To Örebro University

Background: Studies have shown an association between use of azathioprine and increased risk of acute pancreatitis in adult inflammatory bowel disease. However, whether an association exists among paediatric patients is not known. We aimed to investigate whether use of azathioprine is associated with the risk of acute pancreatitis in children with inflammatory bowel disease.

Methods: We did a nationwide register-based cohort study in Sweden (2006-16) and Denmark (2000-16). All paediatric patients (<18 years of age) with inflammatory bowel disease during the study period were identified through hospital records. Episodes of incident azathioprine use and no use of any thiopurine were matched (1:1) using propensity scores, controlling for sociodemographic characteristics, comorbidities, previous treatment, indicators of disease severity, and health care use. Incident acute pancreatitis (physician-assigned diagnosis with ICD-10 code K85) occurring in the 90 days following treatment initiation were identified through outpatient and inpatient hospital records.

Findings: We identified 3574 azathioprine episodes and 18 700 no-use episodes, which resulted in 3374 pairs after propensity score matching; baseline characteristics in the matched cohort were well balanced. Among the matched azathioprine episodes, mean age was 14.3 years (SD 3.1), 1854 (54.9%) were male, 1923 (57.0%) had Crohn's disease, and 1451 (43.0%) had ulcerative colitis or unclassified inflammatory bowel disease. Within the first 90 days following initiation of azathioprine, 40 acute pancreatitis events occurred (incidence rate 49.1 events per 1000 person-years) compared with six events in the no-use group (8.4 events per 1000 person-years). Azathioprine use was associated with an increased risk of acute pancreatitis (incidence rate ratio 5.82 [95% CI 2.47-13.72]; absolute difference 1.0 [95% CI 0.3-2.6] events per 100 patients) during the 90-day risk period.

Interpretation: Use of azathioprine was associated with an increased risk of acute pancreatitis in children with inflammatory bowel disease during the first 90 days following treatment initiation, suggesting the need for regular and rigorous monitoring. The risk of acute pancreatitis needs to be considered when deciding on optimal treatment strategies.