UPDATE 2-Astellas extends tender offer for OSI to Apr 23

* Astellas: extension to give OSI shareholders more time

* Offer terms not changed

* Astellas shares rise 0.5 pct; OSI falls 0.2 pct

(Adds OSI shares, OSI declining to comment)

TOKYO, March 31 (Reuters) - Japan's Astellas Pharma Inc (4503.T) said it has extended its $3.5 billion tender offer for OSI Pharmaceuticals Inc OSIP.O to April 23 from the previous deadline of March 31.

The offer terms, including a $52-per-share offer, have not been changed, Japan's second-biggest drugs firm said, adding that the extension was to give OSI shareholders more time to consider and then apply for the offer.

OSI shares have traded well above the $52-per-share offer as investors expect either a higher price or another suitor.

By March 30, OSI shareholders had tendered 37,858 shares, Astellas said. It said in early March that OSI had around 58 million shares outstanding.

A spokeswoman for OSI declined to comment.

The announcement came two days after the Japanese firm said it would gain a partial look at the financial records of OSI. [ID:nSGE6200CW]

Under a confidentiality agreement, OSI will provide Astellas with access to certain nonpublic information. In return, Astellas will not acquire any shares of OSI under its tender offer until May 15.

OSI shares were down 0.2 percent at $59.92 in morning trading on Nasdaq.

Astellas shares earlier rose 0.5 percent while the benchmark Nikkei 225 average .N225 lost 0.1 percent. (Additional reporting by Lewis Krauskopf in New York, editing by Dave Zimmerman)