Rheumatoid arthritis (RA) is characterized as an autoimmune disorder that primarily affects the synovial joints, leading to systemic inflammation and joint destruction. The autoimmune nature of RA indicates that an inappropriate immune response targets the body's own tissues, particularly the joints. Evidence suggests that genetic factors, such as specific alleles of the major histocompatibility complex (MHC), in conjunction with environmental factors (e.g., infections, tobacco use), can trigger this immune response, resulting in the production of autoantibodies and the activation of autoreactive T and B cells [1,2].

The signs and symptoms of RA are typically the result of synovitis—an inflammation of the synovial membrane of the joints. This inflammatory process involves a complex network of cells and cytokines, particularly in the recruitment, activation, and effector functions of immune cells [3,4]. In addition to the influx of B and T lymphocytes, plasma cells, mast cells, dendritic cells, and neutrophils, fibroblasts also infiltrate the synovial membrane. The hypertrophied synovial layer, known as pannus, contributes to the destruction of bone and cartilage in the joint. Interleukin-1 (IL-1) was the first cytokine detected in the synovial fluid of patients with RA and has been associated with cartilage degradation in vitro [5,6].

Subsequently, levels of tumor necrosis factor-alpha (TNF-α) were also shown to correlate with cartilage degradation in vitro. TNF-α is a potent pro-inflammatory cytokine that induces several other cytokines in the pro-inflammatory cascade, including IL-1, IL-6, IL-8, and granulocyte macrophage-colony stimulating factor (GM-CSF), and increases the levels of various adhesion molecules such as intracellular adhesion molecule (ICAM) and vascular cell adhesion molecule (VCAM) [7,8]. Structural erosions in RA occur at the border between the pannus (the hypertrophied layer of synovial cells) and cartilage, in an area densely populated by TNF-α-producing cells. It has been shown that the neutralization of TNF-α using anti-TNF-α antibodies in mouse models of arthritis results in a significant reduction in joint edema and structural erosions [9].

In RA, pro-inflammatory cytokines released by activated immune cells play a crucial role in mediating the autoimmune response, perpetuating inflammation, and contributing to destructive processes within the joints. This review will focus on elucidating the mechanisms by which these cytokines contribute to RA pathogenesis, as well as the potential side effects of cytokine inhibitors [10,11].

Cytokine Targets

Cytokines have long been explored as potential targets for rheumatoid arthritis (RA) because they are directly involved in the disease process, which can be classified into pro-inflammatory and anti-inflammatory cytokines based on their functions in response to antigens. A seminal preclinical study demonstrated the utility of blocking TNF-α in RA, showing that antibodies against TNF-α significantly reduced high IL-1 production in RA synovial cell cultures [12,13].

Pro-inflammatory cytokines such as TNF-α, IL-1β, IL-6, IL-7, IL-15, IL-17, IL-18, IL-23, IFN-γ, and GM-CSF play critical roles in regulating inflammation and are present at elevated levels in the synovium, synovial fluid, serum, or peripheral blood of RA patients. Certain cytokines, including IL-15, IL-17, IL-23, and GM-CSF, are closely linked to rheumatoid factor (RF), anti-cyclic citrullinated peptide (CCP) seropositivity, and RA activity, making them potential diagnostic biomarkers. Notably, IL-7 is recognized for its role in early RA, with levels varying across different disease stages [14-16].

Table 1. Key Cytokines Involved in Rheumatoid Arthritis and Their Targets

 

Additionally, macrophages secrete various cytokines, including TNF-α, which promotes the proliferation of fibroblast-like synoviocytes (FLS) and synovial cells through the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and ERK-1/2-E26 pathways. This activation leads to the production of inflammatory mediators such as IL-6, matrix metalloproteinase-1 (MMP-1), and MMP-3. Here, the redundancy in cytokine roles becomes apparent; IL-1β enhances MMP production and leukocyte adhesion in RA joints by activating ERK, JNK, AP-1, and NF-κB, while IL-6 induces bone resorption and cartilage degradation via MMPs and receptor activator of nuclear factor kappa-B ligand (RANKL) [17]. IL-7 and IL-15 are implicated in T-cell trafficking and antigen-specific CD4+ T-cell proliferation, respectively, with IL-7 showing potential as a biomarker for early RA. Furthermore, IL-18 and IL-12 jointly stimulate interferon-gamma (IFN-γ) production, further promoting inflammation [18].

Targeting these cytokines with inhibitors has proven effective in reducing symptoms and halting disease progression. For example, adalimumab, a TNF-α inhibitor, prevents TNF-α from binding to its receptors, thereby reducing inflammation and protecting cartilage and bone from destruction. However, the potential side effects of these therapies—including increased susceptibility to infections, malignancies, and infusion reactions—necessitate careful monitoring and consideration in treatment plans to ensure patient safety [19].

Conversely, anti-inflammatory cytokines such as IL-4, IL-10, IL-13, and TGF-β counteract the inflammatory process. Understanding these protective roles is essential for their utilization in targeted therapies. For instance, IL-10, produced by regulatory T cells, suppresses Th17 cells and promotes Treg cell development. Exploring the therapeutic potential of these cytokines could enhance existing treatments and underscore the benefits of personalized medicine strategies.

Introducing IL-4, IL-10, IL-13, or TGF-β as therapeutic agents could harness their anti-inflammatory properties to mitigate the symptoms of rheumatoid arthritis (RA). IL-4 exhibits an anti-angiogenic effect through the inhibition of vascular endothelial growth factor (VEGF), while IL-13 provides cartilage protection via downregulation of Fc gamma receptor I (FcγRI), further supporting their therapeutic potential. This intricate interplay of cytokines underscores the complexity of RA pathogenesis and the potential of targeted cytokine therapies to improve patient outcomes.

IL-13, a cytokine associated with Th2 cell-mediated immune responses, exerts its anti-angiogenic function by activating protein kinase C (PKC) α/β II and ERK-1/2, while simultaneously downregulating the NF-κB/p65 pathway. It may also reduce chondrocyte death and protect cartilage from destruction by lowering FcγRI levels [20,21]. Transforming growth factor (TGF)-β, primarily expressed by macrophages and T lymphocytes, promotes the migration and invasion of fibroblast-like synoviocytes (FLS), inducing epithelial-mesenchymal transition (EMT) via Smad-2/3 activation in RA [22]. Clinical trials have shown that dekavil (an IL-10 agonist) demonstrates significant efficacy in RA patients, as these agents can bind to and initiate receptors, inducing corresponding biological responses.

Thus, cytokines play a fundamental role in the pathogenesis of RA through their regulation of inflammation and interactions with various immune cells. Elevated levels of pro-inflammatory cytokines, such as TNF-α, IL-1β, IL-6, IL-7, IL-15, IL-17, IL-18, IL-23, IFN-γ, and GM-CSF, are found in the synovium, synovial fluid, serum, and peripheral blood of RA patients, highlighting their central role in disease progression [23]. TNF-α, produced by macrophages, stimulates FLS and synovial cells via NF-κB and Erk-1/2-E26 signaling pathways, leading to the secretion of inflammatory mediators like IL-6, MMP-1, and MMP-3, thereby exacerbating inflammation [11].

The treatment of RA has evolved significantly over the past 20 years. Traditional therapies included glucocorticoids and nonsteroidal anti-inflammatory drugs (NSAIDs) for acute inflammation, alongside disease-modifying antirheumatic drugs (DMARDs) such as methotrexate (MTX), hydroxychloroquine, and sulfasalazine for maintenance therapy [24,25]. DMARDs, particularly MTX, have been shown to reduce joint symptoms and extra-articular manifestations while inhibiting radiographic progression, so they continue to be used as first-line therapies. However, although some patients with RA respond to DMARDs alone, a significant proportion do not tolerate this therapy and/or continue to have active disease despite treatment. Additionally, cumulative side effects, particularly from corticosteroids and NSAIDs, can exacerbate comorbidities. This situation created a need for alternative therapies, leading to the emergence of biologic agents for treating moderate to severe RA in the early 1990s [26,27].

Inhibition of inflammatory cytokines, particularly TNF-α, became a major focus of RA clinical research in the 1990s, a trend that continues today. The first open trial using a TNF-α-blocking drug was conducted in the UK in 1992, where 20 patients with RA received infliximab, a chimeric antibody specific to TNF-α. This treatment resulted in a significant reduction in the signs and symptoms of RA, accompanied by a decrease in inflammatory markers [28]. The early promise of anti-TNF-α therapy in RA has been confirmed in several large multicenter trials, revolutionizing the treatment of RA and providing crucial insights into the disease's pathophysiology. These therapies have achieved blockbuster status, generating substantial profits for the pharmaceutical industry. Since the advent of infliximab, other anti-TNF-α agents and cytokine inhibitors have been developed and introduced into clinical practice. Although primarily developed and tested in RA, these drugs have also been used in other chronic inflammatory diseases, sometimes yielding unexpected and divergent results [29,30].

Anti–Tumor Necrosis Factor

Currently, five TNF-α inhibitors are licensed for the treatment of rheumatoid arthritis (RA). These include three full-size monoclonal antibodies (infliximab, adalimumab, and golimumab), one humanized antigen-binding fragment conjugated with polyethylene glycol (certolizumab), and one soluble fusion protein (etanercept). These biologics target and neutralize tumor necrosis factor-alpha (TNF-α), a key cytokine in the inflammatory processes associated with RA.

The American College of Rheumatology (ACR) has established a core set of seven disease activity indicators: swollen joint count, tender joint count, physician's global assessment of disease activity, patient's global assessment of disease activity, patient's assessment of pain, patient's assessment of physical function, and an acute-phase reactant level (either C-reactive protein [CRP] or erythrocyte sedimentation rate [ESR]) [31,32]. These indicators are used to assess treatment efficacy and were instrumental in developing the ACR response criteria, which are standardized measures of clinical improvement. The ACR20 response—indicating a 20% improvement in tender and swollen joint counts along with three of the other five core measures—serves as the standard primary endpoint in RA clinical trials [33]. ACR20 evaluates individual patient improvement rather than average group improvement, typically reported as the percentage of participants achieving this level of improvement. As treatment modalities have advanced, higher thresholds such as ACR50 and ACR70, reflecting 50% and 70% improvement respectively, have also been incorporated into clinical evaluations [34].

Phase III trials for TNF-α inhibitors, including those approved for RA, consistently demonstrated the primary endpoint (ACR20) in patients with active RA who inadequately responded to methotrexate (MTX-IR) [35]. While these studies showed variable times to achieve the primary endpoint, resulting in higher placebo responses in shorter trials, the overall response rates were similar across different TNF-α inhibitors. In MTX-IR patients, approximately 60%, 40%, and 20% achieved ACR20, ACR50, and ACR70 responses, respectively, by week 24. Despite these promising results, significant unmet needs remain, as about 40% of patients do not achieve even a 20% response, and only a small minority reach remission in the short term. A meta-analysis of 6-month ACR response rates from 16 studies comparing various biologics to a placebo-MTX combination estimated the number needed to treat (NNT) to achieve ACR20, ACR50, and ACR70 responses at 3.2, 4.2, and 7.7, respectively. Long-term studies and systematic reviews have further validated the efficacy of TNF-α inhibitors in reducing radiographic progression of erosive damage and improving quality of life, as measured by the Health Assessment Questionnaire (HAQ) [36].

In clinical practice, TNF-α inhibitors like adalimumab function by binding to TNF-α, thereby preventing it from interacting with its receptors and reducing the inflammatory processes associated with cytokines such as MMP-1 and MMP-3. This inhibition helps prevent cartilage and bone destruction. Despite their effectiveness, a significant portion of RA patients do not respond adequately to TNF-α inhibitors, highlighting the need for personalized treatment approaches and ongoing research into alternative therapeutic targets [37]. The development of additional cytokine inhibitors and biologics targeting different aspects of the immune response is crucial for addressing the diverse and complex nature of RA. Furthermore, the role of anti-inflammatory cytokines such as IL-4, IL-10, IL-13, and TGF-β in counteracting RA inflammation presents potential therapeutic avenues. Introducing these cytokines or their agonists could complement existing therapies, providing a multifaceted approach to RA management and improving patient outcomes [38].

Anti-Interleukin-6 (IL-6) drugs and interleukin-1 (IL-1) inhibitors effects and biochemistry

Anti-Interleukin-6 (IL-6) drugs and interleukin-1 (IL-1) inhibitors are crucial for the treatment of rheumatoid arthritis (RA) due to their roles in modulating the immune response and inflammation. IL-6 is a multifunctional cytokine involved in immune regulation, hematopoiesis, and inflammation. It exerts its effects by binding to its receptor, IL-6R, which exists in both membrane-bound and soluble forms [39]. The IL-6/IL-6R complex then associates with the signal-transducing component gp130, triggering downstream signaling pathways, including the JAK-STAT pathway. This activation leads to the transcription of genes involved in inflammatory responses. In RA, IL-6 contributes to synovitis and systemic inflammation by promoting the differentiation of B cells into antibody-producing plasma cells, enhancing T cell activation, and stimulating the production of acute-phase reactants like C-reactive protein (CRP). Elevated IL-6 levels correlate with disease activity and joint destruction [40,41].

Tocilizumab, a humanized monoclonal antibody against the IL-6 receptor, inhibits IL-6 signaling by preventing IL-6 from binding to both soluble and membrane-bound IL-6 receptors. This blockade reduces inflammation, decreases CRP levels, and improves clinical symptoms in RA patients [42]. Tocilizumab has demonstrated efficacy in patients who inadequately respond to traditional disease-modifying antirheumatic drugs (DMARDs) and TNF-α inhibitors, leading to reduced joint damage progression and improved physical function. Another IL-6 inhibitor, sarilumab, also targets the IL-6 receptor and exhibits similar therapeutic effects [43].

Interleukin-1 (IL-1) is a pro-inflammatory cytokine primarily produced by activated macrophages. It exists in two forms: IL-1α and IL-1β, both of which bind to the IL-1 receptor type I (IL-1RI), initiating signal transduction that activates NF-κB and MAPK pathways. These pathways induce the expression of various inflammatory mediators, including additional cytokines, chemokines, and matrix metalloproteinases (MMPs), contributing to synovial inflammation and cartilage degradation in RA. IL-1 also enhances the differentiation of osteoclasts, leading to bone resorption [44].

Anakinra, a recombinant human IL-1 receptor antagonist, competitively inhibits the binding of IL-1 to IL-1RI, thus blocking IL-1-mediated signaling. This inhibition reduces the inflammatory response and slows the progression of joint damage [45,46]. Although effective, anakinra is less commonly used than other biologics due to its daily injection requirement and more modest efficacy compared to alternative options. Additionally, canakinumab, an IL-1β monoclonal antibody, and rilonacept, a soluble decoy receptor for IL-1, provide alternative approaches to IL-1 inhibition. While effective in reducing inflammation and disease activity in RA, their use is more limited compared to other biologics [47].

Both anti-IL-6 drugs and IL-1 inhibitors significantly impact RA by modulating key cytokine pathways involved in the disease's pathogenesis. By targeting these cytokines, these therapies help control systemic and local inflammation, reduce joint damage, and improve patient outcomes [48].

Targeting the Interleukin-17/23 Cytokine Pathway

Targeting the IL-17 and IL-23 cytokine pathway in rheumatoid arthritis (RA) represents an emerging therapeutic strategy due to the significant roles these cytokines play in the disease's inflammatory processes. IL-17, particularly IL-17A, is produced by T helper 17 (Th17) cells and is known to promote inflammation by inducing the production of several pro-inflammatory mediators, including TNF-α, IL-1, and IL-6, as well as chemokines and matrix metalloproteinases (MMPs) [49, 50]. These factors contribute to joint inflammation and destruction.

IL-23, composed of p19 and p40 subunits, is crucial for the differentiation and maintenance of Th17 cells, thereby influencing the production of IL-17 [51]. Clinically, agents that inhibit either IL-17 or IL-23 are being explored for their potential to reduce inflammation and disease progression in RA. Monoclonal antibodies such as secukinumab and ixekizumab specifically bind to IL-17A, preventing it from interacting with its receptor, thereby blocking its pro-inflammatory effects. Clinical trials indicate that these agents can reduce disease activity and improve clinical symptoms in RA patients [52, 53]. However, the response rates observed in RA have been less robust than those seen in other IL-17-mediated conditions, such as psoriasis and ankylosing spondylitis. This suggests that while IL-17 is involved in RA, its role may not be as dominant as in those conditions or that a more comprehensive blockade of the IL-23/Th17 axis might be necessary [54, 55].

Additionally, ustekinumab, which targets the shared p40 subunit of IL-12 and IL-23, indirectly affects the IL-23/Th17 pathway by inhibiting Th17 cell differentiation and IL-17 production. Clinical trials of ustekinumab in RA have shown some efficacy, resulting in improvements in clinical symptoms and reductions in inflammatory markers, although these results have been less pronounced compared to those seen in other autoimmune diseases [56, 57].

Therapeutic methods targeting the IL-17/23 pathway typically involve the subcutaneous administration of these biologic agents at intervals ranging from two to four weeks, depending on the specific drug and patient response. Such therapies are generally considered for patients who have not adequately responded to traditional treatments or TNF-α inhibitors. While the safety profiles of IL-17 and IL-23 inhibitors are generally favorable, ongoing monitoring for potential side effects, such as an increased risk of infections due to immune modulation, is essential [58, 59].

In exploring the therapeutic potential of these cytokines, it is important to note that while IL-17 inhibitors have shown promise, clinical trials have yielded mixed results. Factors contributing to these outcomes may include the complexity of the cytokine networks involved in RA, where IL-17 alone might not dominate the inflammatory landscape as it does in other diseases [66].

Targeting Anti–Granulocyte Macrophage-Colony- Stimulating Factor

GM-CSF is a cytokine that plays a pivotal role in the survival, proliferation, and activation of myeloid cells, including macrophages and neutrophils, which are key players in the inflammatory processes observed in rheumatoid arthritis (RA). GM-CSF enhances the production of other pro-inflammatory cytokines and chemokines, contributing to synovial inflammation, joint destruction, and systemic inflammation [60].

Several monoclonal antibodies targeting GM-CSF or its receptor (GM-CSFR) have been developed and tested in clinical trials for RA. Among these, mavrilimumab and otilimab are two prominent agents. Mavrilimumab is a human monoclonal antibody that binds to the alpha subunit of the GM-CSF receptor, thereby preventing GM-CSF from activating its receptor on immune cells. Otilimab (GSK3196165) targets GM-CSF itself, neutralizing its activity and thereby reducing inflammation [61].

Clinical trials for these anti-GM-CSF therapies have shown promising results. Mavrilimumab has demonstrated significant reductions in disease activity and improvements in clinical outcomes compared to placebo in patients with moderate to severe RA. Patients treated with mavrilimumab showed decreased swollen and tender joint counts, reduced pain and disability scores, and lower levels of acute-phase reactants such as C-reactive protein (CRP). Similarly, otilimab has shown efficacy in reducing RA symptoms and improving physical function, with clinical trials indicating a favorable safety profile [62].

Therapeutic methods involving anti-GM-CSF agents typically include subcutaneous injections administered at regular intervals, such as every two to four weeks. The dosing regimen and duration of treatment can vary based on the specific drug, patient response, and disease severity. These therapies are generally considered for patients who have not responded adequately to traditional DMARDs or other biologic agents, such as TNF-α inhibitors [63].

Lessons from Failed Cytokine Targets in Rheumatoid Arthritis

Another failed target is IL-20. This cytokine has been found to be elevated in patients with rheumatoid arthritis (RA), and preclinical studies suggested that it could contribute to joint inflammation and damage. However, clinical trials involving anti-IL-20 therapies did not demonstrate significant improvements in disease outcomes. Similarly, IL-21 was targeted due to its involvement in B cell differentiation and antibody production, yet anti-IL-21 therapies did not yield the expected clinical benefits. This may be due to redundancy within cytokine networks or compensatory mechanisms that mitigate the effects of IL-21 inhibition [67].

IL-32 was also considered a promising target given its elevated levels in RA synovial tissue and its ability to induce other pro-inflammatory cytokines. However, clinical trials targeting IL-32 have not progressed far, likely due to insufficient efficacy or challenges in developing specific inhibitors. IL-33 and its receptor ST2 have also attracted attention because of their pro-inflammatory roles in RA. Despite preclinical data supporting their involvement in RA pathology, clinical attempts to block IL-33/ST2 signaling have not achieved significant success [68].

Granulocyte colony-stimulating factor (G-CSF) and macrophage migration inhibitory factor (MIF) were also explored as potential targets. G-CSF is involved in the production and activation of neutrophils, which contribute to RA inflammation. However, targeting G-CSF did not result in meaningful clinical improvements, possibly due to its essential role in normal immune function and the risk of impairing host defense. MIF, a cytokine that promotes inflammation and is elevated in RA, was another target that failed to translate into successful clinical outcomes, likely because of the complex interplay of cytokines in RA and compensatory mechanisms that limit the effects of MIF inhibition [69,70].

Inhibition of interleukin-15 (IL-15) was another approach that did not yield the desired results in RA treatment. Although IL-15 is involved in T cell activation and survival, clinical trials with IL-15 inhibitors did not demonstrate significant efficacy, highlighting the redundancy and adaptability of the immune system. Additionally, interleukin-22 (IL-22) was targeted due to its role in tissue remodeling and inflammation; however, clinical efforts to inhibit IL-22 did not produce significant therapeutic benefits in RA [71].

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