Sildenafil: From Heart Failure to Pulmonary Hypertension to the Pediatric Question

A failed angina drug became Viagra, then Revatio, then the most-prescribed off-label treatment in pediatric pulmonary hypertension. Thirty years of one molecule's regulatory journey contains three lessons no modern rare disease program should ignore.

The Hidden Pattern

Sildenafil is the most famous drug repurposing case in modern medicine. Most people know the punchline: a heart drug that didn't work for hearts ended up working for something else entirely. Few people know the second act, the third act, or the regulatory complications that followed.

Behind the punchline is a 30-year story that contains nearly every pattern that defines successful — and failed — drug repurposing. A serendipitous side effect. A mechanism-driven second indication. A pediatric controversy that pitted the FDA against the EMA. A 2026 regulatory landscape that would have made the whole journey faster, cheaper, and more inclusive of children.

This piece walks through the Sildenafil journey not for nostalgia, but to extract three operational lessons for any rare disease program being designed today.

Act One — Angina, Failed

The story begins at Pfizer's Sandwich, UK research facility in the mid-1980s. A team led by Simon Campbell was searching for a treatment for hypertension and angina pectoris (chest pain caused by reduced blood flow to the heart). Their target was the nitric oxide / cyclic guanosine monophosphate (NO/cGMP) signaling pathway, specifically the phosphodiesterase-5 (PDE5) enzyme that breaks cGMP down. By inhibiting PDE5, they reasoned, they could increase cGMP levels, relax vascular smooth muscle, and dilate coronary arteries.

The compound they synthesized was UK-92,480 — later named sildenafil. Phase I trials began in 1991 under Ian Osterloh's direction at Morriston Hospital in Swansea.

The trials failed. Sildenafil had little measurable effect on angina. The drug's half-life was short. By the mid-1990s, the program was on the verge of being killed.

Then the side-effect reports came in.

Phase I clinical trials suggested the drug had little effect on angina, but it could induce marked penile erections.

Pfizer's leadership made the decision that became one of the most-cited examples of drug repositioning in pharmaceutical history. Instead of killing the program, they pivoted it. New trials, new patient population, new indication.

Act Two — Erection, Approved

By 1997, twenty-one separate clinical trials had demonstrated sildenafil's efficacy in treating erectile dysfunction (ED). Pfizer submitted registration dossiers to the FDA and EMEA.

On March 27, 1998, the FDA approved Viagra — the first oral PDE5 inhibitor and the first oral pharmacological treatment for ED. European approval followed in September 1998. Within weeks of US launch, more than one million patients had received prescriptions. The drug's commercial trajectory is well-documented; what matters here is the regulatory architecture.

Pfizer had filed two separate patent applications. One covered sildenafil for cardiovascular disease (the original program). One covered sildenafil for ED (the discovered application). The same molecule, two distinct intellectual property positions, and the option to pursue completely different downstream indications without each undermining the other.

This became the structural foundation for Act Three.

Act Three — Pulmonary Hypertension, Systematically

By the late 1990s, the mechanism of sildenafil was well-understood. PDE5 is highly expressed in lung tissue. The NO/cGMP pathway regulates pulmonary vascular tone. Pulmonary arterial hypertension (PAH) — a devastating disease where the small arteries in the lungs progressively constrict and stiffen — is, mechanistically, exactly the kind of condition where PDE5 inhibition should help.

This was no longer serendipity. It was a hypothesis that followed directly from the established biology. Between 1998 and 2001, mounting evidence demonstrated sildenafil's efficacy in pulmonary vascular disorders, leading to the design of SUPER-1, the pivotal randomized controlled trial.

SUPER-1 enrolled patients with WHO Group I PAH and measured the change in 6-minute walking distance over 12 weeks. The trial succeeded. In 2005, the FDA and EMEA approved sildenafil for the treatment of PAH in adults under a new brand name: Revatio. Same molecule. Different brand. Different dose (20mg three times daily, vs. 50–100mg as needed for ED). Different indication. Different patent.

The Pediatric Question

The most instructive part of the Sildenafil story is not the wins. It is the place the drug got stuck.

Pediatric PAH is rare and devastating. Untreated, children with idiopathic PAH historically had less than one year of survival. Sildenafil had been used off-label in pediatric patients since 2005, and observational data suggested favorable outcomes. The natural next step was a controlled trial.

STARTS-1 (Sildenafil in Treatment-Naïve Children, Aged 1–17, with Pulmonary Arterial Hypertension) enrolled 234 patients across three dose groups plus placebo over 16 weeks. The medium and high doses produced improvements in peak oxygen consumption, mean pulmonary artery pressure, and pulmonary vascular resistance. STARTS-2, the long-term extension, followed the same patients for three years.

Then the data split the regulators.

The European Medicines Agency reviewed STARTS-1 and STARTS-2 in 2011 and approved sildenafil for pediatric PAH (ages 1–17), with caveats on dosing.

The FDA reviewed the same data and reached the opposite conclusion. In August 2012, the agency issued a Drug Safety Communication recommending against the use of sildenafil in pediatric PAH. The warning cited a higher mortality rate in children receiving high-dose sildenafil compared to those on low-dose sildenafil over the three-year STARTS-2 extension.

The pediatric pulmonary hypertension community pushed back hard. The Pediatric Pulmonary Hypertension Network, the American College of Cardiology, the American Heart Association, and the American Academy of Pediatrics jointly raised concerns. Their analysis: most pediatric deaths in STARTS-2 occurred in patients with idiopathic or hereditary PAH (the most severe subtype), the multivariate analysis adjusting for baseline severity reduced the dose-mortality association substantially, and no deaths were considered causally related to sildenafil itself.

In 2014, the FDA clarified that the warning was not an absolute contraindication and that physicians should consider individual benefit-risk balance. But the warning remains in the Revatio label. Sildenafil has never received an FDA pediatric PAH indication.

Today, sildenafil is the most-prescribed drug in pediatric PAH in the United States — and it remains entirely off-label for that use.

What 2026 Changes for the Same Story

If the Sildenafil pediatric PAH question were being raised today, the regulatory landscape would look different in three concrete ways.

First, the Catalyst-era exclusivity question would be moot. Under the post-Catalyst Fix interpretation (see our analysis), an adult PAH approval no longer blocks pediatric programs in the same condition. The same indication-specific scope that always existed in FDA's longstanding interpretation is now codified in statute via Section 6605 of the 2026 Consolidated Appropriations Act.

Second, the Rare Pediatric Disease Priority Review Voucher (PRV) program is now reauthorized through September 2029. Pediatric PAH would qualify for RPD designation, which means a pediatric PAH approval would generate a PRV worth approximately $100–150 million in the secondary market. This non-dilutive value alters the economics of pursuing a dedicated pediatric trial. In 2012, the PRV program existed but was less established and not specifically reauthorized; today, it is a more reliable financial input for any program targeting pediatric rare disease.

Third, sponsors now have FDA-required structures (like PREA enforcement, strengthened by the Mikaela Naylon Act) that compel pediatric study completion when adult indications are pursued in conditions affecting children. A 2026-era Sildenafil program would not be able to leave pediatric questions perpetually unresolved.

None of this rewrites the safety questions raised by STARTS-2. But it does mean a comparable program designed today would have stronger incentives, a clearer regulatory path, and structural pressure to complete pediatric studies — rather than letting them lapse into perpetual off-label use.

Three Lessons for Modern Repurposing

The Sildenafil case is unusual in scope, but the patterns it reveals apply to rare disease repurposing programs being designed today.

Lesson 1: Mechanism explains second indications. Serendipity finds the first.

The Viagra discovery was an accident. The Revatio discovery was a hypothesis derived directly from understood biology. These are different epistemic events, and they require different evaluation frameworks.

Modern computational repurposing (including the work behind approaches like ORPHERA's CREST engine) is designed to find the second kind — drug-disease combinations where mechanism predicts efficacy. The Sildenafil → PAH transition is the canonical proof of concept that this framework works. Once the mechanism is understood, second and third indications become predictable, not lucky.

Lesson 2: Different brands, same molecule, separate regulatory paths.

Pfizer's decision to maintain two patent families and two brand names (Viagra, Revatio) for the same active ingredient was strategically critical. It allowed each indication to develop its own commercial identity, dosing, and safety positioning. It also meant that exclusivity questions for one indication did not directly threaten the other.

For modern rare disease repurposing, the structural lesson is to think of each indication as a distinct product from the start, with its own ODD application, its own 505(b)(2) bridge study, its own pricing strategy, and its own commercial positioning. The Sildenafil case demonstrates this is not just possible — it can be highly successful.

Lesson 3: Pediatric questions don't resolve themselves. They have to be designed in from the beginning.

The pediatric PAH question with sildenafil has now been unresolved for over a decade. Children are still being treated with the drug, but without an FDA approval, with continued safety concerns, with insurance coverage uncertainty, and without the financial incentives that would attract dedicated pediatric trials.

This is the failure mode that the 2026 regulatory landscape is explicitly designed to prevent. A modern rare disease program targeting any condition that affects children should plan pediatric trials, RPD designation, and PRV positioning from the beginning — not as an afterthought once adult approval lands. The financial and regulatory architecture now exists to make this rational.

The Takeaway

Sildenafil is the most successful repurposed drug of the modern era. Three decades, three indications, two brand names, one molecule that has helped tens of millions of people across conditions that share almost nothing on the surface but everything at the level of mechanism.

It is also a case study in what gets lost when regulatory environments do not keep pace with science. Children with PAH have been receiving sildenafil off-label for over a decade because the regulatory pathway to formal pediatric approval was structurally discouraged.

The 2026 landscape changes that. For any company evaluating rare disease repurposing candidates today, the lesson from Sildenafil is that the path from molecule to approved therapy in a new indication is real, navigable, and worth structuring carefully — and that the pediatric component, in particular, should never be deferred.