More details are being released about Japanese drugmaker Eisai’s previously announced five years of job cuts totaling 900 positions. Eisai intends to slice 20% of its work force, or 600 jobs, in the U.S., by April 1.

The cuts are more immediate than Eisai hinted at when announcing its five-year plan. Citing the loss of patent protection on its lead drug, the Alzheimer's treatment Aricept, the company predicted flat sales over the next several years. To handle that lack of growth, Eisai said it would pare back its payroll by 900 positions by 2015. Cuts would be spread over Japanese, U.S. and European operations, it said.

The U.S. cuts are part of the 900-person cutback, a spokeswoman told Bloomberg. Two hundred European positions will go, as will 100 sales jobs in Japan.

"This restructuring is essential to our remaining competitive in this rapidly changing environment," Lonnel Coats, president and CEO of Eisai's U.S. unit, said in a statement. "It will enable us to continually make the necessary investments in our science, our people and ultimately our human health care mission."

One bit of good news for the U.S. is that no primary offices or facilities will be closed, the company said. Eisai's U.S. operations include R&D, manufacturing, sales and marketing and administrative functions, according to a company statement.