"Five years down the road, with some luck, we'll have an Alzheimer's drug that's getting approved," Scangos told Reuters. "I hope we can transform the treatment of MS. By that time, we will have made substantial progress on ALS and other nerve degenerative diseases, spinal muscular atrophy in kids. All that stuff is on our plate," he said.
"I am sure of two things," he added. "Not all of it is going to work, and some of it will."
Investors, excited by the aducanumab news, sent Biogen shares up 9.7 percent on Friday, and the company's stock touched an all-time high of $480.18 during the session.
The small trial showed the treatment significantly slowed cognitive impairment in patients with mild symptoms, a rare bit of good news in a field littered with high profile failures from the likes of Pfizer and Eli Lilly.
Read MoreMoment of truth for Biogen's big Alzheimer's bet
But some patients, especially those with a gene predisposing them to Alzheimer's, developed localized brain swelling. The symptom was most common among patients with the gene who were receiving the highest doses of aducanumab, leading about 1/3 of the participants in that category to discontinue the treatment. The company said the swelling was generally "asymptomatic or with mild, transient symptoms."
Biogen plans to begin a larger trial later this year. "If they can replicate the Alzheimer's data in Phase III, they could conceivably have the biggest drug on the planet," JMP Securities analyst Michael King said. As many as 75 million people are expected to develop the disease by 2030.
For Biogen, which is also working on Alzheimer's drugs that use other mechanisms of action, research on dementia drugs represents an expansion of the company's potential reach.
"If the Alzheimer's thing works, then we're not just an MS company. We are broadly focused on neuro-degenerative diseases," Scangos said.
The company is also developing a drug it hopes will address not just symptoms but the cause of MS, potentially changing the way the disease is treated.