TOKYO, July 11 (Reuters) - The Eisai executive who helped garner full U.S. approval for the Japanese drugmaker's Alzheimer's disease treatment Leqembi will retire at the end of this month, the company said on Tuesday.

Ivan Cheung, aged 46, the global head of the company's Alzheimer's portfolio and chief executive of its U.S. unit, will retire as of July 31, Eisai said in a release.

Keisuke Naito, the son of Eisai Chief Executive Haruo Naito, took over as acting head of the Alzheimer's division as of July 10.

An Eisai spokesperson said that Cheung had met his primary mission in the approval of Leqembi, but will continue to offer some support after his departure.

Cheung had worked at Eisai in a variety of roles since 2005, according to his LinkedIn profile.

Leqembi, co-developed with U.S.-based Biogen, won standard approval from the U.S. Food and Drug Administration (FDA) on July 6. But the FDA placed its strongest "boxed" safety warning on the drug's label, flagging the risk of potentially dangerous brain swelling in it and similar drugs. (Reporting by Shinichi Uchida and Rocky Swift in Tokyo, Julie Steenhuysen in Chicago; Editing by Christina Fincher and Shailesh Kuber)