Peptide-Based Drug Design

178 Jenssen and Aspmo
Some other peptides, e.g., dipeptidyl peptidase IV (8), endopeptidase 3.4.24.16
inhibitor (9), and a copolymer of tyrosine, glutamic acid, alanine, and lysine
(10), have also demonstrated immunomodulatory activity. Thus analogues of
these may some day play an important role in treatment of autoimmune and
inflammatory diseases. It should also be noted that a vast number of naturally
derived peptides also has demonstrated antimicrobial activity toward a wide
range of pathogens (11), resulting in possible templates for further optimization
studies clearly demonstrating the potential importance of peptides as pharmacological
tools.
However, the majority of positive leads identified so far have been isolated
from the natural reservoir of peptides. A possible explanation for this may be that
these peptides have undergone natural selection over billions of years, resulting
in enhanced in vivo stability. Another explanation may be that the majority
of random peptide sequences from chemical and recombinant libraries do not
fold into stable structures (12); however, this may not nessesarily render these
peptides more susceptible to proteolytic degradation.
Peptides have several advantages over small molecular drugs, e.g., they have
higher affinity and specificity to interact with its target, while their toxicity
profile remains low. However, several issues have prevented small proteins and
peptides from becoming a mainstream template in drug design. This is primarily
due to rapid renal clearance and low in vivo stability as a result of protease degradation,
giving them a rather short half-life. Bioavailability and limited access
to intracellular space has also been debated. However, peptide-based antimicrobials
are with no doubt suitable for topical applications (13), offering decreased
potential of resistance induction (14) compared to other antimicrobials.
Synthetic peptides often lack the conformational stability required for a
successful drug; therefore determination of peptide stability in serum constitutes
a powerful and important screening assay for the elimination of unstable
peptides in the pipeline of drug development (see Note 1). Peptide stability in
serum can rather easily be determined by reverse phase–high-performance liquid
chromatography (RP-HPLC) and mass spectroscopy (MS) from both in vitro
and in vivo studies.
2. Materials
2.1. Solid Phase Peptide Synthesis and Purification
1. PAL-PEG resin, Fmoc amino acids and coupling reagent N,N´diisopropylethylamine
(DIPEA) and O-benzotriazole-N,N,N´,N´-tetramethyluronium-hexafluoro-phosphate
KY).
(HBTU) (Advanced ChemTech, Louisville,