Revisiting unexploited natural products in search of new antibacterial drug candidates: the case of the benzoisochromanequinone and dithiolopyrrolone classes

It has become vital to identify antibacterial drug candidates with novel modes of action to address the growing problem of antibiotic resistance. One way of rapidly accessing novel antibiotics is to revisit natural product classes that have previously been shown to possess antibacterial activity, but have never been clinically exploited. The recent introduction of the antibiotics daptomycin, retapamulin, and fidaxomicin to the clinic is a proof of concept that revisiting known of novel natural product scaffolds could achieve clinical benefit. In this thesis, the benzoisochromanequinone (BIQ) and dithiolopyrrolone (DTP) classes of natural products were revisited. In chapter three, the BIQ class actinorhodin was subject to a detailed biological characterization. γ-actinorhodin was found to possess several requisite properties of a useful antibacterial drug candidate; it possessed potent bactericidal anti-Gram-positive activity, which included activity against MRSA and VISA strains (MIC90 of 2 μg/ml), was found to exhibit selective toxicity against prokaryotes, and displayed low resistance potential in vitro. Mode of action studies showed that γ-actinorhodin acts on the bacterial membrane in a manner distinct from other membrane-perturbing agents in clinical use. Preliminary studies of safety and efficacy of γ-actinorhodin in vivo showed potential promise for treatment staphylococcal infection. In chapter four of this thesis, the DPT thiolutin was studied, with an emphasis on understanding its mode of antibacterial action. In E. coli, thiolutin demonstrated preferential inhibition of RNA synthesis in agreement with earlier studies. In S. aureus however, while potent bacteriostatic activity was noted, thiolutin showed an unusual profile in radiolabel incorporation experiments with no inhibitory effect on any macromolecular biosynthetic pathway. Genetic analysis of mutants resistant to thiolutin revealed mutations in the S.aureus thioredoxin and in E. coli glutaredoxin redox systems components suggesting the involvement of these systems in the reductive activation of thiolutin. Inhibition of E. coli transcription and translation could not be detected in vitro, even in the presence of reducing agent suggested that RNA polymerase is not the primary target of thiolutin. The finding from this study and recent publication (Chan et al. 2017) suggest that DTPs exerts their antibacterial activity via a novel mode of action. In summary, the findings of this study with thiolutin and γ-actinorhodin underscore the utility of revisiting unexploited natural product in the search for antibacterials with novel mode of action.