Why Is KPTI Stock Down 69% After-Hours Today?

• The late-stage study tested selinexor as a maintenance treatment against placebo in patients with a specific form of advanced or recurrent endometrial cancer.
• Company officials said they were disappointed by the unexpected outcome and will cut planned spending on the endometrial cancer program.
• Chief Executive Richard Paulson said that the company remains focused on its myelofibrosis program.

Shares of Karyopharm Therapeutics (KPTI) fell 69% in after-hours trading on Thursday after the biotech company reported that its drug selinexor failed to delay disease progression meaningfully in endometrial cancer in a late-stage clinical trial.  

What Did The Trial Show?

The late-stage study tested selinexor as a maintenance treatment against placebo in patients with a specific form of advanced or recurrent endometrial cancer. It did not show a statistically significant delay in disease progression. A trend toward longer progression-free survival was seen in a key patient group—roughly 13 months for those on selinexor versus about 7.5 months on placebo—but the difference did not reach the required threshold for success, the company said.

Safety results matched the drug’s known profile, the company said, with no new concerns identified. Company officials said they were disappointed by the unexpected outcome and will cut planned spending on the endometrial cancer program while completing a full data review for presentation at a future medical meeting. They plan to keep following patients for longer-term survival results.

KPTI’s Steps Ahead

“While the results we are announcing today fell short of our expectations, they do not diminish our confidence in the broader potential of selinexor,” said Chief Executive Richard Paulson. He added that the company remains focused on its myelofibrosis program.

In a separate announcement the same day, Karyopharm said it plans to submit an application to the U.S. Food and Drug Administration in August seeking accelerated approval for selinexor combined with ruxolitinib as a treatment for myelofibrosis. If cleared, the combination could become the first approved dual therapy for the rare blood cancer. The company intends to request priority review, which could lead to a decision about six months after submission.

Paulson called the planned filing a potentially transformative step for patients and for the company.

Selinexor, sold as Xpovio, is already FDA-approved for treating multiple myeloma in certain adult patients. Karyopharm reported U.S. XPOVIO (selinexor) net product revenue of $114.9 million for the full year 2025, accounting for a major portion of the company’s overall revenue.  

However, earlier this year, the FDA withdrew selinexor’s accelerated approval for a type of lymphoma after Karyopharm did not complete a required confirmatory trial, reducing its total addressable market.

How Did KPTI Retail Traders React?

On Stocktwits, retail sentiment around KPTI rose from ‘bearish’ to ‘bullish’ over the past 24 hours, while message volume stayed at normal levels.

A Stocktwits user said that partnerships may help the company continue its pipeline efforts.

Another dismissed the after-hours selloff, noting that the company already has approvals in place and a remaining pipeline.

KPTI stock has fallen 5% year-to-date.

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