Oxidant-Dependent REDOX Responsiveness of Polysulfides

Polysulfide nanoparticles inhibit fibroblast-to-myofibroblast transition via extracellular ROS scavenging and have potential anti-fibrotic properties

This paper is about the effects of reactive oxygen species (ROS) - and of their nanoparticle-mediated extracellular removal - in the TGF-β1-induced differentiation of fibroblasts (human dermal fibroblasts - HDFa) to more contractile myofibroblasts, and in the maintenance of this phenotype. Here, poly(propylene sulfide) (PPS) nanoparticles have been employed on 2D and 3D in vitro models, showing extremely low toxicity and undergoing negligible internalization, thereby ensuring an extracellular-only action. Firstly, PPS nanoparticles abrogated ROS-mediated downstream molecular events such as glutathione oxidation, NF-κB activation, and heme oxidase-1 (HMOX) overexpression. Secondly, PPS nanoparticles were also capable to inhibit, prevent and reverse the TGF-β1-induced upregulation of key biomechanical elements, such as ED-a fibronectin (EF-A FN) and alpha-smooth muscle actin (α-SMA), respectively markers of protomyofibroblastic and of myofibroblastic differentiation. We also confirmed that ROS alone are ineffective promoters of the myofibroblastic transition, although their presence contributes to its stabilization. Finally, the particles also countered TGF-β1-induced matrix- and tissue-level phenomena, e.g., the upregulation of collagen type 1, the development of aberrant collagen type 1/3 ratios and the contracture of HDFa 3D-seeded fibrin constructs. In short, experimental data at molecular, cellular and tissue levels show a significant potential in the use of PPS nanoparticles as anti-fibrotic agents.

ROS-responsive magnesium-containing microspheres for antioxidative treatment of intervertebral disc degeneration

Intervertebral disc degeneration (IVDD) is a degenerative disease characterized by lower-back pain, causing disability globally. Antioxidant therapy is currently considered one of the most promising strategies for IVDD treatment, given the crucial role of reactive oxygen species (ROS) in IVDD pathogenesis. Herein, a ROS-responsive magnesium-containing microsphere (Mg@PLPE MS) was constructed for the antioxidative treatment of IVDD. The Mg@PLPE MS has a core-shell structure comprising poly(lactic-co-glycolic acid) (PLGA) and ROS-responsive polymer poly(PBT-co-EGDM) as the shell and a magnesium microparticle as the core. The poly(PBT-co-EGDM) can be destroyed by H₂O₂ through the H₂O₂-triggered hydrophobic-to-hydrophilic transition, subsequently promoting an Mg-water reaction to produce H₂. Thus, Mg@PLPE MS provides a valuable platform for H₂O₂ elimination and controlled H₂ release. The generated H₂ scavenge for ROS by reacting with noxious •OH. Notably, the Mg@PLPE MS exerted significant antioxidative and anti-inflammatory effects in a disc degeneration rat model and alleviated extracellular matrix degradation and disc cells apoptosis, thereby underlining its efficacy in IVDD treatment. The Mg@PLPE MS also exhibited robust biocompatibility and negligible toxicity, presenting the promise for the antioxidative treatment of IVDD in vivo. STATEMENT OF SIGNIFICANCE: Antioxidant therapy is currently considered one of the most promising strategies for intervertebral disc degeneration (IVDD) treatment, given the crucial role of reactive oxygen species (ROS) in IVDD pathogenesis. Here, ROS-responsive magnesium-containing microspheres (Mg@PLPE MSs) were constructed to alleviate IVDD through controlled release of hydrogen gas. The Mg@PLPE MSs can effectively scavenge overproduced ROS by simultaneously reacting with H₂O₂ and •OH, thus creating a suitable microenvironment for inhibition of ECM degradation. As a result, Mg@PLPE MSs treated IVDD rats exhibit minimal nucleus pulposus decrease, less extracellular matrix degradation, minimal radial fissure of fibrous rings, and higher disc height index. Therefore, the as-prepared Mg@PLPE MSs may shed a new light on clinical treatment of IVDD.

Synthesis of Poly(2-(methylsulfinyl)ethyl methacrylate) via Oxidation of Poly(2-(methylthio)ethyl methacrylate): Evaluation of the Sulfoxide Side Chain on Cryopreservation

Conventional cryopreservation solutions rely on the addition of organic solvents such as DMSO or glycerol, but these do not give full recovery for all cell types, and innovative cryoprotectants may address damage pathways which these solvents do not protect against. Macromolecular cryoprotectants are emerging, but there is a need to understand their structure-property relationships and mechanisms of action. Here we synthesized and investigated the cryoprotective behavior of sulfoxide (i.e., "DMSO-like") side-chain polymers, which have been reported to be cryoprotective using poly(ethylene glycol)-based polymers. We also wanted to determine if the polarized sulfoxide bond (S⁺O^- character) introduces cryoprotective effects, as this has been seen for mixed-charge cryoprotective polyampholytes, whose mechanism of action is not yet understood. Poly(2-(methylsulfinyl)ethyl methacrylate) was synthesized by RAFT polymerization of 2-(methylthio)ethyl methacrylate and subsequent oxidation to sulfoxide. A corresponding N-oxide polymer was also prepared and characterized: (poly(2-(dimethylamineoxide)ethyl methacrylate). Ice recrystallization inhibition assays and differential scanning calorimetry analysis show that the sulfoxide side chains do not modulate the frozen components during cryopreservation. In cytotoxicity assays, it was found that long-term (24 h) exposure of the polymers was not tolerated by cells, but shorter (30 min) incubation times, which are relevant for cryopreservation, were tolerated. It was also observed that overoxidation to the sulfone significantly increased the cytotoxicity, and hence, these materials require a precision oxidation step to be deployed. In suspension cell cryopreservation investigations, the polysulfoxides did not increase cell recovery 24 h post-thaw. These results show that unlike hydrophilic backboned polysulfides, which can aid cryopreservation, the installation of the sulfoxide group onto a polymer does not necessarily bring cryoprotective properties, highlighting the challenges of developing and discovering macromolecular cryoprotectants.

Designing responsive dressings for inflammatory skin disorders; encapsulating antioxidant nanoparticles into biocompatible electrospun fibres

Inflammatory skin disorders are highly prevalent and current treatments are marred by side-effects. Here, we have designed anti-inflammatory fibrous sheets with the potential to treat low exudate inflammatory skin disorders such as psoriasis or atopic dermatitis. Antioxidant and anti-inflammatory nanoparticles composed of crosslinked poly(propylene sulfide) (PPS) were encapsulated in poly(ethylene oxide) (PEO) fibres via electrospinning from an aqueous suspension. The loading of nanoparticles did not adversely effect the homogenous nature of the electrospun fibres; furthermore, nanoparticles retained their morphology, size and anti-inflammatory character after electrospinning. The PPS-nanoparticle-loaded nanofibres were found to be highly cytocompatible when tested on human dermal fibroblasts. These findings suggest they have significant potential to topically treat inflamed tissues that are characterized by high reactive oxygen species (ROS) levels.

Evaluation of Charge-Regulated Supramolecular Copolymerization to Tune the Time Scale for Oxidative Disassembly of β-Sheet Comonomers

A multistimuli-responsive supramolecular copolymerization is reported. The copolymerization is driven by hydrogen bond encoded β-sheet-based charge co-assembly into 1D nanorods in water, using glutamic acid or lysine residues in either of the peptide comonomers. The incorporation of methionine as hydrophobic amino acid supports β-sheet formation, but oxidation of the thioether side-chain to a sulfoxide functional group destabilizes the β-sheet ordered domains and induces disassembly of the supramolecular polymers. Using H₂ O₂ as reactive oxygen species, the time scale and kinetics of the oxidative disassembly are probed. Compared to the charge neutral homopolymers, it is found that the oxidative disassembly of the charged ampholytic copolymers is up to two times faster and is operative at neutral pH. The strategy is therefore an important addition to the growing field of amphiphilic polythioether containing (macro)molecular building blocks, particularly in view of tuning their oxidation induced disassembly which tends to be notoriously slow and requires high concentrations of reactive oxygen species or acidic reaction media.

Main Chain Polysulfoxides as Active 'Stealth' Polymers with Additional Antioxidant and Anti-Inflammatory Behaviour

We present the evaluation of a sulfoxide-based polymer (poly(propylene sulfoxide), PPSO) as a potential 'stealth' macromolecule, and at the same time as a pharmacologically active (anti-inflammatory/anti-oxidant) material. The combination of these two concepts may at first seem peculiar since the gold standard polymer in biomaterials and drug delivery, poly(ethylene glycol) (PEG), is 'stealth' due to its chemical and biological inertness, which makes it hardly biologically active. Polysulfoxides, on the contrary, may couple a substantial inertness towards biomolecules under homeostatic conditions, with the possibility to scavenge reactive oxygen species (ROS) associated to inflammation. Polysulfoxides, therefore, are rather uniquely, 'active' 'stealth' polymers. Here, we describe the synthesis of PPSO through controlled oxidation of poly(propylene sulfide) (PPS), which on its turn was obtained via anionic ring-opening polymerization. In vitro, PPSO was characterized by a low toxicity (IC50 ~7 mg/mL at 24 h on human dermal fibroblasts) and a level of complement activation (in human plasma) and macrophage uptake slightly lower than PEG of a similar size. Importantly, and differently from PEG, on LPS-activated macrophages, PPSO showed a strong and dose-dependent ROS (hydrogen peroxide and hypochlorite)-scavenging activity, which resulted in a corresponding reduction of cytokine production.

Recent Advances on Reactive Oxygen Species-Responsive Delivery and Diagnosis System

Reactive oxygen species (ROS) play crucial roles in biological metabolism and intercellular signaling. However, ROS level is dramatically elevated due to abnormal metabolism during multiple pathologies, including neurodegenerative diseases, diabetes, cancer, and premature aging. By taking advantage of the discrepancy of ROS levels between normal and diseased tissues, a variety of ROS-sensitive moieties or linkers have been developed to design ROS-responsive systems for the site-specific delivery of drugs and genes. In this review, we summarized the ROS-responsive chemical structures, mechanisms, and delivery systems, focusing on their current advances for precise drug/gene delivery. In particular, ROS-responsive nanocarriers, prodrugs, and supramolecular hydrogels are summarized in terms of their application for drug/gene delivery, and common strategies to elevate or diminish cellular ROS concentrations, as well as the recent development of ROS-related imaging probes were also discussed.

Facile synthesis of ROS-responsive biodegradable main chain poly(carbonate-thioether) copolymers

The amphiphilic main chain poly(carbonate-thioether) copolymers were synthesized based on a novel macrocyclic sulfur-substituted carbonate monomer. The assembled nanostructures have rich ROS responsiveness.

Bioinspired Antioxidant Defense System Constructed by Antioxidants-Eluting Electrospun F127-Based Fibers

Cells were continuously exposed to oxidative damage by overproduction of reactive oxygen species (ROS) when they contacted implanted biomaterials. The strategy to prevent cells from oxidative injures remains a challenge. Inspired by the antioxidant defense system of cells, we constructed a biocompatible and ROS-responsive architecture on the substrate of styrene-b-(ethylene-co-butylene)-b-styrene elastomer (SEBS). The strategy was based on fabrication of architectures through reactive electrospinning of mixture including SEBS, acylated Pluronic F127, copolymer of poly(ethylene glycol) diacrylate and 1,2-ethanedithiol (PEGDA-EDT), and antioxidants (AA-2G) and ROS-triggered release of AA-2G from microfibers to detoxify the excess ROS. We demonstrated that the stable and hydrophilic architecture was constructed by phase separation of SEBS/F127 components and cross-linking between polymer chains during electrospinning; the ROS-responsive fibers controlled the release of AA-2G and the interaction of AA-2G with ROS reduced the oxidative damage to cells. The bioinspired architecture not only reduced mechanical and oxidative damage to cells but also maintained normal ROS level for physiological hemostasis. This work provides basic principles to design and develop antioxidative biomaterials for implantation in vivo.

Synthetic/ECM-inspired hybrid platform for hollow microcarriers with ROS-triggered nanoporation hallmarks

Reactive oxygen species (ROS) are key pathological signals expressed in inflammatory diseases such as cancer, ischemic conditions and atherosclerosis. An ideal drug delivery system should not only be responsive to these signals but also should not elicit an unfavourable host response. This study presents an innovative platform for drug delivery where a natural/synthetic composite system composed of collagen type I and a synthesized polythioether, ensures a dual stimuli-responsive behaviour. Collagen type I is an extracellular matrix constituent protein, responsive to matrix metalloproteinases (MMP) cleavage per se. Polythioethers are stable synthetic polymers characterized by the presence of sulphur, which undergoes a ROS-responsive swelling switch. A polythioether was synthesised, functionalized and tested for cytotoxicity. Optimal conditions to fabricate a composite natural/synthetic hollow sphere construct were optimised by a template-based method. Collagen-polythioether hollow spheres were fabricated, revealing uniform size and ROS-triggered nanoporation features. Cellular metabolic activity of H9C2 cardiomyoblasts remained unaffected upon exposure to the spheres. Our natural/synthetic hollow microspheres exhibit the potential for use as a pathological stimuli-responsive reservoir system for applications in inflammatory diseases.

Lipase-catalyzed synthesis of oxidation-responsive poly(ethylene glycol)-b-poly(β-thioether ester) amphiphilic block copolymers

Lipase-catalyzed one-step synthesis of novel oxidation-responsive poly(ethylene glycol)-b-poly(β-thioether ester) diblock copolymers was reported.

Synthesis of thermal and oxidation dual responsive polymers for reactive oxygen species (ROS)-triggered drug release

A PEG-based main chain thermo-responsive polymer with oxidation-triggerable thioether groups: synthesis, characterization and its use for ROS-responsive drug release.

Towards being genuinely smart: ‘isothermally-responsive’ polymers as versatile, programmable scaffolds for biologically-adaptable materials

Thermo-responsive polymers are of broad interest in a range of biomedical and biotechnological fields. This review summaries the use of ‘isothermal’ transitions where thermo-responsive polymers are re-programmed to respond to other stimuli, but with the same outputs, with the aim of making them ‘smarter’.

Redox-sensitive materials for drug delivery: targeting the correct intracellular environment, tuning release rates, and appropriate predictive systems

SIGNIFICANCE

The development of responsive drug delivery systems (DDS) holds great promise as a tool for improving the pharmacokinetic properties of drug compounds. Redox-sensitive systems are particularly attractive given the rich variety of redox gradients present in vivo. These gradients, where the circulation is generally considered oxidizing and the cellular environment is substantially more reducing, provide attractive options for targeted, specific cargo delivery.

RECENT ADVANCES

Experimental evidence suggests that a "one size fits all" redox gradient does not exist. Rather, there are subtle differences in redox potential within a cell, while the chemical nature of reducing agents in these microenvironments varies. Recent works have demonstrated an ability to modulate the degradation rate of redox-susceptible groups and, hence, provide new tools to engineer precision-targeted DDS.

CRITICAL ISSUES

Modern synthetic and macromolecular chemistry provides access to a wide range of redox-susceptible architectures. However, in order to utilize these in real applications, the actual chemical nature of the redox-susceptible group, the sub-cellular location being targeted, and the redox microenvironment being encountered should be considered in detail. This is critical to avoid the over-simplification possible when using non-biological reducing agents, which may provide inaccurate kinetic information, and to ensure these materials can be advanced beyond simple "on/off" systems. Furthermore, a strong case can be made for the use of biorelevant reducing agents such as glutathione when demonstrating a materials redox response.

FUTURE DIRECTIONS

A further understanding of the complexities of the extra- and intracellular microenvironments would greatly assist with the design and application of DDS.

End-group-functionalized poly(N,N-diethylacrylamide) via free-radical chain transfer polymerization: Influence of sulfur oxidation and cyclodextrin on self-organization and cloud points in water

In this work we report the synthesis of thermo-, oxidation- and cyclodextrin- (CD) responsive end-group-functionalized polymers, based on N,N-diethylacrylamide (DEAAm). In a classical free-radical chain transfer polymerization, using thiol-functionalized 4-alkylphenols, namely 3-(4-(1,1-dimethylethan-1-yl)phenoxy)propane-1-thiol and 3-(4-(2,4,4-trimethylpentan-2-yl)phenoxy)propane-1-thiol, poly(N,N-diethylacrylamide) (PDEAAm) with well-defined hydrophobic end-groups is obtained. These end-group-functionalized polymers show different cloud point values, depending on the degree of polymerization and the presence of randomly methylated β-cyclodextrin (RAMEB-CD). Additionally, the influence of the oxidation of the incorporated thioether linkages on the cloud point is investigated. The resulting hydrophilic sulfoxides show higher cloud point values for the lower critical solution temperature (LCST). A high degree of functionalization is supported by ¹H NMR-, SEC-, FTIR- and MALDI–TOF measurements.

Smart drug delivery systems: from fundamentals to the clinic

Smart materials can endow implantable depots, targetable nanocarriers and insertable medical devices with activation-modulated and feedback-regulated control of drug release.

Oxidation-responsive polymers for biomedical applications

This article summarizes recent progress in the design and synthesis of various oxidation-responsive polymers and their application in biomedical fields.

Modular design of redox-responsive stabilizers for nanocrystals

Many potent drugs are difficult to administer intravenously due to poor aqueous solubility. A common approach for addressing this issue is to process them into colloidal dispersions known as "nanocrystals" (NCs). However, NCs possess high-energy surfaces that must be stabilized with surfactants to prevent aggregation. An optimal surfactant should have high affinity for the nanocrystal's surface to stabilize it, but may also include a trigger mechanism that could offer the possibility of altering size distribution and uptake of the NC. This study presents a modular and systematic strategy for optimizing the affinity of polymeric stabilizers for drug nanocrystals both before and after oxidation (i.e., the selected trigger), thus allowing for the optimal responsiveness for a given application to be identified. A library of 10 redox-responsive polymer stabilizers was prepared by postpolymerization modification, using the thiol-yne reaction, of two parent block copolymers. The stabilizing potential of these polymers for paclitaxel NCs is presented as well as the influence of oxidation on size and dissolution following exposure to reactive oxygen species (ROS), which are strongly associated with chronic inflammation and cancer. Owing to the versatility of postpolymerization modification, this contribution provides general tools for preparing triggered-sheddable stabilizing coatings for nanoparticles.

Combination of episulfide ring-opening polymerization with ATRP for the preparation of amphiphilic block copolymers

We report for the first time the combination of ATRP and ring-opening episulfide polymerization as a means to synthesize polysulfide-based low-dispersity amphiphilic block copolymers. The most significant finding is the possibility to perform ATRP under mild conditions using poly(propylene sulfide) macroinitiators, apparently without any significant copper sequestration by the polysulfides. Using glycerol monomethacrylate (GMMA) as a hydrophilic monomer, the polymers self-assembled in colloidal structures with a morphology depending on the PS/GMMA ratio, but also probably on GMMA degree of polymerization. We here also present a new AFM-based method to calculate the average number of amphiphilic macromolecules per micelle.