Axsome Therapeutics said on Monday it would seek marketing approval next year for its oral drug to treat agitation in patients with Alzheimer's disease, after it succeeded in three out of four late-stage studies.

But shares of the New York-based drug developer fell about 8% before the bell, as the treatment, AXS-05, did not show statistically significant results compared to placebo in one of the studies.

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The company tested the drug in more than 1,000 patients with Alzheimer's-related agitation, a symptom that causes emotional distress as well as verbal and physical aggressiveness.

The symptom is reported by up to 70% of patients with the mind-wasting disease, according to the Alzheimer's Association.

The treatment significantly delayed the time to relapse of agitation as measured on a disease severity scale in one study comprising 295 patients and reduced worsening of other symptoms compared to placebo, Axsome said on Monday.

In a trial called ADVANCE-2, the drug did not show statistical significance in delaying relapse time, but the company said results "numerically favored" AXS-05 over placebo on all main and secondary goals.

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The miss will be viewed as "falling short of a very solid efficacy package," said Needham analyst Ami Fadia.

Axsome said AXS-05 was safe across trials, including in a long-term study where patients received the drug for up to 12 months.

The drug modulates the function of certain neurotransmitters that regulate cognitive and behavioral symptoms such as agitation and aggression.

Last year, the U.S. Food and Drug Administration approved Otsuka Pharma's Rexulti, making it the first drug to treat agitation symptoms in dementia patients.

Some prior attempts — such as by Otsuka and BioXcel Therapeutics — to develop an Alzheimer's-related agitation treatment have been unsuccessful or faced issues.