American investment firm Royalty Pharma said it was confident that it would close the deal to acquire Irish drugmaker Elan and accomplish due diligence within 20 days.

The investment company has been planning to conduct meeting with the largest shareholders in Elan, including Johnson & Johnson as well as some investors about the deal, Royalty's chief executive Pablo Legorreta told Reuters.

Legorreta said in a statement that their buyout offer would likely be accepted as they look forward to gain access to the books for due diligence.

Royalty Pharma offered a takeover bid worth $6.6 billion after the Irish drugmaker announced that it will be selling 50% of its stake in in Tysabri, a drug used for treating multiple sclerosis, to Biogen Idec.

Elan declined Royalty's bid and was planning to return $1 billion to its shareholders after the sale.