Particle diameter influences adhesion under flow

Non-Spherical Particles for Targeted Drug Delivery

Nano- and microparticles loaded with various bioimaging contrast agents or therapeutic molecules have been increasingly used for the diagnosis and treatment of diseases and tissue defects. These particles, often a filled or hollow sphere, can extend the lifetime of encapsulated biomedical modalities in circulation and in target tissue. However, there is a great need to improve the drug loading and targeting efficiency of these particles. Recently, several simulation and in vitro experimental studies reported that particle shape plays a pivotal role in the targeted delivery of molecules. To better understand these findings and subsequently expedite the use of particles in biomedical applications, this review paper summarizes the methods to prepare non-spherical nano- and micro-scaled particles. In addition, this review covers studies reporting the effects of particle shape on the loading, delivery and release of encapsulated bioactive cargos. Finally, it discusses future directions to further improve the properties of non-spherical particles.

Platelet-like nanoparticles: mimicking shape, flexibility, and surface biology of platelets to target vascular injuries

Targeted delivery of therapeutic and imaging agents in the vascular compartment represents a significant hurdle in using nanomedicine for treating hemorrhage, thrombosis, and atherosclerosis. While several types of nanoparticles have been developed to meet this goal, their utility is limited by poor circulation, limited margination, and minimal targeting. Platelets have an innate ability to marginate to the vascular wall and specifically interact with vascular injury sites. These platelet functions are mediated by their shape, flexibility, and complex surface interactions. Inspired by this, we report the design and evaluation of nanoparticles that exhibit platelet-like functions including vascular injury site-directed margination, site-specific adhesion, and amplification of injury site-specific aggregation. Our nanoparticles mimic four key attributes of platelets, (i) discoidal morphology, (ii) mechanical flexibility, (iii) biophysically and biochemically mediated aggregation, and (iv) heteromultivalent presentation of ligands that mediate adhesion to both von Willebrand Factor and collagen, as well as specific clustering to activated platelets. Platelet-like nanoparticles (PLNs) exhibit enhanced surface-binding compared to spherical and rigid discoidal counterparts and site-selective adhesive and platelet-aggregatory properties under physiological flow conditions in vitro. In vivo studies in a mouse model demonstrated that PLNs accumulate at the wound site and induce ∼65% reduction in bleeding time, effectively mimicking and improving the hemostatic functions of natural platelets. We show that both the biochemical and biophysical design parameters of PLNs are essential in mimicking platelets and their hemostatic functions. PLNs offer a nanoscale technology that integrates platelet-mimetic biophysical and biochemical properties for potential applications in injectable synthetic hemostats and vascularly targeted payload delivery.

Nanoparticles in drug delivery: mechanism of action, formulation and clinical application towards reduction in drug-associated nephrotoxicity

INTRODUCTION

Over the past few decades, nanoparticles (NPs) have gained immeasurable interest in the field of drug delivery. Various NP formulations have been disseminated in drug development in an attempt to increase efficacy, safety and tolerability of incorporated drugs. In this context, NP formulations that increase solubility, control release, and/or affect the in vivo disposition of drugs, were developed to improve the pharmacokinetic and pharmacodynamic properties of encapsulated drugs.

AREAS COVERED

In this article, important properties related to NP function such as particle size, surface charge and shape are disseminated. Also, the current understanding of how NP characteristics affect particle uptake and targeted delivery is elucidated. Selected NP systems currently used in delivery of drugs in biological systems and their production methods are discussed as well. Emphasis is placed on current NP formulations that are shown to reduce drug-induced adverse renal complications.

EXPERT OPINION

Formulation designs utilizing NP-encapsulated drugs offer alternative pharmacotherapy options with improved safety profiles for current and emerging drugs. NPs have been shown to increase the therapeutic index of several entrapped drugs mostly by decreasing drug localization and side effects on organs. Recent studies on NP-encapsulated chemotherapeutic and antibiotic medications show enhanced therapeutic outcomes by altering drug degradation, increasing systemic circulation and/or enhancing cell specific targeting. They may also reduce the distribution of encapsulated drugs into the kidneys and attenuate drug-associated adverse renal complications. The usefulness of NP formulation in reducing the nephrotoxicity of nonsteroidal anti-inflammatory drugs is an under explored territory that deserves more attention.

Fabrication of biconcave discoidal silica capsules and their uptake behavior by smooth muscle cells

Biconcave discoidal silica microcapsules were fabricated by reaction of tetraethoxysilane (TEOS) on biconcave discoidal Ca(OH)2 templates, followed by core removal. The biconcave discoidal morphology of microcapsules was characterized by confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM). The thickness of silica capsule shell can be tuned by two methods, "Gradient concentration" method and "Multi-step growth" method. Through the latter one, the shell thickness can be controlled more effectively. Compared with spherical microcapsules, the biconcave discoidal ones were internalized into smooth muscle cells (SMCs) with a slower rate.

Characterization of nanoparticle delivery in microcirculation using a microfluidic device

This work focuses on the characterization of particle delivery in microcirculation through a microfluidic device. In microvasculature the vessel size is comparable to that of red blood cells (RBCs) and the existence of blood cells largely influences the dispersion and binding distribution of drug loaded particles. The geometry of the microvasculature leads to non-uniform particle distribution and affects the particle binding characteristics. We perform an in vitro study in a microfluidic chip with micro vessel mimicking channels having a rectangular cross section. Various factors that influence particle distribution and delivery such as the vessel geometry, shear rate, blood cells, particle size, particle antibody density are considered in this study. Around 10% higher particle binding density is observed at bifurcation regions of the mimetic microvasculature geometry compared to straight regions. Particle binding density is found to decrease with increased shear rates. RBCs enhance particle binding for both 210 nm and 2 μm particles for shear rates between 200-1600 s(-1) studied. The particle binding density increases about 2-3 times and 6-10 times when flowing in whole blood at 25% RBC concentration compared to the pure particle case, for 210 nm and 2 μm particles respectively. With RBCs, the binding enhancement is more significant for 2 μm particles than that for 210 nm particles, which indicates an enhanced size dependent exclusion of 2 μm particles from the channel centre to the cell free layer (CFL). Increased particle antibody coating density leads to higher particle binding density for both 210 nm and 2 μm particles.

Abdominal aortic aneurysms targeted by functionalized polysaccharide microparticles: a new tool for SPECT imaging

Aneurysm diagnostic is nowadays limited by the lack of technology that enables early detection and rupture risk prediction. New non invasive tools for molecular imaging are still required. In the present study, we present an innovative SPECT diagnostic tool for abdominal aortic aneurysm (AAA) produced from injectable polysaccharide microparticles radiolabeled with technetium 99m (^99mTc) and functionalized with fucoidan, a sulfated polysaccharide with the ability to target P-Selectin. P-Selectin is a cell adhesion molecule expressed on activated endothelial cells and platelets which can be found in the thrombus of aneurysms, as well as in other vascular pathologies.

Microparticles with a maximum hydrodynamic diameter of 4 µm were obtained by crosslinking the polysaccharides dextran and pullulan. They were functionalized with fucoidan. In vitro interactions with human activated platelets were assessed by flow cytometry that demonstrated a specific affinity of fucoidan functionalized microparticles for P-Selectin expressed by activated platelets. For in vivo AAA imaging, microparticles were radiolabeled with ^99mTc and intravenously injected into healthy and AAA rats obtained by elastase perfusion through the aorta wall. Animals were scanned by SPECT imaging. A strong contrast enhancement located in the abdominal aorta of AAA rats was obtained, while no signal was obtained in healthy rats or in AAA rats after injection of non-functionalized control microparticles. Histological studies revealed that functionalized radiolabeled polysaccharide microparticles were localized in the AAA wall, in the same location where P-Selectin was expressed.

These microparticles therefore constitute a promising SPECT imaging tool for AAA and potentially for other vascular diseases characterized by P-Selectin expression. Future work will focus on validating the efficiency of the microparticles to diagnose these other pathologies and the different stages of AAA. Incorporation of a therapeutic molecule is also considered.

Development of functional biomaterials with micro- and nanoscale technologies for tissue engineering and drug delivery applications

Micro- and nanotechnologies have emerged as potentially effective fabrication tools for addressing the challenges faced in tissue engineering and drug delivery. The ability to control and manipulate polymeric biomaterials at the micron and nanometre scale with these fabrication techniques has allowed for the creation of controlled cellular environments, engineering of functional tissues and development of better drug delivery systems. In tissue engineering, micro- and nanotechnologies have enabled the recapitulation of the micro- and nanoscale detail of the cell's environment through controlling the surface chemistry and topography of materials, generating 3D cellular scaffolds and regulating cell-cell interactions. Furthermore, these technologies have led to advances in high-throughput screening (HTS), enabling rapid and efficient discovery of a library of materials and screening of drugs that induce cell-specific responses. In drug delivery, controlling the size and geometry of drug carriers with micro- and nanotechnologies have allowed for the modulation of parametres such as bioavailability, pharmacodynamics and cell-specific targeting. In this review, we introduce recent developments in micro- and nanoscale engineering of polymeric biomaterials, with an emphasis on lithographic techniques, and present an overview of their applications in tissue engineering, HTS and drug delivery.

Size-Dependent Nanoparticle Margination and Adhesion Propensity in a Microchannel

Intravenous injection of nanoparticles as drug delivery vehicles is a common practice in clinical trials of therapeutic agents to target specific cancerous or pathogenic sites. The vascular flow dynamics of nanocarriers (NCs) in human microcapillaries play an important role in the ultimate efficacy of this drug delivery method. This article reports an experimental study of the effect of nanoparticle size on their margination and adhesion propensity in microfluidic channels of a half-elliptical cross section. Spherical polystyrene particles ranging in diameter from 60 to 970 nm were flown in the microchannels and individual particles adhered to either the top or bottom wall of the channel were imaged using fluorescence microscopy. When the number concentration of particles in the flow was kept constant, the percentage of nanoparticles adhered to the top wall increased with decreasing diameter (d), with the number of particles adhered to the top wall following a d−3 trend. When the volume concentration of particles in solution was kept constant, no discernible trend was found. This experimental finding is explained by the competition between the Brownian force promoting margination and repulsive particle–particle electrostatic forces retarding adhesion to the wall. The 970 nm particles were found to adhere to the bottom wall much more than to the top wall for each of the three physiologically relevant shear rates tested, revealing the effect of gravitational force on the large particles. These findings on the flow behavior of spherical nanoparticles in artificial microcapillaries provide further insight for the rational design of NCs for targeted cancer therapeutics.

Carbon dots functionalized gold nanorod mediated delivery of doxorubicin: tri-functional nano-worms for drug delivery, photothermal therapy and bioimaging

Carbon dots (C-dots) are novel nanomaterials for biological applications owing to their inherent surface decoration by a variety of functional groups, excellent fluorescent properties and biocompatibility under physiological conditions. Gold nanorods (GNRs) are one of the most celebrated nanomaterials for photothermal therapy as well as delivery of therapeutic payloads because of their typical size and shape related features. We have synthesized a unique blend of C-dots and GNRs (C-dots@GNRs) for the controlled release of doxorubicin (DOX) under ideal physiological conditions. Highly fluorescent C-dots were synthesized using microwave assisted heating of gum arabic (GA) and subsequently purified using Sucrose Density Gradient Centrifugation (SDGC). In a modified seed mediated protocol, purified C-dots were added into the growth solution, to make a C-dots@GNR complex. This complex was used for anchoring DOX via covalent and non-covalent pH sensitive chemical bonds. Under physiological conditions, the drug loading capacity of C-dots@GNRs was calculated to be ∼94%. Another beneficial attribute of the complex was found to be synergistic potential in high drug loading and rapid burst of drug release under the influence of near infrared (NIR) radiation (808 nm), thus proving a highly biocompatible thermo-chemotherapy for solid tumors.

Physicochemical characteristics of nanoparticles affect circulation, biodistribution, cellular internalization, and trafficking

Nanoparticles have many potential applications in tumor therapy. Systemically administered nanoparticles should remain in circulation for a long time to increase their accumulation in targeted tissues before being cleared by the reticuloendothelial system, and be effectively internalized by the targeted cells, which can be influenced significantly by the physicochemical characteristics of nanoparticles, such as particle size, surface properties, and particle shape. This review highlights the impact of the main affects of physicochemical properties on nanoparticle transport behavior in blood, their uptake and clearance by macrophages and their consequent biodistribution, as well as their interaction with targeted cells.

Computational study of particle size effects on selective binding of nanoparticles in arterial stenosis

In order to elucidate particle size and wall shear effects on the selective binding of nanoparticles to vessel wall, particle binding to the wall of arterial stenosis was computationally analyzed using a transport and reaction model. The attachment rate constant was modeled as a function of shear rate and particle size. The results showed that it had a positive correlation with the shear rate for particles smaller than 600 nm and a negative correlation with the shear rate for particles larger than 800 nm. Small size particles showed high binding selectivity in the stenosis region for the normal and shear-activated wall, whereas large particles showed high binding selectivity in the low and oscillatory zone for the shear-activated wall.

Margination propensity of vascular-targeted spheres from blood flow in a microfluidic model of human microvessels

Many variants of vascular-targeted carriers (VTCs) have been investigated for therapeutic intervention in several human diseases. However, in order to optimize the functionality of VTC in vivo, carriers' physical properties, such as size and shape, are important considerations for a VTC design that evades the reticuloendothelial system (RES) and successfully interacts with the targeted vessel wall. Nonetheless, little evidence has been presented on the role of size in VTC's interactions with the vascular wall, particularly in the microcirculation. Thus, in this work, we explore how particle size, along with hemodynamics (blood shear rate and vessel size) and hemorheology (blood hematocrit) affect the capacity for spheres to marginate (localize and adhere) to inflamed endothelium in a microfluidic model of human microvessels. Microspheres, particularly the 2 μm spheres, were found to show disproportionately higher margination than nanospheres in all hemodynamic conditions evaluated due to the poor ability of the latter to localize to the wall region from midstream. This work represents the first evidence that nanospheres may not exhibit "near wall excess" in microvessels, e.g., arterioles and venules, and therefore may not be suitable for imaging and drug delivery applications in cancer and other diseases affecting microvessels.

Nanotechnology in therapeutics: a focus on nanoparticles as a drug delivery system

Continuing improvement in the pharmacological and therapeutic properties of drugs is driving the revolution in novel drug delivery systems. In fact, a wide spectrum of therapeutic nanocarriers has been extensively investigated to address this emerging need. Accordingly, this article will review recent developments in the use of nanoparticles as drug delivery systems to treat a wide variety of diseases. Finally, we will introduce challenges and future nanotechnology strategies to overcome limitations in this field.

The Interactions between L-tyrosine based nanoparticles decorated with folic acid and cervical cancer cells under physiological flow

Many anticancer drugs have been established clinically, but their efficacy can be compromised by nonspecific toxicity and an inability to reach the desired cancerous intracellular spaces. In order to address these issues, researchers have explored the use of folic acid as a targeted moiety to increase specificity of chemotherapeutic drugs. To expand upon such research, we have conjugated folic acid to functionalized poly(ethylene glycol) and subsequently decorated the surface of l-tyrosine polyphosphate (LTP) nanoparticles. These nanoparticles possess the appropriate size (100-500 nm) for internalization as shown by scanning electron microscopy and dynamic light scattering. Under simulated physiological flow, LTP nanoparticles decorated with folic acid (targeted nanoparticles) show a 10-fold greater attachment to HeLa, a cervical cancer cell line, compared to control nanoparticles and to human dermal fibroblasts. The attachment of these targeted nanoparticles progresses at a linear rate, and the strength of this nanoparticle attachment is shown to withstand shear stresses of 3.0 dyn/cm(2). These interactions of the targeted nanoparticles to HeLa are likely a result of a receptor-ligand binding, as a competition study with free folic acid inhibits the nanoparticle attachment. Finally, the targeted nanoparticles encapsulated with a silver based drug show increased efficacy in comparison to nondecorated (plain) nanoparticles and drug alone against HeLa cells. Thus, targeted nanoparticles are a promising delivery platform for developing anticancer therapies that overexpress the folate receptors (FRs).

Particle-cell dynamics in human blood flow: implications for vascular-targeted drug delivery

The outcome of vascular-targeted therapies is generally determined by how efficiently vascular-targeted carriers localize and adhere to the endothelial wall at the targeted site. This study investigates the impact of leukocytes, platelets and red blood cells on the margination of vascular-targeted polymeric nanospheres and microspheres under various physiological blood flow conditions. We report that red blood cells either promote or hinder particle adhesion to an endothelial wall in a parallel plate flow chamber depending on the blood flow pattern, hematocrit, and particle size. Leukocytes prevent microspheres - but not nanospheres - from adhering in laminar and pulsatile flows via (1) competition for the available binding space and (2) physical removal of previously bound spheres. In recirculating blood flow, the negative effect of leukocytes on particle adhesion is minimal for large microspheres in the disturbed flow region beyond the flow reattachment. Resting platelets were found to have no effect on particle binding likely due to their dimensions and minimal interaction with the endothelial wall. Overall, the findings of the present work would be critical for designing effective vascular-targeted carriers for imaging and drug delivery applications in several human diseases.

The shape of things to come: importance of design in nanotechnology for drug delivery

The design of nanoparticle (NP) size, shape and surface chemistry has a significant impact on their performance. While the influences of the particle size and surface chemistry on drug delivery have been studied extensively, little is known about the effect of particle shapes on nanomedicine. In this perspective article, we discuss recent progress on the design and fabrication of NPs of various shapes and their unique delivery properties. The shapes of these drug carriers play an important role in therapeutic delivery processes, such as particle adhesion, distribution and cell internalization. We envision that stimuli-responsive NPs, which actively change their shapes and other properties, might pave way to the next generation of nanomedicine.

Ultrasonic imaging of endothelial CD81 expression using CD81-targeted contrast agents in in vitro and in vivo studies

This study is designed to investigate the feasibility for molecular imaging of endothelial CD81 expression in vitro and in vivo using the CD81-targeted ultrasound contrast agents (UCA). In the in vitro study, murine bEnd.3 cells were stimulated with phenazine methosulfate (PMS), an oxidative stress inducer. Changes in CD81 expression after stimulation were confirmed by Western blotting, tracked by using the targeted UCA and further imaged under ultrasound imaging system with 5 MHz transmit frequency. In the in vivo study, expression of endothelial CD81 proteins in murine carotid artery vessels was studied using high-frequency ultrasound system with 40 MHz transmit frequency. Our results showed that endothelial CD81 expression was gradually up-regulated with the increase of PMS concentration. Correspondingly, the accumulation of targeted UCA was gradually improved and could be inhibited significantly upon addition of free anti-CD81 antibodies. The mean video intensity (grey-level) of stimulated cells and vessels from backscatter of the CD81-targeted UCA was 17.2 (interquartile range [IQR] 15.4-19.8) and 27.2 (IQR 22.4-29.8), significantly greater than that of non-stimulated cells with 9.0 (IQR 8.6-10.8) (p < 0.01) and non-stimulated vessels with 11.3 (IQR 10.4-13.2) (p < 0.01), respectively. In conclusion, CD81-targeted UCA allows noninvasive assessment of the expression levels of CD81 on the vascular endothelium and may provide potential insights into early atherosclerotic plaque detection and treatment monitoring.

"Targeting" nanoparticles: the constraints of physical laws and physical barriers

In comparison to the complexities of the body, its organs, its normal and aberrant cells, many nanoparticles will appear to be relatively simple objects. This view is deceptive because the physicochemical properties of nanosystems, although quite well understood on the basis of material science, surface science and colloid theory, are far from simple in practice. While their properties are largely controllable in vitro, often purportedly "designed", their administration by any route changing environments conspires to produce additional layers of complexity. Some of the key physical laws and physicochemical parameters governing the fate of nanoparticles on their journey from point of intravenous administration to desired destinations such as tumors are discussed. Much of the science relevant to nanocarrier based targeting has been elaborated in studying purely physical phenomena, but there can be found therein many analogies with biological systems. These include factors that impede quantitative targeting: diffusion in complex media, aggregation and flocculation, hindered behavior of particles in confined spaces, jamming and dispersion in flow. All of these have the ability to influence fate and destination. Most of the critical processes are particle size dependent but not always linearly so. Virtually all processes in vivo involve an element of probability. Particle size and properties can be controlled to a large extent, but stochastic processes cannot by definition. Progress has been made, but the quantitative delivery of a nanocarrier to defined sites in tumors is neither inevitable nor yet predictable.

The role of particle geometry and mechanics in the biological domain

Nanostructured particulate materials are expected to revolutionize diagnostics and the delivery of therapeutics for healthcare. To date, chemistry-derived solutions have been the major focus in the design of materials to control interactions with biological systems. Only recently has control over a new set of physical parameters, including size, shape, and rigidity, been explored to optimize the biological response and the in vivo performance of nanoengineered delivery vectors. This Review highlights the methods used to manipulate the physical properties of particles and the relevance of these physical properties to cellular and circulatory interactions. Finally, the importance of future work to synergistically tailor both physical and chemical properties of particulate materials is discussed, with the aim of improving control over particle interactions in the biological domain.

Microfabricated particulate drug-delivery systems

Micro- and nanoparticulate drug-delivery systems (DDSs) play a significant role in formulation sciences. Most particulate DDSs are scaffold-free, although some particles are encapsulated inside other biomaterials for controlled release. Despite rapid progress in recent years, challenges still remain in controlling the homogenicity of micro-/nanoparticles, especially for two crucial factors in particulate DDSs: the size and shape of the particles. Recent approaches make use of microfabrication techniques to generate micro-/nanoparticles with highly controllable architectures free of scaffolds. This review presents an overview of a burgeoning field of DDSs, which can potentially overcome some drawbacks of conventional techniques for particle fabrication and offer better control of particulate DDSs.

Aptamer-conjugated nanobubbles for targeted ultrasound molecular imaging

Targeted ultrasound contrast agents can be prepared by some specific bioconjugation techniques. The biotin-avidin complex is an extremely useful noncovalent binding system, but the system might induce immunogenic side effects in human bodies. Previous proposed covalently conjugated systems suffered from low conjugation efficiency and complex procedures. In this study, we propose a covalently conjugated nanobubble coupling with nucleic acid ligands, aptamers, for providing a higher specific affinity for ultrasound targeting studies. The sgc8c aptamer was linked with nanobubbles through thiol-maleimide coupling chemistry for specific targeting to CCRF-CEM cells. Further improvements to reduce the required time and avoid the degradation of nanobubbles during conjugation procedures were also made. Several investigations were used to discuss the performance and consistency of the prepared nanobubbles, such as size distribution, conjugation efficiency analysis, and flow cytometry assay. Further, we applied our conjugated nanobubbles to ex vivo ultrasound targeted imaging and compared the resulting images with optical images. The results indicated the availability of aptamer-conjugated nanobubbles in targeted ultrasound imaging and the practicability of using a highly sensitive ultrasound system in noninvasive biological research.

Overcoming in vivo barriers to targeted nanodelivery

Nanoparticles have been investigated as promising nanocarriers for delivery of imaging and therapeutic agents for several decades, but have met with limited success. Although enormous progress in the fields of nanotechnology and nanoscience has been achieved, basic discoveries have not yet translated into effective targeted therapies. Nanoparticles can potentially improve the pharmacokinetics and pharmacodynamics of drugs; however, the complexity of in vivo systems imposes multiple barriers that severely inhibit efficiency and have to be overcome to fully exploit the theoretical potential of nanoparticles. Here, we address two major challenges to effective systemic nanodelivery. Both limited penetration across the vascular endothelium and uptake by the reticuloendothelial system (RES) substantially impede effectiveness of nanoparticle delivery into tissues. Although the design of nanoparticles with extended circulation half-life is essential, it is not sufficient for effective penetration of nanoparticles across the formidable barrier formed by the vascular endothelium. Current nanodelivery systems rely on passive transvascular exchange and tissue accumulation. They require high dosages to create large concentration gradients that drive nanoparticles passively across the blood-tissue interface. However, passive accumulation has resulted in only a fractional dosage of nanoparticles penetrating into target tissue. This inevitably diminishes therapeutic efficacy and aggravates potential side effects. Although there are multiple ways to augment passive delivery, active delivery of targeted nanoparticles across the vascular endothelium could significantly increase the therapeutic index and decrease side effects of nanoparticle-based drug delivery systems. Use of active transendothelial transport pathways, such as caveolae, may provide an effective solution to both target and deliver nanoparticles.

Targeting therapeutics to the vascular wall in atherosclerosis--carrier size matters

OBJECTIVE

Vascular-targeted imaging and drug delivery systems are promising for the treatment of atherosclerosis due to the vast involvement of endothelium in the initiation and growth of plaque. Herein, we investigated the role of particle size in dictating the ability of vascular-targeted spherical particles to interact with the vascular wall (VW) from pulsatile and recirculating human blood flow relevant in atherosclerosis.

METHODS

In vitro parallel plate flow chambers (PPFC) with straight or vertical step channel were used to examine the localization and binding efficiency of inflammation-targeted polymeric spheres sized from 0.2 to 5 μm to inflamed endothelium from disturbed reconstituted and whole blood flow. Apolipoprotein deficient mice were used to study particle localization and binding to plaque in vivo.

RESULTS

The efficiency of particle binding in disturbed reconstituted blood flow increases as spherical diameter increases from 500 nm to 5 μm. No significant difference was observed between adhesion of 200 nm and 500 nm spheres. Binding efficiency for all particle size was enhanced in disturbed whole blood flow except adhesion of 5 μm in pulsatile whole blood. The adhesion trend in the in vivo model confirmed the binding pattern observed in in vitro assays.

CONCLUSIONS

The presented data shows that the binding efficiency of vascular-targeted drug carriers in blood flow is a function of particle size, wall shear rate, flow type, blood composition and ligand characteristics. Overall, the presented results suggest that micron-sized spherical particles (2 μm), not nanospheres, are optimal for vascular-targeted drug delivery applications in medium to large vessel relevant in atherosclerosis.

Uniform Chitosan Microparticles Prepared by a Novel Spray-Drying Technique

Particle size and morphology are important properties of pharmaceutical particles. Preparation of microparticles with uniform particle size and morphology is necessary in order to systematically relate these properties to the release behavior and other functionalities such as drug encapsulation and dissolution. In this study, we successfully prepared monodisperse, nonagglomerated chitosan microparticles in a single step by a novel spray-drying technique. The control of particle size and morphology of spray-dried microparticles was investigated experimentally. Microparticles with larger particle size can be produced when chitosan precursor of higher concentration was used. Storage time of chitosan precursor, drying temperature, and addition of lactose were shown to be crucial parameters that affect the particle morphology. Appropriate choice of the drying temperature and precursor storage time permitted control of the particle morphology, ranging from nearly spherical to cap-shaped. Surface characteristics of the particles can be finely tuned by the amount of lactose added into the chitosan precursor.

Designer nanoparticles: incorporating size, shape and triggered release into nanoscale drug carriers

IMPORTANCE OF THE FIELD

Although significant progress has been made in delivering therapeutic agents through micro and nanocarriers, precise control over in vivo biodistribution and disease-responsive drug release has been difficult to achieve. This is critical for the success of next generation drug delivery devices, as newer drugs, designed to interfere with cellular functions, must be efficiently and specifically delivered to diseased cells. The chief constraint in achieving this has been our limited repertoire of particle synthesis methods, especially at the nanoscale. Recent developments in generating shape-specific nanocarriers and the potential to combine stimuli-responsive release with nanoscale delivery devices show great promise in overcoming these limitations.

AREAS COVERED IN THIS REVIEW

How recent advances in fabrication technology allow synthesis of highly monodisperse, stimuli-responsive, drug-carrying nanoparticles of precise geometries is discussed. How particle properties, specifically shape and stimuli responsiveness, affect biodistribution, cellular uptake and drug release is also reviewed.

WHAT THE READER WILL GAIN

The reader is introduced to recent developments in intelligent drug nanocarriers and new nanofabrication approaches that can be combined with disease-responsive biomaterials. This will provide insight into the importance of controlling particle geometry and incorporating stimuli-responsive materials into drug delivery.

TAKE HOME MESSAGE

The integration of responsive biomaterials into shape-specific nanocarriers is one of the most promising avenues towards the development of next generation, advanced drug delivery systems.

Flow and adhesion of drug carriers in blood vessels depend on their shape: a study using model synthetic microvascular networks

Development of novel carriers and optimization of their design parameters has led to significant advances in the field of targeted drug delivery. Since carrier shape has recently been recognized as an important design parameter for drug delivery, we sought to investigate how carrier shape influences their flow in the vasculature and their ability to target the diseased site. Idealized synthetic microvascular networks (SMNs) were used for this purpose since they closely mimic key physical aspects of real vasculature and at the same time offer practical advantages in terms of ease of use and direct observation of particle flow. The attachment propensities of surface functionalized spheres, elliptical/circular disks and rods with dimensions ranging from 1microm to 20microm were compared by flowing them through bifurcating SMNs. Particles of different geometries exhibited remarkably different adhesion propensities. Moreover, introduction of a bifurcation as opposed to the commonly used linear channel resulted in significantly different flow and adhesion behaviors, which may have important implications in correlating these results to in vivo behavior. This study provides valuable information for design of carriers for targeted drug delivery.

Potential role of size and hemodynamics in the efficacy of vascular-targeted spherical drug carriers

Targeting of drug carriers to the vascular wall is of interest for localized delivery of therapeutics in many human diseases. Nanometer-sized spherical particles are widely proposed for use as carriers for vascular targeting, yet very little evidence has been presented as to their ability to interact with the vascular wall. Thus, this work focuses on elucidating the effect of particle size along with hemodynamics, blood rheology, and vessel size on the adhesion efficiency of targeted polymeric spheres to inflamed endothelium in vitro via parallel plate flow chamber assays. We find that the binding efficiency of spheres to the endothelium from blood flow generally increased with increasing particle size, wall shear rate and channel height for particle sizes from 100 nm up to 10 microm. However, nano-sized particles showed minimal adhesion to the endothelium from blood flow in horizontal (gravity or anti-gravity direction) and vertical channels on the order of small to medium-sized venules and arteries when compared to micron-sized spheres. Furthermore, adhesion of nanospheres was not enhanced with pulsatility in flow. Overall, the presented data suggests that spheres 2-5 microm in size are optimal for targeting the wall in medium to large vessels relevant in several cardiovascular diseases.

Critical appraisal of targeted ultrasound contrast agents for molecular imaging in large arteries

Molecular imaging may provide new insights into the early detection and development of atherosclerosis before first symptoms occur. One of the techniques in use employs noninvasive ultrasound. In the past decade, experimental and clinical validation studies showed that for the microcirculation targeted ultrasound contrast agents, such as echogenic liposomes, microbubbles and perfluorocarbon emulsions, do improve visualization of specific structures. For large arteries, however, successful application is less obvious. In this review, we will address the challenges for molecular imaging of large arteries. We will discuss the problems encountered in the use of targeted ultrasound contrast agents presently available, mainly based on data obtained in flow chambers and animal studies because clinical studies are lacking. We conclude that molecular imaging of activated endothelium in large- and middle-sized arteries by site-specific accumulation of contrast material is still difficult to achieve due to wall shear stress conditions in these vessels.

An acoustically-driven biochip - impact of flow on the cell-association of targeted drug carriers

The interaction of targeted drug carriers with epithelial and endothelial barriers in vivo is largely determined by the dynamics of the body fluids. To simulate these conditions in binding assays, a fully biocompatible in vitro model was developed which can accurately mimic a wide range of physiological flow conditions on a thumbnail-format cell-chip. This acoustically-driven microfluidic system was used to study the interaction characteristics of protein-coated particles with cells. Poly(D,L-lactide-co-glycolide) (PLGA) microparticles (2.9 +/- 1 microm) were conjugated with wheat germ agglutinin (WGA-MP, cytoadhesive protein) or bovine serum albumin (BSA-MP, non-specific protein) and their binding to epithelial cell monolayers was investigated under stationary and flow conditions. While mean numbers of 1500 +/- 307 mm(-2) WGA-MP and 94 +/- 64 mm(-2) BSA-MP respectively were detected to be cell-bound in the stationary setup, incubation at increasing flow velocities increasingly antagonized the attachment of both types of surface-modified particles. However, while binding of BSA-MP was totally inhibited by flow, grafting with WGA resulted in a pronounced anchoring effect. This was indicated by a mean number of 747 +/- 241 mm(-2) and 104 +/- 44 mm(-2) attached particles at shear rates of 0.2 s(-1) and 1 s(-1) respectively. Due to the compactness of the fluidic chip which favours parallelization, this setup represents a highly promising approach towards a screening platform for the performance of drug delivery vehicles under physiological flow conditions. In this regard, the flow-chip is expected to provide substantial information for the successful design and development of targeted micro- and nanoparticulate drug carrier systems.

Modular Cre/lox system and genetic therapeutics for colorectal cancer

The Cre/lox system is a powerful tool for targeting therapeutic effectors in a wide variety of human disorders. I review a Cre/lox Wnt-targeted system that has shown promise against Wnt-positive colorectal cancer cell lines. In addition to Wnt-specific targeting of cell death inducers, the modular nature of this gene therapy model system can be exploited by designing positive and negative feedback loops to either amplify or inhibit Wnt activity for experimental or therapeutic benefit. I discuss the structural components and performance parameters of the system, the implication of these findings with respect to cancer stem cells, as well as the general applicability of this system to any disorder characterized by differential gene expression. I also consider the issue of gene delivery as well as in vivo testing requirements necessary for the further characterization and development of this system.

Physical approaches to biomaterial design

The development of biomaterials for drug delivery, tissue engineering and medical diagnostics has traditionally been based on new chemistries. However, there is growing recognition that the physical as well as the chemical properties of materials can regulate biological responses. Here, we review this transition with regard to selected physical properties including size, shape, mechanical properties, surface texture and compartmentalization. In each case, we present examples demonstrating the significance of these properties in biology. We also discuss synthesis methods and biological applications for designer biomaterials, which offer unique physical properties.

Synthetic microvascular networks for quantitative analysis of particle adhesion

We have developed a methodology to study particle adhesion in the microvascular environment using microfluidic, image-derived microvascular networks on a chip accompanied by Computational Fluid Dynamics (CFD) analysis of fluid flow and particle adhesion. Microfluidic networks, obtained from digitization of in vivo microvascular topology were prototyped using soft-lithography techniques to obtain semicircular cross sectional microvascular networks in polydimethylsiloxane (PDMS). Dye perfusion studies indicated the presence of well-perfused as well as stagnant regions in a given network. Furthermore, microparticle adhesion to antibody coated networks was found to be spatially non-uniform as well. These findings were broadly corroborated in the CFD analyses. Detailed information on shear rates and particle fluxes in the entire network, obtained from the CFD models, were used to show global adhesion trends to be qualitatively consistent with current knowledge obtained using flow chambers. However, in comparison with a flow chamber, this method represents and incorporates elements of size and complex morphology of the microvasculature. Particle adhesion was found to be significantly localized near the bifurcations in comparison with the straight sections over the entire network, an effect not observable with flow chambers. In addition, the microvascular network chips are resource effective by providing data on particle adhesion over physiologically relevant shear range from even a single experiment. The microfluidic microvascular networks developed in this study can be readily used to gain fundamental insights into the processes leading to particle adhesion in the microvasculature.

Polymeric particles conjugated with a ligand to VCAM-1 exhibit selective, avid, and focal adhesion to sites of atherosclerosis

The increased expression of VCAM-1 on endothelial segments within plaque regions could be used as a target to deliver polymeric drug carriers selectively to sites of atherosclerosis. We probed the hypothesis that polymeric particles conjugated with a ligand for VCAM-1 exhibit selective and avid adhesion to sites of atherosclerosis. Particles made from polystyrene or the biodegradable polymer poly(sebacic acid)-block-polyethylene glycol (PSA-PEG) were conjugated with an antibody to VCAM-1 (alpha-VCAM-1) or IgG (negative control). The particles were injected into the jugular vein of ApoE(-/-) (a murine model of atherosclerosis) or wild type mice and their adhesion to the aorta determined. alpha-VCAM-1 particles exhibited significantly greater adhesion to ApoE(-/-) mouse aorta [32 +/- 5 (mean +/- SEM) particles/mm(2) for polystyrene particles and 31 +/- 7 particles/mm(2) for PSA-PEG particles] compared to the level of adhesion to wild type mouse aorta (18 +/- 1 particles/mm(2) for polystyrene particles and 6 +/- 1 particles/mm(2) for PSA-PEG particles). Within ApoE(-/-) mice, the alpha-VCAM-1 particles exhibited significantly greater adhesion to the aorta (32 +/- 5 particles/mm(2) for polystyrene particles and 31 +/- 7 particles/mm(2) for PSA-PEG particles) compared to the adhesion of IgG particles (1 +/- 1 particles/mm(2) for polystyrene particles and 2 +/- 1 particles/mm(2) for PSA-PEG particles). Detailed analysis of the adhesion revealed that alpha-VCAM-1 particles exhibited focal adhesion to plaque regions, in particular the periphery of the plaques, within the ApoE(-/-) mouse aorta. Combined the data demonstrate that polymeric particles conjugated with a ligand to VCAM-1 exhibit selective, avid and focal adhesion to sites of atherosclerosis providing strong evidence that VCAM-1 ligand bearing polymeric particles could be used for targeting drugs selectively to atherosclerotic tissue.

Intravascular delivery of particulate systems: does geometry really matter?

In cancer therapy and imaging, the systemic passive delivery of particulate systems has relied on the enhanced permeability and retention (EPR) effect: sufficiently small particles can cross the endothelial fenestrations and accumulate in the tumor parenchyma. The vast majority of man-made particulates exhibit a spherical shape as a result of surface energy minimization during their synthesis. The advent of phage display libraries, which are revealing the extraordinary molecular diversity of endothelial cells, and the development of processes for fabricating particles with shapes other than spherical are opening the path to new design solutions for systemically administered targeted particulates. In this paper, the role of particle geometry (i.e., size and shape) is discussed at the tissue and cellular scales. Emphasis is placed on how the synergistic effect of particle geometry and molecular targeting can enhance the specificity of delivery. The intravascular delivery process has been broken into three events: margination, firm adhesion and control of internalization. Predictions from mathematical models and observations from in-vitro experiments were used to show the relevance of particle geometry in systemic delivery. Rational design of particulate systems should consider, beside the physico-chemical properties of the surface coatings, geometrical features as size and shape. The integration of mathematical modeling with in-vitro and in-vivo testing provides the tools for establishing a rational design of nanoparticles.

The effect of shape on the margination dynamics of non-neutrally buoyant particles in two-dimensional shear flows

The margination dynamics of microparticles with different shapes has been analyzed within a laminar flow mimicking the hydrodynamic conditions in the microcirculation. Silica spherical particles, quasi-hemispherical and discoidal silicon particles have been perfused in a parallel plate flow chamber. The effect of the shape and density on their margination propensity has been investigated at different physiologically relevant shear rates S. Simple scaling laws have been derived showing that the number n of marginating particles scales as S(-0.63) for the spheres; S(-0.85) for discoidal and S(-1) for quasi-hemispherical particles, regardless of their density and size. Within the range considered for the shear rate, discoidal particles marginate in a larger number compared to quasi-hemispherical and spherical particles. These results may be of interest in drug delivery and bio-imaging applications, where particles are expected to drift towards and interact with the walls of the blood vessels.

Perfluorohexane-loaded macrophages as a novel ultrasound contrast agent: a feasibility study

PURPOSE

We investigated in vitro the potential of macrophages to act as targeted vehicle for ultrasound molecular imaging.

PROCEDURES

Murine bone marrow-derived macrophages (BMM), incubated for 3 h with different concentrations of perfluorohexane (PFH) emulsions, were monitored by microscopy, flow cytometry, and ultrasound. Effects of PFH loading on BMM adhesion molecule (PSGL-1, VLA-4, Mac-1, LFA-1) expression were analyzed by flow cytometry. Static adhesion of PFH loaded BMM to unstimulated and TNF-alpha stimulated b.End5 endothelial cells was assessed by microscopy.

RESULTS

Incubation of BMM with PFH emulsions resulted in dose-dependent uptake and increased echogenicity (max. 17 dB). Flow cytometry analyses revealed no down-regulation related to PFH loading of BMM adhesion molecule expression. Endothelial adhesion remained functional, even after 24 h, although PFH loading dose-dependently attenuated static adhesion.

CONCLUSION

PFH loaded BMM may potentially serve as ultrasound contrast agent for noninvasive detection of atherogenic hotspots in arteries.

Flow chamber analysis of size effects in the adhesion of spherical particles

The non-specific adhesion of spherical micro- and nano-particles to a cell substrate is investigated in a parallel plate flow chamber. Differently from prior in-vitro analyses, the total volume of the particles injected into the flow chamber is kept fixed whilst the particle diameter is changed in the range 0.5-10 microm. It is shown that: (i) the absolute number of particles adherent to the cell layer per unit surface decreases with the size of the particle as d(-1.7); (ii) the volume of the particles adherent per unit surface increases with the size of the particles as d(+1.3). From these results and considering solely non-specific particles, the following hypothesis are generated (i) use the smallest possible particles in biomedical imaging and (ii) use the largest possible particles in drug delivery.

Particle shape: a new design parameter for micro- and nanoscale drug delivery carriers

Encapsulation of therapeutic agents in polymer particles has been successfully used in the development of new drug carriers. A number of design parameters that govern the functional behavior of carriers, including the choice of polymer, particle size and surface chemistry, have been tuned to optimize their performance in vivo. However, particle shape, which may also have a strong impact on carrier performance, has not been thoroughly investigated. This is perhaps due to the limited availability of techniques to produce non-spherical polymer particles. In recent years, a number of reports have emerged to directly address this bottleneck and initial studies have indeed confirmed that particle shape can significantly impact the performance of polymer drug carriers. This article provides a review of this field with respect to methods of particle preparation and the role of particle shape in drug delivery.

Characterization of the in vitro adherence behavior of ultrasound responsive double-shelled microspheres targeted to cellular adhesion molecules

We have developed novel adhesion molecule-targeted double-shelled microspheres which encapsulate nitrogen. We report in vitro targeting studies utilizing these microspheres conjugated to target-specific antibodies directed towards ICAM-1 and VCAM-1. In static adherence experiments, the adherence patterns of microspheres conjugated to three different monoclonal antibodies (two targeted to ICAM-1 and one to VCAM-1) to their target surfaces were very different. Maximum microsphere adherence at the lowest target and/or ligand densities was observed with the VCAM-1 system. Differences in target-specific adherence were also observed between anti-ICAM-1 and anti-VCAM-1 microsphere conjugates in flow adherence studies. Equilibrium binding studies of the target proteins in solution to the microsphere-bound ligands showed that the affinity constants of two microsphere-bound monoclonal antibodies for their target proteins are similar. Thus, ligand-target affinity is not the only determinant of microsphere adherence to the target surface in our systems. Shear stress was found to have an effect on the mean diameter of adhered microspheres; a decrease in the mean diameter with increasing shear was observed. The magnitude of this effect was dependent on both microsphere-bound ligand and target surface densities, with a more pronounced change at lower densities. Adhered microspheres were readily detectable using ultrasound at the lowest tested surface density of 40 mm(-2).

Microparticle adhesive dynamics and rolling mediated by selectin-specific antibodies under flow

In vitro studies were performed to characterize the relative performance of candidate receptors to target microparticles to inflammatory markers on vascular endothelium. To model the interactions of drug-bearing microparticles or imaging contrast agents with the vasculature, 6 micron polystyrene particles bearing antibodies, peptides, or carbohydrates were perfused over immobilized E- or P-selectin in a flow chamber. Microparticles conjugated with HuEP5C7.g2 (HuEP), a monoclonal antibody (mAb) specific to E- and P-selectin, supported leukocyte-like rolling and transient adhesion at venular shear rates. In contrast, microparticles conjugated with a higher affinity mAb specific for P-selectin (G1) were unable to form bonds at venular flow rates. When both HuEP and G1 were conjugated to the microparticle, HuEP supported binding to P-selectin in flow which allowed G1 to form bonds leading to stable adhesion. While the microparticle attachment and rolling performance was not as stable as that mediated by the natural ligands P-selectin Glycoprotein Ligand-1 or sialyl Lewis(x), HuEP performed significantly better than any previously characterized mAb in terms of mediating microparticle binding under flow conditions. HuEP may be a viable alternative to natural ligands to selectins for targeting particles to inflamed endothelium.

Deformable gas-filled microbubbles targeted to P-selectin

Ultrasound contrast microbubbles have been successfully targeted to a number of intravascular disease markers. We hypothesized that targeted delivery could be improved further, by making the microbubbles deformable, leading to increased microbubble-endothelium adhesion contact area and stabilized adhesion. Activated leukocytes utilize such strategy; they deform after binding to inflamed endothelium in the vasculature. Lipid-shell microbubbles were targeted to the endothelial inflammatory protein P-selectin with a monoclonal anti-P-selectin antibody attached to the microbubble shell. Deformable microbubbles were created by controlled pressurization with partial gas loss, which generated an average excess shell surface area of approximately 30% and the formation of outward-projected wrinkles and folds. Targeted microbubble adhesion and deformability were assessed in the parallel plate flow chamber under shear flow. Sustained adhesion of deformable microbubbles at wall shear stresses between 0.4 and 1.35 dyn/cm(2) was consistently better than adhesion of wrinkle-free microbubbles. Over this shear range, targeted wrinkled microbubbles were deformed by shear flow, unlike wrinkle-free microbubbles. In a murine cremaster inflammation model, a significant improvement of deformable microbubble targeting was observed by intravital microscopy. Overall, the mechanical aspects of adhesion, such as particle shape, deformability and surface microstructure, are important in engineering efficient site-targeted particle-based agents for medical imaging and therapy.

The adhesive strength of non-spherical particles mediated by specific interactions

The specific adhesive interaction between a non-spherical particle and a cell layer under a linear shear flow is analyzed. The effect of the characteristic particle size, expressed in terms of the volume V, and shape, expressed in terms of the aspect ratio gamma, on the adhesive strength is investigated. It is shown that for a fixed shape, there exists an optimal volume V(opt) for which the adhesive strength has a maximum. A surprisingly accurate relationship has been derived between the optimal volume V(opt) and the ratio microS/m(r) (wall shear stress to the receptors surface density) having the form V(opt)=alpha(m(r)/microS)(beta). Also, oblate particles have been shown to adhere more effectively to the biological substrate than classical spherical particles for the same volume V. As a consequence, non-spherical particles can carry a larger amount of drugs and contrast agents than classical spherical particles with the same adhesive strength, improving the therapeutic and imaging efficacy. The formulae and the procedures described in the present work can guide the optimal design of intravascularly injectable micro/nano carriers.