March 6 (Reuters) - U.S. investment firm Royalty Pharma stood by its offer on Wednesday to acquire Irish drugmaker Elan, which has rejected the approach, saying it was ready to move quickly and that it could complete due diligence in 20 days.
It also said it was meeting with Elan shareholders.
Royalty Pharma made a $6.6 billion approach last week after Elan announced last month a $3.2 billion plan to sell its 50 percent stake in multiple sclerosis drug Tysabri to its U.S. partner Biogen Idec.
Elan rejected Royalty Pharma and on Monday sweetened its offer to shareholders, saying that it would give them 20 percent of future royalties from the drug. It had already planned to return $1 billion to shareholders after the drug sale.