Design of Cyclic Peptides - Peptide and Protein Design for Biopharmaceutical Applications - page 161

4.5.2 CXCR4 Antagonists

Chemokine receptors belong to the family of GPCRs and allow cells,
viruses and bacteria to recognize certain chemoattractants or -repellents
and induce migration to or from their source. As the name suggests, their
corresponding ligands belong to the subfamily of cytokines ( chemo tactic
cyto kines ). In the human body this signalling system is initially involved
in stem-cell migration during embryogenesis and later used by the
immune system to guide cells [150,151].
However, this normally vital and benign system has been hijacked by
various diseases like HIV, rheumatoid arthritis (RA), and cancer metasta-
sis. As peptidic antagonists of CXCR4 have already shown anti-HIV
[152], anti-RA [153,154] and anti-metastatic [155,156] activity, they are
a valuable target for medicinal chemistry. In contrast to the RGD motif,
size reduction did not originate from the natural ligand CXCL12 itself but
from biologically active peptides found by screening of natural extracts.
In the late 1980s, four peptides with 17-18 amino acids and con-
strained by two cysteine bridges were found in horseshoe crabs
[157,158]. One of these peptides was optimized after additional biologi-
cal studies showed antiviral activity [159, 160]. Due to these effects the
structure was investigated in this early stage [161]. By converting two
existing phenylalanines into tyrosines and introducing an additional
positive charge by substitution of a valine with lysine, the affinity was
improved by three orders of magnitude (Figure 4.16) [162].
Subsequent modifications included the shortening of the peptide to a
monocyclic strand with 14 amino acids by replacing the central cysteine
bridge and the attached loop region with a bII 0 -inducing dipeptide unit
consisting of D -Lys and L -Pro [163]. After isosteric replacement of Trp 3
with L -3-(2-naphthyl)alanine (Nal) [164], substitution of Lys 12 with citrul-
line (Cit) [165] and removal of the C-terminal amide, a better affinity and
a reduced cytotoxicity to the precursor peptides were observed.
A consecutive Ala-scan [166] showed that four potentially important
amino acids were in close proximity to one another because of the
remaining cysteine bridge, and partly out of the macrocycle, and there-
fore more flexible. These were, together with an additional glycine,
subjected to a scan of cyclic pentapeptides with two orthogonal
libraries consisting of a sequence- and a conformation-based approach
following the spatial screening method, resulting in a small lead com-
pound with equal affinity in the low nanomolar range and high
biostability [167].
Further modifications have been introduced, including retro enantio-
mer libraries, amino acid substitutions and attempts to create