Irish drug company Elan put itself up for sale on Friday, seeking to fend off a hostile bid by Royalty Pharma that shareholders will consider next week.

Elan urged its shareholders not to tender into Royalty's current offer, worth a potential $8 billion, but said the U.S. company would be able to participate in the formal sale process if it so wishes.

"Elan Corporation ... today announced that it is proceeding with a formal sale process in light of the expressions of interest received to date," the company said in a statement.

A source with direct knowledge of the situation said earlier this week that Elan had attracted the interest of a number of mid-sized drug companies and a cash offer of $15.50 per share could be enough to secure its support for a bid.

Royalty's current bid offers $13 in cash per share and added a clause known as a contingent value right (CVR) that could add a further $2.50 per share if blockbuster drug Tysabri hits certain sales milestones.

Seeking to stop Elan pushing through two sets of defensive acquisitions at a meeting of shareholders due to be held on Monday, Royalty last month made its bid subject to them rejecting all resolutions presented at the meeting.

Elan's shares were trading at 10.15 euros ($13.50) at 9:57 a.m. ET on Friday, up 9 percent on the day.