Approaches to peptide synthesis
Stapled peptides. One important collaboration in peptide drug development is between the biopharmaceutical company Aileron Therapeutics and
Roche. In November 2011, Aileron expanded its collaboration with Roche for the discovery, development and commercialisation
of stapledpeptide drugs. The potential $1.1billion drug development collaboration, launched in August 2010, encompasses up
to five programmes with the initial two programmes focusing on oncology, and a third programme being launched to focus on
inflammatory diseases.
Public–private partnerships in biomanufacturing
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Stapled peptides use peptide-stabilisation technology to enhance potency and cell permeability of a drug to address pharmacological
limitations of small molecules and existing biologics in intracellular protein–protein interactions. Although small molecules
can penetrate cells, the large binding surfaces for intracellular protein–protein interactions often make small-molecule modulators
ineffective. As cited previously, peptides cannot permeate cells so act extracellularly (1, 10). Stapled peptides seek to
resolve this problem. Because many undruggable therapeutic targets include those protein–protein interactions, in which alpha-helices
are required in lock-and-key-type mechanisms, an approach is to design alpha-helical peptides that have structural and functional
properties that enable them to penetrate into the cell, bind to the therapeutic target and modulate the biological pathway
(1, 10). Aileron stabilises peptides by "stapling" them with hydrocarbon bonds into an alpha-helix. Once constrained in the
alpha-helix structure, the peptides are protected from degradation by proteases. The stabilised alphahelical peptides can
penetrate cells by energy-dependent active transport and typically have a higher affinity to large protein surfaces (1, 2,
10).
Researchers at the New York Structural Biology Centre recently reported on high-resolution solution nuclear magnetic resonance
techniques with dynamic lightscattering to characterise a family of hydrocarbon-stapled peptides with known inhibitory activity
against HIV-1 capsid assembly to evaluate the various factors that modulate activity. The researchers reported that helical
peptides share a common binding motif, but differ in charge, the length and position of the staple. The research showed that
the peptides share a propensity to self associate into organised polymeric structures mediated predominantly by hydrophobic
interactions between the olefinic chain and the aromatic side-chains from the peptide. The researchers also detailed the structural
significance of the length and position of the staple and of olefinic bond isomerisation in stabilising the helical conformation
of the peptides as potential factors influencing polymerisation (11).
Cyclic peptides. In April 2012, researchers at Carnegie Mellon University (Pennsylvania, USA) reported on their manufacturing method for
producing a synthetic form of a cyclic peptide. Macrocyclic peptides with multiple disulphide crosslinkages, such as those
produced by plants and those found in nonhuman primates, hold potential as drugs because of their broad biological activities
and high chemical, thermal and enzymatic stability (12). Because of their intricate spatial arrangement and elaborate interstrand
crosslinkages, some macrocyclic peptides are difficult to prepare in large quantities and high purity because of the nonselective
nature of disulphide-bond formation (12).
In the current study, the Carnegie Mellon researchers focused on RTD-1, a cyclic peptide held together by three disulphide
bonds. RTD-1 has a broad range of antibacterial, antifungal and antiviral capabilities, and has been shown to inhibit HIV
from entering cells. The researchers created a mimic of RTD-1. While the outside of the mimic peptide maintained the same
amino acids as the original, the researchers replaced the disulphide bonds at its centre with noncovalent Watson–Crick hydrogen
bonds without significantly affecting the biological activity of the peptide. The researchers say the work provides a general
strategy for engineering conformationally rigid, cyclic peptides without the need for disulphide-bond reinforcement (12).
They tested the efficacy of the mimic RTD-1 by mixing the peptide with Escheria coli, Listeria,
Staphylococcus and Salmonella— bacteria that RTD-1 typically protects against. The mimic proved to be effective in killing each of the types of bacteria
tested by the researchers, which included both gram-positive and gram-negative bacterial strains. Furthermore, the mimic peptide
worked by binding to the bacteria's cell membrane, not its DNA or RNA, decreasing the probability that the bacteria could
develop resistance to the peptide. The researchers plan to see if the mimic RTD-1 is effective against other types of pathogens,
including antibiotic-resistant bacteria. They also plan to apply their method to manufacture mimics of other cyclic peptides.
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