From Small Data to Big Decisions: How Clinical Pharmacology Shapes Rare Disease Development.

Rare‑disease drug development is constrained by small and heterogeneous patient populations, limited natural‑history data, and the impracticality of large, randomized trials. Despite increasing regulatory acceptance of totality-of-evidence and mechanism-based development pathways, generating reliable, decision-ready evidence under these constraints remains challenging. This review describes how clinical pharmacology contributes within an evidence‑integration and decision‑support framework through quantitative, model‑informed approaches to address this gap. By integrating nonclinical data, pharmacokinetics, pharmacodynamics, biomarkers, natural‑history information, and clinical efficacy and safety outcomes, and through close collaboration with clinical, statistical, and translational experts, clinical pharmacology supports interpretation of treatment effects and quantitative characterization of uncertainty when conventional evidence is limited. In practice, these approaches inform key development decisions, including dose selection, innovative trial designs, extrapolation and bridging across populations, use of external controls, and evaluation of biomarkers and surrogate endpoints. Importantly, such practices help align regulatory expectations with patient needs, particularly in pediatric and ultra‑rare settings, by enabling appropriate dosing, reduced trial and patient burden, and quantitative assessment of benefit/risk. Examples from rare‑disease programs illustrate how integrated quantitative evidence has supported regulatory decisions, including label expansion and accelerated approval when data may be sparse, heterogeneous, or evolving. Looking ahead, emerging technologies such as artificial intelligence, digital biomarkers, and individualized approaches are expected to further advance rare‑disease drug development. With this evolving landscape, clinical pharmacology is expected to continue playing an important role in evaluating mechanistic plausibility, ensuring analytic rigor, and translating small datasets into meaningful evidence to inform development and regulatory decisions in rare diseases.