I was involved in some recent work published in the Journal of Antibiotics (co-authors from Microbial Screening Technologies, Macquarie University, and Queensland Plant Pathology Herbarium) that describes the isolation, structure elucidation and antimicrobial activity of the talcarpones, which were isolated along with a known naphthazarin, aureoquinone. The paper is available here.

Interesting points:

• These compounds were isolated from Talaromyces johnpittii sp. nov., which was named after John Ingram Pitt (1937–2022), an Australian food microbiologist and mycologist.
• The structures and relative stereochemistry of talcarpones A and B were secured by analysis of MS and NMR data. The absolute configuration of the talcarpones was tentatively assigned as 1 S,10 S,1′S,10′S using ECD calculations.
• The talcarpones and aureoquinone are homologues of hybocarpone and boryquinone, respectively. This is the first report of a homologue of hybocarpone, which had been previously reported from several fungal lichen symbionts. Although the relative configuration of hybocarpone was previously confirmed by X-ray crystallography and total syntheses, the absolute configuration was not assigned.
• Talcarpone A had MICs of 1.6 µg/mL against Candida albicans and Saccharomyces cerevisiae, and 12.5 µg/mL against Bacillus subtilis and Staphylococcus aureus.
• Talcarpone B was shown to convert to talcarpone A in aqueous acetonitrile, which then partially converted to aureoquinone by an unknown mechanism.
• The NMR data of aureoquinone showed that it contained a plane of symmetry. We hypothesised that facile intramolecular proton transfer mediated by proton tunnelling leads to very rapid tautomerisation and averaging of the quinone/quinol carbon resonances in aureoquinone, as has been reported for other symmetrical naphthazarins. Interesting, proton tunnelling is eliminated in asymmetrical environments such as hydroxydroserone.

The culmination of my time consulting for the WHO was the publication of “Analysis of the Clinical Pipeline of Treatments for Drug-Resistant Bacterial Infections: Despite Progress, More Action Is Needed”, which was just announced as one of the Antimicrobial Agents and Chemotherapy (AAC) 2023 Top Cited Collections. Open access here.

I am pleased to announce that I have been selected as a Clarivate Highly Cited Researcher for Pharmacology & Toxicology in 2023. I was also a highly cited researcher in 2016, 2017 and 2022 in Pharmacology & Toxicology and in 2021 in Cross-Field.

The criteria for 2023 was slightly changed from previous years: “Each researcher selected has authored multiple Highly Cited Papers™ which rank in the top 1% by citations for their field(s) and publication year in the Web of Science over the past decade. However, citation activity is not the sole selection indicator. A preliminary list based on citation activity is then refined using qualitative analysis and expert judgement.”

A special thanks to my recent co-authors and supporters!

As part of my work with the Institute for Molecular Bioscience at The University of Queensland, an update to our “Antibiotics in Clinical Trials” reviews that have been published in The Journal of Antibiotics every few years since 2011. The good news is that it’s ‘Open Access’ – click here.

Some key points

• There are 47 direct-acting antibacterials, 5 non-traditional small molecule antibacterials, and 10 β-lactam/β-lactamase inhibitor (BLI) combinations under clinical development as of December 2022 (Total = 62)
• At the start of the pipeline, there are now more than double the number of phase-I candidates (26) compared to 2015 (11), while funding initiatives have also helped to boost the number of phase-II (25) compounds since 2019 (18) (see Figure below).
• Encouragingly, 16/26 (62%) of the compounds in phase-I and 14/25 (56%) in phase-II contain new pharmacophores.
• Two new small molecule antibacterial drugs first approved between 2020 and 2022: levonadifloxacin and its prodrug in India in 2020 and the oxazolidinone contezolid acefosamil in China in 2021. Recently, the sulbactam-durlobactam BLI combination was approved in the USA.
• One ‘non-traditional’ antibacterial, Rebyota, was approved in the USA in 2022 and recently, another, Vowst, was approved in the USA – both for C. difficile.
• Despite the encouraging trends in the early stage of the pipeline, further support will be needed to increase the number of new antibacterial drugs launched onto the market.

Figure. Comparison of the numbers of compounds undergoing clinical development as of 2011, 2013, 2015, 2019 and 2022 by development phase

I wanted to tell you about some work I was involved with that was recently published in Chemical Communications. I was able to solve the structures of two new para-nitrobenzoic acid containing piperazines, which were named brevijanazine A and B, isolated from the fungus Aspergillus brevijanus (NRRL 1935).

The NMR data of the natural products were broad and had multiple sets of resonances due to slow conformational changes of the amide bond(s). The NMR data of the p-nitrobenzoic acid moieties, which are relatively rare in nature, were consistent with those reported for other natural products such as waspergillamide A. This is where the rest of the team (from Microbial Screening Technologies, Macquarie University, The University of Western Australia and Sun Yat-sen University) went into action and the structure was confirmed though X-ray analysis and total synthesis. Probably one of the most interesting aspects was the heterologous biosynthesis, precursor feeding and in vitro microsomal assays that showed that a cytochrome P450 oxygenase converts p-aminobenzoic acid to p-nitrobenzoic acid. It wasn’t too long ago that manipulating the genetics of fungi seemed like a distant dream.

In 2013, my team from MerLion Pharmaceuticals in Singapore, in collaboration with Martin Lear (then at NUS), published the structure of a new acyl tetrapeptide, which was an inhibitor of an antimalarial cysteine protease drug target named falcipain-2, in The Journal of Antibiotics. We named this compound falcitidin. In our work, a small amount of a relatively pure active fraction was secured using bioassay-guided isolation after several fermentation attempts and media changes. The planar structure was elucidated by NMR and MS/MS analysis but attempts to isolate further material for biological testing were hampered by an inconsistent production and a low yield (< 0.1 mg/L). As a consequence, we decided to use an alternative approach to fermentation. First, the absolute configuration was determined by Marfey’s analysis and then the structure was confirmed using total synthesis to be isovaleric acid-D-His-L-Ile-L-Val-L-Pro-NH₂. We also explored some preliminary SAR that was published in Tetrahedron Letters the next year.

This is where the story was left until a paper in ACS Chemical Biology was recently published. This team led by Armin Bauer and Till Schäberle used molecular networking to identify over 30 naturally occurring falcitidin analogues from 25 different strains of the bacteria Chitinophaga sp. The team also investigated their biosynthesis. An example of a more potent analogue was pentacitidin A, which is shown above. Interestingly, synthetic falcitidin was not active when tested at 50 uM in their assays and they hypothesise that this could be due to different assay systems. Please read their in-depth study for further details.

My take home points:

• Bioassay-guided isolation can be difficult on occasions but is important as it can unveil new pharmacophores. Don’t give up easily if the biological signal is clear, but always use dereplication to identify known actives and to group ‘like extracts’.
• Molecular networking is a very powerful tool for identifying analogues as clearly demonstrated in this ACS Chem. Biol. paper.
• Analogue isolation and synthesis are important that led to a deeper understanding of the biological activity and potential utility of these compounds.