Background and Aim There is evidence that shows addition of an immunomodulator (azathioprine or mercaptopurine) to Infliximab (IFX) therapy reduces antidrug antibodies, however, published evidence remains quite limited in paediatric population. We conducted a review to observe whether there is any correlation between the drug level of azathioprine metabolites, that is 6-thioguanine nucleotides (6-TGN) and development of anti-IFX antibodies (Abs) in inflammatory bowel disease (IBD) patients.

Method This is a retrospective study of patients with IBD based on a single tertiary paediatric gastroenterology department that had their levels monitored from March 2016 until March 2020. We defined maximum drug efficacy based on consensus on ESPGHAN management of IBD in paediatric and our lab references (235–450 pmol/8 × 108). In order to maintain consistencies, we included patients on 8 weekly 5 mg/kg of Infliximab infusion regimen who had their azathioprine metabolites measured within 3 months from starting. Fishers test and Pearson correlation were used to test the correlation between the drug level of azathioprine metabolites and development of IFX Abs.

Results 36 (58%) out of 62 patients were included in this study based on the above criteria (median age 14.25). Mean level of 6-TGN was lower in anti-IFX Abs-positive patients compared to anti-IFX Abs-negative patients (316.2 vs 322.8) with 6.607±57.51 (CI -123.5–110.3, p=0.91). There is a positive correlation between positive Anti-IFX abs with lower level of Azathioprine metabolites with coefficient at 0.47 (p=0.05).

Conclusion Our data demonstrates there is positive correlation between lower levels of azathioprine metabolites and positive anti-IFX abs level, hence suggestive of the importance of adherence to treatment to ensure longevity usage infliximab in IBD patients.