Antimicrobial Peptides from Insects.

AMP Therapeutics is developing novel antibiotic peptides as new medicines to treat severe infections, particularly those caused by multi-resistant Gram-negative bacteria. Based on parent compounds isolated from the honey bee (1) and the milkweed bug (2) optimized analogs were designed and tested in vivo and in vitro regarding key properties for pharmaceutical development.

These proline-rich antimicrobial peptides (PrAMPs) from the apidaecin and oncocin families selectively kill Gram-negative bacteria. In contrast to most other antimicrobial peptides that impair the structure of the outer bacterial membrane (3), PrAMPs enter the bacterial cells and inhibit bacterial proteins by specifically blocking their function (4). Consequently, PrAMPs are used as leads for the development of a novel class of antibiotics acting via a new mechanism of action beyond the current antibiotics.

For apidaecins, peptides of approximately 20 residues in size, it was shown that different domains control the uptake into the bacterial cell and the binding to bacterial targets (5). By investigating the structure-activity relationship through medicinal chemistry optimized apidaecins analogs were designed. These apidaecin variants exhibit low MIC values against the major Gram-negative pathogens, including multi-resistant strains of Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. Testing such analogs in a murine protection model of E. coli infection revealed a protective effect at a well-tolerated dose level. It is evident that these analogs kill via a non-lytic mechanism. Neither cytotoxicity or hemolytic activity nor toxicity in mice was observed. A binding model of these apidaecin analogs to the putative bacterial target protein DnaK is proposed (6). Further lead optimization efforts led to second generation apidaecin analogs with improved serum stabilities (7). These lead compounds are currently being tested regarding in vivo proof-of-principle and their suitability for preclinical IND-enabling toxicity testing and a safety pharmacology program to prepare for clinical phase I testing.

The oncocin peptides, AMP Therapeutics’ second proprietary antibacterial peptide family, are of comparable size but differ in their amino acid sequence. Based on the native peptide, improved analogs with significant activity against the major Gram-negative organisms were designed (8). Further lead optimization led to oncocines with significantly increased serum stability, i.e. a higher stability of the peptides towards enzymatic degradation, while retaining their antibacterial activity (9, 10). As in vivo proof-of-principle for this antibacterial peptide class the protective effect of a highly active oncocin analog in a murine infection model of systemic septicaemia was demonstrated (11).

(1) Casteels P et al., EMBO J 1989, 8:2387-2391. (2) Schneider M et al. J Invertebr Pathol 2001, 78:135–140. (3) et al., Immunol Rev 2004, 198:169-184. (4) Scocchi M, Cell Mol Life Sci 2011, 68:2317-2330. (5) Czihal P et al., Int J Peptide Res Ther 2009, 15:157-164. (6) Czihal P et al., ACS Chem Biol 2012, 7:1281-1291. (7) Berthold N et al., Antimicrob Agents Chemother 2012, in press. (8) Knappe D et al., J Med Chem 2010, 53:5240–5247. (9) Knappe D et al., Int J Antimicrob Agents 2011, 37:166–170. (10) Knappe D et al., ChemBioChem 2011, 12:874–876. (11) Knappe D et al., J Antimicrob Chemother 2012, 67:2445-2451.