Synthesis of novel dendrimers having aspartate grafts and their ability to enhance the aqueous solubility of model drugs

Article abstract of DOI:10.1016/j.ejmech.2010.01.069

In this study, a series of aspartate based dendrimers with different cores were synthesized in a convergent approach and well characterized by NMR and MS techniques. The aqueous solubility of the model drugs (L-Histidine, Naproxen, Methotrexate) was measured in the presence of this kind of dendrimers at room temperature in PBS buffers at pH 6, 7 and 8. Results clearly confirmed that the solubility enhancement was due to presence of dendrimers at different pH compared to their corresponding aqueous solubility at different pH. The results indicated that the aspartate based dendrimers could be considered as an effective supplement of PAMAM dendrimers in solubility enhancement and drug delivery. The surface groups played an important role in dendrimer-mediated solubility enhancement.

Full text of DOI:10.1016/j.ejmech.2010.01.069

European Journal of Medicinal Chemistry 45 (2010) 2705–2711
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Preliminary communication
Synthesis of novel dendrimers having aspartate grafts and their ability to enhance
the aqueous solubility of model drugs
,
Liang Ouyang a, Lifang Ma b, Bo Jiang a, Yanhua Li a, Dongsheng He a, Li Guo a
*
a Key Laboratory of Drug Targeting and Drug Delivery System of Education Ministry, Department of Medicinal Chemistry, West China School of Pharmacy, Sichuan University,
Chengdu 610041, PR China
b School of Chemical Engineering, Sichuan University, Chengdu 610065, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, a series of aspartate based dendrimers with different cores were synthesized in a conver-
gent approach and well characterized by NMR and MS techniques. The aqueous solubility of the model
drugs (L-Histidine, Naproxen, Methotrexate) was measured in the presence of this kind of dendrimers at
room temperature in PBS buffers at pH 6, 7 and 8. Results clearly confirmed that the solubility
enhancement was due to presence of dendrimers at different pH compared to their corresponding
aqueous solubility at different pH. The results indicated that the aspartate based dendrimers could be
considered as an effective supplement of PAMAM dendrimers in solubility enhancement and drug
delivery. The surface groups played an important role in dendrimer-mediated solubility enhancement.
Ó 2010 Elsevier Masson SAS. All rights reserved.
Received 13 October 2009
Received in revised form
26 January 2010
Accepted 28 January 2010
Available online 2 February 2010
Keywords:
Dendrimers
Aspartic acid
Solubility enhancement
L-Histidine
Naproxen
Methotrexate
1. Introduction
solubility drugs [7–9]. Generation 0.5 (G0.5) to generation 4 (G4)
PAMAM dendrimers have been used to encapsulate and solubilize
Dendrimers are a novel type of polymeric material that have
highly branched structure and globular shape. The classic chemical
architecture of dendrimers possesses a well-defined core, an inte-
rior region and a large number of end groups [1]. The physical
characteristics of dendrimers, including their monodispersity,
hydrophilicity, encapsulation ability, and large number of functio-
nalizable peripheral groups, make these macromolecules ideal
candidates for evaluation as drug delivery vehicles [2–5].
The family of dendrimers most investigated in drug delivery is
the poly(amido amine) dendrimers (PAMAM). PAMAM dendrimers
are biocompatible, non-immunogenic, water soluble and possess
terminal modifiable amine functional groups for binding various
targeting or guest molecules [6]. The high density of amino groups
and internal cavities in PAMAM dendrimers is expected to have
potential applications in enhancing the aqueous solubility of low
acidic drugs such as Nifedipine [10], Ibuprofen [11] and Indo-
methacin [12]. Further, PAMAM derivatives modified by –OH or
–COOH groups were found to encapsulate small hydrophobic guest
molecules successfully: Twyman et al. [13] converted the ester
terminated PAMAM dendrimers to a more soluble hydroxyl surface
by reacting with TRIS (Tris (hydroxymethyl)aminomethane). The
–OH-terminated dendrimers were found to increase solubility by
weak hydrogen bonding; Neofofistou and Demadis [14] synthesized
the PAMAM dendrimers with carboxy groups in peripheral and used
these kinds of dendrimers as the solubility enhancer of SiO2.
Recently, more dendritic molecules were investigated as solu-
bility enhancers such as poly-lysine (PLS) [15], poly-
propyleneimine (PPI) [16], and poly-citric acid dendrimers (PCA)
[17]. Several previous reviews have covered the early work of the
solubility enhancement with dendrimers. The possible mecha-
nisms of the dendrimer-mediated solubility enhancement are
based on a typical dendrimer’s structure: (a) a central core with
enough internal cavities is available to encapsulate the guest
molecules; (b) branching units provide weak hydrogen bonding to
increase solubility; (c) the surface groups are mainly responsible for
the solubility enhancement by electrostatic interactions [18–20].
Some experts supposed that the hydrophilic surfaces and
Abbreviations: DCC, Dicyclohexylcarbodiimide; HOBt, Hydroxybenzotriazole;
TFA, Trifluoroacetic acid; DCM, Dichloromethane; DIPEA, N,N-Diisopropylethyl-
amine; HBTU, o-Benzotriazol-1-yl-tetramethyluronium hexafluorophosphate.
* Corresponding author. Tel.: þ86 028 85503777; fax: þ86 028 85502629.
E-mail address: rosaguoli2000@yahoo.com.cn (L. Guo).
0223-5234/$ – see front matter Ó 2010 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2010.01.069

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L. Ouyang et al. / European Journal of Medicinal Chemistry 45 (2010) 2705–2711
HOOC
HOOC
hydrophobic interiors made the dendrimers ‘static unimolecular
micelles’ and these unimolecular micelles were able to encapsulate
a number of insoluble drug molecules [21].
HOOC
COOH
COOH
In our previous work, prodrugs with aspartate based den-
drimers were studied [22,23]. A comparative solubility enhance-
ment profile was found when the aspartate based dendrimers were
conjugated to an insoluble drug molecule and the water solubility
was increased by the structures with more polar groups [23]. On
the basis of the mechanisms above and the initial studies, we began
to consider the aspartate based dendrimers could be used as
a solubility enhancer just like the PAMAM dendrimers. One hand,
as the same of PAMAM dendrimers, this kind of structures con-
sisted of amido linkage and the carbon–carbon bond length was
large enough, which created a similar internal microenvironment
with similar polarity, the other hand, opposed to PAMAM den-
drimers and other dendrimers, the surface groups of this kind of
dendrimers were acidic groups: –COOH instead of the alkaline
groups: –NH2. This feature might display new properties in the
acid/base mediated solubility enhancement of some acidic or
alkaline drug molecules.
Anticipated potential of aspartate based dendrimers as solu-
bilizing agents, their similar hydrophobic microenvironment and
different surface groups compared to PAMAM dendrimers and
paucity of reports proving these kinds of dendrimers prompted
us to design a comprehensive study on aspartate based den-
drimers mediated solubilization. It was reported that solubiliza-
tion potential of dendrimers increased with molecular size.
Therefore we designed to utilize different cores to construct
aspartate based dendrimers. The aim of the present work was (1)
to synthesize Generation 1 aspartate based Dendron (G1-Asp-
Dendron) and attach them to different dendritic cores to form
a well-defined highly branched architecture; (2) to study the
potential of aspartate based dendrimers as a solubility enhancer
of the model drugs; (3) to study the effect of pH, concentration,
numbers of surface functional group on interactions between the
dendrimers and the model drugs. Naproxen, Methotrexate and L-
Histidine were used for model drugs because they were clinically
relevant agents and not freely soluble in water at room temper-
ature, furthermore, they were representatives of different acid/
base drugs: weakly acidic Naproxen, –COOH functional group;
weakly acidic and weakly basic Methotrexate, –COOH and –NH2
functional group; weakly basic L-Histidine, imidazole ring side-
chain.
1
2
HOOC
HOOC
HOOC
COOH
COOH
O
O
O
O
O
O
O
O
HOOC
HOOC
COOH
COOH
O
O
3
4
COOH
Fig. 1. The chemical structure of dendritic cores.
The G1-Asp-Dendron (Compound 5) was prepared by the same
method in our previous work [23]. 2-(tert-Butoxycarbonyl)succinic
acid (Boc-Asp) and Dibenzyl 2-aminosuccinate (NH2-Asp-OBzl2)
were connected by amidation reaction using DCC/HOBt (Dicyclo-
hexylcarbodiimide/Hydroxybenzotriazole). The protected group
(Boc–) was removed in TFA/DCM (Trifluoroacetic acid/Dichloro-
methane). The synthetic procedure was outlined in Scheme 1.
In the synthesis of the target dendrimers (T1–T4), for the large
size of the reactant, using DCC/HOBt did not give the product in
acceptable yields. Only a solid phase peptide coupling reagent
HBTU/DIPEA
(o-Benzotriazol-1-yl-tetramethyluronium
hexa-
fluorophosphate/N,N-Diisopropylethylamine) afforded the prod-
ucts in good yields. The synthetic procedure was outlined in
Scheme 2.
3. Results and discussion
3.1. Structural characterization of the synthetic peptide dendrimers
The characteristic data of the synthetic dendrimers was outlined
in Table 1. The high degree of symmetry in these dendrimers
enabled facile confirmation of both structure and purity by NMR
techniques. For example, in the 1H NMR spectrum of dendrimer T3
the core protons observed the resonance signals at 2.37 (brs), 3.2
(s), and 3.55 (brs) ppm were clearly distinguishable from the
resonances arising from the wedges (G1-Asp-Dendron, Compound
5) at 2.65 (m) and 4.72 (m) ppm. Integration of the respective areas
2. Chemistry
of the core protons and
b-methine protons of G1-Asp-Dendron
All the dendritic cores, compounds 1–4 (Fig. 1), were
synthesized by literature methods [24,25]. The synthesis
involved two consecutive chain-forming reactions, the exhaus-
tive Michael addition reaction and the functional group changing
reactions.
confirmed the complete coupling of the central core and peptides.
Furthermore, the structures of these dendrimers were further
verified by ESI MS electrospray ionization mass spectrometry and
MALDI MS matrix-assisted laser desorption/ionization mass spec-
trometry [32,33]. All of the ESI MS spectra displayed a very
O
O
OBzl
OBzl
OBzl
OBzl
O
O
HN
O
O
OBzl
OH
OH
HN
O
TsOH
DCC/ HOBt
O
O
O
TFA/ DCM
O
+
O
O
H
N
OBzl
H
N
DIPEA
H2N
O
N
OBzl
OBzl
O
N
H
OBzl
OBzl
H2N
H
O
O
O
O
O
5
Scheme 1. The synthesis of G1-Asp-Dendron.

L. Ouyang et al. / European Journal of Medicinal Chemistry 45 (2010) 2705–2711
2707
O
O
HO
OH
O
O
O
OH
O
O
NH
O
O
H
N
HO
O O
HN
OH
O
N
H
HO
O
O
HN
NH
H2 / Pd
1, HBTU/HOBt
T1
OH
DIPEA
HO
O
HO
O
HO
O
O
HN
O
O
HO
O
N
O
OH
O
H
N
H
HO
O
O
O
OH
O O
NH
BzlO
BzlO
HN
HO
H
N
OH
O
H2 / Pd
H
N
2, HBTU/HOBt
HN
O
NH
O
HO
O
O
DIPEA
O
O
OH
HN
O
O
H
N
HO
T2
BzlO
BzlO
NH2
O
O
HO
O
O
O
O
HO
OH
O
O
3, HBTU/HOBt
5
HO
O
OH
O
O
O
HN
NH
O
O
H2 / Pd
HO
O
OH
O
DIPEA
N
N
H
O
O
H
N
N
H
H
O
O
O
O
HO
HO
OH
OH
H
H
N
N
O
HN
O
NH
O
O
OH
HO
O
O
O
OH
O
HO
NH
HN
O
O
O
O
T3
OH
HO
O
O
HO
O
O
OH
O
O
HO
O
OH
O
HO
HN
O
O
NH
OH
O
O
O
O
4, HBTU/HOBt
HO
N
H
NH
HN
N
H
OH
H2 / Pd
O
O
DIPEA
O
O
HO
O
OH
O
O
O
O
O
O
O
O
O
HO HN
O O
OH
O
NH
O
H
N
N
H
HO
O
O
OH
O
NH
O
HN
O
HO
H
N
O
OH
HO
NH
H
N
HN
O
OH
O
O
HO
O
HO
HN
O
O
O
O
NH
O
OH
OH
T4
O
HO
O
O
OH
Scheme 2. The synthesis of target dendritic molecules.

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L. Ouyang et al. / European Journal of Medicinal Chemistry 45 (2010) 2705–2711
Table 1
a
2.8
2.6
2.4
2.2
2
The characteristic data of the synthetic dendrimers.
T1
T2
T3
T4
Compound Molecular
formula
Molecular Number of internal Number of terminal
weight
amide groups
carboxyl groups
T1
T2
T3
T4
C28H36N6O22
808.61
C54H63N9O33 1365.12
C65H88N12O48 1805.45 12
C96H126N18O72 2684.11 18
6
9
8
12
16
24
1.8
1.6
1.4
1.2
1
prominent peak corresponding to the dendrimers complexed with
protons or sodium cation. The results of MALDI MS were less
satisfactory, but a molecular ion peak could still be easily found.
These differences between ESI and MALDI, in particular, depended
on the extent of dendrimer–matrix reactions [34]. Moreover,
elemental analysis was also in good agreement with those of the
signed structures.
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of Dendrimers (M)
b
5
4.5
4
3.2. Characterization of Naproxen–dendrimer solubility profiles
T1
T2
T3
T4
Naproxen is one of the acidic non-steroidal anti-inflammatory
drugs (NSAIDs) with a functional group –COOH (pKa ¼ 4.2), so its
solubility in water is pH-dependent [11]. Because the acidity of
naproxen, the inherent solubility was 1.18 mg/ml, pH ¼ 6; 2.24 mg/
ml, pH ¼ 7; 24.45 mg/ml, pH ¼ 8. We measured the solubility of
naproxen in the presence of T1–T4 dendrimers at different acid/
base conditions pH 6, 7, and 8. The solubility profiles were shown in
Fig. 2.
It was observed that the T1 dendrimer did not nearly increase
the aqueous solubility of Naproxen at pH 6 and 7, but at pH 8, there
was a little enhancement: from 24.45 up to 30.78 mg/ml. When
experiments mentioned above are excluded, the other dendrimers
T2–T4 significantly increased the aqueous solubility of Naproxen at
all three pH values. In the presence of T2–T4 dendrimer at all pH
values, the solubility of Naproxen increased in an approximately
linear manner with an increase in dendrimer concentration and the
increased solubility was highest at pH 8, less at pH 7 and least at pH
6. It is clear that the solubility of Naproxen was affected by the
increasing molecular weight and surface functional groups of the
dendrimers T1–T4: take T4 dendrimer for example, after interac-
tions with the dendrimer at concentration of 0.01 mol/L, Naprox-
en’s solubility increased from 1.18 to 2.51 mg/ml at pH 6, from 2.24
to 4.66 mg/ml at pH 7, from 24.45 to 81.92 mg/ml at pH 8.
3.5
3
2.5
2
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of Dendrimers (M)
90
80
70
60
50
40
30
20
T1
T2
T3
T4
c
3.3. Characterization of Methotrexate–dendrimer solubility profiles
Methotrexate is one of the prototype folate antagonist cytotoxic
drugs used in non-malignancy and non-malignant diseases. The
molecule is made up of a heterocyclic portion (a 2,4-diaminosub-
stituted pterine ring) linked to an aminobenzoyl portion, which is,
in turn, an amide bonded to a glutamic acid [27]. Hence, MTX is
a polyelectrolyte carrying two carboxyl groups, with pKa’s of 3.36
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of Dendrimers (M)
Fig. 2. Solubility profiles of Naproxen in the presence of increasing concentrations of
aspartate based dendrimers in different pH: (a) pH ¼ 6; (b) pH ¼ 7; (c) pH ¼ 8.
(a-carboxyl) and 4.70 (g-carboxyl), and a number of potentially
protonated nitrogen functions, the most basic of which, presum-
ably, is the guanidinic N-1 on the pterine ring (pKa 5.71) unit to an
aminobenzoyl. The inherent solubility of methotrexate was
0.76 mg/ml, pH ¼ 6; 5.75 mg/ml, pH ¼ 7; 36.45 mg/ml, pH ¼ 8. The
Methotrexate–dendrimer solubility profiles were shown in Fig. 3.
It was observed that the solubility enhancement was similar to
Naproxen, but the extent was less: take T4 dendrimer for example,
after interacting with the dendrimers at concentration of 0.01 mol/L,
Methotrexate’s solubility increased from 0.76 to 1.04 mg/ml at pH 6,
from 5.75 to 7.62 mg/ml at pH 7, from 36.45 to 51.46 mg/ml at pH 8.
At higher concentrations of the dendrimers, the solubility of
Methotrexate was slightly lower than predicted. This might be
because of precipitation of an insoluble complex at high
concentrations of T1–T4 dendrimers. In fact, we observed yellow
precipitation at higher concentrations in our experiments.
3.4. Characterization of L-Histidine–dendrimer solubility profiles
L-Histidine is one of the 20 standard amino acids presenting in
proteins and it is considered as an essential amino acid in human
body nutritionally. The imidazole side-chain of Histidine has a pKa
of approximately 6, and overall, the amino acid has a pKa of 7.6. So
Histidine is a positively charged weakly basic amino acid under
physiological environment [28]. The inherent solubility of histidine
was 98.84 mg/ml, pH ¼ 6; 40.91 mg/ml, pH ¼ 7; 97.33 mg/ml,

L. Ouyang et al. / European Journal of Medicinal Chemistry 45 (2010) 2705–2711
2709
1.1
1.05
1
a
a
410
370
330
290
250
210
170
130
90
T1
T2
T3
T4
T1
T2
T3
T4
0.95
0.9
0.85
0.8
0.75
0.7
0
0.002
0.004
0.006
0.008
0.01
0.012
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of dendrimers (M)
Concentration of Dendrimers (M)
b 105
8
7.5
7
b
T1
T2
T3
T4
T1
95
T2
T3
T4
85
75
65
55
45
35
6.5
6
5.5
5
0
0.002
0.004
0.006
0.008
0.01
0.012
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of dendrimers (M)
Concentration of Dendrimers (M)
c 105
55
53
51
49
47
45
43
41
39
37
35
T1
c
95
T2
T3
T4
T1
85
75
65
55
45
35
25
T2
T3
T4
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of the dendrimers (M)
0
0.002
0.004
0.006
0.008
0.01
0.012
Concentration of Dendrimers (M)
Fig. 4. Solubility profiles of L-Histidine in the presence of increasing concentrations of
aspartate based dendrimers in different pH: (a) pH ¼ 6; (b) pH ¼ 7; (c) pH ¼ 8.
Fig. 3. Solubility profiles of MTX in the presence of increasing concentrations of
aspartate based dendrimers in different pH: (a) pH ¼ 6; (b) pH ¼ 7; (c) pH ¼ 8.
was well suited for the host–guest interactions and encapsulation
of the biological model drug molecules. More importantly, the
increased surface carboxy groups in periphery and tertiary amines
in internal were available to interact with model drug molecules
[29]. This special structure of dendrimer molecules could act as
hydrogen bond donors and acceptors which could interact with the
atoms of model drug molecules by hydrogen bond formation.
Furthermore, the dendrimer structure had carboxyl groups on the
surface which established electrostatic interactions with different
parts of the drug molecules. So these total interactions were due in
partly to hydrogen and partly to ionic bonds.
pH ¼ 8. The L-Histidine–dendrimer solubility profiles were shown
in Fig. 4.
It was observed that the dendrimer-mediated L-Histidine solu-
bility enhancement was very obvious: the solubility increased from
98.84 to 361.2 mg/ml at pH 6, from 40.91 to 97.33 mg/ml at pH 7,
from 31.75 to 99.12 mg/ml at pH 8.
3.5. Possible mechanism of interactions between aspartate based
dendrimers and model drugs
For the synthesized aspartate based dendrimers (T1–T4), the
order of dendrimer-mediate solubility enhancement was
T4 > T3 > T2 > T1. Compound T4 exhibited the most solubility
enhancement in all model drugs and at all pH conditions, but
compound T1 exhibited almost no increased solubility in some
conditions. It was presumably due to the increase in the size of
dendrimer molecules and the internal architecture of dendrimers
For the different model drugs, the order of dendrimer-mediated
solubility enhancement was L-Histidine > Naproxen > Methotrexate.
The molecular size of L-Histidine was small and it was easy to form
a dendrimer-complex through a simple encapsulation. Further, L-
Histidine had an imidazole ring side-chain which could attribute an
electrostatic interaction between anionic dendrimers and the basic
groups of L-Histidine. Naproxen possessed a carboxyl groups and

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L. Ouyang et al. / European Journal of Medicinal Chemistry 45 (2010) 2705–2711
became considerably polar (negative ion) at the test pH which could
reduce the electrostatic interactions. The solubility enhancement at
the test pH could be ascribed to both hydrophobic interaction and
hydrogen bonding between deprotonated/unionized fractions of
dendrimer/Naproxen or though a simple encapsulation. Methotrexate
exhibited lower level of solubility enhancement most probably due to
the sizeof drug molecules preventing anefficient accessto the internal
cavities (pores) of the dendrimers [30].
As explained earlier, electrostatic interaction between hydro-
phobic drugs and peripheral groups of dendrimers is a major
mechanism responsible for solubility enhancement. Protonation
behaviour and pH of dendrimers affect the solubilizing power of
dendrimers. As the pKa of the –COOH of the aspartic acid was about
4.2, being weakly acid, the surface groups of the dendrimers
became more ionized/protonated with the pH increasing. Cationic
hydrophobic drugs might rarely be entrapped inside these den-
drimers but the electrostatic interaction would be favoured for the
anionic species and the peripheric negative charges of the anionic
dendrimers would attribute the electrostatic interactions with the
drug molecules. So, a pH-dependent solubility enhancement was
observed. The solubility was highest at pH 8 and least at pH 6. This
result further demonstrated that the surface groups played an
important role in dendrimer-mediated solubility enhancement
[31]. It can also be concluded that mechanism of solubilization
largely depends on protonated/deprotonated state of dendrimer.
standard techniques. For solubility studies, water used to prepare
solutions was two-distilled water.
5.2. Synthesis of the compound T1
A sample of succinic acid (0.12 g, 1.0 mmol) dissolved in DCM
(20 ml) was cooled in an ice-water bath and the unprotected G1-
Asp-Dendron 5 (1.50 g, 2.1 mmol) was then added to the reaction
mixture, followed by DIPEA (0.23 g, 2.1 mmol), HBTU (0.80 g,
2.1 mmol) and HOBt (0.27 g, 2.1 mmol). The reaction mixture was
stirred for 3 days. After evaporated under reduced pressure, the
residue was taken up in ethyl acetate (25 ml) and washed with 1 M
HCl (10 ml), brine (10 ml), 1 M NaHCO3 (10 ml). The crude product
was purified by silica gel column chromatography using DCM–
methanol (50:1, v/v) as an eluent to yield a white solid (0.94 g)
Yield: 61%.
The mixture of the obtained solid (0.94 g) and 10% Pd/C
(200 mg) in methanol was stirred at the room temperature under
a
H2 atmosphere. After 24 h the mixture passed through
a membrane filter to remove the catalyst, then evaporated under
reduced pressure. The residue was triturated with pure ethyl ether
to afford T1 as a white powder (0.45 g). Yield: 91%.
5.2.1. Data
1H NMR (400 MHz, D2O): 2.51 (s, 4H, SA-CH2), 2.78–2.96 (m,
12H, Asp-bCH2), 4.79–4.96 (m, 6H, Aspa-CH). ESI MS (m/z): calcd.
for 808.6. obsd. 831.2 ([M þ Na]þ). MALDI TOF MS (m/z): calcd. for
808.6. obsd. 810.3 ([M þ 2H]þ). Anal. calcd. for C28H36N6O22: C,
41.59, H, 4.49, N, 10.39. found: C, 41.77, H, 4.33, N, 10.37.
4. Conclusion
We had prepared a range of aspartate based dendrimers bearing
two to six G1-Asp-Dendron sequences in periphery. The synthetic
dendrimers had the potential to significantly enhance the solubility
of poorly water soluble drugs, such as L-Histidine, Naproxen and
Methotrexate. The increase in drug solubility in aqueous solution
depended on the concentration and molecular size of the den-
drimer, the numbers of surface functional groups and the pH of the
medium. All observations were evidence that the enhancement of
drug solubility was owing to electrostatic interactions and
hydrogen bonding between anionic dendrimers and functional
groups of the model drugs.
From the initial results, we found the aspartate based dendrimer
could be considered as an effective supplement of PAMAM den-
drimers in drug encapsulation and drug solubilization. We are
continuing exploring higher generation of dendritic Asp peptides,
developing controlled release drug delivery systems containing the
drug trapped inside the dendrimers.
5.3. Synthesis of the compound T2
According to the same procedure of preparation T1, the target
molecule T2 (0.36 g, white powder) could be prepared form 2
(0.2 g, 0.6 mmol). Yield: 44% in two steps.
5.3.1. Data
1H NMR (400 MHz, D2O): 2.56–2.96 (m, 18H, Asp-
b
CH2), 3.02–
3.30 (m, 12H, Core-CH2), 4.78–4.92 (m, 9H, Asp
a-CH). ESI MS (m/z):
calcd. for 1366.1. obsd. 1388.8 ([M þ Na]þ). MALDI TOF MS (m/z):
calcd. for 1366.1. obsd. 1387.7 ([M þ Na]þ). Anal. calcd. for
C54H63N9O33: C, 47.48, H, 4.65, N, 9.23. found: C, 47.37, H, 4.79, N,
9.31.
5.4. Synthesis of the compound T3
5. Experimental protocols
According to the same procedure of preparation T1, the target
molecule T3 (0.41 g, white powder) could be prepared form 3
(0.2 g, 0.4 mmol). Yield: 49% in two steps.
5.1. General
The reactions requiring anhydrous conditions were performed
in an Ar or N2 atmosphere. Thin layer chromatography (TLC): silica
gel plates GF254; Compounds were visualized by irradiation with
UV light and/or by treatment with a solution of phosphomolybdic
acid (20 wt.% in ethanol) followed by heating. 1H NMR analysis was
performed by the INOVA VARIAN 400 MHz spectrometer using
CDCl3 or D2O as a solvent at room temperature. Elemental analysis
was performed by Atlantic Microlab, Atlanta, GA, USA. ESI mass
spectrum was recorded with Agilent 1946B ESI-MS instrument,
MALDI TOF MS was recorded with Bruker BIFLEX III (matrix was
sinapic acid SA) and the UV analysis was carried out by the Cary 100
UV–Vis spectrophotometer. Naproxen and Methotrexate were
obtained from Henrui Medicine Co., Ltd. (Jiangsu, China). L-Histi-
dine was purchased from GL Biochem Ltd. (Shanghai, China). The
other chemicals and solvents were A.R. grade and purified by
5.4.1. Data
1H NMR (400 MHz, D2O): 2.37–2.44 (brs, 8H, Core-COCH2),
2.65–2.94 (m, 24H, Asp-
(brs, 8H, Core-OCH2), 4.72–4.91 (m, 12H, Asp
b
CH2), 3.20 (s, 8H, Core-CCH2), 3.55–3.60
-CH). ESI MS (m/z):
a
calcd. for 1805.5 obsd. 1806.8 ([M þ H]þ). MALDI TOF MS (m/z):
calcd. for 1805.5. obsd. 1828.7 ([M þ Na]þ). Anal. calcd. for
C65H88N12O48: C, 43.24, H, 4.91, N, 9.31. found: C, 43.31, H, 4.98, N,
9.18.
5.5. Synthesis of the compound T4
According to the same procedure of preparation T1, the target
molecule T4 (0.29 g, white powder) could be prepared form 4
(0.2 g, 0.3 mmol). Yield: 33% in two steps.

L. Ouyang et al. / European Journal of Medicinal Chemistry 45 (2010) 2705–2711
2711
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1H NMR (400 MHz, D2O): 2.42–2.51 (brs, 12H, Core-COCH2), 2.66–
2.91 (m, 36H, Asp- CH2), 3.37–3.42 (brs, 6H, Core-cycloH), 3.62–3.68
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The aqueous solubility of model drugs was determined at pH 6,
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This work was supported by the National Natural Science
Foundation of China (No.20472055)
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