NEW YORK, Feb 06, 2013 (AFP) - American pharmaceuticals firm Biogen Idec is buying the stake held by Irish partner Elan in the multiple sclerosis drug Tysabri for $3.25 billion, the companies said on Wednesday.

The initial sum will be followed by future royalty payments to Elan, including a minimum 12 percent net share of all Tysabri global sales in the first year.

The firms had been splitting profits on the drug, statements from Biogen Idec and Elan said, but the US firm will now take full control and ownership of Tysabri.

"This transaction enables Elan and its shareholders to realize, upon close, a meaningful percentage of the current value of Tysabri while maintaining long term cash flow realization through the multi-tiered royalty structure of the complete asset," Elan's chief executive Kelly Martin said.

After the initial 12 months, the royalty payments to Elan would rise to 18 percent on up to $2 billion of global net sales of the drug, and to 25 percent on sales beyond the $2 billion figure, the companies said.

Formerly known as Antegren, the drug is prescribed for severe multiple sclerosis, resistant to standard treatment, but it can result in rare cases of very dangerous side effects.

A potentially fatal viral infection of the brain led to a suspension in the marketing of the drug in the United States in 2005 and 2006.