Drug repurposing in oncology and rare diseases: a clinical pharmacology perspective from candidate identification to implementation.

Drug repurposing can shorten selected stages of development by using prior pharmacological and clinical knowledge, but successful translation requires more than biological plausibility. This narrative review critically examines repurposing in oncology and rare diseases from a clinical pharmacology perspective, integrating candidate identification, achievable exposure, clinical evidence and implementation. Representative successful and unsuccessful examples are discussed to illustrate the importance of disease-relevant mechanisms, patient selection, meaningful endpoints and development feasibility. Computational methods and real-world data can support hypothesis generation and prioritisation, but require pharmacological validation and prospective assessment of clinical utility. The review also considers European and Italian regulatory pathways and the economic barriers affecting development of established medicines. We argue that repurposing should be evaluated through the coherence of its evidence package and a feasible development pathway, rather than prior approval status or low acquisition cost alone.Clinical trial number: not applicable.