Does Immunogenicity Have an Influence on the Efficacy of Anti-tumor Necrosis Factor (Anti-TNF) Therapy in Patients With Ankylosing Spondylitis (AS): An Inception Cohort Study

Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that predominantly affects sacroiliac joints and spine. It is the prototype of spondyloarthritides (SpA) and one of the most common rheumatic diseases. Sacroiliitis is the earliest manifestation of disease and accompanying with spinal involvement cause inflammatory back pain (IBP). IBP usually starts insidiously in the early adulthood and typically it is felt deep in the buttock and/or lower lumbar regions. It improves with activity and returns with the rest and is usually accompanied by morning stiffness lasting at least 30 minutes. Involvement of the spine in patients with AS is usually not limited to the sacroiliac joints and lumbar region and usually it extends up to the thoracic and cervical segment. Currently the only sign that is diagnostic for AS is radiographic sacroiliitis. However radiography detects structural changes and take up to ten years to appear unequivocally.

In some patients bone tenderness due to enthesitis may be the primary complaint. Arthritis in the hips and shoulders occur in some patients and is associated with worse prognosis. Typical arthritis pattern in AS patients is asymmetric and usually involves the lower extremity joints. There are several extra-articular features of AS and the most common is acute anterior uveitis.

Until recently treatment options for AS were limited and based on non-steroidal anti-inflammatory drugs (NSAIDs), traditional disease modifying anti-rheumatic drugs (DMARDs) for the rheumatoid arthritis and physical therapy. The last decade witnessed a major advance in AS therapy with the use of anti-tumor necrosis factor (anti-TNF) agents. Anti-TNF agents have a substantial effect not only the axial disease but also in peripheral arthritis, enthesitis and extra-articular features (like psoriasis and inflammatory bowel disease). Currently there are four anti-TNF agents approved for AS: (1) infliximab which is a monoclonal chimeric antibody and given at a dose of 5 mg/kg every 6-8 weeks; (2) etanercept which is human TNF receptor fusion protein and administered subcutaneously at a dose of 50 mg/once a week; (3) adalimumab which is a humanized monoclonal antibody and administered as subcutaneous injection at a dose of 40 mg fortnightly; and (4) golimumab which is a fully human monoclonal antibody and administered subcutaneously at a dose of 50 mg once a month.

Since there is no head-to-head studies comparing the anti-TNF agents in the treatment of AS patients there is no ranking for the prescription of anti-TNF agents. Mixed treatment comparisons (that is statistical model allowing the simultaneous multiple meta-analysis of different pair-wise comparisons) between infliximab, adalimumab and etanercept did not show a statistically significant difference. Indeed similar improvements in Bath ankylosing spondylitis activity index (BASDAI 50) scores and ASAS partial remission (between 45.2% to 51.0% BASDAI 50 and 22.1% to 22.4% for partial remission) have been reported in randomized controlled clinical trials.

Although many AS patients respond very well to anti-TNF therapy, a considerable amount of them do not and additionally a significant proportion of patients have to stop their treatment. In clinical practice the reported 1-year and 2-year drug survival rates for anti-TNF agents are 70-85% and 60-75%, respectively.Moreover in a substantial proportion of patients either increase in the administered dosage or dosing frequency have become necessary. Therefore factors, which can predict the response or related with the primary or secondary non-response for anti-TNF treatment have a growing attention among treating specialists. In a study providing an overview of clinical trials and observational studies showed that increased acute phase reactants, higher disease activity, functional status, younger age, and HLA-B27 positivity were independent baseline predictors of response to anti-TNF treatment and increased acute phase reactants, presence of peripheral arthritis, and male sex were the predictors of long-term drug survival.