Selectivity Modulation of Small Cationic Membrane-Active Cyclic Peptides with Broad-Spectrum Activity against...

Area of Science:

• Medicinal Chemistry
• Microbiology
• Biophysics

Background:

• Antimicrobial resistance necessitates the development of novel therapeutic agents.
• Macrocyclic peptides offer a promising scaffold for new drug discovery.
• Understanding structure-activity relationships is key to optimizing peptide efficacy and safety.

Purpose of the Study:

• To synthesize and evaluate novel macrocyclic peptides with modified ring sizes and backbone flexibility.
• To assess the antimicrobial potency, cytotoxicity, and therapeutic index of designed peptides.
• To elucidate the mechanism of action and conformational properties influencing peptide activity.

Main Methods:

• Peptide synthesis and characterization.
• Antimicrobial susceptibility testing (MIC determination) against Gram-positive and Gram-negative bacteria, and fungi.
• Cytotoxicity assays and therapeutic index calculation.
• Biofilm eradication assays.
• Membrane-disrupting assays (calcein-leakage, ATP leakage).
• Nuclear Magnetic Resonance (NMR) spectroscopy for conformational analysis.
• Plasma stability studies.

Main Results:

• Two optimized peptides, 6b and 10b, exhibited broad-spectrum activity against drug-resistant bacteria and fungi (MICs: 1.5-25 μg/mL).
• These peptides demonstrated significantly improved therapeutic indices (∼400) compared to the lead peptide p1, indicating enhanced safety.
• Peptides 6b and 10b were effective against Gram-negative pathogens resistant to daptomycin, showed rapid bactericidal action, and eradicated biofilms.
• Mechanistic studies confirmed a membranolytic mode of action.
• NMR revealed a unique "sandwich" conformation in peptide 6b, correlating with improved selectivity.
• Both peptides displayed high plasma stability (t1/2 ≈ 6-8 hours).

Conclusions:

• Modification of macrocyclic peptide structure can significantly enhance antimicrobial potency and selectivity.
• Optimized peptides 6b and 10b represent promising candidates for treating infections caused by drug-resistant pathogens.
• The distinct conformation observed in peptide 6b likely contributes to its improved therapeutic profile.
• These findings support the potential of rationally designed macrocyclic peptides as a new class of antimicrobials.