Structure Elucidation, Total Synthesis, Antibacterial In Vivo Efficacy and Biosynthesis Proposal of Myxobacterial Corramycin.

Herein, we describe the myxobacterial natural product Corramycin isolated from Corallococcus coralloides. The linear peptide structure contains an unprecedented (2R,3S)-γ-N-methyl-ß-hydroxy-histidine moiety. Corramycin exhibits anti-Gram-negative activity against Escherichia coli (E. coli) and is taken up via two transporter systems, SbmA and YejABEF. Furthermore, the Corramycin biosynthetic gene cluster (BGC) was identified and a biosynthesis model was proposed involving a 12-modular non-ribosomal peptide synthetase/polyketide synthase. Bioinformatic analysis of the BGC combined with the development of a total synthesis route allowed for the elucidation of the molecule's absolute configuration. Importantly, intravenous administration of 20 mg kg-1 of Corramycin in an E. coli mouse infection model resulted in 100 % survival of animals without toxic side effects. Corramycin is thus a promising starting point to develop a potent antibacterial drug against hospital-acquired infections.

A new class of antibacterials, the imidazopyrazinones, reveal structural transitions involved in DNA gyrase poisoning and mechanisms of resistance.

Imidazopyrazinones (IPYs) are a new class of compounds that target bacterial topoisomerases as a basis for their antibacterial activity. We have characterized the mechanism of these compounds through structural/mechanistic studies showing they bind and stabilize a cleavage complex between DNA gyrase and DNA ('poisoning') in an analogous fashion to fluoroquinolones, but without the requirement for the water-metal-ion bridge. Biochemical experiments and structural studies of cleavage complexes of IPYs compared with an uncleaved gyrase-DNA complex, reveal conformational transitions coupled to DNA cleavage at the DNA gate. These involve movement at the GyrA interface and tilting of the TOPRIM domains toward the scissile phosphate coupled to capture of the catalytic metal ion. Our experiments show that these structural transitions are involved generally in poisoning of gyrase by therapeutic compounds and resemble those undergone by the enzyme during its adenosine triphosphate-coupled strand-passage cycle. In addition to resistance mutations affecting residues that directly interact with the compounds, we characterized a mutant (D82N) that inhibits formation of the cleavage complex by the unpoisoned enzyme. The D82N mutant appears to act by stabilizing the binary conformation of DNA gyrase with uncleaved DNA without direct interaction with the compounds. This provides general insight into the resistance mechanisms to antibiotics targeting bacterial type II topoisomerases.

Studies of antibacterial activity (in vitro and in vivo) and mode of action for des-acyl tridecaptins (DATs).

Tridecaptins comprise a class of linear cationic lipopeptides with an N-terminal fatty acyl moiety. These 13-mer antimicrobial peptides consist of a combination of d- and l-amino acids, conferring increased proteolytic stability. Intriguingly, they are biosynthesized by non-ribosomal peptide synthetases in the same bacterial species that also produce the cyclic polymyxins displaying similar fatty acid tails. Previously, the des-acyl analog of TriA1 (termed H-TriA1) was found to possess very weak antibacterial activity, albeit it potentiated the effect of several antibiotics. In the present study, two series of des-acyl tridecaptins were explored with the aim of improving the direct antibacterial effect. At the same time, overall physico-chemical properties were modulated by amino acid substitution(s) to diminish the risk of undesired levels of hemolysis and to avoid an impairment of mammalian cell viability, since these properties are typically associated with highly hydrophobic cationic peptides. Microbiology and biophysics tools were used to determine bacterial uptake, while circular dichroism and isothermal calorimetry were used to probe the mode of action. Several analogs had improved antibacterial activity (as compared to that of H-TriA1) against Enterobacteriaceae. Optimization enabled identification of the lead compound 29 that showed a good ADMET profile as well as in vivo efficacy in a variety of mouse models of infection.

Optimization of the Antibacterial Spectrum and the Developability Profile of the Novel-Class Natural Product Corramycin.

Corramycin 1 is a novel zwitterionic antibacterial peptide isolated from a culture of the myxobacterium Corallococcus coralloides. Though Corramycin displayed a narrow spectrum and modest MICs against sensitive bacteria, its ADMET and physchem profile as well as its high tolerability in mice along with an outstanding in vivo efficacy in an Escherichia coli septicemia mouse model were promising and prompted us to embark on an optimization program aiming at enlarging the spectrum and at increasing the antibacterial activities by modulating membrane permeability. Scanning the peptidic moiety by the Ala-scan strategy followed by key stabilization and introduction of groups such as a primary amine or siderophore allowed us to enlarge the spectrum and increase the overall developability profile. The optimized Corramycin 28 showed an improved mouse IV PK and a broader spectrum with high potency against key Gram-negative bacteria that translated into excellent efficacy in several in vivo mouse infection models.

Imidazopyrazinones (IPYs): Non-Quinolone Bacterial Topoisomerase Inhibitors Showing Partial Cross-Resistance with Quinolones.

In our quest for new antibiotics able to address the growing threat of multidrug resistant infections caused by Gram-negative bacteria, we have investigated an unprecedented series of non-quinolone bacterial topoisomerase inhibitors from the Sanofi patrimony, named IPYs for imidazopyrazinones, as part of the Innovative Medicines Initiative (IMI) European Gram Negative Antibacterial Engine (ENABLE) organization. Hybridization of these historical compounds with the quinazolinediones, a known series of topoisomerase inhibitors, led us to a novel series of tricyclic IPYs that demonstrated potential for broad spectrum activity, in vivo efficacy, and a good developability profile, although later profiling revealed a genotoxicity risk. Resistance studies revealed partial cross-resistance with fluoroquinolones (FQs) suggesting that IPYs bind to the same region of bacterial topoisomerases as FQs and interact with at least some of the keys residues involved in FQ binding.