Surface-grafted Polyethylene Glycol Research Articles

Opsonin-Deficient Nucleoproteic Corona Endows UnPEGylated Liposomes with Stealth Properties In Vivo.

For several decades, surface grafted polyethylene glycol (PEG) has been a go-to strategy for preserving the synthetic identity of liposomes in physiological milieu and preventing clearance by immune cells. However, the limited clinical translation of PEGylated liposomes is mainly due to the protein corona formation and the subsequent modification of liposomes’ synthetic identity, which affects their interactions with immune cells and blood residency. Here we exploit the electric charge of DNA to generate unPEGylated liposome/DNA complexes that, upon exposure to human plasma, gets covered with an opsonin-deficient protein corona. The final product of the synthetic process is a biomimetic nanoparticle type covered by a proteonucleotidic corona, or “proteoDNAsome”, which maintains its synthetic identity in vivo and is able to slip past the immune system more efficiently than PEGylated liposomes. Accumulation of proteoDNAsomes in the spleen and the liver was lower than that of PEGylated systems. Our work highlights the importance of generating stable biomolecular coronas in the development of stealth unPEGylated particles, thus providing a connection between the biological behavior of particles in vivo and their synthetic identity.

Improvement of surface hydrophilicity and biological sample-compatibility of molecularly imprinted polymer microspheres by facile surface modification with α-cyclodextrin

Molecularly imprinted polymers (MIPs) compatible with complex biological samples hold much promise in many bioanalytical and biomedical applications and development of new strategies for obtaining such advanced synthetic receptors is thus of high interest. Herein, a facile and efficient new approach is presented for the preparation of MIP microspheres capable of selectively recognizing small organic analyte in the complex biological sample, which involves the simple surface modification of MIP particles bearing polyethylene glycol (PEG) brushes with α-cyclodextrin (α-CD). The proof-of-principle was demonstrated by the first synthesis of propranolol-imprinted polymer microspheres bearing PEG brushes of different molecular weights (Mn = 2000 and 5000) via our previously developed reversible addition-fragmentation chain transfer (RAFT) coupling chemistry, their supramolecular inclusion complexation with α-CD, and subsequent chemical crosslinking. The chain length of the surface-grafted PEG brushes showed much influence on the water-compatibility of MIPs. Whereas the MIP particles bearing PEG brushes of Mn = 2000 hardly showed specific template bindings in both pure water and pure milk, those bearing PEG brushes of Mn = 5000 exhibited moderate specific template bindings in pure water. Nevertheless, the latter MIP particles still failed to show enough specific template bindings in pure milk. The inclusion of α-CD onto PEG brushes proved to largely enhance the surface hydrophilicity of MIPs and lead to their much improved water-compatibility. In particular, the MIP microspheres bearing α-CD-modified PEG brushes of Mn = 5000 showed excellent specific template bindings in both pure water and pure milk. The above strategy not only paves the way for efficiently preparing biological sample-compatible MIPs, but also opens the possibility of generating many other advanced biomaterials with high antifouling properties.

Efficient Electrosteric Assembly of Nanoparticle Heterodimers and Linear Heteroassemblies.

Bottom-up approaches to the synthesis of nanostructures are of particular interest because they offer several advantages over the traditional top-down approaches. In this work, we present a new method to self-assemble nanoparticles into controlled heteroaggregates. The technique relies on carefully balancing attractive electrostatic forces with repulsive steric hindrance that is provided by surface-grafted polyethylene glycol (PEG). Two different-sized gold nanoparticles (GNPs) were used as a model system: 13 nm GNPs were functionalized with PEG-thiol and mercapto dodecanoic acid, while 7 nm GNPs were functionalized with PEG-thiol and (11- Mercaptoundecyl)trimethylammonium bromide. When mixed together, these oppositely charged particles self-assemble into stable colloidal structures (i.e., nanoclusters) whose structure depends strongly on the surface concentration of PEG. Smaller structures are obtained as the PEG surface concentration increases because steric hindrance dominates and prevents uncontrolled aggregation. In particular, under the right conditions, we were able to selectively synthesize heterodimers (which are effectively Janus particles) and linear heteroassemblies. This method is scalable, and it provides a step forward in bottom-up synthesis of nanomaterials.

PEGylated Silk Nanoparticles for Anticancer Drug Delivery.

Silk has a robust clinical track record and is emerging as a promising biopolymer for drug delivery, including its use as nanomedicine. However, silk-based nanomedicines still require further refinements for full exploitation of their potential; the application of "stealth" design principals is especially necessary to support their evolution. The aim of this study was to develop and examine the potential of PEGylated silk nanoparticles as an anticancer drug delivery system. We first generated B. mori derived silk nanoparticles by driving β-sheet assembly (size 104 ± 1.7 nm, zeta potential -56 ± 5.6 mV) using nanoprecipitation. We then surface grafted polyethylene glycol (PEG) to the fabricated silk nanoparticles and verified the aqueous stability and morphology of the resulting PEGylated silk nanoparticles. We assessed the drug loading and release behavior of these nanoparticles using clinically established and emerging anticancer drugs. Overall, PEGylated silk nanoparticles showed high encapsulation efficiency (>93%) and a pH-dependent release over 14 days. Finally, we demonstrated significant cytotoxicity of drug loaded silk nanoparticles applied as single and combination nanomedicines to human breast cancer cells. In conclusion, these results, taken together with prior silk nanoparticle data, support a viable future for silk-based nanomedicines.

Preparation of compact nanoparticle clusters from polyethylene glycol-coated gold nanoparticles by fine-tuning colloidal interactions.

Low-molecular weight polyethylene glycol (PEG) has a lower critical solution temperature well outside the boiling point of water at ambient pressure, but it can be reduced at high ionic strengths. We extend this concept to trigger the clustering of gold nanoparticles through the control of colloidal interactions. At high ionic strengths, low-molecular weight (<2000 Da) mPEG-SH-modified gold nanoparticles show clustering with an increase in the solution temperature. The clustering temperature decreases with an increasing ionic strength. The clustering is attributed to the delicate interplay between the high ionic strength and elevated temperature and is interpreted in terms of chain collapse of the surface-grafted PEG molecules. The chain collapse results in a change in the steric interaction term, whereas the high ionic strength eliminates the double-layer repulsion between the particles. The observations are backed by nanoparticle interaction model calculations. We found that the intermediate attractive potential on the order of a few kT allows the experimental fabrication of compact nanoparticle clusters in agreement with theoretical predictions. The approach presented here has the potential to be extended on the externally triggered preparation of nanoparticle clusters with different types of nanoparticles.

Model Glycol-Terminated Surfaces for Adhesion Resistance

ABSTRACT This manuscript introduces a new family of cyclic acetal-terminated alkanethiols used to prepare self-assembled monolayers (SAMs) on gold. The new SAMs, which are designed as variants of the biocompatible protein-repellant surfaces generated from thin films of oligoethylene glycol (OEG) or polyethylene glycol (PEG), were characterized by ellipsometry, contact-angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). A preliminary study of protein adhesion was also performed using fibrinogen as a model protein. The interfacial structure and properties of the new SAMs were compared with those generated from OEG-terminated alkanethiols and from surface-grafted PEG, which have been described previously. The data show that the new adsorbates form well packed and conformationally ordered films with contact angles of water ranging from 67° to 95°, depending on the precise structure of the acetal terminus. As a whole, the new SAMs offer a unique strategy for studying and designing adhesion-resistant biocompatible inferfaces.

Accelerated blood clearance of PEGylated liposomes following preceding liposome injection: Effects of lipid dose and PEG surface-density and chain length of the first-dose liposomes

We recently reported that a second dose of polyethylene glycol (PEG) (M.W. 2000)-modified liposomes (mPEG 2000-liposomes) is rapidly cleared from the blood and accumulates in the liver when injected twice in the same rat or mouse at several-day intervals (referred to as the “accelerated blood clearance (ABC) phenomenon”). In the present study we observed that a high dose (5 μmol/kg) of conventional liposomes (CL: without a PEG-coating) can induce the same phenomenon, while a low lipid dose (0.001 μmol/kg) did not. The induction of the phenomenon by mPEG 2000-liposomes decreased with increasing first dose (0.001–5 μmol/kg). We observed a strong inverse relationship between the dose of initially injected PEG 2000-liposomes and the extent to which the ABC phenomenon was induced: the higher the dose the smaller the phenomenon. Increasing the PEG density at the liposome surface beyond 5 mol% attenuated rather than induced the induction of the phenomenon, but elongation of the PEG chain length up to M.W. 5000, had no effect. In a series of hematological, serum-biochemical and histopathological safety evaluations we observed neither acute toxicity nor any signs of hepatic damage during the induction of the ABC phenomenon. Morphological examination of the liver by transmission electron microscopy (TEM) showed extensive accumulation of the second dose of mPEG 2000-liposomes in the Kupffer cells, even already after 15 min, suggesting that the PEG liposomes had somehow lost the protective effect of the surface-grafted PEG against rapid clearance. The observations reported in this paper may have a considerable impact on the design and engineering of PEGylated liposomal formulations for use in multiple drug therapy.

Experimental and Monte Carlo Simulation Studies of the Thermodynamics of Polyethyleneglycol Chains Grafted to Lipid Bilayers

Experimental measurements of the affinity of binding of fluorescent acylated polyethyleneglycol (PEG) conjugates to bilayers containing varying levels of phosphatidylethanolamine-PEGs (PE-PEGs) have been combined with Monte Carlo simulations to investigate the properties of the polymer chains at a PEG-grafted lipid interface. The affinity of binding of such conjugates to large unilamellar phosphatidylcholine/phosphatidylethanolamine (9:1) vesicles decreases 27-fold as the size of the coupled PEG chain increases from 1 to 114 monomer units. Incorporation of increasing amounts of PE-PEG2000 or PE-PEG5000 into the vesicles progressively reduces the affinity of binding of acylpeptide-PEG2000 or -PEG5000 conjugates. Monte Carlo simulations of surfaces with grafted PEG chains revealed no significant dependence of several characteristic properties of the polymer chains, including the average internal energy per polymer and the radii of gyration, on the grafting density in the range examined experimentally. The average conformation of a surface-grafted PEG2000 or PEG5000 chain was calculated to be fairly extended even at low grafting densities, and the projected cross-sectional areas of the grafted PEG chains are considerably smaller than those predicted on the basis of the estimated Flory radius. The experimental variation of the binding affinity of acylated conjugates for bilayers containing varying mole fractions of PE-PEG2000 or -PEG5000 is well explained by expressions treating the surface-grafted PEG polymers either as a van der Waals gas or as a system of rigid discs described by scaled particle theory. From the combined results of our experimental and simulation studies we conclude that the grafted PEG chains exist in a “mushroom” regime throughout the range of polymer densities examined experimentally and that the diminished affinity of binding of acylated-PEG conjugates to bilayers containing PE-PEGs results from occlusion of the surface area accessible for conjugate binding by the mobile PE-PEG polymer chains.

Liposomal drug formulations. Rationale for development and what we can expect for the future.

Liposomes are versatile drug carriers which can be used to solve problems of drug solubility, instability and rapid degradation. Both hydrophilic and hydrophobic drugs can be associated with liposomes and special techniques have been developed for the efficient loading of weak acids and weak bases into liposomes. Liposomes can function as sustained release systems for drugs and the rate of release can be manipulated. Advantage can be taken of the substantial changes in pharmacokinetics which often accompanies the association of drugs with liposomes. New formulations of liposomes, sterically stabilised with substances like surface-grafted polyethylene glycol have circulating half-lives in humans of up to 2 days. These long circulation times allow concentration of liposomal drug in regions of increased vascular permeability like solid tumours an decreased delivery of drug to normal tissues. Alterations of the biodistribution of drugs, when they are liposomes-associated, in general leads to significant overall decreases in drug toxicity but can also increase toxicity in some tissues. The use of targeting ligands to increase the selectivity of delivery of liposomal drugs to target tissues is currently under development. An understanding of how liposome association can alter drug properties can lead to their rational development in the treatment of many diseases.

Avidin-biotin interactions at vesicle surfaces: adsorption and binding, cross-bridge formation, and lateral interactions

Densely packed domains of membrane proteins are important structures in cellular processes that involve ligand-receptor binding, receptor-mediated adhesion, and macromolecule aggregation. We have used the biotin-avidin interaction at lipid vesicle surfaces to mimic these processes, including the influence of a surface grafted polymer, polyethyleneglycol (PEG). Single vesicles were manipulated by micropipette in solutions of fluorescently labeled avidin to measure the rate and give an estimate of the amount of avidin binding to a biotinylated vesicle as a function of surface biotin concentration and surface-grafted PEG as PEG-lipid. The rate of avidin adsorption was found to be four times less with 2 mol% PEG750 than for the unmodified surface, and 10 mol% PEG completely inhibited binding of avidin to biotin for a 2-min incubation. Using two micropipettes, an avidin-coated vesicle was presented to a biotinylated vesicle. In this vesicle-vesicle adhesion test, the accumulation of avidin in the contact zone was observed, again by using fluorescent avidin. More importantly, by controlling the vesicle membrane tension, this adhesion test provided a direct measure of the spreading pressure of the biotin-avidin-biotin cross-bridges confined in the contact zone. Assuming ideality, this spreading pressure gives the concentration of avidin cross-bridges in the contact zone. The rate of cross-bridge accumulation was consistent with the diffusion of the lipid-linked "receptors" into the contact zone. Once adherent, the membranes failed in tension before they could be peeled apart. PEG750 did not influence the mechanical equilibrium because it was not compressed in the contact zone, but it did perform an important function by eliminating all nonspecific adhesion. This vesicle-vesicle adhesion experiment, with a lower tension limit of 0.01 dyn/cm, now provides a new and useful method with which to measure the spreading pressures and therefore colligative properties of a range of membrane-bound macromolecules.