Amylyx Pharmaceuticals’ drug has become a rallying cause for patients with the deadly neurodegenerative disease ALS, their families and members of Congress who have joined in pushing the FDA to greenlight the drug.

But regulators said in a review that the company’s small study was “unconvincing,” due to missing data, errors in patient recruitment and other issues. On Wednesday, a panel of FDA advisers will hold a nonbinding vote on whether the drug merits approval.

The meeting is being closely watched as an indicator of the FDA’s approach to investigational drugs with imperfect data and its ability to withstand outside pressure.

The FDA’s negative stance on the drug creates a tense scenario at Wednesday’s town hall meeting, where several dozen ALS patients and advocates are scheduled to speak. The agency will consider feedback from its advisory committee before making a final decision on the drug, expected by June.

ALS, or amyotrophic lateral sclerosis, destroys the nerve cells needed to walk, talk, swallow and, eventually, breathe. There is no cure and most people die within three to five years.

Amylyx’s drug is a combination of two older drugs: a prescription drug for liver disorders and a dietary supplement associated with traditional Chinese medicine. Amylyx has patented the combination and claims the chemicals help protect cells from early death.

But FDA reviewers found the drug had “only a modest effect” in slowing disease progression in a mid-term study of 137 patients, who the FDA said was plagued to problems of implementation and analysis. Typically, FDA approval requires two large studies or one study with a “very persuasive” effect on survival.

According to the FDA document, regulators had “strongly suggested” that Amylyx complete a large, late-stage trial to establish the drug’s effectiveness before rolling it out. This study should be completed in 2024.

After further discussion, Amylyx decided to submit its pill for approval based on survival data collected after its initial study. The company said this showed patients taking the drug lived about six months longer than those taking a dummy pill.

But the FDA says the data is unreliable because of problems tracking study participants for several years.

Wednesday’s meeting marks the first time the FDA has convened this advisory group since ignoring its advice last June and approving Biogen’s advice. Alzheimer’s disease drug, Aduhelm. Three panel members resigned over the controversial decision, and congressional investigators have launched an investigation into the FDA’s handling of the drug’s review. As in the current case, the FDA faced intense pressure from patient groups to grant approval.

Currently, there are only two FDA-approved drugs for ALS, the most effective of which prolongs life by several months.

The ALS Association, which helped fund the Amylyx research, said the FDA review does not take into account “the rapidity and severity of ALS and the few treatment options available.”

“It’s a uniformly fatal disease, so any drug that has that benefit – and even if that benefit is modest – is fantastic for patients, who have only one way to go with this disease: death. “said Dr. Jeffrey Rothstein of Johns Hopkins University, who helped enroll patients in the Amylyx study.

Shares of the Cambridge, Mass.-based drugmaker fell more than 50% in morning trading after the FDA review.

The Associated Press Health and Science Department is supported by the Howard Hughes Medical Institute Department of Science Education. The AP is solely responsible for all content.