Letter: comparative safety and efficacy of infliximab vs. adalimumab in Crohn's disease – should one consider disease location? Authors’ reply

We appreciate the interest in our manuscript analysing outcomes of anti-tumour necrosis factor (TNF)-α naive patients with active Crohn's disease (CD) who were treated with infliximab (IFX) or adalimumab (ADA).1, 2 We agree with Dr Srinivas that it is interesting to observe the subgroup analyses from our study, and note that a significant difference in maintenance of clinical remission was reported in our study among patients with ileal CD who were treated with ADA compared to IFX. However, it is important to remember that this was a prospective cohort study, and not a randomised clinical trial, which suggests that there may be differences between the two cohorts that are potentially unmitigated by the lack of randomisation. For instance, patients with ileal disease receiving ADA may not have been similar to those receiving IFX in terms of disease duration or the presence of fibrostenotic disease. These potential differences between the two groups could influence their likelihood of responding to treatment. It is also worthwhile mentioning the utility of subgroup analyses. It is natural to examine for differences in the effect of therapy in clinical subgroups, particularly when there does not appear to be any difference between the two groups in overall outcomes. We must remember, however, that this is a post hoc analysis, which was not postulated before the study, for which the difference is not very large, and which has not been shown in other observational experiences comparing these two drugs. When several subgroup analyses are performed, probability suggests some will demonstrate significant benefit due to chance alone. There have been many examples of misleading findings from subgroup results in otherwise well-done randomised, controlled trials likely on account of chance alone,3 such as beta-blockers being found ineffective for inferior myocardial infarctions,4 only to be refuted by subsequent research. For this reason, subgroup analysis should be mainly used to generate hypotheses for future trials. It will be interesting to see if other investigators report similar experiences in using ADA and IFX for ileal CD, but there is little reason to think one anti-TNFα agent would have significantly better impact in the ileum than another. The authors’ declarations of personal and financial interests are unchanged from those in the original article.2

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