Positive pharmacokinetic effect of azathioprine co-medication on infliximab trough levels is dose-dependent.

BACKGROUND: Thiopurines seem to have positive effect on the pharmacokinetics of anti-tumor necrosis factor biologics. It has been suggested that a reduced dose of thiopurines is sufficient to achieve this synergism. AIMS: To assess the differences of infliximab (IFX) trough levels according to the dose of concomitantly used azathioprine (AZA). PATIENTS & METHODS: All IBD patients treated with IFX (Remicade®) in two IBD centres between November 2015 and April 2017 were eligible. Infliximab trough levels were assessed by ELISA (Ridascreen®, R-Biopharm). The differences in IFX trough levels according to AZA dose were analyzed statistically. RESULTS: In total, 125 patients were included, 42 pts (33.6%) on infliximab monotherapy, 83 pts (66.4%) using combined immune suppression. The respective median IFX levels according to AZA dose were as follows: group 1 (no concomitant AZA) 2.83 µg/ml (range 0-12); group 2 (AZA dose less than 1 mg/kg) 4.91 µg/ml (range 0.09-15.36); group 3 (AZA dose 1 < 2 mg/kg) 5.67 (range 0.16-16.97); group 4 (AZA dose above 2 mg/kg) 7.53 µg/ml (range 1.15-8.73). The differences in IFX trough levels between the respective groups according to AZA dose were statistically significant (p = 0.0159). CONCLUSION: The positive effect of azathioprine on infliximab levels seems to be dependent on the dose of concomitantly used azathioprine.