Evaluation of the efficacy and safety of Iguratimod combined with traditional anti-rheumatic drugs in treating rheumatoid arthritis: a retrospective study

Abstract

Background

Rheumatoid arthritis (RA) is a chronic autoimmune disease that may lead to progressive joint damage and disability. Although methotrexate is a first-line treatment, some patients have inadequate responses or adverse reactions. This study aimed to evaluate the efficacy and safety of methotrexate combined with iguratimod versus methotrexate monotherapy in patients with RA.

Methods

Based on existing medical records, the clinical data of 80 patients with rheumatoid arthritis treated at our hospital between October 2021 and October 2023 were retrospectively analyzed. This was a single-center, retrospective, non-randomized controlled study, and the investigators were not involved in treatment allocation. According to the actual treatment regimens documented in the medical records, patients were divided into a conventional treatment group, which received methotrexate, and a combination treatment group, which received methotrexate plus iguratimod. Treatment efficacy, bone metabolism, CRP, RF, ESR, immune function, and adverse reactions were compared between the two groups. As the treatment regimens were not randomly assigned, the findings were mainly used to evaluate the associations between different treatment regimens and clinical outcomes.

Results

The combination therapy group demonstrated significantly higher response rates for ACR20, ACR50, ACR70 and considerably higher levels of N-MID and T-PINP than the conventional treatment group following treatment. This group demonstrated significantly decreased levels of β-CTX, CRP, RF, and ESR compared to the conventional treatment group (p < 0.05). After treatment, except for the similar changes in CD4 + and CD8 + levels, the levels of IgG, IgA, and IgM in the combination treatment group were significantly lower than the conventional treatment group (p < 0.05). The combination therapy group showed a more significant drop in DAS28 values than conventional treatment group (p < 0.05). However, there was no significant difference in adverse effects (p > 0.05).

Conclusion

In this single-center retrospective study, compared with methotrexate monotherapy, methotrexate combined with iguratimod was associated with higher ACR20, ACR50, and ACR70 response rates, as well as greater improvements in inflammatory markers, bone metabolism indicators, and DAS28 scores, without a significant increase in the incidence of adverse reactions. However, because this study used a non-randomized design and did not include propensity score matching, the findings should be further validated in prospective randomized controlled trials.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s12865-026-00879-9.

Background

Rheumatoid arthritis (RA) is the disease initially targets the synovial membrane, subsequently affecting articular cartilage and bone, ultimately leading to joint deformity, structural damage, and functional impairment. Beyond musculoskeletal manifestations, RA may also involve extra-articular organs such as the heart, lungs, kidneys, and peripheral nerves, resulting in widespread systemic complications [1, 2]. Globally, RA affects approximately 0.5-1% of the population [3], with an estimated prevalence of 0.42% in China, impacting up to 5 million individuals. Notably, RA is three times more prevalent in middle-aged women compared to men [4], and leading cause of major disability in China, with a disability rate of 61.3% [5]. Current treatment guidelines from the American College of Rheumatology (ACR) and the European League Against Rheumatic Diseases (EULAR) recommend the use of conventional synthetic disease-modifying antirheumatic drugs (csDMARDs), with methotrexate (MTX) recommended as the first-line therapeutic agent [6]. Concisely, methotrexate, a folate analog and inhibitor of dihydrofolate reductase, exerts its anti-inflammatory effects by interfering with purine and pyrimidine synthesis, thereby inhibiting the proliferation of immunologically active cells and halting disease progression [7]. However, pertinent research has revealed that adverse reactions have resulted in certain limitations in the therapeutic application of this medication because of significant individual variance [8]. Iguratimod is a synthetic anti-rheumatic drug with anti-inflammatory and immunomodulatory effects. It has been used for the treatment of active RA, particularly in patients with an inadequate response to methotrexate-based therapy. Previous studies have suggested that iguratimod may relieve morning stiffness, joint swelling, and pain [9]. This study was conducted to explore the effect and safety of a combination regimen to provide data support for the choice of treatment regimen for rheumatoid arthritis, particularly active rheumatoid arthritis.

Methods

General information

Patients with rheumatoid arthritis treated in the Department of Rheumatology and Immunology, affiliated with Chaohu Hospital of Anhui Medical University, between October 2021 and October 2023 were enrolled in this study. All participants met the relevant criteria for the study.

Inclusion criteria: (1) participants who fulfilled the diagnostic criteria for active rheumatoid arthritis [10]; (2) Age greater than 18 years; (3) Presence of soft-tissue swelling in at least three joints simultaneously; (4) complete clinical data available for analysis.

Exclusion Criteria included: (1) Presence of other autoimmune diseases, immunodeficiency, or hematologic disorders; (2) Severe hepatic dysfunction, malignancy, or serious infections; (3) Active peptic ulcer or bleeding or low body weight; (4) known allergy to any components of the selected drugs or patients with contraindications; (5) Taking other drugs that can affect the relevant indicators of the study, such as bone metabolism indices; (6) Women who are pregnant, breastfeeding, or have fertility intentions during the treatment period.

This was a single-center, retrospective, non-randomized controlled study. The investigators were not involved in treatment allocation. Whether patients received methotrexate monotherapy or methotrexate combined with iguratimod was mainly determined according to actual clinical records, including factors such as disease condition, disease activity, previous treatment response, drug tolerability, contraindications, and the physician’s clinical judgment. Based on the treatment regimens actually recorded in the medical records, patients were divided into a conventional treatment group (n = 40) and a combination treatment group (n = 40).

To preliminarily assess the comparability of the two groups, baseline variables extracted from the medical records, including age, sex, disease duration, baseline DAS28-ESR score, and baseline RF level, were compared. The results showed no statistically significant differences in the measured baseline variables between the two groups (p > 0.05; Table 1). However, because this study did not use random allocation and did not perform propensity score matching, the potential influence of unmeasured confounding factors and treatment selection bias on the study findings cannot be completely excluded.

Table 1.

Comparison of baseline data between the two groups of patients

Group	Number of cases	Average age(years)	Male/Female ratio	Average course of disease (years)
Conventional treatment group	40	50.85 ± 10.03	6/34	2.55 ± 3.15
Combination therapy group	40	54.33 ± 14.98	12/28	3.32 ± 3.54
t		1.219		1.028
p		0.226	0.108	0.307

Group	Number of cases	Baseline DAS28 score (score)	Baseline RF(IU/ml)
Conventional treatment group	40	5.85 ± 1.63	150.35 ± 24.62
Combination therapy group	40	5.59 ± 1.86	154.12 ± 25.93
t		0.643	0.667
p		0.522	0.507

Baseline anti-CCP titers and prior biologic use were not uniformly recorded in the retrospective medical records and therefore were not included in the baseline comparison

Information on anti-CCP titers, prior biologic DMARD treatment failure, and baseline radiographic erosions was not uniformly recorded in the retrospective medical records and therefore could not be reliably included in the baseline comparison or subgroup analysis.

All procedures performed in studies involving human participants were in accordance with the standards upheld by the Ethics Committee of Affiliated Chaohu Hospital of Anhui Medical University (Approval No: KYXM-202201-012) and with those of the 1964 Helsinki Declaration and its later amendments for ethical research involving human subjects.

Sample size statement

This was a single-center retrospective study. The sample size was determined by the number of eligible patients with complete clinical data during the study period. A total of 80 patients with active rheumatoid arthritis treated at our hospital between October 2021 and October 2023 were included, with 40 patients in the conventional treatment group and 40 patients in the combination therapy group. Because this study was based on existing medical records rather than a prospective trial, no a priori sample size calculation or dropout-rate assumption was applied. All included patients had complete data for the variables analysed. Therefore, the findings should be interpreted as exploratory and require confirmation in larger prospective studies.

Research methods

After enrollment, all patients were given routine physical examination, joint assessments, and laboratory tests, including blood routine, liver and kidney function, and other serological indices tests. During the treatment period, patients were advised to maintain adequate sleep, follow a nutritious and balanced diet, avoid cold exposure to joints, refrain from heavy lifting or vigorous activity, and engage in appropriate functional exercises. Patients in the conventional treatment group received methotrexate tablets (Xinyi, Shanghai Xinyi Pharmaceutical Co., Ltd.; Batch No. 2021031005, 2.5 mg × 16 tablets) orally. The initial dose was 10–15 mg once per week, with a gradual increase of 5 mg every 2–4 weeks, not to exceed 20–30 mg per week.

In the combination treatment group, patients received Iguratimod tablets (Adexin, Jiangsu Xiansheng Pharmaceutical Co., Ltd.; Batch No. 2021061104, 2.5 mg × 14 tablets), administered orally twice daily—one tablet each in the morning and evening—in addition to the methotrexate regimen. Both groups were treated continuously for 6 months.

Observation indicators

Assessment of therapeutic effect

After 6 months of treatment, therapeutic efficacy in the two groups was evaluated using the American College of Rheumatology (ACR) response criteria [10], including ACR20, ACR50, and ACR70.

ACR20 was defined as an improvement of ≥ 20% from baseline in both the tender joint count (TJC) and swollen joint count (SJC), together with an improvement of ≥ 20% in at least three of the following five core measures: (1) patient global assessment; (2) physician global assessment; (3) patient pain score, usually assessed using a visual analogue scale (VAS); (4) physical function assessment, such as the Health Assessment Questionnaire (HAQ) or another functional scale; and (5) an acute-phase reactant, such as erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP).

ACR50 was defined as an improvement of ≥ 50% from baseline in both the tender joint count and swollen joint count, together with an improvement of ≥ 50% in at least three of the five core measures described above.

ACR70 was defined as an improvement of ≥ 70% from baseline in both the tender joint count and swollen joint count, together with an improvement of ≥ 70% in at least three of the five core measures described above.

Bone metabolism index test

Osteocalcin N-terminal intermediate molecule (N-MID), total type I collagen amino-terminal extended peptide (T-PINP), and β-collagen degradation product (β-CTX) were measured using electroluminescence immunoassay.

Detection of other serum indices

Before and after treatment, 5 ml of fasting venous blood was collected from patients, and the plasma was separated by centrifugation (3,500 r/min, 5 min, r = 8 cm) and stored at -20℃. The levels of C-reactive protein (CRP) and rheumatoid factors (RF) were measured by immunoturbidimetric assays, while the erythrocyte sedimentation rate (ESR) was determined using the Westergren method.

Immune function testing

Before and after treatment, serum T lymphocyte subsets and immunoglobulin levels were measured to assess immune function. Concisely, the T lymphocyte subsets, including CD4 + T cells and CD8 + T cells, were assessed using flow cytometry with fluorescence labeling on a Beckman CytoFLEX flow cytometer. The immunoglobulin indicators, including IgG, IgA, and IgM, were measured using the rate scattering turbidity method, with test kits provided by Wuhan Jilide Biotechnology Co., Ltd. The testing process strictly followed the requirements of each specific kit, i.e., CD4 + T Cells: Normal range: Approximately 30% − 60% (of total T cells), CD8 + T Cells: Normal range: Approximately 20% − 40% (of total T cells), Immunoglobulin Normal Values: IgG: Normal range: 7 ~ 16 g/L. IgA: Normal range: 0.7 ~ 4 g/L. IgM: Normal range: 0.4 ~ 2.3 g/L.

DAS28-ESR evaluation criteria and adverse reactions

Both ESR and CRP were measured in this study; however, DAS28-ESR was selected for disease activity assessment because ESR was routinely available in all included patients and was consistently recorded before and after treatment in the retrospective medical records. CRP was analyzed separately as an inflammatory marker and was not incorporated into the DAS28 calculation. Therefore, all DAS28 values reported in this study refer to DAS28-ESR unless otherwise specified.

Disease activity was evaluated using the DAS28-ESR score (Disease Activity Score for 28 joints) criteria [11], which integrates clinical and laboratory data to assess the severity of rheumatoid arthritis.DAS28-ESR components include:

1.  Examination of 28 Joints: Tender Joint Count (TJC28): The number of joints that the patient complains of pain in among 28 joints. Swollen Joint Count (SJC28): The number of swollen or tender joints among 28 joints.

2.  Patient Self-Assessment: General Health Score (GH): Usually assessed using a visual analog scale (VAS) from 0 to 100, where the patient evaluates their health status based on their feelings, with 0 meaning “very poor” and 100 meaning “very good.”

3.  Laboratory Indicators: Acute phase reactants: (1) DAS28-ESR: Uses erythrocyte sedimentation rate (ESR) as an inflammatory marker. (2) DAS28-CRP: Uses C-reactive protein (CRP) as an inflammatory marker.

The formulas for calculating DAS28-ESR are as follows:

DAS28-ESR Formula:

Scoring Criteria: The DAS28-ESR score range is typically from 0 to 10, with higher scores indicating greater disease activity. Low activity: DAS28-ESR < 3.2. Moderate activity: 3.2 ≤ DAS28-ESR < 5.1. High activity: DAS28-ESR ≥ 5.1. Remission: DAS28-ESR < 2.6.

Finally, any adverse drug reactions during the 6-month treatment period were recorded, and severity was graded using the Common Terminology Criteria for Adverse Events (CTCAE); treatment discontinuations attributable to adverse effects were also documented.

Statistical analysis

Statistical analyses were performed using SPSS software, version 23.0 (IBM Corp., Armonk, NY, USA). Continuous variables were tested for normality using the Shapiro–Wilk test. Normally distributed continuous variables were expressed as mean ± standard deviation (SD), and between-group comparisons were performed using the independent-samples t-test. For paired before-and-after comparisons within the same group, the paired-samples t-test was used. When the normality assumption was not met, non-parametric tests were used, and data were presented as median and interquartile range, as appropriate. Homogeneity of variance was assessed using Levene’s test. Categorical variables were expressed as n (%), and comparisons between groups were performed using the chi-square test or Fisher’s exact test, as appropriate.

All tests were two-sided, and a p-value < 0.05 was considered statistically significant. Because this was a retrospective exploratory study with a modest sample size, no adjustment for multiple comparisons was performed, and the results should be interpreted cautiously.

Given the limited sample size and retrospective, non-randomized design, formal subgroup or interaction analyses were not performed. In addition, anti-CCP titers, prior biologic DMARD treatment failure, and baseline radiographic erosions were not uniformly recorded in the medical records; therefore, these variables could not be reliably included in subgroup analyses. Future prospective studies with standardized data collection and larger sample sizes are needed to evaluate these clinically relevant subgroups.

Results

Comparison of efficacy

The clinical response to treatment was evaluated using the ACR20, ACR50, and ACR70 criteria (Table 2). The combination treatment group (methotrexate + iguratimod) demonstrated significantly higher response rates at all three levels compared to the conventional treatment group (methotrexate alone). The between-group differences in response rates were 20.00% for ACR20 (95% CI= (4.21%, 35.62%), 22.50% for ACR50 (95% CI= (1.19%, 41.18%), and 22.50% for ACR70 (95% CI= (1.78%, 40.77%)).

Table 2.

Comparison of efficacy

Group	Number of cases	ACR20	ACR50	ACR70
Combination therapy group	40	38(95.00%)	29(72.50%)	18(45.00%)
Conventional treatment group	40	30(75.00%)	20(50.00%)	9(22.50%)
p		0.012	0.039	0.033

Comparison of bone metabolism-related indices

After 6 months of treatment, both groups showed significant increases in N-MID and T-PINP levels, alongside a notable decrease in β-CTX level, indicating improvements in bone formation and reductions in bone resorption. Notably, the combined treatment group showed significantly increased levels of N-MID and T-PINP compared to the conventional treatment group, however, β-CTX levels were significantly lower (p < 0.05, Tables 3 and 95% Confidence Interval (CI) = (2.7311, 5.5889), 95% CI = (5.6884, 11.6066), 95% CI = (− 0.3899, − 0.1301)).

Table 3.

Comparison of bone metabolic indicators

Group	N-MID(ng/ml)		T-PINP(ng/ml)		β-CTX(ng/ml)
	Before treatment	After treatment	Before treatment	After treatment	Before treatment	After treatment
Conventional treatment group	11.65 ± 3.13	17.28 ± 2.50	21.09 ± 3.08	33.08 ± 6.42	1.02 ± 0.48	0.74 ± 0.16
Combination therapy group	11.66 ± 2.06	21.44 ± 3.77	20.52 ± 2.94	41.73 ± 6.87	1.08 ± 0.30	0.48 ± 0.38
t	0.016	5.830	0.843	5.819	0.600	3.991
p	0.987	< 0.001	0.402	< 0.001	0.551	< 0.001

(Abbreviations N-MID, N-terminal intermediate molecule, T-PINP total type I collagen amino-terminal extended peptide, β-CTX β-collagen degradation product.)

Comparison of CRP, RF, and ESR levels between the two groups

After 6 months of treatment, the CRP, RF and ESR levels in the combined treatment group were significantly lower than those in the conventional treatment group after treatment, with significant differences (p < 0.05, Tables 4 and 95% CI = (-11.1222, -8.2578), 95% CI = (-13.3125, -4.7875), 95% CI = (-14.1428, -9.7922)).

Table 4.

Comparison of CRP, RF and ESR levels between the two groups

Group	CRP(mg/L)		RF(IU/ml)		ESR(mm/h)
	Before treatment	After treatment	Before treatment	After treatment	Before treatment	After treatment
Conventional treatment group	58.82 ± 8.07	23.17 ± 3.23	150.35 ± 24.62	131.99 ± 9.29	70.21 ± 10.93	29.74 ± 5.62
Combination therapy group	60.99 ± 8.39	13.48 ± 3.20	154.12 ± 25.93	122.94 ± 9.85	66.04 ± 10.66	17.78 ± 4.02
t	1.176	13.447	0.667	4.229	1.727	10.943
p	0.243	< 0.001	0.507	< 0.001	0.088	< 0.001

(Abbreviations CRP C-reactive protein, RF rheumatoid factor, ESR erythrocyte sedimentation rate.)

Comparison of immune function between the two groups

Following treatment, both groups exhibited reductions in immune-related indicators compared to their baseline values. While there were no significant differences between the groups in CD4⁺ and CD8⁺ T cell levels after treatment (95% CI = − 4.0090 to 0.2690; − 2.3109 to 0.1709, respectively), the levels of immunoglobulins IgG, IgA, and IgM were significantly lower in the combination treatment group compared to the conventional treatment group. Specifically, the 95% confidence intervals for between-group differences were − 1.8327 to − 1.0023 for IgG, − 0.3915 to − 0.1835 for IgA, and − 0.3832 to − 0.2118 for IgM, with all differences reaching statistical significance (p < 0.05). These results suggest that combination therapy exerts a stronger immunosuppressive effect on humoral immune function than methotrexate alone, while maintaining similar effects on T lymphocyte subsets (Table 5).

Table 5.

Comparison of immune function between the two groups

Group	CD4+（%）		CD8+（%）
	Before treatment	After treatment	Before treatment	After treatment
Conventional treatment group	40.86±5.95	39.23±3.88	40.07±4.71	29.09±3.38
Combination therapy group	41.04±6.60	37.36±5.58	38.07±5.03	28.02±1.99
t	0.130	1.742	1.837	1.721
p	0.897	0.085	0.070	0.089

Group	IgG(g/L)		IgA(g/L)		IgM(g/L)
	Before treatment	After treatment	Before treatment	After treatment	Before treatment	After treatment
Conventional treatment group	13.56±1.45	8.25±0.87	3.49±0.29	1.50±0.28	1.91±0.37	1.64±0.23
Combination therapy group	13.63±1.10	6.84±0.99	3.44±0.44	1.22±0.18	1.88±0.30	1.34±0.15
t	0.238	6.777	0.618	5.231	0.437	6.911
p	0.812	<0.001	0.538	<0.001	0.663	<0.001

(Abbreviations CD4+, cluster of differentiation 4-positive T cells, CD8+ cluster of differentiation 8-positive T cells, IgG immunoglobulin G, IgA immunoglobulin A, IgM immunoglobulin M.)

Comparison of DAS28-ESR scores between the two groups of patients

Before treatment, there was no statistically significant difference in DAS28-ESR between the two groups. The mean DAS28-ESR score was 5.85 ± 1.63 in the conventional treatment group and 5.59 ± 1.86 in the combination therapy group (t = 0.643, p = 0.522), indicating comparable baseline disease activity. However, after six months of treatment, the DAS28-ESR score in the combination therapy group decreased significantly to 2.78 ± 0.69, whereas the conventional treatment group had a post-treatment score of 4.54 ± 1.31. The between-group difference was statistically significant (t = 7.544, p < 0.001, 95% CI = (–2.200, − 1.300)), demonstrating that combination therapy more effectively reduced disease activity.

Comparison of adverse reactions

No serious adverse reactions occurred in either group during the 6-month treatment period. All reported events were mild to moderate (CTCAE grade 1–2), and no treatment discontinuations due to adverse effects were documented. The overall incidence of adverse reactions was 22.50% in the conventional treatment group and 15.00% in the combination therapy group, with no statistically significant between-group difference (p = 0.390; Table 6).

Table 6.

Adverse reactions, severity, and discontinuation during treatment [n(n%)]

Group	elevated transaminases	Diarrhea	N/V	Dizziness/HA	Grade ≥ 3/SAE	CTCAE max	Discont.
Conventional	2 (5.00%)	3 (7.50%)	2 (5.00%)	2 (5.00%)	0 (0.00%)	1–2	0 (0.00%)
Combination	1 (2.50%)	2 (5.00%)	2 (5.00%)	1 (2.50%)	0 (0.00%)	1–2	0 (0.00%)

Discussion

Rheumatoid arthritis is a chronic and recurrent autoimmune disease characterized primarily by polyarticular and symmetrical joint inflammation. Common joint symptoms include morning stiffness, swelling, tenderness, and limited mobility, while systemic symptoms may present with low-grade fever, fatigue, weight loss, and anemia [12]. In advanced stages, the disease can lead to varying degrees of disability. Therefore, the treatment goal for this condition is to control clinical symptoms while delaying disease progression to reduce the rate of disability [13]. Although RA remains incurable, a range of therapeutic options exists, including lifestyle modifications, surgical interventions, physical therapy, and pharmacological treatments. Due to the variability in treatment plans, costs can vary significantly. Currently, medication options include non-steroidal anti-inflammatory drugs (NSAIDs), conventional synthetic disease-modifying anti-rheumatic drugs (csDMARDs), and biological disease-modifying anti-rheumatic drugs (bDMARDs). While these agents are effective in reducing disease activity and controlling inflammation, they also come with certain limitations, such as the risk of infections, adverse effects, and high treatment costs, particularly for biologics [14]. The pathogenesis of RA is complex and remains unclear, although increasing evidence suggests it involves a combination of autoimmune dysregulation, genetic susceptibility, environmental triggers (e.g., smoking), and microbial infections [15, 16].

Methotrexate is the first-line treatment for rheumatoid arthritis worldwide. Initially developed as an anti-folate chemotherapy drug, it has since been used for treating rheumatoid arthritis. Methotrexate primarily exerts its effects by inhibiting the over proliferation of T lymphocytes, thereby reducing immune hyperactivity and correcting immune system dysfunction [17]. Although it is widely used as a first-line medication that can control disease progression, studies indicate that monotherapy has a slow onset of action and may have numerous side effects with long-term use [18]. Clinical scholars advocate for combining other medications with methotrexate to enhance the treatment effectiveness for rheumatoid arthritis.

Iguratimod is a novel synthetic small-molecule drug currently approved for clinical use only in Japan and China. Previous experimental studies have suggested that iguratimod may regulate inflammatory responses partly through the nuclear factor-kappa B (NF-κB) signaling pathway. Studies on macrophages and microglial cells have further demonstrated that Iguratimod suppresses the nuclear translocation of NF-κB p65, thereby reducing the production of pro-inflammatory mediators. These findings provide a possible biological basis for its anti-inflammatory activity, although this pathway was not directly examined in the present clinical study [19]. In certain autoimmune disease models, Iguratimod can reduce the expression of co-stimulatory molecules, including CD54, CD58, CD106, and human leukocyte antigen-DR. Iguratimod also significantly inhibits antigen-specific T cell proliferation mediated by synovial cells [20]. Additionally, Iguratimod inhibits the TNF-α-induced upregulation of IL-6, IL-8, and monocyte chemoattractant protein-1 in rheumatoid arthritis synovial cells in a concentration-dependent manner [21]. However, these mechanistic findings are mainly derived from experimental or previous clinical studies and should not be interpreted as mechanisms directly verified by the present retrospective analysis.

The results of this study indicate that both treatment regimens were associated with improvements in clinical symptoms and disease activity in adult patients with RA. Compared with methotrexate monotherapy, methotrexate combined with iguratimod was associated with higher ACR20, ACR50, and ACR70 response rates in the overall study cohort. This potential benefit may be partly explained by iguratimod’s dual action in suppressing inflammatory responses and modulating immune function. This benefit is biologically plausible. Methotrexate affects folate-dependent purine synthesis and inflammatory signaling. Iguratimod may inhibit NF-κB-mediated inflammatory responses and suppress B-cell immunoglobulin production. These different mechanisms may partly explain the greater improvement observed with combination therapy. However, the present study did not directly examine the NF-κB pathway or related downstream cytokines, and therefore the proposed mechanism should be interpreted as a biologically plausible explanation rather than a mechanism directly demonstrated by this study. Because this study was based on retrospective clinical records, no pathway-level assays, cellular experiments, or serial cytokine profiling were performed. Therefore, the molecular mechanism underlying the observed clinical and laboratory improvements remains to be further clarified. In addition, all three bone metabolism markers improved after treatment in both groups, with greater changes observed in the combination group, including higher N-MID and T-PINP levels and lower β-CTX levels. These findings are consistent with previous reports indicating that rheumatoid arthritis is associated with abnormal bone metabolism and that iguratimod may improve bone metabolism and promote bone formation [22]. Nevertheless, only circulating bone metabolism markers were assessed in this retrospective study. Imaging-based outcomes, such as baseline erosions, progression of bone erosions, or radiographic structural damage, were not systematically evaluated. Therefore, the observed changes in bone metabolism indicators should not be interpreted as direct evidence of improved structural joint outcomes. Moreover, previous meta-analyses have shown that iguratimod-based combination therapy can reduce DAS28 scores and improve inflammatory markers such as CRP, RF, and ESR [23, 24]. Taken together, the present findings suggest that combination therapy may be associated with better short-term clinical response, inflammatory activity, and bone metabolism outcomes than methotrexate alone in the overall adult RA cohort studied.

Responder heterogeneity is expected in RA and the present study was not designed or powered to determine whether the observed associations were consistent across clinically important subgroups. Potential responder subgroups that warrant prespecified evaluation in future studies include anti-CCP-positive patients, patients with high RF titers, individuals with baseline erosive disease, and patients with prior inadequate responses to biologic or targeted synthetic DMARDs. However, because formal subgroup and interaction analyses were not performed in this retrospective cohort, the overall findings should not be interpreted as applying equally to all patient subgroups. Future studies should test treatment-by-subgroup interactions and report subgroup-specific effect estimates to better inform individualized treatment decisions.

In addition, the combination group showed lower post-treatment CRP, RF, and ESR levels than the conventional treatment group, further supporting a stronger anti-inflammatory effect of the combined regimen.

Mechanistic interpretation would be further strengthened by incorporating biomarkers more directly linked to the proposed immunomodulatory pathways, such as serum IL-6, TNF-α, and longitudinal anti-CCP/RF titers, and by examining whether early biomarker changes are associated with subsequent structural outcomes. These analyses were not feasible in the present retrospective dataset and should be addressed in future prospective studies.

A comparison of immunoglobulin and T lymphocyte levels before and after treatment revealed that patients in the combined treatment group exhibited varying degrees of reduction in these immune markers compared to the conventional treatment group, indicating that combination therapy may be associated with additional changes in immune function. Previous studies have suggested that iguratimod may regulate inflammatory cytokine secretion and immune-cell function through pathways such as NF-kB signaling [9]. The observed decrease in immunoglobulin levels (IgG, IgA, and IgM) in this study suggests that Iguratimod may further regulate immune responses by inhibiting B cell activity, which is consistent with previous research results [25]. However, because cytokines such as IL-6 and TNF-α were not measured in this study, the immunomodulatory mechanism could not be directly verified. Further studies measuring cytokine profiles, B-cell-related markers, and NF-kB pathway activity are needed to determine whether these immune changes are directly mediated by iguratimod-related mechanisms.

Because the present cohort included only adult RA patients, the current findings should not be extrapolated to patients with juvenile idiopathic arthritis (JIA). Dedicated pediatric studies are required to evaluate the efficacy and safety of iguratimod-based regimens in JIA.

This study has several limitations that should be acknowledged. First, it was a single-center, retrospective, non-randomized controlled study. Patients were not randomly assigned to different treatment groups but were grouped according to the treatment regimens they had received in routine clinical practice. Treatment selection may have been influenced by factors such as disease activity, previous treatment response, drug tolerability, concomitant medications, economic considerations, and the physician’s clinical judgment; therefore, the possibility of selection bias and confounding bias cannot be excluded. Second, although no statistically significant differences were observed between the two groups in measured baseline variables such as age, sex, and disease duration, the relatively small sample size and incomplete documentation of some potential confounders in the medical records precluded propensity score matching or adequate adjustment for confounding factors. Accordingly, the findings of this study can only suggest an association between methotrexate combined with iguratimod and better short-term clinical efficacy, improved inflammatory markers, and improved bone metabolism indicators, and cannot be used to infer a direct causal relationship. Future multicenter, prospective, prospective randomized controlled trials or larger observational studies with propensity score matching or multivariable adjustment, are needed to further validate the efficacy and safety of the combination regimen.

In addition, no a priori sample size calculation was performed in this study. The sample size was mainly determined by the number of eligible patients with complete data during the study period. Because the sample size was relatively limited, some outcome measures, particularly adverse reactions and comparisons across different ACR response levels, may have been underpowered. Therefore, the findings of this study should be regarded as exploratory.

Anti-CCP titers, prior biologic DMARD treatment failure, and baseline radiographic erosions were not uniformly available in the retrospective medical records. Therefore, these variables could not be reliably included in the baseline comparison, subgroup analysis, or interaction analysis. Residual confounding related to serological status, previous treatment exposure, and baseline structural damage cannot be excluded.

Finally, the follow-up duration was limited to 6 months, and long-term outcomes such as relapse, radiographic progression, and structural joint damage were not assessed. Mechanistic biomarkers, including serum IL-6 and TNF-α, were not routinely measured. In addition, the NF-κB pathway was not directly examined, and imaging-based assessment of bone erosion or radiographic progression was not performed. Thus, the relationship between iguratimod, NF-κB-related inflammatory pathways, bone metabolism markers, and structural joint protection remains inferential. Future prospective randomized controlled trials or larger multicenter observational studies with standardized mechanistic assessments, longer follow-up, and appropriate adjustment methods are needed to clarify the biological mechanisms and further validate the efficacy and safety of the combination regimen.

Conclusion

In this single-center retrospective study, compared with methotrexate monotherapy, methotrexate combined with iguratimod was associated with higher ACR20, ACR50, and ACR70 response rates, as well as greater improvements in inflammatory markers, bone metabolism indicators, and DAS28 scores, without a significant increase in the incidence of adverse reactions. However, owing to the non-randomized design and the absence of propensity score matching, these findings should be further confirmed in prospective randomized controlled studies. These findings should be interpreted cautiously because of the retrospective design, limited sample size, absence of mechanistic cytokine testing, and lack of imaging-based assessment of structural joint outcomes. Further mechanistic studies are required to determine whether these effects are mediated through NF-κB-related inflammatory pathways, immune-cell regulation, or changes in bone remodeling.

Authors’ contributions

Xueqin Chen—designed the study and carried them out; Xueqin Chen, Zongwen Shuai, Jiangli Xia, Xixi Ma, Linwei Hu—supervised the data collection, Xueqin Chen, Zongwen Shuai, Jiangli Xia, Xixi Ma, Linwei Hu—analyzed the data, Xueqin Chen, Zongwen Shuai, Jiangli Xia, Xixi Ma, Linwei Hu—interpreted the data, Xueqin Chen, Zongwen Shuai, Jiangli Xia, Xixi Ma, Linwei Hu—prepared the manuscript for publication and reviewed the draft of the manuscript. All authors have read and approved the manuscript.

Funding

No funding was received for this study.

Data availability

The authors declare that all data supporting the findings of this study are available within the paper and any raw data can be obtained from the corresponding author upon request.

Declarations

Ethics approval and consent to participate

Ethical approval was obtained from the Ethics Committee of Affiliated Chaohu Hospital of Anhui Medical University (Approval no. KYXM-202201-012).

The requirement for written informed consent was waived by the Ethics Committee because this retrospective study used de-identified clinical data and posed minimal risk to participants.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.