The conventional narrative of drug discovery runs in one direction: a novel compound is identified, optimized, tested, and — if everything goes well — approved. The molecule is new. The target is new. The therapeutic indication is new. The development timeline is long, the attrition is high, and the cost is enormous.
Drug repurposing runs the same process in reverse. The molecule already exists. Its safety profile in humans is already known. Its manufacturing process is established. The question is not whether it is safe to give to people — that has been answered — but whether it does something useful in a disease for which it was not originally developed.
This is not a niche strategy. Historical examples include sildenafil citrate transitioning from a cardiovascular compound to an erectile dysfunction treatment, and thalidomide shifting from a sedative to a foundational immunomodulatory agent in multiple myeloma — drugs that found their most important clinical applications not in the indications for which they were designed, but in diseases discovered through observation, serendipity, and scientific curiosity. These are not outliers. They are the clearest illustrations of a principle that the pharmaceutical industry is now pursuing systematically: the biology of existing drugs is richer than their approved labels suggest.
Why Drug Repurposing Has Accelerated
Several converging forces have made drug repurposing more scientifically tractable and more commercially attractive than at any previous point in the industry's history.
The explosion of biological and clinical data available for analysis has transformed what is possible. By leveraging the established safety and efficacy profiles of existing drugs, repurposing can significantly reduce the time, cost, and risk associated with traditional drug development, while providing a valuable pathway for addressing unmet medical needs. But the practical ability to identify repurposing opportunities at scale has historically been limited by the difficulty of systematically mining the relevant data — preclinical pharmacology, clinical adverse event patterns, transcriptomic signatures, network pharmacology relationships — across thousands of approved compounds simultaneously.
Artificial intelligence has changed that. AI-based platforms can analyze gene expression data, protein interaction networks, electronic health records, adverse event databases, and published literature at a scale and speed that no human team can match — identifying pharmacological relationships between approved drugs and disease pathways that would be invisible to conventional analysis. The same generative AI capabilities driving the discovery of new compounds like Rentosertib are increasingly being applied to the repurposing of existing ones.
The COVID-19 pandemic provided the most dramatic demonstration of drug repurposing at accelerated scale. The urgent search for COVID-19 treatments generated an extraordinary volume of repurposing clinical trials — with more than 4,952 clinical trials registered on ClinicalTrials.gov by March 2021, evaluating existing drugs including remdesivir, dexamethasone, baricitinib, and tocilizumab. The outcomes were mixed, but the exercise validated the clinical research infrastructure for rapid repurposing evaluation and produced genuine therapeutic discoveries — dexamethasone's role in reducing COVID-19 mortality being the most consequential.
Most recently, nitisinone — a compound originally developed as a herbicide and later approved for hereditary tyrosinemia type 1 — received FDA approval in 2025 for alkaptonuria, becoming the first targeted therapy for this ultra-rare metabolic disease after 25 years of research. The molecule was not new. The clinical need it addressed was profound and previously unmet.
The Clinical Development Advantage — and Its Limits
The most compelling aspect of drug repurposing from a development perspective is what does not need to be done. Toxicology studies across multiple species. Safety pharmacology assessments. Manufacturing process development. First-in-human dose escalation studies to establish maximum tolerated dose and pharmacokinetic profile.
For an approved drug being evaluated in a new indication, much of this pre-clinical and Phase I work is already complete. The known safety profile means that Phase II proof-of-concept studies can sometimes begin with a level of confidence in the compound's tolerability that a novel molecule cannot offer. The known pharmacokinetics mean that dose selection for the new indication can build on an established human data foundation rather than extrapolating from animal models.
Access to drugs already approved enables off-label clinical studies without the need for new GMP production, lowering trial barriers. The manufacturing supply chain is established. Regulatory submissions can reference the existing safety dossier rather than building a new one from scratch.
These advantages are real and significant. But they come with constraints that define what repurposing clinical programs need to do differently from conventional new drug development.
The indication specificity challenge. An approved drug's safety profile was characterized in a specific patient population, at a specific dose, for a specific duration of use. The new indication may involve a different patient population with different comorbidities, different concomitant medications, and different baseline organ function. The safety data from the original indication cannot be assumed to fully characterize the risk in the new indication. Phase II and III programs for repurposed drugs must include safety evaluation appropriate to the new patient population — not simply reference the existing label.
The dose may be different. The dose that was optimal for the original indication may not be optimal — or even appropriate — for the new one. Sildenafil for pulmonary arterial hypertension is dosed very differently from sildenafil for erectile dysfunction. Thalidomide's immunomodulatory applications require dose regimens that would not have been derived from its original sedative use. The clinical program for a repurposed drug must establish the appropriate dose for the new indication — which may require dose-finding studies that parallel the Phase I/II work done for the original compound.
The mechanism may be different. One of the most scientifically interesting aspects of drug repurposing is that the mechanism of action in the new indication may not be the same as in the original one. Repurposed drugs such as metformin and minoxidil demonstrate the clinical potential of repositioning strategies guided by mechanistic insight, phenotypic screening, and real-world observations — where the observed clinical effect in a new context reveals biology that was not the original pharmacological target. Understanding the mechanism of action in the new indication is important both for clinical development strategy and for regulatory submission — regulators will want to understand why the drug works in the new context, not just whether it does.
Regulatory Pathways for Repurposed Drugs
The regulatory landscape for drug repurposing is more nuanced than the simplified narrative of "already approved, therefore easier to develop" suggests.
In the United States, a repurposed drug seeking a new indication requires a supplemental NDA if the original sponsor is pursuing the new indication, or a full NDA or 505(b)(2) application if a different company is developing the new indication. The 505(b)(2) pathway — which allows reliance on the FDA's existing findings of safety and effectiveness for a previously approved drug — is the most commonly used regulatory mechanism for repurposing by non-originators. It requires the sponsor to demonstrate that the referenced data is scientifically appropriate for the new application and to address any differences in population, dose, route, or formulation.
The FDA's stance on real-world evidence for repurposing has shifted dramatically in recent years. In December 2025, FDA eliminated a major barrier by stating that submissions need not include individual-level patient data from real-world data sources — and a May 2026 initiative opened stakeholder input on using case reports, observational studies, and registry data as components of the repurposing evidence package. This evolution makes the regulatory pathway for repurposing, particularly in rare diseases and underserved indications, meaningfully more accessible than it was even five years ago.
In India, CDSCO's framework for repurposing is evolving. For drugs already approved in India being evaluated for new indications, the regulatory pathway builds on the existing drug master file, with the clinical trial application for the new indication evaluated in the context of the established safety dossier. The January 2026 NDCT amendments, which streamlined several regulatory processes, are beneficial for repurposing programs that involve BA/BE studies or non-clinical testing — reducing the administrative overhead at the early development stages.
India as a Location for Drug Repurposing Clinical Programs
India's clinical research infrastructure offers several specific advantages for drug repurposing programs that are worth understanding explicitly.
Disease prevalence and population diversity. Many of the most interesting repurposing opportunities — metabolic disease, fibrotic conditions, inflammatory disorders, rare genetic diseases, infectious disease — are prevalent in India at high rates, providing the patient access needed for efficient clinical proof-of-concept evaluation. Drug repurposing successes in rare diseases, including nitisinone for alkaptonuria and sirolimus for rare vascular anomalies, illustrate the value of patient populations that are rare globally but may be more accessible in India's large and diverse population.
Cost and speed for proof-of-concept. The Phase II proof-of-concept study is often the most critical and most cost-sensitive stage of a repurposing program — the study that determines whether the investment in a full Phase III program is justified. Conducting these studies in India at 40 to 60 percent of Western costs, with faster site activation and recruitment timelines, substantially improves the economics of repurposing programs that may be pursued by academic groups, patient advocacy organizations, or small biotech companies rather than large pharmaceutical sponsors.
AI-supported target identification. AI algorithms are transforming drug repurposing by revealing new therapeutic targets and mechanisms — and the integration of omics data with computational modeling enhances target identification and validation in repurposing studies. Indian research institutions and bioinformatics groups are increasingly active in this space, creating opportunities for academic-industry collaborations that identify repurposing candidates and partner with CROs to translate them into clinical programs.
What Drug Repurposing Programs Need From a Clinical Research Partner
A CRO supporting a drug repurposing clinical program needs to bring a specific combination of capabilities that differs from conventional new drug development support.
Regulatory strategy for the new indication, building on but not simply referencing the existing approval, requires regulatory expertise in the repurposing pathway — 505(b)(2) in the US, the analogous mechanisms in the EU and India — and the scientific judgment to identify what the new clinical program must demonstrate and what it can appropriately reference from the existing dossier.
Clinical study design for proof-of-concept in the new indication requires understanding of the disease biology, the appropriate patient population, and the endpoints that are validated in the new therapeutic context — which may be quite different from the endpoints used in the original indication's development program.
Safety monitoring designed for the new population — not simply referenced from the existing label — requires pharmacovigilance expertise and the clinical judgment to identify where the safety assumptions from the original indication may not fully apply.
And the biomarker strategy — confirming that the repurposed drug is engaging its target in the new indication and producing the expected pharmacodynamic effect — is often more important in repurposing programs than in conventional development, because the mechanism of action in the new context may not be as well established as in the original indication.
Conclusion
Drug repurposing is not a shortcut. It is a scientifically rigorous and commercially intelligent strategy for accelerating the delivery of medicines to patients who need them — by building on the biological knowledge embedded in existing drugs rather than starting from zero. The savings in time, cost, and pre-clinical work are real. The clinical development work that remains — proving efficacy in the new indication, establishing the appropriate dose, characterizing safety in the new patient population, and navigating the regulatory pathway — is substantial and requires exactly the same scientific and operational rigor as conventional drug development.
The difference is that repurposing programs start from a place of greater biological knowledge. Used well, that head start can transform the economics and the timeline of clinical development in ways that benefit not just sponsors and developers, but the patients waiting for the therapies they need.
At Genelife Clinical Research, we support Phase II and III clinical programs for repurposed small molecules — from regulatory strategy and proof-of-concept study design through clinical execution, statistical analysis, and regulatory submission support — in India and for international markets.
