Run-in periods and treatment outcomes in asthma trials: A narrative review

Background: The run-in period is an important element of randomized controlled trials, and is often used in respiratory disease trials. The design of the run-in period can greatly impact results and data interpretation, and as such should be designed carefully. Methods: In this review, we describe the design of run-in periods across six phase 3A trials of triple therapy in asthma, and discuss how differences in run-in period design (specifically the duration, treatment, and reporting of run-in results) may have the potential to alter the interpretation of study outcomes. Results: We found that the duration of run-in periods ranged between 2 and 7 weeks, with some studies including a combination of screening, run-in and stabilization periods, and others including a run-in period only. Run-in treatment also varied, with some studies running in patients on their previous inhaled corticosteroid/long-acting β2-agonist (ICS/LABA) therapy, and others harmonizing treatment by switching to the same ICS/LABA combination used in the on-treatment phase, or a different ICS/LABA combination entirely. Most of the studies included did not report any changes to study outcomes seen prior to randomization. Conclusion: We discuss the potential implications associated with the various trial designs, and propose that run-in periods should be consciously designed to meet the goals of the specific study. We also propose that standardized reporting of run-in changes would further allow for differentiation between improvements due to improved adherence and true treatment benefits, and aid with comparing data from different clinical trials.