PCSK9: The Gene That Proved a Drug Would Work Before It Existed

The gene

PCSK9 stands for proprotein convertase subtilisin/kexin type 9. It is a protein made primarily in the liver. Its job is to bind to LDL receptors on the surface of liver cells and tag them for destruction. Fewer LDL receptors means less cholesterol gets cleared from the blood. More PCSK9 means higher LDL. Less PCSK9 means lower LDL.

In 2003, Marianne Abifadel, Catherine Boileau, and colleagues identified PCSK9 as the third gene responsible for autosomal dominant hypercholesterolemia - finding gain-of-function mutations in French families with extremely high cholesterol. Too much PCSK9 activity was destroying too many LDL receptors, and cholesterol was piling up.

That discovery told researchers what PCSK9 does. The next one told them what happens when it is missing.

The natural experiment

In 2006, Jonathan Cohen and Helen Hobbs at UT Southwestern published the landmark study that changed the trajectory of cardiovascular medicine. Using data from the Dallas Heart Study and the Atherosclerosis Risk in Communities (ARIC) study, they found people carrying loss-of-function mutations in PCSK9 - people whose bodies naturally produced less of the protein.

The results were striking:

• 2.6% of Black participants carried nonsense mutations in PCSK9. Their LDL was 28% lower than average. Their risk of coronary heart disease was 88% lower.
• 3.2% of White participants carried a different PCSK9 variant. Their LDL was 15% lower. Their CHD risk was 47% lower.

These were not patients on medication. These were people who had lived their entire lives with reduced PCSK9 function. They were healthy. The gene was doing exactly what a drug would need to do - lowering LDL cholesterol - and it had been doing it safely for decades.

The same year, a separate study identified a woman who was a compound heterozygote - she had inherited two different inactivating mutations, one from each parent. Her body produced no detectable PCSK9 protein at all. Her LDL cholesterol was 14 mg/dL. She was a healthy, college-educated aerobics instructor.

No PCSK9. No health problems. An LDL level most cardiologists had never seen.

Why this matters for drug development

Most drugs fail in clinical trials. The most common reason is that the target turns out to be wrong - blocking it either does not produce the expected benefit, or it causes unexpected harm.

Loss-of-function genetics in healthy people inverts this equation. When you find humans who have naturally lived without a protein for their entire lives and are fine, you have the strongest possible evidence that blocking that protein is both safe and effective. No animal model can provide this. No Phase 1 trial can provide this. It is a natural experiment, run over decades, in thousands of people.

PCSK9 became the poster child for this approach. The genetics provided two things simultaneously: proof that the target was safe to block (the woman with zero PCSK9 was healthy) and proof that blocking it would reduce heart disease (88% lower risk in Cohen/Hobbs). Before a single milligram of drug was synthesized, the genetic evidence was already stronger than most Phase 3 trial results.

Every major pharmaceutical company now screens for loss-of-function mutations before committing to a drug target. The approach is called genetic validation - and PCSK9 is the reason it became standard practice.

The drugs that followed

With the genetics in hand, the race to build a PCSK9 inhibitor moved fast. Amgen and Regeneron both developed monoclonal antibodies and reached FDA approval within weeks of each other in 2015. Meanwhile, Alnylam Pharmaceuticals - the company pioneering siRNA therapeutics - partnered with The Medicines Company in 2013 to take a fundamentally different approach: instead of blocking the PCSK9 protein, they would silence the gene that makes it. The siRNA modality was newer and development started later, but the payoff was a drug that lasts six months per dose instead of two to four weeks. Novartis acquired The Medicines Company for $9.7 billion in 2020 to get inclisiran, and won FDA approval for Leqvio in December 2021. LIB Therapeutics brought a fourth option, a fusion protein called Lerochol, approved in December 2025. Four drugs, from four companies, using three different molecular strategies. All targeting the same gene.

Each modality blocks PCSK9 differently. The monoclonal antibodies (Repatha and Praluent) bind and neutralize the PCSK9 protein after it is made. Leqvio uses small interfering RNA to silence the PCSK9 gene in liver cells before the protein is produced - which is why it lasts six months per dose instead of two to four weeks. Lerochol uses an engineered binding domain (an adnectin) fused to human serum albumin, giving it high-affinity PCSK9 binding with a long half-life in a small-volume monthly injection.

If the suffixes look familiar, we decoded them in The Code Hidden in Every Drug Name: evolocumab and alirocumab end in -mab (monoclonal antibody), inclisiran ends in -siran (siRNA).

The order in which these modalities arrived is not accidental. Monoclonal antibodies came first (2015) because they are the proven platform for blocking protein-protein interactions. siRNA came next (2021) - a newer technology that required advances in liver-targeted delivery, but one that offered a fundamentally different duration of action. Fusion proteins (2025) offered a different engineering tradeoff: a smaller molecule with high binding affinity and a long half-life. Each generation solved a different problem.

And a fifth approach is in late-stage development. Merck's enlicitide (MK-0616) is an oral PCSK9 inhibitor - a daily pill. It is a macrocyclic peptide - not a conventional small molecule - because PCSK9's flat binding surface was long considered "undruggable" by oral compounds. It completed Phase 3 trials (CORALreef) with a 56% LDL-C reduction versus placebo, and Merck is expected to file for FDA approval in 2026. If approved, it would be the first oral PCSK9 inhibitor - a shift that could dramatically expand the addressable market, because pills reach patients that injections never will.

How they block PCSK9 - three approaches

The outcomes trial that changed everything

The genetics made the case for PCSK9 inhibition. The FOURIER trial proved it worked in practice.

Published in the New England Journal of Medicine in 2017, FOURIER enrolled 27,564 patients with established cardiovascular disease who were already on statin therapy. Half received Repatha (evolocumab), half received placebo.

Over a median follow-up of 2.2 years, Repatha reduced the risk of the primary composite endpoint - cardiovascular death, heart attack, stroke, hospitalization for unstable angina, or coronary revascularization - by 15%. The key secondary endpoint of cardiovascular death, heart attack, or stroke was reduced by 20%.

The trial did what the genetics predicted: blocking PCSK9 lowered LDL cholesterol, and lowering LDL cholesterol reduced cardiovascular events. It confirmed the target. But it also shifted the commercial trajectory of the drug - from a lipid-lowering agent to a cardiovascular outcomes drug.

Repatha crossed $3 billion in annual revenue in 2025 - more than tripling from its 2020 level. The growth reflects a common pattern with genetically validated drugs: slow initial uptake as payers resist coverage, followed by acceleration once outcomes data (FOURIER) forces broader formulary access.

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The competition

Repatha leads the PCSK9 market, but it is not alone. Praluent launched within weeks of Repatha in 2015, co-commercialized by Regeneron and Sanofi. The two drugs competed head-to-head for years - and the competition went beyond the clinic and into the courtroom.

In 2014, Amgen sued Regeneron and Sanofi, claiming that its patents covered not just evolocumab (Repatha) but essentially all antibodies that bind to PCSK9's receptor-binding region and block its function. If upheld, the patents would have given Amgen exclusive rights over the entire class of PCSK9 antibodies.

The case went all the way to the US Supreme Court. In May 2023, the Court ruled unanimously against Amgen. Justice Gorsuch wrote: "The more one claims, the more one must enable." Amgen had described 26 specific antibodies but claimed rights over a potentially vast number. The ruling invalidated the broad patents and became a landmark decision for biotech patent law - narrowing how broadly companies can claim ownership over biologic drug classes.

Despite surviving the patent challenge, Praluent has remained a distant second commercially. It generated $765 million in 2024 revenue (up 20% from $639 million in 2023), but most of that comes from outside the US - Praluent's international sales ($523 million) are more than double its US sales ($242 million). Repatha, by contrast, is US-dominant and nearly three times larger, at $2.2 billion in the same year.

The more interesting competitive threat is coming from a different direction. Novartis's Leqvio went from $12 million in its 2021 launch year to $1.2 billion in 2025 - a hundredfold increase in four years. Its advantage is not clinical superiority but convenience: two injections per year, administered in a doctor's office, versus monthly or biweekly self-injection at home. For a drug class where adherence is a persistent problem - patients stopping a preventive medication they cannot feel working - Leqvio's dosing schedule may prove decisive.

The pricing story

When Repatha launched in 2015, its list price was roughly $14,000 per year. Payers pushed back hard. Prior authorization requirements were strict. Many patients who could benefit were denied coverage.

In 2018, Amgen cut the list price by roughly 60%, to about $5,850 per year - an unusual move for a biologic in a market where prices typically only go up. The cut was driven by poor uptake: the drug was clinically validated but commercially blocked by payer resistance.

As of 2025, the list price sits around $6,900 per year. Amgen also introduced AmgenNow, a direct-to-patient program offering Repatha at about $2,900 per year for patients who pay out of pocket.

Leqvio (inclisiran) lists at roughly $3,600 per injection - but because it is administered in a physician's office (not self-injected at home), it follows the medical benefit pathway rather than the pharmacy benefit. This changes the coverage and reimbursement dynamics. Leqvio's twice-yearly dosing is also a competitive advantage for adherence - fewer injections, fewer chances to stop.

The pricing trajectory is unusual for biologics. Four approved competitors, an oral option on the way, and list prices that have come down rather than up. Competition on a genetically validated target appears to work differently than competition on targets where clinical differentiation is harder to prove.

What the pipeline looks like

According to our PCSK9 competitive landscape, the combined market - Repatha ($3.0B), Leqvio ($1.2B), and Praluent (~$0.8B) - now exceeds $5 billion annually, with 50 trials still actively recruiting.

The pipeline is shifting in two directions. First, new indications: trials are now testing PCSK9 inhibitors in acute ischemic stroke, in combination with other lipid-lowering agents, and in primary prevention (treating high-risk patients who have not yet had a cardiovascular event). Novartis has two large outcomes trials underway for Leqvio with a combined enrollment of over 31,000 patients.

Second, new modalities. Merck's oral PCSK9 inhibitor (enlicitide) could reshape the market if approved. Every current PCSK9 inhibitor requires injection. A daily pill would lower the barrier to treatment and potentially expand the addressable population significantly.

The playbook PCSK9 created

PCSK9 did not just produce drugs. It produced a template for how to develop them.

The approach - find people with natural loss-of-function mutations, verify they are healthy, then build a drug that mimics what their genetics already do - is now being applied to other targets. Evkeeza (evinacumab) targets ANGPTL3, following the same logic: people with ANGPTL3 loss-of-function have very low lipid levels and no apparent health problems. The drug was approved in 2021 for homozygous familial hypercholesterolemia, a condition where even PCSK9 inhibitors often fall short.

The UK Biobank, the FinnGen study, and large-scale sequencing efforts have made this kind of genetic screening routine. Today, virtually every major pharmaceutical company uses human genetics evidence at the earliest stages of drug development - before a molecule is designed, before a clinical trial is planned. Before committing hundreds of millions to a program, they ask: are there humans already living without this protein? If so, what happened to them?

When the answer is "they are healthy, and they are protected from disease" - that is the strongest signal drug development has. PCSK9 is the proof that the signal works.

One gene. Four approved drugs. A fifth on the way. Three different molecular approaches to blocking the same target. A $5 billion market built on the observation that people born without PCSK9 are not just fine - they are protected. The genetics proved the drug would work before anyone built one.