AstraZeneca and Ionis Pharmaceuticals disclosed Thursday morning that their gene-silencing drug Wainua (eplontersen) failed the largest clinical trial ever conducted in transthyretin-mediated amyloid cardiomyopathy — wiping out what had been one of the most widely anticipated commercial bets in cardiovascular medicine and leaving patients, investors, and scientists with a pointed question: when a stabilizer drug is already keeping the disease in check, does silencing TTR production at its source add anything at all?

The CARDIO-TTRansform Phase 3 trial enrolled 1,432 patients across 130 sites in 20 countries and ran for 140 weeks — the largest ATTR-CM study in history. Yet eplontersen, administered once monthly, failed to meaningfully reduce the composite of cardiovascular deaths and recurrent cardiovascular events compared to placebo. The result sent AstraZeneca shares down roughly 9% on the London Stock Exchange and Ionis shares down approximately 21% in intraday trading, ending the drug’s prospects as a fourth approved ATTR-CM treatment alongside three now-established rivals.

For the estimated 300,000 to 500,000 people living with ATTR-CM worldwide — a progressive, fatal disease caused by misfolded protein deposits that steadily stiffen the heart — the failure means a potentially different treatment option will not be coming. Those who currently have access to approved therapies retain them; the pipeline narrows, not collapses.

Eplontersen Silences TTR — But So Does Amvuttra, and Both Strategies Hit a Wall Together with Stabilizers

To understand what went wrong, it helps to understand what eplontersen is meant to do — and how it differs from the drugs already on the market.

ATTR-CM is caused by the transthyretin (TTR) protein, produced in the liver, that misfolds and accumulates in the heart as amyloid fibrils. Once deposited, those fibrils are essentially insoluble and resistant to degradation. The two main therapeutic strategies both try to reduce the supply of new TTR entering the bloodstream, but they do so in mechanistically distinct ways.

TTR stabilizers — the class that includes Pfizer’s Vyndamax (tafamidis), which surpassed $6 billion in annual sales, and BridgeBio Pharma’s Attruby (acoramidis), approved in late 2024 — work by binding to the thyroxine-binding sites on the TTR tetramer. The tetramer is the normal, functional four-subunit form of the protein. Stabilizers hold it together, preventing it from dissociating into the monomers that misfold and aggregate into amyloid fibrils. They reduce the rate of new fibril formation; they do not remove existing amyloid from the heart.

Eplontersen takes a different approach. It is a Ligand-Conjugated Antisense (LICA) oligonucleotide — a short, chemically modified strand of synthetic DNA conjugated to N-Acetylgalactosamine, a sugar that binds specifically to receptors on liver cells. Once inside a hepatocyte, the drug recruits a cellular enzyme called RNase H, which cleaves the TTR messenger RNA before it can be translated into protein. The result: the liver produces far less TTR in the first place. Eplontersen reduces the TTR supply at its source.

Alnylam Pharmaceuticals’ Amvuttra (vutrisiran), FDA-approved for ATTR-CM in March 2025, uses a related but distinct mechanism called RNA interference. Where eplontersen (an antisense drug) recruits RNase H to cut the TTR mRNA, vutrisiran loads into the cellular RNA-Induced Silencing Complex, where a protein called Argonaute 2 performs the cleavage. Both drugs are GalNAc-conjugated for liver targeting; both deliver TTR knockdown. Vutrisiran is administered quarterly rather than monthly.

Why Adding a Silencer on Top of a Stabilizer May Not Help

The CARDIO-TTRansform trial was designed to study eplontersen on top of whatever standard care patients were already receiving — and the contemporary standard of care has changed dramatically since clinical trials began enrolling patients with ATTR-CM.

When the CARDIO-TTRansform trial was designed, most ATTR-CM patients were untreated or newly beginning tafamidis. By the time the trial enrolled its 1,432 patients, 57% of participants in each study arm were already taking a TTR stabilizer at baseline — and another 24% added one during the trial. In other words, more than 80% of patients in the trial were on a stabilizer at some point during the 140-week study period.

This matters mechanistically. A stabilizer holds the TTR tetramer intact so fewer fibrils form. A silencer reduces how much TTR the liver produces. Both address the upstream supply chain of disease progression. But there is a critical structural gap neither approach solves: the amyloid already deposited in the heart. With stabilizers already reducing new fibril formation, adding a silencer may provide only redundant protection at the supply end while doing nothing about the fibrils that are already there — and it is those existing fibrils that determine how sick the heart is today. In patients whose TTR production is being partially suppressed by stabilizer action, the additional signal from eplontersen may not produce a large enough incremental clinical benefit to show up in a composite outcome endpoint over 140 weeks.

The trial’s own results underscore this interpretation. In the subset of patients who received eplontersen without any concurrent stabilizer — the monotherapy subgroup — the drug did show a nominally statistically significant hazard ratio of 0.71 on the composite cardiovascular endpoint, meaning roughly a 29% reduction in risk compared to placebo. The companies have been careful to describe this as “nominally significant” rather than definitive: a prespecified subgroup analysis from an otherwise failed trial carries substantially less evidentiary weight than a positive primary endpoint, and analysts cautioned against overinterpreting it. But the signal suggests the drug may have measurable biological efficacy when it does not have to compete with an already-active stabilizer background.

“We believe these findings reflect the rapidly evolving treatment landscape, in which contemporary ATTR-CM patients are widely treated with stabilizers,” said Brett P. Monia, Ph.D., chief executive officer of Ionis.

Mathew Maurer, M.D., primary investigator for CARDIO-TTRansform and Arnold and Arlene Goldstein professor of cardiology at Columbia University Irving Medical Center, added that the results “provide important clarity for the field that will help inform future treatment decisions in ATTR-CM.”

Competitors Surge as a Key Rival Exits the Race

The commercial implications rippled outward immediately. AstraZeneca had previously signaled that Wainua could address what it described as a significant commercial market opportunity in cardiovascular medicine — the ATTR-CM space that Pfizer’s Vyndamax already dominates with over $6 billion in annual sales. That scenario no longer applies.

BioPharma Dive reported that Stifel analyst Paul Matteis, writing in a client note Tuesday, said the only “question going in” was how eplontersen might compare to Alnylam’s already-approved Amvuttra — not whether it would succeed. The result “caught Wall Street and investors by surprise,” Matteis wrote. Jefferies analyst Michael Leuchten described the outcome as a “credibility loss” for AstraZeneca, noting that the company is “meant to be able to have exceptionally good trial design ability.”

Shares of Alnylam and BridgeBio, meanwhile, surged by double digits following the announcement — effectively inheriting what eplontersen was supposed to take. Alnylam’s Amvuttra now stands as the only approved RNA-targeting drug for ATTR-CM. The HELIOS-B trial that supported its approval demonstrated a 28% risk reduction in the overall population and a 33% reduction in the monotherapy group — results that look considerably stronger in hindsight given what CARDIO-TTRansform found.

One crucial distinction between the two trials: HELIOS-B enrolled fewer patients and had a somewhat lower stabilizer usage rate during the study period (53% at any point versus CARDIO-TTRansform’s 57% at baseline alone). Whether that difference in stabilizer saturation contributed to vutrisiran’s more favorable result is a question the field will be parsing when the full CARDIO-TTRansform data are presented.

What Remains Available for ATTR-CM Patients

For patients and physicians managing ATTR-CM, the failure does not change existing treatment options. Three approved, active disease-modifying therapies remain on the market.

Vyndamax (tafamidis), approved by the FDA in 2019, demonstrated in the landmark ATTR-ACT trial that it reduced mortality and cardiovascular hospitalizations compared to placebo over 30 months and remains the most widely prescribed ATTR-CM drug globally. Attruby (acoramidis), approved by the FDA in late 2024 and based on the ATTRibute-CM trial, offers an updated TTR stabilizer approach. Amvuttra (vutrisiran), approved in March 2025, provides the only RNA-targeting option now that eplontersen has missed its endpoint.

Wainua retains its existing FDA approval for the polyneuropathy form of TTR amyloidosis (ATTRv-PN) — the nerve disease that was the drug’s first indication and in which it showed convincing efficacy in the NEURO-TTRansform trial. That approval is unaffected by the CARDIO-TTRansform result. The drug will continue to be available for polyneuropathy patients in the U.S. and more than 20 countries where it has been approved.

AstraZeneca’s Cardiovascular Ambitions Take a Hit

The failure lands at an inconvenient moment for AstraZeneca. The company has built an ambitious revenue target for 2030 and had identified its cardiovascular franchise as a meaningful growth driver alongside its oncology portfolio. Eplontersen in ATTR-CM was meant to be a cornerstone of that franchise.

For Ionis, the setback eliminates the prospect of profit-sharing arrangements, commercial royalties, and future milestone payments tied to eplontersen’s ATTR-CM launch. The company’s CEO sought to frame Ionis’s broader situation positively — pointing to other pipeline programs and marketed medicines — but the financial opportunity was real, and it has been foreclosed.

Neither company announced plans to seek regulatory approval based on the top-line data, or to run a follow-on study in ATTR-CM. Pursuing approval on a failed primary endpoint “would seem like a stretch,” Stifel’s Matteis wrote. The full CARDIO-TTRansform dataset, including subgroup analyses, biomarker data, and imaging findings, will be presented at the European Society of Cardiology (ESC) Congress in August 2026. What emerges from those sessions — particularly any pattern in the monotherapy subgroup signal — will determine whether any narrower investigation is scientifically justifiable.

Does Silencing Work in the Heart? A Question for the Field

The deeper scientific question raised by CARDIO-TTRansform is not simply whether eplontersen works, but whether the TTR-silencing strategy as a class of intervention can add meaningful clinical benefit in an era when stabilizers are already standard of care.

Vutrisiran’s HELIOS-B data suggest the answer can be yes — at least in a trial where silencer-only patients constituted a meaningful proportion of the population. But the HELIOS-B population and trial design were different enough that drawing a clean conclusion about mechanism versus design is not possible today. The ATTR-CM field will be watching closely when both datasets are available for side-by-side comparison at ESC.

What cardiologist Ahmad Masri, M.D., cautioned before the results were in — that clinicians should not assume additive benefit from combining stabilizers and silencers without clinical evidence — has now been validated in the most rigorous possible way: by a 1,432-patient randomized controlled trial over 140 weeks.

Frequently Asked QuestionsWhat is ATTR-CM and who is at risk?

ATTR-CM (transthyretin-mediated amyloid cardiomyopathy) is a progressive, fatal heart disease caused by misfolded transthyretin protein that accumulates in the heart’s walls, causing them to stiffen and ultimately fail. It affects an estimated 300,000 to 500,000 people worldwide and is often misdiagnosed as ordinary heart failure. The disease comes in two forms: a hereditary version caused by mutations in the TTR gene, and a wild-type version associated with aging and particularly common in men over 70. Median survival from diagnosis is two to five years without treatment, though disease-modifying therapies have improved those outcomes substantially since tafamidis received FDA approval in 2019.

Why did Wainua fail in ATTR-CM when it succeeded in the polyneuropathy form of TTR disease?

The most likely explanation is mechanistic and clinical, not molecular. In the polyneuropathy form (ATTRv-PN), eplontersen’s TTR-silencing mechanism targets the root cause directly, and patients in the NEURO-TTRansform trial generally were not also receiving TTR stabilizers. In the CARDIO-TTRansform trial, 57% of patients were already on a stabilizer at baseline and another 24% added one during the 140-week study period. Since both stabilizers and silencers reduce the formation of new TTR amyloid — but neither removes existing amyloid deposits from the heart — adding a silencer on top of a stabilizer may not provide enough additional protection to produce measurable outcomes benefit over the 2.5-year study window. The trial’s own monotherapy subgroup, where no stabilizer was being used, did show a nominally significant 29% risk reduction — suggesting the drug has biological activity, but that this activity may be obscured when a stabilizer is already present.

What ATTR-CM treatments are still available for patients?

Three FDA-approved disease-modifying therapies remain available. Pfizer’s Vyndamax (tafamidis), the original and still most widely used treatment, is a TTR stabilizer approved in 2019. BridgeBio Pharma’s Attruby (acoramidis), a second-generation stabilizer, was approved in late 2024. Alnylam Pharmaceuticals’ Amvuttra (vutrisiran), an RNAi therapeutic that silences TTR production via a different mechanism than eplontersen, was approved in March 2025 — and is now the only approved RNA-targeting drug for ATTR-CM following eplontersen’s trial failure. A full comparison of these options should be discussed with a cardiologist familiar with ATTR-CM, as treatment choice depends on disease stage, genotype, and whether a patient has both cardiac and neurological involvement.

How is Wainua (eplontersen) mechanistically different from Amvuttra (vutrisiran), and why does that matter?

Both drugs silence TTR production in the liver, but through different molecular machinery. Eplontersen is an antisense oligonucleotide: a single-stranded DNA-like molecule that recruits a cellular enzyme called RNase H to cleave the TTR messenger RNA before the protein is made. Vutrisiran is a small interfering RNA (siRNA) that enters the RNA-Induced Silencing Complex and uses a different enzyme, Argonaute 2, to achieve the same cleavage. Both drugs are conjugated to GalNAc sugar molecules for liver-targeted delivery. Both reduce TTR production substantially. The clinical difference — if any — comes down not to the molecular mechanism but to trial design, patient population, and dosing schedule. Vutrisiran is administered quarterly; eplontersen monthly. Whether these differences are clinically meaningful, or whether the contrast in trial results reflects patient population differences, will be a major question when the full CARDIO-TTRansform data are presented at ESC Congress in August 2026.