Ionis Pharmaceuticals (IONS) Stock Plummets 20% Following Failed Heart Disease Trial

Key Takeaways

Table of Contents

Shares of Ionis Pharmaceuticals plummeted nearly 20% following disappointing results from the CARDIO-TTRansform Phase 3 clinical trial for eplontersen
Collaboration partner AstraZeneca saw its stock decline approximately 9% following the announcement
The experimental therapy demonstrated no effectiveness in patients already receiving stabilizer medications, who represented most trial participants
A subset of patients not taking stabilizers experienced a 29% reduction in risk, providing limited encouraging data
Competing pharmaceutical companies Alnylam and BridgeBio saw their shares jump over 10% as a significant competitor exits the ATTR-CM market

Shares of Ionis Pharmaceuticals (IONS) tumbled approximately 17-20% Thursday following disappointing news from the company and its partner AstraZeneca (AZN) that their experimental drug eplontersen missed its primary goal in the Phase 3 CARDIO-TTRansform clinical study.

Ionis Pharmaceuticals, Inc., IONS

The clinical study evaluated eplontersen’s ability to lower the risk of cardiac mortality and recurring cardiovascular complications in individuals with transthyretin-mediated amyloid cardiomyopathy (ATTR-CM) versus placebo across a 140-week period. The drug failed to demonstrate this benefit.

IONS shares were down approximately 17.59% Thursday. This extends the stock’s year-to-date decline to 11.78%, although shares remain elevated roughly 99% over the trailing twelve months.

American-listed shares of AstraZeneca dropped about 9% following the identical announcement, highlighting the significant stakes both pharmaceutical firms had in this trial’s success.

The study recruited participants where 57% were already receiving stabilizer therapy at baseline, with an additional 24% initiating such treatment during the trial. Within this patient population, eplontersen failed to show any therapeutic benefit. This represents a significant challenge since stabilizer medications have become standard practice in current clinical care.

Positive Results in Limited Population — Questions Remain

The data contained one bright spot. Among participants not receiving stabilizer drugs, eplontersen demonstrated a 29% decrease in the composite risk of cardiac death and recurring complications. The therapeutic candidate also achieved robust, lasting decreases in transthyretin protein concentrations and successfully met various secondary measures.

However, Wall Street analysts remained skeptical about the partial success providing a viable development pathway. Stifel’s Paul Matteis noted that “trying to approach regulators here on these data would seem like a stretch.” Neither Ionis nor AstraZeneca disclosed intentions to pursue regulatory authorization or initiate an additional clinical study.

Chief Executive Brett Monia expressed disappointment, highlighting the evolving therapeutic environment where increasing numbers of patients begin stabilizer therapy prior to trial enrollment. Complete study results will be unveiled at the European Society of Cardiology Congress in August 2026.

For Ionis particularly, the unsuccessful trial eliminates anticipated profit-sharing arrangements, royalty payments, and additional milestone compensation linked to eplontersen’s commercial success in cardiovascular disease.

Competitor Stocks Rally as Market Competition Narrows

Alnylam Pharmaceuticals and BridgeBio both experienced double-digit percentage gains Thursday. With eplontersen’s prospects in ATTR-CM now uncertain, Alnylam’s Amvuttra appears positioned to maintain its standing as the sole RNA-silencing treatment in this therapeutic area.

Stifel’s Matteis characterized the development as “a huge positive” for Amvuttra. Pfizer’s Vyndamax, currently the dominant product with sales exceeding $6 billion annually, also benefits from reduced competition.

Jefferies analyst Michael Leuchten suggested the greater concern for AstraZeneca extends beyond lost revenue — it’s the damage to reputation. The pharmaceutical giant had approached the data readout with substantial optimism. “AstraZeneca is meant to be able to have exceptionally good trial design ability,” he observed.

Street consensus on IONS before Thursday stood at Strong Buy, featuring an average price objective of $104.61. These recommendations will likely undergo revision.

Trading activity in IONS reached approximately 3 million shares Thursday, significantly exceeding the three-month daily mean of 1.86 million.

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