Shares of Ionis Pharmaceuticals tumbled after the company said a late-stage trial of its drug for a rare type of heart disease failed to meet its primary efficacy endpoint.

The stock sank 23%, to $65.10, in premarket trading Thursday. Through Wednesday's close, shares are up 6% year to date.

Ionis said the treatment, eplontersen, didn't provide a statistically-significant benefit to patients with transthyretin-mediated amyloid cardiomyopathy.

Transthyretin-mediated amyloid cardiomyopathy is an underdiagnosed and potentially fatal disease of the heart muscle in which a protein called transthyretin builds up in the heart, nerves and other organs.

"Although we are disappointed that the study did not meet the primary endpoint, these results have the potential to guide the treatment landscape for ATTR-CM and contribute to advancing future care for patients," Ionis Chief Executive Brett Monia said.

The company noted the drug, which was developed in partnership with AstraZeneca, was well tolerated, with a favorable safety profile consistent with previous results.

Shares of the U.K. drugmaker were also off ahead of Thursday's opening bell, down 8.2% to $173.85.

Write to Connor Hart at connor.hart@wsj.com

(END) Dow Jones Newswires

July 09, 2026 08:04 ET (12:04 GMT)

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