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

Abstract

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.

Combating bacterial resistance by combination of antibiotics with antimicrobial peptides

“…Peptides Indolicidin [80] 16 32 CAMA a [12] 4 -Nisin [12] 8 -A3-APO [89] Not effective 13 Cecropin P1 [70] -16-32 (and 4 d ) Chex1-Arg20 [89] Not an increase in the antimicrobial action [84]. Methods of conjugating peptides with antibiotics depend on the structures of both but include utilizing amide bond formation attached to linking group [85], a disulfide tether [86] and a glutaraldehyde linker [87].…”

“…As solid phase synthesis of peptides can produce an amidated terminus on the peptide, using this to link the peptide to a drug is logical [88]. The covalent conjugation of the cationic AMP, Chex1-Arg20, and several other AMPs, including the magainin analogue MSI-78 with 7-cephalosporanic acid and/or 7-aminodesacetoxycephalosporanic acid, was undertaken by Li et al [89]. This was achieved by attaching the 7-amino group of cephalosporin to the N-terminus of the peptide.…”

“…This was achieved by attaching the 7-amino group of cephalosporin to the N-terminus of the peptide. However, only the MSI-conjugated analogues were found to produce synergistic activity against A. baumannii and multidrug resistant (MDR) A. baumannii 156 [89]. The antibiotic levofloxacin was conjugated to the peptide indolicin by Ghaffar et al [90] using an amide bond or labile ester linkage, which showed little change in activity.…”

Drug delivery systems designed to overcome antimicrobial resistance

“…AMPs have also been linked to conventional β‐lactam antibiotics, and their corresponding conjugates have been shown to enhance the antibacterial activity of their parent drugs. For example, Li et al recently reported the conjugation of functionalized 7‐aminocephalosporanic acid (7‐ACA; 62 ) and 7‐aminodesacetoxycephalosporanic acid (7‐ADCA; 63 ) with MSI‐78 ( 64 ), a synthetic from the magainin analog family (Figure ). In fact, ( 64 ) (a cationic AMP with sequence: GIGKFLKKAKKFGKAFVKILKK‐NH 2 ) previously showed a synergic effect in vitro against bloodstream infections when combined with β‐lactam antibiotics .…”

“…In fact, ( 64 ) (a cationic AMP with sequence: GIGKFLKKAKKFGKAFVKILKK‐NH 2 ) previously showed a synergic effect in vitro against bloodstream infections when combined with β‐lactam antibiotics . In the report by Li and his co‐workers, ( 62 ) and ( 63 ) were 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 ( 64 ) to create the conjugates ( 65 ) and ( 66 ), respectively (Figure ). The obtained MSI‐78‐β‐lactam conjugates were evaluated for antibacterial activity against a panel of six Gram‐negative pathogens, including clinically isolated MDR strains (Table ).…”