Drug Repurposing: Old Drugs, New Uses

What Is Drug Repurposing?

Drug repurposing — also called drug repositioning or drug re-tasking — is the process of finding new therapeutic uses for drugs that have already been developed, approved, or at least tested for safety in humans. Instead of starting drug development from scratch, repurposing leverages existing knowledge about a drug's safety profile, pharmacokinetics, and mechanism of action to accelerate its application to a new disease.

The concept is simple but powerful: a drug that has already passed safety testing for one indication can potentially reach patients for a new indication in years rather than decades, because the early-stage safety work is largely done.

Why Repurposing Makes Sense

Starting a new drug from scratch costs over $2 billion and takes 10–15 years on average, with a 90% failure rate in clinical trials. Most of that failure occurs because of safety problems — unexpected toxicity discovered only after years of investment.

For repurposed drugs, the safety landscape is already mapped. Phase 1 dose-escalation trials (or at least extensive human safety data) already exist. Regulatory agencies recognize this efficiency: repurposed drugs can often enter Phase 2 trials directly if safety data from the original indication is adequate. This compresses the development timeline and reduces cost substantially.

Famous Repurposing Successes

Sildenafil (Viagra): Originally developed as an antihypertensive and anti-anginal agent targeting phosphodiesterase type 5 in blood vessels. Clinical trials found it was only mildly effective for its intended use, but patients reported a striking side effect — erections. Pfizer repurposed it for erectile dysfunction and later pulmonary arterial hypertension (as Revatio).

Thalidomide: Originally used as a sedative and antiemetic in the 1950s (withdrawn after causing severe birth defects). Repurposed in 1998 for erythema nodosum leprosum (a leprosy complication) and later for multiple myeloma, where it targets the tumor microenvironment and immune system.

Minoxidil (Rogaine): Oral antihypertensive that caused unwanted hair growth as a side effect. Reformulated as a topical solution for androgenetic alopecia (male and female pattern baldness).

Metformin: Developed for type 2 diabetes; now in clinical trials for cancer prevention, aging, and polycystic ovary syndrome based on epidemiological observations.

Aspirin: Originally an analgesic; now one of the most widely used cardiovascular protective drugs based on antiplatelet properties discovered decades after initial approval.

Off-Label UseOff-Label Use

The use of a drug for a purpose, population, or dosage not included in its FDA-approved labeling. Off-label prescribing is legal and common — an estimated 20% of all prescriptions are off-label — but

vs. Formal Repurposing

Off-label use is when a physician prescribes an approved drug for a use not specified in its label. This is legal and common — an estimated 20% of all prescriptions in the US are off-label. Off-label prescribing can be the precursor to formal repurposing: if enough physicians use a drug off-label for a condition and outcomes look promising, pharmaceutical companies or academic researchers may pursue formal clinical trials and a new regulatory approval for that indication.

Formal repurposing — obtaining regulatory approval for a new indication — requires clinical trials demonstrating safety and efficacyEfficacy

The maximum therapeutic effect a drug can produce, regardless of the dose given. A drug with higher efficacy can achieve a greater maximum response than one with lower efficacy, even if the latter is

for the new use. The evidence bar is the same as for a new drug; only the starting point (safety data) is different.

How Repurposing Candidates Are Found

Several pathways lead to repurposing hypotheses:

• Serendipitous clinical observation: Doctors notice unexpected effects (as with sildenafil or minoxidil).
• Target-based reasoning: A drug hits a target (receptor, enzyme, pathway) that is later found to be important in another disease.
• Phenotypic screening: Large libraries of approved drugs are tested against disease models in the laboratory.
• Epidemiology: Large healthcare datasets reveal that patients taking a drug for one condition have lower rates of another disease.
• Computational analysis: Network analysis of drug-target-disease interactions identifies potential connections.

Clinical Trials for Repurposed Drugs

Even with existing safety data, repurposed drugs must go through clinical trials for the new indication. Phase 2 and Phase 3 trials are still required to demonstrate efficacy. Sometimes the dosing for the new indication differs substantially from the original — a drug safe at low doses may cause new problems at higher doses needed for a different disease.

Funding is a challenge for repurposing of off-patent drugs. A generic drugGeneric Drug

A medication that contains the same active ingredient, dosage form, strength, and route of administration as a brand-name drug, and has been demonstrated to be bioequivalent. Generic drugs can be mark

cannot be re-patented, so pharmaceutical companies have limited commercial incentive to fund expensive trials. Academic medical centers, government agencies (NIH, BARDA), and non-profit foundations often fill this role.

AI Accelerates Repurposing

AI has become a powerful tool for identifying repurposing candidates by analyzing relationships between drugs, targets, diseases, and patient outcomes across massive datasets. Knowledge graphs — networks connecting thousands of drugs and diseases through shared biological mechanisms — allow algorithms to predict which existing drugs might work for conditions not yet tested.

During the COVID-19 pandemic, several existing drugs were rapidly evaluated as potential treatments using computational repurposing followed by urgent clinical trials — though most repurposed candidates ultimately did not show sufficient efficacy in randomized trials, underscoring that computational prediction still requires clinical validation.

Key Takeaways

• Drug repurposing finds new uses for existing medicines, leveraging completed safety data to accelerate development.
• Famous examples include sildenafil (Viagra), thalidomide, minoxidil, and metformin.
• Off-label prescribing often precedes formal repurposing and can generate real-world evidence for new indications.
• Repurposed drugs still require Phase 2/3 clinical trials for the new indication; only early safety work is simplified.
• AI and computational biology are accelerating the identification of repurposing candidates by analyzing large biomedical datasets.