The clinical trial for an Alzheimer’s treatment from drugmakers Biogen and Eisai was “extremely encouraging” — but more still needs to be done, Biogen Chairman Stelios Papadopoulos told CNBC on Friday.

The companies announced positive results on Thursday from a Phase II study with BAN2401, an anti-amyloid beta protofibril antibody, in 856 patients with early Alzheimer's disease.

The trial demonstrated for the first time that there is an antibody that can clear plaque in the brain, Papadopoulos said on “Closing Bell.”

“More importantly, what this trial showed is that the clearance of plaque was associated with a slowdown of the cognitive decline,” he added, noting it is the first time a rigorous trial has shown that.

However, he pointed out, “It’s not as if we improved the condition or we stabilized even the cognitive decline. We’ve slowed down the cognitive decline.”

“It’s an extremely encouraging first step but it’s only a first step. We need to do a lot more,” said Papadopoulos.