Objective: To study serum infliximab (s-IFX) levels in pediatric patients with inflammatory bowel disease (IBD).
Subjects and methods: s-IFX trough levels were measured in a total of 133 blood samples of 37 pediatric IBD patients (Crohn's disease, 23): 48 during the induction phase (weeks 2 and 6) and 85 during maintenance treatment. Antibodies to infliximab (ATI) were determined in 93 samples (30 patients). s-IFX values were related to fecal calprotectin (FC) and serum markers of inflammation.
Results: During induction (5 mg/kg) and maintenance therapy, the median s-IFX levels were 17.6 μg/ml (range 0-48 μg/ml) and 3.55 μg/ml (range 0-40 μg/ml), respectively. The IFX levels were similar in ulcerative colitis and Crohn's disease (e.g. during maintenance median 3.2 vs. 2.8 μg/ml, p = 0.718), thus the data are pooled. During induction, the s-IFX level was associated with the total dose of IFX, that is, young children with lower body weight had lower levels (p < 0.001 at week 2 and p < 0.05 at week 6). Shorter administration interval resulted in higher trough levels (p < 0.005). All samples with undetectable s-IFX (6.8%) levels presented ATI. High inflammation (FC >1000 µg/g) during induction was associated with lower s-IFX levels (median 4.0 μg/ml, range 0.47-25 compared to median 20 μg/ml, range 0-48 when FC <1000 µg/g, p < 0.005). There was no significant association between the ESR or values of C-reactive protein and s-IFX levels during induction.
Conclusions: In pediatric IBD, lower body weight and higher level of intestinal inflammation are associated to s-IFX levels during induction but relation to therapeutic response is unclear.