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Microbial Membrane-Permeating Peptides and Their Applications 391
XXXX
Toxic
substance XXXXXXXX
di,tri
peptide
Carrier
peptide
ppt
Cell membrane
Cytoplasm
Intracellular target
Toxic
substance
FIGURE 18.5 Generic strategies for peptide-mediated delivery of antimicrobials. The schematic
illustrates a carrier peptide domain attached covalently by a flexible linker to a toxic
domain. Di- and tripeptides could pass through core protein complex or peptide permease
transporters. There are possibilities to use longer permeating peptide to deliver proteins and
other substances into microbes, although the mechanisms involved remain uncertain.
reported for delivery into E. coli and Salmonella typhimurium and the idea was
developed to produce the therapeutic alafosfalin, which showed modest clinical
success. 9,10
A limitation with the oligopeptide permease delivery approach, however, is that
permeases are not able to efficiently transport substrates with cargoes attached,
although certain permeases may be more promiscuous in this respect. 37 Another
limitation is that oligopeptide permeases appear to mutate rapidly to change substrate
specificity; therefore, it seems likely that microorganisms would become resistant
to antimicrobials based on oligopeptide transport.
A related idea is to use cell wall-permeating peptides as vehicles to deliver toxic
substances into microbes (Figure 18.5). At first glance it seems illogical to use
peptides as delivery vehicles because they are relatively large in comparison to most
drugs. Indeed, peptides are normally not considered cell permeable. However, natural
antimicrobial peptides provide a clear precedent for this approach. Many natural
antimicrobial peptides kill target cells not only by permeabilization, but also by
inhibition of intracellular targets, as discussed earlier in this chapter. It seems clear
that certain peptides can permeate cells to reach their main target; it should be
possible to use such carrier domains to deliver attached foreign substances.
Cell-permeating peptides have been used to deliver a wide range of foreign
proteins and oligonucleotides into mammalian cells (see Chapters 1 to 7, 16, and
17) and microbial cell-permeating peptides offer attractive possibilities for antimicrobial
development. The challenge now is to identify peptides that can efficiently
carry attached cargo molecules into cells with a high degree of microbial cell
specificity. In our first attempt to use antimicrobial peptides as delivery vehicles, the
objective was to improve cell uptake and activity of antisense peptide nucleic acids
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