Disulfide bridge based PEGylation of proteins

Receptor selective ruthenium-somatostatin photosensitizer for cancer targeted photodynamic applications

The efficient conjugation of a ruthenium complex and the peptide hormone somatostatin is presented. The resultant biohybrid offers valuable features for photodynamic therapy such as remarkable cellular selectivity, rapid cell uptake by receptor-mediated endocytosis, efficient generation of (1)O2 upon irradiation, potent phototoxicity as well as low cytotoxicity in the "off"-state.

Water-soluble allyl sulfones for dual site-specific labelling of proteins and cyclic peptides

Allyl sulfones as efficient disulfide rebridging agents for site-specific protein modifications with up to two additional functionalities in water.

Water-soluble allyl sulfones provide convenient site-specific disulfide rebridging of native proteins and cyclic peptides. The site-selective functionalization of (a) the peptide hormone somatostatin, (b) the interchain disulfide of bovine insulin and (c) functionalization of the proteins GFP and lysozyme with allyl sulfones proceeds in aqueous solution. Allyl sulfones offer three functionalizable sites that react with thiol containing molecules in a step-wise fashion. Dual labeling of proteins and cyclic peptides is achieved i.e. the attachment of a chromophore and an affinity tag in a single reaction step, which is of great significance for the construction of precise multifunctional peptide and protein conjugates.

Rational design of Raman-labeled nanoparticles for a dual-modality, light scattering immunoassay on a polystyrene substrate

Background

Surface-enhanced Raman scattering (SERS) is a powerful light scattering technique that can be used for sensitive immunoassay development and cell labeling. A major obstacle to using SERS is the complexity of fabricating SERS probes since they require nanoscale characterization and optical uniformity. The light scattering response of SERS probes may also be modulated by the substrate used for SERS analysis. A typical SERS substrate such as quartz can be expensive. Polystyrene is a cheaper substrate option but can decrease the SERS response due to interfering Raman emission peaks and high background fluorescence. The goal of this research is to develop an optimized process for fabricating Raman-labeled nanoparticles for a SERS-based immunoassay on a polystyrene substrate.

Results

We have developed a method for fabricating SERS nanoparticle probes for use in a light scattering immunoassay on a polystyrene substrate. The light scattering profile of both spherical gold nanoparticle and gold nanorod SERS probes were characterized using Raman spectroscopy and optical absorbance spectroscopy. The effects of substrate interference and autofluorescence were reduced by selecting a Raman reporter with a strong light scattering response in a spectral region where interfering substrate emission peaks are minimized. Both spherical gold nanoparticles and gold nanorods SERS probes used in the immunoassay were detected at labeling concentrations in the low pM range. This analytical sensitivity falls within the typical dynamic range for direct labeling of cell-surface biomarkers using SERS probes.

Conclusion

SERS nanoparticle probes were fabricated to produce a strong light scattering signal despite substrate interference. The optical extinction and inelastic light scattering of these probes was detected by optical absorbance spectroscopy and Raman spectroscopy, respectively. This immunoassay demonstrates the feasibility of analyzing strongly enhanced Raman signals on polystyrene, which is an inexpensive yet non-ideal Raman substrate. The assay sensitivity, which is in the low pM range, suggests that these SERS probe particles could be used for Raman labeling of cell or tissue samples in a polystyrene tissue culture plate. With continued development, this approach could be used for direct labeling of multiple cell surface biomarkers on strongly interfering substrate platforms.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-015-0023-y) contains supplementary material, which is available to authorized users.

Bioconjugation – using selective chemistry to enhance the properties of proteins and peptides as therapeutics and carriers

Both peptide and protein therapeutics are becoming increasingly important for treating a wide range of diseases. Functionalisation of theseviasite-selective chemical modification leads to enhancement of their therapeutic properties.

Expansion of bioorthogonal chemistries towards site-specific polymer–protein conjugation

Bioorthogonal chemistries have been used to achieve polymer-protein conjugation with the retained critical properties.

Next-generation disulfide stapling: reduction and functional re-bridging all in one

Herein we present a significant step towards next-generation disulfide stapling reagents. A novel class of reagent has been designed to effect both disulfide reduction and functional re-bridging. The strategy has been applied to great success across various peptides and proteins. Moreover, application to a multi-disulfide system resulted in functional re-bridging without disulfide scrambling.

Polymerization of Ethylene Oxide, Propylene Oxide, and Other Alkylene Oxides: Synthesis, Novel Polymer Architectures, and Bioconjugation

The review summarizes current trends and developments in the polymerization of alkylene oxides in the last two decades since 1995, with a particular focus on the most important epoxide monomers ethylene oxide (EO), propylene oxide (PO), and butylene oxide (BO). Classical synthetic pathways, i.e., anionic polymerization, coordination polymerization, and cationic polymerization of epoxides (oxiranes), are briefly reviewed. The main focus of the review lies on more recent and in some cases metal-free methods for epoxide polymerization, i.e., the activated monomer strategy, the use of organocatalysts, such as N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) as well as phosphazene bases. In addition, the commercially relevant double-metal cyanide (DMC) catalyst systems are discussed. Besides the synthetic progress, new types of multifunctional linear PEG (mf-PEG) and PPO structures accessible by copolymerization of EO or PO with functional epoxide comonomers are presented as well as complex branched, hyperbranched, and dendrimer like polyethers. Amphiphilic block copolymers based on PEO and PPO (Poloxamers and Pluronics) and advances in the area of PEGylation as the most important bioconjugation strategy are also summarized. With the ever growing toolbox for epoxide polymerization, a "polyether universe" may be envisaged that in its structural diversity parallels the immense variety of structural options available for polymers based on vinyl monomers with a purely carbon-based backbone.

PRINT: A Protein Bioconjugation Method with Exquisite N-terminal Specificity

Chemical conjugation is commonly used to enhance the pharmacokinetics, biodistribution, and potency of protein therapeutics, but often leads to non-specific modification or loss of bioactivity. Here, we present a simple, versatile and widely applicable method that allows exquisite N-terminal specific modification of proteins. Combining reversible side-chain blocking and protease mediated cleavage of a commonly used HIS tag appended to a protein, we generate with high yield and purity exquisitely site specific and selective bio-conjugates of TNF-α by using amine reactive NHS ester chemistry. We confirm the N terminal selectivity and specificity using mass spectral analyses and show near complete retention of the biological activity of our model protein both in vitro and in vivo murine models. We believe that this methodology would be applicable to a variety of potentially therapeutic proteins and the specificity afforded by this technique would allow for rapid generation of novel biologics.

Octreotide-Mediated Tumor-Targeted Drug Delivery via a Cleavable Doxorubicin-Peptide Conjugate

Although recent methods for targeted drug delivery have addressed many of the existing problems of cancer therapy associated with undesirable side effects, significant challenges remain that have to be met before they find significant clinical relevance. One such area is the delicate chemical bond that is applied to connect a cytotoxic drug with targeting moieties like antibodies or peptides. Here we describe a novel platform that can be utilized for the preparation of drug-carrier conjugates in a site-specific manner, which provides excellent versatility and enables triggered release inside cancer cells. Its key feature is a cleavable doxorubicin-octreotide bioconjugate that targets overexpressed somatostatin receptors on tumor cells, where the coupling between the two components was achieved through the first cleavable disulfide-intercalating linker. The tumor targeting ability and suppression of adrenocorticotropic hormone secretion in AtT-20 cells by both octreotide and the doxorubicin hybrid were determined via a specific radioimmunoassay. Both substances reduced the hormone secretion to a similar extent, which demonstrated that the tumor homing peptide is able to interact with the relevant cell surface receptors after the attachment of the drug. Effective drug release was quickly accomplished in the presence of the physiological reducing agent glutathione. We also demonstrate the relevance of this scaffold in biological context in cytotoxicity assays with pituitary, pancreatic, and breast cancer cell lines.

PEGylation and its impact on the design of new protein-based medicines

PEGylation is the covalent conjugation of PEG to therapeutic molecules. Protein PEGylation is a clinically proven approach for extending the circulation half-life and reducing the immunogenicity of protein therapeutics. Most clinically used PEGylated proteins are heterogeneous mixtures of PEG positional isomers conjugated to different residues on the protein main chain. Current research is focused to reduce product heterogeneity and to preserve bioactivity. Recent advances and possible future directions in PEGylation are described in this review. So far protein PEGylation has yielded more than 10 marketed products and in view of the lack of equally successful alternatives to extend the circulation half-life of proteins, PEGylation will still play a major role in drug delivery for many years to come.

Programming Bioactive Architectures with Cyclic Peptide Amphiphiles

We present a versatile approach for the synthesis of cyclic peptide amphiphiles of the hormone somatostatin (SST) with tunable lipophilic tails to program bioactive nanoarchitectures. A novel bis-alkylation reagent is synthesized that facilitates the functionalization of SST with a thiol anchor. Different hydrophobic moieties are introduced inspired by a biomimetic palmitoylation approach which opens access to cyclic peptide amphiphiles that display rich self-organization and cell membrane interactions.

Developments and recent advancements in the field of endogenous amino acid selective bond forming reactions for bioconjugation

Bioconjugation methodologies have proven to play a central enabling role in the recent development of biotherapeutics and chemical biology approaches. Recent endeavours in these fields shed light on unprecedented chemical challenges to attain bioselectivity, biocompatibility, and biostability required by modern applications. In this review the current developments in various techniques of selective bond forming reactions of proteins and peptides were highlighted. The utility of each endogenous amino acid-selective conjugation methodology in the fields of biology and protein science has been surveyed with emphasis on the most relevant among reported transformations; selectivity and practical use have been discussed.

Identification of reduction-susceptible disulfide bonds in transferrin by differential alkylation using O(16)/O(18) labeled iodoacetic acid

Stabilization of native three-dimensional structure has been considered for decades to be the main function of disulfide bonds in proteins. More recently, it was becoming increasingly clear that in addition to this static role, disulfide bonds are also important for many other aspects of protein behavior, such as regulating protein function in a redox-sensitive fashion. Dynamic disulfide bonds can be taken advantage of as candidate anchor sites for site-specific modification (such as PEGylation of conjugation to a drug molecule), but are also frequently implicated in protein aggregation (through disulfide bond scrambling leading to formation of intermolecular covalent linkages). A common feature of all these labile disulfide bonds is their high susceptibility to reduction, as they need to be selectively regulated by either specific local redox conditions in vivo or well-controlled experimental conditions in vitro. The ability to identify labile disulfide bonds in a cysteine-rich protein can be extremely beneficial for a variety of tasks ranging from understanding the mechanistic aspects of protein function to identification of troublesome "hot spots" in biopharmaceutical products. Herein, we describe a mass spectrometry (MS)-based method for reliable identification of labile disulfide bonds, which consists of limited reduction, differential alkylation with an O(18)-labeled reagent, and LC-MS/MS analysis. Application of this method to a cysteine-rich protein transferrin allows the majority of its native disulfide bonds to be measured for their reduction susceptibility, which appears to reflect both solvent accessibility and bond strain energy.

A plug-and-play approach to antibody-based therapeutics via a chemoselective dual click strategy

Although recent methods for the engineering of antibody–drug conjugates (ADCs) have gone some way to addressing the challenging issues of ADC construction, significant hurdles still remain. There is clear demand for the construction of novel ADC platforms that offer greater stability, homogeneity and flexibility. Here we describe a significant step towards a platform for next-generation antibody-based therapeutics by providing constructs that combine site-specific modification, exceptional versatility and high stability, with retention of antibody binding and structure post-modification. The relevance of the work in a biological context is also demonstrated in a cytotoxicity assay and a cell internalization study with HER2-positive and -negative breast cancer cell lines.

Antibody–drug conjugates are a class of therapeutic combining the directing ability of antibodies with the cell-killing ability of cytotoxic drugs. Here the authors describe an approach based on click chemistry that enables the rapid assembly of dual-modified antibodies with potential for new therapeutic modalities.

High-Resolution Imaging of Polyethylene Glycol Coated Dendrimers via Combined Atomic Force and Scanning Tunneling Microscopy

Dendrimers have shown great promise as drug delivery vehicles in recent years because they can be synthesized with designed size and functionalities for optimal transportation, targeting, and biocompatibility. One of the most well-known termini used for biocompatibility is polyethylene glycol (PEG), whose performance is affected by its actual conformation. However, the conformation of individual PEG bound to soft materials such as dendrimers has not been directly observed. Using atomic force microscopy (AFM) and scanning tunneling microscopy (STM), this work characterizes the structure adopted by PEGylated dendrimers with the highest resolution reported to date. AFM imaging enables visualization of the individual dendrimers, as well as the differentiation and characterization of the dendrimer core and PEG shell. STM provides direct imaging of the PEG extensions with high-resolution. Collectively, this investigation provides important insight into the structure of coated dendrimers, which is crucial for the design and development of better drug delivery vehicles.

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.

Cancer immunotherapy: nanodelivery approaches for immune cell targeting and tracking

Cancer is one of the most common diseases afflicting people globally. New therapeutic approaches are needed due to the complexity of cancer as a disease. Many current treatments are very toxic and have modest efficacy at best. Increased understanding of tumor biology and immunology has allowed the development of specific immunotherapies with minimal toxicity. It is important to highlight the performance of monoclonal antibodies, immune adjuvants, vaccines and cell-based treatments. Although these approaches have shown varying degrees of clinical efficacy, they illustrate the potential to develop new strategies. Targeted immunotherapy is being explored to overcome the heterogeneity of malignant cells and the immune suppression induced by both the tumor and its microenvironment. Nanodelivery strategies seek to minimize systemic exposure to target therapy to malignant tissue and cells. Intracellular penetration has been examined through the use of functionalized particulates. These nano-particulate associated medicines are being developed for use in imaging, diagnostics and cancer targeting. Although nano-particulates are inherently complex medicines, the ability to confer, at least in principle, different types of functionality allows for the plausible consideration these nanodelivery strategies can be exploited for use as combination medicines. The development of targeted nanodelivery systems in which therapeutic and imaging agents are merged into a single platform is an attractive strategy. Currently, several nanoplatform-based formulations, such as polymeric nanoparticles, micelles, liposomes and dendrimers are in preclinical and clinical stages of development. Herein, nanodelivery strategies presently investigated for cancer immunotherapy, cancer targeting mechanisms and nanocarrier functionalization methods will be described. We also intend to discuss the emerging nano-based approaches suitable to be used as imaging techniques and as cancer treatment options.

Cys(i)-Lys(i+3)-Lys(i+4) triad: a general approach for PEG-based stabilization of α-helical proteins

PEGylation is an important strategy for enhancing the pharmacokinetic properties of protein drugs. Modern chemoselective reactions now enable specific placement of a single PEG at any site on a protein surface. However, few rational structure-based guidelines exist for selecting optimal PEGylation sites. Here, we explore the impact of PEGylation on the conformational stability of α-helices using an α-helical coiled coil as a model system. We find that maleimide-based PEGylation of a solvent-exposed i position Cys can stabilize coiled-coil quaternary structure when Lys residues occupy both the i + 3 and i + 4 positions, due to favorable interactions between the PEG-maleimide and the Lys residues. Applying this Cys(i)-Lys(i+3)-Lys(i+4) triad to a solvent-exposed position within the C-terminal helix of the villin headpiece domain leads to similar PEG-based increases in conformational stability, highlighting the possibility of using the Cys(i)-Lys(i+3)-Lys(i+4) triad as a general strategy for PEG-based stabilization of helical proteins.

A disulfide intercalator toolbox for the site-directed modification of polypeptides

A disulfide intercalator toolbox was developed for site-specific attachment of a broad variety of functional groups to proteins or peptides under mild, physiological conditions. The peptide hormone somatostatin (SST) served as model compound for intercalation into the available disulfide functionalization schemes starting from the intercalator or the reactive SST precursor before or after bioconjugation. A tetrazole-SST derivative was obtained that undergoes photoinduced cycloaddition in mammalian cells, which was monitored by live-cell imaging.

Agile delivery of protein therapeutics to CNS

A variety of therapeutic proteins have shown potential to treat central nervous system (CNS) disorders. Challenge to deliver these protein molecules to the brain is well known. Proteins administered through parenteral routes are often excluded from the brain because of their poor bioavailability and the existence of the blood-brain barrier (BBB). Barriers also exist to proteins administered through non-parenteral routes that bypass the BBB. Several strategies have shown promise in delivering proteins to the brain. This review, first, describes the physiology and pathology of the BBB that underscore the rationale and needs of each strategy to be applied. Second, major classes of protein therapeutics along with some key factors that affect their delivery outcomes are presented. Third, different routes of protein administration (parenteral, central intracerebroventricular and intraparenchymal, intranasal and intrathecal) are discussed along with key barriers to CNS delivery associated with each route. Finally, current delivery strategies involving chemical modification of proteins and use of particle-based carriers are overviewed using examples from literature and our own work. Whereas most of these studies are in the early stage, some provide proof of mechanism of increased protein delivery to the brain in relevant models of CNS diseases, while in few cases proof of concept had been attained in clinical studies. This review will be useful to broad audience of students, academicians and industry professionals who consider critical issues of protein delivery to the brain and aim developing and studying effective brain delivery systems for protein therapeutics.

An amino acid-based heterofunctional cross-linking reagent

We describe the synthesis and characterization of a new lysine-based heterofunctional cross-linking reagent. It carries two readily available aminooxy functionalities and an activated and protected thiol group that is capable of generating reducible disulfides, the former enable bioorthogonal modification of ketones and aldehydes by the formation of an oxime bond. The efficacy of the linker was proven by coupling two doxorubicin molecules to the functionalized amino acid core and the subsequent bioconjugation of this drug conjugate with a thiolated antibody.

RAFT aqueous dispersion polymerization yields poly(ethylene glycol)-based diblock copolymer nano-objects with predictable single phase morphologies

A poly(ethylene glycol) (PEG) macromolecular chain transfer agent (macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate chain-end functionality in a three-step synthesis starting from a monohydroxy PEG₁₁₃ precursor. This PEG₁₁₃-dithiobenzoate is then used for the reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). Polymerizations conducted under optimized conditions at 50 °C led to high conversions as judged by ¹H NMR spectroscopy and relatively low diblock copolymer polydispersities (M_w/M_n < 1.25) as judged by GPC. The latter technique also indicated good blocking efficiencies, since there was minimal PEG₁₁₃ macro-CTA contamination. Systematic variation of the mean degree of polymerization of the core-forming PHPMA block allowed PEG₁₁₃-PHPMA_x diblock copolymer spheres, worms, or vesicles to be prepared at up to 17.5% w/w solids, as judged by dynamic light scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic oligolamellar vesicles were observed at 20% w/w solids when targeting highly asymmetric diblock compositions. Detailed analysis of SAXS curves indicated that the mean number of membranes per oligolamellar vesicle is approximately three. A PEG₁₁₃-PHPMA_x phase diagram was constructed to enable the reproducible targeting of pure phases, as opposed to mixed morphologies (e.g., spheres plus worms or worms plus vesicles). This new RAFT PISA formulation is expected to be important for the rational and efficient synthesis of a wide range of biocompatible, thermo-responsive PEGylated diblock copolymer nano-objects for various biomedical applications.

Advances in Polymer and Polymeric Nanostructures for Protein Conjugation

Linear polymers have been considered the best molecular structures for the formation of efficient protein conjugates due to their biological advantages, synthetic convenience and ease of functionalization. In recent years, much attention has been dedicated to develop synthetic strategies that produce the most control over protein conjugation utilizing linear polymers as scaffolds. As a result, different conjugate models, such as semitelechelic, homotelechelic, heterotelechelic and branched or star polymer conjugates, have been obtained that take advantage of these well-controlled synthetic strategies. Development of protein conjugates using nanostructures and the formation of said nanostructures from protein-polymer bioconjugates are other areas in the protein bioconjugation field. Although several polymer-protein technologies have been developed from these discoveries, few review articles have focused on the design and function of these polymers and nanostructures. This review will highlight some recent advances in protein-linear polymer technologies that employ protein covalent conjugation and successful protein-nanostructure bioconjugates (covalent conjugation as well) that have shown great potential for biological applications.

Ceramic nanocarriers: versatile nanosystem for protein and peptide delivery

INTRODUCTION

Proteins and peptides have been established to be the potential drug candidate for various human diseases. But, delivery of these therapeutic protein and peptides is still a challenge due to their several unfavorable properties. Nanotechnology is expanding as a promising tool for the efficient delivery of proteins and peptides. Among numerous nano-based carriers, ceramic nanoparticles have proven themselves as a unique carrier for protein and peptide delivery as they provide a more stable, bioavailable, readily manufacturable, and acceptable proteins and polypeptide formulation.

AREAS COVERED

This article provides an overview of the various aspects of ceramic nanoparticles including their classification, methods of preparation, latest advances, and applications as protein and peptide delivery carriers.

EXPERT OPINION

Ceramic nanocarriers seem to have potential for preserving structural integrity of proteins and peptides, thereby promoting a better therapeutic effect. This approach thus provides pharmaceutical scientists with a new hope for the delivery of proteins and peptides. Still, considerable study on ceramic nanocarrier is necessary with respect to pharmacokinetics, toxicology, and animal studies to confirm their efficiency as well as safety and to establish their clinical usefulness and scale-up to industrial level.

Mechanistic insights into the stabilization of srcSH3 by PEGylation

Protein PEGylation (attaching PEG chains to proteins) has been widely used in pharmaceuticals and nanotechnology. Although it is widely known that PEGylation can increase the thermodynamic stability of proteins, the underlying mechanism remains elusive. In this Article, we studied the effect of PEGylation on the thermodynamic and kinetic stability of a protein, SH3. We show that the thermodynamic stability of SH3 is enhanced upon PEGylation, mainly due to the slowing of the unfolding rate. Moreover, PEGylation can decrease the solvent-accessible surface area of SH3, leading to an increase of the m-value (the change in free energy with respect to denaturant concentration, which is a measure of the transition cooperativity between corresponding states). Such an effect also causes an enhancement of the thermodynamic stability. We quantitatively measured how the physical properties of PEG, such as the molecular weight and the number of PEGylation sites, affect the stabilization effect. We found that the stabilization effect is largely dependent on the number of PEGylation sites but only has a weak correlation with the molecular weight of the attached PEG. These experimental findings inspire us to derive a physical model based on excluded volume effect, which can satisfactorily describe all experimental observations. This model allows quantitatively calculating the free energy change upon PEGylation based on the change of water excluded zone on the protein surface. Although it is still unknown whether such a mechanism can be extended to other proteins, our work represents a key step toward the understanding of the nature of protein stabilization upon PEGylation.

Comparative binding of disulfide-bridged PEG-Fabs

Protein PEGylation is the most clinically validated method to improve the efficacy of protein-based medicines. Antibody fragments such as Fabs display rapid clearance from blood circulation and therefore are good candidates for PEGylation. We have developed PEG-bis-sulfone reagents 1 that can selectively alkylate both sulfurs derived from a native disulfide. Using PEG-bis-sulfone reagents 1, conjugation of PEG specifically targets the disulfide distal to the binding region of the Fab (Scheme 2 ). PEG-bis-sulfone reagents 1 (10-40 kDa) were used to generate the corresponding PEG-mono-sulfones 2 that underwent essentially quantitative conjugation to give the PEG-Fab product 4. Four Fabs were PEGylated: Fab(beva), Fab'(beva), Fab(rani), and Fab(trast). Proteolytic digestion of bevacizumab with papain gave Fab(beva), while digestion of bevacizumab with IdeS gave F(ab')(2-beva), which after reaction with DTT and PEG-mono-sulfone 2 gave PEG(2)-Fab'(beva). Ranibizumab, which is a clinically used Fab, was directly PEGylated to give PEG-Fab(rani). Trastuzumab was proteolytically digested with papain, and its corresponding Fab was PEGylated to give PEG-Fab(trast). Purification of the PEGylated Fabs was accomplished by a single ion exchange chromatography step to give pure PEG-Fab products as determined by silver-stained SDS-PAGE. No loss of PEG was detected post conjugation. A comparative binding study by SPR using Biacore with low ligand immobilization density was conducted using (i) VEGF(165) for the bevacizumab and ranibizumab derived products or (ii) HER2 for the trastuzumab derived products. VEGF(165) is a dimeric ligand with two binding sites for bevacizumab. HER2 has one domain for the binding of trastuzumab. Binding studies with PEG-Fab(beva) indicated that the apparent affinity was 2-fold less compared to the unPEGylated Fab(beva). Binding properties of the PEG-Fab(beva) products appeared to be independent of PEG molecular weight. Site-specific conjugation of two PEG molecules gave PEG(2×20)-Fab'(beva), whose apparent binding affinity was similar to that observed for PEG-Fab(beva) derivatives. The k(d) values were similar to those of the unPEGylated Fab(beva); hence, once bound, PEG-Fab(beva) remained bound to the same degree as Fab(beva). Biacore analysis indicated that both Fab(rani) and PEG(20)-Fab(rani) did not dissociate from the immobilized VEGF at 25 °C, but ELISA using immobilized VEGF showed 2-fold less apparent binding affinity for PEG(20)-Fab(rani) compared to the unPEGylated Fab(rani). Additionally, the apparent binding affinities for trastuzumab and Fab(trast) were comparable by both Biacore and ELISA. Biacore results suggested that trastuzumab had a slower association rate compared to Fab(trast); however, both molecules displayed the same apparent binding affinity. This could have been due to enhanced rebinding effects of trastuzumab, as it is a bivalent molecule. Analogous to PEG-Fab(beva) products, PEG(20)-Fab(trast) displayed 2-fold lower binding compared to Fab(trast) when evaluated by ELISA. The variations in the apparent affinity for the PEGylated Fab variants were all related to the differences in the association rates (k(a)) rather than the dissociation rates (k(d)). We have shown that (i) Fabs are well-matched for site-specific PEGylation with our bis-alkylation PEG reagents, (ii) PEGylated Fabs display only a 2-fold reduction in apparent affinity without any change in the dissociation rate, and (iii) the apparent binding rates and affinities remain constant as the PEG molecular weight is varied.

Therapeutic anti-methamphetamine antibody fragment-nanoparticle conjugates: synthesis and in vitro characterization

Treatments specific to the medical problems caused by methamphetamine (METH) abuse are greatly needed. Toward this goal, we are developing new multivalent anti-METH antibody fragment-nanoparticle conjugates with customizable pharmacokinetic properties. We have designed a novel anti-METH single chain antibody fragment with an engineered terminal cysteine (scFv6H4Cys). Generation 3 (G3) polyamidoamine dendrimer nanoparticles were chosen for conjugation due to their monodisperse properties and multiple amine functional groups. ScFv6H4Cys was conjugated to G3 dendrimers via a heterobifunctional PEG cross-linker that is reactive to a free amine on one end and a thiol group on the other. PEG modified dendrimers were synthesized by reacting the PEG cross-linker with dendrimers in a stoichiometric ratio of 11:1, which were further reacted with 3-fold molar excess of anti-METH scFv6H4Cys. This reaction resulted in a heterogeneous mix of G3-PEG-scFv6H4Cys conjugates (dendribodies) with three to six scFv6H4Cys conjugated to each dendrimer. The dendribodies were separated from the unreacted PEG modified dendrimers and scFv6H4Cys using affinity chromatography. A detailed in vitro characterization of the PEG modified dendrimers and the dendribodies was performed to determine size, purity, and METH binding function. The dendribodies were found to have affinity for METH identical to that of the unconjugated scFv6H4Cys in saturation binding assays, whereas the PEG modified dendrimers had no affinity for METH. These data suggest that an anti-METH scFv can be successfully conjugated to a PEG modified dendrimer nanoparticle with no adverse effects on METH binding properties. This study is a critical step toward preclinical characterization and development of a novel nanomedicine for the treatment of METH abuse.

Site-specific PEGylation of therapeutic proteins via optimization of both accessible reactive amino acid residues and PEG derivatives

Modification of accessible amino acid residues with poly(ethylene glycol) [PEG] is a widely used technique for formulating therapeutic proteins. In practice, site-specific PEGylation of all selected/engineered accessible nonessential reactive residues of therapeutic proteins with common activated PEG derivatives is a promising strategy to concomitantly improve pharmacokinetics, allow retention of activity, alleviate immunogenicity, and avoid modification isomers. Specifically, through molecular engineering of a therapeutic protein, accessible essential residues reactive to an activated PEG derivative are substituted with unreactive residues provided that protein activity is retained, and a limited number of accessible nonessential reactive residues with optimized distributions are selected/introduced. Subsequently, all accessible nonessential reactive residues are completely PEGylated with the activated PEG derivative in great excess. Branched PEG derivatives containing new PEG chains with negligible metabolic toxicity are more desirable for site-specific PEGylation. Accordingly, for the successful formulation of therapeutic proteins, optimization of the number and distributions of accessible nonessential reactive residues via molecular engineering can be integrated with the design of large-sized PEG derivatives to achieve site-specific PEGylation of all selected/engineered accessible reactive residues.

Green polymer chemistry: Living oxidative polymerization of dithiols

Reduction sensitivity and mild synthetic conditions make disulfide-bonded materials ideal for degradable biomaterial applications. Both the degradation and the synthetic advantages of disulfide-bonded biomaterials have been applied to drug delivery vesicles, protein conjugation, and hydrogel biomaterials, but the synthetic advantages are rarely seen in the creation of biopolymers. A greener and highly efficient oxidative system is presented for the polymerization dithiols to high-molecular-weight poly(disulfide) polymers. The application of this system to 2-[2-(2-sulfanylethoxy)ethoxy]ethanethiol (DODT) produced corresponding degradable poly(disulfide) polymers with molecular weights as high as Mn = 250 000 g/mol and with a polydispersity index (PDI) as low as 1.15.

Advances and trends in the design, analysis, and characterization of polymer-protein conjugates for "PEGylaided" bioprocesses

In addition to their use as therapeutics and because of their enhanced properties, PEGylated proteins have potential application in fields such as bioprocessing. However, the use of PEGylated conjugates to improve the performance of bioprocess has not been widely explored. This limited additional industrial use of PEG-protein conjugates can be attributed to the fact that PEGylation reactions, separation of the products, and final characterization of the structure and activity of the resulting species are not trivial tasks. The development of bioprocessing operations based on PEGylated proteins relies heavily in the use of analytical tools that must sometimes be adapted from the strategies used in pharmaceutical conjugate development. For instance, to evaluate conjugate performance in bioprocessing operations, both chromatographic and non-chromatographic steps must be used to separate and quantify the resulting reaction species. Characterization of the conjugates by mass spectrometry, circular dichroism, and specific activity assays, among other adapted techniques, is then required to evaluate the feasibility of using the conjugates in any operation. Correct selection of the technical and analytical methods in each of the steps from design of the PEGylation reaction to its final engineering application will ensure success in implementing a "PEGylaided" process. In this context, the objective of this review is to describe technological and analytical trends in developing successful applications of PEGylated conjugates in bioprocesses and to describe potential fields in which these proteins can be exploited.

State of the art in PEGylation: the great versatility achieved after forty years of research

In the recent years, protein PEGylation has become an established and highly refined technology by moving forward from initial simple random coupling approaches based on conjugation at the level of lysine ε-amino group. Nevertheless, amino PEGylation is still yielding important conjugates, currently in clinical practice, where the degree of homogeneity was improved by optimizing the reaction conditions and implementing the purification processes. However, the current research is mainly focused on methods of site-selective PEGylation that allow the obtainment of a single isomer, thus highly increasing the degree of homogeneity and the preservation of bioactivity. Protein N-terminus and free cysteines were the first sites exploited for selective PEGylation but currently further positions can be addressed thanks to approaches like bridging PEGylation (disulphide bridges), enzymatic PEGylation (glutamines and C-terminus) and glycoPEGylation (sites of O- and N-glycosylation or the glycans of a glycoprotein). Furthermore, by combining the tools of genetic engineering with specific PEGylation approaches, the polymer can be basically coupled at any position on the protein surface, owing to the substitution of a properly chosen amino acid in the sequence with a natural or unnatural amino acid bearing an orthogonal reactive group. On the other hand, PEGylation has not achieved the same success in the delivery of small drugs, despite the large interest and several studies in this field. Targeted conjugates and PEGs for combination therapy might represent the promising answers for the so far unmet needs of PEG as carrier of small drugs. This review presents a thorough panorama of recent advances in the field of PEGylation.

Strategies for extended serum half-life of protein therapeutics

With a growing number of protein therapeutics being developed, many of them exhibiting a short plasma half-life, half-life extension strategies find increasing attention by the biotech and pharmaceutical industry. Extension of the half-life can help to reduce the number of applications and to lower doses, thus are beneficial for therapeutic but also economic reasons. Here, a comprehensive overview of currently developed half-life extension strategies is provided including those aiming at increasing the hydrodynamic volume of a protein drug but also those implementing recycling processes mediated by the neonatal Fc receptor.