TOKYO ( TheStreet) -- Astellas Pharma , the No. 2 drugmaker in Japan, reached an agreement to acquire OSI Pharmaceuticals ( OSIP) for $4 billion. Astellas increased its offer for OSI to $57.50 a share, up from $52 the Japanese company had offered in March. The offer is a premium of 55% to the $37.02 closing price of OSI on Feb. 26, the last trading day Astellas announced its bid. OSI shares closed Friday at $59.80. OSI's drug line-up includes the blockbuster cancer drug Tarceva, which had $1.2 billion in sales in 2009. "The merger with OSI provides Astellas with a top-tier oncology platform in the U.S. and an expanded product portfolio and pipelines," said Masafumi Nogimori, president and CEO of Astellas in a statement Monday. -- Reported by Joseph Woelfel in New York. Follow TheStreet.com on Twitter and become a fan on Facebook.