Dynamic covalent chemistry-regulated stimuli-activatable drug delivery systems for improved cancer therapy

Drug delivery systems(DDSs) are of paramount importance to deliver drugs at the intended targets,e.g.,tumor cells or tissue by prolonging blood circulation and optimizing the pharmaceutical profiles.However,the therapeutic efficacy of DDSs is severely impaired by insufficient or non-specific drug release.Dynamic chemical bonds having stimuli-liable prope rties are the refore introduced into DDSs for regulating the drug release kinetics.This review summarizes the recent advances of dynamic covalent chemistry in the DDSs for improving cancer therapy.The review discusses the constitutions of the major classes of dynamic covalent bonds,and the respective applications in the tumor-targe ted DDSs which are based on the different responsive mechanisms,including acid-activatable and reduction-activatable.Furthermore,the review also discusses combination strategies of dual dynamic covale nt bonds which can response to the complex tumor microenvironment much more accurately,and then summarizes and analyzes the prospects for the application of dynamic covalent chemistry in DDSs.     

Reducible polyethylenimine hydrogels with disulfide crosslinkers prepared by michael addition chemistry as drug delivery carriers: Synthesis,properties, and in vitro release

Degradable hydrogels crosslinked with disulfide bonds were prepared by Michael addition between amine groups of branched polyethylenimine and carbon–carbon double bonds of N,N′‐bis(acryloyl)cystamine. The influences of the chemical composition of the resulted hydrogels on their properties were examined in terms of morphology, surface area, swelling kinetics, and degradation. The hydrogels were uniformly crosslinked and degraded into water‐soluble polymers in the presence of the reducing agent of dithiothreitol, which improved the control over the release of encapsulated drug. The degradation of hydrogels can trigger the release of encapsulated molecules, as well as facilitate the removal of empty vehicles. Results obtained from in vitro drug release suggested that the disulfide crosslinked hydrogels exhibited an accelerated release of encapsulated drug in dithiothreitol‐containing PBS buffer solution. Moreover, the drug release rate decreased gradually with increasing crosslinking density. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4074–4082, 2009     

Extraordinary Modulation of Disulfide Redox‐Responsiveness by Cooperativity of Twin‐Disulfide Bonds

Disulfide bonds have frequently been incorporated into synthetic materials to promote sensitivity of the systems towards different redox environments. Although many strategies have been developed to rationally tune the stability of disulfide linkers, methods to tune their responsiveness towards different redox environments remain elusive. In this work we have developed and explored a disulfide linker bearing two independent disulfide bonds, referred to as a twin‐disulfide linker. We have demonstrated that the twin‐disulfide linker displays an ultrahigh stability at lower concentrations of reducing agent or in weakly reducing environments without a significant compromise in the sensitivity of its response to highly reducing environments such as cytoplasm, a feature that is in remarkable contrast to the traditional single disulfide bonds. Such an extraordinary responsiveness arises from the cooperativity of the twin‐disulfide bonds, which should be of particular interest for applications such as controlled drug delivery and sensing, because relatively large differences in disulfide stability in different redox environments is desired in these applications.     

Supramolecular Vesicles Coassembled from Disulfide‐Linked Benzimidazolium Amphiphiles and Carboxylate‐Substituted Pillar[6]arenes that Are Responsive to Five Stimuli

Novel supramolecular vesicles based on host–guest systems were coassembled from carboxylate‐substituted pillar[6]arene (CPA[6]) and disulfide‐linked benzimidazolium amphiphiles, and the microstructures of the CPA‐based supramolecular vesicles were clearly elaborated. The supramolecular vesicles showed controlled drug release in response to five stimuli, with glutathione, pH, CO₂, Zn²⁺ ions, and hexanediamine, leading to cleavage of the disulfide bonds, protonation of the carboxylate groups, metal chelation, and competitive binding. This is the first case of a smart pillararene‐based supramolecular vesicle being integrated with five stimuli‐responsive functions to meet the diverse requirements of controlled drug release. Importantly, each of the five stimuli is closely related to microenvironments of tumors and diseases of the human body. The smart stimuli‐responsive supramolecular vesicles have promising applications in drug therapy of tumors and relevant diseases.     

Redox‐ and Temperature‐Controlled Drug Release from Hollow Mesoporous Silica Nanoparticles

A controlled drug‐delivery system has been developed based on mesoporous silica nanoparticles that deliver anticancer drugs into cancer cells with minimized side effects. The copolymer of two oligo(ethylene glycol) macromonomers cross‐linked by the disulfide linker N,N′‐bis(acryloyl)cystamine is used to cap hollow mesoporous silica nanoparticles (HMSNs) to form a core/shell structure. The HMSN core is applied as a drug storage unit for its high drug loading capability, whereas the polymer shell is employed as a switch owing to its redox/temperature dual responses. The release behavior in vitro of doxorubicin demonstrated that the loaded drugs could be released rapidly at higher temperature or in the presence of glutathione (GSH). Thus, the dual‐stimulus polymer shell exhibiting a volume phase transition temperature higher than 37 °C can effectively avoid drug leakage in the bloodstream owing to the swollen state of the shell. Once internalized into cells, the carriers shed the polymer shell because of cleavage of the disulfide bonds by GSH, which results in the release of the loaded drugs in cytosol. This work may prove to be a significant development in on‐demand drug release systems for cancer therapy.     

Tetrel-Centered Exchange Cascades to Decouple Inhibition and Induction of Thiol-Mediated Uptake: Introducing Cell-Penetrating Thiolactones,Focus on Reversible Michael Acceptor Dimers

This study adds thiolactones to the collection of cascade exchangers (CAXs) and reports with dynamic covalent Michael acceptor dimers the first CAXs that rather inhibit than enable thiol-mediated uptake. The superior performance of dimers compared to monomeric Michael acceptors as inhibitors might suggest that top inhibitors move along thiol/ate arrays without losing covalent contact, while cell penetrators exchange along disulfide arrays. This decoupling of inhibition and induction of thiol-mediated uptake is important because the latter is promising for drug delivery, while the former possibly helps to reveal targets associated with antiviral, antithrombotic and antitumor activity, leading to drug discovery. These results are consistent with the working model of thiol-mediated uptake as a general complex network in charge of bringing matter into cells, and thus support its validity.     

Triple Redox Responsive Poly(Ethylene Glycol)‐Polycaprolactone Polymeric Nanocarriers for Fine‐Controlled Drug Release

Stimuli‐responsive nanocarriers with the ability to respond to tumorous heterogeneity have been extensively developed for drug delivery. However, the premature release during blood circulation and insufficient intracellular drug release are still a significant issue. Herein, three disulfide bonds are introduced into the amphiphilic poly(ethylene glycol)‐polycaprolactone copolymer blocks to form triple‐sensitive cleavable polymeric nanocarrier (tri‐PESC NPs) to improve its sensitivity to narrow glutathione (GSH) concentration. The tri‐PESC NPs keep intact during blood circulation due to the limited cleaving of triple‐disulfide bonds, whereas the loaded drug is efficiently released at tumor cells with the increased concentration of GSH. In vitro studies of doxorubicin‐loaded tri‐PESC NPs show that the nanocarriers achieve sufficient drug release in cancerous cells and inhibit the tumor cells growth, though they only bring minimum damage to normal cells. Therefore, the tri‐PESC NPs with triple‐sensitive cleavable bonds hold great promise to improve the therapeutic index in cancer therapy.

     

Dynamic covalent polymers

This Highlight presents an overview of the rapidly growing field of dynamic covalent polymers. This class of polymers combines intrinsic reversibility with the robustness of covalent bonds, thus enabling formation of mechanically stable, polymer‐based materials that are responsive to external stimuli. It will be discussed how the inherent dynamic nature of the dynamic covalent bonds on the molecular level can be translated to the macroscopic level of the polymer, giving access to a range of applications, such as stimuli‐responsive or self‐healing materials. A primary distinction will be made based on the type of dynamic covalent bond employed, while a secondary distinction will be based on the consideration whether the dynamic covalent bond is used in the main chain of the polymer or whether it is used to allow side chain modification of the polymer. Emphasis will be on the chemistry of the dynamic covalent bonds present in the polymer, in particular in relation to how the specific (dynamic) features of the bond impart functionality to the polymer material, and to the conditions under which this dynamic behavior is manifested. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3551–3577.     

Hydrophobic Cysteine Poly(disulfide)‐based Redox‐Hypersensitive Nanoparticle Platform for Cancer Theranostics

Selective tumor targeting and drug delivery are critical for cancer treatment. Stimulus‐sensitive nanoparticle (NP) systems have been designed to specifically respond to significant abnormalities in the tumor microenvironment, which could dramatically improve therapeutic performance in terms of enhanced efficiency, targetability, and reduced side‐effects. We report the development of a novel L ‐cysteine‐based poly (disulfide amide) (Cys‐PDSA) family for fabricating redox‐triggered NPs, with high hydrophobic drug loading capacity (up to 25 wt % docetaxel) and tunable properties. The polymers are synthesized through one‐step rapid polycondensation of two nontoxic building blocks: L ‐cystine ester and versatile fatty diacids, which make the polymer redox responsive and give it a tunable polymer structure, respectively. Alterations to the diacid structure could rationally tune the physicochemical properties of the polymers and the corresponding NPs, leading to the control of NP size, hydrophobicity, degradation rate, redox response, and secondary self‐assembly after NP reductive dissociation. In vitro and in vivo results demonstrate these NPs’ excellent biocompatibility, high selectivity of redox‐triggered drug release, and significant anticancer performance. This system provides a promising strategy for advanced anticancer theranostic applications.     

Observing Confined Local Oxygen-induced Reversible Thiol/Disulfide Cycle with a Protein Nanopore

Disulfide bonds play an important role in thiol-based redox regulation. However, owing to the lack of analytical tools, little is known about how local O₂ mediates the reversible thiol/disulfide cycle under protein confinement. In this study, a protein-nanopore inside a glove box is used to control local O₂ for single-molecule reaction, as well as a single-molecule sensor for real-time monitoring of the reversible thiol/disulfide cycle. The results demonstrate that the local O₂ molecules in protein nanopores could facilitate the redox cycle of disulfide formation and cleavage by promoting a higher fraction of effective reactant collisions owing to nanoconfinement. Further kinetic calculations indicate that the negatively charged residues near reactive sites facilitate proton-involved oxygen-induced disulfide cleavage under protein confinement. The unexpectedly strong oxidation ability of confined local O₂ may play an essential role in cellular redox signaling and enzyme reactions.     

Double‐crosslinked reversible redox‐responsive hydrogels based on disulfide–thiol interchange

We present novel redox‐responsive hydrogels based on poly(N‐isopropylacrylamide) or poly(acrylamide), consisting of a reversible disulfide crosslinking agent N,N′‐bis(acryloyl)cystamine and a permanent crosslinking agent N,N′‐methylenebisacrylamide for microfluidic applications. The mechanism of swelling/deswelling behavior starts with the cleavage and reformation of disulfide bonds, leading to a change of crosslinking density and crosslinking points. Raman and ultraviolet‐visible spectroscopy confirm that conversion efficiency of thiol–disulfide interchange up to 99%. Rheological analysis reveals that the E modulus of hydrogel is dependent on the crosslinking density and can be repeatedly manipulated between high‐ and low‐stiffness states over at least 5 cycles without significant decrease. Kinetic studies showed that the mechanical strength of the gels changes as the redox reaction proceeds. This process is much faster than the autonomous diffusion in the hydrogel. Moreover, cooperative diffusion coefficient (D_coop) indicates that the swelling process of the hydrogel is affected by the reduction reaction. Finally, this reversibly switchable redox behavior of bulky hydrogel could be proven in microstructured hydrogel dots through short‐term photopatterning process. These hydrogel dots on glass substrates also showed the desired short response time on cyclic swelling and shrinking processes known from downsized hydrogel shapes. Such stimuli‐responsive hydrogels with redox‐sensitive crosslinkers open a new pathway in exchanging analytes for sensing and separating in microfluidics applications. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2590–2601     

Dipolar Rotors Orderly Aligned in Mesoporous Fluorinated Organosilica Architectures

New mesoporous covalent frameworks, based on hybrid fluorinated organosilicas, were prepared to realize a periodic architecture of fast molecular rotors containing dynamic dipoles in their structure. The mobile elements, designed on the basis of fluorinated p‐divinylbenzene moieties, were integrated into the robust covalent structure through siloxane bonds, and showed not only the rapid dynamics of the aromatic rings (ca. 10⁸ Hz at 325 K), as detected by solid‐state NMR spectroscopy, but also a dielectric response typical of a fast dipole reorientation under the stimuli of an applied electric field. Furthermore, the mesochannels are open and accessible to diffusing in gas molecules, and rotor mobility could be individually regulated by I₂ vapors. The iodine enters the channels of the periodic structure and reacts with the pivotal double bonds of the divinyl‐fluoro‐phenylene rotors, affecting their motion and the dielectric properties.     

Disulfide‐centered poly(methyl acrylates): Four different stimuli to cleave a polymer

A series of poly(methyl acrylates) incorporating a disulfide center was subjected to four different stimuli to cleave the S S bond, as the weakest member of the chain. Four polymers with molecular weights in the range of 25–93 kDa were synthesized via Cu‐based atom transfer radical polymerization starting from a difunctionalized disulfide‐containing initiator. In tetrahydrofuran, the labile disulfide center was cleaved directly by mechanical force generated by ultrasound irradiation, whereas in chloroform, competing cleavage was also triggered by radical species that were generated by non‐mechanical sonolysis. A reductive cleavage was observed upon treatment with reducing agent, and a clean reversal of the cleavage via oxidation could be obtained if the terminal bromide was first removed via hydrogenolysis. This modified polymer allowed studies on a fourth strategy for the cleavage of the disulfide, namely, dynamic covalent exchange reactions with a fluorine labeled small molecule. While partially based on established protocols, this comparative study underscores the versatility of the disulfide bond for applications in stimuli‐responsive and adaptive materials. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1404–1411     

Synthesis and healing properties of poly(arylether sulfone)–poly(alkylthioether) multiblock copolymers containing disulfide bonds

Thermoplastic elastomers composed of soft and hard segments are important elastic and processable synthetic polymers. The microphase‐separated soft domains show low glass transition temperature and possess sufficient chain mobility at room temperature. In this study, we report the synthesis and healing properties of multiblock copolymers containing disulfide bonds as dynamic covalent bonds. The multiblock copolymers composed of poly(arylether sulfone) and poly(alkylthioether) segments were synthesized by oxidative coupling polymerization of the corresponding thiol‐terminated oligomers. Atomic force microscopy phase images, differential scanning calorimetry, and dynamic mechanical analysis curves indicated the microphase‐separated morphology of the multiblock copolymer. Self‐healing properties of the polymer were evaluated by changes in the elongation at break of the cut/adhered samples. The elongation recovery increased with UV irradiation time, and the multiblock copolymer showed a 93% recovery after UV irradiation for 5 h. The healing efficiency induced by UV irradiation, determined by subtracting the recovery without UV irradiation, was calculated to be 51%. According to the UV spectra and solubility changes after UV irradiation, the main healing factor in this study was the crosslinking reactions caused by thiyl radicals generated from UV irradiation instead of disulfide exchange reactions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3545–3553     

Multiple Disulfide-Bonded States of Native Proteins: Estimate of Number Using Probabilities of Disulfide Bond Formation

The polypeptide backbone of proteins is held together by two main types of covalent bonds: the peptide bonds that link the amino acid residues and the disulfide bonds that link pairs of cysteine amino acids. Disulfide bonds form as a protein folds in the cell and formation was assumed to be complete when the mature protein emerges. This is not the case for some secreted human blood proteins. The blood clotting protein, fibrinogen, and the protease inhibitor, α2-macroglobulin, exist in multiple disulfide-bonded or covalent states in the circulation. Thousands of different states are predicted assuming no dependencies on disulfide bond formation. In this study, probabilities for disulfide bond formation are employed to estimate numbers of covalent states of a model polypeptide with reference to α2-macroglobulin. When disulfide formation is interdependent in a protein, the number of covalent states is greatly reduced. Theoretical estimates of the number of states will aid the conceptual and experimental challenges of investigating multiple disulfide-bonded states of a protein.     

Thermally Adjustable Dynamic Disulfide Linkages Mediated by Highly Air‐Stable 2,2,6,6‐Tetramethylpiperidine‐1‐sulfanyl (TEMPS) Radicals

Intrinsically exchangeable dynamic covalent bonds that can be triggered by readily usable stimuli offer easy incorporation of their dynamic properties in various molecular systems, but the library of such bonds is still being developed. Herein, we report the dynamic covalent chemistry of 2,2,6,6‐tetramethylpiperidine‐1‐sulfanyl (TEMPS) dimers derived from thermally reversible homolytic dissociation of disulfide linkages. High air stability of TEMPS was observed even at 100 °C, affording facile employment of thermal dissociation–association equilibria and adjustable bond exchange properties under atmospheric conditions. We also established an efficient synthetic route for a modifiable derivative of the dimer that enabled incorporation of dynamic properties into linear and network polymer structures. The obtained polymers showed controllable molecular weights, temperature‐dependent swelling properties, healing ability, and recyclability, reflecting the thermally tunable dynamics of the dimer.     

Biomimetic pH/redox dual stimuli‐responsive zwitterionic polymer block poly(L‐histidine) micelles for intracellular delivery of doxorubicin into tumor cells

A series of pH/redox dual stimuli‐responsive poly(2‐methacryloyloxyethyl phosphorylcholine)₂₅‐block‐poly(l ‐histidine)_n (p[MPC])₂₅‐b‐p[His]_n, n = 20, 35, 50, and 75) copolymers consisting of a pH‐responsive p(His)_n block and a biocompatible phospholipid analog p(MPC) block connected by a redox‐responsive disulfide linker have been synthesized. The block copolymers are self‐assembled into uniform micelles (～100 nm) in which doxorubicin (Dox) is efficiently encapsulated. The in vitro release profile shows an enhanced release of Dox at low pH (5.0) in 10 mM glutathione (GSH). The in vitro cell viability assays performed using various cell lines show that the blank hybrid micelles have no acute or intrinsic toxicity. A pH‐dependent cytotoxicity is observed with the Dox‐loaded micelles, especially at pH 5.0. Moreover, confocal microscopy images and flow cytometry results show the pH‐dependent cellular uptake of Dox‐loaded micelles. Therefore, the Dox‐loaded micelles can be considered a good candidate for cancer therapy. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2061–2070     

CO2‐Cross‐Linked Frustrated Lewis Networks as Gas‐Regulated Dynamic Covalent Materials

The design of structurally dynamic molecular networks can offer strategies for fabricating stimuli‐responsive adaptive materials. Herein we first report a gas‐responsive dynamic gel system based on frustrated Lewis pair (FLP) chemistry. Two trefoil‐like molecules with bulky triphenylborane and triphenylphosphine groups are synthesized as complementary Lewis acid and base with trivalent sites. They can together bind CO₂ gas molecules and further form a cross‐linked network via the bonding interactions between FLPs and CO₂. Such CO₂‐bridged dative linkages are shown to be dynamic covalent bonds, which endow the frustrated Lewis network with adaptable behaviors and unprecedented gas‐regulated viscoelastic, mechanical, and self‐healing performance. This study is an initial attempt to apply the FLP concept in materials chemistry, but we believe that this strategy will open a promising future for gas‐sensitive smart materials.     

Dynamic heat capacity and reaction enthalpy during the two-stage network growth in diepoxide–diamine compositions

The real and imaginary components of the dynamic heat capacity, C_p′ and C_p″, respectively, have been measured for a fixed frequency of 5 mHz during the polymerization of various compositions of a diepoxide–diamine, molecular liquid mixture to a network structure. The heat evolved during the polymerization was measured simultaneously. C_p′ decreased in two steps as the covalent bonds formed and the network structure grew. The steps became more separated when the amount of the already excess diepoxide was further increased. C_p″ showed a peak in its plot against the polymerization time, but only in the region where C_p′ showed a second step. This is attributed to the increase in the relaxation time leading to vitrification of the liquid. For the diepoxide-rich compositions, the enthalpy release also occurred in two steps and it was more for the second stage of the network's growth than for the first. Combined measurements of the exothermic effects and C_p′ and C_p″ thus delineated two stages of the network's growth by two chemical reactions. The nature of the second-stage network growth that ultimately vitrifies the stoichiometric liquid mixture is discussed. It is concluded that the second-stage growth is mass-controlled and occurs by an etherification reaction whose thermodynamic consequences have been elusive in past studies.     

Dynamic Bonds Mediate π-π Interaction via Phase Locking Effect for Enhanced Heat Resistant Thermoplastic Polyurethane

Stimulus-responsive polymers containing dynamic bonds enable fascinating properties of self-healing, recycling and reprocessing due to enhanced relaxation of polymer chain/network with labile linkages. Here, we study the structure and properties of a new type of thermoplastic polyurethanes (TPUs) with trapped dynamic covalent bonds in the hard-phase domain and report the frustrated relaxation of TPUs containing weak dynamic bond andπ-πinteraction in hard segments. As detected by rheometry, the aromatic TPUs with alkyl disulfide in the hard segments possess the maximum network relaxation time in contrast to those without dynamic bonds and alicyclic TPUs. In situ FTIR and small-angle scattering results reveal that the alkyl disulfide facilitates stronger intermolecular interaction and more stable micro-phase morphology inπ-πinteraction based aromatic TPUs. Molecular dynamics simulation for pure hard segments of model molecules verify that the presence of disulfide bonds leads to strongerπ-πstacking of aromatic rings due to both enhanced assembling thermodynamics and kinetics. The enhancedπ-πpacking and micro-phase structure in TPUs further kinetically immobilize the dynamic bond. This kinetically interlocking between the weak dynamic bonds and strong molecular interaction in hard segments leads to much slower network relaxation of TPU. This work provides a new insight in tuning the network relaxation and heat resistance as well as molecular self-assembly in stimulus-responsive dynamic polymers by both molecular design and micro-phase control toward the functional applications of advanced materials.