Regulatory and Scientific Frontiers in Drug Repurposing: Accelerating Therapeutic Innovations for Unmet Medical Needs + Examples of Repurposed Drugs

The United States Food and Drug Administration initiated a major regulatory shift on May 11, 2026, by launching a formal program designed to accelerate the clinical repurposing of approved drugs to address chronic and rare diseases. Framed by FDA Commissioner Marty Makary, M.D., M.P.H., as a critical pathway to utilize existing scientific data for underserved patient populations, the initiative focuses heavily on identifying new therapeutic indications or novel target populations for already-approved compounds. By opening public docket FDA-2026-N-4492, which remains active for stakeholder contributions through June 11, 2026, the agency has established a direct channel for clinicians, researchers, and patient advocates to submit data-backed drug candidates. This public solicitation specifically targets clinical sectors with high unmet medical needs and negligible commercial incentives, including metabolic diseases, neurodegenerative conditions, substance use disorders, rare diseases, and specialized men's and women's health conditions.

This policy framework does not operate in isolation; rather, it synthesizes several decades of legislative and regulatory evolution. The program builds upon the statutory foundations of the Best Pharmaceuticals for Children Act and the Making Objective Drug Evidence Revisions for New (MODERN) Labeling Act of 2020, both of which provide mechanisms for updating outdated drug labels when supported by robust scientific literature. It also extends the paradigms of the FDA-led Project Renewal, which historically updated labeling for oncology therapeutics to capture clinical evidence regarding rare cancer subtypes. Furthermore, the program directly implements directives from the September 2025 "Make Our Children Healthy Again" strategy report, which mandated that the FDA and the National Institutes of Health (NIH) jointly establish clinical trial processes and evidence-sharing structures to treat chronic pathology with repurposed generic drugs.

The contemporary data landscape significantly enhances the feasibility of this regulatory initiative. Historically, drug repurposing was driven by serendipitous clinical observations or retrospective case studies. Today, researchers can leverage high-throughput computational tools, machine learning algorithms, and deep clinical data sets - including electronic health records, insurance claims, disease registries, and patient-reported outcomes—to identify therapeutic signals. By combining in silico modeling with real-world evidence, the FDA’s initiative seeks to build an objective pipeline that can validate drug candidates even when traditional pharmaceutical developers lack the patent exclusivity necessary to justify large-scale clinical investments.

The Pharmacoeconomic Foundations of Repositioning Pipelines

Developing a new chemical entity (NCE) through traditional de novo discovery channels is a high-risk, capital-intensive venture. The journey of a novel compound from initial laboratory synthesis to commercial availability constitutes a 10-to-17-year marathon, requiring average capital expenditures that routinely exceed $2.0 to $3.0 billion when accounting for the cost of clinical failures. The attrition rate is high, with less than 10% of candidates that enter Phase I trials successfully obtaining regulatory approval. Conversely, the repurposed drug pipeline offers a compressed, capital-efficient alternative. Because a repurposed candidate enters clinical development with an established human safety profile, developers can largely bypass or drastically shorten preclinical toxicology evaluations and Phase I human safety trials. This "De-Risking Premium" reduces the typical development timeline to a 3-to-12-year range and lowers average development expenditures to approximately $300 million—representing a 50% to 60% reduction in capital requirements and a threefold increase in clinical success probability to approximately 30%. 

Development Metric	De Novo Drug Discovery Pipeline	Repurposed Drug Development Pipeline
Average Timeline	10 to 17 years	3-12 years or 5 tp 7 years (Typically shorter)
Average Capital Cost	$2.0 to $3.0 Billion (Including failure rates)	~ $300 million (a 50% to 60% cost reduction)
Probability of Clinical Success	<10% (From Phase I clinical entry)	~30% (From Phase II clinical entry)
Primary Failure Modes	Unforeseen toxicities and lack of clinical efficacy 9	Primarily restricted to lack of clinical efficacy 9
U.S. Regulatory Pathway	Section 505(b)(1) New Drug Application (NDA)	Section 505(b)(2) NDA (Permits reliance on historical safety data)
Global Market Value	Primary driver of original pharmaceutical pipelines	Valued at $24.4 billion (2015), rising to over $35 billion by 2027

Repurposed therapeutics represent a significant portion of commercial pharmaceuticals, accounting for approximately 30% to 40% of all new drug approvals and generating 25% to 40% of the annual revenue across the global pharmaceutical sector. Structurally, repurposing efforts are classified into on-target and off-target strategies. An on-target profile occurs when a drug interacts with its originally established molecular target to generate a separate therapeutic outcome in a different organ system. This is exemplified by minoxidil, which acts as a potassium channel opener to achieve systemic vasodilation for hypertension, and was later repurposed as a topical treatment for androgenetic alopecia by enhancing microvascular blood flow to hair follicles. Off-target profiling occurs when a molecule exhibits therapeutic activity through unexpected binding interactions with completely different receptor pathways. This distinction shapes the regulatory strategy under the 505(b)(2) pathway, where developers can integrate literature reviews and historical clinical data with targeted bridging studies to secure rapid, low-cost approvals. 

Sildenafil: A Paradigm of Multidirectional Pharmacological Adaptation

The history of sildenafil citrate is a classic example of multidirectional drug repurposing, illustrating how a single chemical entity can be adapted to treat distinct pathological conditions across different organ systems. 

Initial Discovery and Angina Pectoris Research

The sildenafil program began in 1986 at Pfizer's European research facilities in Sandwich, United Kingdom, under a project focused on cyclic guanosine monophosphate (cGMP) and type 5 phosphodiesterase (PDE5). At the time, clinical management of angina pectoris relied on organic nitrates, which release nitric oxide (NO) to stimulate cGMP synthesis, thereby relaxing vascular smooth muscle. However, organic nitrates quickly trigger tachyphylaxis, rendering them ineffective during continuous dosing. 

To bypass this limitation, Pfizer scientists targeted the enzyme responsible for degrading cGMP within vascular smooth muscle and platelets, specifically PDE5. The team synthesized a pyrazolopyrimidine derivative designated UK-92,480, later known as sildenafil, which exhibited high selectivity for PDE5 over other PDE isoforms and had an half-maximal inhibitory concentration (IC₅₀) of 3.5 nM against platelet-derived PDE5. Sildenafil entered clinical trials in 1991, with Phase I safety studies administering single doses up to 200 mg to healthy volunteers. Unfortunately, early clinical data showed that sildenafil produced minimal coronary vasodilation, making it a weak candidate for treating coronary artery disease. 

Repurposing for Erectile Dysfunction

During these early Phase I studies, investigators noted an unusual, recurrent side effect: several male participants reported unexpected penile erections. At the same time, independent academic laboratories published data confirming that nitric oxide serves as the primary neurotransmitter regulating vascular tone within the corpus cavernosum. 

Prior therapies for erectile dysfunction (ED) in the late 1980s were invasive, requiring direct intracavernosal injections of vasoactive agents that often caused painful, non-physiological erections. Recognizing a major clinical opportunity, Pfizer shifted sildenafil's clinical target. Sildenafil does not directly cause an erection; instead, it amplifies endogenous nitric oxide signaling by preventing the breakdown of cGMP, meaning it only works in response to sexual arousal. After 21 clinical trials proved its efficacy, the FDA approved sildenafil as Viagra in March 1998, followed by European approval in September 1998. By 2012, Viagra had secured a commanding share of the ED treatment market, generating over $2.05 billion in annual revenue. 

Repurposing for Pulmonary Arterial Hypertension

Sildenafil's therapeutic evolution continued as researchers explored the distribution of PDE5 in other vascular beds. PDE5 is expressed at exceptionally high levels in pulmonary vascular smooth muscle, far exceeding its concentration in the systemic vasculature or cardiac tissue. Pulmonary arterial hypertension (PAH) is a progressive, fatal vasculopathy characterized by vasoconstriction and muscularization of the small pulmonary arteries, which increases pulmonary vascular resistance (PVR) and eventually leads to right ventricular failure. Early treatments like continuous intravenous prostacyclin required invasive central lines, while other options like the endothelin receptor antagonist bosentan carried up to a 10% risk of liver toxicity.

In the late 1990s, preclinical models using isolated rodent lungs demonstrated that sildenafil could selectively inhibit hypoxic pulmonary vasoconstriction. Unlike non-selective systemic vasodilators, which can cause severe systemic hypotension and worsen ventilation-perfusion mismatching, sildenafil acts primarily in well-ventilated lung areas to improve blood flow and oxygenation. 

Following the successful multicenter SUPER-1 trial, sildenafil was approved in 2005 under the brand name Revatio to treat adult PAH, with a standard dose of 20 mg administered three times daily. The clinical utility of this therapeutic was further validated in 2023, when the FDA expanded Revatio's approval to pediatric patients aged 1 to 17. 

 

In clinical practice, sildenafil requires careful monitoring due to its metabolic and pharmacodynamic profiles. Sildenafil is primarily metabolized in the liver by the cytochrome P450 3A (CYP3A) pathway, meaning it can interact with drugs that inhibit or induce this enzyme. For example, co-administration with the endothelin receptor antagonist bosentan can lower sildenafil plasma concentrations while raising bosentan levels.

Additionally, combining sildenafil with nitrates or nitric oxide donors (such as nitroglycerin) is strictly contraindicated, as it can cause a severe, life-threatening drop in systemic blood pressure. Common side effects, including headaches, facial flushing, nasal congestion, and dyspepsia, are directly linked to its systemic vasodilatory properties. 

Sotatercept: Restoring Homeostasis in Pulmonary Vasculature

Sotatercept, approved by the FDA in March 2024 as Winrevair, is a contemporary example of rational, mechanism-based drug repurposing. It successfully transitioned from a failed bone-density and anemia candidate to a first-in-class, disease-modifying biologic for pulmonary arterial hypertension.

Early Clinical Trajectory: Osteoporosis and Anemia

Originally developed as ACE-011 through a joint venture between Acceleron Pharma and Celgene, sotatercept was designed as a soluble, recombinant fusion protein consisting of the extracellular domain of the human activin receptor type IIA (ActRIIA) linked to the Fc domain of human immunoglobulin G1 (IgG1). The initial therapeutic goal of the ACE-011 program was to treat postmenopausal osteoporosis and other bone-loss disorders by sequestering negative regulators of bone remodeling. 

During Phase I clinical safety studies, investigators noticed a robust, dose-dependent increase in hemoglobin and red blood cell counts. Rather than acting on early erythroid progenitors like traditional erythropoiesis-stimulating agents (ESAs), sotatercept was found to act as a ligand trap. It binds to circulating activins and growth differentiation factors (particularly GDF-11), which normally restrict terminal erythroid maturation. 

This discovery led to several Phase II studies evaluating sotatercept for chemotherapy-induced anemia (CIA)—including study A011-08 in patients with metastatic breast cancer and study ACE-011-NSCL-001 in solid tumors treated with platinum-based chemotherapies. Across these studies, 66.7% of patients treated with a dose of 0.3 mg/kg or a flat dose of 15 mg achieved a hematopoietic response, defined as a hemoglobin increase of >= 1 g/dL. 

Acceleron and Celgene also initiated Phase II studies for transfusion-dependent beta-thalassemia and anemia associated with end-stage renal disease. However, clinical development in hematology was ultimately deprioritized as focus shifted to luspatercept (Reblozyl), a modified ActRIIB-Fc ligand trap that offered superior anemia-targeting properties with fewer systemic side effects. 

The Pivot to Pulmonary Arterial Hypertension

Sotatercept’s clinical development was revived following breakthroughs in understanding the genetic and molecular drivers of pulmonary arterial hypertension (PAH). Genetic studies revealed that familial and idiopathic PAH are heavily driven by loss-of-function mutations in the bone morphogenetic protein receptor type II (BMPR2) gene. 

Under normal physiological conditions, the transforming growth factor-beta (TGF-B) superfamily maintains a balance between anti-proliferative signaling (mediated by BMP/BMPR2 via Smad1/5/8 pathways) and pro-proliferative signaling (mediated by activin/ActRIIA via Smad2/3 pathways). In PAH, the loss of functional BMPR2 signaling leaves the pro-proliferative activin cascade unchecked. This imbalance drives the hyperproliferation of endothelial and smooth muscle cells, causing severe pulmonary vascular remodeling, increased right ventricular afterload, and eventual heart failure.

  

reclinical research published by Yung et al. in 2020 (Sci Transl Med) and Joshi et al. in 2022 (Sci Rep) provided key proof of concept, showing that ActRIIA-Fc could rebalance activin/GDF and BMP signaling to reverse experimental pulmonary hypertension. In vitro models showed that activin-A upregulates Endothelin-1 (ET-1) production in endothelial cells through Smad2/3 signaling. This excessive ET-1 reduces endothelial nitric oxide synthase (eNOS) activity and drives smooth muscle remodeling. Treating these models with follistatin or sotatercept analogs successfully reversed these pathological changes. 

These findings led to Acceleron's 2017 decision to develop sotatercept specifically for PAH, resulting in a clinical trial program that evaluated the drug across several distinct patient populations:

●     PULSAR (Phase II): Enrolled 106 adult patients with WHO Group 1 PAH on stable background therapies. The study showed that adding sotatercept (at doses of 0.3 or 0.7 mg/kg subcutaneously every 3 weeks) significantly reduced pulmonary vascular resistance (PVR) compared to placebo. An open-label extension showed that these improvements in exercise capacity (6MWD), functional class, and NT-proBNP levels were maintained through 18 to 24 months of treatment. 

●     STELLAR (Phase III): The pivotal trial that evaluated 323 patients with longstanding PAH, nearly 60% of whom were on triple background therapy and 40% on continuous parenteral prostacyclin infusions. Adding sotatercept (0.7 mg/kg every 3 weeks) led to a 40.8-meter increase in 6MWD and an 84% reduction in the risk of clinical worsening or death at 24 weeks. 

●     ZENITH (Phase III): Designed for high-risk patients (WHO Functional Class III or IV on maximal medical therapy), ZENITH demonstrated a favorable number needed to treat (NNT) of just 4 to prevent a clinical worsening event.

●     HYPERION (Phase III): Evaluated patients diagnosed within the previous year who were at intermediate-to-high risk. Published in 2025, the trial showed that adding sotatercept to early combination therapy resulted in a 76% risk reduction in the primary composite endpoint of clinical worsening.

●     CADENCE (Phase II): Investigated patients with combined post- and pre-capillary pulmonary hypertension associated with heart failure with preserved ejection fraction (HFpEF), reporting significant PVR reductions and proving the efficacy of activin inhibition in Group 2 disease.

Sotatercept is administered as a weight-based subcutaneous injection every three weeks. The recommended starting dose of 0.3 mg/kg is titrated up to a target dose of 0.7 mg/kg based on tolerability and laboratory monitoring.

Because of its erythropoietic origins, sotatercept requires regular blood counts to monitor for elevated hemoglobin levels and severe thrombocytopenia, which may necessitate dose adjustments or temporary treatment pauses. Other common side effects include headache, epistaxis, skin rash, telangiectasia, dizziness, and localized erythema. 

Expanding the Spectrum of High-Impact Repurposing Successes

To understand the broader impact of systematic drug repurposing, it is helpful to look at other successful examples across different therapeutic classes. 

One of the most dramatic stories is thalidomide.Originally introduced in 1957 as a sedative and morning sickness treatment, it was withdrawn in 1961 after causing severe birth defects in thousands of children. Decades later, researchers discovered its potent immunomodulatory and anti-angiogenic properties. In 1998, the FDA approved thalidomide to treat erythema nodosum leprosum (ENL), a painful complication of leprosy, and in 2006, it was approved for multiple myeloma, which significantly improved survival rates for these patients. 

Another notable example is the transition of oncological and psychiatric medications into immunology and neurology. Methotrexate, originally developed in the 1940s as a high-dose chemotherapy agent for pediatric leukemia, was repurposed at low doses to become the foundational disease-modifying antirheumatic drug (DMARD) for rheumatoid arthritis and psoriasis. 

Similarly, gabapentin and pregabalin, which were originally developed as anticonvulsants to treat epilepsy, are now widely prescribed to manage neuropathic pain and generalized anxiety disorders. 

In the metabolic space, the rise of glucagon-like peptide-1 (GLP-1) receptor agonists is transforming chronic disease management. Active ingredients like semaglutide were originally approved to manage Type 2 diabetes under the brand name Ozempic in 2017. After demonstrating weight-loss effects in clinical trials, the molecule was repurposed as Wegovy in 2021 for chronic obesity management. 

By May 2024, Wegovy was also approved to reduce major adverse cardiovascular events (such as stroke and myocardial infarction) in obese adults, showing how metabolic therapies can be repurposed to address systemic cardiovascular disease. 

Generic Name	Original Trade Name & Indication	Repurposed Trade Name & Indication	Core Molecular Mechanism	Key Clinical Outcome
Sildenafil	Viagra: Angina Pectoris	Revatio: Pulmonary Arterial Hypertension (PAH)	Selective PDE5 inhibitor; prevents cGMP degradation to relax vascular smooth muscle	Extends walk distance (6MWD) and improves cardiopulmonary hemodynamics.
Sotatercept	ACE-011: Osteoporosis & Anemia	Winrevair: Pulmonary Arterial Hypertension (PAH)	Recombinant ActRIIA-Fc fusion protein; traps circulating activins and GDFs	Rebalances TGF-beta signaling to reverse pulmonary vascular remodeling.
Thalidomide	Contergan: Sedative & Morning Sickness	Thalomid: Erythema Nodosum Leprosum & Multiple Myeloma	TNF-alpha inhibitor; exhibits immunomodulatory and anti-angiogenic properties	Achieved a 99% remission rate in ENL and extended survival in Multiple Myeloma.
Minoxidil	Loniten: Refractory Systemic Hypertension	Rogaine: Androgenetic Alopecia (Hair Loss)	Potassium channel opener; relaxes vascular smooth muscle	Repurposed topically to stimulate follicular blood supply and promote hair growth.
Semaglutide	Ozempic: Type 2 Diabetes Mellitus	Wegovy: Chronic Obesity & Major Cardiac Event Prevention	GLP-1 receptor agonist; slows gastric emptying and reduces central appetite	Drives a 15% average reduction in body weight and significantly lowers stroke risk.
Everolimus	Certican: Transplant Rejection Prophylaxis	Afinitor: Tuberous Sclerosis Complex & Subependymal Giant Cell Astrocytoma	Selective inhibitor of mammalian target of rapamycin (mTOR) pathway	Blocks tumor-cell proliferation and reduces seizure frequency in patients.
Nitisinone	Herbicide; Orfadin: Tyrosinemia Type 1	Orfadin: Alkaptonuria (AKU / Black Bone Disease)	Inhibitor of 4-hydroxyphenylpyruvate dioxygenase (HPPD) enzyme	Prevents homogentisic acid accumulation to stop ochronotic arthropathy.

 

Structural Obstacles, Intellectual Property, and Future Horizons

While drug repurpose offers clear advantages in terms of cost and development time, several structural, legal, and economic barriers can prevent these therapies from reaching patients.

The primary challenge is the lack of patentability and market exclusivity for off patent or generic drugs. When a drug's original composition-of-matter patent expires, generic manufacturers can enter the market, which drastically lowers prices. Under standard commercial models, if a developer spends $300 million to clinically validate a new indication for a generic drug, they cannot prevent other generic manufacturers from capturing the market through off-label prescribing. This lack of financial incentive often discourages private investment, leaving clinically promising, generic drug-indication pairings unstudied. 

To address these market failures, public health initiatives like the FDA’s May 2026 program seek to create alternative pathways. By updating labels through statutory tools like the MODERN Labeling Act and forming research partnerships with federal agencies like the NIH, the FDA aims to build a public clinical trials network. These collaborations will allow the clinical research community to run larger, multi-center trials for repurposed generics, using real-world clinical data to update product labels even without a commercial sponsor. 

Additionally, emerging computational technologies are shifting the industry from serendipitous discoveries to systematic, target-based pipelines. AI-driven target discovery, network pharmacology, and automated real-world evidence screening are enabling researchers to rapidly identify existing drugs that can target disease pathways. By addressing both the scientific and economic barriers to drug development, these policies and technological innovations are helping to unlock the full potential of the existing pharmacopeia to treat chronic and rare diseases.

REFERENCE:

FDA Law Blog: Old Drugs, New Tricks: FDA’s Drug Repurposing Initiative