Japanese drug maker Eisai Co. (ESALY) claims it has the right to terminate an Alzheimer's drug partnership with Pfizer Inc. (PFE) once Pfizer closes its purchase of Wyeth (WYE), Pfizer disclosed Friday.

New York-based Pfizer said in a regulatory filing it will oppose any effort by Eisai to terminate the partnership, saying Eisai has no legal basis to do so.

Eisai discovered the Alzheimer's drug Aricept, and Eisai and Pfizer co-promote Aricept in the U.S. and several other countries under a partnership dating to the mid-1990s. Pfizer recorded $482 million in Aricept sales in 2008.

"Eisai has indicated to us that, in its view, upon consummation of the Wyeth acquisition, Eisai will have the right to terminate our Aricept" alliance, Pfizer said in the regulatory filing.

Pfizer agreed in January to acquire Wyeth, Madison, N.J., in a cash-and-stock deal then valued at about $68 billion. One of the attractions for Pfizer was Wyeth's development of experimental Alzheimer's drugs, including bapineuzumab in partnership with Irish drug maker Elan Corp. (ELN). Pfizer expects the acquisition to close at the end of the third quarter or during the fourth quarter.

The basis of Eisai's claim wasn't detailed in Pfizer's regulatory filing.

An Eisai U.S. spokeswoman couldn't immediately be reached. A Pfizer spokeswoman declined to comment on the specifics of the letter from Eisai.

Pfizer shares rose 25 cents, or 1.8%, to $14.41.

-Peter Loftus; Dow Jones Newswires; 215-656-8289; peter.loftus@dowjones.com