U.S. investment company Royalty Pharma is urged by Ireland's Takeover Panel to make a firm offer for Irish drugmaker Elan by May 10.

Elan rejected the indicated approach, worth $11 per Elan share, made by Royalty, but said it is open to the dealine set by the panel.

Royalty expressed disappointment to the refusal of Elan's board to engage with the offer being presented to shareholders. It also said in a statement that the board could deprive Elan shareholders of the chance to conclude on the merits of the possible offer.

In a released statement, the panel announced the a deadline following correspondence with the advisors to both parties.

Royalty has been hoping to add its rights for Tysabri, a drug for multiple sclerosis treatment, to its royalty income.