Introduction

Iguratimod (IGU), also known as T-614, is a novel small molecule anti-rheumatic drug with the effect of non-steroidal anti-inflammatory drug (NSAID) and disease-modifying anti-rheumatic drug (DMARD) and is widely used for rheumatoid arthritis (RA) in China and Japan [1]. IGU has various mechanisms of action (Fig. 1). Firstly, IGU can inhibit prostaglandin E2(PGE2) production in inflammatory tissue by selectively inhibiting both the induction and activity of cyclooxygenase-2(COX-2) [2]. Secondly, IGU can inhibit tumor necrosis factor-α (TNF-α) production; suppress TNF-α induced production of interleukin (IL)-6, IL-8; and interfere with the TNF-α induced translocation of nuclear factor- kappa B (NF-κB) from the cytoplasm to the nucleus [3,4,5]. Thirdly, IGU can inhibit the expression of IL-17 and various proinflammatory factors triggered by it [6, 7]. Besides, IGU can inhibit macrophage migration inhibitory factor (MIF)-induced proinflammatory effects [8], inhibit osteoclastogenesis [9], and promote osteoblastic differentiation [10], etc.

Fig. 1
figure 1

Molecular molecular mechanisms of IGU related to the pathogenesis of AS. IGU inhibits COX-2/PGE2, TNF-α, IL-17 pathway, and MIF, which are important in the pathogenesis of AS. Besides, IGU inhibits RANK-L- mediated osteoclastogenesis and promotes osteoblastogenesis. Note: arrows Arrows with solid line denote stimulatory effect, arrows with dotted line denote inhibitory effect, “?” denotes that effects are unclear so far

Ankylosing spondylitis (AS), also termed radiographic axial spondyloarthritis (axSpa), is the major subtype of spondyloarthritis [11]. The main clinical features of AS include inflammatory back pain caused by sacroiliitis and inflammation at other locations in the axial skeleton, peripheral arthritis, enthesitis, and anterior uveitis [12]. In addition to inflammation, AS is also characterized by new bone formation in the sacroiliac joints (SIJ) and spine [11]. Theories on pathogenesis of AS include misfolding of human leukocyte antigen (HLA)-B27 during its assembly leading to endoplasmic reticulum stress and unfolded protein response (UPR) [13]. Activation of UPR genes results in release of TNF-α, IL-17, which are important in the development of AS [14]. COX-2/PGE2 pathway is also important in the pathogenesis of AS [15]. Besides, current evidence suggests that MIF drives inflammation and bone formation in AS [16]. MIF also interacts with IL-17 and TNF-α pathways by upregulating the expression and secretion of IL-17 [17] and inducing the production of TNF-α [16].

Current drugs for the treatment of AS mainly include non-steroidal anti-inflammatory drugs (NSAIDs), disease-modifying antirheumatic drugs (DMARDs), local steroids, TNF-α blockers, and anti-IL-17 agents [18]. However, these kinds of drugs are limited. Long-term use of NSAIDs can increase risks of adverse cardiovascular and gastrointestinal events [19]. Though conventional DMARDs might have a role in treating peripheral manifestations of AS, these agents are generally not effective for axial manifestations of AS [20]. While local steroids are effective in AS patients with peripheral arthritis, systemic glucocorticoids is not recommended for AS because few data supports the efficacy of these agents in patients with AS [18]. Though TNF-α blockers and anti-IL-17 agents show good efficacy in the treatment of AS, the high drug costs limit their widespread use [21,22,23]. Thus, exploration of new drugs that are effective for AS is necessary. IGU may be effective for the treatment of AS given that its mechanisms of action are closely related to the pathogenesis of AS. In this article, the molecular mechanisms of IGU that are related to the pathogenesis of AS are discussed, highlighting the effect of IGU on COX-2/PGE2, TNF-α, IL-17, MIF, and bone remodeling. In addition, currently available clinical studies of IGU for the treatment of AS are also discussed in the following text.

Molecular mechanisms of IGU related to the pathogenesis of AS

Effect of IGU on COX-2/PGE2 pathway

IGU was initially developed as a novel NSAID and showed anti-inflammatory, analgesic, and antipyretic effect in different animal models [24]. IGU effectively inhibited PGE2 generation by mouse fibroblasts and rat macrophages [25]. Further study found that IGU selectively reduced the COX-2 mRNA levels and inhibited both the induction and activity of COX-2 and thus inhibited PGE2 production in inflammatory tissue [2].

COX-2/PGE2 pathway is important in AS pathogenesis. Genome-wide association study (GWAS) identified a strong association between the PGE2 receptor encoding gene PTGER4 and AS [26]. The expression of COX-2 was found to be elevated in synovial tissues of AS patients [27]. Moreover, COX-2/PGE2 pathway plays a critical role in driving T helper 17 (Th17)-mediated synovial inflammation in an IL-23 and monocyte-independent manner [28]. Intriguingly, PGE2 also induces expression of IL-23R in naïve CD4+ T cells, which initiates activation of STAT3 after exposure to IL-23. Activated STAT3 initiates RORγt expression, which binds to the IL-17 gene promoter region to initiate Th17 cell [29]. Taken together, PGE2 can initiate Th17 both in IL-23-dependent and independent manner. The role of Th17 cell and IL-17 in the pathogenesis of AS will be discussed in detail in the following text.

Therefore, IGU might be effective in the management of AS by acting on COX-2/PGE2 pathway.

Effect of IGU on TNF-α pathway

TNF-α is a pro-inflammatory cytokine inducing NF-κB signaling activation [30]. Study showed that IGU had an inhibitory effect on the production of TNF-α, IL-6, and IL-8 in lipopolysaccharide (LPS)-stimulated human monocytic leukemia cell line (THP-1 cells), and this effect might involve transcriptional regulation through suppression of NF-κB activation without interfering with inhibitor of κB (IκBα) degradation [3]. Kohno et al. found that in cultured human synovial cells, IGU interfered with the TNF-α-induced translocation of NF-κB from the cytoplasm to the nucleus and suppressed TNF-α-induced production of IL-6, IL-8, and monocyte chemoattractant protein 1 [4]. In addition, study in macrophages and microglia also illustrated that IGU inhibited nuclear translocation of NF-κB 65 and downregulated pro-inflammatory response [5].

On the basis of data from basic, translational, and clinical studies, TNF-α has been shown to play an important role in the pathogenesis of AS. Transgenic mice expressing a truncated Peromyscus leucopus TNF-α gene manifest an arthritis resembling ankylosing spondylitis [31]. Mice overexpressed TNF-α consistently developed bilateral erosive sacroiliitis, which are inhibited by blocking of TNF-α [32]. Moreover, high amounts of TNF-α mRNA were detected in cellular infiltrates in the SIJ of patients with active AS [33]. The good efficacy of TNF-α inhibitors in the treatment of AS also provides evidence for a role of TNF-α in the pathogenesis of AS [21].

In conclusion, IGU might have a potential effect in the treatment of AS because of its effect on TNF-α pathway.

Effect of IGU on IL-17 pathway

The cytokine IL-17A is commonly called as IL-17 [34]. Th17 cells are the most common class of IL-17 producing lymphocytes [35]. After treatment with IGU for 28 weeks in RA patients, the levels of Th17 associated inflammatory cytokines and transcription factors were reduced [36]. IGU markedly decreased levels of IL-17 expression in synovial fibroblasts [6]. In addition, IGU suppressed the expression of various proinflammatory factors triggered by IL-17 in cultured fibroblast-like synoviocytes and the inhibition of IL-17 signaling by IGU was further linked to a decrease in mRNA stability of related genes, reduction of MAPK phosphorylation, and disruption of the interaction of Act 1 with TRAF5 and Ikki by targeting Act 1 [7].

IL-17 has been shown to be important in the pathogenesis of AS, including the development of enthesitis and new bone formation. A multitude of IL-17-producing cells have been implicated in AS [13]. Several IL-17-related genes have been identified by GWAS as risk factors for AS development [37]. In addition, animal models such as SKG mice and HLA-B27 transgenic rats also show a role of IL-17 in the development of AS [38, 39]. The IL-17 level is significantly elevated in serum and facet joints of AS patients [40,41,42]. IL-17 acts as an amplifier of enthesitis and induces the production of various cytokines and mediators by resident mesenchymal cells [15]. Blockade of IL-17 function with anti-IL-17 antibodies prevented the development of spontaneous ankylosing enthesitis in mice [43]. IL-17 can contribute to bone erosion, osteitis, and new bone formation, which are the hallmark skeletal features of AS [44]. Specifically, IL-17 promotes osteoclastogenesis directly or indirectly, through producing receptor-activator of nuclear factor-κB ligand (RANKL) or inducting its expression, suggesting that IL-17 might have a catabolic effect on bone [45]. However, the effects of IL-17 on osteoblast differentiation probably depend on the cell type exposed, the differentiation stage of that cell, and the timing and duration of cytokine exposure [45]. Anti-IL-17 agents have shown to be effective in the management of AS, which emphasizes the important role of IL-17 in AS [23, 46].

Based on the above evidences, IGU might be effective in AS treatment.

Effect of IGU on MIF

MIF is an important cytokine that regulates innate and adaptive immune responses [47]. IGU can interact with MIF trimers, inhibit the activity of MIF tautomerase, and inhibit MIF-induced proinflammatory effects, including B cell proliferation and monocyte cytokine release [8]. Moreover, IGU did not suppress systemic inflammation in the absence of MIF, indicating these effects are selective for MIF [8].

Current evidence suggests the importance of MIF in the pathogenesis of AS. MIF upregulates mitogen-stimulated IL-17 expression and secretion [17]. MIF levels are significantly higher in AS patients and there is a significant correlation between Bath Ankylosing Spondylitis Metrology Index (BASMI) and MIF levels in AS patients [48]. In addition, MIF drives inflammation and bone formation and predicts spinal progression in AS [16]. Moreover, a high prevalence of anti-MIF receptor (anti-CD74) autoantibodies has been reported in patients with AS and the presence of autoantibodies of the IgA subclass closely correlates with early disease [49, 50]. Whether these autoantibodies block or stimulate the effect of MIF remains unclear.

In conclusion, IGU might be effective in treating AS given its effect on MIF.

Effect of IGU on bone remodeling

Current evidence suggests that IGU can regulate RANKL/RANK/osteoprotegerin system by decreasing RANKL expression [6, 51, 52]. In addition, IGU can inhibit osteoclast formation, differentiation, migration, and bone resorption in RANKL-induced RAW264.7 cell [9]. Besides, IGU stimulated osteoblastic differentiation of stromal cell line (ST2) and preosteoblastic cell line (MC3T3-E1) in the presence or absence of recombinant human bone morphogenetic protein-2 (rhBMP-2) [10]. Oral administration of IGU to mice also promoted rhBMP-2-induced bone formation by increasing the expression of osterix, an essential transcription factor for osteoblastic differentiation [10]. Taken together, IGU has an anabolic effect on the bone metabolism by inhibiting osteoclastogenesis and promoting osteoblastic differentiation.

Skeletal damage in AS is the result of bone destruction and aberrant bone formation, which may occur simultaneously or in virtual juxtaposition [53]. Similar to other inflammatory joint diseases, AS is associated with systemic bone loss and increased fracture risk [54]. RANKL-mediated osteoclastogenesis is elevated in AS patients [55]. However, new bone formation outweighs bone destruction in AS, leading to syndesmophyte formation [56]. New bone formation in AS is a localized process that begins at specific anatomical sites and is particularly confined to the entheses and the interface between cartilage and bone, predominantly in the SIJs and the spine [57, 58].

In conclusion, bone remodeling in AS is complex and current evidences suggest that IGU has anabolic effect on bone metabolism. However, the effect of IGU on structural bone changes in AS remains unclear and further studies are needed.

Clinical trials of IGU in the treatment of AS

Clinical studies published up to April 2020 were searched in PubMed and Chinese databases WanFang and China National Knowledge Infrastructure (CNKI) using the keywords “ankylosing spondylitis,” “axial spondyloarthritis,” “iguratimod,” and “T-614.” The types of studies include all randomized controlled trials (RCTs), non-RCTs, and case series using IGU as an intervention. Participants were patients with AS who met the 1984 modified New York criteria for AS or 2009 Assessment of SpondyloArthritis International Society (ASAS) classification criteria for axSpa, including patients with refractory AS. A total of 6 articles are retrieved, including 4 RCTs and 2 case series (Table 1). The studies of Luo et al. and Qiu et al. showed that IGU can effectively ameliorate the clinical symptoms of patients with refractory AS,, with increasing percentage of patients meeting ASAS 20 response criteria [59, 60]. In the study of Luo et al., refractory AS patients were defined as patients who had failed therapy on 2 weeks of NSAIDs and with high disease activity, meaning Ankylosing Spondylitis Disease Activity Score (ASDAS) > 2.1 or Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) > 4.0 [59]. While in the study of Qiu et al., refractory AS patients were defined as patients who failed therapy on at least 3 months of regular NSAIDs and DMARDs [60]. In the study of Huang et al., 34 AS patients with BASDAI ≥ 4 had been treated with IGU for 6 months and after 3 and 6 months of treatment, the level of C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and BASDAI, Bath Ankylosing Spondylitis Functional Index (BASFI) score decreased significantly [62]. Other studies showed that after the treatment of IGU for at least 12 weeks, the percentage of patients who met ASAS20 criteria had increased, which was significantly higher than that in traditional DMARD group [62,63,64]. In conclusion, all the studies showed that IGU had good short-term efficacy for AS with few side effects including mild gastrointestinal (GI) side effects, leukocytosis, and elevated transaminase [59,60,61,62,63,64]. It is worth noting that currently available RCTs are all small-scale trials using traditional DMARDs, either salazosulfapyridine (SASP) alone or in combination with methotrexate (MTX) as control drugs, without mentioning the effect of IGU on structural bone changes in AS [60, 62,63,64]. Therefore, current evidences are not strong enough and more clinical studies with larger sample size, longer duration, and more clinical data including radiographic and MRI findings are needed to evaluate the effect of IGU for the treatment of AS.

Table 1 Characteristic of clinical trials of IGU for the treatment of AS

Conclusion

COX-2/PGE2, TNF-α, IL-17, and MIF pathways are shown to be important in the pathogenesis of AS. While IGU can inhibit PGE2, TNF-α, and IL-17 production and inhibit MIF-induced proinflammatory effects, it might have a potential therapeutic effect on AS. Besides, IGU has anabolic effect on the bone metabolism by inhibiting osteoclastogenesis and promoting osteoblast differentiation. However, bone remodeling in AS and the underlying mechanisms are complex. It is hard to predict the effect of IGU on the structural bone changes in AS. To date, there are only several small-scale clinical studies showing the short-term efficacy of IGU for the treatment of AS and none of them studies the effect of IGU on radiographic progression in AS. Therefore, in the future, more studies are needed to evaluate whether IGU is effective in improving the signs and symptoms of AS and the effect of IGU on the structural bone changes in AS.