The board of Irish drugmaker Elan has unanimously rejected a takeover bid from Royalty Pharma, saying the U.S. investment company’s offer of $11.25 a share is much too low.

Royalty last week lowered its bid for Elan from an original $12 per share offer, pricing in the result of a $1 billion share buyback by Elan.

Elan chairman Robert Ingram said Monday that Royalty’s offer “grossly undervalues Elan’s current business platform and our future prospects.”

Royalty’s offer would require backing from at least 90 percent of Elan shareholders.

The buyback complicates that goal. Johnson & Johnson took advantage of the buyback to reduce its stake in Elan from 18 percent to 4.9 percent.

Some investors expect Royalty to improve its offer. Elan shares rose 3.5 percent to 9.21 euros ($12).