Rational Design and Synthesis of Modified Teixobactin Analogues: In Vitro Antibacterial Activity against Staphylococcus aureus, Propionibacterium acnes and Pseudomonas aeruginosa

Abstract

Teixobactin, a recently discovered depsipeptide that binds to bacterial lipid II and lipid III, provides a promising molecular scaffold for the design of new antimicrobials. Herein, we describe the synthesis and antimicrobial evaluation of systematically modified teixobactin analogues. The replacement of the Ile residue with aliphatic isosteres, the modification of the guanidino group at residue 10 and the introduction of a rigidifying residue, that is, dehydroamino acid, into the macrocyclic ring generated useful structure-activity information. Extensive antimicrobial susceptibility assessment against a panel of clinically relevant Staphylococcus aureus and Propionibacterium acnes strains led to the identification of the new lead compound, [Arg(Me) ,Nle ]teixobactin, with an excellent bactericidal activity (minimum inhibitory concentration (MIC)=2-4 μg mL ). Significantly, the antimicrobial activity of several of the teixobactin analogues against the pathogenic Gram-negative Pseudomonas aeruginosa was "restored" when combined with the sub-MIC concentration of the outer membrane-disruptive antibiotic colistin. The antimicrobial effectiveness of a [Tfn ,Nle ]teixobactin (32 μg mL )-colistin (2 μg mL ; 0.5×MIC) combination against P. aeruginosa PAO1 reveals, for the first time, an alternative therapeutic option in the treatment of Gram-negative infections.

Probing key elements of teixobactin–lipid II interactions in membranes

“…The latter observation is consistent with many allo -End10 substituted Txb analogs that retain partial or full antimicrobial activities compared with the wildtype Txb 8–27. Nevertheless, residue 10 of Txb remains an essential residue, regardless of the side chain identity, because of its critical role in phosphate binding by the backbone amide (∼99% probability of donating a hydrogen bond to lipid II, Fig.…”

“…15 and 17 and 27) but not Ile11 15,27. In contrast, some hydrophobic substitutions of Ile11 retain partial activity 15,16,27. Both results indicate that Ile11 is more likely the membrane anchor of the cyclodepsipeptide ring than Ala9 in vivo , matching the membrane binding mode described in System 6.…”

“…This conclusion is further supported by studies of Txb analogs in which polar substitutions can be tolerated for Ala9 (ref. 15 and 17 and 27) but not Ile11 15,27. In contrast, some hydrophobic substitutions of Ile11 retain partial activity 15,16,27.…”

“…In addition, Txb contains an N-terminal methylation and a C-terminal “head-to-side-chain” cyclodepsipeptide ring 6. Most interactions have been interpreted based on the comparison of bacterial growth inhibition activities of numerous chemically synthesized Txb analogs 8–27. In particular, one of the inactive analogs, designated “Ac–Δ 1–5 –Arg 10 –Txb”,15 has been crystallized from solution as a hydrochloride salt, where the chloride anion is coordinated at the center of several backbone amides that form the C-terminal cyclodepsipeptide ring.…”

Dissecting the Binding Interactions of Teixobactin with the Bacterial Cell‐Wall Precursor Lipid II

“…Interest in the peptide's activity and therapeutic potential led to curiosity in synthetic approaches to access teixobactin; with two distinct synthetic routes reported just a year later . Following the initial report, several studies were aimed at understanding the spectrum of antimicrobial activity, structure–activity studies, interrogating the mode of action through modelling, and structural investigations . These studies yielded insight into key residues, modifiable regions, and suspected binding sites of teixobactin to its cellular targets—the bacterial cell wall precursors: lipid II (Figure B) and lipid III.…”