Japan's Astellas Pharma Inc <4503.T> sued OSI Pharmaceuticals Inc to prevent the U.S. biotechnology company from blocking its $3.5 billion hostile takeover attempt with a poison pill or other defense.

The lawsuit, filed in Delaware Chancery Court, intensifies the pressure on OSI just as Astellas on Tuesday commenced its $52-per-share tender offer to the company's shareholders. An OSI spokeswoman declined to comment.

Japan's No. 2 drugmaker aims to gain access to OSI's blockbuster Tarceva cancer treatment as it faces greater generic competition to its own drugs.

The OSI bid is also the second hostile takeover attempt by Astellas Chief Executive Masafumi Nogimori in little more than a year, when he failed in a bid for U.S.-based CV Therapeutics.

Astellas said its offer for OSI, which had been rejected by the company late last month, was open until midnight New York time on March 31.

OSI shares rose 0.6 percent to $56.60, as investors were betting on a management fight that will lead to a higher offer, or a potential rival bid from the likes of Switzerland's Roche Holding AG , its partner in Tarceva.

A senior healthcare banker in London said he doubted Roche would make a counter-bid as it was very reluctant to get involved in hostile deals and would keep its rights to Tarceva even if OSI was bought. Roche has declined to comment.

Citigroup is advising Astellas and Morrison and Foerster LLP is acting as legal counsel.

Astellas shares fell 2.1 percent on Tuesday amid concerns over whether it would get into a bidding war.

A successful bid for OSI would be the fourth-largest overseas acquisition by a Japanese drugmaker after deals in recent years by leader Takeda Pharmaceutical <4502.T>, third-ranked Daiichi Sankyo <4568.T> and No. 4 Eisai <4523.T>.

Astellas said it decided to take its offer directly to OSI shareholders after the company rejected the bid last month, and that it had sought to engage OSI in talks for more than a year. The Japanese company plans to nominate directors at OSI's annual meeting.

OSI said on Monday it previously rejected the offer as too low and advised shareholders not to act. OSI Chief Executive Colin Goddard last month offered to share nonpublic information about the company with Astellas to make a case for a higher value, according to a copy of a letter made public by OSI.

(Reporting by Lewis Krauskopf; additional reporting by Quentin Webb in London; Editing by Michele Gershberg and Maureen Bavdek)