Formylated polyamines as peptidomimetics

Article abstract of DOI:10.1016/j.bmcl.2012.09.008

A new construct for imitating a natural peptide ligand using a modified retro-inverso sequence is described. It is demonstrated through the synthesis of a peptidomimetic derived from the endogenous sequence of leucine enkephalin. The product was active at 400 nM and selective for μ-opioid receptors.

Full text of DOI:10.1016/j.bmcl.2012.09.008

Bioorganic & Medicinal Chemistry Letters 22 (2012) 6580–6582
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Formylated polyamines as peptidomimetics
Sacha Javor a, Aaron Janowsky b, Robert Johnson b, Katherine Wolfrum b, Mitra Tadayoni-Rebek a,
,
⇑
Julius Rebek Jr. a,b,
⇑
a The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
b Research Service, VA Medical Center, The Methamphetamine Abuse Research Center, and the Departments of Psychiatry and Behavioral Neuroscience,
Oregon Health and Science University, 3710 S.W. U.S. Veterans Hospital Rd., Portland, OR 97239, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 June 2012
Revised 29 August 2012
Accepted 4 September 2012
Available online 13 September 2012
A new construct for imitating a natural peptide ligand using a modified retro-inverso sequence is
described. It is demonstrated through the synthesis of a peptidomimetic derived from the endogenous
sequence of leucine enkephalin. The product was active at 400 nM and selective for l-opioid receptors.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Peptides
Peptide mimetic
Retro-inverso peptide
Polyamine
Formylated amines
Peptides mediate a wide variety of biological processes and rep-
resent a major source of inspiration in medicinal chemistry.1 The
specific sequences involved in many relevant biochemical path-
ways are known2 and enormous libraries are readily available
through combinatorial synthesis. However, their limited stability
in vivo and unfavorable pharmacokinetics resulting from their high
polarity, preclude peptides as medicinal candidates per se.3 Rather,
small hydrophobic ligands, often with seemingly unrelated
chemical structures, are generally preferred medicines.4 The ret-
ketomethylene peptide isosteres and have been used in numerous
peptidomimetic applications.6 The replacement of secondary
peptide bonds by the more lipophilic tertiary amides decreases
the polarity7 and improves the pharmacokinetic properties. The
reduced number of HBD’s8 compared with the parent peptide is
expected to improve membrane permeability.9
To illustrate, we perpared the FPA (Scheme 1) of the sequence
for leucine enkephalin (LE).10 LE is a potent endogenous opioid
receptor agonist11,12 involved in the modulation of the perception
of pain and of considerable medicinal interest. The poor cell pene-
tration and high susceptibility to enzyme degradation render exog-
enous LE essentially devoid of practical biological activity in vivo.13
ro-inverso (RI) strategy5—mimicking the natural
the -peptide of the reverse sequence—leads to a structurally re-
L-peptide by using
D
lated peptidomimetic with increased metabolic stability and the
correct presentation of side chains (Fig 1). Even so, these mimetics
do not show improved cell membrane penetration. They also fail to
present the correct hydrogen bond donor (HBD) and acceptor
(HBA) pattern of the peptide backbone, which can be involved in
recognition by the target receptor or enzyme (Fig. 1). We report
here modified structures that overcome these limitations.
The new peptidomimetics are formylated polyamines (FPA).
FPAs are structurally related to RI peptides but feature backbones
that resemble those of the original peptide (Fig. 1). They present
the correct HBA–HBD polyamide pattern by incorporating forma-
mides as homologous carbonyl groups and methylenes to replace
the N–H groups. In peptides, such N?C substitutions lead to
⇑
Corresponding author. Tel.: +1 858 784 2250; fax: +1 858 784 2876.
E-mail addresses: mrebek@scripps.edu (M. Tadayoni-Rebek), jrebek@scripps.edu
Figure 1. Structural features of peptides, retro-inverso mimics and the new
formylated polyamines.
(J. Rebek).
0960-894X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2012.09.008

S. Javor et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6580–6582
6581
Scheme 1. Synthesis of the FPA peptidomimetic 5 from the resin-bound RI peptide 1 and the structure of leucine enkephalin peptide.
The
D
-peptide precursor for the synthesis was the RI peptide 1,
enkephalin mimic. While the amino acid side chains involved in
obtained by classical solid-phase peptide synthesis on Rink-amide
resin followed by N-terminus modification using methanesulfonyl
chloride (Scheme 1). The terminal sulfonamide represents the C-
terminal carboxylic acid of LE as a bioisostere.14 The wholesale
reduction of the backbone amides on-bead15 produced the poly-
amine 2. A similar approach was successfully applied to the con-
struction and decoding of a small on-bead library.16 Structurally
related polyamines showed in vitro activity for opioid receptors17
and more recently, they were also considered for DNA and RNA
gene delivery systems.18 Compound 2 was cleaved from the resin
with acidic treatment in 42% overall yield.19 A terminal amine is
an essential pharmacophore in LE.20 Accordingly, the primary
amine was selectively protected using 2-acetyldimedone.21 Next,
formylation of the remaining secondary amines of 4 was accom-
plished with distilled acetic formic anhydride.22 The FPA 5 was ob-
tained after primary amine deprotection using hydrazine and
purified by RP-HPLC.
this case were tolerant of amide reduction, the ready availability
of amino alcohols as starting materials opens possibilities for syn-
thesis of formylated polyamines corresponding to most naturally-
occurring sequences.
Acknowledgments
We are grateful to the Skaggs Institute, The National Institute
on Drug Abuse/VA Interagency Agreement (A.J.), and the
Department of Veterans Affairs Merit Review and Research Career
Scientist Programs (A.J.) for financial support. We thank Dr. Goran
´
Pljevaljcic and Dr. Daniel Ryan for valuable advice concerning
biological assays. S.J. is a Skaggs Postdoctroal Fellow and was
supported by the Swiss National Science Foundation (SNF).
Supplementary data
Compound 5 was found reasonably stable to degradation in hu-
man serum, showing a half-life of 41 h. Compound 5 also inhibited
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2012.09.008.
radioligand ([3H]DAMGO) binding to recombinant human
l-opi-
oid receptors with a Ki value of about 400 nM (Fig. 2).23 However,
the Ki values for displacement of radioligands from the recombi-
References and notes
nant human
were greater than 5
35S]GTP
S binding, indicating that it is not an agonist at l
j
([3H]DPDPE) and d ([3H]U69,593) opioid receptors
M. In addition, Compound 5 did not alter
-opioid
1. Otvos, L., Jr. In Methods in Molecular Biology, Vol. 494: Peptide-Based Drug Design;
Humana Press: New York, NY, 2008.
2. (a) Duchrow, T.; Shtatland, T.; Guettler, D.; Pivovarov, M.; Kramer, S.;
Weissleder, R. BMC Bioinformatics 2009, 10, 317; (b) Consortium, The. Uni.
Prot. Nucleic Acids Res. 2011, 39, D214.
3. Pauletti, G. M.; Gangwar, S.; Siahaan, T. J.; Aube, J.; Borchardt, R. T. Adv. Drug
Delivery Rev. 1997, 27, 235.
4. (a) Pert, C. B.; Snyder, S. H. Science 1973, 179, 1011; (b) Wexler, R. R.; Greenlee,
W. J.; Irvin, J. D.; Goldberg, M. R.; Prendergast, K.; Smith, R. D.; Timmermans, P.
B. M. W. M. J. Med. Chem. 1996, 39, 625.
5. (a) Goodman, M.; Chorev, M. Acc. Chem. Res. 1979, 12, 1; (b) Chorev, M.;
Goodman, M. Acc. Chem. Res. 1993, 26, 266.
6. (a) Hoffman, R. V.; Tao, J. In Methods in Molecular Medicine, Vol. 23:
Peptidomimetics Protocols; Humana Press Inc.: Totowa, NJ, 1998. pp 103–124;
(b) Rich, D. H.; Bernatowicz, M. S.; Schmidt, P. G. J. Am. Chem. Soc. 1982, 104,
3535.
7. Ghose, A. K.; Viswanadhan, V. N.; Wendoloski, J. J. J. Comb. Chem. 1999, 1, 55.
8. Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Adv. Drug Delivery Rev.
2001, 46, 3.
l
[
c
receptors. Recently, an analogous acylation of secondary amines
was used as a convenient means of introducing additional side
chains into the structure of a peptidomimetic that was found to in-
hibit protein-protein interactions.24 However, in the case of FPAs,
the formamides are only responsible for restoring the hydrogen
bond features and improve the cell permeation. The side chains
come from the RI peptide allowing for a more straightforward
structure-derived approach.
In conclusion, the RI concept was modified to improve bioavail-
ability and enhance molecular recognition features25 of a leucine
9. As guidelines for drug design, for good permeability and absorption, an
octanol–water partition constant (log P) in the À0.4–5.6 range7 and a number
of HBD 6 5 have been proposed.8
120
100
80
60
40
20
0
Naltrexone human δ receptor
Compound 5 human δ receptor
Naltrexone human κ receptor
Compound 5 human κ receptor
Naltrexone human μ receptor
Compound 5 human μ receptor
10. Hughes, J.; Smith, T. W.; Kosterlitz, H. W.; Fothergill, L. A.; Morgan, B. A.;
Morris, H. R. Nature 1975, 258, 577.
´
´
11. Jankovic, B. D.; Maric, D. Ann. N. Y. Acad. Sci. 1988, 540, 691.
12. Holzer, P. Regul. Pept. 2009, 155, 11.
13. (a) Mizuma, T.; Ohta, K.; Koyanagi, A.; Awazu, S. I. J. Pharm. Sci. 1996, 85, 854;
(b) Gwak, H. S.; Cho, M. Y.; Chun, I. K. J. Pharm. Pharmacol. 2003, 55, 1207.
14. Meanwell, N. A. J. Med. Chem. 2011, 54, 2529.
15. (a) Manku, S.; Laplante, C.; Kopac, D.; Chan, T.; Hall, D. G. J. Org. Chem. 2001, 66,
874; (b) Nefzi, A.; Ostresh, J. M.; Houghton, R. A. Tetrahedron 1999, 55, 335.
16. Kappel, J. C.; Fan, Y. C.; Lam, K. S. J. Comb. Chem. 2008, 10, 333.
17. Dooley, C. T.; Houghten, R. A. Biopolymers 1999, 51, 379.
-11 -10 -9
-8
-7
-6
-5
Figure 2. Biological activities with opioid receptors. The average IC50 measured for
5 with the human -opioid receptor was 3.2
M corresponding to a Ki of 400 nM.23
l
l

6582
S. Javor et al. / Bioorg. Med. Chem. Lett. 22 (2012) 6580–6582
18. (a) Levine, M.; Kenesky, C. S.; Zheng, S.; Quinn, J.; Breslow, R. Tetrahedron Lett.
2008, 49, 5746; (b) Zhou, W.; Yerkes, N.; Chruma, J. J.; Liu, L.; Breslow, R. Bioorg.
Med. Chem. Lett. 2005, 15, 1351.
19. The yield was based on the specified loading of the resin and calculated
assuming 3 was a pentakis trifluoroacetic acid salt.
20. Lenz, G. R.; Evans, S. M.; Walters, D. E.; Hopfinger, A. J.; Hammond, D. L. Opiates;
Academic Press: Orlando, FL, 1986. 459.
21. Bycroft, B. W.; Chan, W. C.; Chhabra, S. R.; Hone, N. D. J. Chem. Soc., Chem.
Commun. 1993, 778.
22. Krimen, L. I. Org. Synth. 1970, 50, 1.
23. The Ki values were derived from the IC50 values measured in the ligand
displacement assay using the Cheng–Prusoff equation. See the Supplementary
data for details.
24. Iera, J. A.; Miller Jenkins, L. M.; Kajiyama, H.; Kopp, J. B.; Appella, D. H. Bioorg.
Med. Chem. Lett. 2010, 20, 6500.
25. For an alternative platform, see: Bartfai, T.; Lu, X.; Badie-Mahdai, H.; Barr, A.;
Mazarati, A.; Hua, X.; Yaksh, T.; Haberhauer, G.; Ceide, S.; Kröck, L.; Trembleau,
L.; Somogyi, L.; Rebek, J., Jr. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 10470.