2.1. Nanoformulation and Nanoemulsion as Bioactive Carriers
Due to its avascular nature, the cartilage tissue has a major limitation in its ability to heal or repair, which poses a challenge for drug delivery. Systemic delivery of drugs does not reduce disease symptoms as the damaged tissue lacks vascularity. Hence, localized delivery of drugs via intra-articular administration could be a viable alternative [
17]. The emergence of nanotherapeutics serves as a boon due to its various advantages in drug delivery for OA (
Figure 1).
Compared to conventional therapy, nanoformulations provide the advantages of improving drug delivery and efficacy, reducing cartilage damage, and promoting M2 macrophage polarization and extracellular matrix protein synthesis. Nanoformulations used in drug delivery can be classified based on the type of encapsulation carrier, such as polymer-based (polymer micelles, polymer nanoparticles, dendrimers, and polymer vesicles) or lipid-based (nanostructured lipid carriers, lipid nanocarriers, solid-lipid nanocarriers, liposomes, and nanoemulsions) [
18]. Certain other inorganic nanoformulations have also been reported in OA. For example, magnetic nanoparticles coupled with transient receptor potential vanilloid type 1 (TRPV1) antibodies were found to alleviate OA by preventing chondrocyte ferroptosis and macrophage inflammation via alternating magnetic field stimulation [
19]. Further, the limitations associated with the bioavailability of polyphenols as an alternate therapy in OA have led to a shift in focus towards polyphenolic nanoformulations and drug delivery systems that are widely gaining interest in treating and slowing down disease progression [
20].
Polymeric nanoformulations utilize natural or synthetic polymers that self-assemble to encapsulate drugs using emulsification, nanoprecipitation, and solvent evaporation techniques. These are emerging for their roles in drug delivery due to their inherent advantages like reduced toxicity, target site delivery, prolonged retention, controlled release, increased bioavailability, biodegradable properties, and increased drug solubility [
21]. Polymers used in OA nanotherapy include chitosan, hyaluronic acid, polylactic acid (PLA), polylactic-co-glycolic acid (PLGA), polycaprolactone (PCL) and polyamidoamine (PAA). Chitosan is a cationic polymer that promotes transcellular and paracellular transport of drugs and exhibits pH-dependent drug release due to its ability to solubilize at acidic pH [
22]. Hyaluronic acid is, in contrast, an anionic polymer present in the extracellular matrix of cartilage with a specific binding affinity with the CD44 receptor [
23]. PLGA is a synthetic biodegradable polymer that hydrolyses to form metabolite monomers – glycolic acid and lactic acid [
24]. These polymers are finding emerging applications both individually and synergistically in treating OA.
Lipid-based nanoformulation utilizes lipid biomolecules as a carrier and demonstrates superior targeted drug delivery via ligand-receptor interaction in OA treatment [
25]. They comprise an aqueous region surrounded by a lipid bilayer where drugs are encapsulated and released gradually at the target site [
18]. Liposomes are self-assembling spherical vesicles that elevate drug diffusion across the plasma membrane [
26]. Celecoxib-loaded liposomes conjugated with hyaluronic acid hydrogels were found to be responsive to shear by restructuring, provided lubrication, increased retention, and exhibited targeted delivery of celecoxib (NSAID), thus attenuating OA [
27]. Solid-lipid nanoparticles (SLNs) contain biocompatible lipids like biowaxes, fatty acids, or triglycerides that remain solid at room temperature. Solid-lipid nanoparticles conjugated with chondroitin sulfate and loaded with aceclofenac exhibited increased cellular uptake and extended drug release in an OA model [
28]. Nanostructured lipid carriers (NLC) are different from SLNs due to the use of liquid lipids, which confers their unique properties for drug delivery [
29]. NLC loaded with ibuprofen in the form of a topical gel was found to enhance drug permeation via skin in mice for the treatment of joint inflammation associated with OA [
30]. Nanoemulsions are stable liquid-in-liquid dispersions with increased stability, bioavailability, and cellular uptake [
31]. Nanoemulsion gel of chondroitin sulfate and glucosamine was found to alleviate symptoms of knee OA by attenuating cartilage damage and reducing pain [
32].
2.2. Polyphenolic Nanoformulation and Osteoarthritis
The recent advancement of nanotechnology has significantly impacted disease treatment, particularly in the context of OA, by addressing the bioavailability challenges associated with bioactive polyphenols. Various nanoformulations, including nanoemulsion, lipid nanoparticles, polymeric micelles, polymeric nanoparticles, organic-based nanoparticles, inorganic-based nanoparticles, carbon-based nanoparticles, β-lactoglobulin assembled nanoparticles, and metal oxide nanoparticles are currently under research [
33]. When combined with bioactive compounds, these formulations enhance solubility, improve bioavailability, enable controlled release, and reduce the quantity and frequency of doses, thereby enhancing their therapeutic potential.
Plant polyphenols comprise multiple phenol units that occur naturally as secondary metabolites and have potent anti-inflammatory properties as they act on pro-inflammatory mediators. In addition, polyphenols target oxidative stress by scavenging reactive-oxygen species (ROS) molecules [
34]. Recent data suggests that these polyphenolic compounds inhibit matrix degradation caused during OA pathogenesis by promoting the expression of extracellular matrix components such as aggrecan and type II collagen [
35]. Some polyphenols also directly bind to matrix metalloproteinases (MMPs) and inhibit their activities [
36]. Polyphenols also exhibit their chondroprotective role by activating the Nrf2/ARE pathway [
37]. This redox homeostasis signalling pathway attenuates chondrocyte apoptosis, oxidative stress, and extracellular matrix degradation. Further, polyphenol supplementation in knee OA patients improved physical function and alleviated pain and inflammation [
38]. Various bioactive polyphenols such as methyl gallate, hydroxytyrosol, quercetin and rosmarinic acid showed chondroprotective effects to ameliorate OA, which are possible therapeutics [
39,
40]. Rosmarinic acid effectively suppressed MMP1, MMP3 and MMP13, subsequently upregulating type II collagen [
41,
42].
The beneficial effects of bioactives and their nanoformulation have only recently come to light. For instance, the poor bioavailability of hydroxytyrosol has led to the formulation of a hydrogel containing nanoparticles. These hydroxytyrosol-chitosan nanoparticles, in combination with hyaluronic acid and pluronic micelles, efficiently suppress pro-inflammatory effects and oxidative stress in chondrocytes [
43]. Morin hydrate, a natural antioxidant and anti-inflammatory compound, has been used to formulate a copper-morin-based metal-organic framework (MOF) to mimic metalloenzymes [
44]. These nanoenzymes suppress OA progression by repairing mitochondrial function and modifying the levels of pro-inflammatory markers. Nanofiber microspheres of tannic acid and strontium ions were found to reduce apoptosis, downregulate IL1β, TNFα expression, and inhibit cartilage degradation upon intra-articular treatment [
45]. Quercetin, curcumin, dimethyl curcumin, resveratrol, and oxymatrine are some bioactives encapsulated into liposomes and shown to modulate pro-inflammatory cytokines involved in OA [
46]. Recent data show the beneficial effects of polyphenol nanoformulations in osteoarthritis (
Table 1).
2.3. Epigallocatechin in Osteoarthritis
Epigallocatechin-3 gallate (EGCG) is a polyphenol, predominant in green tea known for its antioxidant and anti-inflammatory effects [
59]. Chondroprotective effects of EGCG were observed in primary osteoarthritic chondrocytes. Several proteins such as IL6, MCP1, IL8, GM-CSF, GROα, GRO, MCP3, IP10, GCP2, NFκB, and NAP2 that were upregulated upon IL1β stimulation were found to be downregulated by EGCG [
60]. EGCG has been reported to exhibit antiarthritic properties by epigenetic modulation of global miRNA expression, thus reducing inflammation in IL1β-induced primary chondrocytes. Further, it was found that EGCG inhibited the expression of ADAMTS5 and COX2, which is upregulated upon IL1β stimulation via the modulation of hsa-miR-140-3p and hsa-miR-199a-3p [
61,
62]. Several nanoformulations used to improve the bioavailability of epigallocatechin-3 gallate have been researched. Copper EGCG nanosheets modulated ROS signaling and downregulated pro-inflammatory cytokines' expression in primary chondrocytes by converting M1 macrophages to M2 macrophages [
63]. EGCG nanodrug, in combination with selenomethionine, significantly ameliorated OA by reducing oxidative stress, accumulation of Fe2+, and promoting activation of glutathione peroxidase 4 [
64]. Nanoparticles of EGCG and glucosamine mixture displayed improved anti-inflammatory potential and higher antiarthritic activity compared to the native mix [
65]. A novel formulation with near-infrared nano-enzyme of EGCG with Au and Ag improves OA treatment by reducing cartilage damage [
55]. Recent work on hydrogels of epigallocatechin-3 gallate seems to be a promising therapy for OA. EGCG, in combination with hyaluronic acid, showed chondroprotective effects by inhibiting IL1β, TNFα, and MMP13, scavenging ROS, and inducing M2 macrophage polarization in chondrocytes [
66,
67].
2.4. Resveratrol in Osteoarthritis
Resveratrol is a polyphenol known for its antioxidant and anti-inflammatory functions, which highlights its chondroprotective effects by improving joint function and reducing cartilage degradation [
68,
69,
70]. The poor solubility and low bioavailability of resveratrol have led to the exploration of nanoformulation. Resveratrol nano-encapsulated in a self-assembling lipid core led to reduced nitric oxide levels in primary human chondrocytes [
71]. Resveratrol nanoemulsions have been found to increase intracellular uptake compared to native resveratrol, minimize toxicity, and decrease oxidative stress [
72]. Further, PLGA nanoparticles of resveratrol could inhibit chondrocyte apoptosis, induce autophagy and subsequently alleviate symptoms of OA [
73]. Due to their sustained release, these nanoparticles showed improved bioavailability with quantifiable concentrations of resveratrol for 35 days. The differentially expressed genes such as CXCL1, IL6, NOX4 and MMP3 were upregulated in primary chondrocytes from OA patients, and those downregulated upon resveratrol treatment [
74]. In addition, targeting MALAT1/miR-9/NFκB1 and caspase-3/MMP13 axis with resveratrol revealed its therapeutic potential in managing OA [
75]. The combined efficacy of resveratrol with meloxicam (NSAID) significantly alleviated symptoms of knee pain in OA and had superior efficacy compared to meloxicam alone [
76,
77]. Hydrogel of resveratrol combined with oxidised hyaluronic acid upregulated SOX9, aggrecan, and type II collagen while suppressing expression matrix metalloproteinases in LPS-induced inflammation in chondrocytes [
78], indicating its further scope in eliminating the disadvantages of bioaccessibility issue of the resveratrol.
2.5. Curcumin in Osteoarthritis
Curcumin, a dietary polyphenol and an active ingredient of
Curcuma longa, has shown potent anti-inflammatory, anti-cancer, antioxidant, and anti-bacterial effects. Evidence suggests anti-arthritic, immunomodulatory and chondroprotective effects of curcumin [
79]. Curcumin has emerged as a safer and more effective supplement for ameliorating OA [
80]. Curcumin alone or co-supplemented with other polyphenols effectively reduced symptoms of OA, as evidenced by improved physical performance and reduced WOMAC (Western Ontario and McMaster Universities Osteoarthritis Index) and VAS (Visual Analog Scale) scores [
81]. Curcumin supplementation reduces the expression and secretion of various serum inflammatory markers such as IL4, IL6, TNFα, and CRP/hs-CRP (high-sensitivity C-reactive protein) [
82,
83,
84,
85]. Despite its potential, poor solubility, limited distribution, restricted bioaccessibility, and inadequate bioavailability of curcumin restrict its therapeutic efficacy. Therefore, nanoformulation approaches are extensively adopted to improve the effectiveness of curcumin.
Curcumin nanoformulations have been designed using various methods to improve solubility, bioavailability, and targeted delivery. For example, self-assembling lipid-core curcumin nanocapsules showed improved solubility and were more effective in reducing nitric oxide in reducing apoptosis in the inflamed chondrocytes [
71]. Further, to avoid any harmful effects of nanocarriers, a carrier-free assembly of curcumin and icariin nanoparticles by π-π stacking formulation exhibited improved cellular uptake, displayed prolonged drug release and synergistic anti-inflammatory effects, thus alleviating OA by protecting the cartilage [
86]. Liposomal formulations of curcuminoids (curcumin and bisdemethoxycurcumin) promoted cellular uptake, improved osteoprotegerin/receptor activator nuclear factor κB (OPG/RANKL) ratio and downregulated IL1β, demonstrating its role in attenuating OA progression and preventing osteoclastogenesis [
87]. Curcumin’s antioxidant potential has been proven to increase upon its encapsulation [
88]. To further enhance its antioxidant potential, a self-assembling ROS-responsive polymeric micelle encapsulated with curcumin displayed extended drug release and ROS-scavenging when triggered with H
2O
2 in chondrocytes and in the OA rat model [
89]. Polymeric micelles in self-assembling acid-activatable curcumin polymer showed chondroprotective effects by downregulating IL1β and TNFα in the acidic microenvironment of monoiodoacetate (MIA)-induced OA [
90,
91]. Similarly, curcumin encapsulated in a pH-responsive cyclic brush zwitterionic polymer exhibited controlled drug release, upregulated type II collagen, and aggrecan, suppressed pro-inflammatory markers such as MMP13, IL1β and improved lubrication in the synovial joints [
92].
The efficacy of curcumin-loaded polylactic-co glycolic acid nanoparticles (PLGA NPs) in MIA-induced OA model exhibited a higher stability, bioavailability and greater chondroprotective effects over native molecules by downregulating MMP1, MMP3, MMP13, IL1β, TNFα and improving locomotor function [
93]. In another study, Cur-PLGA nanoparticles alleviated knee-OA in rats by downregulating TNFα, IL6, IL1β, TGFβ and NFκB [
94]. An analysis of the synergistic effects of hyaluronic acid/chitosan nanoparticles and curcuminoids in the knee-OA model revealed a decrease in chondrocyte apoptosis by upregulating the IκB and thereby inhibiting the NFκB pathway, downregulating the MMP1, MMP13 and promoting the expression of type II collagen [
95]. A similar combination of hyaluronic acid and curcumin-loaded chitosan nanoparticles ameliorated OA due to their prolonged retention in the synovial cavity, inhibiting chondrocyte apoptosis, downregulating pro-inflammatory cytokines and upregulating RUNX2 and AP1 [
96]. Curcumin microgel using poly (ethylene glycol) dimethacrylate efficiently attenuated IL1β-induced inflammatory response in chondrocytes and promoted cartilage repair in the OA model, thus displaying its pro-regenerative potential [
97]. Curcumin was encapsulated into gelatin/silk fibroin microspheres to target localized delivery and studied for its effect on MIA-induced OA. Curcumin microspheres displayed extended anti-inflammatory effects, showed significant histological improvements in rat bone tissue, and reduced apoptosis and serum IL6 [
98]. Another study involving silk fibroin nanoparticles (SFN) demonstrated how curcumin’s efficacy and anti-inflammatory potential are enhanced when encapsulated into SFNs. Curcumin SFNs could provide an environment for controlled drug release and cytocompatibility, improve anti-inflammatory effects by regulating RANTES and IL6, and display antioxidant effects by ROS-scavenging [
99].
Several types of formulations are being developed to improve curcumin’s bioavailability further. For example, next-generation ultrasol curcumin (NGUC) efficiently alleviated OA pathophysiology by downregulating CRP, TNFα, IL6, IL1β, NFκB, COX2 and MMP3 and upregulating GPX, CAT and SOD [
100]. Another formulation, palmitoyl-glucosamine (PGA), co-micronized with curcumin, results in improved bioavailability and reduces pain, tissue damage and paw edema in OA pathophysiology. The PGA-cur downregulated MIA-induced pro-inflammatory cytokines such as IL1β, TNFα, MMP1, MMP3, and MMP9 when integrated into the diet, was found to maintain meloxicam-based pain relief in dogs with OA pain [
101,
102]. A formulation of curcumin in water-dispersible form, was found to facilitate MIA-induced OA by inhibiting chondrocyte apoptosis and cartilage damage, in addition to improving weight-bearing imbalance and downregulating caspase-3, nitrotyrosine, phospho-NFκB, and TNFα [
103]. Several curcumin nanoformulations are reported in clinical trials as a therapeutic against OA, tabulated in
Table 2.
Despite evidence of curcumin’s safety and efficacy, the mechanism of curcumin’s action in ameliorating OA is still unclear. Several mechanisms have been postulated, and curcumin might regulate more than one pathway simultaneously. Curcumin was found to modulate key amino acid metabolism like threonine, serine and glycine, histidine, cysteine and methionine, glycerolipid, and inositol phosphate metabolism [
111]. Curcumin also acts on the NFκB pathway, thereby inhibiting the expression of matrix metalloproteinases and reducing cartilage degradation [
112]. Curcumin may act like a natural COX inhibitor and regulate inflammatory pathways [
113]. Clinical trials with comparative supplementation of curcumin and a NSAID have evidenced similar efficacy [
82,
83,
114]. Further, curcumin has been shown to inhibit the production of pro-inflammatory mediators in an OA model in chondrocytes as well as in the articular cartilage of rats
in vivo [
115,
116]. Despite the current knowledge gap of curcumin’s mode of action, nanoencapsulation and its use as a nutritional intervention exhibited great potential.