Valorisation of the diterpene podocarpic acid - Antibiotic and antibiotic enhancing activities of polyamine conjugates

Antioxidant and Wound Healing Bioactive Potential of Extracts Obtained from Bark and Needles of Softwood Species

Interest in the extraction of phytochemical bioactive compounds, especially polyphenols from biomass, has recently increased due to their valuable biological potential as natural sources of antioxidants, which could be used in a wide range of applications, from foods and pharmaceuticals to green polymers and bio-based materials. The present research study aimed to provide a comprehensive chemical characterization of the phytochemical composition of forest biomass (bark and needles) of softwood species (Picea abies L., H. Karst., and Abies alba Mill.) and to investigate their in vitro antioxidant and antimicrobial activities to assess their potential in treating and healing infected chronic wounds. The DPPH radical-scavenging method and P-LD were used for a mechanistic explanation of the biomolecular effects of the investigated bioactive compounds. (+)-Catechin, epicatechin, rutin, myricetin, 4 hydroxybenzoic and p-cumaric acids, kaempherol, and apigenin were the main quantified polyphenols in coniferous biomass (in quantities around 100 µg/g). Also, numerous phenolic acids, flavonoids, stilbenes, terpenes, lignans, secoiridoids, and indanes with antioxidant, antimicrobial, anti-inflammatory, antihemolytic, and anti-carcinogenic potential were identified. The Abies alba needle extract was more toxic to microbial strains than the eukaryotic cells that provide its active wound healing principles. In this context, developing industrial upscaling strategies is imperative for the long-term success of biorefineries and incorporating them as part of a circular bio-economy.

α,ω-Diacyl-Substituted Analogues of Natural and Unnatural Polyamines: Identification of Potent Bactericides That Selectively Target Bacterial Membranes

In this study, α-ω-disubstituted polyamines exhibit a range of potentially useful biological activities, including antimicrobial and antibiotic potentiation properties. We have prepared an expanded set of diarylbis(thioureido)polyamines that vary in central polyamine core length, identifying analogues with potent methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli, Acinetobacter baumannii and Candida albicans growth inhibition properties, in addition to the ability to enhance action of doxycycline towards Gram-negative bacterium Pseudomonas aeruginosa. The observation of associated cytotoxicity/hemolytic properties prompted synthesis of an alternative series of diacylpolyamines that explored aromatic head groups of varying lipophilicity. Examples bearing terminal groups each containing two phenyl rings (15a-f, 16a-f) were found to have optimal intrinsic antimicrobial properties, with MRSA being the most susceptible organism. A lack of observed cytotoxicity or hemolytic properties for all but the longest polyamine chain variants identified these as non-toxic Gram-positive antimicrobials worthy of further study. Analogues bearing either one or three aromatic-ring-containing head groups were either generally devoid of antimicrobial properties (one ring) or cytotoxic/hemolytic (three rings), defining a rather narrow range of head group lipophilicity that affords selectivity for Gram-positive bacterial membranes versus mammalian. Analogue 15d is bactericidal and targets the Gram-positive bacterial membrane.

Investigation of α,ω-Disubstituted Polyamine-Cholic Acid Conjugates Identifies Hyodeoxycholic and Chenodeoxycholic Scaffolds as Non-Toxic, Potent Antimicrobials

With the increased incidence of antibiotic resistance, the discovery and development of new antibacterials is of increasing importance and urgency. The report of the natural product antibiotic squalamine in 1993 has stimulated a lot of interest in the study of structurally simplified cholic acid-polyamine derivatives. We report the synthesis of a focused set of deoxycholic acid-polyamine conjugates and the identification of hyodeoxycholic acid derivatives as being potently active towards S. aureus MRSA and some fungal strains, but with no attendant cytotoxicity or hemolytic properties. Analogue 7e exhibited bactericidal activity towards a range of Gram-positive bacteria, while preliminary investigation of its mechanism of action ruled out the bacterial membrane as being a primary cellular target as determined using an ATP-release bioluminescence assay.

Discovery and Preliminary Structure-Activity Investigation of 3-Substituted-1H-imidazol-5-yl-1H-indoles with In Vitro Activity towards Methicillin-Resistant Staphylococcus aureus

Antibiotics have been the cornerstone of modern medicine saving lives by virtue of being able to cure infectious diseases and to prevent infections in those who are immune compromised. Their intense use has led to a surging increase in the incidence of antibiotic-resistant bacteria resulting in a desperate need for antibiotics with new mechanisms of action. As part of our search for new antimicrobials we have screened an in-house library of compounds and identified two 3-substituted-1H-imidazol-5-yl-1H-indoles as weak growth inhibitors (MIC 16 µg/mL) against methicillin-resistant Staphylococcus aureus (MRSA). An extensive library of analogues was prepared using the Van Leusen three-component reaction, biological evaluation of which led to the identification of two analogues (26 and 32) with favorable anti-MRSA activity (MIC ≤ 0.25 µg/mL) which also lacked cytotoxic or hemolytic properties. The screening campaign also identified two derivatives, a phenethyl-indole-imidazole 57 and a 5-phenyl-1H-imidazole 111 that were non-toxic selective antifungals towards Cryptococcus neoformans. These results have identified 3-substituted-1H-imidazol-5-yl-1H-indoles and 5-phenyl-1H-imidazoles as new structural scaffolds for further investigation as anti-MRSA and anti-C. neoformans agents, respectively.

Total Synthesis of the Four Stereoisomers of Cyclo(l-Trp-l-Arg) Raises Uncertainty of the Structures of the Natural Products and Invalidates Their Promising Antimicrobial Activities

New therapeutic options to combat the growing incidence of antimicrobial resistance are urgently needed. A 2015 publication reported the isolation and biological evaluation of two diketopiperazine natural products, cyclo(l-Trp-l-Arg) (CDP 2) and cyclo(d-Trp-d-Arg) (CDP 3), from an Achromobacter sp. bacterium, finding that the latter metabolite in particular exhibited strong antibacterial activity towards a range of wound-related microorganisms and could synergize the action of ampicillin. Intrigued by these biological activities and noting inconsistencies in the structural characterization of the natural products, we synthesized the four diastereomers of cyclo(Trp-Arg) and evaluated them for antimicrobial and antibiotic enhancement properties. The detailed comparison of spectroscopic data raises uncertainty regarding the structure of CDP 2 and disproves the structure of CDP 3. In our hands, none of the four stereoisomers of cyclo(Trp-Arg) exhibited detectable intrinsic antimicrobial properties towards a range of Gram-positive and Gram-negative bacteria or fungi nor could they potentiate the action of antibiotics. These discrepancies in biological properties, compared with the activities reported in the literature, reveal that these specific cyclic dipeptides do not represent viable templates for the development of new treatments for microbial infections.