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Broad-Spectrum Activity of Membranolytic Cationic Macrocyclic Peptides Against Multi-Drug Resistant Bacteria and Fungi
The emergence of multidrug-resistant (MDR) strains causes severe problems in the treatment of microbial infections
owing to limited treatment options. Antimicrobial peptides (AMPs) are drawing considerable attention
as promising antibiotic alternative candidates to combat MDR bacterial and fungal infections. Herein, we present
a series of small amphiphilic membrane-active cyclic peptides composed, in part, of various nongenetically
encoded hydrophilic and hydrophobic amino acids. Notably, lead cyclic peptides 3b and 4b showed broadspectrum
activity against drug-resistant Gram-positive (MIC = 1.5–6.2 μg/mL) and Gram-negative (MIC =
12.5–25 μg/mL) bacteria, and fungi (MIC = 3.1–12.5 μg/mL). Furthermore, lead peptides displayed substantial
antibiofilm action comparable to standard antibiotics. Hemolysis (HC50 = 230 μg/mL) and cytotoxicity (>70 %
cell viability against four different mammalian cells at 100 μg/mL) assay results demonstrated the selective lethal
action of 3b against microbes over mammalian cells. A calcein dye leakage experiment substantiated the
membranolytic effect of 3b and 4b, which was further confirmed by scanning electron microscopy. The behavior
of 3b and 4b in aqueous solution and interaction with phospholipid bilayers were assessed by employing nuclear
magnetic resonance (NMR) spectroscopy in conjunction with molecular dynamics (MD) simulations, providing a
solid structural basis for understanding their membranolytic action. Moreover, 3b exhibited stability in human
blood plasma (t1/2 = 13 h) and demonstrated no signs of resistance development against antibiotic-resistant
S. aureus and E. coli. These findings underscore the potential of these newly designed amphiphilic cyclic peptides
as promising anti-infective agents, especially against Gram-positive bacteria.