Theoretical and computational studies of dendrimers as delivery vectors

Equilibrium and dynamical properties of polymer chains in random medium filled with randomly distributed nano-sized fillers

The effect of randomly distributed nano-sized fillers on the equilibrium and dynamical properties of linear polymers is studied by using off-lattice Monte Carlo simulation.

Synthesis and self-assembly behavior of POSS-embedded hyperbranched polymers

We demonstrate a simple approach to prepare POSS-embedded hyperbranched amphiphiles, presenting morphological transition from micelle to vesicle in aqueous solution.

Active targeted drug delivery for microbes using nano-carriers

Although vaccines and antibiotics could kill or inhibit microbes, many infectious diseases remain difficult to treat because of acquired resistance and adverse side effects. Nano-carriers-based technology has made significant progress for a long time and is introducing a new paradigm in drug delivery. However, it still has some challenges like lack of specificity toward targeting the infectious site. Nanocarriers utilized targeting ligands on their surface called 'active target' provide the promising way to solve the problems like accelerating drug delivery to infectious areas and preventing toxicity or side-effects. In this mini review, we demonstrate the recent studies using the active targeted strategy to kill or inhibit microbes. The four common nano-carriers (e.g. liposomes, nanoparticles, dendrimers and carbon nanotubes) delivering encapsulated drugs are introduced.

Photodynamic therapy of oligoethylene glycol dendronized reduction-sensitive porphyrins

OEGylation of porphyrins via a disulfide linkage to form a novel class of dendritic porphyrin photosensitizers (PSs) is presented.

Association of small aromatic molecules with PAMAM dendrimers

Dendrimer pockets enable association by reducing naphthalene hydration even near the dendrimer periphery.

Molecular Modeling to Study Dendrimers for Biomedical Applications

Molecular modeling techniques provide a powerful tool to study the properties of molecules and their interactions at the molecular level. The use of computational techniques to predict interaction patterns and molecular properties can inform the design of drug delivery systems and therapeutic agents. Dendrimers are hyperbranched macromolecular structures that comprise repetitive building blocks and have defined architecture and functionality. Their unique structural features can be exploited to design novel carriers for both therapeutic and diagnostic agents. Many studies have been performed to iteratively optimise the properties of dendrimers in solution as well as their interaction with drugs, nucleic acids, proteins and lipid membranes. Key features including dendrimer size and surface have been revealed that can be modified to increase their performance as drug carriers. Computational studies have supported experimental work by providing valuable insights about dendrimer structure and possible molecular interactions at the molecular level. The progress in computational simulation techniques and models provides a basis to improve our ability to better predict and understand the biological activities and interactions of dendrimers. This review will focus on the use of molecular modeling tools for the study and design of dendrimers, with particular emphasis on the efforts that have been made to improve the efficacy of this class of molecules in biomedical applications.

Emerging concepts in dendrimer-based nanomedicine: from design principles to clinical applications

Dendrimers are discrete nanostructures/nanoparticles with 'onion skin-like' branched layers. Beginning with a core, these nanostructures grow in concentric layers to produce stepwise increases in size that are similar to the dimensions of many in vivo globular proteins. These branched tree-like concentric layers are referred to as 'generations'. The outer generation of each dendrimer presents a precise number of functional groups that may act as a monodispersed platform for engineering favourable nanoparticle-drug and nanoparticle-tissue interactions. These features have attracted significant attention in medicine as nanocarriers for traditional small drugs, proteins, DNA/RNA and in some instances as intrinsically active nanoscale drugs. Dendrimer-based drugs, as well as diagnostic and imaging agents, are emerging as promising candidates for many nanomedicine applications. First, we will provide a brief survey of recent nanomedicines that are either approved or in the clinical approval process. This will be followed by an introduction to a new 'nanoperiodic' concept which proposes nanoparticle structure control and the engineering of 'critical nanoscale design parameters' (CNDPs) as a strategy for optimizing pharmocokinetics, pharmocodynamics and site-specific targeting of disease. This paradigm has led to the emergence of CNDP-directed nanoperiodic property patterns relating nanoparticle behaviour to critical in vivo clinical translation issues such as cellular uptake, transport, elimination, biodistribution, accumulation and nanotoxicology. With a focus on dendrimers, these CNDP-directed nanoperiodic patterns are used as a strategy for designing and optimizing nanoparticles for a variety of drug delivery and imaging applications, including a recent dendrimer-based theranostic nanodevice for imaging and treating cancer. Several emerging preclinical dendrimer-based nanotherapy concepts related to inflammation, neuro-inflammatory disorders, oncology and infectious and ocular diseases are reviewed. Finally we will consider challenges and opportunities anticipated for future clinical translation, nanotoxicology and the commercialization of nanomedicine.

Combined EPR and molecular modeling study of PPI dendrimers interacting with copper ions: effect of generation and maltose decoration

Understanding the early onset of neurodegeneration is crucial to deploy specific treatments for patients before the process becomes irreversible. Copper has been proposed as a biomarker for many neurodegenerative disorders, being the ion released by pathologically unfolded proteins involved in many biochemical pathways. Dendrimers are macromolecules that bind metal ions with a large ion/ligand ratio, thus, allowing a massive collection of copper. This work provides structural information, obtained via molecular modeling and EPR, for the binding sites of copper in polypropyleneimine (PPI) dendrimers, especially in the maltose decorated form that has potential applications in diagnosis and therapies for various types of neurodegenerations. The analysis of the EPR spectra showed that, at the lowest Cu concentrations, the results are well supported by the calculations. Moreover, EPR analysis at increasing Cu(II) concentration allowed us to follow the saturation behavior of the interacting sites identified by the modeling study.

Systemic gene silencing in plants triggered by fluorescent nanoparticle-delivered double-stranded RNA

A cationic fluorescence nanoparticle efficiently enters plants with high transfection efficacy. Applying a mixture of G2/dsRNA to the model plant, Arabidopsis root, leads to significant reduction in the expression of important developmental genes and results in apparent phenotypes. This study reports a non-viral gene nanocarrier which triggers gene silencing in plants and leads to systemic phenotypes.

Charged dendrimers in trivalent salt solutions under the action of DC electric fields

The structural properties and electrophoretic mobility of charged dendrimers in 3:1 electrolyte solutions subjected to direct current electric fields are studied using molecular dynamics simulations. The simulated dendrimer size is studied in zero fields and found to scale as R(g) ∼ N(0.29). The dendrimers exhibit shape distortions when the applied electric field is larger than some critical value, which scales with the number of dendrimer monomers as E(z,crit) ∼ N(0.39(6)). Families of curves, such as the curves of the square of radius of gyration, the asphericity, the degree of prolateness, and the electrophoretic mobility of dendrimers, are shown to collapse to single, master curves in electric fields through appropriate scaling. This reflects the fractal characteristics of these systems. The density profile of the surface monomers and salt cations reveals two pronounced combination effects between the polarization of dendrimer complexes and stripping-off of the condensed salt cations from the dendrimer surface.

Translocation of a nanoparticle through a fluidic channel: the role of grafted polymers

The surface properties of nanoparticles (NPs) are key factors for their design and use in biomedicine; however, our understanding of the effect of surface properties on the translocation of NPs through membranes is still rather poor. Herein, we have used molecular dynamics simulations to study the translocation of a polymer-grafted NP through a fluidic channel. We change the length, number, amount of charge and the charge position of grafted polymers. With the increase of polymer length, the NP flux decreases as a whole due to the increase of NP size, where the -NP translocation fails at the smallest polymer length, because of the strong binding of Na(+). Surprisingly, the NP flux exhibits a maximum with the increase of the polymer number or charge amount, which is co-determined by the NP net charge and size. Owing to the NP-membrane adsorption and NP-ion binding, the NP flux decreases with the decrease of charge position. We also analyze the transport of counterions, which depends on both the NP-ion binding and NP dynamics. Finally, we investigate the effect of electric fields for a given NP type. Our results reveal the important role of grafted polymers in the NP translocation and may have implications in the design of highly efficient NP delivery.

Dendrimer-surfactant interactions

In this article, we reviewed the interactions between dendrimers and surfactants with particular focus on the interaction mechanisms and physicochemical properties of the yielding dendrimer-surfactant aggregates. In order to provide insight into the behavior of dendrimers in biological systems, the interactions of dendrimers with bio-surfactants such as phospholipids in bulk solutions, in solid-supported bilayers and at the interface of phases or solid-states were discussed. Applications of the dendrimer-surfactant aggregates as templates to guide the synthesis of nanoparticles and in drug or gene delivery were also mentioned.

A numerical study of the phase behaviors of drug particle/star triblock copolymer mixtures in dilute solutions for drug carrier application

The complex microstructures of drug particle/ABA star triblock copolymer in dilute solutions have been investigated by a theoretical approach which combines the self-consistent field theory and the hybrid particle-field theory. Simulation results reveal that, when the volume fraction of drug particles is smaller than the saturation concentration, the drug particle encapsulation efficiency is 100%, and micelle loading capacity increases with increasing particle volume fraction. When the volume fraction of drug particles is equal to the saturation concentration, the micelles attain the biggest size, and micelle loading capacity reaches a maximum value which is independent of the copolymer volume fraction. When the volume fraction of drug particles is more than the saturation concentration, drug particle encapsulation efficiency decreases with increasing volume fraction of drug particles. Furthermore, it is found that the saturation concentration scales linearly with the copolymer volume fraction. The above simulation results are in good agreement with experimental results.

The interaction between the outer layer of a mixed ion pair amphiphile/double-chained cationic surfactant vesicle and DNA: a Langmuir monolayer study

The charge density of vesicular bilayers plays an important role in the structure characteristic of the vesicle-DNA complex for gene delivery. In this work, the charge density effect of catanionic vesicle surfaces on the association behavior of the vesicle with DNA was explored with the model Langmuir monolayer approach. The interaction of negatively charged DNA with positively charged Langmuir monolayers composed of catanionic vesicle-forming materials, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) and dihexadecyldimethylammonium bromide (DHDAB), was investigated with surface pressure-area isotherms, area-time relaxation curves and Brewster angle microscope images. The results showed that the adsorption of DNA molecules onto the monolayers was enhanced with an increased DHDAB molar fraction (XDHDAB), which was apparently related to the increased charge density of the monolayers. With XDHDAB being increased up to 0.5, the mixed monolayers with a higher XDHDAB, or higher charge density, possessed a more stable characteristic at high surface pressures, at which the molecular status was close to that in a corresponding vesicular bilayer, due to the DHDAB-improved molecular packing/interaction. It was found that the composition of the mixed HTMA-DS-DHDAB monolayers at high surface pressures would be affected by the adsorbed DNA with the extent depending on XDHDAB. For the formation of stable HTMA-DS-DHDAB monolayer-DNA complexes, a strong electrostatic interaction of DNA with a monolayer of high charge density and a high monolayer stability characteristic resulting from DHDAB-improved molecular packing/interaction were thus required. The finding has an implication for the formulation of catanionic vesicles composed of an ion pair amphiphile, HTMA-DS, with DHDAB in gene delivery applications.

Pharmacoinformatic approaches to understand complexation of dendrimeric nanoparticles with drugs

Nanoparticle based drug delivery systems are gaining popularity due to their wide spectrum advantages over traditional drug delivery systems; among them, dendrimeric nano-vectors are the most widely explored carriers for pharmaceutical and biomedical applications. The precise mechanism of encapsulation of drug molecules inside the dendritic matrix, delivery of drugs into specific cells, interactions of nano-formulation with biological targets and proteins, etc. present a substantial challenge to the scientific understanding of the subject. Computational methods complement experimental techniques in the design and optimization of drug delivery systems, thus minimizing the investment in drug design and development. Significant progress in computer simulations could facilitate an understanding of the precise mechanism of encapsulation of bioactive molecules and their delivery. This review summarizes the pharmacoinformatic studies spanning from quantum chemical calculations to coarse-grained simulations, aimed at providing better insight into dendrimer-drug interactions and the physicochemical parameters influencing the binding and release mechanism of drugs.

Surface-engineered dendrimers with a diaminododecane core achieve efficient gene transfection and low cytotoxicity

Cationic dendrimers are widely used as gene vectors; however, these materials are usually associated with unsatisfied transfection efficiency and biocompatibility. In this study, we used an aliphatic hydrocarbon-cored polyamidoamine (PAMAM) dendrimer as an alternative to traditional cationic PAMAM dendrimers in the design of efficient gene vectors. Diaminododecane-cored generation 4 (C12G4) PAMAM dendrimer showed dramatically higher efficacy in luciferase and EGFP gene transfection than diaminoethane-cored generation 4 (C2G4) and diaminohexane-cored generation 4 (C6G4) PAMAM dendrimers. The viability of cells incubated with C12G4 at transfection concentrations is above 90%. The significantly improved gene transfection efficacy of C12G4 is attributed to the hydrophobic core of C12G4 which increases the cellular uptake of dendrimer/DNA polyplexes. Further modification of C12G4 with functional ligands such as arginine, 2,4-diamino-1,3,5-triazine, and fluorine compounds significantly increase its transfection efficiency on several cell lines. These results suggest that diaminododecane-cored dendrimers can be developed as a versatile scaffold in the design of efficient gene vectors.

Interaction of fullerene chains and a lipid membrane via computer simulations

Coarse-grained molecular dynamics simulations were employed to study the fullerene polymers with various functionalization degrees interacting with the DPPC membrane. Structure, dynamics, and thermodynamics of systems were analyzed.

Size and diffusion of polymer in media filled with periodic fillers

The effect of nanosized fillers on the equilibrium and dynamic properties of a single polymer chain has been studied by using off-lattice Monte Carlo (MC) simulation. Fillers of identical size are arranged periodically in the system and the Lennard-Jones (LJ) interaction is considered between the polymer and fillers. Our results show that the statistical size and dynamic diffusion properties of the polymer are not only dependent on the size of the polymer relative to the size of fillers and the distance between fillers, but also dependent on the interaction between the polymer and filler. The statistical size of the polymer can increase or decrease. Normal diffusion is always observed for long polymers and small fillers, whereas a transition from a desorbed state to an adsorbed state is observed for short polymers and large fillers. Finally, the size and diffusion of the polymer on an infinitely large surface are studied for comparison.

Dynamics of a polymer adsorbed to an attractive homogeneous flat surface

Polymer conformation and statistical sizes change with the surface contact number during the adsorption.

Reciprocal effects of the chirality and the surface functionalization on the drug delivery permissibility of carbon nanotubes

The drug delivery admissibility of nanomaterials such as carbon nanotubes and their uncertain interactions with live tissues and organs have sparked ongoing research efforts. To boost the selective diffusivity of single walled carbon nanotubes (SWCNTs), surface functionalization was adopted in several experimental attempts. Numerous studies had identified polyethylene glycol (PEG) as a bio-compatible surfactant to carbon nanotubes. In this study, a large scale, atomistic molecular dynamic simulation was utilized to disclose the cellular exposure and uptake mechanisms of PEG-functionalized single walled carbon nanotubes (f-SWCNTs) into a lipid bilayer cell membrane. Results showed that with PEGs attached to a SWCNT, the penetration depth and speed can be controlled. Also, the simulations revealed that the adhesion energy between the nanotube and the lipid membrane is affected considerably, in the presence of PEGs, by the chirality of the SWCNTs.

Unique dynamical approach of fully wrapping dendrimer-like soft nanoparticles by lipid bilayer membrane

Wrapping dendrimer-like soft nanoparticles by cell membrane is an essential event in their endocytosis in drug and gene delivery, but this process remains poorly elucidated. Using computer simulations and theoretical analysis, we report the detailed dynamics of the process in which a lipid bilayer membrane fully wraps a dendrimer-like soft nanoparticle. By constructing a phase diagram, we firstly demonstrate that there exist three states in the interaction between a dendrimer and a lipid bilayer membrane, i.e., penetration, penetration and partial wrapping, and full wrapping states. The wrapping process of dendrimer-like nanoparticles is found to take a unique approach where the penetration of the dendrimer into the membrane plays a significant role. The analysis of various energies within the system provides a theoretical justification to the state transition observed from simulations. The findings also support recent experimental results and provide a theoretical explanation for them. We expect that these findings are of immediate interest to the study of the cellular uptake of dendrimer-like soft nanoparticles and can prompt the further application of this class of nanoparticles in nanomedicine.

Orientational relaxation in semiflexible dendrimers

The orientational relaxation dynamics of semiflexible dendrimers are theoretically calculated within the framework of optimized Rouse-Zimm formalism. Semiflexibility is modeled through appropriate restrictions in the direction and orientation of the respective bond vectors, while the hydrodynamic interactions are included via the preaveraged Oseen tensor. The time autocorrelation function M(i)(1)(t) and the second order orientational autocorrelation function P(i)(2)(t) are analyzed as a function of the branch-point functionality and the degree of semiflexibility. Our approach of calculating M(i)(1)(t) is completely different from that of the earlier studies (A. Perico and M. Guenza J. Chem. Phys., 1985, 83, 3103; J. Chem. Phys., 1986, 84, 510), where the expression of M(i)(1)(t) obtained from earlier studies does not demarcate the flexible dendrimers from the semiflexible ones. The component of global motion of the time autocorrelation function exhibits a strong dependence on both degree of semiflexibility and branch-point functionality, while the component of pulsation motion depends only on the degree of semiflexibility. But it is difficult to distinguish the difference in the extent of pulsation motion among the compressed (0 < φ < π/2) and expanded (π/2 < φ < π) conformations of semiflexible dendrimers. The qualitative behavior of P(i)(2)(t) obtained from our calculations closely matches with the expression for P(exact)(2)(t) in the earlier studies. Theoretically calculated spectral density, J(ω), is found to depend on the degree of semiflexibility and the branch-point functionality for the compressed and expanded conformations of semiflexible dendrimers as a function of frequency, especially in the high frequency regime, where J(ω) decays with frequency for both compressed and expanded conformations of semiflexible dendrimers. This decay of the spectral density occurs after displaying a cross-over behavior with the variation in the degree of semiflexibility in the intermediate frequency regime. The characteristic area increases with the increase in the semiflexibility parameter, where the expanded conformations of semiflexible dendrimers record the maximum characteristic area. For the compressed conformations the relative increment of this area is considerably lower than that of the expanded conformations of semiflexible dendrimers.

Enhanced cellular transfection by ternary non-viral gene vectors coupled with adeno-associated virus-derived peptides

The establishment of efficient and safe gene delivery systems is crucial for biomedical applications. To address this objective, novel, ternary hybrid gene vectors are designed with viral capsid peptides in non-viral gene carriers. The viral peptide, TQVGQKT, is coupled with a membrane active peptide, LK15. Which acts as a linker to tag peptide with plasmid DNA. Additionally, polyethylenimine (PEI) is employed to condense the complexes further, thereby forming ternary DNA/TQVGQKT-LK15/PEI complexes. The ternary complexes result in rapid internalization leading to significantly enhanced cellular transfection. The new moiety, TQVGQKT, as well as enhanced cellular transfection, will certainly provide crucial insights for the design of novel non-viral gene carriers with efficient and safe properties.

Design maps for cellular uptake of gene nanovectors by computer simulation

Understanding how nanovectors transport DNA molecules through cell membranes is of great importance in gene therapy. In this paper, we systematically investigate the mechanism of cellular uptake of cationic polymeric nanovectors containing DNA molecules through dissipative particle dynamics simulations. Our results show that the property of polyelectrolyte chains grafted to nanovector and DNA molecules can have important impacts on the endocytosis. Interestingly, it is found that the nanovector can be fully taken up with proper number of DNA molecules on its surface. On the contrary, in the absence of DNA it may become harder to be totally engulfed. Since the adsorption number of DNA is related to external pH, the cellular uptake could exhibit pH-responsive behavior. Further, we also provide insights into the comparison of uptake behaviors between cancer and normal cells, and importantly, we find that the enhanced uptake of gene nanovectors may be an inherent property of cancer cells. The present study may give some significant suggestions on future nanovector design for gene delivery.

Dendrimers in layer-by-layer assemblies: synthesis and applications

We review the synthesis of dendrimer-containing layer-by-layer (LbL) assemblies and their applications, including biosensing, controlled drug release, and bio-imaging. Dendrimers can be built into LbL films and microcapsules by alternating deposition of dendrimers and counter polymers on the surface of flat substrates and colloidal microparticles through electrostatic bonding, hydrogen bonding, covalent bonding, and biological affinity. Dendrimer-containing LbL assemblies have been used to construct biosensors, in which electron transfer mediators and metal nanoparticles are often coupled with dendrimers. Enzymes have been successfully immobilized on the surface of electrochemical and optical transducers by forming enzyme/dendrimer LbL multilayers. In this way, high-performance enzyme sensors are fabricated. In addition, dendrimer LbL films and microcapsules are useful for constructing drug delivery systems because dendrimers bind drugs to form inclusion complexes or the dendrimer surface is covalently modified with drugs. Magnetic resonance imaging of cancer cells by iron oxide nanoparticles coated with dendrimer LbL film is also discussed.

Computational design principles for the discovery of bioactive dendrimers: [s]-triazines and other examples

INTRODUCTION

Chemistry yields dendrimers of many classes and compositions. Translating this synthetic success to bioactivity is significantly aided by the use of computational modeling and our knowledge of the three-dimensional shapes of these macromolecules.

AREAS COVERED

This review discusses the lessons learned during the investigations of [s]-triazine dendrimers. Specifically, the article focuses on the evolving role that computational models have taken in the exploration of these macromolecules. These lessons, furthermore, can be generalized across many dendrimer classes.

EXPERT OPINION

Computational models and the resulting structural data from molecular dynamics simulations provide insights into: shape, solvent penetration, shielding of biolabile linkers, and the density of hydrophobic patches. These models have evolved from artistic representations, through bases for rationalization, to hypothesis-generating tools that drive synthesis. With further advances expected in both software and hardware the answer to the question, 'What does a specific dendrimer look like in solution?' is becoming increasingly clear. Moreover, the authors believe that answer to this question lies at the heart of the design of bioactive dendrimers.

Novel gene delivery systems

Gene therapy is an emerging field in medical and pharmaceutical sciences because of its potential in treating chronic diseases like cancer, viral infections, myocardial infarctions, and genetic disorders. Application of gene therapy is limited because of lack of suitable methods for proper introduction of genes into cells and therefore, this is an area of interest for most of the researchers. To achieve successful gene therapy, development of proper gene delivery systems could be one of the most important factors. Several nonviral and viral gene transfer methods have been developed. Even though the viral agents have a high transferring efficiency, they are difficult to handle due to their toxicity. To overcome the safety problems of the viral counterpart, several nonviral in vitro and in vivo gene delivery systems are developed. Out of these, the most promising and latest systems include polymer-based nonviral gene carriers, dendrimers, and physical means like electroporation, microinjection, etc., Shunning of possible immunogenicity and toxicity, and the feasibility of repeated administration are some of the merits of nonviral gene delivery systems over viral gene delivery. An ideal nonviral gene carrying system should possess all these merits without any compromise to its gene transferring efficiency. The viral gene delivery systems include lytic and nonlytic vectors for drug delivery. Inspite of its toxicity they are still preferred because of their long term expression, stability, and integrity. This review explores the recent developments and relevancy of the novel gene delivery systems in gene therapy.