In vitro anti-Gram-positive and antistaphylococcal biofilm activity of newly halogenated pyrroles related to pyrrolomycins

Bioactive pyrrole-based compounds with target selectivity

The discovery of novel synthetic compounds with drug-like properties is an ongoing challenge in medicinal chemistry. Natural products have inspired the synthesis of compounds for pharmaceutical application, most of which are based on N-heterocyclic motifs. Among these, the pyrrole ring is one of the most explored heterocycles in drug discovery programs for several therapeutic areas, confirmed by the high number of pyrrole-based drugs reaching the market. In the present review, we focused on pyrrole and its hetero-fused derivatives with anticancer, antimicrobial, and antiviral activities, reported in the literature between 2015 and 2019, for which a specific target was identified, being responsible for their biological activity. It emerges that the powerful pharmaceutical and pharmacological features provided by the pyrrole nucleus as pharmacophore unit of many drugs are still recognized by medicinal chemists.

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Pyrrole nucleus is one of the most explored heterocycle in drug discovery.

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Pyrrole derivatives exhibit antitumor, antimicrobial and antiviral activities.

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Targets involved in their biological activities were identified.

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SAR to underline their most important features were discussed.

New Synthetic Nitro-Pyrrolomycins as Promising Antibacterial and Anticancer Agents

Pyrrolomycins (PMs) are polyhalogenated antibiotics known as powerful biologically active compounds, yet featuring high cytotoxicity. The present study reports the antibacterial and antitumoral properties of new chemically synthesized PMs, where the three positions of the pyrrolic nucleus were replaced by nitro groups, aiming to reduce their cytotoxicity while maintaining or even enhancing the biological activity. Indeed, the presence of the nitro substituent in diverse positions of the pyrrole determined an improvement of the minimal bactericidal concentration (MBC) against Gram-positive (i.e., Staphylococcus aureus) or -negative (i.e., Pseudomonas aeruginosa) pathogen strains as compared to the natural PM-C. Moreover, some new nitro-PMs were as active as or more than PM-C in inhibiting the proliferation of colon (HCT116) and breast (MCF 7) cancer cell lines and were less toxic towards normal epithelial (hTERT RPE-1) cells. Altogether, our findings contribute to increase the knowledge of the mode of action of these promising molecules and provide a basis for their rationale chemical or biological manipulation.

Pyrrolomycins Are Potent Natural Protonophores

The escalating burden of antibiotic drug resistance necessitates research into novel classes of antibiotics and their mechanism of action. Pyrrolomycins are a family of potent natural product antibiotics with nanomolar activity against Gram-positive bacteria, yet with an elusive mechanism of action. In this work, we dissect the apparent Gram-positive specific activity of pyrrolomycins and show that Gram-negative bacteria are equally sensitive to pyrrolomycins when drug efflux transporters are removed and that albumin in medium plays a large role in pyrrolomycin activity. The selection of resistant mutants allowed for the characterization and validation of a number of mechanisms of resistance to pyrrolomycins in both Staphylococcus aureus and an Escherichia coli ΔtolC mutant, all of which appear to affect compound penetration rather than being target associated. Imaging of the impact of pyrrolomycin on the E. coli ΔtolC mutant using scanning electron microscopy showed blebbing of the bacterial cell wall often at the site of bacterial division. Using potentiometric probes and an electrophysiological technique with an artificial bilayer lipid membrane, it was demonstrated that pyrrolomycins C and D are very potent membrane-depolarizing agents, an order of magnitude more active than conventional carbonyl cyanide m-chlorophenylhydrazone (CCCP), specifically disturbing the proton gradient and uncoupling oxidative phosphorylation via protonophoric action. This work clearly unveils the until-now-elusive mechanism of action of pyrrolomycins and explains their antibiotic activity as well as mechanisms of innate and acquired drug resistance in bacteria.

Pyrrolomycins as antimicrobial agents. Microwave-assisted organic synthesis and insights into their antimicrobial mechanism of action

New compounds able to counteract staphylococcal biofilm formation are needed. In this study we investigate the mechanism of action of pyrrolomycins, whose potential as antimicrobial agents has been demonstrated. We performed a new efficient and easy method to use microwave organic synthesis suitable for obtaining pyrrolomycins in good yields and in suitable amount for their in vitro in-depth investigation. We evaluate the inhibitory activity towards Sortase A (SrtA), a transpeptidase responsible for covalent anchoring in Gram-positive peptidoglycan of many surface proteins involved in adhesion and in biofilm formation. All compounds show a good inhibitory activity toward SrtA, having IC₅₀ values ranging from 130 to 300 µM comparable to berberine hydrochloride. Of note compound 1d shows a good affinity in docking experiment to SrtA and exhibits the highest capability to interfere with biofilm formation of S. aureus showing an IC₅₀ of 3.4 nM. This compound is also effective in altering S. aureus murein hydrolase activity that is known to be responsible for degradation, turnover, and maturation of bacterial peptidoglycan and involved in the initial stages of S. aureus biofilm formation.

Silver-mediated synthesis of novel 3-CF3/CN/phosphonate-substituted pyrazoles as pyrrolomycin analogues from 3-formylchromones and diazo compounds

Herein, a simple and regioselective synthesis of (2-hydroxyphenyl)(3-(trifluoromethyl/cyano/phosphonate)-1H-pyrazol-5-yl)methanones as pyrrolomycin analogues was reported.

Synthetic small molecules as anti-biofilm agents in the struggle against antibiotic resistance

Biofilm formation significantly contributes to microbial survival in hostile environments and it is currently considered a key virulence factor for pathogens responsible for serious chronic infections. In the last decade many efforts have been made to identify new agents able to modulate bacterial biofilm life cycle, and many compounds have shown interesting activities in inhibiting biofilm formation or in dispersing pre-formed biofilms. However, only a few of these compounds were tested using in vivo models for their clinical significance. Contrary to conventional antibiotics, most of the anti-biofilm compounds act as anti-virulence agents as they do not affect bacterial growth. In this review we selected the most relevant literature of the last decade, focusing on the development of synthetic small molecules able to prevent bacterial biofilm formation or to eradicate pre-existing biofilms of clinically relevant Gram-positive and Gram-negative pathogens. In addition, we provide a comprehensive list of the possible targets to counteract biofilm formation and development, as well as a detailed discussion the advantages and disadvantages of the different current biofilm-targeting strategies.

Current Status and Future Prospects of Marine Natural Products (MNPs) as Antimicrobials

The marine environment is a rich source of chemically diverse, biologically active natural products, and serves as an invaluable resource in the ongoing search for novel antimicrobial compounds. Recent advances in extraction and isolation techniques, and in state-of-the-art technologies involved in organic synthesis and chemical structure elucidation, have accelerated the numbers of antimicrobial molecules originating from the ocean moving into clinical trials. The chemical diversity associated with these marine-derived molecules is immense, varying from simple linear peptides and fatty acids to complex alkaloids, terpenes and polyketides, etc. Such an array of structurally distinct molecules performs functionally diverse biological activities against many pathogenic bacteria and fungi, making marine-derived natural products valuable commodities, particularly in the current age of antimicrobial resistance. In this review, we have highlighted several marine-derived natural products (and their synthetic derivatives), which have gained recognition as effective antimicrobial agents over the past five years (2012–2017). These natural products have been categorized based on their chemical structures and the structure-activity mediated relationships of some of these bioactive molecules have been discussed. Finally, we have provided an insight into how genome mining efforts are likely to expedite the discovery of novel antimicrobial compounds.

Pharmaceutical Potential of Synthetic and Natural Pyrrolomycins

The emergence of antibiotic resistance is currently considered one of the most important global health problem. The continuous onset of multidrug-resistant Gram-positive and Gram-negative bacterial strains limits the clinical efficacy of most of the marketed antibiotics. Therefore, there is an urgent need for new antibiotics. Pyrrolomycins are a class of biologically active compounds that exhibit a broad spectrum of biological activities, including antibacterial, antifungal, anthelmintic, antiproliferative, insecticidal, and acaricidal activities. In this review we focus on the antibacterial activity and antibiofilm activity of pyrrolomycins against Gram-positive and Gram-negative pathogens. Their efficacy, combined in some cases with a low toxicity, confers to these molecules a great potential for the development of new antimicrobial agents to face the antibiotic crisis.

Design, synthesis and antistaphylococcal activity of marine pyrrole alkaloid derivatives

A novel set of 16 hybrids of bromopyrrole alkaloids with aroyl hydrazone were designed, synthesized and evaluated for antibacterial and antibiofilm activities against methicillin-resistant Staphylococcus aureus (MRSA; ATCC 43866), methicillin-susceptible Staphylococcus aureus (MSSA; ATCC 35556) and Staphylococcus epidermidis (SE, S. epidermidis ATCC 35984). Of the 16 tested hybrids, 14 exhibited equal or superior antibiofilm activity against MSSA and MRSA relative to standard vancomycin. Compound 4m showed highest potency with antibiofilm activity of 0.39 µg/mL and 0.78 µg/mL against MSSA and MRSA, respectively. Thus, this compound could act as a potential lead for further development of new antistaphylococcal drugs.