Hyaluronic Acid: A Key Ingredient in the Therapy of Inflammation

Hyaluronic acid (HA) is a natural, unbranched polymer that belongs to a group of glycosaminoglycan heteropolysaccharides (GAGs), which are the main components of the extracellular matrix (ECM) [ 1 , 2 ]. It is a molecule with a simple chemical structure ( Figure 1 ), of a polar (hydrophilic) nature, composed of repeated disaccharide units of D-glucuronic acid and N-acetyl-D-glucosamine, linked by alternating β-1,3 and β-1,4 glycosidic bonds [ 3 , 4 , 5 , 6 , 7 , 8 ]. In its native form, HA appears as a very long polymer, called high-molecular weight HA (HMWHA) [ 2 ]. However, under certain conditions, the molecule can be broken down into small fragments, called low-molecular weight HA (LMWHA) [ 2 ]. HMWHA, usually greater than 1000 kDa, is present in intact tissues and is antiangiogenic and immunosuppressive, while smaller polymers indicate signs of distress and are potent inducers of inflammation and angiogenesis [ 6 , 9 , 10 ]. The literature reports some works, namely, the reviews by Fallacarra et al. [ 1 ] and Litwiniuk et al. [ 2 ], where the structural and biological properties of HA are reported and where medical, pharmaceutical, and cosmetic applications are used, including the role of HA in inflammation and tissue regeneration. This review, in addition to surveying the physicochemical and biological properties (biosynthesis, degradation, hydration, mechanism of action, interaction with membrane receptors, etc.) intends to show the wide potential of HA in therapy for inflammatory diseases, approaching topics including its benefits in pathologies of the joints, intestines, lungs, heart, etc. 1.1. Biosynthesis Most cells in the human body are able to synthesize HA during certain processes in their cell cycle [ 4 , 11 ], although mesenchymal cells are believed to be the predominant source of HA [ 8 , 12 ]. HA differs from other GAGs because it is not sulphated and is not synthesized by Golgi enzymes in association with proteins; it is produced in the inner face of the plasma membrane, without any covalent binding to a protein nucleus, by hyaluronan synthases (HAS-1, HAS -2 and HAS-3) [ 1 , 13 , 14 , 15 ]. These three proteins place the HA molecules in the extracellular space through pore-like structures, along the length of the polymeric chain [ 2 , 8 , 16 , 17 ], repeatedly adding D-glucuronic acid and N-acetyl-D-glucosamine ( Figure 2 ), giving rise to molecules of different sizes [ 11 , 18 ]. This unique synthesis mechanism, which allows the molecule to be secreted during its production, is essential; otherwise, due to its enormous size, the cells would be destroyed [ 19 ]. Incorrect deregulation of the expression of HAS genes results in abnormal HA production and, therefore, an increased risk of pathological events, altered cellular responses to injuries and aberrant biological processes [ 1 , 20 ]. It has also been established that growth factors, such as epidermal growth factor (EGF) or keratinocyte factor, increase the rate of HA synthesis [ 17 ]. As previously reported, mesenchymal cells are believed to be the predominant source of HA; however, some studies report that other cell types are capable of producing large amounts of HA. An example of this is the study described by Sapudom et al., where it is verified that activated fibroblasts are capable of producing large amounts of HA. This study was carried out using 3D collagen matrices, which mimic the ECM of a tumor microenvironment, where the normal dermal fibroblast and activated fibroblasts were incorporated. In addition to verifying that the activated fibroblasts produce a large amount of HA, it was possible to verify that the average weight of HA produced by these cells was around 480 kDa, and that this was produced via HAS-2 [ 21 ].

Due to its molecular structure at a neutral pH, HA attracts water molecules and can contain up to 10,000 times its weight in water [ 9 , 26 , 29 , 33 , 35 ]. As shown in Figure 1 , water molecules are linked by hydrogen bonding, which stabilizes the molecule’s structure. In aqueous solution, due to hydrophobic interactions and intermolecular hydrogen bonding, polymeric chains aggregate, with the formation of loose and elastic matrices, which facilitate cell migration [ 1 , 13 , 16 , 29 , 33 , 37 ]. The organization of the matrices that are formed varies according to the type of tissue and the cellular microenvironment [ 38 ]. Within these HA matrices, pores appear that allow small molecules to diffuse freely, while macromolecules are partially excluded [ 16 , 39 ]. However, HA matrices, by nature, are very dynamic so they move constantly in solution, leading to continuous changes in pore size [ 16 ]. These changes mean that even the macromolecules have the potential to cross the matrix [ 16 ].

HA performs its biological functions according to two basic mechanisms: (I) it can act as a passive structural molecule and (II) as a signalling molecule [ 1 , 40 ]. The passive mechanism is related to the physicochemical properties of HMWHA [ 1 , 40 ]. Due to its macromolecular size, marked hygroscopicity, and viscoelasticity, HA is able to modulate the tissue hydration, osmotic balance and physical properties of ECM, structuring a hydrated and stable extracellular space, where cells, collagen and elastin fibers, among other ECM components, are firmly maintained [ 1 , 40 ]. When interacting with its binding molecules, HA also acts as a signalling molecule [ 1 , 18 , 45 , 46 ]. Depending on its MW, location and specific cell factors (expression of the receptor, signalling pathways and cell cycle), the link between HA and its proteins determines opposing actions: pro- and anti-inflammatory activities, the promotion and inhibition of migration activation and blocking of cell division and differentiation [ 1 , 18 , 45 , 46 ]. Furthermore, HA can interact with specific cytokines, and thus modulate immune cell function. One example is interleukin-8 (IL-8), released by fibroblasts, monocytes/macrophages, endothelial and epithelial cells in the presence of inflammation [ 47 ]. It is known that HA has a weak binding to IL-8, which means its use as a coating compound is possibly advantageous in the inhibition of unwanted immune response [ 48 ]. Although there are studies reporting the interaction between IL-8 and HA, no binding site has been determined. Therefore, it is believed that the interaction between GAGs and IL-8 is not solely an electrostatic process, but that steric interactions as with hydrogen bonds are also crucial to the specificity of the interaction [ 47 ]. Furthermore, these interactions are dependent on the sulfation degree of the GAG and the concentration of IL-8 [ 48 ], so the introduction of sulfate groups to the HA chain significantly improves its binding to IL-8 [ 47 ].

The receptor for HA-mediated cell motility (RHAMM, also known as CD168), exists in several isoforms, which may not only be present in the cell membrane, but also in the cytoplasm and nucleus [ 1 , 2 , 9 , 58 , 59 ]. However, it is poorly expressed in most human tissues [ 26 ]. The cell membrane is anchored to glycosylphosphatidylinositol [ 59 , 60 ], containing two HA-binding domains, in which the molecule binds to CD44 less firmly than in the HA-binding domain [ 38 ]. It is RHAMM that regulates cellular responses to growth factors and plays a role in cell migration, particularly for fibroblasts and smooth cells [ 30 , 60 ]. The interactions of HA-RHAMM play important roles in inflammation and tissue repair [ 1 , 26 ], triggering a variety of signalling pathways, and thus controlling cells such as macrophages and fibroblasts [ 1 ]. On the cell surface, RHAMM interacts with CD44 and modulates cell motility, wound-healing, and signal transduction [ 9 , 51 , 58 ]. It can also perform invasive functions such as CD44 and can even replace its functions [ 9 , 58 ]. At the intracellular level, RHAMM binds to actin filaments, podosomes, centrosomes, microtubules, and mitotic spindle, affecting crucial cellular processes in tumorigenesis [ 9 , 58 ].

The HA-receptor for endocytosis (HARE, also known as Stabilin-2) is capable of binding not only HA, but also other GAGs, and is involved in the release of GAGs into the circulation [ 1 , 2 , 13 ] and in the nuclear factor kappa-light-chain enhancer of activated B cells (NF-кB) [ 13 , 52 ]. It is found on the inner surface of endothelial cells in vascular and lymphatic vessels [ 2 , 19 ]. HARE mediates the systemic clearance of HA from the circulatory and lymphatic systems [ 62 , 63 ]. The lymphatic vessel endothelial receptor 1 (LYVE-1) is an HA-binding protein expressed in the vascular lymph endothelium and macrophages, which controls the renewal of HA by mediating its absorption from the tissues into the lymph. This helps to regulate the moisture level of the tissue [ 1 , 2 , 13 , 51 ]. In this way, LYVE-1 forms complexes with growth factors, prostaglandins and other tissue mediators, which are involved in the regulation of lymphangiogenesis and intercellular adhesion [ 1 ]. LYVE-1 interacts with HA, and the resulting complex can be internalized and targeted in lysosomes [ 51 ]. Toll-Like receptors (TLRs) are involved in the activation of innate and adaptive immune responses and also play roles in tumor progression and inflammation [ 13 , 64 ], being expressed in the membrane of all mammalian cells [ 51 ]. The smaller fragments of HA can bind to the TLR2 and TLR4 receptors of monocytes, dendritic cells and lymphocytes, thus provoking a pro-inflammatory response [ 30 , 51 , 65 ]. These receptors are widely distributed in the gastrointestinal tract, where they play an important role in mediating the response to commensal and pathogenic hosts and bacteria [ 54 ]. Although the direct physical interaction between HA-TLR was not experimentally demonstrated, it is likely that the polyanionic nature of HA mimics the canonical ligands of TLR2 and TLR 4, such as lipopolysaccharide [ 51 ]. The HMWHA-TLR interaction induces cell survival; conversely, the LMWHA-HA interaction induces inflammation and cell death [ 51 ].

Rheumatoid arthritis (RA) is an autoimmune disease that causes bone destruction, hyperplasia of synovial membranes and damage to cartilage. Although the exact mechanism of this pathology remains unknown, abundant activated macrophages in inflamed joints are known to play a crucial role in disease progression through the production of pro-inflammatory cytokines, such as TNF-α, IL-1β and IL-6 [ 66 , 100 ]. Therefore, inhibiting the secretion of pro-inflammatory cytokines from activated macrophages has been the main therapy for RA. CD44 is an adhesion receptor that is often overexpressed on the surface of activated macrophages in patients with RA. Thus, due to its ability to bind to CD44, HA was selected as a target molecule for the development of new RA therapies [ 66 ]. The search for new treatments for this pathology led Zhou and his collaborators to develop a system of targeted prednisolone (PD) delivery, which consists of solid lipid nanoparticles (SLNs) coated with HA (HA-SLN/PD). Studies have shown that the developed system remains stable in circulation for long periods, and accumulates selectively in the inflamed tissue, where PD ends up being selectively released. In addition, in a mouse model with collagen-induced arthritis, it showed good efficacy, based on joint analysis, pannus formation, bone preservation and pro-inflammatory cytokine measurement. It was also demonstrated that the therapeutic effects of HA-SLN/PD are superior to those observed with SLNs/PD and PD alone, which shows the capacity of the system to reach the arthritic joints and selectively release the drugs in these places [ 101 ]. Starting from the same principle, Gouveia et al., developed pH-sensitive liposomes functionalized with HA, where prednisolone disodium phosphate (PDP) was incorporated for intravenous or intra-articular administration. In vitro studies have shown that the liposomes’ ability to respond to pH allowed for a significant increase in the release of the drug over time. Cellular studies have shown that HA-conjugated liposomes were effectively uptaken by macrophages and fibroblasts (by interaction with the CD44 receptor). The results show that this strategy improves the efficiency and increases the bioavailability of PDP in situ [ 102 ]. Based on the activated macrophages and the fact that they overexpressed the CD44 receptor, Yu et al., developed polymeric nanoparticles coated with acid-sensitive HA, using dexamethasone (HAPNPs/Dex) as active ingredient for the intravenous administration of injection. Studies carried out by this group showed that the in vitro release rate of Dex increased significantly with a decrease in pH from 7.4 to 4.5. In cell uptake studies, stronger signals were obtained from the activated macrophages treated with HAPNPs, indicating that these nanodevices could be effective in targeting activated macrophages. On the other hand, non-functionalized nanoparticles had a much lower internalization rate. This indicates that HAPNPs interact favourably with the CD44 receptor. In vivo, therapeutic efficacy studies have shown that the infiltration of prominent inflammatory cells, bone damage and cartilage damage was reduced in the ankle joints of rats with adjuvant-induced arthritis (AIA) when treated with HAPNPs/Dex [ 66 ]. It has been reported that γ-secretase inhibitors can improve RA by suppressing the Notch signaling pathway. Using this information, Heo et al., developed HA nanoparticles (HA-NPs) carrying an γ-secretase inhibitor, N-[N-(3,5-difluorophenacetyl-ι-alanyl)]-S-phenyl-glycine t-butyl ester (DAPT) (DNPs). In vivo, biodistribution studies showed that DNPs were effectively accumulated in the inflamed joint, after systemic administration to mice with collagen-induced arthritis (CIA). When comparing therapeutic efficacy, where clinical scores, tissue damage and neutrophil infiltration were evaluated, the results showed that DNPs were more effective than DAPT. It was also found that DNPs significantly reduced the production of pro-inflammatory cytokines [ 103 ]. Alam and his team developed mineralized nanoparticles (MP-HANPs), composed of PEGylated HA (P-HA) and calcium phosphate, for the administration of MTX. Calcium phosphate acts as a barrier to diffusion, which causes MP-HANPs to release MTX only under acid-neutral conditions. The in vitro results showed that MP-HANPs were internalized by receptor-mediated endocytosis, namely, the CD44, stabilin-2 and RHAMM. The developed structures allowed for a greater accumulation and more controlled release of MTX, but with the paws of CIA mice, with a simultaneous, lower accumulation in the liver, which indicates good biodistribution [ 104 ]. Fan and his team developed HA/curcumin (HA/Cur) nanomicelles. It was shown that the developed nanomicelles have good biocompatibility and promote the proliferation of chondrocytes. After the intra-articular administration of nanomicelles, there was a significant decrease in edema in rats with complete Freund

Tendons are “cords” of connective tissue that connect the muscle to the bone and can withstand stressful situations. During muscle contraction, it is these structures that assist in the movement of bones and joints [ 116 ]. The term tendinopathy describes a clinical condition characterized by pain, swelling and functional limitations of the tendon and nearby anatomical structures [ 117 , 118 ]. HA is actively secreted by the tendon sheath and, similarly to what happens in the joints, as an important component of synovial fluid, it allows for smooth sliding and provides nourishment to the tendon [ 119 ]. Although the literature is still scarce, there are reports that LMWHA is an effective tool in the treatment of various tendinopathies [ 117 ]. The efficacy and safety of peritendinous HA injections (500–730 kDa) in reducing pain in patients affected by lateral elbow, Achille or patellar tendinopathy, was evaluated in a study conducted by Fogli et al. In this study, patients were treated with a cycle of ultrasound-guided peritendinous injections (one injection per week for three consecutive weeks). The results show a significant reduction in the visual analogue scale (VAS) and sagittal thickness in all types of tendinopathies under study. Neovascularization decreased for each tendon at 14 and 56 days, except for the patellar tendon. This shows that peritendinous injections provide significant pain relief and reduced tendon thickness and neovascularization, in a safe and well-tolerated manner [ 120 ]. After surgery on the tendons of the hand, there may be a limitation of hand function caused by postoperative tendon adhesion, which is mainly attributed to tendons that adhere to the surrounding tissues during the healing process, limiting joint mobility. One approach to the prevention of postoperative tendon adhesion is the use of multifunctional nanofiber membranes (NFM), which allow for inhibition of the binding and penetration of fibroblasts. In this context, Chen et al., developed HA nanofiber membranes loaded with different percentages of ibuprofen (20%-HAI20FB, 30%-HAI30FB and, 40%-HAI40FB). The studies conducted by this group show that, except for HAI40FB, all the developed NFMs showed excellent effects in preventing fibroblast binding and penetration, preserving high biocompatibility without influencing cell proliferation. Although HAI40FB shows an improvement in mechanical properties over other NFMs, it did exhibit cytotoxicity. In a rupture model of the flexor tendon of the rabbit HAI30FB, it was shown to function as a physical barrier membrane, simultaneously inhibiting inflammation and allowing adhesion formation, compared with a commercial membrane (Seprafilm ® ) and NFMs without ibuprofen [ 121 ].

In the skin, HA retains and evenly distributes water, thus preserving the volume of the skin and its elastic and flexible properties [ 28 ]. It also plays a protective role as an inhibitor of free radicals, generated upon exposure to solar radiation [ 26 , 28 , 74 ]. It has been reported that about one third of the total amount of HA (5 g) is both the dermis (±0.5 mg/g wet tissue) and the epidermis (±0.1 mg/g wet tissue) [ 16 , 37 ]. In the epidermis, this is metabolized and actively participates in many regulatory processes, such as cell proliferation, migration, and differentiation. In the dermis, it fills the extracellular spaces [ 68 ]. Exposure of the skin to ethanol is a common occurrence in health facilities. In addition to its use in cosmetics, ethanol is used as a vehicle in tests of contact allergies and to increase the skin’s permeability to medicines. Neuman et al., evaluated the physiological levels of damage caused by ethanol in vitro in skin cells (human A431 epidermoid skin cells) and mouse fibroblasts, and the possible repair by HA. The results report that the treatment of cells with ethanol increased cytotoxicity, as well as the release of pro-inflammatory cytokines. On the other hand, HA reduced the amount of pro-inflammatory cytokines released [ 127 ]. The work conducted by Pleguezuelos-Villa et al., aimed to develop mangiferin (natural xanthone) nanoemulsion hyaluronate gels for inflammatory disorders in the absence or presence of transcutol-P (a solubilizer associated with skin-penetration enhancement in topical dosage forms). The results show that the release of mangiferin depends on the MW of the polymer; in addition, the permeability tests (swine epidermis) showed that nanoemulsions with a low molecular weight HA improve permeation, with this effect being more evident in nanoformulations with transcutol- P. In mice inflamed with 12-O-tetradecanoylphorbol-13-acetate (TPA), the topical application of nanoemulsions attenuated of edema and leukocyte infiltration. In summary, the results show that the topical application of these formulations has an appropriate anti-inflammatory effect [ 128 ]. Atopic Dermatitis Atopic dermatitis (AD) is a chronic inflammatory skin disease, characterized by intense itching, rashes and inflammatory reactions [ 129 , 130 , 131 ]. To date, there is no total therapy for the treatment of AD, due to its complex pathogenic interaction between genes that are susceptible to the patient, abnormalities in the skin barrier and immune dysregulation. Several pharmacological and non-pharmacological approaches have been used. Among the various pharmacological interventions, topical corticosteroids (CTs) are the main choice; however, they present a series of local and systemic adverse effects. As an alternative, topical calcineurin inhibitors (TCIs) appear, and can be used for longer periods in an equally effective and safe way [ 130 , 131 ]. The purpose of the work by Zhuo et al., was to provide tacrolimus (TCS) to the deepest layers of the skin for AD treatment. For this purpose, HA-coated nanoparticles (HA-TCS-CS-NPs) were developed. In vitro results show that HA-TCS-CS-NPs follow a sustained release when compared to TCS-CS-NPs. In addition, HA-TCS-CS-NPs show a more effective dermal targeting capacity and increase the TCS that is retained in the epidermis and dermis, showing that the presence of HA improves the therapeutic efficacy of the developed system, probably due to its mucoadhesive properties. It has also been shown that HA-TCS-CS-NPs has pronounced anti-AD efficacy, since it leads to a reduction in transepidermal water loss (TEWL), erythema intensity and dermatitis index [ 131 ].

Inflammatory bowel disease (IBD) mainly consists of Crohn’s disease (CD), and ulcerative colitis (UC) is a chronic disease of the gastrointestinal tract (GIT) [ 140 , 141 , 142 ], caused by dysfunctional or abnormal epithelial and immune responses to intestinal microorganisms [ 140 ]. UC is a condition in which the inflammatory response is confined to the colon. Inflammation is mainly limited to the mucosa, with ulceration, edema and hemorrhage along the colon. Unlike UC, CD can involve any part of the gastrointestinal tract, from the oropharynx to the perianal area [ 143 , 144 ]. Here, inflammation can be transmural, often extending to the serous, resulting in sinus pathways or fistula formation [ 144 ]. It is also characterized by a defect in the integrity of the epithelial barrier, resulting in the translocation of microbial and other antigens, acting as external agents [ 142 ]. Endothelial cells are known to express high levels of CD44 in inflamed IBD sites, which appear to be essential for immune cells to infiltrate inflamed tissues [ 140 ]. Therefore, HA could be an ideal carrier for the targeted delivery of drugs to the inflamed mucosa [ 141 ]. Mesalamine (also known as 5-aminosalicylic acid (5-ASA)) is a first-line drug for patients with IBD, particularly those with mild to moderate UC [ 140 , 142 ]. The study by Chiu et al., investigated the efficacy of its adherence, promoting healing and reducing inflammation both ex vivo and in vivo in rats with UC after combined treatment with HA and 5-ASA (IBD98-M). Treatment with IBD98-M has been shown to significantly reduce intestinal inflammation and promote colon-mucosa-healing in UC induced by 2,4,6-trinitrobenzenesulfonic acid (TNBS). In fact, it is known that the viscoelastic properties of HA can protect mucosal cells, and other anatomical structures, against chemical wounds. Treatment with IBD98-M was found to have a synergistic effect on mucosal wound-healing compared to treatment with HA or 5-ASA alone [ 140 ]. Vafaei et al., used amphiphilic HA conjugates with the ability to form self-assembled nanoparticles (NPs) in aqueous solutions (HANPs) in which budesonide (BDS) has been incorporated. In vitro, BDS-loaded HANPs demonstrated a greater anti-inflammatory effect on the secretion of IL-8 and TNF-α in inflamed cells when compared to the free drug. Increased cell uptake via CD44-receptor-mediated endocytosis was also found in inflamed CACO-2 cells when compared to CACO-2 and NIH3T3 cells [ 141 ]. In the particular case of UC, Xiao et al., developed polymeric nanoparticles that were functionalized with HA, which carried a naturally occurring tripeptide, lysine-proline-valine (KPV), which attenuates the inflammatory responses of colon cells. The obtained nanoparticles (HA-KPV-NPs) successfully mediated the targeted delivery of KPV in the colon epithelial cells and macrophages, showing that they are biocompatible and non-toxic. HA-KPV-NPs have also been shown to have combined effects against UC, accelerating mucosal healing and relieving inflammation. The oral administration of HA-KPV-NPs, encapsulated in a hydrogel based on CS and alginate, exhibited a much stronger ability to prevent mucosal damage and negatively regulate TNF-α, thus showing much better therapeutic efficacy against UC in a mouse model when compared to a KPV-NPs/hydrogel system. These results show that HA-KPV-NP/hydrogel has the ability to release HA-KPV-NP in the colon lumen, and that these nanoparticles subsequently penetrate the colitis tissues and allow the KPV to be internalized in the target cells, acting against UC [ 145 ]. Sammarco et al., demonstrated that intestinal mucosa healing can be accelerated by HMWHA. In the study carried out by this team, after verifying that HA did not present cytotoxicity, it was found that the local application of HA (via enema, in C57BL/6N or BALB/c mice) accelerated the recovery of clinical parameters and improved tissue regeneration, with reduced bleeding and hyperemia, decreased ulcers, granularity and erosions, compared to the group that received saline solution as treatment. They also demonstrated that, in response to mucosal damage, the levels of HAS-2 mRNA were dramatically increased in the affected area, which probably led to the increased synthesis of HA in active disease. However, the newly synthesized HMWHA is probably not enough to prevent the inflammatory response associated with this pathology, so the local application of HMWHA represents an efficient approach [ 146 ]. Chen et al., studied the systemic administration of HMWHA for the treatment of TNBS-induced colitis. In vivo studies with TNBS- and HA-treated C3H/HeN rats showed promising results, namely a reduction in macroscopic damage (ulceration level) and clinical (appearance of diarrhea, signs of fecal blood, perfuse bleeding from anus) and histological alterations (crypt loss, erosions, number of follicle aggregates, edema, and infiltration of cells), when compared to rats t

The oral cavity is defined as the space between the lips until the end of the hard palate. It contains the teeth, the buccal and gingival mucosa, the mandible and the hard palate, as well as the floor of the mouth and the tongue anterior to the circumvented papilla [ 153 ]. It is one of the most complex microenvironments in the human body, where interactions between the host and microbes define health and disease. Teeth are the only functional hard tissues that extend from the inside of the human body, crossing a series of other hard (bones) and soft tissues (connective tissues and epithelia), and surrounded by a rigid biofilm formed by the richest collection of bacteria outside the colon. Here, several zones are created, which work together during the inflammatory responses of the oral cavity [ 154 ]. In the oral cavity, HA supports the structural integrity and homeostasis of tissues that regulate osmotic pressure and tissue lubrication [ 155 ]. The most common forms of inflammation in the oral cavity are gingivitis (inflammation of the gums) and periodontitis (inflammation of the tissues that support the teeth) [ 156 ]. Inflammatory gingivitis generated by plaque is very common and usually recurrent, as it is mistakenly considered trivial. The cyclic recurrence of gingivitis episodes causes irreversible damage to periodontal structures [ 157 ]. On the other hand, periodontitis is characterized by inflammation and an immune response, which generally progresses to the destruction of the periodontium. It is initiated by diverse and complex biofilms that form in the teeth. The substances that are released from this biofilm gain access to the gingival tissue and initiate an inflammatory and immune response, which results in tissue destruction and bone resorption [ 158 ]. The literature reports that in patients with chronic periodontitis, a rapid loss of HMWHA occurs due to the enzymatic digestive processes [ 159 ]. HA acts as a barrier to plaque bacteria and fulfils a variety of extracellular functions that are vital for maintaining healthy gum tissue. Gengigel ® is a 0.2% HA-based gel used to treat gingivitis. Jain Y. evaluated the therapeutic efficacy of Gengigel ® in scaling. The study was carried out on 50 patients diagnosed with plaque-induced gingivitis, divided into two groups (group 1 treated with Gengigel ® ; Group 2 as a control). The results obtained over 4 weeks show a significant improvement in the evaluated parameters (plaque index, gingival index and capillary bleeding), with more advantageous results for Gengigel ® , showing that HA is effective in the treatment of gingivitis [ 157 ]. One of the studies carried out in the area of periodontitis aimed to study the effect of the HA of various molecular weights. To do this, Chen et al., used an HA of 30, 300 and 1300 kDa in the inflammation of P. gingivalis -induced and in wound-healing in human gingival fibroblasts (HGFs). The results indicate that HA of 1300 kDa inhibited the production of IL-1β, IL-6, IL-8, IL-4, and IL-10, while HA of 30 and 300 kDa had no effect. The higher MW HA also inhibited the expression of NF-кB, the degradation of nuclear factor of kappa light polypeptide gene enhancer in the B-cells inhibitor, alpha (IкBα) and the activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinases (p38 MAKP) induced by P. gingivalis . This shows that HMWHA can have beneficial effects on periodontal inflammation and oral wounds [ 158 ]. The effect of 0.8% HA (GENGIGEL ® ) in patients with moderate to severe chronic periodontitis, as an adjunct to scaling and root planning, was assessed by Al-Shammari et al. The study consisted of subgingival application of 0.8% HA over one week. The evaluation was carried out after 6 and 12 weeks and showed an improvement in the studied parameters (plaque index, gingival index, papillary bleeding, periodontal probing depth and clinical attachment loss). The levels of human beta defensin-2 (hBD-2, antimicrobial peptide) were significantly higher at the test sites than at the control sites [ 160 ]. After tooth extraction, alveolar osteitis (AO) is the most common complication, resulting in acute pain and discomfort [ 161 , 162 ]. At the extraction site, fibrin deposition and clot formation occur, which act as a physical barrier and prevent the entry of bacteria. However, the release of tissue kinases can occur during inflammation caused by extraction, which leads to clot destruction (fibrinolysis) [ 163 ]. Partial or complete disintegration of the blood clot leads to a localized inflammatory response [ 161 , 162 ]. The study by Suchánek et al., aimed to evaluate the treatment of AO using a pharmacological device composed of octenidine dihydrochloride (OCD) and HA. In this strategy, the OCD disinfects the wound, making the device absorbable and acting as an analgesic. In turn, HA fixes the device to the mucosa, files the wound, keeps the device stable in the presence of sali

Acute kidney injury (AKI) is a serious kidney disease characterized by rapid loss of kidney function, leading to the accumulation of metabolic waste, electrolyte, and body fluid imbalances. The pathophysiology of this disease is associated with complicated mechanisms, such as the cause of oxidative stress, vascular damage, and inflammation. AKI is characterized, at the pathological level, by severe and even lethal damage to the renal tubules, leading to tubular dysfunction and cell death [ 170 ]. Despite presenting a low solubility in water and, therefore, a low bioavailability in biological systems, curcumin (CUR) is a polyphenol with antioxidant and anti-inflammatory properties, with studies showing that CUR is able to protect against the kidney damage caused by ischemia-reperfusion. To overcome these limitations, a polymeric HA-CUR (HA-CUR) prodrug directed to epithelial cells was developed. Studies have shown that the solubility of HA-CUR was increased compared to free CUR. Cell internalization studies show that HA-CUR was preferentially internalized by tubular epithelial cells compared to free CUR. In terms of biodistribution, the results also show an increase in the accumulation of HA-CUR in the kidneys when compared to free CUR. The levels of pro-inflammatory cytokines (TNF-α and IL-6) and oxidative stress (superoxide dismutase (SOD), glutathione (GSH) and malondialdehyde (MDA)) were reduced in the presence of CUR and HA-CUR, with a better therapeutic efficacy in the presence of HA-CUR [ 170 ].

HA has an excellent potential for a wide range of applications, which comprise much more than the facial treatments with which it is typically associated. Its wide applicability is due to its distinctive biophysical properties, which give it enormous potential. As previously reported, the fact that HA is found in virtually all cell types is indicative of its biological advantages, ranging from its mechanical and swelling properties to lubrification, tissue regeneration and hydration. A relevant aspect for all these biological properties is the cell surface receptors with which HA can interact, including the CD44 receptor, which is widely expressed in activated macrophages (which play a central role in the inflammatory process). It is the interaction with these receptors that enables the targeted delivery to target locations, resulting in greater cellular uptake and, therefore, beneficial results in reducing inflammation. Effectively, what is verified is that HA has been the subject of numerous studies in several pathologies. However, if the focus of HA applications is only inflammatory pathologies, especially OA and RA, it is clear that a great deal of effort has been devoted to the use of this molecule in the search for alternatives to conventional treatments. Although it is clear that HA’s biodegradability and biocompatibility contribute to its widespread use, either alone, in conjunction with other active ingredients, or in the formulation of nanoparticles, more studies should be carried out with the aim of obtaining prolonged circulation and a longer time in situ. A more detailed understanding of the degradation mechanisms of HA will also be essential to optimizing and improving the biomedical applications of this polymer. The development of methods that allow for a simpler administration (such as oral administration) of HA-based products would also be an asset. Similarly, studying how HA acts in extra-articular inflammatory pathologies will also be an essential step towards getting the most out of this powerful molecule. The major complications associated with the delivery of HA are due to its relatively short half-life in biological fluids. This means that it is often necessary to resort to crosslinking techniques, which can lead to greater toxicity compared to “free” HA. The viscosity of HA solutions can also raise some questions, as this can hamper the administration or effectiveness of the developed models. However, the studies reviewed here show promising results for the use of HA in the treatment of inflammatory pathologies.