Reflection on clinical and methodological issues in rare disease clinical trials.

Rare diseases present critical challenges to healthcare systems, patients, and caregivers due to their low prevalence and unique characteristics. Designing clinical trials and developing statistical methodologies for evaluating interventions in rare diseases face several challenges. The "EBStatMax" project, part of the European Joint Programme on Rare Diseases' Demonstration Projects, aimed to address one of these challenges, namely: designing and analyzing longitudinal cross-over data in rare diseases, like Epidermolysis bullosa simplex (EBS). Although the main findings of the project have been published elsewhere, this manuscript reflects on additional hurdles encountered during the project, particularly regarding outcomes and methodological considerations. It explores issues surrounding outcome measurement, statistical methodology, and clinical considerations, emphasizing their broader relevance to methodological advancements in rare disease research beyond this specific case. This manuscript highlights the critical role of international collaboration in rare disease research to enhance evidence quality and aims to inspire further advancements in the field.