Background: LR18 is an alpha(-)helical AMP with high antimicrobial activity, low hemolytic activity, and low cytotoxicity. However, the susceptibility to degradation of the peptidase enzyme and a short half-life hinder its application as a therapeutic agent. Improving the stability and prolonging the half-life of LR18 are crucial to accelerate its application in the treatment of infectious diseases. Methods: A new cyclic peptide, C-LR18, was designed and synthesized through end-to-end cyclization of LR18 via disulfide bonds. The biological activity, half-life, and therapeutic effect of C-LR18 on Escherichia coli(-)infected mice were studied. Results: C-LR18 maintained the characteristics of low cytotoxicity and low hemolytic activity of the original LR18 peptide and had higher antibacterial activity and significantly improved stability. After treatment with 1 mg/mL of trypsin, carboxypeptidase, and papain for 1 h, the MIC of C-LR18 against E. coli ATCC25922 was 4 mu M, while that of LR18 had increased to 128 mu M. After exposure to 50% serum or artificial gut solution for 30 min, the MIC of C-LR18 against E. coli ATCC25922 increased 4-fold, while that of LR18 increased 16-fold. The half-life of C-LR18 in plasma and in rats was extended to 3.37-fold and 4.46-fold, respectively, that of LR18. The acute toxicity of C-LR18 in mice is lower than many AMPs reported so far (LD50 = 37.8 mg/kg). C-LR18 has a therapeutic effect on E.coli-infected mice. Conclusions: The cyclic peptide C-LR18 has higher antibacterial activity and stability and a longer half-life than LR18 in rats in vitro and in vivo. C-LR18 also has a therapeutic effect on KM mice infected with E. coli and is expected to become a therapeutic drug for bacterial diseases and applied to the treatment of human and veterinary diseases.
