Xenon Pharmaceuticals Halts New Enrollment in Depression Drug Trials After Psychosis Cases, Stock Drops 30%

Xenon Pharmaceuticals Inc. (NASDAQ: XENE) told analysts on a conference call after the market closed Friday, September 18, 2026, that it was voluntarily pausing new patient enrollment in two late-stage depression trials after a small number of participants developed neuropsychiatric side effects, including psychosis.

The affected studies are X-NOVA2, testing the drug azetukalner in major depressive disorder, and X-CEED, testing it in bipolar depression, according to Fierce Biotech. Existing patients already enrolled remain on the studies. Only new enrollment stopped.

Christopher Kenney, Xenon's chief medical officer, described the events on the call. "What we're seeing in psychiatry, which is consistent with epilepsy, is a small percentage of patients with confusion, aphasia, ataxia," Kenney said, according to Fierce Biotech. "In a very small number of patients, we have seen an adverse event captured under the broad preferred term of psychosis."

Kenney said the events were "short in duration and reversible, without any long-term sequelae" across both the epilepsy and psychiatry programs, which together have generated more than 1,500 patient-years of safety data on azetukalner, a Kv7 potassium channel opener. He noted the epilepsy program logged just one psychosis case across its Phase 3 trials.

The pause did not show up in Xenon's earlier Phase 2 depression study, according to BiopharmGuy Dive. The company said exposing a much larger group of patients in Phase 3 may simply have surfaced effects that a smaller trial missed. CEO Ian Mortimer told analysts Xenon will use the pause to adjust dosing, which it believes will improve tolerability, and has already notified the FDA.

A depression drug triggering psychotic episodes in trial participants is a legitimate red flag, and patients and their families deserve a straight answer on how common and how severe those episodes were. Xenon has not disclosed exact case counts.

But analysts pushed back on the scale of the market reaction. William Blair analyst Myles Minter called the stock's plunge overblown, noting similar psychosis rates already appear on the labels of blockbuster antidepressants, according to BigGo Finance. TD Cowen's Joseph Thome called the sell-off "overdone," and RBC Capital Markets' Brian Abrahams wrote he "would be buyers on any weakness," estimating azetukalner could still hit $1.7 billion in annual epilepsy sales, per BiopharmGuy Dive.

Not everyone shrugged it off. Needham removed all major depressive disorder and bipolar disorder sales projections from its valuation of Xenon, and Deutsche Bank downgraded the stock to Hold, according to a PR Newswire investor notice carried by Morningstar. Seeking Alpha also downgraded XENE to Hold on September 18, pegging the company's psychiatric drug prospects as effectively worthless in the current price and estimating focal-seizure peak sales at $1 billion to $1.5 billion against a roughly $4 billion market cap and $1.2 billion in cash.

Shares fell as much as 27.6% in after-hours trading following the announcement, according to the PR Newswire notice. BigGo Finance reported a 26% drop in premarket trading. By the close of the next trading session, XENE finished down 30.7% from its pre-announcement level, per the same PR Newswire release.

That release, dated September 22, came from SueWallSt, a service tied to the securities litigation firm Levi & Korsinsky, inviting shareholders who lost money to "submit their losses for review." No lawsuit has been filed and no SEC investigation has been announced. This is a solicitation notice, not confirmation that Xenon did anything wrong.

The depression pause landed the same day Xenon filed a New Drug Application with the FDA for azetukalner in focal seizures, a form of epilepsy affecting one side of the body, according to BigGo Finance. Executives say that filing is unaffected by the psychiatric safety signal, and analysts largely agree, noting other approved epilepsy drugs carry their own serious side-effect profiles.

X-NOVA2 had enrolled about 360 of its planned 450 participants when the pause hit, roughly 80% of target, which Xenon says is enough to detect a clinically meaningful benefit, according to Fierce Biotech. Topline data from that trial are expected in the first quarter of 2027. Whether Xenon's dosing adjustments resolve the psychosis signal, and whether the FDA's ongoing epilepsy review is affected by any of it, remains unanswered. The FDA's decision timeline sits against the backdrop of a federal government shutdown that BiopharmGuy Dive reports has slowed a series of other pending drug reviews, including for Novo Nordisk and Biohaven.