Biogen Idec and Elan Corporation on Monday moved to resuscitate their withdrawn multiple sclerosis drug Tysabri, as the two drugmakers submitted safety reviews to US regulators asking for approval to sell it again.

Biogen Idec, the US group, and Ireland’s Elan asked for a priority review of six months for their approval submission to the Food and Drug Administration. The application included two-year trial data, a new safety assessment and a plan to monitor potential risks.