Why is Ionis Pharmaceuticals stock tumbling today?

Investing.com — Ionis Pharmaceuticals shares dropped 13.1% to $50.51 in pre-market trading, as investors continued to digest the failure of a pivotal clinical trial for pelacarsen, an RNA-targeted cardiovascular drug co-developed with Novartis. On September 4, the companies disclosed that the Phase 3 Lp(a)HORIZON study, which enrolled more than 8,300 high-risk patients over a six-year period, failed to reduce the composite risk of cardiovascular death, heart attack, stroke, or urgent coronary revascularization compared with placebo.

While pelacarsen successfully lowered levels of lipoprotein(a), that biomarker improvement did not translate into a statistically meaningful reduction in hard clinical outcomes, prompting serious questions about the therapeutic hypothesis underlying the entire class of Lp(a)-lowering drugs.

The financial impact on Ionis is direct and substantial. Under the companies' 2019 licensing agreement, Ionis stood to receive tiered royalties ranging from the mid-teens to the low-20% on net sales, in addition to up to $650 million in development, regulatory, and commercial milestones — all of which are now effectively eliminated for the commercial phase.

Citi analysts acknowledged the setback but argued that Ionis's immediate downside is limited given its broader pipeline. Another analyst maintained a Buy rating with a $100 price target, characterizing the Lp(a)HORIZON miss as a contained blow rather than a company-defining failure.

The broader market offered little support for Ionis on the day, with the S&P 500 slipping 0.2% and the Dow Jones declining 0.7%, while the Nasdaq was only marginally higher at 0.1% — a backdrop that provided no meaningful offset to the company-specific selling pressure.

The pelacarsen failure also compounds a difficult stretch for Ionis, which had previously absorbed a sharp sell-off in July 2026 after eplontersen failed a separate cardiovascular Phase 3 trial, leaving the stock well below its 52-week high of $86.74.

Together, the loss of pelacarsen's commercial revenue potential, lingering uncertainty from the earlier eplontersen setback, and cautious broader markets have combined to push Ionis shares toward their 52-week low of $50.01 in pre-market trading. Still, the company's recently approved ZANVASTRO and an upcoming PDUFA catalyst for zilganersen on September 22 offer longer-term pipeline support.

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