X.-J. Chu et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4910–4914
4913
20- and 30-positions of cis-Apc to fully establish the
structure–activity relationship in this region.
rigid templates in achieving good hMC4R agonist
potency and selectivity against hMC1R in linear penta-
peptides.18 Further modification of Apc amino acid,
incorporation of Apc and related analogs into cyclic
peptides, and in vivo studies using Apc-containing line-
ar/cyclic peptides would be reported in due course.
In an attempt to reduce the molecular weight of peptide
21, truncated peptides such as 31 and 32 were prepared.
Tetrapeptide 31, with the Gly residue removed, is signif-
icantly less potent and selective as a hMC4R agonist,
compared to pentapeptide 21. Peptide 32, containing
trans-4-phenylcyclohexane-1-carboxylic acid (trans-Pcc)
(Fig. 2), which could be considered as Apc with its
Acknowledgments
N-cap truncated, is only moderately potent (EC50
=
The authors are grateful to Dr. Christopher S. Framp-
ton (Roche Welwyn, UK) and Dr. Bradford Graves
for X-ray crystallographic study of compound 12 and
to the Physical Chemistry Department for spectroscopic
measurements and interpretations. The authors would
also like to thank Drs. Ramakanth Sarabu and Jefferson
Tilley for their critical reading of the manuscript.
550 nM) as a hMC4R agonist. The reduced hMC4R
agonist activities of peptides 31 and 32 clearly demon-
strate the important roles played by the Gly residue
and N-cap of peptide 21.
The most hMC4R selective peptides described above
were tested in hMC3R and hMC5R agonist assays
and the results are shown in Table 2. The lead pentapep-
tide 1 (Bu-His-DPhe-Arg-Trp-Gly-NH2) was inactive in
both hMC3R and hMC5R agonist assays. The low ago-
nist activities in hMC3R and hMC5R are maintained
when His of peptide 1 was replaced by cis-Apc (peptide
21) and cis-40-EtOApc (peptide 26). The above peptides
were also, in certain cases, tested in hMC1R–hMC5R
binding assays (Table 3). The binding affinities of the
peptides 1, 21, and 26 toward hMC4R, track with their
agonist activities in the order of 21 > 26 > 1. Peptides 21
and 26 bind to hMC1R with micromolar affinity, but
showed no significant agonist activity.
References and notes
1. The Melanocortin Receptors; Cone, R. D., Ed.; Humana:
Totowa, 2000.
2. Fan, W.; Boston, B. A.; Kesterson, R. A.; Hruby, V. J.;
Cone, R. D. Nature 1997, 385, 165.
3. Huszar, D.; Lynch, C. A.; Fairchild-Huntress, V.; Dun-
more, J. H.; Fang, Q.; Berkemeier, L. R.; Gu, W.;
Kesterson, R. A.; Boston, B. A.; Cone, R. D.; Smith, F. J.;
Campfield, L. A.; Burn, P.; Lee, F. Cell 1997, 8, 131.
4. (a) Wessells, H.; Gralnek, D.; Dorr, R.; Hruby, V. J.;
Hadley, M. E.; Levine, N. Urology 2000, 56, 641; (b)
Wessells, H.; Fuciarelli, K.; Hansen, J.; Hadley, M. E.;
Hruby, V. J.; Dorr, R.; Levine, N. J. Urol. 1998, 160, 389;
(c) Martin, W. J.; MacIntyre, D. E. Eur. Urol. 2004, 45,
706.
5. Boyce, R. S.; Duhl, D. M. Curr. Opin. Invest. Drugs 2004,
5, 1063.
6. Cheung, A. W.-H.; Danho, W.; Swistok, J.; Qi, L.;
Kurylko, G.; Rowan, K.; Yeon, M.; Franco, L.; Chu,
X.-J.; Chen, L.; Yagaloff, K. Bioorg. Med. Chem. Lett.
2003, 13, 133.
Hruby et al. proposed that Ôthe position 6 of the synthet-
ic melanocortin ligands is important for enhancing
potency and selectivity at MC3 and MC4 melanocortin
receptorsÕ.15,16 HrubyÕs study was carried out using a
cyclic peptide antagonist as the template (SHU9119)
and the resulting compounds are all hMC4R antago-
nists with no hMC1R biochemical data.15,16 In a study
by Haskell-Luevano et al., 17 histidine surrogates were
introduced into a Ac-His6-DPhe7-Arg8-Trp9-NH2 tetra-
peptide template17 and the tetrapeptide containing ami-
no-2-naphthylcarboxylic acid (Anc) showed good mouse
MC4R agonist potency (EC50 = 21 nM) and good selec-
tivity against mouse MC1R (EC50 = 7900 nM). The
biological activities of these tetrapeptides at human mel-
anocortin receptors were not reported.17 Our work de-
scribed herein showed that linear pentapeptides
7. Phenyl-containing rigid templates investigated include:
O
O
O
N
N
NH
N
N
(Penta-cis-Apc-DPhe-Arg-Trp-Gly-NH2)
containing
N
cis-Apc and substituted Apc are potent hMC4R ago-
nists and they are inactive or weakly active in hMC1R,
hMC3R, and hMC5R agonist assays. This report on
Apc and substituted Apc, together with our earlier re-
port on 5-BrAtc and related Atc,6 demonstrated the
importance of replacing His6 with phenyl-containing
H
O
O
O
rac-
rac-
None of the pentapeptides derived from these amino acids
is as potent or hMC4R selective as peptide 3 (data not
shown).
8. Obrecht, D.; Spiegler, C.; Schonholzer, P.; Muller, K.;
Heimgartner, H.; Stierli, F. Helv. Chim. Acta 1992, 75,
1666.
9. Wysong, C. L.; Yokum, T. S.; Morales, G. A.; Gundry, R.
L.; McLaughlin, M. L.; Hammer, R. P. J. Org. Chem.
1996, 61, 7650.
10. Crystallographic data for compound 12 have been depos-
ited with the Cambridge Crystallographic Data Center as
supplementary publication number CCDC 283241.
11. (a) Chen, L.; Cheung, A. W-H.; Chu, X-J.; Danho, W.;
Swistok, J.; Yagaloff, K. A. WO 0174844, 2001; CAN
CO2H
trans-Pcc
Figure 2. Structure of trans-4-phenylcyclohexane-1-carboxylic acid
(trans-Pcc).