Biomedical Engineering Reference
In-Depth Information
terminus of a peptide can protect it from exopeptidase cleav-
age. Another commonly used approach is to identify the
protease-sensitive sites by alanine or/and by
D
-amino acid
scanning combined with a variety of analytical tools and then
replace the natural amino acids on the sites by nonproteogenic
constrained amino acids or unnatural amino acid such as
N-methyl-
a
-amino acids, or other nonnative amino acids.
Owing to the stereochemical specificity of peptidases, chang-
ing some or even all of
L
-amino acids of a peptide with their
corresponding
D
-amino acids can improve resistance to
proteolytic degradation. In addition,
D
-amino acid peptides
have been found to be significantly less immunogenic than
their corresponding
L
-amino acid peptides [7,8].
The chemical modifications mentioned earlier are limited
to either the terminus of a peptide or the side chain of
residues in the peptide without changing the peptide back-
bone. In many cases, however, modifications of peptide
bonds are necessary. These modifications include replace-
ment of amide bond (amide bond surrogates), attachment of
side chains to the amide nitrogen atoms instead of
a
-carbons
(peptoids), and replacement of
a
-carbons with nitrogen
atoms (azapeptides). Peptides with backbone modifications
such as these are referred to as pseudopeptides [7].
in future drug development. The
a
-helix features 3.6 resi-
dues per complete turn, which places the side chains of
residues i, i
11 on the same face of the
folded structure. The peptide stapling strategy employs
covalent bonds between the i and i
þ
4, i
þ
7, and i
þ
7 side
chain groups to stabilize the helical conformation. Lactam,
disulfide and metal-mediated bridges, and more recently the
all-hydrocarbon bridge using a ring-closing metathesis
reaction have been used to staple helical peptides. Stapling
increases the helical propensity, in particular, stapling with
an all-hydrocarbon cross-linker resulted in significant
improvements in structure, potency, and protease resistance
[11-13].
þ
4ori and i
þ
8.2.4 Peptide Therapeutics Play an Important Role
in Drug Development
Besides the advances in techniques, as discussed earlier,
which improve in vivo stability in peptides, several factors,
including the significant progress of industrial-scale syn-
thetic production, formulation and delivery of peptides,
widespread acceptance of protein therapeutics by physicians
and patients, dramatic increase in the availability of genomic
information, and the development of more sensitive analyti-
cal techniques, have all contributed to peptides, especially
chemically modified peptides, becoming a more important
therapeutic drug resource.
Peptide drugs have drawn increasing interest from the
pharmaceutical industry and enjoyed steady growth since
the 1980s. The average number of peptide drug candidates
entering into the clinical trials increased from 1.2 per year in
the 1970s to 4.6 per year in the 1980s, 9.7 per year in the
1990s, and 16.9 per year in the first decade of this century
[14]. Peptide drugs have been targeted to a wide variety of
indications, and the number of therapeutic categories has
also expanded significantly during this period. In a number
of categories, at least six different compounds progressed
into clinical trials; these categories include metabolic dis-
orders, oncology, allergy and immunology, cardiovascular
diseases, CNS, infections, pain, and endocrinology [14]. In
2008, global sales of six peptide therapeutic drugs, including
Copaxone (random polymer of four amino acids), Lupron
1
,
Zoladex
1
, Sandostatin
1
, Forteo
1
(recombinant para-
thyroid hormone, PTH), and Byetta
1
(synthetic version
of exendin-4 found in the saliva of Gila monster), reached
more than US$750 million [14].
8.2.3 Constraining Peptides to be More Resistant
to Protease Cleavage
Without internal covalent linkages such as disulfide bonding
or binding to a larger protein, for example a receptor,
peptides alone are flexible and rarely retain a stable tight
secondary structure conformation in solution. They take on
an ensemble of extended conformations, which are easily
accessible and digested by proteases as these enzymes
typically bind their substrates in the extended conformation
[9]. However, a group of constrained cyclic peptides have
been discovered in nature, whose amino and carboxyl
termini are linked together with a peptide bond to form a
circular peptide. Some naturally existing cyclic peptides
such as the amatoxins, amanitin, and phalloidin contain
an additional bridge between the side chains of two different
residues within the peptide to form a bicyclic structure. The
covalent linkages between N- and C-termini or between two
side chains of a cyclic or a bicyclic peptide can comprise
either normal peptide bonds such as in Cyclosporin, an
immunosuppressant drug, or nonpeptide bonds such as in
somatostatin, a peptide hormone that is cyclized through a
disulfide bond between two cysteines. When constrained by
cyclization, naturally existing peptides tend to be extremely
resistant to the process of digestion, in some cases enabling
them to survive intact in the human digestive tract. The
strategies of cyclization of biologically active peptides to
stabilize peptides in vivo and extend their serum half-life
have often been used in drug development [10].
The peptide stapling strategy for stabilizing peptide
a
-helices is another approach that has demonstrated promise
8.2.5 Most Peptide Drugs Act as Receptor Agonists
Receptors and their ligands serve as targets for some of the
most successful biological therapeutics on the market,
including mAb drugs such as Remicade, Rituxan, and
Herceptin, receptor-Fc-fusion protein drug such as Enbrel,
and recombinant protein drugs such as Aranesp
1
and Pro-
crit
1
. However, unlike mAb drugs, whose mode of action
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