A Simple and Efficient Method for Synthesis of Carboxymethylated Polyethyleneglycol

A simple and efficient method for the synthesis of carboxymethylated polyethylene glycol (CM-PEG) by the oxidation of the corresponding monomethoxy polyethylene glycol (mPEG) with catalytic amounts of TEMPO and hypobromide as a regenerating oxidant and water as solvent was developed.

Synthesis and Cytotoxic Activity of Conjugates of Monomethoxy-Poly(Ethylene Glycol) End-Capped with Doxorubicin via Ester, Amide, or Schiffs Base Bond

Doxorubicin (adriamycin; ADR) which is one of the most clinically used antitumor agent, has undesirable side-effects. To reduce these side-effects, a macromolecular prodrug of ADR exhibiting high antitumor activity, the conjugate of PEG end-capped with ADR was developed. Tb effect intracellular release of ADR from the conjugate, ester, amide and Schiffs base bonds were employed as modes of bonding ADR to PEG. The MeO-PEG/Schiffs base/ADR conjugate readily released ADR under the lysosomal acidic conditions in vitro and very slowly ADR under physiological conditions. Moreover, the MeO-PEG/Schiffs base/ADR conjugate showed strong cytotoxic activity similar to free ADR against p388D1 lymphocytic leukemia cells in vitro. Fluorescence measurements suggest that the conjugate forms self-aggregates in aqueous solution.

Novel pH-sensitive charge-reversal cell penetrating peptide conjugated PEG-PLA micelles for docetaxel delivery: in vitro study

In order to create a pH-sensitive charge-reversal system for cell penetrating peptides (CPP) to prevent non-specific internalization of the drug; and concomitantly enhance the physical stability and tumor targetability of poly(ethylene glycol)-poly(d,l-lactide) (PEG-PLA) micelles, two sets of novel PEG-PLA micelles were developed. Cell penetrating decapeptide arginine-glycine (RG)5 and a pH-sensitive masking decapeptide histidine-glutamic acid (HE)5 were conjugated at the PEG free end to produce pH sensitive with peptides outside micelles (PHPO), while the pH sensitive with peptides inside micelles (PHPI) are the micelles obtained with the two peptides conjugated to the free end of the PLA block. The polymers were successfully synthesized and characterized by (1)H NMR and GPC. The mixed micelles were prepared and characterized for their loading efficiency, particle size and zeta potential. The surface charge of PHPO was greatly affected by the pH of the solution and (RG)5:(HE)5 ratio at the surface. The pH value of the solution at which the surface charge of PHPO reversed could be manipulated by the feed ratio of (RG)5-PEG-PLA (RGO) and (HE)5-PEG-PLA (HEO), hence, HEO:RGO molar ratio of 45:55 was selected for tumor targeting. Docetaxel (DTX) was sufficiently solubilized by DTX-PHPO with a loading efficiency of 90.18 ± 1.65%. At pH 7.4, DTX loaded mPEG-PLA (DTX-PM) (41.2 ± 0.3 nm), DTX-PHPO (195.3 ± 1.9 nm) and DTX-PHPI (190.9 ± 4.5 nm) showed sustained DTX release of less than 55% within 48 h. However, at pH 6.8 DTX-PHPI released 87.29 ± 0.24%, while DTX-PHPO released 70.49 ± 0.39% of the initial DTX amount within 48 h. Moreover, the physical stability of DTX-PHPO was increased due to the electrostatic interaction of the two peptides. The cellular uptake of DTX-PHPO in SGC-7901 cells and the cell killing effect tested on MCF-7 cells were enhanced by 2 folds at pH 6.8 compared to pH 7.4. Hence, DTX-PHPO is highly pH-sensitive in mildly acidic pH and exhibited higher internalization, but DTX-PHPI exhibited accelerated release. Meanwhile, both formulations displayed low internalization and release at pH greater than 7. This pH sensitive charge reversal design can offer a promising safe carrier using both CPPs and PEG-PLA micelles.

Micellar nanocarriers assembled from doxorubicin-conjugated amphiphilic macromolecules (DOX-AM)

Amphiphilic macromolecules (AMs) have unique branched hydrophobic domains attached to linear PEG chains. AMs self-assemble in aqueous solution to form micelles that are hydrolytically stable in physiological conditions (37 degrees C, pH 7.4) over 4 weeks. Evidence of AM biodegradability was demonstrated by complete AM degradation after 6 d in the presence of lipase. Doxorubicin (DOX) was chemically conjugated to AMs via a hydrazone linker to form DOX-AM conjugates that self-assembled into micelles in aqueous solution. The conjugates were compared with DOX-loaded AM micelles (i.e., physically loaded DOX) on DOX content, micellar sizes and in vitro cytotoxicity. Physically encapsulated DOX loading was higher (12 wt.-%) than chemically bound DOX (6 wt.-%), and micellar sizes of DOX-loaded AMs (approximately 16 nm) were smaller than DOX-AMs (approximately 30 nm). In vitro DOX release from DOX-AM conjugates was faster at pH 5.0 (100%) compared to pH 7.4 (78%) after 48 h, 37 degrees C. Compared to free DOX and physically encapsulated DOX, chemically bound DOX had significantly higher cytotoxicity at 10(-7) M DOX dose against human hepatocellular carcinoma cells after 72 h. Overall, DOX-AM micelles showed promising characteristics as stable, biodegradable DOX nanocarriers.

Synthesis and characterization of hemoglobin conjugates with antioxidant enzymes via poly(ethylene glycol) cross-linker (Hb-SOD-CAT) for protection from free radical stress

Hemoglobin (Hb) conjugated with the antioxidant enzymes (SOD and CAT), by employing dicarboxymethylated poly(ethylene glycol), was designed for protection of hemoglobin against free radicals. In this study, the conjugation process was confirmed by employing SDS-PAGE and SEC techniques. The average molecular weight of the conjugates was estimated to be around 1000 kDa. The enzymatic activities of the SOD and CAT in the conjugates (Hb-SOD-CAT) after conjugation were found to retain greater than 70% and 90% of the original bioactivity. Results show that antioxidant enzymes helped minimize methemoglobin (non-carrier of oxygen) formation during the conjugation process and also during storage at 4°C over a period of 1 month. In summary, the optimized (1:10 Hb/PEG) crosslinked conjugates with antioxidant enzymes showed protective properties from severe free radical stresses when incubated with hydrogen peroxide (0.1 and 1 mM) and xanthine (1 mM)/xanthine oxidase (10 and 20 mU/ml) system.

Modulating the pharmacokinetics of therapeutic antibodies

With the advent of antibody fragments and alternative binding scaffolds, that are devoid of Fc-regions, strategies to increase the half-life of small proteins are becoming increasingly important. Currently, the established method is chemical PEGylation, but more elaborate approaches are being described such as polysialylation, amino acid polymers and albumin-binding derivatives. This article reviews the main strategies for pharmacokinetic enhancement, primarily chemical conjugates and recombinant fusions that increase apparent molecular weight or hydrodynamic radius or interact with serum albumin which itself has a long plasma half-life. We highlight the key chemical linkage methods that preserve antibody function and retain stability and look forward to the next generation of technologies which promise to make better quality pharmaceuticals with lower side effects. Although restricted to antibodies, all of the approaches covered can be applied to other biotherapeutics.

Native PAGE eliminates the problem of PEG–SDS interaction in SDS-PAGE and provides an alternative to HPLC in characterization of protein PEGylation

PEGylation of proteins has become an increasingly important technology in recent years. However, determination and characterization of the PEGylation products are problematic especially for the reaction mixture containing various modified proteins, unreacted PEG, and unmodified protein. A comparative study was carried out with two HPLC methods and two electrophoresis methods for characterization of the reaction mixture in PEGylation of HSA with PEG 5000, 10000, and 20000. RP-HPLC fails to give the correct information about the reaction of PEG 20000. Size-exclusion HPLC (SE-HPLC) produced very poor resolution on the PEG 5000 reaction. SDS-PAGE can run multiple samples of all PEGylation but the bands were smeared or broadened probably due to the interaction between PEG and SDS. On the other hand, native PAGE eliminates the problem of PEG-SDS interaction and provides better resolutions for all samples. Various PEGylated products and unmodified protein migrate differentially in native PAGE under nondenatured conditions. The results demonstrated that native PAGE could be a good alternative to HPLC and SDS-PAGE for the analysis of PEG-protein conjugates especially for characterization of the PEGylation mixture.

Reproducible preparation and effective separation of PEGylated recombinant human granulocyte colony-stimulating factor with novel “PEG-pellet” PEGylation mode and ion-exchange chromatography

A novel preparation for polyethylene glycol (PEG) derivatives and chromatographic separation procedure of the PEGylated recombinant human granulocyte colony-stimulating factor (rhG-CSF) were designed to evaluate the reproducibility and scalability at large laboratory-scale level. The new "PEG-pellet" PEGylation mode was successfully applied to control the pH fluctuation during the conjugation reaction, a general problem in traditional liquid-phase conjugation mode. Moreover, two consecutive ion-exchange chromatography steps were successfully used to separate and purify the PEGylated rhG-CSF. Cation-exchange chromatography was firstly applied to separate PEGylated rhG-CSF from intact rhG-CSF, followed by anion-exchange chromatography to obtain individual PEG-rhG-CSF species (mono-, di- and tri-PEGylated rhG-CSF) and remove the excess free PEG. Furthermore, the molecular weight of individual PEGylated rhG-CSF was identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry and SDS-PAGE, and cell proliferation activity in vitro was detected by MTT assay using NFS-60 cell.

Polyelectrolyte multilayer films with pegylated polypeptides as a new type of anti-microbial protection for biomaterials

Adhesion of bacteria at the surface of implanted materials is the first step in microbial infection, leading to post-surgical complications. In order to reduce this adhesion, we show that poly(L-lysine)/poly(L-glutamic acid) (PLL/PGA) multilayers ending by several PLL/PGA-g-PEG bilayers can be used, PGA-g-PEG corresponding to PGA grafted by poly(ethylene glycol). Streaming potential and quartz crystal microbalance-dissipation measurements were used to characterize the buildup of these films. The multilayer films terminated by PGA and PGA-g-PEG were found to adsorb an extremely small amount of serum proteins as compared to a bare silica surface but the PGA ending films do not reduce bacterial adhesion. On the other hand, the adhesion of Escherichia coli bacteria is reduced by 72% on films ending by one (PLL/PGA-g-PEG) bilayer and by 92% for films ending by three (PLL/PGA-g-PEG) bilayers compared to bare substrate. Thus, our results show the ability of PGA-g-PEG to be inserted into multilayer films and to drastically reduce both protein adsorption and bacterial adhesion. This kind of anti-adhesive films represents a new and very simple method to coat any type of biomaterials for protection against bacterial adhesion and therefore limiting its pathological consequences.

In situ photopolymerization of a polymerizable poly(ethylene glycol)-covered phospholipid monolayer on a methacryloyl-terminated substrate

We have prepared a chemically anchored monolayer of PEG (poly(ethylene glycol)) and phospholipid mixture (PEG/phospholipid) on a methacryloyl-terminated substrate by in situ photopolymerization. Both monoacryloyl phospholipid (acryloyl-PC, 1-palmitoyl-2-[12-(acryloyloxy)dodecanoyl]-sn-glycero-3-phosphocholine) and monoacryloyl PEG (acryloyl-PEG, 12-(acryloyloxy)dodecanoyl-PEG) were synthesized by modifyingphospholipid and PEGwith 12-(acryloyloxy)-1-dodecanoic acid and 12-(acryloyloxy)-1-dodecanol, respectively. The surface pressure-area (pi-A) isotherm showed that acryloyl-PEG molecules were stable in the phospholipid monolayer and that they could be evenly inserted into a phospholipid monolayer at the air/water interface. By adding 10 mol % acryloyl-PEG into phosholipid vesicles, we could produce a PEG/phosholipid monolayer on methacryloyl-terminated substrates using vesicle fusion for 3 h. Then, this polymerizable PEG/phospholipid monolayer was in situ photopolymerized onto a methacryloyl-terminated substrate with eosin Y/triethanolamine as co-initiators. Optimal vesicle fusion and irradiation condition were determined with respect to the vesicle fusion time and duration of irradiation. As confirmed by atomic force microscopy and X-ray reflectivity studies, the polymerized PEG/phosholipid surface formed a PEG-covered phospholipid monolayer with thicknesses of 3 and 6 nm for the base phospholipid monolayer and the covering PEG layer, respectively. The chemical anchoring efficiency ofpolymerized PEG and phospholipid molecules, which was calculated by the relative carbon ratio of each surface before and after methanol washing using X-ray photoelectron spectroscopy, was 98%. This polymerized PEG/phosholipid monolayer showed good stability in organic solution due to firm chemical anchoring to a solid surface.

Poly(L-histidine)-PEG block copolymer micelles and pH-induced destabilization

Poly(L-histidine)-poly(ethylene glycol) diblock copolymers (polyHis-b-PEG) were prepared and used for the construction of polymeric micelles responding to local pH changes in the body. PolyHis was synthesized by ring opening polymerization of L-histidine N-carboxyanhydride, the imidazole amine group of which was protected by the dinitrophenyl group. The resulting polymer (M(n): 5,000 g/mole) was coupled to poly(ethylene glycol) (M(n): 2,000 g/mole) via an amide linkage using the dicyclohexyl carbodiimide and N-hydroxysuccinimide-mediated reaction. The block copolymer in dimethyl sulfoxide formed polymeric micelles on diafiltration against a borate buffer at pH 8. Dynamic light scattering and atomic force microscopy showed the micelles were spherical, diameter approximately 114 nm, with a unimodal distribution. The critical micelle concentration (CMC) at pH 8.0 was 2.3 mg/l. The CMC increased markedly on decreasing the pH of the diafiltration medium below 7.2. Micelles prepared at pH 8.0 were gradually destabilized below pH 7.4, as evidenced by a slight increase in light transmittance, an alteration in size distribution, and a decrease in the pyrene fluorescence intensity. It was concluded that the ionization of the polyHis block forming the micelle core determined the pH-dependent CMC and stability. After further optimization of the pH-sensitivity, pH-sensitive micelles are expected to have application for solid tumor treatment, exploiting the fact that most solid tumors have an acidic extracellular pH.

Bioactive polymers for biohybrid artificial pancreas

To address the solution for some of the obstacles, such as low insulin secretion, limited lifespan and aggregation of transplanted islets, encountered in developing a biohybrid artificial pancreas (BAP), polymeric materials including a reversible polymeric extracellular matrix (ECM), crystallized glucagon-like peptide-1, and oxygen carrying polymers, were prepared and their potential utilities in designing a compact and rechargeable BAP were investigated. For a synthetic, reversible ECM, high molecular weight N-isopropylacrylamide copolymer with a small amount of acrylic acid (2 mole%) was synthesized by conventional radical polymerization in benzene, and its aqueous solution above a critical polymer concentration displayed a sol-gel transition temperature near physiological temperature (33-35 degrees C) without noticeable hysteresis. The physicochemical properties of the gel with islet compatibility proved that the synthetic ECM is an appropriate matrix which can make a BAP rechargeable. Glucagon-like peptide-1 (GLP-1, 7-37) is known to have a strong stimulatory effect on insulin secretion, particularly at high glucose concentrations. When zinc-crystallized GLP-1 was entrapped along with islets in a hollow fiber macrocapsule device, insulin secretion was enhanced at a high glucose concentration (300 mg/dl) with a >85% increase in insulin secretion after an induction period. The cross-linked hemoglobin with difunctional PEO (Hb-C) was prepared to increase the high molecular weight of Hb. This prevents diffusional loss when enclosed in an immunoprotecting membrane. The Hb-C, entrapped in microcapsules, enhanced insulin secretion and improved the viability of microencapsulated islets by promoting oxygen supply to islets. The introduction of the synthetic ECM, crystallized GLP-1, and Hb-C into a BAP may provide a basis for designing a compact and rechargeable BAP (macrocapsule).

Effect of cross-linked hemoglobin on functionality and viability of microencapsulated pancreatic islets

Of many obstacles involved in developing a bioartificial pancreas, which consists of encapsulated and physically immunoprotected islets, for long-term implantation in insulin-dependent diabetic patients, the impaired functionality and decreasing viability of encapsulated islets over time are critical factors in determining the size and longevity of the implant. These factors are closely associated with short oxygen supply to the encaged islets from the implant site. To facilitate oxygen transport to islets in the capsules, we coencapsulated hemoglobin cross-linked with difunctional polyethylene glycol (Hb-conjugate, Hb-C) which is large in size (>100 kDa), thus preventing diffusional loss through the immunoprotecting membrane. The coencapsulation of Hb-C with islets in alginate-poly-L-lysine microcapsules by dissolving Hb-C in an islet-suspended alginate solution at a concentration of 0.25 mM improved the insulin secretion and viability of the islets. At week 0, the islets, coencapsulated with Hb-C, cultured at P(O2) = 40 mmHg (assumed oxygen partial pressure in the most common implant site, the peritoneal cavity), secreted 200% more insulin compared with the control islets without Hb-C at glucose concentrations of both 100 and 300 mg/dL. The Hb-C effect became more significant with time at higher glucose concentrations. After culturing the islets for 8 weeks at 40 mmHg, the insulin secretion was enhanced 200 and 550% at glucose concentrations of 100 and 300 mg/dL as compared with the control, respectively. The results were closely associated with improved viability and suggest that the introduction of Hb-C is an effective approach to maintaining the oxygen supply to encapsulated islets. In addition, Hb-C coencapsulation with pancreatic islets may (1) provide a partial clue to reducing the large size of the biohybrid artificial pancreas, (2) lead to a reduced need for pancreas donation, and (3) prolong the longevity of the biohybrid artificial pancreas in the body.

Chemistry for peptide and protein PEGylation

Poly(ethylene glycol) (PEG) is a highly investigated polymer for the covalent modification of biological macromolecules and surfaces for many pharmaceutical and biotechnical applications. In the modification of biological macromolecules, peptides and proteins are of extreme importance. Reasons for PEGylation (i.e. the covalent attachment of PEG) of peptides and proteins are numerous and include shielding of antigenic and immunogenic epitopes, shielding receptor-mediated uptake by the reticuloendothelial system (RES), and preventing recognition and degradation by proteolytic enzymes. PEG conjugation also increases the apparent size of the polypeptide, thus reducing the renal filtration and altering biodistribution. An important aspect of PEGylation is the incorporation of various PEG functional groups that are used to attach the PEG to the peptide or protein. In this paper, we review PEG chemistry and methods of preparation with a particular focus on new (second-generation) PEG derivatives, reversible conjugation and PEG structures.

Preparation of heterodifunctional polyethyleneglycols: network formation, characterization, and cell culture analysis

Polyethyleneglycols (PEG) are employed extensively in the development of biomaterials; however, the hydroxyl groups in PEG-diols have very limited chemical activity. We developed a synthesis scheme for a library of heterodifunctional PEG (hPEG) with two distinct terminal moieties to improve the reactivity and physicochemical properties of PEG. hPEG were employed in the formulation of polymer networks with various surface physicochemical properties and utilized to study cell-material interaction. Extensive NMR and HPLC analyses confirmed the chemical structure of hPEG. The hydrophilicity of the polymer network was predominantly dependent on the hPEG concentration with the molecular weight and terminal functional group playing lesser roles. Adherent human fibroblast density on the network remained constant with increasing hPEG concentration in the network formulation but decreased rapidly on networks containing 0.8-1.25 g ml(-1) of hPEG. This trend was independent of the hPEG terminal moiety and molecular weight. No adherent cell was observed on all films containing 2.5 g ml(-1) or more of hPEG.

Peptide and protein PEGylation: a review of problems and solutions

The paper discusses general problems in using PEG for conjugation to high or low molecular weight molecules. Methods of binding PEG to different functional groups in macromolecules is reported together with their eventual limitations. Problems encountered in conjugation, such as the evaluation of the number of PEG chains bound, the localisation of the site of conjugation in polypeptides and the procedure to direct PEGylation to the desired site in the molecule are discussed. Finally, the paper reports on more specific methods regarding reversible PEGylation, cross-linking reagents with PEG arms, PEG for enzyme solubilization in organic solvent and new polymers as alternative to PEG.

Interactions and applications of soluble heterobifunctional affinity chelating polymers in immobilized metal affinity chromatography

The interaction of immobilized metal-chelating adsorbents with a dual heterobifunctional soluble polyethylene glycol (PEG) of the form X-PEG-Y is described, where X represents an affinity ligand and Y a chelating agent. The bifunctional PEG derivative used in this study was biotin-PEG-iminodiacetic acid (IDA). Affinity and metal binding constants of this conjugate for copper and avidin were found to be in excellent agreement with the binding affinities of the corresponding unconjugated groups IDA and biotin, respectively. The characteristics of the interaction of this bifunctional derivative is described in terms of its adsorption in immobilized metal affinity chromatographic (IMAC) adsorbents. The results show that this derivative can be reversibly and selectively bound to specific IMAC adsorbents under certain experimental conditions. This immobilized scheme resembles a system where an IMAC adsorbent was transformed into an affinity adsorbent as a result of the interactions of both chelating derivatives, one in solution (biotin-PEG-IDA) and the other on the solid matrix (IMAC adsorbent). Apparently the modified IMAC adsorbents, once the affinity chelating ligands are attached, exhibit characteristics similar to those of covalently bound affinity ligands in affinity chromatographic systems.