How sildenafil found new indications

A candidate first studied for angina

Sildenafil was not originally developed for erectile dysfunction. In the 1980s, researchers at Pfizer were investigating the NO–cGMP pathway as a way to relax blood vessels and treat hypertension and angina. Sildenafil emerged from that work as an inhibitor of PDE5.

PDE5 breaks down cGMP. Inhibiting it prolongs a signal involved in smooth-muscle relaxation. The early clinical results did not support the hoped-for effect in angina, but reports of erections in trial participants prompted a change in research direction.

The observation opened a new development program; it did not complete one. Clinical studies in patients with erectile dysfunction were needed to establish efficacy and safety. Viagra was approved in the United States in 1998.

A biological rationale for pulmonary arterial hypertension

The move into pulmonary arterial hypertension (PAH) followed research into the drug’s mechanism. PDE5 and the NO–cGMP pathway contribute to pulmonary vascular tone, providing a rationale for investigating sildenafil in pulmonary vascular disease.

The SUPER-1 trial assessed measures including the change in six-minute walking distance over 12 weeks in adults with PAH. In 2005, sildenafil was approved for adult PAH under the name Revatio. The active ingredient was the same, but the patient population, dosing, treatment schedule, and endpoints were specific to the new indication.

What pediatric development required

Efficacy in adults does not settle the question of pediatric use. STARTS-1 enrolled 234 children aged 1–17 and compared several sildenafil doses. The long-term extension, STARTS-2, raised concern about mortality in the high-dose group.

Regulators initially responded differently. The EMA approved pediatric use, while the FDA issued a warning in 2012. In 2014, the FDA clarified that the warning should not be interpreted as an absolute contraindication for every pediatric patient.

The subsequent decision matters too. The FDA approved Revatio for PAH in children aged 1–17 in January 2023. Describing this use as still unapproved in the United States is therefore incorrect. The earlier safety debate and the later approval need to be presented in sequence.

A new indication needs its own development plan

Sildenafil illustrates how understanding a mechanism can help identify another indication. A plausible mechanism alone, however, cannot establish clinical efficacy. Researchers still need to select patients and doses and test outcomes that matter clinically.

Viagra and Revatio also show how one ingredient can be developed for different uses. For a subsequent indication, a sponsor needs to determine which existing studies can support the application, what additional evidence is required, and which patents or exclusivities are relevant. Separate brands do not automatically create independent regulatory rights.

Assessing the 2026 policy changes separately

Recent changes to orphan-drug exclusivity and pediatric development incentives may affect new programs. They do not establish that sildenafil’s historical development would necessarily have been faster. Its US pediatric indication was already approved in 2023, and the earlier debate concerned dose and safety as well as regulatory policy.

A new program should review current exclusivity rules and available incentives against its own circumstances. Repurposing an approved ingredient does not automatically qualify a product for a rare pediatric disease priority review voucher. Policy changes can inform a development plan, but they cannot substitute for clinical evidence.

Three questions for candidate assessment

• Mechanism: What evidence connects the drug’s action with the proposed disease?
• Development: Which safety and efficacy findings can be used, and what must still be studied?
• Patients: Does the plan address dosing, safety, and endpoints for the intended population, including children?

The useful lesson is the development process: turning an observation into a hypothesis, testing it clinically, and revising the plan as evidence about the target population accumulates.