Design, synthesis and biological activities of novel pleuromutilin derivatives with a substituted triazole moiety as potent antibacterial agents

Synthesis and biological evaluation of novel 1,2,3,4-tetrahydro-β-carboline derivatives as potential antibacterial agents

The quest for novel antibacterial agents is imperative in the face of escalating antibiotic resistance. Naturally occurring tetrahydro-β-carboline (THβC) alkaloids have been highlighted due to their significant biological derivatives. However, these structures have been little explored for antibacterial drugs development. In this study, a series of 1,2,3,4-THβC derivatives were synthesized and assessed for their antibacterial prowess against both gram-positive and gram-negative bacteria. The compounds exhibited moderate to good antibacterial activity, with some compounds showing superior efficacy against gram-positive bacteria, especially methicillin-resistant Staphylococcus aureus (MRSA), to that of Gentamicin. Among these analogs, compound 3k emerged as a hit compound, demonstrating rapid bactericidal action and a significant post-antibacterial effect, with significant cytotoxicity towards human LO2 and HepG2 cells. In addition, compound 3k (10 mg/kg) showed comparable anti-MRSA efficacy to Ciprofloxacin (2 mg/kg) in a mouse model of abdominal infection. Overall, the present findings suggested that THβC derivatives based on the title compounds hold promising applications in the development of antibacterial drugs.

Discovery of 1,2,3-triazole-based pleuromutilin derivatives as potent gram-positive antibacterial agents

A novel class of pleuromutilin derivatives possessing 1,2,3-triazole as the linker connected to phenyl analogues were designed. The antibacterial properties of the prepared compounds were assessed in vitro against five strains (E. coli, S. aureus, S. epidermidis, and E. faecalis). Most of the tested compounds displayed potent antibacterial activities against gram-positive bacteria and 14-O-[2-(4-((2,4-dinitrophenoxy)-methyl-1H-1,2,3-triazol-1-yl) acetamide)-2-methylpropan-2-yl) thioacetyl]mutilin (7c) exerted antibacterial activities against S. aureus, MRSA and S. epidermidis with MIC values 0.0625 μg/mL, representing 64-fold, 4-fold and 8-fold higher than tiamulin respectively. Compound 6e, 7c and 8c were chosen to carry out killing kinetics, which exhibited concentration-dependent effect. Subsequently, molecular modeling was conducted to further explore the binding of compound 6e, 7a, 7c, 8c and tiamulin with 50S ribosomal subunit from deinococcus radiodurans. The investigation revealed that the main interactions between compound 7c and the ribosomal residues were three hydrogen bonds, π-π, and p-π conjugate effects. Additionally, the free binding energy and docking score of 7c with the ribosome demonstrated the lowest values of -11.90 kcal/mol and -7.97 kcal/mol, respectively, consistent with its superior antibacterial activities.

Rational molecular design converting fascaplysin derivatives to potent broad-spectrum inhibitors against bacterial pathogens via targeting FtsZ

The filamentous temperature-sensitive mutant Z protein (FtsZ), a key player in bacterial cell division machinery, emerges as an attractive target to tackle the plight posed by the ever growing antibiotic resistance over the world. Therefore in this regard, agents with scaffold diversities and broad-spectrum antibacterial activity against Gram-positive and Gram-negative pathogens are highly needed. In this study, a new class of marine-derived fascaplysin derivatives has been designed and synthesized by Suzuki-Miyaura cross-coupling. Some compounds exhibited potent bactericidal activities against a panel of Gram-positive (MIC = 0.024-6.25 μg/mL) and Gram-negative (MIC = 1.56-12.5 μg/mL) bacteria including methicillin-resistant S. aureus (MRSA). They exerted their effects by dual action mechanism via disrupting the integrity of the bacterial cell membrane and targeting FtsZ protein. These compounds stimulated polymerization of FtsZ monomers and bundling of the polymers, and stabilized the resulting polymer network, thus leading to the dysfunction of FtsZ in cell division. In addition, these agents showed negligible hemolytic activity and low cytotoxicity to mammalian cells. The studies on docking and molecular dynamics simulations suggest that these inhibitors bind to the hydrophilic inter-domain cleft of FtsZ protein and the insights obtained in this study would facilitate the development of potential drugs with broad-spectrum bioactivities.

Recent advances in developing modified C14 side chain pleuromutilins as novel antibacterial agents

Owing to the increasing resistance to most existing antimicrobial drugs, research has shifted towards developing novel antimicrobial agents with mechanisms of action distinct from those of current clinical options. Pleuromutilins are antibiotics known for their distinct mechanism of action, inhibiting bacterial protein synthesis by binding to the peptidyl transferase center of the ribosome. Recent studies have revealed that pleuromutilin derivatives can disrupt bacterial cell membranes, thereby enhancing antibacterial efficacy. Both marketed pleuromutilin derivatives and those in clinical trials have been developed by structurally modifying the pleuromutilin C14 side chain to improve their antimicrobial activity. Therefore, this review aims to review advancement in the chemical structural characteristics, antibacterial activities, and structure-activity relationship studies of pleuromutilins, specifically focusing on modifications made to the C14 side chain in recent years. These findings provide a valuable reference for future research and development of pleuromutilins.

Design and Synthesis of Pleuromutilin Derivatives as Antibacterial Agents Using Quantitative Structure-Activity Relationship Model

The quantitative structure-activity relationship (QSAR) is one of the most popular methods for the virtual screening of new drug leads and optimization. Herein, we collected a dataset of 955 MIC values of pleuromutilin derivatives to construct a 2D-QSAR model with an accuracy of 80% and a 3D-QSAR model with a non-cross-validated correlation coefficient (r2) of 0.9836 and a cross-validated correlation coefficient (q2) of 0.7986. Based on the obtained QSAR models, we designed and synthesized pleuromutilin compounds 1 and 2 with thiol-functionalized side chains. Compound 1 displayed the highest antimicrobial activity against both Staphylococcus aureus ATCC 29213 (S. aureus) and Methicillin-resistant Staphylococcus aureus (MRSA), with minimum inhibitory concentrations (MICs) < 0.0625 μg/mL. These experimental results confirmed that the 2D and 3D-QSAR models displayed a high accuracy of the prediction function for the discovery of lead compounds from pleuromutilin derivatives.

Click chemistry-aided drug discovery: A retrospective and prospective outlook

Click chemistry has emerged as a valuable tool for rapid compound synthesis, presenting notable advantages and convenience in the exploration of potential drug candidates. In particular, in situ click chemistry capitalizes on enzymes as reaction templates, leveraging their favorable conformation to selectively link individual building blocks and generate novel hits. This review comprehensively outlines and introduces the extensive use of click chemistry in compound library construction, and hit and lead discovery, supported by specific research examples. Additionally, it discusses the limitations and precautions associated with the application of click chemistry in drug discovery. Our intention for this review is to contribute to the development of a modular synthetic approach for the rapid identification of drug candidates.

Design, Synthesis and Biological Evaluation of Novel Pleuromutilin Derivatives Containing 6-Chloro-1-R-1H-pyrazolo[3,4-d]pyrimidine-4-amino Side Chain

Two series of pleuromutilin derivatives were designed and synthesized as inhibitors against Staphylococcus aureus (S. aureus). 6-chloro-4-amino-1-R-1H-pyrazolo[3,4-d]pyrimidine or 4-(6-chloro-1-R-1H-pyrazolo[3,4-d]pyrimidine-4-yl)amino-phenylthiol were connected to pleuromutilin. A diverse array of substituents was introduced at the N-1 position of the pyrazole ring. The in vitro antibacterial activities of these semisynthetic derivatives were evaluated against two standard strains, Methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300, Staphylococcus aureus (S. aureus), ATCC 29213 and two clinical S. aureus strains (144, AD3) using the broth dilution method. Compounds 12c, 19c and 22c (MIC = 0.25 μg/mL) manifested good in vitro antibacterial ability against MRSA which was similar to that of tiamulin (MIC = 0.5 μg/mL). Among them, compound 22c killed MRSA in a time-dependent manner and performed faster bactericidal kinetics than tiamulin in time-kill curves. In addition, compound 22c exhibited longer PAE than tiamulin, and showed no significant inhibition on the cell viability of RAW 264.7, Caco-2 and 16-HBE cells at high doses (≤8 μg/mL). The neutropenic murine thigh infection model study revealed that compound 22c displayed more effective in vivo bactericidal activity than tiamulin in reducing MRSA load. The molecular docking studies indicated that compound 22c was successfully localized inside the binding pocket of 50S ribosomal, and four hydrogen bonds played important roles in the binding of them.

Design, Synthesis and Biological Evaluation of Novel Indole-piperazine Derivatives as Antibacterial Agents

Herein, a series of novel indole-piperazine derivatives were synthesized. Bioassay results showed the title compounds exhibited moderate to good bacteriostatic efficacy against the test Gram-positive bacteria and Gram-negative bacteria, including methicillin-resistant Staphylococcus aureus (MRSA). Among theses compounds, three remarkable compounds 8f, 9a, and 9h exhibited superior in vitro antibacterial profiles for anti- S. aureus and anti-MRSA to that of gentamicin. Hit compound 9a manifested a rapid bactericidal kinetic effect on MRSA,with no resistance observed after 19 days of sequential passaging. And 8 µg/mL of compound 9a displayed considerable post antibacterial effects to that of ciprofloxacin at the concentration of 2 µg/mL. Cytotoxic and ADMET studies indicated, to some extent, compounds 8f, 9a, and 9h were up to the standard for antibacterial drugs. These results suggest that indole/piperazine derivatives based on the title compounds can serve as a new scaffold for antimicrobial development.

Advances of bioorthogonal coupling reactions in drug development

Currently, bioorthogonal coupling reactions have garnered considerable interest due to their high substrate selectivity and less restrictive reaction conditions. During recent decades, bioorthogonal coupling reactions have emerged as powerful tools in drug development. This review describes the current applications of bioorthogonal coupling reactions in compound library building mediated by the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction and in situ click chemistry or conjunction with other techniques; druggability optimization with 1,2,3-triazole groups; and intracellular self-assembly platforms with ring tension reactions, which are presented from the viewpoint of drug development. There is a reasonable prospect that bioorthogonal coupling reactions will accelerate the screening of lead compounds, the designing strategies of small molecules and expand the variety of designed compounds, which will be a new trend in drug development in the future.

Design, synthesis, antibacterial activity evaluation and molecular docking study of pleuromutilin derivatives bearing amide side chains

A seize of pleuromutilin derivatives containing amide side chains were designed and synthesized as potential antibiotics against Methicillin-resistant Staphylococcus aureus (MRSA). Among all target compounds (compounds 11-30), compound 25 was found to have the strongest antibacterial activity against MRSA (minimum inhibitory concentration = 0.5 μg/ml). The result of the time-kill curves indicated that compound 25 could repress the growth of MRSA in vitro obviously (-3.72 log10 CFU/ml reduction). Furthermore, molecular docking studies demonstrated that compound 25 was localized in the binding pocket of 50S ribosomal subunit (ΔGb = -8.99 kcal/mol). Besides, compound 25 displayed low cytotoxicity to RAW 264.7 cells. The results suggested that compound 25 might be further developed into a novel antimicrobial agent against MRSA.

Design, synthesis and biological evaluation of novel pleuromutilin derivatives possessing 4-aminothiophenol linker as promising antibacterial agents

A series of novel pleuromutilin derivatives containing 4-aminothiophenol moieties have been designed and synthesized as promising antibacterial agents against Methicillin-resistant Staphylococcus aureus (MRSA). The in vitro antibacterial activity of these semisynthetic derivatives against 4 strains of S. aureus (MRSA ATCC 43300, S. aureus ATCC 29213, S. aureus 144 and S. aureus AD3) was evaluated by the broth dilution method. Most of the synthesized derivatives displayed prominent in vitro activity (MIC ≤ 0.5 µg/mL). 12 Compounds possessed superior antibacterial activity against MRSA compared with valnemulin and retapamulin (MIC = 0.0625 µg/mL). Compounds 12, 16a, 16c and 19 exhibited the most effective antibacterial effect against MRSA (MIC = 0.015 µg/mL). Furthermore, the time-kill curves showed compounds 12 and 19 had a certain inhibitory effect against MRSA in vitro. Compounds 12 and 19 possessed longer PAE time (2.74 h and 3.11 h, respectively) than tiamulin (PAE = 2.04 h) against MRSA after exposure at 4 × MIC concentration for 2 h. Compounds 12 and 19 also displayed superior in vivo antibacterial efficacy (-1.20 log10 CFU/mL and -1.21 log10 CFU/mL, respectively) than tiamulin (-0.75 log10 CFU/mL) in reducing MRSA load in the mice thigh infection model. In addition, compound 19 had barely inhibitory effect on RAW 264.7 and 16HBE cells at 8 µg/mL. In molecular docking study, upon docking into the 50S ribosomal subunit, the binding free energy (ΔGb) of compound 12 and 19 was calculated to be -9.02 kcal/mol and -9.89 kcal/mol, respectively.

In Vitro and In Vivo Antibacterial Activity, Toxicity and Resistance Analysis of Pleuromutilin Derivative Z33 against Methicillin-Resistant Staphylococcus aureus

The novel pleuromutilin derivative, which showed excellent in vitro antibacterial activity against MRSA, 22-(2-(2-(4-((4-(4-nitrophenyl)piperazin-1-yl)methyl)-1H-1,2,3-triazol-1-yl)acetamido)phenyl)thioacety-l-yl-22-deoxypleuromutilin (Z33), was synthesized and characterized in our previous work. In this study, the preliminary pharmacodynamics and safety of Z33 were further evaluated. In in vitro antibacterial activity assays, Z33 was found to be a potent bactericidal antibiotic against MRSA that induced dose-dependent growth inhibition and long-term post-antibiotic effect (PAE). The drug-resistance test demonstrated that Z33 possessed a narrow mutant selection window and lower propensities to select resistance than that of tiamulin. Cytochrome P450 (CYP450) inhibition assay determined that the inhibitory effect of Z33 was similar to that of tiamulin against the activity of CYP3A4, and was lower than that of tiamulin on the activity of CYP2E1. Toxicity determination showed that both Z33 and tiamulin displayed low cytotoxicity of RAW264.7 cells. Furthermore, Z33 was found to be a high-security compound with a 50% lethal dose (LD₅₀) above 5000 mg/kg in the acute oral toxicity test in mice. In an in vivo antibacterial activity test, Z33 displayed better therapeutic effectiveness than tiamulin in the neutropenic mouse thigh infection model. In summary, Z33 was worthy of further development as a highly effective and safe antibiotic agent against MRSA infection.

Design, synthesis and antibacterial activity of novel pleuromutilin derivatives with thieno[2,3-d]pyrimidine substitution

A series of novel pleuromutilin derivatives with substituted thienopyrimidines were designed, synthesized, and evaluated for antibacterial act ivity. In this study, the activities of these compounds were investigated using the inhibition circle test, the minimum inhibitory concentration (MIC) test, real-time growth curves, time-kill kinetic assays, cytotoxicity assays, and molecular docking. Most of the tested compounds exhibited moderate antibacterial activity against Staphylococcus aureus, Streptococcus agalactiae, and Escherichia coli. Compound A11 was the most active and displayed bacteriostatic activities against methicillin-resistant S. aureus, with MIC values as low as 0.00191 μg/mL, which is 162 and 32 times lower than that of the marketed antibiotics tiamulin and retapamulin, respectively. Furthermore, the mechanism of action of A11 was confirmed by molecular docking studies.

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.

Discovery of Novel Pleuromutilin Derivatives as Potent Antibacterial Agents for the Treatment of MRSA Infection

A series of novel pleuromutilin derivatives containing nitrogen groups on the side chain of C14 were synthesized under mild conditions. Most of the synthesized derivatives displayed potent antibacterial activities. Compound 9 was found to be the most active antibacterial derivative against MRSA (MIC = 0.06 μg/mL). Furthermore, the result of time-kill curves showed that compound 9 had a certain inhibitory effect against MRSA in vitro. Moreover, according to a surface plasmon resonance (SPR) study, compound 9 (KD = 1.77 × 10-8 M) showed stronger affinity to the 50S ribosome than tiamulin (KD = 2.50 × 10-8 M). The antibacterial activity of compound 9 was further evaluated in an MRSA-infected murine thigh model. Compared to the negative control group, tiamulin reduced MRSA load (~0.7 log10 CFU/mL), and compound 9 performed a treatment effect (~1.3 log10 CFU/mL). In addition, compound 9 was evaluated in CYP450 inhibition assay and showed only moderate in vitro CYP3A4 inhibition (IC50 = 2.92 μg/mL).

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.

The Very First Modification of Pleuromutilin and Lefamulin by Photoinitiated Radical Addition Reactions-Synthesis and Antibacterial Studies

Pleuromutilin is a fungal diterpene natural product with antimicrobial properties, semisynthetic derivatives of which are used in veterinary and human medicine. The development of bacterial resistance to pleuromutilins is known to be very slow, which makes the tricyclic diterpene skeleton of pleuromutilin a very attractive starting structure for the development of new antibiotic derivatives that are unlikely to induce resistance. Here, we report the very first synthetic modifications of pleuromutilin and lefamulin at alkene position C19–C20, by two different photoinduced addition reactions, the radical thiol-ene coupling reaction, and the atom transfer radical additions (ATRAs) of perfluoroalkyl iodides. Pleuromutilin were modified with the addition of several alkyl- and aryl-thiols, thiol-containing amino acids and nucleoside and carbohydrate thiols, as well as perfluoroalkylated side chains. The antibacterial properties of the novel semisynthetic pleuromutilin derivatives were investigated on a panel of bacterial strains, including susceptible and multiresistant pathogens and normal flora members. We have identified some novel semisynthetic pleuromutilin and lefamulin derivatives with promising antimicrobial properties.

Modification of 5-methylphenanthridium from benzothiazoles to indoles as potent FtsZ inhibitors: Broadening the antibacterial spectrum toward vancomycin-resistant enterococci

The death caused by pathogenic bacteria has always been a severe threat to mankind. The prevalence of drug resistance among bacteria underscores an urgent goal for new antibacterial agents with novel mode of action. Here we first designed and synthesized a class of benzothiazolyl-5-methylphenanthridium derivatives and evaluated their antibacterial activity. On this basis, we further designed and synthesized another class of novel indolyl-5-methylphenanthridium derivatives by optimizing the benzothiazolyl-5-methylphenanthridium core and evaluated their antibacterial activity targeting the bacterial cell division protein FtsZ. The results showed that the indolyl-5-methylphenanthridium derivatives had greatly improved activity against various drug-resistant bacterial strains including methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus (VRE). Among them, compound C5 displayed excellent antibacterial activity against susceptible (MIC = 1 μg/mL), methicillin-resistant and clinical isolated S. aureus (MIC = 2 μg/mL). With low hemolytic activity towards mice red blood cells, C5 exhibited good antibacterial effect in vivo in preliminary pharmacodynamic assay. More importantly, C5 was difficult to induce bacterial resistance. Further mechanism studies proved that C5 could inhibit bacterial cell division by promoting FtsZ polymerization, leading to disorderly polymerization and disordered knots. Therefore, our findings suggest that this class of novel indolyl-5-methylphenanthridium derivatives are promising for future antibacterial agents.

Design, synthesis, in vitro and in vivo evaluation against MRSA and molecular docking studies of novel pleuromutilin derivatives bearing 1, 3, 4-oxadiazole linker

A class of pleuromutilin derivatives containing 1, 3, 4-oxadiazole were designed and synthesized as potential antibacterial agents against Methicillin-resistant staphylococcus aureus (MRSA). The ultrasound-assisted reaction was proposed as a green chemistry method to synthesize 1, 3, 4-oxadiazole derivatives (intermediates 85-110). Among these pleuromutilin derivatives, compound 133 was found to be the strongest antibacterial derivative against MRSA (MIC = 0.125 μg/mL). Furthermore, the result of the time-kill curves displayed that compound 133 could inhibit the growth of MRSA in vitro quickly (- 4.36 log10 CFU/mL reduction). Then, compound 133 (- 1.82 log10 CFU/mL) displayed superior in vivo antibacterial efficacy than tiamulin (- 0.82 log10 CFU/mL) in reducing MRSA load in mice thigh model. Besides, compound 133 exhibited low cytotoxicity to RAW 264.7 cells. Molecular docking studies revealed that compound 133 was successfully localized in the binding pocket of 50S ribosomal subunit (ΔGb = -10.50 kcal/mol). The results indicated that these pleuromutilin derivatives containing 1, 3, 4-oxadiazole might be further developed into novel antibiotics against MRSA.

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.

Cprp-An Unusual, Repetitive Protein Which Impacts Pleuromutilin Biosynthesis in the Basidiomycete Clitopilus passeckerianus

Interrogation of an EST database for Clitopilus passeckerianus identified a putative homolog to the unusual stress response gene from yeast; ddr48, as being upregulated under pleuromutilin production conditions. Silencing of this gene, named cprp, produced a population of transformants which demonstrated significantly reduced pleuromutilin production. Attempts to complement a Saccharomyces cerevisiae ddr48 mutant strain (strain Y16748) with cprp were hampered by the lack of a clearly identifiable mutant phenotype, but interestingly, overexpression of either ddr48 or cprp in S. cerevisiae Y16748 led to a conspicuous and comparable reduction in growth rate. This observation, combined with the known role of DDR48 proteins from a range of fungal species in nutrient starvation and stress responses, raises the possibility that this family of proteins plays a role in triggering oligotrophic growth. Localization studies via the production of a Cprp:GFP fusion protein in C. passeckerianus showed clear localization adjacent to the hyphal septa and, to a lesser extent, cell walls, which is consistent with the identification of DDR48 as a cell wall-associated protein in various yeast species. To our knowledge this is the first study demonstrating that a DDR48-like protein plays a role in the regulation of a secondary metabolite, and represents the first DDR48-like protein from a basidiomycete. Potential homologs can be identified across much of the Dikarya, suggesting that this unusual protein may play a central role in regulating both primary and secondary metabolism in fungi.