Astellas Pharma, Japan's No.2 drugmaker, seeks to buy at least 90% of shares in U.S. drugmaker OSI Pharmaceuticals with its tender offer, the company's chief executive said on Monday.

"As for how many shares we may obtain, we will carry out the tender offer, targeting 90% or more," Astellas president Masafumi Nogimori told a news conference.

Astellas, Japan's No.2 drugmaker, agreed to buy the U.S. biotech for $4 billion in cash in a sweetened bid that allows it to add OSI's blockbuster cancer drug Tarceva to its line-up.