Novel RGD peptidomimetics embedding 1,2,3-triazole as central scaffold; Synthesis and αvβ 3 integrin affinity

Article abstract of DOI:10.2174/157018011795514258

Ten new RGD (Arginine-Glycine-Aspartic acid) peptidomimetics have been synthesized and screened for their affinity to α_vβ ₃ integrin receptor. Arginine and Aspartic acid mimetic subunits were connected through 1,2,3-triazole as central scaffold by click chemistry, affording the final products with good yields. Among them, compounds 3f-j exhibited high affinity to the receptor,with IC50 in the low nanomolar range, comparable to that of the reference compound Cilengitide.

Full text of DOI:10.2174/157018011795514258

Letters in Drug Design & Discovery, 2011, 8, 401-405
401
Novel RGD Peptidomimetics Embedding 1,2,3-Triazole as Central
Scaffold; Synthesis and ꢀvꢁ3 Integrin Affinity
Ming Hong Ni1, Emiliano Esposito1, Massimo Castorina2 and Alma Dal Pozzo*,1
1Istituto di Ricerche Chimiche e Biochimiche G. Ronzoni, via G. Colombo, 81, 20133, Milano, Italy
2Area of Oncology, Research and Development Sigma-Tau, Pomezia,Rome, Italy
Received November 12, 2010: Revised January 17, 2011: Accepted February 14, 2011
Abstract: Ten new RGD (Arginine-Glycine-Aspartic acid) peptidomimetics have been synthesized and screened for their
affinity to ꢀvꢁ3 integrin receptor. Arginine and Aspartic acid mimetic subunits were connected through 1,2,3-triazole as
central scaffold by click chemistry, affording the final products with good yields. Among them, compounds 3f-j exhibited
high affinity to the receptor,with IC50 in the low nanomolar range, comparable to that of the reference compound Cilengi-
tide.
Keywords: RGD peptidomimetics, ꢀvꢁ3 integrin, Osteoporosis, 1,2,3-Triazole, Click chemistry.
INTRODUCTION
We focused our interest on a class of structures contain-
ing 2-aminopyridine at the N-terminus and N-ꢀ-
arylsulfonamide-N-ꢁ-diaminopropionic acid at the C-
terminus, which have been developed by others as potential
therapeutics for osteoporosis, because ꢀvꢁ3 is highly ex-
pressed in the osteoclasts but not present in the osteoblasts
[6-10]. Moreover, RGD mimetics containing the amidino-
like aminopyridine in place of arginine have been claimed to
show high receptor affinity and selectivity as well as
bioavailability in rats [11,12]. In this article, we report on the
synthesis and ꢀvꢁ3 affinity of ten RGD mimetics, where the
two pharmacophores are connected through linkers contain-
ing triazole in different positions. During the preparation of
our paper, another group of researchers reported on a number
of RGD analogs, where arginine and aspartic acid mimetics
have been connected by click chemistry [13].
RGD peptides are a well known class of integrin mem-
brane receptor ligands [1]. Among the integrin superfamily,
ꢀvꢁ3 is overexpressed on a variety of cells during prolifera-
tion, and plays a role in several relevant processes based on
cell-cell and cell-matrix adhesion. This integrin recognizes
molecules containing the peptide sequence RGD; therefore,
it is a priviledged target for antagonists containing this se-
quence, which are expected to have utility in the treatment of
several human diseases, like tumor induced angiogenesis,
restenosis and osteoporosis.
During the last two decades, the understanding of the
structural requirements for optimizing the receptor affinity of
RGD analogs has given impetus to the development of a
great number of potent antagonists. Specificity and efficacy
of the molecular recognition process depend upon the con-
formational presentation of the peptide backbone, which
determines the optimal orientation of the two arginine and
aspartic acid pharmacophores for selective binding to the
receptor [2,3].
RESULTS AND DISCUSSION
Synthesis
The building blocks consisting of arginine mimetic
subunits 1a-f were prepared as described in the Scheme 1,
following already described methods: 1a was prepared from
2-amino-5-nitropyridine by reduction, followed by the aro-
matic azide formation [14]. 1b-d were obtained by Sonoga-
shira reaction [15] from 2-amino-4-or 5-bromopyridine or 7-
bromo-azaindole. 1e and 1f were obtained by arylation of
propargylamine with 2-amino-6-bromopyridine (Heck reac-
tion), followed by catalytic transfer hydrogenation and azide
formation [16]. The aspartic acid mimetic building blocks
2a-e were obtained from 2-L-arylsulfonamido-diamino-
propionic acid t-butyl ester 4 or from the glutamic acid de-
rivative 5 by coupling with the desired residues, as described
in the Scheme 2.
Later on, small non-peptide molecules mimicking the
RGD structure have been proposed by several researchers,
with the purpose to obtain drug-like compounds with im-
proved stability and bioavailability comparing to the parent
peptides [4]. Several different heterocyclic scaffolds have
been proposed as a core structure substituting glycine be-
tween the basic and acidic moieties, with the purpose to
guarantee their optimal distance and spatial orientation for
affinity to the receptor.
We have undertaken the synthesis of a series of RGD
peptidomimetics containing 1,2,3-triazole as central scaffold,
considering this heterocycle particularly attractive, since a
diverse array of structural types can be rapidly prepared and
evaluated through the facility and generality of the “click
chemistry” [5].
The assembly of the subunits containing either azido or
alkyne groups to afford the final products 3a-j was per-
formed by the Huisgen 1,3-dipolar cycloaddition [5], as de-
scribed in the Scheme 3. The clean and chemoselective click
chemistry allowed us to easily obtain several analogs with
95% purity and good yields. Physico-chemical data are
shown in Table 1.
*Address correspondence to this author at the Istituto di Ricerche Chimiche
e Biochimiche G. Ronzoni, via G. Colombo, 81, 20133, Milano, Italy; Tel:
+39-02-70600223; Fax: +39-02-70641634; E-mail: dalpozzo@ronzoni.it
1570-1808/11 $58.00+.00
© 2011 Bentham Science Publishers Ltd.

402 Letters in Drug Design & Discovery, 2011, Vol. 8, No. 5
Hong Ni et al.
N
Boc-HN
N
Boc-HN
N
H2N
b
a
N3
NH2
NO2
1a
N
N
H
N
H
H2N
H2N
N
N
N
N
H2N
c
H2N
N
c
c
;
;
Br
Br
1b
1c
1d
Br
NH-Cbz
e, f
R-HN
N
Br
R-HN
N
R-HN
N
N3
d
1e, R = H
1f, R = methyl
Scheme 1. Synthesis of arginine mimetic subunits 1a-f. Reagents and conditions: a, ammonium formate, 10% Pd/C, 1h, 86%; b, i, TMSiN3,
tBuNO2, 48 h, ii, TFA/DCM; c, i, Ethynyltrimethylsilane, (Ph3P)2PdCl2, CuI, 2.5 h; ii, MeOH, K2CO3, 3 h, yields around 50% for two steps;
d, N-Cbz-propargylamine, (Ph3P)2PdCl2, CuI, TEA, 100oC, 3 h, 80.3%; e, same as a, 2 h; f, TfN3, overnight, 78.3% for two steps.
O
O
OtBu
O
COOH
2a,
R = 4-acetylamino or 2,4,6-trimethyl
or 2,6-difluoro or 4-iodo
N
H
HN
R
S
O
O
O
COOH
OtBu
N
O
2b,
2c,
2d,
R = 4-acetylamino
R = 4-acetylamino
H
HN
R
OtBu
a
H2N
S
O
HN
R
O
O
O
S
N3
COOH
O
N3
O
OtBu
OtBu
N
H
4
HN
R
S
O
O
O
COOH
O
N3
N3
N
R = 4-acetylamino
H
HN
R
S
O
O
O
OtBu
O
N
O
OtBu
O
b
OtBu
c
H
H
N
N
NH-Boc
N3
N3
NH2
NH
O
O
O
O
O
O
S
5
O
N
2e
H
Scheme 2. Synthesis of aspartic acid mimetic subunits 2a-e. Reagents and conditions: a, HOAT, DCC, DIPEA, overnight; b, i, 2-
azidoethylamine, DIPEA, 2 h, ii, 4 M HCl in dioxane, 77.4% for two steps; c, 4-acetamidobenzenesulfonyl chloride, DIPEA, overnight,
88.7%.

RGD Peptidomimetics
Letters in Drug Design & Discovery, 2011, Vol. 8, No. 5 403
H2N
N
O
O
a
1a + 2b
OH
N
N
H
NH
N
N
O
O S
O
3 a
N
H
O
a
H2N
N
OH
1b + 2d
H
N
N
NH
O
N
O
N
N
O S
O
3 b
N
H2N
H
a
O
1c + 2c
O
N
OH
N
N
N
H
NH
O
3 c
O
S
O
H2N
N
N
H
a
1c + 2d
O
O
N
OH
N
N
N
H
NH
O
O
S
3 d
O
N
H
H
N
a
N
O
O
1d + 2d
OH
N
N
H
NH
N
N
O
O
S
O
3e
N
H
O
O
H
N
N
a
N
N
R1
1e-f + 2a
OH
N
N
H
NH
3 f-j
O
S
R
O
3f, R = 4-acetylamino;: R1 = H
3g, R = 4-acetylamino;
3h, R = 2,4,6-trimethyl;
3i, R = 2,6-difluoro
3j, R = 4-iodo;
R1 = methyl
R1 = H
R1 = H
R1 = H
Scheme 3. Synthesis of RGD peptidomimetics 3 a-j. Reagents and conditions: a, i, solvent water/t-butylalcohol, 1M sodium ascorbate (0.1
equiv), 0.5 copper sulfate (0.01 equiv), ii, 50% TFA/DCM, total yield > 80%.

404 Letters in Drug Design & Discovery, 2011, Vol. 8, No. 5
Table 1. Physico-Chemical Characteristics of Compounds 3a-3j
Entry
Hong Ni et al.
1H-NMR
HPLC
Esi mass
[M+H+]
Rt, min(a)
3a
9.6 (15)
517.00
(D2O): ꢀ: 8.29 (d, J=2.3Hz, 1H), 8.24, 8.21 (dd, J=2.5and 9.7Hz, 1H), 8.13(s, 1H), 7.73, 7.61
(dd, J=8.8 and 37.4 Hz, 4H), 7.18 (d, J=9.7Hz, 1H), 3.95 – 3.90 (m, 1H), 3.59-3.55 (dd, J=4.5
and 14.1Hz, 1H), 3.30-3.27 (dd, J=8.8 and 14.1Hz, 1H), 2.95 (t, J=7.2Hz, 2H), 2.59- 2.44 (m,
2H), 2.11 (s, 3H).
3b
3c
3d
3e
3f
11.6 (15)
11.4 (15)
11.9 (15)
7.8 (20)
8.3 (18)
531.17
503.15
517.16
541.16
531.16
(D2O): ꢀ: 8.29 (s, 1H), 8.21- 8.18 (dd, J=2.2 and 9.4 Hz, 1H, ), 8.15 (s, 1H), 7.72 -7.61 (dd,
J=8.8 and 33.1 Hz, 4H), 7.10 (d, J=9.2 Hz, 1H), 4.59 (t, J=5.6 Hz, 2H), 3.71 – 3.65 (m, 3H),
2.27 (t, J= 7.3 Hz, 2H), 2.18 (s, 3H), 2.02-1.90 (m, 1H), 1.79-1.68 (m, 1H).
(DMSO-d6+ D2O): ꢀ:8.64 (s, 1H), 7.91(d, J=6.0Hz, 1H), 7.67 (s, 4H), 7.37 (s, 1H), 7.19 (d,
J=6.0Hz, 1H), 5.09 (d, J=6.0Hz, 2H), 3.86 (t, J=6.6 Hz, 1H), 3.43-3.38 (m, 1H), 3.23-3.14 (m,
1H), 2.01 (s, 3H).
(DMSO-d6 + D2O): ꢀ : 8.60 (s, 1H), 7.90 (s, 1H), 7.67 (d, J=4 Hz, 4H), 7.25 (s, 1H), 7.12 (d,
J=5.4 Hz), 4.50 (t, J=6.9 Hz, 2H), 3.83 (t, J=6.8 Hz, 1H), 3.34-3.28 (m, 1H ), 3.15-3.08 (m, 1H),
2.73-2.60 (m, 2H), 2.04 (s, 3H).
(DMSO-d6 + D2O): ꢀ: 8.68 (d, J=1.9 Hz, 1H), 8.40 (s, 1H), 8.34 (d, J=1.6 Hz, 1H), 7.70 (d,
J=4.0 Hz, 4H), 7.47 (d, J= 3.5 Hz, 1H), 6.49 (d, J=3.4 Hz, 1H), 4.53 (t, J=7.0 Hz, 2H), 3.87 (t,
J=6.5 Hz, 1H), 3.37-3.30 (m, 1H), 3.20-3.13 (m, 1H), 2.71-2.64 (m, 2H), 2.05 (s, 3H).
(D2O): ꢀ: 8.19 (s, 1H), 7.74-7.68 (m, 3H), 7.41 (d, J=8.6Hz, 2H), 6.75 (d, J=9.0Hz, 1H), 6.63 (d,
J=7.2Hz, 1H), 4.55 (t, J=6.0Hz, 2H), 4.20-4.16 (m, 1H), 3.79 -3.74 (dd, J=14.2 and 4.0Hz, 1H),
3.51-3.46(dd, J=14.2 and 10.0Hz, 1H), 2.80 (t, J=8.5Hz, 2H), 2.39 (q, J=8.5Hz, 2H), 2.10 (s,
3H).
3g
3h
3i
8.2 (19)
7.5 (30)
8.5 (22)
8.6 (28)
545.19
516.09
510.14
600.04
(D2O): ꢀ: 8.17 (s, 1H), 7.69 (t, J=8.9Hz, 1H), 7.71 (d, J=8.8Hz, 2H), 7.41(d, J=8.8Hz, 2H), 6.72
(d, J=8.9Hz, 1H), 6.59 (d, J=7.1Hz, 1H), 4.55 (t, J=6.0Hz, 2H), 4.20-4.16 (m, 1H), 3.79-3.74
(dd, J=14.2Hz and 4Hz, 1H), 3.51-3.46 (dd, J=14.2 and 10.0Hz, 1H), 2.93 (s, 3H), 2.83 (t,
J=7.2Hz, 2H), 2.40 (q, J=6.6Hz, 2H), 2.15 (s, 1H).
(DMSO-d6): ꢀ: 8.51 (s,1H), 8.26 (t, J=6.0 Hz, 1H), 7.86 (d, 1H), 7.74 (t, J=8.6 Hz, 1H), 6.87
(s,2H), 6.73-6.65 (dd, J=8.6Hz, 2H), 4.47 (t, J=6.9 Hz, 2H), 3.98-3.91 (m, 1H), 3.60-3.51 (m,
1H), 3.45-3.34 (m, 1H), 2.68 (t, J=7.2 Hz, 2H), 2.53 (s, 6H), 2.21 (q, J=7.2Hz, 2H), 2.19 (s,
3H).
(D2O): ꢀ: 8.26 (s,1H), 7.74-7.71 (dd, J=7.3 Hz, 1H), 7.50-7.41 (m, 1H), 6.98 (t, J=9.1Hz, 2H),
6.77 (d, J=7.5Hz, 1H), 6.65 (d, J=7.5Hz, 1H), 4.60 (t, J=6.5Hz, 2H), 4.40-4.35 (m, 1H), 3.85-
3.79 (dd, J=14.1 and 4.2Hz, 1H), 3.64-3.56 (dd, J=14.1 and 9.5Hz, 1H), 2.84 (t, J=7.3Hz, 2H),
2.42 (q, J=7.2 Hz, 2H).
3j
(DMSO-d6), ꢀ: 8.52 (s, 1H), 8.27 (t, J=6.0Hz, 1H), 8.23 (d, J=8.2Hz, 1H), 7.80 (s, 2H), 7.76 (t,
J=8.5Hz, 1H), 7.76 -7.50 (dd, J=8.5 and 84.6Hz, 4H), 6.83 (d, J=8.8Hz, 1H), 6.71 (d, J=7.3Hz,
1H), 4.49 (t, J=6.9Hz, 2H), 4.00 (t, J=6.2Hz, 1H), 3.62-3.50 (m, 1H), 3.40-3.25 (m, 1H), 2.74 (t,
J=7.3Hz, 2H), 2.30 (q, J=7.6Hz, 2H).
aAcetonitrile percentage in the mobile phase is indicated in brackets.
Affinity to ꢀvꢁ3 Integrin
cophore to assume the proper spatial orientation into the re-
ceptor site. As shown in Table 2, these analogs, with the ex-
The series of analogs synthesized can be divided in two
groups. The first group comprises compounds 3a-e, which
have in common the C-terminus, but differ for the N-
terminus, where the triazole scaffold is directly linked to
aminopyridine in different positions. None of these com-
pounds proved to have significant affinity to the receptor.
We observed that, in the structures of this group, the two
heterocycles are very close together; thus, it is plausible that
the steric bulk and/or rigidity of the amidino-like moiety has
restricted the available space in the site of the receptor too
severely, resulting in a mismatched conformation.
ception of the highly insoluble 3j, exhibited excellent affin-
ity toward ꢀvꢁ3 integrin, comparable to Cilengitide,
c(RGDfMeV). Since this integrin has been shown to mediate
adhesion of osteoclasts to the bone matrix, the structures of
this group can be considered interesting leads for further
development of drug-like compounds, potentially useful
against osteoporosis.
MATERIALS AND METHODS
Amino-bromo- or amino-nitro-pyridines, bromo-
azaindole, substituted benzenesulfonyl- chlorides, propiolic
and pentynoic acids, protected glutamic acid were purchased
from Sigma-Aldrich. N-ꢁ-Boc-N-ꢂ-Alloc-L-diaminopro-
pionic acid was purchased from PolyPeptide Laboratories,
and, after protection as t-butyl ester and removal of Boc, was
coupled with the arylsulfonyl chloride to obtain compound 4.
2-methylamino-6-bromopyridine was prepared from the 2-
As a consequence of these considerations, we designed a
second group of compounds, 3f-j, where the triazole is
linked to aminopyridine through a flexible methylene chain,
and they differ from each other for the aryl-substituted C-
terminus. In this case, we hypothesized the simple carbon
chain (similar to that of arginine in the parent peptide) to
create some degrees of freedom, allowing the basic pharma-

RGD Peptidomimetics
Letters in Drug Design & Discovery, 2011, Vol. 8, No. 5 405
containing cyclic pentapeptides as highly potent and selective in-
tegrin ꢁvꢂ3 antagonists. J.Am. Chem. Soc., 1996, 118, 7461-7472.
Haubner, R.; Finsinger, D.; Kessler, H. Stereoisomeric peptide
libraries and peptidomimetics for designing selective inhibitors of
ꢁvꢂ3 integrin for a new cancer therapy. Angew. Chem. 1997, 36,
1374-1389.
Table 2. Inhibition of Echistatin Binding to ꢀvꢁ3 Integrin
Receptor
[3]
Entry
IC50, nM
Cilengitide
18.9 ± 3.1
›1000
[4]
[5]
[6]
Cacciari, B.; Spalluto, G. Non peptidic ꢁvꢂ3 antagonists: recent
developments. Curr. Med. Chem., 2005, 12, 51-70.
3a
3b
3c
3d
3e
3f
Kolb, H.C.; Sharpless, K.B. The growing impact of click chemistry
on drug discovery. Drug Discov. Today 2003, 8, 1128-1137.
Yamamoto, M.; Fischer, J.E.; Gentile, M; Seedor, J.G.; Len, C.T.;
Rodan, S.B.; Rodan, G.A. The integrin ligand echistatin prevents
bone loss in ovarictomized mice and rats. Endocrinology, 1998,
139, 1411-1419.
›1000
›1000
›1000
[7]
Miller, W.H.; Bondinell, W.E.; Cousins, R.D.; Erhard, K.F.; Jakas,
D.R.; Keenan, R.M.; Ku, T.W.; Newlander, K.A.; Ross, S.T.;
Haltiwanger, R.C.; Bradbeer, J.; Drake, F.H.; Gowen, M.;
Hoffman, S.J.; Hwang, S.M.; James, J.E.; Lark, M.W.; Lekhowska,
B.; Rieman, D.J.; Stroup, G.B.; Vasko-Moser, J.A.; Zembryki,
D.L.; Azzarano, L.M.; Adams, P.C.; Salyers, K.L.; Smith, B.R.;
Ward, K.W.; Johanson, K.O.; Huffman, W.F. Orally bioavailable
nonpeptide vitronectin receptor antagonists with efficacy in an os-
teoporosis model. Biorg. Med. Chem. Lett. 1999, 9, 1807-1812.
Pitts, W.J.; Wityak, J.; Smallheer, J.M.; Tobin, A.E.; Jetter, J.W.;
Buynitski, J.S.; Harlow, P.P.; Solomon, K.A.; Corjay, M.H.;
Mousa, S.A.; Wexler, S.; Jadhav, P.K. Isoxazolines as potent an-
tagonists of the integrin ꢁvꢂ3. J. Med. Chem. 2000, 43, 27.40.
Meissner, R.S.; Perkins, J.J.; Duong, L.T.; Hartman, g.d.; Hoffman,
W.F.; Huff, J.R.; Ihle, N.C.; Leu, C.T.; Nagy, R.M. ; Naylor-Olsen,
A.; Rodan, S.B.; Whitman, D.B.; Wesolowski, G.A.; Duggan, M.E.
Nonpeptide ꢁvꢂ3 antagonists. Part 2: constrained glycyl amides de-
rived from the RGD tripeptide. Biorg. Med. Chem. Lett. 2002, 12,
25-29.
Coleman, P.J.; Brashear, K.M.; Hunt, C.A.; Hoffman, W.F.;
Hutchinson, J.H.; Breslin, M.J.; McVean, C.A.; Askew, B.C.;
Hartman, G.D.; Rodan, S.G.; Rodan, G.A.; Leu, C.T.; Libby, L.A.;
Merkle, K.M.; Stump, G.L.; Wallace, A.A.; Lynch, J.J.; Lynch, R.;
Duggan, M.E. Nonpeptide ꢁvꢂ3 antagonists. Part 3: identification of
potent RGD mimetics incorporating novel ꢂ-amino acids as aspar-
tic acid replacements. Biorg. Med. Chem. Lett. 2002, 12, 31-34.
Keenan, R.M.; Miller, W.H.; Barton, L.S.; Bondinell, W.E.; Cous-
ins, R.D.; Eppley, D.F.; Hwang, S.M.; Kwon, C.; Lago, M.A.;
Nguyen, T.T.; Smith, B.R.; Uzinskas, I.N.; Yuan, C.K. Orally
bioavailable nonpeptide vitronectin receptor antagonists containing
2-aminopyridine arginine mimetics. Biorg. Med. Chem. Lett. 1999,
9, 1801-1806.
Duggan, M.E.; Duong, L.T.; Fisher, J.E.; Hamill, T.G.; Hoffman,
W.F.; Huff, J.R.; Ihle, N.C.; Leu, C.T.; Nagy, R.M. ; Perkins, J.J.;
Rodan, S.B.; Wesolowski, G.; Whitman, D.B.; Zartman, A.E.; Ro-
dan, G.A.; Hartman, G.D. Nonpeptide ꢁvꢂ3 antagonists. 1. trans-
formation of a potent, integrin-selective ꢁ IIbꢂ3 antagonist into a po-
tent ꢁvꢂ3 antagonist. J. Med. Chem. 2000, 43, 3736-3745.
Trabocchi, A.; Menchi, G.; Cini, N.; Bianchini, F.; Raspanti, S.;
Bottoncetti, A.;Pupi, A.; Calorini, L.; Guarna, A. Click-chemistry-
derived triazole ligands of arginine-glycine- aspartate (RGD) in-
tegrins with a broad capacity to inhibit adhesion of melanoma cells
and both in vitro and in vivo angiogenesis. J. Med. Chem. 2010, 53,
7119-7128.
Barral, K.; Moorhouse, A.D.; Moses, J.E. Efficient conversion of
aromatic amines into azides. Org. Lett. 2007, 9, 1809-1811.
Chinchilla, R.; Najera, C. The sonogashira reaction; a booming
methodology in synthetic organic chemistry. Chem. Rev. 2007,
107, 874-922.
›1000
14.28 ± 2.3
15.70 ± 0.7
13.60 ± 0.9
17.20 ± 2.1
52.80 ± 0.7
3g
3h
3i
3j
[8]
[9]
amino-6-bromopyridine with paraformaldeyde by reductive
alkylation. Microwave-assisted reactions were performed in
a CEM MW instrument. When necessary, crude products
were purified by flash chromatography on silicagel Merck
230-400 mesh. Purity of compounds was detected by RP-
HPLC (column: Gemini C18, 5 ꢀ, 4.6 X 250, Phenome-
nex®; mobile phase, acetonitrile/water + 0.1% TFA). Mass
spectra were recorded on a Bruker Micro-Tof instrument.
1H-NMR spectra were recorded on a AC300 Bruker instru-
ment.
[10]
[11]
[12]
[13]
Synthesis of Peptidomimetics
General Procedure
The desired couple of building blocks 1a-f and 2a-e, con-
taining an azido or an alkyne functional group, respectively,
were dissolved with a mixture of t-BuOH / water , 0.1 equiv
of sodium ascorbate was added followed by 0.01 equiv of
copper sulfate. In the case of 3a-e, a three fold excess of salts
and repeated microwave irradiation (100 W for two minutes)
were necessary to complete the reaction. After about 2h,
solvent was evaporated and the crude products 3a-j purified
by flash chromatography. Removal of the tert-butyl ester
with 50% TFA in DCM afforded the final products with up
to 80% yields for the two steps, and purity >95% on analyti-
cal RP-HPLC.
[14]
[15]
Binding to ꢀvꢁ3 Integrin
Binding tests were performed according to the method of
Kumar et al. [17], following the procedure already published
[18].
[16]
[17]
Alper, P.B.; Hung, S.C.; Wong, C.H. Metal catalyzed diazo transfer
for the synthesis of azides from amines. Tetrahedron Lett. 1996,
37, 6029-6032.
Kumar, C.C.; Nie, H.; Rogers, C.P.; Malkowski, M.; Maxwell, E.;
Catino, J.J.; Armstrong, J. Biochemical characterization of the
binding of the Echistatin to integrin ꢁvꢂ3 and ꢁvꢂ5 Receptor. J.
Pharmacol. Exp. Ther. 1997, 283, 843-853.
REFERENCES
[18]
Dal Pozzo, A.; Ni, M.H.; Esposito, E.; Dallavalle, S.; Musso, L.;
Bargiotti, A.; Pisano, C.; Vesci, L.; Bucci, F.; Castorina, M.; Fod-
erà, R.; Giannini, G.; Penco, S. Novel tumor-targeted RGD pep-
tide-camptothecin conjugates: synthesis and biological evaluation.
Biorg. Med. Chem. 2010, 18, 64-72.
[1]
Ruoslahti, E.; Pierschbacher, M.D. New perspective in cell adhe-
sion: RGD and integrins. Science, 1987, 238, 491-497.
Haubner, R.; Gratias, R.; Diefenbach, B.; Goodman, S.L.; Jonczick,
A.; Kessler, H. Structural and functional aspects of RGD-
[2]