Drug Repurposing for Rare Diseases: Fast-Tracking Treatments for Underserved Patients

Why Repurposing Matters

Rare diseases collectively affect ~300 million people worldwide, yet >90% lack approved therapies. Drug repurposing—finding new uses for existing approved drugs—offers a faster, cheaper path to treatments by leveraging established safety, pharmacokinetic, and manufacturing data.

Success Stories That Changed Lives

• Metyrapone (adrenal steroidogenesis inhibitor) → repurposed for congenital adrenal hyperplasia.
• Sildenafil (originally for hypertension/angina) → repurposed for erectile dysfunction, then pulmonary arterial hypertension (Revatio).
• Tamoxifen (breast cancer SERM) → found effective in fibrodysplasia ossificans progressiva (FOP), a devastating condition causing heterotopic ossification.
• Sirolimus (transplant immunosuppressant) → repurposed for lymphangioleiomyomatosis (LAM), tuberous sclerosis complex.

Drivers of Modern Repurposing

• Big data & AI: Large-scale genetic/phenotypic datasets + machine learning uncover novel drug–target–disease connections systematically.
• Network pharmacology: Mapping polypharmacology to identify drugs hitting multiple relevant targets.
• Patient-led research: Advocacy groups funding screens, registries, and trials (e.g., Cystic Fibrosis Foundation model).

Persistent Challenges

• Incomplete understanding of rare disease mechanisms.
• Existing drugs may lack relevant preclinical/clinical data for the new indication.
• Regulatory pathways, while streamlined (e.g., FDA's Accelerated Approval, Orphan Drug Act), still require efficacy/safety evidence in the new context.
• Limited commercial incentive due to small patient populations — "valley of death" funding gap.

Collaborative Models Breaking Barriers

• Public-private partnerships: NCATS Drug Repurposing Program, Critical Path Institute.
• Academic networks: Rare Diseases Clinical Research Network (RDCRN), European Reference Networks.
• Patient advocacy orgs: Funding, trial recruitment, natural history data, regulatory engagement.
• Innovative trial designs: Basket trials, platform trials, N-of-1 trials adapted for ultra-rare diseases.

Outlook

Drug repurposing is a pragmatic, high-impact strategy to deliver therapies to underserved populations. Continued investment in data infrastructure, computational biology, regulatory innovation, and patient-centered trial designs will expand its reach and success rate. The next breakthrough for a rare disease may already be sitting in a pharmacy.