Ultrasound-Responsive Nanoparticulate for Selective Amplification of Chemotherapeutic Potency for Ablation of Solid Tumors

Article abstract of DOI:10.1021/acs.bioconjchem.8b00626

Precision medicine requests preferential transportation of the pharmaceutical substances to the pathological site and impartation of localized therapeutic activities to the targeted cells. To accomplish this goal, we attempted a facile nanoscaled ultrasound-responsive delivery system, characterized by doxorubicin assembled with an amphiphilic copolymer (multiple of hydrophobic stearic segments tethered onto the hydrophilic pullulan backbone through ultrasound-labile oxyl-alkylhydroxylamine linkage). As a consequence of the strategically installed ultrasound-labile oxyl-alkylhydroxylamine linkage to elicit the tailored segregation of the hydrophilic pullulan and the hydrophobic stearic segments upon ultrasound impetus, the constructed nanoscaled self-assembly presented distinctive structural destabilization behaviors and afforded spatiotemporal controlled liberation of the cytotoxic drugs. It is worthy to note that the ultrasound was determined to markedly lower the IC₅₀ of the proposed system from over 10 μg/mL to 2.33 μg/mL (approximate 4-fold), thereby serving as a facile impetus to amplify the cytotoxic potency of the proposed drug delivery vehicles. Furthermore, drastic tumor ablation was validated by dosage of the proposed doxorubicin delivery system to T41 tumor-bearing mice accompanied by the tumor-localized ultrasound impetus, while no observable adverse side effect was confirmed. Therefore, the results advocated our ultrasound-responsive delivery vehicle as a tempting strategy for precise spatiotemporal control of the release of the drug cargo, thus affording selectively amplified cytotoxic potency to the ultrasound-imposed site, which should be highlighted as important progress toward precision medicine.

Full text of DOI:10.1021/acs.bioconjchem.8b00626

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Article
Ultrasound-Responsive Nanoparticulate for Selective Amplification
of Chemotherapeutic Potency for Ablation of Solid Tumors
Jing Xia, Jingyun Wang, Xiang Wang, Ming Qian, Liuwei Zhang, and Qixian Chen
Bioconjugate Chem., Just Accepted Manuscript • DOI: 10.1021/acs.bioconjchem.8b00626 • Publication Date (Web): 06 Sep 2018
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Ultrasound-Responsive Nanoparticulate for
Selective Amplification of Chemotherapeutic
Potency for Ablation of Solid Tumors
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Jing Xia,†‡ Jinyun Wang,†‡ Xiang Wang,‡ Ming Qian,‡ Liuwei Zhang,‡ Qixian Chen†‡
*
†State Key Laboratory of Fine Chemicals, Dalian University of Technology, No. 2 Linggong Road,
Dalian 116024, China
‡School of Life Science and Biotechnology, Dalian University of Technology, No. 2 Linggong Road,
Dalian 116024, China
*All correspondence should be addressed to Q. C. (E-mail: qixian@dlut.edu.cn).
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ABSTRACT: Precision medicine requests preferential transportation of the pharmaceutical
substances to the pathological site and impartation of localized therapeutic activities to the targeted
cells. To accomplish this goal, we attempted a facile nanoscaled ultrasound-responsive delivery
system, characterized by doxorubicin assembled with an amphiphilic copolymer (multiple of
hydrophobic stearic segments tethered onto the hydrophilic pullulan backbone through
ultrasound-labile oxyl-alkylhydroxylamine linkage). As a consequence of the strategically installed
ultrasound-labile oxyl-alkylhydroxylamine linkage to elicit the tailored segregation of the hydrophilic
pullulan and the hydrophobic stearic segments upon ultrasound impetus, the constructed
nanoscaled self-assembly presented distinctive structural destabilization behaviors and afforded
spatiotemporal controlled liberation of the cytotoxic drugs. It is worthy to note that the ultrasound
was determined to markedly lower the IC50 of the proposed system from over 10 μg/mL to 2.33
μg/mL (approximate 4-fold), thereby serving as a facile impetus to amplify the cytotoxic potency of
the proposed drug delivery vehicles. Furthermore, drastic tumor ablation was validated by dosage of
the proposed doxorubicin delivery system to T41 tumor-bearing mice in accompanied with the
tumor-localized ultrasound impetus, while no observable adverse side effect was confirmed.
Therefore, the results advocated our ultrasound-responsive delivery vehicle as a tempting strategy
for precise spatiotemporal control of the release of the drug cargos, thus affording selectively
amplified cytotoxic potency to the ultrasound-imposed site, which should be highlighted as an
important progress toward precision medicine.
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KEYWORDS: pullulan, ultrasound-responsive, tumor therapy, drug delivery, doxorubicin
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INTRODUCTION
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The development of nanotechnology has led to appreciation of nanoparticulate-based delivery
strategies, termed as nanomedicine, to be capable of affording targeted transportation of
therapeutic substances to the pathological sites. Particularly, the chemotherapeutic drugs, once
encapsulated into appropriate targeted delivery vehicles, are postulated to remarkably improve the
drug availabilities to the pathological tissues, consequently conducing to improved therapeutic
potency and reduced off-target adverse effect 1-3. To date, a number of nanoparticulates
encapsulating chemotherapeutic drugs were approved by FDA for treatment of intractable cancers,
including Doxil, DaunoXome, Mariqbo, Lipusu and Abraxane 2, 4-5. These drug formulations were
documented to exert appreciable tumor-preferential accumulation activities either by passive
targeting approach based on the tumor-characterized enhanced permeability and retention (EPR)
properties 6-7 or active targeting strategies with aids of specific antibodies or ligands 8-9. Nevertheless,
in subsequence to tumor accumulation, the drug release kinetics from the delivery nanoparticulates
usually relied on the slow drug diffusion rate or the chronic degradation process of the delivery
materials, which substantially restricted the subsequent therapeutic potency 10-11. Spatiotemporal
control of drug release at the targeted location is imperative to advance nanomedicine towards
practical applications. Aiming to promote the therapeutic efficacy at the targeted site, a number of
stimuli-responsive facilities was devised into the delivery nanoparticulates 12-13 to pursue accelerated
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drug release at the tumor sites in response to endogenous stimuli (i.e., pH, glutathione, hypoxia,
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enzymes and other tumor markers)
radiation and particle radiation) 6, 19-20
or exogenous stimuli (i.e., temperature, electromagnetic
.
In view of a variety of stimuli, ultrasound is deemed to be an attractive impetus to seek targeted
amplification of therapeutic efficacy of the nanomedicine with respect to its inherent advantages of
21-23
precise spatiotemporal controllability, noninvasiveness and excellent tissue penetration depth
.
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As opposed to the other exogenous stimulus (ionizing electromagnetic radiation, particle radiation,
electric impulse, etc.), the remarkable safety profile of ultrasound to the human body led to
appreciation of ultrasound as a tempting external impetus. Moreover, the high-frequency ultrasound
to elicit deep tissue penetration is envisioned to render a burst drug release and subsequent potent
cytotoxicity selectively to the ultrasound-localized tumors, thereby minimizing the adverse side
effects to the off-target tissues. Moreover, ultrasound stimuli also appeared to be beneficial in
promoting nanoparticle extravasation through blood capillaries and nanoparticle permeation across
cell membrane24-26. Therefore, ultrasound represented an intriguing alternative stimulus to trigger
the precise controlled drug release at the desired site in a spatially and temporally manner, with the
ultimate aim of improved therapeutic efficacy and minimized adverse effects. Note that the pioneer
research from Dr. J. L. Paris et al.26 has developed a novel ultrasound-responsive doxorubicin (DOX)
delivery formation based on mesoporous silica nanoparticles grafted with copolymers containing
ultrasound-labile chemical bonds on their surface that served as gatekeeper of the silica pores. Upon
ultrasound impetus, the ultrasound-responsive system was subjected to marked molecular
transformation to induce the emergence of abundant avenues and facilitate the release of the DOX
payloads.
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In the present study, we attempted to synthesize a novel pullulan-based drug delivery system,
self-assembled from amphiphilic copolymer with hydrophobic doxorubicin payloads. Particularly, the
pullulan-based amphiphilic polymer was characterized to have a pullulan (referred as P in the
abbreviation of amphiphilic polymer) backbone tethered with a multiple of hydrophobic stearic (SC)
segments through ultrasound-labile linkage of oxyl-alkylhydroxylamine bond (-oa-) (P-oa-SC in
Scheme 1). Noteworthy is pullulan produced by aureobasidium pullulans, which is a
homopolysaccharide consisting of maltotriose units. Due to its excellent biocompatibility and
modifiability, pullulan has been widely employed in biomedical applications27-29. As shown in Scheme
1 (Figure 1), DOX as a hydrophobic anticancer drug was chosen as a model drug and successfully
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loaded into the P-oa-SC self-assembly formation. The obtained pullulan-based ultrasound-triggered
DOX delivery system (P-oa-SC/DOX) is speculated to be capable of employing spatial-controlled
ultrasound impetus to pursue precise amplification of the cytotoxic potency to the tumors, thereby
entitling selectively enhancement of anti-tumor efficacy to the ultrasound-specified tumor site due
to the ultrasound-mediate accelerated drug release and minimal toxicity to the normal tissue due to
the reluctant drug release.
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Scheme 1. Illustration of construction of ultrasound-responsive P-oa-SC/DOX nanoparticulates to
pursue ultrasound-specified chemotherapeutic potency to the tumors.
RESULTS AND DISCUSSION
Polymer Synthesis and Characterizations. The amphiphilic copolymer of P-oa-SC was
synthesized according to the synthetic scheme (Figure 1a). In brief, the precursor pullulan bearing a
variety of chlorides was obtained through acylation reaction between the acryl chloride group of
4-chlorobutyryl chloride and the hydroxyl groups of pullulans. On the other hand,
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4-hydroxyl-2,2,6,6-tetramethyl-1-piperidine oxidanyl radical (HTEMPO) was introduced to the acryl
chloride group of stearic chloride through acylation reaction to yield nitroxide radical functionalized
stearic derivative (OC). The above yielded products were linked based on nitroxide radical coupling
reaction between chlorinated pullulan (P-Cl) and OC to yield P-oa-SC, wherein the -oa- linkage served
as a facile ultrasound-labile linkage.
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Figure 1. Synthetic scheme and characterizations of P-oa-SC. (a): synthetic scheme of P-oa-SC; (b):
1H-NMR spectrum of OC in CDCl3; (c) 1H-NMR spectra of pullulan and its derivatives in DMSO-6.
The yielded products of OC, P-Cl and P-oa-SC were characterized by 1H-NMR measurement. The
representative spectra of OC in CDCl3 was shown in Figure 1b, wherein the peaks at 5.23 ppm and
1.26 ppm could be assigned to the methine (-CH-O) and methyl (-CH3) moieties of piperidine and the
1
peak at 0.88 ppm corresponding to the methyl (CH3) moiety of stearic acid. The H-NMR results
verified the successfully synthesis of OC. On the other hand, the 1H-NMR spectrum of pullulan and its
derivatives was shown in Figure 1c. Note that the broad peak in the range of 3.0–4.0 ppm could be
assigned to the methine and methylene protons (CH-O and CH2-O) of pullulan 17 while the doublet at
approximate 5.5 ppm corresponds to the methylidyne proton (O-CH-O) on glucose units of pullulan30
and the peak at 1.96 ppm belongs to methylene (-CO-CH2-CH2-Cl) of chlorobutyryl chloride. The
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peaks at 0.85 ppm and 1.23 ppm could be assigned to the methyl (-CH3) and methylene (-CH2) on
alkyl segment of stearic chloride. Overall, the number of chlorides in P-Cl was determined to be
approximate 7.21 per 100 glucose units and approximate 4.2 SC segments per 100 glucose units.
To test the feasibility of -oa-cleavage in P-oa-SC responsive to ultrasound impetus, the P-oa-SC
solution was subjected to ultrasound treatment for 5 min, and the reaction solution was transferred
to liquid chromatography and mass spectrometry measurements. As shown in Figure 2b, distinctive
stearic derivative was observed in liquid chromatography (resembling the liquid chromatography
trace of the reference stearic acid). Moreover, mass spectrometry characterization for the product of
Figure 2b(v) affirmed its molecular weight to be approximate 440.4 Da, indicating the production of
stearic alcohol [as shown in the chemical structure of Figure 2b(iv)]. These results validated our
strategically engineering ultrasound-specific labile -oa- linkage to attain ultrasound-mediated
segregation of hydrophilic pullulan and hydrophobic stearic derivatives.
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Figure 2. Characterizations of P-oa-SC and P-oa-SC/DOX. (a): DLS measurement for the P-oa-SC
self-assembly; (b) cleavage of -oa- linkage in P-oa-SC responsive to ultrasound impetus, the liquid
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chromatography of the reference OC (i), P-oa-SC in absence of ultrasound impetus (ii) P-oa-SC in
presence of ultrasound impetus (iii), iv): the inferred chemical structure of OC derivatives as the
product of the ultrasound-mediated P-oa-SC cleavage, v): the mass spectrometry of the peak fraction
in LC iii); (c): DLS measurement for the P-oa-SC self-assembly upon ultrasonication (1.0 MHz, 9.9 W, 3
W/cm2); (d): DLS measurement for P-oa-SC/DOX self-assembly; (e): the fluorescence emission
spectra of free DOX and P-oa-SC/DOX; (f): the cumulative release of DOX from P-oa-SC/DOX in
presence of ultrasound impetus (1.0 MHz, 9.9 W, 3 W/cm2). Data were presented as means ±
standard deviation (S.D.).
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Ultrasound-Stimulated Destabilization of Self-Assembled Nanostructure and Its Implications in
Ultrasound-Stimulated Drug Release. The ultrasound-responsive P-oa-SC nanoparticulates were
prepared by spontaneous self-assembly of amphiphilic P-oa-SC by a dialysis approach, wherein the
nanoscaled formation was achieved as a consequence of hydrophobic interactions between the
tethered SC segments. Of note, the critical micelle concentration (CMC) of P-oa-SC was determined
to be rather low, approximate 32 mg/L based on a pyrene assay (Figure S1). This result indicates the
appreciable colloidal stabilities of P-oa-SC-formulated structure despite in a diluted milieu (e.g. blood
circulation), which is believed to be capable of withstanding premature dissociation and release of
drug payloads post administration. Herein, the self-assembled structure of P-oa-SC was prepared at a
uniform concentration of 5 mg/mL in this study. Its formation was characterized by DLS
measurement, which exhibited an approximate 202.43 ± 3.37 nm hydrodynamic diameter with
unimodal distribution (PDI: approximate 0.1). Moreover, TEM measurement confirmed the
formation of uniform spherical nanoparticulates (Figure S2, left image). Note that appreciable
colloidal stability of the P-oa-SC self-assembly formation was confirmed, as evidenced by constant
DLS size over 15 days incubation in PBS (Figure S3).
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As validated above, the tethered stearic segments appeared to be readily detached from the
pullulan due to facile -oa- linkage breakage upon ultrasound impetus. To explore the impact of the
hydrophobic stearic segment detachment on the structure of P-oa-SC self-assembly, the structural
destabilization of P-oa-SC in responsive to ultrasound impetus was studied by DLS measurement.
Herein, the ultrasound (1.0 MHz, 9.9 W, 3 W/cm2) was adopted as the impetus, which has been
verified as a safety level to the biological species and the biological structures. The average DLS size
of P-oa-SC formation was observed to follow a consistent rise from 202.80 nm to 633.30 nm with
extended ultrasound treatment until 30 min. In consistent with DLS measurement, apparent
structural transformation and secondary aggregation was observed for P-oa-SC self-assembly after
ultrasonication (Figure S2, right). A plausible reason for this observation should be attributable to the
ultrasound-mediated dePEGylation due to cleavage of the -oa- linkage, thereby resulting into the
rearrangement of the self-assembled structure from amphiphilic polymers post the detachment of
hydrophobic segments and aggregative reactions between the dePEGylated self-assemblies. On the
contrary, the DLS size of P-oa-SC nanoparticulates appeared to remain constant without ultrasound
despite extended incubation in PBS (10 mM) (Figure S3). Moreover, appreciable resistance of P-SC
(lack of -oa- linkage) to ultrasound treatment was confirmed (Figure S4), implying the critical role of
-oa- linkage in eliciting structural destabilization rather than the consequences of the mechanical
turbulence of the ultrasonication31. Most likely, the growth in the particle size is conjectured to be as
a result of the breakage of -oa- linkage in P-oa-SC. This breakage could elicit detachment of
hydrophilic pullulan and hydrophobic stearic segments, thereby leading to slough-off of the
hydrophilic pullulan from the tethered hydrophobic stearic segments-formulated nanostructures,
ultimately accounting for structural rearrangement (e.g. secondary aggregation of the
nanostructures). Hence, the results approved the utility of the ultrasound-responsive chemistry for
pursuit of ultrasound-specified structural transformation of self-assembled nanoscaled delivery
systems.
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Furthermore, the hydrophobic drug of DOX was utilized to assemble with P-oa-SC with the aim
of exploring the utility of the ultrasound-specified structural transformation of P-oa-SC self-assembly
in seeking the controlled drug release functionality. Given that DOX is characterized to be an
aggregation-induced quench fluorophore whose fluorescence emission was subjected to markedly
drop when entrapped in an aggregated condition. Herein, the fluorescence emission spectra of DOX
were used to explore the evidence of DOX entrapment into the hydrophobic stearic region of
P-oa-SC self-assembly. As shown in Figure 2e, the DOX characteristic fluorescence emission of
P-oa-SC/DOX appeared to be subjected to drastically drop as compared to the free DOX. A plausible
reason for this aggregation-induced quench of DOX molecules should ascribe to the molecular
aggregation of DOX and hydrophobic stearic segments, thereby rendering an aggregation-induced
quench behavior. This result verified the facile encapsulation of DOX into the self-assembly of
P-oa-SC structure.
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The DOX release profiles from P-oa-SC/DOX were characterized in presence and absence of
ultrasound impetus. As shown in Figure 2f, reluctant drug release was confirmed for P-oa-SC/DOX in
absence of ultrasound impetus, approximate 35 % was determined to release at 24 h. On the
contrary, ultrasound impetus appeared to markedly accelerate the drug release rate, approximate
32 % release within 1 h in presence of ultrasound treatment (1.0 MHz, 9.9 W, 3 W/cm2, 30 min). A
plausible reason for this accelerated drug release should be as a consequence of ultrasound-induced
cleavage of the -oa- linkage. This ultrasound-triggered cleavage is speculated to prompt the
detachment of the hydrophobic stearic segments and hydrophilic pullulan backbone, consequently
accounting for rearrangement of the self-assembled P-oa-SC/DOX architecture. The structural
rearrangement, together with the mechanical impact imposed by ultrasound, could facilitate the
liberation of the DOX payload apart from the self-assembly. To this end, the proposed
ultrasound-stimulated cleavage strategy has been validated as a feasible approach to render
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ultrasound-dictating drug release functionality, which is envisioned to afford selective
ultrasound-dictating cytotoxic potency to the ultrasound-imposed cells.
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Figure 3. In vitro insight into the ultrasound-stimulated drug release and its impact on the cytotoxic
profiles. (a): intracellular distributions of P-oa-SC/DOX in absence and presence of ultrasound
impetus by CLSM observation; (b): quantification of intracellular fluorescence intensity of DOX for
MCF-7 cells treated with P-oa-SC/DOX in absence or presence of ultrasound impetus(1.0 MHz, 9 W, 3
W/cm2, 5 min); (c): cytotoxicity of P-oa-SC/DOX in presence or absence of ultrasound impetus (1.0
MHz, 9 W, 3 W/cm2, 5 min) in Hep-G2 cells. Data were presented as means ± standard deviation
(S.D.). *p < 0.05.
Controlled Intracellular Drug Release and Nucleus-Translocation of Doxorubicin in Responsive
to Ultrasound for Selective Cytotoxicity. Given that the fluorescence intensity of DOX is subject to a
pronounced jump once liberated as the free molecule (Figure 2e), the intracellular fluorescence
intensity of DOX was measured for the cells containing an identical amount of P-oa-SC/DOX in
presence or absence of ultrasound impetus by flow cytometry with aim of exploring the evidence of
the accelerated drug release by ultrasonication. Herein, two identical dishes of MCF-7 cells
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containing a same amount of P-oa-SC/DOX was achieved by incubation at a same concentration of
P-oa-SC/DOX (DOX concentration: 10 μg/mL). At 3 h post incubation, the cells were washed by PBS
and immersed in fresh medium, followed by ultrasound impetus (1.0 MHz, W, 3 W/cm2, 5 min). The
intracellular DOX fluorescence was measured at 2 h and 4 h post ultrasound impetus by flow
cytometry measurement, where the cells in absence of ultrasonication were employed as the control.
In consistent with our speculation, the intracellular level of DOX fluorescence intensity was
significant higher for the ultrasound-imposed cells than those without ultrasonication, as evidenced
by approximate 1.80-fold higher at 2 h and 1.59-fold higher at 4 h (Figure 3b). In consistent with flow
cytometry measurement, CLSM observation also captured a stronger fluorescence signal of
intracellular DOX (red) for the ultrasound-treated cells (Figure 3a). Moreover, significant stronger
fluorescence signal was observed for the cell s at 4 h relative to that at 2 h, implying consistent
release of DOX from P-oa-SC/DOX due to the nanostructural rearrangement of P-oa-SC/DOX induced
by ultrasound-mediated -oa- cleavage. Note that substantial DOX was observed in the nuclei of
ultrasound-treated cells, which is not observed for the cell in absence of ultrasonication. This
observation again approved the functionality of ultrasonication in liberation of DOX as molecular
form, which permitted its translocalization across the nucleus membrane and approach its
therapeutic targets.
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The facilitated drug release and consequent trafficking toward molecular target by ultrasound
impetus is believed to contribute enhanced cytotoxic potency to the affected cells. In this regard, the
cytotoxicity profiles of P-oa-SC in presence and absence of ultrasonication were studied in Hep-G2
cells using MTT assay. Free DOX was employed as a control. As shown in Figure 3c, IC50 of free DOX is
approximate 2.74 μg/ml, yet the cytotoxicity of P-oa-SC was observed to be significant reduced,
whose IC50 is over 10 μg/ml attributable to the reluctant release of DOX. Nevertheless, IC50 of
P-oa-SC/DOX with aids of ultrasound impetus was calculated to be 2.33 μg/mL, approved the
ultrasound-stimulated release to attain ultrasound-controlled cytotoxicity to the internalized cells.
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Note that ultrasound itself is not toxic to the cells (Figure 3c), validating the ultrasound stimulus as a
superior safe alternative than the other stimuli.
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Aside from the targeted cytotoxicity by the proposed ultrasound-based facility, it is also
important to verify the minimal cytotoxic activities to the off-target biological structures. To this
respect, the cytotoxicity and hemolysis activity of the blank P-oa-SC nanoparticulates were
investigated to gain the preliminary aspects of the biocompatibilities of the constructed P-oa-SC
system. Pertaining to the cytotoxic profiles of P-oa-SC nanoparticulates, MCF-7 cells, Hela cells,
Hep-G2 cells and COS-7 cells were used in an MTT assay. Overall, no significant cytotoxicity was
observed for all cell lines treated with P-oa-SC despite ultrasound, as evidence by cell viabilities
exceeding 90% even P-oa-SC at 400 μg/mL to the toxic-sensitive COS-7 cells (Figure S5), indicating
the appreciable safety profile of the constructed P-oa-SC delivery vehicle.
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Hemolysis is acknowledged as an important indicator of cell membrane damage and thus used
to estimate the biocompatibility of P-oa-SC32. At 6 h post incubation, the hemolysis rates of P-oa-SC
formation were determined to be negligible (below 3% even at a high concentration up to 2 mg/mL),
in stark contrast to the hemolysis rates of Tween 80 to be 15.67 % and 71.88 % at the concentration
of 0.5 mg /mL and 2 mg/mL) (Figure S6). The results demonstrated that the proposed P-oa-SC
self-assembly exhibited negligible hemolysis activity, implying its excellent compatibility. Together
with the appreciable negligible cytotoxic profile, these results suggested excellent biocompatibility of
P-oa-SC nanoparticulates, which is believed to be favorable in acquiring high drug bioavailability to
the tumors.
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Figure 4. In vivo anti-tumor efficacy of P-oa-SC/DOX in presence or absence of the tumor-localized
ultrasound impetus. a): schematic illustration of tumor-localized ultrasound to promote the cytotoxic
potency of administrated P-oa-SC/DOX to tumors; b): tumor growth profile; the volumes (c) and
weights (d) of anatomical tumor at day 18 post treatment; (e): H&E staining of tumors and major
organs at day 18 post treatment. Data were presented as means ± standard error (S.E.). *p < 0.05.
Pronounced Anti-Tumor Potency by Virtue of Spatial Ultrasound Impetus to Tumors. In view of
the ultrasound-stimulated selective cytotoxicity to the affected cells, we attempted a localized
ultrasonication to apply the tumors of 4T1-bearing mice post intratumor administration of
P-oa-SC/DOX nanoparticles. The anti-tumor efficacy was estimated by measuring the tumor volumes
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up to 18 days. As opposed to the limited inhibitory effect on the tumor growth from solely
ultrasound treatment (comparable to the PBS control group, approving the safety profile of
ultrasound impetus), P-oa-SC/DOX exhibited distinctive suppression to the growth of tumors, which
is believed to be as result of slow drug diffusion or degradation of the delivery materials eliciting the
DOX release from the delivery vehicles to impart chemotherapeutic efficacy. Noteworthy was the
potent eradication of solid tumor by P-oa-SC/DOX with aids of ultrasound impetus, approving the
strategic use of ultrasonication for pursuit of amplified chemotherapeutic potency of the
ultrasound-responsive delivery vehicles. Meanwhile, considerable inhibitory efficacy was also
obtained from the mice treated by the free DOX, nevertheless, severe systemic toxicity was observed
as evidenced by the marked drop in the body weight of the treated mice (Figure S7) and histology
analysis of diverse organs (particularly for spleen) by H&E staining (Figure 4e). In contrast, no
observable toxicity was confirmed for histology of the major organs (aside from the tumors) from
treatment of P-oa-SC/DOX. This result approved the appreciable safety profile of P-oa-SC/DOX.
Pertaining to the histology of the tumors of P-oa-SC/DOX-treated group, apparent pycnosis and
karyolysis patterns were observed, particular for ultrasound-imposed P-oa-SC/DOX group (abundant
necrosis and extensive hemorrhagic inflammation). To this end, the ultrasound stimuli-facility in
design of DOX delivery vehicles was validated to confer selective amplification of therapeutic
potency to the targeted on-demand tumor sites and minimize the potential off-target adverse effect.
In view of the wide availability of ultrasound as a spatial precisely localized impetus and the
feasibility to attain deep tissue penetration, our elaborated system could be emphasized to develop
further to find a broad utility in precision medicine applications.
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CONCLUSIONS
In this study, we have successfully tailored an ultrasound-labile -oa- linkage between the
hydrophobic stearic segments and the hydrophilic pullulan amphiphilic polymer. The developed
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pullulan-based amphiphilic polymer was determined to be able to self-assemble with hydrophobic
drugs (DOX) into nanoscaled delivery vehicles. The subsequent investigations revealed the proposed
pullulan-based delivery nanoparticles (P-oa-SC) possessed excellent biocompatibility and particularly
presented facile ultrasound-stimulated drug release characters. As a consequence of
ultrasound-stimulated cleavage of -oa- linkage, the constructed nanostructures have been validated
to present ultrasound-accelerated drug delivery profile, which allowed utility of ultrasound as the
drug release impetus to acquire selective on-demand chemotherapeutic potency to the targeted
cells. Furthermore, dosage of P-oa-SC/DOX supplemented with tumor-localized ultrasound exhibited
potent anti-tumor efficacy but minimized toxicities to the normal tissues despite the possibility of
non-specific distributions of P-oa-SC/DOX in the normal tissues. Consequently, the precise
spatiotemporal controlled nanomedicine utilizing ultrasound as a convenient external impetus to
amplify the cytotoxic potency could serve as an intriguing strategy for maximized inhibition of tumor
growth and minimized systemic toxicity.
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EXPERIMENTAL SECTION
Materials. Pullulan (MW = 100 kDa) was purchased from Shandong Zhongqing Biotechnology
Company (Zibo, China). 4-chlorobutyryl chloride, pyridine, N, N-dimethylformamide (DMF), 2,2,6,6-
HTEMPO, triethylamine (TEA), anhydrous-dichloromethane, stearyl chloride, magnesium sulfate,
2,2’-biopyridine, cuprous bromide, pyrene, 1-methyl-2-pyrrolidinone, dimethyl sulfoxide (DMSO),
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), colchicine, chlorpromazine
(CPM) were all obtained from Sigma-Aldrich (St. Louis, USA). Doxorubicin hydrochloride (referred as
DOX hereafter) was purchased from Dalian Meilun Biotechnology Co. Ltd. (Dalian, China). All the
other chemicals and solvents were purchased from Sigma-Aldrich (St. Louis, USA). The cell lines,
including human breast cancer cell (MCF-7), human cervical cancer cell (HeLa) and human hepatoma
cell (Hep-G2), mouse mammary tumor cell (4T1) and monkey kidney cell (COS-7), were purchased
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from Cell Bank of Shanghai Institute of Biochemistry and Cell Biology (Shanghai, China). Dulbecco's
modified Eagle's medium (DMEM) and fetal bovine serum (FBS) were purchased from Hyclone
(Logan, MA). Female Bal b/c mice (5 weeks old) were purchased from the animal experiment center
of Dalian Medical University (Dalian, China). All animal experimental protocols were established
according to the guidelines of the Animal Committee of Dalian University of Technology.
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Synthesis and Characterizations of P-oa-SC. The synthetic procedure of ultrasound-responsive
P-oa-SC was illustrated in Scheme 1. In brief, the chloride-functionalized pullulan (P-Cl) was obtained
by reaction of 4-chlorobutyryl chloride and pullulan, wherein pullulan (1.62 g, 10 mmol) in 60 mL
DMF was titrated with 4-chlorobutyryl chloride (1.72 mL, 15.37 mmol) and small-amount of pyridine.
At 36 h post reaction at room temperature, the reaction mixture was poured into 600 mL ethyl
alcohol to precipitate P-Cl. The yielded P-Cl was dissolved in 15 mL deionized water and transferred
to dialysis (MWCO: 3,500 Da) in deionized water for 5 times. The solid P-Cl product was obtained by
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lyophilization as white powder. The product was transferred to H-NMR measurement for
quantification of chloride composition in the yielded P-Cl.
Furthermore, the piperidinyl stearate (OC) was synthesized according to the procedures as
previously reported33. Briefly, HTEMPO (1.034 g, 6 mmol) and triethylamine (0.83 mL, 6 mmol) were
dissolved in 15 mL anhydrous-dichloromethane and stirred for 30 min in ice-water bath prior to
reaction. Stearic chloride (1.81 mL, 6 mmol) 10 mL anhydrous-dichloromethane was added to
aforementioned dichloromethane solution in argon atmosphere, followed by 30 °C reaction for 24 h.
The crude product was subjected to extraction in water to remove unreacted HTEMPO. The organic
phase collected and subjected to sequential treatments with magnesium sulfate, filtration over filter
paper, evaporation and column chromatography (petroleum ether: ethyl acetate=9:1) to yield OC as
dark solid.
Finally, P-oa-SC was synthesized based on nitroxide radical coupling reaction according to the
procedures as previously reported34. In brief, P-Cl (340 mg, nCl = 0.14 mmol), OC (42.5 mg, 0.1 mmol)
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and 2,2’-biopyridine (62.4 mg, 0.4 mmol) were dissolved in 30 mL 1-methyl-2-pyrrolidinone, followed
by stirring in argon atmosphere at 25 °C for 30 min with aim of eliminating unfavorable oxygen.
Furthermore, cuprous bromide (0.0286 g, 0.2 mmol) was added the aforementioned
1-methyl-2-pyrrolidinone solution and subjected to reaction for 24 h at 40 °C. The crude product of
P-oa-SC was obtained by precipitation in anhydrous ethanol, followed by dialysis (MWCO: 3500 Da)
in deionized water for 5 times. The P-oa-SC solid was obtained by lyophilization. The number of SC
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segments per P-oa-SC was estimated based on H-NMR measurement (Advance II 400, Bruker,
Switzerland).
Preparation and Characterizations of P-oa-SC Self-Assembly. P-oa-SC self-assembly was
prepared according to a dialysis method10. Briefly, the synthesized P-oa-SC (10 mg) was dissolved in
DMSO (3 mL), which was transferred in a dialysis tube (MWCO: 3500 Da) in deionized water or PBS
(10 mM, pH 7.4) for 5 times at 4 °C to afford the self-assembly formation of P-oa-SC.
A pyrene assay was utilized in pertinent to the critical micelles concentration (CMC) of the
synthesized amphiphilic P-oa-SC35. Herein, a constant concentration of pyrene (6.0 × 10-7 mol/L) was
mixed with a class of P-oa-SC solution with varied concentrations, ranging from 5 × 10-4 mg/mL to 1
mg/mL). The fluorescence excitation spectra of pyrene (6 × 10-7 mol/L) were recorded (ex: 334 nm,
em: 340-450 nm). The index of the fluorescence intensity at 372 nm (I372) and 383 nm (I383) was
calculated to infer the behaviors of pyrene molecules trapped into the stearic-aggregated
hydrophobic regions of the self-assembled formation.
Furthermore, the particle size and zeta potential of nanoparticles were characterized by
dynamic light scattering (DLS, Zetasizer Nano-ZS90, Malvern instruments, UK) measurement. All of
the DLS measurements were performed at 25°C. Of note, the concentration of the self-assembly was
adjusted at a uniform concentration (1.0 mg/mL) in deionized water or PBS (10 mM, pH 7.4).
The ultrasound-stimulated disassembly behaviors of P-oa-SC formation was studied by
monitoring the hydrodynamic diameter change under ultrasound treatment. Briefly, P-oa-SC
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self-assembly (1.0 mg/mL) were placed inside an acoustic power system (Huis bio engineering
technology Co. Ltd., City, China) and subjected to ultrasound treatment (1.0 MHz, 9.9 W, 3 W/cm2,
10 min, 20 min or 30 min) at 37 °C.
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Moreover, the aforementioned solution post ultrasound treatment was transferred to Liquid
Chromatography Mass Spectrometry measurement (1100HPLC&6130MSD, Agilent, Waters, Santa
Clara, CA) measurement to explore evidence of the cleavage of the -oa- linkage in P-oa-SC.
Preparation and Characterizations of DOX-Loaded P-oa-SC/DOX Delivery System. DOX-loaded
P-oa-SC (P-oa-SC/DOX) nanoparticulates were prepared by following a dialysis method as previously
reported17. In brief, DMSO solution containing P-oa-SC (5 mg/mL) and varied concentrations of DOX
was transferred to dialysis (MWCO: 3500 Da) in deionized water for 5 times.
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DOX Release Behaviors Under Ultrasound Impetus. The DOX release profiles from
P-oa-SC/DOX formation under ultrasound were characterized to verify the strategical use of -oa-
linkage for pursuit of ultrasound-stimulated release. In brief, P-oa-SC/DOX formation in PBS (10 mM,
pH 7.4) were subjected to ultrasound (1.0 MHz, 9.9 W, 3 W/cm2, 37 °C) for a 10 min, 20 min or 30
min period. Note that the periodic ultrasound schemes alternative to continuous ultrasound were
employed: a maximum of 90 s continuous ultrasound followed by 3 min interval (appreciable safety
profiles with negligible cytotoxicity confirmed for the periodic ultrasound scheme). The
aforementioned solutions were injected into dialysis bags (MWCO: 3500 Da), followed by dialysis in
100 mL PBS (10 mM, pH 7.4) on top of a shaking incubator at 37 °C under dark. At predetermined
time intervals, 3 mL of external solution was collected for quantification of DOX by fluorescence
measurement (ex: 505 nm, em: 561 nm). Of note, 3 mL of fresh PBS (10 mM, pH 7.4) was fused back
to the dialysis phase.
Cytotoxicity. The cytotoxicity of P-oa-SC nanoparticle was studied by an MTT assay against
Hep-G2 cells, MCF-7 cells, COS-7 cells and Hela cells. In brief, Cells were seeded onto a 96-well plate
(104 cells/well) and cultured in 100 μL DMEM medium at 37°C, 5% CO2. At 24 h post incubation, the
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cell culture medium was replaced by the fresh one containing varied concentrations of P-oa-SC
nanoparticle. Following another 48 h incubation, the cell viability was measured by following MTT
protocol36. Note that the untreated cells were used as the control. Cell viability was expressed as a
percentage to the control.
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Intracellular Drug Releasing. The intracellular drug releasing profile of nanoparticles was
investigated by CLSM (OLYMPUS FV1000, Olympus, Japan) and Flow cytometry (FACS Canto TM, BD,
USA). Herein, MCF-7 cells (105 cells/well) were seeded onto glass bottom dishes and cultured in
DMEM medium supplemented with 10% FBS (37 °C, 5% CO2). At 24 h post incubation, the cell
medium was replaced with fresh one containing P-oa-SC/DOX sample. At 30 min post incubation, the
cells were washed with PBS and incubated in fresh medium. Furthermore, the cells treated with
P-oa-SC/DOX sample were either scheduled to ultrasound impetus (1.0 MHz, 9.9 W, 5 min: two
circles of 150 s ultrasound treatment with 150 s interval) and additional 1.5 h or 3.5 h incubation or
directly incubated in fresh medium for 2 h or 4 h. Prior to CLSM observation, the cells were washed
three times with PBS, and stained with Hoechst 33342. The fluorescence images were observed by
CLSM (Hoechst 33342: Ex = 405 nm, Em = 460-500 nm; DOX: Ex = 488 nm, Em = 560-600 nm). Aiming
for precisely quantification of intracellular fluorescence intensities of DOX with or without
ultrasound, the cells were rinsed with PBS for three times and collected by sequential treatments of
trypsinization, centrifugation and resuspension in 300 μL PBS. The fluorescence intensity of DOX in
each sample was analyzed with PE filter set (Ex: 485 nm, Em: 520-560 nm) by a flow cytometer (BD
FACSCanto II, Franklin Lakes, NJ).
In Vitro Chemotherapeutic Potency. The in vitro chemotherapeutic potency of P-oa-SC/DOX
were studied against the cancerous cell lines of Hep-G2 cells and MCF-7 cells by an MTT assay. In
brief, Hep-G2 or MCF-7 cells were seeded onto a 96-well plate (104 cells/well) and cultured in 100 μL
DMEM medium at 37°C, 5% CO2. At 24 h post incubation, the cell culture medium was replaced by
the fresh one containing varied concentrations of P-oa-SC/DOX. At 4 h post incubation, the cells
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were washed by PBS and incubated in a fresh medium, followed by 5 min ultrasound (1.0 MHz, 9.9
W, …W/cm2, two circles of 150 s ultrasound treatment with 150 s interval) and additional 24 h
incubation. The cell viabilities were measured by MTT method. Note that the untreated cells were
used as a control. The Cell viabilities were expressed as a percentage of the control.
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In Vivo Anti-Tumor Efficacy. 4T1 cells (2 × 106) in PBS solution (100 μL) were subcutaneously
injected into both flanks of female nude Balb/c mice (aged 6-8 weeks). Once the tumor size
approaching approximately 50 mm3, the tumor-bearing mice were randomly assigned into the
following five groups (n=3): (1) control (only injected with PBS); (2) PBS with tumor-localized
ultrasound impetus (3.0 MHz, 2 W/cm2, 3 min); (3) free DOX; (4) P-oa-SC/DOX without
tumor-localized ultrasound; (5) P-oa-SC/DOX with tumor-localized ultrasound (3.0 MHz, 2 W/cm2, 3
min). The dosage of DOX was based on the weight of mice (4 mg/kg). All the mice were subject to
intratumor injection every other day and subjected to ultrasound impetus for 3 min at 4, 8 and 24 h
post injection. Note that the periodic ultrasound schemes were employed to avoid the potential
ultrasound-induced thermal consequences to the tumors: a maximum of 90 s continuous ultrasound
followed by a minimum of 180 s interval (appreciable safety profiles without observable cytotoxicity
confirmed for the proposed periodic ultrasound scheme in Figure 4b). The therapeutic efficacy was
estimated by measuring the tumor volume, body weight in each group every three days in the
duration of 18 days. Tumor volume was calculated according to the following equation: tumor
volume (cm3) = length (width)2/2. Relative tumor volume and relative body weight of mice were
calculated as V/V1, and W/W1, respectively. (V1 and W1 represent the tumor volume and body weight
at day 0). The mice were sacrificed and the tumors were collected after treatments at day 18.
Ex Vivo Histological Staining. The 4T1-cell bearing mice were sacrificed at 18 days post dosage.
The major functional organs including hearts, livers, spleens, lungs, kidneys, and tumors were
dissected and transferred to fixation in a 4% paraformaldehyde solution at room temperature.
Furthermore, the treated tissue samples were embedded in paraffin and subjected to section into
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5-8 μm thickness. H&E staining (BBC Biochemical, Mount Vernon, WA) was performed for
subsequent observation of tissue sections using an IX-83 bright-field/fluorescence microscope
(Olympus, Tokyo, Japan).
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Statistical Analysis. All the experiments were performed three times unless specifically
mentioned and the data were presented as means ± standard deviation (S.D.). Statistical analysis was
conducted with the Student’s t-test and one-way analysis of variance (ANOVA) analysis by Origin
software. The statistical significance was concluded when p value < 0.05 (*) and p value < 0.01 (**).
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Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
Estimation of critical micelles concentration of P-oa-SC, colloidal stability of P-oa-SC self-assembly,
TEM morphologies, cell viabilities of the blank P-oa-SC self-assembly, hemolysis activities of P-oa-SC
self-assembly, and body weight post diverse treatment.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: qixian@dlut.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was funded by National Natural Science Foundation of China (No. 21376039,
U1608222). and the funding support from the Fundamental Research Funds for the Central
Universities [No. DUT17RC(3)059].
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