Ankylosing spondylitis (AS) is a chronic inflammatory musculoskeletal disease that mainly affects the spine and sacroiliac joints, but may also affect peripheral joints and entheses. This disease generally appears in the third decade of life with a higher prevalence in men. The pathogenesis of AS remains unclear. Interactions between genetic factors, innate immunity, lifestyle, and environmental factors are thought to be responsible.1,2 In addition to these factors, oxidative stress (OS) has become an area of interest in AS pathogenesis and progression.

OS is a condition caused by an imbalance between increased production and accumulation of reactive oxygen species (ROS) and the ability of the antioxidant defense to detoxify these ROS. The presence of excessive and uncontrolled levels of ROS leads to cell and tissue damage (oxidative stress).3,4 Increased OS is implicated in the pathogenesis of some diseases such as cardiovascular and neurodegenerative diseases, but also in the pathogenesis of some inflammatory diseases such as rheumatoid arthritis and inflammatory bowel disease.5–7

In the literature, there have been few studies concerning oxidative metabolism in patients with AS and the relation between OS and the characteristics of this disease. Authors suggest that increased OS may be implicated in the pathogenesis of AS, as well as in the progression and exacerbation of this disease.7,8 Further studies on this subject are needed.

The primary aim of our study was to investigate serum oxidative and anti-oxidative status in patients with AS and to compare them with that of age- and sex-matched healthy control. Additionally, relationships between the OS markers and disease indices were assessed.

This study included 44 patients with AS (mean age: 44.9±14.12 years, sex-ratio M/F: 2.14) and 46 healthy controls (mean age: 44.62±10.74 years, sex-ratio M/F: 2.28). In patients with AS, the mean disease duration was 21.11 months [range: 2–54] and the mean age at symptom onset was 36.8±9.2 years. The AS was axial in 63.6% (n=28), axial and peripheral in 34.1% (n=15) and only peripheral in 2.3% (n=1). Hip joint involvement was observed in 20.4% of cases (n=9), and thoracic wall involvement in 25% (n=11). Extra-articular manifestations were reported in 50% of patients (n=22), including: anterior uveitis (22.7%, n=10), restrictive respiratory syndrome (18.2%, n=8), cardiac conduction disorders (9.1%, n=4), aortic insufficiency (4.5%, n=2), and IgA mesangial nephropathy (2.3%, n=1). HLA-B27 antigen was identified in 61.4% of patients (n=27).

The mean BASDAI score was 4.52±1.45. The disease was active (BASDAI ≥4) in 68.2% of patients (n=30) and inactive (BASDAI <4) in 31.8% (n=14). The mean BASFI score was 4.64±2.52.

The demographic, clinical and laboratory data of the subjects (patients and controls) are summarized in Table 1. No significant differences were found between the two groups regarding age, sex, BMI, blood pressure, smoking, alcohol, glucose, lipid levels, uric acid, total bilirubin and homocystein level. Inflammatory biomarkers (ESR and CRP) were significantly higher in patients than in healthy controls (p<0.001).

Regarding the oxidative profile in our patients, significantly higher concentrations of protein oxidative products (AOPP) and lipid peroxidation products (MDA) were noted in AS patients in comparison to the control group (57.62±11.72 vs. 42.64±9.18μmol/l, p<10−6; and 55.69±16.37 vs. 28.71±10.85μmol/mg, p<10−6, respectively) (Table 2).

On the other hand, catalase (CAT) enzyme and glutathione peroxidase (GPx) activities and reduced glutathione (GSH) level were significantly lower in AS patients than in those of healthy controls (3.47±1.82 vs. 4.75±3.17mM/min/mg; 1.94±0.85 vs. 2.75±0.91mM/min/mg; and 0.48±0.23 vs. 1.11±0.41nmol/mg, respectively; p<10−6) (Table 2).

In AS patients, AOPP levels were positively correlated with disease activity (BASDAI, p=0.001) and functional impairment (BASFI, p=0.006) (Fig. 1). No significant correlation was found between MDA concentrations and BASDAI and BASFI scores. Among antioxidant markers, GPx activity correlated negatively with BASDAI (p=0.001) and BASFI (p=0.01) (Fig. 2). No significant correlations were observed between CAT activity or GSH levels and BASDAI or BASFI scores.

Fig. 1.

Positive correlation of AOPP levels with BASDAI score and BASFI score.

Fig. 2.

Negative correlation of GPx activity with BASDAI score and BASFI score.

No statistically significant correlations were found between oxidant/antioxidant parameters (AOPP, MDA, CAT, GPx, and GSH) and inflammatory biomarkers (ESR, CRP).

In the current study, the plasma levels of oxidant markers (AOPP and MDA) in AS patients were significantly higher than those of the healthy control subjects. Additionally, the levels of anti-oxidant molecules (GPx and CAT activities, GSH) in AS patients were significantly lower than those of the healthy control subjects. This indicates an imbalance between oxidant and anti-oxidant molecules in the plasma of patients with AS.

Although many studies have examined oxidative metabolism in inflammatory diseases such as rheumatoid arthritis and inflammatory bowel diseases,5,6,17,18 there are few studies in the available literature concerning the oxidative status in patients with AS.

In our study, in agreement with the studies previously published, increased markers of oxidative stress, especially AOPP and MDA, were reported in patients with AS. Similarly, several studies have documented higher levels of AOPP,19,20 MDA,21–23 and total oxidant status (TOS)7,8,24 in AS patients compared with healthy controls.

Regarding the anti-oxidant status in AS, data from previous studies are controversial and inconsistent. Some studies, consistent with our findings, reported a decrease in antioxidant defenses in AS patients compared with healthy controls.8,19,25,26 Conversely, other studies documented increased activities of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and GPx.21,23,24 These increases were attributed to a compensatory antioxidative response to excessive oxidative stress.

The exact mechanisms involved in the increased OS observed in AS remain unknown. In general, OS is defined as a condition resulting from an imbalance between the production of reactive oxygen species (ROS) – primarily generated from oxygen within human cells, particularly in the mitochondria – and the availability of antioxidants in the body. As a consequence, the body is no longer able to effectively control the excessive and prolonged presence of toxic ROS, which are potentially harmful and responsible for cellular damage and impaired cellular function.27 Various endogenous and exogenous factors can stimulate and induce excessive ROS production, leading to OS. Exogenous sources include smoking, alcohol consumption, excessive sun exposure (ultraviolet radiation), intense physical exercise, stress, an unbalanced diet (rich in fats), pollution, and exposure to chemicals and toxic substances. Endogenous sources include obesity, hyperglycemia, dyslipidemia, hyperhomocysteinemia, and iron overload.3,27 In our study, most of these risk factors were assessed and compared between the two groups, and no significant differences were found between patients and controls regarding age, BMI, smoking and alcohol consumption, blood glucose levels, lipid profile, and homocysteine levels. No significant difference between the two groups was found regarding uric acid and total bilirubin levels, both of which have antioxidant properties. However, certain risk factors for OS were not assessed in our study, such as physical and sports activity, sun exposure, diet, geographical origin, and exposure to toxins and pollution.

In patients with AS, the increase of OS is attributed mainly to the excessive production of reactive oxygen species (ROS) by inflammatory cells as activated neutrophils during the inflammatory process.28 Moreover, the inflammatory cells infiltrating the joints in AS release pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α). TNF-α has in recent years been shown to activate many pathways involved in OS via nuclear translocation of NF-κB.29 A number of studies have reported a decreased in the oxidant markers in patients with a TNF-α targeting agent than in those with conventional drugs.8,30

These notions explain the significant correlation between the levels of oxidative markers levels and the disease activity score (BASDAI) observed in many previous studies7,20,21 and in our study i.e. SO is more pronounced in patients with active disease compared to patients in remission. In the current study, a significant positive correlation between AOPP levels and BASDAI score (p=0.001) and a significant negative correlation between GPx activity and BASDAI score (p=0.001) were found.

Finally, as in the current study, many researchers have hypothesized that OS may contribute to the exacerbation and the progression of the disease.7,25 Indeed, in our study, markers of OS (AOPP and GPx) were significantly correlated with the BASFI score i.e. with the importance of the functional disability caused by the disease, suggesting the association of the increased OS with a more severe and a more impairing disease in AS patients. Our study is not the only one to report such findings; other studies, such as those by Solmaz et al.7 and Nazıroğlu et al.,25 have also demonstrated a significant association between OS biomarkers and disease severity scores, particularly the functional disability score (BASFI) and the spinal mobility assessment score (BASMI).

Beyond its impact on quality of life and functional ability, increased OS in AS has been shown to potentially contribute to a higher cardiovascular risk and may itself represent an additional cardiovascular risk factor in these patients. Indeed, OS – mainly through the oxidation of low-density lipoproteins (LDL) and the deposition of oxidized LDL in the arterial wall – is responsible for endothelial dysfunction, which plays a major role in the pathogenesis of cardiovascular diseases and the formation of atherosclerotic plaques.31,32

The main limitation of our study and all other published studies is the small population size and the cross-sectional nature. Further studies with a greater number of patients are required to clarify the role of OS in the etiopathogenesis and the evolution of AS.

Based on these findings, it can be suggested that novel therapies targeting OS pathways and enhancing the body's antioxidant defense systems could be beneficial in the management of AS. Such approaches might improve the prognosis of this disease and contribute to better management of the increased cardiovascular risk. Future studies are needed to further document and validate these concepts. In this context, it is important for AS patients to maintain optimal antioxidant defenses. Moderate and regular physical activity, weight reduction when necessary, and a healthy diet low in fats and rich in fruits and vegetables – particularly those high in antioxidants such as nuts, whole grains and seeds, spinach, carrots, tomatoes, and citrus fruits – should always be encouraged in patients with AS.

Our results demonstrated the imbalance between oxidant and anti-oxidant molecules in the plasma of patients with AS, with significantly increased levels of oxidant markers and significantly decreased levels of anti-oxidant molecules than those in the healthy control subjects, suggesting that OS might play a crucial role in the onset and the development of the disease. These findings require confirmation through studies with larger populations and controlled trials. In this context, new therapies targeting OS pathways could be valuable in treating patients with AS.

ZG: Conception, writing, data collection; HF: Supervision; MM and AE: Analysis and interpretation; AF: Data collection; MK, SB and KC: Revision; FA: Supervision and validation.

This study was conducted in accordance with the ethical principles outlined in the Declaration of Helsinki. All participants were fully informed about the study's purpose, and procedures. Written informed consent was obtained from each participant. Participation was voluntary, and participants were assured that they could withdraw from the study at any time.

There are no financial disclosures or statements of support by any author.

None declared.