MELVILLE. N.Y. ( TheStreet) -- OSI Pharmaceuticals ( OSIP) shares soared almost 52% Monday after it said it would review an unsolicited takeover proposal from Japanese drugmaker Astellas Pharma with its advisers.

But the chances of OSI accepting the bid are low as it rejected a similar oral proposal from Astellas in February.

Astellas on Monday launched a hostile tender offer of $3.5 billion for OSI, the maker of the lung cancer drug Tarceva. Astellas' offer of $52 a share was a 40% premium over OSI's closing price Friday of $37.02.

The tender offer is expected to begin Tuesday.

OSI shares gained $19.23 on Monday to close at $56.25, in anticipation that OSI could receive a higher bid.

Astellas said its hostile tender offer for OSI comes after numerous previous attempts to buy the U.S. drug company were turned away.

In a press release Monday, following news of the Astellas bid, OSI released a copy of a letter dated Feb. 22 in which it rejected an oral proposal of $52 a share from Astellas, saying it wasn't "interested in undertaking a sale of OSI at that price, since it believes Astellas' proposal very significantly undervalues the company."

In responding to the February proposal, OSI said it offered to provide Astellas with non-public information which it said was fundamental to its valuation of OSI. The response to that letter was Astellas' unsolicited proposal, OSI said.

-- Reported by Joseph Woelfel in New York.

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