Click Chemistry for Biofunctional Polymers: From Observing to Steering Cell Behavior

Click chemistry has become one of the most powerful construction tools in the field of organic chemistry, materials science, and polymer science, as it offers hassle-free platforms for the high-yielding synthesis of novel materials and easy functionalization strategies. The absence of harsh reaction conditions or complicated workup procedures allowed the rapid development of novel biofunctional polymeric materials, such as biopolymers, tailor-made polymer surfaces, stimulus-responsive polymers, etc. In this review, we discuss various types of click reactions─including azide-alkyne cycloadditions, nucleophilic and radical thiol click reactions, a range of cycloadditions (Diels-Alder, tetrazole, nitrile oxide, etc.), sulfur fluoride exchange (SuFEx) click reaction, and oxime-hydrazone click reactions─and their use for the formation and study of biofunctional polymers. Following that, we discuss state-of-the-art biological applications of "click"-biofunctionalized polymers, including both passive applications (e.g., biosensing and bioimaging) and "active" ones that aim to direct changes in biosystems, e.g., for drug delivery, antiviral action, and tissue engineering. In conclusion, we have outlined future directions and existing challenges of click-based polymers for medicinal chemistry and clinical applications.

Redox-Responsive Core-Cross-Linked Micelles of Miktoarm Poly(ethylene oxide)-b-poly(furfuryl methacrylate) for Anticancer Drug Delivery

The physiological instability of nanocarriers, premature drug leakage during blood circulation, and associated severe side effects cause compromised therapeutic efficacy, which have significantly hampered the progress of nanomedicines. The cross-linking of nanocarriers while keeping the effectiveness of their degradation at the targeted site to release the drug has emerged as a potent strategy to overcome these flaws. Herein, we have designed novel (poly(ethylene oxide))₂-b-poly(furfuryl methacrylate) ((PEO_2K)₂-b-PFMA_nk) miktoarm amphiphilic block copolymers by coupling alkyne-functionalized PEO (PEO_2K-C≡H) and diazide-functionalized poly(furfuryl methacrylate) ((N₃)₂-PFMA_nk) via click chemistry. (PEO_2K)₂-b-PFMA_nk self-assembled to form nanosized micelles (mikUCL) with hydrodynamic radii in the range of 25∼33 nm. The hydrophobic core of mikUCL was cross-linked by a disulfide-containing cross-linker using the Diels-Alder reaction to avoid unwanted leakage and burst release of a payload. As expected, the resulting core-cross-linked (PEO_2K)₂-b-PFMA_nk micelles (mikCCL) exhibited superior stability under a normal physiological environment and were de-cross-linked to rapidly release doxorubicin (DOX) upon exposure to a reduction environment. The micelles were compatible with HEK-293 normal cells, while DOX-loaded micelles (mikUCL/DOX and mikCCL/DOX) induced high antitumor activity in HeLa and HT-29 cells. mikCCL/DOX preferentially accumulated at the tumor site and was more efficacious than free DOX and mikUCL/DOX for tumor inhibition in HT-29 tumor-bearing nude mice.

Reduction-responsive disulfide linkage core-cross-linked polymeric micelles for site-specific drug delivery

Reduction-responsive disulfide linkage core-cross-linked polymeric micelles (CLM@DOX) were developed for effective site-specific doxorubicin delivery.

Synthesis of crosslinkable diblock terpolymers PDPA-b-P(NMS-co-OEG) and preparation of shell-crosslinked pH/redox-dual responsive micelles as smart nanomaterials

Crosslinked polymer nanomaterials have attracted great attention due to their stability and highly controllable drug delivery; herein, a series of well-defined amphiphilic PDPA-b-P(NMS-co-OEG) diblock terpolymers (P1–P3) were designed and prepared via RAFT polymerization and were self-assembled into non-cross-linked (NCL) nanomicelles, which were further prepared into shell-cross-linked (SCL) micelles via cystamine-based in situ shell cross-linking. Using P3 as an optimized polymer, SCL-P3 micelles were prepared, which demonstrated remarkable pH/redox-dual responsive behaviour. For drug delivery, camptothecin (CPT)-loaded SCL-P3 micelles were prepared and showed much higher CPT-loading capability than their NCL-P3 counterparts. Notably, the SCL-P3 micelles showed good synergistic pH/redox-dual responsive CPT release properties, making them potential “smart” nanocarriers for drug delivery.

A series of well-defined amphiphilic PDPA-b-P(NMS-co-OEG) diblock terpolymers were prepared via RAFT polymerization and self-assembled into non-cross-linked nanomicelles, and then shell-cross-linked micelles via cystamine-based in situ shell cross-linking.

Recent advances in stimuli-responsive polymeric micelles via click chemistry

Stimuli-responsive polymeric micelles via click chemistry are divided into six major sections (temperature, light, ultrasound, pH, enzymes, and redox).

Advances in Stimuli‐Responsive Polypeptide Nanogels

Polymer nanogels, which are nanosized particles of hydrogels formed by physically or chemically crosslinking polymers, are one of the most promising nanoplatforms for use in biomedical applications, including drug delivery, gene therapy, and bioimaging. Polypeptide nanogels exhibit many advantages for applications in biomedical fields, including stable 3D structures, high drug‐loading efficiency, excellent biocompatibility, stimuli responsiveness, and so forth. More specifically, smart polypeptide nanogels undergo suitable transitions under endogenous (e.g., reduction, reactive oxygen species, pH, and enzymes) or exogenous stimuli (e.g., light, temperature, voltage, and magnetic fields) for on‐demand drug delivery. Here, a comprehensive introduction to the preparation and applications of diverse stimuli‐responsive polypeptide nanogels is given. Moreover, the opportunities and challenges of polypeptide nanogels for the development of biomedicine are briefly predicted.

Synthetic polypeptides: from polymer design to supramolecular assembly and biomedical application

Synthetic polypeptides from the ring-opening polymerization of N-carboxyanhydrides (NCAs) are one of the most important biomaterials. The unique features of these synthetic polypeptides, including their chemical diversity of side chains and their ability to form secondary structures, enable their broad applications in the field of gene delivery, drug delivery, bio-imaging, tissue engineering, and antimicrobials. In this review article, we summarize the recent advances in the design of polypeptide-based supramolecular structures, including complexes with nucleic acids, micelles, vesicles, hybrid nanoparticles, and hydrogels. We also highlight the progress in the chemical design of functional polypeptides, which plays a crucial role to manipulate their assembly behaviours and optimize their biomedical performances. Finally, we conclude the review by discussing the future opportunities in this field, including further studies on the secondary structures and cost-effective synthesis of polypeptide materials.

Selenylsulfide Bond-Launched Reduction-Responsive Superparamagnetic Nanogel Combined of Acid-Responsiveness for Achievement of Efficient Therapy with Low Side Effect

With the objective to achieve in-between reduction-responsive drug release, selenylsulfide bond was first explored as a reduction cleavable linkage, compared with the most commonly employed disulfide and diselenide bonds. The reductive nanogel, with a combination of superparamagnetic and acid responsiveness, was fabricated. The expected release profiles were testified. Cytotoxicity assays illustrated the remarkable inhibition to the growth of HeLa cells, in contrast, high tolerance to L02 cells. In vivo investigation exhibited the obvious shrinkage in tumor but a healthy appearance. Hematoxylin-eosin staining and histological examination revealed the lower toxicity. The complex nanogels would have appeared highly promising in cancer therapy.

Dual-sensitive and biodegradable core-crosslinked HPMA copolymer–doxorubicin conjugate-based nanoparticles for cancer therapy

The nanoplatform of biosafe crosslinked copolymer-NPs efficiently delivers anticancer drugs to tumor cellsviablood circulation.

Synthesis and self-assembly of diblock glycopolypeptide analogues PMAgala-b-PBLG as multifunctional biomaterials for protein recognition, drug delivery and hepatoma cell targeting

PMAgala-b-PBLG glycopolypeptide analogues might serve as redox-responsive, highly biocompatible multifunctional biomaterial platforms for practical applications.

Photo, pH and redox multi-responsive nanogels for drug delivery and fluorescence cell imaging

A light, pH and redox triple-responsive spiropyran-based nanogel is prepared and applied for the efficient delivery of anticancer drugs and fluorescence cell imaging for the strong emission of merocyanine photoisomers.

Photosensitizer cross-linked nano-micelle platform for multimodal imaging guided synergistic photothermal/photodynamic therapy

The multifunctional nano-micelle platform holds great promise to enhance the accuracy and efficiency of cancer diagnosis and therapy. In this work, an amphiphilic poly[(poly(ethylene glycol) methyl ether methacrylate)-co-(3-aminopropyl methacrylate)]-block-poly(methyl methacrylate) (P(PEGMA-co-APMA)-b-PMMA) block copolymer was synthesized by successive RAFT polymerizations and subsequent chemical modification. Then the multifunctional micelles with high solubility in physiological environments were developed by a self-assembly and crosslinking processes. The photosensitizer segment, 5,10,15,20-tetrakis (4-carboxyphenyl) porphyrin (TCPP), serves as a tetra-functional cross-linker, photodynamic agent, fluorescence indicator, as well as magnetic resonance (MR) contrast agent after labelling with manganese ions (Mn(2+)), while IR825 simultaneously locating in the interior of the fabricated micelles contributed to the photoacoustic (PA) imaging ability and the photothermal effect. The prepared nanoparticles show great stability in a physiological environment with uniform morphology and diameters of around 80 nm as disclosed by stability investigation, TEM and DLS analysis. IR825@P(PEGMA-co-APMA)-b-PMMA@TCPP/Mn nanoparticles displayed high in vivo tumor uptake with a long blood circulation half-life (∼3.64 h) by the EPR effect after intravenous (i.v.) injection, as revealed by fluorescence, MR and PA imaging models. In vivo anti-tumor effects were achieved via a combined photothermal and photodynamic therapy without noticeable dark toxicity, and this strategy was able to induce a remarkably improved synergistic therapeutic effect to both superficial and deep regions of tumors under mild conditions compared with either single photothermal or photodynamic mechanisms.

One-Step "Click Chemistry"-Synthesized Cross-Linked Prodrug Nanogel for Highly Selective Intracellular Drug Delivery and Upregulated Antitumor Efficacy

Polymeric prodrugs formed by the conjugation of drugs onto polymers have shown great promise in cancer therapy because of the enhancement of water solubility, elimination of premature drug release, and the improvement of pharmacokinetics. To integrate the two advantages of upregulated stability during circulation and selective release of drug in cancer cells, a pH and reduction dual-sensitive prodrug nanogel (CLP) was synthesized via a simple one step "click chemistry". CLP was spherically shaped with a uniform diameter of 60.6 ± 13.7 nm and exhibited great stability in size against large volume dilution, high salt concentration, and long-time incubation in phosphate-buffered saline. Owing to the presence of hydrazone-bonded doxorubicin (DOX) and disulfide cross-linker, CLP released minimal amount (7.8%) of drug under normal physiological pH (i.e., 7.4) condition. But it released 85.5% of the loaded DOX at endosomal pH (i.e., 5.5) plus the presence of 5.0 mM GSH in 120 h. CLP could be effectively internalized by tumor cells and subsequently release DOX in the intracellular environment, resulting in effective proliferation inhibition of HeLa and MCF-7 cells. Furthermore, compared with free DOX and non-cross-linked prodrug micelle (NCLP), CLP accumulated more in tumor site but less in the normal organs, so that CLP performed the enhanced antitumor efficiency and reduced side-toxicities toward the MCF-7 human breast cancer xenograft nude mouse model. With convenient fabrication, favorable stability, controlled release properties, optimized biodistribution, and enhanced suppression of tumor growth, CLP held great potential for optimal antitumor therapy.

Synthesis and Characterization of Cleavable Core-Cross-Linked Micelles Based on Amphiphilic Block Copolypeptoids as Smart Drug Carriers

Amphiphilic block copolypeptoids consisting of a hydrophilic poly(N-ethyl glycine) segment and a hydrophobic poly[(N-propargyl glycine)-r-(N-decyl glycine)] random copolymer segment [PNEG-b-P(NPgG-r-NDG), EPgD] have been synthesized by sequential primary amine-initiated ring-opening polymerization (ROP) of the corresponding N-alkyl N-carboxyanhydride monomers. The block copolypeptoids form micelles in water and the micellar core can be cross-linked with a disulfide-containing diazide cross-linker by copper-mediated alkyne-azide cycloaddition (CuAAC) in aqueous solution. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis revealed the formation of spherical micelles with uniform size for both the core-cross-linked micelles (CCLMs) and non-cross-linked micelles (NCLMs) precursors for selective block copolypeptoid polymers. The CCLMs exhibited increased dimensional stability relative to the NCLMs in DMF, a nonselective solvent for the core and corona segments. Micellar dissociation of CCLMs can be induced upon addition of a reducing agent (e.g., dithiothreitol) in dilute aqueous solutions, as verified by a combination of fluorescence spectroscopy, size exclusion chromatography (SEC), and (1)H NMR spectroscopic measurement. Doxorubicin (DOX), an anticancer drug, can be loaded into the hydrophobic core of CCLMs with a maximal 23% drug loading capacity (DLC) and 37% drug loading efficiency (DLE). In vitro DOX release from the CCLMs can be triggered by DTT (10 mM), in contrast to significantly reduced DOX release in the absence of DTT, attesting to the reductively responsive characteristic of the CCLMs. While the CCLMs exhibited minimal cytotoxicity toward HepG2 cancer cells, DOX-loaded CCLMs inhibited the proliferation of the HepG2 cancer cells in a concentration and time dependent manner, suggesting the controlled release of DOX from the DOX-loaded CCLMS in the cellular environment.

Preparation of pH-sensitive zwitterionic nano micelles and drug controlled release for enhancing cellular uptake

Zwitterionic copolymers have exhibited high resistance to nonspecific protein adsorption and have wide applications in drug delivery systems. Herein, a pH-responsive poly(Lysine-alt-N,N'-bis(acryloyl) diaminohexane) was synthesized through the Michael addition polymerization between N, N'-bis(acryloyl) diaminohexane and lysine. Subsequently, nano micelles (NMs) were formed by self-assembly of the copolymer in an aqueous solution. The NMs showed a slightly negative charge in blood environment, but a positively charged surface in extracellular pH of tumor. This feature could be used to enhance permeability and retention effect, and reinforce tumor cell uptake. Vitro release studies revealed that the release of DOX from the DOX-loaded NMs was evidently faster at pH 5.0 than at pH 7.4. MTT assays revealed that NMs were nontoxic. Thus, these smart NMs were feasible candidates and could be potentially used in cancer chemotherapy.

Reduction-responsive polypeptide nanogel delivers antitumor drug for improved efficacy and safety

Chemotherapy plays an irreplaceable role in the treatment of various malignant tumors today. The traditional drug formulations lack of selectivity, cause serious damage to normal tissues, and can't achieve a desired therapeutic efficacy. For this situation, a facilely prepared reduction-responsive polypeptide nanogel was employed for targeting intracellular delivery of antitumor drug in this study. Doxorubicin (DOX) as a model drug was loaded into nanogel through a sequential dispersion and dialysis approach with a drug loading efficiency (DLE) of 56.8wt.%. The loading nanogel, i.e., NG/DOX, exhibited a medium hydrodynamic radius of 56.1±3.5nm, glutathione-accelerated DOX release, and efficient cellular uptake and proliferation inhibition. Moreover, NG/DOX exhibited upregulated intratumoral accumulation and improved antitumor efficacy toward HepG2 hepatoma-xenografted BALB/c nude mouse model compared with free drug. The enhanced tumor suppression of NG/DOX was further confirmed by the histopathological and immunohistochemical analyses. Furthermore, the excellent in vivo security of NG/DOX was systematically demonstrated by the variation detection of body weight, histopathological assay, levels of bone marrow cell micronucleus rate (BMMR) and white blood cells (WBCs), and detection of clinical parameters in corresponding organs and serum. With controllable large-scale preparation and fascinating properties in vitro and in vivo, the reduction-responsive polypeptide nanogel is revealed to exhibit great potential for on-demand intracellular delivery of antitumor drugs, and shows a good prospect for clinical chemotherapy.

STATEMENT OF SIGNIFICANCE

The traditional drug formulations lack of selectivity, cause serious damage to normal tissues, and can't achieve a desired therapeutic effect. For this situation, a facilely prepared reduction-responsive polypeptide nanogel is employed for targeting intracellular delivery of antitumor drug in this study. The laden nanogel keeps structural integrity and less drug release in the circulatory system after intravenous injection, releases the payload triggered by the intracellular high concentration of GSH, and exhibits the excellent tumor inhibition and security in vivo. Furthermore, the other hydrophobic antitumor drugs can also be on-demand delivered by the smart nanogel. All of the above advantages confirm the bright prospect of reduction-responsive nanogel on the road of malignancy chemotherapy.

Synthesis, characterization and applications of selenocysteine-responsive nanoprobe based on dinitrobenzene sulfonyl-modified poly(carbonate) micelles

Dinitrobenzenesulfonyl-modified micelles can be used for selenocysteine detection in cells and tissues.

Effect of hydrophobicity of core on the anticancer efficiency of micelles as drug delivery carriers

Recently, micelles, which are self-assembled by amphiphilic copolymers, have attracted tremendous attention as promising drug delivery systems for cancer treatment. Thus, the hydrophobic core of the micelles, which could efficiently encapsulate small molecular drug, will play a significant role for the anticancer efficiency. Unfortunately, the effect of hydrophobicity of micellar core on its anticancer efficiency was rarely reported. Herein, the amphiphilic diblock polymers of poly(ethylene glycol) and polyphosphoester with different side groups (butyl, hexyl, octyl) were synthesized to tune the hydrophobicity of the micellar core. We found that the in vitro cytotoxicity of the DOX-loaded micelles decreased with the increasing hydrophobicity of micellar core due to the drug release rate. However, following systemic delivery, the DOX-loaded micelles with the most hydrophobic core exhibited the most significant inhibition of tumor growth in a MDA-MB-231 tumor model, indicating the importance of hydrophobicity of core on the antitumor efficacy of drug delivery systems.

Reduction-responsive polymeric micelles and vesicles for triggered intracellular drug release

SIGNIFICANCE

The therapeutic effects of current micellar and vesicular drug formulations are restricted by slow and inefficient drug release at the pathological site. The development of smart polymeric nanocarriers that release drugs upon arriving at the target site has received a tremendous amount of attention for cancer therapy.

RECENT ADVANCES

Taking advantage of a high reducing potential in the tumor tissues and in particular inside the tumor cells, various reduction-sensitive polymeric micelles and vesicles have been designed and explored for triggered anticancer drug release. These reduction-responsive nanosystems have demonstrated several unique features, such as good stability under physiological conditions, fast response to intracellular reducing environment, triggering drug release right in the cytosol and cell nucleus, and significantly improved antitumor activity, compared to traditional reduction-insensitive counterparts.

CRITICAL ISSUES

Although reduction-sensitive micelles and polymersomes have accomplished rapid intracellular drug release and enhanced in vitro antitumor effect, their fate inside the cells including the mechanism, site, and rate of reduction reaction remains unclear. Moreover, the systemic fate and performance of reduction-sensitive polymeric drug formulations have to be investigated.

FUTURE DIRECTIONS

Biophysical studies should be carried out to gain insight into the degradation and drug release behaviors of reduction-responsive nanocarriers inside the tumor cells. Furthermore, novel ligand-decorated reduction-sensitive nanoparticulate drug formulations should be designed and explored for targeted cancer therapy in vivo.

Spiropyran-decorated light-responsive amphiphilic poly(α-hydroxy acids) micelles constructed via a CuAAC reaction

Light-responsive amphiphilic poly(α-hydroxy acids) with pendent spiropyran chromophore was synthesized and the resultant micelles assembled in aqueous solution presented excellent light-response.

Emulsion click microspheres: morphology/shape control by surface cross-linking and a porogen

Click chemistry was applied to prepare emulsion microspheres (MSs), to realize in situ cross-linking and bioconjugation in the same system. The morphology/shape of the MSs can be adjusted by applying surface click cross-linking and a porogen simultaneously.

Biodegradable multiblock polyurethane micelles with tunable reduction-sensitivity for on-demand intracellular drug delivery

Biodegradable polyurethanes bearing varied amounts of disulfide linkages in the backbone can rapidly enter tumor cells and efficiently transport the encapsulated payloads into cytosol, resulting in controlled inhibition effects against cancer cells. The nanocarriers are promising candidates for on-demand intracellular drug delivery applications.

Photo-responsive reversible micelles based on azobenzene-modified poly(carbonate)s via azide–alkyne click chemistry

We provide a convenient method to construct photo-responsive poly(carbonate)s via ring-opening polymerization of cyclic carbonates followed by azide–alkyne click chemistry.

Facile synthesis of a reduction-responsive amphiphilic triblock polymer via a selective thiol–disulfide exchange reaction

A reduction-responsive amphiphilic triblock polymer was prepared via polycondensation between a dithiol and dipyridyl disulfide, followed by a selective thiol–disulfide exchange reaction.

pH and reduction dual responsive cross-linked polyurethane micelles as an intracellular drug delivery system

pH- and reduction-responsive disulfide-crosslinked polyurethane micelles (CL-PUMs) were developed. The CL-PUMs remained stable in normal PBS while selectively swelled or decomposed in response to weakly acidic environment or reducing agent.