Dendrimers as Non-Viral Vectors for siRNA Delivery

Abstract

There is a tremendous interest in moving siRNA therapeutics into a clinical setting for the treatment of various diseases. This in itself however depends largely on the availability of safe and efficient siRNA delivery systems. In this context, dendrimers have attracted considerable attention as siRNA vectors due to their well-defined structures and multivalent features. The present review offers a brief overview of the current status of dendrimers as siRNA delivery vectors, focusing on the different dendrimers investigated for their siRNA delivery ability and the related structural alterations employed to improve their safety and efficiency for this purpose.

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256 New J. Chem., 2012, 36, 256–263 This journal is
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Cite this:
NewJ.Chem
., 2012, 36, 256–263
Dendrimers as non-viral vectors for siRNA deliveryw
Xiaoxuan Liu,
abc
Palma Rocchi
c
and Ling Peng*
a
Received (in Montpellier, France) 10th May 2011, Accepted 18th July 2011
DOI: 10.1039/c1nj20408d
There is a tremendous interest in moving siRNA therapeutics into a clinical setting for the
treatment of various diseases. This in itself however depends largely on the availability of safe and
efficient siRNA delivery systems. In this context, dendrimers have attracted considerable attention
as siRNA vectors due to their well-defined structures and multivalent features. The present review
offers a brief overview of the current status of dendrimers as siRNA delivery vectors, focusing on
the different dendrimers investigated for their siRNA delivery ability and the related structural
alterations employed to improve their safety and efficiency for this purpose.
Introduction
RNA interference (RNAi) was first reported by Fire and Mello
et al. in 1998.
1
It denotes a sequence-specific gene silencing
process triggered by small interfering RNAs (siRNAs) (Fig. 1).
2
In the RNAi process, endogenous long double strand RNA
molecules are cleaved into small interfering RNAs of about
21–23 nucleotides by an endonuclease called Dicer.
3
The
resulting siRNA molecule is incorporated into a nuclease
complex, namely an RNA-induced silencing complex (RISC),
where the siRNA duplex is unwound, and one of which
(the antisense strand) remains bound to the RISC complex.
This active RISC form then binds to mRNA with a complementary
sequence via Watson–Crick base-pairs and induces its degradation,
consequently preventing the translation of the corresponding
mRNA into protein (Fig. 1).
4
The whole process is catalytic
because, after mRNA degradation, the active RISC form is
regenerated and can participate in another cycle of mRNA
binding, cleavage and regeneration.
5
On the basis of RNAi, synthetic siRNA can be designed and
applied to target any gene with a known sequence, hence it can
be harnessed to silence disease genes with a therapeutic aim.
6
Compared to antisense oligonucleotides, siRNAs are more
resistant to nuclease degradation. In addition, siRNAs show a
more potent and prolonged therapeutic effect, thanks to the
catalytic mechanism. The promise that siRNA can efficiently
and specifically down-regulate genes with any known sequence
has attracted much attention among scientists wishing to
develop siRNA therapeutics to treat diseases through the
silencing of the corresponding disease-related genes. This has
a
De
´
partement de Chimie, Centre Interdisciplinaire de Nanoscience de
Marseille, CNRS UPR 3118 CINaM, 163, 13288 Marseille cedex 09,
France. E-mail: ling.peng@univmed.fr; Fax: 0033 4 91 82 93 01;
Tel: 0033 4 91 82 91 54
b
State Key Laboratory of Virology, Wuhan University, Wuhan 430072,
P. R. China
c
INSERM U624, 163, avenue de Luminy, 13288 Marseille, France
w This article is part of the themed issue Dendrimers II, guest-edited
by Jean-Pierre Majoral.
Xiaoxuan Liu
Xiaoxuan LIU received her
co-tutorship PhD in 2010 from
Wuhan University in China and
Universite
´
de la Me
´
diterrane
´
ein
France under the co-direction of
Prof. Fanqi Qu and Dr Ling
Peng. She is currently a post-
doctoral researcher in the group
ofDrPalmaRocchiinthe
INSERM laboratory U624 and
workingonaEuropeanproject
to develop multi-functional
dendrimers as nanovectors for
siRNA delivery under the
coordination of Dr Ling Peng.
Palma Rocchi
Dr Palma ROCCHI is a group
leader at Inserm in Marseilles.
Her principal research interest
is focused on drug discovery for
the treatment of castration-
resistant prostate cancer. In
her three years as post-doctoral
fellow in the group of
Dr Gleave at the Prostate
Centre (Vancouver, Canada),
Palma Rocchi developed a drug
(OGX-427) able to down-
regulate Hsp27 using an
oligonucleotide antisense and
siRNA approach. OGX-427 is
now in clinical trials phase II in
Canada and United States in patients with prostate cancer.
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New J. Chem., 2012, 36, 256–263 257
created a completely new era of RNA therapeutics, which has
the potential to revolutionize traditional methods in medicine
in the search for drug candidates for various diseases, especially
those currently deemed incurable or difficult to cure due to drug
resistance and/or the absence of efficacious drug candidates.
Any breakthrough in RNAi technology for therapeutic
application requires, however, safe and easy-to-handle siRNA
delivery systems able to protect the siRNA during extra- and
intra-cellular delivery and bring it safely to the site of interest.
Over the past decade, tremendous efforts have been made to
develop carriers for siRNA delivery.
6,7
The main requirements
for siRNA carriers include the ability to: (1) protect siRNA
from degradation during systemic circulation; (ii) deliver
siRNA to the target sites and avoid nonspecific delivery; (iii)
promote cellular uptake and subsequent endosomal escape;
(iv) release siRNA and make siRNA readily accessible to the
cellular RNAi machinery thus permitting the RNAi process
and resulting in an effective gene silencing effect.
Both viral and non-viral vectors have been explored as
siRNA carriers.
6
While viral vectors show high efficacy, they
are hampered by serious concerns over safety, high production
cost and short shelf life etc. Non-viral delivery systems on the
other hand do address the safety risk and manufacturing costs
but remain less effective than the viral vectors. These non-viral
vectors are usually divided into two main classes—cationic
lipids and polymers.
7
In both cases, the positively charged
vectors are able to assemble the siRNA into nanoparticles and
form either lipoplexes or polyplexes, respectively. These
nanoparticles can encapsulate siRNA and thus protect the
siRNA from degradation and promote cellular uptake via
endocytosis (Fig. 2). Nanoparticles with a size of around
100 nm are preferable. This is because, typically, clathrin-
mediated endocytosis, which generates vesicles with a size of
about 100 nm in diameter,
8
is responsible for the uptake of
many macromolecules from extracellular medium. After
internalization, the siRNA/vector complexes are escaped from
the endosomes and release siRNA in the cytoplasm to undergo
the RNAi process and consequently silence the targeted gene
(Fig. 2).
One special family of polymeric vectors that are emerging as
promising non-viral vectors for siRNA delivery are cationic
dendrimers.
9,10
Dendrimers are synthetic macromolecules with
a ‘‘dendritic structure’’.
11
The name ‘‘dendrimer’’ originates
from the Greek word ‘‘d

endron’’ (pronounced dendron) mean-
ing ‘‘tree’’. Their particular architecture constitutes three dis-
tinct domains (Fig. 3): (1) a central core, (2) repetitive branch
units organized in geometrically radiated progression, with each
successive branching unit being named generation, and (3) a
large number of terminals on the outer surface. Unlike traditional
polymers, the structures of dendrimers can be precisely controlled
during their stepwise synthesis achieved either via divergent or
via convergent strategies.
11
Divergent dendrimer growth
implies that the synthesis starts at the reactive center of the
dendrimer core and undergoes construction with branching
units towards the surface. Convergent dendrimer growth on
the other hand begins at the surface of the future dendrimer
and continues through gradual addition of monomers to the
surface units, which will eventually attach to the core and yield
the corresponding dendrimer. Under rigorously controlled
synthesis, the obtained dendrimers have a well-defined
structure and narrow polydispersity in addition to the unique
structural geometry and multivalent property, and are thus
expected to make an ideal drug delivery platform.
Up to now, a multitude of dendrimers have been explored
for siRNA delivery, including poly(amidoamine) (PAMAM)
dendrimers,
9,12–20
poly(propylene imine) (PPI) dendrimers,
21,22
poly(L-lysine) dendrimers,
23–25
carbosilane dendrimers,
26–28
triazine dendrimers
29
and polyglycerol dendrimers
30,31
etc.
These dendrimer vectors often bear positively charged amine
Fig. 1 The mechanism of RNA interference. Long double-strand
RNA (dsRNA) is cleaved into small interfering RNA (siRNA) by
the enzyme Dicer. Then siRNA is incorporated into an RNA-induced
silencing complex (RISC) and unwound into two single strands. The
sense strand is degraded, while the antisense strand which remains on
the RISC complex guides this active RISC complex to bind to mRNA
with a complementary sequence and induces the cleavage of mRNA,
leading to the silencing of the targeted gene. The active RISC complex
can then be recycled for the destruction of identical mRNA targets.
Ling Peng
Dr Ling Peng is a research
director at the Interdisciplinary
Center on Nanoscience in
Marseille. She studied chemistry
at Nanjing University in China,
achieved her PhD program with
Prof. Albert Eschenmoser at
ETH Zurich in Switzerland,
and then carried out three-year
postdoctor al research with Prof .
Maurice Goeldner in Strasbourg
before being r ecruited by
CNRS in France in 1997. Her
current research is focused on
dendrimers for nucleic acid
delivery and chemical probes
for exploring biological events and drug discovery.
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258 New J. Chem., 2012, 36, 256–263 This journal is
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The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
functionalities at the dendrimer surface under physiological
conditions and are responsible for ionic condensation with
negatively charged siRNA molecules. The siRNA/dendrimer
complexes are able to bind to the cell surface, enter into the
cells, where the siRNA molecules can be then released into the
cytosol. Dendrimers harboring tertiary amines in their interior
can preferentially promote siRNA release via the ‘‘proton
sponge’’ effect.
32
The released siRNA molecules eventually
join the RNAi machinery where gene silencing can occur.
Here, we will highlight the current status of various dendrimer
vectors investigated for their siRNA delivery capacity.
Poly(amidoamine) dendrimers
Poly(amidoamine) (PAMAM) dendrimers were first synthesized
by Tomalia using a divergent synthetic strategy
33,34
and have
been extensively investigated as non-viral vectors for DNA
delivery.
35–39
Today a large panel of PAMAM dendrimers
with different core structures and terminal groups is commercially
available. Commercial kits such as PolyFect
s
and SuperFect
s
based on PAMAM dendrimer ingredients were initially
developed for DNA delivery. These reagents have since been
shown to be able to deliver siRNA also.
12,13
The active ingredient in these commercial delivery kits is
fractured and degraded PAMAM dendritic polymers. These
are obtained from the intact dendrimers either by thermal
degradation or by alkaline hydrolysis, and the precise control
over the structure is lost during this degradation process.
In addition, the preparation of intact dendrimers is time-
consuming, and requires strictly controlled stepwise synthesis
and laborious purification. Therefore, deliberately degrading a
meticulously synthesized perfect dendrimer is neither chemically
rational nor economically beneficial. With this in mind, Peng
et al. developed a series of structurally flexible triethanolamine
(TEA) core PAMAM dendrimers (Fig. 4A).
14
The TEA core is
considerably more extended compared to the conventional
NH
3
core (Fig. 4B), and therefore expected to give the
corresponding PAMAM dendrimers more space to accommodate
the branching units and thus render them structurally more
flexible. These TEA-core dendrimers have been shown to bind
siRNA to form stable nanoscale complexes
15
able to protect
siRNA from degradation and promote cellular uptake.
16
Higher generations of these flexible PAMAM dendrimers
(generation Z 5) were effective vectors for the delivery of
siRNA, both in a luciferase model
9
and in a prostate cancer
model, able to knock-down heat-shock protein 27 (Hsp27) and
produce caspase-dependent apoptosis-induced anticancer
activity.
16
Recently, these dendrimers have been demonstrated
to efficiently deliver siRNA into human T cells and primary
PBMC cells and produce an effective gene silencing effect.
17
Moreover, these dendrimers were able to deliver, via systemic
administration in a HIV-infected humanized RAG-hu mice
model, a cocktail of siRNA targeting both HIV replication
and host HIV infection.
17
The resulting siRNA gene silencing
led to extraordinary anti-HIV activity, with significant
suppression of viral loads by several orders of magnitude,
the effective prevention of host CD4 T-cell depletion and of
viral escape.
17
These results demonstrate the extremely
promising potential of this family of dendrimers for further
clinical applications to deliver siRNA therapeutics.
With the aim of reducing the cytotoxicity of PAMAM
dendrimers relating to their positive surface charges, partial
Fig. 2 Non-viral vectors mediated siRNA delivery and gene silencing in mammalian cells.
Fig. 3 General structure of a dendrimer.
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New J. Chem., 2012, 36, 256–263 259
acetylation of the terminal amines was realized. While the so
obtained dendrimers did indeed show reduced toxicity, their
siRNA delivery efficiency was also diminished thus meaning
that only a modest fraction of the terminal amines could be
modified in order to maintain the delivery.
18
Masking
the positive charges on the dendrimer surface could decrease
the binding and interaction capacity of these dendrimers
towards the negatively charged siRNA, and consequently
compromised their delivery efficiency. The relative number
of terminal amines on the dendrimer surface should therefore
allow a critical balance between a reduced toxicity and
conserved activity.
Cyclodextrins (CD) are a family of cyclic oligomers of glucose
bound together in a ring. They have no toxicities, do not elicit
immune responses, and are able to promote the interaction with
biomembrane constituents such as phospholipids.
40
So
Tsutsumi et al. conjugated CD to generation 3 PAMAM
dendrimers with the aim of increasing the membrane affinity
in order to maintain the delivery ability while reducing the
cytotoxicity.
19
These conjugates were shown to improve the
intracellular uptake of CD–dendrimer/siRNA complexes and
siRNA release, leading to a potent RNAi effect with negligible
cytotoxicity in various cell lines.
19
An important concern with non-viral vectors is their lack of
cell-specificity. To address this, targeting moieties such as
antibodies or ligands have been introduced into the dendrimer
structures for targeted delivery. Based on the over-expression
of epidermal growth factor receptors in multiple human solid
tumors, Yang et al. tested an epidermal growth factor (EGF)
conjugated PAMAM dendrimer for its tumor targeting
siRNA delivery capacity.
20
The corresponding EGF/dendrimer
conjugate allowed a considerable knockdown of gene
expression in a receptor-dependent manner, offering promising
perspectives for targeted delivery.
Poly(propylene imine) dendrimers
Poly(propylene imine) dendrimers (PPI) (Fig. 5) are another
class of dendritic macromolecules that have attracted considerable
attention for their siRNA delivery potential.
21,22
In order to
increase their biocompatibility and promote their targeting
capacity, Minko et al.
21
coated the corresponding siRNA/
PPI dendrimer nanoparticles with the PEG polymer, followed
by their decoration with luteinizing hormone-releasing
hormone (LHRH) peptide as the cancer targeting moiety.
This strategy effectively increased the siRNA stability in
serum, promoted tumor-specific targeting and uptake, and
resulted in effective gene silencing.
21
Further in vivo distribution
data also confirmed that the targeted dendrimer and siRNA
were accumulated mainly in the tumor, while the non-targeted
dendrimer and siRNA were found predominantly in liver and
kidney and only at trace levels in the tumor.
21
Later, with the view to synchronizing the detection of tumor
progression and therapeutic responses in situ, Minko et al.
prepared a delivery system comprising siRNA, PPI dendrimer
and SPIO (Superparamagnetic Iron Oxide).
22
SPIO is a kind
of Magnetic Resonance Imaging (MRI) contrast reagent which
enhances the imaging of biological molecules. The siRNA/PPI/
SPIO nanoparticles were subsequently formulated, PEGylated
and then conjugated with the LHRH peptide for cancer
targeting.
22
This led to the successful simultaneous delivery
of both siRNA and MRI-contrast agents specifically to tumor
Fig. 4 Structurally flexible triethanolamine (TEA) core PAMAM
dendrimer (A) and traditional amine (NH
3
) core PAMAM dendrimer
(B) of generation 2.
Fig. 5 Poly(propylene imine) dendrimers (PPI) of generation 4.
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cells, allowing the suppression of cellular resistance via siRNA-
mediated gene silencing and at the same time, MRI imaging of
primary tumor or metastases.
22
This multifunctional delivery
system had the advantage of being able to deliver siRNA
therapeutics for cancer treatment and simultaneously monitor
in situ the therapeutic responses with minimal medical intervention
or adverse side effects of the treatment on healthy organs.
Unfortunately, toxicogenomics investigation showed that
PPI dendrimers could alter the expression of endogenous
genes mainly involved in immuno-defense responses, cell
proliferation and apoptosis.
41
These results imply that PPI
dendrimers could intrinsically impact inherent gene function
in humans, which may limit their further clinical applications.
Poly(L-lysine) dendrimers
Dendritic poly(L-lysine) compounds (Fig. 6) have also been
studied as potential delivery vectors for siRNA.
23–25
Being
composed of naturally occurring
L-lysines as construction
units, poly(
L-lysine) dendrimers have the appealing advantage
of being biocompatible and biodegradable in the presence of
acids and enzymes.
38
Although poly(L-lysine) dendrimers were
shown to bind and interact strongly with nucleic acids, they
alone showed little capacity to efficiently deliver siRNA for
gene silencing.
23
This might be due to their lack of tertiary
amines in the dendrimer interior which would otherwise have
provided a proton sponge effect to facilitate the intracellular
release of siRNA. Hence the siRNAs remain complexed with
the dendrimers and are unavailable for the RNAi machinery
and thus gene silencing.
23
In combination with Endo-Porter, a
weak-basic amphiphilic peptide, the transfection efficiency of
poly(
L-lysines) is significantly improved.
23
This is probably
because both the weak basic and amphiphilic properties of
Endo-Porter can help promote the intracellular release of the
siRNA thus increasing the delivery and gene silencing efficiency.
Later, Niidome et al. found that intravenous delivery of
generation 6 dendritic poly(
L-lysine) in complex with siRNA
specifically targeting apolipoprotein B (ApoB) could lead to
the effective knockdown of ApoB in mice, while no notable
toxicity was observed.
24
The good delivery ability of
the poly(
L-lysine) dendrimer in vivo could be ascribed to its
in vivo degradable properties after systemic administration.
Further conjugation with oleic acids at the terminals of a
smaller generation 3 poly(
L-lysine) dendrimer also resulted in
efficient siRNA delivery and effective gene silencing in vitro
and in vivo with no significant cytotoxicity.
25
This could be
attributed to the synergistic benefit originating from the lipid
and polycation content of the poly(
L-lysine) dendrimer, which
may increase the delivery efficiency and gene silencing effect.
Carbosilane dendrimers
Water-soluble carbosilane (CBS) dendrimers have been
recently studied as siRNA delivery vehicles.
26–28
The delivery
ability of a generation 2 ammonium-terminating carbosilane
dendrimer (Fig. 7) has been tested on many cell types including
astrocytes, dendritic cells, lymphocytes, macrophages and
human peripheral blood mononuclear cells, that range from
Fig. 6 Dendritic poly(L-lysine) dendrimer of generation 3.
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New J. Chem., 2012, 36, 256–263 261
immortalized adherent cells to suspension cells and primary
cells. Effective gene silencing with the resulting reduction
of HIV replication was observed in PBMC, SupT1 and
U-87-MG cells with siRNA targeting specifically GAPDH
and COX-2 respectively.
26,27
However, the concentration of
siRNA used in these studies was relatively elevated, often
reaching 250 nM or higher. This may raise concerns over
off-target effects and related toxicity hence limiting their
further in vivo application.
In a separate study, Cen
˜
a et al. showed that this carbosilane
dendrimer could efficiently transfect siRNA to primary
neurons and achieve more than 80% reduction in protein
synthesis of hypoxia-inducible factor following the silencing
of the corresponding gene.
28
This represented the first report
of a dendrimeric non-viral vector able to attain such an
effective down-regulation of a functional protein following
siRNA delivery to neuron cells. Further in vivo studies are
necessary to validate the potential use of this carbosilane
dendrimer for siRNA delivery in the disease models.
Triazine dendrimers
Triazine dendrimers (Fig. 8) are so-named as they are
composed of 1,3,5-triazine rings as branching units with the
spacer bearing amine groups at both ends. Recently, a series of
triazine dendrimers with different core structures, generation
numbers and surface functionalities were evaluated for their
siRNA delivery capacity.
29
Both the core structure and the end
group functionality were found to critically influence the
delivery efficiency: dendrimers with a rigid structure together
with arginine-like terminals or hydrophobic terminals showed
the most effective siRNA gene silencing effects in a luciferase
model on Hela cells.
29
In addition, the corresponding siRNA/
dendrimer complexes were stable in circulation and exhibited
passive targeting to the lung after intravenous administration.
29
However, no reports have yet been made on any disease models
concerning the potential therapeutic application of this family of
dendrimers in delivering siRNA and mediating gene silencing.
Much hope is currently being held for these dendrimers.
Polyglycerol dendrimers
Haag et al. developed polyglycerol (PG) dendrimers (Fig. 9)
containing various cationic amine terminals such as a star-like
oligoamine shell.
30,31
These dendrimers have neutral bio-
compatible aliphatic polyether cores and multiple amine terminals
for siRNA binding and complexation. Of these, polyglycerol
pentaethylenehexamine (PG-PEHA) (Fig. 9A) showed the most
effective gene silencing effects, whereas polyglycerol amine
(PG-NH
2
) (Fig. 9B) exhibited the best balance between
transfection efficiency and toxicity.
30
The favorable primary
amines in the 1,2-orientation within PG-NH
2
may be responsible
for its observed efficient siRNA delivering properties.
30
Furthermore, PG-NH
2
could deliver siRNA in vivo via
intravenous administration and accomplish effective gene
silencing with low levels of toxicity,
31
demonstrating the potential
use of this dendrimer to deliver siRNA therapeutics.
30,31
Conclusion and perspectives
Over the past decade, dendrimers have attracted increasing
attention for their potential as non-viral vectors for siRNA
delivery. Their nanometric size (1–10 nm) and well-defined
structure with a spherical architecture bearing unique radiating
branching units in the interior and numerous end groups on the
surface make them ideal drug delivery platforms. Increasing
efforts have been made to develop various dendrimers and
dendritic structures for siRNA delivery. In parallel, various
chemical alterations have been required to increase bio-
compatibility and delivery efficiency, reduce toxicity and
promote targeted delivery via surface group modification,
the introduction of cleavable linkage and the conjugation of
targeting moieties. While considerable basic knowledge has
been acquired and significant advances made in this regard,
further translation to the clinic setting represents a challenging
road ahead. Much attention will be oriented to developing
multifunctional biodegradable dendrimeric delivery platforms
for siRNA delivery aimed at maximizing the efficacy and
specificity of delivery and minimizing the complicated side
effects. A successful outcome will undoubtedly depend on the
close and constant interaction as well as multidisciplinary
cooperation between chemists and biologists.
Fig. 7 An ammonium-terminatin g carbosilane dendrimer of generation 2.
Fig. 8 Structure of a triazine dendrimer of generation 2.
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Acknowledgements
We acknowledge financial support from the international
ERA-Net EURONANOMED European Research project
DENANORNA, National Mega Project on Major Drug
Development (2009ZX09301-014), Association Franc¸ aise
contre les Myopathies (No. 13074, 10793), Wuhan University,
CNRS and INSERM. We thank WANG Qi for the help with
some figures. We apologize to the authors whose original
publications have been not cited due to the reference
limitation.
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