Astellas Pharma and OSI Pharmaceuticals have come to a definitive agreement under which the Tokyo-based drugmaker will acquire OSI, a biotech company based in Melville, N.Y., in a cash transaction valued at about $4 billion. The deal follows Astellas’ agreement to increase its bid to $57.50 per share from the $52.00 per share the company floated in a hostile offer earlier this year. The new bid represents a 55% premium over the closing price of OSI’s shares on Feb. 26, the last trading day before Astellas’ first announcement. OSI, which focuses on oncology, has one drug on the market, Tarceva, codeveloped with Roche. The Swiss firm markets the product, which accounted for approximately 80% of OSI’s 2009 revenues of $428 million. With the acquisition, Astellas will add oncology to its North American operations, which include cardiology, dermatology, infectious diseases, neuroscience, transplants, and urology. OSI agreed to the higher bid after a 15-month wrangle with the Japanese firm that became public in March. “We believe today’s announcement recognizes the significant value we have built for our stockholders,” OSI CEO Colin Goddard says.