The antimicrobial peptide SET-M33. Characterization of back-up molecules for the setup of novel antibiotics and development of a medical device for removing bacterial toxins from blood

The increasing frequency of multidrug-resistant (MDR) and extensively drug- resistant pathogens creates an urgent need for new antimicrobial drugs and new strategies for the treatment of infectious diseases The peptide SET-M33 is currently under preclinical development for the set-up of a new antibacterial agent to treat infections caused by Gram-negative pathogens. It is a cationic non-natural peptide synthetized in a tetra-branched form that makes it more resistant to degradation in biological fluids. SET-M33 has broadly shown high antimicrobial activity, both in vitro and in vivo, anti-inflammatory activity through selective LPS neutralization, low haemolytic activity, lack of immunogenicity and ability to eradicate biofilms. Here we report the construction of a new SET-M33-based device for the selective removal of bacterial toxins such as lipopolysaccharide (LPS) and lipoteichoic acid (LTA) from a biological fluid of patient with sepsis. In such method the peptide is covalently attached to a solid support through its C- terminus, optionally with the interposition of a linker, and is used to capture the toxins. In an attempt to produce back-up molecules, SET-M33 was synthesized with amino acids in D configuration instead of traditional L residues (SET-M33D). This isomeric version shows a therapeutic activity in vitro and in vivo and immunomodulatory activity in vitro against pro-inflammatory cytokines induced by Gram-negative E. Coli and Gram-positive S. aureus bacteria. Furthermore, the results of peptide’s toxicity in vitro and in vivo suggest that SET-M33D exhibits a low selectivity for eukaryotic cells while it is able to target bacteria cell efficaciously. Lastly, we report the synthesis of the peptide in the two-branched form (SET- M33DIM) and its characterization in terms of stability, antimicrobial toxicity and efficacy in vitro, ex vivo and in vivo, anti-inflammatory proprieties and mechanism of action. Compared to the tetra-branched molecule, SET- M33DIM shows a similar antibacterial activity in vitro for strains of K. pneumonia and E. coli, a moderate activity against P. aeruginosa, and no activity against strains of the Gram-positive S. aureus. In vivo the peptide is able to delay significantly signs of acute infection in animal lethally infected with P. aeruginosa. Its toxicity profile results improved respect to SET-M33L more than 20 fold when tested on eukaryotic cells and a haemolysis degree not more than 22%, even using a concentration which is 200-fold respect to the MIC. Moreover, SET-M33DIM exhibits a strong capability to neutralize LPS, inhibiting the expression of inflammatory cytokines and enzymes in macrophages.