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.

New polycyclic tetramate macrolactams with antimycobacterial activity produced by marine-derived Streptomyces sp. KKMA-0239.

During our screening for anti-mycobacterial agents against Mycobacterium avium complex (MAC), two new polycyclic tetramate macrolactams (PTMs), named hydroxycapsimycin (1) and brokamycin (2), were isolated along with the known PTM, ikarugamycin (3), from the culture broth of marine-derived Streptomyces sp. KKMA-0239. The relative structures of 1 and 2 were elucidated by spectroscopic data analyses, including 1D and 2D NMR. Furthermore, the absolute configuration of 1 was confirmed by a single-crystal X-ray diffraction analysis. Compounds 2 and 3 exhibited moderate antimycobacterial activities against MAC, including clinically isolated drug-resistant M. avium.

Efficient assembly and anti-tumor evaluation of novel polycyclic [1,2-a]-fused indoles.

Structurally diverse cyclopenta[4,5]pyrrolo[1,2-a]indoles heterocycles were smoothly constructed in good to excellent yields (up to 99 %) with excellent diastereoselectivities (>19:1 dr) through a novel and facile strategy based on BF3-catalyzed Friedel-Crafts alkylation/Aldol/Dehydrative cyclization cascade reaction. The anti-proliferative activity of these newly synthesized polycyclic indoles was screened, and all the functionalized reductive derivatives exhibited favorable anti-tumor activity. Notably, compound 4ae displayed the remarkable inhibitory activity against MCF-7 and HeLa cells with IC50 values of 4.62 µM and 7.71 µM, respectively. Mechanistically, the representative compound 4ae could effectively induce apoptosis of MCF-7 cells in crediting to up-regulate the relative expression of apoptotic protein BAX/Bcl-2, subsequently activate Pro-caspase 9 and cleave PARP, simultaneously block the cell cycle through down- and up-regulate the expression of cyclin B1 and p53, respectively. Moreover, compound 4ae also exhibited promising antineoplastic efficacy in subcutaneous MCF-7 xenograft mice which manifest significant shrunken tumors conspicuous nuclear apoptotic signal and minimal systemic toxicity. This strategy not only established a novel and efficient method for the assembly of structurally complex indole heterocycles, but also provided a series of compounds possessing attractive anti-cancer activity, which holds immense potential for future biomedical applications.

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.

Schizandrin A induces non-small cell lung cancer apoptosis by suppressing the epidermal growth factor receptor activation.

OBJECTIVE: The purpose of this study is to explore the biological mechanism of Schizandrin A (SchA) inducing non-small cell lung cancer (NSCLC) apoptosis. METHODS: The reverse molecular docking tool "Swiss Target Prediction" was used to predict the targets of SchA. Protein-protein interaction analysis was performed on potential targets using the String database. Functional enrichment analyses of potential targets were performed with Gene Ontology and Kyoto Encyclopedia of Genes and Genomes. The conformation of SchA binding to target was simulated by chemical-protein interactomics and molecular docking. The effect of SchA on the expression and phosphorylation level of EGFR was detected by Western blot. Lipofectamine 3000 and EGFR plasmids were used to overexpress EGFR. Apoptosis was tested with Annexin V-FITC and propidium iodide staining, and cell cycle was detected by propidium iodide staining. RESULTS: The "Swiss Target Prediction" database predicted 112 and 111 targets based on the 2D and 3D structures of SchA, respectively, of which kinases accounted for the most, accounting for 24%. Protein interaction network analyses showed that molecular targets such as ERBB family and SRC were at the center of the network. Functional enrichment analyses indicated that ERBB-related signaling pathways were enriched. Compound-protein interactomics and molecular docking revealed that SchA could bind to the ATP-active pocket of the EGFR tyrosine kinase domain. Laboratory results showed that SchA inhibited the phosphorylation of EGFR. Insulin could counteract the cytotoxic effect of SchA. EGFR overexpression and excess EGF or IGF-1 had limited impacts on the cytotoxicity of SchA. CONCLUSIONS: Network pharmacology analyses suggested that ERBB family members may be the targets of SchA. SchA can inhibit NSCLC at least in part by inhibiting EGFR phosphorylation, and activating the EGFR bypass can neutralize the cytotoxicity of SchA.

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.

A comprehensive review on Schisandrin and its pharmacological features.

Schisandrin stands as one of the primary active compounds within the widely used traditional medicinal plant Schisandra chinensis (Turcz.) Baill. This compound exhibits sedative, hypnotic, anti-aging, antioxidant, and immunomodulatory properties, showcasing its effectiveness across various liver diseases while maintaining a favorable safety profile. However, the bioavailability of schisandrin is largely affected by hepatic and intestinal first-pass metabolism, which limits the clinical efficacy of schisandrin. In this paper, we review the various pharmacological effects and related mechanisms of schisandrin, in order to provide reference for subsequent drug research and promote its medicinal value.

A Review of Extraction and Purification, Biological Properties, Structure-Activity Relationships and Future Prospects of Schisandrin C: A Major Active Constituent of Schisandra Chinensis.

Since ancient times, China has used natural medicine as the primary way to combat diseases and has a rich arsenal of natural medicines. With the progress of the times, the extraction of bioactive molecules from natural drugs has become the new development direction for natural medicines. Among the numerous natural drugs, Schisandrin C (Sch C), derived from Schisandra Chinensis (Turcz.) Baill. It has excellent potential for development and has been shown to possess various pharmacological properties, including hepatoprotective, antitumor and anti-inflammatory activities. Based on the biological properties of hepatoprotection, scholars have explored Sch C and its synthetic products in depth; some studies have shown that pentosidine has the effect of improving the symptoms of liver fibrosis and reducing the concentration of alanine transaminase (ALT) and aspartate aminotransferase (AST) in the serum of rats, which is an essential inspiration for the development of anti-liver fibrosis drugs. But more in vivo and ex vivo studies still need to be included. This paper focuses on Sch C's extraction and synthesis, biological activities and drug development progress. The future application prospects of Sch C are discussed to perfect its development work further.

Selective Activation of a TRPC6 Ion Channel Over TRPC3 by Metalated Type-B Polycyclic Polyprenylated Acylphloroglucinols.

Selective modulation of TRPC6 ion channels is a promising therapeutic approach for neurodegenerative diseases and depression. A significant advancement showcases the selective activation of TRPC6 through metalated type-B PPAP, termed PPAP53. This success stems from PPAP53's 1,3-diketone motif facilitating metal coordination. PPAP53 is water-soluble and as potent as hyperforin, the gold standard in this field. In contrast to type-A, type-B PPAPs offer advantages such as gram-scale synthesis, easy derivatization, and long-term stability. Our investigations reveal PPAP53 selectively binding to the C-terminus of TRPC6. Although cryoelectron microscopy has resolved the majority of the TRPC6 structure, the binding site in the C-terminus remained unresolved. To address this issue, we employed state-of-the-art artificial-intelligence-based protein structure prediction algorithms to predict the missing region. Our computational results, validated against experimental data, indicate that PPAP53 binds to the 777LLKL780-region of the C-terminus, thus providing critical insights into the binding mechanism of PPAP53.

Synthesis and evaluation of novel pleuromutilin derivatives targeting the 50S ribosomal subunit for antibacterial ability.

Multidrug-resistant bacteria, particularly methicillin-resistant Staphylococcus aureus, have become a major global public health concern. Therefore, developing new antibiotics that do not possess cross-resistance for the currently available antibiotics is critical. Herein, we synthesized a novel class of pleuromutilin derivatives containing substituted triazine with improved antibacterial activity. Among these derivatives, 6d, which contains 4-dimethylamino-1,3,5-triazine in the side chain of pleuromutilin, exhibited highly promising antimicrobial activity and mitigated antibiotic resistance. The high antibacterial potency of 6d was further supported by docking model analysis and green fluorescent protein inhibition assay. Additionally, cytotoxicity and acute oral toxicity evaluation and in vivo mouse systemic infection experiments revealed that 6d possessed tolerable toxicity and promising therapeutic efficacy.

Inhibitory effects of Schisandrin C on collagen behavior in pulmonary fibrosis.

Pulmonary fibrosis (PF) is a serious progressive fibrotic disease that is characterized by excessive accumulation of extracellular matrix (ECM), thus resulting in stiff lung tissues. Lysyl oxidase (LOX) is an enzyme involved in fibrosis by catalyzing collagen cross-linking. Studies found that the ingredients in schisandra ameliorated bleomycin (BLM)-induced PF, but it is unknown whether the anti-PF of schisandra is related to LOX. In this study, we established models of PF including a mouse model stimulated by BLM and a HFL1 cell model induced by transforming growth factor (TGF)-ß1 to evaluate the inhibition effects of Schisandrin C (Sch C) on PF. We observed that Sch C treatment decreased pulmonary indexes compared to control group. Treatment of Sch C showed a significant reduction in the accumulation of ECM as evidenced by decreased expressions of α-SMA, FN, MMP2, MMP9, TIMP1 and collagen proteins such as Col 1A1, and Col 3A1. In addition, the expression of LOX in the lung tissue of mice after Sch C treatment was effectively decreased compared with the MOD group. The inhibition effects in vitro were consistent with those in vivo. Mechanistic studies revealed that Sch C significantly inhibited TGF-ß1/Smad2/3 and TNF-α/JNK signaling pathways. In conclusion, our data demonstrated that Sch C significantly ameliorated PF in vivo and vitro, which may play an important role by reducing ECM deposition and inhibiting the production of LOX.

Changes in Temperature Alter the Toxicity of Polycyclic Aromatic Compounds to American Lobster (Homarus americanus) Larvae.

Polycyclic aromatic compounds (PACs) present in the water column are considered to be one of the primary contaminant groups contributing to the toxicity of a crude oil spill. Because crude oil is a complex mixture composed of thousands of different compounds, oil spill models rely on quantitative structure-activity relationships like the target lipid model to predict the effects of crude oil exposure on aquatic life. These models rely on input provided by single species toxicity studies, which remain insufficient. Although the toxicity of select PACs has been well studied, there is little data available for many, including transformation products such as oxidized hydrocarbons. In addition, the effect of environmental influencing factors such as temperature on PAC toxicity is a wide data gap. In response to these needs, in the present study, Stage I lobster larvae were exposed to six different understudied PACs (naphthalene, fluorenone, methylnaphthalene, phenanthrene, dibenzothiophene, and fluoranthene) at three different relevant temperatures (10, 15, and 20 °C) all within the biological norms for the species during summer when larval releases occur. Lobster larvae were assessed for immobilization as a sublethal effect and mortality following 3, 6, 12, 24, and 48 h of exposure. Higher temperatures increased the rate at which immobilization and mortality were observed for each of the compounds tested and also altered the predicted critical target lipid body burden, incipient median lethal concentration, and elimination rate. Our results demonstrate that temperature has an important influence on PAC toxicity for this species and provides critical data for oil spill modeling. More studies are needed so oil spill models can be appropriately calibrated and to improve their predictive ability. Environ Toxicol Chem 2023;42:2389-2399. © 2023 SETAC.

Assessing Boron-Pleuromutilin AN11251 for the Development of Antibacterial Agents.

Pleuromutilins are a group of antibiotics derived from the naturally occurring compound. The recent approval of lefamulin for both intravenous and oral doses in humans to treat community-acquired bacterial pneumonia has prompted investigations in modifying the structure to broaden the antibacterial spectrum, enhance the activity, and improve the pharmacokinetic properties. AN11251 is a C(14)-functionalized pleuromutilin with a boron-containing heterocycle substructure. It was demonstrated to be an anti-Wolbachia agent with therapeutic potential for Onchocerciasis and lymphatic filariasis. Here, the in vitro and in vivo PK parameters of AN11251 were measured including PPB, intrinsic clearance, half-life, systemic clearance, and volume of distribution. The results indicate that the benzoxaborole-modified pleuromutilin possesses good ADME and PK properties. AN11251 has potent activities against the Gram-positive bacterial pathogens tested, including various drug-resistant strains, and against the slow-growing mycobacterial species. Finally, we employed PK/PD modeling to predict the human dose for treatment of disease caused by Wolbachia, Gram-positive bacteria, or Mycobacterium tuberculosis, which might facilitate the further development of AN11251.

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 pleuromutilin derivatives containing 1,2,4-triazole linker.

A series of thioether pleuromutilin derivatives containing 1,2,4-triazole on the side chain of C14 were designed and synthesized. The in vitro antibacterial activities experiments of the synthesized derivatives showed that compounds 72 and 73 displayed superior in vitro antibacterial effect against MRSA minimal inhibitory concentration (MIC = 0.0625 µg/mL) than tiamulin (MIC = 0.5 µg/mL). The results of time-kill study and postantibiotic effect study indicated that compound 72 could inhibit the growth of MRSA quickly (-2.16 log10 CFU/mL) and showed certain postantibiotic effect (PAE) time (exposure to 2 × MIC and 4 × MIC for 2 h, the PAE was 1.30 and 1.35 h) against MRSA. Furthermore, the binding mode between compound 72 and 50S ribosome of MRSA was explored by molecular docking and five hydrogen bonds were formed between compound 72 and 50S ribosome.

Novel pyridinium cationic pleuromutilin analogues overcoming bacterial multidrug resistance.

A series of pyridinium cation-substituted pleuromutilin analogues were designed, synthesized and evaluated for their antibacterial activities in vitro and in vivo. Most derivatives showed potent antibacterial activities, especially e4 that displayed the highest antibacterial activity against multi-drug resistant bacteria and was subjected to time-kill kinetics, resistance studies, cytotoxicity and molecular docking assays. Molecular docking results, scanning electron microscopy and o-nitrophenyl-ß-galactopyranoside tests showed that e4 not only inhibited bacterial protein synthesis but also disrupted bacterial cell walls. Compound e4 showed an ED50 of 5.68 mg/kg against multi-drug resistant Staphylococcus aureus in infected mice model. In in vivo and in vitro toxicity tests, e4 showed low toxic effects with an LD50 of 879 mg/kg to mice. These results suggest that compound e4 may be considered as a new therapeutic candidate for bacterial infections.

Antibacterial activity evaluation of pleuromutilin derivatives with 4(3H)-quinazolinone scaffold against methicillin-resistant Staphylococcusaureus.

Growing antibiotic resistance is causing a health care crisis, leading to an urgent need for new antibiotics to tackle serious hospital and community infections. Pleuromutilin, a naturally occurring product with moderate antibacterial activity, has a unique structure that has attracted great efforts to modify its scaffold to obtain lead compounds. Herein, we report the synthesis of a series of novel pleuromutilin derivatives with a scaffold of 4(3H)-quinazolinone or its analogues at the C-14 side chain and investigated their in vitro activity against Staphylococcus aureus and Staphylococcus epidermidis as well as Gram-negative bacteria (Escherichia coli and Salmonella enterica subsp. enterica serovar pullorum). Structure-activity relationship (SAR) studies showed that the substituents on the benzene ring of 4(3H)-quinazolinone was not as important as the substituted position to improve antibacterial activity while the substituted groups on the N-3 position of 4(3H)-quinazolinone had strong impact on the efficacy. The replacement of the benzene moiety of 4(3H)-quinazolinone with other rings (pyridine, pyrrole, thiophene, or cyclopentyl) also showed high antibacterial efficacy, meaning the benzene ring was dispensable for exerting powerful antibacterial properties. In vitro pharmacokinetics investigations and cytotoxicity assays indicated that 2-mercapto-4(3H)-quinazolinone scaffold was superior to 2-(piperazin-1-yl)quinazolin-4(3H)-one. Among this series of pleuromutilin analogues, compound 23 with a structure of 2-mercapto-3H-pyrrolo[2,3-d]pyrimidin-4(7H)-one displayed the best in vitro antibacterial activity against MRSA (MIC = 0.063 µg/mL) and low cytotoxicity to RAW 264.7 cells (IC50＞100 µM) and was demonstrated to inhibit MRSA effectively in a mouse thigh infection model, outperforming the comparator, tiamulin.

Molecular networking assisted discovery and combinatorial biosynthesis of new antimicrobial pleuromutilins.

Pleuromutilins, the unique fungal metabolites possessing 5/6/8 tricyclic skeleton, are potent antibacterial leading compounds for the development of new antibiotics. We applied the MS/MS molecular networking technique and the combinatorial biosynthesis approach to discover new pleuromutilin analogues. Ten pleuromutilin derivatives including seven new compounds (1-7) were obtained from the solid culture of Omphalina mutila. The gene cluster for the biosynthesis of pleuromutilins in the mushroom of O. mutila was identified and further expressed in yeast. Nine pleuromutilin-type diterpenes including three new "unnatural" pleuromutilins (16-18) were generated in a GGPP-engineered Saccharomyces cerevisiae. The antimicrobial bioassays indicated that compounds 3, 9, 10, 15, and 17 exhibited potent inhibition against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus faecalis (VRE). Several pleuromutilins were found to show immunomodulatory activities by promoting the cell viability, enhancing the ROS and NO production, or increasing the levels of proinflammatory cytokines IL-6 and TNF-α in the macrophage RAW 264.7. The structure-activity relationship for pleuromutilins was analyzed.

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, biological evaluation and molecular docking studies of novel pleuromutilin derivatives containing nitrogen heterocycle and alkylamine groups.

A series of pleuromutilin derivatives containing alkylamine and nitrogen heterocycle groups were designed and synthesised under mild conditions. The in vitro antibacterial activity of these semisynthetic derivatives against four strains of Staphylococcus aureus (MRSA ATCC 43300, S.aureus ATCC 29213, S.aureus AD3, and S.aureus 144) were evaluated by the broth dilution method. Compound 13 was found to have excellent antibacterial activity against MRSA (MIC = 0.0625 µg/mL). Furthermore, compound 13 was further studied by the time-killing kinetics and the post-antibiotic effect approach. In the mouse thigh infection model, compound 13 exhibited superior antibacterial efficacy than that of tiamulin. Meanwhile, compound 13 showed a lower inhibitory effect than that of tiamulin on RAW264.7 and 16HBE cells at the concentration of 10 µg/mL. Molecular docking study revealed that compound 13 can effectively bind to the active site of the 50S ribosome (the binding free energy = -9.66 kcal/mol).