Releasable Conjugation of Polymers to Proteins

Controlling rates and reversibilities of elimination reactions of hydroxybenzylammoniums by tuning dearomatization energies

Hydroxybenzylammonium compounds can undergo a reversible 1,4- or 1,6-elimination to afford quinone methide intermediates after release of the amine. These molecules are useful for the reversible conjugation of payloads to amines. We hypothesized that aromaticity could be used to alter the rate of reversibility as a distinct thermodynamic driving force. We describe the use of density functional theory (DFT) calculations to determine the effect of aromaticity on the rate of release of the amine from hydroxybenzylammonium compounds. Namely, the aromatic scaffold affects the dearomatization reaction to reduce the kinetic barrier and prevent the reversibility of the amine elimination. We consequently synthesized a small library of polycyclic hydroxybenzylammoniums, which resulted in a range of release half-lives from 18 minutes to 350 hours. The novel mechanistic insight provided herein significantly expands the range of release rates amenable to hydroxybenzylammonium-containing compounds. This work provides another way to affect the rate of payload release in hydroxybenzylammoniums.

Tailored Branched Polymer-Protein Bioconjugates for Tunable Sieving Performance

Protein-polymer conjugates combine the unique properties of both proteins and synthetic polymers, making them important materials for biomedical applications. In this work, we synthesized and characterized protein-branched polymer bioconjugates that were precisely designed to retain protein functionality while preventing unwanted interactions. Using chymotrypsin as a model protein, we employed a controlled radical branching polymerization (CRBP) technique utilizing a water-soluble inibramer, sodium 2-bromoacrylate. The green-light-induced atom transfer radical polymerization (ATRP) enabled the grafting of branched polymers directly from the protein surface in the open air. The resulting bioconjugates exhibited a predetermined molecular weight, well-defined architecture, and high branching density. Conformational analysis by SEC-MALS validated the controlled grafting of branched polymers. Furthermore, enzymatic assays revealed that densely grafted polymers prevented protein inhibitor penetration, and the resulting conjugates retained up to 90% of their enzymatic activity. This study demonstrates a promising strategy for designing protein-polymer bioconjugates with tunable sieving behavior, opening avenues for applications in drug delivery and biotechnology.

Enzyme-Polymer Conjugates with Photocleavable Linkers for Control Over Protein Activity

Reversible conjugation of polymers to proteins is important for a variety of applications, for example to control protein activity. Light is often employed as an external trigger to allow for spatio and temporal control over release of a payload. In this report, we demonstrate preparation of photocleavable poly(polyethylene glycol) acrylate)-lysozyme (pPEGA-Lys) conjugates via ortho-nitrobenzyl linkages. The conjugates were made by both grafting-to and grafting-from in order to compare and contrast the two synthetic approaches. First, a lysine-reactive ortho-nitrobenzyl atom transfer radical polymerization (ATRP) initiator was synthesized. For the grafting-to strategy, the initiator was employed in the ATRP of PEGA, and the subsequent polymer was conjugated to the lysine residues of lysozyme. For the grafting-from strategy, lysozyme was modified first with the photocleavable initiator, and the purified macroinitiator was then subjected to polymerization conditions to synthesize the protein-polymer conjugate. The polymer was cleaved from the protein via UV light, and activity before and after polymer removal was evaluated, showing 83% recovery. This work provides evidence that reversing conjugation is successful for activity modulation for ortho-nitrobenzyl linked protein-polymer conjugates.

Noncovalent PEGylation of protein and peptide therapeutics

Clinical applications of protein therapeutics-an advanced generation of drugs characterized by high biological specificity-are rapidly expanding. However, their development is often impeded by unfavorable pharmacokinetic profiles and largely relies on the use of drug delivery systems to prolong their in vivo half-life and suppress undesirable immunogenicity. Although a commercially established PEGylation technology based on protein conjugation with poly(ethylene glycol) (PEG)-protective steric shield resolves some of the challenges, the search for alternatives continues. Noncovalent PEGylation, which mainly relies on multivalent (cooperative) interactions and high affinity (host-guest) complexes formed between protein and PEG offers a number of potential advantages. Among them are dynamic or reversible protection of the protein with minimal loss of biological activity, drastically lower manufacturing costs, "mix-and-match" formulations approaches, and expanded scope of PEGylation targets. While a great number of innovative chemical approaches have been proposed in recent years, the ability to effectively control the stability of noncovalently assembled protein-PEG complexes under physiological conditions presents a serious challenge for the commercial development of the technology. In an attempt to identify critical factors affecting pharmacological behavior of noncovalently linked complexes, this Review follows a hierarchical analysis of various experimental techniques and resulting supramolecular architectures. The importance of in vivo administration routes, degradation patterns of PEGylating agents, and a multitude of potential exchange reactions with constituents of physiological compartments are highlighted. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Recent Advancements on Self-Immolative System Based on Dynamic Covalent Bonds for Delivering Heterogeneous Payloads

The precisely spatial-temporal delivery of heterogeneous payloads from a single system with the same pulse is in great demand in realizing versatile and synergistic functions. Very few molecular architectures can satisfy the strict requirements of dual-release translated from single triggers, while the self-immolative systems based on dynamic covalent bonds represent the "state-of-art" of ultimate solution strategy. Embedding heterogeneous payloads symmetrically onto the self-immolative backbone with dynamic covalent bonds as the trigger, can respond to the quasi-bio-orthogonal hallmarks which are higher at the disease's microenvironment to simultaneously yield the heterogeneous payloads (drug A/drug B or drug/reporter). In this review, the modular design principles are concentrated to illustrate the rules in tailoring useful structures, then the rational applications are enumerated on the aspects of drug codelivery and visualized drug-delivery. This review, hopefully, can give the general readers a comprehensive understanding of the self-immolative systems based on dynamic covalent bonds for delivering heterogeneous payloads.

Insights into self-degradation of cysteine esters and amides under physiological conditions yield new cleavable chemistry

An unprecedented H₂S release from cysteine esters and amides (CysO/NHR) under physiological conditions was discovered and the plausible mechanism was proposed. Alkylation of the amino moiety of cysteine esters enables the H₂S release to be tuned and further provides support to the mechanistic insights. This discovery not only provides new insights into several fundamental science issues including non-enzymatic H₂S-produced pathways, but also inspires new tunable cleavable motifs for sustained release of arylthiols and even for prodrug design.

Customized Reversible Stapling for Selective Delivery of Bioactive Peptides

We have developed a new concept for reversible peptide stapling that involves macrocyclization between two amino groups and decyclization promoted via dual 1,4-elimination. Depending on the trigger moiety, this strategy could be employed to selectively deliver peptides to either intracellular or extracellular targets. As a proof of concept, a peptide inhibitor targeting a lysine-specific demethylase 1 (LSD1) was temporarily cyclized to enhance its stability and ability to cross the cell membrane. Once inside the cells, the biologically active linear peptide was released under reducing environment. Moreover, we have developed reversibly stapled peptides using antimicrobial peptides (RStAMPs) whose bioactive helical conformation can be temporarily destabilized by stapling the peptide backbone. The resulting helix-distorted RStAMPs are nontoxic and highly resistant to protease hydrolysis, while at the infection site, RStAMPs can be rapidly activated by the overproduced H₂O₂ through the dual 1,4-elimination. The latter restored the helical structure of the native peptide and its antimicrobial activity. This work illustrates a highly valuable macrocyclization strategy for the peptide community and should greatly benefit the field of peptide delivery.

Traceless pH-Sensitive Antibody Conjugation Inspired by Citraconic Anhydride

We introduce a pH-sensitive amide bond, inspired by citraconic anhydride, for the reversible conjugation of polymers to the lysine residues of proteins and antibodies. The pH sensitivity arises from a conformation lock at the end of the polymer, which we introduce by means of a Diels-Alder reaction, that positions a carboxylic acid close to the amide after conjugation occurs. The amide is stable over weeks at pH 7.4 but sensitive to hydrolysis at pH 5.5 and below, returning the amine to its original state. The pH sensitivity can be tuned by positioning secondary amide groups nearby. We use this approach to PEGylate an antibody to human serum albumin at high dilution and demonstrate successful recovery of the activity after hydrolysis at pH 5.5. These results offer a convenient and traceless approach to protein and antibody functionalization.

Rational Control of Protein-Protein Interactions with Protein-ATRP-Generated Protease-Sensitive Polymer Cages

Protease-protease interactions lie at the heart of the biological cascades that provide rapid molecular responses to living systems. Blood clotting cascades, apoptosis signaling networks, bacterial infection, and virus trafficking have all evolved to be activated and sustained by protease-protease interactions. Biomimetic strategies designed to target drugs to specific locations have generated proprotein drugs that can be activated by proteolytic cleavage to release native protein. We have previously demonstrated that the modification of enzymes with a custom-designed comb-shaped polymer nanoarmor can shield the enzyme surface and eliminate almost all protein-protein interactions. We now describe the synthesis and characterization of protease-sensitive comb-shaped nanoarmor cages using poly(ethylene glycol) [Sundy, J. S. Arthritis Rheum. 2008, 58(9), 2882-2891]methacrylate macromonomers where the PEG tines of the comb are connected to the backbone of the growing polymer chain by peptide linkers. Protease-induced cleavage of the tines of the comb releases a polymer-modified protein that can once again participate in protein-protein interactions. Atom transfer radical polymerization (ATRP) was used to copolymerize the macromonomer and carboxybetaine methacrylate from initiator-labeled chymotrypsin and trypsin enzymes, yielding proprotease conjugates that retained activity toward small peptide substrates but prevented activity against proteins. Native proteases triggered the release of the PEG side chains from the polymer backbone within 20 min, thereby increasing the activity of the conjugate toward larger protein substrates by 100%. Biomimetic cascade initiation of nanoarmored protease-sensitive protein-polymer conjugates may open the door to a new class of responsive targeted therapies.

Room-Temperature Grafting from Synthesis of Protein-Polydisulfide Conjugates via Aggregation-Induced Polymerization

The reversible modification of proteins with lipoic acid (LPA)-derived polydisulfides (PDS) is an important approach toward the transient regulation and on-demand recovery of protein functions. The in situ growth of PDS from the cysteine (Cys) residue of a protein, however, has been challenging due to the near-equilibrium thermodynamics of the ring-opening polymerization of LPA. Here, we report the protein-mediated, aggregation-induced polymerization (AIP) of amphiphilic LPA-derived monomers at room temperature, which can be performed at a concentration as low as ∼2% of the equilibrium monomer concentration normally needed. The aggregation of monomers increases the effective monomer concentration in aqueous solutions to the degree that the polymerizations behave similarly to those in bulk. The PDS conjugation enhances the thermostability, protease resistance, and tolerance to freeze-thaw treatments of the target proteins. Moreover, the PDS conjugation allows rapid and convenient purification of Cys-bearing proteins by taking advantage of the liquid-liquid phase separation of the protein-PDS conjugates and the full recovery of native proteins under mild reducing conditions. This AIP effect may shed light on facilitating other polymerizations with a similar near-equilibrium character. The PDS conjugation can open up new avenues to protein delivery, dynamic and reversible protein engineering, enzyme preservation, and recycling.

Synthesis and Application of Trehalose Materials

Trehalose is a naturally occurring, nonreducing disaccharide that is widely used in the biopharmaceutical, food, and cosmetic industries due to its stabilizing and cryoprotective properties. Over the years, scientists have developed methodologies to synthesize linear polymers with trehalose units either in the polymer backbone or as pendant groups. These macromolecules provide unique properties and characteristics, which often outperform trehalose itself. Additionally, numerous reports have focused on the synthesis and formulation of materials based on trehalose, such as nanoparticles, hydrogels, and thermoset networks. Among many applications, these polymers and materials have been used as protein stabilizers, as gene delivery systems, and to prevent amyloid aggregate formation. In this Perspective, recent developments in the synthesis and application of trehalose-based linear polymers, hydrogels, and nanomaterials are discussed, with a focus on utilization in the biomedical field.

Self-Immolative Hydroxybenzylamine Linkers for Traceless Protein Modification

Traceless self-immolative linkers are widely used for the reversible modification of proteins and peptides. This article describes a new class of traceless linkers based on ortho- or para-hydroxybenzylamines. The introduction of electron-donating substituents on the aromatic core stabilizes the quinone methide intermediate, thus providing a platform for payload release that can be modulated. To determine the extent to which the electronics affect the rate of release, we prepared a small library of hydroxybenzylamine linkers with varied electronics in the aromatic core, resulting in half-lives ranging from 20 to 144 h. Optimization of the linker design was carried out with mechanistic insights from density functional theory (DFT) and the in silico design of an intramolecular trapping agent through the use of DFT and intramolecular distortion energy calculations. This resulted in the development of a faster self-immolative linker with a half-life of 4.6 h. To demonstrate their effectiveness as traceless linkers for bioconjugation, reversible protein-polyethylene glycol conjugates with a model protein lysozyme were prepared, which had reduced protein activity but recovered ≥94% activity upon traceless release of the polymer. This new class of linkers with tunable release rates expands the traceless linkers toolbox for a variety of bioconjugation applications.

Modulating Boronic Ester Stability in Block Copolymer Micelles via the Neighbor Effect of Copolymerized Tertiary Amines for Controlled Release of Polyphenolic Drugs

The traceless and pH-sensitive properties of boronic esters are attractive for the synthesis of polymer-drug conjugates, but current platforms suffer from both low stability under physiologically relevant conditions and synthetically demanding optimization to tune drug release profiles. We hypothesized that the high catechol affinity and stability of Wulff-type boronic acids could be mimicked by copolymerization of phenyl boronic acid with a tertiary amine and subsequent micellization. This strategy yielded a versatile platform for the preparation of reversible polymer-drug conjugates, which more than doubled the oxidative stability of encapsulated polyphenolic drug cargo at physiologically relevant pH and enabled simple and incremental tuning of drug release kinetics. Moreover, we validated, with ¹⁹F NMR, that these copolymers exhibit uniquely high catechol affinity that could not be replicated by combinations of similarly functionalized small molecules. Overall, this report demonstrates that copolymerization of boronic acid and tertiary amine monomers is a powerful and modular approach to improving boronic ester chemistry for drug delivery applications.

Nanohybrids as Protein-Polymer Conjugate Multimodal Therapeutics

Protein therapeutic formulations are being widely explored as multifunctional nanotherapeutics. Challenges in ensuring susceptibility and efficacy of nanoformulation still prevail owing to various interactions with biological fluids before reaching the target site. Smart polymers with the capability of masking drugs, ease of chemical modification, and multi-stimuli responsiveness can assist controlled delivery. An active moiety like therapeutic protein has started to be known as an important biological formulation with a diverse medicinal prospect. The delivery of proteins and peptides with high target specificity has however been tedious, due to their tendency to aggregate formation in different environmental conditions. Proteins due to high chemical reactivity and poor bioavailability are being researched widely in the field of nanomedicine. Clinically, multiple nano-based formulations have been explored for delivering protein with different carrier systems. A biocompatible and non-toxic polymer-based delivery system serves to tailor the polymer or drug better. Polymers not only aid delivery to the target site but are also responsible for proper stearic orientation of proteins thus protecting them from internal hindrances. Polymers have been shown to conjugate with proteins through covalent linkage rendering stability and enhancing therapeutic efficacy prominently when dealing with the systemic route. Here, we present the recent developments in polymer-protein/drug-linked systems. We aim to address questions by assessing the properties of the conjugate system and optimized delivery approaches. Since thorough characterization is the key aspect for technology to enter into the market, correlating laboratory research with commercially available formulations will also be presented in this review. By examining characteristics including morphology, surface properties, and functionalization, we will expand different hybrid applications from a biomaterial stance applied in in vivo complex biological conditions. Further, we explore understanding related to design criteria and strategies for polymer-protein smart nanomedicines with their potential prophylactic theranostic applications. Overall, we intend to highlight protein-drug delivery through multifunctional smart polymers.

Design of a Thiol-Responsive, Traceless Prodrug with Rapid Self-Immolation for Cancer Chemotherapy

Prodrugs can be formed by chemical modification of the existing active pharmaceutical ingredients (APIs); however, this often sacrifices their functional efficacy. Self-immolative linkers have recently attracted attention, as they can be designed to release pristine APIs. Herein, we report a strategy to generate a self-immolative prodrug (SIP) that can release pristine doxorubicin (DOX). Compared to conventional linkers, the key SIP DOX (KSIP-DOX) developed here can rapidly and quantitatively release the API due to its strong leaving group after reduction by thiol groups, which are present in tumors. KSIP-DOX has enhanced cellular uptake and improved anticancer efficacy, demonstrating its utility for cancer treatment.

Predictive Platforms of Bond Cleavage and Drug Release Kinetics for Macromolecule–Drug Conjugates

Macromolecule–drug conjugates (MDCs) occupy a critical niche in modern pharmaceuticals that deals with the assembly and combination of a macromolecular carrier, a drug cargo, and a linker toward the creation of effective therapeutics. Macromolecular carriers such as synthetic biocompatible polymers and proteins are often exploited for their inherent ability to improve drug circulation, prevent off-target drug cytotoxicity, and widen the therapeutic index of drugs. One of the most significant challenges in MDC design involves tuning their drug release kinetics to achieve high spatiotemporal precision. This level of control requires a thorough qualitative and quantitative understanding of the bond cleavage event. In this review, we highlight specific research findings that emphasize the importance of establishing a precise structure–function relationship for MDCs that can be used to predict their bond cleavage and drug release kinetic parameters.

Attenuation of the Reaction of Michael Acceptors with Biologically Important Nucleophiles

β-Elimination of drugs tethered to macromolecular carbamates provides a platform for drug half-life extension. However, the macromolecular Michael acceptor products formed upon drug release can potentially react with biological amines and thiols and may raise concerns about safety. We desired to mitigate this possibility by developing linkers that have predictable rates of β-elimination but suppressed rates of nucleophilic addition to their Michael acceptor products. We prepared Michael acceptor products of β-eliminative linkers that contained a methyl group at the Cβ carbon or a gem-dimethyl group at the Cγ carbon and studied the kinetics of their reactions with the most prevalent biological nucleophiles-amine and thiol groups. Aza-Michael reactions with glycine are slowed about 20-fold by methylation of the β-carbon and 175-fold with a gem-dimethyl group at the γ-carbon. Likewise, addition of the glutathione thiol to γ-gem-dimethyl Michael acceptors was retarded 7-24-fold compared to parent unsubstituted linkers. It was estimated that in an in vivo environment of ∼0.5 mM macromolecular thiols or ∼20 mM macromolecular amines-as in plasma-the reaction half-life of a typical Michael acceptor with a γ-gem-dimethyl linker could exceed 3 years for thiols or 25 years for amines. We also prepared a large series of γ-gem-dimethyl β-eliminative linkers and showed excellent structure-activity relationships of elimination rates with corresponding unsubstituted parent linkers. Finally, we compared the first-generation unsubstituted and new gem-dimethyl β-eliminative linkers in a once-monthly drug delivery system of a 39 amino acid peptide. Both linkers provided the desired half-life extension of the peptide, but the Michael acceptor formed from the gem-dimethyl linker was much less reactive. We conclude that the γ-gem-dimethyl β-eliminative linkers provide high flexibility and greatly reduce potential reactions of Michael acceptor products with biologically important nucleophiles.

A traceless linker for aliphatic amines that rapidly and quantitatively fragments after reduction

Reduction sensitive linkers (RSLs) have the potential to transform the field of drug delivery due to their ease of use and selective cleavage in intracellular environments. However, despite their compelling attributes, developing reduction sensitive self-immolative linkers for aliphatic amines has been challenging due to their poor leaving group ability and high pK_a values. Here a traceless self-immolative linker composed of a dithiol-ethyl carbonate connected to a benzyl carbamate (DEC) is presented, which can modify aliphatic amines and release them rapidly and quantitatively after disulfide reduction. DEC was able to reversibly modify the lysine residues on CRISPR–Cas9 with either PEG, the cell penetrating peptide Arg₁₀, or donor DNA, and generated Cas9 conjugates with significantly improved biological properties. In particular, Cas9–DEC–PEG was able to diffuse through brain tissue significantly better than unmodified Cas9, making it a more suitable candidate for genome editing in animals. Furthermore, conjugation of Arg₁₀ to Cas9 with DEC was able to generate a self-delivering Cas9 RNP that could edit cells without transfection reagents. Finally, conjugation of donor DNA to Cas9 with DEC increased the homology directed DNA repair (HDR) rate of the Cas9 RNP by 50% in HEK 293T cell line. We anticipate that DEC will have numerous applications in biotechnology, given the ubiquitous presence of aliphatic amines on small molecule and protein therapeutics.

Reduction sensitive linkers (RSLs) have the potential to transform the field of drug delivery due to their ease of use and selective cleavage in intracellular environments.

Polymer nanomedicines

Polymer nanomedicines (macromolecular therapeutics, polymer-drug conjugates, drug-free macromolecular therapeutics) are a group of biologically active compounds that are characterized by their large molecular weight. This review focuses on bioconjugates of water-soluble macromolecules with low molecular weight drugs and selected proteins. After analyzing the design principles, different structures of polymer carriers are discussed followed by the examination of the efficacy of the conjugates in animal models and challenges for their translation into the clinic. Two innovative directions in macromolecular therapeutics that depend on receptor crosslinking are highlighted: a) Combination chemotherapy of backbone degradable polymer-drug conjugates with immune checkpoint blockade by multivalent polymer peptide antagonists; and b) Drug-free macromolecular therapeutics, a new paradigm in drug delivery.

Proteins Conjugated with Sulfoxide-Containing Polymers Show Reduced Macrophage Cellular Uptake and Improved Pharmacokinetics

The conjugation of hydrophilic polymers to proteins is an effective approach to prolonging their circulation time in the bloodstream and, hence, improving their delivery to the target region of interest. In this work, we report the synthesis of protein-polymer conjugates using a highly water-soluble sulfoxide-containing polymer, poly(2-(methylsulfinyl)ethyl acrylate) (PMSEA), through a combination of "grafting-to" and "grafting-from" methods. Oligomeric MSEA was synthesized by conventional reversible addition-fragmentation chain transfer (RAFT) polymerization and subsequently conjugated to lysozyme to produce a macromolecular chain transfer agent. This was followed by a visible light-mediated chain extension polymerization of MSEA to obtain a lysozyme-PMSEA conjugate (Lyz-PMSEA). It was found that the Lyz-PMSEA conjugate exhibited much reduced macrophage cellular uptake compared with unmodified and PEGylated lysozyme. Moreover, the Lyz-PMSEA conjugate was able to circulate longer in the bloodstream, demonstrating significantly improved pharmacokinetics demanded for pharmaceutical applications.

Disulfide-Based Self-Immolative Linkers and Functional Bioconjugates for Biological Applications

It is of vital importance to reversibly mask and selectively activate bioactive agents for advanced therapeutic and diagnostic purposes, aiming to efficiently suppress background interferences and attenuate systemic toxicity. This strategy has been involved in diverse applications spanning from chemical/biological sensors and diagnostics to drug delivery nanocarriers. Among these, redox-responsive disulfide linkages have been extensively utilized by taking advantage of extracellular and intracellular glutathione (GSH) gradients. However, direct conjugation of cleavable triggers to bioactive agents through disulfide bonds suffers from bulky steric hindrance and limited choice of trigger-drug combinations. Fortunately, the emergence of disulfide self-immolative linkers (DSILs) provides a general and robust strategy to not only mask various bioactive agents through the formation of dynamic disulfide linkages but also make it possible to be selectively activated upon disulfide cleavage in the reductive cytoplasmic milieu. In this review, recent developments in DSILs are focused with special attention on emerging chemical design strategies and functional applications in the biomedical field.

Emerging PEGylated non-biologic drugs

Introduction: PEGylation is a well-established technology for improving the therapeutic value of drugs by attaching polyethylene glycol (PEG). The first PEGylated enzyme products appeared on the market in the early 1990s; currently, more than 18 PEGylated products have been approved by Food and Drug Administration, which encompass various classes of drug molecules, such as enzymes, interferons, granulocyte colony-stimulating factors, hormones, antibody fragments, coagulation factors, oligonucleotide aptamers, synthetic peptides, and small organic molecules. Areas covered: While PEGylated products mainly comprise biologic drugs, such as recombinant proteins and enzymes, non-biologic drugs have recently emerged as a target for PEGylation. This review focuses on the recent development of PEGylated non-biologic drugs, such as small organic molecules, synthetic peptides, and aptamers. Expert opinion: Several PEGylated versions of anti-cancer drugs, opioid agonists, glucagon-like peptide-1 receptor agonists, and oligonucleotide aptamers are in active development stage, and it is likely that they will have a dramatic impact on the market. Although some safety concerns about PEG in clinical trials have been recently issued, PEGylation is still a commercially attractive proposition as a half-life extension technology for long-acting drug development.

Modularly engineered injectable hybrid hydrogels based on protein-polymer network as potent immunologic adjuvant in vivo

Lymphoid organs, which are populated by dendritic cells (DCs), are highly specialized tissues and provide an ideal microenvironment for T-cell priming. However, intramuscular or subcutaneous delivery of vaccine to DCs, a subset of antigen-presenting cells, has failed to stimulate optimal immune response for effective vaccination and need for adjuvants to induce immune response. To address this issue, we developed an in situ-forming injectable hybrid hydrogel that spontaneously assemble into microporous network upon subcutaneous administration, which provide a cellular niche to host immune cells, including DCs. In situ-forming injectable hybrid hydrogelators, composed of protein-polymer conjugates, formed a hydrogel depot at the close proximity to the dermis, resulting in a rapid migration of immune cells to the hydrogel boundary and infiltration to the microporous network. The biocompatibility of the watery microporous network allows recruitment of DCs without a DC enhancement factor, which was significantly higher than that of traditional hydrogel releasing chemoattractants, granulocyte-macrophage colony-stimulating factor. Owing to the sustained degradation of microporous hydrogel network, DNA vaccine release can be sustained, and the recruitment of DCs and their homing to lymph node can be modulated. Furthermore, immunization of a vaccine encoding amyloid-β fusion proteinbearing microporous network induced a robust antigen-specific immune response in vivo and strong recall immune response was exhibited due to immunogenic memory. These hybrid hydrogels can be administered in a minimally invasive manner using hypodermic needle, bypassing the need for cytokine or DC enhancement factor and provide niche to host immune cells. These findings highlight the potential of hybrid hydrogels that may serve as a simple, yet multifunctional, platform for DNA vaccine delivery to modulate immune response.

A guide to maximizing the therapeutic potential of protein-polymer conjugates by rational design

Proteins are an important class of therapeutics that have advantages including high target specificity, but challenges to their use include rapid clearance and low physical stability. Conjugation of synthetic polymers is an effective approach to address the drawbacks and enhance other properties such as solubility. In this review, we present various considerations in synthesizing protein-polymer conjugates for therapeutic applications with a focus on the choice of polymer, protein, and conjugation method, as well as characterization and evaluation of the resulting conjugate in order to maximize the therapeutic potential of the protein drug.

Solid-phase synthesis of protein-polymers on reversible immobilization supports

Facile automated biomacromolecule synthesis is at the heart of blending synthetic and biologic worlds. Full access to abiotic/biotic synthetic diversity first occurred when chemistry was developed to grow nucleic acids and peptides from reversibly immobilized precursors. Protein-polymer conjugates, however, have always been synthesized in solution in multi-step, multi-day processes that couple innovative chemistry with challenging purification. Here we report the generation of protein-polymer hybrids synthesized by protein-ATRP on reversible immobilization supports (PARIS). We utilized modified agarose beads to covalently and reversibly couple to proteins in amino-specific reactions. We then modified reversibly immobilized proteins with protein-reactive ATRP initiators and, after ATRP, we released and analyzed the protein polymers. The activity and stability of PARIS-synthesized and solution-synthesized conjugates demonstrated that PARIS was an effective, rapid, and simple method to generate protein-polymer conjugates. Automation of PARIS significantly reduced synthesis/purification timelines, thereby opening a path to changing how to generate protein-polymer conjugates.

Releasable and traceless PEGylation of arginine-rich antimicrobial peptides

This study reports a strategy to temporarily mask arginine residues within antimicrobial peptides (AMPs) with methoxy poly(ethylene glycol) (mPEG). PEGylation protects AMPs from serum proteases, and can be released at a pharmaceutically-relevant rate. Fully active and unmodified (i.e., native) AMPs are released with time.

Arginine-rich antimicrobial peptides (AMPs) are emerging therapeutics of interest. However, their applicability is limited by their short circulation half-life, caused in part by their small size and digestion by blood proteases. This study reports a strategy to temporarily mask arginine residues within AMPs with methoxy poly(ethylene glycol). Based on the reagent used, release of AMPs occurred in hours to days in a completely traceless fashion. In vitro, conjugates were insensitive to serum proteases, and released native AMP with full in vitro bioactivity. This strategy is thus highly relevant and should be adaptable to the entire family of arginine-rich AMPs. It may potentially be used to improve AMP-therapies by providing a more steady concentration of AMP in the blood after a single injection, avoiding toxic effects at high AMP doses, and reducing the number of doses required over the treatment duration.

Modulated Fragmentation of Proapoptotic Peptide Nanoparticles Regulates Cytotoxicity

Peptides perform a diverse range of physiologically important functions. The formulation of nanoparticles directly from functional peptides would therefore offer a versatile and robust platform to produce highly functional therapeutics. Herein, we engineered proapoptotic peptide nanoparticles from mitochondria-disrupting KLAK peptides using a template-assisted approach. The nanoparticles were designed to disassemble into free native peptides via the traceless cleavage of disulfide-based cross-linkers. Furthermore, the cytotoxicity of the nanoparticles was tuned by controlling the kinetics of disulfide bond cleavage, and the rate of regeneration of the native peptide from the precursor species. In addition, a small molecule drug (i.e., doxorubicin hydrochloride) was loaded into the nanoparticles to confer synergistic cytotoxic activity, further highlighting the potential application of KLAK particles in therapeutic delivery.

Automated Synthesis of Well-Defined Polymers and Biohybrids by Atom Transfer Radical Polymerization Using a DNA Synthesizer

A DNA synthesizer was successfully employed for preparation of well-defined polymers by atom transfer radical polymerization (ATRP), in a technique termed AutoATRP. This method provides well-defined homopolymers, diblock copolymers, and biohybrids under automated photomediated ATRP conditions. PhotoATRP was selected over other ATRP methods because of mild reaction conditions, ambient temperature, tolerance to oxygen, and no need to introduce reducing agents or radical initiators. Both acrylate and methacrylate monomers were successfully polymerized with excellent control in the DNA synthesizer. Diblock copolymers were synthesized with different targeted degrees of polymerization and with high retention of chain-end functionality. Both hydrophobic and hydrophilic monomers were grafted from DNA. The DNA-polymer hybrids were characterized by SEC and DLS. The AutoATRP method provides an efficient route to prepare a range of different polymeric materials, especially polymer-biohybrids.

Peptides for tumor-specific drug targeting: state of the art and beyond

This review outlines the most recent advances in peptide-mediated tumor-targeting and gives insight into the direction of the field.

Use of polymer conjugates for the intraperoxisomal delivery of engineered human alanine:glyoxylate aminotransferase as a protein therapy for primary hyperoxaluria type I

Alanine:glyoxylate aminotransferase (AGT) is a liver peroxisomal enzyme whose deficit causes the rare disorder Primary Hyperoxaluria Type I (PH1). We now describe the conjugation of poly(ethylene glycol)-co-poly(L-glutamic acid) (PEG-PGA) block-co-polymer to AGT via the formation of disulfide bonds between the polymer and solvent-exposed cysteine residues of the enzyme. PEG-PGA conjugation did not affect AGT structural/functional properties and allowed the enzyme to be internalized in a cellular model of PH1 and to restore glyoxylate-detoxification. The insertion of the C387S/K390S amino acid substitutions, known to favor interaction with the peroxisomal import machinery, reduced conjugation efficiency, but endowed conjugates with the ability to reach the peroxisomal compartment. These results, along with the finding that conjugates are hemocompatible, stable in plasma, and non-immunogenic, hold promise for the development of polypeptide-based AGT conjugates as a therapeutic option for PH1 patients and represent the base for applications to other diseases related to deficits in peroxisomal proteins.

Stability Enhancing N-Terminal PEGylation of Oxytocin Exploiting Different Polymer Architectures and Conjugation Approaches

Oxytocin, a cyclic nine amino acid neurohypophyseal hormone therapeutic, is effectively used in the control of postpartum hemorrhaging (PPH) and is on the WHO List of Essential Medicines. However, oxytocin has limited shelf life stability in aqueous solutions, particularly at temperatures in excess of 25 °C and injectable aqueous oxytocin formulations require refrigeration (<8 °C). This is particularly problematic in the hot climates often found in many developing countries where daytime temperatures can exceed 40 °C and where reliable cold-chain storage is not always achievable. The purpose of this study was to develop N-terminal amine targeted PEGylation strategies utilizing both linear PEG and polyPEG "comb" polymers as an effective method for stabilizing solution formulations of this peptide for prolonged storage in the absence of efficient cold-chain storage. The conjugation chemistries investigated herein include irreversible amine targeted conjugation methods utilizing NHS ester and aldehyde reductive amination chemistry. Additionally, one reversible conjugation method using a Schiff base approach was explored to allow for the release of the native peptide, thus, ensuring that biological activity remains unaffected. The reversibility of this approach was investigated for the different polymer architectures, alongside a nonpolymer oxytocin analogue to monitor how pH can tune native peptide release. Elevated temperature degradation studies of the polymer conjugates were evaluated to assess the stability of the PEGylated analogues in comparison to the native peptide in aqueous formulations to mimic storage conditions in developing nations and regions where storage under appropriate conditions is challenging.

Readily adaptable release kinetics of prodrugs using protease-dependent reversible PEGylation

Protein and peptide therapeutics with good in vitro activities often fail due to poor bioavailability, circulation lifetime, and immunogenicity. PEGylation, i.e. conjugation of polyethylene glycol (PEG), significantly improves serum stability and renal clearance besides reducing the immunogenicity and thus enhances pharmacokinetics and tolerance in vivo. Several PEGylated drugs are marketed including several top-selling blockbusters. However, PEGylation can mask the binding site, especially in peptides, and thereby reduce the activity drastically, which is only rarely compensated by the improved bioavailability. Prodrug strategies using temporary PEGylation, i.e. the authentic drug is released from a PEG-linked precursor by hydrolysis or enzymatic degradation, can overcome these weaknesses. Recently, we reported a strategy coupling PEG via a peptide linker cleaved C-terminally by trypsin-like proteases in blood to release the unmasked therapeutic peptide. Here, we designed twelve short peptide linkers (four or five residues) to tune the release-rates of oncocin Onc112, a proline-rich antimicrobial peptide. In 25% aqueous mouse serum, Onc112 was released with half-life times from 0.5 to 12h. When elongated N-terminally with 5kDa ɑ-methoxy-ω-mercapto PEG as thioether, the half-life times of the prodrugs ranged from 7 to 42h in full mouse serum. Conjugation of a 20kDa instead of the 5kDa PEG increased the half-life times more than twofold, whereas longer peptide linkers up to twelve residues increased them only slightly. In all cases, Onc112 was released continuously providing stable peptide levels for at least 16h. The kinetics will allow the specific design of PEG-linker-drug-combinations for optimizing the pharmacokinetics of promising peptide therapeutics.