Lp(a) HORIZON Trial: Pelacarsen Cut Lp(a) by 78 Percent but Did Not Prevent Heart Attacks

Imagine a detective story in which the prime suspect is caught, locked up for nearly five years and the crimes keep happening at exactly the same rate. That, roughly, is what cardiologists heard in Chicago on Saturday. Results from the Lp(a) HORIZON trial, presented at the American Heart Association’s Scientific Sessions 2026 and published at the same time in the New England Journal of Medicine, showed that the experimental drug pelacarsen slashed levels of lipoprotein(a) by about 78 percent. Heart attacks, strokes and cardiovascular deaths did not budge.

If you have never heard of lipoprotein(a), you are not alone. That is part of why this result matters. It is one of the most common inherited heart risk factors in the world. It is also the one most people have never been tested for.

What is Lp(a) and why should you care?

Lipoprotein(a), usually written Lp(a) and pronounced “L P little a”, is a cholesterol-carrying particle that shares some features with LDL, the cholesterol you probably know as the bad one. According to the American Heart Association, about one in five people worldwide, roughly 1.4 billion, are thought to have elevated levels. Unlike LDL, your Lp(a) level is largely set by your genes and stays fairly stable throughout life. Diet, exercise and statins have little effect on it.

The risk is real. The AHA notes that levels of 125 nmol/L or higher are linked to about 1.4 times the long-term risk of heart attack or stroke, while levels of 250 nmol/L or higher are linked to at least double the risk. Currently no drug approved by the US Food and Drug Administration lowers Lp(a). The 2026 multi-society dyslipidemia guideline cited by the AHA recommends measuring it at least once in adulthood.

What the Lp(a) HORIZON trial tested

The trial was large and unusually long. More than 8,300 adults at 797 sites in 42 countries took part. Everyone already had established cardiovascular disease, such as a previous heart attack, a stroke caused by a clot or narrowed leg arteries. Their Lp(a) level had to be at least 70 mg/dL. Participants received a monthly injection of pelacarsen 80 mg or a placebo, on top of their usual care. That care was already excellent: TCTMD reports that around 78 percent were on high-intensity statins and the median LDL cholesterol was about 65 mg/dL.

Pelacarsen is an antisense drug, a synthetic genetic molecule that enters liver cells and blocks production of Lp(a). It did that job very well. At 12 months, levels in the pelacarsen group had fallen by 77.7 percent compared with placebo.

The result nobody expected

Then came the part that matters. After a median follow-up of 4.7 years, major cardiovascular events occurred in 12.3 percent of patients on pelacarsen and 12.1 percent on placebo, according to TCTMD. The hazard ratio was 1.02, with a 95 percent confidence interval from 0.90 to 1.15. In plain language: no difference. The result held among patients with the highest starting levels and across the individual outcomes, including cardiovascular death.

Novartis, which funded the study, had already announced in September 2026 that the trial missed its main goal. The full data now confirm it. The company says it is discontinuing the pelacarsen programme in atherosclerotic cardiovascular disease. Senior author Steven Nissen of the Cleveland Clinic did not dress it up. “It didn’t work,” he said in the AHA release, calling the results completely neutral across multiple analyses.

Side effects also counted against the drug. Flu-like symptoms, injection-site reactions and allergic reactions were more common with pelacarsen. The AHA reports 19 cases of anaphylaxis in the pelacarsen group and none with placebo, with no deaths from allergic reactions. Roughly one in five patients on pelacarsen stopped the medication because of side effects, compared with about one in thirteen on placebo. Serious adverse events overall were slightly lower in the pelacarsen group, at 37.3 percent versus 41.1 percent.

Why might it have failed?

This is where the science gets interesting and where you should be careful to separate data from hypotheses. Nobody yet knows why lowering Lp(a) this much did not prevent events. At the meeting, Donald Lloyd-Jones of Boston University raised several possibilities, as summarized by TCTMD. Perhaps the participants’ arteries were already too diseased to benefit. Perhaps their Lp(a) was not high enough at the start, or what remained after treatment was still too high. Perhaps the drug’s side effects offset part of any gain.

The trial also has limits its authors acknowledge. About 78 percent of participants identified as white and fewer than 4 percent were of African ancestry, even though people of African ancestry have on average about twice the Lp(a) levels of white people. Some events happened during the COVID-19 pandemic. The researchers could not rule out that some were COVID-related.

Crucially, experts are not abandoning Lp(a) as a target. Lloyd-Jones said he would not give up on it as a causal risk factor. The guideline advice to test once in adulthood still stands. Karol Watson of UCLA, the trial’s discussant, pointed to at least four ongoing outcome trials of other Lp(a)-lowering drugs: two with olpasiran, one with lepodisiran and one with the oral drug muvalaplin. Several of these are small interfering RNA drugs, which Nissen noted may lower Lp(a) further with better tolerability. He also urged people enrolled in those trials not to drop out because of this result.

What it means for you

If you had pinned your hopes on a quick fix for a family history of early heart attacks, this is a frustrating week. But the real lesson is broader. A drug that improves a number on a lab report is not automatically a drug that keeps you out of the hospital. Medicine learns this lesson again and again. It is exactly why large outcome trials exist.

For now, the practical message from the guideline is unchanged. Knowing your Lp(a) level once in your life can help you and your doctor understand your overall risk and focus on the factors you can influence, such as LDL cholesterol, blood pressure, smoking and diabetes. The next chapter of this story will be written by the trials still running. Until then, the suspect remains on file, just not convicted.

This article is for general information only and does not replace advice from a doctor. If you have questions about your heart health or cholesterol, please talk to a qualified healthcare professional.