Nanopharmacy: inorganic nanoscale devices as vectors and active compounds. Pharmacol Res

Abstract

In this review we would like to aim at pharmaceuticals engineered on the nanoscale, i.e. pharmaceuticals where the nanomaterial plays the pivotal therapeutic role or adds additional functionality to the previous compound. Those cases would be considered as nanopharmaceuticals. The development of inorganic systems is opening the pharmaceutical nanotechnology novel horizons for diagnosis, imaging and therapy mainly because of their nanometer-size and their high surface area to volume ratios which allow for specific functions that are not possible in the micrometer-size particles. This review will focus on pharmaceutical forms that are based on inorganic nanoparticles where the nanosize of the inorganic component provides unique characteristics to the pharmaceutical form. Several examples of these systems that are either in pre-clinical investigation and under examination by the Food and Drug Administration (FDA) or that have been already approved by the FDA and are in clinical practice today like Gastromark, NanoTherm, Colloidal Gold for Lateral Flow tests, HfO-NPs, BioVant will be described and reviewed.

Full text

Pharmacological Research 62 (2010) 115–125
Contents lists available at ScienceDirect
Pharmacological Research
journal homepage: www.elsevier.com/locate/yphrs
Review
Nanopharmacy: Inorganic nanoscale devices as vectors and active compounds
Pilar Rivera Gil∗, Dominik Hühn, Loretta L. del Mercato, Daniel Sasse, Wolfgang J. Parak
Fachbereich Physik and Wissenschaftliches Zentrum für Materialwissenschaften, Philipps Universität Marburg, Renthof 7, 35037 Marburg, Germany
article info
Article history:
Received 28 December 2009
Received in revised form 14 January 2010
Accepted 15 January 2010
Keywords:
Inorganic nanoparticles
Market
Nanoscale medicines
Nanopharmacy
Pharmaceutical preparations
Medical devices
abstract
In this review we would like to aim at pharmaceuticals engineered on the nanoscale, i.e. pharmaceuticals
where the nanomaterial plays the pivotal therapeutic role or adds additional functionality to the previ-
ous compound. Those cases would be considered as nanopharmaceuticals. The development of inorganic
systems is opening the pharmaceutical nanotechnology novel horizons for diagnosis, imaging and ther-
apy mainly because of their nanometer-size and their high surface area to volume ratios which allow for
specific functions that are not possible in the micrometer-size particles. This review will focus on pharma-
ceutical forms that are based on inorganic nanoparticles where the nanosize of the inorganic component
provides unique characteristics to the pharmaceutical form. Several examples of these systems that are
either in pre-clinical investigation and under examination by the Food and Drug Administration (FDA)
or that have been already approved by the FDA and are in clinical practice today like Gastromark®,
NanoTherm®, Colloidal Gold for Lateral Flow tests, HfO-NPs, BioVantTM will be described and reviewed.
© 2010 Elsevier Ltd. All rights reserved.
Contents
1. The era of Nanoscience............................................................................................................................... 115
2. Physical, biochemical and biological properties of inorganic nanoparticles ........................................................................ 116
2.1. Iron oxide nanoparticles (FeO-NPs) .......................................................................................................... 116
2.2. Gold nanoparticles (Au-NPs) ................................................................................................................. 117
2.3. Hafnium oxide (HfO2) nanoparticles (HfO-NPs) ............................................................................................. 117
2.4. Calcium phosphate nanoparticles (CaP-NPs) ................................................................................................ 117
3. Inorganic nanoparticles as API ....................................................................................................................... 118
3.1. In vivo imaging—FeO-NPs formulations for MRI ............................................................................................. 118
3.2. Thermal cancer therapy—FeO-NPs formulations for hyperthermia ......................................................................... 118
3.3. Photothermal cancer therapy—gold shell NPs for thermal ablation......................................................................... 119
3.4. Ionizing radiation cancer therapy—radiotherapy with HfO-NPs ............................................................................ 120
4. Inorganic NPs as vectors or with an enabling function .............................................................................................. 121
4.1. In vitro diagnosis—lateral flow devices based on fluorogenic Au-NPs conjugated to specific antibodies (Abs) ............................ 121
4.2. Delivery systems—CaP-NPs (a) for drug delivery and (b) as implantable biomaterials ..................................................... 122
4.3. Vaccination—CaP-NPs as vaccine adjuvants ................................................................................................. 122
5. Conclusions .......................................................................................................................................... 123
Acknowledgement ................................................................................................................................... 123
References ........................................................................................................................................... 123
1. The era of Nanoscience
Nanotechnologies are nowadays gaining in commercial use.
After almost thirty years of basic and applied research, the num-
ber of commercial products advertised as containing nanoparticles
∗Corresponding author.
E-mail address: pilar.riveragil@physik.uni-marburg.de (P. Rivera Gil).
(NPs) is increasing rapidly. Within all the fields to which nanotech-
nology can be applied, the medical field is one of the prominent
directions which attract continuous investment and financial
support. Nanomedicine could be defined as the application of nan-
otechnology to health care [1] [2] or more concretely, the use of
nanoscale or nanostructured materials in medicine that according
to their structure have unique medical effects [3]. Nanomedicine
involves nanomaterials in a submicron size range of a few to a
few hundreds of nanometers which are on purpose designed to
1043-6618/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.phrs.2010.01.009

116 P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125
result in new medical effects due to their unique physico-chemical
properties that differ from their macroscopic counterparts. These
materials also require novel manufacturing and characterization
techniques. Different applications of Nanomedicine within the
health care include the use of NPs (i) as active pharmaceutical ingre-
dient (API), where the main role is played by the nanomaterial, i.e.
for the purpose of therapy, diagnostics, imaging; (ii) as vectors (a
solid carrier that introduces the active ingredient into a recipient
or host organism) or with an enabling function. In the latest appli-
cation, the NPs add a new functionality to the pre-existing product,
e.g. NPs for target delivery or as biomaterials.
Based on the definition of Nanomedicine and that of Pharmacy
(science concerned with the preparation, dispensing and effec-
tive use of pharmaceuticals), in this review we would referred
to Nanopharmacy as an interdisciplinary science concerned with
the preparation, dispensing and effective use of nanoscale-based
pharmaceuticals, i.e. active compounds used in the treatment,
cure, prevention or diagnosis of diseases. A proposed defini-
tion describes nanopharmacy as “decreasing the particle size of
sparingly soluble drugs down to nanometric regime and conjuga-
tion with appropriate excipients”[4]. However, in this review we
would like to aim at pharmaceuticals engineered on the nanoscale,
i.e. pharmaceuticals where the nanomaterial plays the pivotal
therapeutic role or adds additional functionality to the previous
compound. Those cases would be considered as nanopharmaceu-
ticals. The most common nanopharmaceutical forms today are
organic platforms that include polymeric NPs and dendrimers,
liposomes and other lipid assemblies, and engineered viral NPs
mostly for drug/gene delivery applications [5]. Nevertheless, the
development of inorganic systems is opening the pharmaceu-
tical nanotechnology novel horizons for diagnosis, imaging and
therapy mainly because of their nanometer-size and their high
surface area to volume ratios which allow for specific func-
tions that are not possible in the micrometer-size particles
[6,7].
This review will focus on pharmaceutical preparations that
are based on inorganic NPs where the nanosize of the inorganic
component provides unique characteristics to the pharmaceuti-
cal form. Several examples of these systems that are either in
pre-clinical investigation and under examination by the Food and
Drug Administration (FDA) or that have been already approved
by the FDA and are in clinical practice today like Gastromark®,
NanoTherm®, Colloidal Gold for Lateral Flow tests, will be described
and reviewed.
2. Physical, biochemical and biological properties of
inorganic nanoparticles
2.1. Iron oxide nanoparticles (FeO-NPs)
FeO-NPs are commonly composed of an inorganic magnetic
core and a biocompatible surface coating that provides chemical
stability under physiological conditions, dispersibility in aque-
ous solution, and reduced toxicity [8–10]. Inorganic cores of
4–15 nm size made up of iron oxides such as magnetite (Fe3O4)or
maghemite (␥-Fe2O3) dominate the field of biomedical applications
due to their superior stability and the lower toxicity compared to
other metal oxides [11,12]. FeO-NPs can naturally be broken down
within the Fe-metabolism in the spleen and liver and due to their
small size are more difficult to recognize by the immune system
[13–15]. When the particle has only one magnetic domain (like in
case of small NPs) thermal excitations can flip the direction of mag-
netization in relation to the particle axis at temperatures higher
than the so called blocking temperature. Thus, superparamagnetic
NPs (SPIOs) show no magnetism above the blocking temperature
in the absence of an external magnetic field, as their respective
magnetizations are randomly oriented. This is termed the super-
paramagnetic state [16]. A magnetic field can effectively magnetize
the particles, as the magnetic moments of the particles are ori-
ented in this field. When the field is removed the magnetization
disappears completely, unlike in larger particles or bulk material
in which residual magnetism (also known as remanence) can be
observed. Depending on the sort of magnetic field applied, SPIOs
exhibit different properties useful for medical applications, i.e. (i)
as heat-producing agents for treatment or (ii) as contrast agents
for imaging. (i) When exposed to varying magnetic fields created
by alternating currents (AC) SPIOs heat up. The underlying heating
mechanisms by which the field energy is converted into thermal
energy depends on the magnetic properties of the particles and
thus on their size [17]. Hysteresis heating under AC magnetic fields
can only be observed in ferro- or ferri-magnetic materials, but not
in superparamagnetic particles [17]. The heat originated by SPIOs
is produced because the magnetic moments align under the influ-
ence of the magnetic field (but the particles as a whole do not
physically rotate) and relax to their equilibrium orientation when
the field is turned off. This effect is referred to as Néel relaxation
[17]. During cyclic alignment energy is dissipated. In contrast, heat-
ing by physical rotation of the particle as a whole is referred to
as Brownian rotation and possible in both cases [17]. SPIOs show
remarkably higher specific absorption rates (SAR [W/g], the mea-
sure of the heat generating capacity of the magnetic substance in
the alternating magnetic field) than larger particles. Thus, by apply-
ing the right quantity of field energy and for the right time heat can
be efficiently produced to cause thermal-mediated cell death. This
makes SPIOs a good alternative for clinical applications of hyper-
thermia, in particular for cancer therapy [18]. (ii) When exposed
to static magnetic fields SPIOs are magnetized, which creates a
local magnetic field gradient. This finally leads to improved sig-
nals for the visibility of the internal structure where the NPs are
accumulated. Diagnostic imaging in principle depends on the sig-
nal contrast between normal and pathologic tissue: the higher the
signal contrast, the more advantageous the conditions for resolving
anatomic structures and pathologic changes. In general, contrast
agents can mark selective regions such as the gastrointestinal tract,
for example to enhance the distinction from other adjacent organs
and tissues [20]. In essence, MR image contrast is mainly due to
distinct spin relaxation times of different types of tissue and the
local proton density. The spin is a fundamental quantum mechan-
ical property, which has no classical analogue and is therefore not
exactly illustrative. As a basic concept, the spin can be perceived
as a vector that points in a respective direction. Within the MRI
scanner a strong, static magnetic field causes the nuclear spins of
hydrogen protons to precess around an axis parallel to the magnetic
field lines. The precessing spins are like small magnets that orien-
tate themselves on the sides of a cone parallel or antiparallel to the
field lines. For energetic reasons, nuclei with parallel orientation
predominate and create a constant magnetization. The precession
frequency of the spins (Larmor frequency) depends on the strength
of the applied field, and because the field is applied with a spa-
tial gradient, the precession frequency of the spins depends on
their position within the field. A transverse radio frequency (RF,
an electromagnetic wave) pulse, tuned to the protons Larmor fre-
quency of a certain layer within the magnetic gradient field, is used
to deflect the spins, and thereby the magnetization is tilted. The
magnetization can be decomposed into a component along field
lines of the static magnetic field (longitudinal magnetization), and
a perpendicular component (transverse magnetization). After the
perturbation through the RF pulse both the longitudinal and the
transverse magnetization relax independently to their state of equi-
librium by spin–lattice and spin–spin interaction, respectively. The
realignment of the magnetic moments induces small voltages in

P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125 117
surrounding inductors that are monitored to obtain the MR image.
The time constants of the longitudinal (T1) and transverse (T2)
relaxation are tissue specific and can additionally be altered by
the accumulation of contrast agent in tissue [21]. The SPIOs cause
local field gradients within a magnetic field that strongly reduce
both T1 and T2 relaxation times [14]. How much the proton relax-
ation rate is increased by a contrast medium is described by its
relaxivity, which is the reciprocal of the relaxation time [14]. The
relaxivity further depends on temperature, field strength and sol-
vent [22]. Conventional contrast agents rely on paramagnetic metal
(usually gadolinium) chelates that primarily shorten the T1 relax-
ation of the regions nearby [23]. They are called positive contrast
agents because they increase the signal intensity in T1-weighted
sequences and thus appear bright [22]. Free Gd3+ is acutely toxic,
but the tolerance of gadolinium chelates is basically excellent [24].
However, nephrogenic systemic fibrosis was recently associated
with gadolinium-containing contrast agents in patients with severe
renal impairment [25]. Depending on the diagnostic question, the
contrast medium is either orally administered or directly injected
into veins, arteries or joints to make, e.g. the brain, kidney, liver,
blood vessels or the gastrointestinal tract and potential damages
visible. In many cases, pathology can be diagnosed by varied signal
enhancement kinetics [26]. As an example, hypervascular tumor
tissue is often seen to enhance earlier than normal tissue, thus
the tumor-to-normal tissue contrast is transiently increased [27].
Nowadays, novel formulations can be found in the market that are
based on SPIOs that, unlike conventional contrast agents, provide
T2 pronounced contrast enhancement by shortening T2 relaxation
times [22,28,29]. The crystalline FeO cores coated with dextran
(in ferumoxide and ferumoxtran) or siloxane (in ferumoxsil) [30]
are referred to as “negative contrast agents” because they origi-
nate shorter T2 times that appear dark in T2 weighted images.
The mean crystal diameters of particles used for MRI (volume
weighted distribution) are listed in the range of 4.6–5.6 nm for
ferumoxides, 4.3–6.2 nm for ferumoxtran and 7.9–9.5 nm for fer-
umoxsil. Differences in the values depend on the measurement
technique and the definition of “mean”, i.e. number average, sur-
face area weighted or volume weighted average diameter [30].
Therefore, within a magnetic field SPIOs are able to create local
inhomogeneities what makes them suitable for MRI applications
[14].
2.2. Gold nanoparticles (Au-NPs)
The synthesis and functionalization of inorganic colloidal Au-
NPs comprises well-known reproducible methods [31–33] which
makes these NPs suitable for commercial purposes. Au-NPs show
high colloidal stability upon wrapping the inorganic Au core within
a shell of stabilizing molecules that also acts as an anchor for further
functionalization upon attachment of biomolecules [34]. Au-NPs
have unique optical properties due to their nanometer size [35]. Au-
NPs are able to strongly absorb light if the corresponding frequency
matches with their surface plasmon resonance (SPR) frequency.
This absorption band causes the color of a colloidal Au-NP solu-
tion. The SPR frequency is mainly influenced by shape and size of
the Au-NPs. For example 20 nm Au-NPs typically absorb at about
520 nm what causes the (observable by eye) red color of the solu-
tion. SPR is basically a collective oscillation of the free electron gas
in the metal upon stimulation [36]. On a NP surface such a stimulus
leads to the formation of evanescent surface waves with longitudi-
nal electronic oscillations parallel to the metal surface. Coupling of
the electron gas to the atomic lattice transfers the absorbed energy
from the electron gas to the crystal lattice (resistive heating). Finally
the heat diffuses into the surrounding medium [37–41]. These pho-
tothermal properties of Au-NPs make them suitable for medical
applications related to temperature-sensitive phenomena. Exam-
ples comprise the removal of tissue, such as endometrial ablation,
the destruction of tumors by increasing the temperature of the can-
cerous cells to lethal levels, as done in hyperthermia or thermal
ablation [42,43]. Therefore pharmaceutical preparations that sup-
port these therapies (mostly auxiliary therapies) are very promising
in the field of nanopharmacy.
Because gold is a noble metal and therefore slack of chemi-
cal reactions it causes almost no inherent toxicity, though several
aspects of cytotoxicity have to be verified. The nanoscopic dimen-
sions could cause cytotoxic effects if Au-NPs are taken up and stored
by cells, considering the fact that nano-sized gold particles may
turn into effective catalysts also depending on their shape [44].
Of course also different properties like cell type [45,46], surface
chemistry (including functionalized Au-NPs by side chains) [47],
and Au-NP size [48] influence the cytotoxic behavior. How Au-
NPs interact with cells, cell compartments and especially DNA has
been investigated in several studies. In vitro studies agree that the
most favorable reason for cytotoxicity is concentration-dependent
[49,50]. In the case of the interaction of Au-NPs with DNA, there
are two dominant reasons. On one hand NPs with the right size
(about 1.4 nm) match nearly with that of the major DNA grooves
and on the other hand gold as the most electronegative metal eas-
ily attracts to those grooves due to their negative environment.
Thereby small Au-NPs cause a strong toxicity towards different cell
lines as opposed to 18–20 nm Au-NPs [51]. Recently, in vivo studies
confirmed size-dependent toxicity [52,53]. Besides also the form of
uptake might play a role, thus oral ingestion and injection showed
different safety profiles [54,55]. Regarding the studies concerning
cytotoxicity of Au-NPs one can say that this important factor is not
explored enough yet and that their use should be always consider-
ing the possibility of potential cytotoxicity.
2.3. Hafnium oxide (HfO2) nanoparticles (HfO-NPs)
Hf is an inert transition element corrosion resistant and with a
high melting point that forms a stable crystalline oxide which tends
to be insoluble in water. The low ionization energy values (the least
required energy to release a single electron from the atom into vac-
cum) for Hf together with its low electronegativity (ability to draw
electrons relative to other elements) make this metal easily ioniz-
able and thus a good electron donor upon light irradiation. Due to
these physicochemical properties, HfO appears as a potent oxidant
agent that could be useful for redox-related medical applications as
radiotherapy. The mechanism underlying the action of Hf for radio-
therapy is explained as follows. When X-rays are absorbed by HfO,
the Hf atoms become ionized and can act as electron donors, which
can create free radicals. In the same direction, when excited HfO
particles relax to the ground state, they emit UV light [56], which in
turn can produce free radicals. In this way, X-ray excitation of HfO in
contact with water and oxygen can produce reactive oxygen species
(ROS). These include free radical and non-radical reactive interme-
diates such as O2−,OH
−, and H2O2. In biological environment, high
concentrations of these reactive metabolites are able to cause cell
death [57]. Although, HfO has been mainly investigated as thin films
[58,59], inorganic microparticles made of hafnium germinate and
HfO as by-products with excellent X-ray absorption properties due
to their high density have also been described recently [60]. How-
ever, neither the crystalline structure, nor the size distribution, nor
the stability in solution of these particles, is yet well controlled.
Then they strongly vary on the synthesis procedures, which are not
yet well characterized.
2.4. Calcium phosphate nanoparticles (CaP-NPs)
Calcium phosphate is one of the principle building blocks of hard
tissues in the body such as bones, teeth and tendons [61]. Moreover

118 P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125
Fig. 1. Schematics of a multifunctional CaP-NP for both imaging and drug delivery.
Imaging agents (green) and active therapeutic agents (red) can be encapsulated
within the matrix of the particle whereas low-fouling molecules (i.e. polyethylene
glycol (PEG)) and targeting molecules (antibodies or other recognition agents) can
be functionalized onto the particle surface. Figure adapted from reference [67] (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article).
its biodegradation products, Ca2+ and PO43-, are found in relatively
high concentrations (about 1–5 mM) in the bloodstream [62,63].
Because of their natural occurrence, CaP-NPs are classified as bio-
compatible and safe systems by the FDA [61] and are widely used
in pharmaceutical technology [64–68]. Nevertheless, World Health
Organization and the European Pharmacopoeia recommend their
use at concentrations lower than 1.3 mg of calcium per dose [69],
probably because even small concentrations of calcium might dis-
rupt the delicate chemical balance of the organism. The synthesis of
CaP-NPs can be performed by using a wide variety of precipitation
strategies [70–73]. However, the major part of these procedures
produces particles with diameters above 100 nm and with some
agglomeration [71,72]. Because the formation of CaP-NPs occurs
during a precipitation reaction, the active agent (hydrophilic or
hydrophobic) can be easily encapsulated within the CaP matrix
of the particle by simply adding it during the particle forma-
tion process [68,74]. However, special attention must be taken by
selecting the precipitation strategy, since composition and adsorp-
tion properties of CaP-NPs must be highly controlled in order to
obtain a safety calcium/phosphorus molar ratio and enough cargo
entrapped, respectively [69]. Encapsulation preserves not only the
pharmacological activity of the cargo from biological degradations,
but also reduces the potential toxicity of the cargo versus non-
target sites during circulation within the body. CaP formulations are
typically highly stable in physiological conditions over extended
times (several weeks) [67]. The release mechanism of encapsu-
lated material is mainly based on the pH-dependent solubility of
CaP-NPs. In fact, the CaP embedding matrix is relatively insolu-
ble at physiological pH (pH 7.4) and becomes increasingly soluble
below pH 6 [64,65,68,75,76]. The pH changes occurring during cel-
lular uptake, lead to dissolution of CaP-NPs and subsequent release
of encapsulated agents [74]. In this way, it can be concluded that
CaP-NPs appear as a promising carrier (Fig. 1) due to their biocom-
patibility and their ability to use the natural environment of the cell
to release the cargo without the necessity of an external trigger.
3. Inorganic nanoparticles as API
In this section, some pharmaceutical preparations that make use
of the above introduced NPs that are already marketed or under
clinical studies will be described and referred to their medical appli-
cations.
3.1. In vivo imaging—FeO-NPs formulations for MRI
The clinical use of SPIOs is most advanced in the field of
MRI. MR images are obtained by measuring the relaxation of
hydrogen spins. Contrast agents (like FeO-NPs) can strongly influ-
ence the relaxation of nearby hydrogen spins. Upon a strong,
static magnetic field the magnetic moments of the nuclei of the
hydrogen atoms are aligned to the magnetic field. Then the pre-
cessing of the spins is disturbed upon radio frequency pulses,
after which the spins can finally relax to their original orientation.
This re-alignment of the magnetic moments induces currents in
conduction coils which are the signals detected by MR tomogra-
phy. FeO-NPs have gained market maturity [3]. Approved contrast
agents based on SPIOs are currently used to improve imaging of
abdominal structures (GastroMARK®/Lumirem®and Abdoscan®).
The median diameter (coating included) of FeO-NPs is greater than
50 nm. GastroMARK (ferumoxil, agent: AMI-121) is an aqueous
suspension intended for oral administration, with particle sizes
reported between 200 and 400 nm. However these values cor-
respond to the overall size of the FeO formulation and not to
the inorganic core, which is in the range of a few nanometers.
After administration, GastroMARK flows through and darkens the
bowel, which makes it easier to distinguish the intestinal loops
from adjacent tissues and organs, as pancreas and anterior kid-
ney. Other SPIO-based contrast agents approved for the imaging
of the liver and spleen like Endorem/FERIDEX I.V. and Resovist
has been currently removed from the market. One of the limita-
tions of FeO-NPs is that they tend to suffer from strong problems
of aggregation during the synthesis as well as inside the cells.
Therefore, special attention must be taken to control the colloidal
and chemical stability of FeO-NPs in different environments. This
involves for example low/high pH and interaction with blood-
occurring proteins. The contrast enhancement of NPs is strongly
affected by size, surface properties and degree of aggregation
[85].
Another example of FeO-NPs being currently developed as blood
pool agents for vascular angiography and for lymph node imag-
ing are the ultra small superparamagnetic iron oxide nanoparticles
(USPIOs) with diameters less than 50 nm [14]. The most advanced
formulation in this field seems to be Sinerem®, an USPIO-based
contrast agent developed for tumor detection in lymph nodes. In
2007, Guerbet (www.guerbet.com) withdrew its marketing autho-
rization application for Sinerem®, whose efficiency could not be
demonstrated statistically.
3.2. Thermal cancer therapy—FeO-NPs formulations for
hyperthermia
Thermal cancer therapy is basically the use of heat from different
sources, such as electromagnetic waves or ultrasound, to kill tumor
cells mainly by means of cytoplasmic and membrane protein denat-
uration [86]. The idea of hyperthermia as the artificially induced
elevation of temperature above the therapeutic threshold of 41 ◦C
up to 46 ◦C inside the body exists since decades [18,87]. Hyper-
thermia is usually applied in combination with other therapies like
chemotherapy or radiotherapy [88]. The underlying mechanism
of thermal radiosensitization is that heat produces vasodilatation
(increase in the diameter of the vessels) and subsequent reduc-
tion of the velocity of the blood flow in order to maintain the flux
constant as well as raising the pressure in the vessel. In case of
tumor tissue connected to the blood supply by their own vessels,
the resulting low velocity and differences in pressure favors the gas
exchange between the extracellular and the intratumoral space and
could lead to higher O2levels in the hypoxic cancerous tissue, thus
increasing the efficacy of the radiotherapy (see Section 3.4). Simi-
larly, chemosensitization occurs due to the increased delivery of the

P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125 119
chemotherapeutic drug. However, these two processes only occur
in the first phase of the hyperthermia (<43 ◦C). When the tempera-
ture increases to lethal levels (>43 ◦C), the hyperthermia can starve
out the tumor cells by raising the hypoxia and the acidosis of the
tumor because of a decreased blood flow [86]. Recently, superficial
hyperthermia as a palliative therapy for solid surface and sub-
surface tumors was approved by the FDA (www.bsdmedical.com).
However this technology based on the simple application of heat is
limited to the location and depth of the tumor and by the side effects
related to the direct heating upon tissue. The use of NPs could help
to overcome these problems. In the field of nanotechnology both
Au and FeO-NPs can be aimed at hyperthermia [89–92]. Au-NPs
can be heated by absorption of light, whereby the absorbed light
energy is converted into thermal energy. Upon exposure of FeO-NPs
to alternating magnetic fields, heat is generated by Néel relaxation
and Brownian rotation [10,93]. SPIOs show remarkable higher SAR
than the larger particles show [18]. Ideally, biofunctionalized NPs
can be targeted (via passive or active targeting) to the tumor, where
they accumulate. If now an external stimulus is applied the temper-
ature of cells close to the particles is raised faster than that of more
distant cells. In this way cells in the vicinity of the particles can be
selectively killed and a reduction of the exposure time of the organ-
ism to the external stimuli is achieved. Nevertheless, hyperthermia
by photoinduced heating of Au-NPs will work best for tissue close
to the skin since the light intensity diminishes among penetration.
For deeper tissues heating with magnetic particles is favorable. On
one hand, the magnetic fields are more difficult to focus than the
light beam, thus unnecessarily exposing the healthy tissue to heat.
On the other hand, the overall increase in the temperature stim-
ulates the immune system which could help fighting against the
tumor.
In the field of nanotechnology, MagForce Nanotechnologies
GmbH (Berlin) (www.magforce.de) with their technology currently
in the final phases of clinical trials seems to be pioneers in this
field. NanoTherm®magnetic fluid is an aqueous colloidal disper-
sion of FeO-NPs (≈111 mg/ml Fe concentration). The FeO-NPs are
synthesized in magnetite phase (Fe3O4). The resulting NPs have
an average size that ranges from 10 to 15 nm and the cores are
coated with aminosilanes. NanoTherm®is currently under clinical
studies within Phase I (feasibility studies) or Phase II (efficacy stud-
ies) for esophageal cancer and various local recurring or residual
tumors or for malignant brain tumors (glioblastoma multiforme)
and prostate carcinomas, respectively. The NPs are brought to the
tumor by instillation (lat. instillare =to drip) which implies the slow
administration of the liquid (3–10 ml) drop by drop with the help
of a cannula. The tumor cells can incorporate the NPs upon expo-
sure of the tumor tissue to the solution. Now the magnetic field
applicator is positioned around the overheating region (around
20 cm) and a spool current of 100 kHz oscillating cycle continu-
ously adjustable from 100 to 500 A is applied. In response the NPs
generate heat (Fig. 2). Since the SAR of the NPs is known, the exact
quantity of magnetic fluid necessary for a thermotherapy proce-
dure can be determined. Thus, the magnetic energy conversion into
heat can be calculated from the density distribution which is mea-
sured in a computer tomography. However, high SAR values are
pivotal for an effective energy transfer under clinical conditions
because the strength and the frequency of the applied magnetic
AC field are (clinically and technically) limited [17]. Furthermore,
differences in thermal conductivity among the tissues or inhomo-
geneous distribution of the NPs within the tissue could result in
very non-uniform heat (temperature) distribution and thus inef-
ficiency of the technique since the thermal dose and the clinical
outcome are significantly correlated. Additionally, hyperthermic
cell death can be disrupted if the temperature increase is not main-
tained high and long enough to enter the exponential phase (>43 ◦C)
where irreversible cytotoxicity is induced [86]. Possible reasons are
because of applying inadequate exposure times (too short) or due
to thermoresistance, i.e. increase and decrease of the temperature
above the threshold for irreversible damage between different heat
shocks originate a thermotolerance [94].
3.3. Photothermal cancer therapy—gold shell NPs for thermal
ablation
Both, hyperthermia and thermal ablation are therapies related
to the use of heat as a way of action to kill pathophysiological tis-
sue. The difference relies mainly on the range of rising temperature
applied. Whereas hyperthermia is associated with temperatures
between 41 and 46 ◦C where the cells are more sensitive to other
therapies, thermal ablation involves higher temperature raising
up to 70–80 ◦C that causes irreparable damage to the cells [95].
Thermal ablation whereby a needle attached to an electrode is posi-
tioned into a tumor has been studied in many forms, including
microwaves, radiofrequencies, laser irradiation, and high-intensity
focused ultrasound [91]. Depending on the size of the tumor, ther-
mal ablation can itself be the primary therapy or an auxiliary
therapy to the traditional ones, i.e. surgery, chemo- and radiother-
apy. The so-called Plasmonic Photothermal Therapy makes use of
the physico-chemical properties of Au-NPs and has been found
to be orders of magnitude more intense and powerful in tumor
destruction than conventional phototherapy (irradiation of dis-
eased tissue without NPs) [96]. Au-NPs-based formulations make
use of different forms of laser excitation (near infrared, NIR or
visible light) to cause a light-to-heat energy conversion [97–99].
By tuning the shape of the NPs, the excitation wavelengths can
be shifted to the NIR region of the light spectrum which is more
Fig. 2. FeO-NPs-induced hyperthermia for cancer therapy. FeO-NPs (blue dots) are brought to the tumor site (esophageal cancer). An alternating magnetic field (˚(t)) of
sufficient strength is applied to heat up the particles via the conversion of magnetic field energy to thermal energy. Thus, a permanent energy flow into the surrounding
tissue, that irreversibly increases the temperature to lethal levels can be obtained resulting in hyperthermal-induced cell death (the necrotic tumor tissue is represented in
black color) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article).

120 P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125
Fig. 3. Photo-induced thermomechanical damage mediated by Au-NPs. Light (h) irradiation of Au-NPs induces thermal explosion. Upon absorption of light, Au-NPs transfer
the energy into heat which is then transported by the water-based medium to the surrounding areas. The increase in the temperature of water causes vapor bubbles that
can mechanically damage the surrounding medium due to the formation of pressure waves with the elevated temperatures obtained by heating the Au-NPs. Figure adapted
from reference [102].
permeable in tissue compared to light of smaller wavelengths.
Combining thermal ablation and the photothermal properties of
Au-NPs, Nanospectra Biosciences (www.nanospectra.com) devel-
oped a core-shell intravenous formulation (AuroShell) for particle
assisted laser ablation of solid tumors which is currently the sub-
ject of a clinical pilot study concerning refractory and/or recurrent
tumors of the head and neck. AuroShell consists of a silica core
(80–150 nm) with an Au shell of 10–20 nm that has been PEGy-
lated (functionalized with poly(ethylene)glycol, PEG). PEG was
presumably used to increase the colloidal stability of the NPs in
the blood stream. The uptake of the AuroShell NPs by tumor cells
after intravenous injection underlies the enhanced permeability
and retention effect (EPR) concerning solid tumors. For irradiation
a laser probe gets inserted into the solid tumor and upon excita-
tion of the Au-NPs the tumor cells are intended to be destroyed.
The main advantage of this NP-based device relies on the ability
to localize the heating in the tumor where the NPs are located and
therefore to shorten the time of irradiation applied. However, as
thermal ablation, it is a localized destructive technique and this
limits the number and the size of the tumors that can be treated.
Furthermore, this system is a matter of unspecific/non-controllable
way of delivery that makes use of the intrinsic properties of tumors
to accumulate the NPs [100] and therefore with the associated
disadvantages of accumulation of the NPs in healthy tissue. This
undesired effect could be abrogated by controlling the accumula-
tion of the NPs via active targeting or if possible by direct injection
of the NPs to the tumor site. Despite active or passive targeting,
it would be possible that the diffusion of the heat from the NPs
to the surrounding environment does not reach all areas of the
tumor, since the temperature elevation compared to the surround-
ing medium decreases reciprocally with 1/r(r= distance to the NP
center) [101]. Therefore not all the cancer cells will be destroyed.
Another possibility that could enhance/supplement thermal abla-
tion mediated by Au-NPs is offered by the use of light induced
thermal explosion of Au-NPs enabled by short laser pulses. For this
a strongly absorbing target has to generate heat more rapidly than
the heat can diffuse away in the surrounding medium. The effect of
explosion would lead to the formation of air/water vapor bubbles
and acoustic or shock waves which cause an intense mechani-
cal damage of the surrounding cancer tissue (Fig. 3). This method
promises also the protection of healthy tissue due to the sensi-
tive adjustability of parameters like the laser pulse duration and
wavelength [102].
3.4. Ionizing radiation cancer therapy—radiotherapy with
HfO-NPs
Radiation therapy is hereby referred to as the treatment of
diseases (e.g. cancer) with ionizing radiation, such as X-rays. It
involves the formation of ROS, i.e. free radicals or non-radical reac-
tive metabolites of the ionized/excited molecules, that leads to
(programmed) cell death (necrosis/apoptosis) [103]. In oncology,
radiotherapy is mainly used with curative intent (as an adjuvant
treatment that follows the primary therapy, normally surgery,
when the risk for relapse is high or concomitantly to other thera-
pies) or for palliation. One of the major limitations of radiotherapy is
the low levels of molecular oxygen (state called hypoxia) present in
the tumor tissue. Oxygen is a potent radiosensitizer that induces the
formation of ROS upon its reduction by ionized/excited molecules
(i.e. water, metals). Vascular abnormalities or intratumoral pres-
sure gradients are parameters involved in generating a hypoxic
environment underlying solid tumors that adversely influences
the outcome of radiotherapy [104]. HfO-NPs (nbtxr3) represent
another example of inorganic NPs as therapeutic sources in this
case for radiotherapy. Nbtxr3 NPs are synthesized by Nanobiotix
(www.nanobiotix.com)[60] for the treatment of radiosensitive and
radioresistant tumors. These NPs are still under pre-clinical devel-
opment and not even yet validated for clinical trials. Nbtxr3 consists
of a suspension of inert crystalline NPs of hafnium oxide with an
average size of 70 nm, stabilized in water with a coating agent. The
formulation is for injection directly at the tumor site. Due to their
inert nature, the NPs do not cause any chemical reaction unless

P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125 121
Fig. 4. Radiotherapy based on HfO-NPs. Water soluble HfO (red and white, respectively)-NPs are directly injected to and thus localized at the tumor site (e.g. brain tumor).
Radiotherapy based on HfO-NPs depends on the ability of Hf atoms to ionize surrounding tissue by the absorption of X-rays (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of the article).
they are ionized with an external beam of X-rays. Once injected
intratumorously, the interaction between the X-rays and the NPs
results in ionization (Fig. 4). This promotes the composition of free
radicals, which ultimately kill the tumor cells. Probably Nanobi-
otix tries to make use of HfO-NPs to mimic the action of oxygen
in order to overcome the hypoxia-associated decreased radiosen-
sitivity and therefore to increase the effectiveness of a given dose
of ionizing radiation. HfO-NPs would then serve as radiosensitizers
agents to sensitize tumor cells to radiation. The main advantage
of this system relies in the ability to diminish the exposure time
of the organism to the ionizing radiation and to accumulate in the
target tumor tissue higher levels of radiation due to the increased
absorption by the NPs. Nevertheless, the still unsolved problem is
that water molecules also strongly absorb X-rays and therefore the
healthy tissue is also exposed to free radicals. Furthermore like
in other kinds of radiotherapy, the dose delivered to the patient
should be verified with in vivo measurements in order to control
the amount of energy imparted to or absorbed by the body and
to avoid unnecessary harm to health. This should be taken into
special account when involving NPs in the treatment. Inhomoge-
neous distribution of the particles within the tumor could lead to
non-uniform patterns of energy deposition.
4. Inorganic NPs as vectors or with an enabling function
In this section some inorganic NPs-based formulations as car-
riers to guide the active ingredient into a recipient or host
organism, with an enabling function to add a new functionality or
as implantable biomaterials will be described.
4.1. In vitro diagnosis—lateral flow devices based on fluorogenic
Au-NPs conjugated to specific antibodies (Abs)
The system in this chapter consists of Au-NPs conjugated to Abs
for the detection of molecules in vitro. Though this system fits bet-
ter into the definition of medical device (as a diagnostic agent) and
not on the definition of pharmaceutical preparation, it is a very well
established application of Au-NPs on the market and thus interest-
ing to describe. It is also worthy to mention that although the heart
of the system could be considered being the Abs as the detection
component, the Au-NPs also play a pivotal role as signal genera-
tor and read out device. Since strictly thought the Au-NPs are the
carriers for the Abs, we have decided to classify this device under
chapter 4 (inorganic NPs as vectors).
Lateral flow assays are also known as immunochromatographic
strip tests. Lateral flow devices are based on a strip format that uses
(nitro)cellulose, paper or a plastic support to detect a single or mul-
tiple analytes based on fluid migration or capillary action. A great
variety of lateral flow devices for the detection of analytes under
circumstances where a rapid test is required can be found in the
market rapid test is required. They are easy and quickly to read out
but they are not the most sensitive devices. The principle works
upon an antigen-Ab binding in a sort of sandwich immunoassay.
Therefore, two different Abs that are able to detect distinct epi-
topes of an analyte molecule are used (e.g. for pregnancy, drugs).
One is immobilized in a line onto a nitrocellulose membrane. The
other one is labeled with a signal (e.g. color) generator (e.g. Au-
NPs) and positioned within a glass fiber membrane. The latter is
dissolved upon exposure with the test analyte (antigen) in an aque-
ous solution. The newly created complex (Au-NP-Ab-analyte) flows
along the substrate upon capillary forces until the analyte becomes
detected when reaching the line/zones on the nitrocellulose mem-
brane pretreated with the immobilized Abs. In positive samples,
there is a high accumulation of immobilized complexes, and the
line/zone exhibits a colored band due to the coupling of plasmons
in nearby Au-NPs [105] that is easily detectable with the eyes. It
consists of an on/off test for qualitative measurements. However,
quantitatively results can be obtained by determining the optical
density via scanning photometry or charge coupled device (CCD).
To give an example, Nymox Pharmaceutical Corporation devel-
oped a commercially available lateral flow test for recent smoke
exposure called TobacAlertTM. The analyte to be tested is in this
case cotinine, a metabolite of nicotine (“cotinine” is an anagram of
“nicotine”). The test strip works upon the above mentioned princi-
ple with the extension that not only one site is equipped with the
immobilized antibody, but several “traps” are placed on the strip
one behind the other. Thus, the Au-NP-Ab-analyte complex flows
further on if a trap is satisfied and therefore the smoke exposure
corresponds to the number of labeled traps. As mentioned before,
the main disadvantage of these assays is their reduced sensitiv-

122 P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125
ity. Hereby a compromise is found between sensitivity and fast
response. Another critical point would be the lack of control of the
number of attached Abs to the colored reagent (Au-NPs) and thus
the possibility of a falsely negative result (e.g. if a lot of analyte binds
to an insufficient amount of Au-NPs). This is the current operation
procedure that should be improved towards enhanced sensitivity,
i.e. optimization of the selected Ab and the raw material that affect
the flow rate of the complex and thus the time of interaction with
the immobilized Ab.
4.2. Delivery systems—CaP-NPs (a) for drug delivery and (b) as
implantable biomaterials
(a) Binding of a pharmaceutical to a particulate drug deliv-
ery system is a strategy widely investigated to induce sustained
drug delivery [106,107]. Bioceramics, such as CaP, represent an
interesting class of materials suitable for the use as carriers for
drugs, non-viral gene delivery, antigens, enzymes, and proteins.
Moreover, CaP can be produced at a low cost and are simple
to manufacture (see Section 2.4). So far the use of CaP as drug
carrier for localized pharmaceutical treatments has been demon-
strated by injections [108] or surgical placement of disks, pellets
or particulates [109]. Thanks to the localized release of drugs
from these CaP-based systems, low concentrations of drugs were
required compared to the typically high concentrations required
in the bloodstream and other organs to achieve therapeutic out-
comes. Nowadays several companies are working to develop CaP
particulate systems suitable for clinical applications. BioSante Phar-
maceuticals Company (www.biosantepharma.com) announced in
august 2009 the development of a novel therapeutic drug delivery
platform (BioAirTM/BioOralTM ) based on the CaP nanotechnology.
BioAirTM/BioOralTM are formulations for delivering proteins via
buccal and pulmonary routes using biodegradable CaP-NPs as basis
(a carrier, composed of one or more excipients, for the active sub-
stance(s) in semi-solid and solid preparations) for controlled drug
release of proteins and peptides. Thanks to the improved bioavail-
ability and to the encapsulation of the cargo within a carrier, less
protein is needed per dose. The Company completed pre-clinical
tests showing that biodegradable CaP-NP basis enhance and extend
the hypoglycemic effect of proteins when administered subcuta-
neously, buccally and pulmonarily. It is expected that CaP NPs
might be used to deliver different types of proteins, such as human
Factor IX for hemophiliacs, Interferon alpha-2b for chronic hep-
atitis B treatment, a1-antitrypsin for the prophylaxis of venous
thromboembolism, and peptides. However, the successful use of
nanosized CaP particles as drug carrier for localized treatment
strongly depends on the ability to efficiently load and release the
drug in a controlled manner. Moreover, it is also imperative that the
drug released is chemically active and effective over a long period
of time. Thus, additional efforts must be made to avoid a “burst”
release profile of the drug which is due to the naturally occurring
degradation of CaP compounds in the body which would strongly
interfere with the kinetics of cargo release and thus effectiveness
of the drug.
(b) CaP biomaterials became the most interesting artificial
bone grafts when bioglass (CaP containing glass) and hydroxya-
patite ceramics (HA) were found to be osteoconductive (in the
1970s–1980s). So far, the main goals in this task have consisted
of adding additional properties, such as osteoinductivity, resorba-
bility, simplicity of handling and mechanically resistance, in order
to implement CaP biomaterials according to the clinical require-
ments. However, recently a different approach which is based on
the use of CaP as particulate formulation has been suggested by the
company Angstrom Medica Inc. (www.angstrommedica.com) for
creating structural, injectable, and programmable medical devices
that can act standing alone or as a carrier for pharmokinetic agents
and orthobiologic materials. The formulation NanOssTM is a highly
osteoconductive biomaterial able to remodel over time into human
bone with applications in sports medicine, trauma, spine and gen-
eral orthopedics markets. In biomaterial science osteoconduction
means bone formation towards implants from host bone bed as
well as guided bone formation on material surfaces resulting in
bone binding. The details of the process used to create NanOssTM
are currently a trade secret. However, the company has developed
a new process for forming nanostructured CaP utilizing a patented
precipitation process. The result is a precipitate of CaP nanocrystals
with a predetermined size, shape, and composition. The nanocrys-
tals are then combined into building blocks that are ultimately
used to assemble complex devices whose morphology is inspired
to the most commonly used devices in orthopedic surgery. Because
the main goal in several fields of nanotechnology for biomedical
application is to create novel materials able to mimic the func-
tions of the naturally occurring materials, the strategy based on this
formulation appears to be quite promising. In fact, the key prop-
erties of these CaP-based devices are that they are bioactive and
that they reproduce the shape and the size of the bones. Moreover
they are able to induce bone cell attachment and proliferation. All
these properties combined together allow for using these Ca-NPs
as implantable systems for directing the growth of the body cells
around the nanocrystal matrixes. These cells are then able to engulf
the NPs, break them down and remodel them into real bone. It is
clear that particulate implants whose activities are based on the
above described mechanism, represent ideal devices because once
injected into the target-damaged tissue they turn into the integral
part of the future regenerated bone. Due to their natural occurrence,
CaP nanoparticulate systems are very well tolerated and easily
absorbed and might constitute a good alternative as implantable
bioresorbable materials to the classic bioscaffolds [110,122,123].
4.3. Vaccination—CaP-NPs as vaccine adjuvants
Adjuvants (immune potentiators or immunomodulators) are
used to improve the immune response to vaccine antigens against
an infectious disease. The incorporation of adjuvants into vaccine
formulations is aimed at enhancing, accelerating and prolonging
the specific immune response towards the desired response to
vaccine antigens. The main routes by which different kinds of adju-
vants exert their activities include: (i) presentation of the antigen,
i.e. adsorbants and particulate adjuvants, emulsions; (ii) immune
potentiation/modulation, i.e. microbial, synthetic and endogenous
adjuvants, mineral salts (aluminium hydroxide (Al) or CaP gels);
(iii) the protection of the antigen from degradation and elimination,
i.e. emulsions; (iv) targeting to specific cells, i.e. T-helper cells 2 (T-
lymphocytes against bacterial infection). In case of the commonly
used adjuvants based on mineral salts (i.e. Al or CaP gels), they are
associated to the vaccine by adsorption processes and are able to
enhance the immunogenicity of the vaccine by promoting the for-
mation of an inflammatory focus at the site of injection, which is
a disadvantage if it ends up in an allergic reaction. A novel adju-
vant formulation based on CaP technology takes advantage of the
biofriendly nature of CaP and the nanotechnology to make CaP-
based nanocarriers that incorporate the antigen molecules in their
interior. In this system, the vaccine/adjuvant association is mainly
encapsulation and the mechanism of action is achieved by protect-
ing the antigen from degradation [69,108,111,112]. Thus, a higher
local tolerance is achieved. Furthermore, a common problem for
adjuvants lies in the ineffectiveness against immunogenic-weak
antigens, i.e. recombinant proteins or viruses. In this context, CaP-
NPs have demonstrated to enhance vaccine immune responses
to viral antigens as well [108]. Additionally, CaP-NPs based adju-
vants are easy to manufacture on an industrial scale, and show

P. Rivera Gil et al. / Pharmacological Research 62 (2010) 115–125 123
less variation in quality and physicochemical properties between
batches than the aluminum compounds [113,114]. Together with
the introduction of BioAirTM/BioOralTM , BioSante Pharmaceuticals
also announced novel vaccine adjuvants (BioVantTM) based on the
CaP nanotechnology. A synthetic analog of CaP is used as a basis
for BioVantTM to build a CaP-NP-adjuvanted matrix for vaccines
against different viruses. Pre-clinical studies have demonstrated
that the system elicits positive immune response to vaccines and
may sustain higher antibody levels over a longer period than
both aluminum-formulated vaccines and non-adjuvanted vaccines
leading to the use of lower vaccine dosage, while maintaining
or improving effectiveness and offering an improved safety pro-
file. However, these results are based on pre-clinical trials and
BioSante’s achievements in this area are still on a research and
development level. In particular, the BioVant formulation is in
Phase I development under the U.S. Food and Drug Administra-
tion (FDA) SPA (Special Protocol Assessment). Remarkably, both
the geometry and the mechanism of action of this system are
poorly understood and the company does not bring clearness, espe-
cially regarding the association vaccine/adjuvant, which directly
influences the efficiency of vaccination. The main advantage of
the NP-based systems relies on the ability of nanotechnology to
enhance the uptake of vaccine/adjuvant formulations by appropri-
ate cells through manipulation of their surface chemistry.
5. Conclusions
Materials on the nanosize scale have unique characteristics
compared to their macrosized counterparts that derivate from their
nanosize. Additionally, because of their enormous surface area rel-
ative to their total volume only NPs are able to produce a large
number of binding sites between cells and target molecules. The
coating of the inorganic crystal mainly confer the NPs stability in
water solutions [115] but can also be used to functionalize the sur-
face of the NPs with different biomolecules for different purposes
[116]. For example, for in vivo applications PEG (poly(ethylene
glycol)) molecules increase the colloidal stability and reduce recog-
nition of the NPs by the mononuclear-phagocyte system due to
their low-fouling properties (the attachment of unspecific serum
protein to the surface of the NPs which will make the NPs easily
detectable by the immune system is reduced). Specific molecules
like DNA, antibodies, aptamers or ligands (transferring, folic acid)
transfer the NPs molecular recognition properties to detect tar-
get cells or cellular components [117,118]. Other kind of chemical
functionalities like cell penetrating peptides are postulated to allow
the internalized NPs to escape from the intracellular compartment
where they are trapped [119–121]. Although inorganic NPs are for-
eign bodies and potentially immunogenic, their small sizes (below
100 nm) confine them a sort of acute protection against recogni-
tion by the mononuclear-phagocyte system and thus, clearance
from the organism is delayed. Furthermore, an elevate number
of NPs can accumulate inside cells [121] thus increasing the effi-
ciency of their function. Despite the incoming potential of NPs in the
field of medicine and pharmacy, NPs also have their limitations (as
described in this review) and therefore should not be considered as
a panacea. NPs are rather novel tools to increment the pre-existing
tool kit for diagnosis and treatment of diseases and they are not
necessarily aimed at substituting current methods.
Acknowledgement
This project was supported by the European Union (grant
NANOGNOSTICS).
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