Nanomedicine delivers promising treatments for rheumatoid arthritis

Abstract

An increased understanding in the pathophysiology of chronic inflammatory diseases, such as rheumatoid arthritis, reveals that the diseased tissue and the increased presence of macrophages and other overexpressed molecules within the tissue can be exploited to enhance the delivery of nanomedicine. Nanomedicine can passively accumulate into chronic inflammatory tissues via the enhanced permeability and retention phenomenon, or be surface conjugated with a ligand to actively bind to receptors overexpressed by cells within chronic inflammatory tissues, leading to increased efficacy and reduced systemic side-effects. This review highlights the research conducted over the past decade on using nanomedicine for potential treatment of rheumatoid arthritis and summarizes some of the major findings and promising opportunities on using nanomedicine to treat this prevalent and chronic disease.

Introduction to chronic inflammatory disease

Inflammation is a normal response that protects tissues from infection or injury. The normal cycle of acute inflammation includes the activation of inflammatory mediators and recruitment of monocytes from circulation to remove foreign pathogens at the inflammation site. The resolution of inflammation consists of downregulation of proinflammatory mediators, release of anti-inflammatory mediators and the removal or clearance of apoptotic cells by phagocytes (i.e., efferocytosis) [1–4]. Acute inflammation promotes tissue repair (through the production of anti-inflammatory mediators), removes damaging pathogens and restores normal tissue functions; however, when the inflammatory trigger is not cleared or is persistent, or when the inflammation is nonresolving for other reasons, chronic inflammation can arise. Chronic inflammation involves the ongoing induction of proinflammatory mediators, infiltration of monocytes into the tissue and ultimately leads to tissue damage. This non-resolving inflammatory response, which may include overexpressed anti-inflammatory mediators, damaged tissues, necrotic monocytes that are not cleared through the lymphatic system and other factors, can become an inflammatory trigger in itself, and can result in an adaptive immune response [5–7]. Genetic factors, environmental triggers as well as adaptive immune response can lead to chronic inflammatory disease [8–10].

Rheumatoid arthritis (RA), inflammatory bowel disease (IBD), chronic obstructive pulmonary disease (COPD) and systemic lupus erythematosus (SLE) are examples of chronic inflammatory diseases. In this review, RA and its therapies will be highlighted. RA affects approximately 1.0% of the population in developed countries, affecting women three-times more than men [11]. The costs associated with disease management in the USA have been reported as US$19.3–39.2 billion per year for RA, with approximately 30% of that costs covered by patients [12–17]. The range of costs can be associated with direct and indirect medical expenses, costs of therapy chosen and the duration of disease. The CDC estimates 15,600 RA hospitalizations annually, further illustrating the prevalence and extent of care required for managing this disease [18].

The clinical presentation of RA is characterized by synovial inflammation, which can lead to deformation, bone erosion as well as loss of joint function. Other clinical manifestations often include the presence of rheumatoid factor (RF) and anticitrullinated protein antibody (ACPA) in the blood, muscle soreness and joint tenderness, as well as increased risk for cardiovascular, pulmonary and skeletal disorders [8]. The etiology of the disease is not completely understood; however, connections have been made to the HLA genes as well as other risk factors [4,19].

Most chronic inflammatory diseases can be controlled, but not cured, with currently available therapies. The current standard of care (SOC) for RA consists of: anti-inflammatory drugs, including nonsteroidal anti-inflammatory drugs (NSAIDs) and corticosteroids; disease-modifying antirheumatic drugs (DMARDs); biologic agents and surgery as needed. NSAIDs are primarily used to reduce minor inflammation and relieve pain; however, they do not reduce joint damage and are associated with significant side-effects including gastrointestinal bleeding, fluid retention and increased risk of heart disease. Corticosteroids have beneficial anti-inflammatory and immunosuppressive properties, making them applicable for bridging therapy with other agents, as low-dose therapy for continuous treatment, as high-dose therapy for the treatment of flares or administered intra-articularly for symptomatic relief. Their use, however, is greatly limited by adverse effects such as adrenal suppression, glucose intolerance and increased susceptibility to infections. DMARDs include methotrexate (MTX), hydroxychloroquine, sulfasalazine and leflunomide. Early introduction of these agents has been shown to provide a more favorable outcome in patients, and they are known to reduce disease progression. However, these agents are not without side-effects, namely gastrointestinal complications, liver toxicity and hematologic adverse events. Biologic agents directly target components of the immune response, including proinflammatory cytokines and immune cells. Examples of these proinflammatory cytokines include TNF-α, which is the therapeutic target of infliximab, etanercept, adalimumab, golimumab, certolizumab; IL-1, the therapeutic target of anakinra; and IL-6, the therapeutic target of tocilizumab. Biologics also target B cells (rituximab) and block the costimulatory signal required for T-cell activation (as seen with abatacept). Despite their efficacy, biologics are also associated with significant adverse effects, specifically increased risk of infections (e.g., tuberculosis) and certain types of cancers [20,21].

To impact the progression of disease, the pathogenesis of disease must be understood. Although the etiology of disease remains unknown, great strides in research have led to the identification of agents involved in the initiation and propagation of the inflammatory response. Proinflammatory cytokines TNF-α, IL-1 and IL-6 have been the targets of biologic therapies such as Humira (adalimumab), a TNF-α inhibitor used for the treatment of RA, Crohn’s disease and ulcerative colitis [20–22]. Intracellular signal pathways have also been studied, with a focus on protein kinases. Kinase inhibitors block protein phosphorylation, thereby preventing the activation of transcription factors that control the release of proinflammatory mediators, including the aforementioned TNF-α, IL-1 and IL-6. Protein kinases including JAK, Syk, P13K and p-38 MAPK have been identified in the signal transduction pathway for RA [20,21,23–25]. Xeljanz (tofacitanib), the JAK inhibitor, has been approved for use in patients with RA.

Overview of nanomedicine

Nanomedicine is defined as the application of nanotechnology in the diagnosis, treatment or prevention of disease. Nanomedicines may include drug-loaded liposomes, nanoparticles, polymeric micelles, nanogels and nanocapsules [26]. In addition, polymer–drug conjugates, polymer–protein conjugates and antibodies are all classified as nanomedicines [26]. Nanomedicines can be designed to: protect the therapeutic agent from degradation, remain in blood circulation longer, be tailored for macrophage uptake or targeted to certain receptors and permeate through certain diseased tissues as interendothelial cell gaps are generally 1–2 nm in healthy tissues [27–30], but can be up to 600 nm in diseased tissues, such as inflamed joints [31,32].

Nanomedicines can be prepared with polymers, lipids as well as inorganic nanostructures. Lipid-, polymer-, and hybrid lipid–polymer-based nanomedicines are often utilized for intravenous (iv.) administration and include, but are not limited to, liposomes, PEGylated liposomes, polymersomes, micelles, dendrimers and hydrogel nanoparticles. Nanoparticle system should be designed considering: the properties of the active molecule to be delivered, the biological target, the environment prior to reaching the target and the environment at the target sites [33,34]. Hydrophobic and hydrophilic properties of polymers and lipids can be manipulated to encapsulate hydrophobic or hydrophilic active moieties. The types of polymers and lipids used can i) be determined based on compatibility with the active drugs or ii) modify drug-release property. Ligands or antibodies can be conjugated onto the nanoparticles to facilitate active targeting, if a known receptor is overexpressed in diseased tissues, which will be discussed in more details later. Furthermore, PEG chains can be covalently conjugated onto nanoparticles, also known as PEGylation, to decrease clearance of the nanoparticles when in circulation by providing a hydrophilic and steric barrier against opsonization [34,35].

Inflammatory tissue as a target for drug delivery

There has been a surge in the use of nanomedicine for drug delivery in the treatment of cancer, as solid tumor environment consists of ‘leaky’ vasculature from gaps in the endothelial cell lining and fenestrations that allow for higher permeability of relatively large molecules and particles. Additionally, the presence of other mediators in the diseased tissue can also increase permeability. For example, TNF-α elicits monocyte recruitment from circulation and stimulates additional pores in the endothelial lining. This enhanced permeability paired with an impaired lymphatic drainage system in tumor tissues is known as the enhanced permeability and retention (EPR) effect [34,36]. Utilizing the EPR phenomenon for targeted drug delivery may increase the efficacy and reduce the systemic toxicities of potent anticancer agents [37,38].

The process of monocyte recruitment from circulation and the development of endothelial gaps that facilitate plasma leakage into the injured site are characteristic of inflamed tissues. In chronic inflammatory conditions, inflammatory mediators can be overexpressed and persistent, leading to ‘leaky’ vasculature similar to that seen in solid tumors [39,40]. In addition to increased permeability, inflamed tissues also have more activated macrophages or other monocytes that can be utilized as targets for site-specific drug delivery. Moreover, it has been shown that certain cell adhesion molecules (CAMs) are overexpressed on endothelial cells in IBD [41] and that vasoactive intestinal peptide (VIP) receptors are overexpressed in activated synoviocytes in patients with RA [42]. Ligands specific to those overexpressed molecules can be conjugated to nanomedicine to actively target drug to inflammatory tissues.

Current landscape of nanomedicine for RA

Adalimumab (Humira), etanercept (Enbrel) and infliximab (Remicade) are currently among the top ten best-selling drugs in the USA. They are biologics used for the treatment of RA and other inflammatory diseases. However, significant adverse effects may occur with the use of these biologics, leaving patients vulnerable to serious infections such as tuberculosis [43]. The use of nanocarriers allows for increased site-specific drug delivery to inflamed tissues, by utilizing the disease state including, but not limited to, enhanced permeability or changes in pH of inflamed tissues and by utilizing monocytes as active targets for drug delivery. For RA, iv., intra-articular (ia.) and subcutaneous (sc.) administration routes will be reviewed as these allow for systemic delivery to circulation with access to ‘leaky’ vasculature, or local administration directly to diseased tissues; thereby, allowing for maximum drug action. Note that this may differ depending on the disease state; for example, when treating IBD, nanoparticles may be dosed orally because the intestine is the diseased tissue for this condition.

Studies of nanomedicines for potential treatment of RA are summarized in Table 1. These include in vitro, in vivo and clinical studies utilizing nanomedicines for targeted drug delivery to diseased tissues in RA animal models or patients. This summary includes information from searches of multiple databases of scientific literature, including PubMed and ScienceDirect as well as for clinical trials [44]. These searches were limited to publications and clinical applications within the last 10 years. These studies evaluated the use of passive or active targeting for drug delivery, as well as the ability to increase the efficacy of existing therapies by utilizing nanomedicines.

Table 1.

Nanomedicines for the treatment of rheumatoid arthritis: in vitro, in vivo and clinical trials.

Type of therapy	Drug	Nano-DDS	Route	Targeting mechanism	Phase	Model†	Ref.
NSAID	Piroxicam	Liposomes	NA	Macrophage uptake	Preclinical, in vitro	RAW264.7 macrophages	[45]
NSAID	Nimesulide	Polymeric nanoparticles	ia.	Passive	Preclinical	NA	[46]
Gold salts	Gold salts	Nanoparticles	NA	Macrophage uptake	Preclinical, in vitro	RAW264.7 macrophages	[47]
Corticosteroid	Prednisolone	PEGylated liposomes	iv.	Passive	Phase II	NA	[48–50]
Corticosteroid	Prednisolone	PEGylated liposomes	iv.	Passive	Preclinical, in vivo	AIA mice; CIA mice	[51,52]
Corticosteroid	Methylprednisolone, betamethasone	PEGylated liposomes	iv., sc.	Passive	Preclinical, in vivo	AA rats	[53,54]
Corticosteroid	Methylprednisolone	Polymeric nanoparticles	iv.	Passive	Preclinical, in vivo	CIA mice	[55]
Corticosteroid	Betamethasone	PEGylated polymersomes	iv.	Passive	Preclinical, in vivo	AA rats; AbIA mice	[56]
Corticosteroid	Dexamethasone	Liposomes	iv.	Passive	Preclinical, in vivo	AA rats; CIA mice	[57,58]
Corticosteroid	Dexamethasone	PEGylated liposomes	sc.	Active (αvβ3 receptor)	Preclinical, in vivo	AA rats	[59]
Corticosteroid	Dexamethasone	PEGylated liposomes, polymeric micelles, polymer conjugates	iv.	Passive	Preclinical, in vivo	AA rats	[60]
Corticosteroid	Dexamethasone	PEGylated liposomes; liposomes	NA	Passive	Preclinical, in vitro	Leukocytes, fibroblasts, hepatocytes, macrophages	[61]
Folic acid antagonist (immunosuppressant)	Methotrexate	Dendrimers	iv.	Active (folate receptor)	Preclinical, in vivo	CIA rat	[62]
Folic acid antagonist (immunosuppressant)	Methotrexate	Lipid nanoemulsions	ia.	Passive	Preclinical, in vivo	AIA rabbits	[63]
Folic acid antagonist (immunosuppressant)	Methotrexate	Polymeric nanoparticles	NA	Active (anti-CD64 antibody)	Preclinical, in vitro	RAW264.7 macrophages	[64]
Folic acid antagonist (immunosuppressant)	Methotrexate	Polymeric nanocomplexes	NA	Cellular uptake	Preclinical, in vitro	HepG2	[65]
Folic acid antagonist (immunosuppressant)	Methotrexate	Polymeric nanoparticles; silica nanoparticles	ip.	Passive	Preclinical, in vivo	PIA rats	[66]
Immunosuppressant	Cyclosporine	Polymeric micelles (polysialic acid)	NA	Passive	Preclinical, in vitro	SW982 cells	[67]
TNF-α inhibitor (immunosuppressant)	Fragment TNF-α inhibitor	PEGylated nanomolecule	sc.	Passive	Phase III	NA	[44]
Immunomodulatory agent	TRAIL	PEGylated nanocomplexes	sc.	Active (ligand, DR 5 receptor)	Preclinical, in vivo	CIA mice	[68]
Immunomodulatory agent	TRAIL	Liposomes (conjugated)	ia.	Passive	Preclinical, in vivo	AIA rabbits	[43]
Immunomodulatory agent	Curcumin	Nanoemulsions	NA	Macrophage uptake	Preclinical, in vitro	RAW264.7 macrophages	[69
Angiogenesis inhibitor	Fumagillin	Lipid nanoparticles	iv.	Active (αvβ3 receptor)	Preclinical, in vivo	KRN mice	[70]
γ-secretase inhibitor	DAPT	Polymeric nanoparticles (hyaluronic acid)	iv.	Passive	Preclinical, in vivo	CIA mice	[71]
Topoisomerase inhibitor (cytotoxic)	Camptothecin	PEGylated lipid micelles	sc.	Passive and active (VIP)	Preclinical, in vivo	CIA mice	[42]
Gene therapy	tgAAC94 (TNF-α silencing)	Adeno-associated virus (AAV)	ia.	Passive	Phase II	NA	[44,72]
Gene therapy	siRNA (TNF-α silencing)	PEGylated liposomes	iv.	sPLA2 catalysis	Preclinical, in vitro	HeLa cells	[73]
Gene therapy	siRNA (TNF-α silencing)	Polymeric nanoparticles	ip.	Passive	Preclinical, in vivo	CIA mice	[74]
Gene therapy	siRNA (TNF-α silencing)	Liposomes	iv.	Passive	Preclinical, in vivo	CIA mice	[75]
Gene therapy	siRNA (TNF-α silencing)	Liposomes	iv.	Passive, surface charge (cationic)	Preclinical, in vivo	CIA mice	[76]
Gene therapy	siRNA (TNF-α silencing)	Polymer-lipid nanoparticle hybrid	ia.	Passive	Preclinical, in vivo	CIA mice	[77]
Gene therapy	siRNA (STAT1 silencing)	Polymeric nanoparticles	iv.	Passive	Preclinical, in vivo	CIA mice	[78]
Gene therapy	siRNA (IL-2/15Rβ silencing)	Polymeric nanoparticles	iv.	Passive	Preclinical, in vivo	AA rats	[79]
Gene therapy	IL-1 receptor antagonist plasmid DNA	Polymeric nanoparticles	iv.	Active (folate receptor)	Preclinical, in vivo	AA rats	[80]
Biologic	Etanercept	Polymeric nanocomplexes	sc.	Passive	Preclinical, in vivo	CIA mice	[81]
Biologic	VIP	PEGylated lipid micelles	sc.	Active (VIP)	Preclinical, in vivo	CIA mice	[82]

^†Model for in vitro or in vivo evaluation.

AA: Adjuvant arthritis; AbIA: Antibody-induced arthritis; AIA: Antigen-induced arthritis; CIA: Collagen-induced arthritis; DAPT: N,(N-[3,5-difluorophenacetyl]-L-alanyl)-S-phenylglycine t-butyl ester; DDS: Drug delivery system; ia.: Intra-articular; ip.: Intraperitoneal; iv.: Intravenous; KRN: Serum-induced arthritis; NA: Not applicable; NSAID: Nonsteroidal anti-inflammatory drug; PIA: Pristane-induced arthritis; sc.: Subcutaneous; TRAIL: TNF-related apoptosis inducing ligand; VIP: Vasoactive intestinal peptide.

Taking advantage of enhanced permeability

The passive targeting of nanomedicines to inflamed tissues based on enhanced permeability has been supported by various in vivo biodistribution studies [51,56,59,63,70]. Ishihara et al. showed that PEGylated polymersomes encapsulated with the glucocorticoid betamethasone preferentially accumulated in inflamed joints in a mouse model of antibody-induced arthritis. The high accumulation correlated with reduction in arthritic score, as well as reduced expression of proinflammatory cytokine, IL-6. In vivo imaging showed that the accumulation of the polymersomes in the joints maintained for up to 96 h, which led to a sustained therapeutic effect for 8 days [56].

Glucocorticoids are often utilized for patients with RA, and are considered potent anti-inflammatory agents; however, the exact mechanism of action of this class of drugs is not completely understood. Encapsulation of them into liposomes or polymersomes allows for more local delivery and accumulation to inflammation sites due to the EPR effect, thereby reducing systemic side-effects and enhancing therapeutic efficiency. Hofkens et al. showed that prednisolone phosphate encapsulated in PEGylated liposomes was able to downregulate the activation of proinflammatory macrophages and upregulate anti-inflammatory macrophages in vitro; however, only the downregulation of proinflammatory macrophages was observed in vivo [51]. The authors also conducted biodistribution studies to confirm that after iv. or sc. administration, the liposomes extravasate through leaky vasculature into synovial tissues and are engulfed by macrophages within the inflamed tissues [50], further supporting the utilization of the enhanced permeability for targeted delivery of anti-inflammatory agents. After macrophage uptake, significant reductions were seen in the expression of proinflammatory cytokines including TNF-α, IL-1β, IL-8 as well as CD86 protein, giving insight into the mechanism of action of the prednisolone phosphate [51]. Because of the promising in vivo results, safety studies were conducted for repeat dosing of the liposomal prednisolone phosphate, as well as dose range finding. It was concluded that the safety profile of the glucocorticoid benefited from the liposomal formulation, and that the effective dose and dose frequency of the glucocorticoid could be reduced in animal models by as much as tenfold; showing comparable efficacy with four daily injections of 10 mg/kg of free drug to a single dose of 1 mg/kg prednisolone phosphate in the liposomes [50]. The ability of nanoparticle formulations of glucocorticoids to suppress proinflammatory cytokines such as TNF-α at a lower effective dose and dose frequency may be advantageous to decrease the broader immunosuppression seen with many biologic TNF-α inhibitors on the market. A Phase II clinical study with liposomal prednisone has been conducted, confirming the safety and increased efficacy of the liposomal prednisone relative to free drug.

Ulmanksey et al. used an adjuvant arthritis (AA) rat model to evaluate two formulations of PEGylated liposomes, one containing methylprednisolone and the other betamethasone, against free drug as well as biologic TNF-α inhibitors, etanercept and infliximab. Their in vivo study results showed that the liposomal formulations led to a significant decrease in proinflammatory mediators and a longer duration of therapeutic effect when compared with free drugs or to the biological therapies; indicating a promising path forward toward a more effective and less frequent therapy using glucocorticoid–nanomedicines as an alternative to the immunosuppressant biologic therapies [53].

Selective biodistribution in inflamed tissues due to enhanced permeability and the resultant lower effective dose and longer duration of drug action leading to decreased dose frequency are a recurring theme with nanomedicines in RA models [45,50,53,55,57,58]. For example, Ulmansky et al. reported a longer duration of drug action in vivo with a liposomal glucocorticoid formulation, resulting in weekly administration, as opposed to daily injections of the free drug as well a decrease in effective dose ranging from two- to 25-fold [53,54]. Hwang et al. showed that delivery of α-methylprednisolone using a conjugated cyclodextrin polymer-based nanoparticle formulation reduced the dosing frequency from daily to weekly and the effective dose by up to 100-fold, with a comparable or superior suppression of RA symptoms [55].

Active targeting for increased site-specific delivery

Although the accumulation of the nanocarriers in inflamed tissues due to enhanced permeability can help to decrease effective dose and dose frequency, increased knowledge of the disease tissues could lead to even more discriminating biodistribution. Chronic inflammation is characterized by a persistent inflammatory response that includes the infiltration of macrophages, lymphocytes and plasma from circulation eventually leading to tissue damage. In the case of RA, tissue damage is mainly seen in the joints, leading to bone erosion and deformation. The affected joint tissue is characterized by an increased expression of adhesion molecules and chemokines, which can be exploited for active drug targeting. Recent studies indicate that folate receptor-β (FR-β) expression is elevated on activated macrophages in inflamed joints in RA [83]. With this information, Thomas et al. prepared MTX dendrimer nanoparticles with folic acid (FA) as a targeting ligand. MTX is one of the most commonly prescribed DMARDs, alone or in combination with biologic therapy; however, many patients are intolerant to the medication due to significant side-effects. Targeted delivery of MTX may help reduce these side-effects. Thomas et al. showed that the FA-conjugated dendrimer nanoparticles selectively bound to the activated macrophages in vitro and led to significant disease suppression in vivo [62]. Additionally, it was shown that the maximum-tolerated dose (MTD) of the MTX in nanoparticles was 7.5-fold higher than that of the free MTX [62]. The FA as a targeting ligand was also used in chitosan-DNA nanoparticles to enhance the delivery of a plasmid that encodes IL-1Ra, a receptor antagonist and natural blocker of IL-1, and the FA-conjugated chitosan-DNA nanoparticles were shown to have less cytotoxic effects than the FA-free nanoparticles, likely due to the active uptake of the particles by activated macrophages, reducing toxicity to other cells [80].

Vascular endothelial cells (VECs) are involved in the recruitment of monocytes from circulation into inflamed tissues during inflammation. They are also involved in angiogenesis during tissue repair. VECs have certain cell adhesion molecules (CAMs) or growth factor receptors that may be overexpressed in inflamed tissues and could be used as targets for active drug delivery [74,84–86]. Koning et al. evaluated the delivery of dexamethasone with PEGylated liposomes surface-conjugated with a specific peptide ligand to target the α_vβ₃ receptor, an integrin overexpressed on VECs in inflammation sites. In an in vivo AA rat model, the targeted liposomes showed a threefold increase in accumulation in the inflammation site, compared with the nonconjugated liposomes, which correlated with a significant decrease in arthritic severity score and a longer therapeutic effect [59]. This increased localization was interesting, as the conjugated liposomes were cleared significantly faster from circulation than the nonconjugated liposomes, showing how effective the targeted liposomes were, even with a shorter circulation time [59]. The same receptor was also targeted for the delivery of an angiogenesis inhibitor, fumagillin and the targeted fumagillin showed a higher affinity to inflamed tissues, decreased leukocytes recruited into the tissues, and suppressed inflammation [70]. In addition, the effective dose of the optimized targeted fumagillin nanoparticle formulation was decreased by eightfold [70].

VIP is a hormone active in the resolution of inflammation; therefore, even though resolution of inflammation is not achieved, VIP receptors tend to be overexpressed in activated macrophages and proliferating synoviocytes in RA. The conjugation of VIP to nanoparticles would allow for active targeting of overexpressed receptors; they could also potentially provide therapeutic value by downregulating proinflammatory cytokines and upregulating anti-inflammatory cytokines [87]. Koo et al. utilized VIP as a targeting ligand to deliver camptothecin (CPT), an anticancer drug that induces cell death, to the overproliferating synoviocytes [42]. In their study, the VIP was utilized at a very low dose and it alone had no therapeutic effect. CPT, which is a topoisomerase inhibitor, is limited in clinical use due to its systemic toxicity, including hematologic toxicity. Koo et al. reported that low doses of CPT in PEGylated lipid micelles that were conjugated with VIP showed a significant reduction in arthritic score compared with micelles that did not include the VIP ligand, as well as free CPT at threefold higher dose [42]. Histology analysis also showed a significant reduction of macrophages and synoviocytes in the previously inflamed tissue after treatment with CPT in the VIP-conjugated PEGylated micelles [42]. The dose of CPT used was approximately 100-fold lower than used for cancer therapy, and no systemic toxicity was observed in the CIA mouse model, making this a potentially promising new agent for the treatment of RA [42].

Advancing the SOC

The application of nanoparticles in molecular therapy is a growing area of research, as seen in Table 1. A clinical study using ia.-injected adeno-associated virus (AAV) to deliver a TNF agonist gene showed the proof of concept for the use of gene therapy in RA; however, the AAV therapy also resulted in significant side-effects, including joint swelling and discomfort at the injection site [72,88]. siRNA that specifically inhibits the expression of certain genes, such as TNF-α, that are critical in RA development and progression is an interesting alternative to biologic agents such as anti-TNF-α antibodies, because it has the potential to selectively inhibit the expression of proinflammatory genes in targeted cells, while avoiding or minimizing the systemic side-effects associated with current biologics. However, siRNA is largely ineffective when given alone in vivo, and is thus generally formulated into nanocarriers such as liposomes or polymeric nanoparticles to protect the siRNA and deliver it to inflamed sites [89–91]. For example, Scheinman et al. encapsulated STAT1 siRNA into nanoparticles prepared with poly(lactic-co-glycolic) acid (PLGA) and showed a fourfold increase in protection of the siRNA from serum nuclease when compared with naked siRNA after 20 h of incubation in bovine serum [78]. Importantly, the STAT1 siRNA nanoparticles caused partial disease regression in a mouse model of RA, a notable achievement as most therapies act to stop disease progression or to ameliorate symptoms, pointing to the potential of using siRNA in RA therapy [78]. Komano et al. encapsulated TNF-α siRNA using Wrapsomes consisting of a cationic lipid bilayer core that was surrounded by the siRNA complex. The siRNA-cationic lipid bilayer core was then encapsulated by a neutral lipid bilayer and this outer layer was PEGylated to reduce systemic clearance [75,92]. The TNF-α siRNA-Wrapsomes showed therapeutic effect in a mouse model of collagen-induced arthritis when administered at the onset of disease [75]. It was also showed that most of the siRNA in the Wrapsomes was delivered into CD11b⁺ cells, including macrophages and neutrophils, in inflamed synovium [75]. Khoury et al. complexed TNF-α siRNA with cationic liposomes and showed that the siRNA-cationic lipo-some complexes significantly reduced TNF-α secretion, ranging from 50 to 70% inhibition, over the course of 3 weeks and significantly ameliorated the disease in an experimental arthritis model [76].

With increased understanding of disease pathophysiology and the ability to deliver drugs directly to diseased tissues, more diverse agents and ligands can be evaluated now. For example, the TNF-related apoptosis inducing ligand (TRAIL) is a protein that binds to death receptors (DR) overexpressed on cancer cells, namely DR4 and DR5, inducing apoptosis in these cells without affecting normal, healthy cells [93–95]. This very specific activity makes it a very promising agent for the treatment of cancer, where the SOC is extremely toxic to normal cells, and clinical trials are in progress or completed using TRAIL or its agonistic antibodies in renal cancer, non-Hodgkin’s lymphoma and nonsmall cell lung cancer [44,96–99]. Synoviocytes in RA also overexpress DR5, making it a candidate for TRAIL-induced apoptosis. Kim et al. utilized PEGylated nanocomplex formulations to deliver TRAIL in a CIA mouse model, showing that the PEGylated nanoparticles increased the half-life of the protein by 13-fold and sustained delivery of TRAIL resulted in better efficacy against the disease [68]. Martinez-Lostao et al. used TRAIL conjugated to liposomes for in vivo evaluation in an AA rabbit model. The liposome formulation resulted in significant reductions in synovial hyperplasia, to almost normal values, and reduced angiogenesis and joint inflammation [43].

Limitations of nanomedicine

Nanomedicines are not without their limitations, especially for use in chronic conditions. The safety of the nanomaterials must be determined, in addition to the therapeutic agent itself. This may be a costly exercise, reducing the speed or number of nanomedicines that are translated into clinical trials. Additionally, it should also be shown that the nanoparticles or nanocarriers do not themselves incite an inflammatory response. The use of biocompatible, nonimmunogenic and biodegradable materials may be the key to avoiding this type of adverse reactions [90,100].

Conclusion

There has been increased interest and applications of nanomedicines in treating RA and other chronic inflammatory diseases. The ongoing development of biocompatible nanomaterials and delivery systems for current antirheumatic agents may ultimately lead to lower effective doses, reduced dose frequency and more effective therapies with less systemic side-effects. Increased understanding of the pathophysiology of RA will not only prompt new ideas to enhance the localization of antirheumatic agents in inflamed joints but also open the door to new agents such as siRNA, for which nanocarriers are generally needed to be effective in vivo.

Future perspective

In the near future, more anti-inflammatory agents targeting proinflammatory cytokines will likely be developed to treat RA and other chronic inflammations. Various nanomedicines that lower the dose and dosing frequency of existing anti-inflammatory agents, and thus reducing their side-effects, will likely move to clinical trials and/or clinics. Moreover, increased understanding of the mechanisms underlying RA and other chronic inflammatory diseases will lead to the identification of new drug targets and new drugs.

Executive summary.

Rheumatoid arthritis & its current treatments

• Chronic inflammations, including rheumatoid arthritis (RA), are diseases that can be controlled, but not cured.

• All current therapies for RA are associated with serious unwanted side-effects, especially after long-term usage.

• Macrophages and cytokines produced by them play a critical role in RA development and progression, and are the targets of many antirheumatic agents.

• Chronic inflammation site has leaky vasculature and impaired lymphatic drainage.

Nanomedicine in RA therapy

• Nanomedicine shows promise in the treatment of many diseases, including RA.

• Both small molecular anti-inflammatory agents and large molecules such as siRNA and proteins were delivered using nanocarriers.

• Delivery of anti-inflammatory agents using nanocarriers generally lowers the dose and dosing frequency of the agents, and reduces their side-effects.

• Nanomedicine is not without limitations, and biocompatible materials should be used to prepare nanocarriers to minimize carrier-induced side-effects.