No abstract

The emergence of multiple-drug-resistant (MDR) bacterial pathogens in hospitals (nosocomial infections) presents a global threat of growing importance, especially for Gram-negative bacteria with extended spectrum β-lactamase (ESBL) or the novel New Delhi metallo-β-lactamase 1 (NDM-1) resistance. Starting from the antibacterial peptide apidaecin 1b, we have optimized the sequence to treat systemic infections with the most threatening human pathogens, such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. The lead compound Api88 enters bacteria without lytic effects at the membrane and inhibits chaperone DnaK at the substrate binding domain with a K(D) of 5 μmol/L. The Api88-DnaK crystal structure revealed that Api88 binds with a seven residue long sequence (PVYIPRP), in two different modes. Mice did not show any sign of toxicity when Api88 was injected four times intraperitoneally at a dose of 40 mg/kg body weight (BW) within 24 h, whereas three injections of 1.25 mg/kg BW and 5 mg/kg BW were sufficient to rescue all animals in lethal sepsis models using pathogenic E. coli strains ATCC 25922 and Neumann, respectively. Radioactive labeling showed that Api88 enters all organs investigated including the brain and is cleared through both the liver and kidneys at similar rates. In conclusion, Api88 is a novel, highly promising, 18-residue peptide lead compound with favorable in vitro and in vivo properties including a promising safety margin.

dProline-rich antimicrobial peptides (PrAMPs) from insects and mammals have recently been evaluated for their pharmaceutical potential in treating systemic bacterial infections. Besides the native peptides, several shortened, modified, or even artificial sequences were highly effective in different murine infection models. Most recently, we showed that the 18-residue-long peptide Api88, an optimized version of apidaecin 1b, was efficient in two different animal infection models using the pathogenic Escherichia coli strains ATCC 25922 and Neumann, with a promising safety margin. Here, we show that Api88 is degraded relatively fast upon incubation with mouse serum, by cleavage of the C-terminal leucine residue. To improve its in vitro characteristics, we aimed to improve its serum stability. Replacing the C-terminal amide by the free acid or substituting Arg-17 with Lornithine or L-homoarginine increased the serum stabilities by more than 20-fold (half-life, ϳ4 to 6 h). These analogs were nontoxic to human embryonic kidney (HEK 293), human hepatoma (HepG2), SH-SY5Y, and HeLa cells and nonhemolytic to human erythrocytes. The binding constants of all three analogs with the chaperone DnaK, which is proposed as the bacterial target of PrAMPs, were very similar to that of Api88. Of all the analogs tested, Api137 (Gu-ONNRPVYIPRPRPPHPRL; Gu is N,N,N=,N=-tetramethylguanidino) appeared most promising due to its high antibacterial activity, which was very similar to Api88. Positional alanine and D-amino acid scans of Api137 indicated that substitutions of residues 1 to 13 had only minor effects on the activity against an E. coli strain, whereas substitutions of residues 14 to 18 decreased the activity dramatically. Based on the significantly improved resistance to proteolysis, Api137 appears to be a very promising lead compound that should be even more efficient in vivo than Api88.

Proline-rich antimicrobial peptides (PrAMPs) freely penetrate through the outer membrane into the periplasm of Gram-negative bacteria, before they are actively translocated by a permease/transporter-mediated uptake into the cytoplasm where they are reported to inhibit chaperone DnaK. Here we have studied the PrAMP apidaecin 1b, which is produced in honey bees in response to bacterial infections, and optimized apidaecin analogs for their bacterial uptake. The peptides were labeled with 5(6)-carboxyfluorescein and their internalization in Escherichia coli and Klebsiella pneumoniae was visualized by fluorescence microscopy and quantified by flow cytometry for four different time points over an incubation period of 4 h. Apidaecin 1b entered only 40% to 50% of the cells at detectable quantities, whereas designer peptides Api88, Api134 and Api155 entered more than 95% of the bacteria within 30 min at around fourfold higher quantities than the native peptide. Interestingly, a shortened version designated as (1-17)Api88, bound DnaK as efficiently as the 18-residue long Api88 and entered the bacteria at similar kinetics as Api88, but was unable to inhibit the bacterial growth. Similar conflicts with currently proposed mechanisms of PrAMPs were also obtained for some Ala-substituted analogs and reverse apidaecin sequences. Although peptides with C-terminal amides enter the cells much more efficiently than homologous C-terminal acids, this improved cell penetration does not improve the antibacterial activities. These studies suggest that PrAMPs utilize additional modes of action to kill sensitive organisms.

A novel concept to release peptidic drugs systemically by serum proteases from a PEGylated precursor makes it possible to tune release kinetics to fit the medical needs. Drug release depends on the size of the PEG polymer and the sequence and length of the peptide linker. The antimicrobial activities of the prodrugs were even better than those of the free peptides, whereas direct PEGylation abolished the peptide activity.