Journal of Biomaterials and Nanobiotechnology, 2011, 2, 510-526
doi:10.4236/jbnb.2011.225062 Published Online December 2011 (
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems
Nuno Martinho1, Christiane Damgé2, Catarina Pinto Reis1*
1CBIOS—Laboratory of Nanoscience and Biomedical Nanotechnology, Faculty of Sciences and Health Technologies (FCTS), Lu-
sophone University of Humanities and Technologies (ULHT), Lisbon, Portugal; 2Institute of Physiology, Faculty of Medicine,
University of Strasbourg, Strasbourg, France.
E-mail: *
Received October 3rd, 2011; revised November 12th, 2011; accepted November 20th, 2011
Drug targeting to specific organs and tissues has become one of the critical endeavors of the century since the use of
free drugs in conventional dosage forms generally involves difficulties in achieving the target site at the appropriate
dose after or during a proper time period. Consequently, the search for new drug delivery approaches and new modes
of action represent one of the frontier research areas. New drug delivery systems include lipidic, proteic and polymeric
technologies to provide new sustained drug delivery with better body distribution, drug protection from the harsh ex-
ternal environment and avoidance of drug clearance. Many of these technologies have reached the market therefore
proving the benefits of these new carriers. This review covers the generalities of those new carriers and their new ad-
vances in drug delivery.
Keywords: Nanomedicine, Nanoparticles, Liposomes, Cyclodextrins, Dendrimers, ADEPT
1. Introduction
Drug targeting to specific organs and tissues has become
one of the critical endeavors of the new century. The
search for new drug delivery approaches and new modes
of action represent one of the frontier areas which in-
volves a multidisciplinary scientific approach to provide
major advances in improving therapeutic index and
bioavailability at site specific-delivery [1-4]. The hard to
target tissues such as blood-brain barrier permeation
limitation can now be overcome allowing the use of
therapies otherwise excluded by conventional dosage
forms [5]. These new systems can hinder solubility prob-
lems, protect the drug from the external environment
such as photodegradation and pH changes, while reduc-
ing dose dumping by controlling the release profile [3,4].
Moreover, controlled targeting at the site of action and
reduced time of exposure at non-targeting tissues in-
creases the efficacy of treatments and reduce toxicity and
side effects [6] thus improving patient compliance and
Biocompatibility is one of the major pre-requisites for
pharmaceutical use, and designing a formulation to fit the
physicochemical properties of the drug poses the chal-
lenge to new dosage forms. Nowadays, the versatility
and biodegradability of polymers such as poly(D-L-lac-
tide-co-glycolide) (PLGA) constitute a leading approach
to new dosage forms to avoid physiological and patho-
logical hurdles encountered in developing targeting stra-
tegies. This approach can improve the pharmacokinetic
profiles of numerous drugs through the delivery of a
higher dose at the site-specific organs by using ligands [7]
while conferring a controlled release and degradation to
non-toxic products. Meanwhile, oral administration is the
most convenient route for drug delivery and the focus of
recent research concerns the development of carriers that
can cross biological barriers such as the gastrointestinal
(GI) tract. In such a way it is necessary for the carrier to
protect the drug against the hostile and degrading milieu
of the GI tract while increasing the residence time (e.g.
bioadhesion) and target specific cells to enhance absorp-
tion which will most likely require less frequency regi-
A number of drug delivery systems are currently under
investigation to circumvent the limitation commonly
found in conventional dosage forms and improve the
potential of the respective drug. On the other hand, there
has been a focus on the microenvironment of the cells
and their interaction with these new dosage forms [8]. As
a result, these new technologies have prompted the old
concept of the magic bullet proposed by Paul Ehrich’s
vision [1].
Recent Advances in Drug Delivery Systems511
2. Type of New Drug Carriers Systems
Microencapsulation has been important to the develop-
ment of new therapeutics and has been used to produce
microspheres containing both hydrophilic and hydropho-
bic drugs entrapped within biocompatible polymers [9].
The purpose of using these carriers is to obtain a con-
trolled release thus maintaining therapeutic drug levels
over a specified time period while reducing systemic
abso rpt ion [9]. These systems have been used in food and
cosmetic industry [4] and drug [10] and gene delivery [11].
Microparticles are a generic term to mention micro-
capsules and microspheres which can be made of poly-
mers or lipids (liposomes) with sizes ranging from 1 to
250 µm (ideally <125 µm and exceptionally 1000 µm)
[12,13]. This technology is very important in drug deliv-
ery. Reduced doses due to higher absorption and pro-
longed absorption time by using adhesion properties of
microparticles have been envisioned [14]. On the other
hand, good in vitro/in vivo correlations have been ob-
served [14]. Biodegradable microparticles are easily
cleared by physiological systems thus avoiding the pos-
sible cytotoxicity caused by accumulation in cells and
tissues. Active substances may be either adsorbed at the
surface of the polymer or encapsulated within the particle.
Furthermore, controlled release can be achieved by pH-
sensitive (especially useful in intravenous delivery) and/
or thermo-sensitive microparticles. Microparticles have
been used to encapsulate several peptides (e.g. calcitonin
and insulin), anesthetics, anti-viral drugs, hypertension
and anticancer drugs [12,14], among others. There are
several methods for the preparation of microparticles
including the polymerization of synthetic monomers and
synthesis from preformed polymers [14].
However, sub-micron size particles have shown to of-
fer marked advantages over microparticles [15,16]. For
example PLGA micro- and nanoparticles were compared
for their uptake in caco-2 cells and revealed a higher up-
take from nanoparticles (41% vs. 15%) [17]. Moreover,
targeting to specific tissues such as inflamed and can-
cerous tissues may be limited on ly to nanoparticles [18].
2.1. Microsponges
Microsponges are biologically porous inert particles that
are made of synthetic poly mers with the capacity to store
a volume of an active agent up to their own weight [19].
They can protect the drug from the environment and pro-
vide a controlled release. Market products are available
such as Retin-A micro® for acne vulgaris and Carac®
containing fluorouracil for actinic keratosis treatments.
2.2. Nanotechnology
The use of nanotechnology for drug delivery rapidly
produced commercially available products and the term
nanomedicine emerged. Nanomedicine is the application
of nanometer scale materials in an innovative way to
develop new approaches and therapies. At this scale,
materials display different physicochemical properties
due to their small size, surface structure and high surface
area [2]. These properties allow nanoparticulate systems
to overcome current limitatio ns of conventional formula-
tion as they facilitate the intracellular uptake to specific
cellular targets. Thus, nanotechnology has been adopted
in several fields such as drug/gene delivery [20,21], im-
aging [22] and diag no st i cs [23 ] .
2.3. Immunoconjugates
Antibody drug-conjugates or immunoconjugates are re-
combinant antibodies covalently bound through a linker
to a drug [24]. The idea behind this technology is to tar-
get potent drugs to th e specific site by using the specific-
ity of monoclonal antibodies (mAb) thus avoiding non-
targeted organs toxicity [24,25]. These immunoconju-
gates can be used across a wide spectrum of diseases by
selecting the appropriate molecular domains [26]. How-
ever, initial works showed some limitations such as shor t
half-lives, immunogenicity or even lack of efficient in-
teraction [25,26]. To avoid this limitation strategies such
as PEGylation, conjugation with proteins such as albu-
min or the use of chimeric humanized and fully human
mAbs has been envisioned [26]. As a result, the first ap-
proved immunoconjugate (Mylotarg, gemtuzumab ozo-
gamicin) was used for the treatment of acute myeloid
leukemia [25]. Several other immunoconjugates are on
the pipeline and in ongoing phase 3 clin ical trials such as
Naptumomab estafenatox for the treatment of advanced
renal disease or Brentuximab vedotin for the treatment of
Hodgkin lymphoma [27]. On the other hand, new strate-
gies have been developed to use antibodies attached on
nanoparticles and liposomes (so called immunonanopar-
ticles and immunoliposomes, respectively) [22,28,29].
These systems can be applied to encapsulate multiple
drugs while protecting from the external environment
and exert a controlled release. Moreover, they can target
hard-to-target tissues such as blood-brain barrier (BBB)
by targeting transferrin, insulin or glutathione receptors,
triggering their activation and consequent internalization
2.4. Virus
Viruses are potential vehicles for drug and gene therapies
due to their natural ability to infect specific cells and
transport genomic material to the nucleus [30-32]. Using
recombinant virus can improve transfection efficiency
[31] while evading degradation by lysosomes [32] thus
enhancing drug delivery. The main difficulties involve
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems
creating viral vectors lacking replication machinery
while maintaining the ability to infect mammalian cells
[32]. Various viruses have been tested and the most
common used are lentivirus, retrovirus and adenovirus
[21,32]. However, the use of viruses raises concerns re-
lated to their safety due to the risk of insertional mistakes
and activation of proto-oncogenes, viral replication and
strong immune responses [30]. Moreover, retroviruses
have size loading limitation as they can only infect di-
viding cells therefore they are most used for ex vivo de-
livery. Lentivirus on the other hand can deliver gene into
nondividing cells as well as adenovirus (the virus re-
mains extrachromosomal which reduces the chances of
disrupting cellular genome) [30]. These systems are most
likely to be applied in cytotoxic gene therapy [30,33]. In
contrast to these, nonviral vectors such as liposomes (vi-
rosomes) and nanoparticles have rapidly increased due to
their low immune response and ease of synthesis [34].
However, limitation of inefficient transfer and low gene
expression have been reported and have to be overcome
somes are liposomes with increased flexibility due to the
addition of ethanol and surfactants, respectively [3,40,
41]. Niosomes are a non-ionic surfactant vesicles made
up from polyoxyethylene alkyl ethers, polyoxyethylene
alkyl esters or saccharose diesters [3]. These systems are
specially designed for skin delivery (ethanol is a known
permeability enhancer) due to their facilitated fu sion and
malleability (transferosomes are ultradeformable) with
membranes and have shown that they can be modulated
from superficial skin (e.g. treatment of Herpes virus) to
full dermal penetration (e.g. required for transdermal
delivery of insulin) [40,41] overcoming limitation com-
monly found in liposomes [41]. The other type of lipo-
somes are classified as virosomes which are liposomes
carrying viral proteins removed from virus on their sur-
face. This strategy has been proposed to immunization
[34] and can be administered via mucosal (nasal, vaginal,
etc.), intradermal and intramuscular routes. Those sys-
tems can incorporate a variety of molecules and can be
designed to imp rove th e up tak e by dend ritic cells th rough
different receptor-mediated routes [31]. Furthermore,
cochleates are stable particles (more than other lipidic
structures) derived from liposomes composed mainly of
charged phosphatidylserine in the presence of divalent
counter ion such as Ca2+ which forms a continuous large
lipid bilayer sheet with no internal aqueous space [35,42,
43]. Cochleate delivery has shown potential use for am-
photericin B, factor VIII delivery, proteins, peptides and
DNA [43,44]. Finally, there are cubosomes. Because of
their multilayer structure of continuous lipid bilayer cu-
bosomes are similar to cochleates but they are considered
as novel lipid delivery systems. They have self-assembly
cubic-like appearance, are biocompatible and show bio-
adhesive properties ideal for oral administration [45,46].
Example, the oral administration of cubosomes loaded
with insulin resu lted in a hypo glycemic effect in rats [47].
More recently, the problems associated with the use of
ultrasound in liposomes was overcome and a new kind of
liposomes named eLiposomes were produced [6]. The
eLiposome can be used as drug carriers which can be
induced to vaporize and cavitate when exposed to ultra-
sound being useful in several applications such as in
cancer therapy [6]. A variety of commercially available
products constituted from liposomes are available such as
Pevaryl® containing econazole which have been used to
treat dermatomycosis, Diclac® for therapy of osteoarthri-
tis and Daylong® containing UV filters for patients with
high risk of actinic keratosis.
2.5. Vesicular Systems
2.5.1. Liposomes, Transferosomes, Ethosomes,
Niosomes, Virosomes, Cochleate, Cuboso mes
These are phospholipid based vehicles composed of a
bilayer membrane that can be divided into small unila-
mellar vesicles (or SUV from 20 nm to 100 nm), large
unilamellar vesicles (LUV from 100 to 500 nm) and
multilamellar vesicles (MVL exceeding 500 nm) [3].
These systems have the ability to encapsulate both lipo-
philic drugs within their membrane and hydrophilic
drugs inside or outside the aqueous core and the mem-
brane of these carriers can be altered and tuned [6]. Li-
posomes which are most commonly produced with phos-
phatidylcholine show great compatibility, ease of prepa-
ration, wide range of drug compatibilities, increased so-
lubility of drugs (e.g. cycloporin A [35]), tuned pharma-
cokinetic profile and improved oral absorption. Com-
monly, they present difficulties when orally delivered
due to the poor stability o f the vesicles under the physio-
logical conditions typically found in the GI tract [4,35,
36]. Liposomes can also act as a drug depot injected
subcutaneously and intact vesicles were found after 96h.
However, liposomes are metastable systems and their
pharmaceutical use may be limited due to content leak-
age with poor controlled release, low encapsulation effi-
ciency and loading. Moreover, weak chemical and phy-
sical protection of sensitive drugs, aggregation into large
particles and hydrolysis with formation of oxidation
products with difficulties in industrial scale production
and stability problems during storage have been also de-
scribed [3,37-39]. As a result, ethosomes and transfero-
2.5.2. Solid Lipid Nanoparticles (SLN) and
Nanostructure Lipid Carriers (NLC)
Solid lipid nanoparticles (SLN) are made up from lipids,
solid at room and body temperature, such as glycerol
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems513
behenate, glycerol palmitostearate, lecithin, triglycerides
and tristearin glyceride [4,35]. Contrary to liposomes,
SLN have shown to be stable for a long period, protect
labile compounds from chemical degradation and can be
processed up to large-scale production. However, they
still present problems related to their loading efficiency
due to the formation of a lipid crystal matrix and possib le
changes of the physical state of the lipids [3,35,39]. To
overcome this limitation, a novel structure composed of a
mixture of lipids solid and fluid at room temperature
(semi-liquid formulations) named nanostructured lipid
carriers (NLC) were produced [3]. This system shows
high encapsulation efficiency and loading capacity due to
the formation of less ordered lipid matrix, and th ey show
long term stability with a controlled release and without
burst effect. These colloidal carriers have emerged as a
potential alternative to other recent colloidal systems like
polymeric nanoparticles [35].
2.6. Microemulsions and Nanoemulsions
Micro- and nanoemulsions are isotropic mixtures of oil/
water stabilized by surfactants frequently in combination
with co-surfactants [3,4,41]. They have shown high solu-
bilization and dissolution properties, thermodynamic sta-
bility and the stabilizers prevent particle agglomeration
and/or drug leakage. Thus, they have improved permea-
tion enhancement ideal for transdermal delivery as they
act in synergy [41]. Microemulsions may work by en-
hanced disruption of skin-lipid structure or by improving
the stability of the drug in the formulation.
2.7. Cyclodextrins
Cyclodextrins are cyclic oligosaccharides containing at
least 6 D-(+)-glucopyranose units attached by α-1,4-
linkage. Three types of cyclodextrins are found in the
nature named α (6 units), β (7 units) and γ-cyclodextrins
(8 units). β-Cyclodextrin is ideal for drug delivery due to
the cavity size, efficiency drug complexation and loading,
availability and relatively low cost [41,48]. They can
prevent the drug degradation, improve the drug stability
and solubility resulting on an higher bioavailability [4,
48]. An example of cyclodextrins in drug delivery system
is the derivate 2-hydroxylpropyl (HPβCD) which is a
powerful solubilizer and has a hydrophilic outside and
hydrophobic inside [48]. For absorption in the GI tract,
the complexes must contact with the surface thus pro-
moting dissociation and drug permeation across the
membrane [41]. Moreover, cyclodextrins can work syn-
ergistically as permeation enhancers to improve their ab-
sorption across th e skin.
2.8. Metal Nanoparticles and Quantum Dots
Inorganic nanoparticles have emerged a few years ago as
drug and gene delivery systems, imaging agents and di-
agnostic biosensors [22,49]. Magnetic drug targeting
(such as the use of iron) is characterized by conjugating a
magnetic material under the action of the external mag-
netic field, which can accumulate in target tissue areas
under the action of the external magnetic field [4,23,50].
However, magnetic particles alone are not suited for drug
vehicles because of limitations in the controlled release.
A mixed composition of a magnetic nucleus and a poly-
meric shell could take advantage of the two components
Quantum dots are colloidal cores surrounded by one or
more surface coatings that reduce leaching of metals
from the core. These nanoparticles are of extreme im-
portance for diagnosis.
Furthermore, titanium dioxide and zinc oxide demon-
strate the potential of nanoparticles to improve therapeu-
tic/prevention performance being particularly useful as
sunscreen agents [51]. The micronization of these com-
pounds to nanometer range removes the opacity charac-
teristic associated with them and increases the UV pro-
tection [51,52].
Finally, gold nanoparticles have shown a selective
transportation of drugs to cancer cell nucleus specially
when incorporated with conjugated arginine-glycine-
aspartic acid peptide (RGD) and PEG [53]. When reach-
ing the tumor cells, they can induce hyperthermia using
non-invasive r a di o frequency.
2.9. Polymers
2.9.1. Dendrimers
Dendrimers are tree-like branched synthetic polymer
macromolecular nanoparticles in a dendron-like structure
which can be designed to target specific structures [54].
They have a remarkable well-defined control over size
(comparable size to proteins) with narrow polydispersity
[54-56]. In addition, they have a large surface fun ctional-
ity providing a wide range of applications such as drug
[57] and gene delivery [58], biological adhesives [59],
imaging agents (e.g. MRI) [56]. Thus, they can be used
for oral, transdermal, ocular and intravenous deliveries
[60,61]. Moreover, dendrimers have shown that they can
easily cross cell barriers by both paracellular and tran-
scellular pathways [56]. Dendrimers can be structurally
modified. This modification can be made to the nature of
the core and the scaffold giving polyfunction capacity to
the dendritic structure. This can be copulated to an anti-
body and its production can be through divergent and
convergent routes or other techniques such as self-as-
sembling synthesis, lego chemistry and click chemisty
[54,57]. Their size, molecular weight and number of sur-
face functional groups can be modulated through the
increase in generation number ( 1 nm per generation) [56,
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems
57]. In general, dendrimers are terminated with amine
surface groups (G1, G2, G..) but can also be terminated
with carboxylate (G1.5, G2.5, G..) [62]. Moreover, the
interior is characterized by the availability of a wide
amount of solvent-filled void space that can accommo-
date the drug [56]. Additionally, dendrimers are non-im-
munogenic and are small enough to escape the vascula-
ture and target tumor cells. Their size can be tailored to
be below the threshold for renal filtration [55]. There are
several systems available such as poly(amidoamine)—
PAMAM, poly(etherhydroxylamine)-PEHAM and poly
(propyleneneimine)—PPI, and phosphorous containing
dendrimers [56]. PAMAM is the most used dendrimer
due to the fact that it provides a large range of reactive
sites for the conjugation for drug or other chemical moi-
ety complexation [55,57]. Dendrimers provide a high
loading capacity with controlled release which can be
modulated to actively release the agent by pH-triggering
cleavage. The rate of drug release from the matrix is in-
fluenced by the nature of the linking bond or spacer be-
tween the drug and scaffold and the targeted physiologi-
cal domain for intended release. The surface ligands can
also control the release from the dendrimers such as in-
creased steric hindrance of mannose and folate.
A novel concept that enables simultaneous release of
all functional groups by a single stimulus has been re-
ported which has been named cascade-release dendri-
mers (or dendrimer disassembly or self-immolative den-
drimers). However, this system raises concerns about
drug release at the wrong time and place which can raise
toxicity profiles [56]. Several dendrimer-based diagnos-
tic and/or in vitro technologies are already in the market
such as Stratus CS which is a dendrimer-coupled anti-
body reagents [63], Superfect (activated dendrimer tech-
nology for DNA transfection into a broad range of cell
lines) [64] and PriofectTM which is a transfection reagent
[65]. Priostar™ and STARBURST® have also been de-
signed to be used as targeted diagnostic and therapeutic
delivery systems for a wide variety of drugs to cancer
cells and other diseases [66,67]. As well, Vivagel® is a
microbicide for prevention of HIV and HSV and it is
based on dendrimers [68].
2.9.2. Natural and Synthetic Polymeric Nanoparticles
Drug/gene encapsulation can be achieved by embedding
into the matrix or ab sorbed onto th e surface of nanoparti-
cles homogenously dispersed or not. As for the micro-
particles, the term nanoparticles is a collective name for
both nanospheres and nanocapsules [16]. Nanoparticles
are solid carriers that can be either made up of natural or
synthetic polymers and whether or not biodegradable
[16]. Nanoparticles have received more attention than
have liposomes because of their therapeutic potential and
greater stability in biological fluids as well as during
storage [69]. Nanoparticles are advantageous in many
ways since they use the unique micro-anatomy of the
inflamed tissue blood capillaries, which have gaps be-
tween the lining of endothelial cells causing vessel
leakiness. Moreover, they show high encapsulation effi-
ciency and protection of instable drugs against degrada-
tion of the external environment in comparison to lipo-
somes [3,70].
Several methods have been described and nanoparti-
cles can be obtained by polymerization of a monomer or
from pre-formed polymers [16] but recent methods make
use of safe solvents with industrial applicatio n.
The nanoparticles properties can be tailored by using
different polymers and co-polymers or proteins. The new
strategies use new biodegradable synthetic polymers and
modified polymers from natural products such as chito-
san and albumin. Chitosan has been shown to be rela-
tively safe and is used as a food additive. Moreover, chi-
tosan is widely used due to its biocompatibility, muco-
adhesiveness and permeability enhancing properties [35,
71,72] and its derivates have shown improved character-
istics. Albumin is a natural carrier of hydrophobic mole-
cules such as fatty acids, hormones and fat-soluble vita-
mins. Albumin has been extensively used as it is non-
toxic and non-immunogenic.
However, natural polymers raise concerns in purity
and stability and thus synthetic polymers have been ap-
plied. Synthetic polymers from the ester family such as
poly(lactic acid) (PLA), poly(cyanoacrylates) (PACA),
poly(acrylic acid), poly(anhydrides), poly(amides), poly
(ortho esters), poly(ethylene glycol), and poly(vinyl al-
cohol) (PVA) and other like poly(isobutylcynoacrylate)
(PIBCA), poly(ethylene oxide) (PEO), poly(ε-caprolac-
tone) (PCL) are suitable for drug delivery due to their
biodegradability. They can be conjugated between them
to form different structures with different properties such
as controlled release profiles and strong cell biocompati-
bility. In fact, PLGA, another synthetic polymer, has
been extensively used in medical applications such as
suture materials [73] and bone fixation nails and screws
[74] as well as in diverse drug delivery applications [20,
75,76]. It is biocompatible and biodegradable forming
compatible moieties of lactic acid and glycolic acid
which are further removed by the citric acid cycle [2]. As
this process is slow it does not affect normal cell function
Recently, poly(β-amino ester) (PbAE) has emerged in
the spotlight because it demonstrates a pH sensitive re-
lease [7,77,78] in which at acid pH it rapidly releases its
contents. This polymer has shown to be less toxic than
other cationic polymers such as poly(ethyleneimine) and
poly(L-lysine) (PLL) [78]. PbAE are insoluble at phy-
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems515
siological pH but become instantly soluble in aqueous
media when the pH of th e solution is reduced below 6.5.
These agents are useful for therapeutics in the vicinity of
tumor mass [77,78] and for others they must escape en-
dosomal compartmentalization prior to fusion with ly-
sosomes [78].
3. General Mechanisms Consideration
3.1. Surface Functionalization, Controlled
Release and Tissue-Targeting Design
A number of methods have been investigated to target
drugs to a specific site of interest either by passive (in-
creased accumulation due to passive physiological fac-
tors) or active diffusion (use of ligan ds to specific target)
Surface modification of drug carriers with bioactive
molecules that can be adsorbed, coated, conjugated or
linked to them which interact with cell receptors demon-
strate a selective affinity for a specific cell or tissue type
and can subsequently enhance drug uptake (Figure 1). The
modified-coating (e.g. combined albumin and chitosan)
can also be used to prevent enzymatic degradation both
on the GI tract and plasma [72]. Monoclonal antibodies
(or fragments) or non-antibody ligands like carbohy-
drates specific for cell surface such as lectins have been
investigated [79]. Also, most recently small molecules or
peptides agonists/substracts or antagonists/inhibitors for
receptors that are overexpressed on cell surface of spe-
cific tissue (e.g. folate, transferrin as well as galactosa-
mine) have shown promising results [6,54,79,80]. Sev-
eral considerations have to be taken as the use of target-
ing ligands which can enhance distribution to secondary
target sites of non-intended tissues [54]. In fact, the dis-
advantage of using non-antibody ligands is their non-
selective expression [79]. On the other hand, immuno-
conjugates poses problems related to immunogenicity
and retention in the reti culoendothelial system (RES) [55] .
The carrier surface modification can also incorporate
coatings to chan ge the lipophilicity/hydrophilicit y profile,
prevent the uptake by immune cells and improve cell
recognition (e.g. the synergy between the distribution and
signaling of antibodies). As a result, per example, once
IV injection occurs, nanoparticles are cleared from the
plasma within a few minutes due to opsonization and
subsequent phagocytosis by the cells of the RES [81].
Opsonization can be reduced by applying some surface
ligands. An example is PEG, a hydrophilic polymer,
which promotes the resistance to the binding of plasma
proteins and prevents aggregation induced by salts and
proteins in the serum [21]. This fact prevents opsoniza-
tion and recognition from phagocytes and thus avoiding
immune responses. Moreover, PEG can also reduce the
access of enzymes to dendrimers scaffold and can there-
fore reduce their degradation [54]. In fact, in vivo nano-
particles and liposomes coated with PEG increase circu-
lation time from several minutes to many hours and en-
hance residence times up to 200-fold in humans [82-84].
On the other hand, the effectiveness of PEG depends on
surface density, chain length [85] and ability to avoid the
liver uptake. However, PEG carriers are intended for in-
tracellular penetration and sometimes PEG prevents nor-
mal interactions of the carrier with cells. Also,
PE-Gylated nanocarrier systems have shown to induce an
immune response, known as the accelerated blood clear-
ance (ABC phenomenon) after repeated injection with
subsequent increased accumulation on the liver and
spleen [86].
Thus, new strategies have been pursued such as re-
placing PEG with polyamino acid polyhydroxyethyl-L-
aspargine (PHEA). This strategy demonstrated favorably
long circulation times and reduced ABC phenomenon
compared to PEG [86]. Moreover, using a hydrazone-
cholesteryl hemisuccinate linkage to the PEG which
could be cleaved by esterases showed pH-response at pH
5.5 (with a t1/2 of 6.7 h) and was stable at physiological
pH (with a t1/2 of 40.9 h) and thus can be used for tumor
targeting [86]. Additionally, PEO (poly(ethylene glycol)
and its derivates also promote stealth shielding and pro-
longed circulation [7,78,87] and have been extensively
used as biomaterials due to excellent biocompatibility
and low toxicity [78]. PLA and PLGA have also demon-
strated some stealth shielding [16]. All these syste ms can
also be used as prom o t ers of GI absorption [88] .
Another key factor to improve the carrier targeting is
the surface charge (zeta potential). This determines the
interaction with plasma proteins, cell membranes and
surface, thus ultimately affecting clearance and distribu-
tion patterns [54,79,89]. For instance, cationic surfaces
(obtained per example by chitosan coating) demonstrate
a strong interaction with cell membranes and surfaces
due to their overall anionic charge [54]. However, PLGA
nanoparticles are slightly negative at the surface and this
tends to limit their interaction with both negatively
charged plasmids and their intracellular uptake [71].
After reaching the cell, the drug release can be made
from two different mechanisms including release from
the carrier as well as absorption from the cell and the
carrier can be taken into the cell and slowly release its
contents [79]. Nanoparticles are absorbed by different
mechanisms but endocytosis is the most significant con-
tributor to cell entry [54,90]. Caveolae-mediated endo-
cytosis is thought to be the primary uptake mechanism
for particles above 200 nm [90] but also lipid raft-asso-
ciated receptors [90], actin and clathrin, microtubules,
and cholesterol-dependent process might be implied in
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems
Copyright © 2011 SciRes. JBNB
the nanoparticle uptake mechanisms [54,90]. The surface
charge is a key factor as anionic dendrimers were endo-
cytosed by clathrin dependent processes (with promotion
of tight junction opening) but were independent of cave-
olin-mediated endocytosis.
Earlier research on internalization of PAMAM den-
drimers showed that the interaction was mediated by
electrostatic interaction between the cationic primary
amine surface groups and the negatively charged pro-
teoglycans displayed on the surface of mammalian cells
which trigger macropinocytosis and clathrin-mediated
endocytosis [57].
When considering this uptake by cells, they have to be
designed to avoid the acidic environment of lysosomes
which are the common degradation pathway of nanopar-
ticles inside the cell [90]. Moreover, once dendrimers
saturated the lysosomal pathway, they were eventually
found in endosomes [54]. Recent advances taking ad-
vantages of the microenvironment of the cell have been
investigated and new carriers can reach the nucleus (gene
therapy) or other organelles involved in the disease state
leading to a direct release of the loading drug in consid-
erable concentrations at the specific site [90].
Another approach is the use of an external stimulus to
increase cellular ability for drug uptake such as ultra-
sounds to temporarily increase the junctions between
cells. Here eLiposomes may find a potential use [6]. Pas-
sive targeting refers to the increased accumulation of
drug or drug-carrier at a particular site due to passive
physiological factors. For cancer therapy, this typically
includes taking advantage of enhanced permeability and
retention (EPR) [6].
On the other hand, a major problem associated with
the new drug carriers is their release profile because
these can be associated with burst releases [91]. While
burst releases are useful in dermal and systemic delivery,
they may lead to a significant and unpredictable toxicity
especially for potent drugs and treatments of chronic
diseases. Several strategies have been proposed and con-
trolled release can be achieved by bi-association of lipo-
somes encapsulated inside a polymeric particle [92] as
well as chitosan associated with alginate which demon-
strate a controlled release and prevention of burst release
[93]. Similar strategies can be achieved by using mi-
croparticles containing nanoparticles [91]. Also changing
some technological features such as production method
[91] and use of surfactant can promote different control
release [91].
Other technological features include deficient hetero-
geneous drug distribution (e.g. surface-associated drug),
temperature of solvent removal, the physicochemical
nature of the polymeric matrix (use of non water soluble
polymers to avoid water uptake), porosity and recovery
method as well as the concentration of drug incorporated
[94-97], among others.
3.2. Combined Therapy by Simultaneously
Encapsulated Drugs
These are systems that have the potential to deliver more
than one drug at once. For example, PLGA nanoparticles
were simultaneously loaded with vincristine sulfate and
verapamil hydrochloride to deliver the effective chemo-
therapeutic agents while inhibiting P-gP efflux system.
This system allows overcoming tumor lack of sensitivity
and increases therapeutic index. As a result, the same
strategy was planned for delivery of doxorubicin and
cyclosporine A [98,99]. However, recent studies suggest
that PLGA-PEG interact with P-gP [20] and this could
improve the efficacy of this system. The design of these
types of systems has to take into consideration the char-
acteristics of the drugs to be encapsulated. Example, hy-
drophobic drugs are more likely to be encapsulated in
hydrophobic polymers and vice-versa [95]. To overcome
this limitation, synthesis of new polymers such as (PLA-
PEG-PLA)n or PCL-PEG can be produced [95] and reti-
noic acid (hydrophobic) and calf thymus DNA (hydro-
Figure 1. Different possibilities for nanoparticles specific targeting. In the left nanoparticle functionalized with: (A) protective
polymer with targeting ligand/probe copulated; (B) Antibody; (C) Enzyme; (D) Complexation with DNA; (E) protective
polymer; (F) ligand; In the right: nanoparticles can either release their content after cell internalization or near the cell after
argeting a specific receptor. t
Recent Advances in Drug Delivery Systems517
philic) were both encapsulated in this system with satis-
factory loading [95]. Another strategy is to have two dif-
ferent release rates of the two drugs to improve treat-
ments (such as cancer treatment). In fact, a paclitaxel and
a C6-ceramide were encapsulated in a controlled blend
polymer of PLGA-PbAE to effectively overcome the
cancer drug resistance mechanisms [7].
3.3. Carrier Distribution
As stated above the RES mainly in the liver and spleen
are the major obstacles to carrier systems due to their
ability to internalization and removal from systemic cir-
culation [16]. In fact, after IV administration of PLGA
nanoparticles, the majority were found in the liv er (about
40%) followed by kidney (26%), heart (12%) and brain
(13%) and only a small amount was found in the plasma
[2]. Similar results were obtained with PLGA-PbAE [7].
The route of administration is also important for the
distribution pattern as after IP injection for all types of
charged particles due to lymphatic clearance [49]. More-
over, lipophilicity of carriers influence the uptake from
cells and as a result more hydrophilic particles may be
rapidly eliminated [16].
The medium pH modulates surface charge thus chang-
ing cellular uptake and subsequently the distribution of
the carrier through the system [71]. The nanoparticles
charge surface and route of administration were further
explored by using 10 nm Gold nanoparticles functional-
ized with different groups aiming different zeta potential
(neutral, negative, positive and zwittteronic) by IV and
IP administrations. Following IV injection, a 10 fold
lower peak plasma concentration was observed with
positive charged particles and clearance within 15 min-
utes was more pronounced for negative and positively
charged particles [49]. On the other hand, after IP injec-
tion low concentrations of both negative and positive
charged particles were found [49]. These results evidence
that neutral and zwitteronic nanop articles show enhanced
circulation. These marked differences in bioavailability
could be primarily due to opsonization of the nanoparti-
cles with antibodies for recognition by resident macro-
phages [49] and the same effect was observed in den-
drimers [54].
Nanocarriers can be modulated to deliver drugs to
specific tissues and organs. Branching size of dendrimers
can be modulated to determine their distribution and
elimination throughout the body. Thus, they can avoid
renal clearance with a cut-o ff of 40 - 60 kDa which is ap -
proximately the G7 [54,56]. From G1 to G5 the den-
drimers are rapidly cleared to the kidneys/bladder and
from G3 - G7 they are mainly seen in circulation while
G8 are found in the lymph node and, finally, superior to
G9 are found in the liver [56].
As stated previously, PEG influences the distribution
of the carrier in the body. In general, as the molecular
weight of the PEGylated dendrimers increases, uptake
from the injection site into the lymph becomes a more
important contributor to the overall absorption profile,
revealing potential drug delivery systems as well as
improved lymphatic system imaging agents [54].
4. Pharmaceutical Applications
4.1. Brain Delivery
The blood brain barrier (BBB) is an extraordinary gate-
keeper toward exogenou s substan ces be ing estimated that
98% of all drug never reach the brain in therapeutic con-
centrations [23]. There have been several experimental
strategies to address these problems and enhance brain
bioavailability of existing therapeutics into the CNS
[100]. These included injecting drugs directly into the
brain or CSF (intraparenchymal or intracerebral admini-
stration), various implants or convection-enhanced de-
livery, slow-release devices, transient disruption of the
BBB such as MRI-guided focused ultrasound and che-
mical or osmotic modulation of tight junctions—with the
use of hyperosmotic solutions of saccharides (e.g. man-
nitol) or vasoactive compounds (e.g. RMP-7) [84,101].
Overall, the idea of using an appropriate drug carrier
to delivery across the BBB is reinforced. Nanovectoring
with tissue-specific targets is an ideal pathway since it
delivers both hydro-and lipophilic drugs, as well as mac-
romolecules such as peptides and genes through a con-
trolled release profile over an extended period of time [5,
81,102]. Since nanoparticles are small in size, they easily
penetrate into small capillaries and through the physical
restrictions presented by the brain interstitial space.
Consequently, they can be transported within cells, al-
lowing an efficient drug accumulation at targeted sites in
the body [81,102]. However, nanoparticles cannot freely
diffuse through the BBB and require receptor-mediated
transporters [103]. Hence, the use of the specific peptid es
for targeting the receptor-mediated transcytosis across
BBB can be a successful strategy for improving drug
delivery to the brain [5]. In this way, promising results
have been achieved by directly delivering drugs to the
brain interstitium through the design of polymer-based
drug delivery systems [102,104].
Different approaches have been pursued and in recent
researches using the combination of two techniques such
as improvement of target-specificity and bioavailability
[5,101]. Antibodies for different receptors, chimeric pep-
tides fused molecules [101], pro-drugs resembling the
natural ligands, viral vectors [105] and nanoparticles are
the most common techniques [5,103].
In nanoparticle field, dalargin or loperamide-loaded
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems
PBCA nanoparticles coated with polysorbate 80 showed
a pronounced analgesic effect in comparison with that of
free drug [106]. Several mechanisms were proposed and
endocytosis and transcytosis mediated by carriers were
evidenced, as nanoparticles were overcoated with Apo-A,
B, C, E or J. The effect was only achieved when ApoB or
E were in the coating surface. In this study, polysorbates
(and also poloxamers) can act as an anchor for several
Apo which are then able to interact with the LRP recap-
tor, before being taken up by the microvessel endothelial
cells via receptor-mediated endocytosis [80,85,107,108].
Another approach to LRP receptor is by the use of a
series of peptides called “angiopep” at the surface of
nanoparticles which have shown specific targeting of the
LRP [109,110]. The most used is angiopep-2 as it shows
enhanced transcytosis across the brain and it has been
effective against glioblastoma [109,111].
Other receptors that have been proposed to targeting
are insulin, albumin, transferrin, lactoferrin [110] and
more recently, the glutathione receptor [20]. In the last
case, liposomes were coated with glutathione-conjugated
PEG (G-Technology®) and they successfully delivered
free drug (doxorubicin or ribavirin) [83,101]. For all the
reasons stated, in 2010, EMA granted an orphan designa-
tion (EU/3/10/781) for the GSH-PEG liposomal doxoru-
bicin hydrochloride for the treatment of glioma [112].
4.2. Mucosal Drug Delivery
The oral route is the most desirable route for the admini-
stration of drugs as it is simple and free from complica-
tions arising from more invasive methods. When design-
ing such formulation, several parameters have to be ac-
cessed as charges from the carrier system and content,
the solubility of the drug carrier, among others. Those
factors will ultimately alter their uptake from mucosal
membranes. Moreover, mucosal surfaces are typically
efficiently removing the drugs by mucus clearance me-
chanisms and the GI tract acts as a physiological and
chemical barrier posing several challenges. Also, the
drug can cause irritatio n and limit its use by this route.
To overcome these limitations, several methods have
been investigated and nanoparticles are also a useful tool
in mucosal delivery. It has been shown that they can
protect protein and peptide drugs from enzymatic degra-
dation and increase their low permeability across the
intestinal epithelium and circumvent efflux processes [54,
62,69,72,113]. Specifically, nanoparticles can be taken
up by increased residence time in the enterocytes [16], by
targeting to M cells [114,115] or by specific targeting
receptors at the surface. In this area, polymers play a
crucial function. Example, chitosan possesses marked
mucoadhesive properties to the mucosal surface and can
transiently open the tight junctions between mucosal
cells (an effect also observed for PAMAM dendrimers)
[62,99,113]. As a result, nanoparticles composed of chi-
tosan loaded with insulin have been able to enhance in-
testinal absorptio n in vivo [72,116].
Furthermore, oral vaccination has gained new insights
and several studies have been performed [117]. Oral im-
munization has been making use of live attenuated or-
ganisms [118] or the use of peptides [114] and recently
based on DNA vaccines [119]. However, there are still
limitations for effective oral immunization such as the
failure to swallow the vaccine, inactivation in the GI tract
or interference with gut flora [69]. Promising results have
been obtained and humoral and cellular in vivo respon ses
have been observed through the use of specific ligands
(e.g. RGD peptide) to target M cells [114,115]. In addi-
tion, immunization with carriers may be ideal when the
antigen of interest is not immunogenic enough. PbAE
microparticles on their own can activate dendritic cells
[77] acting as an immune-stimulating complex and as a
result, PLGA-PbAE microparticles were able to induce
antigen-specific rejection of transplanted synergetic tu-
mor cells.
Furthermore, nasal vaccination has been investigated
as a promising route for vaccination. PLGA blended with
different stabilizers [76] and cochleates [44] demon-
strated good in vivo vaccination, capable of overcoming
nasal cell membranes. As well, genetic vaccination has
the potential to treat and prevent several diseases for
which conventional vaccines are ineffective and limited
[77]. The use of a carrier to specifically target APC cells
may show promising advantages while protecting the
encapsulated genetic material [77].
Gene delivery to desired cells involves the concept of
delivery the gene for expression (e.g. production of pro-
teins that play a role in drug) or the use of siRNA (to
target a specific mRNA expression) [30,120]. The use of
carriers allows the entry of these genetic materials to the
cells that otherwise would be destroyed by enzymes and
due to their small size and high density are easier to
transfer into cells. As stated, viral vectors are effective
although raising certain concerns, but synthetic systems
have higher flexibility and safety profiles [30]. An ideal
gene delivery carrier would be a system that can safely
transport the genetic materials without exhibiting any
toxicity and immune responses as well as being able to be
produced on large scale [120]. Using virosomes, these can
be processed b y APC cel ls whic h ensure s present ation via
MHC I or II resulting in humoral and cellular responses
[34]. A commercially available product is Epaxal® [121]
which compared to conventional aluminum-adsorbed
hepatitis A vaccine, the virosome-based vaccine may
provide enhanced protection and cause fewer local ad-
verse effects [34]. Other products based on the same
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems519
concept are PEVION’s virosomes for influenza virus
[122]. Using cationic modified particles (such as chitosan
and derivates such as mannosylated chitosan), genetic
material can bind and condense through electrostatic in-
teractions [123]. Recently, PEI polymer has been exten-
sively used both in vitro and in vivo. PEI is the most ef-
fective nonviral carrier with high transfection capacity
and ability to escape from endosomes [31,120,124]. In
fact, ExGen 500TM technology (linear PEI condensed
with genetic material) has been shown to interact with
cell surface proteoglycans resulting in internalization by
endosomes after which PEI acts as an effective proton
sponge buffer protecting the genetic material from ly-
sosomal degradation [32,124]. This proton sponge effect
is due to PEI primary and secondary amines that lead to
an influx of counter ions (chloride) that enhances os-
motic pressure and eventually burst the endocytic vesicle.
Other mechanism has been proposed in that PEI weak-
ened the endosome membrane thus preventing fusion
with lysosomes [124].
Another area of interest is the antibiotics and antiviral
therapy using those drug carrier systems. Again, nano-
carriers are ideal, as they can overcome the defense
mechanisms, can result in higher uptake to the site and
exert better results than the free drug. They may also be
suitable to target drugs where the free drug would not
permeate into (e.g. BBB). Anticancer drug using gold
nanoparticles have shown antibacterial and antifungal
activity against gram-negative bacteria [50]. Ampho-
tericin B PLGA nanoparticles, taken both orally and IV,
were more e ffective in re ducing the lun g burden i n murine
models of pulmonary and disseminated aspargilosis than
commercial available formulations [125]. AmBiso me® is
a liposomal preparation of amphotericin B that is admin-
istered by IV injection and has been effective in the
treatment of cryptococcal meningitis in HIV-infected
patients, Aspergillus Candida and/or Cryptococcus spe-
cies infections refractory to amphotericin B deoxycholate
[36]. Rece ntly, VivaGel ® by Starp harma Pty Ltd. has bee n
investigated as a vaginal m icrobicide for the prevention of
HIV and HSV infections [68,126] using dendrimer tech-
nology (SPL7013). The highly charged surface allows
SPOL7013 to attach to targets on viruses, blocking viral
attachment and/or adsorption to cells thereby preventing
infection. In the case of HIV, SPL7013 is though t to bind
gp120 proteins on the surface of the virus [126]. It has
been able to inhibit by <99% HIV infection of human
macrophages in vitro, and to protect Vero cells from
HSV-2 infection, and effectively to block vaginal trans-
mission of SHIV [126].
4.3. Pulmonary Drug Delivery
The pulmonary route requires a suitable design as the
deposition of the nanoparticles differs according to the
particle size [69]. On the other hand, the mucus may re-
strain the entry of nanoparticles. PSA-PEG nanoparticles
were able to penetrate and diffuse in sputum expectorate
from lungs of cystic fibrosis patients and this system
could be used to improve drug therapies in various mu-
cosal surfaces [88].
4.4. Skin Drug Delivery
Application to the skin desires two effects: transdermal
and topical effects. The transdermal delivery has gained a
significant importance fo r systemic treatment as it is able
to avoid first-pass metabolism and major fluctuations of
plasma levels typical of repeated oral administration.
SLN, due to an initial burst release followed by water
evaporation, proved to penetrate human and pig skin ex
vivo more rapidly and to a higher extent than conven-
tional dosage forms and a nanoemulsion [3]. The same
results were observed for SLN and NLC incorporating
red nile (4 fold enhancing). The rapid degradation of
those systems may promote contact with the skin and the
occlusion may promote drug uptake. Additionally, SLN
were able to induce epidermal drug targeting for pred-
nicarbate [127] and podophyllotoxin [128]. Other drug
carriers have been used in skin drug delivery. Example,
transfersomes with ketoprofen (Diractin®) [129] were
applied as a transdermal system in a multicentre, ran-
domized, double-blind trial and showed similar efficacy
in relief of knee osteoarthritis compared to celecoxib. In
addition, liposomes tend to fuse at the skin surface [3]
and marked changes can be induced in the horny layer
depending on the phospholipids used as intercellular
deposition can occur and destroy lipid membranes [3].
Antifungal drugs are of special interest and although
current formulations cure the majority of th e problems an
econazole liposome formulation in vitro has shown better
cure rates [3]. In general, percutaneous drug absorption
appears to be increased via association with dendrimers
due to their ability to interact with lipid bilayers in the
skin [54]. Moreover, targeting specific areas of the skin
can be tailored. As an example, OMC-coated PCL nano-
particles were found 3.4 fold greater in the stratum
corneum compared to an emulsion.
Finally, the role of skin appendages is sometimes ne-
glected due to the fact that they only represent 0.1% of
the skin surface [130]. It was found that microparticles
ranging from 3 - 10 µm selectively penetrate the follicu-
lar ducts, whereas particles larger than 10 µm remain
randomly distributed in the hair follicles and stratum
corneum [130]. Moreover, 5 µm PLGA microparticles
were visualized in the follicular ducts, while 1 µm was
randomly distributed into the stratum corneum and hair
Copyright © 2011 SciRes. JBNB
Recent Advances in Drug Delivery Systems
Copyright © 2011 SciRes. JBNB
Recently, a new study performed a similar experiment
and 200 nm were able to aggregate and penetrate along
the follicular duct and this was the major penetration
pathway [131] displaying an increasing interest for a
potential vaccination therapy. On the other hand, 40 nm
nanoparticles were able to enter Langerhans cell in the
hair follicles while 750 and 1500 nm could not. The 40
nm nanoparticles were able to penetrate deep into the
hair follicle while 750 and 1500 nm aggregate in the in-
fundibulum of human hair follicles and could not target
the pilosebaceous unit [132]. Smaller molecules with a
ranging diameter between 7 and 20 nm were tested in
skin permeation and were found almost exclusively in
the hair follicle infundibulum and below. On the other
hand, polystyrene nanoparticles ranging from 20 to 200
nm were found in the follicle openings [133]. Further-
more, liposomes showed that they can target the pilose-
baceous unit rich in Langerhans cells and gained interest
in immunizations.
4.5. Cancer Delivery
Cancer delivery presents a challenging obstacle for every
dosage forms. Targeting cancer cells while avoiding
damage to other cells is the main endeavor of cancer
therapy. Major clinical obstacles raised to chemothera-
peutic agents are due to large body distributions, mul-
tidrug resistance mechanism (MDR), poor absorption,
increased metabolism and excretion while having poor
diffusion through the tumor mass which constitutes the
impaired delivery [7,89]. Herein, the concept of en-
hanced permeability and retention (EPR) in the solid
tumor [134] and the microenvironment of the tumor
(physiological drug resistance) [8] plays a vital role to
the enhancement of nanoparticles’ uptake.
PEG has been a key agent in long circulation of carri-
ers and has shown the ability to passively accumulate in
tumor tissue via EPR effect [7,8,49,54]. In addition, the
lower pH observed (pH 6.5) in some tumors create a pH
gradient that hinders the permeation of drugs and consti-
tutes one of the causes of chemotherapy failure. PbAE-
PEO, a pH sensitive polymer shows a 5.2 fold higher
concentration of Paclitaxel when compared to the aque-
ous solution [87]. Thus, it can be considered as an ideal
carrier to overcome the pH gradient barrier. Moreover,
using PLGA-PbAE nanoparticles with ceramide incur-
porating paclitaxel demonstrates higher accumulation
within the tumor [7].
Another way to achieve selectivity of tumor cells is by
using antibody-directed enzyme prodrug therapy (ADEPT).
This technique involves a two-step approach to cancer
therapy in which an immunoconjugate composed of a
mAb-enzyme is administered to be localized within the
tumor mass. Then, it is allowed to clear from the sys-
temic circulation over time and once the ratio of tumor/
non-tumor is sufficiently high a prodrug (anticancer
agent) is given (Figure 2). After reaching the tissue, it is
mostly converted in tumor cells and consequently exerts
local effects [25].
On the other hand, fenestration within the new vascu-
lature (angiogenesis) is observed exhibiting pores of 200
- 400 nm [7] or with an upper limit of 12 nm in the
blood-brain tumor barrier [135]. So, dendrimers nanopar-
ticles ranging between 7 - 10 nm were developed to de-
liver therapeutic concentrations across the tumor blood
brain barrier protecting from leakage to normal tissues
Using specific ligands is another approach to target
tumor cells and the high-affinity folate receptor, known
as the folate-binding protein, has been used as a target for
the delivery of a carrier containing folate at the surface to
target drugs to cancer tissue. The folate receptor is over-
expressed in breast, ovary, endometrium, kidney, lung,
head and neck, brain and myeloid cancers [55]. In vivo,
liposomes as well as PAMAM dendrimer conjugated
with folate acid have shown higher efficacy (10-fold) and
lower toxicity compared to those of free dr u g [55].
Other ligands have been used to target more specifi-
cally such as LHRH coupled carrier to deliver siRNA to
Figure 2. Antibody-directed enzyme prodrug therapy (ADEPT). Higher concentrations are found in cells overexpressing a
receptor. The antibody localizes the enzyme at the tumor mass and after the intake of a prodrug it will be converted near the
umor cells. t
Recent Advances in Drug Delivery Systems521
cancer cells [21] and N-acetylgalactosamine (NAcGal)
coupled to G5 dendrimers to hepatic cancer cells [57],
among others.
The use of drug carriers systems can also be used for
topical tumor cancer such as melanoma. Example, 5-
fluorouracil niosomes increased the penetration in the
stratum corneum by 8 fold. The cytotoxicity for the
melanoma increased resulting in more efficiency and less
irritancy than when incorporated to microsponges [3].
5. Concluding Remarks
As seen, the effort to produce these new drug carrier
systems is clearly high. Undoubtedly, those carriers pro-
vide the hope to treat and diagnose several diseases.
Several technologies have advanced into clinical studies
and are nowadays market products that have been shown
favorable results. It was also shown in this review that
these recent drug carriers are a promising set of tech-
nologies that already penetrated the cancer area and they
likely have a strong impact in this field in the future. In
fact, the rationale development of anticancer carriers will
provide new ways of treatment, circumventing current
limitations for conventional dosage forms. However,
there are some issues that need to be understood in order
to ensure their safety and effectiveness. Nevertheless, in
the future, new entities will become available and re-
sponsive and “clever” polymers will offer new perspec-
tives for the treatment of diseases.
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