Atara's stock plummets 55% following inconclusive phase 2 MS analysis

Atara Biotherapeutics’ stock plunged following an interim analysis that deemed phase 2 data for its progressive multiple sclerosis (MS) candidate to be inconclusive.

The South San Francisco-based biotech has been assessing ATA188—its cell therapy candidate for MS made of T cells that target and attack Epstein-Barr virus (EBV)-infected B cells—in a phase 2 trial dubbed EMBOLD. The therapy hinges on the theory that EBV is involved in the development of MS and can even increase risk of the disease. Recent studies have found EBV in the brains of patients with MS at higher rates than in the average population.

An interim analysis was designed to assess whether ATA188 resulted in an improvement for patients on the expanded disability status scale—a method of quantifying disability in MS—after six months of treatment compared to placebo. This could then be used to predict an improvement over 12 months of treatment.

But the verdict from the analysis was that the six-month data weren’t sufficient to draw conclusions about whether ATA188 would deliver a benefit over 12 months. The independent data and safety monitoring committee that assessed the results "believes the six-month interim endpoint may be an inaccurate measure of the potential of this intervention in this condition," the company said in a release late Tuesday afternoon. 

The markets responded accordingly. Atara’s stock had risen during the day in anticipation of the data release, opening at $7.43 per share Tuesday morning and closing at $8.66. But the stock plummeted more than 50%, sitting at $4.13 when the market opened Wednesday. 

Based on the committee's recommendation, the biotech will continue with the study without adjusting its sample size. The committee didn’t highlight any safety concerns, and Atara still anticipates sharing top-line data form the trial in October 2023.

On a subsequent conference call Tuesday night, analysts and investors appeared mostly confused as the majority of questions were answered with the same response: Atara can't disclose that information at this time. 

When asked whether the results of the interim analysis would be released, analysts received the same response.

“We're trying to protect the integrity of the placebo-controlled study and ensure that we don't introduce potential bias by disclosing publicly the interim results,” Jakob Dupont, M.D., head of Atara’s global R&D operations, said on the call.

What’s mystifying is that Atara said it enrolled enough patients, with 34 total patients at six months and 15 at the 12-month mark. However, half of those patients received placebo, leaving the biotech without enough information to confirm or deny whether ATA188 made a statistical difference.  

An Atara representative said the company's cash runway is expected to take the biotech into the fourth quarter of 2023, beyond ATA188’s anticipated readout date.

This article was updated at 1 pm ET Wednesday. A previous version of publication misquoted Dupont as saying "buyers" instead of "bias."