In Vitro and In Vivo Characterization of Novel Cathelicidin-Based Peptides with Antimicrobial Activity Against .

: Infections caused by multidrug-resistant (MDR) are steadily increasing, thus the discovery and development of new and effective agents are needed. Antimicrobial peptides (AMPs) are a heterogeneous group of innate defense system peptides with broad antimicrobial activity. In this study, 17 AMPs were tested, identifying CAP-18, a cathelicidin-based compound, as the most active. CAP-18 was optimized by synthesizing structural derivatives, which were selected for further studies based on their activity against a collection of MDR and colistin-resistant strains. : AMPs collection was initially tested against different strains, identifying CAP-18 as the most active. CAP-18 derivatives were synthetized and assessed by the Minimum Inhibitory Concentration (MIC), time-kill kinetics, cytotoxicity against human cell lines, hemolytic activity, and therapeutic index (IC/MIC). The mechanism of action was assessed by Transmission Electron Microscopy (TEM), and in vivo efficacy was determined through a murine skin infection model. : CAP-18 and D-CAP-18 had a MIC of 4 and 2 μg/mL, respectively, whereas CAP-18 and D-CAP-18 presented MIC values of 16 mg/L. The shorter derivatives of CAP-18 showed a lower activity. Time-kill curves revealed a fast bactericidal effect. These derivatives showed low toxicity against different human cell lines and low hemolysis, resulting in a wide therapeutic index (IC/MIC), with D-CAP-18 having the best therapeutic index (137.4). TEM provided insight into the mechanism of action, revealing bacterial membrane damage. In vivo studies of both CAP-18 and D-CAP-18 showed good activity with a 3 log decrease compared to the infected control group. : Among the investigated four peptides, D-CAP-18 is the most promising candidate to treat skin infections caused by MDR since it shows potent activity both in vitro and in vivo, and a high therapeutic index.