Bisphosphonate-functionalized cyclic Arg-Gly-Asp peptidomimetics

Article abstract of DOI:10.3998/ark.5550190.0014.215

We report the synthesis of three new conjugates between a cRGD integrin ligand and alendronic acid as a bisphosphonate anchor. The selected ligand is an RGD peptidomimetic, carrying the conformationally constrained RGD sequence on an azabicycloalkane scaf

Full text of DOI:10.3998/ark.5550190.0014.215

Issue in Honor of Prof Richard R. Schmidt
ARKIVOC 2013 (ii) 185-200
Bisphosphonate-functionalized cyclic Arg-Gly-Asp peptidomimetics
Carmelo Drago,a Daniela Arosio,b Cesare Casagrande,c and Leonardo Manzoni*b
a
Università degli Studi di Milano, Centro Interdipartimentale Studi Biomolecolari
e Applicazioni Industriali, Via Fantoli 16/15, Milano, I-20138, Italy
Consiglio Nazionale delle Ricerche, Istituto di Scienze e Tecnologie Molecolari, Via Golgi 19,
b
Milano, I-20133, Italy
c Università degli Studi di Milano, Dipartimento di Chimica, Via Golgi 19, I-20133 Milano, Italy
E-mail: Leonardo.manzoni@istm.cnr.it
Dedicated to Professor Richard R. Schmidt on the occasion of his 78th anniversary
Abstract
We report the synthesis of three new conjugates between a cRGD integrin ligand and alendronic
acid as a bisphosphonate anchor. The selected ligand is an RGD peptidomimetic, carrying the
conformationally constrained RGD sequence on an azabicycloalkane scaffold and endowed with
high affinity for integrin ανβ3. Since integrin ανβ3 is involved in the adhesion of osteoprogenitor
cells and of osteoblasts, these constructs could prove useful as new tools for biofunctionalization
of materials in biomedical application and for development of modern implants.
Keywords: Arg-Gly-Asp, integrins, bisphosphonate, peptidomimetics
Introduction
Osseointegration of implants is known to be a biological process that occurs by formation of a
direct structural and functional connection between ordered living bone and the surface of a load
carrying implant, without intervening soft tissue.1 Surface modifications have been shown to
enhance osseointegration at early implantation times in terms of both velocity and intensity of
bone formation. For example, rough surfaces can stimulates differentiation, growth and
attachment of bone cells, and increases mineralization. The main methods reported in the
literature to create implant roughness are acid etching, sandblasting, titanium plasma spraying
and hydroxyapatite (HA) coating.2 Furthermore, the biofunctionalization of implant surfaces, by
adding different substances to improve their biological characteristics, appear an important tool
to enhance the capacity of osseointegration.3 A common theme in this field is the modification of
the material in order to promote selectively interactions with a specific cell type through
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biomolecular recognition events. Typically, peptides containing the cell-binding domains found
in the extracellular matrix proteins are immobilized on the material to promote cell adhesion via
ligand receptor interaction.4 Integrins are an example of cell adhesion receptors that bind to
specific amino acid sequences, such as the RGD (Arg-Gly-Asp) sequence that is found in type I
collagen, fibronectin, osteopontin and bone sialoprotein. Studies of collagen-coated surfaces5
have confirmed the improved ability of extra cellular matrix-like surfaces to integrate implants
into tissues, and the relevance of the RGD-motif in the adhesion of osteoprogenitor cells and of
osteoblasts. The results have prompted research for the incorporation of RGD features into
synthetic polymers suitable for implant coating6 and also for the direct linking of RGD-carrying
sequence to the implant material,7 overcoming technological and biological problems of natural
proteins. In particular, some papers explore the use of non-peptidic integrin ligands,8 or of cyclic
peptide structure carrying phosphonate groups on titanium or bone surfaces.
Recently, we have designed and synthesized cyclic peptidomimetic ligands which contain the
RGD recognition sequence embedded into novel azabicycloalkane scaffolds.9 These studies have
produced some specific ligands with nanomolar affinity for αvβ3 and αvβ5 integrins, which are
currently being profiled as anti-angiogenetic agents.10 Among them, compound DB58 (Figure 1)
contains a site suitable for conjugation to different functional units,11 which has been derivatized
for application in medical diagnosis and therapy.12
Our aim was the development of new coatings able to conjugate the αv-specific cyclic-RGD
peptide to titanium implants or to hydroxyapatite-like objects or bone cements, using a
methylene-bis-phosphonate moiety as anchor system. For practical reasons, such as commercial
availability, biological characterization, and the presence in the same molecule of both the
anchor and the conjugable functionalities, we chose alendronic acid (alendronate) as source of
diphosphonic moiety. Sodium alendronate is a commonly used antiresorptive pharmacological
agent that has been shown to be effective in osteopenic women for reducing bone resorption,
increasing bone density, and decreasing fracture incidence.13 Furthermore, studies on the effect
of alendronate on implant surface has been already reported in the literature.14 The capacity of
phosphonic acid groups to bind strongly over a large pH range (pH 1-9) to TiO2,15 and the
presence of two phosphonate groups can improve the binding of the coating molecule to the Ti
surface.
In this experimental study the alendronate will be bridged with the cyclic-RGD peptide by
spacers that differ for length, hydrophilicity or hydrophobicity, flexibility, as well as the type of
conjugation.
Results and Discussion
The cyclic RGD peptidomimetic DB58 (Figure 1) which binds to αvβ3 and αvβ5 integrin receptors
was developed by our group previously.11b The replacement of the
D
-Phe-Val dipeptide in the
lead structure cyclo(RGDfV)16 with an azabicycloalkane scaffold showing reverse-turn mimetic
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properties, constrains the RGD sequence into pre-organized conformations and provides the
required affinity and selectivity for integrin antagonism. The functionalization of the scaffold
with an heteroalkyl substituent enabled us to conjugate the RGD containing ligand with different
diagnostic or therapeutic entities without losing activity.12
O
O
Arg
Gly
HN
N
HO
Arg
Gly
O
O
HN
HN
Asp
Asp
c(RGDfV)
DB58
Inhibition of biotinylated
vitronectin binding
IC50 αvβ3 22.6 3.5 nM
OH
OH
O
+
P
HO
+
IC50 αvβ3 205 33.5 nM
H2N
OH
P
OH
O
1 Alendronic acid
HN
7
8
9
O
6
3
NH2
5
4
N
N
H
HN
X
10
O
2 X = N3
3 X = NH2
O
HN
HN
H
N
O
O
O
OH
Figure 1. Integrin ligands and bisphosphonate anchor moiety.
As mentioned above, we selected alendronic acid 1 as the anchor and prepared three
compounds differing for the distance between the anchor moiety and the targeting groups, for the
type of conjugation and for the hydrophilicity of the linkers. In compound 6 the anchor group is
directly conjugated to the RGD-carrying moiety, whereas in compounds 12 and 16 two linkers of
different length are introduced. Compound 6 is accessible in a three step synthesis starting from
4-pentynoic acid 4, activated as succinimidyl ester followed by coupling with alendronic acid 1.
The regioselective copper-catalyzed Huisgen 1,3-dipolar cycloaddition between an azide and a
terminal alkyne, often referred to as “click reaction”, has been reported to proceed in very good
yields in t-butanol/water, a solvent mixture that appeared ideal for dissolution of both linker and
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cyclopeptide. Thus, the “click” reactions of azide 2, with the terminal alkyne present in
compound 5, (CuSO4, Na
-ascorbate, t-BuOH, H2O)17 proceeded without drawback reaching
L
completion within 18 hours to give the corresponding triazole conjugates 6 in 80 % yield after
RP-HPLC purification. (Scheme 1)
Scheme 1. i) NHS, DIC, DCM dry, r.t. 18 h, 1, H2O, CH3CN, NaOH until pH 7.2-7.4, 0 °C, 18
h, 70% over two steps; ii) Na-L-ascorbate, t-BuOH, H2O, CuSO4, r.t., 18 h, 80%.
In the case of compound 12, an appropriate linker has been synthesized. The commercially
available 4,7,10-trioxa-1,13-tridecanediamine 7 has been monoprotected at the nitrogen atom as
the trityl derivative which was reacted with 4-pentynoic acid 4 using standard coupling
procedures to give 8 in 55% yield over two steps. Compound 8 was treated with TFA in the
presence of iPr3SiH affording the deprotected compound. The resulting free amine was coupled
with diglycolic anhydride to give the carboxylic acid 9 in 71% yield over two steps (Scheme 2).
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Scheme 2. i) TrCl, Et3N, DCM dry, r.t., 18 h; ii) 4, DIC, DCM dry, r.t., 18 h, 55% over two
steps; iii) iPr3SiH, TFA, r.t., 4 h; iv) diglycolic anhydride, pyridine, DMF, r.t., 4 h, 71% over two
steps.
Scheme 3. (i) 9, Na L-ascorbate, t-BuOH, H2O, CuSO4, r.t., 18 h; (ii) NHS, DIC, DCM dry, r.t.,
18 h, 1, CH3CN, H2O, NaOH until pH 7.2-7.4, 0 °C, 18 h, 55% over two steps; (iii) TFA,
thioanisole, iPrSiH, ethanediol, phenol, water, r.t., 18h, 84%.
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With compound 9 in hand, the next task was to couple the linker and the integrin ligand to
create the desired triazolyl-linked conjugates. An attempt to perform the click reaction between
compound 9 and the unprotected integrin ligand 2 afforded the desired product in poor yield.
Better results were obtained using the protected integrin ligand. Thus, completely protected azide
10 has been “clicked” with linker 9 to give the corresponding triazolyl conjugate. Activation of
the carboxylic acid with N-hydroxysuccinimide (NHS) and N,N′-diisopropylcarbodiimide (DIC)
followed by the addition of alendronate 1 gave the desired conjugate protected compound 11 in
55% over three steps. Finally, the side chain protecting group has been removed by TFA in
presence of scavengers (thioanisole, phenol, iPr3SiH) to give the desired compound 12 in 84%
yield (Scheme 3).
Scheme 4. (i) 14, HOBt, EDC, DIPEA, DCM dry, r.t. 2h; (ii) H2, Pd/C, MeOH, r.t., 2h; (iii)
diglycolic anhydride, DMF, pyridine, r.t., 4h, 52% over three steps; (iv) NHS, DIC, DCM dry,
r.t., 18h, 1, CH3CN, H2O, NaOH until pH 7.2-7.4, 0°C, 18 h, 84% over two steps; (v) TFA,
thioanisole, iPrSiH, ethandiol, phenol, water, r.t., 18h, 93%.
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In the case of the last compound, the known linker 1412d was linked with the amino
derivative integrin ligand 1311b using N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide
hydrochloride (EDC), 1-hydroxybenzotriazole (HOBt) and N,N-diisopropylethylamine (DIPEA)
as coupling agents, followed by hydrogenation on Pd/C under standard condition to remove the
benzyloxy carbonyl protecting group on the nitrogen. The resulting free amine was reacted with
diglycolic anhydride to obtain the carboxylic acid 15 in 52% yield over three steps, ready for
following reactions. Coupling of the intermediate 15 and alendronic acid 1 using standard
reagents, followed by side chain protecting groups removal by TFA in presence of scavengers,
gave the final product 16 in 78% yield over three steps and after RP-HPLC purification (Scheme
4).
Receptor binding assay
The derivatives 6, 12, and 16 were examined in vitro for their abilities to inhibit the binding of
biotinylated vitronectin to the isolated, immobilized αvβ3 and αvβ5 integrin receptors (Table 1).
Affinities of commercially available compound c(RGDfV) and our starting compound DB58
were determined as references, in the same assays. The functionalized cyclic pentapeptides 6, 12,
and 16 displayed a binding affinity toward αvβ3 integrin in the nanomolar range, comparable or
slightly lower than unconjugated DB58.
Although some differences in the binding affinities, probably due to the different nature and
length of the linkers, all the three synthesized compounds still interact well with integrin αvβ3,
thus confirming that the functionalization of the scaffold does not influence the binding activity.
Table 1. Inhibition of biotinylated vitronectin binding to αvβ3 and αvβ5 receptorsa
Entry
Compound
IC50 (nM) ± SD for αvβ3
3.0 ± 1.6
IC50 (nM) ± SD for αvβ5
17.4 ± 3.3
1
2
3
4
5
c(RGDfV)
DB58
6
22.6 ± 3.5
205 ± 33.5
50.3 ± 13.4
475 ± 44
242.6 ± 62.9
20.5 ± 15.3
1979 ± 835
721 ± 99
12
16
a
IC50 values were calculated as the concentration of compound required for 50% inhibition of
biotinylated vitronectin binding as estimated by GraphPad Prism 5 program. All values are the
mean (± standard deviation) of triplicate determinations.
Conclusions
In conclusion: we have synthesized three new conjugates between an integrin ligand and a
methylene-bis-phosphonate anchor, choosing alendronic acid, to improve binding to the surface
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of appropriate materials, such as titanium or hydroxyapatite. On the basis of the results obtained
in the solid phase receptor binding assay, the conjugation of a biologically relevant entity on the
appendage of the azabicycloalkane scaffold does not affect the affinity of the RGD ligand toward
the integrin receptors.
Thus, these new conjugates could be used for biofunctionalization of materials in biomedical
applications, and could find application in the development of modern implants. Biological
evaluation in vitro and in vivo of these new compounds will be reported in due course.
Experimental Section
General. All chemicals and solvents were of reagent grade and were used without further
purification. Solvents were dried by standard procedures and reactions requiring anhydrous
1
13
conditions were performed under nitrogen or argon atmosphere. H and C NMR spectra were
recorded at 300 K on a Bruker Avance-600 or Bruker Avance-400 spectrometer. Chemical shifts
1
13
31
δ for H and C are expressed in ppm relative to internal Me4Si as standard. P NMR spectra
were recorded on a 400 MHz spectrometer operating at 162 MHz, with complete proton
31
decoupling. P NMR spectroscopic chemical shifts are reported in ppm (δ) relative to external
85% H3PO4 at 0 ppm. Signals were abbreviated as s, singlet; bs, broad singlet; d, doublet; t,
triplet; q, quartet; m, multiplet. Mass spectra were obtained with Agilent 1100 analytical HPLC
equipped with diode array detector and Bruker ion-trap Esquire 3000+ with ESI. Thin layer
chromatography (TLC) was carried out with pre-coated Merck F254 silica gel plates. Elemental
analyses were performed by the staff of the microanalytical laboratory of our department. Flash
chromatography was carried out with Macherey-Nagel silica gel 60 (230-400 mesh) or using SP1
(single-column) Biotage flash purification system (with C18 cartridges). Preparative HPLC was
performed using MS-based preparative HPLC system Waters. Semi-preparative HPLC was
carried out on a Waters SymmetryPrep C18-7µm 7.8 × 300 mm column or a Waters Atlantis C18
OBD 5µm 19 mm × 10 cm.
Synthesis of compound 5. 4-Pentynoic acid (98.1 mg, 1.0 mmol) was dissolved in dry CH2Cl2
(10 ml), and then N-hydroxysuccinimide (NHS) (115.09 mg, 1.0 mmol) and N,N′-
diisopropylcarbodiimide (DIC) (387 µl, 2.5 mmol) were sequentially added. The reaction
mixture was stirred at room temperature overnight. After reaction completion, the solvent was
removed under reduced pressure. The crude was left under high vacuum. The crude product was
diluted in CH3CN (20 ml) and the corresponding solution was slowly added to a solution of
alendronic acid trihydrate (975 mg, 3 mmol) (solubilized with water (8 ml) and the pH of the
corresponding solution was adjusted to pH 7.2-7.4 adding a 0.1 M water solution of NaOH,
finally to the mixture was added CH3CN (24 mL)) at 0 °C using a syringe pump (40 µl/min). The
reaction mixtures was left under stirring 18 h at 0 °C. After reaction completion, the crude
product was precipitated by addition of ethanol to obtain 5 as a dirty white solid (277 mg, 0.84
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mmol, 70 %). 1H NMR (400 MHz, D2O): δ: 1.85-1.75 (m, 2H), 1.95-1.85 (m, 2H), 2.34 (t, J 2.0
13
Hz, 1H), 2.41 (m, 4H), 3.18 (t, J 6.8 Hz, 2H); C NMR (100 MHz, CDCl3): 174.6, 83.6, 70.3,
40.2, 34.6, 31.2, 23.4, 14.6. Anal. Calcd for C9H17NO8P2 (329.04): C, 32.84; H, 5.21; N, 4.26%.
Found: C, 32.88; H, 5.30; N, 4.26%.
Synthesis of compound 6. A 0.9 M water solution of sodium ascorbate (45 µl, 0.4 mmol) and a
0.3 M water solution of CuSO4 (65 µl, 0.02 mmol) were sequentially added to stirred solution of
compound 5 (32.9 mg, 0.10 mmol) and compound 2 (56.4 mg, 0.10 mmol) in a 1:1 mixture of
H2O/t-BuOH (300 µl). The reaction mixture was stirred overnight at room temperature and then
the solvent was removed under reduced pressure. Finally, the residue was purified by
chromatography on a C18 reversed phase semi-preparative hplc column and then lyophilized.
HPLC eluant conditions: from 98% of H2O (0.1% TFA) and 2% of CH3CN (0.1% TFA) to 70%
of H2O (0.1% TFA) and 30% of CH3CN (0.1% TFA), flow rate 12 ml/min., 30 min. runs, giving
pure 6 (71.4 mg, 0.08 mmol, 80%).
1H NMR (400 MHz, D2O + 5% TFA): δ: 1.12 (m, 1H, H-5), 1.32-1.57 (m, 4H, 2 Hγ-Arg, Hβ-
Arg, H-7), 1.63 (m, 2H, NHCH2CH2CH2C(OH)(PO3H2)2), 1.68-1.98 (m, 4H, H-8, CH2CH2-
C(OH)(PO3H2)2, Hβ-Arg), 2.22-2.40 (m, 3H, H-5, H-7, H-8), 2.45-2.60 (m, 3H, COCH2CH2-
triazole, Hβ-Asp), 2.91 (m, 1H, Hβ-Asp), 2.93-3.10 (m, 6H, COCH2CH2-triazole, 2 Hδ-Arg,
NHCH2CH2CH2C(OH)(PO3H2)2), 3.25 (m, 1H, H-4), 3.36 (d, J 16 Hz, 1H, Hα-Gly), 3.91 (m,
1H, H-6), 4.10 (d, J 16 Hz, 1H, Hα-Gly), 4.15 (m, 1H, H-9) 4.18-4.36 (m, 3H, 2 H-10, Hα-Asp),
13
4.38-4.47 (m, 2H, H-3, Hα-Arg), 8.10 (s, 1H, H triazole); C NMR (100 MHz, D2O): δ: 174.7,
174.0, 173.0, 171.7, 169.3, 156.8, 62.2, 55.8, 52.9, 51.6, 51.3, 42.4, 40.4, 39.7, 36.2, 35.2, 32.7,
32.2, 30.1, 27.0, 24.4, 21.1; 31P-NMR (161.9 MHz, D2O): δ: 21.4. Anal. Calcd for
C31H50N12O15P2 (892.30): C, 41.71; H, 5.65; N, 18.83%. Found: C, 41.75; H, 5.64; N, 18.84%;
MS (ESI+) m/z: 893.5 (M+H+), 447.3 (M+2H+).
Synthesis of compound 8. Compound 7 (11.02 g, 50.0 mmol) was dissolved in dry CH2Cl2 (50
ml), and then TEA (209 µl, 1.5 mmol) and TrCl (278.8 mg, 1.0 mmol) were sequentially added.
The reaction mixture was stirred at room temperature 18 h. After reaction completion, the
mixtures was washed with a saturated solution of NaHCO3 (3 x 20 ml). The organic layer was
dried over Na2SO4, and then the solvent removed under reduced pressure. The crude product was
purified by flash chromatography (BiotageTM eluant conditions: 1% of MeOH and 99% of
CH2Cl2 to 10% of MeOH and 90% of CH2Cl2). Yield 85% (393 mg, 0.85 mmol) of pure
compound. Anal. Calcd for C29H38N2O3 (462.29): C, 75.29; H, 8.28; N, 6.06%. Found: C, 75.28;
H, 8.30; N, 6.05%.
The compound (462.6 mg, 1.0 mmol) was dissolved in dry CH2Cl2 and then pentynoic acid
(147.2 mg, 1.5 mmol) and DIC (310 µl, 2.0 mmol) were sequentially added. The reaction
mixture was stirred at room temperature overnight. After reaction completion, the solvent was
removed under reduced pressure and the crude product was purified by flash chromatography
(BiotageTM eluant conditions: 1% of MeOH and 99% of CH2Cl2 to 10% of MeOH and 90% of
CH2Cl2). Yield 65% (353 mg, 0.65 mmol) of pure 8.
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1H NMR (400 MHz, CDCl3): δ: 1.71 (m, 4H, -CH2CH2CH2-), 1.88 (s, 1H, C≡CH), 2.12 (bs, 2H,
Ph3NHCH2-), 2.27 (t, J 7.2 Hz, 2H, -CH2C≡CH), 2.44 (m, 2H, -CH2CO-), 3.31 (dd, J 12.0, 5.6
Hz, 2H, -CH2NHCO-), 3.6-3.35 (m, 12H, -CH2-), 6.30 (bs, 1H, -NHCO), 7.10 (t, J 7.2 Hz, 3H,
13
Ph), 7.20-7.15 (m, 6H, Ph), 7.39 (d, J 7.6 Hz, 6H, Ph); C NMR (100 MHz, CDCl3): δ: 170.8,
146.3, 128.7, 127.7, 126.2, 83.1, 70.6, 70.3, 70.3, 70.1, 70.0, 69.1, 41.0, 38.2, 38.0, 35.5, 35.4,
30.6, 28.7, 14.9. Anal. Calcd for C34H42N2O4 (542.31): C, 75.25; H, 7.80; N, 5.16%. Found: C,
75.35; H, 7.79; N, 5.15%. MS (ESI+) m/z: 543.4 (M+H+).
Synthesis of compound 9. Compound 8 (543 mg, 1.0 mmol) was treated with iPr3SiH (410 µl,
2.0 mmol) and TFA (153 µl, 2.0 ml). The reaction mixture was stirred at room temperature for 2-
4 hours. After reaction completion, TFA was removed under reduced pressure and the crude
product was diluted with water and the mixures was exctracted with iPr2O (3 X 20 ml). Water
was removed under reduced pressure and the crude product was used on the following step
without any further purification.
The crude product was dissolved in DMF and then diglycolic anydride (232 mg, 2.0 mmol) and
pyridine (323 µl, 4.0 mmol) were sequentially added. The reaction mixture was stirred at room
temperature for 4 hours. After reaction completion, water was added to the reaction mixture.
Finally, the solvent was removed under reduced pressure and the crude product was purified by
flash chromatography on Biotage using a C18 reverse column (BiotageTM eluant conditions: 1%
of CH3CN and 99% of H2O to 100% of CH3CN and 0% of H2O). Yield 71% (295 mg, 0.71
mmol) of pure 9.
1H NMR (400 MHz, CDCl3): δ: 1.82 (m, 4H, -CH2CH2CH2-), 2.02 (m, 1H, C≡CH), 2.46 (m, 2H,
-CH2C≡CH), 2.54 (m, 2H, -CH2CO), 3.41 (m, 4H, -CH2NH), 3.67-3.55 (m, 12H, -CH2-),4.11 (d,
J 3.6 Hz, 2H, -CO-CH2O-),4.19 (d, J 4 Hz, 2H, -CO-CH2O-), 6.77 (bs, 1H, -NH), 7.64 (bs, 1H, -
13
NH); C NMR (100 MHz, CDCl3): δ: 172.0, 171.8, 1697, 82.9, 71.7, 70.4, 70.3, 70.2, 70.0,
69.9, 69.5, 69.4, 69.3, 42.6, 38.2, 36.9, 35.3, 29.0, 28.7, 14.9. Anal. Calcd for C19H32N2O8
(416.22): C, 54.80; H, 7.74; N, 6.73%. Found: C, 54.85; H, 7.75; N, 6.72%. MS (ESI+) m/z:
417.3 (M+H+).
Synthesis of compound 11. A 0.9 M water solution of sodium ascorbate (45 µl, 0.4 mmol) and a
0.3 M water solution of CuSO4 (65 µl, 0.02 mmol) were sequentially added to stirred solution of
compound 10 (83.2 mg, 0.10 mmol) and of the alkyne 9 (41.6 mg, 0.10 mmol) in a 1:1 mixture
of H2O/tBuOH (300 µl). The reaction mixture was stirred 18 h at room temperature and then the
solvent was removed under reduced pressure. Finally, the residue was purified by BiotageTM
flash chromatography on a C18 reverse column (BiotageTM eluant conditions: 1% of CH3CN and
99% of H2O to 100% of CH3CN and 0% of H2O). Yield 65% (81.2 mg, 0.65 mmol).
Anal. Calcd for C55H85N13O18S (1247.59): C, 52.91; H, 6.86; N, 14.59%. Found: C, 52.95; H,
6.87; N, 14.61%. MS (ESI+) m/z: 1249.1 (M+H+), 625.4 (M+2H+).
The above compound (81.2 mg, 0.065 mmol) was dissolved in dry CH2Cl2 (1 ml), and then NHS
(7.5 mg, 0.065 mmol) and DIC (25.2 µl, 0.1625 mmol) were sequentially added. The reaction
mixture was stirred at room temperature overnight. After reaction completion, the solvent
removed under reduced pressure. The crude was left under the high vacuum. The crude product
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was diluted in CH3CN (2 ml) and the corresponding solution was slowly added to a solution of
alendronate (63.4 mg, 0.195 mmol, was solubilized with 4 mL of water and the pH of the
corresponding solution was adjusted to pH 7.2-7.4 adding a 0.1 M water solution of NaOH,
finally to the mixture were added 12 mL of CH3CN) at 0 °C using a syringe pump (40 µl/min).
The reaction mixtures was left under stirring over night at 0 °C. After reaction completion, the
solvent was removed under reduced pressure and the crude product was purified by HPLC using
a C18 reverse column (HPLC eluant conditions: from 90% of H2O (0.2% TFA) and 10% of
CH3CN (0.2% TFA) to 0% of H2O (0.2% TFA) and 100% of CH3CN (0.2% TFA), flow rate 12
ml/min., 30 min. runs). Yield 84% (80.8 mg, 0.0546 mmol) of pure protected compound.
1H NMR (400 MHz, D2O): δ: 1.24 (m, 1H, H-5), 1.32-1.48 (m, 12H, 2 Hγ-Arg, Hβ-Arg,
COC(CH3)3), 1.58 (m, 1H, H-7), 1.64 (m, 2H, OCH2CH2CH2NH), 1.71-1.87 (m, 6H,
OCH2CH2CH2NH, NHCH2CH2CH2C(OH)(PO3H2)2, H-8, Hβ-Arg), 1.97 (m, 2H, NHCH2CH2-
CH2C(OH)(PO3H2)2), 2.06 (s, 3H, CH3 Mtr), 2.25-2.38 (m, 3H, H-5, H-7, H-8), 2.50 (s, 3H, CH3
Mtr), 2.53-2.64 (m, 6H, CH3 Mtr, COCH2CH2-triazole, Hβ-Asp), 2.92 (dd, J 7.6 Hz, J 16.4 Hz,
1H, Hβ-Asp), 2.98 (m, 2H, COCH2CH2-triazole), 3.07-3.19 (m, 4H, 2 Hδ-Arg,
OCH2CH2CH2NH), 3.19-3.23 (m, 5H, H-4, NHCH2CH2CH2C(OH)(PO3H2)2, OCH2CH2-
CH2NH), 3.43 (t, J 6.2 Hz, 2H, OCH2CH2CH2NH), 3.48 (m, 1H, Hα-Gly), 3.51 (t, J 6.2 Hz, 2H,
OCH2CH2CH2NH), 3.54-3.66 (m, 8H, OCH2), 3.81 (s, 3H, OCH3 Mtr), 3.95 (m, 1H, H-6), 4.05
(s, 2H, OCH2CO), 4.06 (s, 2H, OCH2CO), 4.12-4.17 (m, 3H, H-9, H-10, Hα-Gly), 4.32-4.43 (m,
13
4H, H-10, Hα-Asp, H-3, Hα-Arg), 6.75 (s, 1H, H Mtr), 7.85 (s, 1H, H triazole); C NMR (100
MHz, D2O): δ: 174.7, 173.7, 172.0, 171.6, 169.2, 112.8, 83.1, 70.0, 69.6, 69.4, 68.6, 68.4, 62.2,
55.8, 52.9, 52.1, 51.3, 42.5, 39.4, 36.4, 36.3, 36.2, 34.9, 34.7, 32.7, 32.3, 30.8, 29.7, 28.3, 27.3,
23.3, 23.1, 20.7, 11.4. Anal. Calcd for C59H96N14O24P2S (1478.59): C, 47.90; H, 6.54; N,
13.25%. Found: C, 47.87; H, 6.55; N, 13.28%; MS (ESI+) m/z:1480.0 (M+H+), 741.0 (M+2H+).
Synthesis of compound 12. A mixture composed of TFA (5 ml), 5% of thianisole (250 µl) 5%
triisopropylsilane (250 µL), 5% ethandiol, 5% Phenol (275 mg) and 5% water (250 µL) was
added to a stirred solution of compound 11 (80.8 mg, 0.0546 mmol). The reaction mixtures were
left stirring at room temperature overnight and then concentrated under reduced pressure. The
crude product was solubilized with water (10 ml) and washed with EtOAc (3 X 10 ml), Et2O (3
X 10 ml), iPr2O (3 × 10 ml), hexane (3 × 10 ml), DCM (3 × 10 ml), CHCl3 (3 × 10 ml), iPr2O (3
× 10 ml). Finally, water was removed under reduced pressure. When the crude was not
sufficiently pure, the residue was purified by chromatography on a C18 reversed phase semi-
preparative hplc column and then lyophilized. HPLC eluant conditions: from 98% of H2O (0.1%
TFA) and 2% of CH3CN (0.1% TFA) to 70% of H2O (0.1% TFA) and 30% of CH3CN (0.1%
TFA), flow rate 12 ml/min., 30 min. runs. Yield 84% (55.5 mg, 0.0459 mmol) of pure 12.
1H NMR (400 MHz, D2O): δ: 1.26 (m, 1H, H-5), 1.42-1.60 (m, 4H, 2 Hγ-Arg, Hβ-Arg, H-7),
1.64 (m, 2H, OCH2CH2CH2NH), 1.70-1.88 (m, 5H, OCH2CH2CH2NH, NHCH2CH2-CH2-
C(OH)(PO3H2)2, H-8), 1.90-2.08 (m, 3H, NHCH2CH2CH2C(OH)(PO3H2)2, Hβ-Arg), 2.27-2.47
(m, 3H, H-5, H-7, H-8), 2.54 (m, 2H, COCH2CH2-triazole), 2.66 (dd, J 17.2 Hz, J 6.8 Hz, 1H,
Hβ-Asp), 2.95 (m, 2H, COCH2CH2-triazole), 3.02, (dd, J 17.2 Hz, J 7.6 Hz, 1H, Hβ-Asp), 3.07-
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3.18 (m, 4H, 2 Hδ-Arg, OCH2CH2CH2NH), 3.18-3.30 (m, 5H, H-4, NHCH2CH2-
CH2C(OH)(PO3H2)2, OCH2CH2CH2NH), 3.40 (t, J 6.2 Hz, 2H, OCH2CH2CH2NH), 3.47 (m, 1H,
Hα-Gly), 3.51 (t, J 6.2 Hz, 2H, OCH2CH2CH2NH), 3.54-3.66 (m, 8H, OCH2), 4.00 (m, 1H, H-6),
4.05 (s, 2H, OCH2CO), 4.06 (s, 2H, OCH2CO), 4.14-4.28 (m, 3H, H-9, H-10, Hα-Gly), 4.33-4.47
13
(m, 3H, H-10, Hα-Asp, H-3), 4.54 (m, 1H, Hα-Arg), 7.81 (s, 1H, H triazole); C NMR (100
MHz, D2O): δ: 174.7, 174.6, 174.5, 173.9, 173.0, 171.6, 169.2, 156.8, 124.5, 70.0, 69.6, 69.4,
68.5, 68.3, 62.2, 55.8, 52.9, 51.8, 51.7, 51.3, 42.4, 40.4, 39.4, 36.4, 36.3, 36.2, 34.9, 33.3, 32.7,
31
32.3, 30.9, 30.1, 28.3, 27.0, 24.4, 23.3, 20.8; P-NMR (161.9 MHz, D2O): δ: 20.4. Anal. Calcd
for C45H76N14O21P2 (1210.48): C, 44.63; H, 6.33; N, 16.19%. Found: C, 44.68; H, 6.34; N,
16.24%; MS (ESI+) m/z 1211.7 (M+H+), 606.7 (M+2H+).
Synthesis of 15. Compound 13 (34.1 mg, 0.1 mmol) was dissolved in dry CH2Cl2 (1 ml), and
then EDC (23 mg, 0.12 mmol), HOBt (16.2 mg, 0.12 mmol) and DIPEA (70 µl, 0.4 mmol) were
sequentially added. The reaction mixture was stirred at room temperature for 15 min. before
adding the compound 12 (100.7 mg, 0.125 mmol). The reaction mixture was stirred at room
temperature overnight, and then the solvent was removed under reduced pressure, and the
residue was purified by BiotageTM flash chromatography on a C18 reverse column (BiotageTM
eluant conditions: 1% of CH3CN and 99% of H2O to 100% of CH3CN and 0% of H2O). Yield
74% (83,6 mg, 0.074 mmol).
1H NMR (400 MHz, CDCl3): δ: 8.13 (bs, 1H), 7.69 (bs, 1H), 7.27-7.15 (m, 5H), 7.00 (bs, 1H),
6.45 (s, 1H), 6.32 (m, 1H), 5.57 (m, 1H), 5.02 (m, 2H), 4.57 (m, 1H), 4.38 (m, 2H), 4.19 (m,
2H), 4.13 (m, 2H), 3.75 (s, 3H), 3.57 (m, 6H), 3.44 (m, 2H), 3.41 (m, 1H), 3.31 (m, 2H), 3.26
(m, 1H), 3.12 (m, 2H), 2.90 (m, 1H), 2.60 (s, 3H), 2.52 (s, 3H), 2.48 (m, 1H); 2.32 (m, 3H), 2.04
(s, 9H), 1.87 (m, 3H), 1.70-1.45 (m, 4H), 1.35 (s, 9H), 1.15 (m 1H); 13C NMR (100 MHz,
CDCl3): δ: 173.2, 171.4, 170.6, 169.8, 136.6, 128.5, 128.1, 111.8, 83.1, 70.9, 70.5, 70.3, 70.0,
66.7, 62.5, 55.7, 55.5, 40.8, 36.2, 34.9, 33.5, 30.4, 28.3, 28.0, 24.1, 18.3, 12.0. Anal. Calcd for
C52H76N10O16S (1128.52): C, 55.31; H, 6.78; N, 12.40%. Found: C, 55.40; H, 6.77; N, 12.41%.
Catalytic amount of 10% Pd-C was added to solutions of the above compounds (56.5 mg, 0.05
mmol) in MeOH (3 ml). The resulting mixture was stirred at room temperature for ca. 2 hours
under hydrogen (1 atm). After reaction completion, the mixture was filtered through a Celite pad,
and then washed with MeOH (3 × 1 ml). The combined organic solution was concentrated under
reduced pressure. The crude product was dissolved in DMF dry (1 mL) and then diglycolic
anydride (23.2 mg, 0.2 mmol) and pyridine (32 µl, 0.4 mmol) were sequentially added. The
reaction mixture was stirred at room temperature for 4 hours. After reaction completion, water
was added to the reaction mixture. Finally, the solvent was removed under reduced pressure and
the crude product was purified by flash chromatography on Biotage using a C18 reverse column
(BiotageTM eluant conditions: 1% of CH3CN and 99% of H2O to 100% of CH3CN and 0% of
H2O). Yield 70% (38.9 mg, 0.035 mmol) of pure 15.
1H NMR (400 MHz, CDCl3): δ: 1.27 (m, 1H, H-5), 1.43 (s, 9H, COC(CH3)3), 1.43-1.68 (m, 4H,
2 Hγ-Arg, Hβ-Arg, H-7), 1.79-1.97 (m, 2H, H-8, Hβ-Arg), 2.05 (s, 3H, CH3 Mtr), 2.20-2.40 (m,
3H, H-5, H-7, H-8), 2.40-2.58 (m, 4H, CH3 Mtr, Hβ-Asp), 2.60 (s, 3H, CH3 Mtr), 2.75 (m, 1H,
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H-4), 2.92 (dd, J 6.6 Hz, J 16.6 Hz, 1H, Hβ-Asp), 3.00-3.12 (m, 2H, H-10, Hδ-Arg), 3.18 (m,
1H, Hδ-Arg) 3.27 (m, 1H, H-10), 3.33-3.48 (m, 3H, OCH2CH2NH, Hα-Gly), 3.52 (t, J 4.8 Hz,
2H, OCH2CH2NH), 3.54-3.69 (m, 8H, OCH2), 3.75 (s, 3H, OCH3 Mtr), 3.92 (s, 2H, OCH2CO),
3.98-4.09 (m, 3H, H-6, OCH2CO), 4.04 (s, 2H, OCH2CO), 4.11-4.24 (m, 2H, H-9, Hα-Gly), 4.39
(m, 1H, H-3), 4.49 (m, 1H, Hα-Asp), 4.55 (m, 1H, Hα-Arg), 6.10-6.41 (m, 3H, NH
guanidinium), 6.45 (s, 1H, H Mtr), 7.15 (m, 1H, NHCH2azabicycloalkane), 7.30 (m, 1H,
NHazabicycloalkane), 7.41 (m, 1H, NH-Arg), 7.60 (m, 1H, NH linker), 7.74 (m, 1H, NH-Gly),
8.29 (m, 1H, NH-Asp); 13C NMR (100 MHz, CDCl3): δ: 173.2, 171.5, 170.6, 169.8, 136.6,
111.8, 83.1, 71.2, 70.9, 70.4, 70.3, 70.2, 70.0, 69.6, 69.0, 62.4, 55.7, 55.5, 38.8, 36.0, 34.9, 33.5,
30.4, 28.3, 24.1, 18.3, 12.0. Anal. Calcd for C48H74N10O18S (1110.49): C, 51.88; H, 6.71; N,
12.60%. Found: C, 51.87; H, 6.72; N, 12.61%. MS (ESI+) m/z 1111.9 (M+H+).
Synthesis of 16. Compound 15 (111.1 mg, 0.1 mmol) was dissolved in dry CH2Cl2 (1 ml), and
then NHS (11.5 mg, 0.1 mmol) and DIC (39 µl, 0.25 mmol) were sequentially added. The
reaction mixture was stirred at room temperature 18 h. After reaction completion, the solvent
removed under reduced pressure. The crude was left under the high vacuum overnight. The crude
product was diluted in CH3CN (2 ml) and the corresponding solution was slowly added to a
solution of alendronate 1 (97 mg, 0.3 mmol, was solubilized with water (4 ml) and the pH of the
corresponding solution was adjusted to pH 7.2-7.4 adding a 0.1 M water solution of NaOH,
finally to the mixture was added CH3CN (12 mL)) at 0 °C using a syringe pump (40 µl/min). The
reaction mixtures was left under stirring over night at 0 °C. After reaction completion, the
solvent was removed under reduced pressure and the crude product was purified by HPLC using
a C18 reverse column. HPLC eluant conditions: from 90% of H2O (0.2% TFA) and 10% of
CH3CN (0.2% TFA) to 0% of H2O (0.2% TFA) and 100% of CH3CN (0.2% TFA), flow rate 12
ml/min., 30 min. runs. Yield 84 % (124 mg, 0.084 mmol) of pure protected compound.
1H NMR (400 MHz, CDCl3): δ: 7.65 (bs, 1H), 6.72 (s, 1H), 4.46 (t, J 7.2 Hz, 1H), 4.33 (m, 2H),
4.18 (m, 2H), 4.04 (d, J 4.4 Hz, 4H), 3.96 (s, 2H), 3.93 (m, 1H), 3.78 (s, 3H), 3.64 (d, J 6.0 Hz,
8H), 3.59 (t, J 5.2 Hz, 2H), 3.41 (m, 3H), 3.20 (t, J 6.8 Hz, 3H), 3.08 (m, 3H), 2.90 (m, 1H), 2.74
(m, 1H), 2.56 (s, 3H), 2.47 (s, 3H), 2.47-2.25 (m, 3H), 2.02 (s, 3H), 1.95 (m, 2H), 1.90-1.55 (m,
13
5H), 1.38 (s, 9H), 1.22 (m, 2H); C NMR (100 MHz, CDCl3): δ: 174.8, 174.5, 173.7, 172.8,
172.0, 171.8, 171.6, 171.5, 170.1, 112.7, 83.1, 70.4, 70.0, 69.7, 69.6, 69.4, 68.8, 62.1, 55.9, 55.7,
52.9, 51.5, 42.5, 40.0, 39.3, 38.6, 35.7, 34.8, 33.3, 32.9, 32.5, 30.9, 30.2, 29.7, 27.3, 26.8, 23.3,
23.1, 17.6, 11.4.
A mixture composed of TFA (5 ml), 5% of thioanisole (250 µl) 5% triisopropylsilane (250 µL),
5% ethandiol, 5% Phenol (275 mg) and 5% water (250 µL) was added to a stirred solution of
protected compound (0.084 mmol). The reaction mixtures were left stirring at room temperature
overnight and then concentrated under reduced pressure. The crude product was solubilized with
water (2 ml) and washed with EtOAc (3 ×2 ml), Et2O (3 × 2 ml), iPr2O (3 × 2 ml), hexane (3 × 2
ml), DCM (3 × 2 ml), CHCl3 (3 × 2 ml), iPr2O (3 × 2 ml). Finally, water was removed under
reduced pressure. When the crude was not sufficiently pure, the residue was purified by
chromatography on a C18 reversed phase semi-preparative hplc column and then lyophilized.
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HPLC eluant conditions: from 98% of H2O (0.1% TFA) and 2% of CH3CN (0.1% TFA) to 70%
of H2O (0.1% TFA) and 30% of CH3CN (0.1% TFA), flow rate 12 ml/min., 30 min. runs. Yield
93 % (83.8 mg, 0.078 mmol) of pure 16.
1H NMR (400 MHz, D2O): δ: 1.26 (m, 1H, H-5), 1.42-1.70 (m, 4H, 2 Hγ-Arg, Hβ-Arg, H-7),
1.73-1.89 (m, 3H, NHCH2CH2CH2C(OH)(PO3H2)2, H-8), 1.90-2.10 (m, 3H,
NHCH2CH2CH2C(OH)(PO3H2)2, Hβ-Arg), 2.34-2.51 (m, 3H, H-5, H-7, H-8), 2.69 (dd, J 17.0
Hz, J 7.0 Hz, 1H, Hβ-Asp), 2.78 (m, 1H, H-4), 3.00-3.30, (m, 7H, Hβ-Asp, 2 Hδ-Arg, 2 H-10,
NHCH2CH2CH2C(OH)(PO3H2)2), 3.37-3.53 (m, 3H, OCH2CH2NH, Hα-Gly), 3.60 (m, 3H,
OCH2CH2NH), 3.63-3.74 (m, 8H, OCH2), 3.92-4.04 (m, 3H, OCH2CO, H-6), 4.07 (s, 4H,
OCH2CO), 4.20-4.32 (m, 2H, H-9, Hα-Gly), 4.36 (d, J 8.4 Hz, 1H, H-3), 4.48 (t, J 7.0 Hz, 1H,
Hα-Asp), 4.55 (m, 1H, Hα-Arg); 13C NMR (100 MHz, D2O): δ: 174.8, 174.5, 174.0, 173.0,
172.8, 171.9, 171.6, 170.2, 156.4, 70.3, 70.0, 69.7, 69.6, 69.4, 68.8, 62.2, 56.0, 53.0, 51.7, 51.5,
31
42.5, 40.4, 40.0, 39.4, 38.6, 35.7, 33.3, 32.9, 32.5, 30.8, 30.2, 26.9, 24.5, 23.3; P-NMR (161.9
MHz, D2O): δ: 20.7. Anal. Calcd for C38H65N11O21P2 (1073.38): C, 42.50; H, 6.10; N, 14.35%.
Found: C, 42.60; H, 6.11; N, 14.38%. MS (ESI+) m/z:1074.6 (M+H+).
Biology
Solid-phase receptor binding assay
Purified αvβ3 and αvβ5 receptors (Chemicon International, Inc., Temecula,CA, USA) were
diluted to 0.5 µg/mL in coating buffer containing 20 mM Tris-HCl (pH 7.4), 150 mM NaCl, 1
mM MnCl2, 2 mM CaCl2, and 1 mM MgCl2. An aliquot of diluted receptors (100 µL/well) was
added to 96-well microtiter plates (NUNC MW 96F Medisorp Straight) and incubated overnight
at 4°C. The plates were then incubated with blocking solution (coating buffer plus 1% bovine
serum albumin) for an additional 2 h at room temperature to block nonspecific binding, followed
by 3 h incubation at room temperature with various concentrations (10-5–10-12 M) of test
compounds in the presence of biotinylated vitronectin (1 µg/mL). Biotinilation was performed
using an EZ-Link Sulfo-NHS-Biotinylation kit (Pierce, Rockford, IL, USA). After washing, the
plates were incubated for 1 h at room temperature with biotinylated streptavidin–peroxidase
complex (Amersham Biosciences, Uppsala, Sweden) followed by 30 min incubation with 100
µL/well Substrate Reagent Solution (R&D Systems, Minneapolis, MN) before stopping the
reaction with the addition of 50 µL/well 2N H2SO4. Absorbance at 415 nm was read in a
SynergyTM HT Multi-Detection Microplate Reader (BioTek Instruments, Inc.). Each data point
represents the average of triplicate wells; data analysis was carried out by nonlinear regression
analysis with GraphPad Prism software. Each experiment was repeated in triplicate.
Acknowledgements
The authors thank CNR and MUR (FIRB RBNE03LF7X and PRIN 2006030449 research
projects) for financial support.
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