Controlled Stimulus-Responsive Delivery Systems for Osteoarthritis Treatment

ROS are highly reactive ions and free radicals, including superoxide (O₂⁻), hydroxyl radical (·OH), hypochlorite ion (OCl⁻), hydrogen peroxide (H₂O₂), and monoclinic oxygen (¹O₂) [34,88]. ROS play a crucial role in physiological functions, including the regulation of protein function, production of multiple hormones, modulation of cell signaling, mediation of inflammation, and elimination of pathogens. In general, low levels of ROS regulate cell signaling pathways and promote cell proliferation [89]. However, excessive ROS can disrupt normal signal transduction and homeostasis [90]. Based on the elevated levels of ROS specific to certain diseases, polymeric nanocarriers with distinct ROS-responsive properties may enhance targeted drug delivery for the treatment of these conditions [91]. To address the problem that the dense extracellular matrix of cartilage is poorly targeted and rapidly cleared from the cartilage surface, which prevents NP penetration and requires repetitive high-dose administration within the joint cavity, Wu et al. proposed the use of the ROS-responsive material poly(ethylene glycol diacrylate) (PEGDA)-1,2-ethylenedithiol (EDT) copolymer (PEGDA-EDT) and reduced graphene oxide (rGO) as graphene-based nanomaterials to prepare a smart ROS-responsive poly (lactic acid) (PLA)/PEGDA-EDT@rGO-fucoxanthin (PPGF) nanofibrous membrane as a DDS for OA therapy. PPGF nanofibrous membranes were prepared by introducing PEGDA-EDT as a ROS-responsive motif, rGO as a drug carrier, and fucoxanthin (Fx) as an antioxidant and anti-inflammatory agent in PLA to achieve ROS-responsiveness and long-term drug release for OA therapy. The novel nanofibrous membrane DDS could “turn on” in response to excessive ROS, prolonging the residence time of Fx in the joint cavity, promoting effective drug concentration, and improving the therapeutic efficiency of OA [71]. Inspired by oxidative stress in the pathogenesis of OA, Jiang et al. first demonstrated the antioxidant capacity of the small molecule compound oltipraz (OL) on IL-1β-treated chondrocytes. Then, a functional ROS-responsive nanocarrier, i.e., mesoporous silica NPs (MSNs) modified with methoxy polyethylene glycol-thioketal (TK) and loaded with OL, was constructed (Figure 5a). In this study, by constructing a new type of nanocarrier, the antioxidant can be brought into chondrocytes, and the drug can be intelligently released in a high-ROS environment [72]. Zhang et al. reported an innovative ROS-responsive therapeutic polymer NP capable of loading hydrophobic drugs and self-reporting the payload release upon ROS stimulation. The NPs consist of an amphiphilic block copolymer composed of PEG and an oxidatively responsive hydrophobic block containing pendant phenylboronic pinacol ester groups and a small portion of a 1,8-naphthimide dye. Since the naphthimide dye is covalently coupled to the polymer, it is possible to fluorescently track the polymer carrier inside the cell [73]. Wu et al. synthesized and assembled a hydrogen peroxide (H₂O₂, which belongs to the ROS)-sensitive nanomicelle loaded with the anti-inflammatory drug dexamethasone (DEX) and the cartilage differentiation factor chondrocyte-derived photoproduced protein-1 (CDMP-1). This low-toxicity, ROS-responsive NP (DLNP) capable of eliminating joint inflammation and inducing cartilage repair was constructed. H₂O₂ was used as a positive control, and the NPs had -SeSe- groups as the ROS-responsive component, with DEX and CDMP-1 as the main pharmacophore. The drug-carrying NPs were delivered directly to the arthritic lesions by intra-articular injection. Using the high concentration of oxygen radicals at the arthritic lesion, -SeSe- breakage and slow release of DEX and CDMP-1 were observed. The results showed that the drug-loaded micelles effectively inhibited the proliferation of activated macrophages, induced macrophage apoptosis, and had anti-inflammatory effects and led to the differentiation of BMSCs to chondrocytes [74]. Shen et al. developed a multifunctional ROS-activated therapeutic polymer NP capable of loading hydrophobic drugs and self-reporting the payload release upon ROS stimulation. The NPs consisted of a Cy5.5-modified cartilage-targeting peptide (CAP, DWRVIIPPRPSA) and a PEG-modified oxidative-responsive TK ligand hydrophobic block containing black hole quencher 3 (BHQ-3) as a quencher for Cy5.5, which was then encapsulated with DEX to form the TKCP@DEX NPs. TKCP@DEX NPs specifically targeted articular cartilage via CAP and responded to high levels of ROS due to high levels of ROS in inflamed tissues leading to sulfhydryl bond breakage, resulting in the gradual breakdown of the polymer and release of Cy5.5 and the drug (Figure 5b). The results show that the nanoprobes can be intelligently “switched on” in response to excessive ROS and “switched off” in normal joints. Especially for cartilage, TKCP@DEX can effectively respond to ROS and slow-release DEX to significantly reduce cartilage damage in OA joints. An in vivo experiment was conducted to evaluate the therapeutic effect of TKCP@DEX on OA. By observing the macroscopic appearance and scores of the cartilaginous femoral condyles at 2 and 4 weeks after treatment, the results showed that the TKCP@DEX group reduced the damage by 65% and 57.78%, respectively, which demonstrated its effective restorative effect in the treatment of OA (Figure 5c) [75]. Tang et al. reported an innovative self-reporting drug delivery platform based on ROS-responsive random copolymers (P1) capable of visualizing drug release kinetics through activation of an integrated fluorophore. P1 is synthesized by co-polymerization of boronic acid pinacol, PEG, and naphthalene dicarbonyl imide monomers, which endow ROS sensitivity, hydrophilicity, and fluorescence signals, respectively [76]. ROS-responsive materials have been the focus of numerous studies as one of the triggering drug release mechanisms for responsive polymeric DDSs. ROS are overexpressed at high concentrations at many disease sites. The differences between normal and pathological tissues make ROS-responsive delivery systems with great potential for many disease applications [92].

Due to the presence of an acidic microenvironment in a variety of pathophysiological conditions, pH-responsive materials have been extensively investigated for the site-specific delivery of different therapeutic agents in formulations across multiple scales [93]. For example, in diseased tissues such as cancer, bacterial infections, and inflammation, the pH is different from the physiological pH 7.4, and therefore, pH-responsive DDSs are becoming increasingly popular, which can take advantage of this pH difference to specifically release encapsulated drugs into diseased tissues [58]. Thus, by designing nanomaterials to release drugs at a specific pH, drugs can be selectively delivered to the site of inflammation and released to produce a therapeutic effect, and one way to design pH-responsive polymeric nanomaterials is to use polymers with pH-sensitive functional groups. These functional groups can change conformation or decompose in response to changes in pH, resulting in the release of the drug from the nanomaterials [94]. There are two mechanisms for the release of pH-sensitive drug carriers: the first stems from changes in hydrophobicity or charge of the carrier induced by protonation or deprotonation due to changes in the ambient pH, and the other is caused by dynamic chemical bond breaking in pH-sensitive biomaterials [95]. Xiong et al. developed a pH-responsive metal–organic frameworks (MOFs) system modified by HA and loaded with the anti-inflammatory protocatechuic acid (PCA), named MOF@HA@PCA, for the treatment of OA. The results showed that MOF@HA@PCA was able to respond intelligently to the acidic conditions in the OA microenvironment and gradually release the PCA, thereby significantly reducing IL-1β-induced synovial inflammation in chondrocytes and OA joints. MOF@HA@PCA also downregulated the expression of OA inflammatory markers and promoted the expression of cartilage-specific genes [77]. He et al. designed a nanoscale pH-responsive DDS for OA therapeutics by using modified MSNs and pH-responsive poly (acrylic acid) (PAA) loaded with andrographolide (AG) to form AG@MSNs-PAA nanoplatforms. The NPs have uniform size (∼120 nm), high drug loading efficiency (22.38 ± 0.71%), and pH-responsive properties, which are beneficial for slow release in OA environments [78].

Zhang et al. designed a super-lubricating circulating brush amphoteric polymer (CB). Specifically, the cyclic polymer poly-2-hydroxyethyl methacrylate (c-p(HEMA)) was used as a core template for the statistical copolymerization of [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) (SBMA) and N,N-dimethylaminoethyl methacrylate (DMAEMA) by “one-step” atom transfer radical polymerization to prepare pH-responsive cyclic electrically brushed amphiphilic polymers (cb-P(HEMA-g-P(DMAEMA-st-SBMA)), CB). In particular, the special structure of the ring–brush polymer can encapsulate both the anti-inflammatory hydrophobic agent curcumin (Cur) and the hydrophilic agent loxoprofen (LXP) to achieve a high drug loading capacity and then achieve a fast release of LXP and a slow release of Cur based on the pH-responsive behavior for long-term OA therapy [79]. In the case of OA, there is a decrease in synovial fluid pH in different regions of the synovial cavity, and in order to take advantage of these pH differences. Zerrillo et al. prepared PLGA NPs and ammonium bicarbonate (NH₄HCO₃)-encapsulated pH-responsive PLGA NPs. These NPs were loaded with HA as a possible model drug for OA and with NIR dyes for use in molecular imaging through the visualization of the NPs. This is because the porous surface of PLGA NPs allows small molecules such as H₂O and H₃O⁺ to enter. Under low pH conditions, high concentrations of hydrated hydrogen ions (H₃O⁺) react with NH₄HCO₃ loaded in the NPs to produce NH₄⁺, CO₂, and H₂O, leading to pH neutralization. Furthermore, it was shown that pH-responsive NPs exhibited extracellular burst release behavior and higher chondrocyte viability [80]. In addition, a pH-responsive NP drug with high delivery efficiency was constructed by loading celastrol into hollow HMSNs using chitosan as a coating, which also showed potential against OA [81].

Enzymes play an important role as catalysts in a variety of biochemical reactions, and certain enzymes are upregulated under pathological conditions. These enzymes overexpressed in disease states have attracted scientists to develop enzyme degradable DDSs. In addition, enzyme-responsive materials are highly specific and selective with fast response rates compared to other stimulus-sensitive systems [96]. Since enzymes are highly selective and efficient, a variety of modes of drug release behavior, such as surface ligand activation and chemical bond breaking, can be achieved by being introduced into biomaterials functional groups that are sensitive to enzymes that are aberrantly expressed in the OA microenvironment [97]. Joshi et al. developed an arthritic flare-responsive hydrogel platform by self-assembling small molecule amphiphilic triglyceride monostearate (TG-18). The corticosteroid triamcinolone acetonide (TA) was loaded onto the TG-18 hydrogel as a model drug, and the drug reservoir released the drug in response to enzymes overexpressed in inflamed joints [82]. Yi et al. developed injectable hydrogels with MMP-responsive and sustained dexamethasone sodium phosphate (DSP)-releasing behavior for OA treatment. Oxidized hyaluronic acid (OHA) was prepared by the specific oxidation of HA by sodium periodate, and then, Col-OHA hydrogels loaded with DSP were prepared based on the Schiff base reaction between type I collagen and OHA, as well as the self-assembly of collagen. In addition, Col-OHA hydrogels showed a sustained release of MMP-responsive DSP, which had the ability to provide the sustained and effective relief of OA symptoms in vivo and had no significant effect on liver and kidney functions [83].

Thermosensitive hydrogels are designed based on the temperature differences between the human body and the external environment, as well as the variations among different tissues and organs [21]. When the temperature rises, the polymer chains dehydrate, facilitating the formation of hydrophobic domains. Driven by the increase in entropy, this process ultimately converts the hydrated liquid into a hydrogel network. This hydrogel can rapidly gel at cartilage defects during the transition from room temperature to physiological temperature (37 °C) [23]. Due to the interactions between its hydrophilic and hydrophobic domains, temperature-responsive in situ gels have garnered significant attention in tissue engineering. Seo et al. synthesized amphiphilic poly(organophosphazene) with temperature-dependent NP formation and sol–gel transition behavior when dissolved in aqueous solution for tretinoin (TCA) delivery. Since the hydrophobic portion of the polymer could interact with the hydrophobic portion of TCA, TCA was encapsulated into self-assembled polymer NPs (TePNs) that were well dispersed in aqueous solution at room temperature. The TePN solution can be injected in a solution state at around room temperature and converts directly to a hydrogel upon intra-articular injection due to body temperature. The encapsulated TCA is then slowly released by loosening the TePN hydrogel network and degradation of the polymer. Finally, the prolonged release of TCA treats OA by inhibiting MMP expression, decreasing pro-inflammatory cytokine expression, and increasing anti-inflammatory cytokine expression within the cartilage (Figure 6a) [84].

Zhu et al. developed a multifunctional composite thermosensitive hydrogel (HPP@Cu gel) consisting of a mixture of poloxamer 407 (P407) and HA used as a gel matrix, which was then physically mixed with copper nanodots (Cu NDs) and platelet-rich plasma (PRP). The HPP@Cu hydrogel was injected into the joint as a solution, which undergoes a phase change to a hydrogel at body temperature, slowly releasing the Cu NDs, HA, and PRP and prolonging the duration of pharmacological effects (Figure 6b) [85]. Li et al. designed a flurbiprofen thermosensitive gel based on poly(ε-caprolactone-co-lactide)-b-poly(ethylene glycol)-b-poly(ε-caprolactone-co-lactide) (PCLA-PEG-PCLA) triple-block copolymers for continuous intra-articular drug delivery. The copolymer system presents a flowable sol at room temperature to achieve its injectability and rapidly converts to a stationary gel at body temperature to form a drug reservoir that serves as a slow-release matrix for intra-articular drug delivery. In vitro drug release studies have demonstrated the sustained release of flurbiprofen from thermosensitive gels for more than three weeks. The intra-articular administration of flurbiprofen was effective in prolonging analgesia and significantly reduced the inflammatory response in a rat model of OA by downregulating the expression of inflammatory factors [86]. Jung et al. showed a novel intra-articular injectable thermosensitive hydrogel for long-term delivery of the nonsteroidal anti-inflammatory drug piroxicam (PX) that was prepared by a simple physical mixing of HA and Pluronic F-127 (HP) in aqueous solution. Pluronic F-127, as a biocompatible polymer, exhibits a rapid thermos-reversible sol–gel transition behavior and the ability to stabilize hydrophobic drugs in aqueous solution (Figure 6c), and biocompatible polysaccharide HA was used as a carbohydrate additive for controlled drug release to reduce the concentration of Pluronic F-127 required for gelation. Both in vitro and in vivo results showed that PX-loaded HP hydrogels not only exhibited excellent mechanical strength but also induced sustained drug release behavior [87].

Due to the presence of photosensitive groups, photo-responsive hydrogels can respond to light with a specific wavelength (NIR, UV, or visible) under phase change. Remote, non-contact, and precise control of photo-responsive hydrogels can be achieved by light irradiation [21]. Photo-responsive DDS is a nano-DDS that enables drug release through photo-responsive materials under light irradiation at specific wavelengths. Compared to other internal stimulation methods, photo-stimulation has many advantages such as convenient operation, easy accessibility, and the ability to achieve precise control in time and space. In addition, external light stimulation can avoid individual differences caused by patients themselves [110]. Xue et al. prepared microspheres of mesoporous polydopamine (MPDA) wrapped in a metal–organic skeleton (MOF) and modified type II collagen-targeting peptide (WYRGRL) on their surface. A novel nanodrug-carrying system for OA treatment was obtained by loading rapamycin in the core layer of MPDA and bilirubin in the shell layer of MOF to realize the delivery of dual drugs (Figure 7a). The experimental results showed that the dual-DDS had excellent NIR laser-stimulated responsive drug release, which could sequentially achieve Br scavenging of cellular free radicals and enhancement of autophagic activity via Rap. Briefly, the rapid release of Br from MOF shells exhibited excellent ROS scavenging ability and antiapoptotic effects but responsively reduced autophagic activity to some extent. Meanwhile, Rap was rapidly released from the MPDA core under NIR irradiation, further enhancing autophagy activation and chondrocyte protection [101], since molybdenum-based polyoxometalate clusters (POMs), often described as nanoclusters of transition metals and oxygen atoms, have been shown to be promising candidates for ROS-related diseases [111].

In addition, several studies have shown that POM has excellent NIR response properties [112]. Shi et al. developed a NIR-responsive POM to treat an OA mouse model primarily by enhancing ROS scavenging. The results showed that NIR-responsive POM exhibited excellent antioxidant activity and anti-inflammatory effects, which could effectively alleviate the clinical symptoms of OA mice. Meanwhile, the progression of OA was slowed down by reducing the production of inflammatory cytokines and catabolic proteases [102]. He et al. developed a photothermal therapy injectable hydrogel for OA treatment, which was achieved by binding HA-thiourea (NCSN) solution to Cu²⁺ in a single-step process. The cross-linked structure of the hydrogel is based on the dynamically reversible chelation between NCSN and Cu²⁺. Notably, these cross-linking sites have significant photothermal properties. In addition, unchelated NCSN helps to neutralize excess ROS. When the hydrogel was used in combination with NIR therapy (HSC/NIR), it effectively promoted chondrocyte anabolism while reducing IL-1β-induced catabolism and inflammatory responses [103]. Li et al. innovatively constructed mitochondria-targeted and SOD-mimicking Mn₃O₄@PDA@Pd-SS31 nanoenzymes with good biocompatibility, NIR responsiveness, and a synergistic cascade for the scavenging of mROS for the treatment of OA. Briefly, Mn₃O₄ was used as the core of the nanoenzymes, encapsulated with PDA to enhance its biocompatibility and loaded with Pd to synergistically enhance the enzyme catalytic activity of the nanoenzymes. In addition, the mROS were precisely scavenged by modifying the mitochondria-targeting peptide SS31 on its surface to achieve a mitochondria-targeting ability. The results showed that the nano-enzymes accelerated the release of Pd and Mn₃O₄, enhanced the activities of SOD and CAT mimetic enzymes, reversed mitochondrial dysfunction, promoted mitochondrial autophagy, effectively scavenged mROS in chondrocytes, regulated the microenvironment of oxidative stress, and ultimately inhibited inflammatory responses under NIR irradiation (Figure 7b) [104]. Xue et al. prepared neutrophil and erythrocyte hybrid membrane-encapsulated dexamethasone sodium phosphate (Dexp)-loaded hollow copper sulfide NPs (D-CuS@NR NPs) for OA treatment (Figure 7c). Overall, synergistic treatment with mild heating, extended circulation, and targeted delivery was achieved in this system. The biomimetic NPs exhibited excellent photothermal conversion and controlled drug release behavior under a 1064 nm NIR laser (Figure 7d). Notably, D-CuS@NR NPs combined with 1064 nm NIR treatment had stronger anti-inflammatory effects at the cellular level compared to free Dexp and D-CuS@NR NPs. Meanwhile, in vivo fluorescence imaging showed that D-CuS@NR NPs could target inflammatory joint sites due to the activation of CD11a-containing neutrophil membranes. Importantly, D-CuS@NR NPs treated with NIR could effectively inhibit cartilage degeneration through photothermal therapy and the downregulation of synovial inflammation [105].

Magnetic particles are nanostructures in the size range of 1–100 nm, usually containing a central magnetic core and a surface coating adjacent to a functional coating. Magnetic particles are generally composed of pure metals such as iron, cobalt, nickel, manganese, etc., and the main ones used for biomedical applications are iron oxide NPs [113]. Furthermore, magnetic particles can be used to overcome the problems of conventional DDSs and deliver drugs to the desired target area, maintaining the NPs in a specific location during drug release when combined with an external magnetic field [114]. Jiang et al. constructed a magnetically guided biodegradable nanocarrier system (MNP) and synthesized KGN-loaded magnetic NPs (KGN-MNPs) by encapsulating KGN on the surface of iron oxide NPs using poly(propylene lactone) (PLA). This system enabled intrachondral delivery of KGN to promote chondrogenic differentiation of bone marrow MSCs. In addition, the application of MNPs and an external magnetic field significantly increased the retention of KGN in the joint matrix, facilitated the penetration of more KGN-MNPs into the cartilage, avoided the rapid clearance of KGN from the joint, and improved the utilization of KGN [106].

Liu et al. developed mesoporous polydopamine NPs (DAMM NPs) doped with arginine and manganese ions (Mn) to deliver DEX for OA treatment (Figure 8a). Firstly, the mesoporous structure may enable AMM with a high specific surface area and abundant pores to load DEX more efficiently and prolong its release to inhibit synovial macrophage-induced inflammation and directly ameliorate OA progression. Secondly, arginine-doped PDA NPs can directly reduce ROS-induced chondrocyte apoptosis. In addition to these therapeutic properties, the incorporation of Mn²⁺ into arginine-doped MPDA NPs displays magnetic resonance imaging (MRI)-sensitive signals, enabling real-time visualization of damaged articular cartilage [107]. Yang et al. developed a magnetic polysaccharide hydrogel microsphere (MPM) consisting of modified natural polysaccharide hyaluronic acid (HAMA) and chondroitin sulphate (CSMA), which was prepared using microfluidic electrospray and freezing processes. Magnetic NPs with a spine-like structure capable of trapping stem cell exosomes (Exo) were encapsulated in a microcarrier, along with the anti-inflammatory drug DS (Figure 8b). The presence of magnetic NPs gives the microcarriers magnetic control (Figure 8c). In addition, DS and Exo released from the microcarriers had a synergistic effect on relieving OA symptoms and promoting cartilage repair. The results of in vitro and in vivo experiments demonstrated the excellence of microcarriers in OA treatment [108].

Ultrasound is the “remote control” and “trigger” of smart composite biomaterials. It is mainly based on cavitation, microfluidics, scattering, and acoustic radiation force and other basic technical features to play a role in realizing the precise regulation of ultrasound-responsive biological components in smart composite biomaterials, such as driving, sensing, payload transfer, chemical or biological process initiation, etc. [115]. Ultrasound technology as a non-invasive method of precision medicine in the range of 0 kHz–50 MHz in combination with biomaterials has attracted great attention and has been widely used in the medical field [116]. Due to its non-invasiveness, ease of control, and high spatial and temporal precision, ultrasound can deliver drugs to blood vessels, tumors, and bone tissue, among others. Currently, ultrasound-triggered DDSs include micelles, liposomes, and hydrogels [117]. Jahanbekam et al. designed an injectable thermionic triggered drug carrier based on Pluronic^® F-127, HA, and gelatin. The optimized hydrogel was used in combination with four different surfactants, which could act as a reservoir for the drug to be released in a controlled manner to better regulate the release rate and provide a burst release when triggered by ultrasound [45].

Vinikoor et al. presented an injectable, biodegradable piezoelectric hydrogel made of short electrostatically spun poly-L-lactic acid nanofibers embedded within a collagen matrix, which can be injected into joints. In vitro experimental data showed that the piezoelectric hydrogel under ultrasound enhanced cell migration, induced stem cells to secrete TGF-β1, and promoted cartilage formation. In vivo, rabbits with osteochondral critical size defects receiving ultrasound-activated piezoelectric hydrogels showed increased subchondral bone formation, improved hyaline cartilage structure, and good mechanical properties close to healthy natural cartilage [118]. Yuan et al. reported PLGA MPs loaded with KGN (MPs@KGN) prepared by the premixed membrane emulsification (PME) method, and ultrasound transducer was utilized to sonicate them. In addition, carboxymethyl chitosan oxidized CS (CMC-OCS) hydrogels were prepared by embedding MPs by the Schiff base reaction to prepare CMC-OCS/MP scaffolds. The cumulative release of KGN from the MPs exhibited a slow rate, and after ultrasound treatment, the MPs appeared to be significantly collapsed, allowing KGN to maintain a continuous concentration for at least 28 days [109].