Xenon Stock Craters on Drug Trial Setback. Its Failure Could Be This Rival’s Gain.

Xenon Pharmaceuticals said it had submitted a New Drug Application for its lead molecule, azetukalner, in focal seizures while pausing trials for major depressive disorder and bipolar depression. (Photograph by Louis Reed)

What should have been a milestone for Xenon Pharmaceuticals was quickly derailed by a clinical setback—and one that could prove a windfall for a similarly pressured rival.

Xenon said late Thursday that it had submitted a New Drug Application for azetukalner in focal seizures, a type of seizure affecting only one side of the brain and body. The development brings the biopharmaceutical company one step closer to its first commercial product.

But what should have been a celebratory moment was overshadowed by Xenon’s announcement that it had paused new patient enrollment in ongoing studies for major depressive disorder and bipolar depression.

The decision follows reports of “neuropsychiatric adverse events” in a larger psychiatric dataset, including dizziness, sleepiness, confusion, and psychosis. While these events aligned with azetukalner’s known safety and tolerability profile, they hadn’t appeared in earlier clinical testing, Xenon said.

The drugmaker described the enrollment pause as a likely temporary, “precautionary measure,” saying it was looking at potential dosing modifications to resolve issues.

Still, shares cratered 23% to $44 in premarket trading Friday, deepening a slump that began in after-hours trading Thursday. If these losses hold, Xenon will suffer its largest single-day drop since March 2017, when the stock plunged a record 53%.

Wall Street was split on the news. Deutsche Bank’s David Hoang cut his rating on Xenon shares to Hold from Buy, arguing the update weakens the drug’s case as a leading treatment for focal epilepsy. Wells Fargo’s Benjamin Burnett disagreed, reiterating an Overweight rating on the stock while noting he sees no negative read-through for focal onset seizures.

One clear takeaway has emerged from the uncertainty: This stumble could work in favor of Biohaven , a rival developer of epilepsy treatments. Last week, the biotech confirmed the Food and Drug Administration had placed a partial clinical hold on its lead candidate, opakalim.

Unlike Xenon’s voluntarily decision to halt enrollment, regulators forced the trial pause due to “insufficient information” about a metabolite found during rodent testing. It is unclear how the small molecule affects human subjects, and Biohaven has pledged to submit additional data.

While the news sent shares tumbling into double digits at the time, Citi analyst Samantha Semenkow now sees a favorable setup for Biohaven in light of Xenon’s setback. While Semenkow believes Xenon’s epilepsy franchise remains unaffected, she expects the news to sharpen investor focus on safety differentiation between opakalim and azetukalner.

Both opakalim and azetukalner belong to a class of drugs known as KV7 channel activators, which open potassium channels in the body to calm overactive nerve and muscle cells. Semenkow believes the news “could leave Biohaven favorably positioned.”

Investors seem hesitant now to buy into either story—whether Biohaven can actually benefit or Xenon can limit the damage. Biohaven stock traded slightly lower on Friday while Xenon paced toward its lowest close in six months.

Making the case for Xenon’s lead drug may be the toughest sell of all. Needham analyst Serge Belanger cast doubt on azetukalner’s psychiatric potential, warning that adjusting the dose to curb side effects might compromise its efficacy.

Belanger has stripped depression sales from his financial model and lowered his price target on the shares to $60 from $78 to account solely for the drug’s prospects in epilepsy.

Write to Mackenzie Tatananni at mackenzie.tatananni@barrons.com

Copyright ©2026 Dow Jones & Company, Inc. All Rights Reserved. 87990cbe856818d5eddac44c7b1cdeb8