harnessing the collective properties of nanoparticle …with light, thus enabling superior imaging...
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Nano Res
1
Harnessing the collective properties of nanoparticle
ensembles for cancer theranostics
Yi Liu1,2
, Jun-Jie Yin2, Zhihong Nie
1()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0541-9
http://www.thenanoresearch.com on July 10, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0541-9
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Harnessing the Collective
Properties of Nanoparticle
Ensembles for Cancer
Theranostics
Yi Liu1,2, Jun-Jie Yin2, Zhihong Nie1*
1University of Maryland, United States 2U.S. Food and Drug Administration,
United States
The self-assembly of NP ensembles from NP building blocks and their application in
cancer imaging and therapy are summarized. Because of the new and advanced collective
properties, the NP ensembles show many advantages over existing individual NP-based
theranostic systems.
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Nano Res.
Harnessing the collective properties of nanoparticle
ensembles for cancer theranostics
Yi Liu1,2
, Jun-Jie Yin2, Zhihong Nie
1()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
vesicle; nanoparticle;
self-assembly; cancer
theranostics
ABSTRACT
Individual inorganic nanoparticles (NPs) have been widely used in fields of
drug delivery, cancer imaging and therapy. There are still many hurdles that
limit the performance of individual NPs for these applications. The utilization
of highly ordered NP ensembles opens a door to resolve these problems, as a
result of their new or advanced collective properties. The assembled NPs show
several advantages over individual NP-based system, such as improved cell
internalization and tumor targeting, enhanced multimodality imaging
capability, superior combination therapy arising from synergistic effect,
possible complete clearance from the whole body by degradation of assemblies
into original small NP building blocks, and so on. In this Perspective, we
discuss the potential of utilizing assembled NP ensembles for cancer imaging
and treatment by taking plasmonic vesicular assemblies of Au NPs as an
example. We first summarize the recent development on the self-assembly of
plasmonic vesicular structures of NPs from amphiphilic polymer-tethered NP
building blocks. We further review the utilization of plasmonic vesicles of NPs
for cancer imaging (e.g., multi-photon induced luminescence, photothermal,
and photoacoustic imaging), and cancer therapy (e.g., photothermal therapy,
and chemotherapy). Finally, we outline current challenges and our perspectives
along this line.
1 Introduction
Although the past few years have witnessed an
unprecedented revolution in cancer diagnostic and
therapeutic, the clinical outcomes for cancer patients
have largely remained disappointing. The extremely
dismal statistics arises, mainly due to the lack of
suitable tools for early detection of cancer (in terms
of type, classification, and location) and ineffective
therapeutic strategies [1]. Many conventional
imaging techniques are available for the detection
and characterization of tumors, such as X-ray,
positron emission tomography, and magnetic
resonance imaging. These techniques, however, do
not provide contrast, sensitivity, dynamic range, and
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spatiotemporal resolution sufficient enough in many
situations [1]. As to therapeutic strategies, tumors at
initial stages are usually removed by surgery if
possible at the present time [2]. Unfortunately, the
surgical treatment has severe limitations and show
relatively poor long-term clinic outcomes.
Chemotherapy and radiotherapy (or following
surgery sometimes) are considered as other effective
alternative strategies. However, systemic drug
delivery is often failed to deliver sufficient amount of
chemotherapeutic drugs specifically to tumor sites to
suppress cancer metastases [3, 4]. The severe side
effect arising from non-specificity often makes the
patient extremely weak and even results in death.
Moreover, because of the poor efficacy of
non-specific chemotherapy, drug resistance will
develop over times (after about one year on average)
for most of anticancer drugs in virtually all patients
[5]. For these reasons, it is necessary to develop more
effective diagnostic and therapeutic strategies for
cancer therapy.
The utilization of inorganic nanoparticles (NPs)
as theranostic agents holds the potential to offer
unique solutions to current issues in combating
cancers [6, 7]. Inorganic NPs have shown intrinsic
optical, electronic, and magnetic properties that are
dependent on their size, shape, and composition.
This enables their biomedical application by
integrating effective imaging, diagnosis, and therapy
in one system [8-10]. Generally, NPs in an individual
or simple clustered form are used as delivery
vehicles for this application. For example, Au NP
based theranostic platform uniquely combines
targeting, multimodal imaging including
multi-photon induced luminescence (MAIL),
photothermal (PT) [11], and photoacoustic (PA)
imaging, and combination therapy including
chemotherapy and PT ablation (PTA) of cancer cells
[6, 12, 13]. Despite promising future, significant
hurdles still remain at this frontier [6, 14]. Firstly, the
efficiency with which individual NPs absorb and
scatter light requires further improvement, in order
to achieve optimal imaging. Particularly, our inability
to tune the interactions of light with a collection of
NPs for theranostic severely limits our capability of
further improving current individual NP-based
theranostic systems. Secondly, the loading capacity of
therapeutic and imaging agents by individual
vehicles (mostly through surface immobilization) is
relatively low, which limits the delivery efficiency of
these active agents to specific tumor areas. Last but
not least, long-term safety in terms of toxicity,
degradation, and clearance of NPs has to be
improved and fully understood before the translation
of this technology to the clinical realm [15]. Clinical
translation of inorganic NPs is fundamentally limited
because of the inherent difference in their uptake and
clearance from the body. Smaller NPs are eliminated
quickly from the body, but they are either less likely
to be taken up by passive targeting via such as
enhanced permeable retention (EPR) or do not offer
satisfactory physical properties for diagnosis.
Scheme 1. Schematic illustration of biodegradable Au vesicles
assembled from amphiphilic Au NPs for cancer theranostics. The
absorption of the plasmonic vesicles can be tuned within NIR
wavelength range. The biodegradable Au vesicles loaded with
therapeutic agents will accumulate at cancer cells via improved
EPR effect and passive targeting. The release of payloads can be
triggered by NIR light. After effective detection and treatment of
cancer cells, the vesicles will degrade into original small NP
building blocks for ease elimination from the body. The strong
plasmon coupling between Au NPs within membranes will
endow biodegradable Au vesicles significantly enhanced imaging
capability and combination therapy.
Although individual NPs are no doubt exciting,
ensemble of interacting NPs can exhibit a rich variety
of novel and extremely useful collective properties
that can be radically different from their individuals
[16-20]. These new synergistic properties arise from
the coupling interactions between metallic,
semiconductor or magnetic NPs within the ensemble.
The successful use of NP ensembles has been
demonstrated in the field of such as energy,
biosensing, metamaterials, and optoelectronics
[21-23]. It is expected that harvesting the collective
properties of NP ensembles would enable full
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realization of the enormous potential of inorganic
NPs in cancer imaging and treatment. Such
expectation is also originated from at least the
following factors:
i) Harvesting the collective properties of NP
ensembles will maximize our capability of
manipulating the interaction of a collection of NPs
with light, thus enabling superior imaging (i.e, PT
and PA imaging) and combination cancer therapy
[24-27].
ii) The ease in fine tailoring the
physicochemical properties of NP ensembles by
controlling individual building blocks will dictate
their enhanced blood stability, biodistribution, and
specificity of targeted delivery, as the superstructures
of NP ensembles interact with cells and tissues as a
whole [28].
iii) After treatment, the degradation of NP
ensembles into original building blocks, which can be
eliminated quickly from the whole-body, will
guarantee their long-term safety in terms of clearance
and toxicity, when the surface of NPs is proper. This
strategy naturally resolve current challenge in
achieving high specificity and efficiency of tumor
targeting via passive targeting (which is generally
more effective for NPs with a diameter of ~30-50 nm),
while promoting the clearance of NPs from the body
(which is more favorable for small NPs) [28, 29].
iv) The synergistic cytotoxic effect (e.g., the
combination of PTA and chemotherapy) may arise
from a collection of NPs carrying imaging or
therapeutic agents, thus leading to better tumor
control for patients [24].
In this Perspective article, we will convey the
concept of utilizing NP ensembles for cancer
theranostics by taking plasmonic vesicular
assemblies of noble metal NPs as an example
(Scheme 1). Unless otherwise specified, vesicular
assemblies of NPs will be written as NP vesicles
briefly herein. First, we will highlight current
advances in assembling NPs into vesicular ensembles
with programmable coupling between NPs with the
assemblies. Second, we will discuss the utilization of
NP vesicles in cancer imaging and treatment. Finally,
we will outline current challenges and our
perspectives along this line.
2 Construction of NP vesicles
Amphiphilicity-driven self-assembly of lipids
gives rise to liposomes with a bilayer structure which
is analogous to the structure of cell membrane [30-33].
Similarly, block copolymers (BCPs) can assemble into
polymersomes with tailored mechanical and
structural properties through self-aggregation of
their hydrophobic tails in an aqueous medium [34,
35]. Inspired by the well-established self-assembly of
these natural and synthetic molecular amphiphiles,
the concept of amphiphilic colloidal NPs has been
recently proposed. Amphiphilic NPs are generally
made by decorating the surface of NPs with
hydrophilic and/or hydrophobic molecules. Driven
by the directional interactions induced by molecular
tethers, amphiphilic NPs can spontaneously organize
into the desired entities.
The amphiphilicity of NPs can be achieved by
modifying a hydrophilic (or hydrophobic) NP with
hydrophobic (or hydrophilic) polymer brushes, while
selectively exposing part of the NP surface to
surrounding media. For instance, selective end
functionalization of hydrophilic cetyl
trimethylammonium bromide-coated Au nanorods
with hydrophobic polystyrene (PS) brushes generates
a new class of fascinating one-dimensional NP
amphiphiles [36]. These amphiphilic nanorods can
assemble into a wide range of nanostructures (e.g.,
nanochains, rings, bundles, and vesicles) with
tunable optical properties. Weller and Förster studied
the self-assembly of CdSe/CdS core-shell NPs
modified with a brush-like layer of poly(ethylene
oxide) (PEO) chains into spherical, cylindrical, and
vesicular structures in dilute solution [37].
A common feature of the amphiphilic NPs
described above is that, unlike true block copolymers,
only one of the chemical constituents is polymeric;
hence the comparable rigidity of the other section
may limit the inherent complexity of assemblies due
to packing constraints. A closer block copolymer
analogue can be produced by functionalizing NPs
with mixed brushes of hydrophilic and hydrophobic
chains, such that both of the incompatible chemical
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sections exhibit conformational flexibility. For
example, metallic NPs covalently attached with
amphiphilic V-shaped PS-b-PEO molecules on their
surface can assemble into one-dimensional tubular
structures [38]. Ultra-small Au NPs asymmetrically
functionalized with a single amphiphilic triblock
copolymer chain per NP assembled into micelles,
vesicles, rods, and large compound micelles [39]. In
these cases, the amphiphilic NPs could be viewed as
one type of Janus NPs, where NP core served as the
junctions of hydrophobic and hydrophilic
homopolymers.
Figure 1 (a) Schematic illustration of self-assembled NP vesicles
and tubules from amphiphilic NPs. (b), (c) Representative SEM
images of NP vesicles (b) and tubules (c) respectively. Inset in (b)
is the FFT pattern of SEM images. Scale bars: 200 nm in (b), (c).
Copyright 2012, American Chemical Society. (d)-(g)
Representative SEM images of Janus-like vesicles with spherical
(d), hemispherical (e), and disk-like (f), (g) shapes. SEM images
of patchy vesicles (h) and heterogeneous vesicles (i). Scale bars:
400 nm in (d), 150 nm in (e), 200 nm in (f), (h), (i), and 600 nm
in (g). Copyright 2014, American Chemical Society.
Other than NPs with asymmetrical
functionalization, NPs tethered by a mixture of
hydrophilic/hydrophobic polymer chains can also
self-assemble into NP vesicles. Recently, Duan et al.
demonstrated the assembly of plasmonic vesicular
nanostructures from Au NPs carrying a mixture of
hydrophilic poly(ethylene glycol) (PEG) and
hydrophobic poly(methyl methacrylate) (PMMA)
polymer brushes [40]. Moffitt and co-workers
showed the synthesis and self-assembly of CdS NPs
decorated with a mixture of hydrophobic PS and
ionizable hydrophilic (polyacid) polymer chains [41].
Tethering a mixture of hydrophilic/hydrophobic
polymer brushes on the surface of NPs is an effective
strategy for driving the self-assembly of NPs.
However, the intrinsic difference in bonding
strengths and absorption kinetics of different
polymers onto NPs makes it relatively difficult to
quantitatively control or predict the relative density
of each type of polymer, and hence their assembly
structures [42-44]. In contrast, BCP tethers offer
greater control over the chemical functionality and
composition (i.e., relative volume of
hydrophilic/hydrophobic moieties) as well as
architectural complexity of polymer chains on NP
surface. The chemical incompatibility and
conformational flexibility of isotropically grafted
BCPs gives rise to spontaneous anisotropy and
directional interactions to the colloidal building
blocks, their unique assembly behaviors. Based on
this concept, Nie et al. presented a new class of
amphiphilic NPs composed of inorganic NPs
tethered with amphiphilic linear BCPs (e.g.,
poly(2-(2-methoxyethoxy)ethyl
methacrylate)-b-polystyrene (PMEO2MA-b-PS) or
PEO-b-PS) [26]. As shown in Fig. 1(a)-(c), driven by
the conformational changes of tethered BCP chains,
such amphiphilic NPs can self-assemble into
well-defined nanostructures in selective solvents.
They include unimolecular micelles, clusters with
controlled number of NPs, tubular and vesicular
nanostructures comprising a monolayer shell of
highly ordered, hexagonally packed NPs [25]. This
strategy is applicable to the assembly of NPs with
various sizes, shapes (i.e., nanorods), and
compositions. More importantly, the interparticle
distance between Au NPs in the assemblies can be
tuned to achieve control over the plasmonic
properties of assembled structures by varying the
molecular length of hydrophobic blocks. This opens a
door to the utilization of the collective properties of
NP ensembles for biomedical applications.
Most recently, Nie et al. achieved the assembly of
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NP vesicles with well-defined shape, morphology,
and surface pattern. The concurrent assembly of
amphiphilic BCP and NP building blocks produced
patchy vesicles with multiple small amphiphilic NP
domains surrounded by BCP phase, Janus-like
vesicles with distinguished BCP and NP halves, and
heterogeneous vesicles with uniform distribution of
NPs [45] (Fig. 1(d)-(i)). The formation of
nanostructures with various morphologies arises
from the delicate interplay between the dimension
mismatch of the two types of amphiphiles, the
entanglement of polymer chains, and the mobility of
amphiphilic NPs. It is interesting to note that the
entropic attraction between amphiphilic NPs, as a
result of the maximization of the conformational
entropy of BCP chains, plays a dominant role in
controlling the phase-separation of the two types
amphiphiles in the membranes.
3 NP vesicles for cancer theranostics
Vesicles of Au NP ensembles will not only
preserve the intrinsic properties of polymeric and Au
components, but also acquire new or advanced
functionalities, such as encapsulation, local release,
tunable absorption, enhanced PT conversion
efficiency, and biodegradation. These unique features,
which are mostly not attainable by individual NPs,
facilitate their performance in cancer theranostics.
Cellular Internalization and Colocalization. The
vesicular assemblies of NP ensembles interact with
cells or tissues as a whole system. The
physicochemical properties (i.e., size, surface
chemistry, surface topology, and mechanical property)
of NP ensembles can be precisely engineered by
controlling the physical and chemical features of
individual NP building blocks. The unprecedented
control over the trait of NP assemblies offers us new
opportunities of enhancing the targeting and
biodistribution of theranostic vehicles. Recent
preliminary in vitro studies by Nie et al. showed that
the NP vesicles interact with cells as one system, and
colocalized within the lysosomes of MDA-MB-435
breast cancer cells [24]. LysoTracker was used to stain
lysosome and the subcellular localization of NP
vesicles encapsulated with photosensitizer Ce6 was
traced by confocal laser scanning microscopy (CLSM).
The fluorescent signals from LysoTracker and
vesicles matched very well, which suggests that
majority of vesicles colocalized within lysosomes.
Furthermore, the uptake of vesicles is internalized
via an energy-dependent endocytosis mechanism. In
another study, Duan et al. also revealed that
Herceptin conjugated NP vesicles could quickly
bound to HER2-positive SKBR-3 breast cancer cells
and be uptake by the cells through the endocytic
pathway [46]. In contrast, after incubating with NP
vesicles, only sparsely distributed vesicles can be
found in HER2-negative MCF-7 breast cancer cells. In
addition, Ijiro confirmed that NP vesicles showed
twice the level of cellular uptake compared with that
of dispersed NPs (same surface chemistry), which
indicates that NP vesicles can be efficiently
internalized into cells as their size is suitable for
endocytosis [47]. All these results together
demonstrate the potential of using bioconjugated
plasmonic NP vesicles to recognize and target
specific types of cancer cells.
Enhanced in vitro and in vivo cancer imaging
and treatment. The manipulation of the interactions
between light and a collection of NPs offers us a
powerful tool to achieve optimal design and physical
properties of nanostructures. This enables the
superior performance, which is beyond the capability
of individual NP-based theranostic platform, of NP
ensembles in cancer imaging and treatment. The
coupling between Au NPs within the vesicular
membranes resulted in a drastic red-shift of the
localized surface plasmon resonance (LSPR) peak
and a significant enhancement of LSPR absorption in
the near-infrared (NIR) range. These features enable
their superior performance in biomedical application.
Recent studies showed that Au NP vesicles exhibit
enhanced imaging capability originated from the
coupling between Au NPs within vesicular
membranes [25, 29]. First, NP vesicles show a ~7-fold
enhancement in MAIL imaging of 4T1 breast cancer
cells, compared to individual Au NPs with the same
quantity and diameter [25]. MAIL imaging of cells
internalized with vesicles was achieved by exciting
with 800 nm light and recording in a wide spectra of
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470-600 nm. It is reasonable to believe that
optimization of the system can further improve
MAIL signal.
Figure 2 (a) Thermal images of MDA-MB-435 tumor-bearing
mice exposed to 808 nm laser for 5 min at post-injection of PBS
or biodegradable Au vesicles (BGVs). (b) Heating curves of
tumors upon laser irradiation as a function of irradiation time. (c)
PA signals of BGVs and Au nanorods (GNRs) as a function of
optical density. (d) In vivo 2D and 3D ultrasonic (US) and PA
images of tumor tissues at pre-injection and post-injection of
BGVs. Arrows indicate the location of BGVs. (e) PA intensities
of tumor tissues with intratumoral administration of the same
amount of regular Au vesicles (GVs) or BGVs. Copyright 2013,
Wiley-VCH.
Second, the coupling between Au NPs within
vesicular membranes leads to a strong NIR
absorption and a significant increase in PT
conversion efficiency (η) of the assemblies, hence
improving their performance in PT/PA imaging and
PTA (Fig. 2) [48]. For example, biodegradable Au
vesicles composed of poly(ethylene
glycol)-b-poly(ε-caprolactone) (PEG-b-PCL) tethered
Au NPs showed an ultra-strong plasmonic coupling
effect, due to the small interparticle distance of NP
building blocks [29]. The η of biodegradable Au
vesicles (~150 nm in diameter, ~800 nm absorption)
with strong-coupling 25-nm NPs is 37%, which is
much higher than 18% and 22% for that of regular Au
vesicles and Au nanorods (~10 nm in diameter and
~42 nm in length), at equal optical density (OD) at
808 nm (Fig. 2(a), (b)). The increase in η of NP
vesicles significantly enhanced their performance in
PT and PA imaging in vitro and in vivo (Fig. 2(c), (d)).
In vivo study showed that the PA signals in the
tumor region was ~10-fold stronger than control
group without Au vesicles injection. Even compared
with Au nanorods, PA signals from biodegradable
Au vesicles are almost 7-fold stronger when the OD
at 808 nm is 1.0. Furthermore, the η and resulting
imaging capability of NP vesicles is strongly
dependent on the coupling strength between Au NPs
within membranes (Fig. 2(e)). The vivo PA signal
doubled when the distance between Au NPs within
assemblies decreased.
Because of the higher η, the biodegradable Au
vesicles also exhibited significantly enhanced PT
therapeutic efficacy as compared with Au nanorods
and regular Au vesicles [29]. After laser irradiation,
all the tumors injected with biodegradable Au
vesicles were effectively ablated, leaving black scars
at their original sites without showing reoccurrence
within ~4 weeks. The mice were tumor-free and
survived over 30 days without a single death or
tumor reoccurrence. In contrast, Au nanorods and
regular Au vesicles administration/irradiation groups
showed slower delay in tumor growth or tumor
regression, all mice showed average life spans of
14~20 days since treatment started. Hematoxylin and
eosin (H&E) staining of tumor slices was also carried
out for tumors collected immediately after laser
irradiation. Significant cancer cell damage was
observed in the Au vesicle (with strong
coupling)-treated group, but not in groups treated
with Au nanorods or Au vesicles (with weak
coupling).
Improved Delivery of Therapeutic Agents for
efficient therapy. Organic vesicles (i.e., liposomes)
have made the greatest clinical impact in drug
delivery, because of their unique ability to
encapsulate and deliver hydrophilic and/or
hydrophobic compounds simultaneously [30-35]. NP
vesicles mimic the function of organic vesicles for
encapsulation, while showing significantly improved
ability of retaining drugs in the cavity. The loading of
therapeutic or imaging agents within vesicular
cavities overcomes the surface-area-limited loading
of drugs on Au NPs by conventional approaches
[49-52]. The kinetically trapping of Au NPs in the
members possibly minimizes the leakage of mostly
toxic drugs during circulation. Moreover, the
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spatiotemporal release of drugs from NP vesicles can
be activated specifically at target sites by NIR light or
chemical triggered breakup of the vesicular
membranes [27, 53, 54]. These unique features of NP
vesicles may drastically improve chemotherapy (or
other therapies such as photodynamic therapy)
results and minimize side-effects of non-specific drug
delivery in cancer treatment.
Figure 3 (a) UV-vis spectra of Au vesicles (GVs) (black), Ce6
(blue) and Au vesicle-Ce6 (GV-Ce6) (red). The arrows indicate
characteristic Q-bands of Ce6; (b) Ce6 loading efficiency of
GV-Ce6 as a function of Ce6 concentration; (c) the changes of
fluorescence intensity at the characteristic peaks of SOSG and
Ce6 (528 and 662 nm) as a function of laser irradiation time. (d)
Tumor growth curves of different groups of tumor-bearing mice
after treatment. Tumor volumes were normalized to their initial
sizes. Error bars represent the standard deviations of 4-6 mice per
group. Asterisk indicates P < 0.05. Copyright 2013, American
Chemical Society.
Most recently, Nie and co-workers reported the
design of multifunctional photosensitizer Ce6-loaded
plasmonic Au vesicles for trimodality
fluorescence/thermal/PA imaging guided synergistic
photothermal/photodynamic therapy (PTT/PDT)
cancer treatment [24] (Fig. 3). The Au vesicles
showed a strong absorbance in the NIR range of
650-800 nm, as a result of the plasmonic coupling
between neighboring Au NPs in the vesicular
membranes. This enables the use of 671 nm laser
irradiation to simultaneously excite both Au vesicles
and Ce6 to produce heat and singlet oxygen, ablating
cancer cells (Fig. 3(a)). When the weight ratio of Ce6
to Au vesicles (mCe6/mv) is 60%, the Ce6 loading
efficiency is up to 18.4 wt %; and this value can be
further increased with increasing mCe6/mv (Fig. 3(b)).
In contrast, the loading efficiency of Ce6 onto Au
nanorods through electrostatic interaction saturated
at ~9 wt%. The efficient loading of Ce6 in Au vesicles
significantly increases the accumulation of Ce6 in
cancer cells. The heating effect upon laser irradiation
dissociates the Ce6-loaded Au vesicles to release
payloads (Fig. 3(c)). The tumor tissues visualized by
the fluorescence, thermal and PA signals from
Ce6-loaded Au vesicles can be selectively destroyed
in a noninvasive manner by the illumination of 671
nm laser. Both in vitro and in vivo therapeutic
efficacies of Ce6-loaded Au vesicles were enhanced
compared to either individual PTT or PDT alone, or
the simple summation of PTT/PDT due to the
synergistic effect (Fig. 3(d)). In another example,
Doxorubicin (DOX) was encapsulated in Au NP
vesicles with surface-conjugated with targeting
moiety such as monoclonal antibody and folate. The
platform can selectively enter various cancer cells
including SKBR-3 breast cancer cells and
MDA-MB-435 breast cancer cells, and the release of
payloads was achieved by pH or photo-stimulation
[46, 55]. Moreover, the dramatic change in scattering
properties and SERS signals upon the dissociation of
the entity of vesicles allows one to trace the
intracellular drug delivery by plasmonic imaging
and SERS spectroscopy.
Pharmacokinetics and clearance of NP vesicles.
As mentioned earlier, the realization of inherent
contradictory processes ─ the improved passive
targeting (e.g., through ERP effect) and the clearance
of NP from the body ─ is very much fundamentally
limited. One fascinating concept to solve the
challenge is to construct NP ensembles with desired
physicochemical properties for delivering the
vehicles efficiently to targeted regions, while the NP
ensembles can dissociate into much smaller original
NP building blocks which can be effectively
eliminated from kidney and the whole body. In a
recent fascinating example, DNA was used to initiate
the assembly of NPs into larger core-satellite
structure with controlled biological delivery and
elimination properties [28]. Through burying DNA
inside the inner and using NPs as scaffolds, the
assembled structure decrease their accessibility to
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cellular interactions but increase the density of PEG
coverage above the DNAs. This design can reduce
assembled structure’s uptake and sequestration by
macrophages, improve their accumulation into
tumors. More importantly, the complex variety of
hydrolytic enzymes in cells will quickly hydrolyse
the DNA linkages that connect the NPs, facilitate the
elimination of NPs from the body. In a word, the
elaborate design of the core-satellite structure not
only improves the tumor-targeting efficiency,
avoiding uptake in the reticuloendothelial system,
but also maintains the clearance of injected NPs in
living animals.
With respects to vesicular assemblies of NP
ensembles, the system will not only enable efficient
delivery of imaging and therapeutic agents, but also
can degrade into small building blocks and hence
potentially clear out from the body. The
biodistribution and pharmacokinetics of
intratumorally injected NP vesicles was studied [29].
After the utilization of NP vesicles assembled from 25
nm Au NPs for theranostics, the degradation and
redistribution of Au NPs in different organs was
quantified using inductively coupled plasma mass
spectrometer (ICP-MS) analysis. The concentration of
Au element in different organs was measured at 1, 2
and 8 day (n=3/group). Their studies indicates that
after the laser irradiation, the biodegradable Au
vesicles using PEG-b-PCL as polymer tethers
would degrade into original NP building blocks and
leak into circulation and prominently accumulate in
the reticuloendothelial system (RES) including liver
and spleen at 2 day. After 8 days, vesicles are partly
cleared from the RES. The biodistribution of
biodegradable Au vesicles after intratumoral
injection showed that biodegradable Au vesicles
would leak into circulation and prominently
accumulate in the RES including the liver and spleen
on day 2 and cleared from the RES on day 8. It is
worthy to note that the laser irradiation speeds up
the degradable of NP vesicles into individual NPs.
However, if without laser irradiation, the
biodegradable Au vesicles always stayed in the
tumor tissues. Together these results suggest that it
is promising to improve the clearance of NP vesicles
by breaking down the assemblies into individual NPs
with proper surface, although more systematic study
is required.
4 Conclusions and outlook
Harvesting the new or advanced physical and
chemical properties of NP ensembles holds the
promise to offer new solutions to existing problems
in the field of cancer theranostics. This is reflected by
current exciting advances in the construction and
biological application of plasmonic NP vesicles.
Assemblies made from other types of NPs (e.g.,
quantum dots and magnetic NPs) or the combination
of multiple types of NPs are also attractive for a
range of biomedical applications, due to the exciton
coupling, plasmon-exciton coupling, and
magnetic-magnetic interactions between NP subunits.
These NPs have also been integrated in vesicular
membranes of polymers or lipids for cancer
diagnosis, imaging and treatment, although more
efforts should be made to utilize their collective
properties [56-58].
The utilization of NP vesicles for theranostics
possess at least three advantages over existing
individual NP-based theranostic systems: i) The
physicochemical properties of NP vesicles can be
easily tailored by controlling individual building
blocks; ii) The ensemble structures of NP vesicles
facilitate the uptake by targeted tumor cells, and the
degradable property guarantees their long-term
safety; iii) The fascinating collective properties
enhance their cancer imaging and treatment
capability.
Although these results are encouraging, new
breakthroughs are still needed. First of all, the in vivo
biodistribution and long-term toxicological effects of
these NP vesicles administrated via intravenous
injection must be carefully assessed before their use
in clinical daily practice. Further studies should
systematically evaluate the risks of using NP vesicles
for in vivo imaging and treatment with regards to
organ distribution, accumulation and clearance, and
immunogenicity and inflammation at the whole
organism level, as well as to quantitatively assess
their imaging and therapeutic performance in animal
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Nano Res.
models. Secondly, greater efforts should be made to
understand how light (or magnetic field when
magnetic NPs are used) interacts with a collection of
inorganic NPs. Fundamental understanding of the
system through both experimental and
computational studies will enable us to achieve
optimal design of NP ensembles with desired
properties for these applications. For instance, what
would be ideal size of NP building blocks that will
offer sufficient coupling strength between NPs, while
not sacrificing their elimination from the body?
Finally, challenges remain in precisely engineering
NP ensembles with precisely tunable physical and
chemical properties (e.g., size, size distribution,
interparticle distance, surface), as many factors are
intertwined. For instance, the diameter of NP vesicles
can be reduced down to ~50 nm, when small Au NPs
(~5 nm in diameter) with relatively short ligands are
used [59, 60]. However, the use of smaller Au NPs
significantly reduces the plasmonic coupling
between neighboring NPs, which makes it difficult to
tune the optical property of the NP ensembles. New
advances in both self-assembly and theoretical
prediction of properties are required to achieve better
design of NP ensembles for various biomedical
applications.
Acknowledgements
This work is supported by NSF career award
(DMR-1255377), startup funds from the University of
Maryland, and the Joint Institute for Food Safety and
Nutrition, University of Maryland, College Park, MD
through a cooperative agreement funded by FDA,
Grant No. 5U01FD001418 (Yi Liu). This article is not
an official U.S. FDA guidance or policy statement. No
official support or endorsement by the U.S. FDA is
intended or should be inferred.
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