Introduction The approved dose for infliximab (IFX) in the treatment of ankylosing spondylitis (AS) is 5 mg/kg body weight every 6 weeks. Several studies have shown the 3 mg/kg dose to be effective in a subgroup of AS patients but there is few published evidence regarding other dose-reduction regimens, namely adjusting the interval between doses and individualised dose adjustment. We analysed AS patients disease activity upon increasing IFX administrations intervals on an individual basis.

Methods The Rheumatic Diseases Portuguese Register (Reuma.pt) was used to select all patients diagnosed with AS, under IFX therapy for ≥4 months, followed at Santa Maria Hospital. All patients received IFX 5mg/kg at 0,2 and 6 weeks and thereafter at variable intervals, between 6 and 11 weeks, on an individual basis, determined by clinical judgement. Clinical remission was defined as an ASDAS < 1.3 for ≥4 months and recurrence as an ASDAS ≥ 1.3 during 2 or more consecutive visits.

Results 50 patients were followed for a mean time of 57 ± 35 months. 72% were males and the mean age at baseline was 45 years. 11 patients (22%) were maintained on IFX every 6 weeks, 12 (24%) increased the interval between doses immediately after week 6 and 23 (54%) increased interval between doses later than week 6. The mean time between starting infliximab and the decision to increase dose intervals was of 18.2 ( ± 11.1) months. 10 patients (26% of those who increased dose intervals) had an ASDAS < 1.3 when the decision to increase dose intervals was taken. 21 patients (42%) further achieved remission, with the physician determined regime, 21.5 ± 28.1 months after starting IFX. Regarding these patients, the majority 16 (76%) showed persistent remission, while 5 (24%) had recurrence of activity, on average 12.9 months after remission. At the last visit 19 (90%) were in remission.

Conclusion This study confirms that increasing IFX administration intervals can be performed in clinical practice in a subgroup of patient without worsening of disease activity. A high percentage of patients achieved remission (42%) and maintained it through follow-up (32%) based in an individual adjustment regime, determined by the physician decision.