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Small molecules that can restore the action of legacy antibiotics toward drug-resistant bacteria represent an area of ongoing research interest. We have previously reported indole-3-glyoxylamido and indole-3-acetamido-polyamine conjugates that exhibit intrinsic activity toward bacterial and fungal species, and the ability to enhance the action of doxycycline toward the Gram-negative bacteria Pseudomonas aeruginosa; however, these desirable activities were commonly associated with unfavorable cytotoxicity and/or red blood cell hemolytic properties. In this paper, we report the synthesis and biological investigation of a new class of α,ω-di(indole-3-carboxamido)polyamine derivatives, leading to the identification of several analogues that exhibit antimicrobial- and antibiotic-potentiating activities without detectable cytotoxic or hemolytic properties. 5-Bromo-substituted indole analogues 3 and 12–18 were generally more broad-spectrum in their activity than others in the set, with 13b (polyamine PA-3-6-3) being particularly notable for its anti-Staphylococcus aureus, Acinetobacter baumannii, and Cryptococcus neoformans activities (MIC ≤ 0.28 µM). The same analogue also restored the action of doxycycline toward P. aeruginosa with a 21-fold enhancement, while the corresponding 5-bromo-indole-3-carboxamide-PA3-7-3 analogue was able to enhance the action of both doxycycline and erythromycin toward P. aeruginosa and Escherichia coli, respectively. The analogue 13b was capable of disrupting the bacterial membrane of both S. aureus and methicillin-resistant S. aureus (MRSA) and the outer membrane of P. aeruginosa, suggesting that membrane perturbation could be a mechanism of action of both intrinsic antimicrobial activities and antibiotic potentiation.

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.

A library of naturally occurring and semi-synthetic discorhabdins was assessed for their effects on Merkel cell carcinoma (MCC) cell viability. The set included five new natural products and semi-synthetic compounds whose structures were elucidated with NMR, HRMS, and ECD techniques. Several discorhabdins averaged sub-micromolar potency against the MCC cell lines tested and most of the active compounds showed selectivity towards virus-positive MCC cell lines. An investigation of structure–activity relationships resulted in an expanded understanding of the crucial structural features of the discorhabdin scaffold. Mechanistic cell death assays suggested that discorhabdins, unlike many other MCC-active small molecules, do not induce apoptosis, as shown by the lack of caspase activation, annexin V staining, and response to caspase inhibition. Similarly, discorhabdin treatment failed to increase MCC intracellular calcium and ROS levels. In contrast, the rapid loss of cellular reducing potential and mitochondrial membrane potential suggested that discorhabdins induce mitochondrial dysfunction leading to non-apoptotic cell death.

The widespread incidence of antimicrobial resistance necessitates the discovery of new classes of antimicrobials as well as adjuvant molecules that can restore the action of ineffective antibiotics. Herein, we report the synthesis of a new class of indole-3-acetamido-polyamine conjugates that were evaluated for antimicrobial activities against a panel of bacteria and two fungi, and for the ability to enhance the action of doxycycline against Pseudomonas aeruginosa and erythromycin against Escherichia coli. Compounds 14b, 15b, 17c, 18a, 18b, 18d, 19b, 19e, 20c and 20d exhibited strong growth inhibition of methicillin-resistant Staphylococcus aureus (MRSA) and Cryptococcus neoformans, with minimum inhibitory concentrations (MIC) typically less than 0.2 µM. Four analogues, including a 5-bromo 15c and three 5-methoxyls 16d–f, also exhibited intrinsic activity towards E. coli. Antibiotic kill curve analysis of 15c identified it to be a bactericide. While only one derivative was found to (weakly) enhance the action of erythromycin against E. coli, three examples, including 15c, were found to be strong enhancers of the antibiotic action of doxycycline against P. aeruginosa. Collectively, these results highlight the promising potential of α,ω-disubstituted indole-3-acetamido polyamine conjugates as antimicrobials and antibiotic adjuvants.

The marine natural product ianthelliformisamine C is a bis-cinnamido substituted spermine derivative that exhibits intrinsic antimicrobial properties and can enhance the action of doxycycline towards the Gram-negative bacterium Pseudomonas aeruginosa. As part of a study to explore the structure–activity requirements of these activities, we have synthesized a set of analogues that vary in the presence/absence of methoxyl group and bromine atoms and in the polyamine chain length. Intrinsic antimicrobial activity towards Staphylococcus aureus, methicillin-resistant S. aureus (MRSA) and the fungus Cryptococcus neoformans was observed for only the longest polyamine chain examples of non-brominated analogues while all examples bearing either one or two bromine atoms were active. Weak to no activity was typically observed towards Gram-negative bacteria, with exceptions being the longest polyamine chain examples 13f, 14f and 16f against Escherichia coli (MIC 1.56, 7.2 and 5.3 µM, respectively). Many of these longer polyamine-chain analogues also exhibited cytotoxic and/or red blood cell hemolytic properties, diminishing their potential as antimicrobial lead compounds. Two of the non-toxic, non-halogenated analogues, 13b and 13d, exhibited a strong ability to enhance the action of doxycycline against P. aeruginosa, with >64-fold and >32-fold enhancement, respectively. These results suggest that any future efforts to optimize the antibiotic-enhancing properties of cinnamido-polyamines should explore a wider range of aromatic ring substituents that do not include bromine or methoxyl groups.

As part of our search for new antimicrobials and antibiotic enhancers, a series of naphthyl- and biphenyl-substituted polyamine conjugates have been synthesized. The structurally-diverse library of compounds incorporated variation in the capping end groups and in the length of the polyamine (PA) core. Longer chain (PA-3-12-3) variants containing both 1-naphthyl and 2-naphthyl capping groups exhibited more pronounced intrinsic antimicrobial properties against methicillin-resistant Staphylococcus aureus (MRSA) (MIC ≤ 0.29 µM) and the fungus Cryptococcus neoformans (MIC ≤ 0.29 µM). Closer mechanistic study of one of these analogues, 20f, identified it as a bactericide. In contrast to previously reported diarylacyl-substituted polyamines, several examples in the current set were able to enhance the antibiotic action of doxycycline and/or erythromycin towards the Gram-negative bacteria Pseudomonas aeruginosa and Escherichia coli. Two analogues (19a and 20c) were of note, exhibiting greater than 32-fold enhancement in activity. This latter result suggests that α,ω-disubstituted polyamines bearing 1-naphthyl- and 2-naphthyl-capping groups are worthy of further investigation and optimization as non-toxic antibiotic enhancers.

Antibiotic resistance is a growing global health threat, requiring urgent attention. One approach to overcome antibiotic resistance is to discover and develop new antibiotic enhancers, molecules that work with legacy antibiotics to enhance their efficacy against resistant bacteria. Our previous screening of a library of purified marine natural products and their synthetic analogues led to the discovery of an indolglyoxyl-spermine derivative that exhibited intrinsic antimicrobial properties and was also able to potentiate the action of doxycycline towards the difficult to treat, Gram-negative bacterium Pseudomonas aeruginosa. A set of analogues have now been prepared, exploring the influence of indole substitution at the 5- and 7- positions and length of the polyamine chain on biological activity. While limiting cytotoxicity and/or hemolytic activities were observed for many analogues, two 7-methyl substituted analogues (23b and 23c) were found to exhibit strong activity towards Gram-positive bacteria with no detectable cytotoxicity or hemolytic properties. Different molecular attributes were required for antibiotic enhancing properties, with one example identified, a 5-methoxy-substitiuted analogue (19a), as being a non-toxic, non-hemolytic enhancer of the action of two tetracycline antibiotics, doxycycline and minocycline, towards P. aeruginosa. These results provide further stimulation for the search for novel antimicrobials and antibiotic enhancers amongst marine natural products and related synthetic analogues.

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.

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.

Isolates of a variety of fungal plant pathogens (Alternaria radicina ICMP 5619, Cercospora beticola ICMP 15907, Dactylonectria macrodidyma ICMP 16789, D. torresensis ICMP 20542, Ilyonectria europaea ICMP 16794, and I. liriodendra ICMP 16795) were screened for antimicrobial activity against the human pathogenic bacteria Acinetobacter baumannii, Pseudomonas aeruginosa, Escherichia coli, Mycobacterium abscessus, and M. marinum and were found to have some activity. Investigation of the secondary metabolites of these fungal isolates led to the isolation of ten natural products (1–10) of which one was novel, (E)-4,7-dihydroxyoct-2-enoic acid (1). Structure elucidation of all natural products was achieved by a combination of NMR spectroscopy and mass spectrometry. We also investigated the antimicrobial activity of a number of the isolated natural products. While we did not find (E)-4,7-dihydroxyoct-2-enoic acid (1) to have any activity against the bacteria and fungi in our assays, we did find that cercosporin (7) exhibited potent activity against Methicillin resistant Staphylococcus aureus (MRSA), dehydro-curvularin (6) and radicicol (10) exhibited antimycobacterial activity against M. marinum, and brefeldin A (8) and radicicol (10) exhibited antifungal activity against Candida albicans. Investigation of the cytotoxicity and haemolytic activities of these natural products (6–8 and 10) found that only one of the four active compounds, radicicol (10), was non-cytotoxic and non-haemolytic.

There is a real and urgent need for new antibiotics able to kill Mycobacteria, acid-fast bacilli capable of causing multiple deadly diseases. These include members of the Mycobacterium tuberculosis complex, which causes the lung disease tuberculosis (TB) as well as non-tuberculous Mycobacteria (NTM) a growing cause of lung, skin, soft tissue, and other infections. Here we describe a medium-throughput bioluminescence-based pipeline to screen fungi for activity against Mycobacteria using the NTM species Mycobacterium abscessus and Mycobacterium marinum. We used this pipeline to screen 36 diverse fungal isolates from the International Collection of Microorganisms from Plants (ICMP) grown on a wide variety of nutrient-rich and nutrient-poor media and discovered that almost all the tested isolates produced considerable anti-mycobacterial activity. Our data also provides strong statistical evidence for the impact of growth media on antibacterial activity. Chemical extraction and fractionation of a subset of the ICMP isolates revealed that much of the activity we observed may be due to the production of the known anti-mycobacterial compound linoleic acid. However, we have identified several ICMP isolates that retained their anti-mycobacterial activity in non-linoleic acid containing fractions. These include isolates of Lophodermium culmigenum, Pseudaegerita viridis, and Trametes coccinea, as well as an unknown species of Boeremia and an isolate of an unknown genus and species in the family Phanerochaetaceae. Investigations are ongoing to identify the sources of their anti-mycobacterial activity and to determine whether any may be due to the production of novel bioactive compounds.

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.

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.

The structure of the sesquiterpene onchidal (6), a component of the defensive secretion of the shell-less mollusc Onchidella binneyi, contains a masked α,β-unsaturated 1,4-dialdehyde moiety, the presence of which has been proposed to be the cause of the feeding deterrent activity exhibited by the mollusc. We have found onchidal acts as an electrophile, reacting rapidly with the model nucleophile n-pentylamine forming diastereomeric aminated pyrrole adducts. Somewhat surprisingly, no reaction was observed between onchidal and n-pentanethiol. Structurally simplified n-pentyl 11–13 and cyclohexylmethyl 15–17 analogues of onchidal were prepared and demonstrated similar amine-selective reactivity. Onchidal and analogues reacted with the model protein lysozyme, forming covalent adducts and leading to protein cross-linking. These results provide preliminary evidence supporting the molecular mechanism of biological activity exhibited by onchidal.

Screening of several fungi from the New Zealand International Collection of Microorganisms from Plants identified two strains of Penicillium, P. bissettii and P. glabrum, which exhibited antimicrobial activity against Escherichia coli,Klebsiella pneumoniae, and Staphylococcus aureus. Further investigation into the natural products of the fungi, through extraction and fractionation, led to the isolation of five known polyketide metabolites, penicillic acid (1), citromycetin (2), penialdin A (3), penialdin F (4), and myxotrichin B (5). Semi-synthetic derivatization of 1 led to the discovery of a novel dihydro (1a) derivative that provided evidence for the existence of the much-speculated open-chained form of 1. Upon investigation of the antimicrobial activities of the natural products and derivatives, both penicillic acid (1) and penialdin F (4) were found to inhibit the growth of Methicillin-resistant S. aureus. Penialdin F (4) was also found to have some inhibitory activity against Mycobacterium abscessus and M. marinum along with citromycetin (2).

Fungi have become an invaluable source of bioactive natural products, with more than 5 million species of fungi spanning the globe. Fractionation of crude extract of Neodidymelliopsis sp., led to the isolation of a novel polyketide, (2Z)-cillifuranone (1) and five previously reported natural products, (2E)-cillifuranone (2), taiwapyrone (3), xylariolide D (4), pachybasin (5), and N-(5-hydroxypentyl)acetamide (6). It was discovered that (2Z)-cillifuranone (1) was particularly sensitive to ambient temperature and light resulting in isomerisation to (2E)-cillifuranone (2). Structure elucidation of all the natural products were conducted by NMR spectroscopic techniques. The antimicrobial activity of 2, 3, and 5 were evaluated against a variety of bacterial and fungal pathogens. A sodium [1-13C] acetate labelling study was conducted on Neodidymelliopsis sp. and confirmed that pachybasin is biosynthesised through the acetate polyketide pathway.

Antimicrobial bioassay-guided fractionation of the endophytic fungi Neofusicoccum australe led to the isolation of a new unsymmetrical naphthoquinone dimer, neofusnaphthoquinone B (1), along with four known natural products (2–5). Structure elucidation was conducted by nuclear magnetic resonance (NMR) spectroscopic methods, and the antimicrobial activity of all the natural products was investigated, revealing 1 to be moderately active towards methicillin-resistant Staphylococcus aureus (MRSA) with a minimum inhibitory concentration (MIC) of 16 µg/mL.

We have isolated a filamentous fungus that actively secretes a pigmented exudate when growing on agar plates. The fungus was identified as being a strain of Epicoccum nigrum. The fungal exudate presented strong antifungal activity against both yeasts and filamentous fungi, and inhibited the germination of fungal spores. The chemical characterization of the exudate showed that the pigmented molecule presenting antifungal activity is the disalt of epipyrone A—a water-soluble polyene metabolite with a molecular mass of 612.29 and maximal UV–Vis absorbance at 428 nm. This antifungal compound showed excellent stability to different temperatures and neutral to alkaline pH.

Natural products have been a great source for drug leads, due to a vast majority possessing unique chemical structures. Such an example is the protoilludane class of natural products which contain an annulated 5/6/4-ring system and are almost exclusively produced by fungi. They have been reported to possess a diverse range of bioactivities, including antimicrobial, antifungal and cytotoxic properties. In this review, we discuss the isolation, structure elucidation and any reported bioactivities of this compound class, including establishment of stereochemistry and any total syntheses of these natural products. A total of 180 protoilludane natural products, isolated in the last 70 years, from fungi, plant and marine sources are covered, highlighting their structural diversity and potential in drug discovery.

Antimicrobial bioassay-guided fractionation of Microcera larvarum led to the isolation of a γ-lactone with a furo[3,4-b]pyran-5-one bicyclic ring system (1) and three known compounds, (3S,4R)-4-hydroxymellein (2), (3S,4S)-4-hydroxymellein (3) and 7-hydroxy-3-(1-hydroxyethyl)isobenzofuran-1(3H)-one (4). Structure elucidation was conducted by NMR spectroscopic methods. Absolute configuration of 1 (2R, 3S, 5S, 7S, 8R) was established using the chiral derivatizing agent MPA and was fully supported by calculated specific rotation and ECD spectra. The spectroscopic data observed for 1 were identical to those previously reported for theissenolactone A (7), necessitating a correction of the latter (from C-5/C-8 trans ring fusion to cis). Compounds 1–4 were evaluated for antimicrobial activity against a panel of pathogens.

The cytotoxic marine natural product discorhabdin C contains a 2,6-dibromo-cyclohexa-2,5-diene moiety, previously proposed to be a critical feature required for biological activity. We have determined that the dienone-ring of discorhabdin C is indeed electrophilic, reacting with thiol and amine nucleophiles, affording debrominated adducts. In the case of reaction with 1-aminopentane the product contains an unusual C-2/N-18 ring closed, double-hydrate moiety. This electrophilic reactivity also extends to proteins, with lysozyme-discorhabdin C adducts being detected by ESI mass spectrometry. These results prompted further examination of an extract of discorhabdin C-producing sponge, Latrunculia (Latrunculia) trivetricillata, leading to the isolation and characterisation of a new example of a C-1/N-13 linked discorhabdin dimer that shared structural similarities with the 1-aminopentane-discorhabdin C adduct. To definitively assess the influence of the dienone moiety of discorhabdin C on cytotoxicity, a semi-synthetic hydrogenation derivative was prepared, affording a didebrominated ring-closed carbinolamine that was essentially devoid of tumour cell line cytotoxicity. Antiparasitic activity was assessed for a set of 14 discorhabdin alkaloids composed of natural products and semi-synthetic derivatives. Three compounds, (-)-discorhabdin L, a dimer of discorhabdin B and the discorhabdin C hydrogenation carbinolamine, exhibited pronounced activity towards Plasmodium falciparum K1 (IC50 30–90 nM) with acceptable to excellent selectivity (selectivity index 19–510) versus a non-malignant cell line.

In an effort to gain more understanding on the structure activity relationship of pseudoceratidine 1, a di-bromo pyrrole spermidine alkaloid derived from the marine sponge Pseudoceratina purpurea that has been shown to exhibit potent biofouling, anti-fungal, antibacterial, and anti-malarial activities, a large series of 65 compounds that incorporated several aspects of structural variation has been synthesised through an efficient, divergent method that allowed for a number of analogues to be generated from common precursors. Subsequently, all analogues were assessed for their antibacterial activity against both Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria. Overall, several compounds exhibited comparable or better activity than that of pseudoceratidine 1, and it was found that this class of compounds is generally more effective against Gram-positive than Gram-negative bacteria. Furthermore, altering several structural features allowed for the establishment of a comprehensive structure activity relationship (SAR), where it was concluded that several structural features are critical for potent anti-bacterial activity, including di-halogenation (preferable bromine, but chlorine is also effective) on the pyrrole ring, two pyrrolic units in the structure and with one or more secondary amines in the chain adjoining these units, with longer chains giving rise to better activities.

The first total synthesis of the unusual aromatic sesquiterpene panicein A2 is reported and the structure of the natural product has been confirmed. When tested by the NCI against a range of human cancer cell lines, it was found that panicein A2 exhibits very little antiproliferative activity at 10 μM – an observation that is at odds with the earlier report that stated panicein A2 exhibits in vitro cytotoxicity against a number of tumour cell lines.

In an effort to more accurately define the mechanism of cell death and to establish structure-activity relationship requirements for the marine meroterpenoid alkaloids thiaplidiaquinones A and B, we have evaluated not only the natural products but also dioxothiazine regioisomers and two precursor quinones in a range of bioassays. While the natural products were found to be weak inducers of ROS in Jurkat cells, the dioxothiazine regioisomer of thiaplidiaquinone A and a synthetic precursor to thiaplidiaquinone B were found to be moderately potent inducers. Intriguingly, and in contrast to previous reports, the mechanism of Jurkat cell death (necrosis vs. apoptosis) was found to be dependent upon the positioning of one of the geranyl sidechains in the compounds with thiaplidiaquinone A and its dioxothiazine regioisomer causing death dominantly by necrosis, while thiaplidiaquinone B and its dioxothiazine isomer caused cell death via apoptosis. The dioxothiazine regioisomer of thiaplidiaquinone A exhibited more potent in vitro antiproliferative activity against human tumor cells, with NCI sub-panel selectivity towards melanoma cell lines. The non-natural dioxothiazine regioisomers were also more active in antiplasmodial and anti-farnesyltransferase assays than their natural product counterparts. The results highlight the important role that natural product total synthesis can play in not only helping understand the structural basis of biological activity of natural products, but also the discovery of new bioactive scaffolds.

Seven new adociaquinone derivatives, xestoadociaquinones A (1a), B (1b), 14-carboxy-xestoquinol sulfate (2) and xestoadociaminals A–D (3a, 3c, 4a, 4c), together with seven known compounds (5–11) were isolated from an Indonesian marine sponge Xestospongia sp. Their structures were elucidated by extensive 1D and 2D NMR and mass spectrometric data. All the compounds were evaluated for their potential inhibitory activity against eight different protein kinases involved in cell proliferation, cancer, diabetes and neurodegenerative disorders as well as for their antioxidant and antibacterial activities.

Pure compound screening has previously identified the indolglyoxy lamidospermidine ascidian metabolites didemnidine A and B (2 and 3) to be weak growth inhibitors of Trypanosoma brucei rhodesiense (IC50 59 and 44 μM, respectively) and Plasmodium falciparum (K1 dual drug resistant strain) (IC50 41 and 15 μM, respectively), but lacking in selectivity (L6 rat myoblast, IC50 24 μM and 25 μM, respectively). To expand the structure–activity relationship of this compound class towards both parasites, we have prepared and biologically tested a library of analogues that includes indoleglyoxyl and indoleacetic “capping acids”, and polyamines including spermine (PA3-4-3) and extended analogues PA3-8-3 and PA3-12-3. 7-Methoxy substituted indoleglyoxylamides were typically found to exhibit the most potent antimalarial activity (IC50 10–92 nM) but with varying degrees of selectivity versus the L6 rat myoblast cell line. A 6-methoxyindolglyoxylamide analogue was the most potent growth inhibitor of T. brucei (IC50 0.18 μM) identified in the study: it, however, also exhibited poor selectivity (L6 IC50 6.0 μM). There was no apparent correlation between antimalarial and anti-T. brucei activity in the series. In vivo evaluation of one analogue against Plasmodium berghei was undertaken, demonstrating a modest 20.9% reduction in parasitaemia.

Much is known regarding the antibiotic susceptibility of planktonic cultures of Mycobacterium tuberculosis, the bacterium responsible for the lung disease tuberculosis (TB). As planktonically-grown M. tuberculosis are unlikely to be entirely representative of the bacterium during infection, we set out to determine how effective a range of anti-mycobacterial treatments were against M. tuberculosis growing as a biofilm, a bacterial phenotype known to be more resistant to antibiotic treatment. Light levels from bioluminescently-labelled M. tuberculosis H37Rv (strain BSG001) were used as a surrogate for bacterial viability, and were monitored before and after one week of treatment. After treatment, biofilms were disrupted, washed and inoculated into fresh broth and plated onto solid media to rescue any surviving bacteria. We found that in this phenotypic state M. tuberculosis was resistant to the majority of the compounds tested. Minimum inhibitory concentrations (MICs) increased by 20-fold to greater than 1,000-fold, underlying the potential of this phenotype to cause significant problems during treatment.

The occurrence of sulfated steroids and phenolics in marine organisms is quite widespread, being typically reported from Echinoderms. In contrast, alkane and alkene aliphatic sulfates are considerably rarer with examples being reported from a diverse array of organisms including echinoderms, sponges and ascidians. While no ecological roles for these metabolites have been proposed, they do exhibit a diverse array of biological activities including thrombin inhibition; the ability to induce metamorphosis in larvae; antiproliferative, antibacterial and antifungal properties; and metalloproteinase inhibition. Of particular interest and an avenue for future development is the finding of antifouling properties with low or nontoxic effects to the environment. This review focuses on alkyl sulfates and related sulfamates, their structures and biological activities. Spectroscopic and spectrometric techniques that can be used to recognize the presence of sulfate groups are also discussed, data for which will enhance the ability of researchers to recognize this class of chemically- and biologically-interesting marine natural products.

Sample configurations of distaminolyne A (1a); isolated from the ascidians Pseudodistoma opacum and P. cereum, and collected at different sites in New Zealand, were investigated by two methods: Exciton coupled electronic circular dichroism (EC ECD) of the corresponding N,O-dibenzoyl derivative 1b; and chiral reagent derivatization of 1a with (S)- and (R)-α-methoxyphenylacetic acid (MPA), followed by 1H-NMR analysis. Configuration and optical purity of 1a (%ee) was found to vary depending on the geographic distribution of ascidian colonies. An improved method for preparing N,O-diarenoyl derivatives of 1a was optimized. The EC ECD method was found to be complementary to the MPA-NMR method at different ranges of %ee.

Pure compound screening has identified the dioxothiazino-quinoline-quinone ascidian metabolite ascidiathiazone A (2) to be a moderate growth inhibitor of Trypanosoma brucei rhodesiense (IC50 3.1 μM) and Plasmodium falciparum (K1 dual drug resistant strain) (IC50 3.3 μM) while exhibiting low levels of cytotoxicity (L6, IC50 167 μM). A series of C-7 amide and Δ2(3) analogues were prepared that explored the influence of lipophilicity and oxidation state on observed anti-protozoal activity and selectivity. Little variation in anti-malarial potency was observed (IC50 0.62–6.5 μM), and no correlation was apparent between anti-malarial and anti-T. brucei activity. Phenethylamide 7e and Δ2(3)-glycine analogue 8k exhibited similar anti-Pf activity to 2 but with slightly enhanced selectivity (SI 72 and 93, respectively), while Δ2(3)-phenethylamide 8e (IC50 0.67 μM, SI 78) exhibited improved potency and selectivity towards T. brucei rhodesiense compared to the natural product hit. A second series of analogues were prepared that replaced the quinoline ring of 2 with benzofuran or benzothiophene moieties. While esters 10a/10b and 15 were once again found to exhibit cytotoxicity, carboxylic acid analogues exhibited potent anti-Pf activity (IC50 0.34–0.035 μM) combined with excellent selectivity (SI 560–4000). In vivo evaluation of a furan carboxylic acid analogue against P. berghei was undertaken, demonstrating 85.7% and 47% reductions in parasitaemia with ip or oral dosing respectively.

A series of N-14 sidechain substituted analogues of styelsamine (pyrido[4,3,2-mn]acridine) and cystodytin (pyrido[4,3,2-mn]acridin-4-one) alkaloids have been prepared and evaluated for their DNA binding affinity and antiproliferative activity towards a panel of human tumor cell lines. Overall it was found that styelsamine analogues were stronger DNA binders, with the natural products styelsamines B and D having particularly high affinity (Kapp 5.33 × 106 and 3.64 × 106 M−1, respectively). In comparison, the cystodytin iminoquinone alkaloids showed lower affinity for DNA, but were typically just as active as styelsamine analogues at inhibiting proliferation of tumor cells in vitro. Sub-panel selectivity towards non-small cell lung, melanoma and renal cancer cell lines were observed for a number of the analogues. Correlation was observed between whole cell activity and clogP, with the most potent antiproliferative activity being observed for 3-phenylpropanamide analogues 37 and 41 (NCI panel average GI50 0.4 μM and 0.32 μM, respectively) with clogP ~4.0–4.5.

Indole derivatives including bromoindoles have been isolated from the South Pacific marine sponges Rhopaloeides odorabile and Hyrtios sp. Their structures were established through analysis of mass spectra and 1D and 2D NMR spectroscopic data. Their potential inhibitory phospholipase A2 (PLA2), antioxidant and cytotoxic activities were evaluated. The new derivative 5,6-dibromo-l-hypaphorine (9) isolated from Hyrtios sp. revealed a weak bee venom PLA2 inhibition (IC50 0.2 mM) and a significant antioxidant activity with an Oxygen Radical Absorbance Capacity (ORAC) value of 0.22. The sesquiterpene aureol (4), also isolated from Hyrtios sp., showed the most potent antioxidant activity with an ORAC value of 0.29.

Elements of the hypoxia inducible factor (HIF) transcriptional system, a key regulator of the cellular hypoxic response, are up-regulated in a range of cancer cells. HIF is fundamentally involved in tumor angiogenesis, invasion, and energy metabolism. Inhibition of the transcriptional activity of HIF may be of therapeutic benefit to cancer patients. We recently described the identification of two marine pyrroloiminoquinone alkaloids with potent activity in inhibiting the interaction between the oncogenic transcription factor HIF-1α and the coactivator protein p300. Herein, we present further characterization data for these two screening hits: discorhabdin H (1) and discorhabdin L (2), with a specific focus on their anti-angiogenic and anti-tumor effects. We demonstrated that only discorhabdin L (2) possesses excellent anti-angiogenic activity in inhibiting endothelial cell proliferation and tube formation, as well as decreasing microvessel outgrowth in the ex vivo rat aortic ring assay. We further showed that discorhabdin L (2) significantly inhibits in vivo prostate tumor growth in a LNCaP xenograft model. In conclusion, our findings suggest that discorhabdin L (2) represents a promising HIF-1α inhibitor worthy of further drug development.