Biogen Inc. (NASDAQ: BIIB) announced today that the outcome of its large clinical trial consisting of 856 patients regarding an Alzheimer’s drug produced positive results. Together with its Japanese partner, Eisai Co., the experimental drug BAN2401 was tested over a period of 18 months.

Patients were randomly assigned to five different dose regimens. Patients who received the highest dosage, 10 mg/kg biweekly, saw a slower progression of Alzheimer’s. The results of the trial mark a monumental step in Alzheimer’s treatment, a category in which over 100 experimental drugs have failed to produce positive results. The success of BAN2401 relies on the details of the data, which will be presented at a medical meeting, according to Bloomberg Intelligence analyst, Sam Fazeli.

According to a statement from Biogen, “Further, the highest treatment dose of BAN2401 began to show statistically significant clinical benefit as measured by ADCOMS as early as 6 months, including at 12 months.”

The results of this trial exceeded investor expectations, after a December trial failed, and other recent Alzheimer’s studies also produced poor result.

This is the first late-stage anti-amyloid antibody study to successfully achieve statistically significant results at 18 months, further validating the amyloid hypothesis,” said Lynn Kramer, M.D., Chief Clinical Officer and Chief Medical Officer, Neurology Business Group, Eisai.

Following the announcement, Biogen shares soared over 16% at market opening.