RNA Drugs: Has the "Golden Narrative" Collapsed?

A blockbuster RNA drug that once sent Alnylam's market capitalization soaring by billions of dollars in a single day is now undergoing a belated stress test.

Recently, the CARDIO-TTRansform study published in the New England Journal of Medicine showed that the ASO drug eplontersen (brand name Waiuna), developed by AstraZeneca and Ionis, failed to significantly reduce the risk of cardiovascular events or death in patients with transthyretin-mediated cardiomyopathy (ATTR-CM) on top of standard of care.

Following the announcement, Alnylam's stock fell as much as 5%, bringing its cumulative decline since the data were first disclosed to approximately 30%. AstraZeneca and Ionis shares fell about 1% and 2%, respectively.

The market's sharp reaction was not simply due to a negative trial, but because a deeper question has surfaced: When standard of care is already robust enough, how much incremental value can RNA silencing agents still demonstrate?

Chapter 1: Two Types of "Silencing," One Common Pathway

ATTR-CM is a fatal heart disease caused by misfolding of transthyretin (TTR), leading to the formation of amyloid fibrils that deposit in cardiac tissue.

TTR is synthesized by the liver and normally circulates in the blood as a tetramer, transporting thyroxine and retinol-binding protein. When tetramers dissociate into monomers, the monomers misfold and aggregate, eventually depositing as amyloid fibrils in the myocardial interstitium, causing ventricular wall thickening, diastolic dysfunction, and progressive heart failure.

ATTR-CM is divided into hereditary (caused by TTR gene mutations) and wild-type (associated with aging, no gene mutation). Wild-type ATTR-CM is far more common in the elderly population than previously recognized, and was long underdiagnosed due to diagnostic difficulties.

For a long time, the mainstay of clinical treatment has been drugs that stabilize TTR tetramers to slow disease progression.

Pfizer's Vyndamax (tafamidis) was the first disease-modifying therapy approved for ATTR-CM. Its pivotal ATTR-ACT study enrolled 441 patients and showed that tafamidis reduced all-cause mortality risk by 30% and cardiovascular-related hospitalization risk by 32%.

BridgeBio's Attruby (acoramidis) was approved in 2024. Its Phase III ATTRibute-CM study showed a significant improvement in the composite endpoint of all-cause mortality or cardiovascular hospitalization at 30 months, with serum TTR levels stabilized early after treatment.

These "stabilizers" do not reduce TTR production; instead, they bind to the thyroxine-binding site on the tetramer, enhancing tetramer stability and reducing dissociation into pathogenic monomers.

Alnylam's Amvuttra (vutrisiran) and Ionis/AstraZeneca's Waiuna (eplontersen) take a different approach: using nucleic acid drugs to silence TTR mRNA, reducing TTR protein synthesis at the source.

Vutrisiran is a small interfering RNA (siRNA) that guides target mRNA degradation through RNA interference mechanisms. Eplontersen is an antisense oligonucleotide (ASO) that cleaves target mRNA through an RNase H-dependent mechanism. Though they belong to different technology platforms, both employ GalNAc conjugation for liver-targeted delivery, and both can reduce serum TTR levels by more than 80%. Vutrisiran is administered subcutaneously once every three months, while eplontersen is given monthly. Both have been approved for hereditary ATTR polyneuropathy.

In theory, stabilizers and silencers have complementary mechanisms. Stabilizers "guard" existing TTR tetramers, while silencers "shut down" TTR production. The combination is thought to more thoroughly block amyloid formation — a logic that was preliminarily validated in the 2024 HELIOS-B study.

HELIOS-B was a Phase III randomized double-blind trial in ATTR-CM patients, enrolling 655 patients randomized to receive vutrisiran or placebo, with the primary endpoint being a composite of all-cause mortality and recurrent cardiovascular events.

The results showed that vutrisiran reduced the risk of the primary endpoint by 28% in the overall population and by 33% in the monotherapy subgroup (patients not also on a stabilizer), and reduced all-cause mortality risk by 36% in the overall population. These results directly drove FDA approval of vutrisiran for ATTR-CM in 2025, and sent Alnylam's market capitalization soaring by billions of dollars on the day the data were announced.

Chapter 2: Two Trials, Two "Eras"

However, the negative results from CARDIO-TTRansform have poured cold water on this narrative.

This Phase III trial had a design highly comparable to HELIOS-B: double-blind, randomized, placebo-controlled, with the same primary endpoint of cardiovascular death and recurrent cardiovascular events — but on a larger scale. The trial enrolled more than 1,400 ATTR-CM patients already on standard of care, with follow-up extending to 140 weeks, far exceeding HELIOS-B in both sample size and observation duration.

With this level of statistical power, the results showed that 29.4% of patients in the eplontersen group experienced the primary endpoint event, compared to 32.2% in the placebo group — an absolute risk difference of only 3 percentage points, with a hazard ratio of 0.89 that did not reach statistical significance. Key secondary endpoints, including cardiovascular death, heart failure hospitalization, and stroke/TIA, also did not show consistent benefit trends.

The most striking difference lies in the baseline characteristics. In the HELIOS-B study, only 53% of enrolled patients were on concomitant stabilizers; in the CARDIO-TTRansform study, that proportion was 81%.

The patients recruited in these two trials actually belong to two different treatment eras. When designing CARDIO-TTRansform, AstraZeneca and Ionis deliberately enrolled more patients aligned with contemporary clinical practice — where stabilizers are widely used, heart failure management is more aggressive, and disease is diagnosed earlier.

This was a prudent attempt to make the evidence more reflective of real-world practice, but it also significantly compressed the incremental value space for the silencer. In the New England Journal of Medicine, the researchers listed several possible reasons for eplontersen's failure, including that heart failure management "evolved" during the trial and that the use of stabilizers and other therapies "increased over time."

The mechanism of action of silencers is to reduce de novo TTR synthesis and thereby limit new amyloid deposition. If a patient is already on a stabilizer, the production of toxic monomers has already been significantly reduced; if the disease has progressed to an advanced stage with substantial irreversible amyloid deposits in the heart, the marginal benefit of further lowering TTR levels diminishes.

In other words, the value of silencers is highly dependent on the magnitude of "residual risk" — the stronger the background therapy, the smaller the residual risk, and the narrower the space for silencers to demonstrate incremental benefit.

This does not mean that silencers have no future in ATTR-CM. A more likely scenario is that these drugs will shift toward specific subpopulations: those who cannot tolerate stabilizers, those who respond poorly to stabilizers, those in early-stage but rapidly progressing disease, and carriers of specific genetic mutations in hereditary ATTR-CM.

Alnylam has already attempted to delineate these populations in post-hoc subgroup analyses from HELIOS-B, but post-hoc analyses offer limited evidence to support regulatory decisions. What can truly answer this question are prospectively designed studies.

Chapter 3: Beyond Placebo

If we zoom out from a single trial to the broader competitive landscape of TTR cardiomyopathy, the negative CARDIO-TTRansform result reveals an even more complex picture. The RNA silencers now face a market very different from a decade ago, when stabilizers were the only option. It is now a crowded field with multiple technology platforms advancing in parallel and background therapy continuously strengthening.

The most upstream challenge comes from gene editing. Intellia Therapeutics' NTLA-2001 is the world's first in vivo CRISPR/Cas9 therapy to enter pivotal clinical trials. It uses lipid nanoparticles to deliver Cas9 mRNA and guide RNA, directly knocking out the TTR gene in the patient's liver.

Early data show that a single dose reduces serum TTR levels by over 90%, with effects lasting at least one year. Once approved, this "one-time treatment, lifetime benefit" model would fundamentally change the treatment logic for ATTR. Although gene editing still faces multiple challenges — long-term safety, delivery efficiency, pricing — its potential to dilute the commercial value of existing nucleic acid drugs should not be underestimated.

Alnylam itself is also accelerating iteration. Vutrisiran is dosed once every three months, and the company has already lined up a next-generation TTR siRNA aimed at extending the dosing interval to once every six months or even once a year. But in the context of external skepticism about HELIOS-B evidence, such internal competition may also raise more questions in the market about the long-term value of the current-generation product.

The stabilizer camp has not stood still either. Tafamidis faces generic competition in multiple markets after patent expiration, and price declines may actually increase its accessibility, further raising stabilizer coverage as background therapy. Acoramidis is actively building real-world evidence post-approval, aiming to demonstrate that stabilizer monotherapy is sufficient for most patients. As background therapy becomes stronger and cheaper, the incremental value of silencers will be further compressed.

A number of Chinese biotech companies have also advanced TTR-targeting nucleic acid drugs into clinical stages.

Biosciences' TTR-targeting siRNA candidate BW-50218 has entered Phase I in Australia. Shenogen Biopharma is leveraging its GalNAc liver delivery platform for RNAi therapies, and in February 2026, entered into a global exclusive licensing agreement with Roche's Genentech for an RNAi drug. Additionally, Beite Pharmaceutical's TTR-targeting siRNA BPR-30221616 has received clinical approval in China for the treatment of ATTR-CM.

Unlike U.S. companies that pursue global first-in-class and premium pricing, Chinese companies tend to favor fast-follow and cost-control strategies. Once these products are validated and launched in China, they will not only create price competition within China's large patient pool, but may also enter price-sensitive emerging markets such as Southeast Asia, the Middle East, and Latin America through licensing or export.

The story of TTR cardiomyopathy is a microcosm of RNA drugs moving from a "disruptor" narrative to that of a "combination therapy tool." It does not negate the scientific value of RNA silencing mechanisms, but it reveals a harsher commercial reality: in a rapidly evolving treatment landscape, a single trial rarely defines the fate of a class of drugs, but it can certainly recalibrate market value expectations. And the influx of global competitors makes this value recalibration even more dramatic.

References:

1. AstraZeneca, Ionis data spark debate about RNA drugs' impact on deadly heart disease; BioPharma Dive

2. AstraZeneca-partnered small nucleic acid new drug fails Phase III; DXY Insight Database