Side chain cyclization based on serine residues: Synthesis, structure, and activity of a novel cyclic analogue of the parathyroid hormone fragment 1-11

Article abstract of DOI:10.1021/jm1008264

The N-terminal region of the parathyroid hormone (PTH) is sufficient to activate the G-protein-coupled PTH receptor 1 (PTHR1). The shortest PTH analogue displaying nanomolar potency is the undecapeptide H-Aib-Val-Aib-Glu-Ile-Gln- Leu-Nle-His-Gln-Har-NH₂ that contains two helix-stabilizing residues (Aib^1,3). To increase the helical character and proteolytic stability of this linear peptide, we replaced Gln^6,10 with (a) Lys⁶ and Glu¹⁰ to introduce a lactam bridge and (b) Ser^6,10 to form a diester bridge upon cross-linking with adipic acid. These cyclopeptides were, respectively, 468-fold less and 12-fold more potent agonists than the linear analogue. Despite their different potencies, all three analogues adopted similar α-helix structures, as shown by NMR and molecular dynamics studies. However, the diester bridge could better mimic the orientation and chemical properties of the side chains of Gln⁶ and Gln¹⁰ in the linear PTH analogue than the lactam moiety. This is apparently important for efficient receptor activation and provides further insights into the receptor-bound ligand conformation.

Full text of DOI:10.1021/jm1008264

8072 J. Med. Chem. 2010, 53, 8072–8079
DOI: 10.1021/jm1008264
Side Chain Cyclization Based on Serine Residues: Synthesis, Structure, and Activity
of a Novel Cyclic Analogue of the Parathyroid Hormone Fragment 1-11†
Andrea Caporale,*,‡ Mattia Sturlese,‡ Lorenzo Gesiot,‡ Fabrizio Zanta,‡,§ Angela Wittelsberger, and Chiara Cabrele*,§
‡Dept. of Chemical Sciences, University of Padova, Institute of Biomolecular Chemistry, CNR, via Marzolo 1, 35131 Padova, Italy,
§
€
Fakultat fur Chemie und Biochemie, Ruhr-Universitat Bochum, Universitatsstrasse 150, 44801, Bochum, Germany, and
Dept. of Physiology, Tufts University School of Medicine, 136 Harrison Avenue, Boston, Massachusetts 02111, United States
€
€
€
Received July 3, 2010
The N-terminal region of the parathyroid hormone (PTH) is sufficient to activate the G-protein-coupled
PTH receptor 1 (PTHR1). The shortest PTH analogue displaying nanomolar potency is the undecapep-
tide H-Aib-Val-Aib-Glu-Ile-Gln-Leu-Nle-His-Gln-Har-NH2 that contains two helix-stabilizing resi-
dues (Aib1,3). To increase the helical character and proteolytic stability of this linear peptide, we replaced
Gln6,10 with (a) Lys6 and Glu10 to introduce a lactam bridge and (b) Ser6,10 to form a diester bridge upon
cross-linking with adipic acid. These cyclopeptides were, respectively, 468-fold less and 12-fold more
potent agonists than the linear analogue. Despite their different potencies, all three analogues adopted
similar R-helix structures, as shown by NMR and molecular dynamics studies. However, the diester
bridge could better mimic the orientation and chemical properties of the side chains of Gln6 and Gln10 in
the linear PTH analogue than the lactam moiety. This is apparently important for efficient receptor
activation and provides further insights into the receptor-bound ligand conformation.
Introduction
R-helical segments that possess limited stability due to low
nucleation probability.16,17
Many cellular processes are controlled by receptor-binding
peptides and hormones; therefore, the development of new
peptide analogues and peptidomimetics that can target biolo-
gically relevant receptors and modulate their activation is of
great importance in biology and medicine. Moreover, the
prevalence and diversity of bioactive peptides as well as their
role in crucial physiological functions have renewed the interest
in the application of peptides and peptide derivatives as ther-
apeutic agents.1,2
One of the most common strategies to improve the biological
activity, proteolytic stability, and pharmacokinetic properties of
peptides is based on the introduction of conformational con-
straints, for example, by backbone or side chain cyclization.3-9
The nonbiogenic cyclizations via lactam formation between side
chains of ω-amino and ω-carboxyl R-amino acids,5 via ring-
closing metathesis10-13 of ω-alkenyl-containing amino acid
building blocks or via click chemistry,14,15 have been shown to
be suitable for the mimicry and stabilization of R-helices that play
a major role in mediating ligand-protein and protein-protein
interactions. This is especially important in the case of short
Parathyroid hormone (PTH)a is an 84-residue long ligand of
the class B G-protein-coupled receptor PTHR118 and plays a
role in the regulation of Ca2þ levels in blood.19 The N-terminal
PTH segment 1-34 maintains full activity20 and increases bone
mineral density and strength; hence, it is considered to be one of
the most effective treatments of osteoporosis: As a matter of
fact, recombinant PTH-(1-34) (teriparatide) is the first bone
formation agent approved for marketing.21-23
About a decade ago, Gardella and co-workers found that
the N-terminal PTH fragment 1-14 is sufficient to induce
PTHR1-mediated cAMP production, but it is a very weak
agonist (EC50 = 100 μM).24 However, the following structure-
activity relationship (SAR) studies on PTH-(1-14) have led
to the identification of analogues with superior potency, such
as H-[Ala3,12,Gln10,homoarginine (Har11),Trp14]-PTH-(1-14)-
NH2 (EC50=0.12 μM), and even reduced length, such as H-
[Ala3,Gln10,Har11]-PTH-(1-11)-NH2 (EC50 = 3.1 μM).25
Although the receptor-bound conformation of PTH is not
known, structural investigations of PTH-(1-34) indicate the
presence of a R-helix starting from residue 3.26,27 Therefore,
the formation of a stable, amphipatic helix should be a
prerequisite of the ligand for strong receptor binding and activa-
tion. Accordingly, the substitution of native residues with helix-
inducing CR,R-dialkyl residues resulted in analogues of PTH-
(1-11/14) with potencies in the nanomolar range: Examples
are H-[Aib1,3,Gln10,Har11,Ala12,Trp14]-PTH-(1-14)-NH2 and
H-[Aib1,3,Gln10,Har11]-PTH-(1-11)-NH2 (EC50 = 1.1 and
4.0 nM, respectively).28 All together, these data support the
idea that analogues of the PTH fragments 1-11/14 with
improved helix and metabolic stability represent attractive
drug candidates for the treatment of osteoporosis.29 Herein,
we propose a chemical approach toward constrained analogues
†Dedicated to Prof. E. Peggion, Department of Chemical Sciences,
University of Padova, on the occasion of his retirement.
*To whom correspondence should be addressed. Tel: þ39-049-
8275740. E-mail: caporaleandrea74@gmail.com (A.C.). Tel: þ49-234-
3227034. E-mail: chiara.cabrele@rub.de (C.C.).
a Abbreviations: Aib, 2-aminoisobutyric acid; CD, circular dichroism;
DIPEA, N,N-diisopropylethylamine; DMF, N,N-dimethylformamide; Har,
homoarginine; HATU, 1-[bis(dimethylamino)methylene]-1H-7-azabenzo-
triazolium hexafluorophosphate 3-oxide; HBTU, 1-[bis(dimethylamino)-
methylene]-1H-benzotriazolium hexafluorophosphate 3-oxide; HOAt, 1-hy-
droxy-7-azabenzotriazole; HOBt, 1-hydroxybenzotriazole; Hse, homoserine;
Nle, norleucine; NMP, N-methyl-2-pyrrolidone; PTH, parathyroid hor-
mone; rPTH, rat PTH; TFA, trifluoroacetic acid; TIS, triisopropylsilane;
TMS, tetramethylsilane.
r
pubs.acs.org/jmc
Published on Web 10/28/2010
2010 American Chemical Society

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 22 8073
of PTH-(1-11), which combines the use of CR,R-dialkyl
amino acid residues with i - i þ 4 side chain-to-side chain
cyclization via a macrolactam or macrolactone linkage. Their
structural and biological properties are presented in the
following sections.
Results
Peptide Design and Synthesis. On the basis of a previous
study by Gardella and co-workers,30 there is some evidence that
the receptor-bound conformation of PTH displays a close
contact between the side chains of the residues at positions 6
and 10. This supports the hypothesis that the N terminus of the
ligand must adopt a helical structure upon receptor binding.
SAR studies aiming at the minimization of the PTH length
showed that substitutions of the native PTH residue Gln6 are
generally not tolerated, whereas the replacement of the native
residue Asn10 with Gln leads to an analogue with superior
potency.25 Gardella and co-workers30 investigated the effect of
a covalent linkage between the side chains at positions 6 and 10
on the potency of H-[Ala3,12,Gln10,Har11,Trp14]-rPTH-(1-14)
that was appropriately modified by the substitutions Lys6/Gln
and Glu10/Gln and successively subjected to lactamization.
Both the linear and the cyclic forms of the double-substituted
analogue are much less potent than the starting analogue (130
and 21 μM, respectively, vs 0.12 μM). However, it is interesting
to observe that the cyclic peptide is considerably more potent
than the linear one, which suggests that close proximity of the
side chains of residues 6 and 10 might favor the binding to the
receptor and its activation.
On the basis of the structural and functional data described
above and with the aim to find novel small-sized PTH agonists
with high potency, we designed two novel analogues of PTH-
(1-11) by combining the use of the helix-inducing 2-aminoiso-
butyric acid (Aib) residues with the side chain-to-side chain
cyclization at positions 6 and 10. The Aib-containing linear
peptide H-[Aib1,3,Gln10,Har11]-PTH-(1-11)-NH2, which was
previously shown to have nanomolar potency,28 was chosen as
the starting scaffold and was further modified by replacing Met8
with norleucine (Nle) to avoid sulfur oxidation31,32 (peptide 1 in
Figure 1).
Figure 1. Structures of the linear (1) and cyclic (2 and 3) analogues
of PTH-(1-11).
Table 1. Analytical and Biological Data of the Analogues of
PTH-(1-11) 1-3
þa
þb
c
d
peptide
no.
M þ Hcalcd
M þ Hobsd
tR
(min)
EC50
(nM)
(Da)
(Da)
1
2
3
1318.8
1301.8
1346.8
1318.8
1301.8
1346.7
13.2
14.0
15.0
2.4 ( 0.3
1122 ( 78
0.2 ( 0.1
a Monoisotopic mass calculated for M þ Hþ. b Obtained by API-
TOF-MS. c Obtained with a gradient of 10-90% B in 20 min (A, 0.1%
TFA in water; B, 0.1% TFA in acetonitrile/water 90:10), a flow rate of
1.5 mL/min, and UV detection at 215 nm. d Adenylyl-cyclase activity
obtained with a luciferase assay on HEK293 cells expressing PTHR1
(the data were obtained by the combination of two independent experi-
ments, each performed in triplicate).
We decided to introduce a cyclic motif by using a lactam
bridge between Lys6 and Glu10 (peptide 2), in analogy to the
scaffold previously applied by Gardella and co-workers,30 or
alternatively by cross-linking the side chains of Ser6 and Ser10
with a dicarboxylic acid (peptide 3). Peptide macrocycli-
zation based on Ser residues has been reported by Grubbs
and O’Leary13 who performed ring-closing metathesis on an
R-/310-helical model heptapeptide33 containing either O-allyl-
serine, Ser(All), or O-allyl-homoserine, Hse(All), at positions
2 and 6 [Boc-Val-Xaa-Leu-Aib-Val-Xaa-Leu-OMe, Xaa =
Ser(All) or Hse(All)]. The resulting 21-/23-membered ring
system allowed the peptide backbone to adopt a helical con-
formation similar to that of the linear precursor, although the
two-atom shorter ring induced a kink of the helix at the Aib
residue. On the basis of this structural study, we opted for a 23-
membered cyclic scaffold to prepare the new constrained PTH
analogue: Consequently, the side chains of Ser6 and Ser10 were
cross-linked by using adipic acid.
(DIPEA) or 1-[bis(dimethylamino)methylene]-1H-7-azaben-
zotriazolium hexafluorophosphate 3-oxide (HATU)/1-hy-
droxy-7-azabenzotriazole (HOAt)/collidine.34,35 The cycliza-
tion step was carried out on the solid support as well. For
the synthesis of 2, the alloc and allyl protecting groups were
chosen for the side chains of Lys6 and Glu10, respectively.36
The peptide chain was assembled until position 6, and then,
the alloc and allyl groups were selectively removed with Pd(0)
in the presence of phenylsilane as nucleophile. The lactam
bridge was formed upon in situ activation with HBTU/HOBt/
DIPEA. Then, the peptide chain was further elongated. After
acidic cleavage of the crude product from the resin and
simultaneous removal of the remaining protecting groups,
the purified cyclic peptide was obtained by preparative HPLC
and characterized by analytical HPLC and API-TOF-MS
(Table 1).
The cyclic analogue 3 was obtained by acylation of the
β-hydroxyl groups of Ser6 and Ser10 with adipic acid (Scheme 1).
Again, the peptide chain was assembled until position 6. After
selective removal of the trityl groups from the Ser side chains
with 1% trifluoroacetic acid (TFA) in dichloromethane, the
two hydroxyl groups were bridged with adipic acid upon in
situ activation with HATU/HOAt/collidine. Unfortunately, the
Peptide chain assembly was performed by manual solid
phase synthesis, using NR-Fmoc-protected amino acids and
the coupling reagent mixtures 1-[bis(dimethylamino)methylene]-
1H-benzotriazolium hexafluorophosphate 3-oxide (HBTU)/
1-hydroxybenzotriazole (HOBt)/N,N-diisopropylethylamine

8074 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 22
Caporale et al.
Scheme 1. Synthetic Route for the Preparation of Cyclopeptide 3
following elongation of the cyclic intermediate with Ile failed, as
indicated by persistent positive ninhydrin test37 despite multiple
couplings and confirmed by mass analysis of the peptide ob-
tained by a small-scale TFA cleavage. For this reason, in an
alternative synthetic route, the peptide chain was assembled
until position 5, followed by selective deprotection/cross-linking
of the Ser side chains. After peptide chain elongation and acidic
treatment to cleave the resin and the remaining protecting
groups, the cyclic peptide 3 was purified by preparative HPLC
and identified by analytical HPLC and API-TOF-MS (Table 1).
The linear peptide, used as a reference, was synthesized
according to a previous work.31 The analytical data of the
three synthesized peptides are summarized in Table 1.
Circular Dichroism (CD) Spectroscopy. The CD spectra of
peptides 1-3 were recorded under the same conditions used
in previous works for other PTH analogues (TFE/water 20:80,
v/v, at 25 °C).38 The three peptides adopted a helical structure, as
indicated by a positive band at 192 nm and two negative ones at
208 and 222 nm (Figure 2). On the basis of the ratio (R) of the
ellipticity values at 222 and 208 nm, which was close to 0.7 for all
three peptides, it could be assumed that both R- and 310-helix
structures were present. Indeed, the R value was intermediate
between the typical values for R-helices (0.85-0.9) and those
for 310-helices (0.2-0.4).39 Moreover, it is known that short
sequences containing CR,R-dialkyl amino acid residues are prone
to form a 310-helix.40,41 The CD curve of the cyclic analogue 2
containing the lactam bridge between Lys6 and Glu10 was very
similar to that of the linear peptide 1. Its R-helix content was
estimated to be 56% by the algorithm CONTIN.42,43 In con-
trast, the cyclic analogue 3 containing the bridged residues Ser6
and Ser10 was characterized by a significantly more intense CD
Figure 2. CD spectra of the linear (1) and cyclic (2 and 3) analogues
of PTH-(1-11). The peptide concentration was 200 μM for 1 and 2
and 160 μM for 3 in TFE/water (20:80, v/v).
spectrum, which indicated a superior content of ordered struc-
ture with respect to the linear peptide 1 and also to the cyclic
peptide 2 (70% R-helix content was estimated42,43). No con-
centration dependency of the CD curves was observed for any of
the three analogues (data not shown).
Two-Dimensional (2D) NMR Spectroscopy. The 2D NMR
spectra of the three peptides were recorded at the concentra-
tion of 2.2 (1) or 2.4 mM (2 and 3) in TFE/water (20:80, v/v)
at 298 and 310 K. The chemical shifts of the linear peptide did
not show significant changes at the two temperatures. In
contrast, the chemical shifts of the cyclic peptides were found
to be temperature-dependent. This observation suggested
that the temperature might influence the conformation of the

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 22 8075
Figure 3. Comparison of the CSI patterns of the linear (1) and
cyclic (2 and 3) analogues of PTH-(1-11). As the temperature effect on
the random coil CR-H chemical shifts is negligible in the range of
298-310 K,48 the chemical shift differences of the CR-H protons were
calculated using the corresponding random coil values at 298 K. The
prefix “cy” is used for the side-chain cyclized residues.49
constrained peptides, which led us to choose the temperature
used for the biological tests (310 K) to determine their struc-
tures. A complete proton resonance assignment was per-
formed using the standard procedure.44 The spin systems of all
amino acid residues were identified using magnitude correlation
spectroscopy (COSY)45 and CLEAN-total correlation spectros-
copy (TOCSY)46 spectra. The sequence-specific assignment was
accomplished using rotating frame Overhauser effect spectros-
copy (ROESY)47 spectra. The peptides were found to adopt an
R-helix spanning over residues 2-11, as indicated by the nega-
tive pattern of their chemical shift index (CSI) for the CR-H
protons (Figure 3). Interestingly, the CSI values of the CR-H
protons of the cross-linked residues Ser6 and Ser10 resulted to be
positive. This was attributed to the chemical modification of the
Ser side chains: Indeed, the conversion of the β-hydroxyl group
into an ester group altered the inductive effect of the β-oxygen,
leading to a shift of the NMR signals of the CR-H protons
toward lower fields. Furthermore, the different extent of the shift
observed for the two side-chain acetylated Ser residues was pro-
bably due to a difference in the relative orientation between the
ester groups and the CR-H protons (diamagnetic anisotropy).
Molecular Dymanics. The structures of peptides 1-3 were
obtained with a simulated annealing protocol, using the
distance restraints extrapolated by the ROE peak intensities
(137 for 1, 158 for 2 and 147 for 3). Superposition of the
10 best structures of each analogue was characterized by small
rmsd values for the backbone-atom positions from Val2 to
Figure 4. Frontal (left) and lateral (right) view of the lowest energy
structures of the linear (1) and cyclic (2 and 3) analogues of
PTH-(1-11) (from the top). In the frontal view, the residue numera-
tion is reported along the CR-atom direction. In the lateral view, the
residue number is close to the CR-atom.
Interestingly, the hydrogen bond acceptor character of the two
γ-carbonyl groups of Gln6 and Gln10 in peptide 1 was nicely
mimicked by the two ester functional groups in analogue 3
(Figure 4). This was obviously not the case of the lactam bridge
in analogue 2, which could display the γ-carbonyl oxygen of the
amide group as a single hydrogen bond acceptor. Moreover, the
spatial region occupied by the single amide group of the lactam
bridge in analogue 2 did not overlap with any of the two regions
occupied by the amide groups of Gln6 and Gln10 in analogue 1.
Biological Activity. The agonistic activity of peptides 1-3
was assessed by using a luciferase assay on HEK293 cells
stably expressing the PTHR1 (Figure 5). The linear peptide 1
acted as an agonist with an EC50 value of 2.4 nM (Table 1). The
cyclic analogue 2 was 468-fold less potent than the linear peptide,
displaying an EC50 value of 1.1 μM. In contrast, the cyclic peptide
3 not only was 12-fold more potent than the linear analogue 1
(EC50 = 0.2 nM), but it also showed higher efficacy in activating
the receptor, as indicated by the dose-response curve that rose
above the 100% activity threshold.
cy-Ser/cy-Glu/Gln10 (0.282 A for 1, 0.207 A for 2, and 0.378 A
for 3), which indicated the presence of a unique conformation
for each peptide. In contrast, Har11 displayed different possible
conformations. The values of the dihedral angles were found to
lay in the typical R-helical region of the Ramachandran plot (see
the Supporting Information), thus confirming the helix con-
formation of all three analogues. The side chain cyclization
seemed to slightly modify the geometry of the helical segment
4-10: In particular, the distance between the CR-atoms of the
cyclized residues at positions 6 and 10 was dependent from
the size of the bridge. The average distance calculated for the
˚
˚
˚
Discussion and Conclusions
Stabilization of the N-terminal helical structure of the PTH
fragments 1-11 and 1-14 represents a promising strategy for
the development of new antiosteoporosis drugs. In the last years,
the most used approach has been the substitution of Ala1 and
Ala3 with conformationally constrained CR,R-dialkyl amino
acids like Aib and 1-aminocycloalkane-carboxylic acids.28,38,50
Moreover, Gardella and co-workers have claimed the presence
˚
family of the 10 lower energy structures was 6.42 A in the
˚
reference peptide 1, whereas it was smaller in peptide 2 (6.22 A)
˚
and higher in peptide 3 (6.71 A). This implicated a different
spatial distribution of the CR-atoms as well as of the side chains.
Indeed, the rmsd values for the Cβ-atoms of peptides 2 and 3
˚
versus the linear peptide 1 were 1.150 and 1.430 A, respectively.

8076 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 22
Caporale et al.
presented in this work was obtained by side chain acylation of
Ser6 and Ser10 with adipic acid, which results in a 23-mem-
bered ring. Although this cyclic motif is just two atoms larger
than the lactam bridge, the helix segment 4-11 is significantly
less compact than that in the lactam-containing analogue:
Indeed, the distance between the CR-atoms at positions 6 and
˚
˚
10 is 6.71 A in peptide 3 and 6.22 A in peptide 2. Moreover,
comparison of the frontal and lateral views of the three
peptide structures in Figure 4 clearly shows that the diester
bridge is a much better surrogate of the two Gln side chains in
peptide 1 than the lactam bridge, as it matches their orienta-
tion and additionally mimic the two γ-carbonyl groups. This
superior mimicry ability of the Ser-based cyclization reflects
the potency of the cyclic analogue 3 that is, respectively,
>5600-fold and 12-fold higher than that of the poor agonist
2 and potent agonist 1. Interestingly, cyclopeptide 3 produced
a greater maximal response than the ligand used as positive
control in the luciferase assay, namely, PTH-(1-34)-NH2.
Therefore, the cyclic peptide 3 represents a small-sized, con-
formationally constrained PTH agonist with subnanomolar
potency (0.2 nM) and major efficacy in activating PTHR1-
mediated signal transduction in vitro when compared to
PTH-(1-34)-NH2 (activity ratio of about 1.6:1). Such biolo-
gical properties make of peptide 3 a very interesting PTHR1
ligand, and future investigations will be carried out to deeply
understandits mode ofreceptor bindingand activation, which
might somehow differ from that of the linear peptide 1, as the
different shape of its dose-response curve, in particular its
slope (0.34 against 1), would let presume. To this regard, it has
been reported that different PTH analogues may have different
affinities for the two putative, pharmacologically distinguish-
able GTPγS-resistant (R0) and GTPγS-sensitive (RG) recep-
tor conformations.51,52 For example, PTH-(1-34), which is
hypothesized to interact both with the N-terminal extracellular
and transmembrane/extracellular loop domains of the recep-
tor, can almost equally bind both receptor states, whereas
H-[Aib1,3,Gln10,Har11,Ala12,Trp14]-PTH-(1-14)-NH2, which
should interact only with the transmembrane/extracellular
loop domain, binds preferentially the RG state.52 In addition,
it has been shown that R0-preferring ligands produced re-
markably prolonged signaling responses in bone and kidney
PTH target cells.52
Figure 5. Dose-responsecurves for the linear(1) and cyclic (2 and 3)
analogues of PTH-(1-11). The values for maximal response and
slope were extrapolated by fitting the data with a sigmoidal model.
Maximal response: 90 ( 2 (1), 98 ( 2 (2), and 159 ( 13% (3). Slope:
1.1 ( 0.1 (1), 0.9 ( 0.1 (2), and 0.34 ( 0.10 (3).
of close contacts between the side chains of residues 6 and 10 in
the receptor-bound conformation of the ligand, being Gln6 and
Asn10 in the native PTH sequence.30 Such a structural motif
would correspond to a well-defined R-helix turn, in which the
i - i þ 4 residues would be located on the same face of the
amphipatic helix. Therefore, the introduction of a covalent
linkage between the i - i þ 4 side chains is expected to stabilize
the R-helix, provided that the chemical and structural changes
imposed by the linkage are tolerated. In the case of the N-term-
inal region of PTH, SAR studies have shown that mutations of
Gln6 are badly tolerated, whereas the substitutions of Asn10 with
Ala, Asp, Glu, and Gln not only are fully tolerated, but they even
enhance the receptor activation in comparison to the unmodified
sequence, being Gln10/Asn the most potent mutation.25 Also, a
D-amino-acid scan of the linear analogue 1 underlined the
importance of the orientation of the side chain at position 6:
Indeed, the D-Gln6-containing peptide was 40-fold less potent
than the D-Gln10-containing one.31 All together, these observa-
tions suggest that both positions 6 and 10, but especially position
6, prefer to be occupied by side chains displaying hydrogen bond
donors and/or acceptors.
To improve peptide potency and stability, we have presented
here an approach that combines both the advantages deriving
from the introduction of conformationally constrained amino
acids and the cyclic structures. The cyclic analogue 2 displays a
21-membered macrolactam obtained upon the cyclization of
Lys6 with Glu10. This bridge forces the peptide segment 4-11 to
adopt a compact helix motif, as indicated by the minor distance
between the CR-atoms at positions 6 and 10, relative to the linear
In conclusion, we have presented the effect of cycliza-
tion of the side chains at positions 6 and 10 on the structure
and function of an Aib-containing, linear analogue of
PTH-(1-11). The NMR and in vitro studies suggest that
the capacity of the ligand to form a stable helix is a structural
prerequisite for high receptor activation, provided that other
factors, like the presence of key structural/functional groups
and their appropriate spatial distribution, are fulfilled. For
example, on the basis of this work, the importance of the
γ-amide groups of Gln6 and Gln10 seems to be related more
with their H-bond acceptor rather than donation function.
Furthermore, the successful design of cyclopeptide 3 shows
that side chain-to-side chain cyclization based on the cross-
linking of two β-hydroxyl groups with dicarboxylic acids
offers a valuable alternative to the commonly used lactam
linkages, especially in those cases, in which it is necessary to
mimic the presence of two carbonyl groups. The fact that the
diester-bridged cyclopeptide 3 is stable in basic buffered
aqueous solutions (the peptide degradation in phosphate
buffer, pH 8, was 9% after 24 h at room temperature, as
detected by HPLC analysis) supports the potential applica-
tion of such a bridging tool for peptide mimicry.
˚
analogue 1(6.22 vs 6.42 A), and as shown in Figure 4. Moreover,
although the lactam bridge points into the same direction of the
Gln6 and Gln10 side chains in the linear peptide, it is obviously
unable to act as surrogate of the two amide groups. This may
explain the dramatic loss of potency of cyclopeptide 2 in com-
parison to the linear peptide reference. Our results confirm
the negative effect of the substitution of Gln6 and Gln10 with a
lactam bridge between Lys6 and Glu10, as previously observed
for an analogue of PTH-(1-14).30
In contrast to the classical lactam formation based on the
Lys and Glu/Asp side chains, the cross-linking of two Ser side
chains by a dicarboxylic acid offers the possibility to form a
bridge of variable length and containing two ester bonds as
potential hydrogen bond acceptors. The cyclic analogue 3

Article
Journal of Medicinal Chemistry, 2010, Vol. 53, No. 22 8077
Experimental Section
Fmoc-Ile5-Ser-Leu-Nle-His-Ser-Har11-NH2. To cleave the Trt
protecting group from the Ser side chains, the peptide resin was
preswelled in CH2Cl2 and then treated with the mixture TFA/
TIS/CH2Cl2 (1:5:94) for 2 min. This step was repeated 20 times.
Cyclization was carried out in NMP/DMF (1:1) (4 mL) by using
adipic acid (7.7 mg, 0.053 mmol, 0.7 equiv), HATU (162 mg,
0.43 mmol, 5.9 equiv), HOAt (58 mg, 0.43 mmol, 5.9 equiv), and
collidine (102 μL, 0.77 mmol, 10.5 equiv) for 2 h. This treatment
was repeated a second time, and the reaction completion was
controlled by analytical HPLC after a small-scale TFA cleavage.
After cyclization, the peptide chain was elongated with the
remaining amino acids by using the procedure described for
peptide 2.
Peptide Deprotection/Cleavage from the Resin and Purifica-
tion. The peptide resins were treated with the deprotection and
cleavage mixture TFA/TIS/water (95:2.5:2.5) for 2.5 h. After
filtration, the crude peptides were recovered by precipitation from
methyl tert-butyl ether, subjected to preparative HPLC purifica-
tion, lyophilized, and, finally, characterized by analytical HPLC
and API-TOF-MS. All three peptides were >95% pure (based on
HPLC analysis). Yields: 1, 34%; 2, 4%; and 3, 7%.
CD Spectroscopy. CD measurements were carried out on a
JASCO J-715 spectropolarimeter (Tokyo, Japan). The CD
spectra were acquired and processed using the J-700 program for
Windows. All experiments were carried out at room temperature
using HELLMA quartz cells with Suprasil windows and optical
path lengths of 0.01 and 0.1 cm. All spectra were recorded over the
wavelength range 190-260 nm, using a bandwidth of 2 nm, a time
constant of 8 s, and a scan speed of 20 nm/min. The signal-to-noise
ratio was improved by recording eight accumulations. The peptides
were dissolved in TFE/water (20:80, v/v), and their concentrations
were in the range 0.07-1.07 mM, as estimated by the UV absor-
bance at 210 nm.53 The spectra were reported in terms of mean
residue molar ellipticity (deg cm2 dmol-1).
Materials and Methods. Rink amide MBHA resin (0.73 mmol/g
loading) was obtained from Inalco-Novabiochem (Milan, Italy).
HBTU, HOBt, HATU, HOAt, and most of the Fmoc-protected
natural amino acids were obtained from GL Biochem (Shanghai,
China). Fmoc-Aib-OH was purchased from NeoMPs (Strasbourg,
France). Collidine was purchased from Carlo Erba Reagenti
(Rodano-Milano, Italy). N,N-Dimethylformamide (DMF) dried
over molecular sieves (H2O <0.01%) and N-methyl-2-pyrrolidone
(NMP) were obtained from Iris Biotech GmbH. DIPEA, piper-
idine, adipic acid, Fmoc-Lys(Alloc)-OH, Fmoc-Glu(OAll)-OH,
Pd(PPh3)4, and PhSiH3 were obtained from Fluka (Taufkirchen,
Germany). DMEM, fetal bovine serum, Opti-Mem I, and PBS
were from Life Technologies, Inc. (Gaithersburg, MD). FuGENE
6 transfection reagent was purchased from Roche Diagnostics
(Indianapolis, IN). Passive lysis buffer was from Promega Corp.
(Madison, WI). D-Luciferin was obtained from Molecular Probes
(Eugene, OR). HPLC peptide purification was performed on a
Shimadzu LC-8A instrument (Kyoto, Japan), equipped with a
Shimadzu SPD-6A UV detector, using a Waters Delta-Pak silica
˚
HPLC column (C18-100 A, 300 mm ꢀ 19 mm, 15 μm). The flow
rate was 17 mL/min with a linear gradient of 20-45% (v/v) B over
20 min (A, 0.1% TFA in water; B, 0.1% TFA in 90% acetonitrile/
water 90:10). Homogeneity of the products (>95% pure) was
assessed by analytical reversed-phase HPLC using a Vydac C18
column packed with a polymerically bonded end-capped n-octade-
˚
cyl reversed phase based on 300 A TP silica (218TP54, 250 mm ꢀ
4.6 mm, 5 μm), with a linear gradient of 10-90% (v/v) B in 20 min,
a flow rate of 1.5 mL/min and UV detection at 215 nm. Molecular
masses were determined on a Perseptive Biosystems MARINER
API-TOF mass spectrometer (Foster City, CA) by using the low-
flow electrospray mode (instrument resolution/accuracy: 5000/100
ppm) and the external calibration method.
2D NMR Spectroscopy. NMR spectra of 2.2 (1) or 2.4 mM
(2 and 3) peptide solutions in TFE/water (20:80, v/v) were recorded
at 298 and 310 K on a BRUKER AVANCE DMX-600 spectro-
meter. The water signal was suppressed by presaturation during the
relaxation delay. The spin systems were determined using magni-
tude-COSY45 and CLEAN-TOCSY46 spectra. In the latter case,
the spin-lock pulse sequence was 70 ms long. The specific sequence
assignment was accomplished by using a ROESY47 spectrum and a
mixing time of 150 ms. All spectra were acquired by collecting
512 experiments, each one consisting of 16 (COSY), 32 (TOCSY),
or 56 (ROESY) scans and 2048 data points in the F2 dimension.
Spectral processing was carried out using TOPSPIN 1.2. Spectra
were calibrated against the tetramethylsilane (TMS) signal. Inter-
proton distances were obtained by integration of the signals from
the ROESY spectra after correction of the volumes according to the
intensity ratio method.54 Peak assignment and volumes integration
were performed by using the CARA software (downloaded from
http://cara.nmr.ch).55
Peptide-Chain Assembly. The synthesis of the linear analogue
1 was previously described.31 A similar protocol was applied to
assemble the C-terminal hexapeptides as precursors for the
syntheses of the cyclic analogues 2 and 3 (see the Supporting
Information for detailed synthetic schemes). Briefly, peptide
chain assembly was performed manually on Rink amide MBHA
resin (0.073 mmol). Single couplings (2 h) were carried out with
the reaction mixtures Fmoc-Xaa-OH/HBTU/HOBt/DIPEA
(4:4:4:8 equiv) in DMF, where Xaa was Har(Pbf), Glu(OAll),
Ser(Trt), His(Trt) for peptide 2 or His(Boc) for peptide 3, Nle,
Leu, and Lys(Alloc). The Fmoc group was removed with
piperidine/DMF (20:80, v/v) for 45 min.
Peptide 2. The protecting groups alloc and allyl were removed
from Lys6 and Glu10 by treating the peptide resin, which had been
previously swelled in CH2C12, with a solution of Pd(PPh3)4 (21 mg,
0.018 mmol, 0.25 equiv) and PhSiH3 (216 μL, 1.75 mmol, 24 equiv)
in CH2C12 (2 mL) (3 ꢀ 30 min under nitrogen). Then, the peptide
resin was washed with CH2C12 (3 ꢀ 3 mL, 2 min), DMF (3ꢀ 3 mL,
1 min), and again with CH2C12 (3 ꢀ 3 mL, 2 min). Cyclization was
performed in NMP/DMF (1:1) (4 mL) by using HBTU (166 mg,
0.44 mmol, 6 equiv), HOBt (59 mg, 0.44 mmol, 6 equiv), and
DIPEA (150 μL, 0.88 mmol, 12 equiv) for 2.5 h. This treatment was
repeated a second time, and the reaction completion was confirmed
by the ninhydrin test. Then, the Fmoc group was removed with
piperidine/DMF (1:4) for 45 min, and the peptide chain was
elongated by using the reaction mixtures Fmoc-Xaa-OH/HATU/
HOAt/collidine (4:4:4:8 equiv) in DMF, where Xaa was Ile, Glu-
(OtBu), Aib, and Val. Single couplings (2 h) were used for Ile5 and
Glu4, whereas double couplings (2 ꢀ 2 h) were applied for Aib3,
Val2, andAib1 (the latter was coupled as Boc-protected amino acid).
Peptide 3. After double coupling (2 ꢀ 2 h) of Fmoc-Ile-OH
(4 equiv) with HBTU/HOBt/DIPEA (4:4:8 equiv) in DMF, a
small-scale cleavage (5 mg of peptide resin) was carried out with
TFA/triisopropylsilane (TIS)/H2O (95:2.5:2.5) for 2 h, and the
cleaved peptide was characterized by MS and analytical HPLC.
The data confirmed the high homogeneity of the sequence
Molecular Dynamics. The peptide structures were calculated
using X-PLOR-NIH 2.22. The simulated annealing (SA) pro-
tocol was performed using r-6 averaged values for distances
involving equivalent or nonstereo-assigned protons. The SA
protocol consisted of two parts: In the first part, 1000 structures
for each peptide were obtained by 50 steps of initial energy
minimization, followed by 30 ps of high-temperature dynamics
at 2000 K and 3000 steps of cooling from 2000 to 100 K. The
obtained rough structures were optimized through 200 cycles
of energy minimization using a restraint force constant of
˚
50 kcal/(mol A). In the second part, each structure was refined
by heating up to 1000 K and cooling for 20000 steps. Then, the
structures were optimized again through 200 cycles of energy
minimization. The 10 lower energy structures containing no
˚
ROE distance restraints violation (<0.5 A from the integration
value) were chosen for conformational studies. The generated
structures were visualized and analyzed using the program
VMD (1.8.6.) and MOE2008.10.

8078 Journal of Medicinal Chemistry, 2010, Vol. 53, No. 22
Caporale et al.
Biological Activity. Human embryonic kidney (HEK293) cells
stably transfected with recombinant PTHR1 (HEK293/C21 cell
line) were used.56 The cellular cAMP response element (CRE) of
luciferase was transfected using the CRE-Luc plasmid. HEK293/
C21 cells were cultured at 37 °C in DMEM supplemented with 10%
fetal bovine serum in a humidified atmosphere of 95% air and 5%
CO2. The cells were seeded at 105 cells/well in 24-well, collagene-
coated plates, and on the following day, they were treated with
FuGENE 6 transfection reagent (1 μL/well), CRE-Luc plasmid
(0.2 μg/well) in 0.5 mL/well Opti-Mem I, serum-free medium,
according to the manufacturer’s recommended procedure.
(6) Hurevich, M.; Tal-Gan, Y.; Klein, S.; Barda, Y.; Levitzki, A.;
Gilon, C. Novel Method for the Synthesis of Urea Backbone Cyclic
Peptides Using New Alloc-protected Glycine Building Units.
J. Pept. Sci. 2010, 16, 178–185.
(7) Adessi, C.; Soto, C. Converting a Peptide into a Drug: Strategies to
Improve Stability and Bioavailability. Curr. Med. Chem. 2002, 9,
963–978.
(8) Craik, D. J. Seamless Proteins Tie Up Their Loose Ends. Science
2006, 311, 1563–1564.
(9) Clark, R. J.; Jensen, J.; Nevin, S. T.; Callaghan, B. P.; Adams, D. J.;
Craik, D. J. The Engineering of an Orally Active Conotoxin for the
Treatment of Neuropathic Pain. Angew. Chem. Int. Ed. 2010, 49,
6545–6548.
Luciferase Assay. About 18 h after CRE-Luc plasmid trans-
fection, the cells were rinsed with PBS, and the transfection
medium was replaced with 225 μL/well DMEM. Aliquots of the
peptide solutions at different concentrations in PBS supplemented
with 0.1% bovine serum albumin were then added to the wells and
incubated at 37 °C for 4.5 h. After this time, the medium was
aspirated, and the cells were lyzed by gentle shaking with 200 μL/
well passive lysis buffer. The cells were transferred to labeled low-
binding Eppendorf tubes and centrifuged for 2 min, and then,
80 μL/tube of supernatant was transferred into individual sample
glass tubes. The luciferase activity was measured using a Lumat LB
9507 luminometer (EG&G Berthold). All the CRE-Luc experi-
ments were carried out in triplicate. Calculations and data analysis
were performed using nonlinear regression.
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Covalently Cross-linked Peptide Helices by Ring-closing Metath-
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Chao, J. A.; Steinmetz, W. E.; O’Leary, D. J.; Grubbs, R. H. Ring-
closing Metathesis of Olefinic Peptides: Design, Synthesis, and
Structural Characterization of Macrocyclic Helical Peptides.
J. Org. Chem. 2001, 66, 5291–5302.
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chain-to-side-chain Cyclization by Click Reaction. J. Pept. Sci.
2009, 15, 451–454.
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Marchi, R. D.; Rovero, P.; Halperin, J. A.; D’Ursi, A. M.; Papini,
A. M.; Chorev, M. Synthesis and Conformational Analysis of a
Cyclic Peptide Obtained via i to iþ4 Intramolecular Side-chain-to-
side-chain Azide-alkyne 1,3-Dipolar Cycloaddition. J. Org. Chem.
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10652.
Acknowledgment. We thank Dr. Thomas Burgemeister
(University of Regensburg, Germany), Prof. Stefano Mammi,
and the team of the Magnetic Resonance Laboratory CNR-
ICB at the University of Padova for their support during the
NMR experiments. We are grateful to Dr. Barbara Biondi
(University of Padova) for her kind help in mass analysis and to
Prof. Stefano Moro and the Molecular Modeling Section of the
Department of Pharmaceutical Sciences (Univeristy of Padova)
for computational support. We acknowledge Sandhya Sharma
for her kind help in the biological tests carried out at the
Department of Physiology of the Tufts University School of
Medicine, Boston. This work was partly supported by the
Deutsche Forschungsgemeinschaft (DFG Grant CA296) and
by the Italian Ministry of Education and University.
€
(18) Juppner, H.; Abou-Samra, A.-B.; Freeman, M.; Kong, X.-F.;
Schipani, E.; Richards, J.; Kolakowski, L. F., Jr.; Hock, J.; Potts,
J. T., Jr.; Kronenberg, H. M.; Segre, G. V. A G Protein-linked
Receptor for Parathyroid Hormone and Parathyroid Hormone-
related Peptide. Science 1991, 254, 1024–1026.
€
(19) Gardella, T. J.; Juppner, H. Interaction of PTH and PTHrP with
their Receptors. Rev. Endocr. Metab. Disord. 2000, 317–329.
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Niall, H. D.; Reit, B.; Parsons, J. A.; Potts, J. T., Jr. Bovine
Parathyroid Hormone: Minimum Chain Length of Synthetic Pep-
tide Required for Biological Activity. Endocrinology 1973, 93,
1349–1353.
(21) Neer, R. M.; Arnaud, C. D.; Zanchetta, J. R.; Prince, R.; Gaich,
G. A.; Reginster, J. Y.; Hodsman, A. B.; Eriksen, E. F.;
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Parathyroid Hormone (1-34) on Fractures and Bone Mineral
Density in Postmenopausal Women with Osteoporosis. N. Engl.
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Supporting Information Available: Schemes of the peptide
syntheses, analytical HPLC profiles of the purified peptides,
degradation data of cyclopeptide 3 at pH 8 at 25 and 37 °C,
tables of the chemical shifts of the peptides and of the random
coil CR-H protons used to obtain the CSI patterns, Ramachandran
plots of the 10 lower energy structures for each peptide, and
summary of the cross-peaks found in the ROESY spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(22) Tashjian, A. H., Jr.; Gagel, R. F. Teriparatide [Human PTH-
(1-34)]: 2.5 Years of Experience on the Use and Safety of the
Drug for the Treatment of Osteoporosis. J. Bone Miner. Res. 2006,
21, 354–365.
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