Biogen Inc's experimental Alzheimer's drug gave disappointing results at an international conference. Shares of the Cambridge company fell 4.34% to $391.74 yesterday after the results were declared.

Biogen presented its findings on aducanumab at the Alzheimer’s Association International Conference in Washington, DC. Aducanumab blocks formation of the protein thought to cause the brain plaques that are a hallmark of the disease.

It has even been considered as a potential breakthrough in March when Biogen presented results which showed that the drug improved cognition, particularly at the highest dose, but that it also had side-effects that included swelling of the brain.

Investors thought that the data released yesterday would show that another dose of 6 mgs would be as effective after one year as the 10-mg dose without the side-effects. But the study researchers presented that the 6-mg dose failed to significantly slow cognitive decline.

John Boris, an analyst with Suntrust Robinson Humphrey who attended the conference, said, “If they’re getting brain swelling at 10 milligrams and their 6-milligram dose is showing limited efficacy, it draws into question Biogen’s program”.

Biogen spokeswoman Catherine Falcetti said the study used only 166 patients, and said that it is not unusual for a trial with that small number of patients to not show statistical significance.

She said in a statement that what they paid much attention to was safety and tolerability which they saw at 6 milligrams.

Now it is said that Phase III trials, the results of which the company will submit to federal regulators, will entail two studies, each with 1,350 people who will be given aducanumab for 18 months.