Copolymer Micelles: A Focus on Recent Advances for Stimulus-Responsive Delivery of Proteins and Peptides

Historically used for the delivery of hydrophobic drugs through core encapsulation, amphiphilic copolymer micelles have also more recently appeared as potent nano-systems to deliver protein and peptide therapeutics. In addition to ease and reproducibility of preparation, micelles are chemically versatile as hydrophobic/hydrophilic segments can be tuned to afford protein immobilization through different approaches, including non-covalent interactions (e.g., electrostatic, hydrophobic) and covalent conjugation, while generally maintaining protein biological activity. Similar to many other drugs, protein/peptide delivery is increasingly focused on stimuli-responsive nano-systems able to afford triggered and controlled release in time and space, thereby improving therapeutic efficacy and limiting side effects. This short review discusses advances in the design of such micelles over the past decade, with an emphasis on stimuli-responsive properties for optimized protein/peptide delivery.

Self-assembling nanocarriers from engineered proteins: Design, functionalization, and application for drug delivery

Self-assembling proteins are valuable building blocks for constructing drug nanocarriers due to their self-assembly behavior, monodispersity, biocompatibility, and biodegradability. Genetic and chemical modifications allow for modular design of protein nanocarriers with effective drug encapsulation, targetability, stimuli responsiveness, and in vivo half-life. Protein nanocarriers have been developed to deliver various therapeutic molecules including small molecules, proteins, and nucleic acids with proven in vitro and in vivo efficacy. This article reviews recent advances in protein nanocarriers that are not derived from natural protein nanostructures, such as protein cages or virus like particles. The protein nanocarriers described here are self-assembled from rationally or de novo designed recombinant proteins, as well as recombinant proteins complexed with other biomolecules, presenting properties that are unique from those of natural protein carriers. Design, functionalization, and therapeutic application of protein nanocarriers will be discussed.

Nanocarriers for intracellular co-delivery of proteins and small-molecule drugs for cancer therapy

In the past few decades, the combination of proteins and small-molecule drugs has made tremendous progress in cancer treatment, but it is still not satisfactory. Because there are great differences in molecular weight, water solubility, stability, pharmacokinetics, biodistribution, and the ways of release and action between macromolecular proteins and small-molecule drugs. To improve the efficacy and safety of tumor treatment, people are committed to developing protein and drug co-delivery systems. Currently, intracellular co-delivery systems have been developed that integrate proteins and small-molecule drugs into one nanocarrier via various loading strategies. These systems significantly improve the blood stability, half-life, and biodistribution of proteins and small-molecule drugs, thus increasing their concentration in tumors. Furthermore, proteins and small-molecule drugs within these systems can be specifically targeted to tumor cells, and are released to perform functions after entering tumor cells simultaneously, resulting in improved effectiveness and safety of tumor treatment. This review summarizes the latest progress in protein and small-molecule drug intracellular co-delivery systems, with emphasis on the composition of nanocarriers, as well as on the loading methods of proteins and small-molecule drugs that play a role in cells into the systems, which have not been summarized by others so far.

End Group Dye-labeled Polycarbonate Block Copolymers for Micellar (immuno-)Drug Delivery

Defined conjugation of functional molecules to block copolymer end groups is a powerful strategy to enhance the scope of micellar carriers for drug delivery. In this study, we have established an approach to access well-defined polycarbonate-based block copolymers by labeling their end groups with single fluorescent dye molecules. Following controlled polymerization conditions, the block copolymers' primary hydroxy end group can be converted into activated pentafluorophenyl ester carbonates and subsequently aminolyzed with fluorescent dyes that are equipped with primary amines. During a solvent evaporation process, the resulting end group dye-labeled block copolymers self-assemble into narrowly dispersed 26 nm sized micelles and simultaneously encapsulate hydrophobic (immuno-)drugs. The covalently attached fluorescent tracer can be used to monitor both uptake into cells and stability under biologically relevant conditions, including incubation with blood plasma or during blood circulation in zebrafish embryos. By encapsulation of the TLR7/8 agonist CL075, immune stimulatory polymeric micelles are generated that get internalized by various antigen presenting dendritic cells and promote their maturation. Generally, such end group dye-labeled polycarbonate block copolymers display ideal features to permit targeted delivery of hydrophobic drugs to key immune cells for vaccination and cancer immunotherapy. This article is protected by copyright. All rights reserved.

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.

Self-assembly of protein-polymer conjugates for drug delivery

Protein-polymer conjugates are a class of molecules that combine the stability of polymers with the diversity, specificity, and functionality of biomolecules. These bioconjugates can result in hybrid materials that display properties not found in their individual components and can be particularly relevant for drug delivery applications. Engineering amphiphilicity into these bioconjugate materials can lead to phase separation and the assembly of high-order structures. The assembly, termed self-assembly, of these hierarchical structures entails multiple levels of organization: at each level, new properties emerge, which are, in turn, influenced by lower levels. Here, we provide a critical review of protein-polymer conjugate self-assembly and how these materials can be used for therapeutic applications and drug delivery. In addition, we discuss central bioconjugate design questions and propose future perspectives for the field of protein-polymer conjugate self-assembly.

Supramolecular cancer nanotheranostics

Among the many challenges in medicine, the treatment and cure of cancer remains an outstanding goal given the complexity and diversity of the disease. Nanotheranostics, the integration of therapy and diagnosis in nanoformulations, is the next generation of personalized medicine to meet the challenges in precise cancer diagnosis, rational management and effective therapy, aiming to significantly increase the survival rate and improve the life quality of cancer patients. Different from most conventional platforms with unsatisfactory theranostic capabilities, supramolecular cancer nanotheranostics have unparalleled advantages in early-stage diagnosis and personal therapy, showing promising potential in clinical translations and applications. In this review, we summarize the progress of supramolecular cancer nanotheranostics and provide guidance for designing new targeted supramolecular theranostic agents. Based on extensive state-of-the-art research, our review will provide the existing and new researchers a foundation from which to advance supramolecular cancer nanotheranostics and promote translationally clinical applications.

Biological responses to physicochemical properties of biomaterial surface

Biomedical scientists use chemistry-driven processes found in nature as an inspiration to design biomaterials as promising diagnostic tools, therapeutic solutions, or tissue substitutes. While substantial consideration is devoted to the design and validation of biomaterials, the nature of their interactions with the surrounding biological microenvironment is commonly neglected. This gap of knowledge could be owing to our poor understanding of biochemical signaling pathways, lack of reliable techniques for designing biomaterials with optimal physicochemical properties, and/or poor stability of biomaterial properties after implantation. The success of host responses to biomaterials, known as biocompatibility, depends on chemical principles as the root of both cell signaling pathways in the body and how the biomaterial surface is designed. Most of the current review papers have discussed chemical engineering and biological principles of designing biomaterials as separate topics, which has resulted in neglecting the main role of chemistry in this field. In this review, we discuss biocompatibility in the context of chemistry, what it is and how to assess it, while describing contributions from both biochemical cues and biomaterials as well as the means of harmonizing them. We address both biochemical signal-transduction pathways and engineering principles of designing a biomaterial with an emphasis on its surface physicochemistry. As we aim to show the role of chemistry in the crosstalk between the surface physicochemical properties and body responses, we concisely highlight the main biochemical signal-transduction pathways involved in the biocompatibility complex. Finally, we discuss the progress and challenges associated with the current strategies used for improving the chemical and physical interactions between cells and biomaterial surface.

Nanomedicine-mediated alteration of the pharmacokinetic profile of small molecule cancer immunotherapeutics

The advent of immunotherapy is a game changer in cancer therapy with monoclonal antibody- and T cell-based therapeutics being the current flagships. Small molecule immunotherapeutics might offer advantages over the biological drugs in terms of complexity, tissue penetration, manufacturing cost, stability, and shelf life. However, small molecule drugs are prone to rapid systemic distribution, which might induce severe off-target side effects. Nanotechnology could aid in the formulation of the drug molecules to improve their delivery to specific immune cell subsets. In this review we summarize the current efforts in changing the pharmacokinetic profile of small molecule immunotherapeutics with a strong focus on Toll-like receptor agonists. In addition, we give our vision on limitations and future pathways in the route of nanomedicine to the clinical practice.

Straightforward Access to Glycosylated, Acid Sensitive Nanogels by Host-Guest Interactions with Sugar-Modified Pillar[5]arenes

The introduction of specific targeting units to polymer nanogels usually requires tedious chemical modifications, which limits flexibility in the design of combinatorial approaches. Here, we present a straightforward and versatile method to reversibly introduce various carbohydrate-based targeting units to a pH-sensitive nanogel via host-guest interactions. Glucose-, mannose-, or fructose-modified pillar[5]arenes can adaptably and conveniently be introduced to the surface of the nanogel. Binding studies between these nanogels and the lectin Concanavalin A revealed a high selectivity and strong interaction with only the mannose-modified nanogels. With the addition of other pillar[5]arenes, the interaction can be influenced proving a dynamic exchange of the targeting units. In comparison with common covalent modifications of polymer nanostructures, the presented combination of straightforward precipitation polymerization and supramolecular interactions promises convenient access to adaptable nanostructures for high-throughput screening of targeted delivery systems.

pH-Responsive protein nanoparticlesviaconjugation of degradable PEG to the surface of cytochromec

A protein nanoparticle system based on cytochromecwas modified with acid-degradable polyethylene glycol (PEGylation). Vinyl ether moieties distributed in the polyether backbone, enabled particle degradation at slightly acidic pH.

Polymer-drug conjugate therapeutics: advances, insights and prospects

Polymer-drug conjugates have long been a mainstay of the drug delivery field, with several conjugates successfully translated into clinical practice. The conjugation of therapeutic agents to polymeric carriers, such as polyethylene glycol, offers several advantages, including improved drug solubilization, prolonged circulation, reduced immunogenicity, controlled release and enhanced safety. In this Review, we discuss the rational design, physicochemical characteristics and recent advances in the development of different classes of polymer-drug conjugates, including polymer-protein and polymer-small-molecule drug conjugates, dendrimers, polymer nanoparticles and multifunctional systems. Current obstacles hampering the clinical translation of polymer-drug conjugate therapeutics and future prospects are also presented.

Smart pH-Sensitive Nanogels for Controlled Release in an Acidic Environment

The encapsulation of therapeutic compounds into nanosized delivery vectors has become an important strategy to improve efficiency and reduce side effects in drug delivery applications. Here, we report the synthesis of pH-sensitive nanogels, which are based on the monomer N-[(2,2-dimethyl-1,3-dioxolane)methyl]acrylamide (DMDOMA) bearing an acid cleavable acetal group. Degradation studies revealed that these nanogels hydrolyze under acidic conditions and degrade completely, depending on the cross-linker, but are stable in physiological environment. The best performing system was further studied regarding its release kinetics using the anticancer drug doxorubicin. In vitro studies revealed a good compatibility of the unloaded nanogel and the capability of the doxorubicin loaded nanogel to mediate cytotoxic effects in a concentration and time-dependent manner with an even higher efficiency than the free drug. Based on the investigated features, the presented nanogels represent a promising and conveniently prepared alternative to existing carrier systems for drug delivery.

Transiently thermoresponsive polymers and their applications in biomedicine

The focus of this review is on the class of transiently thermoresponsive polymers. These polymers are thermoresponsive, but gradually lose this property upon chemical transformation - often a hydrolysis reaction - in the polymer side chain or backbone. An overview of the different approaches used for the design of these polymers along with their physicochemical properties is given. Their amphiphilic properties and degradability into fully soluble compounds make this class of responsive polymers attractive for drug delivery and tissue engineering applications. Examples of these are also provided in this review.

Synthesis and Assembly of Laccase-Polymer Giant Amphiphiles by Self-Catalyzed CuAAC Click Chemistry

Covalent coupling of hydrophobic polymers to the exterior of hydrophilic proteins would mediate unique macroscopic assembly of bioconjugates to generate amphiphilic superstructures as novel nanoreactors or biocompatible drug delivery systems. The main objective of this study was to develop a novel strategy for the synthesis of protein-polymer giant amphiphiles by the combination of copper-mediated living radical polymerization and azide-alkyne cycloaddition reaction (CuAAC). Azide-functionalized succinimidyl ester was first synthesized for the facile introduction of azide groups to proteins such as albumin from bovine serum (BSA) and laccase from Trametes versicolor. Alkyne-terminal polymers with varied hydrophobicity were synthesized by using commercial copper wire as the activators from a trimethylsilyl protected alkyne-functionalized initiator in DMSO under ambient temperature. The conjugation of alkyne-functionalized polymers to the azide-functionalized laccase could be conducted even without additional copper catalyst, which indicated a successful self-catalyzed CuAAC reaction. The synthesized amphiphiles were found to aggregate into spherical nanoparticles in water and showed strong relevance to the hydrophobicity of coupled polymers. The giant amphiphiles showed decreased enzyme activity yet better stability during storage after chemical modification and self-assembly. These findings will deepen our understanding on protein folding, macroscopic self-assembly, and support potential applications in bionanoreactor, enzyme immobilization, and water purification.

Accelerating the acidic degradation of a novel thermoresponsive polymer by host–guest interaction

Carboxylate modified pillar arenes can not only shift the LCST of acetalized polymers but can also accelerate their hydrolysis under acidic conditions.

Tyrosine-Triazolinedione Bioconjugation as Site-Selective Protein Modification Starting from RAFT-Derived Polymers

The electrophilic aromatic substitution (S_EAr) reaction of triazolinediones (TADs) with the phenol moiety of tyrosine amino acid residues is a potent method for the site-selective formation of polymer-protein conjugates. Herein, using poly(N,N-dimethylacrylamide) (pDMA) and bovine serum albumin (BSA) as model reagents, the performance of this tyrosine-TAD bioconjugation in aqueous solutions is explored. At first, reversible addition-fragmentation chain transfer (RAFT) polymerization with a functional urazole, a precursor for TAD, chain transfer agent is used for the synthesis of a TAD end-functionalized pDMA. Eventually, the BSA ligation efficiency and selectivity of this polymer was evaluated in different aqueous solvent mixtures using SDS-PAGE and mass spectroscopy after trypsin digestion.

Nano-thin walled micro-compartments from transmembrane protein-polymer conjugates

The high interfacial activity of protein-polymer conjugates has inspired their use as stabilizers for Pickering emulsions, resulting in many interesting applications such as synthesis of templated micro-compartments and protocells or vehicles for drug and gene delivery. In this study we report, for the first time, the stabilization of Pickering emulsions with conjugates of a genetically modified transmembrane protein, ferric hydroxamate uptake protein component A (FhuA). The lysine residues of FhuA with open pore (FhuA ΔCVF^tev) were modified to attach an initiator and consequently controlled radical polymerization (CRP) carried out via the grafting-from technique. The resulting conjugates of FhuA ΔCVF^tev with poly(N-isopropylacrylamide) (PNIPAAm) and poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA), the so-called building blocks based on transmembrane proteins (BBTP), have been shown to engender larger structures. The properties such as pH-responsivity, temperature-responsivity and interfacial activity of the BBTP were analyzed using UV-Vis spectrophotometry and pendant drop tensiometry. The BBTP were then utilized for the synthesis of highly stable Pickering emulsions, which could remain non-coalesced for well over a month. A new UV-crosslinkable monomer was synthesized and copolymerized with NIPAAm from the protein. The emulsion droplets, upon crosslinking of polymer chains, yielded micro-compartments. Fluorescence microscopy proved that these compartments are of micrometer scale, while cryo-scanning electron microscopy and scanning force microscopy analysis yielded a thickness in the range of 11.1 ± 0.6 to 38.0 ± 18.2 nm for the stabilizing layer of the conjugates. Such micro-compartments would prove to be beneficial in drug delivery applications, owing to the possibility of using the channel of the transmembrane protein as a gate and the smart polymer chains as trigger switches to tune the behavior of the capsules.

Core/shell protein-reactive nanogels via a combination of RAFT polymerization and vinyl sulfone postmodification

AIM

A promising nanogel vaccine platform was expanded toward antigen conjugation.

MATERIALS & METHODS

Block copolymers containing a reactive ester solvophobic block and a PEG-like solvophilic block were synthesized via reversible addition-fragmentation chain-transfer polymerization. Following self-assembly in DMSO, the esters allow for core-crosslinking and hydrophilization by amide bond formation with primary amines. Free thiols were accessed at the polymer chain ends through aminolysis of the reversible addition-fragmentation chain-transfer groups, and into the nanogel core by reactive ester conversion with cysteamine. Subsequently, free thiols were converted into vinyl sulfone moieties.

RESULTS

Despite sterical constraints, nanogel-associated vinyl sulfone moieties remained well accessible for cysteins to enforce protein conjugation successfully.

CONCLUSION

Our present findings provide a next step toward well-defined vaccine nanoparticles that can co-deliver antigen and a molecular adjuvant.

Grafting PNIPAAm from β-barrel shaped transmembrane nanopores

The research on protein-polymer conjugates by grafting from the surface of proteins has gained significant interest in the last decade. While there are many studies with globular proteins, membrane proteins have remained untouched to the best of our knowledge. In this study, we established the conjugate formation with a class of transmembrane proteins and grow polymer chains from the ferric hydroxamate uptake protein component A (FhuA; a β-barrel transmembrane protein of Escherichia coli). As the lysine residues of naturally occurring FhuA are distributed over the whole protein, FhuA was reengineered to have up to 11 lysines, distributed symmetrically in a rim on the membrane exposed side (outside) of the protein channel and exclusively above the hydrophobic region. Reengineering of FhuA ensures a polymer growth only on the outside of the β-barrel and prevents blockage of the channel as a result of the polymerization. A water-soluble initiator for controlled radical polymerization (CRP) was consecutively linked to the lysine residues of FhuA and N-isopropylacrylamide (NIPAAm) polymerized under copper-mediated CRP conditions. The conjugate formation was analyzed by using MALDI-ToF mass spectrometry, SDS-PAGE, circular dichroism spectroscopy, analytical ultracentrifugation, dynamic light scattering, transmission electron microscopy and size exclusion chromatography. Such conjugates combine the specific functions of the transmembrane proteins, like maintaining membrane potential gradients or translocation of substrates with the unique properties of synthetic polymers such as temperature and pH stimuli handles. FhuA-PNIPAAm conjugates will serve as functional nanosized building blocks for applications in targeted drug delivery, self-assembly systems, functional membranes and transmembrane protein gated nanoreactors.

Enhancing Conjugation Yield of Brush Polymer-Protein Conjugates by Increasing Linker Length at the Polymer End-Group

Polymers with oligoethylene glycol side chains are promising in therapeutic protein-polymer conjugates as replacements for linear polyethylene glycol (PEG). Branched PEG polymers can confer additional stability and advantageous properties compared to linear PEGs. However, branched PEG polymers suffer from low conjugation yields to proteins, likely due to steric interactions between bulky side chains of the polymer and the protein. In an effort to increase yields, the linker length between the protein-reactive functional end-group of the polymer chain and branched PEG side chain was systematically increased. This was accomplished by synthesizing four well-defined poly(poly(ethylene glycol methyl ether) acrylates) (pPEGA) with pyridyl disulfide end-groups by reversible addition-fragmentation chain transfer (RAFT) polymerization mediated by chain transfer agents (CTAs) with different linker lengths. These, along with linear PEG and poly(N-isopropylacrylamide) (pNIPAAm), were conjugated to two model proteins, bovine serum albumin (BSA) and beta-lactoglobulin (βLG). The conjugation yields were determined by gel electrophoresis. The length of the linker affected conjugation yield for both proteins. For BSA, the conjugation yield step increased from 10% to 24% when the linker was altered from 1 ethylene glycol (EG) unit to 3, with no additional increase for 4 and 6 EG units. In the case of βLG, the yield gradually increased from 9% to the 33% when the linker length was increased from 1 to 6. PEG and pNIPAAm reacted with yields as high as 75% further emphasizing the effect of steric hindrance in lowering conjugation yields.

Squaric ester amides as hydrolysis-resistant functional groups for protein-conjugation of RAFT-derived polymers

We report on the synthesis of amine-reactive polymers, for the purpose of protein conjugation.