Molecular hydrogelators consist of Taxol and short peptides/amino acids

Article abstract of DOI:10.1039/c2jm32203j

Injectable molecular hydrogels hold big potential for the local delivery of anti-cancer drugs for chemotherapy. We recently reported on a molecular hydrogelator of two complementary anti-cancer drugs of dexamethasone and Taxol. In this study, we study in detail the structure-gelation property of Taxol derivatives. We found that even the conjugates of Taxol and an amino acid such as glutamic acid, serine, and arginine with hydrophilic side chains were efficient molecular hydrogelators. The six gels reported in this paper were characterized by rheology and TEM and the release profile of Taxol from gels were also studied. Gels reported in this study possessed high drug (Taxol) loading percentages in self-assembled nanofibers, exhibited constant and sustained release of Taxol from gels, and were injectable. Our results demonstrated that Taxol derivatives were efficient gelators and the injectable gels formed by them might be developed into local delivery systems for chemotherapy. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm32203j

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PAPER
Molecular hydrogelators consist of Taxol and short peptides/amino acids†
a
Huaimin Wang, Linna Lv, Guangyang Xu, Chengbiao Yang, Jiangtao Sun and Zhimou Yang
a
b
a
b
a
*
*
Received 9th April 2012, Accepted 26th June 2012
DOI: 10.1039/c2jm32203j
Injectable molecular hydrogels hold big potential for the local delivery of anti-cancer drugs for
chemotherapy. We recently reported on a molecular hydrogelator of two complementary anti-cancer
drugs of dexamethasone and Taxol. In this study, we study in detail the structure–gelation property of
Taxol derivatives. We found that even the conjugates of Taxol and an amino acid such as glutamic acid,
serine, and arginine with hydrophilic side chains were efficient molecular hydrogelators. The six gels
reported in this paper were characterized by rheology and TEM and the release profile of Taxol from
gels were also studied. Gels reported in this study possessed high drug (Taxol) loading percentages in
self-assembled nanofibers, exhibited constant and sustained release of Taxol from gels, and were
injectable. Our results demonstrated that Taxol derivatives were efficient gelators and the injectable gels
formed by them might be developed into local delivery systems for chemotherapy.
which may further push forward the practical application of
molecular hydrogels for chemotherapy.6–8
1. Introduction
Cancer is one of the leading causes of death in the world. Up to
now, surgery in combination with systemic chemotherapy is still
the most effective way to save cancer patients and prolong their
life.1 However, the clinically used anti-cancer drugs such as Taxol
and camptothecin are limited due to their low water solubility
and the systemic chemotherapy will cause serious adverse side
effects to patients. In order to improve the solubility of hydro-
phobic anti-cancer drugs and reduce side effects of systemic
chemotherapy, many drug delivery systems composed of nano-
sized vehicles and anti-cancer drugs have been reported and some
of them have been used in clinic.2,3 For example, nanoparticles of
albumin can improve the solubility of Taxol and the resulting
Abraxane (protein-bound paclitaxol) has been approved by the
FDA for the treatment of metastatic breast cancers. The Wang
group also designed a dual pH-sensitive nanoparticle system for
the highly selective delivery of the anti-cancer drug to cancer cells
including cancer stem cells.3 Besides these successful examples of
drug delivery systems, molecular hydrogels have attracted
considerable attention for the delivery of anti-cancer drugs in
recent years.4 The self-assembled nanofibers in molecular
hydrogels can be used as carriers for the delivery of hydrophobic
anti-cancer drugs.5 What’s more, molecular hydrogels of anti-
cancer drug derivatives have been reported with high drug
loading capacities and are carrier-free drug delivery systems,
Molecular hydrogels,9 formed by the self-assembly of small
molecules,10 have shown large potentials in fields of drug
delivery,11 3D cells culture,12 etc. The first example of a molecular
hydrogel formed by a derivative of a clinically used anti-cancer
drug was reported by Xu and co-workers.6 The gels were formed
by the conjugate of Taxol and a short peptide derivative of Nap-
FFKY and it exhibited sustained release profiles of Taxol/Taxol
derivatives. Stimulated by their results, our group has combined
Taxol and the tumor targeting folic acid in a molecular hydro-
gelator that can self-assemble into nanospheres by the enzyme of
phosphatase.7 Recently, we reported on molecular hydrogels of
two synergetic drugs, dexamethasone (Dex) and Taxol/campto-
thecin.8 The preliminary results showed that the above reported
molecular hydrogelators exhibited similar efficacies to inhibit
cancer cell growth to original anti-cancer drugs and the gels can
sustain the release original anti-cancer drugs or drug derivatives.
However, preparing precursors of these molecular hydrogelators
of anti-cancer drugs requires multiple synthetic steps, which leads
to low yields and might hinder their practical applications. Thus,
it will be beneficial to develop simpler molecular hydrogelators of
anti-cancer drugs by easy synthetic pathways with high yields. In
this paper, we reported several novel molecular hydrogelators
consisting of Taxol and short peptides/amino acids. We found
that even conjugates of Taxol and an amino acid with a hydro-
philic side chain were efficient molecular hydrogelators.
aState Key Laboratory of Medicinal Chemical Biology and College of Life
Sciences, Nankai University, Tianjin 300071, P. R. China. E-mail:
yangzm@nankai.edu.cn
bSchool of Pharmaceutical Engineering & Life Science, Changzhou
University, Changzhou 213164, P. R. China. E-mail: jtsun08@gmail.com
† Electronic supplementary information (ESI) available: characterization
of the compounds, rheology, congress curve of cell inhibition, and optical
images of gels post injection. See DOI: 10.1039/c2jm32203j
2. Experimental
2.1 Materials and general methods
Fmoc-amino acids were obtained from GL Biochem (Shanghai).
Taxol was purchased from Baoman Biotechnology (Shanghai).
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Succinic acid was obtained from Sigma. All the other starting
materials were obtained from Alfa. Commercially available
reagents and solvents were used without further purification,
unless noted otherwise.
mixture was stirred overnight and the title products were purified
by reverse phase HPLC.
2.4 Characterization of precursors of gelators
1
The synthesized compounds were characterized by H NMR
(Bruker ARX-400) using DMSO-d6 as the solvent and ESI-MS
spectrometric analyses were performed at the Thermo Finnigan
LCQ AD System. HPLC was conducted at LUMTECH HPLC
(Germany) system using a C18 RP column with MeOH (0.05% of
TFA) and water (0.05% of TFA) as the eluents. TEM samples
were prepared as follows: a copper grid coated with a thin layer
of carbon was dipped into the hydrogel. 10 seconds later, 20 mL
of doubly distilled H2O was used to wash the copper grid three
times. A 6 mL solution of Uranyl acetate was used to stain the
sample for 5 seconds, the staining solution was removed by a
filter paper, and the copper grid was kept in a desiccator for
about 30 minutes. The prepared sample was performed at the
JEM100CXII system, operating at 100 kV. LC-MS was con-
ducted at the LCMS-20AD (Shimadzu) system, HR-MS was
performed at the Agilent 6520 Q-TOF LC/MS.
Dex-K(Taxol)E-ss-EE. 1H NMR (400 MHz, DMSO-d6) d
7.97–8.06 (m, 4H), 7.85–7.93 (m, 3H), 7.72–7.76 (m, 1H), 7.65–
7.69 (m, 2H), 7.55–7.61 (m, 1H), 7.47–7.52 (m, 2H), 7.44–7.46
(m, 3H), 7.30 (d, J ¼ 10.28, 1H), 7.18–7.20 (m, 1H), 6.21 (s, 1H),
6.23 (d, J ¼ 10.20, 1H), 6.01 (s, 1H), 5.79–5.85 (m, 1H), 5.51–5.60
(m, 2 H), 5.33–5.42 (m, 3H), 5.16 (s, 1H), 5.02–5.07 (m, 1H),
4.90–4.94 (m, 2H), 4.76–4.81 (m, 1H), 4.64 (s, 1H), 4.25–4.31 (m,
1H), 4.09–4.17 (m, 5H), 3.98–4.04 (m, 2H), 3.57–3.59 (m, 1H),
3.40–3.44 (m, 1H), 3.17 (m, 1H), 2.84–3.06 (m, 3H), 2.72–2.78
(m, 3H), 2.60–2.68 (m, 4H), 2.22–2.39 (m, 16H), 2.08–2.19 (m,
4H), 1.88–2.00 (m, 3H), 1.70–1.82 (m, 7H), 1.58–1.67 (m, 4H),
1.48–1.54 (m, 7H), 1.23–1.36 (m, 6H), 0.99–1.03 (m, 6H), 0.88 (s,
2H), 0.78 (d, J ¼ 6.74, 3H), HR-MS: calcd M+ ¼ 2176.82, obsvd
(M + H)+ ¼ 2177.8274.
Ac-K(Taxol)E-ss-EE. 1H NMR (400 MHz, DMSO-d6) d 7.98–
8.00 (m, 2H), 7.85 (d, J ¼ 7.63, 2H), 7.72–7.75 (m, 1H), 7.64–7.69
(m, 2H), 7.55–7.59 (m, 1H), 7.44–7.51 (m, 6H), 7.20–7.21 (m, 1H),
6.30 (s, 1H), 5.82–5.87 (m, 1H), 5.54–5.59 (m, 2H), 5.34–5.43 (m,
2H), 4.91–4.93 (m, 2H), 4.65 (s, 1H), 4.09–4.15 (m, 1H), 3.99–4.02
(m, 2H), 3.59 (d, J ¼ 6.58, 1H), 3.52 (s, 1H), 3.33 (s, 11H), 2.95–
2.99 (m, 2H), 2.79–2.81 (m, 4H), 2.59–2.68 (m, 2H), 2.29–2.37 (m,
2H), 2.24 (s, 3H), 2.11 (s, 3H), 1.79–1.86 (m, 1H), 1.55–1.76 (m,
9H), 1.49–1.54 (m, 5H), 1.20–1.27 (m, 3H), 1.00–1.14 (m, 10H),
HR-MS: calcd M+ ¼ 1744.63, obsvd (M + H)+ ¼ 1745.6378.
2.2 Synthesis of the peptide
All the peptides were prepared by standard solid-phase peptide
synthesis (SPPS) using 2-chlorotrityl chloride resin and the cor-
responding Fmoc-protected amino acids with side chains prop-
erly modified. The first amino acid was loaded onto the resin at
about 0.7 mmol gꢀ1 of resin. After loading the first amino acid to
the resin, the capping regent (DCM : MeOH : DIPEA
¼
17 : 2 : 1) was used to ensure that all the active sites of the resin
were protected.A solution of 20% piperidine in DMF was used to
remove the Fmoc group, next the Fmoc-protected amino acid
was coupled to the free amino group using HBTU as the coupling
reagent. The growth of the peptide chain followed the established
Fmoc SPPS protocol. The crude peptides were collected using the
TFA-mediated cleavage method: the peptide derivative was
cleaved using 95% of trifluoroacetic acid with 2.5% of TIS and
2.5% of H2O for 30 minutes. 20 mL per gram of resin of ice-cold
diethyl ether was then added to cleavage the reagent. The
resulting precipitate was centrifuged for 10 min at room
temperature at 10 000 rpm. Afterward the supernatant was
decanted and the resulting solid was dissolved in DMSO for
HPLC separation. HPLC was used to separate the peptides.
Taxol-K(ac)E-ss-EE. 1H NMR (400 MHz, DMSO-d6) d 7.94–
8.00 (m, 3H), 7.84–7.86 (m, 2H), 7.72–7.78 (m, 2H), 7.65–7.69
(m, 2H), 7.54–7.59 (m, 1H), 7.44–7.52 (m, 6H), 7.19–7.22 (m,
1H), 6.30 (s, 1H), 5.80–5.85 (t, 1H), 5.52–5.57 (t, 1H), 5.42 (d, J ¼
6.99, 1H), 5.36 (d, J ¼ 8.65, 1H), 4.90–4.94 (m, 2H), 4.65 (s, 1H),
4.25–4.31 (m, 1H), 4.11–4.22 (m, 4H), 3.99–4.04 (m, 2H), 3.59 (d,
J ¼ 6.97, 1H), 3.35–3.52 (m, 10H), 3.24–3.34 (m, 4H), 2.96–3.01
(m, 2H), 2.72–2.80 (m, 4H), 2.60–2.67 (m, 2H), 2.30–2.47 (m,
2H), 2.24–2.34 (m, 14H), 2.11 (s, 3H), 1.87–2.00 (m, 3H), 1.77 (m,
10H), 1.55–1.66 (m, 2H), 1.50 (s, 4H), 1.33–1.37 (m, 2H), 1.22–
1.24 (m, 2H), 1.00–1.03 (m, 6H). HR-MS: calcd M+ ¼ 1744.63,
obsvd (M + H)+ ¼ 1745.6368.
Taxol-E-ss-EE. 1H NMR (400 MHz, DMSO-d6) d 7.99 (d, J ¼
7.55, 2H), 7.86 (d, J ¼ 7.27, 2H), 7.72–7.76 (m, 1H), 7.64–7.69 (m,
2H), 7.54–7.59 (m, 1H), 7.44–7.52 (m, 6H), 7.18–7.22 (m, 2H),
6.21 (s, 1H), 5.81–5.85 (t, 1H), 5.53–5.58 (t, 1H), 5.42 (d, J ¼ 6.82,
1H), 5.34–5.37 (m, 1H),4.90–4.93 (m, 1H), 4.66 (s, 1H), 4.25–4.30
(m, 1H), 4.11–4.19 (m, 3H), 3.99–4.04 (m, 2H), 3.58–3.60 (m,
2H), 3.41–3.57 (m, 9H), 3.28–3.32 (m, 4H), 2.74–2.78 (m, 4H),
2.60–2.67 (m, 2H), 2.45–2.47 (m, 2H), 2.21–2.34 (m, 14H), 2.11
(s, 3H), 1.78–2.00 (m, 8H), 1.50–1.73 (m, 7H), 1.00–1.03 (m, 5H),
HR-MS: calcd M+ ¼ 1574.52, obsvd (M + H)+ ¼ 1575.5303.
2.3 Preparation of different precursors
100 mg (0.10 mmol) of Taxol-SA was dissolved in 10 mL of
dichloromethane (DCM), followed by 1.1 equiv. (13.2 mg, 0.115
mmol) N-hydroxysuccinimide (NHS) and 25.9 mg (0.126 mmol)
of N,N0-dicyclohexylcarbodiimide with a catalytic amount of 4-
dimethylamiopryidine were added. After being stirred at room
temperature for 3 h, the solution was filtered by a filter paper to
remove the precipitation. The filtrate was evaporated under
reduced pressure to yield a white power, which was used directly
for the next step. After the white powder obtained above was
dissolved in 5 mL of N,N-dimethylformamide, 1.3 equiv. of
corresponding peptide or amino acid was then added with 3
equiv. N-diisopropylethylamine (DIPEA). The resulting reaction
Taxol-R-ss-EE. 1H NMR (400 MHz, DMSO-d6) d 7.86 (d, J ¼
7.56, 2H), 7.71–7.77 (m, 1H), 7.64–7.70 (m, 1H), 7.46–7.60(m,
8H), 7.19–7.21 (m, 2H), 6.29 (s, 1H), 5.80–5.85 (t, 1H), 5.51–5.55
(t, 1H), 5.42 (d, J ¼ 7.27, 1H), 5.34–5.36 (d, 1H), 4.92 (d, J ¼
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7.86, 2H), 4.64 (s, 1H), 4.24–4.30 (m, 1H), 4.07–4.20 (m, 3H),
3.99–4.05 (m, 2H), 3.56–3.59 (m, 1H), 3.06–3.08 (m, 2H), 2.72–
2.81 (m, 4H), 2.61–2.68 (m, 2H), 2.20–2.35 (m, 12H), 2.11 (m,
3H), 1.85–1.99 (m, 2H), 1.61–1.82 (m, 8H), 1.40–1.52 (m, 7H),
0.99–1.05 (m, 6H), HR-MS: calcd M+ ¼ 1601.58, obsvd (M +
H)+ ¼ 1602.5892.
compounds were then added into the cells. 72 hours later, we
replaced the medium with fresh medium supplemented with
15 mL of MTT reagent (5 mg mLꢀ1). After another 4 hours, the
medium containing MTT was removed and DMSO (100 mL per
well) was added to dissolve the formazan crystals. The optical
density of the solution was measured at 490 nm, using a micro-
plate reader (Bio-RAD iMarkTM, America). Cells without the
treatment of the compounds were used as the control. The
experiment was repeated for 3 times. The cell viability percentage
was calculated by the following formula: The cell viability
percentage (%) ¼ ODsample/ODcontrol ꢃ 100%. Prism 5.0 software
was used to calculate the IC50.
1
Taxol-S-ss-EE. H NMR (400 MHz, DMSO-d6) d 7.97–7.80
(m, 3H), 7.86 (d, J ¼ 7.64, 2H), 7.24–7.76 (m, 1H), 7.64–7.69 (m,
2H), 7.55–7.59 (m, 1H), 7.45–7.52 (m, 6H), 7.18–7.21 (m, 1H),
6.30 (s, 1H), 5.81–5.86 (t, 1H), 5.53–5.58 (t, 1H), 5.42 (d, J ¼ 7.00,
1H), 5.34–5.37 (m, 2H), 4.90–4.93 (m, 1H), 4.25–4.31 (s, 1H),
4.08–4.22 (m, 3H), 3.99–4.04 (m, 2H), 3.58–3.60 (m, 1H), 3.49–
3.54 (m, 2H), 3.29–3.37 (m, 4H), 2.73–2.77 (m, 4H), 2.60–2.68 (m,
3H), 2.20–2.34 (m, 12H), 2.09–2.14 (m, 3H), 1.88–2.01 (m, 3H),
1.75–1.83 (m, 5H), 1.55–1.74 (m, 3H), 1.50 (s, 3H), 0.97–1.04 (m,
6H), HR-MS: calcd M+ ¼ 1532.51, obsvd (M + H)+ ¼ 1533.5204.
3. Results and discussion
3.1 Gelation ability
Our previous results indicated that Dex-FFFK(Taxol)E-ss-EE
(Dex, F, K, E, and -ss- represented dexamethasone, phenylala-
nine, lysine, glutamic acid, and disulfide linker, respectively)
could form hydrogels both at room temperature and 37 ꢁC after
being converted to Dex-FFFK(Taxol)E-s by glutathione
(GSH).8 However, the structure of Dex-FFFK(Taxol)E-ss-EE is
very complicated and the resulting hydrogels are only stable for
about two weeks. Therefore, we opt to refine the chemical
structure of Dex-FFFK(Taxol)E-ss-EE to develop structurally
simpler molecular hydrogelators of Taxol.
2.5 Preparation of hydrogels
5 mg of a precursor of the gelator was dissolved in 0.49 mL of PBS
buffer solution (pH ¼ 7.4). 4.0 equiv. of glutathione (GSH) in
10 mL of PBS buffer (pH ¼ 7.4, adjusted by Na2CO3) was then
added to the above solution. A gel would form after the solution
was kept at room temperature (22–25 ꢁC) or 37 ꢁC for 30 minutes.
2.6 Determination of release profile of Taxol from gels
We firstly removed tripeptide of FFF from Dex-FFFK(Taxol)
E-ss-EE to make Dex-K(Taxol)E-ss-EE (Scheme 1). We
observed the formation of a molecular hydrogel (DexK(Taxol)E-
gel in Fig. 1A) by adding 4 equiv. of GSH to a phosphate buffer
solution (PBS, pH ¼ 7.4) containing 1.0 wt% of Dex-K(Taxol)E-
ss-EE within 10 minutes. Similar to gels formed from solutions of
Dex-FFFK(Taxol)E-ss-EE, the gels from Dex-K(Taxol)E-ss-EE
were not very stable either and only stable for 3 days at 37 ꢁC. We
then removed the part of Dex from Dex-K(Taxol)E-ss-EE to
make Ac-K(Taxol)E-ss-EE (Scheme 1). Surprisingly, Ac-
K(Taxol)E-ss-EE could also form hydrogels (AcK(Taxol)E-gel
in Fig. 1B) by GSH. These results indicated that both Dex and
the tripeptide of FFF were not requisites for building molecular
hydrogelators of Taxol.
0.25 mL of gels (1.0 wt%, 8 hours after the addition of GSH) was
treated with 0.25 mL of fresh PBS buffer solutions (pH ¼ 7.4).
0.2 mL of the upper buffer solution was taken out to run LC-MS
and 0.2 mL of fresh PBS buffer solution was added back each
time. The areas of the peaks in LC-MS spectra were used to
determine the percentage of Taxol from their corresponding gels.
The experiment was conducted in 3 parallel experiments. The
experiment was conducted at 37 ꢁC.
2.7 Rheological measurement
All rheological experiments were performed at 37 ꢁC ꢂ 0.1 ꢁC in
different modes (dynamic time sweep, dynamic frequency sweep,
dynamic strain sweep) using an AR 1500ex rheometer (TA
company, America) by 40 mm parallel plates. To minimize
evaporation, a solvent trap was employed and a low viscosity
mineral oil was applied around the sample. Time sweep experi-
ment data were collected at 1.0 rad sꢀ1 frequency and 1.0% strain.
Dynamic frequency sweep experiments were performed in the
range of 0.1–100 rad sꢀ1 at 1.0% strain. To study the injectable
properties of the hydrogel, time sweep experiments were per-
formed at 1.0 rad sꢀ1 frequency and 1.0% strain after the hydrogel
had been injected through a 1 mL syringe to the rheometer.
There are many gelators of lysine (K) derivatives with
hydrophobic molecules attached on the 3-position amine group
of K.13 In order to study the influence of the position of Taxol on
the two amine groups of K, we prepared the compound of Taxol-
K(Ac)E-ss-EE (Scheme 1). It could also form gels (Taxol-K(Ac)
E-gel in Fig. 1C) by GSH. We then removed K(Ac) from Taxol-
K(Ac)E-ss-EE to make Taxol-E-ss-EE. It was interesting that
Taxol-E-ss-EE could also gel aqueous solutions after being
converted to Taxol-E-s (Taxol-E-gel in Fig. 1D). We then
changed E to other amino acids with different charges such as
arginine (R) with a positive side chain and serine (S) with a
neutral side chain. Both solutions of Taxol-R/S-ss-EE could also
be converted to gels by adding GSH (Taxol-R/S-gel in Fig. 1E
and 1F). These results indicated that conjugates of Taxol and
even a single amino acid were efficient molecular hydrogelators.
We found that the Taxol-E-gel was stable at room temperature
2.8 Determination of IC50 values
The cytotoxicity of different peptides and the IC50 values of
Taxol, Taxol-succ, precursors of gelators, and gelators were
measured by the MTT cell viability test. The HepG2 cells were
seeded in 96-well plates at a density of 2000 cells per well with a
total medium volume of 75 mL and incubated for 24 hours. 25 mL
of the solutions containing serials of concentrations of above
ꢁ
and 37 C for at least 6 months. However, other five gels were
only stable for less than 7 days and would change to precipitates
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3.2 Rheology
We studied the viscoelasticity of all hydrogels, which reflected the
mechanical property of them. Dynamic time sweeps could
provide useful information about the process from the flow state
to gel state and indicate the gelling point when the storage
modulus (G0) was bigger than the loss modulus (G00). According
to Fig. S-13,† six hydrogels could form within 30 min at room
temperature after 4 equiv. of GSH had been added to reduce the
disulfide bond and convert precursors to their corresponding
gelators. Dynamic frequency sweeps (Fig. 2) were then used to
compare the mechanical properties of the resulting gels formed
2 hours after the addition of GSH, all hydrogels exhibited similar
G0 values of around 1000 Pa, except the Taxol-S-gel whose G0
value was above 10 000 Pa. Both G0 and G00 values of all gels
showed weak frequency dependencies at the region of 0.1 to
100 rad sꢀ1, suggesting elastic networks in the hydrogels.
3.3 TEM images of hydrogels
Negative-stained transmission electron microscopy (TEM) images
of hydrogels were recorded to study the self-assembled structures
in hydrogels. As shown in Fig. 3, we observed networks of nano-
fibers in all hydrogels. However, the density and morphology of
fibers were different. Dex-K(Taxol)E-gel (Fig. 3A), Taxol-R-gel
(Fig. 3E), and Taxol-S-gel (Fig. 3F) exhibited dense networks of
fibers with the width of around 10–30 nm, suggesting heavy
aggregations of self-assembled structures in these gels. Compared
with these three gels, both AcK(Taxol)-E-gel (Fig. 3B) and Taxol-
K(Ac)-E-gel (Fig. 3C) also showed dense networks of fibers of
Scheme
1 Chemical structures of precursors of molecular
hydrogelators.
Fig. 1 Optical images of hydrogels formed by treating PBS solutions
containing 1.0 wt% of different precursors with 4 equiv. of GSH. (A)
Dex-K(Taxol)E-gel (B) AcK(Taxol)E-gel, (C) Taxol-K(Ac)E-gel, D)
Taxol-E-gel, (E) Taxol-R-gel, and (F) Taxol-S-gel.
in the end. Except the Taxol-S-gel, the other five as-formed
hydrogels were injectable. As shown in Fig. S-15 and S-16,† three
hydrogels (AcK(Taxol)E-gel, Taxol-K(Ac)E-gel, and Taxol-E-
gel) could re-form within 5 minutes after injection through a
1.0 mL syringe (Becton Dickinson Consumer Products). Both
Dex-K(Taxol)E-gel and Taxol-R-gel could not re-form until
after at least 12 h. The Taxol-S-gel changed to a partial gel with
precipitation after injection. The phenomena suggested that the
as-formed hydrogels, except Taxol-S-gel, were injectable,
which is beneficial for their future applications. We then char-
acterized these hydrogels by different techniques including
rheology and TEM.
Fig. 2 Rheological measurements in the dynamic frequency sweep mode
for all the gels formed 2 h after the addition of 4 equiv. of GSH at a strain
of 1.0% (initial precursor concentration ¼ 1.0 wt%): (A) Dex-K(Taxol)E-
gel, (B) AcK(Taxol)E-gel, (C) Taxol-K(Ac)E-gel, (D) Taxol-E-gel, (E)
Taxol-R-gel, and (F) Taxol-S-gel.
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about 10–20 nm in width but the fibers were more evenly distrib-
uted. Taxol-E-gel exhibited a totally different morphology of
nanofibers to other five gels in where we observed well-distributed
uniform nanofibers of about 20 nm. The aggregation degree was
followed by the order of Dex-K(Taxol)E-gel ꢄ Taxol-R-gel ꢄ
Taxol-S-gel > AcK(Taxol)E-gel ꢄ Taxol-K(Ac)E-gel > Taxol-E-
gel, which interpreted the stability of these gels that Dex-K(Taxol)
E-gel, Taxol-R-gel, and Taxol-S-gel were only stable for three
days, AcK(Taxol)E-gel and Taxol-K(Ac)E-gel were stable for 5–
7 days, and Taxol-E-gel was stable for more than 6 months.
3.4 Release profile of Taxol from gels
Fig. 4 (top) showed the release profiles of Taxol from different
hydrogels at physiological temperature condition (37 ꢁC). All
hydrogels released original Taxol molecule but not Taxol
derivatives by the ester bond hydrolysis. Dex-K(Taxol)E-gel
released Taxol at a constant rate of about 0.48 mg mLꢀ1 per hour
from 2 to 24 hours. AcK(Taxol)E-gel exhibited a two stage
release profile—it released Taxol at a rate of ꢄ6.0 mg mLꢀ1 per
hour in the first 12 hours, followed by a rate of ꢄ2.6 mg mLꢀ1 per
hour in the next 12 hours. Similar to AcK(Taxol)E-gel, Taxol-
K(Ac)E-gel also showed a two stage release profile but the rates
were different—1.6 and 1.13 mg mLꢀ1 per hour in the first 8 hours
and the following 16 hours, respectively. Both Taxol-R-gel and
Taxol-S-gel showed burst release profiles in the first 12 hours and
possessed much slower release speeds in the following 12 hours.
Interestingly, Taxol-E-gel exhibited a totally different release
profile to the other five gels. There were no burst releases of
Fig. 4 (top) Accumulative release profile of Taxol from different kinds
of hydrogels at 37 ꢁC in 100 mM PBS buffers (pH ¼ 7.4) and (bottom)
accumulative release profile of Taxol from Taxol-E-gel at 37 ꢁC in PBS
and PBS containing different concentration of BSA.
Taxol from the Taxol-E-gel, and the Taxol-E-gel released Taxol
at a constant rate of about 2.3 mg mLꢀ1 per hour throughout the
entire measurement period of 24 hours. We could not conclude a
relationship between the release profiles of Taxol from gels and
other characterizations of the gels. We believed that the constant
release profile of Taxol-E-gel could be beneficial to its future
practical applications.
The presence of surfactant or serum would affect the release
profile of drug molecules from hydrogels because surfactant or
serum could serve as a solubilising agent to increase the solubility
of the hydrophobic drug molecules in aqueous solution.14 We also
obtained the release profile of Taxol from Taxol-E-gel with bovine
serum albumin (BSA) as the solubilising agent. As shown in Fig. 4
(bottom), the addition of BSA enhanced the release and changed
the release profile of Taxol from the gel. Taxol-E-gel exhibited a
constant release in the 24 hour experimental period in the absence
of BSA (Fig. 4 (top)), while it showed a two stage release profile in
the presence of BSA (Fig. 4 (bottom)). For example, Taxol-E-gel
released Taxol at a rate of ꢄ8.15 mg mLꢀ1 per hour in the first
10 hours, following by a rate of ꢄ1.72 mg mLꢀ1 per hour in the next
14 hours when PBS containing 10% of BSA was used.
Fig. 3 Negative-stained TEM images of the gels: (A) Dex-K(Taxol)E-
gel, (B) AcK(Taxol)E-gel, (C) Taxol-K(Ac)E-gel, (D) Taxol-E-gel, (E)
Taxol-R-gel, and (F) Taxol-S-gel.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 16933–16938 | 16937

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Acknowledgements
This work is supported by NSFC (21172023), A Project Funded
by the Priority Academic Program Development of Jiangsu
Higher
Education
Institution,
and
Tianjin
MSTC
(11JCZDJC17200).
Notes and references
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4. Conclusions
In summary, we have developed a series of hydrogelators based
on Taxol and short peptides/amino acids with simple synthetic
strategies and high yields (more than 75%). The gels showed
high weight percentages of Taxol in the self-assembled struc-
tures (about 50.0%) and most of them were injectable. These
gels exhibited different stabilities and release profiles of Taxol
due to the different chemical structures of gelators. Though we
were unable to conclude a relationship between the chemical
structures and the release profiles/stabilities of the hydrogels,
we have found the gel of Taxol-E-gel, with an extraordinary
stability, could release original Taxol at a constant rate without
any burst releases. This was a detailed study on the gelation
ability of Taxol conjugates with short peptides and amino
acids, which could provide useful information for the devel-
opment of Taxol-hydrogels for practical chemotherapy
applications.
16938 | J. Mater. Chem., 2012, 22, 16933–16938
This journal is ª The Royal Society of Chemistry 2012