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internalization of nanoparticles under flow conditions.
tion half-life of their cargo, and to deliver a sufficient payload of therapeutics to the diseased regions.
by the RES , or an overall inability to reach the designated target in effective dosage [2–4].
of the commonly reported drug-carrier systems (reviewed in detail in ) are briefly outlined below.
Corresponding author: Dr. Iwona Cicha, Section of Experimental Oncology and Nanomedicine (SEON), Glückstr.
g. PLGA nanoparticles have been tested as drug carriers for tissue plasminogen activator (mouse model of thrombosis ). 31].g. 22]. in contrast to their broad utility as imaging agents [25–29]. pitavastatin (rat model of myocardial ischemia-reperfusion ). or drug carriers. In animal models. e. superoxide dismutase (mouse model of stroke ). a PEGylated liposomal formulation of doxorubicin (Doxil® ) has been the first FDA-approved nanodrug indicated for the treatment of several types of cancer. the successful development of novel intravascular nanosystems for disease-specific imaging and drug delivery requires extensive studies in vitro and in vivo. recurrent ovarian cancer. commercial availability and overall low immunogenicity . Among their advantages as a drug delivery platform are the ease of prepa- ration. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow Liposomes are composed of a lipid bilayer consisting of amphipathic phospholipids (primarily phos- phatidylcholine) that enclose an interior aqueous space . Being synthesized from naturally occurring phospholipids. which is expected to enable safe and repeated administration. However. as well as biosensors. the effects of shear forces on the liposomal bilayer integrity are largely unknown. the circulating blood is separated from the surrounding tissues by a biolog- ical barrier consisting of endothelial cell monolayer. Endothelial functions in health and disease In the mammalian body. 7]. which allows conjugation of antibodies or other targeting ligands . relatively few reports addressed the use of SPIONs for vascular drug delivery thus far [30. 2. thus increasing therapeutic success of intravascular nanosystems. as well as their application for hyperthermia- therapy of cancer . liposomes have relatively low toxicity [6. The favourable safety profile of PLGA results from its physiological degradation to easily metabolized products. Such hybrid nanosystems with adjustable characteristics are expected to improve the future clinical utility and safety of nanother- apies.g. Thus far. Apart from considering the disease mechanisms and pathological processes. namely lactic acid and glycolic acid . including AIDS- related Kaposi’s sarcoma.192 I. often coated with organic materials such as fatty acids. However. or polymers [21. metastatic breast cancer and multiple myeloma (reviewed in ). biocompatibility or contrast enhancement. including the control of vasomotor tone. is a highly metabol- ically active organ. However. PLGA is the most common biodegradable polymer approved for use in humans. Still. as well as carriers of several anti-cancer agents [11. these novel therapeutic approaches would greatly benefit also from the knowledge of endothelial biology and endothelial transport mechanisms to ensure adequate safety and effective drug delivery. endothelial cells are involved in many physiological processes. but are easily cleared from the circulation by RES. the above-described materials can be freely combined in order to achieve the desired charac- teristics of the nanoparticles. or photothermal ablation therapy. Gold nanoparticles consisting of a dielectric core of silica coated with a metallic layer of gold. an expansive cell monolayer covering a total surface area of 4000–7000 m2 in an average-sized human . polyethylene glycol (PEG)). The stability and circulation time of liposomal formulations can be greatly improved by con- jugation of the head groups of phospholipids with polymerizable moieties (e. the important concern related to gold particles is their potential cytotoxicity and a slow elimination resulting in a long-term persistence within many organ systems. . Liposomes are often functionalized with maleimide. for optical imaging. Notably. SPIONs consist of iron oxide core. The magnetic properties of SPIONs allow the remote control of their accumulation by means of external magnetic field . 15]. leukocyte adhesion and trafficking. Gold nanoparticles have been utilized for experimental delivery of anti-cancer agents [17–19] and for delivery of inotropic agent (levosimendan) in a rat model of heart failure . are tunable to various sizes and forms  and can be used for e. frequently used in form of nanoshells or nanocarriers. polysac- charides. Apart from fulfilling the barrier function. The endothelium.
without a microvascular network. In response to inflammation and injury. In physiological angiogenesis. Similar observations have been reported for atherosclerotic plaque neovasculature. the neo-plasms and neo-tissues cannot grow beyond a critical size. The former approach exploits the enhanced permeability of the endothelium. as the compromised endothelial barrier results in leaky vessels with intercellular gap sizes of 100 nm to 2 ␮m depending on the tumor type [40–42]. intracytoplasmic vacuoles. Additionally. in order to protect the tissues from harmful pathogens. The resulting thin-walled. which are initiated by the subendothelial accumulation of blood lipids and inflammatory cells. the capillary leakage improves the possibility of drug accumulation in the tumor. the immaturity of the tumor or plaque neovessels improves the chances of efficient drug delivery . so that the nanoparticles which enter the interstitium via immature vessels can be entrapped in the tumor [43. drugs and nanoparticles that cross blood-tumor tissue barrier have higher retention times than in normal tissues. with excessive branches and shunts . involving the basement membrane formation and pericyte recruitment. tumors lack a proper lymphatic drainage system. In terms of the therapeutic approaches. and the mural cells are missing. To date. Furthermore. The basement membrane is discontinuous or absent. increased vascular permeability may play a deleterious role. including pas- sive and active targeting. In the medium and large arteries. 36]. Nanoparticles that prolong the circulation half-life of the carried drugs are therefore . which results in an increased interstitial pressure at the center of tumors relative of the tumor periphery . particularly in inflammatory diseases and cancer. where the extravasation of nano-sized particles is facilitated . it is important to consider the effects of hemodynamic forces on nanoparticle behaviour in circulation and adhesion to the endothelium. and basement membrane detachment . open intercellular junctions. 45]. Oxygen and nutrients supply is maintained in almost all tissue types by a vast network of blood capillaries distributed not further apart than the diffusion limit of oxygen. leakage and hemorrhage [38. Sprouting from the pre-existing vessels is also the main route for tumor angiogenesis which. however. contributing to the formation of atherosclerotic plaques. Without blood vessels. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow 193 thrombocyte adhesion and hemostasis. 3. including their basic physicochemical characterization and the biological effects of nanoparticles on the vascular cells and blood components. endothelial barrier func- tion in microvessels is reduced: Increased capillary permeability allows the migration of immune cells into the diseased or injured tissues. damaged cells or irritants . Therefore. Intravascular drug delivery strategies In order to design safe carriers. hypoxia is thus a driving stimulus for angiogenesis. 39]. however. thorough preclinical analyses of the candidate nanosystems intended for intravascular administration are necessary. These ves- sels are also leaky with numerous endothelial fenestrae. where intraplaque microvascular endothelial cells showed numerous abnormalities. lacks the coordinated temporal and spatial regulation. In contrast to normal vascular network. wounds. permeable sprouts undergo a tightly-regulated maturation process. two main strategies are proposed to deliver nanocarriers to the vascular wall. increased microvessel permeability and matrix degradation are fol- lowed by endothelial cell migration and proliferation [35. tumour vessels are tortuous and unevenly dilated. This compromised structural integrity of the intraplaque microvessels can lead to enhanced leukocyte infiltration. such as membrane blebs. or atherosclerotic plaques alike. This combination of leaky vasculature and poor lymphatic drainage results in so-called Enhanced Permeation and Retention (EPR) effect . I. In tumours. leading to struc- tural and functional abnormalities and vessel immaturity. Moreover. vesicles and transcellular holes and widened intercellular junctions. which is 200 ␮m . For example. the delivery of anti-cancer drugs in effective quantities to the center of the tumour would not be possible.
5–6 dyn/cm2 in the veins. 4. is based on so-called “magnetic drug targeting”. in particular cationic charge . Moreover. apart from inflammatory status . exert a profound anti-apoptotic effect on endothelial cells [69–71]. Another promising strategy of drug delivery. which results in increased drug payloads in the target tissue. Therefore. Passive targeting can also utilize nanoparticle properties. cancer or neurodegenerative diseases . G proteins. in combination with an external magnetic field is used to target the nanoparticles to the diseased vasculature regions as demonstrated by the studies in a rabbit model of cancer [56–58]. increasing the systemic dose in attempt of achieving the sufficient drug efficacy is often impossible due to numerous adverse effects. The wall shear stress in humans varies between 0. 80–82]. In terms of nanocarrier- mediated drug delivery to microvasculature (e. a mouse model of thrombosis . the hemodynamic forces may be of lesser relevance. In contrast. 52] and integrins [53.g. the glycocalyx becomes stiffer and loses its buffering function [75–77]. and 10–15 dyn/cm2 in the central arteries (e. the efficacy of drugs and passively-targeted drug carriers applied via intravenous route is often insufficient for a meaningful clin- ical improvement. This targeting approach has been shown to allow a better control of nanoparticle biodistribution and to enhance their therapeutic efficacy (see below). The active targeting of nanoparticles to vascular wall is achieved by grafting the surface or the shell of the nanocarriers with specific ligands or antibodies to molecules expressed on the endothelium. conjugation of superparamagnetic particles with drugs. as the single file flow in microvessels and capillaries  increases the contacts of nanoparticles with the vessel wall. caveolae. including vascular endothelial growth factor receptors [51. at the same time reducing their systemic dose and toxicity. However. the carotid artery) . 79].g. thus further increasing the endothelial permeability . tyrosine kinase receptors. the exposure to disturbed. shear stress-activated intracellular processes may represent the major modulators of the nanoparticle . upon exposure to non-uniform shear stress. see Fig. the brachial or femoral artery). to increase non-specific cell targeting of negatively-charged endothelial glycocalyx [47–50]. In parallel. the interactions and binding dynamics of drug carriers targeted to the endothelium in medium and large-diameter arteries may. various endothelium- expressed molecules have been tested as targets for nanoparticles. the signaling pathways activated by laminar shear stress (including ion channels. For this reason. as also discussed in a dedicated chapter below. despite the presence of leaky vessels in tumors and the compromised blood-brain barrier in many brain disorders. and increases the permeability of monolayer to immune cells and blood components [74. Depending on the disease context. In this approach. 1). 3–7 dyn/cm2 in the peripheral arteries (e. nanoparticulate carriers that can be functionalized for actively-targeted drug delivery  constitute an attractive alternative for therapy of e. non-uniform shear stress renders endothelial cells prone to inflammatory activa- tion. 68]. largely depend also on local hemodynamics and blood rheology that govern both the endothelial responsiveness and the behavior of blood-borne cells and particles. However. endothelial glycocalyx thickness strongly depends on the shear stress patterns. a mouse model of cardiac ischemia  and several mouse models of cancer [61–63].g. growth factor receptors. as briefly discussed below. integrins.194 I. and the cytoskele- ton components (reviewed in [67. Additionally.g. In the arteries. pulmonary circulation). and prevent cytokine-induced expression of pro-inflammatory genes and endothelial adhesion molecules [72–74]. Regulation of endothelial function and nanoparticle interactions by the ﬂow patterns The specific hemodynamic conditions characteristic of the target vasculature region are expected to modulate the particle internalization and therapeutic efficacy. 54] in cancer (reviewed in ) as well as adhesion molecules in inflammation and atherosclerosis (reviewed in ). Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow expected to increase the payload of drugs reaching the target site. Laminar shear stress promotes glycocalyx formation in ECs [75–77] and thick and negatively charged endothelial glycocalyx improves the barrier function [78.
but penetrated the endothelial cell cytoplasm and localized near the nucleus under flow conditions .5 dyn/cm2 .g. The majority of the existing studies. VEGFR2. NP50 did not enter the cytoplasm of cells grown either in static or flow conditions. which is a pre-requisite to estimate the cellular responses in physiological-like settings. In contrast.7 nm CdTe quantum dots (QD) and 50 nm silica nanoparticles (NP50) with HUVECs using a microfluidic platform (channel dimensions: 600 ␮m width × 120 ␮m height × 20 mm length). Nanoparticle internalization under ﬂow conditions Although endothelial cells represent the first-contact cells for nanoparticles administered via intravas- cular route and are often the key target for therapeutic nanocarriers. Below. platelet-endothelial cell adhesion molecule 1. Moreover. the rheologi- cal behavior of blood cells in the arterial flow differs from that in the microvessels [84–86]. 1. Erythrocyte accumulation in the center of the lumen and the formation of rouleaux. Shear stress-activated signalling pathways in endothelial cells. The maximal uptake of QDs was observed at 0. PECAM-1. QD4. internalization by arterial endothelial cells. vascular endothelial cadherin. Samuel et al. utilize static cell culture conditions to assess the endothelial toxicity of nanoformulations and their cellular interactions. atherosclerosis). 0. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow 195 Tyrosine Apical membrane kinase receptors G-proteins Cell-cell Caveolae junctions Ion channels VEGFR2 Cytoskeleton Cytoskeleton PECAM-1 Extracellular matrix VE-cadherin Integrins Fig. several recent attempts to investigate the influence of shear stress on the uptake of circulating nanoparticles and their effects on endothelial cells in vitro and in vivo are discussed. or NP50 (50 nm) suspended in PBS for 20 minutes. this can critically affect the efficacy of the drug delivery systems or contrast agents accumulation in the diseased regions in vivo. 5.  compared the short-term effects of shear stress (0. QDs did not show internalization within 20 min under static conditions. however. may strongly affect the margination of nano-sized particles . From the existing reports.7 nm). with a decrease at 5 dyn/cm2 . it is clear that the effects of shear stress on the particle uptake is to a large degree dependent on the particle type and the experimental settings. and 5 dyn/cm2 ) on the interactions of negatively charged. VE-cadherin.7 (2. leaving a cell-free layer at the vessel wall. I. In terms of nanoparticle applicability in certain diseases (e.5. thioglycolic acid-coated 2.7 nm). 1. including the selected shear stress magnitude and the exposure time. but were bound to the cell membranes under flow conditions. due to the larger size of the vessels. relatively few studies have investigated nanoparticle-endothelial interactions under flow conditions.7 (4.7 nm and 4. vascular endothelial growth factor 2. and was associated with shear stress-induced . Cell monolayers were exposed to 3 ␮M negatively charged QD2.
in liver sinusoidal vessels (0.0 dyn/cm2 were selected. that the presented data were obtained on not fully confluent cells. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow cytoskeleton reorganization and formation of membrane ruffles. they must be internalized by endothelial cells also in static conditions upon prolonged incubation: Whereas NP50 did not cause any cytotoxic effect on cultured HUVECs even after 24 h of exposure. . 6. whereas under static conditions 20% PDMAEMA showed the highest uptake.6 × 105 . 205 nm. Three types of polymeric nanoparticles of similar size were compared: Negatively charged poly((methyl methacrylate)-co-(methacrylic acid)) with 3% methacrylic acid (3% PMAA. as described for the acute endothelial response to shear stress exposure [89–91]. all tested nanoparticles were mainly cleared by Kupffer cells in the liver. Similar to the in vitro results. representing polymers with pH-dependent anionic and cationic charges . Under shear stress of 0. the estimated uptake of 3% PMAA was markedly lower that other particle types. 0. indicating a slower kinetics of particle internalization . that although QD are not taken up within the first 20 min of exposure. with no major differences between static versus flow conditions.5 dyn/cm2 and at 5 ␮L/min (0. poly((methyl methacrylate)-co-(methacrylic acid)) with 13% methacrylic acid (13% PMAA. –38. which may affect both the cell behavior and interactions with nanoparticles. 207 nm.7 treatment.3. Regretfully. 196 nm. A slight increase in the amount of internalized nanoparticles was observed in cells exposed to 10.7. 8.7 were well tolerated for up to 4 h. Different effects of shear stress on particle internalization were observed. +31. followed by a significant decrease in the number of cells at 8 and 24 h. width 2 mm and height 150 ␮m) and continuously perfused with nanoparticle suspensions.0. The cytotoxicity studies carried in static conditions. Furthermore. a significant reduction of in the number of treated cells was detected. The dynamic cell culture at shear stresses of 3 dyn/cm2 and higher enhanced the uptake of 13% PMAA particles in a shear stress-dependent manner.g.e. and poly((methyl methacrylate)-co-(2-dimethylamino ethyl-methacrylate)) with 20% 2-dimethylamino ethyl-methacrylate (20% PDMAEMA. 4. however.196 I. A different approach in vitro was applied by Rinkenauer et al.  compared the effects of gold nanoparticles stabilized with sodium citrate on human umbilical vein endothelial cells cultured in static conditions versus cells grown a single-channel microfluidic device (length 4 cm. but they were also internalized by the liver-specific endothelial cells to a lesser extent.7 particle uptake: Whereas no uptake was observed within the first 20 min. This model was compared with the static cell culture and a mouse model. who utilized a chip-based dynamic cell culture model to pre-expose endothelial cells to different levels of shear stress prior to the incubation with nanoparticles.7 dyn/cm2 ) and the shear stress values observed in human circulation.1 dyn/cm2 ). without a clear shear-stress dependent pattern.1 dyn/cm2 . the number of nanoparticles per cell was lower by an order of magnitude than in static conditions (2. 3.3 mV). –43. Fede et al. but it was not statistically significant . in order to assess its capability to predict the in vivo responses to the methacrylate-based nanoparticles. HUVECs seeded in rhombic chamber chips (120 ␮L chamber volume) were exposed to shear stress for 24 h. the presented results of long-term static exposure indicate. which is unlikely to occur in the absence of particle internalization.3 mV). indicating a significant toxicity . whereby the low-charged particles (3% PMAA) showed the minimal uptake. and 24 h) after the QD4. significant differences were observed in gold nanoparticle accumulation after 24 h exposure in cells grown under flow versus static conditions.0 dyn/cm2 . it must be noted. Shear stress values of 0. representing basal nutrient exchange flow with minimal mechanical stimulation observed e. The authors utilized laminar flow at 30 ␮L/min. whereas the uptake of 13% PMAA was somewhat lower than 20% PDMAEMA . In vivo. However.3 mV). QD2.. neither the consequences of the exposure to QDs under flow conditions on cell viability. In a recent study. Despite these relatively low shear stress values. stand in a contradiction with the data on QD4. nor the effects of prolonged exposure under flow on particle internalization and their cytotoxicity were reported. a significant reduction in the number of treated cells was observed at all time points (i. the internalization of these cationic particles was decreased under flow conditions. correspond- ing to about 0. Interestingly however. followed by 60 min perfusion with nanoparticles at a concentration of 200 ␮g/mL.0 and 10.
our data indicated that in case of many types of nanoparticles. namely their sedimentation. the fraction of viable cells was approximately 20% higher at 5 × 1011 nanoparticles/mL.38 ␮m diameter) in flowing blood suspensions can be even 3-fold greater than in the central region of the flow . Collectively. nor induced cell detachment due to shear stress exposure. However. negatively affecting endothelial viability at concentrations of 100 ␮g/mL and higher. Consequently. 6. 96]. Whole blood models Prior work demonstrated that platelets accumulate within the cell free layer near the vessel walls [95. the concentrations affecting cell growth and viability in static conditions (100 ␮g/mL) also induced cell death under flow conditions . relatively large (245 nm) and characterized furthermore by a tendency to aggregate.6–5 dyn/cm2 ) . To be able to draw conclusions about the effect of physiologic shear stress on nanoparticle internalization. In contrast. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow 197 versus 2. As shown in our studies. reduced endothelial viability at about 100 ␮g/mL in static conditions. Mathemati- cal studies reported by Lee et al. Both the liposomes and the dextran T6-coated iron oxide nanoparticles showed excellent biocompatibility at static and flow conditions. of the cytostatic and cytotoxic effects observed after 24 h incubation below the concentrations of 200 ␮g/mL . respectively).9 × 106 nanoparticles per cell. cytotoxic effect was induced from the concentration of 50 ␮g/mL in static conditions. elongated particles exhibit larger propensity to laterally drift and marginate in laminar flow and that ellipsoidal micrometer- sized particles display stronger hydrodynamic margination under flow than sub-micrometer and . In the case of poly(isobutylcyanoacrylate) nanoparticles coated with 90% carboxymethyl-dextran and 10% fucoidan. comprehensive standardised studies are necessary that would compare the effects of a broad range of shear stress levels (low vs high). These features may contribute to an increased endothelial uptake even under high shear stress conditions. suggested that compared with spherical particles. these particles were well tolerated by the cells exposed to flow up to 400 ␮g/mL. Regarding the existing reports. as well as iron oxide nanoparticles coated with lauric acid and albumin. Similar results were obtained in our recent studies utilizing the bifurcating flow channels to compare the nanoparticle toxicity on endothelial cells exposed to laminar shear stress (10 dyn/cm2 ) or non- uniform shear stress (spatial range 0. it is acutely clear that no meaningful comparisons of the obtained results are possible. Only for one nanoparticle type (poly(isobutylcyanoacrylate) nanoparticles coated with 80% dextran T70. patterns (uniform vs non-uniform) and duration (acute vs chronic) on the particle uptake by endothelial cells in these conditions. whereas under flow conditions (0. these nanoparticles at 100 ␮g/mL did not dramatically affect endothelial viability under flow. as each group utilizes different flow models with differing shear stress magnitudes and durations. 2 types of polyaccrylate particles and 2 types of iron oxide nanoparticles. 10% diethylaminoethyl- dextran and 10% fucoidan). the viability of HUVECs exposed to gold nanoparticles in static conditions was significantly reduced at concentrations higher than 5 × 1010 nanoparticles/mL (gold concentration 1. This may be related to the fact that these particles are positively-charged. All lipid nanoparticles. and that the near-wall concentration of platelet-sized latex beads (2. including two types of liposomes. and 1 × 1012 nanoparticles/mL . but not all.6 × 10–3 mg/mL). which occurs over time and leads to increased effective concentrations of nanoparticles in the nearest vicinity of cell monolayer. This results from the inherent property of nanoparticles.1 dyn/cm2 ). lipid nanoparticles of 3 different sizes. and did not affect endothelial cell morphology. the cytotoxic effect being observed in the laminar and non- uniform shear stress region at very high concentrations (400 ␮g/mL). the longer-term cell culture assays under static conditions may overestimate the potential toxicity. The HUVEC monolayers were perfused for 18 h with medium containing 100 or 400 ␮g/mL of different types of nanoparticles. I. this effect was responsible for the majority.
hematocrit. 2A). the authors investigated in detail the effects of geometrical parameters (volume. The above-discussed findings were to a large extent confirmed by a series of comprehensive ex vivo investigations concerning the endothelial interactions with nano. axis length) on the margination efficacy of spherical and ellipsoid particles. Strategies to enhance endothelial interactions with nanoparticles The above-discussed in vitro and ex vivo results indicate that many types of nanocarriers may not be adequate for vascular applications in medium and large human arteries due to their small size and/or insufficient margination from the bloodstream. sub-micrometer and nanometer particles of any shape within the circu- lation can only oscillate around their trajectory. This effect is even more valid for lighter particles. Molecular targeting Conjugating nanoparticles to specific ligands that target endothelial activation markers may serve as a useful approach to enhance the internalization of particles under arterial flow (Fig. shape. the detailed studies which would verify these findings in animal models are missing. showing that although margination of rod- shaped microparticles with high aspect ratios was significantly improved as compared to spheres of equal volume. including shear rate. within the tumor microcirculation.198 I. Considering silica particles of 500 nm diameter. shear rate of 103 /s) with no external forces would be 7 ␮m for silica. the modelled contributions of the inertial and gravitational forces under physiological con- ditions are negligibly small. among them vascular cell adhesion molecule-1 (VCAM-1). These authors utilized spherical particles conjugated with sialyl Lewisa (sLea ). reported by the group of Eniola-Adefeso [99. intercellular adhesion molecule-1 (ICAM-1). including medium and large vessels.5 ␮m for iron oxide and 2 ␮m for gold particles. showing that nanoparticles (100–500 nm) displayed minimal margination from human blood flowing in chambers of varying heights (125–700 mm) towards endothelial monolayer. From this model. 7. density. a significantly higher bind- ing of intermediately-sized microspheres (2–5 ␮m) was detected in this model . Extensive efforts are therefore focused on identifying efficient targeting approaches that could enhance the binding of nanoparticles to vascular endothelium at the disease-specific regions. and erythrocyte aggregability . confirming the theoretical predictions of Lee et al. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow nanometer particles . These mathematical predictions indicate that. the nanorods did not display enhanced margination compared to that of nanospheres . the margination of nanoparticles was not enhanced. preventing their margination.e. and the nanoparticle behav- ior under flow is expected to differ in dependence of inherent particle properties (size. platelet-endothelial cell adhesion molecule-1 (PECAM-1). although microparticle attachment to the endothelium was 2 to 4-fold increased under pulsatile blood flow com- pared to laminar flow. The model predicts that under pathological conditions. Notably.g. the gravitational force may dominate leading to the sedimenta- tion of larger particles in horizontal capillaries . with diameters ranging from 100 nm to 10 ␮m. a ligand spe- cific to the endothelial-expressed selectins. in the absence of external forces. as well as endothelial selectins. such as polymeric or lipid-based beads. In contrast. 7. Thus far. however. 100]. aspect ratio. Several endothelial adhesion molecules have thus far been tested as molecular targets in vitro and in vivo. the minimum equivalent radii for observing margination under normal hemodynamic conditions (i. . 3. or stiffness) as well as blood rheological parameters. .and microparticles suspended in human whole blood. In a further study .1. e. The authors concluded that both nanorods and nanospheres show no margination in the presence of erythro- cytes in vitro. Extensive in vivo investigations at all physiological ranges of shear stress and vessel diameter are thus urgently needed to characterize the ability of particles with different physicochemical properties to deliver drugs to the specific vascular beds.
In the case of anti-VCAM-conjugated Fe3 O4 @SiO2 nanoparticles. vascular cell adhesion molecule-1.3 dyn/cm²) . (A) Molecular targeting. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow 199 A. vascular endothelial growth factor 2. vascular endothelial cadherin. intercellular adhesion molecule-1. [102. nanoparticles. VCAM-1. platelet-endothelial cell adhesion molecule 1. I. SPIONs. VE-cadherin. VEGFR2.4 dyn/cm² for 3 min as compared to cells grown under lower shear stress (6. Molecular targeting Flow direction NPs conjugated with antibodies or ligands ICAM-1 ICAM-1 VCAM-1 VCAM-1 P-selectin E-selectin PECAM-1 VEGFR2 VE-cadherin Extracellular matrix B. Yang et al. the . but was 3–5 fold decreased in cells exposed to 10. NP. 2. ICAM-1. (B) Magnetic targeting. PECAM-1. Magnetic targeting Flow direction circulatingSPIONs PECAM-1 VEGFR2 VE-cadherin Extracellular matrix magnet Fig.57 ␮m) and fluorescent core-shell Fe3 O4 @SiO2 nanoparticles (355 nm) under short-term flow exposure in vitro. 103] investigated the influence of anti-VCAM antibodies on endothelial uptake of ultrasound microbubbles (3. superparamagnetic iron oxide nanoparticles. Targeted microbubble adhesion to LPS-activated endothe- lial cells increased dose-dependently with increasing surface antibody densities. Active targeting of nanoparticles to endothelial cells.
In accordance with the in vitro data and the theoretical predictions. also monoclonal antibodies against VCAM-1 conjugated to iron oxide microparticles were used to target atherosclerotic lesions of ApoE-deficient mice. Collec- tively. 5. further increased their affinity to VCAM-1 in the aortic roots of ApoE-deficient mice . As an example. Conjugating nanoparticles to yet another peptide homologous to VLA-4. and their affinity to the endothelium could be further significantly improved by adding an additional P-selectin-targeting moiety . internalization of PECAM-conjugated nanocarriers was not . The authors concluded that actin recruitment to stress fibers which control the cell shape under flow may delay the uptake of ICAM-targeted nanoparticles by interfering with actin reorgani- zation required for CAM-mediated endocytosis.  utilized fluorescent iron oxide nanoparticles conjugated with a peptide containing sequence homology to the alpha-chain of very late antigen-4 (VLA-4.15. the authors con- cluded that the formation of actin stress fibers interferes with endothelial endocytic pathways . Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow degree of nanoparticle adhesion to activated HUVECs decreased significantly relative of static con- ditions when the cells were exposed to increasing shear stress levels (1. Kelly et al. Since the acute induction of actin stress fibers in the absence of flow had similar suppressive effect on nanoparticle internalization. a known ligand for VCAM-1). cationic lipoparticles containing anti-miR-712 and coated with VCAM-1 peptide ligand accu- mulated specifically in inflamed mouse endothelium and effectively prevented atherosclerotic plaque formation in mice. The endocytosis of polystyrene nanoparticles conjugated to anti-ICAM-1 antibodies (∼180 nm diameter) was investi- gated in endothelial-like cells (EAhy926 cells) and primary HUVECs pre-exposed to laminar shear stress at 4 dyn/cm2 for 24 h . These peptides showed 12-fold higher target-to-background ratios as compared with VCAM-1 monoclonal antibodies and successfully identified VCAM-1-expressing endothelial cells in a murine model of inflammation and in atherosclerotic lesions of apolipoprotein E (ApoE)-deficient mice . . relatively slow. In contrast to ICAM-targeted particles. With increasing exposure time (0. these results indicate that both monoclonal antibodies and small peptide ligands to VCAM-1 can significantly improve endothelial targeting of nanoparticles in vivo and allow disease-dependent particle accumulation even in the large vessels. and 9. 1. the particles were internalized via a non-classical pathway. Both in static conditions and under flow. These studies are in agreement with the reported in vivo approaches to VCAM-1 targeting. 5. Endothelial targeting with nanocarriers conjugated to anti-ICAM and anti-PECAM antibodies under flow conditions was also addressed in detail by the group of Muzykantov [108. This pattern was observed for both non-targeted and VCAM-1-targeted nanoparticles.94 dyn/cm2 ) for 3 min.15 dyn/cm2 was furthermore reduced. The impact of acute and chronic flow conditions on the CAM-mediated endothelial internalization of PECAM-targeted nanospheres (180 nm) was investigated by Han et al.1. Compared to cells under static cell culture. In vivo. but effective endocytosis of ICAM-targeted nanoparticles was detected in mouse pulmonary endothelium after intravenous injec- tion. 109]. The formation of actin stress fibers upon flow-adaptation (5 dyn/cm2 for 16 h) inhibited the uptake of anti-PECAM nanoparti- cles. which was accelerated by the treatment with LPS. These nanocon- structs were capable of detecting activated endothelial cells. whereas acute flow without stress fiber formation (1 dyn/cm2 for 30 min) stimulated the uptake. In a recent study by Kheirolomom et al. the adhesion of nanoparticles to HUVEC monolayer at 5. . CAM-mediated endocytosis. nanoparticle uptake was slightly faster in capillaries with lower shear stress . in contrast to control unconjugated nanoparticles which did not bind to endothelial cells. whereby the accumulation of VCAM-targeted nanoparticles was nearly 2-fold more effective under all tested conditions . 10 min). Apart from small peptide sequences. about 35% reduction in uptake of ICAM-targeted nanoparticles was observed in flow-adapted endothelial cells.200 I. which was in accordance with the in vivo results showing lower rates of anti-PECAM nanoparticle endocytosis in arterial compared to capillary vessels.
and their uptake at increasing shear stress levels was significantly higher than that of untargeted particles. indicating that the E-selectin-targeted probe detects specific pattern of vascular inflammation . Intraarterial infusion of nanoparticles in an ex vivo rat model of carotid balloon injury over 3 minutes demonstrated that 2-fold more GPIb-conjugated nanoparticles adhered to the injured arterial wall. so the nanoparticles targeted to P-selectin are not specific to activated endothelial cells. Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow 201 induced in activated HUVECs. indicating that the GPIb-conjugated PLGA nanoparticles can effectively deliver drugs at the site of vascular injury . constitute another potential target for molecular imaging and drug delivery to endothelium. the extracellular fragment of platelet glycoprotein Ib␣ (GPIb␣).g. several recent studies utilized platelet- mimicking approach to nanoparticle functionalization in order to improve endothelial targeting. but numbers of internalized particles were only 30% smaller than in static conditions . which were subsequently confirmed by histologic analysis. Lin et al. if the uptake of PECAM-targeted nanocarriers differs between cells exposed to various patterns of shear stress. PECAM is not cytokine-induced. the cell-associated unconjugated particles were decreased by 80% at 25 dyn/cm2 and localized mostly to extracellular spaces. Selectins. leading to nearly abolished uptake at 15 dyn/cm2 . PECAM was shown to promote atherosclerotic lesion formation in regions of disturbed flow . GPIb-nanoparticles were strongly internalized by endothelial cells. as compared with free docetaxel .  as an intravenous docetaxel delivery platform in a rat model of coronary restenosis. these findings suggest that nanoparticles mimicking the interactions of platelets with activated endothelial cells/subendothelial matrix can bind to the arterial wall under physiologic flow conditions. arterioles/arteries) or pathological conditions (e. I. In a mouse model of inflammation. this effect was prevented by the presence of GPIb␣ on the particles. In a further study from the same group . inflammation). Compared to static conditions. dexamethasone-loaded PLGA nanoparticles (220 nm) were conjugated with GPIb␣ and compared with untargeted nanoparticles under varying levels of shear stress (0–25 dyn/cm2 ) after 30 minutes of flow. rapidly upregulated on endothelial cells upon their activation. PLGA particles cloaked in platelet membrane vesicles were also utilized by Hu et al. The platelet-like functionality of these particles was demonstrated by selective binding to the damaged vasculature in a rat model of angioplasty-induced arterial damage. an anti-E-selectin monoclonal antibody was conjugated to ultrasmall SPION for targeting E-selectin in vivo . as compared to control particles. . It is unknown. Injection of targeted nanoparticles resulted in distinct changes in R2 relaxation rate (1/T 2) characteristics in inflamed regions as compared with control regions. Relative of static conditions. P-selectin is expressed both by platelets and activated endothelium. however. Collectively. dual targeting of P-selectin and VCAM-1 with iron oxide microparticles has been successfully employed for MR imaging of atherosclerotic plaques in ApoE-deficient mice . Moreover. but is abundantly expressed also by quiescent cells and primarily localized to cell-cell junctions. GPIb␣-conjugated nanoparticles showed a slightly diminished uptake under 5 dyn/cm2 and a more pronounced reduction of internalization at 15 dyn/cm2 . with minimal accumulation within the cells. The platelet-mimicking particles furthermore showed a superb therapeutic efficacy in a rat model of coronary stenosis. On the contrary. in terms of their uptake by human aortic endothelial cells under physiological flow conditions (shear stress between 0 dyn/cm2 to 15 dyn/cm2 ). The above-discussed studies demonstrate that the regulation of targeted-nanoparticle internalization by flow conditions and/or endothelial activation may strongly modulate drug delivery into endothelium exposed to differ- ent physiological hemodynamic patterns (capillaries vs.  compared unconjugated polystyrene NPs (100 nm) with nanoparticles conjugated with glycocalicin. Whereas cellular uptake of untargeted nanoparticles after 30 minutes of flow was strongly decreased with the increase of shear stress magnitude. Being involved in shear stress-mediated mechanotransduction . Unlike ICAM. However.
Chao et al.202 I. both in terms of the amounts of delivered drug and the therapeutic outcome has been demonstrated in several studies on tumor- bearing rabbits treated with mitoxantrone-loaded SPIONs applied intraarterially under the guidance of an external magnet [56. Conclusions and perspectives Nanotechnology-based strategies are expected to have a great clinical impact on the diagnostics and therapy of human diseases in the future. the parenterally-administered particles should be able to achieve an increased circulation half-life and a high margination rate. rat iliac artery embolic model .  utilized PEG-ylated iron oxide/gold nanoparticles loaded with doxorubicin (22 nm) for magnetic drug targeting. The above-described studies indicate that active targeting using the magnetic field enhances the specific drug delivery to the tumor vasculature and increases its therapeutic efficacy. who employed carbon-coated iron carbide (Fe5 C2 ) nanoparticles functionalized with bovine serum albumin and loaded with doxorubicin.2. . although ex vivo studies indicate that accumulation of flowing SPIONs in the arterial wall is easily achievable under the guidance of a sufficiently strong external magnet. It must be noted. rat femoral artery . Magnetic targeting “Magnetic drug targeting” utilizes an external magnetic field to target drugs conjugated with SPIONs to the diseased vasculature regions (Fig. however. several targeting strategies are employed to ensure the delivery of a sufficient payload of drug to the vascular regions under physiologic shear stress conditions. The external epicardial magnet- enhanced targeting resulted in a strong VEGF gene expression in the ischemic region and improved cardiac repair . Compared with passive targeting. Magnetic targeting was furthermore effective in a rat model of myocardial infarction reported by Zhang et al. a rational design of nanoparticulate contrast agents and drug carriers is necessary. Upon intravenous administration of doxorubicin-loaded particles to hepatoma cell tumor-bearing mice. whereby the most promising experimental results have thus far been reported with endothe- lial adhesion molecule-targeting and magnetic drug targeting approaches. Passive targeting can be used to deliver nanotherapeutics to the diseased regions. 8. Therefore.g. where the externally-controlled magnetic nanobeads conjugated to adenoviral vectors-encoded human VEGF gene were administered intravenously. The efficacy of this approach. a single intravenous application of nanoparticles combined with magnetic targeting provided a higher accumulation of drug in tumor tissue after 24 h post-application in a mouse model of Ehrlich carcinoma. 2B). In arterial circulation. In a recent report. 57]. Effective site-specific magnetic targeting of intravenously-administered nanoparticles by placing a magnet above the tumor in mice was also shown by Yu et al. shear stress-activated processes may significantly affect the nanoparticle internalization by endothelial cells. that thus far the experimental attempts to magneti- cally target the larger arteries have been relatively scarce (e. To ensure the clinical safety and feasibility of these entirely novel approaches. but its efficacy is limited to the vascular beds where the blood-tissue barrier is compro- mised. contributing to subsequent tumor growth inhibition and reduced side effects in healthy tissues . Cicha / Strategies to enhance nanoparticle-endothelial interactions under ﬂow 7. . Further studies in vitro . large arteries and smaller arterial branches in primates ). a significantly increased accumulation of doxorubicin in tumors was achieved with external magnetic force . and allow enhanced interactions with endothelial cells in the target region. leading to a significant tumor volume reduction as compared with free drug and non- magnetically targeted particles. Ideally.  investigated the tumor targeting and therapeutic efficacy of a magnet-enhanced delivery in vivo using PEG-modified iron oxide/gold nanoparticles (50 nm) loaded with doxorubicin. Elbialy et al.
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Identification and Control of Fungi Associated With the Post-harvest Rot of Solenostemon Rotundifolius (Poir)J.K. Morton in Adamawa State of Nigeria.

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