Background: The ability of serum infliximab level to predict clinical outcome in infliximab therapy in Crohn's disease is unclear. Here, we aimed to clarify the correlation between the timing of loss of response (LOR) to treatment and a decrease in serum infliximab level, and, in addition, to identify an indicator of infliximab level.
Methods: The study used data from a previous clinical study of infliximab for Crohn's disease, in which infliximab was initially given at 0, 2, 6 weeks at 5 mg/kg, and then at 8-week intervals to 62 week-10 responders. Of these 62, here we analysed data from 57 in whom Crohn's disease activity index and serum infliximab level were evaluated at week 14.
Results: Twelve patients showed a clinical response despite an infliximab level <1 μg/mL at week 14; of these, 8 (67 %) experienced LOR by week 54. A decrease in infliximab level preceded LOR in 6 (75 %). In receiver operating characteristic curve analysis, C-reactive protein (CRP) showed better performance in detecting an infliximab level <1 μg/mL. Infliximab level was <1 μg/mL in 60-80 % of patients with CRP >0.5 mg/dL. Time to LOR (median: 22.0 weeks) was significantly longer than that to a decrease in infliximab level to <1 μg/mL (14.0 weeks, p < 0.05) or to an increase in CRP to >0.5 mg/dL (14.0 weeks, p < 0.01).
Conclusions: A decrease in serum infliximab level preceded LOR, and was easily detected by an increase in CRP. The CRP may be an indicator of serum infliximab level in predicting LOR.