Synthesis and changes in affinity for NOP and opioid receptors of novel hexapeptides containing β2-tryptophan analogues

Article abstract of DOI:10.1016/j.bmcl.2013.05.064

We report the synthesis and the biological activity of new analogues of Ac-RFMWMK-NH₂ and Ac-RYYRWK-NH₂, modified in position 4 and 5, respectively, with incorporation of newly synthesized β²-tryptophan analogues. Trp was

Full text of DOI:10.1016/j.bmcl.2013.05.064

Accepted Manuscript
Synthesis and changes in affinity for NOP- and opioid receptors of novel hex‐
apeptides containing β 2-tryptophan analogues
Rositsa Zamfirova, Nikola Pavlov, Petar Todorov, Polina Mateeva, Jean
Martinez, Monique Calmès, Emilia Naydenova
PII:
S0960-894X(13)00659-8
DOI:
http://dx.doi.org/10.1016/j.bmcl.2013.05.064
BMCL 20525
Reference:
To appear in:
Bioorganic & Medicinal Chemistry Letters
Received Date:
Revised Date:
Accepted Date:
12 April 2013
15 May 2013
18 May 2013
Please cite this article as: Zamfirova, R., Pavlov, N., Todorov, P., Mateeva, P., Martinez, J., Calmès, M., Naydenova,
E., Synthesis and changes in affinity for NOP- and opioid receptors of novel hexapeptides containing β 2-tryptophan
analogues, Bioorganic & Medicinal Chemistry Letters (2013), doi: http://dx.doi.org/10.1016/j.bmcl.2013.05.064
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Synthesis and changes in affinity for NOP- and opioid receptors of novel
hexapeptides containing ꢀ2-tryptophan analogues
a
b
b
a
c
Rositsa Zamfirova , Nikola Pavlov , Petar Todorov , Polina Mateeva , Jean Martinez ,
Monique Calmès c, Emilia Naydenova b,*
a Institute of Neurobiology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
b
Department of Organic Chemistry, University of Chemical Technologies and Metallurgy,
Sofia, Bulgaria, e-mail: e_naydenova@abv.bg, tel. +35928163425
c Institut des Biomolécules Max Mousseron (IBMM) UMR 5247 CNRS-Université Montpellier
1 et Université Montpellier 2, France
ABSTRACT
We report the synthesis and the biological activity of new analogues of Ac-RFMWMK-NH2
and Ac-RYYRWK-NH2, modified in position 4 and 5, respectively, with incorporation of
newly synthesized ꢀ2-tryptophan analogues. Trp was substituted by the (S)-2-(1-methyl-1H-
indol-3-yl)propionic residue or by (S)-2-(5-methoxy-1H-indol-3-yl)propionic residue. The
biological activity (pEC50 and Emax) of these compounds was tested on electrically stimulated
preparations of rat vas deferens. The 5-methoxy ꢀ-tryptophan group reverses the affinity of
the compounds.
Keywords: Nociceptin analogue, Nociceptin/orphanin FQ - NOP receptor, ꢀ-Tryptophan
analogues, Rat vas deferens
The numerous endogenous opioid peptides (nociceptin, enkephalins, endomorphines
etc.) exert their effects by activating ꢀ, ꢁ, ꢂ and ꢃ.receptors. The opioids have numerous
pharmacological effects. Both receptor-specific opioid agonists and antagonists are useful
pharmacological tools and have potential as therapeutic agents.
Nociceptin (N/OFQ) is an endogenous hexadecapeptide1,2 that belongs to the family of
opioids, but it does not interact with the classical ꢁ, ꢂ and ꢃ receptors.3,4 There are close
structural similarities between N/OFQ and opioid peptides (especially with dynorphyn A5,6),
as well as between nociceptin receptor (NOP) and opioid receptors.7-9 Moreover, activating
1

NOP receptor (coupled to the same G-protein-mediated second messenger system, as the
opioids), decreases cAMP production,1,2 inhibits voltage-sensitive calcium channels,10 and
activates the inward potassium conductance,11 intracellular processes that result in inhibition
of cellular excitability. Despite these similarities, there are a lot of pharmacological and
functional differences between N/OFQ/NOP and the classical opioid systems.12,13 The N/OFQ
and its NOP receptor modulate a variety of biological functions, both at central and peripheral
level. The understanding of its role depends upon the development of selective and highly
potent peptide and non-peptide agonists and antagonists. The ligands for NOP receptors could
be summarized in the following groups: a) non-peptide ligands like J-11339714, SB-612111,
and Ro 64-619815; b) nociceptin-related peptides identified by structure-activity studies16 and
N/OFQ(1-13)NH217; c) hexapeptides. The latter are small synthetic peptides with general
formula Ac-RYY-R/K-W/I-R/K-NH2, isolated and characterized by Dooley et al. in 1997 by
combinatorial chemistry.18 They are selective agonists and antagonists with a very high
affinity to NOP receptors. In other study, the same group19 identified other hexapeptides,
strong inhibitors of ꢁ- and ꢃ-opioid receptors, named acetalins.
Based on these findings, we report in the present study the synthesis and the biological
screening of new analogues of Ac-RFMWMK-NH2 and Ac-RYYRWK-NH2, modified in
position 4 and 5, respectively, with incorporation of newly synthesized ꢀ2-tryptophan
analogues:
Ac-Arg-Tyr-Tyr-Arg-X-Lys-NH2
Ac-Arg-Phe-Met-X-Met-Lys-NH2
O
C
O
C
NH CH2 CH
NH CH2 CH
H3CO
Where X is:
N
CH3
HP3 - HP4
N
H
HP5 - HP6
These novel hexapeptides have been synthesized by including ꢀ2-tryptophan
analogues20 in position 4 and 5, respectively, using SPPS by Fmoc (9-fluorenylmethoxy-
carbonyl) chemistry (see Supplementary data). The analytical data of the synthetic peptides
are shown in Table 1. In order to estimate the relationship, we prepared the compounds HP3-
HP6 by replacement of the natural tryptophan with an enantiopure ꢀ2-tryptophan analogue. In
2

general, ꢀ-amino acids are more resistant than ꢄ-amino acids to enzymatic degradation21 and
they have often-relevant roles in medicinal chemistry.22-24
Table 1. Analytical data of synthetic peptides.
c ESI MS: (MH)+
b tR, min
a
[α]D (o)
ꢁ
Peptides
calculated
found
Ac-RYYRWK-NH2
Ac-RFMWMK-NH2
Ac RYYR NH CH2 CH CO
-23
-18
-21
4.90
5.20
7.15
1011.5 1012.2
938.5 939.6
1025.6 1025.9
HP1
HP2
HP3
K NH2
N
H3C
MK NH2
Ac RFM NH CH2 CH CO
HP4
-19
9.11
952.5
953.0
N
H3C
CH CO K NH2
OCH3
-25
-28
11.45 1042.1 1042.6
HP5
HP6
Ac RYYR NH CH2
HN
CH CO MK NH2
Ac RFM NH CH2
10.80
968.9
969.1
OCH3
HN
a
o
b
c
Optical rotation in methanol (c = 1) at 20 C; tR is the retention time determined by HPLC; The
mass ion (MH+) was obtained by electrospray mass spectrometry.
HP1 (Ac-RYYRWK-NH2) is a well-known hexapeptide, isolated and characterized by
Dooley et al.18 It was found to be a short-chain peptide ligand with a very high affinity for
NOP receptors. In our experiments, it was synthesized and used like a reference compound
and a template for hexapeptides HP3 and HP5. Tested on rat vas deferens (rvd), HP1 exerted
effects similar to those reported previously by us25 and: by Ho et al.26 on musculus
anococcygeus. The onset of peptide-evoked inhibition of ES-induced smooth-muscle
contractions occurs in a concentration of 1x10-11M, which is lower than that of N/OFQ(1-13)-
NH2. Compared to the latter, the maximal effect of HP1 was reached at the same
concentration (1x10-6M), but it evoked only about 40% inhibition of the contractions (Fig.
1A, Table 2). The effect was not influenced by naloxone (Nal, a blocker of opioid receptors) and
3

was completely prevented by naloxone benzoylhydrazone (Nal-B, a blocker of opioid and NOP
receptors), confirming that HP1 interacts with NOP receptors only.
Table 2. Effects of the newly synthesized peptides on LFES-Evoked Contractions of Rat Vas
Deferens.
Peptide alone
Peptide + Nal
Peptide
pEC50
Emax (%)
pEC50
Emax (%)
-31
-14 17
7.80±0.38
5.99±0.24
6.50±0.24
5.91±0.18
5.85±0.50
5.78±0.58
-36 6
-52 9
-36 7
-50 10
-52 11
-34 5
8.21±0.44
5.94±0.17
6.21±0.24
5.79±0.10
5.78±0.22
6.26±0.40
6
HP 1
HP 2
HP 3
HP 4
HP 5
HP 6
-17
-15
9
5
-12 12
-14
9
The maximal inhibitory effect of HP1 is reached at a concentration of 1x10-6M. Compared to HP1, the
other tested peptides have shown lower activity. With them, the initial effect, as well as the maximal one,
developed at higher concentrations. Therefore, the presented pEC50 and Emax (compounds HP2 – HP6)
are calculated for concentration 1x10-5M. Data represent mean values ±S.E.M. of 6 separate experiments.
120
100
80
60
40
20
0
120
100
80
60
40
20
0
120
100
80
60
40
20
0
A
B
C
HP3
HP1
HP5
HP1 + Nal
HP1 + Nal-B
N/OFQ(1-13)NH2
HP3 + Nal
HP3 + Nal-B
N/OFQ(1-13)NH2
HP5 + Nal
HP5 + Nal-B
N/OFQ(1-13)NH2
-11 -10 -9 -8 -7 -6
-9
-8
-7
-6
-5
-11 -10 -9 -8 -7 -6 -5
log conc. (M)
log conc. (M)
log conc. (M)
Figure 1. Rat vas deferens. Concentration-response curve of N/OFQ(1-13)NH2 , HP1 (A), HP3 (B)
and HP5 (C) on the contractions induced by LFES before and after naloxone or naloxone
benzoylhydrazone. The data are means SEM of six experiments.
4

In our experiments, HP1 induced a strong tachyphylaxis, as reported by Dooley et al.18
Because of this, each dose-response curve (without or after blockade of opioid or NOP
receptors) was created on a separate preparation (see Supplementary data).
HP3, in which Trp in position 5 is substituted by (S)-2-(1-methyl-1H-indol-3-
yl)propionic residue exhibited lower affinity (pEC50=6.5), compared to the parent molecule.
The maximal inhibitory effect (Emax), almost the same as that of HP1, was produced by the
peptide in a concentration of 1x10-5M. The effect of HP3 was strongly reduced by pre-
incubation of smooth muscles with Nal and completely prevented by Nal-B (Fig. 1B). It is
evident, that inclusion of N-methyl ꢀ2-tryptophan analogue in position 5 results in a peptide
(HP3) that possesses affinity for both opioid- and NOP receptors.
Concentrations higher than 1x10-5M (data not shown), markedly decreased vas
deferens contractions, which is not due to specific interactions with NOP or opioid receptors.
The effect of HP5 could be divided in 2 components. In low concentrations (1x10-11M), the
compound weakly decreased the electrically evoked contractions, expressing a plateau at
concentrations 1x10-10M – 1x10-7M. Applied in higher concentrations (1x10-6M – 1x10-5M),
HP5 strongly and concentration-dependently depressed the evoked muscle contractions, the
maximal inhibitory effect being almost equal (but in 10-fold higher concentration) to that of
N/OFQ(1-13)NH2 (pEC50= 7.29, Emax= -73%). When the peptide was applied after Nal-
blockade of opioid receptors, this inhibitory effect was not observed, suggesting that HP5 is
an opioid agonist (Fig. 1C).
The opioid antagonist HP2 (Ac-RFMWMK-NH2) and its derivative HP4 showed
dynamic properties similar to those of HP5 (Fig. 2A, Fig. 2B).
Low concentrations of the peptides (1x10-11M) moderately diminished the smooth-
muscle contractions. The higher concentrations did not change (1x10-10M -1x10-9M) or even
slightly enhanced (1x10-8M-1x10-7M) the contractions. The blockade of opioid receptors by
1x10-6M Nal completely prevented the effects of HP2 and HP4 in the concentration range
1x10-11M-1x10-6M, showing that their effects are a result of interaction with µ-receptors. Only
in very high concentrations, it is possible to presume also an activation of NOP receptors, as
far as Nal-B is more effective than Nal in preventing the inhibitory action of HP2 and HP4 (1x10-
6M-1x10-5M). Both peptides showed similar affinity and efficacy (Table 2).
The effects of low concentrations of HP6 differ from that of HP2 and HP4, displaying
a slight inhibition of smooth muscle contractions, due probably to interaction with NOP
receptors (Fig. 2C). This effect (up to concentration of 1x10-5M) was blocked by Nal-B only.
It appears that the insertion of (S)-2-(5-methoxy-1H-indol-3-yl)propionic residue in position 4
5

reversed the affinity of the original peptide Ac-RFMWMK-NH2 from ꢁ- to NOP receptors. In
the concentration range 1x10-6M-1x10-5 M, HP6 most probably activates opioid receptor, too.
120
100
80
60
40
20
0
120
100
80
60
40
20
0
120
100
80
60
40
20
0
B
C
A
HP4
HP2
HP6
HP4 + Nal
HP4 + Nal-B
N/OFQ(1-13)NH2
HP2 + Nal
HP2 + Nal-B
N/OFQ(1-13)NH2
HP6 + Nal
HP6 + Nal-B
N/OFQ(1-13)NH2
-11 -10 -9 -8 -7 -6 -5
-11 -10 -9 -8 -7 -6 -5
-11 -10 -9 -8 -7 -6 -5
log conc. (M)
log conc. (M)
log conc. (M)
Figure 2. Rat vas deferens. Concentration-response curve of N/OFQ(1-13)NH2 , HP2 (A), HP4 (B)
and HP6 (C) on the contractions induced by LFES before and after naloxone or naloxone
benzoylhydrazone. The data are means SEM of six experiments.
In the present paper, we studied the changes in peptide profile evoked by structural
modification in position 5 of the hexapeptide HP1, an agonist of NOP receptors, and in
position 4 of the hexapeptide HP2, an antagonist of opioid receptors.
There are, but only a few data in the literature, concerning the role of particular amino
acids and their position in biologically active hexapeptides, activating NOP-receptors. At
present, the research efforts of several groups have proved some structure requirements for
biological activity of hexapeptide ligands with affinity to NOP receptors.
It was found that acetyl group at N-terminal of hexapeptides is required for high
affinity to NOP receptors and also that presence of positively charged Arg in position 1,27,28 as
well as Arg in position 4,29 could be decisive for short-chain peptide affinity and biological
activity.
Binding studies29 on N-terminally modified hexapeptides reveal that such compounds
possess very high receptor affinity, but changed intrinsic activity. Similar data have been
established about acetelins; Arg in position 1 is a prerequisite19 for their activity, while the
presence of N-terminal acetyl group does not change their affinity. This gave us reason to
suggest, that the remaining part of the hexapeptide molecule, has a role for the selective
binding to NOP- or ꢁ-receptors. Studying the structure-activity requirements for NOP-
activating hexapeptides, Ambo et al.30 have found that the amino acid residue in position 5
6

plays a key role in agonist/antagonist activity. In some respect, our study extends these
findings showing that replacement of Trp in position 5 by ꢀ2-tryptophan analogs also affects
the selectivity of hexapeptides.
Based on the test of HP2 (Ac-RFMWMK-NH2) on guinea-pig ileum, Dooley et al.18
described this hexapeptide like a potent and selective opioid antagonist, binding with a high
affinity to ꢃ3 and µ-receptors. They also found, that the effects of HP2 depend on the species
and it is relatively weak antagonist in mouse vas deferens. On rat vas deferens used in our
experiments, the short-chain peptide in low concentrations decreased (by about 20%) the
smooth-muscle contractions, suggesting that in this case HP2 is rather weak agonist, than
antagonist of opioid receptors. Since rat vas deferens is a ꢁ-specific tissue30, it could be
concluded that HP2 is an agonist of ꢁ-receptors in this tissue. Moreover, it has been suggested
that rat vas deferens is a good model “to assess the ꢁ-receptor efficacy of opioid agonists
proving a more physiological environment for a ligand-receptor interaction than other efficacy
measuring methods”.31
Our present results showed, that the presence of N-methyl ꢀ2-tryptophan residue in
position 5 in Ac-RYYRWK-NH2, modified the selectivity of the referent peptide – HP3
interact with both NOP- and opioid-receptors. The same group in position 4 did not change
the properties of Ac-RFMWMK-NH2 - HP4 possess the same affinity and efficacy as the
parent substance. The 5-methoxy ꢀ2-tryptophan residue in position 5 in Ac-RYYRWK-NH2
(HP5) or in position 4 of Ac-RFMWMK-NH2 (HP6) reverse the affinity of the compounds –
HP5, a derivative of NOP-agonist apparently has low affinity to NOP-receptors and activates
opioid receptors, while HP6, a derivative of µ-receptor antagonist, exhibits properties of a
weak NOP-receptor agonist. These data give us ground to suggest that in this case not only
the position of modification, but also the nature of the incorporated group, lead to significant
changes in peptide’s selectivity and affinity.
Acknowledgments
This study was supported by Grant DTK_02/61 of the National Research Fund, Bulgaria.
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9

Graphical abstract
Ac-Arg-Tyr-Tyr-Arg-X-Lys-NH2
Ac-Arg-Phe-Met-X-Met-Lys-NH2
O
C
O
NH CH2 CH
NH CH2 CH
C
H3CO
Where X is:
N
CH3
HP3 - HP4
N
H
HP5 -HP 6
10