Outcome strategies for clinical trials in Neuropaediatric rare diseases.

Neuropaediatric Rare Diseases (NRDs) impose a profound and multidimensional burden on patients, families, and healthcare systems. Persistently low clinical trial success rates reflect an unmet methodological need as much as a therapeutic one. A fundamental bottleneck is the absence of fit-for-purpose clinical outcome assessments. In fact, instruments validated for non-rare or adult populations fail to capture clinically meaningful change in heterogeneous, small, and developmentally complex NRD populations. Building directly on a systematic catalogue of methodological challenges in NRD outcome research published by our group (Acosta et al., 2025), this paper presents a structured set of actionable outcome strategies proposed by a large multidisciplinary expert group convened under the auspices of the European College of Neuropsychopharmacology (ECNP) and the International Society for CNS Clinical Trials and Methodology (ISCTM). Using a structured, iterative expert-opinion approach, each identified challenge served as a prompt for developing one or more candidate strategies, each mapped one-to-one to its corresponding barrier. Strategies are presented across four thematic domains: (1) innovative methodologies to enhance ecological validity and reduce rater context effects; (2) novel or adapted outcomes and endpoints that preserve clinical meaningfulness under conditions of high heterogeneity and limited sample sizes; (3) the purposeful use of natural history resources; and (4) approaches to support comparability and synthesis across programmes. Additional considerations address caregiver expectancy bias and recruitment, stakeholder alignment, maturational confounding, and preclinical-clinical connectivity. Collectively, these strategies constitute a practical, challenge-mapped "living" toolbox for clinical scientists designing NRD trials. Each strategy is already in use or validated in analogous rare-disease contexts. Realising their potential at scale requires institutional programmes, pre-competitive co-validation platforms, systematic stakeholder co-design, and early engagement with regulatory agencies as scientific partners in endpoint development.