Sildenafil
Sildenafil citrate
A failed heart drug became the most famous pivot in pharmaceutical history.
The clinical trial side effect that created a $2 billion market.
Key Facts
- 1Originally developed as an anti-angina drug for chest pain
- 2Failed its primary endpoint in clinical trials for heart disease
- 3Male volunteers famously refused to return their leftover tablets
- 4Generated over $2 billion in annual peak sales for Pfizer
- 5Later approved for pulmonary arterial hypertension as Revatio
The Problem
Why This Molecule Was Needed
In the late 1980s, Pfizer's Sandwich (UK) research site was hunting for new treatments for angina pectoris, the chest pain caused by insufficient blood flow to the heart. The prevailing theory was that inhibiting phosphodiesterase type 5 (PDE5), an enzyme that degrades cyclic GMP, would relax vascular smooth muscle and improve coronary blood flow. Angina affected millions, and existing treatments had significant limitations, so a selective PDE5 inhibitor seemed like an elegant molecular solution.
The chemistry team, led by Andrew Bell, Simon Campbell, and David Roberts, began optimising a pyrazolopyrimidinone scaffold for PDE5 selectivity. They needed a compound that would potently inhibit PDE5 while sparing the closely related PDE6 (found in the retina) and PDE3 (found in the heart). The molecular challenge was achieving selectivity across enzyme subtypes that share a highly conserved catalytic site.
The Discovery
How It Happened
Sildenafil synthesised
The team synthesises sildenafil, choosing a pyrazolopyrimidinone core for its structural resemblance to cGMP, the natural PDE5 substrate.
UK-92,480 (sildenafil) selected
After testing hundreds of analogues, the team selects UK-92,480 for its excellent PDE5 potency (IC50 = 3.5 nM), good selectivity over PDE3 (>1000-fold), and acceptable oral pharmacokinetics.
Phase I and the unexpected observation
Phase I trials show sildenafil is safe and well-tolerated, with modest cardiovascular effects. However, clinical staff begin receiving reports of an unexpected side effect: penile erections, with some volunteers notably reluctant to return unused tablets.
The pivot: from heart to erectile dysfunction
Phase II trials for angina show disappointing efficacy, and Pfizer pivots the programme to erectile dysfunction, a condition with no oral treatment and enormous unmet need.
FDA approval as Viagra
Sildenafil is approved by the FDA in March 1998, becoming the fastest-selling drug in history at that time with $1 billion in first-year sales.
The Molecule
Up Close
Tap any region or group card to explore key structural features.
The Takeaway
Substrate mimicry and the art of the pivot
Sildenafil teaches two lessons. The scientific lesson is substrate mimicry: designing a molecule that looks enough like the natural substrate (cGMP) to fool the enzyme, but different enough to block rather than be processed. The strategic lesson is equally important: clinical serendipity is only useful if you recognise it. Pfizer could have dismissed the ED side effect as a distraction, but instead they pivoted the entire programme around an unexpected observation.
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