pleuromutilin analogs containing substituted 1,2,3-triazole moieties were designed, synthesised and assessed for their in vitro antibacterial activity against Methicillin-resistant Staphylococcus aureus (MRSA). Initially, the in vitro antibacterial activities of these derivatives against 4 strains of S. aureus (MRSA ATCC 43300, ATCC 29213, AD3, and 144) were tested by the broth dilution method. Most of the synthesised pleuromutilin analogs displayed potent activities. Among them, compounds 50, 62, and 64 (MIC = 0.5∼1 µg/mL) showed the most effective antibacterial activity and their anti-MRSA activity were further studied by the time-killing kinetics approach. Binding mode investigations by surface plasmon resonance (SPR) with 50S ribosome revealed that the selected compounds all showed obvious affinity for 50S ribosome (KD = 2.32 × 10-8∼5.10 × 10-5 M). Subsequently, the binding of compounds 50 and 64 to the 50S ribosome was further investigated by molecular modelling. Compound 50 had a superior docking mode with 50S ribosome, and the binding free energy of compound 50 was calculated to be -12.0 kcal/mol.

Design, synthesis and biological evaluation of novel pleuromutilin derivatives as potent anti-MRSA agents targeting the 50S ribosome

A series of novel pleuromutilin derivatives were designed and synthesized with 1,2,4-triazole as the linker connected to benzoyl chloride analogues under mild conditions. The in vitro antibacterial activities of the synthesized derivatives against four strains of Staphylococcus aureus (MRSA ATCC 43300, ATCC 29213, AD3 and 144) were tested by the broth dilution method. Most of the synthesized derivatives displayed potent activities, and 22-(3-amino-2-(4-methyl-benzoyl)-1,2,4-triazole-5-yl)-thioacetyl)-22-deoxypleuromutilin (compound 12) was found to be the most active antibacterial derivative against MRSA (MIC = 0.125 μg/mL). Furthermore, the time-kill curves showed compound 12 had a certain inhibitory effect against MRSA in vitro. The in vivo antibacterial activity of compound 12 was further evaluated using MRSA infected murine thigh model. Compound 12 exhibited superior antibacterial efficacy than tiamulin. It was also found that compound 12 had no significant inhibitory effect on the proliferation of RAW264.7 cells. Compound 12 was further evaluated in CYP450 inhibition assay and showed moderate inhibitory effect on CYP3A4 (IC50 = 3.95 μM). Moreover, seven candidate compounds showed different affinities with the 50S ribosome by SPR measurement. Subsequently, binding of compound 12 and 20 to the 50S ribosome was further investigated by molecular modeling. Three strong hydrogen bonds were formed through the interaction of compound 12 and 20 with 50S ribosome. The binding free energy of compound 12 and 20 with the ribosome was calculated to be -10.7 kcal/mol and -11.66 kcal/mol, respectively.

Discovery of a Potent Adenine-Benzyltriazolo-Pleuromutilin Conjugate with Pronounced Antibacterial Activity against MRSA

Conjugation of pleuromutilin is an attractive strategy for the development of novel antibiotics and the fight against multiresistant bacteria as the class is associated with low rates of resistance and cross-resistance development. Herein, the preparation of 35 novel (+)-pleuromutilin conjugates is reported. Their design was based on a synthetically more efficient benzyl adaption of a potent lead but still relied on the Cu(I)-catalyzed alkyne-azide [3 + 2] cycloaddition for conjugation onto pleuromutilin. Their antibacterial activity was evaluated against the multiresistant Staphylococcus aureus strain USA300 for which they displayed moderate to excellent activity. Compound 35, bearing a para-benzyladenine substituent, proved particularly potent against USA300 and additional strains of MRSA and displayed as importantly no cytotoxicity in four mammalian cell lines. Structure-activity relationship analysis revealed that the purine 6-amino is essential for high potency, likely because of strong hydrogen bonding with the RNA backbone of C2469, as suggested by a molecular model based on the MM-GBSA approach.

Antibacterial properties and clinical potential of pleuromutilins

Covering: up to 2018Pleuromutilins are a clinically validated class of antibiotics derived from the fungal diterpene (+)-pleuromutilin (1). Pleuromutilins inhibit bacterial protein synthesis by binding to the peptidyl transferase center (PTC) of the ribosome. In this review we summarize the biosynthesis and recent total syntheses of (+)-pleuromutilin (1). We review the mode of interaction of pleuromutilins with the bacterial ribosome, which involves binding of the C14 extension and the tricyclic core to the P and A sites of the PTC, respectively. We provide an overview of existing clinical agents, and discuss the three primary modes of bacterial resistance (mutations in ribosomal protein L3, Cfr methylation, and efflux). Finally we collect structure-activity relationships from publicly available reports, and close with some forward looking statements regarding future development.

Synthesis and antibacterial activities of novel pleuromutilin derivatives bearing an aminothiophenol moiety

We synthesized a series of novel thioether pleuromutilin derivatives incorporating 2-aminothiophenol moieties into the C14 side chain via acylation reactions under mild conditions. We evaluated the in-vitro antibacterial activities of the derivatives against methicillin-resistant Staphylococcus aureus (MRSA, ATCC 43300), Staphylococcus aureus (ATCC 29213) and Escherichia coli (ATCC 25922). The majority of the synthesized derivatives possessed moderate antibacterial activities. Compound 8 was found to be the most active antibacterial derivative against MRSA. We conducted docking experiments to understand the possible mode of interactions between compounds 8, 9b, 11a and 50S ribosomal subunit. The docking results proved that there is a reasonable correlation between the binding free energy and the antibacterial activity. Compound 8 was evaluated for its in-vivo antibacterial activity and showed higher efficacy than tiamulin against MRSA in mouse infection model.

Conjugates of Classical DNA/RNA Binder with Nucleobase: Chemical, Biochemical and Biomedical Applications

Among the most intensively studied classes of small molecules (molecular weight < 650) in biomedical research are small molecules that non-covalently bind to DNA/RNA, and another intensively studied class is nucleobase derivatives. Both classes have been intensively elaborated in many books and reviews. However, conjugates consisting of DNA/RNA binder covalently linked to nucleobase are much less studied and have not been reviewed in the last two decades. Therefore, this review summarized reports on the design of classical DNA/RNA binder - nucleobase conjugates, as well as data about their interactions with various DNA or RNA targets, and even in some cases protein targets are involved. According to these data, the most important structural aspects of selective or even specific recognition between small molecule and target are proposed, and where possible related biochemical and biomedical aspects were discussed. The general conclusion is that this, rather new class of molecules showed an amazing set of recognition tools for numerous DNA or RNA targets in the last two decades, as well as few intriguing in vitro and in vivo selectivities. Several lead research lines show promising advancements toward either novel, highly selective markers or bioactive, potentially druggable molecules.

Effect of uridine protecting groups on the diastereoselectivity of uridine-derived aldehyde 5'-alkynylation

The 5'-alkynylation of uridine-derived aldehydes is described. The addition of alkynyl Grignard reagents on the carbonyl group is significantly influenced by the 2',3'-di-O-protecting groups (R1): O-alkyl groups led to modest diastereoselectivities (65:35) in favor of the 5'R-isomer, whereas O-silyl groups promoted higher diastereoselectivities (up to 99:1) in favor of the 5'S-isomer. A study related to this protecting group effect on the diastereoselectivity is reported.

Design, synthesis, and structure-activity relationship studies of novel pleuromutilin derivatives having a piperazine ring

A series of novel pleuromutilin derivatives possessing piperazine moieties were synthesized under mild conditions. The in vitro antibacterial activities of these derivatives against Staphylococcus aureus and Escherichia coli were tested by the agar dilution method. Structure-activity relationship studies resulted in compounds 11b, 13b, and 14a with the most potent in vitro antibacterial activity among the series (minimal inhibitory concentration = 0.0625-0.125 μg/mL). The binding of compounds 11b, 13b, and 14a to the E. coli ribosome was investigated by molecular modeling, and it was found that there is a reasonable correlation between the binding free energy and the antibacterial activity.

Site-Specific Labeling of MicroRNA Precursors: A Structure-Activity Relationship Study

Functionalized oligoribonucleotides are essential tools in RNA chemical biology. Various synthetic routes have been developed over recent years to conjugate functional groups to oligoribonucleotides. However, the presence of the functional group on the oligoribonucleotide backbone can lead to partial or total loss of biological function. The limited knowledge concerning the positioning of functional groups therefore represents a hurdle for the development of oligoribonucleotide chemical tools. Here we describe a systematic investigation of site-specific labeling of pre-miRNAs to identify positions for the incorporation of functional groups, in order not to hinder their processing into active mature miRNAs.

Synthesis and Biological Evaluation of Triazolyl 13α-Estrone-Nucleoside Bioconjugates

2′-Deoxynucleoside conjugates of 13α-estrone were synthesized by applying the copper-catalyzed alkyne–azide click reaction (CuAAC). For the introduction of the azido group the 5′-position of the nucleosides and a propargyl ether functional group on the 3-hydroxy group of 13α-estrone were chosen. The best yields were realized in our hands when the 3′-hydroxy groups of the nucleosides were protected by acetyl groups and the 5′-hydroxy groups were modified by the tosyl–azide exchange method. The commonly used conditions for click reaction between the protected-5′-azidonucleosides and the steroid alkyne was slightly modified by using 1.5 equivalent of Cu(I) catalyst. All the prepared conjugates were evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cell lines (HeLa, MCF-7 and A2780) and the potential inhibitory activity of the new conjugates on human 17β-hydroxysteroid dehydrogenase 1 (17β-HSD1) was investigated via in vitro radiosubstrate incubation. Some protected conjugates displayed moderate antiproliferative properties against a panel of human adherent cancer cell lines (the protected cytidine conjugate proved to be the most potent with IC₅₀ value of 9 μM). The thymidine conjugate displayed considerable 17β-HSD1 inhibitory activity (IC₅₀ = 19 μM).

DNA Three Way Junction Core Decorated with Amino Acids-Like Residues-Synthesis and Characterization

Construction and physico-chemical behavior of DNA three way junction (3WJ) functionalized by protein-like residues (imidazole, alcohol and carboxylic acid) at unpaired positions at the core is described. One 5'-C(S)-propargyl-thymidine nucleotide was specifically incorporated on each strand to react through a post synthetic CuACC reaction with either protected imidazolyl-, hydroxyl- or carboxyl-azide. Structural impacts of 5'-C(S)-functionalization were investigated to evaluate how 3WJ flexibility/stability is affected.

3'-O- and 5'-O-Propargyl Derivatives of 5-Fluoro-2'-Deoxyuridine: Synthesis, Cytotoxic Evaluation and Conformational Analysis

A series of new 3'-O- and 5'-O-propargyl derivatives of 5-fluoro-2'-deoxyuridine (1-4) was synthesized by means of propargyl reaction of properly blocked nucleosides (2,4), followed by the deprotection reaction with ammonium fluoride. The synthesized propargylated 5-fluoro-2'-deoxyuridine analogues (1-4) were evaluated for their cytotoxic activity in three human cancer cell lines: cervical (HeLa), oral (KB) and breast (MCF-7), using the sulforhodamine B (SRB) assay. The highest activity and the best SI coefficient in all of the investigated cancer cells were displayed by 3'-O-propargyl-5-fluoro-2'-deoxyuridine (1), and its activity was higher than that of the parent nucleoside. The other new compounds exhibited moderate activity in all of the used cell lines.

Flexible double-headed cytosine-linked 2'-deoxycytidine nucleotides. Synthesis, polymerase incorporation to DNA and interaction with DNA methyltransferases

New types of double-headed 2'-deoxycytidine 5'-O-triphosphates (dC(XC)TPs) bearing another cytosine or 5-fluorocytosine linked through a flexible propargyl, homopropargyl or pent-1-ynyl linker to position 5 were prepared by the aqueous Sonogashira cross-coupling reactions of 5-iodo-dCTP with the corresponding (fluoro)cytosine-alkynes. The modified dC(XC)TPs were good substrates for DNA polymerases and were used for enzymatic synthesis of cytosine-functionalized DNA by primer extension or PCR. The cytosine- or fluorocytosine-linked DNA probes did not significantly inhibit DNA methyltransferases and did not cross-link to these proteins.

Three new double-headed nucleotides with additional nucleobases connected to C-5 of pyrimidines; synthesis, duplex and triplex studies

In the search for double-coding DNA-systems, three new pyrimidine nucleosides, each coded with an additional nucleobase anchored to the major groove face, are synthesized. Two of these building blocks carry a thymine at the 5-position of 2'-deoxyuridine through a methylene linker and a triazolomethylene linker, respectively. The third building block carries an adenine at the 6-position of pyrrolo-2'-deoxycytidine through a methylene linker. These double-headed nucleosides are introduced into oligonucleotides and their effects on the thermal stabilities of duplexes are studied. All studied double-headed nucleotide monomers reduce the thermal stability of the modified duplexes, which is partially compensated by using consecutive incorporations of the modified monomers or by flanking the new double-headed analogs with members of our former series containing propyne linkers. Also their potential in triplex-forming oligonucleotides is studied for two of the new double-headed nucleotides as well as the series of analogs with propyne linkers. The most stable triplexes are obtained with single incorporations of additional pyrimidine nucleobases connected via the propyne linker.

Synthesis and Antibacterial Activity of Novel Pleuromutilin Derivatives

In this study we describe the design, synthesis, and antibacterial activity of novel pleuromutilin analogs. A series of new compounds containing piperazine and alkylamino or arylamino groups was synthesized. The new compounds were characterized via (1)H-NMR, (13)C-NMR, Fourier transform (FT)-IR and MS, and were further evaluated for their in vitro activity against seven Gram-positive, and one Gram-negative, pathogens. Antibacterial data revealed that all compounds exhibited moderate to good antibacterial activities against sensitive Gram-positive pathogens. Specifically, 9d displayed the best activity: its activity to Staphylococcus aureus (ATCC25923) is 0.125 µg/mL, which is equal to the control compound tiamulin. The antibacterial activities of 9d to Streptococcus suis (minimum inhibitory concentration (MIC) of 2 µg/mL), Streptococcus agalactiae (MIC of 0.5 µg/mL), and Streptococcus dysgalactiae (MIC of 0.5 µg/mL) were also excellent compared with the control drug erythromycin (MIC of >128 µg/mL). The binding modes of these compounds with active sites were calculated using the programs of Molecular Operating Environment (MOE) and Pymol.

Double-coding nucleic acids: introduction of a nucleobase sequence in the major groove of the DNA duplex using double-headed nucleotides

A series of double-headed nucleosides were synthesized using the Sonogashira cross-coupling reaction. In the reactions, additional nucleobases (thymine, cytosine, adenine, or guanine) were attached to the 5-position of 2'-deoxyuridine or 2'-deoxycytidine through a propyne linker. The modified nucleosides were incorporated into oligonucleotides, and these were combined in different duplexes that were analyzed by thermal denaturation studies. All of the monomers were well tolerated in the DNA duplexes and induced only small changes in the thermal stability. Consecutive incorporations of the monomers led to increases in duplex stability owing to increased stacking interactions. The modified nucleotide monomers maintained the Watson-Crick base pair fidelity. Stable duplexes were observed with heavily modified oligonucleotides featuring 14 consecutive incorporations of different double-headed nucleotide monomers. Thus, modified duplexes with an array of nucleobases on the exterior of the duplex were designed. Molecular dynamics simulations demonstrated that the additional nucleobases could expose their Watson-Crick and/or Hoogsteen faces for recognition in the major groove. This presentation of nucleobases may find applications in providing molecular information without unwinding the duplex.

Design, synthesis, and structure-activity relationship studies of novel thioether pleuromutilin derivatives as potent antibacterial agents

A series of novel thioether pleuromutilin derivatives incorporating various heteroaromatic substituents into the C14 side chain have been reported. Structure-activity relationship (SAR) studies resulted in compounds 52 and 55 with the most potent in vitro antibacterial activity among the series (MIC = 0.031-0.063 μg/mL). Further optimization to overcome the poor water solubility of compound 55 resulted in compounds 87, 91, 109, and 110 possessing good in vitro antibacterial activity with increased hydrophilicity. Compound 114, the water-soluble phosphate prodrug of compound 52, was also prepared and evaluated. Among the derivatives, compound 110 showed moderate pharmacokinetic profiles and good in vivo efficacy in both MSSA and MRSA systemic infection models. Compound 110 was further evaluated in CYP450 inhibition assay and displayed intermediate in vitro inhibition of CYP3A4.

A click chemistry approach to pleuromutilin derivatives. Part 3: extended footprinting analysis and excellent MRSA inhibition for a derivative with an adenine phenyl side chain

Five promising pleuromutilin derivatives from our former studies, all containing adenine on various linkers, were supplemented with two new compounds. The binding to Escherichia coli ribosomes was verified by extensive chemical footprinting analysis. The ability to inhibit bacterial growth was investigated on two Staphylococcus aureus strains and compared to the pleuromutilin drugs tiamulin and valnemulin. Three of the compounds show an effect similar to tiamulin and one compound shows an excellent effect similar to valnemulin.

Multicomponent click synthesis of new 1,2,3-triazole derivatives of pyrimidine nucleobases: promising acidic corrosion inhibitors for steel

A series of new mono-1,2,3-triazole derivatives of pyrimidine nucleobases were synthesized by one-pot copper(I)-catalyzed 1,3-dipolar cycloaddition reactions between N-1-propargyluracil and thymine, sodium azide and several benzyl halides. The desired heterocyclic compounds were obtained in good yields and characterized by NMR, IR, and high resolution mass spectrometry. These compounds were investigated as corrosion inhibitors for steel in 1 M HCl solution, using electrochemical impedance spectroscopy (EIS) technique. The results indicate that these heterocyclic compounds are promising acidic corrosion inhibitors for steel.

Synthesis and biological evaluation of novel 1,2,3-triazolonucleotides

A general procedure for the preparation of 1,2,3-triazole analogs of nucleosides from diethyl 2-azidoethoxymethyl- and 2-azidoethoxyethylphosphonates was elaborated. The application of microwave irradiation shortened the reaction time to 10 min in comparison to ca. 48 h when 1,3-dipolar cycloadditions were performed under standard conditions. All compounds were evaluated in vitro for inhibitory activity against a broad variety of DNA and RNA viruses. None of the compounds were antivirally active at subtoxic concentrations. Compound 17k exhibited moderate inhibitory effects on the proliferation of human T-lymphocyte cells (IC50=64 µM for CEM).

Design, synthesis, antiviral, and cytotoxic evaluation of novel phosphonylated 1,2,3-triazoles as acyclic nucleotide analogues

The 1,3-dipolar cycloaddition of diethyl 2-azidoethyl-, 3-azidopropyl-, 2-azido-1-hydroxyethyl-, 3-azido-2-hydroxypropylphosphonates with selected N-propargyl nucleobases gave a series of the phosphonylated 1,2,3-triazole acyclonucleosides in which the phosphonate residue and nucleobases were linked by three- and four-carbon chains. Under standard conditions (TMSBr, ethanol), all synthesized O,O-diethylphosphonates were transformed into the respective phosphonic acids. All compounds were evaluated in vitro for activity against a broad variety of DNA and RNA viruses. Unfortunately, no antiviral activity was observed at 100 μM.

Synthesis and biological properties of C-2 triazolylinosine derivatives

O(6)-(Benzotriazol-1H-yl)guanosine and its 2'-deoxy analogue are readily converted to the O(6)-allyl derivatives that upon diazotization with t-BuONO and TMS-N(3) yield the C-2 azido derivatives. We have previously analyzed the solvent-dependent azide·tetrazole equilibrium of C-6 azidopurine nucleosides, and in contrast to these, the O(6)-allyl C-2 azido nucleosides appear to exist predominantly in the azido form, relatively independent of solvent polarity. In the presently described cases, the tetrazole appears to be very minor. Consistent with the presence of the azido functionality, each neat C-2 azide displayed a prominent IR band at 2126-2130 cm(-1). A screen of conditions for the ligation of the azido nucleosides with alkynes showed that CuCl in t-BuOH/H(2)O is optimal, yielding C-2 1,2,3-triazolyl nucleosides in 70-82% yields. Removal of the silyl groups with Et(3)N·3HF followed by deallylation with PhSO(2)Na/Pd(PPh(3))(4) gave the C-2 triazolylinosine nucleosides. In a continued demonstration of the versatility of O(6)-(benzotriazol-1H-yl)purine nucleosides, one C-2 triazolylinosine derivative was converted to two adenosine analogues via these intermediates, under mild conditions. Products were desilylated for biological assays. The two C-2 triazolyl adenosine analogues demonstrated pronounced antiproliferative activity in human ovarian and colorectal carcinoma cell cultures. When evaluated for antiviral activity against a broad spectrum of DNA and RNA viruses, some of the C-2 triazolylinosine derivatives showed modest inhibitory activity against cytomegalovirus.

A click chemistry approach to pleuromutilin derivatives, part 2: conjugates with acyclic nucleosides and their ribosomal binding and antibacterial activity

Pleuromutilin is an antibiotic that binds to bacterial ribosomes and thereby inhibit protein synthesis. A new series of semisynthetic pleuromutilin derivatives were synthesized by a click chemistry strategy. Pleuromutilin was conjugated by different linkers to a nucleobase, nucleoside, or phenyl group, as a side-chain extension at the C22 position of pleuromutilin. The linkers were designed on the basis of the best linker from our first series of pleuromutilin derivatives following either conformational restriction or an isosteric methylene to oxygen exchange. The binding of the new compounds to the Escherichia coli ribosome was investigated by molecular modeling and chemical footprinting of nucleotide U2506, and it was found that all the derivatives bind to the specific site and most of them better than pleuromutilin itself. The effect of the side-chain extension was also explored by chemical footprinting of nucleotide U2585, and the results showed that all the compounds interact with this position to varying degrees. Derivatives with a conformational restriction of the linker generally had a higher affinity than derivatives with an isosteric exchange of one of the carbons in the linker with a hydrophilic oxygen. A growth inhibition assay with three different bacterial strains showed significant activity of several of the new compounds.

Synthesis and Antibacterial Activities of Pleuromutilin Derivatives with Thiazole-5-Carboxamide and Thioether Moiety

Seven novel pleuromutilin derivatives with thiazole-5-carboxamide and thioether moiety in the C14 side chain were designed and synthesised. The antibacterial activities of the target compounds were tested via agar-well diffusion method in vitro. The results showed that three target compounds still had antibacterial activity against Staphylococcus aureus ATCC26112 and Staphylococcus aureus SC at a low concentration of 0.05 μg mL−1.

Azide-tetrazole equilibrium of C-6 azidopurine nucleosides and their ligation reactions with alkynes

Facile syntheses of C-6 azidopurine ribonucleosides and 2'-deoxyribonucleosides have been developed. For silyl- and acetyl-protected as well as unprotected nucleosides, access to the azido derivatives could be readily attained via displacement of BtO(-) from the O(6)-(benzotriazol-1-yl)inosine nucleosides by azide anion. Use of diphenylphosphoryl azide/DBU as a simple route to the acetyl-protected azido nucleosides was also evaluated, but this proved to be inferior. Since these azido nucleosides can exist in an azide.tetrazole equilibrium, the effect of solvent polarity on this equilibrium was investigated. Subsequently, a detailed analysis of Cu-mediated azide-alkyne ("click") ligation was undertaken. Biphasic CH(2)Cl(2)/H(2)O medium proved to be best for the ligation reactions, suppressing the undesired azide reduction that was competing. Interestingly, although the tetrazolyl isomer predominates (ca. 80%) in CD(2)Cl(2) and in CD(2)Cl(2)/D(2)O, the Cu-catalyzed click reactions proceed smoothly with the silyl-protected ribo- and 2'-deoxyribonucleosides, leading to the C-6 triazolyl products in good to excellent yields. Thus, depletion of the azido form from the reaction mixture shifts the azide.tetrazole equilibrium, eventually resulting in complete consumption of azide and tetrazole. In several cases, major and minor azide-alkyne ligation products were observed, and characterization data are provided for both. In order to confirm the regiochemistry leading to the major isomer, one product was crystallized and evaluated by X-ray crystallography. The Cu-catalyzed azide-alkyne ligation is clearly efficient and significantly superior to thermal reactions, which were slow. Biological evaluation showed low cytotoxicities for the agents, suggesting their usefulness as biological probes.

The A-Z of bacterial translation inhibitors

Protein synthesis is one of the major targets in the cell for antibiotics. This review endeavors to provide a comprehensive "post-ribosome structure" A-Z of the huge diversity of antibiotics that target the bacterial translation apparatus, with an emphasis on correlating the vast wealth of biochemical data with more recently available ribosome structures, in order to understand function. The binding site, mechanism of action, and modes of resistance for 26 different classes of protein synthesis inhibitors are presented, ranging from ABT-773 to Zyvox. In addition to improving our understanding of the process of translation, insight into the mechanism of action of antibiotics is essential to the development of novel and more effective antimicrobial agents to combat emerging bacterial resistance to many clinically-relevant drugs.

Single 23S rRNA mutations at the ribosomal peptidyl transferase centre confer resistance to valnemulin and other antibiotics in Mycobacterium smegmatis by perturbation of the drug binding pocket

Tiamulin and valnemulin target the peptidyl transferase centre (PTC) on the bacterial ribosome. They are used in veterinary medicine to treat infections caused by a variety of bacterial pathogens, including the intestinal spirochetes Brachyspira spp. Mutations in ribosomal protein L3 and 23S rRNA have previously been associated with tiamulin resistance in Brachyspira spp. isolates, but as multiple mutations were isolated together, the roles of the individual mutations are unclear. In this work, individual 23S rRNA mutations associated with pleuromutilin resistance at positions 2055, 2447, 2504 and 2572 (Escherichia coli numbering) are introduced into a Mycobacterium smegmatis strain with a single rRNA operon. The single mutations each confer a significant and similar degree of valnemulin resistance and those at 2447 and 2504 also confer cross-resistance to other antibiotics that bind to the PTC in M. smegmatis. Antibiotic footprinting experiments on mutant ribosomes show that the introduced mutations cause structural perturbations at the PTC and reduced binding of pleuromutilin antibiotics. This work underscores the fact that mutations at nucleotides distant from the pleuromutilin binding site can confer the same level of valnemulin resistance as those at nucleotides abutting the bound drug, and suggests that the former function indirectly by altering local structure and flexibility at the drug binding pocket.