June 23 (Reuters) - Japanese company Eisai Co (4523.T) and partner Biogen Inc (BIIB.O) said on Wednesday the U.S. Food and Drug Administration had granted breakthrough therapy designation to their experimental therapy, lecanemab, for patients with early Alzheimer"s. The drug works in a similar manner to Biogen"s Aduhelm, which was approved earlier this month. It removes sticky deposits of a protein called amyloid beta from the brains of patients in the earlier stages of Alzheimer"s in order to stave off its impact, including memory loss and the inability to take care of oneself. Eisai completed enrollment in March in a pivotal 18-month study of lecanemab in symptomatic early Alzheimer"s patients. Lecanemab is also being studied in a large trial on people with evidence of amyloid in their brains, who do not yet have symptoms of the disease. The FDA grants breakthrough therapy designation in order to expedite the development and review of medicines for serious or life-threatening conditions.