To combat the serious issue of increasing global antibiotic resistance, new antimicrobial therapies are urgently required. As one alternative, previously used antibiotics are being investigated for use in combination with more modern antibiotics including antimicrobial peptides. Towards this goal, 7-aminocephalosporanic acid, the precursor to the conventional β-lactam antibiotic, cephalosporin, and the related compound, 7-aminodesacetoxycephalosporanic acid, were each chemically modified to enable their use in solid phase peptide synthesis for covalent conjugation via their 7-amino group and a glycolic linker to the N-terminus of a series of cationic antimicrobial peptides, MSI-78, CA(1-7)M(2-9)NH2 and des-Chex1-Arg20. Chemically functionalized Cα-Fm-protected 7-aminocephalosporanic acid and 7-aminodesacetoxycephalosporanic acid building blocks were separately prepared and attached by their 7-amino group to the N-terminus of each peptide on the solid phase via a glutaric anhydride linker. The resulting conjugated AMPs were assessed for antibacterial activity against a panel of six Gram-negative bacteria, including clinically isolated multi-drug resistant (MDR) pathogens. Only the conjugated MSI-78 analogues displayed significant activity against Acinetobacter baumannii and MDR A. baumannii 156 and enhanced activity against Klebsiella pneumoniae. Further work is required to optimize the conjugation of AMPs to 7-cephalosporanic acid and/or 7-aminodesacetoxycephalosporanic acid to universally induce an enhanced effect on killing of drug sensitive and MDR bacterial strains that are common causes of hospital-acquired infections.

Covalent conjugation of cationic antimicrobial peptides with a β-lactam antibiotic core

The β-lactams constitute a well-known class of compounds having tremendous biological significance. Besides being a motif of biological interest, they serve as versatile synthons in organic chemistry. In fact, their easy accessibility in the laboratory by several methods combined with inherent reactivity of the β -lactam ring due to ring-strain places it among the most sought for substrate in the arsenal of synthetic organic chemists. Several chemical reagents, heat, and light promote its ring-opening, ring-expansions and rearrangement reactions yielding a wide variety of biologically relevant nitrogen-containing acyclic and heterocyclic compounds. In recent years, the reactivity of differently functionalized β-lactam rings towards diverse kinds of reagents has been investigated. These investigations exploit selective bond cleavage of the β-lactam nucleus via N1-C2, C3- C4, C2-C3 or N1-C4 bond cleavage using simple reagents. The reduction of amide carbonyl group, thionation, and pyrolysis/photolysis have also been explored. These investigations have led to the discovery of many easy synthetic methods for biologically important classes of compounds such as β-amino acids, β-amino esters, amino sugars, amino alcohols, peptides, azetidines, and other heterocyclic compounds. This article discusses the advances made in the studies on the reactivity of β- lactam ring during the last ten years.

Recent Progress in Chemistry of β-Lactams

The review covers work published in the calendar year 2012. The synthesis of four-membered oxygen- and nitrogen-containing heterocycles is reviewed. New developments in the reaction chemistry of four-membered heterocycles, especially β-lactams, are covered.

/pdf/four-membered-ring-systems-37q08bs7lk.pdf

Four-Membered Ring Systems

Novel enantiopure pseudopeptide models containing a central -(β-lactam)-(Aa)- scaffold characterized by the combined presence of an α-alkyl-α-amino-β-lactam (i+1) residue and a α-substituted (i + 2) amino acid have been readily synthesized from α-alkyl serines. The conformational analysis of such β-lactam pseudopeptides conducted in CDCl(3) and DMSO-d(6) solutions using 1D- and 2D-NMR techniques revealed an equilibrium between β-II turn and γ-turn conformers, which was ultimately modulated by the relative configuration of the -(β-lactam)-(Aa)- residues. Long-range chiral effects on the α-lactam pseudopeptide conformers were also found when two (i) and (i + 3) chiral residues were attached to the termini of a central -(β-lactam)-(Aib)- segment. In such mimetics, heterochiral (i) and (i + 3) residues reinforced a β-II turn conformer, whereas homochiral corner residues stabilized an overlapped β-II/ β-I double turn motif. No β-hairpin nucleation was observed in any instance. In good agreement with the conformers found in solution, β-turned and open structures were also characterized by X-ray crystallography. Relative stabilities of the different conformers were estimated computationally at a B3LYP/6-31++G** calculation level, and finally, a conformation equilibrium model based on steric inter-residual interactions around the -(β-lactam)-(i + 2)- segment was proposed to account for the observed chiral effects.

/pdf/chirality-driven-folding-of-short-b-lactam-pseudopeptides-22t57cvg0y.pdf

Chirality-driven folding of short β-lactam pseudopeptides.

A click-chemistry-based approach was implemented to prepare peptidomimetics designed in silico and made from aromatic azides and a propargylated GIGI-mimicking platform derived from the altered Melan-A/MART-126(27L)-35 antigenic peptide ELAGIGILTV. The CuI -catalyzed Huisgen cycloaddition was carried out on solid support to generate rapidly a first series of peptidomimetics, which were evaluated for their capacity to dock at the interface between the major histocompatibility complex class-I (MHC-I) human leucocyte antigen (HLA)-A2 and T-cell receptors (TCRs). Despite being a weak HLA-A2 ligand, one of these 11 first synthetic compounds bearing a p-nitrobenzyl-triazole side chain was recognized by the receptor proteins of Melan-A/MART-1-specific T-cells. After modification of the N and C termini of this agonist, which was intended to enhance HLA-A2 binding, one of the resulting seven additional compounds triggered significant T-cell responses. Thus, these results highlight the capacity of naturally circulating human TCRs that are specific for the native Melan-A/MART-126-35 peptide to cross-react with peptidomimetics bearing organic motifs structurally different from the native central amino acids.

/pdf/synthesis-and-biological-evaluation-of-hapten-clicked-4pcic1w3lb.pdf

Synthesis and Biological Evaluation of Hapten-Clicked Analogues of The Antigenic Peptide Melan-A/MART-126(27L)-35.

β-Lactam peptides were envisioned as conformational constraints in antigenic peptides (APs). Three different β-lactam tripeptides of varying flexibility were prepared in solution and incorporated in place of the central part of the altered melanoma associated antigenic peptide Leu27-Melan-A26–35 using solid phase synthesis techniques. Upon TFA cleavage from the solid support, an unexpected opening of the β-lactam ring occurred with conservation of the amide bond. After adaptation of the solid phase synthesis strategy, β-lactam peptides were successfully obtained and both opened and closed forms were evaluated for their capacity to bind to the antigen-presenting class-I MHC HLA-A2 protein system. None of the closed β-lactam peptides bound to HLA-A2, but their opened variants were shown to be moderate to good HLA-A2 ligands, one of them being even capable of stimulating a Melan-A-specific T cell line.

/pdf/design-synthesis-and-evaluation-of-b-lactam-antigenic-gvasvmr9wn.pdf

Design, synthesis and evaluation of β-lactam antigenic peptide hybrids; unusual opening of the β-lactam ring in acidic media

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Design, synthesis and evaluation of β-lactam antigenic peptide hybrids; unusual opening of the β-lactam ring in acidic media