"The prospect of being able to offer meaningful disease-modifying therapies to individuals suffering from this terrible disease is both exciting and humbling," Biogen's chief medical officer, Alfred Sandrock, said in the release.
Eisai said it would discuss next steps with regulators, as another large trial would typically be required to validate the findings.
Shares of the Japanese company surged 19 percent, their daily limit, in Tokyo, while Biogen shares were up 18 percent in late-morning trading in New York on Friday.
Expectations for the study involving more than 800 patients were held in check after a December review failed to achieve its targets.
BAN2401 goes after what could be the most harsh version of a protein called beta amyloid, believed to be involved in the disease as its deposits start forming in the brain years ahead of symptoms emerging.